Combustion characteristics of coal, biomass and their chars in air and oxy-fuel environments

Carbon capture and storage (CCS) is expected to play an important role in mitigating the effects of climate change. The focus of this work is to determine how the change of combustion environment in an oxy-fuel CCS plant affects the combustion behaviour of coal, biomass and a torrefied biomass. The industrially relevant fuels selected were analysed to determine their fundamental composition and combusted in air and a range of oxy-fuel environments (5-30% O2/CO2) using a thermogravimetric analyser (TGA). The key temperatures and kinetic parameters of both the devolatilisation and char combustion stages were investigated to determine how the shift to an oxy-fuel combustion environment effects overall combustion behaviour. The changes in devolatilisation behaviour were determined through the derivation of apparent first order kinetics and no noticeable difference between combustion in air and 21% O2/CO2 atmospheres were observed. The increase in oxygen concentration in the oxy-fuel environments resulted in linear increases in kinetic parameters which were then used to develop fuel specific empirical equations that relate the devolatilisation rate to the oxygen concentration. The devolatilisation of the biomass fuels were shown to be more sensitive to the change in combustion atmosphere than the coals. Chars were produced using ballistic heating rates in a TGA (1000 K min-1) and it was found that the coals exhibited similar mass loss behaviour in N2 and CO2 environments during char production. The biomass and torrefied biomass samples showed enhanced devolatilisation in CO2 atmospheres which leads to differences in the char combustion behaviour between the coal and biomass fuels. The char combustion behaviour was determined through the determination of apparent mth order kinetics, from which, fuel specific nth order kinetic models were derived to describe char combustion accurately over the full range of oxy-fuel combustion atmospheres. The kinetic parameters determined highlighted the similarity between the N2 and CO2 produced coal chars and the difference between the biomass chars. The coal chars were found to be more sensitive to the change in combustion atmosphere. The work in this thesis gives a good understanding of the differences between conventional air and oxy-fuel combustion atmospheres using industrially relevant fuels. Several useful kinetic models have been derived for both the devolatilisation and char combustion stages that lend themselves to computational fluid dynamics and process optimisation while the fundamental characterisation lends itself to life cycle analysis of CCS systems