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

摘要

The application of advanced technologies employing combustion/co-firing of coal andbiomass is seen as a promising approach to minimising the environmental impact andreducing CO2 emissions of heat/power production. The existing uncertainties in thecombustion behaviour of such fuel mixes and the release of alkali metals with otherelements during the combustion (or co-firing) of many bio-fuels are some of the mainissues that are hindering its application. The potential presence of high levels of alkalichlorides and low levels of sulfates in the deposits formed on heat exchanger can causeenhanced corrosion and/or limit the heat transfer between the hot combustion gases andthe water/steam system within the process plant.This work has investigated the detailed gas compositions and deposition characteristicsof the combusted gas streams produced from fossil and biomass fuels pure and/or blendin a pilot-scale combustors (PF and FBC) at Cranfield University. Combustion gasanalysis were obtained on-line by a high resolution multi-component Fourier TransformInfra-Red (FTIR) gas analyser and deposits samples were collected from the flue gasusing air-cooled probes with surface temperatures of about 500, 600, 700oC andanalysed using SEM-EDX and XRD techniques. Fuels included several biomass fuels(cereal co-product (CCP) straw, miscanthus (pulverised), oil seed rape straw (againststored pellets), miscanthus (pellets), willow, fast pyrolysis bio-oil) and twocommercially-used coals (El-cerrejon and Daw Mill). The results of the experimentalstudies have been compared with thermodynamic equilibrium predictions.High combustion efficiency was maintained throughout the range of fuel mixes. TheSO2 and HCl levels were low in pure biomass combustion and increased as the biomassfraction of the fuel decreased when co-fired with these coals. However, the NOx outputremained stable except for Miscanthus:Daw Mill mixtures and OSR stored pelletcombustion. The deposition flux was highest on the coolest probes for each fuel. Thelowest deposition fluxes were found for the combustion of either fast pyrolysis bio-oilor coppiced willow. There is evidence of significant differences deposition fluxesbetween El-cerrejon coal and Daw Mill coal mixed with CCP and/or miscanthus. Thepresence of chlorine was identified in deposits produced from combustion of purebiomass or high biomass mixes. The lowest levels found here in fast pyrolysis bio-oilcombustion and only detected at higher shares (≥ 80 %) of biomass co-fired with DawMill coal, whereas, mixed biomass with El-cerrejon coal produced Cl in deposits at alow % biomass share.The application of thermodynamic equilibrium modelling has been found to be usefultool for providing a qualitative understanding of elements present and/or control by hotgas in modern combustion processes.