Investigations of the Role of Coal Thermoplastic Properties and Coke Structure in Gasification Processes

Coal classification schemes have been developed over many years from studies of the physical and chemical properties of coals as a method of dividing coals into various categories according to their behaviour during carbonisation and combustion. Although there is no universally accepted scheme, most coal classification schemes are based on two main types of parameters:a)volatile matter, and b)caking and swelling properties. The limited scope and empirical nature of the techniques used in classification schemes and their variation with experimental conditions means that classification schemes have only limited applicability when comparing coal behaviour in high pressure gasifiers where the nature of the process is a primary consideration and the experimental conditions are vastly different from those used in the tests. The thermoplastic properties of coal, as well as characterising the initial agglomeration processes, are important in determining both porous structure and carbon texture of the coke product. Both the coal and coke structures are significant factors in determining gasification behaviour. The coke strength and reactivity during gasification are of considerable significance in fixed bed gasifiers. The agglomerated coke is broken up by the stirrer to facilitate the countercurrent flow of gases and solids(1) and hence, the mechanical strength of the coke may be an important factor. In the case of weak cokes, a wide size range may be produced and increased levels of dust may occur causing a loss of efficiency and throughput. Coke structure is important in determining reactivity towards oxidising gases which is a main gasification step. The objective of this study is to provide information on the dependence of coal and coke properties under experimental conditions found in fixed bed gasifiers. The direct link between measurements of coal thermoplasticity and coke structure obtained by examining the structure of the carbonised residues from the high pressure thermoplasticity measurements is considered to be an important aspect of the investigation since it eliminates any possible ambiguities in comparing various sets of data. From these considerations and other experiments under simulated gasifier conditions, comparisons with gasifier operational data can be made which should make it possible to develop optimum methods for predicting coal performance in a particular process.