Pyrolysis converts organic materials by heating in the absence of oxygen to produce a number of products; bio-oil, char, and noncondensable gas (Bridgwater, 2012a,b). There are a number of types of pyrolysis, with the main two being slow pyrolysis and fast pyrolysis. Details on each pyrolysis residence time, heating rate, operating temperature range, and product spectrum are given in Table 14.1 (Bridgwater and Bridge, 1991). Bio-oil refers to the volatile components from fast pyrolysis after condensation and much current research aims to upgrade this bio-oil into a more acceptable heat and power resource and also biofuels and chemicals. Upgrading is mostly based on catalytic processing supplemented by feed modification and process improvement (Banks et al., 2014; Jenkins et al., 1996; Raveendran et al., 1995; Tan and Wang, 2009; Harmsen et al., 2010).

The homogeneity and much higher yield of liquid from fast pyrolysis mean that this is the preferred technique for liquid products (bio-oil). There are a number of essential features of a fast pyrolysis process:

There are no standard decomposition processes for biomass components: cellulose, hemicellulose, and lignin. The varying proportions of cellulose, hemicellulose, and lignin in biomass influence the fast pyrolysis product distribution. The thermal decomposition of cellulose has been studied extensively as it is the major component of wood and has a less complex structure compared to hemicellulose and lignin. Fast pyrolysis conditions (biomass particle size, pyrolysis temperature, and hot vapor residence time) have an effect on fast pyrolysis products, as well as biomass component composition, poor thermal conductivity of biomass, high reactivity of volatiles, and the catalytic effect of char and alkali metals contained in ash (Nowakowski and Jones, 2008; Nowakowski et al., 2007; Nik-Azar et al., 1997; Patwardhan et al., 2010). Fig. 14.1 shows the general distribution of fast pyrolysis products from specific biomass components. Cellulose and hemicellulose primary decomposition components are condensable vapors (bio-oil) and gas. Lignin's primary decomposition components are bio-oil, solid (char), and noncondensable gases. Also found in biomass are extractives which contribute to bio-oil and noncondensable gas yields and mineral content (such as alkali metals) are entrained in char.

Catalytic upgrading offers the possibility of upgrading bio-oil to a product with more favorable properties (Bridgwater, 2012a,b). Fast pyrolysis produces bio-oil by rapidly heating biomass up to a controlled temperature of between 400 and 600°C (Scott et al., 1988). Bridgwater and Peacocke (2000) reported that fast pyrolysis produces maximum yields at processing temperatures around 500°C. The feed material is usually specified as less than 10 wt.% water content (Jahirul et al., 2012) as all feed water ends up in the hydrophilic bio-oil as well as the water of reactions (Demirbas, 2004). The essential features of fast pyrolysis for producing liquids are very high heating (Fred and Peter, 1977) and heat transfer rates (Bridgwater and Bridge, 1991) which require a feed of an appropriate particle size which is usually less than 3 mm (Papadikis et al., 2010; Wang et al., 2005; Di Blasi, 2002). The pyrolysis temperature should be carefully controlled as reported above. The vapor phase temperature has to be carefully monitored and controlled to at least 425°C to minimize tar condensation leading to blockages and not above 460°C to minimize thermal cracking reactions. Fast pyrolysis has a short hot vapor residence time typically less than 1 s (Bridgwater et al., 1999). The residence time has to be kept as short as possible to prevent secondary reactions taking place which will convert the condensable fast pyrolysis vapors into permanent gases, water vapor, and char (Liden et al., 1988; Scott et al., 1999). Typical liquid yields are around 75% from clean wood (Bridgwater et al., 1999).

The conversion of lignocellulosic materials and waste materials into bio-oil using pyrolysis technology is one of the most promising technologies to convert solid feedstocks into liquid products. However, substituting bio-oil for conventional liquid fossil fuels is problematic due to high viscosity, high oxygen content, and thermal instability. Therefore catalysts are being utilized in the pyrolysis reaction system to upgrade pyrolysis vapor to obtain a bio-oil product with decreased oxygen and polymerization precursor content to improve its heating value and thermal stability. Catalytic pyrolysis has many advantages over other conversion processes (Carlson et al., 2011). These are:

The liquid produced from pyrolysis contains a variety of different chemical species which can amount to over 300 compounds with oxygen content usually around 35–40% (Czernik and Bridgwater, 2004). Bio-oil properties are greatly affected by its composition which is made up of numerous functional groups (acids, esters, aldehydes, amines, ethers, alcohols, furans, aliphatic hydrocarbons, and aromatic hydrocarbons). The high number of functional groups decreases the stability of the oil, which leads to an increase in viscosity over time which raises concerns on its feasibility as a substitute for fossil fuel. High water and oxygen content in bio-oil reduces the heating value. Due to these properties the bio-oil has to be upgraded to make it a viable option for fossil fuel replacement.

Bio-oil contains a significant amount of components with OH groups that require dehydration. Dehydration is the removal of oxygen in the form of water. A wide range of studies have been carried out with oxygenates ranging from low-molecular-weight alcohols (eg, methanol and ethanol) to phenolic compounds. Even though phenolic compounds are more abundant in bio-oil, studies have focused on low-molecular-weight alcohols for insight of the chemistry behind dehydration reactions.

The presence of a considerable amount of acids (eg, acetic and formic) in bio-oil results in a low pH (pH value of 2–3) (Zhang et al., 2007). This low pH presents application problems due to its corrosive nature (Laird et al., 2009; Aubin and Roy, 1990) and instability. Acids increase the oxygen to carbon ratio, which decreases the heating value, therefore making it a less desirable alternative to fossil fuel. The removal of oxygen in the form of carbon dioxide to form a carboxylated compound is known as decarboxylation, a general equation is given as:

The presence of aldehydes and ketones (carbonyl groups found in their structure) in bio-oil results in a reduced heating value and instability. The removal of the carbonyl group from aldehydes and ketones can result in improved bio-oil properties for fossil fuel replacement. The removal of oxygen in the form of carbon monoxide is known as decarbonylation, a general equation is given as:

Activated alumina is manufactured by dehydroxylation of aluminum oxide hydroxide (Boehmite) which produces a highly porous substance, surface area typically in the range of 150–380 m² per gram. The term “activated alumina” refers to the activation due to calcination. The pore structure is resistant to high temperatures and abrasion, which have been a reason for activated alumina being used as a thermochemical catalyst. Both Brönsted and Lewis acid sites are contained in activated alumina catalysts which are required for catalytic cracking. Catalytic pyrolysis of miscanthus using activated alumina did not contribute to any considerable improvement in bio-oil composition where oxygenated compounds were dominant (Yorgun and Şimşek, 2008). Even though the bio-oil was dominated by oxygenated compounds there was a marked increase in aromatic hydrocarbons notably improving the quality of the pyrolysis oil. A study by Ateş et al. (2005) had similar results with the aromatic fractions showing less prominent change compared to noncatalyzed pyrolysis liquid fractions, but an increase in lighter hydrocarbons was observed. The increased yield of lighter hydrocarbons can be a consequence of poor cracking and hydrogenation ability of activated alumina (Wang et al., 2008). The larger pore size of activated alumina compared to ZSM-5 zeolite catalyst contributed to increased formation of coke (Williams and Horne, 1995) as larger coke precursors, such as polyaromatic hydrocarbons (PAHs), can enter the pore structure. From the studies above the main reaction mechanism performed by activated alumina appears to be dehydroxylation of hydrocarbons (Yorgun and Şimşek, 2008).

Zeolites are porous solids commonly used in the petroleum industry as a catalytic material. The main components of zeolites are alumina and silica. Due to their shape and cage-like structure they have unique properties which have a wide range of chemical applications (Wilson and Clark, 2000). Zeolites are normally referred to as solid acid catalysts. Zeolites have a three-dimensional tetrahedral structure which consists of silica and alumina atoms linked through oxygen anions. By sharing the oxygen atoms they can form rigid open 3D frameworks and form regular intercrystalline networks. These frameworks consist of channels and pores of certain molecular dimensions (pore sizes around 2–10 Å); this is one of the reasons why they are highly selective (Wilson and Clark, 2000; Jae et al., 2011). The rigid open intercrystalline structure allows for the zeolite structure to retain its shape after many reactions without the pore structure collapsing, contracting, or swelling.

Porous materials have been studied with regards to technical applications as catalysts and catalyst supports. Porous materials are divided into three classes: microporous (pore size <2 nm), mesoporous (2–50 nm), and macroporous (>50 nm) materials. Zeolites have attracted strong attention in pyrolysis vapor upgrading technologies, as described above, but zeolites only with micropores impose severe mass transfer limitations which result in poor catalytic performance of high-molecular-weight substrate catalytic reactions. To improve diffusion of reactants the pore size can be increased (Davis et al., 1988) or the use of mesoporous catalysts can be used. Mesoporous catalysts have relatively large pores which allow for mass transfer of larger compounds and have a high surface area (high concentration of active sites per mass of catalyst).

Fluid catalytic cracking (FCC) is the most important conversion process used in the petroleum industry, used to convert high-boiling high-molecular-weight hydrocarbon fractions to more valuable gasoline fuel grades (Beavon, 1959; Corma and Wojciechowski, 1985). The petroleum industry originally cracked petroleum hydrocarbons by thermal cracking which has now been replaced by catalytic cracking. Catalytic cracking has increased yields of petrol fuel products and also produced a gaseous byproduct with higher olefin content. The basic fluid catalytic cracking unit consists of two main sections: the reactor and the regenerator. In the reactor hot catalyst particles are contacted with a hydrocarbon feed producing cracked products and spent coked catalyst. The coked catalyst is separated from the cracked products and then regenerated by burning the coke in a regenerator (650–750°C at around 2 atm.). The hot catalyst is then recycled to the reactor for additional cracking.

An important use of transition metals is as catalysts for a variety of industrial processes, such as polymer and petroleum industries. Transition metal catalysts can be divided into two groups: homogeneous (dissolved in the reaction mixture) and heterogeneous (a solid-phase catalyst which is separate and insoluble from the reaction mixture). Catalytic dehydrogenation of light alkanes to obtain alkenes for polymerization and other organic synthesis has been thoroughly investigated (Carrà and Forni, 1972; Cimino et al., 1989). Chromium oxide supported catalyst has been used for many decades in industry for alkane dehydrogenation (Marcilly and Delmon, 1972; Frey and Huppke, 1933), by transforming saturated hydrocarbons (alkanes) to unsaturated organic compounds (olefins). Heterogeneous chromium containing catalysts have shown importance in hydrogenation, dehydrogenation, and polymerization reactions (Cimino et al., 1989; Udomsak and Anthony, 1996; Derossi et al., 1994; De Rossi et al., 1993). Iron- and nickel-based catalysts have been shown to crack large molecules into smaller fractions resulting in their use in gasification for hydrogen production (Azhar Uddin et al., 2008; Di Felice et al., 2011). Zhang et al. (2015) catalytically pyrolyzed rice husk with Ni and Fe catalysts to maximize gas yield and high-quality syngas production. Ni and Fe catalysts were used to crack the heavy compounds produced from pyrolysis (oxygenated organics) to obtain lighter gases. In addition, zinc oxide has been used for production of hydrogen via catalytic stream reforming of biomass-derived oxygenates (Wang et al., 1996). Nokkosmaki et al. (2000) studied the conversion of pyrolysis vapors of pine sawdust with zinc oxide catalysts. It was found that zinc oxide acted as a mild catalyst only decomposing a small fraction of water-soluble anhydrosugars and polysaccharides which led to an increase in oil stability (due to a reduction in oxygen containing compounds). Wang et al. (2008) have found that biomass pyrolyzed with CoMo and NiMo, supported on alumina, produced light aromatic hydrocarbons such as benzene, toluene, xylene, and naphthalene. The use of CoMo produced the greatest yield of light aromatics compared to NiMo, but NiMo produced the highest yield of methane (Wang et al., 2008). The increase in methane is expected as CoMo has a milder hydrogenation activity compared to NiMo. The study also showed that higher pyrolysis temperatures lead to a decrease in light aromatic yields (increase in methane yield). CoMo and NiMo have been used as hydroprocessing catalysts, via hydrodeoxygenation (HDO), for the conversion of different bio-oils (Elliott, 2007; French et al., 2011) to improve stability and compatibility with conventional fuels. Renewable fuels and chemicals have been produced from the pyrolysis of triglyceride materials with the incorporation of transition metal catalysts (Maher and Bressler, 2007; Idem et al., 1997; Li et al., 2009; Ito et al., 2012). It was shown that transition metal catalysts greatly increased the yield of hydrocarbons via decarboxylation of saturated fatty acid chains.

The final group of catalysts used in pyrolysis for improved oil generation or production of higher-value chemicals are the carbonates. Carbonate-derived catalysts are readily available and cheap, this allows for the catalysts to be disposed of when their activity has decreased, avoiding the cost of catalyst regeneration. He et al. (2010) studied the production of syngas from pyrolysis of municipal solid waste in the presence of calcined dolomite. The study showed that dolomite had a great impact on product yields by increasing gas yields and decreasing oil and char yields compared to noncatalytic process. These results have been shown to be representative of other studies (Conesa and Domene, 2015; Waheed et al., 2015). As dolomite has been shown to enhance the yield and quality of product gas and decrease tar yield by cracking and reforming the high-molecular-weight organics, it has been extensively used and investigated in gasification processes (Conesa and Domene, 2015; Chaudhari et al., 2003; Hu et al., 2006; Sutton et al., 2001). Sodium carbonate supported on alumina was shown to have excellent deoxygenation activity (Nguyen et al., 2013). Bio-oil produced via catalytic pyrolysis of wood chips using sodium carbonate as a catalyst had a much lower oxygen content (12.30 wt.%) compared to noncatalytic oil (42.10 wt.%). A negative effect of sodium carbonate was highlighted by Nguyen et al. (2013a,b) as decarboxylation of the pyrolysis vapors led to the formation of carbonyls which correlates to lower bio-oil stability. Catalytic pyrolysis of vegetable oils using sodium carbonate as a catalyst has produced varying results. Konwer et al. (1989) have shown that high yields of aromatics can be achieved by using sodium carbonate as a catalyst for seed oil pyrolysis. Seed oil was pyrolyzed at 500°C in the presence of 1% sodium carbonate and produced a liquid fraction similar to crude oil containing a high amount of aromatic hydrocarbons. Contrary to previous research (Konwer et al., 1989), Dandik and Aksoy (1998) pyrolyzed sunflower oil in the presence of sodium carbonate to produce a liquid product containing very low concentrations of aromatics. Catalytic pyrolysis of pine wood in the presence of potassium carbonate demonstrated a reduction in liquid yield with an increase in the yields of gas and char when compared to noncatalytic pyrolysis (Wang et al., 2010). This indicates a strong catalytic effect on the secondary reaction of the liquid product shown by increased gas release (H₂, CO, CO₂) and higher yields of alkanes and phenols.

Catalysts can lose their activity as the reaction proceeds due to many unwanted effects and this is known as catalyst deactivation. A number of reasons can result in catalyst deactivation and are described below.

Catalytic pyrolysis using an in situ process mode occurs in a single reactor where feedstock and catalyst are mixed thoroughly to achieve high catalyst and primary pyrolysis vapor contact. Primary pyrolysis vapors almost instantly diffuse into the catalyst pores where they are catalytically cracked/reformed. Pure catalyst or a catalyst–sand mixture replaces the typical heat carrier of silica sand used in noncatalytic pyrolysis. When large-scale units are continuously operated, such as auger reactors or circulating fluidized bed reactors, the char accumulated in the reactors should be separated from the catalyst before any catalyst regeneration. This is an important process as it prevents the build-up of char-bound alkali metals (ash components) on the catalyst surface which can result in permanent deactivation of the catalyst or negative effects on the bio-oil yield and quality from further catalyst usage.

For the optimal catalytic fast pyrolysis process resulting in sufficient aromatic hydrocarbon yields, careful optimization is required:

Support through the EPSRC SUPERGEN Challenge project Development of fast pyrolysis based advanced biofuel technologies for biofuels No. EP/K036548/1 is particularly acknowledged.