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), 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.
Source: Graham, R.; Bergougnou, M.; Mok, L.; and Lasa, H. DE. (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.
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 affect 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 speed 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 favor the formation of char - and higher for small particles, which favor the formation of tars and liquids.