Considerable experimental work has been done with cellulose to clarify the kinetics of biomass pyrolysis. Most kinetic studies on cellulose pyrolysis have

built on the multistep model proposed in the early work with cellulose and described the evolution of volatiles by a single, pseudo-first-order reaction of the type (с/. Broido, 1976; Bradbury, Sakai, and Shafizadeh, 1979; Zaror and Pyle, 1982; Lai, and Krieger-Brockett, 1993; Antal and Varhegyi, 1993)

dm/dt = A exp (— E/RT)(1 — m),

where A is the preexponential factor (time~ *), E is the apparent activation energy (J/mol), R is the ideal gas constant (J/mol-K), T is the absolute tempera­ture (K), and m is the fraction of volatiles produced at time, t. This expression should be quite useful for designing advanced pyrolysis systems, but unfortu­nately, the reported kinetic factors vary widely and several results are in conflict. However, it was found that experimental thermogravimetric data obtained on prolonged, low-temperature, isothermal treatment of pure cellu­lose fit a slightly modified “Broido-Shafizadeh” model (Antal and Varhegyi, 1995). Under conditions of commercial interest, small samples of pure cellulose are reported to undergo thermal decomposition by a single, first-order, high- activation-energy, rate-determining step. The activation energy is estimated to be 238 kj/g-mol. As our knowledge of biomass pyrolysis expands, it is expected that detailed kinetic parameters for more biomass components will be eluci­dated and applied to fine-tune practical biomass pyrolysis systems and designs.

Most of the experimental evidence accumulated over many years of study to understand biomass pyrolysis indicates that there are four basic kinds of processes. All of them involve the formation of large amounts of water vapor. One type is dominant at low heating rates and relatively low pyrolysis tempera­tures below about 250 to 300°C; chars, tars, and dehydration products are the primary products and some volatiles and gases are formed. One type is domi­nant at conventional pyrolysis conditions of intermediate heating rates and temperatures in the 300 to 600°C range; chars, tars, volatiles, and gases are formed in reasonable yields. The third type is dominant at fast heating rates and temperatures in the range 450 to 600°C; volatile liquids are the primary products, but some chars, tars, and gases are formed. The fourth type of pyrolysis occurs at temperatures above 600°C where gasification reactions begin to dominate. Devolatilization of biomass is important in each of these pyrolysis types.