Biomass Pyrolysis

Insights into Biomass Pyrolysis Process Pyrolys Pyrolysis is is the thermal thermal decom decompos position ition of bioma biomass ss occurr occurring ing in the absence absence of oxygen 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 density of the biomass biomass resource resource and reduce transport transport and handling handling costs. Heat transfer 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.

 A wide range range of biomass feedstocks feedstocks can be used used in pyrolysis pyrolysis processes. processes. The pyrolysis pyrolysis process 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 produces dust instead instead of oil. High-moisture High-moisture waste streams, such as sludge sludge and meat processing 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.

On Biochar and Bio-oil

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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 CO 2, pyrolysis can be used to sequester sequester the carbon and remove remove circulating circulating carbon dioxide from the atmosphe atmosphere re and stores stores it in virtually permanent soil carbon pools, making it a carbon-negative carbon-negative process.  According  According to Johannes Johannes Lehmann Lehmann of Cornell Cornell University, University, biochar biochar sequestra sequestration tion could 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, robust, clean clean and simple simple produ productio ction n techno technolog logy. y. The The use of pyrolys pyrolysis is also also provid provides es an opportunity for the processing of agricultural residues, wood wastes and municipal solid waste into useful useful clean energy. energy. Although some organic organic matter is necessary for agricultural 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:

produces produces dust instead instead of oil. High-moisture High-moisture waste streams, such as sludge sludge and meat processing 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.

On Biochar and Bio-oil

Bioc Biocha harr

sequ seques estr trat atio ion n

is

cons consid ider ered ed

carb carbon on

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 CO 2, pyrolysis can be used to sequester sequester the carbon and remove remove circulating circulating carbon dioxide from the atmosphe atmosphere re and stores stores it in virtually permanent soil carbon pools, making it a carbon-negative carbon-negative process.  According  According to Johannes Johannes Lehmann Lehmann of Cornell Cornell University, University, biochar biochar sequestra sequestration tion could 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, robust, clean clean and simple simple produ productio ction n techno technolog logy. y. The The use of pyrolys pyrolysis is also also provid provides es an opportunity for the processing of agricultural residues, wood wastes and municipal solid waste into useful useful clean energy. energy. Although some organic organic matter is necessary for agricultural 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:

1. Very high heating heating and heat transfer rates, which often require require a finely ground biomass biomass feed

2. Carefully controlled reaction temperature of around 500 C in the vapour phase and residence o

time of pyrolysis vapours in the reactor less l ess than 1 s 3. Quenching (rapid cooling) of the pyrolysis vapours vapours to give give the bio-oil product. 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 compounds and speciality chemicals.

Introduction to Biomass Pyrolysis  April 13, 2012 6:09 am Leave a Comment Salman Zafar  ⋅

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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-oi bio-oill and gases gases includ including ing metha methane, ne, hydro hydrogen gen,, carbo carbon n monox monoxide ide,, and and carbon 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 g ases 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. 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.

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.

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.

Primary Biomass Conversion Technologies – Thermochemical  April 21, 2009 12:47 am Leave a Comment Salman Zafar  ⋅

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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

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

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 (> 5 seconds) and solids (minutes to days) residence times. Flash pyrolysis is characterized by moderate temperatures (400600oC), rapid heating rates (> 2°C per second), and short gas residence times (< 2 seconds). Fast pyrolysis(thermolysis) involves rapid heating rates (200 to 105°C 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

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

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 (< 300oC), and to volatile compounds (tar and organic liquids, predominantly levoglucosan) at high temperatures (> 300oC) (Funakuzuri, 1986). The yield of light hydrocarbons (i.e., C1 - C4) is negligible below 500°C but increases substantially at high temperatures (Scott et al., 1988). At temperatures above 600°C, tar yields drop, gas yields increase, and the pyrolysis of cellulose is complete (Hajaligol, 1982; Bradbury, 1979; Funazukuri, 1986; and Piskorz, 1986). Hemicellulose is the most reactive component of biomass and decomposes between 200 and 260oC (Koufopanos, 1989). The decomposition of hemicellulose is postulated to occur in two steps—the breakdown of the polymer into water soluble fragments followed by conversion to monomeric units and decomposition into volatile compounds (Soltes and Elder, 1981). Hemicelluloses produce more gases and less tar than cellulose, and no levoglucosan. They also produce more methanol and acetic acid than cellulose. Lignin is a highly linked, amorphous, high molecular weight phenolic compound which serves as cement between plant cells and is the least reactive component of biomass. The time required to pyrolyze biomass resources is controlled by the rate of pyrolysis of lignin under operating conditions. Decomposition of lignin occurs between 280°C and 500°C, although some physical and/or chemical changes (e.g., depolymerization, loss of some methanol) may occur at lower temperatures (Koufopanos, 1989). At slow heating rates, lignin loses only about half of its weight at temperatures below 800°C (Wenzel, 1970). Pyrolysis of lignin yields more char and tar than cellulose (Soltes and Elder, 1981). For wood, the decomposition of the major components occurs separately and sequentially with the hemicellulose decomposing first and the lignin last. Up to 200°C, moisture is removed, volatile products such as acetic acid and formic acid are released, and non-condensable gases such as CO and CO2 are produced. Between 200 and 280°C, further decomposition of  the char and wood occur resulting in the release of pyroligneous acids, water and non-condensable gases. Separation of tar occurs. Between 280 and 500°C, release of combustible volatile products (CO, CH4, H2, formaldehyde, formic acid, methanol, and acetic acid) occurs. Char formation decreases and the carbon content of the char increases. Condensable tar is released. Above 500°C, carbonization is complete. Secondary reactions begin if the materials are not removed from the reaction zone as quickly as they form. When cooled, some volatile compounds produced during the pyrolysis of biomass resources condense to form a liquid called bio-oil. Bio-oil consists of 20-25% water, 25-30% pyrolytic lignin, 5-12% organic

acids, 5-10% non-polar hydrocarbons, 5-10% anhydrosugars, and 10-25% other oxygenated compounds. Due to large amounts of oxygenated compounds, bio-oil is polar and does not mix readily with hydrocarbons (such as petroleum-derived fuels). It contains less nitrogen than petroleum, and almost no metal or sulfur. Bio-oil is acidic (pH of 2 to 4) due to the creation of organic acids (e.g., formic and acetic acid) when biomass degrades and is corrosive to most metals except stainless steel. Hydrophilic bio-oils contain 15 to 35 percent water by weight which cannot be removed by conventional methods like distillation. High water content decreases its viscosity which aids in transport, pumping and atomization, improves stability, and lowers the combustion temperature which reduces NOx emissions. Some additional water can be added, but only up to a point before phase separation occurs which prevents bio-oils from being dissolved in water. Bio-oil is relatively unstable compared to fossil fuels due to the presence of more polymeric compounds. Table 1 summarizes select properties of bio-oil derived from the pyrolysis of wood.

A number of studies have examined factors that affect the kinetics of biomass pyrolysis reactions. Studies that have examined temperature and heat rate interactions include Scott, 1988 (maple wood); Aarsen, 1985 (wood); Ayll’on, 2006 (meat and bone meal); Koufapanos, 1989 (sawdust); Nunn, 1985 (wood and cellulose); Utioh, 1989 (wheat grain); Sadakata, 1987. These studies indicate that temperature is more important than rate of heating with respect to the mix of products, and that at any given temperature and heat rate, bio-oil and char are the dominant products. Bio-oil yields increase up to temperatures between 550°C and 680°C and then decline. As temperatures increase, char production decreases (to a steady level above 650°C) and the carbon content of the char increases. Hydrocarbon gas yields (e.g., C2H6, C3H6) increase up to about 660°C and then decline, probably due to thermal

cracking. The time required to obtain a given conversion level decreases with increasing temperature. Biomass weight loss is higher at lower pressures (Ward and Braslaw, 1985). At any given temperature, char residues increase pressure. Cellulose displays the strongest pressure dependency and lignin the lowest--the pressure effect is observable at temperatures above 350°C. The higher pressure increases the residence time of the volatile compounds resulting in higher yields of low molecular weight gases and lower yields of  tar and liquid products (Blackadder and Rensfelt, 1985). The presence of inorganic materials (either as additives or as the natural ash content of the biomass resource) affects the mix of pyrolysis products. The impacts are measured using thermogavimetry (TG), thermal evolution analysis (TEA), and differential thermal analysis (DTA). Alkaline compounds have a more pronounced effect than do acidic compounds. Alkaline catalysts increase gas yields and char production and decrease tar yields; reduce the decomposition temperature; increase weight loss; and increase reaction rates (Utioh, 1989; Roberts, 1970; Tsuchiya and Sumi, 1970). Acid catalysts cause transglycosylation reactions in small quantities, and dehydration of the anhydrosugars in larger quantities. Acidic catalysts enhance the condensation of intermediate compounds and affect char oxidation. Inorganic salts reduce CO, H2, and hydrocarbon gases, but increase CO2; decrease tar; increase water yields; and increase char yields (Nasser, 1986). The presence of catalysts are most significant for wood and cellulose pyrolysis but negligible for lignin pyrolysis(Nassar and MacKay, 1986). Pyrolysis Reactions

 The sequence and rate at which pyrolysis reactions occur and the factors that influence the rate are described by the kinetics of the reaction. The kinetics of fast pyrolysis reactions are characterized by Equation 1,

(Equation 1)

where Wt is the particle weight after reaction time (in grams), t is the pyrolysis time (in seconds), K o is the frequency factor (in seconds),W∞ is the ultimate particle weight (in grams), R is the universal gas constant (in Joule per grams Kelvin), E is the activation energy (in  Joule per grams), and T is the temperature (in degrees Kelvin). The reported value of E varies substantially (ranging from 40 to 250 kJ/mole) depending on the operating conditions and the type of material used. Factors that affect the kinetics of pyrolysis reactions include the heat rate (length of heating and intensity), the prevailing temperature, pressure,

the presence of ambient atmosphere, the existence of catalysts, and the chemical composition of the fuel (e.g., the biomass resource). Pyrolysis reactions occur over a range of temperatures, and products formed earlier in the process tend to undergo further transformations in a series of consecutive reactions. Control of these factors determines the yield and mix of products formed. Figure 1 presents a schematic of pyrolysis reactions. During pyrolysis, two main types of reactions occur—dehydration reactions and fragmentation reactions.

Dehydration reactions occur under conditions of slow heat rates, low temperatures (< 310°C), and long residence times. During these reactions, the molecular weight of the fuel is reduced (in part due to the elimination of water) and char and water vapor are formed. As the heat rate and temperature increase, free radicals and low molecular weight (< 105) volatile compounds such as hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2), are formed. Increasing temperatures reduce char formation and alter the chemical composition of the char. Conversion of non-aromatic hydrocarbons to aromatic hydrocarbons (i.e., carbon compounds that are unsaturated (contain few hydrogen compounds) and that show low reactivity) occurs at temperatures between 300 and 400°C. Dehydration reactions are typical of slow pyrolysis.

Fragmentation reactions occur at > 310°C. During these reactions, the fuel is de-polymerized to form levoglucosan (an anhydrosugar derived from cellulose) and tar. The tars undergo secondary reactions depending

on heat rate, temperature, and pressure which affects the residence time of compounds. Under conditions of medium temperatures (200 to 600°C), high pressure, and long residence times, the volatile compounds and light tars are recombined to form stable secondary tars. Under conditions of  rapid heat rates, high temperature, and low pressure, tars vaporize and produce transient oxygenated fragments which are further cracked to yield olefins (alkenes—organic chemicals characterized by double bonds between carbon atoms), CO, N2, and other hydrocarbons such as acetol, furfural, and unsaturated aldehydes. If high temperatures are maintained for an extended period of time (long residence times), the olefins are converted to permanent hydrocarbon gases (e.g., C2H6, C3H6), condensable aromatic vapors (e.g., benzenoid and non-benzenoid hydrocarbons), and carbon black (mixture of partially burned hydrocarbons). Rapid quenching of intermediate products (i.e., short residence times) is needed to recover the ethylene-rich gases (olefins) used to produce alcohols, gasoline, and bio-oil. Fragmentation reactions are typical of fast pyrolysis.

Ambient atmosphere affects the heat rate and the nature of the secondary reactions and may be a vacuum, an inert surrounding, or a reactive surrounding. In a vacuum, primary products are rapidly removed in the gas phase and are unavailable for further reactions. Water or steam speeds up the breakdown of molecules (hydrothermolysis) and may be catalyzed by acid or alkali reagents. The presence of inorganic salts and acid catalysts can lower the process temperature, increase char formation, and alter char properties.

 The chemical and physical properties of the fuel are key variables in the pyrolysis kinetics and thus significantly affect the yields and product mix. The heat rate is a function of the fuel size and type of pyrolysis equipment. Heat rates are lower for large particle sizes which favors the formation of char and higher for small particles which favors the formation of tars and liquids. Pyrolysis Technology Variant Tech.

Residence Heating Temperatur Products time rate e °C Carbonation days very low 400 charcoal Conventiona 5-30 min low 600 oil, gas, l char Fast 0.5-5s very high 650 bio-oil Flash-liquid