Pyrolysis Reactions

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Pyrolysis Reactions

The sequence and rate at whichpyrolysisreactions occur and the factors that influence the rateare described by the kinetics of the reaction. The kinetics of fastpyrolysisreactions are

characterized by Equation 1,

(Equation 1)

where Wt is the particle weight after reaction time (in grams), t is thepyrolysistime (in

seconds), Ko 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 Jouleper 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 ofpyrolysisreactions 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., thebiomassresource).Pyrolysisreactions 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 ofpyrolysisreactions. Duringpyrolysis, two main types of

reactions occurdehydration reactions and fragmentation reactions.

Dehydration reactions occur under conditions of slow heat rates, low temperatures (< 310C),

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 compoundssuch 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) occursat temperatures between 300 and 400

C. Dehydration reactions are typical of slowpyrolysis.

Fragmentation reactions occur at > 310C. During these reactions, the fuel is de-polymerized

to form levoglucosan (an anhydrosugar derived from cellulose) and tar. The tars undergosecondary reactions depending on heat rate, temperature, and pressure which affects the

residence time of compounds. Under conditions of medium temperatures (200 to 600C), high

pressure, and long residence times, the volatile compounds and light tars are recombined toform 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 (alkenesorganic chemicals characterized by double bonds between carbonatoms), CO, N2, and other hydrocarbons such as acetol, furfural, and unsaturated aldehydes. If
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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 (mixtureof 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 fastpyrolysis.

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 orsteam 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 thepyrolysiskinetics and

thus significantly affect the yields and product mix. The heat rate is a function of the fuel size

and type ofpyrolysisequipment. Heat rates are lower for large particle sizes which favors theformation of char and higher for small particles which favors the formation of tars and liquids.

Pyrolysis of Biomass Resources

Allbiomass resourcesare composed primarily of cellulose (typically 30 to 40 percent of dryweight), 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 amongbiomass resources.

This heterogeneity creates variability in the yields ofpyrolysisproducts.

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) isnegligible below 500C but increases substantially at high temperatures (Scott et al., 1988). At

temperatures above 600C, tar yields drop, gas yields increase, and thepyrolysisof cellulose is

complete (Hajaligol, 1982; Bradbury, 1979; Funazukuri, 1986; and Piskorz, 1986).

Hemicellulose is the most reactive component ofbiomassand decomposes between 200 and

260oC (Koufopanos, 1989). The decomposition of hemicellulose is postulated to occur in

two stepsthe 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 moremethanoland acetic acid than cellulose.

Lignin is a highly linked, amorphous, high molecular weight phenolic compound whichserves as cement between plant cells and is the least reactive component ofbiomass. The

time required to pyrolyzebiomass resourcesis controlled by the rate ofpyrolysisof ligninunder operating conditions. Decomposition of lignin occurs between 280C and 500C,

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, ligninloses only about half of its weight at temperatures below 800C (Wenzel, 1970).Pyrolysisof
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lignin yields more char and tar than cellulose (Soltes and Elder, 1981).

For wood, the decomposition of the major components occurs separately and sequentiallywith the hemicellulose decomposing first and the lignin last. Up to 200C, 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 280C, furtherdecomposition of the char and wood occur resulting in the release of pyroligneous acids, waterand non-condensable gases. Separation of tar occurs. Between 280 and 500C, 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 500C, 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 thepyrolysisofbiomass resourcescondense 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 polarand 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) whenbiomassdegrades and is corrosiveto 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 thecombustiontemperature which reduces NOx emissions. Some additional water can beadded, 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

pyrolysisof wood.

A number of studies have examined factors that affect the kinetics ofbiomasspyrolysisreactions. Studies that have examined temperature and heat rate interactions include Scott,

1988 (maple wood); Aarsen, 1985 (wood); Ayllon, 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 550C and 680C and

then decline. As temperatures increase, char production decreases (to a steady level above650C) and the carbon content of the char increases. Hydrocarbon gas yields (e.g., C2H6,

C3H6) increase up to about 660C and then decline, probably due to thermal cracking. Thetime required to obtain a given conversion level decreases with increasing temperature.

Biomassweight loss is higher at lower pressures (Ward and Braslaw, 1985). At any given

temperature, charresiduesincrease pressure. Cellulose displays the strongest pressure
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dependency and lignin the lowest--the pressure effect is observable at temperatures above

350C. 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 thebiomassresource) affects the mix ofpyrolysisproducts. The impacts are measured usingthermogavimetry (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 thedecomposition 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 cellulosepyrolysisbut negligible for ligninpyrolysis(Nassarand MacKay, 1986).

Sumber: http://bioweb.sungrant.org/Technical/Biopower/Technologies/Pyrolysis/Default.htm
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Pyrolysis Reactions