Hydrothermal Carbonization (HTC) pellets quality assessment: combustion kinetics, efficiency and emissions

In current times, climate change due to greenhouse gas (GHG) emissions and rising fossil fuel prices create the need to focus more on renewable energy to become energy independent. The combustion of solid fuels such as biomass is a reliable option for thermal energy production and for combined heat and power production. This work focuses on the quality analysis of carbon-rich solid fuel produced by Hydrothermal Carbonization (HTC) of green waste (such as leaves, small branches, and grass). The combustion and emission performances of the HTC pellets are compared to those of conventional softwood pellets. Thermogravimetric analysis, kinetics assessment, and final use tests on a fixed bed boiler were carried out for both fuels. Combustion efficiency, boiler efficiency, and emissions factors were experimentally determined, computed and compared. Thermogravimetric analyses of the fuels show some qualitative and quantitative differences, in particular a third peak of burning rate corresponding to the oxidation of solid carbon, with a high activation energy. For similar parameters of boiler operation with HTC pellets, a 7% increase in combustion efficiency and a 7% increase in boiler efficiency were observed. However, higher particulate matter and CO emissions were observed. Results suggest that the air/fuel ratio can be further optimized for the use of HTC pellets for this type of boiler. This makes HTC pellets a promising fuel to help tackle climate change.