Life cycle approach for energy and environmental analysis of biomass and coal co-firing in CHP plant with backpressure turbine

Abstract

Generation of near CO2 free energy (electricity and heat) in existing large co-generation technologies can be achieved by partial substitution of fossil fuels with biomass commonly regarded as CO2-neutral fuel. Co-firing of biomass with fossil fuels aims at reduction of greenhouse gases (GHG) emissions and non-renewable fuel resources depletion. Life cycle analysis was carried out for hard coal and two biomass sorts of different origin (willow chips and residual wood chips) requiring a diverse approach for their upstream inventory of non-renewable energy resources depletion and GHG emission. These fuels are co-combusted in a combined heat and power plant (CHP plant) for generation of electricity and heat. As in the case of cogeneration process, it was necessary to allocate the harmful effects between both energy carriers. Allocation of the burdens basing on the principle of the avoided process has been proposed. The empirical correlations concerning the electricity consumption of boiler auxiliaries' and boiler energy efficiency along with the increasing share of biomass in the fuel blend were applied in the calculations. The functional unit chosen to compare the results was 1 TJ of heat. Sensitivity analysis was carried out to bring the final conclusions and recommendations. Both environmental burdens (non-renewable resources depletion and the greenhouse effect) are dependant on the share of biomass in the combusted blend. It was proved, that the partial substitution of coal with biomass (in the considered range of 0–20% on the thermal basis) leads to the decrease of the total life-cycle non-renewable energy resources depletion and cumulative GHG emissions for generation and supply of 1 MJ of heat (functional unit) and 1 MWh of electricity generated during multifunctional process of combined heat and power generation. Residual biomass shows its advantages over dedicated energy crops (on the example of willow biomass). Additionally, it has been proved, that the share of operational component dominates in the total system burdens concerning the regarded life cycle.