Acerca de

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, liquefaction, pyrolysis, 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 as pyrolysis & gasification are better suited for homogeneous waste types after removing non-combustibles, recyclables, and inert materials from the waste stream [SD, 2022].  A summary of these techs is given below:

• 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 relatively low temperatures (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.  It is a complex system, which is greatly influenced by conditions of temperature, pressure, heating rate, and residence time.  Biomass pyrolysis is usually conducted at temperatures around 500 °C, often in an inert atmosphere, providing enough heat to deconstruct the strong bio-polymers [USDA, 2021].  It is the only thermal process that produces fuels in the three states of matter (solid, liquid, and gaseous).  There is also the catalytic pyrolysis, aiming at the production of other products, such as naphtha for crackers & basic chemicals.

• 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 vapor, CO2, and ash) under high-temp (750-1100 °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