Evaluation Of Co-firing Of Coal And Biomass Systems Based On Emergy Theory

Co-firing of coal and biomass systems is traditionally evaluated from efficiency, economy or environmental impact. In order to meet the requirements of economic and environmental sustainability, a reasonable evaluation should be coped with the analysis based on the environmental impact and economic benefit for sustainability assessment. Considering for reflecting comprehensive performances, based on emergy theory, four emergy ratios(emergy yield ratio, environmental loading ratio, emergy sustainable indices and transformity, namely, EYR, ELR, ESI and Tr) are adopted to estimate sustainabilities of co-firing of coal and biomass systems with different capacities. The main contents are listed as follows:(1) The existing calculation of emergy indexes are summarized, and the emergy model of co-firing of coal and biomass systems is established.(2) Based on emergy theory, taking 100 MW, 200 MW and 300 MW coal-fired power generation/cogeneration systems as research objects, and their sustainable performances are analyzed. The results show that although all the systems have good emergy yield ratio, but all of them are unsustainable systems, and the transformities of all coal-fired cogeneration systems are less than the coal-fired power generation systems.(3) The sustainablitiy of co-firing of coal and biomass systems with mixing ratio 10% are compared. Results indicate that the EYR of all systems is slightly changed, and ELR is much lower than the original systems, so all of the systems evolve from unsustainable systems into sustainable systems. Emergy indexes of the systems are counted with the change of mixing ratio. The results indicates that the ratio of 15% is the optimal mixing ratio from the aspect of of economy, on the other hand, the mixing ratio is bigger, the better the environmenal benifit.(4) Emergy indexes are calculated for all systems mixed same biomass based on emergy theory. The results show that for different types with the same capacity of the system, emergy yield ratio, environmental loading ratio and emergy sustainable indices of the cogeneration syetem are close to those of the power generation system, but its transformity is far lower than that of the power generation system. Therefore, the small-capcity coal-fired cogeneration system burned with biomass has good flexibility and economic benifit.