Gasification: Difference between revisions

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Gasification can be considered a process between [[Pyrolysis|pyrolysis]] and conventional [[Combustion]] in that it involves the partial oxidation of a substance. This means that oxygen is added but the amounts are not enough to allow the fuel to be completely oxidised and full combustion to occur. The temperatures employed are typically above 650°C. Generally, the [[Syngas|syngas]] generated from Gasification will have a [[Net Calorific Value]] ([[NCV]]) of 4-10MJ/Nm3 <ref>[[DEFRA]], 2013. [https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/221035/pb13888-thermal-treatment-waste.pdf Advanced Thermal Treatment of Municipal Solid Waste.] London. </ref>.

==Principles of Gasification==

Most of the main gasification reactions are endothermic. An exception is the oxidation of char or combustible gases by oxygen. Also the water gas shift towards hydrogen and the pyrolysis in certain temperature windows, which are exothermic as well. Therefore, there is a need to supply to or generate energy within the gasifier to balance the overall conversion.

===Autothermal Gasifiers===

Autothermal (direct) gasifiers provide the necessary heat of conversion by adding an oxidant to achieve partial oxidation of the fuel within the gasification reactor. This releases energy directly in the reactor where it is consumed. Autothermal conditions are easy to achieve using air or oxygen. Overall the complexity of the process is reduced compared to allothermal gasifiers, however the heat release occurs in the zone of contact between the oxidant and a combustible which requires a good internal heat transfer to even out the temperature or causes a temperature gradient inside the gasifier.

===Allothermal Gasifiers===

Allothermal (indirect) gasifiers are characterized by the fact that heat is provided from an external source (the processes of heat production and heat consumption are physically separated). The heat is generated by combustion and transferred to the gasification with a heat carrier (e.g. circulating bed material) or a heat exchanger (e.g. heat pipe exchanger)<ref>[https://www.sciencedirect.com/science/article/pii/B9780128155547000076 Waste Gasification Process for SNG Production]</ref>. In contrast to autothermal gasifiers where only one product gas stream is produced, allothermal gasifiers generally produce two separate gas streams: a medium calorific product gas stream having a low content of nitrogen from the gasification reactor and a flue gas stream from the combustion reactor. Both streams need to be cleaned to the standard required for the gas end user (product gas), or for release to the stack (flue gas), respectively.

There are two main types of allothermal reactor. One uses a solid heat carrier (sand or larger aggregates) that is circulated between the gasification and combustion reactors, respectively. The hot energy carrier coming into the gasification reactor releases heat to drive the gasifier reactions, and when leaving to the combustion reactor also withdraws a major part of the remaining solid residue/char (a). The second type is the heat-integrated gasifier, where part of the product gas or char residues are separated from the product gas and are burnt, and via some form of indirect heat exchanger the energy in the hot flue gas is transferred to the gasifier by a combination of radiation and convective heat transport (b).

==Kiln/Reactor Types in Gasification ==

~~Autothermal (direct) gasifiers provide the necessary heat of conversion by adding an oxidant to achieve partial oxidation of the fuel within the~~ gasification ~~reactor. This releases energy directly in the reactor where it is consumed~~. ~~Autothermal conditions~~ are ~~easy~~ to ~~achieve using air or oxygen. Overall the complexity of the process is reduced compared to allothermal gasifiers, however the heat release occurs in the zone of~~ contact ~~between the oxidant and a combustible which requires a good internal heat transfer to even out the temperature or causes a temperature gradient inside the gasifier~~.

There are several configurations for gasification kilns/reactors. Gasifiers are typically grouped according to their flow pattern and gas-solid contact.

Types ~~of autothermal gasifier~~:

Gasifier Kiln/Reactor Types:

#Co-current flow (downdraft) moving bed gasifier

#Counter flow (updraft) moving bed gasifier

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<ref name="ref1">[https://www.ieabioenergy.com/wp-content/uploads/2019/01/IEA-Bioenergy-Task-33-Gasification-of-waste-for-energy-carriers-20181205-1.pdf Gasification of Waste for Energy Carriers (IEA Bioenergy)]</ref>

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~~=== Allothermal Gasifiers ===~~

Allothermal (indirect) gasifiers rely on the fact that the heat necessary for running the gasification reactions is delivered to the gasification reactor from an external source. The heat is generated by combustion and transferred to the gasification with a heat carrier (e.g. circulating bed material) or a heat exchanger (e.g. heat pipe exchanger)<ref>[https://www.sciencedirect.com/science/article/pii/B9780128155547000076 Waste Gasification Process for SNG Production]</ref>. There are two main types of allothermal reactor. One uses a solid heat carrier (sand or larger aggregates) that is circulated between the gasification and combustion reactors, respectively. The hot energy carrier coming into the gasification reactor releases heat to drive the gasifier reactions, and when leaving to the combustion reactor also withdraws a major part of the remaining solid residue (char). The second type is the heat-integrated gasifier, where part of the product gas or char residues are separated from the product gas and are burnt, and via some form of indirect heat exchanger the energy in the hot flue gas is transferred to the gasifier by a combination of radiation and convective heat transport.

Allothermal/ indirect fluidized beds can have different combinations of fluidization types in the two beds, such as two stationary fluidized bed reactors, one stationary fluidized bed gasifier and circulating bed combustor, one circulating fluidized bed gasifier and one stationary bed combustor or two circulating fluidized beds. Indirect double fluidized beds function in the same way as a fluidized bed, the difference being that there is no oxidant, instead typically steam is added to the gasifier, since the energy required is instead provided by hot sand bed material being transferred at high rate from the second, combustor bed.

~~<ref name="ref1" />~~