5.1.2. Reactions & Transformations
The chemistry of gasification is quite complex and is accomplished through a series of physical transformations and chemical reactions within the gasifier. Some of the major chemical reactions are shown in the diagram below. In a gasifier, the carbonaceous feedstock undergoes several different processes and/or reactions:
- Dehydration – Any free water content of the feedstock evaporates, leaving dry material and evolving water vapor which may enter into later chemical reactions.
- Pyrolysis – This occurs as the feedstock is exposed to rising temperature in the gasifier. Devolatization and breaking of the weaker chemical bonds occurs, releasing volatile gases such as tar vapors, methane, and hydrogen, along with producing a high molecular weight char which will undergo gasification reactions.
- Combustion – The volatile products and some of the char react with limited oxygen to form carbon dioxide (CO2), carbon monoxide (CO), and in doing so, provide the heat needed for subsequent gasification reactions.
- Gasification – The remaining char reacts with CO2 and steam to produce CO and hydrogen (H2).
- Water-gas-shift and methanation – These are separate reversible gas phase reactions taking place simultaneously based on gasifier conditions. These are minor reactions which play a small role within in the gasifier. Depending on the desired product, the syngas may undergo further water-gas shift and methanation processing downstream from the gasifiers.
- Gasification [Second Edition] (2008)
Christopher Higman and Maarten van der Burgt, Gulf Professional Publishing, ISBN: 978-0-7506-8528-3