Analysis of co-firing biomass with South African coal in pulverised coal boilers
Date
2011-05-12
Authors
Pokothoane, Palo Sidwell
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Abstract
In this study, the effect on the extent of emissions reduction with co-combustion of
small proportions (0-20%) of biomass with coal in a pulverised fuel combustor, is
investigated. Such emissions include CO2, SO2 and NOx. South African coal has
high ash content and low volatile content that affects its ignitability. The effect of
biomass co-firing on ignitability and slagging is also investigated. While higher
percentages of biomass could reduce emissions more and improve ignitability,
biomass tends to increase the undesirable fouling and slagging propensities, hence
limiting the amount of biomass that can be co-fired.
Two types of biomass, namely grass and sawdust (calorific value 16-18 MJ/kg), and
one coal (calorific value 21 MJ/kg) were used in this study. Combustion tests were
carried out using the Eskom 1 MW Pilot Scale combustion Test Facility (PSCTF).
The coal chosen was representative of an average coal burned at the Eskom’s coalfired
power stations. For each of the types of biomass, three blends of biomass and
coal were used, resulting in seven different feed fuels including coal alone. The
ratios of biomass to coal, on an energy basis, in the three blends were 10%:90%,
15%:85% and 20%:80%. Seven tests were also carried out with the same fuels
using a drop tube furnace (DTF) to determine the reaction kinetics of the baseline
coal and its three different blends with each type of biomass. The reaction
parameters obtained from these tests were used as input data in numerical
simulations of the tests. Simulation using CFD software was used to predict
combustion characteristics of each fuel in the PSCTF, which in turn can be
extrapolated to predict the performance in a full scale commercial boiler. The
simulation results were validated by the experimental data from the PSCTF;
comparison of the combustion and emissions characteristics with experimental data
from the PSCTF showed that the simulation procedure was capable of predicting
these characteristics with generally good accuracy. The results coming out of this work are positive. Co-firing with grass at the PSCTF
was found to reduce the emissions by between 13% and 50% for NOX, between 12%
and 23% for CO2 and between 21% and 29% for SO2 as the proportion of biomass
increased from 10% to 20%. The maximum emissions reduction for sawdust
occurred at 20% co-firing ratio; these are, 29% for NOx, 17% for CO2 and 15% for
SO2.
For grass based co-firing, the combustion efficiency of the coal used was improved
by 0.55% and 0.62% for 15% and 20% co-firing ratios respectively; whereas that of
sawdust was between 0.78% and 0.38% as co-firing ratio was increased from 10%
to 20%. Combustion efficiency for 10% grass dropped by 0.65%. The DTF results
indicate that both grass and sawdust were able to improve the ignitability of the coal
used in a temperature range of 1000°C to 1200°C. The combustion efficiency
determined from DTF results for this range indicate an improvement of between
0.1% and 1.4% for grass and between 0.8% and 2.45% for sawdust. The QEMSCAN
analysis of slag deposits when co-firing biomass indicated that they should generally
be weak enough to be handled by soot blowing equipment.
In conclusion grass (herbaceous) biomass resulted in greater emissions reductions
than sawdust (ligneous). It was also found that slagging would be less of a problem
when using grass, because the slag deposits are more friable than those of sawdust.
At higher co-firing ratios grass based co-firing was found to improve coal ignitability
better than that can be achieved with sawdust. The optimum co-firing ratio with grass
would appear to be about 15% on an energy basis.
This project was carried out to obtain fundamental technical information on some of
the salient effects of using biomass co-firing with pulverised power station coal.
Before any implementation can be carried out, the next step would be to conduct a
detailed technical and economic feasibility study into possible large-scale
applications, using full systems engineering principles.