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

LES and RANS of air and oxy-coal combustion in a pilot-scale facility: predictions of radiative heat transfer

The development of carbon capture and storage (CCS) technology is important to permit the use of fossil fuels while honouring commitments to curb greenhouse gas emissions. Coal is a valuable global resource, which is widely available around the world, however its detrimental e ect on climate change will limit its use in a future with strict controls over carbon emissions. Oxyfuel combustion is a promising CCS technology that is being actively pursued in the development of large scale demonstration projects. Under the oxyfuel process for CCS, the combustion gas is replaced with a mixture of recycled ue gas and enriched oxygen. The resulting combustion environment can vary signi cantly from traditional air- red combustion. The development of modelling capabilities will greatly improve the optimisation process to develop oxyfuel technology into an economically viable prospect. This study evaluates the use of large eddy simulation (LES) and Reynoldsaveraged Navier Stokes (RANS) models on the prediction of thermal radiation during coal combustion for both air- red and oxyfuel operation in a pilot-scale 250 kWth furnace. The furnace is part of the UKCCSRC Pilot-scale Advanced Capture Technology (PACT) facilities and was designed for detailed analysis of the combustion process. Two radiation models were evaluated during the RANS calculations, the widely used weighted sum of grey gases (WSGG) andthe full-spectrum correlated k (FSCK) model, while the LES case was calculated using the FSCK radiation model. The results show that the LES solutions are in better agreement with measured values than the RANS predictions for both air- red and oxyfuel coal combustion, however LES demands considerably more computational resources

Prediction of the radiative heat transfer in small and large scale oxy-coal furnaces

Predicting thermal radiation for oxy-coal combustion highlights the importance of the radiation models for the spectral properties of gases and particles. This study numerically investigates radiation behaviours in small and large scale furnaces through refined radiative property models, using the full-spectrum correlated k (FSCK) model and Mie theory based data, compared with the conventional use of the weighted sum of grey gases (WSGG) model and the constant values of the particle radiation properties. Both oxy-coal combustion and air-fired combustion have been investigated numerically and compared with combustion plant experimental data. Reasonable agreements are obtained between the predicted results and the measured data. Employing the refined radiative property models achieves closer predicted heat transfer properties to the measured data from both furnaces. The gas-phase component of the radiation energy source term obtained from the FSCK property model is higher within the flame region than the values obtained by using the conventional methods. The impact of using non-grey radiation behaviour of gases through the FSCK is enhanced in the large scale furnace as the predicted gas radiation source term is approximately 2-3 times that obtained when using the WSGG, while the same term is in much closer agreement between the FSCK and the WSGG for the pilot-scale furnace. The predicted total radiation source term (from both gases and particles) is lower in the flame region after using the refined models, which results in a hotter flame (approximately 50-150 K higher in this study) compared with results obtained from conventional methods. In addition, the predicted surface incident radiation reduces by using the refined radiative property models for both furnaces, in which the difference is relevant with the difference in the predicted radiation properties between the two modelling techniques. Numerical uncertainties resulting from the influences of combustion model, turbulent particle dispersion and turbulence modelling on the radiation behaviours are discussed

Entrained Metal Aerosol Emissions from Air-Fired Biomass and Coal Combustion for Carbon Capture Applications

Biomass energy with CO2 capture could achieve net negative emissions, vital for meeting carbon budgets and emission targets. However, biomass often has significant quantities of light metals/inorganics that cause issues for boiler operation and downstream processes; including deposition, corrosion, and solvent degradation. This study investigated the pilot-scale combustion of a typical biomass used for power generation (white wood) and assessed the variations in metal aerosol release compared to bituminous coal. Using inductively coupled plasma optical emission spectrometry, it was found that K aerosol levels were significantly greater for biomass than coal, on average 6.5 times, with peaks up to 10 times higher; deposition could thus be more problematic, although Na emissions were only 20% of those for coal. Transition metals were notably less prevalent in the biomass flue gas; with Fe and V release in particular much lower (3–4% of those for coal). Solvent degradation may therefore be less severe for biomass-generated flue gases. Furthermore, aerosol emissions of toxic/heavy metals (As/Cd/Hg) were absent from biomass combustion, with As/Cd also not detected in the coal flue gas. Negligible Cr aerosol concentrations were found for both. Overall, except for K, metal aerosol release from biomass combustion was considerably reduced compared to coal