An interpretation procedure for thermogravimetric analysis on South African coals

Abstract

Coal is a chemical compound with a complex composition. Proximate and ultimate analysis as well as physical and mechanical tests which are available do not furnish all the information required by industry. Information regarding e.g. the burning properties of coal is required by furnace and boiler designers. Thermal analysis can be used for studying these properties using small masses of coal. Differential thermal analysis (DTA) where temperature variations are measured which result from reactions which occur when coal is heated and thermogravimetric analysis, (TGA) where similarly mass variations are measured, are regarded as important. Results from eg. TGA tests are usually interpreted without unified and formally accepted prescripts. It is desirable to investigate how the best use can be made of thermal analysis of coal, using DTA and TGA, and to devise a procedure for interpreting the results. The problem is approached by studying DTA and TGA results obtained from a specially selected group of coals for which other analytic data are available. The samples were prepared according to the standard procedure for the laboratory analysis of coal. At first, related information published by other researchers were integrated and parameters used by them identified and assessed in terms of fundamental scientific principles. Special experiments were conducted to probe the situation regarding orthodox kinetics of reactions and its related theory and observed discrepancies with regard to coal and its decomposition characteristics. It was found, along with other observers, that eg. activation energy values which are derived from TGA data should be used with extreme caution. In general an empirical approach for interpretation of results has much to offer. A system is developed by which various thermal techniques, executable by a single instrumental arrangement can be identified by expressing them in matrix format using as indexes, heating arrangement, reactant type and reaction environment. 5(1,1,2) e.g. designates a linearly heated mass of coal in air at atmospheric pressure. To enhance the comparison of TG characteristics of similar coals they should be classified according to a matrix arrangement C(g,t,r) which is based on three index parameters namely, grade ie. the ash content, type ie. the vitrinite content and rank. A TG test provides the basic requirements for computing the indexes using formulae provided by the originators. Information which can only be derived from a TG experiment e.g. 5(1,1,2), are referred to as specification parameters. The information consists of e.g. a series of temperatures which identify specific reaction changes in the coal. Several other parameters can be derived. Coals are assessed by comparing their specification parameters. Only two of the S(i,j,k) techniques are described in detail namely TG dispersion 5(1,1,2) and drop furnace burnout 5(3,1,2). The dispersion technique requires a linear increasing furnace temperature while the near sample temperature is also measured. This technique shows the decomposition stages of coal from dehydration to complete burnout. The drop furnace technique, involves a sudden exposure of the coal to a hot furnace environment with free air access: Actual burnout of coal is emulated. The model is not perfect for combustion but the best that can conveniently be acquired for comparing the burning responses of coals. To illustrate the type of information which can be obtained with other S(i,j,k) techniques experimental results of eg. a devolatilization test are presented. Brief reference is made to the role that particle size plays during combustion and the importance of the composition of volatiles which are released is indicated by presenting relevant information from external sources. The chemical composition and structure of the coals could not be determined but relevant information from an external source was presented which has a bearing on the practical analytic procedure which was described. After a study of the group TG characteristics of the various coals and discussion of the conclusions which are justified a practical procedure is described according to which the equipment can be used both for routine or special purpose analysis of coal.