Biomass Gasification
About - Biomass gasification
Biomass gasification means incomplete combustion of biomass resulting in production of combustible gases consisting of Carbon monoxide (CO), Hydrogen (H2) and traces of Methane (CH4). This mixture is called producer gas. Producer gas can be used to run internal combustion engines (both compression and spark ignition), can be used as substitute for furnace oil in direct heat applications and can be used to produce, in an economically viable way, methanol – an extremely attractive chemical which is useful both as fuel for heat engines as well as chemical feedstock for industries. Since any biomass material can undergo gasification, this process is much more attractive than ethanol production or biogas where only selected biomass materials can produce the fuel.
Process of Biomass gasification
The main method of producing syngas from biomass feedstocks is called gasification. Although gasification reactions can take many forms, these processes are defined by cranking up the temperature to between 650 and 1,400oCelsius. There are two approaches to achieving these elevated temperatures: direct heating and indirect heating.
In direct heating, a relatively small amount of oxygen is added to the reactor. If this gas is made up of more than 90 percent oxygen, the resulting syngas will be rich in carbon monoxide and hydrogen. The contrasting approach uses various means of indirect heat transfer to achieve high operating temperatures, including hot sand circulation and exotic alloy heat exchangers The least expensive approach to biomass gasification is the direct approach, which adds air not pure oxygen to the system with simple blower technology. The gas released from this approach is called producer gas because although this method is a money saver, nitrogen from the air becomes a major component of the gas. A gasification system consists of four main stages
- Feeding of the feedstock
- Gasifier reactor where the actual gasification occurs
- Cleaning of the resultant gas
- Utilisation of combustible gas
1. Combustion zone
The combustible substance of a solid fuel is usually composed of elements carbon, hydrogen and oxygen. In complete combustion carbon dioxide is obtained from carbon in fuel and water is obtained from the hydrogen, usually as steam. The combustion reaction is exothermic and yields a theoretical oxidation temperature of 14500C14. The main reactions, therefore, are:
C + O2 = CO2 (+ 393 MJ/kg mole) (1)
2H2 + O2 = 2H2 O (- 242 MJ/kg mole) (2)
2. Reaction zone
The products of partial combustion (water, carbon dioxide and uncombusted partially cracked pyrolysis products) now pass through a red-hot charcoal bed where the following reduction reactions take place.
C + CO2 = 2CO (- 164.9 MJ/kg mole) (3)
C + H2O = CO + H2 (- 122.6 MJ/kg mole) (4)
CO + H2O = CO + H2 (+ 42 MJ/kg mole) (5)
C + 2H2 = CH4 (+ 75 MJ/kg mole) (6)
CO2 + H2 = CO + H2O (- 42.3 MJ/kg mole) (7)
Reactions (3) and (4) are main reduction reactions and being endothermic have the capability of reducing gas temperature. Consequently the temperatures in the reduction zone are normally 800-10000C. Lower the reduction zone temperature (~ 700-8000C), lower is the calorific value of gas.
3. Pyrolysis zone
Wood pyrolysis is an intricate process that is still not completely understood. The products depend upon temperature, pressure, residence time and heat losses. However following general remarks can be made about them. Upto the temperature of 2000C only water is driven off. Between 200 to 2800C carbon dioxide, acetic acid and water are given off. The real pyrolysis, which takes place between 280 to 5000C, produces large quantities of tar and gases containing carbon dioxide. Besides light tars, some methyl alcohol is also formed. Between 500 to 7000C the gas production is small and contains hydrogen. Thus it is easy to see that updraft gasifier will produce much more tar than downdraft one. In downdraft gasifier the tars have to go through combustion and reduction zone and are partially broken down.
Finally in the drying zone the main process is of drying of wood. Wood entering the gasifier has moisture content of 10-30%. Some organic acids also come out during the drying process. These acids give rise to corrosion of gasifiers.
Differences between Gasification & Pyrolysis - There are two main differences between them. First, pyrolysis is undertaken at an equivalence ratio (actual air/air required for complete combustion) of less than 0.2 (typically below 0.15), whereas gasification usually has an operating ratio of between 0.2 and 0.4. Secondly, the processing temperatures for pyrolysis are lower. Biomass gasification or pyrolysis offers the most attractive alternative energy system for agricultural purposes. Biomass gasification technology is also environment-friendly, because of the firewood savings and reduction in CO2 emissions.
Design of Gasifier
Design of gasifier depends upon type of fuel used and whether gasifier is portable or stationary. Gas producers are classified according to how the air blast is introduced in the fuel column. History of gasification reveals serveral designs of gasifiers. The most commonly built gasifiers are classied as :
- Updraft gas producer
- Downdraft gas producer
- Twin-fire gas producer
- Crossdraft gas producer
- Other gas producer
Conversion of solid biomass into combustible gas has all the advantages associated with using gaseous and liquid fuels such as clean combustion, compact burning equipment, high thermal efficiency and a good degree of control. In locations, where biomass is already available at reasonable low prices (e.g. rice mills) or in industries using fuel wood, gasifier systems offer definite economic advantages. Biomass gasification technology is also environment-friendly, because of the firewood savings and reduction in CO2 emissions.
Biomass gasification technology has the potential to replace diesel and other petroleum products in several applications, foreign exchange.
Other advantages of biomass gasification/pyrolysis include:
- Easy to operate and maintain
- Provides energy security
- Generates local employment
- Gasifiers can be designed for rural areas
Thermal applications: cooking, water boiling, steam generation, drying etc.
Motive power applications: Using producer gas as a fuel in IC engines for applications such as water pumping Electricity generation: Using producer gas in dual-fuel mode in diesel engines/as the only fuel in spark ignition engines/in gas turbines.
Disadvantages
- Gasification is quite complex and sensitive process. There exists high level of disagreement about gasification among engineers, researchers, and manufacturers. Many manufacturers claim that their unit can be operated on all kinds of biomass. But it is quite questionable fact as phyiscal and chemical properties varies fuel to fuel.
- Operation of diesel or gasoline engine is simple. Engine starts immediately and there is no trouble within the run. Handling of liquid fuel is also easy task. Anybody expecting something similar will be disappointed with operation of gasifier. It requires atleast half an hour or more to start the system. Fuel is bulky and frequent refuelling is often required for continuous running of the system. Handling residues such as ash, tarry condensates is time consuming and dirty work. Driving with producer gas feuled vehicles requires much more and frequent attention than gasoline or diesel fueled vehlicles
- Getting the producer gas is not difficult, but obtaining in the proper state is the challenging task. Gasoline and diesel have quite homogenous property. The physical and chemical properties of producer gas such as energy content, gas composition and impurities vary time to time. All the gasifiers have fairly strict requirements for fuel size, moisture and ash content. Inadequate fuel preparation is an important cause of technical problems with gasifiers.
- Gasifier is too often thought of as simple device that can generate a combustible gas from any biomass fuel. A hundred years of reseach has clearly shown that key to successful gasification is gasifier specifically designed for a particular type of fuel.Those interested in this technology must remember that it requires hard work and tolerence. Although technology is inconvenient, it is economical at many places and may lead to self-reliance in fuel crisis.
Babcock & Wilcox Volund
A Denmark-based company, a subsidiary of Babcock & Wilcox Power Generation Group, Inc. is significantly growing the global reach of its renewable power generation technology. The company has signed an agreement potentially worth up to EU100million (U.S. $156 million) to supply biomass plants that use its combined cycle gasification (CCG) technology to Advanced Renewable Energy Ltd. (ARE) in Italy. Under the terms of the 10-year agreement, B&WV will be the exclusive supplier to ARE using CCG technology in as many as 25 small-scale, biomass-fueled power plants. B&W Volund anticipates orders for six plants in 2008, with the first scheduled for commercial operation in the first quarter of 2010. B&WV has previously licensed its gasification technology in Japan and continues to pursue licensing opportunities around the world. Each of the power plants, to be built in Italy, will be designed to produce 4-megawatts (MW) of electricity using wood chip biomass to produce gas fuel for engines that will drive electric generators (August, 2008)
Southern Research Institute
The Institute announced they have signed a five-year agreement with ThermoChem Recovery International (TRI) to assemble and operate a biomass gasification pilot plant for TRI within its Carbon-to-Liquids (C2L) Development Center in North Carolina. Southern Research engineers and technicians, in collaboration with TRI Engineers, will test various types of biomass, syngas cleaning systems, catalysts, and other operational features that support for production of clean power or biofuels. The specific terms of the contract were not released (April 2008)
Coskata
A biofuel startup in Illinois can make ethanol from just about anything organic for less than $1 per gallon. Coskata uses existing gasification technology to convert almost any organic material into synthesis gas, which is a mix of carbon monoxide and hydrogen. Rather than fermenting that gas or using thermo-chemical catalysts to produce ethanol, Coskata pumps it into a reactor containing bacteria that consume the gas and excrete ethanol. The company says the process yields 99.7 percent pure ethanol. The company plans to have its first commercial-scale plant producing up to 100 million gallons of ethanol a year by 2011 (Jan 2008).
Choren
The key element in the Choren technology is the Carbo-V Process Technology The Carbo-V Process is a three-stage gasification process involving the following subprocesses:
· Low temperature gasification,
· High temperature gasification and
· Endothermic entrained bed gasification.
During the first stage of the process, the biomass (with a water content of 15 – 20 %) is continually carbonized through partial oxidation (low temperature pyrolysis) with air or oxygen at temperatures between 400 and 500 °C, i.e. it is broken down into a gas containing tar (volatile parts) and solid carbon (char). During the second stage of the process, the gas containing tar is post-oxidized hypostoichiometrically using air and/or oxygen in a combustion chamber operating above the melting point of the fuel’s ash to turn it into a hot gasification medium. During the third stage of the process, the char is ground down into pulverized fuel and is blown into the hot gasification medium. The pulverized fuel and the gasification medium react endothermically in the gasification reactor and are converted into a raw synthesis gas. Once this has been treated in the appropriate manner, it can be used as a combustible gas for generating electricity, steam and heat or as a synthesis gas for producing SunDiesel.
Primenergy
A large gasification system supplier is Primenergy, L.L.C. an Oklahoma corporation with principal offices located in Tulsa, Oklahoma. Primenergy has a plant in Stuttgart,Arkansas that gasifies over 500 tons per day of rice hulls with electrical power generation, via steam turbine, of over 12 MW, with the ability to extract up to 100,000 pounds per hour of medium pressure process steam.14 Primenergy has other plants within and outside the U.S.
Enerkem
Enerkem’s gasification, sequential gas conditioning and catalysis technology converts sorted municipal solid waste and forest residues into cellulosic ethanol and other biofuels.The company has operated a pilot plant since 2003 and is currently building an industrialscale cellulosic ethanol production plant in Canada. It is also participating, in partnership with world-class organizations, in other projects which are in various stages of development.
Solena Group
Solena Group is a US company that uses a highly cost-effective and technically efficient thermal depolymerization/gasification process. Solena's Plasma Technology and Algae Biomass Eliminates Produces Electricity CO2 Emissions Solena develops, builds, owns and operates renewable bio-energy plants. Using its patented plasma technology and algae systems, Solena’s plants produce clean, reliable electricity, with no CO2 emissions.
Flambeau River Paper Mill
The pulp and paper industry is on the verge of a fundamental shift. Pulp and paper producers have an infrastructure in place for processing biomass, a readily available wood resource, and they could be in the most suitable position to think about expanding their product base to include liquid fuels or bioenergy. This becomes possible primarily owing to the Gasification / F-T combination. Flambeau River Paper Mill is one company that has taken a lead in this direction. Flambeau River Biofuels, LLC announced plans to build a modern, integrated, biofuels project using “forest residue” feedstock. The biomass will be fed into a steam reformer and gasified. The resulting syngas will then be fed through a gas-to-liquids FT catalytic process. An existing oil company has expressed interest in an off-take agreement to purchase the oil products for their refinery.
Stora Enso/Neste
NSE Biofuels Oy Ltd. is a 50-50 joint venture between Stora Enso Oyj and Neste Oil Corporation focused on the production of synthetic diesel from wood residues. It plans to use a circulating fluidized-bed (CFB) biomass gasifier. The plant utilizes a fuel-flexible circulating fluidized-bed gasification technology to convert a wide spectrum of biomass into a clean syngas to be used in a gas to liquids (Fischer-Tropsch) process to produce feedstock for renewable diesel from biomass/wood residue-based gas. The gasification and syngas cleaning system will be part of NSE’s new-generation renewable diesel demonstration plant at Stora Enso’s Varkaus Mill in Finland. The plant is expected to start up in early 2009 and will be integrated into the energy infrastructure of the Stora
Enso Varkaus Mill.
- Currently building a demonstration plant at the Stora Enso Varkhaus paper mill,expected startup is 2008
- Plan to subsequently build a 100,000 TPA commercial unit.
- Varkaus, Finland
