Biomass pyrolysis refers to thermal decomposition in the presence of little or no oxygen, while biomass gasification involves pyrolysis and partial oxidation in a well-controlled oxidizing envi­ronment. Depending on the process variables, such as the reactor temperature and residence time, the biomass pyrolysis yields various amounts of gaseous, liquid, and solid products of varying compositions. For instance, conventional pyrolysis, which has been utilized for thousands of years, involves lower temperatures and longer residence times with the principal product being the solid char. In contrast, fast pyrolysis involves moderately high temperatures (~500°C) and short residence times (~2 s), with the main product being a dark brown liquid or bio-oil along with other gaseous, liquid and solid products, including char. This process is much more com­monly used at present compared to conventional pyrolysis. While most agricultural and forestry residues can be used in fast pyrolysis, most work has focused on wood-based feedstock, including hemicellulose, cellulose and lignin. The pyrolysis process generally requires about 15% of the energy available in the feed, which can be provided by the combustion of char or a combination of char gasification and combustion of resulting producer gas. Note that char and gas are the two main by-products of pyrolysis, which typically contain about 25 and 5% of the energy in the

feed, respectively. Other means of supplying the required energy may include the combustion of bio-oil, fresh biomass, or fossil fuel, depending upon the reactor design and regional conditions.

Bridgwater (2011) and Mohan et al. (2006) provide reviews on fast pyrolysis and the properties of bio-oils generated from this process. The effects of various process parameters on the overall reaction rate, volatile yields and products formed are extensively discussed in these reviews. Such parameters include the biomass composition and structure, reactor temperature, heating rate, and residence time. Various gaseous and liquid fuels produced from bio-oils are also discussed. As stated earlier, fast pyrolysis in general involves high heating rates with a reaction temperature of around 500°C, rapid cooling of the pyrolysis vapors to yield bio-oil, which is the main product, and a rapid removal of product char to minimize cracking of vapors. It is characterized by the strongly coupled processes of heat and mass transport, phase change, and chemical kinetics. As discussed by Bridgwater (2011), a critical factor is to bring the reacting biomass particles to an optimum temperature and minimize their exposure to lower temperatures that favor the formation of charcoal. While there have been studies on the kinetic and thermal decomposition mechanisms for the pyrolysis of plant biomass, various processes associated with fast pyrolysis are generally not well understood.

The major product of pyrolysis is a dark brown liquid or bio-oil, which has approximately the same elemental composition as the original biomass. It consists of a complex mixture of oxygenated hydrocarbons with a varying but appreciable amount of water from both the original moisture and reaction product. Note that the presence of water makes bio-oils immiscible with petroleum-derived fuels. The physical properties of bio-oils are discussed in Czernik (2004). Proximate analysis of the bio-oil gives a chemical formula of CH19O07. The typical heating value of bio-oils is about 17MJ/kg, which is about 40-45% of that of hydrocarbon fuels. Figure 2.3a from Bridgwater (2011) shows typical organics yields from different feedstocks and their variation with temperature, while Figure 2.3b shows the temperature dependence of the four main products, namely organics, char, gas, and water, from a typical feedstock.

In addition, Bridgwater (2012) lists the physical properties of a representative wood-derived bio-oil. The pyrolysis chemistry of different biomass feed stocks is discussed inBridgwater (2012). Bio-oils can be utilized in several different ways to produce energy, fuels, and chemicals. They have been used directly as fuels in stationary applications, especially for electricity generation. A more sustainable approach is to produce conventional fuels for transportation and power generation using either an integrated facility or a decentralized operation. Such fuels include diesel, gasoline, kerosene, methane, liquefied petroleum gas, and others. An integrated facility involves a refinery­like operation with biomass pyrolysis followed by preprocessing, deoxygenation, and refining of

bio-oils. A schematic of such a facility is depicted in Figure 2.4 from Mohan (2006). As discussed in this reference, the decentralized operation has received much interest in recent years. In such an operation, bio-oils or bio-oil-char slurries produced from biomass pyrolysis are transported to a central processing plant for gasification and synthesis of hydrocarbon transport fuels, such as Fischer-Tropsch (FT) fuels and alcohols. While there is some energy penalty associated with transportation and additional bio-oil gasification, it may be compensated by the economy of scale that can be achieved in a gasification and fuel synthesis plant on a commercial scale.

The modeling of pyrolysis processes is extremely complex (Niksa, 2000) due to the wide variation in biomass composition and the amount and number of products formed. Most previous work has focused on developing empirical or global kinetic models for predicting the rate of production of various species, including char, bio-oil, liquids, and other liquid and gas species, formed during pyrolysis. Varhegyi et al. (2011) performed thermo-gravimetric experiments to examine the pyrolysis of different feedstocks, and reported a distributed activation energy model using three pools of reactants. Brown et al. (2001a, b) studied experimentally and numerically the chemistry of biomass and cellulose pyrolysis in a laminar entrained-flow reactor using a molecular — beam mass spectrometer. Computational fluid dynamics (CFD) simulations were performed to model the transport and chemical processes in the reactor. It was observed that the primary cellulose pyrolysis products underwent subsequent secondary reactions. A rate law was developed to describe the thermal conversion of these products. While such studies have provided valuable information on the overall pyrolysis kinetics, there is scope for more fundamental research using surrogate mixtures to examine the transport and thermochemical processes associated with the biomass pyrolysis and subsequent conversion of bio-oil to fuels. Significant research is also needed on catalytic processes for the production of various gaseous and liquid fuels.

In summary, the potential of using biomass pyrolysis and subsequent refining of bio-oils to produce second-generation biofuels is increasingly being recognized. Similar to a petroleum refinery, a biorefinery concept may provide a sustainable and value-added approach for the use of biomass to produce energy, fuels and chemicals. This concept is particularly attractive for biomass because of its chemical heterogeneity and regional variability. However, the chemical composi­tion of biomass, approximately (CH2O)„, is quite different from that of petroleum, (CH2)„, and, therefore, the range of primary chemicals derived from biomass and petroleum will be different. In (Bridgwater, 2011) a schematic of an integrated pyrolysis-based biorefinery concept is shown. It indicates that bio-oils produced from pyrolysis can be processed to provide various gaseous and liquid fuels. These fuels are mostly compatible with conventional fuels, but are cleaner.

Consequently, they can be deployed without significant changes to existing infrastructure. More­over, as discussed in the next section, the biomass gasification can be used to make syngas, a mixture of H2 and CO, for subsequent synthesis of hydrocarbons, alcohols and other chemicals. However, this route may be quite energy intensive, and its cost effectiveness and environmental benefits need to be examined. It may be more economical to use syngas directly for electricity generation.