Pyrolysis or thermal decomposition is the molecular breakdown of organic materials such as hydrocarbons via cleavages of chemical bonds at elevated temperatures without the involvement of oxygen or air. Typical chemical bond cleavages during pyrolysis involve the C-C and C-H bonds at most operating temperatures, whereas double bonds of C=C, C=O are substan­tially more difficult to break at most practical conditions. As can be imagined by the chemical bonds that are typically broken during pyrolysis, the pyroly­sis reaction starts at a temperature as low as 150-200°C, where the intrinsic reaction rate is very slow and the extent of reaction is far from completion in any reasonable time. It should be clearly noted that this low temperature is not a typical operating temperature for pyrolysis, considering that the pyro­lytic decomposition reactions are still present at this low temperature, even though not active at all. Most practical pyrolysis of hydrocarbons without using any catalyst is pursued at a temperature higher than 450°C. Pyrolysis involves the concurrent change of chemical compositions and physical phases, and the process is irreversible.

If high molecular weight hydrocarbons are pyrolyzed in an oxygen — deprived environment at a temperature of 450-650°C, lighter hydrocarbons (reduced carbon numbers and lower molecular weights), hydrogen, and solid char would be typically formed. Lighter hydrocarbons nearly always involve methane (CH4), ethylene (C2H4), ethane (C2H,), and other fragmented hydro­carbons, of which methane is most dominant. Depending upon the pyroly­sis conditions, liquid range hydrocarbons, C4 — C15, are also obtained. Char formation is believed to be via a route similar to the formation of polycy­clic aromatic hydrocarbons (PAHs) which involves polymerization of highly reactive free radicals of fragmented hydrocarbons and unsaturated hydro­carbon species. Hydrogen formation during pyrolysis is via cleavage reac­tions of C-H chemical bonds of the original and intermediate hydrocarbon molecules and some of the hydrogen molecules formed in such a manner are recombined with methyl radicals and ethylene thus producing methane and ethane. As illustrated, pyrolysis of hydrocarbons can yield materials of all three different phases (i. e., solid, liquid, and gas) as its end products depend­ing upon the treatment conditions. Furthermore, the actual number of chem­ical reactions involved and the number of final and intermediate chemical species are countless. Therefore, pyrolysis collectively represents a class of chemical reactions taking place as thermochemical decomposition. A more generalized chemical reaction equation for hydrocarbon pyrolysis may be written as follows:

CHb ^ c ■ CH4 + CdHe + CfHg + h ■ H2
a = c + d + f
b = 4c + e + g + 2h

CuHvOw ^ CkH°wl + CmHn°w2 + p ‘ H2O + q ■ H2 + r ‘ CO2 + s ‘ CO
u = k + m + r + s
v = l + n + 2p + 2q
w = wl + w2 + p + 2r + s

In the first reaction expression, methane is explicitly written in the product side, because methane is always a dominant hydrocarbon product species of hydrocarbon pyrolysis.

Biomass chemical compounds are far more oxygenated than straight hydrocarbons and biomass also contains a high level of moisture. Therefore, thermal decomposition or pyrolysis of biomass also generates carbon oxides in addition to the aforementioned pyrolysis products of condensable hydro­carbons, methane, and hydrogen. If biomass is microbially degraded in anaerobic conditions, it generates a product gas rich in methane and carbon dioxide. This product gas is called biogas, or landfill gas. A process system developed for exploiting this biogas is called an anaerobic digester, which can produce methane rich gas from waste materials on a small-scale unit.

The scientific definition of pyrolysis presented in this section precludes oxygen involvement in its mechanistic reaction steps. This was necessary in defining pyrolysis as a thermochemical reaction by itself. However, it should be clearly noted that the actual pyrolysis reaction can also occur as a component reaction of many reactions simultaneously taking place in many different chemical process environments, including both oxidative as well as reducing environments. In such environments, pyrolytic decomposition reactions compete with other chemical reactions also occurring in the sys­tem and as such the reaction environment becomes that much more complex in terms of both the nature and the total number of simultaneous reactions taking place in the system.

Of course, it is also true that pyrolysis alone in the absence of oxygen can be targeted in certain process environments, as is the case with fast pyrolysis of biomass. Typical fast pyrolysis processes are operated at a temperature that is substantially lower than typical gasification temperatures of steam gasification, Boudouard reaction, hydrogasification, and partial oxidation. Hence, fast pyrolysis as a transformation process is more or less strictly a combination of devolatilization and pyrolytic decomposition reactions in an oxygen-deprived environment. Furthermore, it must be clearly understood that the principal targeted product of fast pyrolysis of biomass is liquid- phase bio-oil, not gaseous synthesis gas.

Biomass can be gasified without any gasifying agent additionally intro­duced into the reactor and this type of gasification is called pyrogasification.

Pyrolytic gasification or pyrogasification of biomass takes advantage of both pyrolysis and gasification and it can be carried out both catalytically [31, 32] and noncatalytically. In pyrogasification, no separate gasifying medium or oxygen (or air) is introduced, it is expected that a gasifying reactant such as steam has to be in situ provided from biomass pyrolysis. In pyrogasification, biomass pyrolysis also produces biochar and this biochar reacts with steam via steam gasification to generate product gas.

Pyrolytic gasification of wood using a stoichiometric nickel aluminate catalyst was carried out by Arauzo and coworkers [32] in a fluidized bed reactor and near-equilibrium yields of products were obtained above 650°C. Although they obtained 85-90% gas yields, tar production was not detected. They further tested the process using a modified nickel-magne­sium aluminate stoichiometric catalyst and also an addition of potassium as a promoter. They found that magnesium addition to the catalyst crystal­line lattice enhanced attrition resistance of the catalyst with a minor loss of gasification activity and an increased production of coke. However, they found little effect from an addition of potassium component. Catalyst foul­ing by carbon deposition, that is, surface coverage by coke, was shown and regeneration of magnesium-containing catalyst by carbon burn-off was demonstrated.

Asadullah et al. [31] comparatively evaluated the catalytic performances of Rh/CeO2/SiO2, steam reforming catalyst G-91, and dolomite for a number of different biomass gasification modes, including pyrolytic, CO2, O2, and steam. With respect to the biomass conversion to product gas and selectivity of useful gaseous species, Rh/CeO2/SiO2 has shown superior results in all gasification modes. In the pyrogasification case, about 79% of the carbon in biomass was converted to the product gas at 650°C. There was no tar detected in the effluent gas stream. The gasifier used for their experiment was a lab — scale continuously fed fluidized bed reactor.