Biomass Pyrolysis

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<p>Insights into Biomass Pyrolysis ProcessPyrolysis is the thermal decomposition of biomass occurring in the absence of oxygen. It is the fundamental chemical reaction that is the precursor of both the combustion and gasification processes and occurs naturally in the first two seconds. The products of biomass pyrolysis include biochar, bio-oil and gases including methane, hydrogen, carbon monoxide, and carbon dioxide. Depending on the thermal environment and the final temperature, pyrolysis will yield mainly biochar at low temperatures, less than 450 0C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 800 0C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is bio-oil. Pyrolysis can be performed at relatively small scale and at remote locations which enhance energy density of the biomass resource and reduce transport and handling costs. Heat transfer is a critical area in pyrolysis as the pyrolysis process is endothermic and sufficient heat transfer surface has to be provided to meet process heat needs. Pyrolysis offers a flexible and attractive way of converting solid biomass into an easily stored and transported liquid, which can be successfully used for the production of heat, power and chemicals.</p> <p>A wide range of biomass feedstocks can be used in pyrolysis processes. The pyrolysis process is very dependent on the moisture content of the feedstock, which should be around 10%. At higher moisture contents, high levels of water are produced and at lower levels there is a risk that the process only</p> <p>produces dust instead of oil. High-moisture waste streams, such as sludge and meat processing wastes, require drying before subjecting to pyrolysis. Biomass pyrolysis has been attracting much attention due to its high efficiency and good environmental performance characteristics. It also provides an opportunity for the processing of agricultural residues, wood wastes and municipal solid waste into clean energy. In addition, biochar sequestration could make a big difference in the fossil fuel emissions worldwide and act as a major player in the global carbon market with its robust, clean and simple production technology.</p> <p>On Biochar and Bio-oil</p> <p>Biochar</p> <p>sequestration</p> <p>is</p> <p>considered</p> <p>carbon</p> <p>negative as it results in a net decrease in atmospheric carbon dioxide over centuries or millennia time scales. Instead of allowing the organic matter to decompose and emit CO2, pyrolysis can be used to sequester the carbon and remove circulating carbon dioxide from the atmosphere and stores it in virtually permanent soil carbon pools, making it a carbon-negative process. According to Johannes Lehmann of Cornell University, biochar sequestration could make a big difference in the fossil fuel emissions worldwide and act as a major player in the global carbon market with its robust, clean and simple production technology. The use of pyrolysis also provides an opportunity for the processing of agricultural residues, wood wastes and municipal solid waste into useful clean energy. Although some organic matter is necessary for agricultural soil to maintain its productivity, much of the agricultural waste can be turned directly into biochar, bio-oil, and syngas. Pyrolysis transforms organic material such as agricultural residues and wood chips into three main components: syngas, bio-oil and biochar (which contain about 60 per cent of the carbon contained in the biomass. The two main methods for biochar production are fast pyrolysis and slow pyrolysis. The biochar yield is more than 50% in slow pyrolysis but it takes hours to complete. On the other hand, fast pyrolysis yields 20% biochar and takes seconds for complete pyrolysis. In addition, fast pyrolysis gives 60% bio-oil and 20% syngas. The essential features of a fast pyrolysis process are:</p> <p>1. Very high heating and heat transfer rates, which often require a finely ground biomass feed</p> <p>2. Carefully controlled reaction temperature of around 500 C in the vapour phase and residenceo</p> <p>time of pyrolysis vapours in the reactor less than 1 s 3. Quenching (rapid cooling) of the pyrolysis vapours to give the bio-oil product. Bio-oil is a dark brown liquid and has a similar composition to biomass. It is composed of a complex mixture of oxygenated hydrocarbons with an Bio-oil has a much higher density than woody materials (three to six times, depending on form), which reduces storage and transport costs. Bio-oil is not suitable for direct use in standard internal combustion engines. Alternatively, the oil can be upgraded to either a special engine fuel or through gasification processes to a syngas and then bio-diesel. Bio-oil is particularly attractive for co-firing because it can be more readily handled and burned than solid fuel and is cheaper to transport and store. Since the oil has a density of about 1200 kg m-3, it can be conveniently transported over long distances. Current end-use possibilities are as a boiler fuel for stand-alone heat or in combined heat and power (CHP) using the steam cycle after either diesel or gas turbine electricity generation. The majority of these options have been found to be technically feasible. In addition, bio-oil is also a vital source for a wide range of organic compounds and speciality chemicals.</p> <p>Introduction to Biomass PyrolysisApril 13, 2012 6:09 am Leave a Comment Salman Zafar</p> <p>Pyrolysis is the thermal decomposition of biomass occurring in the absence of oxygen. It is the fundamental chemical reaction that is the precursor of both the combustion and gasification processes and occurs naturally in the first two seconds. The products of biomass pyrolysis include biochar, bio-oil and gases including methane, hydrogen, carbon monoxide, and carbon dioxide. Depending on the thermal environment and the final temperature, pyrolysis will yield mainly biochar at low temperatures, less than 450 0C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 800 0C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is bio-oil.</p> <p>Pyrolysis can be performed at relatively small scale and at remote locations which enhance energy density of the biomass resource and reduce transport and handling costs. Pyrolysis offers a flexible and attractive way of converting solid biomass into an easily stored and transported liquid, which can be successfully used for the production of heat, power and chemicals. A wide range of biomass feedstocks can be used in pyrolysis processes. The pyrolysis process is very dependent on the moisture content of the feedstock, which should be around 10%. At higher moisture contents, high levels of water are produced and at lower levels there is a risk that the process only produces dust instead of oil. High-moisture waste streams, such as sludge and meat processing wastes, require drying before subjecting to pyrolysis. The efficiency and nature of the pyrolysis process is dependent on the particle size of feedstocks. Most of the pyrolysis technologies can only process small particles to a maximum of 2 mm keeping in view the need for rapid heat transfer through the particle. The demand for small particle size means that the feedstock has to be sizereduced before being used for pyrolysis.</p> <p>Pyrolysis processes can be categorized as slow pyrolysis or fast pyrolysis. Fast pyrolysis is currently the most widely used pyrolysis system. Slow pyrolysis takes several hours to complete and results in biochar as the main product. On the other hand, fast pyrolysis yields 60% bio-oil and takes seconds for complete pyrolysis. In addition, it gives 20% biochar and 20% syngas.</p> <p>Bio-oil is a dark brown liquid and has a similar composition to biomass. It has a much higher density than woody materials which reduces storage and transport costs. Biooil is not suitable for direct use in standard internal combustion engines. Alternatively, the oil can be upgraded to either a special engine fuel or through gasification processes to a syngas and then bio-diesel. Bio-oil is particularly attractive for co-firing because it can be more readily handled and burned than solid fuel and is cheaper to transport and store. Bio-oil can offer major advantages over solid biomass and gasification due to the ease of handling, storage and combustion in an existing power station when special start-up procedures are not necessary. In addition, bio-oil is also a vital source for a wide range of organic compounds and speciality chemicals.</p> <p>Primary Biomass Conversion Technologies ThermochemicalApril 21, 2009 12:47 am Leave a Comment Salman Zafar</p> <p>A wide range of technologies exists to convert the energy stored in biomass to more useful forms of energy. These technologies can be classified according to the principal energy carrier produced in the conversion process. Carriers are in the form of heat, gas, liquid and/or solid products, depending on the extent to which oxygen is admitted to the conversion process (usually as air). The three principal methods of thermochemical conversion corresponding to each of these energy carriers are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air. Conventional combustion technologies raise steam through the combustion of biomass. This steam may then be expanded through a conventional turbo-alternator to produce electricity. A number of combustion technology variants have been developed. Underfeed stokers are suitable for small scale boilers up to 6 MWth. Grate type boilers are widely deployed. They have relatively low investment costs, low operating costs and good operation at partial loads. However, they can have higher NOx emissions and decreased efficiencies due to the requirement of excess air, and they have lower efficiencies. Fluidized bed combustors (FBC), which use a bed of hot inert material such as sand, are a more recent development. Bubbling FBCs are generally used at 10-30 MWth capacity, while Circulating FBCs are more applicable at larger scales. Advantages of FBCs are that they can tolerate a wider range of poor quality fuel, while emitting lower NOx levels. Gasification of biomass takes place in a restricted supply of oxygen and occurs through initial devolatilization of the biomass, combustion of the volatile material and</p> <p>char, and further reduction to produce a fuel gas rich in carbon monoxide and hydrogen. This combustible gas has a lower calorific value than natural gas but can still be used as fuel for boilers, for engines, and potentially for combustion turbines after cleaning the gas stream of tars and particulates. If gasifiers are air blown, atmospheric nitrogen dilutes the fuel gas to a level of 10-14 percent that of the calorific value of natural gas. Oxygen and steam blown gasifiers produce a gas with a somewhat higher calorific value. Pressurized gasifiers are under development to reduce the physical size of major equipment items. A variety of gasification reactors have been developed over several decades. These include the smaller scale fixed bed updraft, downdraft and cross flow gasifiers, as well as fluidized bed gasifiers for larger applications. At the small scale, downdraft gasifiers are noted for their relatively low tar production, but are not suitable for fuels with low ash melting point (such as straw). They also require fuel moisture levels to be controlled within narrow levels. Pyrolysis is the term given to the thermal degradation of wood in the absence of oxygen. It enables biomass to be converted to a combination of solid char, gas and a liquid bio-oil. Pyrolysis technologies are generally categorized as fast or slow according to the time taken for processing the feed into pyrolysis products. These products are generated in roughly equal proportions with slow pyrolysis. Using fast pyrolysis, bio-oil yield can be as high as 80 percent of the product on a dry fuel basis. Bio-oil can act as a liquid fuel or as a feedstock for chemical production. A range of bio-oil production processes are under development, including fluid bed reactors, ablative pyrolysis, entrained flow reactors, rotating cone reactors, and vacuum pyrolysis</p> <p>Pyrolysis is the thermal decomposition of organic fuels (e.g., biomass resources, coal, plastics) into volatile compounds (e.g., gases and bio-oil) and solids (chars) in the absence of oxygen and usually water. Pyrolysis types are differentiated by the temperature, pressure, and residence (processing) time of the fuel which determines the types of reactions that dominate the process and the mix of products produced. Slow (conventional) pyrolysis is characterized by slow heating rates (0.1 to 2oC per second), low prevailing temperatures (around 500oC), and lengthy gas (&gt; 5 seconds) and solids (minutes to days) residence times. Flash pyrolysis is characterized by moderate temperatures (400600oC), rapid heating rates (&gt; 2C per second), and short gas residence times (&lt; 2 seconds). Fast pyrolysis(thermolysis) involves rapid heating rates (200 to 105C per second), prevailing temperatures usually in excess of 550oC, and short residence times. Currently, most of the interest in pyrolysis focuses on fast pyrolysis because the products formed are more similar to fossil fuels currently used. Of particular interest is the production of bio-oil which can be used for heating and to produce transportation fuels and organic chemicals. Pyrolysis of Biomass Resources</p> <p>All biomass resources are composed primarily of cellulose (typically 30 to 40 percent of dry weight), hemicellulose (25 to 30 percent of dry weight), and lignin (12 to 30 percent of dry weight), but the percent of each compound differs significantly among biomass resources. This heterogeneity creates variability in the yields of pyrolysis products. Cellulose is a straight and stiff molecule with a polymerization degree of approximately 10.000 glucose unite (C6 sugar) Hemicellulose are polymers built C5, C6 sugars with a polymerisation degree of about 200 sugar units. Both cellulose and hemicellulose can be vapored with negligible char formation at temperatures above 500 "C. Lignin is a three dimensional branched polymer composed phenolic units. Due to the aromatic content of lignin, it degrades slowly on heating and contributes to a major fraction of the char formation. In addition to the major cell wall composition like cellulose, hemicellulose and lignin, biomass often contains varying amounts</p> <p>of species called "extractives". These extractives, which are soluble in polar or no polar solvents, consists of terpenes, fatty acids, aromatic compounds and volatile oil. The composition of various biomass materials is present. Cellulose is converted to char and gases (CO, CO2, H2O) at low temperatures (&lt; 300oC), and to volatile compounds (tar and organic liquids, predom...</p>