Gasification
From Wikipedia, the free encyclopedia
- For the water carbonator, see Gasogene.
Gasification is a process that converts carbonaceous materials, such as coal, petroleum, petroleum coke or biomass, into carbon monoxide and hydrogen.
In a gasifier, the carbonaceous material undergoes three processes:
- The pyrolysis (or devolatilization) process occurs as the carbonaceous particle heats up. Volatiles are released and char is produced, resulting in up to 70% weight loss for coal. The process is dependent on the properties of the carbonaceous material and determines the structure and composition of the char, which will then undergo gasification reactions.
- The combustion process occurs as the volatile products and some of the char reacts with oxygen to form carbon dioxide and carbon monoxide, which provides heat for the subsequent gasification reactions. Pyrolysis and combustion are very rapid processes.
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The gasification process occurs as the char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen. The resulting gas is called producer gas or syngas (or wood gas when fueled by wood) and may be more efficiently converted to energy such as electricity than would be possible by direct combustion of the fuel, as the fuel is first combusted in a gas turbine and the heat is used to produce steam to drive a steam turbine. Also, corrosive ash elements such as chloride and potassium may be refined out by the gasification process, allowing high temperature combustion of the gas from otherwise problematic fuels.
Gasification of char
The gasification process was originally developed in the 1800s to produce town gas for lighting and cooking. Natural gas and electricity soon replaced town gas for these applications, but the gasification process has been utilized for the production of synthetic chemicals and fuels since the 1920s.
Wood gasifiers, called Gasogene or Gazogène, were used to power motor vehicles in Europe during World War II fuel shortages.[1]
It is now recognized that gasification has wider applications; in particular the production of electricity, ammonia and liquid fuels (oil) using Integrated Gasification Combined Cycles (IGCC), with the possibility of producing methane and hydrogen for fuel cells. IGCC is also a more efficient method of CO2 capture as compared to conventional technologies. IGCC demonstration plants have been operating since the early 1970s and some of the plants constructed in the 1990s are now entering commercial service.
Within the last few years, gasification technologies have been developed that use plastic-rich waste as a feed. In a plant in Germany such a technology - on large scale - converts plastic waste via producer gas into methanol.[2]
Gasification relies on chemical processes at elevated temperatures >700°C, contrary to biological processes such as anaerobic digestion that produce biogas.
Breakdown of hydrocarbons into syngas is done by carefully controlling the amount of oxygen present while heating the hydrocarbons to extreme temperatures.
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[edit] Gasification processes
Four types of gasifier are currently available for commercial use: counter-current fixed bed, co-current fixed bed, fluid bed and entrained flow.[3][4][5]
The counter-current fixed bed ("up draft") gasifier consists of 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. The ash is either removed dry or as a slag. The slagging gasifiers require a higher ratio of steam and oxygen to carbon in order to reach temperatures higher than the ash fusion temperature. The nature of the gasifier means that the fuel must have high mechanical strength and must be non-caking so that it will form a permeable bed, although recent developments have reduced these restrictions to some extent. The throughput for this type of gasifier is relatively low. Thermal efficiency is high as the gas exit temperatures 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 or recycled to the reactor.
The co-current fixed bed ("down draft") gasifier is 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 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.
In the fluid bed gasifier, the fuel is fluidized in oxygen (or air) and steam. 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 is rather low, so recycle or subsequent combustion of solids is necessary to increase conversion. Fluidized bed gasifiers are most useful for fuels that form highly corrosive ash that would damage the walls of slagging gasifiers. Biomasses generally contain high levels of such ashes.
In the 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 temperatures 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 bed 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 pulverised, which requires somewhat more energy than for the other types of gasifiers. By far the most energy consumption related to entrained bed gasification is not the milling of the fuel but the production of oxygen used for the gasification.
[edit] Gasification process examples
[edit] High Temperature Conversion of Waste
High Temperature Conversion of Waste (HTCW) is a high temperature melting downdraft gasification process which gasifies the feed material within a controlled and limited oxygen supply. Combustion of the feed material is prevented by the limited oxygen supply.
The temperature within the HTCW reactor reaches 2700°C, at which point molecular dissociation takes place. The pollutants that were contained within the feed waste material such as dioxins, furans, as well as pathogens are completely cracked into harmless compounds.
All metal components in the waste stream are converted into a castable iron alloy/pig iron for use in steel foundries. The mineral fraction is reduced to a non-leaching vitrified glass, used for road construction and/or further processed into a mineral wool for insulation. All of the organic material is fully converted to a fuel quality synthesis gas which can be used to produce electrical energy, heat, methanol, or used in the production of various other chemical compounds. The resultant syngas, with a H2/CO ratio of nearly 1:1, is being further investigated for use in the production of Fischer-Tropsch fuels. Under certain conditions, heat from the reactor could be used for district heating, industrial steam production or water desalination plants.
The HTCW was designed by the K.B.I. Group GMBH to convert any types of waste except for radioactive material. The HTCW reactor works on the principle of negative pressure downdraft gasification. This means that there are no emissions from HTCW reactor itself. The syngas is drawn down through the high temperature zone and then to the the gas cleaning system also designed by the K.B.I Group. The clean gas may then be utilized as already mentioned, or exported to an end user such as a power station.
The HTCW process was examined by the Environment Agency, an organization that examines all available methods for the environmentally sound treatment of waste on behalf of the British Government. The resulting case study was published in their online databank and recognizes the HTCW technology to be a suitable process for the treatment of waste.[6]
[edit] See also
- FutureGen zero-emissions coal-fired power plant
- Fluidized bed
- Fluidized bed combustion
- List of solid waste treatment technologies
- Plasma arc waste disposal
[edit] External links
[edit] References
- ^ Gas Generator Project History of the Gasogene technology.
- ^ Converting waste to methanol
- ^ Beychok, M.R., Process and environmental technology for producing SNG and liquid fuels, U.S. EPA report EPA-660/2-75-011, May 1975
- ^ Beychok, M.R., Coal gasification for clean energy, Energy Pipelines and Systems, March 1974
- ^ Beychok, M.R., Coal gasification and the Phenosolvan process, American Chemical Society 168th National Meeting, Atlantic City, September 1974
- ^ KBI Process Review, www.environment-agency.gov.uk/wtd, Retrieved 28.12.06
