Because of the broad scope of direct biomass pyrolysis, the basic technologies and principal products are tabulated in Table 8.12 to facilitate easy comparison. The conversion conditions and major products shown in this table are typical, but subject to considerable variation. There are several commonalities among the different pyrolysis methods. Pyrolysis time and temperature are clearly the key operating parameters that have the most influence on product yields and distributions. Moderate but optimized temperatures are needed at short residence times to maximize liquid yields, whereas long residence times and

low temperatures are needed to maximize char yields. Biomass gasification, which can involve pyrolysis at the higher temperatures, is treated in Chapter 9. With few exceptions, the selectivities of specific pyrolysis products are poor. Essentially all of the product liquids produced by direct pyrolysis are highly oxygenated, acidic, generally unstable over time, and contain many com­pounds, and the product gases are low-energy gaseous mixtures. Some specialty and commodity chemicals can be extracted from the product liquids for market. The oils can often be used directly as fuel for power and steam production. The charcoals can be readily separated from the product mix for sale or captive use, and the product gases can be used as fuel and in some cases as sources of chemicals. Overall, however, most biomass pyrolysis plants will have to deal with complex product separations, waste disposal, and further product refining if pure chemicals and other products are required. The costs of the additional operations will have to be justified based on the markets for the various products that can be manufactured by biomass pyrolysis. Multiproduct slates will dominate most of these plants. This can be quite beneficial from a revenue standpoint when markets fluctuate provided the plant operating conditions can be readily changed to take advantage of the markets for specific products, much like a petroleum refinery.

Up to the 1930s, biomass pyrolysis processes were utilized on a large scale to produce several commodity chemicals from pyrolytic oils. These processes have since been largely replaced by nonpyrolytic processes based on petroleum and natural gas feedstocks. Note that only a few building blocks from fossil feedstocks—ethylene, propylene, butadiene, benzene, toluene, xylene, and synthesis gas—are used to manufacture the vast majority of the organic chemi­cals sold today (Chapter 13). Since the First Oil Shock in 1973, hundreds and perhaps thousands of research projects have been carried out to perfect and develop biomass pyrolysis technologies for the production of petroleum substi­tutes. The history of commercializing modern biomass pyrolysis systems in industrialized countries, however, since the First Oil Shock is not very encoura­ging. In the United States, for example, a few advanced design, biomass pyroly­sis plants were built in the 1980s and then closed down, generally because of operating problems and poor economics.

The properties of pyrolytic liquids from biomass such as their high oxygen contents and acidity generally limit their fuel uses to heating oils. The emphasis over the past several years has therefore been to develop methods for upgrading them to more hydrocarbon-like liquids for use as motor fuels and motor fuel components. One approach has focused on catalytic hydrogenation. Hydrogen, which can either be generated from the biomass feed or the conversion prod­ucts, or be obtained from an independent source, is reacted directly with the pyrolytic liquids or intermediate process streams at elevated pressures and temperatures to yield substitute fuels with higher hydrogen-to-carbon ratios.

In theory, highly oxygenated feedstocks should be capable of reduction to liquid and gaseous fuels at any level between the initial oxidation state of the feed and methane:

R(OH)x + у H2 -» RH/OH)^ + у H20 R — R’ + H2 —> RH + R’H.

For a cellulosic material containing hydroxyl groups, the reactions might consist of dehydroxylation and depolymerization by hydrogenolysis, during which there is a transition from solid to liquid to gas. In early work to produce hydrocarbon fuels, hydroliquefaction of biomass or wastes was achieved by direct hydrogenation of wood chips on treatment at 10,132 kPa and 340 to 350°C with water and Raney nickel catalyst (Boocock and Mackay, 1980). The wood is completely converted to an oily liquid, methane, and other hydrocar­bon gases. Batch reaction times of 4 h give oil yields of about 35 wt % of the feedstock. The oil still contains substantial oxygen, about 12 wt %, but has a heating value of about 37.2 MJ/kg. Distillation yields a major fraction that boils in the same range as diesel fuel and is completely miscible with it.

Catalytic hydrogenation of the pyrolytic liquids at elevated pressures and temperatures and deoxygenation with molecular sieve catalysts that yield hy­drocarbon liquids with higher hydrogen-to-carbon ratios than the liquid feed­stocks have been studied (с/. Elliott and Baker, 1987; Rajai et ah, 1991; Bakhshi, Kaitikaneni, and Adjaye, 1995; Laurent, Maggi, and Delmon, 1995; Horne, Nugranad, and Williams, 1995). Several systems are quite effective for convert­ing pyrolytic liquids to hydrocarbons suitable for use as motor fuels or motor fuel additives. Catalytic hydrogenation is well-developed commercial technol­ogy in the petroleum industry and can be applied to pyrolytic oils to obtain partial or complete reduction to hydrocarbons. Deoxygenation with molecular sieve catalysts has the advantage that a source of hydrogen is not needed and BTX and light olefins can be produced by direct passage of the pyrolytic vapors over zeolite catalysts, although conversion conditions must be carefully controlled to avoid formation of undesirable by-products such as coke. High yields of BTX and olefins approaching the theoretical limits can be obtained at high weight hourly space velocities and low steam-to-biomass ratios. This route to olefins appears to offer an economic route to methyl t-butyl ether (MTBE) and ethyl t-butyl ether (ETBE) from the light olefins for use as oxygen­ates in gasolines (Bain et ah, 1993).

It would seem that a more practical approach to the upgrading of pyrolytic liquids from biomass is to utilize what is already on hand, namely, the oxygen­ated product liquids. Instead of conversion to hydrocarbons, which usually requires severe reaction conditions, why not convert the liquids by simple chemistry to other liquids that are suitable for use as motor fuels or additives? Although not directly related to pyrolysis, this approach has been pursued in the development of oxygenated diesel fuels (biodiesel) with natural biomass- derived oils, as will be discussed in Chapter 10. With the possible exception of the lignin components in biomass, the overall thermal efficiencies of convert­ing a highly oxygenated organic feedstock in which the majority of the carbon atoms are bonded to oxygen atoms to other oxygenates should be much more favorable, and lower in cost, than conversion to hydrocarbons. However, there may even be exceptions to this rationale, as will be shown in the following section. Also, there are several opportunities to produce chemical additives suitable for use in modern gasolines as oxygenates by thermochemical process­ing of biomass (Chapter 13).