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Thermochemical pathways for biofuel production involve the application of heat to convert biomass into energy or higher-value products through pyrolysis and gasification

Summary​

The thermochemical pathway utilizes heat to convert biomass into energy or higher-value products through processes like pyrolysis and gasification. Pyrolysis involves heating biomass without the presence of oxygen to produce materials such as bio-oil, charcoal, and other gaseous products depending on the temperature of the reaction. Gasification, an older technology originating from coal processing, converts biomass into syngas (carbon monoxide and hydrogen) or producer gas through partial combustion reactions. This pathway offers advantages like bypassing pretreatment needs and handling diverse carbonaceous feedstocks, including municipal solid waste. The resulting syngas can be further processed into drop-in fuels using Fischer-Tropsch synthesis.

The thermochemical pathway utilizes heat to induce chemical reactions that result in recoverable energy or conversion of the biomass product into another higher value product. Direct combustion is the most basic example of the thermochemical pathway, as it is a reaction between the biomass (fuel) and oxygen to produce more heat 1. However, when referring to the thermochemical pathway for second generation biofuels, the term is most often used to describe Gasification and Pyrolysis. A key benefit of the thermochemical pathway is the ability to bypass the need for pretreatment.

Pyrolysis​

Pyrolysis is the process of heating biomass in the absence of oxygen. This thermal degradation process usually results in a solid residue like charcoal, a liquid residue known as bio-oil, as well as other gaseous products 1. Pyrolysis can be broken down into three main categories

Conventional pyrolysis This form of pyrolysis utilizes a slow heating rate up to lower temperatures of between 527 - 1,247°F (275 - 675°C) 1. Throughout the heating time, three stages of pyrolysis occur as the material breaks down to form the solid char and bio-oil 1. The traditional production of charcoal is considered a crude form of conventional pyrolysis where the liquid and gas products are not recovered.

Fast pyrolysis Fast pyrolysis occurs at a higher temperature between 1,067 - 1,785°F (575 - 975°C) with a higher heating rate than conventional pyrolysis and shorter residence time (time spent in the reactor) 1. Fast pyrolysis is more efficient in the production of liquid and gaseous products than conventional pyrolysis 1.

Flash pyrolysis Flash pyrolysis occurs at an even higher temperature 1,427 - 1,877°F (775 - 1025°C) and faster heating rate than fast pyrolysis and is typically used in the production of bio-oil 1.

Gasification​

Gasification is a relatively old technology that had its origin in the 1700s with the gasification of coal via heating in the United Kingdom to produce syngas (short for synthesis gas) or producer gas, that was first commercially used for city street lighting 2. In the past half century, gasification has been explored as a process to convert biomass of almost any kind into syngas. Syngas is a mixture of predominantly carbon monoxide (CO), hydrogen (H₂), whereas producer gas consists of the previously mentioned chemicals plus carbon dioxide (CO₂), methane (CH₄), and nitrogen gas (N₂). Gasification can occur in two different ways, either with or without a catalyst. When no catalyst is used, the gasification process occurs at temperatures as high as 2,372°F (1,300°C), but when a catalyst is used, temperatures can be reduced to around 1,652°F (900°C) 1.

The gasification process initially involves a pyrolysis step where biomass is thermally degraded, after which a small amount of oxygen (O₂) is provided to induce partial combustion that bonds the carbon (C) that was released during the pyrolysis step with oxygen shown in the equations below

C+O2↔CO2\begin{align*} \mathrm{C + O_2 \leftrightarrow CO_2} \end{align*} C+12O2↔CO\begin{align*} \mathrm{C + \frac{1}{2}O_2 \leftrightarrow CO} \end{align*}

The actual gasification reaction occurs between the carbon contained in the sold char (C) and oxygen or steam (H₂O) that is introduced into the reactor. This produces a syngas that is rich in carbon monoxide and hydrogen as shown in the following equation:

C+CO2↔2CO\begin{align*} \mathrm{C + CO_2 \leftrightarrow 2CO} \end{align*} C+H2O↔CO+H2\begin{align*} \mathrm{C + H_2O \leftrightarrow CO + H_2} \end{align*}

One of the main advantages of gasification is that it can utilize a very wide range of feedstocks so long as they are carbonaceous (composed of carbon), including lignocellulosic biomass as well as municipal solid waste (MSW), which can give rise to processes that are considered waste-to-energy. After syngas is produced, it can be further processed into usable drop-in fuels via Fischer–Tropsch (F-T) synthesis.

Sources​

Footnotes​

  1. Naik, S. N., Goud, V. V., Rout, P. K., & Dalai, A. K. (2010). Production of first and second generation biofuels: A comprehensive review. Renewable and Sustainable Energy Reviews, 14(2), 578–597. https://doi.org/10.1016/j.rser.2009.10.003 ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8

  2. National Gas Museum. (2025). The early days. https://www.nationalgasmuseum.org.uk/discover/the-early-days/ ↩