Biomass from forestry residues such as thinning and harvest operations is often cited as an underutilized resource that could be mobilized for energy production. In many cases the residues of normal forestry activities that cannot be utilized in an economically viable way, are left to decay naturally on the forest ground. To some extent this practice is beneficial and necessary also, to maintain the productivity and the biodiversity of forests. Nevertheless, if approached in a sustainable way, it can still be compatible with the energetic valorisation of a large part of those residues.
Researchers from Linnaeus University have analysed the long-term effects on climate change of producing electricity in large-scale power plants, comparing the use of biomass from forest residues with the use of coal. In doing so, they have elaborated different scenarios, considering the different conversion efficiencies of average, state of the art and emerging technologies for biomass and for coal. The study has also considered the impact on CO2 emissions of the application of CCS systems (carbon capture and storage) both to coal and biomass plants. CCS could contribute significantly to climate change mitigation by capturing CO2 from the plants and storing it in long-term underground storage. Thus, the use of forest residues that otherwise would decay on the forest floor, in an energy conversion facility equipped for CCS, can result in a net reduction of CO2 in the atmosphere, even though CCS processes require energy in large amounts. The study estimated the primary energy use, the annual CO2 emission, the annual change in CO2 concentration in the atmosphere, and the cumulative radiative forcing (CRF) of different systems producing 1 MWh of electricity for 40 years. CRF is a parameter based on the difference of sunlight absorbed by the earth and the energy radiated back to space, and is used to measure the Global Warming Potential over a given time horizon.
The study assumed that biomass plants have lower conversion efficiencies compared to coal plants, therefore biomass systems would require more fuel at the conversion plant and have a higher consumption of primary energy. However, when considering the full fuel cycle and not only the combustion phase, coal-based systems require more primary energy than the corresponding bioenergy systems. Likewise, combustion plants equipped with CCS technology require more primary energy than those without CCS. In coal-based system, the emission from the natural decay of forest slash must be added to the fossil emission from the power plant. The result is that cumulative emissions of a coal-based system increase with time, as the unused biomass decays in the forest. When considering the changes in CO2 in the atmosphere over time, the cumulative emissions from bioenergy systems are significantly lower than emissions from coal power plants.
By comparing these scenarios in terms of cumulative radiative forcing, that is the long-term effect these systems have on climate change, the study demonstrated that along with the application of CCS, using forest slash instead of coal for electricity production, could strongly reduce long-term CO2 emissions, and could even result in global cooling effects.
The benefits of replacing coal with biomass in term of radiative forcing are continually increasing over time, even though the replacement occurred only for a certain period. While it’s true that the application of CCS technologies at power plants requires additional fuel for the process, it can strongly reduce the CO2 emission at power plants. the effects are larger if this technology is used for biomass-based power generation. Since the efficiency of power plants strongly influences the use of primary energy and the resulting CO2 emission, shifting from today’s average technologies to emerging technologies for electricity production such as integrated gasification combined cycle (IGCC) could halve the CO2 emissions.
This post is based on the paper “Primary Energy and Climate Change Effects of Forest Residues and Fossil Coal for Electricity Production With and Without Carbon Capture and Storage“, by Nguyen Le Truong et al. Linnaeus University, presented at the 24th European Biomass Conference and Exhibition.