Table 15.1 shows some results from recent studies on the pyrolysis of lignocellulosic biomass in the presence or absence of catalysts. Earlier, catalysts such as carbon­ates and hydroxides were mainly tested for the catalytic pyrolysis of lignocellulosic biomass. The use of alkaline compounds like NaOH, Na2CO3 and Na2SiO3 resulted in bio-oils rich in acetol and to some extent favored H2 formation. The use of Fe2(SO4)3 as a catalyst favored the formation of furfural and 4-methyl-2-methoxy — phenol (Chen et al., 2008). Lu et al. revealed that SOl~/SnO2 were an effective catalyst to yield 5-methyl furfural (Lu et al., 2009). The selectivity varied signifi­cantly once the catalyst support was altered. For example, SO4~/TiO2 catalyst favored the formation of furfural; SO4/ZrO2 catalyst favored the formation of furan (Lu et al., 2009).

Liquid acids such as H2SO4, hydrochloric acid, phos­phoric acid and solid acids such as ZSM-5, Al-MCM-41 are also used as catalysts (Table 15.1), in addition to their uses in the pretreatment of lignocellulosic biomass (Lu et al., 2011). The typical products of liquid acid catalytic

pyrolysis of biomass are levoglucosenone, furfural and levoglucosane (0.1 wt% sulfuric or polyphosphoric acid as catalyst) (Dobele et al., 2005; Kawamoto et al., 2007). Taking the separation between catalysts and liquid products and corrosive into consideration, the catalytic pyrolysis of lignocellulosic biomass over zeolites and many other solid catalysts have recently received much attention. Microporous zeolite and mesoporous materials such as ZSM-5, Al-MCM-41 have been used catalysts in catalytic pyrolysis of ligno — cellulosic biomass. In particular, hydrocarbons can be produced in considerable quantities by fast pyrolysis of biomass over these catalysts (Pattiya et al., 2008). Ola — zar et al. conducted the fast pyrolysis of pine sawdust catalyzed by ZSM-5 in a spouted-bed reactor using nitrogen as the carrier gas. A 12% yield of aromatic com­pounds was obtained (carbon) (Olazar et al., 2000). French et al. revealed that over nickel, cobalt, iron, or gallium-substituted ZSM-5 at 673—873 K and with a catalyst-to-biomass ratio of 5—10 by weight, lignocellu — losic biomass was pyrolyzed to give an approximately 16 wt% yield of hydrocarbons (French and Czernik,

2010) . The solid catalysts have advantages over the liquid acid catalysts. However, for solid catalysts at high temperatures, the cracking and deoxygenating activity decreased with time because of the coke formed on them (Carlson et al., 2008).

Reactors

Several types of reactors have been designed for fast pyrolysis of biomass, and the reactor is very crucial to fast pyrolysis of biomass. There are entrained-flow reac­tors (EFRs), fluid bed reactors, rotating cone reactors, and ablative reactors.

Entrained-Fow Reactors

In EFRs, schematically shown in Figure 15.1, biomass particles are usually fed into the reactor in a stream of hot, inert gas. Reaction is typically completed at 973—1073K within a residence time of a few seconds. Dupont et al. used the mixture of two softwoods (sylvester pine and spruce) as a model of biomass to pyrolyze in an EFR (Dupont et al., 2008). The influence of the particle size (0.4 and 1.1 mm), temperature (1073—1273 K), the presence of steam in the gas atmo­sphere (0 and 20 vol%) and the residence time (between 0.7 and 3.5 s for gas) on conversion and selectivity were studied. Results showed that the particle size was the most crucial parameter that influenced decomposition and more than 70 wt% of gas was produced.

Ablative Reactors

An ablative pyrolysis reactor is considered as a possible alternative to an EFR. The surface is heated by

hot flue gas produced by combustion of pyrolysis gases or char and rotates while biomass is pressed onto the hot surface (873K).

However, in general, an ablative pyrolysis reactor has difficulty in getting sufficient heat transfer from hot gases to the ablative surface and in contacting feed­stock of diverse morphologies (particle shape, struc­ture, and density) with the ablative surface. In practice, relatively few feedstocks would be suitable for ablative pyrolysis.

Bubbling Fluid Bed Reactor and Circulating Fluidized Beds

A fluid bed reactor is very suitable for fast pyroly­sis, as the biomass is rapidly heated and there are high heat and mass transfer rates between gas, parti­cles and catalysts and any other objects in the reactor. Vapor and solid residence time are controlled by the fluidizing gas flow rate. Bubbling fluidized beds (Figure 15.2(a)) are usually referred to simply as fluid­ized beds, which provide good temperature control and very efficient heat transfer to biomass particles due to high density of solids in the bed. Jung et al. pyrolyzed rice straw and bamboo sawdust in a bubbling fluidized bed equipped with a char separa­tion system (Jung et al., 2008). They found that the maximum bio-oil yield was above 70 wt% and a higher feed rate and a smaller feed size were more favorable to the production of bio-oil.

While circulating fluid bed reactors have similar features to bubbling fluid bed reactors. A main differ­ence between them is the amount of gas used to flui­dize the bed. In the circulating fluid bed reactor (Figure 15.2(b)), the gas flow is intentionally set high enough to transport particles out of the bed, which are recovered by gas cyclones and then returned to the fluidized bed. All char is burned in the secondary reactor to reheat the circulating sand or is separated as a fine powder.

The circulating fluidized bed can be divided into two zones: pyrolysis zone; and reduction and cracking zone (Wu et al., 1992). In the pyrolysis zone, biomass is loaded into the bed and pyrolyzed very quickly to form char, tar, H2O, and gas (CO2, CO, CH4, CnHm and H2). In the reduction and cracking zone, pyrolysis char of contributes to secondary cracking in the vapor
phase. For example, with the circulating fluidized bed as reactor, Dai et al. pyrolyzed wood powder at 773K (Dai et al., 2000). The main effects were: (1) the higher temperature and longer residence time contributed to the secondary reactions and then lead to less liquids; (2) the lower heating rate favored the carbonization and reduced the liquid production; and (3) most com­pounds in bio-oil were nonhydrocarbons and alkanes, aromatics, and asphalt were relatively less.

Rotating Cone Reactor

Rotating cone reactor is designed to achieve the intense mixing and heat transfer between biomass and heat carrier without the use of a large amount of fluidizing gas (Figure 15.3). Gas is needed for char burn-off in a secondary bubbling fluid bed combustor and sand transport recirculated to the pyrolyzer. Flash

pyrolysis of wood dust was processed in a rotating cone reactor by Wagenaar et al. the cone geometry was spec­ified by a top angle of p/2 radians and a maximum diameter of 650 mm (Wagenaar et al., 1994). The rotating cone reactor model included the description of the particle flow behavior, the particle conversion and the gas-phase cracking of tar vapors. It appeared that the product distribution was affected by the gas — phase reaction kinetics and residence time and the gas-phase residence time was determined by the avail­able reactor volume and the feed rate of the wood particles.

New Systems

Recently, emerging technology is to couple a pyroly­sis reactor with other catalytic reactors such as steam reformer and hydrogenation. For example, the technol­ogy of hydroprocessing is intended to convert bio-oil to petroleum-refinery compatible feedstock (Elliott et al., 2012). The combination can also be used to build a microscale pyrolysis reactor coupled to the molecular-beam mass-spectrometer (Bahng et al.,

2009) . It can be used as a very efficient tool for studying mechanisms of thermal and catalytic processes and to optimize process conditions for different products from a variety of feedstocks.