RENEWABLE METHANE FROM
ANAEROBIC DIGESTION OF BIOMASS
David P. Chynoweth, University of Florida,
Gainesville, FL
John Owens, University of Florida, Gainesville,
FL
Robert Legrand, Radian International,
Austin, TX
Dick Glick, Corporation for Future Resources,
Tallahassee, FL
Paper Presented at
The Seventh National BIOENERGY '96
September 15-20, 1996, Nashville, TN
WHY RENEWABLE ENERGY?
In the 1970's, the U.S. and other developed countries
experienced an energy crisis. We were told by government officials and energy
companies that fossil fuels were soon to be depleted and that we must reduce
energy consumption and very rapidly move to alternative energy supplies
with emphasis on renewable sources such as solar and biomass. A dramatic
personal imposition of this crisis was the scarcity of gasoline needed to
satisfy my addiction to our two automobiles and several other motor vehicles,
including a tractor, truck, snowmobile, and motorcycle. I remember waiting
in lines for 30 or more minutes just to get a tank of gasoline. I recall
that energy prices more than doubled and numerous government programs were
initiated to stimulate research and development and startup of businesses
in the renewable energy fields. Folks became conscious of energy-conserving
practices such as turning down the thermostat, insulating, and even giving
up the huge American gas guslers for the small high gas-mileage Japanese
models. In fact, I left the University of Michigan to become involved in
engineering aspects and commercialization of processes for production of
methane from biomass and wastes. Our lab was booming, with a research budget
in the range of 1-2 million dollars per year. Then as fast as this crisis
appeared, it disappeared in the early 1980's. Gas prices plummeted, numerous
renewable energy businesses folded, and funding for renewable energy disappeared.
Did the basis for the energy crisis go away? I don't think so. Rather, conservation
practices and new discoveries of fossil fuels relieved the immediacy of
the problem and special interest groups encouraged revival of the fossil
fuel addiction.
A revived interest in renewable energy and related conversion
technologies is emerging again. Although the eventual depletion of fossil
fuels lurks in the background as a long-term incentive for development of
sustainable energy forms, the increased dependency of the U.S. and other
countries on foreign imports is not healthy to their economies. More urgent
incentives to reemphasize renewable energy are related to global environmental
quality. The first concern to emerge was release of toxic compounds and
oxides of nitrogen and sulfur resulting from combustion of fossil fuels.
These air pollutants contribute globally to health and environmental problems
the most common of which is referred to as acid rain. The greatest concern,
however, is the threat of global warming related to increasing concentrations
of carbon dioxide and other upper atmospheric pollutants resulting from
anthropogenic activities. Use of renewable biomass (including energy crops
and organic wastes) as an energy resource is not only "greener" with respect
to most pollutants, but its use represents a closed balanced carbon cycle
with respect to atmospheric carbon dioxide. It also could mitigate atmospheric
carbon dioxide levels through replacement of fossil fuels. A third concern
is the recognized need for effective methods for treatment and disposal
of large quantities of municipal, industrial, and agricultural organic wastes.
These wastes not only are a major threat to environmental quality, but also
represent a significant renewable energy resource.
WHY METHANE?
Biomass can be converted to a variety of energy forms
including heat (via burning), steam, electricity, hydrogen, ethanol, methanol,
and methane. Selection of a product for conversion is dependent upon a number
of factors, including need for direct heat or steam, conversion efficiencies,
energy transport, conversion and use hardware, economies of scale, and environmental
impact of conversion process waste streams and product use. Under most circumstances
methane is an ideal fuel. Currently it represents about 20% of the U.S.
energy supply. Related to this, an extensive pipeline distribution system
and a variety of hardware are in place for its domestic, municipal, and industrial
use. Compared to other fossil fuels, methane produces few atmospheric pollutants
and generates less carbon dioxide per unit energy. Because methane is comparatively
a clean fuel, the trend is toward its increased use for appliances, vehicles,
industrial applications, and power generation. Although most uses require
high purity methane, it can be used in a variety of stages of purity and
efficiencies of transport and use are good compared to electricity. Other
fuels such as methanol and hydrogen are not well developed commercially for
production and use and are more difficult to produce from biomass. Ethanol
is becoming a popular biomass-derived fuel. Although it has the advantage
of easy storage and transport, the fermentation process for its production
requires extensive feedstock pretreatment and pure culture maintenance,
and energy requirements associated with feed processing and product separation
result in overall low process efficiencies. These problems are not characteristic
of processes for biological conversion of biomass to methane.
CONVERSION PROCESSES
Methane can be produced from biomass by either thermal
gasification or biological gasification (commonly referred to as anaerobic
digestion). Economic application of thermal processes is limited to feeds
with either a low water content (< 50%) or those having the potential
to be mechanically dewatered inexpensively. This limitation is linked to
energy needed for evaporation of water in order to achieve high temperatures
required for the process. Thermal processes for methane production also
are only economic at large scales and generate a mixture of gaseous products
(e.g., hydrogen and carbon monoxide) that must be upgraded to methane. This
paper emphasizes biological gasification which is a low-temperature process
that can convert wet or dry (with added water) feeds economically at a variety
of scales. The product gas is composed primarily of methane and carbon dioxide
with traces of hydrogen sulfide and water vapor. The major limitation of
biological gasification is that conversion is usually incomplete, often
leaving as much as 50% of the organic matter unconverted. However, land
application of these compost residues is compatible with topsoil maintenance
and related sustainable use of the land for growth of biomass. Process rates
are significantly lower than those of thermal processes and the bacteria
involved require a balanced diet of nutrients that may not be available
In some feed stocks .
PRINCIPLES OF ANAEROBIC DIGESTION
Anaerobic digestion is an application of biological methanogenesis
which is an anaerobic process responsible for degradation of much of the
carbonaceous matter in natural environments where organic accumulation results
in depletion of oxygen for aerobic metabolism. This process, which is carried
out by a consortium of several different microorganisms, is found in numerous
environments, including sediments, flooded soils, animal intestines, and
landfills. Anthropogenic (man-caused) stimulation of methane formation and
release into the atmosphere is of recent concern because, like carbon dioxide,
it is also a significant greenhouse gas. The major sources of concern are
flooded soil crops, domestic animals with rumens, landfills, and animal
waste handling facilities.
In a generalized scheme for anaerobic digestion, feedstock
is harvested or collected, coarsely shredded, and placed into a reactor
which has an active inoculum of microorganisms required for the methane
fermentation. A conventional reactor is mixed, fed once or more per day,
heated to a temperature of 35°C, and operated at a hydraulic retention time
of 15 to 20 days and loading rate of 0.1 lb VOS (organic matter as ash-free
dry weight) ft-3 day-1. Under these conditions, about 50% reduction in organic
matter is achieved corresponding to a methane yield of 4.0 ft3 lb-1 VOS
(ash-free dry wt.) added. The biogas composition is typically 60% methane
and 40% carbon dioxide with traces of hydrogen sulfide and water vapor.
Solid residues may be settled and/or dewatered by other means and used as
a compost. The product gas can be used directly or processed to remove carbon
dioxide and hydrogen sulfide.
This conventional design is being replaced by more innovative
designs influenced primarily by feed suspended solids content. The objectives
of most of these advanced designs are to increase solids and microorganism
retention, decrease reactor size, and reduce process energy requirements.
For dilute low solids (<1 %) wastes such as food processing wastes, attached-film
reactors are employed. Attachment of organisms on to inert media permits
low retention times without washout. In designs for feeds with intermediate
solids (5 - 10%) content (e.g. sewage sludges or aquatic plants), solids
and organisms are recycled following settling in the digester or in a separate
secondary digester. For high solids feedstocks, high-solids (>10%) stirred
digesters or leachbed batch systems are being used. These improved designs
have increased possible loading rates 20-fold, reduced residence times,
and process stability. Study of the biochemical methane potential of numerous
biomass feedstocks has shown that >80% of some feedstocks, such as sorghum,
is possible.
The earliest use of anaerobic digestion was the treatment
of domestic and animal wastes. Underdeveloped countries like China and India
have employed smallscale digesters for treatment of sewage and animal wastes.
In these applications the methane produced is used for cooking, lighting,
and operation of small engines and the residues are applied to fields as
compost. The process kills disease-causing organisms resulting in reduced
health problems related to fecal contamination. In the U.S. and other developed
countries, commercial application of the process was previously limited
primarily to domestic sludges. As greater cost and impending depleting of
fossil fuels became apparent in the 1970's and early 1980's, the search
for renewable alternative fuels resulted in an expanded interest in anaerobic
digestion to include industrial wastes, municipal solid waste, and biomass
energy crops as feedstocks. During this period several novel high-rate digester
designs were commercialized for industrial wastes, predominantly in the
food industry. These industries realized the benefits of treating their
wastes by a process that eliminates the costly aeration requirement and
generates a fuel that can off-set a portion of the energy requirements of
their operations. Although a few animal waste digesters were placed into
operation in the developed countries, the absence of strict environmental
regulations for these wastes and prevailing low energy prices stifled their
development. Research on anaerobic digestion of municipal solid waste also
blossomed, resulting in new digester designs for high solids feedstocks.
Although small demonstration plants representing these designs were built
and operated, low tipping fees and plunging energy prices stifled further
commercialization.
RENEWABLE METHANE FROM BIOMASS
Several research programs investigated energy crops (aquatic
and marine plants, grasses, and woods) coupled with anaerobic digestion
for generation of renewable substitute natural gas. These programs integrated
research on crop production and harvesting, conversion to methane by anaerobic
digestion, and systems analysis. Resource potential estimates for these
feedstocks (Table 1 ) have been reported at 7 quadrillion (quad = 1015 )
for wastes and 30 quads for terrestrial biomass (grasses and woods). Estimates
in Table 1 indicate that the potential from land-based biomass is about
22 quads. As shown in Table 2, our energy demands could be met using 102%
of existing cropland. The potential for marine biomass is huge at greater
than 100 quads. All of the U.S. energy needs could be supplied by marine
macroalgae grown on about 600 million acres (one million square miles) of
ocean. However, this optimistic estimate has many uncertainties related
primarily to design of offshore farms. The cost of methane from these renewable
energy systems was significantly higher (510 times) than fossil-derived
energy and interest in their continued funding dwindled with continuation
of energy gluts and depressed prices in the 1980's.
Because biomethanogenesis decomposes organic matter with
production of a useful energy product, anaerobic digestion of organic wastes
is receiving increased attention. With increased levels of waste production,
limited area for landfilling or application, and increased awareness of
environmental impact, alternative methods for treatment of solid and agricultural
wastes are being sought. Currently these wastes release undesired methane
into the atmosphere due to anaerobic conversion in landfills, lagoons, or
stock piles. Treatment and recovery of this gas in reactors would reduce
this source of atmospheric methane. An attractive option for treatment of
the organic fraction of these wastes is to separately treat the organic
fraction by composting and apply the stabilized residues on land as a soil
amendment. The residues would reduce water needs and prevent erosion. The
compost from treatment of wastes from a population of 100,000 could be applied
on a sustained basis on less than 1000 acres of land. This scheme, however,
requires effective separation of undesired components such as metals, glass,
plastics, and toxic compounds which affect the quality of residues more
than the conversion process. In European countries which lead in this field,
the most effective method of separation is source separation, resulting
in compost with sufficiently low levels of contaminants for land disposal.
Although aerobic composting continues to be a more popular process for stabilization
of these wastes, anaerobic digestion has the advantages of methane production
and lack of need for aeration or mixing. Several full-scale anaerobic composting
plants are in operation in France, Belgium, and Denmark.
Biomethane has lost favor to bioethanol as a desired
product from renewable biomass. This is mainly related to the ease of use
of ethanol as a transformation fuel. Use of biomethane should be reconsidered
since the use of methane powered vehicles is increasing in the U.S. Furthermore,
data shown in Table 3 illustrate that processes for production of biomethane
have higher feed energy recovery, lower system energy requirements, and
lower costs than bioethanol production processes. Furthermore, methane yields
and kinetics would be improved significantly if the same drastic depolymerization
pretreatment steps employed for conversion to bioethanol were used in conjunction
with anaerobic digestion.
The major incentive for reconsideration of energy crops
for conversion to methane is the environmental impact of fossil fuel use.
The severity of this impact has led to international discussions of imposing
a carbon tax in the range of 50-100 dollars per ton of carbon released as
carbon dioxide. The impact of such a tax is illustrated in Figure 1. Considering
this tax and the cost of its removal during combustion, biomass will readily
become a viable option. Furthermore the long term depletion of fossil fuel
resources and reduced dependency on foreign imports provide strong additional
incentives for rapid development of renewable energy resources.
CONCLUSION
As population increases and technology development begin
to result in significant resource depletion and environmental deterioration,
we must take a global view on the ground rules for sustaining our species
in a manner that is compatible with preservation of the biosphere. This
will require production of feed, food, and energy by technologies that are
indefinitely sustainable and which have minimal environmental impacts. This
will involve a major shift to renewable resources for energy; sustainable
agricultural practices for production of food, feed, and energy; recycle
of all nonrenewable resources, e.g. minerals, metals, etc.; and elimination
of discharge of anthropogenic materials and compounds into the environment,
e.g. plastics and toxic chemicals. Derivation of methane from energy crops
and organic wastes could play a major role toward this objective.
REFERENCES
Bird, K. T., Benson, P. H., eds., Seaweed Cultivation for
Renewable Resources, Elsevier Applied Science Publishers, London, 1987.
Chynoweth, D. P., "Biomass Conversion Options," in Smith,
W. H. and Reddy, R., eds., Aquatic plants for wafer Treatment and Resource
Recovery, pp. 621-642, Magnolia Publ. Inc., Orlando, FL,1987.
Chynoweth, D.P., "Environmental Impact of Biomethanogenesis,"
Envir. Monitoring and Assessment, 42, 3-18, 1996.
Chynoweth, D. P. and Isaacson, H. R., eds., Anaerobic Digestion
of Biomass, Elsevier Applied Science Publishers, Ltd., New York, NY, 1987.
Chynoweth, D. P., Turick, C. E., Owens, J. M., Jerger,
D. E., and Peck, M. W., "Biochemical Methane Potential of Biomass and Waste
Feedstocks," Biomass and Bioenergy, 5, 95-11, 1993.
Legrand, R.. "Methane From Biomass Systems Analysis and
CO2 Abatement Potential," Biomass and Bioenergy, 5, 301-316, 1993.
Legrand, R. and Warren, C. S., "Biogas Generation From
Community-Derived Wastes and Biomass in the U.S.," Paper presented at the
Tenth Annual Energy-Sources Technology Conf. and Exhib., ASME, Dallas, TX,
1987.
Spencer, D. F., "A Preliminary Assessment of Carbon Dioxide
Mitigation Options," Ann. Rev. Energy Environ., 16, 259-273,1991.
Van Haandel, A.C. and Catunda, P.F.C., "Profitability Increase
Of Alcohol Distilleries By The Rational Use Of Byproducts, Wat. Sci. Tech.,
26, 117-124. 1994.
Table 1. Energy Potential of Biomass and Wastes in the
United States*
Resource EJ/yr
Municipal Solid Waste 1.5
Sewage Sludge and Sludge Grown Biomass 0.8
Biodegradable Industrial Wastes 4.1
Crop Residues 0.3
Logging Residues 0.4
Energy Crops
land-based payment-in-kind land (32 million hectares 22.0
32 million additional hectares marine >100
Total (excluding marine) 29.5
*Sources: Legrand and Warren (1987); Chynoweth et al. (1987)
Table 2. U.S. Cropland Needed to Displace Fossil Fuel Energy
Supplies
Fossil Fuel Weight of CO2 -C Produced in 1988 1015 g %
1987 Cropland Needed to Displace Fuel
Petroleum 2.5 49
Natural Gas 1.1 28
Coal 2.2 2
Total 5.8 103
Ref: Legrand (1993)
Table 3. Costs and energy yields for bioethanol and biomethane.
Bioenergy System Energy Yield, Product/Feed, % System Energy
Requirement, % of Product Energy Costs, $/GJ
Ethanol From Sugar Cane 381 17.31 $12.92
Ethanol With Bagasse Hydrolysis 633 ND ND
Methane From Sorghum4 69.8 7.9 $6.17
1 From Van Haandel and Catunda (1994)
2 Based on $1.20 per gal. Ethanol
3 Estimated from (1)
4 R. Legrand, Personal Communication
