Many dehydration, cracking, isomerization, dehydrogenation, aromatization, coking, and condensation reactions and rearrangements occur during pyrolysis. The products are water, carbon oxides, other gases, charcoal, organic com­pounds (which have lower average molecular weights than their immediate precursors), tars, and polymers. When cellulose is slowly heated at about 250 to 270°C, a large quantity of gas is produced consisting chiefly of carbon dioxide and carbon monoxide. Table 8.1 illustrates how the carbon oxides, hydrocarbons, and hydrogen in the product gas vary with increasing tempera­ture as the slow, dry distillation of wood progresses (Nikitin et aL, 1962). Initially, small amounts of hydrogen and hydrocarbon gases and larger amounts of carbon oxides are emitted. The hydrocarbons in the product gas then increase with further temperature increases until hydrogen is the main product. The carbon oxides and most other products owe their formation to secondary and further reactions.

Pyrolysis of cellulose yields the best-known of the 1,6-anhydrohexoses, /J-glucosan or levoglucosan (1,6-anhydro-jS-D-glucopyranose), in reasonably good yields (Shafizadeh, 1982) (Fig. 8.1). A novel technique based on flash devolatilization of biomass and direct molecular-beam, mass-spectrometric analysis has shown that levoglucosan is a primary product of the pyrolysis of pure cellulose (Evans and Milne, 1987a, 1987b, 1988). However, the yield of levoglucosan on pyrolysis of most biomass is low even though the cellulose

TABLE 8.1 Composition of Gases from the Slow, Dry Distillation of Wood”

Process (°С)

Elimination of water 155-200

Evolution of carbon oxides 200-280

Start of hydrocarbon evolution 280-380

Evolution of hydrocarbons 380-500

Dissociation 500-700

Evolution of hydrogen 700-900 “Nikitin et al. (1962). “HCs” are hydrocarbons.

content is about 50 wt %. Also, when pure cellulose is treated with only a small amount of alkali, levoglucosan formation is inhibited and a different product slate composed of furan derivatives is produced.

Levoglucosan is also obtained directly on pyrolysis of glucose and starch. The compound has the same empirical formula as the monomeric building block of cellulosic polymers, (C6HI0O5). Some investigators suggest that these observations support a mechanism wherein the initial pyrolysis reaction yields glucose as an intermediate. This is equivalent to the sequential hydrolysis of cellulose by addition of water to form glucose, and elimination of water by dehydration of glucose to form the anhydride. It seems more probable that if levoglucosan is the initial intermediate, a thermally induced, depolymerization — internal displacement reaction occurs to form the pyranose directly by a con­certed mechanism.

In early work on the mechanisms and kinetics of biomass pyrolysis, measure­ment of the weight change as a function of time over a 1000-h period during the pyrolysis of pure cellulose at temperatures up to 260°C in a vacuum led to a multistep mechanism consistent with the experimental data (Broido, 1976). A two-path mechanism was proposed in which one involved depolymerization and led to completely volatile products, and the other involved a sequence of steps leading to char formation. Most investigators now generally recognize at least two pathways for cellulose pyrolysis (Fig. 8.2). One involves dehydra­tion and charring reactions via anhydrocellulose intermediates to form chars, tars, carbon oxides, and water, and one involves depolymerization and volatil­ization via levoglucosan intermediate to form chars and combustible volatiles (с/. Zaror and Pyle, 1982; Antal and Varhegyi, 1995). The first pathway would be expected to occur at lower temperatures where dehydration reactions are dominant. The second pathway results in the formation of oligomeric species as well as their degradation products, which immediately enter the vapor phase (Antal et ai, 1996). If permitted to quickly escape the reactor, the vapors form condensed oils and tars. If held in contact with the solid biomass undergoing devolatilization within the reactor, the vapors degrade further to form chars, various gases, and water. The two competitive pathways help to explain the effects of pyrolysis conditions on product yields and distributions. Note that although these pathways may be dominant, there are undoubtedly many other pathways that are operative with actual biomass species. Thermal treatment converts hemicelluloses to furanoses and furans, the lignins to mononuclear and condensed aromatic and phenolic compounds, and the proteins to a wide range of nitrogen-containing aliphatic and heterocyclic compounds.