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Combustion is one of the oldest ways to convert fuel to useful energy. Combustion of biomass is a process in which oxygen reacts with carbon in the fuel and produces carbon dioxide, water and heat. Gaseous combustion products include also nitrogen oxidants, carbon monoxide and aromatic compounds. Solid products include charcoal and ash, for example. (Loo & Koppejan 2008)
Adequate input for the combustion process includes wood waste, pellets and bio waste with relatively low moisture content. In a combustion reactor or furnace, raw material reacts with oxygen in high temperature (> 800 °C). Usually, an initial step of combustion is drying. The second and third steps include pyrolysis and gasification. The final step is combustion, in which overall efficiency is highly dependent on temperature, available oxygen and raw material properties. Combustion can use either natural or forced draught. (Loo & Koppejan 2008) (MicrE 2011)
In general, combustion processes can be divided to batch and continuous processes. In households, wood-stove is a conventional batch combustion process, when larger continuous combustion reactors, with higher technological properties and larger scale, are more common in industry. These combustion reactors can be divided to fixed bed, bubbling fluidized bed, circulating fluidized bed and pulverized fuel reactors. Furthermore, several types of wood-fired stoves are also available commercially. (Loo & Koppejan 2008)
Combustion can be utilized to produce heat for households and for industrial processes. In addition, hot gases from the process can also heat up water in heating boilers in order to produce electricity. In some cases combustion can also produce gaseous and liquid fuels. Ash from the process can be utilized as fertilizer. (Loo & Koppejan 2008)
Permissions for the installation of a combustion plant vary depending on the scale of the plant. In small-scale applications, such as stoves, no significant requirements are needed. Small-scale combustion stoves usually need only construction permission. After construction, the stove needs to be verified by a rescue authority. (FINLEX 2011)
In case of larger biomass combustion plants, the input raw material defines the suitable process. As well, the type of output, heat or combined heat and electricity production, characterizes the technology needed. In larger scale plants, agreements will need to be made with possible raw material suppliers. Some raw materials may require pre-treatment in order to reach good combustion efficiency, as well as satisfy the requirements of emission levels and bottom ash quality. (FINLEX 2011)
Larger scale combustion plants also require a construction plan and land use plan. City plans will have to be taken into account as well. In addition, storages, pipes and other process equipment will need construction permission. Stability and fire safety regulations are compulsory to follow. (Ministry of Environment 2011a)
Environmental permits are compulsory for larger combustion plants. Air emissions, such as CO2 as well as output ash amounts, are significant. Environmental Impact Assessment will have to be conducted to estimate overall emissions and impacts to environment. Moreover, the solid waste streams from the plant will have to be evaluated by environmental authorities. Combustion plants also fall under the IPPC Directive; therefore, best available technologies will have to be adopted in order to protect theenvironment. (Ministry of Environment 2011a, FINLEX 2011)
The combustion process produces mainly heat, however, when electricity is also generated, the excess electricity can be sold to electricity companies. Before distributing electricity or heat, agreements and permits will have to be done with receiving companies. Standards and regulations related to possible boilers, turbines and CHP-unit are compulsory to follow. (FINLEX 2011)
In larger combustion plants, rescue plan, hazard identification and risk evaluation are necessary to compose, since there are hazardous chemical compounds and high temperature present in the process. Safe working environment is also ensured by following these safety requirements. (Ministry of Environment 2011a)
Legislation and regulations guiding a combustion process is largely dependent on the scale of the process. In the case that heat production of the combustion plant is 20 MW or more, the emission trade directive is taken into account. Co-combustion may require stricter requirements for combustion circumstances. Legislation and regulations for the combustion process may also have national varieties. (Ministry of Environment 2011a)
In small-scale applications, the main safety-related issues originate from the spillage or backdraft of exhaust gas, which should be led outside. Carbon monoxide (CO) is one of the most hazardous compounds from the combustion process. CO forms when combustion temperature is low and available oxygen levels are low. CO is an odorless, tasteless and initially non-irritating and, therefore, difficult to detect. Yet even at relatively low concentrations, CO can cause lightheadedness and confusion. A CO detector, adequate ventilation and appropriate combustion conditions are essential to avoid problems with CO. (EREC 2008, DeKieffer 1995)
If the exhaust gas is not properly ventilated, there can be problems with moisture. Moisture may condensate on the walls and windows, increasing humidity levels. High humidity levels can greatly contribute to mold formation, leading to health hazard. Too high humidity levels can also cause wood deterioration and structural problems of the house. Exhaust gas contains also nitrogen oxides, which can be a health hazard. (EREC 2008)
The high temperature of the combustion process can also lead to hot surfaces. Hot steam can also be dangerous. In the case of mishandling or malfunction of the combustion process, fire can spread to the ambient environment. In addition, gas leaks from the process can set up fires or cause an explosion. (DeKieffer 1995)
To ensure the safe operation of a combustion plant, especially in larger units, proper ventilation system, automatic fuel turn-off valve, gas detectors and flame sensing devices are valuable. These devices need to be inspected and tested at times. Furthermore, accidents prevention may also require education of staff and clients. It is also necessary to clean process equipment at times and check for possible structural damage. (EREC 2008)
If corrosion happens in the process, it can cause structural problems in the heat exchangers. Spillages from heat exchangers can be very hot and dangerous. Also malfunction of boiler, electricity and machinery can cause a safety hazard. (Loo & Koppejan 2008)
Raw material may also require chemical or mechanical pre-treatment, which can also be a source of safety hazards from machinery to hazardous chemicals. These are dependant on raw material properties pre-treatment processes. (Loo & Koppejan 2008)
Properties of raw material have a great impact on combustion process efficiency. At first, moisture content is significant factor. Increasing moisture content can reduce the maximum temperature of the combustion and increase also retention time. High moisture content leads to incomplete combustion and high amounts emissions. Drying of raw material may be needed to decrease moisture content of raw material. (Loo & Koppejan 2008)
Appropriate temperature (more than 800 °C) is important to maintain due to its significant influence on reaction rates. Temperature is also important to optimize in order to reduce emissions from the combustion process. Higher temperatures can be reached also by improving the insulation of the combustion chamber. (Loo & Koppejan 2008)
The amount of available oxygen can restrict the combustion process. Due to this, excessive air ratio is used, but it is necessary to optimize it. Too high oxygen content can decrease the temperature of combustion. In large-scale applications, it is important to ensure sufficient mixing of excess air and ensure also the amount of forced draught to the combustion process. In small-scale applications, the problem of inefficient combustion can be inefficient natural drought. (Loo & Koppejan 2008)
Fuel type and properties, such as density, porosity, size and surface area, can affect significantly the combustion process. Larger particle sizes requires longer retention times, while more porous and finer materials have better reactivity. It is not recommended to use manure and municipal wet organic wastes in the combustion process because they can inhibit it. Also impregnated and painted woods are not suitable for the combustion process. In addition excessive fuel load can also inhibit a small-scale combustion process. (Loo & Koppejan 2008)
In small-scale combustion systems, too large glass area can cause heat losses, because heat radiates easily through it. Adequate retention time is also necessary to maintain, especially in batch processes. In large-scale applications air preheating may also be needed to raise the temperature of the process. (Loo & Koppejan 2008)
Ash from biomass combustion process can contain high alkali and heavy metal concentrations, causing corrosive effects to a boiler. Moreover, ash and slag can foul surfaces, causing harm especially for heat exchange systems. Agglomeration of ash particles can also inhibit the combustion equipment and lead to poor combustion conditions, but high ash levels can affect also downstream processes. These conditions will lead to inefficient combustion productivity, therefore, process equipment must be cleaned at times, and adequate combustion conditions are important to maintain. (Loo & Koppejan 2008)
In large-scale systems electrical and machinery malfunctions can have numerous unpredictable consequences to the combustion process. For example malfunction of forced air system in large-scale applications can disturb the system significantly. Boilers, CHP-units and turbines can also foul and corrode. Therefore, a proper maintenance schedule needs to be upheld. (Loo & Koppejan 2008)
References
DeKieffer Rob (1995) Combustion Safety Checks: How Not to Kill Your Clients. Home Energy Magazine. [Internet pages]. [Cited 6 July 2011]. Available at: http://www.proctoreng.com/articles/rob.html
Energy Efficiency and Renewable Energy Clearinghouse (EREC)(2008) Combustion Equipment Safety. Available from:http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/26464.pdf
Loo Sjaak van & Koppejan Jaap (2008) The Handbook of Biomass Combustion and Co-firing. London, United Kingdom. Earthscan. 465 p. ISBN: 978-1-84407-249-1.
Ministry of Environment 2011a. Environmental permits [Internet pages]. [Cited 26 June 2011]. Available at: =http://www.ymparisto.fi/default.asp?node=96&lan=fi |
