Coal characterisation and combustion

Abstract

There are three related studies within this thesis that examine the relationships between the properties of coals and the characteristics of the chars produced during rapid pyrolysis in a laboratory based Entrained Flow Reactor (EFR) which simulates the rapid rates of heating (104 to 105 °C s-1 ) typical of pulverised fuel boilers. The first study, using a suite of nine coals, investigates the influence of coal rank upon the generation of specific types of char, their respective physical and structural characteristics and their relative combustabilities. The second study, using a suite of twenty-two coals, examines various coal characterisation techniques and the correlations between those techniques and the associated char, and specifically investigates petrographic techniques as a means of characterising coal feedstock. The third study, using freshly mined coals, investigates the effects of oxidation (100°C, air) and weathering (ambient) upon standard analytical techniques and relates such changes to the physical, structural and combustion characteristics of the associated char. There is a common relationship between the elemental oxygen content of the parent coal and the generation of specific types of char for both vitrinite rich coals of differing coal rank and for the oxidised or weathered coals. There is also an inverse relationship (R 2 = 0.97) between the elemental oxygen content of a vitrinite rich coal and the proportion of cenospheres generated by pyrolysis at 1000°C using the EFR. Furthermore, the enhancement of char combustion at 1000°C (in an EFR) is related to the physical and structural characteristics of the char, i.e. the presence or absence of porosity (visible using SEM and TEM), the CO2 surface area and optical texture. A relationship exists (R2 = 0.83) between the morphology of a char (1000°C / N2) and the petrographic composition of the parent coal. The new term microlithotype, is an amalgamation of various vitrinite rich microlithotype classes that simplifies the nomenclature. A combination of calorific value, microlithotype, and coal rank (vitrinite reflectance) illustrates the influence of petrographic composition upon calorific value and also suggests a 'Province' dependency amongst the Cretaceous/Tertiary and Carboniferous coals studied. The coal properties calorific value, microlithotype, and coal rank can be related (R 2 = 0.91) to the proportion of porous chars for the Cretaceous/Tertiary suite of coals, illustrating the use of multivariate analyses when characterising coal feedstock. The effects of oxidation and weathering upon vitrinite fluorescence is also reported. The oxidation of coal at 100°C produces rims of quenched fluorescence which are not apparent within the weathered coals. Furthermore, the intensity of fluorescence at 650 nm (1650) decreases due to progressive oxidation or weathering, but decreases at a rate that is dependent upon the severity (temperature) of the conditions employed. The proposed oxidation quotient (0/Q = I65W%Romax) is a sensitive indicator of the oxidative conditions up to 100°C.