Co-firing fossil fuels and biomass: combustion, deposition and modelling

The application of advanced technologies employing combustion/co-firing of coal and biomass is seen as a promising approach to minimising the environmental impact and reducing CO2 emissions of heat/power production. The existing uncertainties in the combustion behaviour of such fuel mixes and the release of alkali metals with other elements during the combustion (or co-firing) of many bio-fuels are some of the main issues that are hindering its application. The potential presence of high levels of alkali chlorides and low levels of sulfates in the deposits formed on heat exchanger can cause enhanced corrosion and/or limit the heat transfer between the hot combustion gases and the water/steam system within the process plant. This work has investigated the detailed gas compositions and deposition characteristics of the combusted gas streams produced from fossil and biomass fuels pure and/or blend in a pilot-scale combustors (PF and FBC) at Cranfield University. Combustion gas analysis were obtained on-line by a high resolution multi-component Fourier Transform Infra-Red (FTIR) gas analyser and deposits samples were collected from the flue gas using air-cooled probes with surface temperatures of about 500, 600, 700 o C and analysed using SEM-EDX and XRD techniques. Fuels included several biomass fuels (cereal co-product (CCP) straw, miscanthus (pulverised), oil seed rape straw (against stored pellets), miscanthus (pellets), willow, fast pyrolysis bio-oil) and two commercially-used coals (El-cerrejon and Daw Mill). The results of the experimental studies have been compared with thermodynamic equilibrium predictions. High combustion efficiency was maintained throughout the range of fuel mixes. The SO2 and HCl levels were low in pure biomass combustion and increased as the biomass fraction of the fuel decreased when co-fired with these coals. However, the NOx output remained stable except for Miscanthus:Daw Mill mixtures and OSR stored pellet combustion. The deposition flux was highest on the coolest probes for each fuel. The lowest deposition fluxes were found for the combustion of either fast pyrolysis bio-oil or coppiced willow. There is evidence of significant differences deposition fluxes between El-cerrejon coal and Daw Mill coal mixed with CCP and/or miscanthus. The presence of chlorine was identified in deposits produced from combustion of pure biomass or high biomass mixes. The lowest levels found here in fast pyrolysis bio-oil combustion and only detected at higher shares (≥ 80 %) of biomass co-fired with Daw Mill coal, whereas, mixed biomass with El-cerrejon coal produced Cl in deposits at a low % biomass share. The application of thermodynamic equilibrium modelling has been found to be useful tool for providing a qualitative understanding of elements present and/or control by hot gas in modern combustion processes.