The increase of global carbon emissions by 2.7% in 2018 from the 2015 level indicates that the utilisation of fossil fuel, particularly coal, as the primary energy source is still very significant worldwide. Many countries have made efforts to reduce their carbon emissions by replacing coal with biomass. The UK government has promoted the gradual transition from coal-firing to biomass-firing to achieve complete coal power plant shutdown by 2025. This transition program is also supported with several incentive schemes to encourage the current and planned future coal-firing power plants to utilise biomass.
However, several disadvantages are attributed to the utilisation of biomass as an energy source. Significant exploitation of woody biomass contributes to flooding, increasing carbon debts, and food security issues due to deforestation. Additionally, the use of herbaceous biomass causes operational problems in power plants due to severe ash deposition and corrosion.
Treated wood is abundantly available and is known to complement virgin biomass in combustion to mitigate the disadvantages associated with woody and herbaceous biomass combustion. In the EU, 52.9 million tonnes of waste wood is produced annually, 54% of which is utilised to produce electricity. However, treated wood contains trace elements (TE), e.g. arsenic, cadmium, copper, lead, nickel, etc. sourced from prior anthropogenic treatment, e.g. preservatives and paints. These elements are relatively volatile at high temperatures and are released to the atmosphere during combustion. Ingestion of these elements may harm human metabolism and pose other health risks to wildlife.
This thesis aims to develop a comprehensive model to predict the fate and the occurrence of trace elements (TEs) and ash-forming elements (AFEs) in combustion of treated wood and virgin wood. The combustion process considered is based on the biomass combustion operation in the 250-kW entrained-flow boiler in the UKCCSRC Pilot-scale Advanced Capture Technology (PACT) facility in Sheffield, UK. The developed model is designed to predict the downstream concentrations, the fate profiles, and the occurrence profiles each AFE and TE. The modelled downstream concentrations as the element concentrations in uncooled downstream gas are well validated by the experimental downstream concentrations measured with the online inductively-coupled plasma – mass spectrometer (ICP-MS). The modelled fate and occurrence profiles of each AFE and TE are in good agreement with the experimental results pertaining to the respective elements.