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Biomethane from Biomass, Biowaste, and Biofuels

Biomethane from Biomass, Biowaste, and Biofuels

Bioenergy Edited by J. Wall et al. © 2008 ASM Press, Washington, DC

Chapter 16

Biomethane from , Biowaste, and

ANN C. WILKIE

Biofuel production from agricultural, municipal, and in- (Wilkie et al., 2004). However, production is dustrial wastes is efficiently accomplished through con- not limited to conversion of animal . version to biogas, a mixture of mostly methane (CH4) can be made from most biomass and waste materials and (CO2), via . regardless of the composition and over a large range of Anaerobic digestion is a process by which a complex moisture contents, with limited feedstock preparation. mixture of symbiotic transforms or- The feedstocks for this omnivorous process can be ganic materials under -free conditions into bio- composed of carbohydrates, lignocellulosics, , , nutrients, and additional cell matter, leaving salts , or mixtures of these components. The process is and refractory . In practice, microbial suitable for conversion of liquid, slurry, and solid anaerobic conversion to methane is a process for both wastes; it can even be employed for the conversion of effective waste treatment and sustainable pro- gaseous products (synthesis gas) from ther- duction. In waste treatment, this process can provide a mochemical gasification systems. In addition, methane source of energy while reducing the pollution and odor production can be effectively applied to improve energy potential of the substrate. Unlike fossil , use of re- yields from other production processes includ- newable methane represents a closed carbon cycle and ing bioethanol, , and production. thus does not contribute to increases in the atmos- Implementation of digestion technology at agricultural, pheric concentration of carbon dioxide (Wilkie, 2005). municipal, and industrial facilities allows efficient de- Microbial methane production has the potential centralized energy generation and distribution to local for reducing the demand for fossil fuels like coal, oil, markets. While traditionally applied to wastes and and that have provided the power for de- wastewaters, the anaerobic digestion of energy crops veloping and maintaining the technologically advanced can also be employed in a sustainable system. modern world. However, fossil resources are finite, and their continued recovery and use significantly impact our environment and affect the global climate. Short- ANAEROBIC MICROBIOLOGY ages of oil and gas are predicted to occur within our lifetimes or those of our children. To prepare for a Methane is the end product of anaerobic metabo- transition to more sustainable sources of energy, viable lism—a metabolic sequence carried out by communities alternatives for conservation, supplementation, and re- of hydrolytic bacteria and fungi, acid-producing inter- placement must be explored, posthaste. mediary organisms, and finally, methanogenic Archae- Biogas production from agricultural, municipal, bacteria. Methane-producing communities are very sta- and industrial wastes can contribute to sustainable en- ble and resilient, but they are also complex and largely ergy production, especially when nutrients conserved in undefined. the process are returned to agricultural production Buswell and Sollo (1948) demonstrated the treat- (Fig. 1). Little energy is consumed in the process, and ability of a range of wastes and emphasized the concept consequently the net energy from biogas production is of an acid phase versus a methane phase, showing the high compared to other conversion technologies. The importance of volatile organic acids as intermediates in technology for methane production is scalable and has the process. They also demonstrated the applicability been applied globally to a broad range of organic of a stoichiometric equation that balanced carbon, hy- waste feedstocks, most commonly animal manures drogen, and oxygen (equation 1) to predict the amount

Ann C. Wilkie • Soil and Science Department, University of Florida—Institute of Food and Agricultural Sciences, Gainesville, FL 32611.

195 196 WILKIE

Figure 1. Biogas cycle

of methane and carbon dioxide evolved from conver- teria ferment the larger acids to a combination of acetic sion of organic compounds with a known empirical acid, one-carbon compounds, , and carbon formula. Later, 14C tracers were used to show that ac- dioxide. The homoacetogenic bacteria synthesize acetic etate was indeed cleaved to form methane and carbon acid by utilizing hydrogen/carbon dioxide or one- dioxide, suggesting that acids were important interme- carbon compounds or by hydrolyzing multicarbon com- diates in the conversion process. pounds. The methanogenic Archaebacteria uniquely catab- ϩ Ϫ Ϫ → CnHaOb (n a/4 b/2)H2O olize acetic acid and one-carbon compounds to meth- Ϫ ϩ ϩ ϩ Ϫ (n/2 a/8 b/4)CO2 (n/2 a/8 b/4)CH4 (1) ane. The are obligate anaerobes that can pick up electrons from dead-end , through Of great importance to the understanding of an- interspecies hydrogen transfer, and shuttle these elec- aerobic microbiology was the discovery of Bryant trons through a unique form of respiration which re- et al. (1967), through isolating the elusive “S-organism” sults in the reduction of carbon dioxide to methane. from Methanobacterium omelianskii, that the conver- The organisms that use hydrogen to reduce CO2 are sion of to methane was accomplished with commonly regarded as the earliest life forms due to a mixed culture. The discovery of other cocultures their chemoautotrophic abilities. quickly followed, and the number of species isolated in All morphological forms are represented among pure culture increased. With the identification of the methanogens including rods, cocci, spirals, sarci- closely coupled syntrophic cocultures of methanogens nae, and filamentous organisms. Surprisingly, this di- and other species, the earlier hypothesis of an acid phase verse group of organisms is known to metabolize a very followed by a methanogenic phase developed into a limited number of substrates including acetate, for- more descriptive scheme that embraces the importance mate, , acetone, methylamines, carbon monox- of hydrogen as an intermediate in the process. ide, and H2/CO2. The substrates for di- First the fermentative, or hydrolytic, bacteria and vide these organisms into groups, two of which are fungi hydrolyze complex organic polymers and ferment notably important in active digesters: the aceticlastic me- them to organic acids, hydrogen (or formate), and - thanogens, which cleave acetic acid, and the hydrogen- bon dioxide. The hydrogen-producing acetogenic bac- utilizing methanogens, which utilize hydrogen and one- CHAPTER 16 • BIOMETHANE FROM BIOMASS, BIOWASTE, AND BIOFUELS 197 carbon compounds. However, this distinction is not al- Table 1. Theoretical methane yield of biomass components ways useful since some species may metabolize both Component Methane yield (liter/g of VS) substrates. Carbohydrates ...... 0.35 For aceticlastic methanogens, low levels of acetate Proteins (leucine) ...... 0.57 (Ͻ50 mg/liter) favor the growth of more-filamentous Fats () ...... 0.95 organisms (e.g., Methanosaeta) that must rely on a larger surface-to-volume ratio in order to improve sub- strate diffusion rates. High levels of acetate favor the The natural assemblages in the mixed culture have predominance of clusters of aceticlastic methanogens evolved to form robust and stable cultures with ex- (e.g., Methanosarcina), which have lower surface-to- tremely broad substrate utilization capabilities. There is volume ratios that serve to protect them from the in- no requirement for genetically modified organisms to hibitory nature of high organic acid concentrations. extend catabolic activity, so sterilization of process re- Differences in maximum growth rate and substrate uti- siduals is not necessary. Although the free energy avail- lization affinities can be exploited to select for predom- able from anaerobic conversion of substrates to meth- inant methanogens. Organisms such as Methanosarcina ane is low, causing low microbial-growth rates, the should be favored for selection if high conversion rates activity and turnover rates of substrates are higher than of high-strength wastes are the primary goal, whereas for aerobic metabolism. Also, anaerobic respiration of Methanosaeta should be favored if low effluent bio- methanogens results in the production of a nonin- chemical oxygen demand is more important. In addi- hibitory product, methane, that moves into the gaseous tion, these attributes can be exploited together by stag- phase, which contrasts with other proc- ing an anaerobic process with the first stage favoring esses that produce inhibitory final products (e.g., etha- high conversion rates and the next stage favoring efflu- nol) that remain in solution. This gives the process a ent quality. distinct advantage for either continuous or batch con- version of substrates to energy products. Of significance for the application of anaerobic di- PROCESS gestion is the high level of energy recovery in the biogas compared to the energy content of the substrate uti- In practice, anaerobic digestion is the engineered lized. The efficiency of this conversion is directly re- methanogenic decomposition of organic matter, carried lated to the low level of free energy of reaction avail- out in reactor vessels, called digesters, that may be able for microbial synthesis. Rather than transforming mixed or unmixed and heated or unheated. The process the energy in waste into sludge as in aerobic processes, uses a mixed culture of ubiquitous organisms, and due a minimal amount of this energy is consumed by anaer- to its mixed-culture nature, there are no requirements obic cell synthesis and the rest is retained in the for feedstock sterilization and no contamination con- methane end product. This also explains the low rates cerns. Stable digester operation requires that the bacter- of microbial growth in anaerobic systems compared to ial groups be in dynamic equilibrium, as some of the aerobic processes. intermediate metabolites (hydrogen, propionate, am- Chemical oxygen demand (COD) is a convenient monia, and sulfide) can be inhibitory and the pH of the measurement to estimate the organic content in a waste- system must remain near neutral. Also, methane is water or biomass sample and, theoretically, 0.35 liter of sparingly soluble, such that end product recovery is effi- methane is formed from 1 g of COD digested. In aerobic cient and economical as the gas separates itself from the processes such as the activated-sludge process, the sludge aqueous phase and is easily removed from the digester yield can be as high as 0.5 kg of dry solids per kg of through piping that conveys it to storage for final use. COD utilized, whereas the sludge yields for anaerobic Current commercial-scale methods of methane processes range from 0.03 to 0.15 kg of dry solids per kg production yield from 50 to 97% conversion of sub- of COD consumed depending on the substrate. Sludge strate to methane on an energy basis, depending on the by-product from aerobic processes requires further feedstock. The mean oxidation state of the feedstock treatment for stabilization in order to reduce odor and determines the stoichiometry of the end products. pollution potential. Furthermore, after stabilization, the Carbohydrate substrates yield 50% methane and 50% sludge still requires final disposal. Anaerobic treatment carbon dioxide, while more-reduced feedstocks (e.g., processes, in contrast, produce a relatively small amount ) yield higher proportions of methane. The theo- of sludge by-product which is more stable, less capable retical methane yield of carbohydrates, proteins, and of causing odor or pollution problems, and ready for fats is given in Table 1. Also, carbohydrate-rich sub- final disposition in sustainable crop production. Also, strates yield more methane than do feedstocks with anaerobic treatment results in pathogen decimation high concentrations of lignocellulose. through microbial competition and starvation. 198 WILKIE

Nutrients contained in the organic matter are con- 15 to 60 days and moderate organic loading rates (OLR), served and mineralized to more soluble and biologically typically expressed as weight of organic matter (volatile available forms, providing a more predictable biofertil- solids [VS] or COD) per culture volume of reactor per izer. Since sludge production in anaerobic digestion is day. The maximum OLR and minimum HRT that can be minimal, virtually all of the and applied are dependent on operating temperature, waste contained in the original waste is retained in the treated characteristics, and reactor type. effluent. By recycling the treated effluents back to pro- Feeds with low concentrations of suspended solids ductive agricultural lands at appropriate rates, the crops (Ͻ2%) can be digested in high-rate immobilized reac- benefit from the presence of these important plant nutri- tors such as the upflow anaerobic sludge-bed digester ents. Where insufficient cropland is available, other nu- (UASB), anaerobic filter (AF), and fixed-film systems. trient recovery technologies may be employed to reduce These reactors retain high concentrations of immobi- the nutrient content of the digested wastewater. lized microorganisms, permit low HRT without organ- ism washout, and are particularly suited for treatment of soluble wastewaters. The tendency of microbial con- DESIGNS sortia to adhere to surfaces and grow as a biofilm spurred the development of both the aerobic trickling The construction of anaerobic digesters for biogas filter and the AF reactor (also called a packed bed). production has little in common with that of industrial While the principle of filling a reactor with a packing fermentors. The low value of energy products com- media is straightforward, the selection of packing ma- pared with fermentation products necessitates low-cost terial and operational strategies may have significant construction and materials. While industrial fermenta- effects on performance and costs. Media used for pack- tion vessels are often jacketed stainless steel tanks, with ing have included natural materials such as stones, clay, baffles, agitators, and clean-in-place systems, and con- wood, bamboo, and reeds, as well as polymers made of structed on elevated stands, anaerobic digesters are , polyethylene, and polypropylene. often simple insulated concrete or carbon steel tanks Polymers shaped as rings, bio-balls, and oriented mod- constructed with low-cost materials either on or below ular media have been used in various applications. In the surface. Without the need for efficient aeration, the some cases, AFs rely on trapping microbial solids within requirement for mixing must only meet the needs for the media rather than using a true biofilm for microbial microbial contact with substrate, uniform temperature, activity. Thus, the term “fixed-film digesters” should be and prevention of solids accumulation. Since sterility is used to designate true biofilm reactor designs. not a concern, no clean-in-place systems or provisions to In immobilized reactors where a highly degradable, prevent microbial contamination are required. Unlike high-COD wastewater (Ͼ20 g/liter) is fed, effluent recy- other fermentations, no specific process or equipment is cling can be employed to overcome localized acid- required for product recovery, since methane is rela- ification of the microbial biomass. In addition, highly tively insoluble and therefore separates spontaneously. acidic wastewater can benefit from effluent recycle to Anaerobic digesters must be essentially gas-tight minimize the requirement for added alkali by using the vessels with a provision for introducing feedstock and alkalinity of the effluent. Phased digestion is often em- removing effluent and biogas. The classical anaerobic ployed for highly degradable waste, where a primary aci- digester is essentially a chemostat. Tanks with rigid tops dogenic reactor is operated at short HRT to form inter- must have provisions for pressure and vacuum relief, mediate acids, which are then fed into a methanogenic and biogas piping must meet safety standards. Tank reactor. This approach can control sharp pH swings, en- tops may also be floating rigid tops or flexible mem- hance biofilm and granular sludge activity, and lower brane materials. Simple heat exchangers may be placed overall process HRT. A further refinement involves stag- internally or external to the tank, and mixing can em- ing, where reactors are employed in series to achieve ploy agitators, simple recirculation of the mixed liquor, higher treatment efficiencies. The first reactor is opti- or injection of compressed biogas. mized to maximize biogas production (higher OLR), There are two broad classifications of digesters, whereas the second reactor is optimized for treatment ef- those that rely on suspended growth of microorganisms ficiency (lower OLR). and those that employ a mechanism for immobilization to retain active microbial biomass within the vessel. With feedstocks containing high levels of suspended solids, INOCULATION nonimmobilized designs are generally used including cov- ered anaerobic lagoons, complete-mix reactors, plug-flow The use of a source high in anaerobic microbes reactors, and anaerobic contact reactors. These digesters (e.g., digester effluent) to start up an anaerobic system require relatively long hydraulic retention times (HRT) of is called inoculation. The quality and quantity of in- CHAPTER 16 • BIOMETHANE FROM BIOMASS, BIOWASTE, AND BIOFUELS 199 oculum are critical to the performance, time required, cell growth are 12 and 2%, respectively, of the volatile and stability of biomethanogenesis during commission- solids converted to cell biomass. If 10% of the degrad- ing (start-up) or restart of an anaerobic digester. Much able solids are converted into microbial biomass, this agricultural processing occurs on a seasonal basis, and at would be equivalent to a requirement of 1.2 and 0.2% the start of a new campaign, the anaerobic treatment op- of the biodegradable volatile solids, respectively, for ni- eration must be restarted after a period of being idled. In trogen and phosphorus. is also an important addition, a digester may need inoculation after mainte- contributor to the buffering capacity in digesters but nance operations. In manures and some wastes, the mi- can be toxic to the process at high levels. crobes needed for digestion may be already present in Methane production and volatile acid utilization the waste in small numbers, albeit sufficient to act as an may be enhanced when micronutrients are added to inoculum, and will develop into a fully functional bacte- nutrient-deficient substrates. Requirements for several rial population if the right conditions are provided, in- micronutrients have been identified, including iron, cop- cluding a suitable temperature and retention time. Other per, manganese, , molybdenum, nickel, and vana- wastes, especially from industry, may be relatively sterile dium (Wilkie et al., 1986; Speece, 1996). Available and require the addition of inoculum. With batch and forms of these nutrients may be limiting because of plug flow designs, inoculum must be added with the feed their ease of precipitation and removal by reactions with and low inoculum levels may lead to imbalance due to phosphate and sulfide. Limitations of these micronutri- the more rapid growth rate of acid-forming bacteria ents have been demonstrated in reactors in which the (compared to methanogens) and depression of pH. analytical procedures failed to distinguish between Depending on the buffering capacity (alkalinity), a di- available and sequestered forms. Other nutrients needed gester may be able to overcome low inoculum rates. in intermediate concentrations include sodium, potas- Granular sludge, the microbial by-product from sium, , magnesium, and . Combining UASB reactors, has been shown to be a practical source wastes is an effective means of overcoming nutrient for inoculum due to its stability in storage, microbial den- limitations. Codigestion with often enhances sity, and availability. Granular sludge may be used to en- the conversion of other biomass and waste feedstocks hance populations for start-up of complete- through balancing micronutrients. mix reactors and anaerobic filters as well as UASBs. Start-up of immobilized systems requires that biofilm or granule growth be optimized to achieve design perfor- CONTROL/TOXICITY mance quickly. During start-up, performance parameters (methane gas content, ratio of acids-to-alkalinity, and Biological methanogenesis has been reported at tem- pH) should be carefully monitored to ensure that per- peratures ranging from 2°C (in marine sediments) to over formance is not deteriorating. In many applications, a 100°C (in geothermal areas). Most applications of this high inoculation rate is not feasible or digester effluent is fermentation have been performed under ambient (15 to not available. Under such circumstances, one must obtain 25°C), mesophilic (30 to 40°C), or thermophilic (50 inoculum from an anaerobic environment (anaerobic sed- to 60°C) temperatures. In general, the overall process iments or animal manure) and gradually develop and ac- kinetics doubles for every 10-degree increase in operating climate the inoculum to the level needed. The major ob- temperature, up to some critical temperature (about stacle to overcome is the fact that, during growth toward 60°C) above which a rapid drop-off in microbial activity a mature population, acid formers may grow faster than occurs. Most commercial anaerobic digesters are oper- methanogens, leading to an increase in volatile organic ated at mesophilic or ambient temperatures. A higher op- acids, reduced pH, and loss of methane production. This erating temperature permits reduced reactor size. can be prevented by buffering the system and/or reducing Thermophilic digesters exhibit some differences the feed loading rate. compared to mesophilic digesters. The microbial popu- lations operating in the thermophilic range are geneti- cally unique, do not survive well at lower temperatures, NUTRITION and can be more sensitive to temperature fluctuations outside their optimum range. Also, ammonia is more Nitrogen and phosphorus are the major nutrients toxic in thermophilic digesters due to a higher propor- required for anaerobic digestion. These elements are tion of free ammonia. Although thermophilic digesters building blocks for cell synthesis, and their require- have higher energy requirements, heat losses can be ments are directly related to the microbial growth in minimized through effective insulation and use of heat anaerobic digesters. An empirical formula for a typical exchangers to reduce effluent heat losses. Thermophilic anaerobic bacterium is C5H7O2NP0.06 (Speece, 1996). operation is practiced when the reduced reactor size Thus, the nitrogen and phosphorus requirements for justifies the higher energy requirements and added 200 WILKIE effort to ensure stable performance, when process purities such as hydrogen sulfide. Biogas can be used wastewater is already hot, or when pathogen removal readily in all applications designed for natural gas such is of greater concern. as direct combustion for absorption heating and cool- Biomethanogenesis is sensitive to several groups of ing, cooking, space and , and drying. inhibitors including alternate electron acceptors (oxy- Biogas can also be upgraded to natural gas specifica- gen, nitrate, and sulfate), sulfides, heavy metals, halo- tions and injected into the existing network of natural genated hydrocarbons, volatile organic acids, ammo- gas pipelines. Biogas may also be catalytically trans- nia, and cations. The intermittent presence of microbial formed into hydrogen, ethanol, or methanol. inhibitors in the wastewater stream can lead to serious If is employed in the biogas conver- process upsets and failure. The toxic effect of an in- sion system, heat normally wasted may be recovered hibitory compound depends on its concentration and and used for hot water production. In gas turbines, the the ability of the bacteria to acclimate to its effects. The may be used to make steam and drive an inhibitory concentration depends on different variables, additional steam turbine, with the final waste heat go- including pH, HRT, temperature, and the ratio of the ing to hot water production. This is termed a combined toxic substance concentration to the bacterial mass cycle cogeneration system. Combining hot water recov- concentration. Antagonistic and synergistic effects are ery with electricity generation, biogas can provide an also common. Methanogenic populations are usually in- overall conversion efficiency of 65 to 85%. fluenced by dramatic changes in their environment but For smaller biogas installations, shaft horsepower can be acclimated to otherwise toxic concentrations of and electrical generation are most effectively achieved many compounds. by the use of a stationary internal combustion engine. Organic acids, pH, and alkalinity are related pa- Adequate removal of hydrogen sulfide is important to rameters that influence digester performance. Under reduce engine maintenance requirements. If compressed conditions of overloading and the presence of inhibitors, for use as an alternative transportation in light and methanogenic activity cannot remove hydrogen and or- heavy-duty vehicles, biogas can use the same existing ganic acids as fast as they are produced. The result is technique for fueling as currently used for compressed- accumulation of acids, depletion of buffer, and depres- natural-gas vehicles. In many countries, biogas is viewed sion of pH. If uncorrected via pH control and reduc- as an environmentally attractive alternative to diesel tion in feeding, pH will drop to levels that stop the fer- and for operating and other local transit mentation. Independent of pH, extremely high volatile vehicles. The exhaust fume emissions from methane- acid levels (Ͼ10,000 mg/liter) also inhibit performance. powered engines are lower than the emissions from The major alkalis contributing to alkalinity are ammo- diesel and gasoline engines. Also, the sound level gener- nia and bicarbonate. The most common chemicals for ated by methane-powered engines is generally lower pH control are , lime, magnesium hy- than that generated by diesel engines. droxide, and sodium bicarbonate. Lime produces cal- cium bicarbonate up to the point of solubility of 1,000 mg/liter. Sodium bicarbonate adds directly to the bicar- WASTE RESOURCES bonate alkalinity without reacting with carbon dioxide. However, precautions must be taken not to add this Recently, anaerobic wastewater pretreatment has chemical to a level of sodium toxicity (Ͼ3,500 mg/liter). attained extensive acceptance for a variety of industrial Currently, the control of feed rate to an anaerobic di- wastewaters associated with food processing, beverages, gester most often relies on off-line measurements of breweries, distilleries, and most recently pulp and paper volatile organic acids to prevent process upset through production. Batch operation of the production sequence manual intervention. Several investigators have advo- is common in these industries, producing a wastewater cated control schemes based on biogas production rate, of variable strength and quantity, complicating the alkalinity, liquid-phase hydrogen, pH, and digester sub- operation of a continuous biological treatment system. strate concentration. A few examples of agricultural and industrial waste streams are identified in Table 2. Traditionally, treat- ment of manures and municipal sludge have been the BIOGAS USE most prominent applications of anaerobic digestion, and there is currently a resurgence in the promotion of Biogas is a flexible form of that on-farm biogas production from animal manure (see may be used directly for process heat and steam or con- the Agstar Program, U.S. Environmental Protection verted to electricity in reciprocating engines, gas tur- Agency; http://www.epa.gov/agstar/). Anaerobic diges- bines, or fuel cells. Biogas is composed mostly of tion of municipal sludge is applied at many municipal methane, as is natural gas, but may contain some im- plants. However, the pretreat- CHAPTER 16 • BIOMETHANE FROM BIOMASS, BIOWASTE, AND BIOFUELS 201

Table 2. Examples of agricultural and industrial wastewater strength nitrogen, to improve digestion rates. Household or Feed source Wastewater COD (mg/liter) other waste streams can be blended with manure to im- Beef processing ...... 7,500 prove the microbial diversity and contribute essential Beverage ...... 1,600 micronutrients. The organic fraction of MSW is suit- Brewery...... 1,900–2,400 able for codigestion with farm and industrial wastes, Clam...... 3,500 and many successful examples can be found in Europe Confectionery ...... 9,500 Dairy ...... 1,900–5,260 (see the European Anaerobic Digestion Network; Distillery ...... 95,000 http://www.adnett.org/). Conversion of agricultural, Ice cream...... 29,063 municipal, and industrial wastes to biogas offers a sus- Municipal...... 200 tainable means for biofuels production, yet the role of Pharmaceutical ...... 9,985 biogas production in the production of other biofuels Pork processing ...... 1,572 ...... 2,000–10,500 (e.g., alcohols, biodiesel, hydrogen, and ) is also Pulp and paper ...... 1,600–16,400 an application worthy of exploitation. Rendering ...... 8,800 Sauerkraut...... 10,000 Starch ...... 8,800–11,400 refining...... 5,000–20,000 Vegetable ...... 2,300–10,000 METHANE IN BIOETHANOL PRODUCTION Whey ...... 8,900 Yeast...... 30,000 The production of biomethane at bioethanol pro- duction facilities can contribute to the energy require- ments of ethanol production or to increasing the energy ment of municipal wastewater by high-rate anaerobic yields from substrates for sale to local markets as fuel or treatment offers a new application of biogas produc- electricity. Depending on the feedstock and process de- tion in municipal wastewater treatment works ( sign, ethanol production results in several by-products Haandel and Lettinga, 1994). Conversion of soluble (Fig. 2) which may include crop residues, stillage, evapo- biochemical oxygen demand in municipal wastewater rator condensate, condensed solubles, spent cake and/or to biogas avoids much of the costs of aeration and the distillers’ grains, all of which have a high potential for production of residual sludge requiring disposal. methane production (Table 3). Stillage, a residual of the The organic fraction of municipal solid wastes distillation of ethanol from fermentation liquor, con- (MSW) also has a high potential for biogas production. tains a high level of biodegradable COD as well as nutri- A majority of MSW is disposed of in landfills, many of ents and has a high pollution potential (Wilkie et al., which are implementing biogas recovery systems. How- 2000). Up to 20 liters of stillage may be generated for ever, nutrients contained in MSW are sequestered in each liter of ethanol produced. Conversion of stillage landfills and the land area for these operations is often to biogas and application of effluent to croplands re- not suitable for economic development. Separation of sults in a more sustainable ethanol production system. the organic fraction of MSW and conversion to biogas Many ethanol plants minimize effluent discharges can produce residuals that are suitable for by evaporation of the stillage to produce evaporator crop production, which results in a sustainable solid- condensate (used partially for makeup water) and con- waste recycling system. densed solubles. The evaporator condensate contains volatile fermentation products that can inhibit . Anaerobic digestion can remove these CODIGESTION fermentation products and provide a liquid more suit- able for process recycling. The distillers’ grains and con- Digestion of a given waste can often benefit from densed solubles are normally blended for use in animal codigestion with other waste streams that are locally feed as dried distillers’ grains and solubles. However, the available. There are many reasons for considering codi- current rapid expansion of ethanol production could gestion, including the potential to reach a more favor- lead to saturation of the feed market with dried dis- able economy of scale due to materials handling or op- tillers’ grains and solubles, affecting the sale value of this timal production and utilization of biogas. Codigestion by-product. Thus, there is an opportunity for biogas may provide increased revenues from tipping fees as production from these by-products to offset facility well as from enhanced biogas production. Very dry energy requirements. In production, feedstocks may be blended with wastewaters to facili- nonfermentable hydrolysis products can also be con- tate handling and digestion. Waste high in , verted to methane. Finally, crop residues may also be which could suffer from ammonia toxicity, can be harnessed for biogas production, which can greatly im- blended with lignocellulosic materials, which are low in prove the energy yield from ethanol production. 202 WILKIE

Figure 2. Potential biogas feedstocks from bioethanol production.

METHANE IN are consumed, yielding around 75 liters of biodiesel and 25 liters of crude . The washing process pro- Biodiesel is normally produced from either virgin duces another 30 liters of biodiesel washwater. Both the plant oils or waste vegetable oils through a catalytic crude glycerol and the biodiesel washwater have signifi- transesterification process. The typical biodiesel produc- cant methane production potential. When tion process uses an alkaline hydrolysis reaction to con- is pressed from seeds (or ), there is also a press vert vegetable oil into biodiesel by using methanol, cake by-product along with crop residues from harvest- , and heat. A transesterification ing that are both amenable to biogas production (Table reaction splits the glycerol group from the 3). Conversion of biodiesel by-products to methane of- oils, producing methyl esters (biodiesel) and glycerol by- fers a sustainable treatment solution, while also provid- product (Fig. 3). To purify the biodiesel, a washing proc- ing additional energy. Methane can also be converted to ess is employed to remove soaps, free fatty acids, and methanol, an ingredient used in biodiesel production. excess methanol, producing a washwater by-product. Also, digester effluent could be used to grow oleaginous While process yields and inputs depend largely on oil algae for biodiesel production. type and quality, for every 100 liters of oil, approxi- mately 25 liters of methanol and 0.8 kg of KOH/NaOH METHANE IN THE

Table 3. COD of some bioenergy by-products Hydrogen is often considered as a long-term solu- Feedstock COD (g/kg) tion to dwindling supplies and the environ- Ethanol thin stillage from corn ...... 56.0–64.5 mental consequences of petroleum use in the trans- Ethanol stillage from beet molasses ...... 55.5–116.0 portation sector. However, hydrogen production and Ethanol stillage from juice...... 22.0–45.0 storage are still very expensive. Since water is the pri- Ethanol stillage from cane molasses...... 22.5–118.0 mary product of H2 combustion, the fuel is viewed as a Ethanol stillage from cellulosics...... 19.1–140.0 means to eliminate CO emissions. Yet, if H produc- Evaporator condensate ...... 2.6–5.7 2 2 Condensed solubles ...... 724 tion is from fossil sources, it will still result in signifi- Dried distillers’ grains...... 368 cant CO2 emissions. Only the production of H2 from Crude glycerol from biodiesel production ...... 1,800–2,600 renewable energy sources can result in reduced green- Washwater from biodiesel...... 40.1–161.0 house gas emissions. One means of renewable H2 pro- Press cake from oil crops ...... 1,570 duction is through fermentation of organic matter. CHAPTER 16 • BIOMETHANE FROM BIOMASS, BIOWASTE, AND BIOFUELS 203

Figure 3. Potential biogas feedstocks from biodiesel production.

However, theoretically, only 33% of the energy in car- biomass. Wastes and biomass crops can be gasified in a bohydrates is available for microbial H2 production reduced atmosphere combustion process to convert the due to the requirement to regenerate metabolic reduc- biomass into a mixture of CH4, CO2, CO, and H2. ing potential (Angenent et al., 2004; Hungate, 1974). While catalytic conversion of syngas to methanol has This means that 66% of the carbohydrate feedstock re- historical application for producing “wood alcohol,” mains in the fermentation effluent and requires further the H2, CO2, and CO in syngas can be used as a feed- processing. Anaerobic digestion can easily convert this stock in methane production. Currently, catalysts for residual carbon to biomethane, and the methane could conversion of syngas to mixed higher alcohols (ethanol, then be converted to hydrogen catalytically. Still, the propanol, and butanol) are in development, but in any efficiency of conversion for methane production sug- of the catalytic processes, H2S is problematic for cata- gests that it is easier to convert all of the carbohydrate lyst longevity. Anaerobic digestion could serve as a directly to methane rather than suffer the low yields of process for syngas cleanup to convert the mixture to microbial hydrogen production. This methane could be CH4 (Sipma et al., 2006) and allow more-efficient cat- upgraded to natural gas or converted into electricity, alytic conversion to further products (H2, ethanol, or both of which are easier to transport than H2. methanol). Pure-culture fermentation of syngas to etha- There are other means by which methane factors nol is also in development (Younesi et al., 2005), a into the hydrogen economy. First, the of process which also generates acetate that may in turn H2 is four times less than that of CH4 on a molar or be converted to CH4 via anaerobic digestion. volume basis, suggesting that methane could serve as a more efficient storage vector for hydrogen. Secondly, there is an existing infrastructure of pipelines for trans- ENERGY CROPS porting CH4 that are not suitable for moving H2. Capitalizing on this network of pipelines, methane Meeting the demand for alternative fuels from sea- could be transported to regions of demand and con- sonal crops grown for bioenergy is potentially tenuous. verted to H2 locally as required. Renewable methane, Storage of crops can result in losses of carbohydrates therefore, is an appropriate energy vector even if hy- available for fermentation to ethanol. Direct methane drogen is a desirable replacement fuel. production from energy crops can overcome these losses because of the ability of the anaerobic digestion process to use fermentation intermediates as substrates. SYNTHESIS GAS Harvested crops can be ensiled to preserve overall en- ergy content, using technology with which farmers are Another renewable fuel source that could integrate already familiar. Further, any improvement in conver- with methane production is the production of synthesis sion efficiency that enhances cellulosic ethanol yields is gas (syngas) through thermochemical gasification of equally applicable for biomass conversion to methane. 204 WILKIE

Sugarcane, a power crop, has a long growing sea- Table 4. Ranges of biochemical methane a son in tropical and subtropical climates, and because it potential yield for biomass energy crops is a C4 plant, is one of the best plants for Sample Methane yield (at STPb) (liter/g of VS) collecting and harvesting . While conver- Kelp (Macrocystis) ...... 0.39–0.41 sion of the soluble fraction of sugarcane into ethanol Sorghum ...... 0.26–0.39 has been implemented on a large scale in , etha- Sargassum ...... 0.26–0.38 Napiergrass ...... 0.19–0.34 nol production facilities are capital intensive, requiring Poplar ...... 0.23–0.32 several unit processes and significant energy consump- Water hyacinth...... 0.19–0.32 tion. However, the soluble fraction of sugarcane can Sugarcane ...... 0.23–0.30 also be converted into biogas. The production of biogas Willow ...... 0.13–0.30 requires much less investment, little energy is consumed Laminaria ...... 0.26–0.28 ...... 0.20–0.22 in the process, and the potential feedstock is not lim- Avicel cellulose...... 0.37 ited to the sugars but can use the whole sugarcane plant aModified from Chynoweth et al., 1993. as well as other energy crops. Further, nutrients con- bSTP, standard temperature and pressure. tained in the cane are conserved in the process and can be returned to the fields to maintain a sustainable pro- duction cycle with minimal synthetic inputs. heating, electrical production, and vehicular fuel. Cane juice can be digested directly to produce Biogas can be upgraded and injected into natural gas methane, without the need for alcohol fermentation, pipelines, leveraging the existing distribution infra- centrifugation and distillation, and the consumption structure. Liquids, slurries, solid wastes, and gaseous of high-grade energy associated with these processes. waste can all be processed by anaerobic digestion to Some 47% of the total energy in cane would be present form biogas. Several digester designs have been devel- in the biogas produced. The remaining could oped to optimize processing of different feedstocks. still be used for energy production through combus- Digester size can be scaled to match the application, and tion, as currently implemented in the sugar industry. A centralized plants, codigesting a mixture of wastes, can further reduction of investment and operational costs be utilized to achieve economies of scale and improved and an increase in energy output could be obtained by performance. Methane production can be integrated subjecting not only the juice but the whole cane to into biorefineries since by-products from production of anaerobic digestion. Assuming that 70% of the bagasse bioethanol, biodiesel, biohydrogen, and syngas are also can be converted into methane, which is a realistic figure suitable for anaerobic digestion, thus increasing net for a low-lignin (only 6.3%) plant such as sugarcane, energy yields and recycling valuable nutrients for crop then the energy conversion efficiency would increase to production. Finally, processing of terrestrial and ma- 80% of the energy content of cane (Chynoweth et al., rine energy crops to biomethane can result in higher 1993; Pate et al., 1984; van Haandel, 2005). By com- energy yields than that of other biofuels. Given the parison, only 40% of the energy content of cane is ac- diversity of feedstocks and ease of product recovery, tually converted into alcohol, consuming 24% of the methane from organic wastes and energy crops offers a cane energy in the process, while 12% is discharged as major solution that is renewable, wastewater (stillage) and 24% remains in the excess carbon dioxide neutral, and locally based, thereby pro- bagasse (van Haandel, 2005). tecting the environment, creating jobs, and strengthen- Corn has also undergone whole-plant conversion ing local economies. to methane. Methane yields for corn at varying harvest REFERENCES times have ranged from 268 to 366 liters/kg of VS Amon, T., B. Amon, V. Kryvoruchko, W. Zollitsch, K. Mayer, and L. (Amon et al., 2007). Table 4 gives ranges of methane Gruber. 2007. Biogas production from and dairy cattle yield for various terrestrial and crops. manure—influence of biomass composition on the methane yield. 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