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In thermochemical conversion of biomass, bonds btw adjacent carbon, hydrogen, and oxygen molecules are broken down to release their chemical energy stored during the process of photosynthesis, process in which plants use sunlight, water, and CO2 to create O2 & energy in the form of sugar.  Thermochemical conversion includes combustion, pyrolysis, liquefaction, and gasification [SD, 2018]. 

The thermochemical conversion w/ excess of oxygen is known as combustion, the only one that produces heat & electricity, whereas the conversion w/o O2 supply is known as pyrolysis.  Techs such pyrolysis & gasification are better suited for homogeneous waste types after removing non-combustibles, recyclables, and inert materials from the waste stream [SD, 2022].

Liquefaction is a process that generates a liquid from a solid or a gas (e.g., liquefaction of coal to produce liquid hydrocarbon fuels) using low temperature (250–350 °C) and high pressure (5–20 MPa) [ACS, 2011].

Pyrolysis is the heating of an organic material, such as biomass or plastic waste, in the absence of oxygen.  Biomass pyrolysis is usually conducted at or above 500 °C, often in an inert atmosphere, providing enough heat to deconstruct the strong polymers [USDA, 2021].  It is the only thermal process that produces fuels in the three states of matter (solid, liquid, and gaseous fuels).

Gasification is the process of conversion of organic compounds (solid or heavy liquid feedstock) into syngas (a mix of H2 and CO, in addition to other components such as water vapour, CO2, and ash) under high-temp (750°C to 1100°C), from atmospheric pressure to a few bars, and controlled oxygen levels.  It transforms carbon-containing products (e.g., the products from pyrolysis) into a primarily gaseous product [FS, 2023].

Figure 1 shows some thermochemical processes.  Table 1 shows some main features of thermochemical processes [SD, 2018].

Figure 1: Thermochemical processes