Gasification: Difference between revisions

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[[File:Circulating fluidised bed.png|150px|right]]

In a circulating fluidized bed, the gas velocity is higher than for a stationary bed and the bed material or loose clusters of bed material are carried up in the gasifier shaft. By a radial transport some of this material is moved to the wall and transported back to the bottom by gravity as part of the wall layer sliding down (the flow in a large diameter bed establishes a core-annulus type of flow, with the gas-solid suspension flowing upwards and the wall layer moving sand bed moving downwards). The remainder of the solid suspension is carried out by the gas to an external primary particulate separator (typically a cyclone) from which it is returned to the bottom of the gasifier by means of a recycle line with a moving bed of solids.

===== Entrained flow gasifier =====

Most entrained flow gasifiers are analogous to the combustion of a solid or liquid fuel or slurry in a burner firing into a combustion chamber. For this reason, the fuel must either be a pumpable liquid that can be dispersed to droplets in the burner nozzle or small particles (< 1 mm) that can be consistently fed by means of dense phase transport to the fuel register of the burner.

Entrained flow gasifier also typically operates in an ash melting mode, i.e. the ash is removed as slag in the bottom of the reactor. This requires operation at very high temperature (1500°C) to reduce the slag viscosity and make it free flowing to avoid build-up of slag in the reactor. The refractory lining of the gasifier should however not be exposed to molten slag. Instead, a protecting layer of solid slag should be formed at the slightly lower temperatures of the wall. This is sometimes assisted by cooling the reactor or parts of it. To reach such high temperatures with low energy content fuels and still retain a significant heating value in the product gas is not possible with air, it requires the use of oxygen mixed with steam.

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 [[File:Circulating fluidised bed.png|150px|right]]
 In a circulating fluidized bed, the gas velocity is higher than for a stationary bed and the bed material or loose clusters of bed material are carried up in the gasifier shaft. By a radial transport some of this material is moved to the wall and transported back to the bottom by gravity as part of the wall layer sliding down (the flow in a large diameter bed establishes a core-annulus type of flow, with the gas-solid suspension flowing upwards and the wall layer moving sand bed moving downwards). The remainder of the solid suspension is carried out by the gas to an external primary particulate separator (typically a cyclone) from which it is returned to the bottom of the gasifier by means of a recycle line with a moving bed of solids.
+===== Entrained flow gasifier =====
+[[File:Entrained flow.png|140px|right|Entrained flow gasifier. All rights reserved.]]
+Most entrained flow gasifiers are analogous to the combustion of a solid or liquid fuel or slurry in a burner firing into a combustion chamber. For this reason, the fuel must either be a pumpable liquid that can be dispersed to droplets in the burner nozzle or small particles (< 1 mm) that can be consistently fed by means of dense phase transport to the fuel register of the burner.
+Entrained flow gasifier also typically operates in an ash melting mode, i.e. the ash is removed as slag in the bottom of the reactor. This requires operation at very high temperature (1500°C) to reduce the slag viscosity and make it free flowing to avoid build-up of slag in the reactor. The refractory lining of the gasifier should however not be exposed to molten slag. Instead, a protecting layer of solid slag should be formed at the slightly lower temperatures of the wall. This is sometimes assisted by cooling the reactor or parts of it. To reach such high temperatures with low energy content fuels and still retain a significant heating value in the product gas is not possible with air, it requires the use of oxygen mixed with steam.