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bioweb.sungrant.org » Technical » Biopower » Technologies » Pyrolysis

Pyrolysis
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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 (400-600oC), rapid heating rates (> 2°C per second), and short gas residence times (< 2 seconds). Fast pyrolysis (thermolysis) involves rapid heating rates (200 to 10C 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 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), Ko is the frequency factor (in seconds), Wis 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 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 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 can not 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). 

 
Biomass Pyrolysis Reactors

Substantial differences in the temperature of the biomass resource and the reactor temperature may affect the heat rate. A number of different kinds of pyrolysis reactors are available. Pyrolysis is a precursor to gasification and combustion, and the same reactors used for gasification (i.e., fixed bed and fluidized bed reactors) can be used for pyrolysis.  Bubbling fluidized bed reactors are simpler to design and construct than other reactor designs, and have good gas to solids contact, good heat transfer, good temperature control, and a large heat storage capacity. High liquid yields (60 to 75 percent weight of wood on a dry basis) can be typically achieved. Small fuel particle sizes are needed (< 2 -3 mm) to ensure high heat rates. The rate of particle heating is the rate limiting factor.  Figure 2 shows a fluidized bed pyrolysis reactor developed by Daugaard (2003).

 

 

  


In the BioTherm reactor (Figure 3), fluidized sand in a zero-oxygen environment quickly heats the fuel to 450oC where the fuel is decomposed into solid char, gas, vapors and aerosols. After exiting the reactor zone, these products pass through two sequential cyclones where most of the solid char particles are removed and collected. The scrubbed gases, vapors and aerosols enter a direct quenching system where they are rapidly cooled (< 50oC) with a liquid immiscible in bio-oil. The bio-oil is condensed and collected and the quench liquid is recovered (in a heat exchanger) and recycled. Non-condensable gas and residual bio-oil aerosol droplets enter a precipitator that electrostatically removes particulates and aerosols. The clean, inert gas is then recycled back to the bubbling fluidized bed reactor. The excess non-condensable gas (a medium Btu gas) is combusted to provide heat to the reactor sand.

 

  

 

Circulating fluidized bed pyrolysis reactors are similar to bubbling fluidized bed reactors but have shorter residence times for chars and vapors which results in higher gas velocities, faster vapor and char escape, and higher char content in the bio-oil. They have higher processing capacity, better gas-solid contact, and improved ability to handle solids that are difficult to fluidize than bubbling fluidized bed reactors, but are less commonly used. The heat supply typically comes from a secondary char combustor.

 

Ablative pyrolysis reactors function on the premise that, while under pressure, heat transferred from a hot reactor wall will soften feedstock in contact with it allowing the pyrolysis reaction to move through the biomass in one direction. The feedstock is mechanically pushed through the reactor.  High rates of pressure significantly affect the rate of the reaction and the velocity of the feedstock on the heat exchange surface. Rather than limited by the rate of heat transfer through the biomass particle, the reaction rate is limited by the rate of heat supply to the reactor and thus larger particles can be pyrolyzed. Inert gases are not required resulting in smaller processing equipment and more intense reactions. However, the process is dependent on surface area so scaling to larger facilities is costly and the use of mechanical drivers is more complex.

 

In a rotating cone pyrolysis reactor, room temperature biomass particles and hot sand are introduced near the bottom of the cone, mixed, and transported upwards by the rotation of the cone. Pressures are slightly above atmospheric levels. Rapid heating and short gas phase residence times can be achieved. Figure 4 presents a schematic of a rotating cone pyrolysis reactor.

 

    

 
References

Aarsen, F., C. Beenackers and Van Swaaij. 1985. Wood pyrolysis and carbon dioxide char gasification kinetics in a fluidized bed. In: Fundamentals of Thermochemical Biomass Conversion. R.P. Overend, T.A. Milne and L.K. Mudge (eds). Elsevier Applied Science Publishers, London. pp. 691-715.

Ayll´on, M., M. Aznar, J. S´anchez, G. Gea and J. Arauzo. 2006. Influence of temperature and heating rate on the fixed bed pyrolysis of meat and bone meal. Chemical Engineering Journal 121: 85–96.

BioTherm. 1999. BiothermTM A system for continuous quality, fast pyrolysis bio oil. Fourth Biomass Conference of the Americas, Oakland, California. September. 1999.

Blackadder, W. and E. Rensfelt. 1985. A pressurized thermo-balance fro pyrolysis and gasification studies of biomass, wood and peat. In: Fundamentals of Thermochemical Biomass Conversion. R.P. Overend, T.A. Milne and L.K. Mudge (eds). Elsevier Applied Science Publishers, London. pp. 747-759.

Bradbury, A., Y. Sakai and F. Shafizadeh. 1979. A kinetic model for pyrolysis of cellulose. J. Appl. Poly. Sciene. Vol. 23: 3271 – 3280.

Bridgwater, A.V.; Czernik, S.; Piskorz, J. (2002). The Status of Biomass Fast Pyrolysis. In: Fast Pyrolysis of Biomass: A Handbook Volume 2. Edited by Bridgwater. Cpl Press. Online Bookshop.

Daugaard, D. 2003. The Transport Phase of Pyrolytic Oil Exiting a Fast Fluidized Bed Reactor. Unpublished Ph.D. Dissertation. Iowa State University.

Funazukuri, T., R. Hudgins and P. Silveston. 1986. Product distribution in pyrolysis of cellulose in microfluidized bed. J. Anal. Pyrolysis. Vol. 9: 139-158.

Graham, R., M. Bergougnou, L. Mok, and H. DE Lasa. 1985. Fast Pyrolysis (Utrapyrolysis) of biomass using solid heat carriers. In Fundamentals of Thermochemical Biomass Conversion. R. Overend., T. Milne and L. Mudge. Elsevier Applied Science Publications, London and New York.

Hajaligol, M., J. Howard, J. Longwell and A. Peters. 1982. Product composition and kinetics of rapid pyrolysis of cellulose. Ind. Eng. Chem. Process Des. Dev. Vol. 21: 457-465.

Koufopanos, C. A., G. Maschio and A. Lucchesi. 1989. Kinetic modeling of the pyrolysis of biomass and biomass components. Canadian Journal of Chemical Engineering. Vol. 67. pp. 75-84.

Nassar, M. and G. MacKay. 1984. Studies on the mechanism of flame retarding. Thermochimica. Vol. 81: 9 – 14.

Nassar, M., A. Bilgesu, G. MacKeay. 1986. Effects of inorganic salts on product composition during pyrolysis of black spruce. Wood Fiber Science, Vol. 18 (1): 3 – 10.

Nunn, T. R., J. B. Howard, J.P. Longwell and W. A. Peters. 1985. Studies of the rapid pyrolysis of sweet gum hardwood. In Fundamentals of Thermochemical Biomass Conversion. R. Overend., T. Milne and L. Mudge. Elsevier Applied Science Publications, London and New York.

Piskorz, J., D. Radlein and D. Scott. 1986. On the mechanism of rapid pyrolysis of cellulose. J. Analy. Appl. Pyrolysis. Vol. 9: 121-137.

Prins, W. and BM. Wagenaar. 1997. In Biomass Gasification and Pyrolysis. Eds. Kalschmitt, MK and AV. Bridgwater, pp 316-326. (CPL Press)

Probestin, R. and R. Hicks. 1982. Syntthetic fuels. Chapter 8. McGraw-Hill, New York.

Roberts, A. 1970. A review kinetics data for the pyrolysis of wood and related substances. Combustion and flame. Vol 14: 261-272.

Sadakata, M., K. Takahashi, M. Saito and S. Takeshi. 1987. Production of gas fuel and char from wood, lignin and holocellulose by carbonization. Fuel, 66: 1667-1671.

Scott, D.S, J. Piscorz, M.A. Bergougnou, R. Graham and R.P. Overend. 1988. The Role of Temperature in Fast Pyrolysis of Cellulose and Wood. Industrial Engineering Chemical Research., 27: 8-11.

Soltes, E. and T. Elder. 1981. Pyrolysis. In Organic Chemicals from Biomass. Goldstein IS. (ed.) CRC Press, Florida. 63-100.

Tsuchiya, Y. and K. Sumi. 1970. Thermal decomposition products of cellulose. Journal of Applied Poly. Sci. Vol. 14: 2003-2013.

Utioh, A.C, N.N. Bakshi and D.G MacDonald. 1989. Pyrolysis of grain screening in a batch reactor. Canadian Journal of Chemical Engineering, 67: 439-442.

Ward, S. and J. Braslaw. 1985. Experimental weight loss kinetics of wood pyrolysis under vacuum. Combustion and Flame. Vol. 61: 261-269.

Wenzl, H. 1970. The Chemical Technology of Wood. Academic Press, New York.


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      Author:  Samy Sadaka (edited by Marie Walsh)   Reviewed: 4/2007
Last Modified: 4/25/2011
Link to Author's Manuscript
  
Copyright © 2007 Sun Grant Initiative and the University of Tennesee.  Full disclaimer and guide to usage available here.