Pyrolysis
What is Pyrolysis?
Pyrolysis is the decomposition of organic materials by heating without oxygen or other agents, although steam is used sometimes. Pyrolysis can be used to convert chemicals such as ethylene dichloride to polyvinyl chloride (PVC). Biomass or waste can be converted into other useful products such as syngas using pyrolysis.
Anhydrous pyrolysis happens when a solid organic material is heated without oxygen. Frying, baking, toasting and roasting are examples of pyrolysis, as the outer layer of the food changes as it is heated, but the inside doesn’t. Charcoal is produced by the pyrolysis of wood. Flames in a wood fire are caused by gases combusting as a result of pyrolysis, not the act of burning wood itself. As a result of this, pyrolysis is importing for those involved in fire safety.
Hydrous Pyrolysis takes place using water. Using this method of pyrolysis, waste matter can be transformed into a liquid with similar characteristics to light crude oil.
In vacuum pyrolysis the organic matter is heated in a vacuum to lower the boiling point and avoid unwanted chemical reactions.
Fast pyrolysis is required in order to obtain the most liquid, and can be done using several methods. Several methods are currently used to provide the required heat with varying degrees of success. Using partial combustion of biomass using air injection can result in poor quality products. Using direct heat transfer with a hot gas requires ensuring that there is enough heat and that the gas flows properly which can be difficult. Indirect heat transfer uses walls or tubes, however it can be difficult to make sure the heat transfer is efficient on both sided of the heat exchange surface. Direct heat transfer using circulating solids is the most efficient but complex method, and works by transferring heat from the solids between a burner and pyrolysis reactor.
There are different methods for producing biomass pyrolysis. Fixed beds that were traditionally used to make charcoal have been used but produce a relatively small amount of liquid. Augers technology is adapted from a process for coal gasification, but it is not really suitable for large scale applications. Ablative processes require the particles of biomass to be moved at high speed against a hot metal surface. This can be difficult to upscale as the ratio of the wall surface to reactor volume decreases as the size of the reactor increases. With the Rotating Cone method, a mixture of sand and biomass use centrifugal force to spin. Fine particles are required in order to produce a viable amount of liquid. Fluidised Bed Technology involved introducing biomass particles to a bed of hot sand which is fluidized by gas. The biomass particles are then rapidly heated. This method is difficult to expand so it can be used for commercial purposes due to the quality of the bio oil and the size of the pyrolysis reactor. Using Circulating Fluidised Beds, in which biomass particles are introduced to a rotating bed of hot sand, is the most viable option for commercial pyrolysis. This technology can be scaled up for commercial use. The process is more complex and the end products are more diluted, meaning that obtaining the liquid is more difficult.
Pyrolysis can be used to create synthetic diesel fuel, which helps to reduce the reliance on fossil fuels, and, as it is renewable, it is much more environmentally friendly. Bio oil can be produced using pyrolysis which can then be used as a fuel, or as a food additive or in pharmaceuticals. Bio oil can’t be used in most car engines without additional requirements. Syngas, which can be used in vehicles, can be created from pyrolysis. Char which can be burned for energy, or used as a fertilizer, is also produced using pyrolysis.
As the pyrolysis technologies improve and the quest for suitable alternative and renewable energy sources continues, pyrolysis will play a bigger role in reducing the reliance on fossil fuels.
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