Hydrotreating aims to remove the oxygen through a family of de-oxygenation

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This is a carbon limited system and gives a maximum stoichiometric yield of 58% by weight on liquid bio-oil or a maximum energetic yield of about 50% wt. ignoring the significant hydrogen generation requirement which is discussed below.

A summary of the activities in catalytic hydrotreating of pyrolysis and liquefaction products is given in Table 6.5.

Table 6.5 Organisations Involved in Catalytic Upgrading of Pyrolysis and Liquefaction Oils since 1980 by Hydrotreatment and Related Processes

Battelle Pacific Northwest Laboratory (PNL), USA

64,65,66,67,68,69,70,

Colorado School of Mines, USA

Institute Nationale Recerche Scientifique (INRS), Canada Imperial College, University of London, UK Institute of Wood Chemistry, Germany Lawrence Berkeley Laboratory, CA, USA National Renewable Energy Laboratory, USA Saskatchewan Research Council Technical University of Berlin, Germany Technical University of Compiegne, France

Texas A&M University, USA 33, 86,

University of Chalmers, Sweden

University of Louvain (UCL), Belgium 95, 96, 97, 98, 99,100,101,

University of Rouen

University of Sassari, Italy

University of Waterloo, Canada

University of Saskatchewan, Canada

University of Toronto, Canada

Veba Oel, Germany

VTT (Technical Research Centre of Finland), Finland

The essential processing features for pyrolysis bio-oil hydrotreating are:

• high pressures of 70 to 200 bars to provide a high hydrogen partial pressure,

• a two stage process: an initial stabilisation reactor or reaction zone operating

at about 250-275°C followed by a more conventional hydrotreating process operating at 350-400°C. The initial stabilisation consumes little hydrogen but is essential to avoid polymerisation at the higher temperatures of hydrotreating. Without this initial stabilisation, the pyrolysis liquid rapidly polymerises and cokes the catalyst (110, 112). A temperature stepped single upflow continuous fixed bed reaction has been successfully employed for this purpose in both the USA (61, 69) and Germany (110, 112).

In practice, for pyrolysis liquids, hydrotreating yields of about 35% wt on wet liquids, based on better than 98% de-oxygenation, have been achieved from pyrolysis oils in a continuously operated carbon limited system which is about 70% of the theoretical maximum (62, 63, 116).

Conventional sulphided CoMo hydrotreating catalysts that are utilised commercially for hydro-desulphurisation have been found to be quite effective, although there is much potential for catalyst development both with conventional catalysts and with novel catalysts. Liquefaction oils do not require the initial stabilisation step and may be conventionally processed.

Hydrotreating is based on a modification or extension of well-established refinery practice for hydro-desulphurisation and involves a family of reactions including hydrogenation, de-oxygenation and cracking. The pyrolysis or liquefaction material being processed is made up of a wide range of classes of organic compounds, which all behave differently under different reaction conditions. The hydrotreating process, therefore, has to be "customised" to a particular feed and a specified product. It has been successfully demonstrated at a continuous laboratory scale on pyrolysis products (64, 65), liquefaction products (78, 84), selective extracts from liquid products (82, 107), lignin (75, 80, 81, 82) and black liquor (114, 115). Optimisation is required to establish the best catalyst system for the highly oxygenated liquids and the optimum process parameters.

Conventional sulphided cobalt/molybdenum (Co/MoS) hydrotreating catalysts have proved successful. A variety of related catalysts including Ni, Co, Mo in oxide and sulphide forms on silica and alumina supports have also given quite good results in work at Battelle Pacific Northwest Laboratory, the University of Louvain (UCL) and the Institute of Wood Chemistry (see Table 5 for full citations), although catalyst activity of the oxide forms is significantly lower. However the absence of sulphur and sulphided catalysts may be an advantage to be exploited if longer term tests show that sulphur retention is a problem with the essentially sulphur-free pyrolysis oil feedstock.

There is uncertainty over the catalyst lifetime as few extended runs have been reported. At least one continuous run of 8 days has been completed but with substantial deterioration in activity (110, 112). There is no information on the cause of this loss in activity, although physical examination of the catalyst afterwards suggests that the water in the bio-oil may have attacked the support and drastically reduced the surface area through agglomeration. Other hypotheses include alkali attack, coking, and loss of sulphide from the catalyst. There is also uncertainty over catalyst stability and activity with regard to the sulphur and possible contamination of the product oil. One of the concerns is sulphur stripping from the catalyst which

would seriously impair its effectiveness. The water product may have an adverse effect on conventional catalysts, and they may require modification to improve their water tolerance. Other contaminants may also have an adverse effect but this has not yet been explored and long term testing and detailed catalyst examination is necessary if this route is to be further developed.

Other catalytic environments have also been tested including the use of hydrogen donor solvents (89, 90, 97) and less conventional catalyst systems including oxide forms of CoMo and NiMo and modified NiMo which have so far proved to be less active than sulphided catalysts (97). Soltes employed platinum and palladium catalysts in a hydrogen donor solvent, claiming superior results to other catalysts (88).

Basic work has been carried out on model compounds (75, 101) and constituents of wood, notably lignin to devise a more effective method for utilisation of wastes from the pulp and paper industry (75, 80, 81, 101). This has included hydrotreating black liquor (113 — 115). Use of model compounds has some value in establishing pathways and kinetics as well as establishing minimum and optimum reaction parameters, but the pyrolysis liquid is so complex that this approach may be of only limited use in the short term. There will, however, be potential benefits from this approach in developing partial hydrotreating to derive a stable but only partially de — oxygenated product and in the production of chemicals rather than fuels.

Integrated conversion and upgrading has been employed in a number of configurations and processes including a first stage of hydrolysis with hydrotreating (76) and solvolysis followed by hydrotreating (79).

Analysis of products is always an important contribution to improve understanding of the processes involved so that they can be optimised and scaled up. Most of the organisations listed in Table 5 have developed appropriate analytical techniques, and some publications have focused on these analytic methods (87, 95, 98)

One of the useful side-effects of full hydrotreating, i. e. less than 2% wt oxygen in the final product, is that the product water readily separates from the hydrocarbon product and is relatively clean. The water will also tend to strip out the alkali metals present, for example, in wood ash and resolve one of the problems in using biomass derived fuels in turbines.