Combustion and Emission Characteristics of HTC-Coals in Pulverized Fuel Combustion Test Rig

摘要

Hydrothermal carbonisation is a thermochemical conversion process to convert biomass to a carbon rich solid product in an elevated temperature (180°C-250°C) and pressure (2-10 MPa) in presence of water for several hours. Hydrothermal carbonatisation converts biomass to carbon rich 'lignite like' product, here referred as 'HTC-Coal'. These types of bio-fuels are also referred as 'bio-coal' and 'hydro-char' in many literatures. Hydrothermal carbonisation is a biomass pre-treatment technology which can convert new range of biomass feedstocks to solid bio-fuels. Biogenic residues/wastes are more suitable feedstock for hydrothermal carbonisation than conventional biomass like wood and straw. HTC-Coal from feedstock sources namely; spent grains from brewing industry (referred as HTC-SG), municipal bio-waste (referred as HTC-BW) and horse manure and digested slurry (referred as HTC-DS) are investigated during this study. This research work has been performed under the frame work of European Union FP7 project 'BioBoost'. This experimental study characterizes the combustion and emission behaviour of various HTC-Coals in a pulverized fuel combustion test rig. The combustion tests are performed in a 20 kW lab scale and 500 kW pilot scale pulverized fuel combustion test rig. The combustion conditions/settings are chosen to represent the real application scenario. Both mono-firing and co-firing scenarios are tested for the selected HTC-Coal. The volatile combustion behaviour of HTC-Coal is significantly different and exhibit considerable impact on the flame characteristics. The HTC-Coal flames were substantially longer and shows strong tendency to form soot (as black clouds penetrating outside the flame) which influences the heat transfer and temperature profile inside the furnace as well as emission behaviour (especially, NO_x emission). The fuel-N to NO_x conversion for HTC-Coals was lower in comparison to conventional solid fuels (coal, lignite and wood). Despite of higher fuel nitrogen content in HTC-Coals, the NO_x emission was substantially lower. The general understanding of HTC-Coal as 'lignite like' fuel is not enough to explain the early stage combustion behaviour and NO_x formation/destruction behaviour, neither the HTC Coals can be represented by the conventional biomass fuels like wood. SO_2 emission shows a direct correlation with the amount of sulfur content in the respective fuel. Some HTC-Coals have higher calcium (CaO) content in ash however the self-capture of SO_2 in ash was negligible. The ash and deposition issues with HTC-Colas are discussed in comparison with conventional biomass fuels, like wood/straw. The deposit and ash issues are closely related to the ash composition and vary according to the original biomass feedstock for the respective HTC-Coal. It clearly shows that the alkali (esp. potassium) and chlorine that are the most problematic species in conventional biomass is not an issue for most HTC-Coals. The HTC-Coal occupies a new space of renewable and CO_2 neutral solid fuels which needs further research and development work to increase the 'know how' on various aspect of HTC-Coals as fuel and its utilization in production of heat and power.