Gasification: Difference between revisions

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==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.

Most of the main gasification reactions are endothermic. These are reactions that absorb heat energy from their surroundings. An exception is the oxidation (the loss of electrons during a reaction) of char or [[Combustion|combustible]] gases by oxygen and also the water gas shift towards hydrogen and the [[pyrolysis]] in certain temperature windows, which are exothermic (reactions that release heat energy into their surroundings). 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~~.

Autothermal (direct) gasifiers provide the necessary heat of conversion by adding an oxidant (a reactant that removes electrons from other reactants) to achieve partial oxidation (when oxygen is fed at a below level of which is required for complete 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 (a reactant that removes electrons from other reactants) and a [[Combustion|combustible]] which requires a good internal heat transfer to even out the temperature.

===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.

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 reactor 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 nitrogen content from the gasification reactor and a [[Flue Gas|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|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).

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/char residues are separated from the product gas and are burnt. Then via an indirect heat exchanger, the energy in the hot [[Flue Gas|flue gas]] is transferred to the gasifier by a combination of radiation and convective heat transport (b).