Advanced oxy-fuel combustion for carbon capture and sequestration

This dissertation assesses the contribution of CCS in mitigating climate change, investigates Computational Fluid Dynamics (CFD) in aiding the development of CCS technology, and presents the results of air and oxy-fuel combustion experiments conducted in a 250 kW furnace.
Coal combustion was investigated using non-preheated and preheated air. Preheating increased the heat input to the flame and the radiative heat transfer near the flame region, enhancing flame stability and burnout. Radiative and convective heat transfer measurements showed that the total heat transfer is mainly influenced by thermal radiation, data on which is essential in validating newly developed radiation models.
Oxy-fuel experiments produced flue gas with over 90% CO2 concentration (allowing CCS without chemical scrubbing). Exit concentrations of NO and SO2 increased with reduced recycle ratio, largely due to the reduction in dilution. However, total NO emissions reduced by ~50% compared to air-firing, which was attributed to low levels of atmospheric N2 in the oxidiser and significant reductions in fuel NO formation.
Air and oxy-fired peak radiative heat transfer corresponded to a range typical of coal-fired boilers. For the oxy-cases, in-furnace temperatures and heat flux increased with total O2 concentration. Radiative heat transfer increased with higher gas emissivity. The results indicated that the air-fired temperature profiles can be matched when retrofitting to oxy-firing by modifying the recycle ratio, and the optimum ratio lies between the investigated cases of 27% and 30% O2 concentrations (using a dry recycle). The radiative heat flux profiles can also be adjusted. Temperature and heat flux measurements indicated delayed combustion due to the higher heat capacity of CO2 and delayed mixing between the Primary and Secondary/Tertiary streams.
CFD modelling was undertaken on 250 kW and 2.4 MW coal-fired furnaces under air-firing conditions, and a 500MWe utility boiler firing coal, a biomass blend, and 100% biomass under air and oxy-fuel conditions. Using wet recycle, the optimum total O2 concentration lies between 25 and 30%, where air-fired heat transfer characteristics can be matched without significant modifications when firing coal or the biomass blend, but not 100% biomass.