CURRENT DEVELOPMENTS IN THE CONVERSION OF WOOD TO LIQUID FUELS J. I. ZERBE

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1 CURRENT DEVELOPMENTS IN THE CONVERSION OF WOOD TO LIQUID FUELS J. I. ZERBE B. Sc. (Pennsylvania State U.) M. Sc. & Ph.D (New York State College of Environmental Science & Forestry) Program Manager Forest Products Laboratory, Madison, Wisconsin Synopsis This report covers the possibilities for making liquid fuels from wood with particular emphasis on a two-stage process for producing ethanol. The process is dependent on dilute acid hydrolysis. Other processes and other liquid fuels are described, including methanol derived from wood by gasification. Zerbe, J. I. Current developments in the conversion of wood to liquid fuels. In: Symposium on Forest Products Research International--Achievements and the Future; Vol April 22-26; Pretoria, Republic of South Africa. South African Council for Scientific and Industrial Research, National Timber Research Institute; 1985: 10p

2 1 INTRODUCTION Today, with abundant supplies of oil and fairly stable energy prices worldwide, liquid fuels from wood are not the popular discussion topics as they were in 1973 and 1979, when world oil supplies were curtailed. Nonetheless, much progress in technology for developing liquid fuels from wood was made during the last 12 years, and the potential for further progress through research is promising. Fossil fuel supplies are finite and exhaustible. Although supplies of coal in the U. S. are predicted to last for hundreds of years, the longrange prospects for oil are much worse. And the trend is certainly for escalation in oil costs relative to other fuels. Therefore, liquid fuels from wood may relieve some of the stress caused by dwindling petroleum supplies. WOOD RESOURCE FOR ENERGY Wood could make the best possible contribution to diminishing oil supplies by substituting for oil where oil is now used In direct burning applications such as boiler fuel. Estimates indicate that in the United States about 546 million dry metric tonnes (600 million dry tons) of wood in excess of the drain for consumer products are available for use now. Using just half of these residues for fuel could supply seven percent of our nation s current energy consumption; the equivalent of about 900 million barrels of oil. However, the future shortfall in oil will be felt most acutely in high costs of liquid vehicular fuels. in this case, liquid fuels derived from wood are the most satisfactory oil substitutes, provided the conversion of solid wood to liquid fuel is efficient and economical. The impact of liquid fuel from wood could be greater with more emphasis on production of wood for chemical feedstocks for making the fuel. Short-rotation intensive culture is being considered as an alternative for increasing production of wood for energy. The U.S. Department of Energy has a major effort underway to produce high yields of tree biomass in short rotations. At the University of Washington a hybrid cottonwood produced about 22.5 dry tonnes per hectare-year (10.0 dry tons per acre-year) over a 3-year period--more than double normal productivity. Even higher productivity rates are being achieved in Florida and Hawaii with species of eucalyptus. Further studies are needed as basic biotechnology advances from agriculture into forestry. The incorporation of genetic-engineeringadvanced propagation techniques into systems with effective herbicides, fertilizer, and irrigation holds promise for great increases in yield in many regions of the United States and the world. Even so, for most parts of the United States, we have found that short-rotation energy plantations are not economical at today s prices. However, conditions in Hawaii could lead to developments that may change this statement. There, sugar growers use bagasse in boilers to provide steam for electrical generation. The electricity produced, in part, is sold to the local utility. At this time, demand for sugar is low and not enough bagasse is available for power generation. Excess sugar-growing land is

3 2 bring used on a pilot basis to grow fast-growing eucalyptus to produce fuel to supplement bagasse and generate the needed electricity. Currently in the U.S., for most areas sufficient residue wood exists that a switch to more expensive wood from intensive culture is unattractive and unlikely for the next few decades. Cull wood and waste wood together with growth of commercial wood in excess of that harvested amounts to about 4 times as much wood as that which is used. The figures suggest that more wood could be used for less demanding purposes such as alcohol or other liquid fuels without depleting the resource or reducing the amount or quality of the wood needed for the forest products industry. Actually, forest quality could be improved through relief of crowding and competition for soil nutrients and sunlight by removal of noncommercial growths. ETHANOL The leading contenders for liquid fuels from wood are alcohols, principally ethanol and methanol. Fuel ethanol has been and is now being made from wood. However, subsidies are generally necessary to enable this fuel to compete with gasoline. Seventy years ago two plants made ethanol from wood in the U.S. using a dilute sulfuric acid hydrolysis process. But these plants closed because supplies of wood residues decreased and their costs increased. In addition, the cost of black strap molasses, the main competitor as a feedstock for alcohol production, decreased. (Baker, 1981). Forty to fifty years ago, other ethanol from wood plants were built in different countries. In the U.S., the War Production Board constructed a dilute sulfuric acid wood hydrolysis plant for the production of ethanol at Springfield, OR. The plant was completed under sponsorship of the U.S. Department of Agriculture in Production trials indicated that, with modifications, the plant could operate profitably. However, two different companies that leased the plant were unable to operate the plant successfully. A sulfite pulpmill operator in Bellingham, WA, also constructed a plant at that time for the production of ethanol by fermentation of sugar obtained as a byproduct from pulp manufacture. In this plant, which together with other similar plants around the world is still operating, the spent-sulfite pulping liquor is treated to remove sulfur dioxide and lignosulfonate byproducts prior to fermentation. In this process, 95 liters (25 gallons) of 95 percent ethanol are produced per ton of sulfite pulp. Other than fermentation of sugars from sulfite waste liquor, the process that has been most successful for conversion of wood to ethanol is dilute acid hydrolysis. Such planes operated in Germany and Switzerland during World War II, and some such plants are still operating in the Soviet Union. In the December 1984 World Wood, it is reported that a new wood hydrolysis plant in a wood processing complex has been built in eastern Siberia. Another type of plant is concentrated acid hydrolysis. One of these plants also operated in Germany. It was less competitive than dilute acid hydrolysis because it required the use of much more acid, and the

4 3 acid could not be recovered effectively. An advantage of concentrated acid over dilute acid hydrolysis is that cellulose from wood could be hydrolyzed to glucose, needed for fermentation to alcohol, in much higher yields. More recently there has been developmental work on a new approach to cellulose hydrolysis with enzymes. This emanated from studies by the U.S. Army into the nature of biological cellulose degradation end the means of protecting cellulosic materials from decay in tropical regions. Over 14,000 fungi active in degrading cellulose, wool, leather, and other materials were accumulated and studied. This led to selection of an enzyme based on an organism, Trichoderma, that is effective in hydrolyzing cellulose. It is used in processes proposed for commercialization that can convert cellulose to glucose in high yields. Disadvantages are that wood must be pretreated before the enzyme can act on the cellulose, and the cost of the enzyme is high. There are pilot plants and commercial plants operating or under consideration for these three types of processes today; but, dilute acid hydrolysis is most highly proven, and, probably, best suited for implementation in a full-scale plant. Several types of dilute acid hydrolysis processes have been used. The original process in the U.S. was a single-stage batch type. It produced 69 liters of ethanol per tonne (20 gallons per ton) of dry wood. Later, a percolation process, known as the Scholler process, was developed in Germany. In the 1940's, this process was modified by the Forest Products Laboratory and the Madison process (Figure 1) was developed. The Madison process proceeded more rapidly than the Scholler process because it removed the sugars produced by the hydrolysis more rapidly. The Madison process produced 222 liters of 95 percent ethanol from a tonne (64.5 gallons per ton) of dry bark-free Douglas-fir wood waste. This process is similar to the one used in plants that are operating in the Soviet Union; and the technology has recently been used in the construction of a new alcohol-from-wood plant in Brazil. More recent research at the Forest Products Laboratory is aimed at an improved dilute acid hydrolysis process for use with softwoods. In 1980 we began work on a two-stage hydrolysis process first prepared by the Stora Kopparberg Company in Sweden about 1944 (Cederquist, 1954). Hemicellulose is hydrolyzed in the first stage and the more refractory cellulose is hydrolyzed in the second stage. The two-stage process is particularly appropriate for hardwoods, since it facilitate recovering both 5-and 6-carbon sugars from wood particles. Single stage processes concentrate mainly on 6-carbon sugars, but the 5-carbon sugars are a significant fraction in hardwoods. They may be used as raw material for valuable chemical products or animal feeds. We have just completed a major report for the research on the two-stage process, We believe that it, as well as the percolation-process, can be considered reasonably near the point of commercial exploitation. Originally, we undertook the two-stage process work on the supposition that many of the disadvantages inherent in the percolation process could be overcome. This has proved to be true, but to a lesser extent than we

5 4 hoped. A significant advantage of the two-stage process is the high concentration of the sugar product solutions. If suitable processing equipment can be developed, the concentrations of sugars from the two-stage process are at least double those obtained from percolation. The energy consumption and equipment size for subsequent processing steps are roughly halved by two-stage processing. However, the capital requirements for the hydrolysis itself are not greatly reduced. In the percolation process, the entire operation is done in a single vessel. In two-stage operation, two reactors and two sets of washing equipment are needed. The net difference for both equipment and energy cost for hydrolysis is only slightly in favor of the two-stage process. For ethanol production, the energy requirements of two-stage operation should be about 40% less than the percolation process, and equipment cost, about 25% less. The purity of the solutions generated by the two-stage process is considerably better than obtained by percolation. The advantages of the two-stage process are offset to some extent by the higher yields of the percolation process. The yield advantage depends both on the type of wood being considered and the subsequent processing of the solution. Unfortunately, available data on the percolation process relate primarily to softwoods, and yields are given either as total sugars obtained or as recovered ethanol. No data are available on the changing concentration and composition of the product solution. The carbohydrate yields from each of the component fractions of the wood-- hemicellulose and cellulose--are unknown. Because of this, the yield comparisons given below are uncertain estimates, but some points for consideration are brought out. The southern red oak used in this study contains 37.8% anhydroglucose and 18.4% anhydroxylose. On hydrolysis, these yield 42.0% glucose and 20.9% xylose, a total of 62.9% potential sugars, on the basis of the wood, Complete conversion to ethanol would result in 420 liters of 190-proof ethanol/tonne of wood 280 liters from glucose and 140 liters from the xylose. When only glucose is considered to be fermentable to ethanol, the ethanol produced in the two-stage process is 8.70 kg/100 kg of process wood, which is equivalent to 114 liters/tonne. The ethanol yield Is 40.7% of that theoretically available from the potential glucose of the wood. By the percolation process, the estimated ethanol production is 124 liters/tonne, about 9% greater than that obtained by two-stage processing. The yield by percolation Is 44% of that theoretically available. The efficiency of saccharification in percolation is actually much greater than this, probably about 55-60%, but much of the glucose produced accompanies the hemicellulose stream and could not be economically processed to ethanol. The percolation method, operating on hardwood and producing ethanol from the glucose-rich fraction only, would have a yield advantage over two-stage processing. If it is assumed that xylose as well as glucose can be fermented to ethanol with equal efficiency, the percolation process has a much greater yield advantage. This is because the xylose-fermenting organism would also utilize glucose, and the poor separation of the sugars is inconsequential. Using this assumption, the percolation-process yield

6 5 is estimated at 267 liters/tonne and two-stage operation 234 liters/tonne, a 14% yield advantage for percolation. Yields for two-stage processing are considerably higher if it is assumed that the reversion material formed from glucose can be converted to ethanol. If this were the case, 133 liters/tonne could be obtained solely from the glucose fraction. This increase of 19 liters/tonne from the oligomers reverses the ranking and gives two-stage processing a 7.3% advantage under the supposition that xylose is not fermentable to ethanol. If it is assumed that all carbohydrates (xylose, glucose, and reversion products) are fermentable, the percolation process has a 5.5% greater ethanol yield. The Tennessee Valley Authority is now adopting the research on the two-stage process to the design and construction of a pilot plant (Fig. 2). It is anticipated that they will be making sugars from first-stage prehydrolysis some time during the summer of The costs of production of alcohol by dilute acid hydrolysis are difficult to estimate. Ultimately in an actual plant the costs of alcohol production will likely be reduced through production and sale of byproducts. Because of different ways of handling byproduct credits, and assumption of different feedstock costs, the estimated costs of ethanol made from wood very widely. I believe it is reasonable to assume $0.40 to $0.70 per liter of ethanol. METHANOL Since this overview program is on gasification, methanol production from wood has a closer tie-in than ethanol. The most feasible route per production of methanol from wood is through gasification (Figure 3). Although ethanol has been the preferred fuel for blending with gasoline, in the U.S. some methanol is used to accomplish octane enhancement because it is less expensive than ethanol. If methanol is used above several percentage points in gasoline, cosolvents such as methyl-tertiary-butyl ether, tertiary butanol, or ethanol are required. Another approach is to use methanol as a neat fuel. Some fleets of cars In the U.S. are operating solely on methanol, and much research on the use of methanol unmixed with other fuels has been conducted in Germany. Natural gas is the preferred starting material for methanol, but as with petroleum, shortages of natural gas and significant increases in natural gas prices are constant threats. Much consideration has also been given to coal as a feedstock for methanol. Wood has some distinct advantages over coal as a source of methanol. Because wood doesn't have significant amounts of sulfur or heavy metals, is easy to pyrolyze, and has a low ash content, it can be shown that for plants of the same capacity it is cheaper to make methanol from wood than from coal. A case could also be made to favor wood over coal. because of environmental and safety concerns. On the other hand coal has advantages of being available in many places in high concentrations at low moisture content and high energy density.

7 6 The main advantage always claimed for coal is economy of scale. Thus, a plant using 20,000 tonnes of coal per day would be more economical on a per liter of methanol basis than a plant using 1,000 tonnes of wood per day. With coal concentrated in large deposits, and supply of a methanol plant close to a mine site, a capacity of 20,000 tonnes per day should be possible, but supplying the equivalent in wood per day would be extremely difficult, if not impossible. However, investment in such a large coal-to-methanol plant would require so large an investment that a private company would be unlikely to risk putting it into operation. No wood gas to methanol plants have been built, but plants have been proposed in Canada and Brazil. There is at least one coal to methanol plant in the U.S. It is at Kingsport, Tenn. A key to improving the chances for making methanol from wood competitively is to improve the efficiency of gasification, i.e., increase CO and H contents of the gas 2 produced and thereby reduce CO 2 and hydrocarbons (Rowell, 1977). PYROLYSIS AND HYDROGENATION Other approaches to liquid fuels from wood are production of oils by pyrolysis or hydrogenation and extraction of natural oils and resins from trees. Processes for production of oils by pyrolysis have been developed by Georgia Institute of Technology, Battelle Northwest, Texas A&M University, and others. However, only pilot-scale plants have been built and operated. For several years, the U.S. Department of Energy built and operated an experimental plant in Albany, Oregon, to manufacture oil from wood chips and other biomass materials through a hydrogenation process. The process is difficult to operate, because high pressures (about kpa) are required. EXTRACTIVES Plants sometimes produce more highly reduced natural substances with properties like hydrocarbons. Research by the Forest Service, USDA, has shown that the application of the herbicide paraquat to certain pine species increases the production of oleoresins by a factor of ten. Turpentine from such oleoresins was used in Japan as a base for gasoline. It has higher energy density than gasoline (Reed, 1976). CONCLUSION I believe the most likely wood-to-liquid fuel plant for competing In today's market would be a dilute acid hydrolysis plant of a scale to use 1000 dry tonnes of wood per day to produce 100 million liters of ethanol per year. However, in the U.S. and most other countries, it would be impossible for such a plant to compete without subsidy. In the U.S. about 1500 million liters of biomass ethanol are produced annually for blending with gasoline at the raio of 1 part ethanol to 9 parts gasoline.

8 7 This is mostly from maize with high subsidies by the Federal Government and as high or higher subsidies from States in which the blended gasoline is sold. An equivalent amount of alcohol might be produced from wood, and the same subsidies would apply. Baker, Andrew, J.; Jeffries, Thomas W. Status of wood hydrolysis for ethanol production. Unpublished report to: USDA, FS for U.S. AID, Development Support Bureau, Office of Energy (TMR Authorization No ); 1981: 66 p. Hokanson, A. E.; Rowell, R. M. Methanol from wood waste: A technical and economic study. Gen. Tech. Rep. FPL 12. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory; p.

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10 Current Developments in the Conversion ofwood to Liquid Fuels 9

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Ethanol From Cellulose: A General Review

Reprinted from: Trends in new crops and new uses. 2002. J. Janick and A. Whipkey (eds.). ASHS Press, Alexandria, VA. Ethanol From Cellulose: A General Review P.C. Badger INTRODUCTION The use of ethanol