Biomass Refining of Scrap Wood for Fuel Ethanol and Byproducts
Wood2Chem has melded new and old technology for a process that should be highly profitable for converting scrap
wood to fuel alcohol and/or industrial chemicals. Grandiose ideas about solving the U.S. need for transportation fuels
and the appalling unfavorable trade balance that it causes are not our immediate concern. Our analysis shows that a
focus on only fuel alcohol requires high prices and/or government subsidies. However, a modest program that includes
valuable byproducts as well as fuel ethanol appears to be a wonderful investment opportunity without any special
legislation, tax breaks, or government hand outs. The initial factory will use simple, established technology based on
steam explosion and hydrolysis with dilute sulfuric acid. Ultimately, commercial wood chips can be the main
feedstock because the coproducts from lignin and hemicellulose of wood will be major additional sources of revenue.
The early version of the process will have only fuel alcohol and crude sugar syrup for sale, and scrap wood will be
essential to profitability.
Converting scrap wood to fuel alcohol is a good, low-risk investment opportunity because:
acid hydrolysis is old established technology
low-value scrap wood changes shaky economics to certain profits,
our process has almost no waste to endanger the environment;
environmentalists should be supportive and not hostile,
future coproducts will greatly enhance revenues,
a low-cost, flexible factory can enlarge and accept new technology incrementally.
Processes invented during World War II treat wood with acid to release sugars that are fermented to alcohol by yeast.
The unreacted wood and the sludge from neutralizing the spent acid constitute a manageable but troublesome
disposal problem. Wood impregnated with steam and decompressed explosively is easily separated into its components
- cellulose, hemicellulose, and lignin. Each of these leads to products, and there is almost no waste except for a
neutralization sludge that has very little of the wasted wood from other processes.
Although other groups favor expensive, high-technology enzymatic hydrolysis, this is expensive and has problems for
scale-up to a full-sized refinery. Steam explosion is an ideal pretreatment for hydrolysis of cellulose by enzymes, but
these enzymes are very expensive to produce. A factory that initially uses acid hydrolysis will have much lower capital
and operating cost but inferior yields. Phasing in enzymatic hydrolysis is much less risky than starting with this
expensive and only partially proven high technology.
A factory based on steam explosion of scrap wood and fractionation of the wood components has many future options.
Most important is an opportunity to develop lignin as a product with more potential value than the ethanol. Another
advantage is converting hemicellulose to a sugar solution with immediate use for feeding cattle and more valuable
future prospects as a feedstock for bioconversion to several possible products such as calcium-magnesium acetate
(CMA) for deicing of roads and for treating polluted air. The attractive economics of the initial factory switch to
outstanding profits as these additional products come on line.
BACKGROUND
A process developed at Rensselaer Polytechnic Institute by Professor Henry Bungay takes elements from various new
and old processes to constitute a reliable, relatively inexpensive method for producing fuel ethanol, lignin, and crude
syrup. It draws heavily from the Iogen process, currently at the pilot plant stage at Ottawa, Canada. Our process could
add the essential features of the Iogen process by constructing the sections for making and using enzymes, but our
initial operations will avoid all of their patent claims. The most important feature of this process is flexibility in terms
of incorporating new technology. Most of the equipment in the RPI process is columns and tanks that can be modified
as better process steps are perfected or used simply as columns and tanks at suitable places in any process.
A report prepared by Arthur D. Little, Inc. for the New York State Energy Research and Development Authority
(NYSERDA) and the National Renewable Energy Laboratory (formerly the Solar Energy Research Institute, SERI)
analyzed the Iogen process for biomass refining and found that coproduct credits were crucial (Nystrom, et al., 1985).
A plant that used commercial wood chips would have to sell fuel alcohol at unrealistic prices when the only value
derived from the other components is adding them to boiler fuel. On the other hand, refineries that sell lignin at prices
that seem reasonable for applications such as manufacture of adhesives would be profitable even if the alcohol were
given away. This A. D. Little analysis is quite conservative; other more recent analyses paint a better economic picture
of biomass refining. When the feedstock is scrap wood, the return on investment becomes highly attractive.
Revenues from a lignin coproduct will not come immediately, and the value is uncertain. A thorough analysis of lignin
processes and markets (Frank, et al., 1989) concluded that there would be a modest demand for specialty derivatives at
high prices, but lignin at 10 cents/pound would have sizable markets for adhesives and the like. This low estimate for
value of lignin is justified in the report, but others feel that the potential value of lignin should be high because it
substitutes for phenol that sells for more than 30 cents/pound or replaces components of phenol-formaldehyde
adhesives that sell for more than 60 cents/pound.
The approximate composition of wood is shown in Figure 1.
Cellulose is a linear polysaccharide that liberates glucose when hydrolyzed with acid or with enzymes. Fermenting the
glucose to alcohol is ancient technology. Hemicellulose is a polymer of several sugars, predominantly pentoses, and the
sugar in greatest concentration is xylose. There is no commercial technology for fermenting these sugars to alcohol.
Lignin is a polymer of separate aromatic rings with ether linkages, carbon-carbon bonds, side chains similar to
propanol, and free phenolic groups.
Figure 1 Composition of Wood
In New York and other states, waste wood from construction, demolition, pallets, crates, dimension lumber, and the
like is sent to landfills or incinerated. This wood has been viewed as a resource for burning, but biomass refining can
provide much greater financial rewards. Mobile equipment exists for chipping scrap wood at the dumps and for
removal of nails and staples. When all of the wood at a site has been processed, the equipment is moved to a new site.
The output is at least 50 tons per hour, and the top rate of chipping approaches 100 tons per hour. The input for a
refinery large enough to realize the economies of scale is 500 to 1000 tons per day. This means that the current scale of
chipping of scrap wood can keep up and surpass the factory needs to the extent that a modest storage lot would provide
chips while the equipment was being moved. Of course, there can be more than one supplier of chips.
Another feedstock for a biomass refinery is inferior hardwood trees that would otherwise be left in the forests. By the
time that business expands so that scrap wood becomes an insufficient resource, new trees will be affordable because
operating experience will lead to higher refinery yields and the byproduct markets will be well established. The current
price for chips from these trees is $15 to $25 per ton on a wet basis.
BASE-LINE PROCESS
Our process impregnates wood chips with steam, and they disintegrate as the pressure is released suddenly. This is
much like the old Masonite process. Final products are ethanol, pentose syrup, and a lignin containing some
unhydrolyzed cellulose. The RPI process has clean separation of the lignin and cellulose and employs acid hydrolysis
rather than enzymatic hydrolysis. The key to the IOGEN process is hydrolysis of cellulose with enzymes, and their
patents protect the time and temperature of the steam explosion step. The patents for common steam explosion not
directed at enzymatic hydrolysis were granted in the 1930's and have long expired. Acid hydrolysis was patented in the
1940's and only some new reactor designs that are not essential to the RPI process are protected.
Steam explosion weakens cellulose, destroys hemicellulose, and melts lignin. The chips are transformed to a wet
brown powder that is nicely sized for further processing. Explosion cellulose is relatively easy to hydrolyze with
enzymes. Hemicellulose is hydrolyzed to sugars during the steaming and explosion, but the reactions continue on to
resinous and polymeric compounds. Yields of sugars from hemicellulose are only about 50 percent of theoretical with
severe conditions of steam explosion. Adding a trace of sulfuric acid catalyzes hydrolysis during the steam explosion
so that milder conditions are used, and the yield of sugars from hemicellulose is 70 to 80 percent.
Our first separation step is extraction with water. Exploded wood will be conveyed with water, and careful recycle and
management will provide semi-countercurrent operation to minimize dilution. The second separation step is extraction
with hot ethanol. This differs from solvent pulping in that no catalysts are added, and recent patents on solvent pulping
do not apply. Solvent extraction of wood is very old technology. The water washing will be carried out in columns,
and flow will be switched to ethanol at an appropriate time. Boiling ethanol extracts about 95 percent of the lignin
from exploded wood. In contrast to solvent pulping, where the organic solvent must be recovered from both the treated
wood and from the extracts, the baseline process recovers ethanol only from a relatively clean lignin extract. The
lignin is left behind when the alcohol is distilled. Alcohol with the spent solids that are mostly cellulose, works its way
back into liquid streams as the cellulose is further treated and hydrolyzed. There is only a small amount of final solid
residue from this refinery, and this retains a negligible amount of ethanol. We propose that this residue be burned in
wood-fired boilers that power the refinery.
The acid hydrolysis in the RPI processes is addition of partially purified cellulose to dilute sulfuric acid in portions as
previous portions dissolve. A countercurrent process developed at SERI (Wright, et al., 1985) should substitute for the
ancient method we are now using as soon as possible because the yields are better.
The baseline process is intended to create new opportunities because of the fractionation of the main biomass
components. This is a poor process when the lignin is burned and when the crude syrup is little more than a waste.
Drawbacks of the initial configuration of our process are:
1 - less ethanol because some is used for extraction of lignin, and 2 - less ethanol because glucan forms of
hemicellulose are hydrolyzed during steam explosion to glucose that goes with the crude syrup rather than finding its
way into the ethanol fermentation. There must be compensation for these losses. While there are some processing
advantages and step yield improvements because of fractionation, these are insufficient to justify the losses of ethanol.
The justification is excellent lignin and prospects for fermentation of the crude syrup to generate new revenues that will
make the process more profitable. In other words, accepting scrap wood should make several different biomass refining
processes profitable. Our process has much less initial investment than competing processes and has provisions for the
ideal mix of potentially profitable coproducts.
SYRUPS TO ADD TO ANIMAL FEED
Syrups composed of mixed sugars from hemicellulose were available when a washing step was added to the Masonite
explosion process. This syrup was not as good as molasses for cattle feeding and was not marketed aggressively.
Nevertheless, feeding of cattle is a proven option if no better use of these sugars can be devised.
Developing markets for lignin and for crude sugar syrups both require technical support. There is no doubt that the
syrup can be sold at some price. The value is not based mainly on the sugar content but on such properties as viscosity,
flavor, ability to bind animal feeds, tackiness, feel, flavor, and odor. These properties can be manipulated to some
extent, but prices depend on competition, customer service, and suitability.
WASTE TREATMENT
One of the most crucial factors for new and old factories today is environmental protection. A biomass refinery is
inherently low in pollution. One troublesome waste is the gypsum sludge formed when spent sulfuric acid from the
hydrolysis step is neutralized with lime, but this is a very common type of waste for many different chemical plants.
Disposal of this sludge by old, established methods will not be a serious problem.
Other wastes from biomass refining are spent steam from explosion and residues (stillage) from the distillation of
ethanol. The spent steam condenses to a solution containing acetic acid and other chemicals formed during explosion
of the wood. The stillage has no toxic or hazardous constituents and could be considered for irrigation of forest lands
near the factory. However, the yeast, traces of unreacted sugars, and organic chemicals in the stillage have nutritional
value. If stillage is used to wash the exploded wood, these materials will end up in the crude syrup and should not
hurt its value. Condensed steam from explosion might also be used in the washing step.
If the spent aqueous streams are used for washing, there will be almost no liquid for waste treatment. The savings in
capital cost and operating cost would be significant because some designs of biomass refineries show that up to 40 per
cent of the cost is for the section for waste treatment. However, development of a successful bioconversion of crude
syrup to valuable biochemicals may depend on reducing the contamination of the syrup, and the stillage and condensate
would then go to waste treatment. The beer from the new fermentation step for crude syrup would also require
treatment following product recovery. The priority for finding better uses for crude syrup is not high if feeding to
animals solves our waste treatment problems.
ECONOMIC ANALYSIS
Economic details have been deleted from this version of this document but will be furnished to responsible persons in
confidence. Our analysis confirms the A. D. Little report that the prices are unrealistic when converting commercial
wood chips to ethanol, but coproduct credits and accepting scrap wood make our process immediately profitable with
ethanol sold at competitive prices. Tax credits for fuel ethanol are not needed. The value for waste wood is not exact in
terms of dry mass of wood. While trees have a fairly constant moisture of about 50 per cent, waste wood is relatively
dry. The moisture content will be low, but each batch of scrap wood would need a moisture measurement to know the
prices of its components.
IMPLEMENTATION PLAN
Lignin recovery is not needed for about one year after plant start up because markets will have to be developed. The
RPI process has excellent lignin as one of its main advantages, but this advantage would be wasted initially. The main
disadvantage of the RPI process is consumption of alcohol for recovery of lignin, but the initial factory should not
implement the lignin option until it is needed. This will lower capital cost and divert more alcohol to final product.
The initial factory should be sized and designed for the full process but would consist only of steam explosion,
washing, acid hydrolysis, fermentation of sugars to alcohol, and recovery of the alcohol by distillation. There would
have to be evaporators because the sugars from just washing are too dilute for transportation to the formulation plants
for cattle feed. Equipment for the other steps would be held in reserve and purchased as the markets for byproducts
develop. For example, pure carbon dioxide from the alcohol fermentation should be collected, compressed, and sold.
There is a strong demand for carbon dioxide in the Northeastern U.S., and the equipment for its recovery can be added
in increments as sales develop.
Without the lignin steps, the RPI process closely resembles the type of wood-to-ethanol process invented during World
War II and operated today in the former Soviet Union. In fact, a St. Petersburg plant runs a process on scrap wood and
sawdust.
CONCLUSION
There is sufficient technology available for devising a profitable process for biomass refining. However, each method
has its advantages and disadvantages. The RPI process has old, proven technology with some new twists. The process
is quite simple and is highly flexible so that new technology could be added without making the old equipment
obsolete.
Private investment was attracted to refining of lignocellulosic biomass several years ago when results in the laboratory
were good. Now almost every step has been improved, and pilot plants are in operation. The first factories will have
some types of lignin suited for specialty, high-value uses and can get additional revenue by making acetate salt for
deicing of highways (CMA) from the sugars from hemicellulose. At the scale required for fuel alcohol to have
national significance, biomass refining will represent a large capital investment, many new jobs, and will have a major
impact on national economies.
REFERENCES
Anon., "Biomass Research Program Update", NTIS TVA/NFERC/BIO-91/3
Frank, M.E., R.L. Mednick, and K.M. Stern, "Use and value of reactive lignin", Report prepared for New York State
Energy Research and Development Authority by CHEM SYSTEMS, INC. 944-ERER-ER-87 (1988)
Nystrom, J.M., Greenwald, C.G., Hagler, R.W., and Stahr, J.J., [un]Technical and Economic Feasibility of Enzymatic
Hydrolysis for Ethanol Production from Wood[noun], Final Report for NYSERDA and SERI, Arthur D. Little, Inc.,
June 1985
Wright, J.D., Bergeron, P.W., Werdene, P.J. (1985). "Progressing Batch Reactor Acid Hydrolysis Experiment",
SERI/TP-232-2803, 373-387.