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Gasification is a thermochemical process, which uses biomass as a feedstock in order to produce syngas and other outputs. In other words, gasification packs energy into chemical bonds in the product gas leading to higher energy density. Possible feedstock for gasification includes wood, wood residues, bark, shrubs, sawdust, energy crops and other wood-based raw materials. Wastes, such as agricultural wastes and crop residues, are also suitable raw materials.
(MicrE 2011, Basu 2010)
A conventional gasification process consists of biomass drying, pyrolysis, oxidation and reduction steps. In the pyrolysis chamber, large hydrocarbon molecules of biomass break up into smaller molecules in the absence of oxygen. Therefore, relatively volatile compounds of the biomass are separated from the char. Temperature in the pyrolysis chamber varies between 400–650°C.
(Basu 2010, MicrE 2011)
Endothermic pyrolysis and gasification reactions occur in the oxidation chamber at temperatures between 900–1200°C. Syngas is formed in the reduction chamber through several reactions. Gasification processes can be divided to updraft and downdraft gasifiers. Furthermore, reactors can be roughly classified to moving bed reactor, fluidized bed reactor and entrained flow reactor.
(Basu 2010, MicrE 2011)
The most significant impurities, such as tars and particulates are separated from the final product. Conventional separation processes for tar and particulate removal are usually cyclones, filters, electrostatic precipitators and scrubbers. Tar can also be treated by catalyst or thermal cracking. Companies have been developing also a plasma technique for the gasification process. (Basu 2010)
The final product, syngas (a mixture of hydrogen and carbon monoxide) can be utilized as a fuel in the internal combustion engine or to run a gas turbine. Ash from the process can be utilized as a fertilizer or as an additive in construction materials. (MicrE 2011)
Construction and operation of a gasification plant is connected to several different requirements and legislative systems. Before making further applications for building a plant, it is necessary to determine the raw material input (biomass or waste etc.), so the equipment, circumstances and type of the process can be defined. In addition, the use of the end product is to be thought over, so the possible needs and regulations of CHP-unit or agreements with bio fuel companies can be considered. (Gasification guide 2009)
Firstly, a construction permit is needed for a plant. Regulation of land use planning is taken into account in order to find an appropriate place for the gasification plant. It is also possible that city plan is also taken into consideration here. (Ministry of Environment 2011a)
Environmental permit for the gasification plant is essential. To this end, an environmental impact assessment is necessary to conduct in order to ascertain possible impacts to the environment such as emissions to the atmosphere, noise pollution, soil protection, waste generation and possible waste water discharges. Obligations related to the IPPC Directive vary from country to country and can depend on the size of the gasification plant. (Gasification guide 2009, Ministry of Environment 2011a)
Rescue plan, hazard identification and risk assessment for the plant is also composed and the ATEX directive is followed. Health and safety at work must be ensured by following the regulations related to fire and explosion hazards. Moreover the regulations involved to electrical and pressure equipment and machinery are also necessary to consider. Adequate handling, transportation and storing of several hazardous substances is essential to avoid accidents. Special monitoring may be required for certain installations. Fire safety and stability regulations of the plant building are essential. (Gasification guide 2009, Ministry of Environment 2011a)
The use of energy may need requirements connected to feeding electricity to the power grid. In this case, a possible buyer for the electricity is to be selected and the terms of electricity transactions are agreed upon. (Gasification guide 2009)
Legislative systems and regulations related to installation of pipes and storages are also required. Transportation of waste is also regulated. The produced syngas can be sold to companies, which must be agreed upon with the companies. In addition, agreements with raw material suppliers can be made. In this case the possible seasonality of raw material supply is to be considered. (Malla2 2006)
Differences in national and regional regulations can exist. To avoid the major problems with legislative systems and regulations, it is necessary to discuss with local environmental authorities at an early stage of the gasification plant planning process. (Ministry of Environment 2011a)
The gasification process is inwardly related to production, utilization and handling of toxic and flammable compounds. Carbon monoxide (CO) is a very poisonous compound, which can be dangerous for life. Very small concentrations of CO can cause headache, dizziness and nausea. Therefore, an alarm system for too high CO concentrations is necessary. In addition, due to possible CO leaks, a ventilation system is necessary. (Gasification guide 2009)
Explosion hazard can occur, if there is spark available for ignition and the concentrations of CO2, H2 and O2 are suitable. Moreover specific concentration of dust and source of ignition can cause a dust explosion. Product gas can also auto-ignite in temperatures of 600-650 °C. Glowing particles, gases and explosions can also start the fire in the plant. During repairs, an explosion can occur, if there is still gas inside the reactor. During the storage and transportation of product gas it is also necessary to follow safety regulations. (Gasification guide 2009)
Overpressure can also lead to gas escaping, leading to gas intoxication. In addition to CO, also other compounds from the process can be hazardous. For example Polycyclic aromatic hydrocarbon (PAH) compounds are toxic and carcinogenic, and can leak from the process. Inadequate control system programming can cause severe consequences. Other malfunction, for example in the reactor and pumps, can cause gas leaks leading to mentioned outcomes. (Gasification guide 2009)
Fluctuating and too high pressure can cause material damage to process equipment and hence it can be a safety hazard, causing leakages, for example. Also too high temperature can cause problems to process material and cause malfunctions or even self-ignition of some gas mixtures. In addition, occupational health and safety hazards have to be considered as a part of risk assessment. Possible occupational safety and health hazards are also: noise emissions, exhaust gas, heat stress, hot surfaces and electricity. Electrical hazards can lead to static electricity build up and sparks, causing, in the worst case, an explosion. (Gasification guide 2009)
The best way to avoid these safety hazards is to follow the ATEX Directive. Gasification facilities should be constructed by following international standards. Also having appropriate equipment to monitor concentration of chemicals is essential. In addition, the education of staff is mandatory. (Gasification guide 2009)
There are several chemical and physical factors affecting the yield of the product gas. Firstly, high moisture content is essential to be removed, by drying the raw material input in a specific drying chamber. If the biomass entering the pyrolysis chamber has too high moisture content (over 30 %), it can inhibit the gasification process and lead to decreased thermal efficiency. In addition, an adequate particle size is important factor to get good yield of gasification. Specific hydrogen-to-carbon ratio of the raw material, among others, affects the gasification yield, especially in the pyrolysis chamber. (Basu 2010)
Tar can cause troubles to the gasification process, when the tar-containing gas is cooled. Tar will condense on cooler surfaces or remaining in small aerosol drops. Tar can condense also on cooler pipeline surfaces, causing blockage, but it can also block engines and filters. Formation of liquid tar can be avoided by keeping the temperature above the dew point of tar before a tar separation unit. Without separation of tar, it will greatly inhibit the subsequent use of syngas, for example in an internal combustion engines. (Basu 2010)
Heavy metals, such as lead, copper and zinc, especially as chlorides, inhibit the gasification process. Most of heavy metals slow down gasification reactions, leading to longer retention time. In addition, some alkali metals can be harmful for the gasification process, because they can foul heat transfer surfaces and react with other inorganic compounds, causing corrosion. (Chartier et al. 1996)
If the gasification chamber is fed by too high oxygen concentration, it can lead to combustion, and thus weaken the yield of product gas. To this effect, an adequate temperature and pressure is necessary to maintain. Appropriate retention time of the process affects greatly on the result. In addition, too high ash content (over 15 %) can inhibit the process. Raw materials possessing low ash content can also minimize disposal issues. (Basu 2010)
Product gas yield can be low due to inefficient catalyst. Catalysts need to be recovered at times and the surface of the catalyst can foul. In case of electrical and machinery malfunctions, a professional mechanic will need to fix the problem. Such malfunctions can cause unknown and diverse consequences in the process, so it is hard to consider them in advance. (Basu 2010)
References
Basu Prabir (2010) Biomass Gasification and Pyrolysis. Elsevier Science Publishing Co Inc . 376 p. ISBN: 978-0-12-374988-8
Chartier P, Ferrero G.L, Henius U.M, Hultberg S, Sachau J, Wiinblad M (1996) Biomass for energy and environment. Volume 2. Copenhagen, Denmark. 1473 p. ISBN: 008-0428495
Gasification guide 2009. Guideline for Safe and Eco-friendly Biomass Gasification. European Commission 2009. Available at: http://www.gasificationguide.eu/gsg_uploads/documenten/D10_Final-Guideline.pdf
Ministry of Environment 2011a. Environmental permits [Internet pages]. [Cited 26 June 2011]. Available at: =http://www.ymparisto.fi/default.asp?node=96&lan=fi
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