Syngas consists primarily of carbon monoxide and hydrogen and has less than half the energy density of natural gas.
It is combustible and often used as a fuel source or as an intermediate in the production of other chemicals. It contains small amounts of particulates and various other elements including sulphur which must be removed before use. This is not only to ensure a clean exhaust but to avoid damage to the gas turbine itself.
For use as a fuelit is most often produced by gasification of carbon-based feedstocks such as coal or municipal waste. Gasification is a process that converts carbonaceous materialssuch as coalpetroleumpetroleum coke or biomassinto carbon monoxide and hydrogen. It was originally developed in the 1800s to produce town gas for lighting and cooking. This was replaced by natural gas and electricity but the gasification process is still used for the production of synthetic chemicals and fuels. It could also be used to produce methane and hydrogen for fuel cells.
The breakdown of hydrocarbons into syngas is made by controlling the amount of oxygen present while heating the hydrocarbons to extreme temperaturesaround 1600°C. This melts the inert materialwhich flows to the bottom of the gasification vessel where it is cooled into a glass-like non-leachable inert slag. This slag is used primarily as aggregate in road gravel or concrete applications.
Syngas is most efficiently used in a combined cycle power plant where the waste heat from the gas turbine is used to make steam to generate additional electricity via a steam turbineincreasing the overall efficiency of power generation. An IGCC plantintegrated gasification combined cycleis a combined cycle plant that is supplemented by a front-end coal gasification plant for generation of syngas.
In a gasifierthe carbonaceous material undergoes three processes: pyrolysiswhich releases volatiles and charresulting in up to 70percent weight loss for coal; combustionas the volatile products and some of the char reacts with oxygen to form carbon dioxide and carbon monoxide; and gasificationwhere the char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen.
Pyrolysis is a process of producing fuels by heating feedstock in an oxygen-deficient atmosphere to a very high temperature where the atoms vibrate apart at random positionsso the process is one of thermal decompositionnot combustion.
The resulting gas is called producer gas or syngas and may be more efficiently converted to energy such as electricity than would be possible by direct combustion of the original fuel. Alsocorrosive ash elements such as chloride and potassium may be refined out by the gasification processallowing high temperature combustion of the gas from otherwise problematic fuels.
Gasifier types
Several types of gasifier are currently available for commercial use: the up-draft gasifier consists of a fixed bed of carbonaceous fuel through which the gasification agentsteam and air or oxygen flows counter to the fuel flow; the down-draft version is similar but the gasification agent gas flows co-currentlyor downwardswith the fuel; the fluid bed gasifierwhere the fuel is fluidised in oxygenor airand steam; and entrained flow gasifierswhich remove the major part of the ash as a slag as the operating temperature is well above the ash fusion temperature.
In practicean IGCC plant comprises an air separation unit (ASU)a gasification planta gas clean-up system and a combined cycle power plant. The ASU separates air into its component parts and sends the gasifier a stream of pure oxygen. After gasificationthe syngas is then piped through environmental control processes where pollutants and particulates are removed. Then it can be cleanly burned in a turbine.
The integration of the gasification plant with a combined cycle power plant is the most efficient method currently available to convert solid fuel into electricity. An IGCC plant needs 10 to 20percent less fuel than a large-scale standard coal fired power plant and up to 35percent less than a small-scale industrial version. IGCC plants also use about 30percent less water than a coal fired power plant as gas turbines do not require cooling.
IGCC plants are considerably smaller in physical size and footprint than a standard coal fired power plant. The buildings are much smallerwith the outside facilities consisting of mostly vessels and pipes. The gas turbine exhaust stack is the tallest structure anddepending on the local terrain and dispersion modellingis typically 100m tallabout half the size of a standard coal fired power plant.
The emissions from IGCC plants are below those of even the most advanced conventional coal power plants. Sulphur scrubbing is in excess of 98percentwith 99.9percent scrubbing levels reached in certain circumstances. Particulates from the combustion of syngasboth PM10 and PM2.5are almost non-existent. The primary particulate emissions are from the handling of bulk materials and the movement of people on the plant site.
NOx emissions are also dramatically lower. In a standard coal plantlimits of 25ppm are common whereas a gasification power plant can meet limits of 15ppm without scrubbing and can be reduced to below 5ppm.
Virtually no metals or hazardous air products are emitted and instead are captured as inert slag or as small amounts of inert fly ash. Equipment vendors guarantee 90percent mercury scrubbing efficiency and in practice virtually 100percent is actually recovered. And future trends call for complete capture of CO2 from power plants with sequestration in underground caverns.
Commercial aspects
For companies wishing to move from natural gas to syngasFoster Wheeler’s Global Power Group points out that the degree of integration between the gas turbine and the air separation unit is an important design aspect that can bring substantial benefits in performance efficiency and capital outlay. But the best degree of integration is strongly dependent on the characteristics of the selected gas turbine frame.
In the case of total integration100percent of the air required by the ASU is supplied by bleeding some of the airgenerally not more than 15percent of total flowfrom the discharge of the gas turbine compressor. Depending on the gas turbine frame this air is available at 10–15barso the air separation plant has to be a high-pressure typedelivering oxygen and nitrogen at 3–4bar.
Oxygen is recompressed and used in gasificationwhile nitrogen is recompressed and reinjected in the syngas to replenish the mass deficit caused by the air bleedingandat the same timereduce N0x formation during combustion by lowering the flame peak temperature.
Alternatively the air separation plant can be stand-aloneor non-integrated. In this case a low-pressure air separation plant is neededwith its own air compressor delivering air to the process at the minimum pressure required to meet the energy demand of the unit. In this casesyngas humidification is generally preferred to nitrogen addition for N0x controlbecause of the large nitrogen compression energy consumption.
The company also points out that plant designed for natural gas needs significant changes when moving to syngas. Different burners have to be installed while the greater fuel flow of syngas can overload the turbine blades.
Foster Wheeler’s Global Power Group offers advancedcost-effective and environmentally friendly energy solutions to customers around the world. It is expanding its manufacturing facility in China to meet increased demand for steam generators and replacement pressure parts at substantial savings.
In a pointer to the futureSiemens is acquiring a Swiss companySustec-Groupfor its expertise in converting coal to electricity. This acquisition also brings a 50percent stake in a joint venture in Chinaa country heavily committed in the long term to coal as a fuelsupported by its vast coal reserves.
Siemens also plans to build a large-scale1000MWcoal gasification plant in Saxony. The syngas produced will be used for power generation and production of roughly 600000tons of methanol per year. It is scheduled to start commercial production about three years from now. The company says an attractive growth potential is now emerging for advanced coal gasification technology.
CurrentlyGE is one of the world’s leading providers of both gasification and power generation technologies. It provides long-term license agreements that include the gasification process design packageprocess and instrumentation.
EfficiencyMap is a performance monitoring and optimisation systemdesigned for use with simple cycle gas turbinescombined cyclesconventional fossil steam plants and the steam cycle of nuclear plants. It helps plants operate more efficiently.
And the American government says thatbecause of deregulationrapidly changing market demandsfluctuations in natural gas pricesand increased environmental concernsgasification will become the centrepiece of tomorrow’s advanced power plants.
To boost take-up of the technologythe US Department of Energy (DOE) is sponsoring a broad spectrum of research and demonstration projects for gasification technologies such as the $1 bn10-year demonstration FutureGen project at its National Energy Technology Laboratory (NETL). This is aimed at creating the world’s first coal-firedgasification-basednear-zero emissions electricity and hydrogen production power plant.
For more informationvisit www.fwc.comwww.gepower.comwww.siemens.com
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