Faculty of Engineering and the Built Environment
School of Chemical and Metallurgical Engineering
IMPROVING LOCAL CLASS A TO OIL WELL
CEMENT USING PET PLASTIC WASTE
MSc (50/50) Research Report
Prepared by:
Name: Msizi Mkhize
Student Number: 1475631
Submitted to:
School of Chemical and Metallurgical Engineering, Faculty of Engineering and
the Built Environment, University of the Witwatersrand,
Johannesburg, South Africa.
Supervisor: Prof Diakanua B. Nkazi
Prof Sunny E. Iyuke
September 2022
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Table of Contents
ABSTRACT ................................................................................................................................... 3
1. INTRODUCTION ................................................................................................................. 4
Problem Statement:.................................................................................................................. 7
Aims and Objectives:............................................................................................................... 8
2. LITERATURE REVIEW ..................................................................................................... 9
2.1. Fundamentals in Sustainable Manufacturing ............................................................... 9
2.1.1. Processing Routes ..................................................................................................... 9
2.1.2. Selection of Raw Materials ..................................................................................... 11
2.1.3. Raw Materials ......................................................................................................... 12
2.1.4. Proportioning .......................................................................................................... 13
2.1.5. Kiln Fuels Selection ................................................................................................ 17
2.2. Process Variables Management ..................................................................................... 21
2.2.1. Temperatures and Heat of Reactions ...................................................................... 22
2.2.2. Clinker Quality Parameters ..................................................................................... 25
2.3. Energy Efficient Processing ........................................................................................... 26
2.4. Additives Management .................................................................................................. 27
2.4.1. Accelerators ............................................................................................................ 29
2.4.2. Retarders ................................................................................................................. 31
2.5. Proposed Sustainable Production Processes .................................................................. 34
3. EXPERIMENTAL PROCEDURE .................................................................................... 39
3.1. Materials ......................................................................................................................... 39
3.2. PET Additives Preparations ........................................................................................... 40
3.2.1. Additive I- Pure PET Fibres ................................................................................... 40
3.2.2. Additive II-Irradiated PET Fibres ........................................................................... 41
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3.2.3. Additive III- Depolymerized PET Plastic Powder Additive................................... 41
3.3. Oil Well Cement Slurry Formulations and Sample Preparations .................................. 43
3.3.1. Slurry Base Design ................................................................................................. 44
3.3.2. Preparations of Sample Slurries/Specimens ........................................................... 45
3.4. Assessing PET Additives ............................................................................................... 46
3.4.1. Failure Parameter Tests........................................................................................... 46
3.4.2. Rheological Property Tests ..................................................................................... 47
4. RESULTS AND DISCUSSIONS ........................................................................................ 50
4.1. Local Class A Cement Composition .............................................................................. 50
4.2. Preparation of Bis(2-Hydroxyethyl) Terephthalate........................................................ 55
4.3. Outcomes of Oil Well Cement Slurry Formulations and Sample Preparations ............. 60
4.4. Outcomes of Assessing PET Additives .......................................................................... 66
5. CONCLUSIONS .................................................................................................................. 82
6. REFERENCES .................................................................................................................... 83
7. APPENDICES ...................................................................................................................... 92
APPENDIX A ........................................................................................................................... 92
APPENDIX B ........................................................................................................................... 93
APPENDIX C ........................................................................................................................... 95
APPENDIX D ......................................................................................................................... 125
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ABSTRACT
Addition of polyethylene terephthalate (PET) waste plastic in cement mixtures mostly negatively
affect cement matrix properties. Mainly, decreasing the compressive strength, while also
affecting slurry properties of the cement mixtures. However, recent findings for concrete cement
mixtures show that through prior pretreatment of the plastic waste material, via irradiation
technique or through oxidizing solutions, the strength of cement mixtures is regained. Promoting
sustainable practices, this work considered the utilization of local class A cement, improved into
oil well cement, and then further investigates the prospect of using PET plastic waste in
cementing oil and gas wells. Local class A was formulated into standard oil well cement utilizing
chemical additives, whereby their compatibility and interactions were explored. A temperature
of 38 ℃ and atmospheric pressure were utilized as bottom hole circulating conditions. Three
unique PET derivatives as additives, counting pure PET fibres, irradiated PET fibres, and powder
Bis(2-hydroxyethyl) Terephthalate (BHET) synthesized through PET glycolysis, were each then
added in 0.2%, 1.0%, and 1.8% BWOC dosages. Varyingly, their addition increased the oil well
cement compressive strengths, each optimally, by 22.05%, 19.34%, and 81.82%, respectively.
Overall, plastic viscosities increased with increasing incorporation dosages, with slip effect
resulting due to PET fibres incorporation. Addition of a superplasticizer among the additives is
crucial in controlling rheological behavior and most importantly in improving compressive
strength of PET plastic incorporated oil well cements. PET fibres have a potential to be used as
reinforcements while BHET can be readily used as an oil well cement additive.
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1. INTRODUCTION
The hydration reaction of oil well cement involves multiple physico-chemical mechanisms,
covering diffusion, nucleation, growth, complexation, as well as adsorption mechanisms. A
sequence of reactions takes place simultaneously and consecutively between water and cement
constituents, with chemical additives in some cases, thereby leading to the setting and hardening
of the oil well cement slurry. Strengths of hardened oil well cement mainly define the reliability
and the ability to resist deformation when loads are applied. Deficiency in set cement’s
compressive strength (Huwel, 2014) signal increased chances of casing failure accompanied
with a decrease in lifespan of a well, while according to Alp and Akin (2013) increased
compressive strengths, additionally to increased durability, are further associated with decreased
porosity. Aside from ensuring adequate mechanical properties of the set cement after the oil and
gas well cementing operation, oil well cement additives also play a crucial role in controlling
slurry properties including rheological behavior, thickening time, and fluid loss.
The utilization of polymer waste, counting rubbers (Ahmed et al., 2020) and plastics (Jassim,
2017), has been of interest lately, with researchers struggling to find widely applicable and
sustainable alternative methods to handle this abundant non-biodegradable polymer waste.
Nonetheless, several studies for application of these polymer wastes in cementing jobs have been
carried out. Various studies show that Polyethylene Terephthalate (PET) plastic waste can be
incorporated or added in concrete as aggregate, powder, or fibre. Further, these studies involving
PET plastic incorporation in cement products have been a success to some extent. As fibres and
coarse aggregates, PET in concrete has been reported to improve the workability of the concrete
(Choi et al., 2005). Furthermore, it improves impact resistance, enhances cracking properties,
and leads to increased flexural strength, especially in fibre forms as used in the studies by Kim et
al. (2010); Koo et al. (2014); and Pelisser et al. (2012).
Hence, these studies further commonly reported that the compressive strength was reduced with
the incorporation of PET waste. Additional effects apart from the slight decrease in density
involve the decrease in elastic modulus with the increase in PET fibre incorporation volume
reported by Kim et al. (2010), and the PET plastic’s increased sensitivity to alkaline
environments. Given the PET plastic’s increased sensitivity to alkaline environments, the
improvements brought about by adding recycled PET fibres in a study by Pelisser et al. (2012)
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deteriorated after 150 days, accompanied by increasing porosity at about 365 days. Koo et al.
(2014) together with Schaefer et al. (2017) further emphasised that the use of PET enhanced
concrete under aggressive alkaline conditions must be thoroughly evaluated given the increased
possibility of degrading durability perfomance under these conditions. Contrarily, the
incorporation of PET waste in concrete as fine powder, as in the study of Umasabor and Daniel
(2020), where PET plastic waste was pulverized into powder first, prior to mixing it with the
concrete, reported varying results.
One remarkable study by Rai et al. (2012) set forth that the reductions in compressive strength
and workability which resulted when different percentages by volume of plastic flakes partially
replaced sand in concrete were minimal. Further, these reductions can be countered by adding a
superplasticizer. Hence, the workability of the waste plastic mix concrete was reported to have
increased by around 10 − 15%, while the compressive strength increased by approximately 5%
following the addition of a superplasticizer to the cement mix.
Lee et al. (2019) discovered that chemically treating PET plastic before incorporation in concrete
as the replacements of coarse aggregate, especially using the strong oxidizing calcium
hypochlorite solution (Ca(ClO)2), yielded an improvement in bond strength between
cementitious matrix and PET plastic aggregates with further reduction in the gap at the
interfacial transition zone. As a result of such PET plastic pretreatment, the compressive strength
of the concrete containing chemically oxidized PET plastic coarse substitute aggregates
increased compared to the control PET aggregate which was treated with water while porosity
and permeability decreased.
Major breakthroughs of using PET plastic waste to achieve notable improvements in cement
related operations involved pretreatment of plastic before it is applied as a concrete additive,
either in the form of aggregate or powder. Based on a number of different studies, the gamma
irradiation of PET and its thermal depolymerization through glycolysis have emerged as leading
techniques to be utilized for transforming the waste polyethylene terephthalate plastics for use in
cement mixtures. Gamma irradiation results effects lead to direct changes in physical properties.
Polyethylene terephthalate, like the many other polymers, undergoes major crosslinking and
minor oxidative degradation on gamma radiation. However, with its sterilization through gamma
radiation readily occurring in both film and fiber forms, withstanding at least 1000 kGy of
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gamma radiation even though discoloration takes place at lower doses (Plester, 1973).
Furthermore, both radiation-induced crosslinking and chain scission can result in improved PET
plastic strength (Schaefer et al., 2017).
A study by Cota et al. (2007) reported that high density polyethylene (HDPE) mechanical
strength properties improve with increasing gamma irradiation dose. Further, the application of
radiation at lower dose rates led to effectively obtaining lower doses that necessarily provide a
similar change in resistance parameters as high doses, meaning that gamma radiation affects
HDPE in a more efficient way at lower dose rates.
In the case of irradiated PET as sand substitute in concrete, from the study of Martínez-Barrera et
al. (2015) it was concluded that irradiated PET-enhanced concrete shows greater mechanical
strength. It was reported that radiation-induced crosslinking improved the mechanical strength of
PET enhanced concrete for doses up to 150 kGy, with greater compression strength exhibited by
samples irradiated at a dose of 100 kGy.
Schaefer et al. (2017) also explored the effectiveness of gamma irradiated PET plastic when used
as an additive in cement paste comprising of supplementary cementitious materials counting
silica fume and Class F fly ash. This study of Schaefer et al. (2017) involved irradiation doses
of 10 kGy for the low dose plastic additive and 100 kGy for the high dose plastic additive. It was
then found that irradiating plastic at a high dose (100 kGy) was the best, recouparating strength
lost due to the addition of plastic in cement paste. Furthermore, it was clarified that adding a high
dose irradiated PET plastic rather than regular, non-irradiated or low dose irradiated PET plastic
in various concretes caused an increase in compressive strength accompanied by a decrease in
porosity.
Meanwhile, thermal depolymerization of polyethylene terephthalate through glycolysis to yield
Bis(2-hydroxyethyl) Terephthalate (BHET) has been covered in numerous studies over the past
decades. Recently, counting the studies by Bartolome et al. (2014) covering the recent
developments; by Raheem et al. (2018) simulating the process of BHET and its recovery using
two–stage evaporation systems; and by Lalhmangaihzuala et al. (2020) where the catalysis of
PET depolymerization through glycolysis is significantly improved in an environmentally
friendly way, hence the developed bio-derived solid heterogeneous orange peel ash catalyst.
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In the study by Mendivil-Escalante et al. (2014), it was found that BHET synthesized through
glycolysis from PET plastic waste has a great potential to be applied as a concrete mixture
additive, although further research was recommended, particularly on resulting mechanical as
well as durability performances.
In one remarkable study by Simsek (2020), it was found that cementitious composites of BHET
have higher calcium hydroxide content and lower porosity together with water absorption
percentages in comparison to cementitious composites of dioctyl terephthalate. It was reported
that hydrogen bonds are formed in BHET cementitious composites, and those hydrogen bonds
crucially prevent cement particles from agglomerating because of electrostatic attraction. They
futher significantly contribute in healing cracks through formation of more hydrated products.
This project, unlike these previous studies, focuses on investigating the prospect of using waste
PET plastics in cementing of oil and gas wells. Effects of the PET derived additives are
investigated in significantly dispersed oil well cement slurries following highest amounts of the
superplasticizer addition, hence its benefits to strength in PET containing concrete mixtures. PET
was used in the form of pure fibres, irradiated fibres, and powdered BHET synthesized through
its glycolysis (thermal depolymerization), while shallow oil well conditions were assumed.
Problem Statement:
Given the introduction of plastic waste without any pretreatment generally decreasing the quality
of cement matrices, would the curing of PET containing cement mixtures at oil well temperature
and pressure conditions have a unique effect, or would the proposed irradiation and
depolymerization techniques for pretreatment of PET plastic waste before incorporation be able
to effectively maintain or improve the slurry and matrix properties of the oil well cement.
The viscosity of polymer PET plastic waste at higher temperatures decreases, and polymer
incorporation in oil well cement mix for utilization at increased temperatures, usually > 110℃,
as of Abbas et al. (2013), could see cement slurry/matrix properties being consequently
inadequate. Due to the lowering viscosity of the incorporated polymer additive, fluid loss
together gas migration might occur during application. At this point should the issue of the
polymer viscosity emerge, seemingly the option could be to rapidly increase the incorporation
concentration of the polymer to encounter the problematic decreasing viscosity at high
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temperatures (Abbas et al., 2013), or to settle for the lower temperature and pressure conditions
where the PET incorporated oil well cement would probably function outstandingly. However,
the leading solution could be adjustments in other blended cement additives dosages, which their
compatibility with the incorporated PET plastic waste additive is also in question.
Aims and Objectives:
The aim of investigating the prospect of using PET plastic waste in cementing of shallow oil
wells was carried out through the following objectives:
• Developing oil well cement slurry base design using local class A cement with oil well
cement additives including a superplasticizer, retarder, expanding agent, fluid loss additive,
and a defoamer at varying amounts; then assessing compatibility of these added additives,
slurry mixability at a specified density, its settling and final strength.
• Incorporating PET plastic waste in fibre form on the oil well cement slurry to investigate the
effects of PET plastic addition on the slurry’s initial states and final hardened state.
• Pretreating PET plastic waste by gamma irradiation, to yield irradiated PET fibre additive,
and by thermal depolymerization yielding BHET as the powdered additive, then investigate
the effects of these PET plastic derived additives on the oil well cement slurry initial states
and final hardened state.
• Cure oil well cement slurry samples at 38 ℃ and atmospheric pressure to correlate the curing
conditions of prepared cement slurries to typical shallow oil well conditions.
• Prepare oil well cement slurries or samples varying by amount of PET plastic waste and/or
additives added to roughly determine the optimum addition amounts.
• Thoroughly investigate the effects brought about by addition of BHET powder, irradiated,
and non-irradiated PET plastic waste fibres on the oil well cement mixture by performing
rheological measurements and failure parameter tests to determine each mixture’s rheological
properties (yield point and plastic viscosity) and compressive strength.
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2. LITERATURE REVIEW
2.1. Fundamentals in Sustainable Manufacturing
A stepwise production of Portland clinker involves the crushing and grinding of raw materials,
blending of the raw materials in the correct proportions, and burning the prepared mix in a kiln.
Chemically, cement production begins with a calcination process where calcium carbonate is
decomposed at a temperature of approximately 900 ℃, thereby leaving calcium oxide (burnt
lime) while gaseous carbon dioxide is released, usually into the air. Thereafter, the clinkering
process follows, clinker is produced, which consists of silicates, ferrites, and alumina of calcium
that are given rise to by reacting calcium oxide from the calcination process with silica, alumina,
and ferrous oxide at increased temperatures typically ranging between 1400 ℃ and 1500 ℃
(Nilsson et al., 2007).
The World Business Council for Sustainable Development (2014) reported that limestone, a
source of calcium carbonate, is heated at a temperature of 1450 ℃ in a kiln containing small
quantities of other raw materials including clay, sand and iron. As a result clinker is produced,
with this process consuming large fuel amounts. Moreover, the produced clinker is just a hard
intermediate product, it is further ground and milled at the same time with gypsum into fine
powder, thereby the ordinary Portland cement is produced. However, more cement chemical
additives other than the already mentioned gypsum can be added, and together with the produced
clinker would be ground and milled to produce the unique desired oil well cement product.
2.1.1. Processing Routes
Main cement production processes involve the dry process, where raw materials are crushed and
thereafter dried to raw meal as flowable powder, followed by feeding it into the kiln where it will
be preheated or precalcined; the semi-dry process, where the dry raw meal is formed into pellets
by combining it with water, followed by feeding those formed pellets into a grate preheater then
into the kiln, alternatively straight to a long kiln equipped with crosses; the semi-wet process,
where the slurry is initially dewatered in the filter presses, then the resulting filter cake is forced
out into pellets followed by being fed either into a grate preheater, alternatively straight to a filter
cake drier for raw meal production; and the wet processes, where raw materials, usually of high
moisture levels, are crushed in water thereby forming a pumpable slurry which is either initially
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fed to a slurry drier then into the kiln, or fed straight into the kiln (European Commission, 2001;
Nilsson et al., 2007).
Given the environmental concerns including climate change and global warming together with
the fact that the manufacturing of cement results to emission of carbon dioxide, a greenhouse gas
regarded as the primary driver of global climate change, cement manufacturing industries can be
advised to consider using more environmentally friendly cement production processes.
According to Nilsson et al. (2007), approximately 5 % of the global carbon dioxide emissions
back then came from cement production, with contributing factors counting the calcination
process, a thermal treatment process in presence of air or oxygen in the kiln, and the immense
demand and use of cement. Zhang et al. (2010) further reported that other deeply worrying
emissions released during the manufacturing of cement include nitrogen oxides (NOX) and
sulphur dioxide (SO2), which together with carbon dioxide are mainly emitted from the pre-
calciner kiln system. Nonetheless, additional emissions like dust which is usually controlled
using mechanical collectors or dust collectors, electrostatic precipitators, and fabric filters, was
also highlighted as alarming. Furthermore, Nilsson et al. (2007) included the releases of carbon
monoxides, volatile organic compounds, noise, together with dibenzofurans and polychlorinated
dibenzodioxins as releases that pose relatively minor treats.
Taking into consideration a statement by Nilsson et al. (2007) which implied that the process
route to manufacture cement is predominantly decided by whether the raw materials are dry or
wet. One may be advised to use any manufacturing process that work best for their interests,
either dry, semi-dry, semi-wet, or wet. However they would be strongly encouraged to employ a
reasonable division of the combustion environment in the calciner since according to Zhang et al.
(2010) a reasonable division of the combustion environment in the calciner can be regarded as
the principal method to manage the formation of pollutant gases. In the contrary, one can be
advised to specifically employ the use of a dry process route and add a reasonable division of the
combustion environment in the calciner during cement production since according to both the
European Commission (2001) and Zhang et al. (2010) the production of cement clinker via a dry
process kiln with multi-stage suspension preheating and precalcination was then by far the best
cement manufacturing method. Further demonstration was given by Zhang et al. (2010), where
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the dry process cement production technology referred to as the pre-calcining technology indeed
led to the reduction in the formation of heat loads and gas pollutants within the rotary kiln
system, sorely by transferring of heavy raw material decomposition tasks to outside the kiln.
Moreover, with wet cement manufacturing process routes known for the increased energy
consumption, one would indeed be greatly advised to employ the use of dry processing routes as
much as they can to manufacture cement, thereby cutting in energy costs as the utilization of
fuels will be lowered, and energy resources will consequently be reserved. According to
European Commission (2001); Nilsson et al. (2007); and Zhang et al. (2010) cement
manufacturing not only give emission and pollution issues, other underlying core issues involve
the extreme high energy amounts consumed, with the energy costs usually ranging at about 30 −
40% of production costs, and the enormous raw material amounts required for utilization, with a
rough scale taken from Europe stating that about 1 𝑡𝑜𝑛 of clinker is produced by consuming raw
materials with an average weight of 1.57 𝑡𝑜𝑛𝑛𝑒𝑠. Hence, the energy intensiveness and enormous
raw material amounts consumed, one can clearly see that a proper choice of the manufacturing
route alone is not enough to achieve a sustainable level. As a result, one may be encouraged to
explore the use of alternative raw materials and fuels, together with more productive and
environmentally friendly ways to produce clinker and cement as a whole.
2.1.2. Selection of Raw Materials
Cement manufacturing requires careful control when it comes to the chemistry of principal
ingredients which are, namely, CaO, SiO2, Fe2O3, and Al2O3, together with minor constituents
including sulfites (SO3), potassium oxide (K2O), sodium oxide (Na2O), titanium dioxide (TiO2),
and phosphorous pentoxide (P2O5) (Ghoroi and Suresh, 2007; Hıdıroğlu, 2017; Hokfors, 2014).
The World Business Council for Sustainable Development (2014) wrote that the use of
alternative waste raw materials and fuels, as natural raw materials and conventional fuels
replacements, in optimum quantities with varyingly decreased proportions of natural raw
materials or conventional fuels may also result in achieving the desired optimal balance of
material composition in the clinker, although in a more sustainable way.
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According to Oliveira et al. (2015) when calculations are done, prior the proportioning of raw
materials, they must take into consideration the argillaceous or siliceous materials that might be
contained in increased proportions within various limestones, as well as from the ash generated if
coal is used to fire the kiln. Minor impurities in the raw material also must be taken into account,
as they can have a significant effect on cement performance. Procter (2014) further wrote that
during the course of selecting replacements for natural or conventional resources as raw
materials or fuels to be used during cement production, it must be ensured that the alternative
replacement fuels and raw materials fulfil the quality specifications equally as the natural raw
materials and frequently used conventional resource fuels.
2.1.3. Raw Materials
Oil cements are the modifications of Portland cement, and Portland cement is produced by
burning and grinding a mixture of calcareous and argillaceous materials (Morga, 1958).
Calcareous materials act as a source of calcium carbonate, normally are natural occurring
calcareous deposits, and they include the likes of limestone, marl, and chalk. On the other hand,
several minerals and ores (argillaceous raw materials) including iron ore, shale, clay, and sand
provide cement chemical necessities counting iron oxide, silica, and alumina (Morga, 1958;
Nilsson et al., 2007).
Imbabi et al. (2013) with Snellings et al. (2012) reported that instead of the total usage of the
above mentioned argillaceous natural raw materials including clay and shale, the partial use of
process residues counting power station ash or fly ash, and blast furnace slag as partial natural
raw materials replacements has been widely adopted. These process residues similarly contain
significant quantities of clay-like components, resembling the partially replaced natural raw
materials. According to Yi (2019) alkali waste usage as partial natural raw material replacement
for calcareous materials is also possible. Moreover, Procter (2014) insisted that waste and by-
products containing useful minerals including calcium, silica, alumina and iron may be utilized
as natural raw material replacements in the cement kiln. Another remarkable study by Al Dhamri
(2020) reported that oil based mud cuttings can be sucessfully recycled and reused as a natural
raw materials replacement during the production of clinker as they contain silica, calcium and
alumina.
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The option of partially replacing natural cement raw materials with the resulting process residues
comes with advantages including the cut in raw material costs, costs related to their pre-
processing or mining if needed, which would come with a cut in energy costs too since
preprocessing and mining are generally energy intensive operations. Given also the alarming
negative climate changes worldwide, this option is encouraged as it comes with further benefits
of saving energy through the use of process residues and waste materials as raw materials, while
lowering the concerning consumption of natural resources (cement making natural raw
materials), and possibly decreasing environmental impacts.
Also, the utilization of potential pozzolanic materials including rice husks, lime sludge, and
broken bricks may significantly reduce cement manufacturing costs and further reduce the
release of pollutants, hence these materials are self-calcining as they generally burn on their own.
2.1.4. Proportioning
Raw materials are first powdered, followed by mixing in specific proportions, and this raw
material proportioning is key in obtaining a specific cement product with the required properties.
Raw materials (also considering fuel waste) are proportioned accordingly in such a way that the
cement’s main mineral compositions including tricalcium aluminate, tetracalcium aluminoferrite,
tricalcium silicate, and dicalcium silicate are uniquely combined to give rise to specific Portland
clinker and further to certain specific class of oil cement (API cement). Hence, the compositions
as displayed in Table 1 are different for each API cement class, and each cement class would
further have unique fineness and varying water to cement ratio. Moreover, the difference in these
API cement classes is motivated by the fact that they at least each would be exposed to different
well conditions in the form of cement slurries, and those well conditions include varying high
pressures and temperatures, as well as corrosive underground fluids and varying degrees of
sulfate attacks (Hıdıroğlu, 2017).
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Table 1: API Oil Well Cement Classes with Respective Typical Clinker Phase Compositions,
Well Depth and Temperature Ranges, Together with Associated Degrees of Sulfate Resistance
(1. Pikłowska, 2017; 2. Mixhaux et al., 1989; 3. Teodoriu et al., 2019; 4. Nelson and Guillot,
2006, 5. Yi , 2019; 6. Hıdıroğlu, 2017)
API
CLASS
TYPICAL POTENTIAL
PHASE COMPOSITION
(%)
DEPTH
(𝐦)
TEMPERATURE
(℃)
SULPHATE
RESISTANCE
𝐂𝟑𝐒 𝐂𝟐𝐒 𝐂𝟑𝐀 𝐂𝟒𝐀𝐅
A 50.33 25 9 81,2,6 0 − 18301,3,5,6 32.45 − 82.451,3,5 Normal
B 46 31.67 5 12.331,2,6 0 − 18301,3,5,6 32.45 − 82.451,3,5 Moderate & High
C 56.33 17 9 8.331,2,6 0 − 18301,3,5,6 32.45 − 82.451,3,5 Normal, Moderate,
& High
D 26.67 52.33 2.67 121,2,6 1830
− 30501,3,5,6
82.45 − 143.551,3,5 Moderate & High
E 30 50.33 2.67 111,2,6 1830
− 42701,6
94 − 1441 Moderate & High
G 50 30 5 121,2,6 0 − 24401,3,5 44 − 1111 Moderate & High
H 50 30 5 121,2,6 0 − 24401,3,5 44 − 1111 Moderate & High
As displayed on Table 1 above, major constituent of the Portland clinker for every API cement
grade is the silicate phases, and according to Nelson and Guillot (2006),- these phases generally
account for about 80% of the total materials. Silicate phases found in oil well cements include
tricalcium silicates (3CaO ∙ SiO2), which is abbreviated as C3S in Table 1, it is the major
Portland clinker component as it produces more strength while also being behind the early
bonding of cement, together with dicalcium silicate (2CaO ∙ SiO2), which is abbreviated as C2S,
and it is responsible for late binding of cement, hence its slow hydration or medium activeness
would have an effect on bonding time as well as on the final cement stone strength, consequently
a small gradual gain in strength over a longer duration would be promoted.
Given the fact that increased tricalcium silicate amounts in cements provide even more heat
required for cement hardening, as well as the exhibition of rapid strength and increased
endurance build-ups, all because of the tricalcium silicate’s high calorific value, thereby cements
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with high tricalcium silicate amounts could further be distinguished by having increased
hydration heat and shrinkage (Pikłowska, 2017). According to Mixhaux et al. (1989), magnesium
and sulfates in downhole brines generally react with the hydration products of cement and
consequently lead to loss of compressive strength. The development of cements having low
amounts of tricalcium aluminate (3CaO ∙ Al2O3), abbreviated as C3A in Table 1, is key for oil
well cements as such cements are less likely to experience sulfate attacks meaning the increased
tricalcium aluminate amounts make cement more susceptible to sulfate attack (Mixhaux et al.,
1989; Nelson and Guillot, 2006).
In Table 1, as the well depth goes deeper with increasing temperatures, the possibility of sulfate
attacks also increases, thereby the ability to resist sulphate attacks (sulfate resistance) for that
particular cement to be used in such depths and temperatures must also increase. Hence, API
cement classes D and E are to be used in wells with greater well depth ranges and more extreme
temperatures due to their low tricalcium aluminate contents. However, API cement classes D and
E are not the only API cements with decreased tricalcium aluminate contents, API class B
cements also, but they are used at the same well depth range of 0 − 1830 m as API cement
classes A and C with much higher tricalcium aluminate contents. This is because just like API
cement classes A and C, API class B cements contain way more increased tricalcium silicate
amounts which promote early strength, thereby it is not ideal or possible to use such cements in
way more deeper wells due to early hydration and setting times, rather one may be strongly
advised to use API class B cements in wells where formations contain extreme high sulfates
contents, still between a 0 − 1830 m well depth range.
Furthermore, tricalcium aluminate affects the bonding speed of a cement slurry, where it
provides increased heat levels thereby accelerating bonding of a slurry (Nelson and Guillot,
2006). Tricalcium aluminate further has a power of corroding the installed casing, and it
dissolves easily in water, particurlarly in sulphated water, making it to have limited contribution
to high sulphate resistance cements (Pikłowska, 2017). On the other hand, tetracalcium
aluminoferrite (4CaO ∙ Al2O3 ∙ Fe2O3), abbreviated as C4AF on Table 1, has an effect on
strength over time, where during hydration it provides minor amounts of heat as it has a
significant calorific value, thereby promoting low-heat hydration (Pikłowska, 2017).
P a g e | 16
Hence, rapid hydration, and early strength development promoted by tricalcium aluminate and
tricalcium sulfate, respectively, thereby API cement classes D and E as displayed in Table 1
must have lowest concentrations of these rapid hydrating compounds. Reason being, because
these API cements are to be specifically used in greater depths (1830 − 3050 m for API cement
class D and 1830 − 4270 m for API cement class E) with increased temperatures (82.45 −
143.55 ℃ for API cement class D and 94 − 144 ℃ for API cement class E) which would result
in unwanted extreme rapid hydration of the slurry and potentially make it impossible to pump or
work with. Therefore, by having these compounds at such very low concentrations would extend
the duration of hydration and further prolong time of pumping the slurry, thereby making it
easier to use API cement classes D and E at these greater depths and higher temperatures.
API cement classes A, B, and D are manufactured for use in wells with depths below 1830 m.
As displayed on Table 1, these cement classes have different compositions, thus each cement
class function varyingly, with API class A cements used when no special necessities are
required. API cement class B as briefly discussed above has low tricalcium aluminate amounts
meaning it is more sulfate resistant, thereby it is to be specifically used in case where moderate
to high sulfate resistance is needed, hence, more tricalcium aluminate in cement leads to sulfate
attack susceptibility (Mixhaux et al., 1989; Nelson and Guillot, 2006; Teodoriu et al., 2019).
Furthermore, API class C cements with significantly increased amounts of tricalcium silicate,
and apparently being finely ground, they are specifically developed for use given a cement slurry
with high early compressive strength is needed, hence, the significantly high tricalcium silicate
amounts would mainly promote early rapid hydration (Pikłowska, 2017; Teodoriu et al., 2019).
While API cement classes D and E as shown on Table 1, are to be specifically used where
moderately high temperatures and pressures are experienced due to having decreased tricalcium
silicate and tricalcium aluminate amounts, which are early strength development, and rapid
hydration promoting principal compounds, respectively. Cement classes G and H on the other
hand have strict manufacturing specifications as well as behaviours that are easily predicted,
furthermore they can be used along with additional chemical additives, counting accelerators and
retarders, thereby covering an even wider range of well depths and temperatures (even way more
than the 0 − 2440 m typical well depth range that these cement classes can cover without
additional chemical additives). Even the entire wide range of well depth coverage is possible
P a g e | 17
using these API cement classes along with chemical cement additives (Pikłowska, 2017;
Teodoriu et al., 2019). Moreover, as displayed on Table 1, API class G cements and API class H
cements have similar composition, the only difference according to Nelson and Guillot (2006) is
that API class H is coarser than API class G cement. Given the fact that the coarser grind lead to
prolonged hydration, thereby API class G cements probably have more rapid hydration than the
coarser API class H cements, given no chemical additives have been added on both cement
classes.
2.1.5. Kiln Fuels Selection
Fuel burning taking place in the cement kiln system provides heat (thermal energy), thereby
leading to the occurrence of a chemical reaction between the pre-blended raw materials and fuels'
ash, and in turn this chemical reaction results into having clinker as a product (Ishak et al., 2016).
However, when it comes to modern kilns, increased fuel amount is needed in the calciner
compared to the kiln, since according to Hassold (2018) the calcination process consumes most
energy apart from energy losses, thereby leading to about 60 -70 % of the fuel being burned in
the precalciner. Furthermore, Hassold (2018) clarified that adequate energy density is required
for the kiln fuel, and when that fuel with adequate energy density is fired it results to the partial
melting of the feed (Typically at a temperature around 1450 ℃), while it further set about quality
nodulized clinker, steady coating for long campaign life, and close contact reactants as well as
their rapid reaction.
A range of fuels burned in cement making kilns include coal, historically the main fuel burned in
cement making kilns; natural gas, which together with coal are burned or utilized in their natural
forms; and other fossil fuels counting petroleum fuels, bituminous and shale sands. Well-known
and frequently utilized fuels for the provision of increased temperatures in modern cement kilns
include coal, natural gas, and petroleum fuels, moreover these fuels are not only fired in cement
making kilns but they are also major sources of energy worldwide till to date (Youn et al., 2019).
Hence, these widely consumed cement kiln fuels, one may be advised to use only coal and
natural gas as primary fuels to fire the kiln.
This advice would be based on the fact that both coal and natural gas are used in their natural
forms while petroleum fuels, bituminous sands, and shale sands need to be distilled and/or
refined prior to their usage as fuels (Chinyama, 2011; Youn et al., 2019). With coal and natural
P a g e | 18
gas not requiring no prior distillation and/or refining before firing in the kiln, thereby no extra
pre-processing costs are to be involved meaning decreased energy expenses. On the other hand,
one may be advised to specifically use natural gas only among the previously mentioned kiln
fuels, even though coal provides cheaper energy as well. This advice would sorely be based on
the fact that when natural gas is burned to acquire energy, it results in fewer emissions of almost
all forms of air pollutants, including carbon dioxide, compared to when coal or petroleum fuels
are burned for the production of the same energy amount.
Considering the effects that come about due to the overuse of natural energy resources, counting
the extinction of resources as they are mostly non-renewable as well as pollution of the
environment briefly through emissions and associated acid rains together with global warming,
the exploration and utilization of alternative fuels to fire the cement kilns has been receiving
growing attention. Ptasinski et al. (2007) demonstrated in their study, referred to as the exergetic
evaluation of biomass gasification, that fossil fuels can be successfully substituted by renewable
fuels. There, they experimented with solid biofuels counting straw, untreated and treated wood,
grass, and plants, together with liquid biofuels counting vegetable oil which were to be coal
replacements as gasification feedstock.
According to Chinyama (2011), alternative fuels that can be fired to produce high temperatures
during cement manufacturing include gaseous fuels counting pyrolysis gas, refinery waste gas,
and landfill gas; the liquid fuels including wax suspensions, waste solvents, used oils and oil
sludge, petrochemical waste, paint waste, distillation residues, chemical waste, as well as tar; and
lastly the solid fuels which include, rice hulks, petroleum coke, refuse derived fuels, pulp and
sewage sludges, paper waste, wood waste, plastic waste, rubber residues, used tyres, nut shells,
battery cases, and domestic refuse.
When it comes to choosing fuels to be fired during cement production, one may surely be
strongly advised to employ the use of alternative waste fuels as much as they can rather than the
complete use of conventional fossil fuels, considering the facts that alternative waste fuels are
relatively inexpensive, thereby cuts in energy costs. Moreover, the combustion of waste in
cement kiln systems rather than in incinerators results in reduced carbon dioxide emissions,
which is not only good for the environment but also good for the industry since there will be a
reduction in carbon dioxide emission penalties (Chinyama, 2011).
P a g e | 19
A closer look into the burning of alternative waste fuels within the cement kiln systems. Given
the combustion process in a calciner occurring at lower temperatures of around 900 ℃, coupled
with the fact that lower grade heat of less than 18 𝑀𝐽/𝑘𝑔 is more appropriate for the calciner,
thereby it would be advisable for one to use as much as they can of the low-grade alternative
waste fuels to fire the calciner, hence more fuel is also needed in the calciner than in the kiln
(Chinyama, 2011; Hassold, 2018). Moreover, on the account that the net energy requirement for
calcination is much higher than the net energy requirement of producing clinker in the kiln, due
to the exothermic formation reaction of tricalcium silicate (C3S), one may indeed come to a solid
conclusion that the burning of alternative waste fuels should mainly take place in the calciner.
According to Ptasinski et al. (2007) and Nørskov et al. (2012) there are diverse combinations of
renewable biomass resources and various waste materials to produce waste derived fuels
including refuse derived fuel and solid recovered fuel, which would obviously have
comparatively higher calorific values (in comparison to individual waste fuels) depending on
how they were synthesized. During the production of these waste derived fuels, there are
variations in degree of waste size reduction as well as variations in the removal of organic and
inert material. These production differences together with how refined and processed the final
product is, mainly distinguish refuse derived fuels and solid recovered fuels (Psomopoulos,
2014). Moreover, solid recovered fuels are a biodegradable waste or residues fraction produced
from various types of non-hazardous waste including municipal solid waste, industrial waste, and
commercial waste, while the refuse derived fuels are crude materials mainly made from domestic
non-hazardous waste. Although there usually are metals removal systems when waste derived
fuels are produced, but pollutants including chlorine, sulphur and heavy metals remain
significantly present in finished fuel products, however in minimized propotions (Nørskov et al.,
2012).
Given one has to select a substitute waste fuel to be fired within the cement kiln system and they
have to choose between untreated non-hazzardous waste as fuel and the refuse derived fuel, they
would deffinitely be encouraged to employ the use of refuse derived fuel in this case. This advice
is based on facts that refuse derived fuels generally contain relatively lower bulk density as well
as lower content of ash, while having relatively higher heating value (Psomopoulos, 2014). On
the other hand, the untreated non-hazardous waste not only has relatively lower calorific value,
P a g e | 20
meaning it would release a relatively lower energy amount per 1 𝑘𝑔 of fuel burned, it also has a
great potential to release increased amounts of pollutants counting heavy metals, sulphur, and
chlorine due to not being pretreated.
According to Gawlik et al. (2007), solid recovered fuels unlike refuse derived fuels, meet a series
of standards, environmentally and process-relevant standards, and those standards include the
corroding capacity, biodegradable fraction as well as the required minimal contents of various
critical trace elements counting mercury, thallium, and cadmium. Hence these standards which
clearly display that refuse derived fuels are less refined than solid recovered fuels, coupled with
the fact that refuse derived fuels generally are less efficient as kiln system’s fuels when
compared to solid recovered fuels which typically have a guaranteed quality average calorific
value, thereby one may be encouraged to employ the use of solid recovered fuels rather than
using refuse derived fuels. However, one would also be strongly advised to consider using these
waste derived fuels correspondingly, for instance, given parts of the system or the whole system
that require much higher energy amounts and refuse derived fuels do not contain that much of
contaminants, although they are not refined, furthermore they have a satisfying calorific value,
thereby in such a case refuse derived fuels can be utilized since they have would be having
higher calorific values, thereby more efficient and economically viable in that particular system,
while solid recovered fuels may then be burned in systems that require slightly decreased
amounts of energy.
Note, the choice of opting for refuse derived fuels instead of solid recovered fuels in the previous
case would only be based on extreme limited additional minor contaminants thereby minimal
more emissions are to be released, in a sacrifice to save on refining or further waste
preprocessing costs. Given solid recovered fuels and untreated non-hazardous waste as optional
fuels in the cement kiln systems, one would be advised to use solid recovered fuels instead of
untreated non-hazardous waste. This advice is based on facts that solid recovered fuels have
relatively higher calorific values, they have a more homogeneous form which is tyically in
pellets, and they have decreased content of moisture, thereby burning more efficiently than
untreated nonhazardous waste (Gawlik et al., 2007).
Hence, with alternative waste fuels mainly being used as fuel in the calciners during the
production of cement, moreover, with lower grade heat of less than or equal to 18 MJ/kg being
P a g e | 21
more appropriate for the calcination of raw materials, one would be greatly encouraged to
employ the use of solid recovered fuels (since they have typical calorific values ranging around
16 − 22 𝑀𝐽/𝑘𝑔), as a sustainable alternative to fossil fuel consumption during cement
manufacturing rather than using untreated nonhazardous waste or refuse derived fuels
(Chinyama, 2011; Hassold, 2018; Nasrullah et al., 2014). Furthermore, disregarding the limiting
factors that come about when alternative waste fuels are utilized (limiting factors counting
significant contents of heavy metals, chlorine, and sulfur pollutants present within these
alternative waste fuels), the use of waste derived fuels is encouragable since by their utilization a
sustainable alternative for waste material which could not be recycled because of their economic
inefficiency is also achieved (Gawlik et al., 2007; Nasrullah et al., 2014).
2.2. Process Variables Management
According to Hokfors (2014), although the formation of intermediate products also releases
certain amounts of heat, more heat is required for melting to produce final clinker product,
specifically for evaporating water and for the decomposition of calcium carbonates, clays, as
well as magnesium carbonates. Thereby, temperatures within the kiln form part of the process
variables that need to be managed in order to produce quality clinker. Typical heat of reactions
for the involved reactions of raw meal feed pyro-processing to produce cement clinker are also
estimated and discussed as part of the crucial process variables. These change in enthalpies of the
involved chemical reactions occurring at constant pressures are basically thermodynamic units of
measurements that are key when it comes to calculations of each reaction’s released or produced
energy amounts per mole. With a positive value of a change in enthalpy (heat of reaction)
indicating that products have greater enthalpy, and the reaction is thereby endothermic, meaning
heat is required; and with a negative value of a change in enthalpy indicating that reactants have
greater enthalpy, the reaction is thereby exothermic, meaning heat is produced. One last process
variable included below is the flow of gases and solids within the cement kiln system, with heat
transfer between these phases also being briefly highlighted.
In the following discussions, it is clear the main controlled variables in a cement manufacturing
kiln system only include the raw meal pyro-processing temperatures and the temperature of the
P a g e | 22
exhaust gases, while the manipulated variables would include rates of fuel and kiln feed.
Moreover, flow, the content of oxygen, as well as the pressure within the system are all usually
controlled by occasionally adjusting the speed of fans.
2.2.1. Temperatures and Heat of Reactions
According to Hassold (2018); Manias (2004); and Nelson and Guillot (2006), the quickly rising
and falling of the temperature within the kiln system promotes good formation of tricalcium
silicate and dicalcium silicate crystals, with tricalcium silicate having to decompose during the
cooling stage, and to counter the decrease of tricalcium silicate, cooling rates have to be
increased. Hence, an increase in cooling rates brings about an increase in maintained tricalcium
silicate amounts. Benefits of rapidly rising the temperatures within the kiln, followed by rapid
cooling further include the formation of clinker that can be easily grinded meaning cement would
have an increased milling capacity, which comes about because of the produced tiny but highly
stressed crystals of tricalcium silicate. Hence, the highly reactive crystals of tricalcium silicate
present in increased amounts due to rapid heating and cooling, thereby quality cement strength
would be achieved since high tricalcium silicate amount produces more strength in cement
products.
According to Wang (2006) depending on the temperature requirements needed for each cement
clinker principal component to be formed, the preferred dry processing route applied to the
modern rotary kiln system leads to the rotary kiln system being splitted into four kiln zones
counting, the decomposition zone, transition zone, sintering zone, and the cooling zone. In
addition to the different formation temperatures of cement clinker components, Table 2 further
displays crucial raw meal feed pyroprocessing or clinker formation reactions as well as the
associated typical heat of the chemical and mineralogical reactions (reaction enthalpies) occuring
within the dry cement rotary kiln zones.
P a g e | 23
Table 2: Dry Cement Rotary Kiln Zones with Relative Reactions as well as Typical Reaction
Conditions (Temperatures and Heat of Reactions) Required for the Formation of Quality Clinker
Product. (1. Wang et al., 2006; 2. Rodrigues et al., 2017; 3. Manias, 2004; 4. Benhelal et al.,
2012; 5. Hokfors, 2014; 6. Ghoroi and Suresh, 2007)
Kiln Zones Clinker Formation Reactions & Reaction Conditions
Decomposition
Zone
Chemical Reactions ➢ CaCO3 → CaO + CO2
➢ CaO + Al2O3 → CaO ∙ Al2O3
➢ CaO + Fe2O3 → CaO ∙ Fe2O3
➢ CaO + CaO ∙ Fe2O3 → 2CaO ∙ Fe2O3
➢ 3(CaO ∙ Al2O3) + 2CaO → 5CaO ∙ 3Al2O3
Main Reaction CaCO3 Decomposition
Average Temperature (℃) 850 1,2 or 550 − 960 4,5
Average rxn Heat (kJ/kg) 1750.19 1,2,4,5
Transition Zone
Chemical Reactions ➢ 2CaO + SiO2 → 2CaO ∙ SiO2
➢ 3(2CaO ∙ Fe2O3) + 5CaO3. 3Al2O3 + CaO →
3(4CaO ∙ Al2O3 ∙ Fe2O3)
➢ 5CaO ∙ 3Al2O3 + 4CaO → 3(3CaO ∙ Al2O3)
Main Reaction 2CaO ∙ SiO2 Formation
Average Temperature Range (℃) 800 − 1250 1,2,3,4,5
Average rxn Heat (kJ/kg) −698.71𝟏,𝟐,𝟒,𝟓
Main Reaction 4CaO ∙ Al2O3 ∙ Fe2O3 Formation
Average Temperature Range (℃) 999 − 1259 1,2,3,4
Average rxn Heat (kJ/kg) −95.75 1,2,4,5
Main Reaction 3CaO ∙ Al2O3 Formation
Average Temperature Range (℃) 999 − 1259 1,2,3,4
Average rxn Heat (kJ/kg) −25.67 1,2,5
Sintering Zone
Chemical Reactions ➢ 2CaO ∙ SiO2 + CaO → 3CaO ∙ SiO2
Main Reaction 3CaO ∙ SiO2 Formation
Average Temperature Range (℃) 1305 − 1439 1,2,3,5
Average rxn Heat (kJ/kg) −490.33 1,2,5
Cooling Zone Main Reaction Cooling of Clinker
P a g e | 24
Between the temperatures 550 − 960 ℃ the decomposition of calcium carbonate as part of the
raw meal feed takes place, starting from the preheaters, being intense in the precalciner, and
completed in the kiln’s decomposition zone. Additionally,-within this zone, besides this
decalcination reaction, further formation of minor quantities of CaO ∙ Al2O3; CaO ∙ Fe2O3; 2CaO ∙
Fe2O3; and 5CaO ∙ 3Al2O3 compounds also take place (Benhelal et al., 2012; Hokfors, 2014).
Within the transition zone, an exothermal zone and a zone of rapidly increased temperatures,
typically at temperatures ranging from 800 ℃ to 1250 ℃, dicalcium silicate components form,
followed by tetracalcium aluminoferrite formation at temperatures ranging from 999 ℃
to 1259 ℃, and lastly, the formation of tricalcium aluminate also at temperatures ranging from
999 ℃ to 1259 ℃. These formation reactions within the transition zone strictly take place in the
kiln.
Before reaching the cooling zone, the material undergoing pyroprocessing reaches the rotary
kiln’s sintering zone where it is further subjected to even higher temperatures ranging from
about 1305 ℃ to approximately 1439 ℃, thereby leading to the formation of tricalcium silicate
components (Benhelal et al., 2012; Hokfors, 2014; Manias, 2004; Rodrigues et al., 2017; Wang,
2006). In the transition zone, a zone further known for liquid phase formation, the significantly
increased reaction heat amounts (−698.71 kJ/kg) are released with an aim of elevating the feed
material’s temperature from about 960 ℃ to approximately 1259 ℃. Moreover, tricalcium
silicate formation from dicalcium silicate and calcium oxide in the sintering zone is an
exothermic reaction (−490.33 kJ/kg), meaning it mainly occurred at a temperature greater than
1430 ℃, roughly at a temperature ranging between1430 ℃ and 1439 ℃. Hence, according to
(Jons, 1980), this tricalcium silicate formation reaction is exothermic under temperatures greater
than 1430 ℃ and endothermic under temperatures below 1430 ℃.
After all these reactions, at any temperature of more than 1259 ℃, the solid clinker would then
melt and consequently a perfectly nodulized clinker mixture would be produced. Following the
leaving of the produced clinker from the high temperature transition zone, the produced clinker
proceeds to the cooling zone and to the air cooler where it is cooled. The cooling of clinker
mainly takes place in the air cooler than in the dry rotary kiln’s cooling zone, thereby the cooling
zone in the dry rotary kiln would be extremely short (Wang, 2006).
P a g e | 25
2.2.2. Clinker Quality Parameters
According to Fadayini et al. (2020) and Winter (2005), the chemical parameters are based on the
oxide composition, and they are crucial in detailing the characteristics of finished clinker
product.
First, is the Lime Saturation Factor (LSF), a ratio of calcium oxide to other main oxides,
particularly estimates the ratio of 3CaO ∙ SiO2 to 2CaO ∙ SiO2 in the clinker. With clinker having
increased LSF expected to be comprised of increased 3CaO ∙ SiO2 to 2CaO ∙ SiO2 proportion in
comparison to clinker with lower LSF. On the other hand, LSF values exceeding 1.0 would be
signaling an increased possibility of having unreacted lime within the clinker.
𝐿𝑆𝐹 =
𝐶𝑎𝑂
2.8𝑆𝑖𝑂2+1.2𝐴𝑙2𝑂3+0.65𝐹𝑒2𝑂3
𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 1
This LSF calculation is also applicable to ordinary Portland cement (comprised of only clinker
and gypsum) with a modification of subtracting (0.7 × SO3) from the CaO content. However,
this calculation still does not account for fine limestone in cement, even slag together with fly
ash given they are used as substitute raw materials are left unaccounted for, therefore the
presence of these fine raw materials would require a more complicated calculation of LSF.
According to Winter (2005), by estimating the amount of carbon dioxide and further accordingly
adjusting the formula, limestone could be quantified. However, the presence of either or both fly
ash and slag as substitute raw materials may see this clinker LSF calculation being highly
inconvenient, practically.
Following, is the Silica Ratio or Silica Modulus abbreviated as SR, with its’ increase signaling
that the clinker is comprised of increased amounts of calcium silicates and low amounts of
ferrites and aluminates. The typical silica ratio ranges from 2.0 to 3.0 (Fadayini et al., 2020 and
Winter, 2005).
𝑆𝑅 =
𝑆𝑖𝑂2
𝐴𝑙2𝑂3 + 𝐹𝑒2𝑂3
𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 2
Lastly, is the alumina ratios abbreviated as AR, a measure of aluminate and ferrite phases in the
clinker. In Portland clinker it typically ranges from 1.0 to 4.0, with clinker products having a
P a g e | 26
higher AR values signaling proportionally increased aluminate together with decreased ferrite
contents (Fadayini et al., 2020 and Winter, 2005).
𝐴𝑅 =
𝐴𝑙2𝑂3
𝐹𝑒2𝑂3
𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 3
The fourth and unpopular parameter as detailed by Winter (2005) is the Lime Combination
Factor abbreviated as LCF. It resembles the LSF parameter however with a subtraction of free
lime content from the whole CaO amount. Moreover, given its’ value is 1.0 for instance, that
would mean the greatest silica portion is in the form of 3CaO ∙ SiO2.
2.3. Energy Efficient Processing
During the production of cement, particularly in the pyro-processing stage, cement kilns play a
very critical part, and their improvements towards sustainability and optimum manufacturing
technologies have since been the centre of attention. Dating back in 1930's, when the Germans
made attempts of redesigning the kiln system with an aim of reducing fuel amounts that were
being wasted, and consequently coming up with remarkable grate preheater and gas suspension
preheater technological developments.
According to Manias (2004), main types of cement kilns include the rotary kilns as well as the
vertical cement kilns. Amongst these cement kiln types only rotary kilns are widely used for
clinker production as of late, and therefore the following discussions will be based on
productions via rotary kilns. Hence, with the dry processing routes being more preferrable and
with modern cement plants rapidly adopting the use of them, the discussions in this paper will be
more focusing on a dry method of processing.
The success of the rotary kiln in the cement industry came about due to the decreased kiln size
while the kiln dimensions remained unchanged, and the increased production capacity which
resulted in operational costs reductions accompanied by a break-through in efficient energy
consumptions due to modern preheating and precalcining installations according to Telschow
(2012). With the cement kilns being the hearts of the cement production processes, the rotary
kilns which are known for the continuous production of large cement or clinker quantities while
maintaining uniform and high-quality products, have to further fulfil their other purpose of
P a g e | 27
maintaining low operating costs with clinker production at its maximum rate. However, saving
on costs is not only the area of focus. Given the looming resource depletion and climate crises,
technologies including efficient coolers, precalcination, together with proper modeling of the
kiln and successful search for accurate heat transfer phenomena may prove to be even more
useful in saving energy, broadening and increasing sustainable alternative fuel types that can be
utilized, as well as reducing the associated greenhouse gas emissions.
2.4. Additives Management
The manufacturing of oil well cements undergo in a way that they must meet the desired physical
and chemical standards, standards that will enable this manufactured cement to be successfully
applied on a certain type of well formation. In most cases, additional or corrective components
must be added to produce optimal compositions. Moreover, in relation to the application of that
particular cement, multiple cement additives may be included. Joel (2009) and Fink (2015)
reported that these cement additives are the blended chemicals and materials, blended into the
base cement slurries where they enhance cement performance. They are added under normal
conditions in the manufactured clinker after its thorough grinding and milling, thereby leading to
the production of a cement product with optimal compositions (Nilsson et al., 2007).
When these cement additives are added into the cement formulations, they are added with
regards to various aspects (Fink, 2015). Depending on each type of cement additive, cement
additives are generally added for the following reasons; dispersing cement particles, modifying
cement setting time while facing a challenge of the wells’ extreme temperature and pressure
conditions, controlling filtration loss of the liquid from the cement slurry, indemnifying for
shrinkage of the cement while it undergoes setting and hardening, ensuring improvements of
interfacial bonding between the cement and casing, and for controlling the formation fluids
inflow into the cement column as the oil cement sets.
While formulating the appropriate cement slurry for any cementing operation, appropriate
selection of chemical additives and their optimum quantity is a must. Types of cement additives
include retarders, accelerators, extenders, fluid loss additives, lost circulation material,
dispersants, special additives such as antifoams, and weighting agents. Given that they are
developed to allow the use of Portland based cement in diversified oil and gas well applications,
P a g e | 28
each cement additive major type also has different types of chemical additives which perform
nearly the same function when designing cement slurry, and in this part few of the best in each
category are going to be reviewed (Anaele and Otaraku, 2020; Fink, 2015; and Oliveira et al.,
2015).
Loss of circulation issue when applying primary cementing would later lead to the requirement
of remedial cementing, however with proper utilization of lost circulation additives, loss of
whole fluids before and during a cementing job would be prevented. The addition of antifoams in
cement slurries on the other hand would be aimed at reducing foaming as well as minimising air
entrainment during mixing. While expansion additives would be added with an aim of improving
the cement to pipe and formation bond by causing the exterior dimensions of cement that is
setting to grow slowly when the cement is exposed to down-hole fluids, weighting agents are
added with an aim of increasing the density of the cement slurry, and light weight additives are
added with an aim of lowering hydrostatic pressure as well as improving slurry economy
(Mixhaux et al., 1989; Nelson and Guillot, 2006).
Fluid loss control additives on the other hand reduce the rate at which cement slurry filtrate is
lost to a permeable formation given a cement slurry is being applied across a permeable
formation, especially under pressure. They viscosify the mix water or plug the pore throat in the
filter cake with long polymer chains (Nelson and Guillot, 2006). Given that failure to control
fluid loss of the cement would result to early slurry dehydration thereby early cement hardening
and further secondary cementing might be required since the speedy escape of fluid from cement
would form holes, therefore one may indeed be advised to make use of fluid loss additives
(Broni et al., 2015).
Friction reducers, commonly known as dispersants, act on the surface of cement grains making
cement slurries less viscous thereby they would be mixed and pumped easily. Considering the
fact that dispersants not only decrease the rate of turbulent flow but also often exhibit secondary
retardation and further improve fluid loss control, thereby it is also advisable for one to make use
of them accordingly with quality understanding of the end results (Nelson and Guillot, 2006).
Extenders reduce slurry density thereby also reducing the hydrostatic pressure during cementing
operation and avoiding lost circulation that may come about due to breakdown of weak
formations. In the meantime, they may also increase slurry yield where the cement quantity
P a g e | 29
needed for production of a targeted volume of set product would be reduced (Nelson and Guillot,
2006). Given the facts that extenders not only lighten cement slurries to protect weak formations,
but they also have a possibility of being used to increase slurry yields, thereby one may indeed
be advised to consider using them where necessary since the increased slurry yields would lead
to reduction in costs and further to cuts in budgets while also it would mean less cement was
used thereby less clinker amounts utilized and consequently less concerning pollutant emissions
would have been released during the production of reduced clinker amounts.
2.4.1. Accelerators
Accelerators as cement additives are added with an aim of cutting down the time it takes for
cement to set, generally by speeding up the reaction rate between water and cement, in some
cases also increasing early strength development (Myrdal, 2007). This means the cements’
thickening time shortens given the accelerated compressive strength development of cement,
thereby the costly operating oil rig times consequently shorten as a result the rigs operating costs
are also reduced. According to Myrdal (2007), with some accelerators accelerating either setting
or hardening and some accelerating both setting and hardening, actually the starting point or the
initial action of the accelerator takes place in the plastic state of the cement paste, and in the
latter stage the accelerator mainly take action in the hardened state. Even though accelerators
function differently and incompatibly, Myrdal (2007) implied that a significant number of
accelerators have an effect on both the setting and hardening of cement, occasionally based on
the amount of the accelerator put.
Types of accelerators include soluble inorganic salts and soluble organic salts as well as
compounds, with most accelerators on the market being the mixtures of compounds from both
soluble organic and inorganic salts. Soluble organic salts and compounds include a group of
alkanolamines, and a group of carboxylic and hydroxycarboxylic acids as well as their salts.
Moreover, triethanolamine, and calcium formate, respectively, are the most commonly used
accelerators among their groups (Myrdal, 2007). One can be encouraged to use triethanolamine
when they have it readily available and need to accelerate the hydration of aluminate-containing
phases, particularly tricalcium aluminate hydration kinetics, while retarding the hydration of
tricalcium silicate. Moreover, the addition of triethanolamine as an accelerator might result into
it reacting with the ferrite phase of Portland cement, thus resulting into having a chelating effect,
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and the chelating effect of triethanolamine with ferrite ions either accelerates or retards the
hydration of cement depending on the applied dosage (Myrdal, 2007; Yohannes et al., 2017).
Calcium formate functions as expected for an accelerator, it decreases the setting times, while
increasing the compressive strength. Moreover, according to Heikal (2004) it decreases total
porosity and initiate the unloosening of calcium hydroxide. Lower porosity, also given that there
is sufficient binding material content, means the cement has higher strength. Furthermore, the
unloosened calcium hydroxide, which is one of the major constituents of hydrated Portland
cement is a good indication for evolution of cement hydration reactions. Another
recommendation would be that of combining both triethanolamine and calcium formate to use
their mixture for accelerating cement hydration process, hence according to Myrdal (2007) the
combination of these two accelerators results into a synergistic effect, meaning their mixture
accelerates cement hydration at an even greater rate compared to these individual accelerators.
Soluble inorganic salts include alkali and alkali earth metals salts of hydroxide, chloride,
bromide, fluoride, nitrite and nitrate, carbonate, thiocyanate, sulphate, thiosulphate, perchlorate,
silicate, and aluminate (Myrdal, 2007). When it comes to participating in accelerating the
reaction rate, both the ions of the anion and the metal’s cation take part. According to both
Myrdal (2007) and Tamas (1966), calcium chloride is the most effective and widely utilized
accelerator, in such a way that nearly all manufactured accelerators that are in the market are
comprised of calcium chloride. Looking at the possibilities of an accelerator reacting with the
ferrite phase of Portland cement, according to Tamas (1966) such hypotheses were raised and
ruled out. These hypotheses referred to calcium chloride accelerating the cement hydration
process by chemically reacting with the aluminate and/or the cement’s ferrite phases or with
calcium hydroxide formed during hydration, while leaving the silicate phases unaffected. Tamas
(1966) further clarified that it is highly unlikely that these processes or reactions are needed
during the action of calcium chloride, given that aluminates and ferrites are only a minor part of
commercial Portland cement and their hydration products not only are small in absolute amount
but also have specific surfaces that are negligible in comparison with the hydration products of
the silicate phases.
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On the contrary of using the best of soluble organic salts and compounds as accelerators,
particularly the mixture of triethanolamine and calcium formate, one may instead be encouraged
to use calcium chloride to accelerate the process of cement hydration. Reasons for this
recommendation is the lower costs of calcium chloride meaning saving in costs, calcium
chloride’s readily availability meaning it can be acquired easily, and lastly but not least calcium
chloride has the predictable performance characteristics as it has spent decades being used and
studied as an accelerator (Myrdal, 2007; Tamas, 1966).
2.4.2. Retarders
Retarders as cement chemical additives are added with an objective of decreasing the speed of
cement hydration, by slowing down the setting time of the resulting cement slurry. Given the fact
that most of the time oil cements utilized in wells of different formations generally lack extended
fluid life for operating at bottom hole circulating temperatures of more than 38 ℃ which means
decreased thickening time, thereby the retarders play a role of hydration inhibition and delaying
the setting, consequently providing enough duration for slurry to be properly placed in deeper
and higher temperature wells, meaning the thickening time of pumping the cement slurry in
place would be increased (Anaele and Otaraku, 2020). Furthermore, the effectiveness of a
retarder in delaying cement hydration is dependent on the dosage of the additive as well as on
curing conditions, as a result the prediction of the Bottom Circulation Temperature (BHCT) must
also be accurate in order for the correct concentration of the retarder to be used, thereby avoiding
fast setting while also avoiding over retardation.
Even though the mechanism for cement retardation through the use of retarders is still
controversial, according to Nelson and Guillot (2006) existing theories, which are not fully
descriptive on their own, include the adsorption theory, where the retarder is believed to be
adsorbing onto the hydration product surfaces and as a result hindering contact with water; the
precipitation theory, where the retarder is believed to be reacting with ions of calcium, hydroxyl,
or both in the aqueous phase, and leading to the formation of a layer that is insoluble and
impermeable surrounding grains of cement; the nucleation theory, where the retarder is believed
to be adsorbing onto the hydration products’ nuclei and inhibiting the upcoming growth; as well
as the complexation theory, where the retarder is believed to act as a chelating agent, chelating
the ions of calcium and hinder the formation of nuclei. Factors counting the cement product
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phase (either aluminate or silicate phase) where the retarder shall be acting upon, as well as the
retaders’ chemical nature, are the main factors that must also be closely considered (Nelson and
Guillot, 2006).
Some of the most utilized chemical additives as the oil well cement retarders include
lignosulphonates which are the readily available cement retarders extracted from wood pulp and
as an aqueous solution they are known as the lignin liquor coming in a form of calcium and
sodium salts of lignosulphonic acids, several saccharides and natural gums; as well as cellulose
extracts counting carboxymethyl hydroxyethyl cellulose and hydroxyethyl cellulose (Mixhaux et
al., 1989; Nelson and Guillot, 2006).
The addition of lignosulphonate retarders in concentrations of about 0.1 −
1.5% by weight of cement generally lead to the retardation process in cement slurries being
effective until temperatures of about 122 ℃ are reached (Mixhaux et al., 1989; Nelson and
Guillot, 2006). Given one has to choose a cement retarder to be used at much greater depths of
temperatures higher than 122 ℃ and they only have lignosulphonates, they would be greatly
encouraged to use those lignosulphonates and further treat them with borax, hence according to
Anaele and Otaraku (2020), lignosulphonates mixed with chemicals including borax can be
utilized at well temperatures adding up to 315 ℃. Calcium lignosulfonate is regarded as the best
lignosulphonate retarder and it has guaranteed effectiveness at temperatures above 93 ℃
according to Bediako and Joel (2016).
Between the mentioned cellulose extracts or derivatives which are for use in well cementing jobs
as retarders, one may be advised to specifically utilize hydroxyethyl cellulose when they further
want to ensure constant maintenance of water/solid ratio in cement slurries downhole, hence
hydroxyethyl cellulose is also used as fluid loss additive (Nelson and Guillot, 2006).
Hydroxyethyl cellulose as a fluid loss additive would be controlling the rate of water that could
be lost to adjacent permeable zones through suitable viscosification by simply making the
cement slurry avoid losing water by filtration into permeable zones (Broni et al., 2015).
However, one may be advised to rather choose using carboxymethyl hydroxyethyl cellulose
instead of hydroxyethyl cellulose given the facts that carboxymethyl hydroxyethyl cellulose is
more effective as a retarder at temperatures of about up to 110 ℃ BHCT while hydroxyethyl
cellulose is only effective as a retarder at temperatures of about up to 52 ℃ BHCT where the
https://www.sciencedirect.com/topics/engineering/permeable-zone
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thickening duration in freshwater slurry would only be increased by roughly two hours (Anaele
and Otaraku, 2020). Given that the cement thickening time must be equal to mixing, pumping
and displacing cement job time plus another 1 − 2 hours duration as a reasonable safety factor,
and according to Lake (2006) most cementing jobs (mixing, pumping and displacing) are
completed in a duration of about 90 minutes or less (which is seemingly too much time given
only two hours extension is provided by hydroxyethyl cellulose as a retarder in freshwater
slurries), therefore one may indeed be advised to rather make use of carboxymethyl hydroxyethyl
cellulose as a retarder, especially in wells with greater depths as it seemingly guarantees longer
retardation periods even at higher bottom hole circulation temperatures.
One may still doubt the use of carboxymethyl hydroxyethyl cellulose instead of hydroxyethyl
cellulose, arguing that hydroxyethyl cellulose would be a better fit for their intended applications
as it would also guarantee that there would be no fluid losses. Nonetheless, they would still be
advised to choose carboxymethyl hydroxyethyl cellulose as a cellulose derivative retarder
appropriate for use since carboxymethyl hydroxyethyl cellulose also provide quality fluid loss
control while they also have been utilized for decades as the only cellulose derivative retarder,
meaning greater exposure on its ups and lows as well as more developed experiences in its usage
as retarders.
Given the review by Bediako and Joel (2016) that carboxymethyl hydroxyethyl cellulose as a
retarder, still at temperatures ≥ 110 ℃ BHCT and at equal concentrations as calcium
lignosulphonate retarder, these two retarders function the same, therefore should one have them
both readily available the only advice would be to choose either. However, they should consider
using calcium lignosulphonate in squeeze cementing since according to Rike (1973)
lignosulphonates used as retarders in squeeze cementing is more appropriate given they provide
consistent quality while the resulting dispersing effect also produces a more even slurry of gel
cements.
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2.5. Proposed Sustainable Production Processes
Below is Figure 1, a separate line preheater precalciner kiln system employing a dry
clinker/cement production process with an installed waste heat recovery system, showing the
proposed cement production process. The proposed dry processing route to produce cement
clinker utilizes a rotary kiln equipped with a single precalciner, four cyclone preheaters in each
of the two cyclone preheater towers, tertiary air duct, bypass system, as well as specifically low
NOx burners to fire the kiln system and the grate cooler for quality cooling of the produced
clinker product. Furthermore, waste heat recovery system is also installed for the recovery of
waste heat from the hot exhaust gases and convert it into electricity to be utilized in powering the
plants necessities including operating equipment counting induced draft fans (ID fans) among
them. Also, there is a proposed use of at least 60% waste derived fuels (refuse derived fuels or
solid recovered fuels) as precalciner fuel while raw materials would also be diversely utilized
accordingly.
The installed waste heat recovery power generation system utilizes the heat from the rotary kiln
released through the two preheater towers in the form of hot kiln exhaust gases as well as the
heat from the grate clinker cooler also in the form of hot exhaust air. The recovery of heat from
the preheater towers goes through the preheater boiler while from the clinker cooler it goes
through the clinker cooler boiler. Each boiler then release heat in the form of steam, followed by
the combining of those released steam streams into one stream of high-pressure steam before it
reaches the steam turbine generator which will then generate electricity simply by extracting
thermal energy from the pressurized steam then utilize it to carry out mechanical work on a
rotating output shaft. Given that the generation of electricity via the installed waste heat recovery
system comes about without any additional fuels in the kiln or in the precalciner, thereby it is
clear that this recovery system may have a significant impact on saving the plant’s energy costs.
P a g e | 35
Dust Collection
Dust collection BYPASS
Hot Exhaust Air
S
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Hot Exhaust Gases
Low NOx Burners
Clinker
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Preheater
Boiler
High Pressure Steam
Condensate
Pump
Condensate
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Condenser
Steam Turbine Generator
~
Kiln Exhaust
Gases to
Desulphurization
Clinker Cooler
Exhaust Air to
Desulphurization
Grate Cooler
WASTE HEAT RECOVERY SYSTEM
B
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Dust Bin
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Induced
Draft Fan
Treated Dust to In-Plant Recycling
P a g e | 36
Figure 1: A Separate Line Preheater Precalciner Rotary Kiln System Employing a Dry
Clinker/Cement Production Process as well as the Waste Heat Recovery System (Sourced from
(Atmaca and Yumrutaş, 2015; Bayuaji et al., 2016; Harder, 2002; Huang et al., 2005;
Tsamatsoulis, 2016).
In the grate cooler, as the cooling air would be flowing through the grate plate which moves from
the zone of effective cooling to the zone of recuperating heat, it is important that these zones
within a grate cooler are separated enough and they are individually optimized, thereby
improving heat transfer while at the same time ensuring quality clinker cooling.
Considering that the performance of the grate cooler can also play a significant role in waste heat
recovery, where it recovers heat from the hot clinker exiting the kiln, thereby the transfer of heat
within it would also be managed accordingly for the optimum recovery of waste heat as well as
for quality cement clinker formation conditions. Improved heat transfer within the grate cooler
would result to improved waste heat recovery. Hence, the grate cooler’s speed would then be
lowered thereby increasing the thickness of the cement clinker bed on the grate, particularly on
the zone of heat recovery that is separated from the zone of effective cooling within the grate
cooler, as a result the transfer of heat in the grate cooler would be further improved. The grate
plate for the installed grate clinker cooler would further have air outlets positioned horizontally
together with an increased pressure drop, thereby improving the transfer of heat within the grate
cooler even more and consequently improving the recovery of waste heat (Radwan, 2012).
The recovery of waste heat energy in this proposed cement production process would further be
ensured by the reduction of primary air entering the kiln system through the low NOX burners,
while ensuring the increase of secondary air and tertiary air from the grate cooler. The use of low
NOX burners would specifically contribute to the lowering of pollutant NOX emissions since the
proposed separate line preheater precalciner rotary kiln system tend to release an increased
amount of nitrogen oxide pollutants due to having much higher peak temperatures at times
(Hassold, 2018).
This proposed rotary kiln system equipped with waste heat recovery system is also expected to
generate waste cement kiln dust in increased amounts, and the increased dust amounts would
P a g e | 37
specifically pose an issue on the waste heat recovery system and to the performance of the kiln,
mainly in preheaters. Moreover, due to high chances that a considerable amount of heat can be
lost through the rotary kiln’s shell, fuel amounts would be reduced in the main kiln burner while
they are increased in the precalciner. However, this way of reducing the rotary kiln’s thermal
load may possibly lead to the formation of cement kiln dust that cannot be recycled for reuse in
the process since more alternative or substitute waste fuels (especially refuse derived fuels) are
intended to be used to fire the precalciner, and the resulting cement kiln dust would be composed
of increased chlorine metal contents. Hence, the use of more and more of waste derived fuels
increases the amounts of chlorine metals in the system’s generated dust and gases. As a result,
the bypass system is introduced, and its’ installation is indeed necessary although it would
contribute to loss of considerable heat amounts as some of the precalciner/kiln hot exhaust gases
would exit along with the hot removed dust, meaning the preheater systems’ thermal efficiency
would also be reduced by the reduction of internal circulating dust in the preheater system.
Also, given that dust from the grate cooler dust is usually hard and abrasive, while the dust from
the kiln is usually sticky and fine powdered, thereby the introduced bypass system from the
precalciner is mainly installed to recover fine powdered and obviously high alkaline cement kiln
dust, hence waste derived fuels would mainly be fired in the precalciner. The bypassed chlorine
concentrated dust would then be treated where the expected high chlorine amounts would be
removed, and thereafter the treated dust would be recycled to the raw mixture for reuse as a raw
material for the same cement production process. Moreover, this bypass system would be
ensuring that chlorine build-up in the to be released exhaust gases is minimised as much as
possible. In addition, the efficiency of cyclone preheaters when it comes to separation would be
optimized if it needs be (possibly by pressure drop). Also, the dust removers are already installed
for constant removal of dust in the hot exhaust gases leaving the grate cooler and the preheater
towers hence large amounts of dust are expected to come along. Furthermore, to control the
effect of dust that might enter the installed waste heat recovery system with hot exhaust gases,
the boilers could be further equipped with a cleaning device (Radwan, 2012).
Quality pyro-processing of the raw materials would be inevitable given a more effective
combustion environment within the proposed separate line preheater precalciner rotary kiln
system. To ensure higher enough degree of raw meal calcination for the production of good
P a g e | 38
quality cement clinker, the raw meal’s residence time in the kiln system would be mainly
controlled by the installed kiln inlet seals which also minimize the induced false that may
negatively affect the performance of the kiln and the recovery of heat energy in the grate cooler.
Further with effective solid gas separation, efficient circulation of dust, as well as the quality
precalcined raw materials due to proper modeling of the kiln system coupled with easily
regulated kinetic behavior, a higher enough degree of calcination would surely be obtained
(Manias, 2004).
Although an exhaust gas heat exchanger is not included in the proposed cement clinker
production process, its’ installation is highly recommended. It would be installed between the
boiler and the exit point of exhaust gases coming from the boiler, while being connected to pure
cycle unit and further to the already existing cooling tower. The exhaust gas heat exchanger
would lead to reduction in maintenance costs, since with its installation the costs of dismantling
the entire waste heat recovery system to replace the boilers would also be eliminated.
The exhaust gas heat exchanger would be using water to recover heat, thereby additional to heat
recovery it would serve to filter sizeable portions of pollutants out of the exhaust gases,
especially carbon dioxide pollutant gases. This would further decrease the strain on the filter
systems that are also highly recommended for installation in this proposed production process.
However, the installation and operation of the installed bypass duct (leading to the chlorine/dust
bypass system) together with the operation of the recommended exhaust gas heat exchanger or
the already installed waste heat recovery system would individually bring an increase in the
system’s pressure. As a result, induced draft fans are installed to maintain minimal pressure
(pressure drop) across both the heat recovery and bypass systems, and the more the pressure in
the system the more fan power would be required.
Furthermore, the released exhaust gases, especially the exhaust gases from the preheater systems,
they would be desulphurized via the dry lime scrubbing technique, where lime would be injected
directly into the exhaust gas for the removal of not only sulphur dioxides but also acidic gases of
hydrochloric acid. The rest of the pollutant gases including carbon dioxide, nitrogen oxides as
well as the unrecycled dust would further be managed sustainably.
P a g e | 39
3. EXPERIMENTAL PROCEDURE
Experimental work carried out for this project involved prior analysis of the local class A
cement, preparations of the PET/PET plastic waste based additives, and the base design.
Preparations of oil well cement slurries blended with the prepared PET/PET based additive(s)
followed, accompanied by the determination of relative compressive strengths and rheological
behaviours.
Composition of the used local class A cement was determined through XRF analysis using the
X-ray Florescence (XRF) spectroscopy.
3.1. Materials
The under investigation local class A cement (42.5R AfriSam high strength cement) shown in
Figure 2 was provided by AfriSam South Africa. It is for use under normal conditions, when
there are no particular properties specified for any other type are required (For general use).
Figure 2: Picture of the Utilized Local Class A Cement.
Utilized oil well cement additives included the polycarboxylate based superplasticizer (PCE)
provided by Sika South Africa, citric acid as a retarder, calcium oxide as an expanding agent,
polyvinyl alcohol as a fluid loss additive, and polyethylene glycol (PEG 6000) as a defoamer.
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Calcium oxide, polyvinyl alcohol, and polyethylene glycol (PEG 6000), and citric acid were all
sourced from Sigma Aldrich, together with lead acetate (used as a catalyst during PET
depolymerisation) and ethylene glycol (≥ 99.5%). Also used as a solvent in depolymerisation of
PET to yield the BHET additive. These chemicals did not undergo any further processing or
purification; they were used in their original sourced state.
Polyethylene Terephthalate (PET) was sourced from waste PET plastic bottles, which were
washed and dried before they were processed accordingly.
3.2. PET Additives Preparations
Waste PET plastic bottles were washed using tap water, dried, and cut into flakes with
dimensions of about 1 cm × 1 cm, with the harder top and bottom parts of the bottles being left
out. Thereafter, the flakes were uniquely prepared or processed as explained and shown below to
yield the respective PET derived additives.
3.2.1. Additive I- Pure PET Fibres
After the cut PET plastic flakes with dimensions of about 1 cm × 1 cm were obtained, they were
further cut into short and thin sized PET fibres using a scissor. Final length of the fibres was
limited to 1 cm while the thickness was limited to less than 1180 μm, with the help of sieve
analysis the fibres that did not pass through the sieve of 1180 μm by size were eliminated or cut
further as represented in Figure 3.
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Figure 3: (a) Waste PET Plastic Bottles (b) Cut PET Plastic Flakes (c) Prepared PET Fibre
Additive (d) Sieve Through to Eliminate Thicker Fibres.
3.2.2. Additive II-Irradiated PET Fibres
Washed (using tap water), dried, and cut waste PET plastics with dimensions of about 1 cm ×
1 cm were sent to an irradiation facility where they were irradiated at radiation doses of 5.8 Gy/
min up to a total dose of 10 kGy using a cobalt-60 gamma irradiator.
Irradiated PET plastic flakes were then further cut into short and thin sized PET fibres using a
scissor. Final length of the fibres was limited to 1 cm while the thickness was limited to less than
1180 μm, with the help of sieve analysis the fibres that did not pass through the sieve of
1180 μm by size were eliminated or cut further.
3.2.3. Additive III- Depolymerized PET Plastic Powder Additive
PET plastic was depolymerized through glycolysis to form Bis(2-Hydroxyethyl) terephthalate
(BHET) as follows.
PET plastic flakes weighing 20.132 grams and 80 mL of ethylene glycol (a solvent) were
introduced into a 250 mL round bottom flask, followed by the addition of 0.406 g of lead acetate
as a catalyst for this chemical depolymerization of PET reaction. The prepared mixture was then
refluxed at a temperature of 190 ℃ in a closed system for 90 minutes, using a glass condenser
and continously running tap water for cooling. As the reaction continued under reflux, the
mixture changed from being clear with visible continously dissolving flakes of PET to being
(d) (c) (b) (a)
P a g e | 42
uniform, viscous and grayish liquid, which further turned into an opaque gray coloured semi-
solid when cooled to room temperature.
After the mixture was heated under reflux and allowed to cool, unreacted ethylene glycol was
separated from the refluxed mixture by adding 40 mL volume of distilled water to dissolve it,
followed by heating the resulting mixture while stirring using a magnetic stirrer hot pla