A major disadvantage of hydrotreating is the significant hydrogen requirement of around 700 l/kg pyrolysis oil (62), (31 g moles/kg; 62 g/kg) for full hydrotreating, while Veba suggest 600 — 1000 l/kg bio-oil (110, 112). These should be compared to the theoretical yields of 380 !/kg for CoMo catalysts and 440 l/kg for NiMo catalysts (97). An excess of typically 100% is thus required for processing due to hydrogenation of components such as light aromatics.

The hydrogen can be generated in a number of ways including recovery and/or regeneration from the spent gases. These options are summarised in Table 6.6 which includes approximate relative costs, and depicted in the flowsheet of Figure 2. These alternatives have not been evaluated, but will significantly affect the economics. A 1000 t/d biomass processing plant will require about 50 t/d hydrogen for complete hydrotreating which, if generated from biomass by gasification and CO shifting would require up to 800 t/d additional biomass at a significant economic and efficiency penalty. A potentially attractive route is to utilise the surplus hydrogen which is available at fuel value in many refineries. Product yields and process costs are discussed below.

6.4.2 Zeolites

Early zeolite cracking research on biomass derived pyrolysis liquid was carried out on a conventional mono-functional ZSM-5 catalyst, although more, recently specialist catalysts have been developed by modification of the structure in an unspecified way (117). The reaction follows the representative equation below with a maximum stoichiometric yield of 42% on liquid, or a maximum energetic yield of about 50% wt.:

Об He O4 —> 4.6CH1.2 + 1.4C02 + 1.2H20

This is a hydrogen limited reaction with the aromatic product being limited by the availability of hydrogen for aromatics and water formation and oxygen is rejected as both CO2 and H2O.

BIOMASS

One conjectural way of overcoming the hydrogen deficit is through modification of the catalyst to include a shift component to generate in-situ hydrogen from product gases through the water-gas shift reaction. This might be carried out in a bi­functional or multi-functional catalyst that can operate in a carbon limited environment rather than a hydrogen limited one. This is depicted in the equations below which would give a maximum stoichiometric yield of 55% weight on liquid:

3.6CO+ З.6Н2О —> З.6Н2 + З.6СО2

Сб H8 O4 + З.6Н2 —> 6 CH1.2 + 4H20

This is now a carbon limited reaction with the aromatic product being limited by carbon availability assuming that there is sufficient CO available in the product gas for production of hydrogen. This second route represents an optimum but has not been attempted.

Thus on this simplistic stoichiometry, there is little difference in yields conceptually in either the upgrading route of hydrotreating or the zeolite cracking one, although aromatics have a higher potential value than naphtha particularly as a chemical feedstock.

Organisations who have been, or are involved, in zeolite processing are listed in Table 6.7.

The processing conditions for zeolite cracking are atmospheric pressure, temperatures from 350°C up to 600°C and hourly space velocities of around 2. Earlier work focused on the lower temperatures, but as catalyst development continued, better results are understood to have been obtained with modified catalysts at higher temperatures with much lower coke formation and longer catalyst life (118). The mechanisms of cracking and reforming are not well understood, but there appears to be a combination of cracking on the catalyst "surface" followed by synthesis of aromatics in the catalyst pores. There are a number of potential advantages over hydrotreating including low pressure (atmospheric) processing; temperatures similar to those preferred for optimum yields of bio-oil; and a close coupled process. These offer significant processing and economic advantages over hydrotreating.

Actual yields of 15% aromatics in a hydrogen limited environment have been achieved, with projections of 23% if the olefinic gas by-products are utilised through alkylation (133). These correspond to 40% and 60% of the theoretical maxima respectively. Most work has been carried out in the vapour phase acting as quickly as possible on the freshly produced primary pyrolysis vapours to take advantage of the special conditions under which they are formed. Some early but limited work has been carried out on liquid phase processing, including Occidental (140), GTRI (119), INRS (122) and Mobil who mixed methanol with the condensed bio-oil before spraying it into a fluid bed (124). Liquefaction oil has also been processed in the liquid phase with aromatics yields of up to 37% when co-processed with water (155).

Related work on specific chemicals and model compounds derived from biomass including sugars and carbohydrates has been carried out by INRS (120), and later extended to pyrolysis oils (121) and liquefaction oils (122). Poor yields of hydrocarbons were obtained with high coke and tar and rapid deactivation of the catalyst. Other work carried out on model compounds includes that of Milne et al. at NREL (125, 126, 128) who have carried out fundamental R&D for about 15 years on both pyrolysis and zeolite cracking (see Table 7), and University of Laval (143).

The most extensively reported work is that by NREL (see Table 7). Milne et al. have carried out screening studies on zeolite catalysts in the NREL molecular beam mass spectrometer which is particularly well suited to this type of work (125, 126, 128, 129). Diebold et al. (131-135) have carried out more applied and larger scale work by close coupling a zeolite cracking unit to the NREL vortex flash pyrolyser. Early work was carried out on a slip stream of pyrolysis vapours from the reactor, while more recently a substantial fixed bed of catalyst was added to process all the pyrolysis product. An assessment of the resultant technology for production of gasoline has recently been completed by the lEA Bioenergy Agreement (159). Optimum catalysts and conditions have yet to be determined, but there is a trade-off between olefins and aromatics and process conditions in which catalyst life and catalyst regeneration are significant factors. Data from various researchers are insufficiently compatible to draw conclusions.

In addition to work on bio-oil or pyrolysis liquids, the process has also been applied to wastes from the wood, pulp and paper industry including tall oil (151 — 153), and biomass derived materials using synthetic clays (160).

An unusual project in the context of this review is the synthesis of zeolite catalysts from rice hulls through thermal processing at the Indian Institute of Technology although specific applications and efficacy of the resultant catalysts are unclear (161,162).

Zeolite cracking seems to be less well developed than hydrotreating and there is a much wider range of possibilities to be explored with mono — and multi-functional catalysts. In this respect the screening methods of NREL are particularly valuable. There is not only less information available from specific work carried out, but there are no directly analogous refinery processes from which data may be extrapolated, as well as an enormous variety of catalysts from which to choose. Conversely, the inherent nature of a one-step integrated process for synthesis of hydrocarbons is very attractive and offers much potential for fuels and chemicals.