CO2 emission mitigation through fuel transition on Danish CHP and district heating plants

• Published in: Global change biology. Bioenergy 2021-07, Vol.13 (7), p.1162-1178
• Main Authors: Nielsen, Anders Taeroe, Nord‐Larsen, Thomas, Bentsen, Niclas Scott
• Format: Article
• Language: English
• Subjects: bioenergy; carbon emissions; climate impact; fuel transition; indirect GHG effect; indirect land use change; wood chips; wood pellets
• ISSN: 1757-1693; 1757-1707
• DOI: 10.1111/gcbb.12836

The study analysed how carbon dynamics were influenced by the transition from coal or natural gas to forest biomass on a number of district heat and combined heat and power plants in Denmark. For 10 plants, we calculated the cumulative net carbon emissions over time (t) from the fuel transition (CCE(t)) and carbon payback time (CPT), a measure of the time it takes for a fuel transition to biomass to reduce the amount of carbon emitted to the atmosphere relative to a continuation of using fossil fuels. Subsequently, we derived the relative cumulative net carbon emissions (RCCE(t)), as a measure of the carbon emission savings/costs induced by the fuel transition. Finally, we performed sensitivity analyses of key parameters, with special focus on emissions from indirect/market mediated effects. For fuel transitions from coal to biomass, CPT ranged from 0 to 13 years indicating that carbon emission benefits were achieved at the latest after 13 years. Relative cumulative net carbon emissions 30 years after the fuel transition (RCCE(30)) ranged from 0.29 to 0.85 corresponding to emission savings of 15–71% relative to continued use of coal. For fuel transitions from natural gas to biomass, CPT ranged from 9 to 34 years and RCCE(30) from 0.81 to 1.03. Sensitivity analyses showed that the use of truly residual biomass (harvest residues or industrial residues with no alternative use), biomass harvest from productive forests and short transport distances are instrumental in achieving a short carbon payback time and large emission savings. The quantification of indirect or market mediated GHG emissions is controversial and uncertain. We analysed additional carbon emissions related to indirect land use change (iLUC), indirect wood use change (iWUC) and indirect fuel use change (iFUC). Including iLUC added 1–4 years, iWUC added 1–3 years and iFUC added 1 year to the mean CPT. The rapid growth of biomass used for energy has spurred scientific, public, and political concern over the climate benefits of forest biomass for energy, over sustainability and sustainability verification, and over GHG accounting of bioenergy as carbon neutral. This study analysed carbon dynamics of ten recent fuel transitions on CHP or district heating plants from coal or natural gas to forest biomass in Denmark and found that forest biomass is not carbon neutral per se, but in most cases, the transition from fossil to biomass fuel has been beneficial in reducing CO2 emissions to the atmosphere.