Editing Gasification (section)

==Processes==
[[File:Gasifier types.svg|500px|thumb|Main gasifier types]]
Several types of gasifiers are currently available for commercial use: counter-current fixed bed, co-current fixed bed, [[fluidized bed]], entrained flow, plasma, and free radical.<ref name="nnfcc"/><ref>Beychok, M.R., ''Process and environmental technology for producing SNG and liquid fuels'', U.S. EPA report EPA-660/2-75-011, May 1975</ref><ref>Beychok, M.R., ''Coal gasification for clean energy'', Energy Pipelines and Systems, March 1974</ref><ref>Beychok, M.R., ''Coal gasification and the Phenosolvan process'', American Chemical Society 168th National Meeting, Atlantic City, September 1974</ref>

===Counter-current fixed bed ("up draft") gasifier===
A fixed bed of carbonaceous fuel (e.g. coal or biomass) through which the "gasification agent" (steam, oxygen and/or air) flows in counter-current configuration.<ref>Thanapal SS, Annamalai K, Sweeten J, Gordillo G, (2011), “Fixed bed gasification of dairy biomass with enriched air mixture”. Appl Energy, doi:10.1016/j.apenergy.2011.11.072</ref> The ash is either removed in the dry condition or as a [[slag]]. The slagging gasifiers have a lower ratio of steam to carbon,<ref>{{Cite conference|title=Development Status of BGL-Gasification|url=http://www.iec.tu-freiberg.de/conference/conference_05/pdf/21_Picard.pdf|last1=Kamka|first1=Frank|last2=Jochmann|first2=Andreas|date=June 2005|conference=International Freiberg Conference on IGCC & {{ill|XTL fuel|lt=XtL|de|XtL-Kraftstoff}} Technologies|others=speaker Lutz Picard|access-date=2011-03-19|url-status=dead|archive-url=https://web.archive.org/web/20110719095948/http://www.iec.tu-freiberg.de/conference/conference_05/pdf/21_Picard.pdf|archive-date=2011-07-19}}</ref> achieving temperatures higher than the ash fusion temperature. The nature of the gasifier means that the fuel must have high mechanical strength and must ideally be non-caking so that it will form a permeable bed, although recent developments have reduced these restrictions to some extent.{{Citation needed|date=June 2014}} The throughput for this type of gasifier is relatively low. [[Thermal efficiency]] is high as the temperatures in the gas exit are relatively low. However, this means that tar and methane production is significant at typical operation temperatures, so product gas must be extensively cleaned before use. The tar can be recycled to the reactor.

In the gasification of fine, undensified biomass such as [[rice husks|rice hulls]], it is necessary to blow air into the reactor by means of a fan. This creates very high gasification temperature, as high as 1000 C. Above the gasification zone, a bed of fine and hot char is formed, and as the gas is blow forced through this bed, most complex hydrocarbons are broken down into simple components of hydrogen and carbon monoxide.<ref>{{cite journal |last1=Khan |first1=Muhammad Mueed |last2=Amjad |first2=Abdul Basit |title=Hydrogen production from municipal waste and low grade lignite blend |journal=Results in Engineering |volume=24 |pages=103495 |year=2024 |doi=10.1016/j.rineng.2024.103495 |doi-access=free }}</ref>

===Co-current fixed bed ("down draft") gasifier===
Similar to the counter-current type, but the gasification agent gas flows in co-current configuration with the fuel (downwards, hence the name "down draft gasifier"). Heat needs to be added to the upper part of the bed, either by combusting small amounts of the fuel or from external heat sources. The produced gas leaves the gasifier at a high temperature, and most of this heat is often transferred to the gasification agent added in the top of the bed, resulting in an [[Thermal efficiency|energy efficiency]] on level with the counter-current type. Since all tars must pass through a hot bed of char in this configuration, tar levels are much lower than the counter-current type.

===Fluidized bed reactor===
[[File:Gasification facility "Advanced Methanol Amsterdam".jpg|thumb|371x371px|Visualisation of proposed fluidized bed gasification facility in Amsterdam designed to convert waste materials into biofuels<ref>{{Cite web|url=https://www.gidara-energy.com/advanced-methanol-amsterdam|title=Advanced Methanol Amsterdam}}</ref>]]

The fuel is [[fluidization|fluidized]] in oxygen and steam or air. The ash is removed dry or as heavy agglomerates that defluidize. The temperatures are relatively low in dry ash gasifiers, so the fuel must be highly reactive; low-grade coals are particularly suitable. The agglomerating gasifiers have slightly higher temperatures, and are suitable for higher rank coals. Fuel throughput is higher than for the fixed bed, but not as high as for the entrained flow gasifier. The conversion efficiency can be rather low due to [[elutriation]] of carbonaceous material.  Recycle or subsequent combustion of solids can be used to increase conversion. Fluidized bed gasifiers are most useful for fuels that form highly corrosive ash that would damage the walls of slagging gasifiers. Biomass fuels generally contain high levels of corrosive ash.

Fluidized bed gasifiers uses inert bed material at a fluidized state which enhance the heat and biomass distribution inside a gasifier.  At a fluidized state, the superficial fluid velocity is greater than the minimum fluidization velocity required to lift the bed material against the weight of the bed. Fluidized bed gasifiers are divided into Bubbling Fluidized Bed (BFB), Circulating Fluidized Bed (CFB) and Dual Fluidized Bed (DFB) gasifiers.

===Entrained flow gasifier===
A dry pulverized solid, an atomized liquid fuel or a fuel slurry is gasified with oxygen (much less frequent: air) in co-current flow. The gasification reactions take place in a dense cloud of very fine particles. Most coals are suitable for this type of gasifier because of the high [[operating temperature]]s and because the coal particles are well separated from one another.

The high temperatures and pressures also mean that a higher throughput can be achieved, however thermal efficiency is somewhat lower as the gas must be cooled before it can be cleaned with existing technology. The high temperatures also mean that tar and methane are not present in the product gas; however the oxygen requirement is higher than for the other types of gasifiers. All entrained flow gasifiers remove the major part of the ash as a slag as the operating temperature is well above the ash fusion temperature.

A smaller fraction of the ash is produced either as a very fine dry [[fly ash]] or as a black colored fly ash slurry. Some fuels, in particular certain types of biomasses, can form slag that is corrosive for ceramic inner walls that serve to protect the gasifier outer wall. However some entrained flow type of gasifiers do not possess a ceramic inner wall but have an inner water or steam cooled wall covered with partially solidified slag. These types of gasifiers do not suffer from corrosive slags.

Some fuels have ashes with very high ash fusion temperatures. In this case mostly limestone is mixed with the fuel prior to gasification. Addition of a little limestone will usually suffice for the lowering the fusion temperatures. The fuel particles must be much smaller than for other types of gasifiers. This means the fuel must be pulverized, which requires somewhat more energy than for the other types of gasifiers. By far the most energy consumption related to entrained flow gasification is not the milling of the fuel but the production of oxygen used for the gasification.

===Plasma gasifier===
In a [[Plasma gasification|plasma gasifier]] a high-voltage current is fed to a torch, creating a high-temperature arc. The inorganic residue is retrieved as a glass like substance.