Determination and characterization of sulphur in South African coal

Abstract

Sulphur determination is of priority in coal investigations due to its associated environmental pollution. There are generally various forms of sulphur in coal such as organic and inorganic forms both of which need to be characterized. However, it is the organic sulphur present in coal that is not well understood and studied. In this work, total and various forms of sulphur have been identified in raw coal samples from Camden, Majuba, Lethabo, Kriel, Duvha and Thuthuka power stations in South Africa. Organic sulphur compounds (2- methyl thiophene, 3-methyl thiophene, 2-ethyl thiophene and dibenzothiophene) were characterized from these coal samples in order to determine the quality of South African coal. Organic sulphur compounds were extracted from coal samples by means of ultrasonic bath. The extraction of organic sulphur compounds was first optimised by using various organic solvents such as dichloromethane, toluene and hexane followed by quantification using gas chromatography with a flame ionisation detector. The extraction time was also optimized from 0 to 75 minutes while spiked standard concentrations were varied from 1700 to 17000 mg kg-1 to control the process. Ultrasonic extracts were analyzed by GC equipped with SPB-1 Sulphur column and flame ionisation detector. Microwave assisted extraction was used to extract various forms of sulphur in coal. During sequential extraction, sulphate and pyrite sulphur were extracted from coal samples using hydrochloric acid and nitric acid respectively. Total organic or total sulphur and selected metals in coal were extracted by means of a mixture of concentrated hydrofluoric acid, hydrochloric acid and boric acid. Inductively coupled plasma-optical emission spectroscopy allowed the analysis of all total and other inorganic sulphur forms as well as selected metals from Microwave assisted extraction system after calibration set with standards. Total sulphur compounds and other sulphur forms were also determined using elemental analyzer for carbon, hydrogen, nitrogen and sulphur (CHNS). The standard calibration for CHNS determination was assessed by the use of Sulfamethazine, a certified reference material. 2-methyl thiophene, 3-methyl thiophene, 2-ethyl thiophene and dibenzothiophene were identified in coal samples. The organic sulphur compounds found in most coal samples were methyl thiophenes. Extraction time effect on the recovery of organic sulphur compounds showed that the recovery of organic sulphur investigated was up to 88% with 75 minutes ultrasonic bath extraction. Spiked concentration effect on the recovery of the target compounds was observed at 75 minutes of extraction and the recovery of extraction was generally in the range of 64.42-70.11%. However, an average of 66% recovery of target compounds was obtained from the highest spiked concentration. Duvha raw coal samples were found with more target organic sulphur compounds compared to other samples. Dibenzothiophene was found in highest concentration (15.5 mg kg-1) among the compounds identified. Products from preliminary pyrolysis of coal samples were identified by coupling pyrolysis furnace with GC-FID. The chromatograms showed four peaks identified in pyrolysis products from blank and spike coal samples. Those compounds proved to be the fragmentation products of target organic sulphur compounds. CHNS results showed that carbon, hydrogen, nitrogen and sulphur contents in coal had average values of 66.20%, 2.98%, 1.15% and 0.92% respectively. Those values agreed with figures reported by certified coal samples and confirmed the rank of the coal samples which is of the bituminous type. Microwave assisted extraction followed by Inductively coupled plasma-optical emission spectroscopy results showed that the total sulphur in coal samples had an average value of 8758 mg kg-1 and each sample showed that the total sulphur content is less than 20000 mg kg-1 (2%). Results also showed that the average value of sulphate content (600 mg kg-1) is lower than pyrite (4500 mg kg-1) and organic (3600 mg kg-1) forms. However organic and pyrite forms competed for their presence in coal samples. Microwave assisted extraction followed by Inductively coupled plasma-optical emission spectroscopy results of the various sulphur forms were in the range of values reported in certified material. Coal sulphur content is considered low from 0 to 20000 mg kg-1 and high when it is beyond 20000 mg kg-1. This means that South African coal generally has low sulphur content as referred to by the results obtained in this work. Sulphur compounds were found in coal samples as sulphate, pyrite and organic forms. Thiopene compounds found within the organic forms, moisture, ash and CHNS contents from this study showed that South African coal is generally bituminous type.