Desulphurisation of high sulphur coal to improve industrial utilisation

Abstract

Coal from northeastern India is considered to be a very good quality coking coal owing to its low ash content (<8%). It has an excellent caking property, and after carbonisation it can provide a very good CSR (coke strength, after reaction with CO₂). On the other hand, its metallurgical application is not so encouraging as a result of its high sulphur content (3-7%). Normally the acceptable sulphur content in metallurgical coke is 0·7% maximum. About 90% of the sulphur is present in organic form, and the remaining 10% as pyrites and sulphates. Removal of sulphur from the organic types is somewhat complicated as it cannot be done by any physical process such as benificiation or flotation, whereas sulphur in pyrites and sulphates can be removed by physical means. With a view to lowering the sulphur content of the coal, desulphurisation studies were carried out at elevated temperatures ranging from 400 to 950°C under the flow of various reducing gases such as coke oven gas, water gas, steam and pure hydrogen. The retention time at each temperature of experimentation was usually varied at 15, 30 and 45 min. It was observed that the sulphur content in the coal was decreased in every experiment with the various reducing gases. The extent of sulphur removal in treatment of the coal with coke oven gas was up to 78·1%, that with steam 83·1% and that with water gas 85·7%. The minimum sulphur content found in desulphurised coal was 0·86%, which could not be used for metallurgical purposes unblended. In the case of experiments with steam, a severe loss of carbon (15-20%) was observed because of reaction with the steam, whereas no such losses were observed in experiments with the other two reducing gases. Hydrogen lowered the sulphur content at a faster rate in comparison with the other gases, but could not increase the degree of reduction. The present study may help to establish the technoeconomic feasibility of the processes; however, large scale experimental studies are required for confirmation. I&S/1721