Gasification: Difference between revisions

Line 28:

===== Fluidised bed gasifier =====

In fluidized bed gasifiers, the reaction space contains a sand-like bed material that is fluidized or entrained by the oxidant gas (air or oxygen), steam or mixtures thereof being added in the bottom. Autothermal or direct gasifiers use an oxidant, and allothermal or indirect gasifiers use steam without an oxidant being fed to the gasifier section.

Stationary (bubbling) fluidized bed material is kept suspended by the gas in a defined bed volume through which gas in the form of interstitial gas and bubbles pass. Above the bed there is a freeboard section used for disengagement of particles mainly ejected by bubbles erupting on the bed surface. This gives a density profile in the reactor of high and uniform density from the fluidizing gas injection level to the top of the bed, and a low density close to the gas density in the freeboard section from the upper bed level to the gas exit.

Line 39:

[[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.

Line 65:

Grate type gasifiers that resemble updraft moving bed gasifiers, i.e. the fuel is moved as a bed on an inclined or moving grate, or on a belt in a furnace tunnel while the oxidant is added from below, such that drying and pyrolysis occurs close to the feed point and burn-out of the char at the ash exit point. This is a kind of cross-flow arrangement where the product gases from all sections in an updraft gasifier is generated separately and then mixed, unlike the counter current passage of the gas from one section through the next in an updraft gasifier.

<ref>[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>

<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>

Line 75:

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.