Fundamentals of Thermochemical Biomass Conversion

The increasing populations of mankind with their expectations of rising living standards will cause shortages of energy and resources in the future. The shortages of large trees for conversion to wood for constructional and high quality uses will become a crucial problem if present patterns of forest use continue.1Serious shortages of fuelwood already exist in some global regions. Greater availability and more efficient utilization of trees and other biomass will be required to meet the demands.

According to the organizers, this lecture should place emphasis on cellulose, hemicellulose, and other important polysaccharides and on extractives in lignocellulose-rich materials, such as wood and straw and related agricultural residues, and not include the often more costly starch-rich and sugar-rich materials. The latter materials are important sources for biotechnical and organo-chemical conversion, rather than thermochemical biomass conversion. The lignocellulose polymers cellulose, hemicellulose and lignin represent the major portion of the enormous amount of biomass annually produced on land.

Lignin is deposited by woody plants for the purpose of providing the growing plant with mechanical support, of sealing a water conducting system which links roots with leaves (needles),1–3 and for protecting wood against degradation by weathering4 and biodegradation.5 Lignified plants contain between 20 and 30% lignin, depending on species, age, juvenile vs. mature wood, sapwood vs. hardwood, and normal vs. reaction wood.1,6 The 20–30% lignin content by weight corresponds to approximately 35–45% by energy content since lignin is the polymeric cell wall component with the highest energy content, consisting of more than 60% carbon and around 30% oxygen. This is contrasted by hemicellulose and cellulose with carbon contents of less than 50% and oxygen contents of nearly 50%.7

Developers of microbiological processes to convert biomass into chemicals and fuels recognized long ago that pretreatments were needed prior to fermentation or enzyme reactions, if high rates and yields are to be achieved.1,2 Microorganisms and enzymes have difficulty in depolymerizing crystalline polymers, such as cellulose, and hydrophobic molecules, such as lignin and triglycerides. Furthermore, biomass constituents frequently conspire to form complexes that are difficult for microbes or enzymes to break apart. Because of these problems, numerous pretreatment methods have been developed to aid in microbiological processing by disrupting the lignocellulose complex, facilitating the depolymerization of polymers, and removing protective extractives.

The thermal dehydration kinetics of cellobiose have been studied previously in the 170–250 °C temperature range.1 A comparison with the dehydration kinetics of sucrose suggested that the eliminated water is responsible for its liquefaction at 238 °C and that this so-called melting process is a high temperature aqueous dissolution of the partially dehydrated cellobiose. The kinetics of dehydration appear to be strongly influenced by the fact that each hydroxyl hydrogen atom is hydrogen bonded, either inter- or intramolecularly, to a neighboring hydroxyl oxygen atom.2

As one of the ultimate renewable resources, forest products will inevitably become a major source of fuels and organic chemicals. Any technological process for converting biomass to fuels must involve the break-down of complex organic structures and stabilization of the decomposition products. The first step in conventional physicochemical routes to biomass gasification and/or liquefaction usually involves severe decomposition of the feed at temperatures above 600 °C (e.g. pyrolysis, partial oxidation, steam reforming). In pyrolysis, high temperatures are required to provide sufficiently rapid kinetics. However, under such conditions, an appreciable fraction of the feed is converted to char, which complicates materials handling and decreases yield of fluid fuels.

Approximately 40% of the wood harvested in the US today is for pulp and paper production.1 This high grade use will certainly continue in the years to come. Thus our approaches to biomass utilization have emphasized systems which complement cellulose production and incorporate schemes for total utilization of the woody stem. This means more efficient utilization of wood industry wastes and byproducts. Our plan is oriented to both the energy and materials crises.

In order to avoid problems caused by solids handling, char formation and tar formation during the gasification of wood and cellulosic materials, the use of a low temperature molten salt system was investigated. This approach was adopted in order to achieve dissolution of the biomass material in the salt prior to pyrolysis, thus enabling the pyrolysis to be carried out in the liquid phase throughout.

Several thermochemical methods for converting biomass into chemicals, fuels and synthetic gases are currently under development. Various reactor configurations, catalysts and reaction conditions are employed depending primarily upon the desired product. Steam gasification is one method being developed for the production of synthetic gases from biomass.

The precarious nature of the energy supply has encouraged research on renewable sources of energy. The conversion of biomass and organic wastes to gaseous and liqud fuels shows good potential. A Biomass Conversion Program was initiated at Brookhaven National Laboratory early in 1981 with the main objective of obtaining process chemistry and design information on the production of liquid and gaseous fuels from wood via flash pyrolysis. Flash pyrolysis is a fast, gas-phase decomposition of carbonaceous materials over a wide range of process conditions in the presence of an inert gas.

The biomass or commonly available lignocellulosic materials could be converted to different types of fuel and chemical feedstock by a variety of thermochemical processes. Each of these processes involves two highly significant and inter-related general aspects: first, material and energy transformations that could be explored and understood through the discipline of chemistry; second, material transport and heat transfer that could be investigated and designed through the disciplines of process engineering. Variations of the reaction conditions and the processing design, which are closely related, are often investigated in a sporadic or empirical manner in order to define the optimum conditions that provide high yields of operational efficiency.

The potential for energy and chemicals production from renewable resources has re-established interest in thermal conversion of biomass. If the devolatilization of a single wood particle can be predicted accurately and optimized, process design and scale-up of biomass conversion processes can benefit from the methodology developed for heterogeneous catalysis(e.g. ref. 1). Although using small particles might be advantageous for certain gasification processes, size reduction of fibrous wood chips is difficult and expensive.2 Therefore, large particles or a distribution of particle sizes will likely be employed. Knowledge of the effects of particle size, particle thermal properties and wood anisotropy on pyrolysis product distribution and volatiles release rate will be important in preparing an optimal biomass feedstock for any thermal conversion process.

Pyrolysis is a reaction common to all thermochemical conversion reactions of biomass. When conducted under reduced pressure conditions, the thermal decomposition reactions are of fundamental interest since this permits one to get an insight into the primary chemical reactions which occur during the initial breakdown of the biomass macromolecules.

The profitable conversion of lignin to low-molecular-weight products is hindered by the complexity of both its chemical structure and also its conversion product spectra. These complexities not only mask the actual fundamental events occurring during lignin processing but render design, scale-up and optimization difficult. Each of these tasks and also potential catalyst screening and future research guidance would be facilitated by the development of a fundamentals-based conversion simulation model. The present communication reports on our attempt at such a description of Kraft lignin pyrolysis reactions.

Gasification of wood and related biomass fuels provides a means of converting these renewable resources to an energy form that can be stored, transported and burned more cleanly and efficiently. The availability of biomass materials, particularly in industries such as the food processing and forest products industries, and their lower nitrogen, sulfur and ash content compared to conventional fuels such as coal, offer distinct advantages for the use of biomass as gasification substrates. Furthermore, gasification can be used in combination with other thermal processes such as pyrolytic saccharification of wood1 to recover the fuel value of the energy-rich char which is a byproduct of this process.

Many previous studies on biomass pyrolysis, including several kinetic investigations, are reviewed by: Molton and Demmitt,1 Lewellen et al.2 and Hajaligol3 (cellulose); Allan and Mattila4 and Klein5 (lignin); Roberts6 and Wenzl7 (wood); and Milne8 and Antal9 (different forms of biomass). Much remains to be learned about the quantitative effects of temperature, heating rate, solids residence time, volatiles residence time, pressure, gaseous atmosphere, sample dimension and biomass type on product yields, compositions and rates of production. Improved understanding of the role of primary decomposition and volatiles secondary reactions in pyrolysis is also needed. Better information in these areas would advance basic understanding of biomass thermal conversion pathways, and facilitate development of models of the thermochemical conversion and combustion of biomass.2

For the utilization of solid fuels at a low coalification stage, such as peat and biomass, processes are in development which in a number of cases involve rapid heating, short solids residence times and high pressure. For the simulation of the above process conditions in the laboratory different types of microreactors have been developed.1,2

Lignin, a major component of biomass found in all vascular plants, is an important potential source of fuels and chemicals. Pyrolysis of lignin is important in itself for the potential production of tars and liquids which can contain valuable chemicals. In addition, understanding pyrolysis has a broad importance based on the assumption that thermal decomposition is an important initial step in the combustion, hydrocracking or donor-solvent liquefaction of lignin, just as in the case of coal.

The thermal decomposition behavior of biomass, and in particular cellulose, has been the subject of experimental investigations for a considerable time. Most of the early work, and current pertinent publications, examine pyrolysis of biomass with respect to the propagation or retardation of flames. A great deal of useful kinetic information has thus resulted; but the models and rate constants so obtained primarily describe the behavior of cellulose at conditions of temperature and pressure less severe and gas compositions much simpler than are likely to exist in a gasifier. Hence, there is a need to obtain kinetic information which describes the behavior of the entire biomass sample when it is subjected to temperatures and pressures conducive to the production of usable gaseous fuels such as methane and syngas mixtures.

One of the important gasification research needs identified in our comprehensive survey of biomass gasification1 is a more detailed understanding of the molecular processes and kinetics involved in the pyrolysis of biomass and its constituents. Of particular interest is the fast pyrolysis of biomass, which can accomplish nearly charless gasification to valuable intermediates.2 Improved understanding of the details of fast pyrolysis requires a versatile sampling and analytical technique. Such a technique should allow observations of gases, condensible species and reactive intermediates in realtime, sampled from realistic laboratory fast-pyrolysis reactors. Bench-scale studies3,4 indicate that millisecond processes are occurring and that tars and oils are likely precursors to the final product slate of lighter hydrocarbons.

The rates for decomposition of both cellulose and wood (Douglas fir sawdust) have been examined by monitoring the transient formation of gaseous species. Since no significant difference was detected in the rate constants as determined by monitoring separately all major gaseous products,1 recent work has concentrated on measurement of only carbon monoxide, which is one of the principal gaseous reaction products over the temperature range studied. The rates obtained for cellulose decomposition by measuring carbon monoxide compare favorably with those determined by Min2 when measuring total volatile formation. Weight loss data, obtained by Bradbury et al.3 under vacuum conditions, showed faster reaction rates, indicating that pressure (in this case atmospheric) may suppress the volatiles forming reaction.

Some of the best features of conventional pyrolysis and air/oxygen gasification can be incorporated into the development of a successful fast pyrolysis process without many of the associated disadvantages. Conventional biomass pyrolysis and flash pyrolysis research, which tend to maximize the production of multi-component condensibles, has clearly illustrated the need to characterize and upgrade the liquid products, and to identify viable industrial applications prior to commercial development.

Standard methods of wood analyses frequently require that the material be reduced (comminuted) to a specified size range prior to testing. It is recommended that a particle size smaller than 250/μm be utilized in determinations of gross heat of combustion by adiabatic oxygen bomb calorimetry.1 Wood and bark calorific values have been determined using various particle sizes.2–4 In most cases, the materials are oven-dried before testing.

Finland has considerable peat and wood resources which are extensively used as fuels and there is a need to investigate the pyrolysis of those fuels. Boilers are not perfect and their efficiency can probably be improved. Burning profiles of fuels provide information for that aim.1

Reactions of carbohydrates form practically a separate branch of organic chemistry, but the study of the pyrolysis of carbohydrates was neglected for a long time and only a few research groups have been pursuing this research area. Increasing oil prices, however, have created more interest in alternative energy resources. Carbohydrate based biomasses have the best potential to supplement our energy and chemical feedstuff need which is presently derived from fossil fuel. During the past few years, considerable efforts were made on the thermal reactions of various biomasses. Unfortunately, most of the attempts were directed toward developing reactors for special biomasses and not for the basic reactions of those carbohydrates which are the main components of biomasses. For this reason, our knowledge is quite limited on the various simultaneous and consecutive reactions involved in carbohydrate pyrolysis even for such simple compounds as hexoses and pentoses. This paper reports the first step of a mathematical approach to develop more information about these reactions.

The disposal of rice crop residues has received increasing attention over a number of years from those both inside and outside the field of agriculture. The residues present a growing and costly management problem as grain yields increased by improved technology are accompanied by increasing straw yields. The traditional method of economical disposal, namely burning, faces growing public opposition and legal regulation, as air pollutant levels from many sources increase and state and local controls are established. To continue utilizing this practice, cleaner burning techniques must be developed. However, to alter the rice straw’s burning mode effectively a better understanding must be obtained of its overall thermal degradation mechanism. Although cellulose, one of the components of rice straw, has been studied extensively by numerous researchers, 1 – 13 rice straw itself has not been given the same consideration.

The production of mixtures of carbon oxides and hydrogen from biomass is attractive since such ‘synthesis gas’ can be converted subsequently to a range of relatively high-grade fuels, such as methanol and methane, by established chemical process routes. Over the last decade, much research on the thermal processing of different types of biomass has been carried out and reports in which this work was summarized and reviewed have been prepared for the Energy Technology Support Unit of the UK Department of Energy by Ader Associates and staff at the University of Aston.1 Following publication of these reports a programme of experimental investigations and design studies was initiated: the overall objectives of this programme were (a) to determine for specific biomass feedstocks the optimum conditions for their gasification to fuels and (b) to select and design reactor systems for assessment in a 40 kg/h test facility designed, constructed and operated by Foster Wheeler Power Products Ltd, UK. This programme has been running since the beginning of 1980 and the following sections of the paper describe some of the concepts and methods used by the Aston research team.

There is increasing interest in the conversion of coal and biomass to gaseous fuels through the use of both high temperature and low temperature gasification processes. In gasification at high temperatures, the exothermic C-0₂ reaction supplies the heat to drive the endothermic C-H₂0 reaction and CO and H₂ are the primary fuel gases produced. Gasification occurs under conditions (Zone b or III in Fig. 1) where reaction rates are limited in part or completely by mass transport of reacting gas to carbon sites. Concentration of carbon active sites or the presence of gasification catalysts are not of primary importance in determining gasification rates. The extent of utilization (η) of active sites is small; thus the apparent activation energy for carbon gasification (E_a is also small. In the limit, gasification rates are independent of starting organic precursor.

Upon heating, biomass materials undergo solid phase pyrolysis at relatively low temperatures (> 300 ºC), forming reactive volatile matter, a few permanent gas species and solid char. Unlike the various coals and peats, biomass materials typically lose 70% or more of their weight by the solid phase pyrolysis reactions. This transformation of the bulk of the biomass material from the solid to the vapor phase suggests the important role of vapor phase chemistry in the pyrolysis of biomass materials. Recognizing the highly reactive nature of the major constituents of the volatile matter, the significance of the vapor phase chemistry becomes even more apparent.

The heating of biomass in an oxygen-deficient situation is known as pyrolysis and has been used for centuries to produce charcoal, tars, wood alcohol and other solvents. The traditional slow heating of biomass produces about equal amounts of gases, char and tarry liquids. These tarry liquids have been promoted as boiler fuels, but they are not thought to be suitable for use in internal combustion (IC) engines. Current state-of-the-art dictates that the liquid fuels used for IC engines be either a low boiling hydrocarbon material, an alcohol, or a mixture of the two.

The gasification of wood chars with CO₂ and steam is an important process step in the conversion of biomass to fuel and synthesis gases. Wood fuels can be gasified in a wide variety of sizes, shapes and densities. Commonly, chips and pellets are suitable for fixed and fluidized beds while sawdust is preferred for entrained units. In terms of fuel preparation, handling and feeding, whole tree chips and pelletized sawdust are most desirable: therefore fixed and fluid-beds are widely used with wood fuels.

Biomass—forestry and agricultural residues—can locally be directly utilized for combustion. However, conversion of these solid materials to gaseous products is of great importance to make transport of energy practicable.

Direct use of biomass as a fuel offers a limited field of applications. Conversion of biomass to gases and liquids is of great importance to make transport of energy cheaper, gases and liquids being more easily handled than solid forestry and agricultural residues. Moreover, this conversion makes the utilization of biomass energy possible in the transport sector.

The use of catalysts in biomass gasification has been suggested for a long time.1 Fung and Graham found that potassium carbonate and calcium oxide have catalytic influences on the gasification rate and the product yield.2 This article does not, however, deal with catalysts for the primary gasification. The objective has been to find catalysts for refinement and upgrading of the gas already produced.

Gasification is a complex, but versatile process. There is a wide variety in the physical and chemical processes involved, the possible reactor types and operating conditions, and the potential feedstock materials, with their different structure, properties and behaviour. A generalized treatment of the process is therefore precluded.

Since pyrolysis is the first step in the gasification process of a solid material, rates and temperatures at which pyrolysis occur together with pyrolysis product yields are critical parameters in the design of a gasification process.

The world oil crisis stimulated interest in alternative sources of energy for industry, and the gasification of biomass to produce a clean fuel gas is gaining support as an attractive alternative in particular situations. In Canada several types of gasifiers at different stages of development, ranging from pilot scale to prototype units, are being developed by industry with government assistance.

In the gasification of biomasses for production of methanol it is desirable to reduce the contents of methane and other higher hydrocarbons in the gas.

Since the oil crisis of 1973 there has been an ever-increasing interest in biomass, wood and peat for the production of gas (synthetic gas and fuel gas).

To contribute toward increasing combustion intensity in fireboxes for particulate materials, the effects of several parameters on burning time of single wood cubes were investigated in 1978 and reported in an earlier paper.3 That paper discussed the effect of size, moisture content, furnace wall temperature and air temperature on burning time of small wood cubes.

Rising energy costs have prompted many industries, and particularly wood-related industries, to examine biomass fuel sources as replacements for oil and gas. Several studies1–4 have indicated a considerable resource in unused wood and wood waste. One factor limiting the efficient use of this resource is a lack of basic combustion data. This limitation has made industrial boiler design empirical and subject to much guesswork. Trajectory analysis of burning fuel particles in the furnace or boiler should be an important design tool; however, much of the basic data have not been available. The average fuel particle size is on the order of centimeters but may range as high as 0–3 m and as small as 100 μm.3 Fuel particle moisture content may also vary widely. Hence, one can see that, without knowledge of mass burning, heat release rate and burn-out times under specific operating conditions, it is very difficult to size and design an efficient boiler.

The problem of fuel selection is becoming a more difficult one for the combustion engineer, primarily because of the rapidly increasing number and variety of new fuels available.1 These manifold new fuels are being identified or developed in response to the dual world problem of energy availability and maintenance of environmental air quality.1,2 Nations around the world will place an increased reliance on the utilization of biomass resources, particularly wood, to partially meet short term energy demands. Although the increased usage of virgin wood or wood wastes seems feasible, detailed combustion performance characteristics of pulverized wood (ignition and burnout times; pollutant emissions) as a function of fuel properties (species; moisture; particle size) and combustor operating conditions (preheat; excess air) are virtually unknown. Development of second-generation technology is necessary for increased wood utilization. Combustion research is required to supply well-defined data on biomass flames, and a fundamental understanding of pulverized- fuel combustion.

Substantial attention has been focused recently on energy production from biomass and synthetic biomass-based fuels. Several studies report in detail the pyrolysis of cellulose/lignin fuels; for example, see ref. 1. However, little detailed study has been directed to the reaction of biomass pyrolysis products, or synthetic biomass-based fuels in combustion processes. In the present research, model compounds typical of the products of wood devolatilization, or pyrolysis, are burned under well-controlled, well- aerated conditions in a laboratory stirred reactor. The intermediate condensed and gas phase products are analyzed, and the amount of material remaining unburned is determined as a function of increasing temperature. Limited results are obtained for combustion of wood pyrolysis gas, which is supplied to the stirred reactor from a separate ‘stick’ pyrolyzer.

In Sweden, a country without any substantial coal resources, there is an interest in using peat and biomass as raw materials for fuel conversion processes.1,2 The present investigation, which is sponsored by the National Swedish Board for Energy Source Development, was undertaken to obtain data for comparison of coal, peat and biomass in such direct liquefaction processes where the raw material is heated and liquefied in a slurrying vehicle.

Research directed to the combustion of peat pellets is not plentiful, because the idea of processing peat to pellets is quite new. In this work, an attempt was made to determine the most important flame characteristics of pellet burners in order to obtain some information about the combustion of pellets.

Catalysts play a major role in conventional hydrocarbon processing but only recently have they been applied to conversion of biomass materials. Pacific Northwest Laboratory (PNL) is studying catalytic steam gasification of biomass for producing specific gas products. These studies are sponsored by the Biomass Energy Systems Division of the US Department of Energy.

A project has been under development at Arizona State University (ASU) since 1975 with the goal of producing quality liquid hydrocarbon fuels from cellulosic and waste polymer materials. An indirect liquefaction approach is used, i. e. gasification to synthesis gas followed by liquefaction of the synthesis gas. The primary virtue of an indirect liquefaction approach for cellulosic type feedstocks is that the oxygen contained in the materials is easily separated. Thus the hydrocarbon liquid is free of oxygenated compounds and can therefore be tailored to match transportation fuel products currently derived from petroleum (e. g. diesel, jet fuels, high octane gasoline).

Over the past decade, a number of studies have been conducted on the fluidized bed gasification of biomass materials. Several factors influence gasifier performance, the most important being temperature. Several investigators have noted that different types of feed materials produce different results when gasified under apparently similar condition.1–5 Some of these investigators have postulated that the differences observed are due to differences in the cellulose content of the feed material.

Fixed bed and moving bed gasifiers for the production of low Btu gas from wood and charcoal were widely used in Sweden and other countries during the World War II era. The Swedish experience was compiled by the Swedish Academy of Engineering, and this work was recently translated by Reed and Jantzen.1 After the war, the need for gasifiers dwindled although the Swedes continued their development efforts.

The methane yield in biomass gasification is always of great importance. The yield must be high in fuel gas production and it has to be small in synthesis gas production for methanol or ammonia. A fundamental understanding of the methane producing and consuming reactions are therefore a prerequisite for optimal design of biomass gasifiers. This article gives a new contribution to this understanding. Results are presented and discussed from an investigation where these reactions are studied over a broad range of pressures and where the influence of biomass char and ash has been eliminated.

Previous studies of the influence of catalysts on the thermal gasification of biomass have shown that alkali carbonates are among the most active catalysts for the gasification of biomass with steam.1–3 Most of these studies have investigated gasification at temperatures above 550 °C and have concentrated on gasification of the char formed after initial volatilization. When considering single reactor production of high-Btu gas from biomass, or any other organic substrate, one is faced with the contrasting reaction conditions favoring gasification and methane formation. The basic dilemma is the choice between a high temperature, low pressure reaction system which favors the breakdown of biomass to gases, and a low temperature, high pressure system which favors methane formation. In this article, we report laboratory investigations of the influence of sodium carbonate in the presence of a supported nickel metal catalyst with the intent of maximizing methane production from biomass at low temperature in a pressurized gasification system.

The controlled propagation of flames is an important foundation of our modern technology—from the humble gas flames in our stoves and the combustion of gasoline in our cars, to the powdered coal flames in our largest power plants.

The conversion of biomass materials into oils has a precise objective:

To transform a carbonaceous solid material which is originally difficult to handle, bulky and of low energy concentration, into pumpable oils having physico-chemical characteristics which permit storage, transferability through pumping systems, and further use either in direct combustion furnaces or as feedstocks for hydrotreatment leading to specific fuels and chemicals.

This paper describes the results of the analytical effort dealing with the products from the United States Department of Energy, Biomass Liquefaction Experimental Facility at Albany, Oregon. This facility, which was operated under contract to DOE by Wheelabrator Cleanfuel produced approximately 80 barrels of wood-derived oil in the period from August 1979 to March 1981.1,2 The oil was produced by two variations of the basic CO-steam process.3 One variation, the PERC process, involved recycle of a portion of the product so that a wood-flour-in-oil slurry system was used. The other variation, the LBL process, was a once-through process wherein the wood was prehydrolyzed and then processed in a water slurry mode. Both variations operated at about 340 °C and 3000 psig with aqueous sodium carbonate as the catalyst and CO/H₂ as the cover gas. Both processes were tested in a number of reactor configurations. The actual plant operation time is therefore broken into a number of test runs; products from runs 7 through 12 will be discussed in this paper. A detailed description of the equipment configuration and processing conditions for each test run is given in refs 1 and 2.

The work presented in this paper is a part of the Oil from Peat and Biomass project sponsored by the National Swedish Board for Energy Source Development.

Wood is converted to a dense liquid fuel by thermal treatment at 300–360 °C under a pressure of water and reducing gas in the presence of aqueous alkali. The product includes both an oil phase and an aqueous phase rich in dissolved organics. Previous work1 showed that the oil phase has little in common with petroleum fuels, and the standard methods of petroleum chemistry are therefore largely inapplicable. Thus there existed a need for new methods for the fractionation of biomass oils into constituent chemical classes for the purpose of assessing comparative product quality and evaluating the best end uses and upgrading options.

Considerable interest has developed over the last decade concerning renewable alternatives for petroleum-based fuels. A likely candidate for such renewable fuels may be produced by the liquefaction of biomass under aqueous, alkaline conditions.

In a period when fossil hydrocarbons are likely to become scarcer, methods to convert renewable biomass to useful fuels and chemical feedstocks are increasingly attractive. Such methods would permit the conversion of the very large solar energy reserves trapped as lignocellulosic materials into a form which would have the advantages of being initially sulfur-free and readily portable. All such biomass is composed of two vastly chemically different components: the carbohydrate-based holocellulose (cellulose and hemicelluloses) and the aromatic based lignin, which together have a much lower heating value than fossil fuel due to their high oxygen content. The production of a fuel compatible with natural oil must therefore involve significant oxygen removal. Any one such process may not be suitable for conversion of both the holocellulose and the lignin and an optimization of reaction conditions may be required.

Thermochemical liquefaction involves conversion of biomass at high temperatures and pressures to liquid products. These reactions occur in an aqueous or organic carrier liquid, often in the presence of other reactants and a catalyst. A wide range of lignocellulosic feedstocks are possible, and reactions are relatively rapid.

It is timely to describe the application of electrochemical methods for the transformation of organic compounds; the method is powerful and non-thermal and its possible advantages are being increasingly explored for the conversion of biomass-derived organic compounds. Specific problems concerned with the conversion of biomass will be discussed but first it is necessary to describe the reactive intermediates which result from electron transfer, to explain the link with biomass problems, and to outline the requirements for operation on a large scale.

One of the broad classes of renewable biomass materials of interest for conversion either to liquid fuels or to chemical feedstocks are the underutilized low-grade hardwoods such as aspen and other poplar species, southern hardwoods, etc.1 Such lignocellulosic materials consist of three major biopolymers—cellulose, hemicelluloses and lignins—and a small amount of extractives. The quantities of the various components are a function of the species.2 The chemical and physical properties of the individual polymers, mainly the lignin fraction, are such that their response to chemical or microbiological treatments is variable. In order to simplify the conversion of biomass to chemicals and fuels, the separation (pretreatment) of the major polymeric species is necessary, except where thermal processes such as gasification or pyrolysis are concerned. The separation of the lignin component from wood in the manufacture of paper has long been known and practised in industry4 by a variety of methods. In the context of the conversion of biomass into fuels and chemicals, a number of pretreatments have been considered.3,5 Such treatments can lead to low-molecular-weight cellulosic and hemicellulosic polymers that can be more easily converted into alcohols or other chemicals by chemical or microbiological processes.

Lignin is a three-dimensional, randomly linked macromolecule comprising methoxylated phenylpropane units. While lignins from grasses, softwoods and hardwoods differ in terms of the level of phenyl group linkages and degree of methoxylation, the principal structural features involve ether linkages α and β to the phenylpropane aromatic ring. These C-O bonds comprise approximately 65% of monomer-monomer bonding and include alkyl-O-aryl bonds linking phenylpropane units, alicyclic ethers of the tetrahydrofuran type and diaryl-ethers.1 By selectively cracking these bonds one could fragment lignin into mono- and diaryl phenols which could serve as an alternative source of phenolic feedstocks or high-value speciality chemicals.

An important event of the Estes Park meeting was the first attempt to arrive at a specification for standard biomass materials for thermochemical research. Prior to the conference, the National Research Council of Canada retained a wood chemist, Mr George Barton, to survey a number of institutions and their researchers (listed in Appendix A) as to their viewpoints on the need for what could loosely be described as ‘standard wood’. The various standards of TAPPI and ASTM were reviewed and along with the survey results were used as a foundation for a round table panel presentation and discussion.

In recent years there has been a growing interest in using wood, bark and foliage for chemical biomass conversion studies.1 The reasons are understandable since forests represent one of the largest sources of renewable biomass still available to mankind. Also, the Forest Product Industries provide a constant, collected source of potential thermochemical conversion material such as tops, limbs, bark and foliage not required for lumber or pulp. This potential will increase dramatically if plans to introduce whole tree logging materialize. Unfortunately, many scientists have been attracted to this potential biomass who are unfamiliar with the wide variation among and between tree species. To many wood is wood and they make little attempt to define the sample on which valuable scientific research is done. Borrowing a sentence from the Basic Coal Sciences Project Advisory Report2 and substituting wood for coal, the following statement emerges and describes the current situation concisely: ‘Considerable basic research has been done on a wide range of wood samples for various purposes, yet much of this previous research cannot be correlated since little, if any comparisons can be drawn from the samples used’. The Estes Park conference unanimously endorsed the need for reviewers and authors to be aware of tree variability and to define all research samples in such a way that other scientists may be able to calibrate and correlate their own research results.