Environmental performance assessment of retrofitting existing coal fired power plants to co-firing with biomass: carbon footprint and emergy approach

To reduce the emission of greenhouse gasses, developed countries tend to increase the use of environmentally friendly renewable energy sources. Retrofitting of existing coal fired condensing power plants to co-firing with biomass is a generally accepted method for decreasing the dependency on fossil fuels and carbon-dioxide emission reductions. To determine if the co-firing is an environmentally friendly solution, two methods are used to cover all significant aspects of electricity production process that may influence the environment: carbon footprint and emergy evaluation. These environmentally accounting approaches were chosen to determine the maximum supply distance of biomass that allows the co-firing of coal and biomass to be more environmentally efficient than the pure coal combustion. Furthermore, geological origin of the coal combusted is taken into account, considering that the environmental inputs for feedstock creation varied throughout the history. The results of the study showed that the addition of approximately 20% biomass to the mass of the combustion mixture causes the decrease in carbon-dioxide emissions for nearly 11–25% and total emergy flow for 8–15%. However, further results indicate that the co-firing process is environmentally acceptable if the biomass supply stocks are within the area determined by maximum supply distances. Nevertheless, the supply area radius resulting from the emergy evaluation is 49–62% shorter depending on the coal type combusted. Furthermore, the emergy loading ratio of co-firing was lower than for the pure coal firing (10.65 compared to 12.39, respectively) indicating that the co-firing process causes less pressure on the ecosystem.