Thermochemical treatment of biomass can convert biomass into solid, liquid, and gaseous fuel products whose compositional distribution is governed by the imposed process treatment conditions. The solid product is usually char (or biochar), the liquid product is bio-oil, and the gaseous product is biosyn­gas. The process also involves formation of the unwanted product of tar. The thermochemical conversion process involves heating of the biomass feed­stock, which triggers a series of parallel and consecutive reactions including

devolatilization of volatile matter, pyrolytic decomposition of hydrocarbons and other carbonaceous matters, gas-solid type gasification reactions, coke and char formation, tar and its precursor formation, and more. Simply speak­ing, depending upon the processing temperature and reactor residence time, thermochemical treatment of biomass can be regrouped into three basic types of process treatment, namely carbonization, fast pyrolysis, and gasifi­cation. As shown in Table 5.5, the principal product, or intended product, of fast pyrolysis is a liquid fuel, whereas the desired product of gasification is a gaseous fuel [23].

Even though it is not listed in Table 5.5, indirect liquefaction via the biosyn­gas route is also a viable option for liquid fuel production, as well demon­strated in the fields of coal and natural gas syngas [5, 14, 24, 25]. As the name implies, indirect liquefaction goes through two stages of process treatment, viz., gasification followed by liquefaction by which liquid hydrocarbon fuels such as methanol, dimethylether (DME), higher alcohols, gasoline, diesel, and jet fuel are synthesized using the syngas produced by the gasification stage. In this case, biomass syngas is a thermochemical intermediate for the next stage synthesis of liquid fuel.

5.1.4 Analysis of Biomass Feedstock and Product Compositions

Fast pyrolysis of biomass generates a wide variety of organic and inorganic chemical compounds and the product compositions vary significantly depending upon the types of feedstock as well as the process treatment conditions to which the biomass is subjected. Therefore, studies of pro­cess modeling and technoeconomic analysis are often carried out using model compounds carefully chosen for the specific process and typical feedstock [26]. The analysis of corn stover samples used by Mullen et al. [27] for their fast pyrolysis study is presented in Table 5.6, as an example of the compositional analysis of biomass feedstock. They carried out the fast pyrolysis in a bubbling fluidized bed of quartz sand at a temperature of 500°C.

Inorganic elemental composition of corn stover used for the aforemen­tioned pyrolysis determined by x-ray fluorescence (XRF) is given in Table 5.7. Also compared in the same table are the XRF analysis data for corn cobs which were also tested for fast pyrolysis by Mullen et al. (2010) [27]. As can be seen, the elemental composition between corn cob and corn stover are quite different. The most abundant element in corn cob was K, whereas Si was the most abundant species in corn stover. High levels of K and P in both samples are expected, although the high levels of Ca, Mg, Al, Fe, and Mn in corn stover are noteworthy. Mineral matters in the biomass feedstock can reappear as contaminants or trace elements in bio-oils and biosyngas, which can potentially affect the catalytic activity of the downstream processing by fouling or poisoning.

The yield data of the USDA corn stover fast pyrolysis by Mullen et al. [27] is shown in Table 5.8. The pyrolysis product distribution in terms of the product phases was bio-oil 61.7%, biochar 17.0%, and noncondensable gas (NCG) 21.9%. As explained earlier and also summarized in Table 5.5, the principal product of fast pyrolysis of biomass, that is, corn stover in this example, is bio-oil.

From the product compositions, the following observations are deemed significant:

1. The gaseous effluent of fast pyrolysis has a heating value of only 6.0 MJ/kg. The gas composition is dominated by carbon oxides (CO and CO2), followed by methane and hydrogen.

2. High levels of oxygen in the effluent gas show that the gaseous efflu­ent served at least as an outlet for deoxygenation of biomass.

3. Bio-oil also showed a very high level of oxygen and its heating value was seriously affected. In order to enhance the fuel quality of bio-oil as well as to enhance the fast pyrolysis process, a systematic rejec­tion of oxygen from the products’ molecular structures (i. e., deoxy­genation) would become crucially important.

4. Biochar showed a heating value nearly as high as that of bio-oil, even though it contained a high level of ash.

5. Biochar showed a high C/H ratio, which is indicative of its lack of volatile hydrocarbons. Thus, biochar is a useful by-product of the fast pyrolysis process of biomass.