In contrast to the pyrolysis conditions needed to increase charcoal yields, the conditions for increasing the yields of organic liquid products would be ex­pected to involve short heat-up and reaction times and rapid removal and quenching of the organic volatiles before they are carbonized. Further, if the pyrolysis temperature at which maximum devolatilization occurs is chosen, but which is insufficient to gasify the volatiles (i. e., convert them to light fuel gases), more liquid products should be produced. When the reaction temperature is too low, less devolatilization and more char and tar formation occur, and when the temperature is too high, gasification reactions are domi­nant. Intuitively, pyrolysis at short reaction times and intermediate tempera­tures would be expected to promote higher organic liquid yields at the expense of other products. Char and gas yields should be low under these conditions. Numerous studies have indeed demonstrated that short-residence-time pyroly­sis, or flash pyrolysis, can be performed with biomass feedstocks to maximize liquid yields (с/. Bridgwater and Bridge, 1991; Bridgwater and Peacocke, 1995).

In some of the early work on the continuous flash pyrolysis of biomass at atmospheric pressure, it was shown that at optimum temperatures, liquid yields are maximized (Scott and Piskorz, 1983). With entrained flow injection of biomass feedstock of —250 jam to +105 ju, m particle size into a mini — fluidized-bed reactor with sand heat carrier and vapor residence times of 0.44 s, it was found that the maximum yield of liquid products occurs at the optimum temperature, and that yield drops off sharply on both sides of this maximum. Pure cellulose was found to have an optimum temperature for production of liquids at 500°C, whereas the wheat straw and wood species tested had optimum pyrolysis temperatures for maximum liquids at 600°C and 500°C, respectively. The yields of organic liquids were of the order of 55 to 65% of the dry weight of the biomass fed. The liquids contained relatively large quantities of organic acids. Pilot plant tests verified these observations. As research progressed on the conversion of biomass by flash pyrolysis, the optimum conditions for maximum liquid yields were found to be temperatures within the range 400 to 600°C, vapor residence times within the range 0.1 to 2.0 s, particle sizes less than 2 mm and a maximum of 5 mm for wood feeds, and an oxygen-free gaseous atmosphere such as recycled flue gas in the pyrolysis zone (Scott, Piskorz, and Radlein, 1993). Any reactor that can be operated under these conditions and that provides for biomass heating so that the particle temperatures can exceed about 450°C before 10% weight loss occurs can be used as a flash pyrolysis reactor. Such designs include fluidized — bed reactors, circulating fluid beds, transport or entrained flow reactors with or without a solid heat carrier, ablative reactors, and reduced-pressure reactors. Maximum organic liquid yields were projected to be 55 to 65 wt % of woody feedstocks and 40 to 65 wt % of grass feedstocks. The product yields from the flash pyrolysis of softwood (white spruce) and hardwood (poplar) are shown in Table 8.10. The total pyrolytic liquid yields are about the same for each feed, 66.5 and 65.7% of the feed dry weight, but they include several sugars and polysaccharide derivatives that are normally solids at ambient conditions. About 50% of the pyrolytic liquid product is water soluble. The largest fraction is the pyrolytic lignin, the insoluble fraction that remains after water extraction of the pyrolytic liquid. In other research, it has been shown that because of differences in oxidation rates between the anhydrosugar and aromatic lignin-derived pyrolysis products, which are formed in the fast pyroly­sis of deionized or prehydrolyzed wood, it is possible to carry out flash pyrolysis with controlled levels of oxygen to selectively oxidize lignins with relatively little effect on the anhydrosugar yields (Piskorz et ah, 1995). The recovery of the anydrosugars for use as fermentable sugars or as chemicals is expected to be simplified because their concentrations in the pyrolysis liquid are then significantly increased. The complex nature of the products and the poor selectivity of pyrolysis are evident, but with suitable refining, a wide range of chemicals could be manufactured from the products. Indeed, similar technol­ogy has already been commercialized for the production of fuels and chemicals.

A commercial, flash pyrolysis plant (RTP™) built in the United States in 1989 is believed to be the first successful plant in the world based on fast pyrolysis (Graham, Freel, and Bergougnou, 1991). This plant had a capacity of 100 kg/h of particulate hardwood feedstocks and is an upflow design that incorporates complete recirculation of the solid heat carrier in a reactor system capable of operating between 450 and 600°C at a residence time in the range of 0.6 to 1.1 s. The plant is used for the production of boiler fuel and specialty chemicals such as flavorings and natural colorings. The liquid is pourable and pumpable at room temperature and has an HHV ranging from 15 to 19 MJ/ kg, including the contained moisture, or approximately the same heating value as the feedstock entering the conversion unit. Typical liquid yields from representative hardwoods at 10-15% moisture content are about 73 wt % of the feedstock, including contained moisture (Graham and Huffman, 1995). In general, the yield increases slightly with an increase in the cellulose content of the feedstock and decreases slightly with an increase in feedstock lignin. However, the energy yield is almost constant since lignin-derived liquids have a higher energy content than cellulose-derived liquids. The liquids are pro­duced as a single phase unlike the heavy tars produced by conventional biomass pyrolysis. Experimentation has shown that by manipulating the vapor con­denser operating conditions, the product liquid can be tailored for chemicals and boiler, diesel, or turbine fuel applications without altering the pyrolysis

process itself. Since 1989, larger commercial plants of capacity up to 70 green t/day of feedstock have been built in North America, demonstration plants have been completed in Europe, and others that range in feedstock capacity from 100 to 350 t/day are in the advanced planning stages (Graham, Freel, and Kravetz, 1996).

In the mid-1990s, the economics of the RTP process for a 100-t/day plant were viable at a product fuel oil price of $5.27/GJ if the biomass feedstocks were available at zero cost. This can occur in several situations, especially with captive sources of waste biomass. The fuel oil price covers all fixed and variable operating costs, the annual capital costs to finance debt (75% of total capital), and an acceptable rate of return on the equity investment (25% of total capital). It is assumed that wood is available in the form of wet chips, and that the drying and grinding equipment and pyrolysis unit are included in the capital cost estimates. Expressed in other terms, the cost of converting 1 dry tonne of feedstock to fuel oil by this process is about $60, which means that the cost of the fuel oil is then about $10.54/GJ if the cost of the biomass feedstock is $60/dry t. For zero-cost fuel oil, a tipping fee of $60/dry t of waste feedstock is required. All of this means that the economics can be favorable in locations where there is an abundance of feedstock at zero or negative cost. In other words, the technology is very site specific, or the cost of competitive products, petroleum fuel oils in this case, must be high enough to justify commercial plants.

An example of one of the first flash pyrolysis processes developed for waste biomass is shown in Fig. 8.4 (U. S. Environmental Protection Agency, 1975; Preston, 1976). In this process, MSW is separated by a sequence of steps to obtain refuse-derived fuel (RDF) and recyclables. The sequence consists of shredding of MSW and air classification to obtain the RDF, magnetic separation of the ferrous metals, screening and froth flotation to recover a glass cullet, and aluminum separation by an aluminum magnet. The RDF is dried in a rotary kiln to about 4 wt % moisture content, and finely divided to a particle size of which 80% is smaller than 14 mesh (1200 /u. m). The feed, about 0.23 kg of recycled char preheated to 760°C per kilogram of this finely divided material, is rapidly passed through the pyrolysis reactor at atmospheric pres­sure. The raw product mixture, which consists of product gas and liquid, the char fed to the reactor, and new char formed on pyrolysis, leaves the reactor at about 510°C. Separation of the gas and liquid from the char and rapid quenching to about 80°C yields the liquid fuel. The remaining gas is passed through a series of cleanup steps for in-plant use. Part of the gas is used as an oxygen-free solids transport medium for pyrolysis and part of it as fuel. The raw product yields are about 10 wt % water, 20 wt % char, 30 wt % gas, and 40 wt % liquid fuel. The product char has a heating value of about 20.9 MJ/kg, contains about 30 wt % ash, and is produced at an overall yield

of about 7.5 wt % of the dry feed. The corresponding values for the liquid fuel are about 24.4 MJ/kg, 0.2 to 0.4% ash, and 22.5 wt % of dry feed as received (approximately 1 bbl/ton of raw refuse). This product was proposed for use as a heating oil; its properties are compared with those of a typical No. 6 fuel oil in Table 8.11. It is apparent that some major differences exist, but successful combustion trials in a utility boiler with the liquid fuel were performed. A plant designed to process 181 t/day of MSW was built in the United States in 1977, operated for one year, and then after producing several thousand liters of oil, was mothballed because of operating problems in the MSW separation unit and flash pyrolysis reactor.

Another flash pyrolysis process (GTEFP process) operating at atmospheric pressure was also developed that affords liquid yields in excess of 60% on a dry basis from hardwoods (O’Neil, Kovac, and Gorton, 1990). Yields as high as 70% were projected for commercial plants. The GTEFP process was devel­oped in bench-scale studies and a large-scale PDU in which the particulate feed is entrained in hot combustion gases and pyrolyzed at millisecond resi­dence times and temperatures in the range 500°C. Typical higher heating values of the product oils were 22 MJ/kg.

Short-residence-time pyrolysis of biomass at reduced pressure has been found to improve the yields of liquid products (Roy et al, 1985, 1990). In this research, a large-scale, electrically heated, multiple-hearth PDU afforded pyrolytic oil yields in the range of about 50 wt % of the feedstock with a wide range of wood species. Pyrolysis temperatures in the last hearth were about 450°C. The optimum temperature range was found to be between 350 and 400°C at residence times of about 2 to 30 s (с/. Bridgwater and Bridge, 1991). A yield of 60 wt % on a dry, ash-free basis of pyrolytic oil was obtained at an average heating rate of 10°C/min at a total system pressure of 5 to 41 kPa. The process was operated at feed rates of 30 kg/h in a multiple-hearth pilot plant, which was shown to offer advantages in product separation and fraction­ation because of the primary condensing units attached to each hearth. How­ever, the char and gas yields still comprised about one-third of the products, so it is probable that the liquid yields could be improved further at shorter residence times. A multiple-hearth reactor may not be suitable for biomass pyrolysis at millisecond reaction times.

Still another fast biomass pyrolysis configuration for increasing liquid yields involves what is termed ablative pyrolysis (Diebold et al, 1987, 1990). In this system, biomass particles are entrained tangentially into a vortex tube with a jet of carrier gas at velocities over 100 m/s. This causes the solid particles to be centrifuged to the hot wall of the vortex reactor, where very rapid heat transfer occurs to the surface of the particles. Ablative or surface pyrolysis takes place at high rates essentially independent of the feedstock particle size. This type of conversion process favors chain-cleavage reactions to form oxygenated, organic vapors rather than chars and gases, and is expected to make it possible to design small reactors having high throughput rates. With temperatures of 625°C on the reactor walls, 60 to 70 wt % of the primary vapor products are composed of oxygenated organic compounds and polymer fragments. They condense to form acidic, water-soluble liquids that have nearly the same elemental composition as the feedstock. For softwood feeds, the vortex reactor produces about 58 to 67 wt % of the dry feedstock as primary pyrolysis oils, 10 to 12 wt % as char, 10 to 14 wt % as gases, and 13 to 16 wt % as water. A plant for conversion of 33 t/day of wood wastes has been designed for construction in the Midwest to evaluate this process (Johnson, Tomberlin, and Ayres, 1991; Johnson et al, 1993). Dry wood feedstock (13 mm) is discharged from a hopper and pneumatically transported by recy­cled pyrolysis gases to a vortex reactor enclosed in a furnace designed to burn pyrolysis gas and natural gas. Natural gas is used for startup. The vapor-rich gases leave the vortex reactor along with the char particles entrained in the gas stream and pass through a cyclone, which is designed to remove 99.5% of the solids. The vapors are condensed at the rate of 11 L/min (dry). This plant is projected to cost $1.5 million, to produce 870 L/h (wet) of oil and 0.21 t/h of char, and to have a net annual revenue before taxes of $194,000, presuming the tipping fee for accepting the waste wood is $ll/t, and the oil and char can be sold for $0.11/L and $88/t on the open market. Larger plants having capacities of 90 and 227 t/day are being designed to gain economies of scale.

Other fast biomass pyrolysis techniques for liquid products have been exam­ined wherein a reactive atmosphere is present during conversion to attempt to affect yields and product compositions. Examples are flash pyrolysis in an atmosphere of hydrogen (hydropyrolysis) (Bodle and Wright, 1982; Sundaram, Steinberg, and Fallon, 1984) and methane (methanolysis) (Steinberg, Fallon, and Sundaram, 1983). In hydrogen atmospheres, hydrogenation might be expected to occur, at least to some extent, to yield liquids with lower oxygen and higher hydrogen contents. The experimental data indicated only moderate improvements in product yields and compositions under the test conditions used in an entrained flow pyrolysis reactor. However, substantial changes were observed in methane atmospheres. Flash methanolysis of dried wood particles at residence times of 1 to 2.8 s in an entrained flow pyrolysis reactor at pressures of 138 to 1379 kPa and temperatures between 800 and 1050°C

afforded benzene, toluene, and the xylenes (BTX), a heavy oil, ethylene, and carbon monoxide. As much as 12% of the available carbon in the wood feed­stock was converted to BTX, 21% to ethylene, and 48% to carbon monoxide at 345 kPa and 1000°C. The maximum heavy oil yield was observed at 345 kPa and 800°C. Data obtained with a methane blanket alone under the same pyrolysis conditions showed that no products were formed. It was concluded from this research that the optimum conditions for maximum production of both BTX and ethylene are a reactor pressure of 345 kPa, a temperature of 1000°C, a residence time of less than Is, and a methane-to-wood feed ratio of about 5.