Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 471-477
47 1
SYMPOSIA SECTION
I.
Symposium on Chemicals from Cellulosic Materials M. Chang, Chairman 179th National Meeting of the American Chemical Society Houston, Texas, March 1980
Methane Production from Agricultural Residues. A Short Review Yud-Ren Chen,' Vincent H. Varel, and Andrew G. Hashlmoto Roman L. Hruska U.S.Meat Animal Research Center, Agricultural Research, Science and Education Admlnlstration, US. Department of Agriculture, Clay Center, Nebraska 68933
This paper summarizes the methanogenesis process, the environmental requirement, kinetics, energy requirements, and methane production cost of methane fermentation systems. A great deal of knowledge of microbiology of anaerobic fermentation was obtained from the previous studies of the anaerobic treatment of sewage sludge. Available data of biodegradability of the residue and kinetic equations can be used to predict the methane production under different operating conditions. The optimum condition for fermenting beef cattle residue is operating at a thermophlllc temperature (55 "C) with an influent concentration of 80 g of VSIL. This produces yields of 3.96 L of CHJL fermenterday at 5 days retention time. It is apparent that the anaerobic fermentation process is technically feasible. However, only at plant sizes larger than 300 Mg TSIday will the anaerobic fermentation system produce methane gas comparable to the current natural gas price. I f the effluent can be used as a feed supplement for livestock, the anaerobic fermentation system for livestock residue will be economically feasible at a plant size between 3 and 6 Mg TSIday. This corresponds to beef cattle feedlots between 1000 and 2000 head.
Introduction Recent concern over dwindling natural resources has prompted the development of technologies that are responsive to energy shortages, resource conservation, and environmental degradation. Anaerobic fermentation of livestock and crop residues to produce methane has drawn interest because the methane produced from fermentation can be sold or used within the livestock and farm operations, the effluent can be used as fertilizer or be fed back to livestock, and the fermentation of livestock wastes provides pollution, odor, and pest controls. Van Dyne and Gilbertson (1978), surveying 3050 counties in the United States, estimated that 56.8 X lo9 kg (dry weight basis) of manure was produced annually from beef cattle, 23.9 X lo9 kg from dairy cattle, 8.97 X lo9 kg from swine, and 6.98 X lo9 kg from poultry in 1974. Because only 52% of these manures is collectable, the potential methane production from these livestock wastes is estimated to be about 13.6 X lo9 m3 (STP) per year. This is equivalent to an energy of 0.506 X lo9 GJ per year (0.479 QUADS). The annual total crop residue (corn stalk and wheat straw) production was estimated to be 400 x lo9 kg (Anderson, 1972). Because only one-half of these residues is collectable, the potential methane production from this resource is estimated to be 50 X log m3 (STP),which is equivalent to 1.87 X lo9 GJ (1.77 QUADS) of energy production annually. The total potential methane production from agricultural residues is around 63.6 X lo9 m3 (STP),which is equivalent to a production of 2.38 x IO9 GJ of energy annually. This is about 2.2% of the total
estimated energy demand in the United States in 1980 (Bois, 1977). Although the amount of crop residue produced each year in the United States is over four times the amount of livestock residue produced, the crop residues are also used as roughage in cattle and sheep rations, bedding for livestock, and soil conditioner to improve fertility and reduce soil erosion. For these reasons, interest has concentrated mostly on developing the technology and evaluating the feasibility of converting livestock residue to methane. However, the microbial reactions, process schemes, kinetics, and equipment and labor requirements are similar for livestock wastes and crop residue. Anaerobic Fermentation Anaerobic fermentation is a biological process by which organic matter is decomposed to yield gaseous methane and carbon dioxide in the absence of air. At present, four different bacterial groups are recognized as being responsible for carrying out the fermentation (Figure 1). The hydrolytic bacteria (group I) catabolize carbohydrates, proteins, lipids, and other minor components of biomass to fatty acids, H2,and CO,; hydrogen-producingacetogenic bacteria (group 11) catabolize certain fatty acids and neutral end products to acetate, C02, and H,; homoacetogenic bacteria (group III) synthesize acetate using H2, C02,and formate, or hydrolyze multicarbon compounds to acetic acid; and methanogenic bacteria (group IV) then utilize acetate, C02 and H2 to produce methane (Zeikus, 1979; Wolfe, 1979). The coordinated activity of these
This article not subject to US. Copyright. Published 1980 by the American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 4, 1980 COMPLEX ORGANIC MATTER
ACElOOENIC BACTWA
I
IV. METWNWENIC BACTERIA
CH4. C%
Figure 1. Illustration showing the four bacterial groups involved in the anaerobic fermentation of organic matter.
bacterial groups as a whole ensures process stability during anaerobic fermentation. Environmental Requirements Some environmental factors that influence the bacterial degradation and volume of gas produced during methanogenesis are pH, alkalinity, volatile acids, temperature, nutrients, and toxic materials. The methanogenic and acetogenic bacteria are sensitive to the pH of fermenter liquor. The pH in turn is a function of the bicarbonate alkalinity of the system, the COzpartial pressure in the fermenter, and the concentration of volatile acids. McCarty (1964a) reports that methane production proceeds quite well as long as the pH is maintained between 6.6 and 7.6, with an optimum range between 7.0 and 7.2. At pH values below 6.2, acute toxicity occurs. High pH can be a problem with methane production from animal wastes because of the high levels of ammonia generated at high organic loading rates (Jewel1 et al., 1976). Alkalinity is a measure of the buffering capacity of the fermenter contents and consists of the bicarbonate, carbonate, ammonia, and hydroxide components. Organic acids and acid salts may also contribute to the buffering capacity (Am. Public Health ASSOC.,1975). McCarty (1964a) indicates that a bicarbonate alkalinity in the range of 2.5 to 5.0 g of CaC03/L (25 to 50 mM) provides a safe buffering capacity for anaerobic treatment of waste. Sievers and Brune (1978) and Kroeker et al. (1979) reported on the importance of ammonia in buffering animal waste fermentations. The relatively low carbon:nitrogen ratio of animal manures was reported to be a major factor in the stability experienced in fermenting animal manures. Ammonia was reported to contribute to the process stability by increasing the bicarbonate buffering capacity and elevating the solution pH. McCarty and McKinney (1961) found that organic acid levels should remain below 2.0 g of acetate/L (33 mM) for efficient fermentation. Above this level, the acids were toxic. This apparently holds true for thermophilic temperatures also, as Varel et al. (1977) reported less efficient methane production from cattle waste when the level of organic acids rose above 2.0 g/L. Kroeker et al. (1979) showed acute methanogenic toxicity at unionized volatile acid concentrations between 30 and 60 mg/L as acetic acid. This corresponded to total volatile acid concentrations between 1.65 and 2.6 g/L as acetic acid. Temperature is an important environmental parameter in anaerobic digestion processes. Faster rates of fermentation and minimization of bacterial and viral pathogens are some benefits obtained with higher temperatures (Pfeffer, 1974; Cooney and Wise, 1975). Using shredded municipal refuse, Pfeffer (1974) established two optimum temperatures. The optimum in the mesophilic and thermophilic range was 42 and 60 "C, respectively. He also
concluded that it was less expensive to produce methane at the higher temperature. Our study of anaerobic fermentation of beef cattle manure ranging from 30 to 65 "C, however, did not show an optimum at 42 OC. Instead, the reaction rate increases from 30 to 50 OC, maintains fairly constant at 50 to 60 "C, and has a sharp drop at 65 "C (Chen et al., 1979). A definite acclimation period was required to initiate fermentation in the thermophilic temperature range. Buhr and Andrews (1977) indicated that although the literature is contradictory, it does indicate that minor fluctuations in temperature can cause problems for thermophilic fermenters and this may require further study. Golueke (1958) found that the total volatile acids increased with increases in temperatures between 35 and 65 "C. Another important environmental condition is the presence of the nutrients such as nitrogen, phosphorus, and sulfur, and trace nutrients needed by bacteria (Bryant, 1974; Bryant et al., 1971; McCarty, 1964a). Animal manures usually contain d the required nutrients in adequate quantities. McCarty (1964b) reports that other elements which have stimulatory effects at low concentrations may include sodium, potassium, calcium, magnesium, and iron. All of these elements may exhibit inhibitory effects at higher concentrations. In general, the bacteria involved in methanogenesis that have been studied have simple nutrient requirements and, while various individual species may require organic materials such as B-vitamins, fatty acids, or a small number of amino acids for growth, these are supplied by other bacterial species (Bryant, 1974; Bryant et al., 1971). Thus, inorganic minerals are the nutrients of main concern. Other environmental factors involve toxicities resulting from excessive quantities of many organic or inorganic substances. The threshold toxic levels of inorganic substances vary depending on whether these act singly or in combination. Certain combinations have synergistic effects, whereas others display antagonistic effecta (McCarty, 1964b; Kugelman and McCarty, 1965). A number of investigators have implicated high concentrations of sulfate as retarding the production of methane, but recently, Bryant et al. (1977) and Winfrey et al. (1977) have independently proposed that competition for available H2 is the mechanism by which sulfate inhibits methanogenesis in natural ecosystems. The sulfate-reducing bacteria apparently are able to scavenge the available H2 faster than the CHI bacteria. Ammonia inhibition is a significant problem with some high rate fermentation processes, particularly when ammonia-rich wastes from swine and poultry are fed and a proper acclimation period is not permitted (Lapp et al., 1975; Stevens and Schulte, 1979; Sievers and Brune, 1978; Kroeker et al., 1979; Converse et al., 1977a). McCarty (1964b) reported that at concentrations between 1.5 and 3.0 g/L of total ammonia nitrogen and at a pH greater than 7.4, the ammonia concentration may become inhibitory. At concentrations above 3.0 g/L, ammonia becomes toxic regardless of pH. However, Lapp et al. (1975), Converse et al. (1977) and Fischer et al. (1979) have reported stable CH4 production with ammonia concentrations in excess of 3.0 g/L (2.2 to 8.0 g/L). Kroeker et al. (1979) used a urea and acetic acid substrate to investigate the effect of ammonia inhibition on CHI production. They concluded that CHI was progressively inhibited as the ammonia nitrogen concentration increased above 2 g/L; however, toxicity (i.e., complete cessation of CHI production) did not occur even at ammonia nitrogen concentrations of 7.0 g/L.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 4, 1980
Fermenting livestock residues that have been fed antibiotics may also face toxicity problems. Hashimoto et al. (1979b) reported that chlortetracycline had no adverse effect on methanogenesis; however, monensin nearly doubled the lag time for CHI production in batch fermentation, and only at very long retention times (30 to 40 days) could a stable fermentation in a continuous system be maintained. Fischer et al. (1978) also reported that the fermentation was unstable when the swine residues for the swine fed lincomycin for controlling dysentery was fermented. Other common forms of toxicity include those of heavy metals, but the concentrations and conditions under which heavy metals are toxic are ill-defined (McCarty, 1964b; Kugelman and Chin, 1971). Fermentation Kinetics Kinetic Models. Several kinetic models have been wed to describe the anaerobic fermentation process. The Monod (1950) kinetic model has been adapted to describe the anaerobic digestion kinetics of sewage sludge (O'buke, 1968,Lawrence and McCarty, 1969; Andrews and Pearson, 1965) and animal manures (Morris, 1976; Hill and Barth, 1977). The advantages of the Monod-type model are that the kinetic parameters (the microorganism maximum specific growth rate and half-velocity constant) have deterministic connotations which describe the microbial processes, and the model is able to predict the conditions for maximum biological acticity, and when activity will cease (i.e., wash-out). Disadvantages of the Monod model are that one set of kinetic parameters cannot describe the biological process a t short and long retention times (Garrett and Sawyer, 1952; Chiu et al., 1972a,b) and that the kinetic parameters vary with the influent concentration (Morris, 1976) and sometimes cannot be obtained for certain complex substrates (Pfeffer, 1974). To overcome the disadvantages of the Monod model, various forms of the first-order kinetic model have been used (McKinney, 1962; Eckenfelder, 1963; Grau et al., 1975; Grady et al., 1972; Pfeffer, 1974; Morris, 1976). The advantages of the first-order models are that they are simple to use and give good fit of experimental data. Their disadvantages are that they do not predict the conditions for maximum biological activity and system failure. The Contois (1959) kinetic model has these advantages and generally avoids the disadvantages inherent in the Monod model. The Contois model was adapted to describe the kinetics of CHI fermentation (Chen and Hashimoto, 1978; Chen et al., 1979)
where yv = volumetric CH4 production rate, L of CH4/L of fermentepday; Bo = ultimate CHI yield, L of CH4/g VS added as 6 m, So = influent total volatile solids (VS) concentration, g/L; 6 = retention time, day; pm = maximum specific growth rate of microorganisms, day-'; and K = kinetic parameter, dimensionless. Equation 1states that for a given loading rate (&/e), the daily volume of CHI qroduction per volume of fermenter depends on the retention time (e), the biodegradability of the material (Bo) and the kinetic parameters pm and K. Ultimate Methane Yield (Bo).Equation 1shows that the amount of CH, produced is directly proportional to the ultimate CHI yield (Bo). Theoretically, a reduction of 1g of chemical oxygen demand (COD) is equivalent to the production of 0.35 L of CH4 at standard temperature (0 "C) and pressure (1atm) (McCarty, 1974). The ultimate
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473
Table I. Ultimate Methane Yield of Livestock Manures Bo, L of data used species ration CH4/g of VS 9% corn silage 0.32 Chen et al. (1979) 91.5% corn silage 0.17 Hashimoto e t al. (197913) 40.0% corn silage 0.23 Hashimoto e t al. (197913) 7% corn silage 0.29 Hashimoto e t al. (197913) 20% roughage 0.28 Vareiet al. ' (1977) 58-68% silage 0.24 Morris (1976) 72% roughage 0.17 Bryant et al. (1976)
-
-
07-
0 21
-B
0.6
.
A
w 0.5 .
h
I 04k
03-
0 IL
f
oe-
25
30
35 40 45 50 55 FERMENTATION TEMPERATURE ( C'l
60
€5
Figure 2. Effect of fermentation temperature on the maximum specific growth rate of microorganisms (steady-state results).
CH, yield is, therefore, a measure of the biodegradability of the waste. For livestock wastes, Bo depends on the species, ration, the age of the manure, the collection and storage method, and the amount of foreign material such as dirt and bedding incorporated into the manure but not fermentation temperature. Table I shows some values of Bo for cattle wastes of different rations. Since the cattle used in the experiments reported by Varel et al. (1977), Bryant et al. (1976), and Morris (1976), were all dairy breeds, the variation of Bo may have been attributed to the different rations fed to the cattle. Table I shows that the waste from cattle fed higher grain rations had greater Bo values than that from animals fed higher roughage rations. The beef cattle manure with Bo values of 0.29 and 0.32 L of CH4/g of VS was from cattle fed a high concentrate ration. Our studies show that Bo does not depend on temperature ranging from 30 to 60 "C (Chen et al., 1979, Hashimoto et al., 1979~). Maximum Specific Growth Rate (pm). Figure 2 shows the effect of temperature on the maximum specific growth rate for fermentation of beef cattle manure (Chen et al., 1979). The data do not show peak rates at 42 and 60 "C, as reported by Pfeffer (1974),but a gradual increase in with temperature to 60 "C then a decline. Based on the previous reported pm on anaerobic fermentations of sewage sludge, municipal refuse, and livestock wastes, Hashimoto et al. (1980) have further expressed the pm as a linear function of the fermentation temperature for temperatures ranging from 30 to 60 "C pm = 0.013(t) - 0.129 (2) where t is temperature in "C. Kinetic Parameter ( K ) . Figure 3 shows the effect of influent volatile solids (VS) concentration on K for beef
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-
10.0
0 DAIRY RESIWE. 60. C
iI
0 DAIRY R E S I M , 52.5' C
8.0
.
A B E 5 RESIWE. 60. C
Y
K
k! 6.0 -
[ 0
4.0
-
2.0
.
Q
'I
LIVESTOCK M A W S PILOT- AND FULL-SCALE FERMENlVRS
B
/
RELATIVE STANDARD
I
w +
H
0.0 I
0
20
40
60
80
100
I20
140
INFLUENT VOLATILE SOLIDS g/L
Figure 3. Effect of influent volatile solids concentrations on the kinetic parameter K.
PREDICTED VOLUMETRIC METHAN PRODUCTION RATE ( L CH4/L fermentw/doy)
Figure 5. Comparison of the predicted volumetric methane production rates from the pilot- and full-scale fermenter resulta (beef, dirt lota, V; beef, confinement, A; dairy, 0;swine, 0).
TEMPERATURE. 60. C
LOADING RATE, g VS/L fennenlor.day
Figure 4. Effect of loading rate on volumetric methane production rate.
and dairy residue fermented at 32.5 and 60 "C. The value of K for 60 "C is fairly constant up to an influent VS concentration of about 80 g/L; then it increases exponentially as the VS concentration increases above 80 g/L. Figure 3 also shows that there may be an effect of temperature on K at high influent VS concentrations. The values at 32.5 "C are close to those at 60 "C at low influent VS concentrations, but higher at VS concentrations above 60 g/L. At 80 g/L, K may be as high as 1.7 for 35 "C (Hashimoto et al., 1980). Equation 1 shows that when Bo, So, 0, and p m are constant and K is increased, the CHI production rate (yv) decreases. Thus, the increase in K indicates some type of inhibition which may be caused by one or more of the following: overloading (i.e., more substrate is being added to the system than the bacteria can effectively utilize); inhibitory substances (e.g., volatile acids, ammonia, heavy metals, and salts) exceeding threshold levels at that concentration; or reduced mass transfer of substrate and products due to the higher solids concentration. The Effect of Loading Rate on Methane Production. Figure 4 is a plot of yv vs. VS loading rate assuming a constant So. For a given VS loading rate, higher CHI production rates are possible at higher influent VS concentrations and longer retention times than at lower VS concentrations and shorter retention times. However, this holds true only when K is a constant. At influent VS concentration above 80 g/L for 60 "C, yv decreases due to inhibition (higher K ) . Application of the Kinetic Model. Equation 1 was used to predict yv of various pilot- and full-scale systems fermenting livestock manures between 30 and 60 "C (Table
11). Equation 2 was used to estimate p m at each temperature. The data in Figure 3 were used to estimate K at each So (K = 0.9 up to So= 80 g of VS/L, and K = 1.6 for So = 104 g of VS/L for the data of Jewell, 1979). Ultimate CHI yields (Bo)were assumed to be 0.25,0.20, and 0.50L of CH4/g of VS added for beef (dirt lots), dairy, and swine manure, respectively. Value of Bo was assumed to be 0.34 L of CH4/g of VS for the confinement beef manure used for 0 = 20, 12, 6, and 4 day and 0.38 L of CH4/g of VS for the beef manure used for 0 = 7 and 5 day since these values were reported previously for these manures (Hashimoto and Chen, 1979a). The difference in biodegradability was attributed to the difference in rations fed to the cattle and the difference in manure collection practices (Hashimoto and Chen, 1979a). Figure 5 shows a plot of the experimental vs. the predicted yv. The points are remarkably close to the line-of-perfect fit, considering that pm, K , and Bowere obtained independent of the experimental data (i.e., these values were not adjusted to fit the experimental data). Until more values of Bo are experimentally derived (as described in the section of Bo), we recommend the following Bovalues in L of CH4/g of VS added beef (dirt lot, 0.25 f 0.05; beef (confinement), 0.35 f 0.05; dairy, 0.20 f-0.05;and swine, 0.50 f 0.05.
Biogas The major constituents of biogas and their concentrations generated from stable fermentations are given in Table 111. Methane comprises about 50 to 65% of the biogas produced during anaerobic fermentation. Methane is a colorless, odorless, flamable gas with an energy value of 37.3 MJ/m3. The critical temperature and pressure of CHI are -82 "C and 4.6 MPa, respectively. Carbon dioxide is the other major constituent of biogas. The predominant effect of COz in biogas is that it dilutes the energy value of biogas and increases the volume of biogas to be handled and stored. Water vapor in the biogas is an important contaminant that must be removed before most applications. Condensed water tends to accumulate in gas handling equipment and meters. This causes malfunctions such as frozen pipes and corrosion of metal parts when combined with hydrogen sulfide (H2S)present in the gas. Pigg (1977) reported an average HzS concentration of 1.9 g/m3 (range: 1.7 to 2.1 g/m3) in biogas from dairy manure fermented at 35 "C. Converse et al. (1977) reported Ha concentrations ranging from 4.0 to 12.5 g/m3 in biogas from poultry manure fermented at 35 "C. Our data showed an average H2S concentration of 1.52 g/m3 (standard deviation = 0.64) and no effect of fermentation
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 4, 1980 475
Table 11. Experimental and Predicted Volumetric Methane Production Rates y v , L of CH,/L.day teomp, ST,Ie, kg Of C VS/m3.day e , day exptl pred species beef dirt 3.1 20 0.69 0.67 35 20 1.07 confinement 55 1.06 3.42 confinement 5.20 12 1.54 1.51 55 confinement 6 2.85 55 11.4 2.72 4 3.03 confinement 55 14.9 2.76 confinement 55 7 3.48 11.8 3.34 confinement 55 5 4.41 17.0 4.13 confinement 55 5 4.35 16.8 4.09 manure wlbedding feces feces manure w/straw manure w/straw manure w/straw manure w/straw manure manure manure manure" manure wlwood chips manure w/strawb manure wlstraw b
35 35 35 35 35 35 35 35 35 35 35 35 35 35
6.40 4.26 4.23 4.31 4.28 6.12 9.90 6.22 10.51 3.45 3.23 3.71 3.51 6.94
dairy 12 15 15 15 15 10.4 6.2 10.2 6.2 30 30 30 30 15
confinement confinement confinement confinement confinement confinement confinement confinement confinement confinement
35 35 35 35 35 35 35 35 40 40
2.10 2.10 1.05 1.05 2.90 2.61 3.12 4.0 4.81 2.40
swine 15 15 30 30 15 15 15 15 6 12
Hourly-fed.
Plug-flow.
reference Burford e t al. (1977) Hashimoto et al. (1979a) Hashimoto et al. (1979a) Hashimoto et al. (1979a) Hashimoto et al. (1979a) Hashimoto et al. (1979a) Hashimoto et al. (1979a) Hashimoto et al. (1979a)
0.77 0.83 0.56 0.67 0.53 0.77 1.07 0.94 1.41 0.66 0.72 0.47 0.61 0.72
0.98 0.69 0.69 0.70 0.70 0.89 1.06 0.91 1.13 0.58' 0.55' 0.63' 0.60' 0.99'
Coppinger et al. (1978) Converse et al. (1977) Converse et al. (1977) Converse et al. (1977) Converse et al. (1977) Converse et al. (1977) Converse et al. (1977) Converse et al. (1977) Converse et al. (1977) Jewell (1979) Jewell (1979) Jewell (1979) Jewell (1979) Jewell (1979)
0.95 0.89 0.57 0.50
0.86 0.86 0.48 0.48 1.18 1.06 1.27 1.63 1.41 0.96
Kroeker et al. (1979) Kroeker et al. (1979) Kroeker et al. (1979) Kroeker et al. (1979) Fischer et al. (1978) Fischer et al. (1978) Fischer et al. (1978) Fischer et al. (1978) Stevens and Schulte (1979) Stevens and Schulte (1979)
1.08 1.07 1.17 1.36 1.42 0.91
K = 1.6.
Table 111. Composition of Biogas constituent
concentration
methane carbon dioxide moisturea hydrogen sulfide
50-65% 35-50% 30-160 g/m3 1.52-12.5 g/m3
a Assumes saturation between 35 and 60 "C. pH between 7 and 8.
Assumes
temperature (range 45 to 65 "C) on the concentration of H2S. Ashare et al. (1978) evaluated available methods to clean biogas to meet natural gas pipeline standards. Their assessment showed that water scrubbing was the most economical process which is used commercially. Two other processes, the phosphate buffer and membrane separation processes were comparable in cost to the water-scrubbing process. However, neither of these processes has been extensively field tested at this time. Water-scrubbing is a relatively simple process in which the H2S-free biogas is compressed and then flows countercurrent to water in a pressurized packed column. The scrubbed C 0 2 is then vented to the atmosphere and the water recycled back to the stripping column. The scrubbed biogas is then dehumidified to meet the natural gas pipeline standards. One of the simplest and most common methods to remove H2S is the iron oxide (or iron sponge) process. In this process, H 2 0 reacts chemically with ferric oxide to form ferric sulfide. The sponge is regenerated by passing
oxygen through it to oxidize t,he ferric sulfide to elemental sulfur and ferric oxide. The ferric oxide is usually impregnated on wood shavings as a low-cost medium. Periodically, the ferric oxide must be recharged because the sulfur accumulation tends to reduce activity and increase the pressure drop through the bed. One cubic meter of iron sponge can clean about 33 OOO m3of biogas containing 2 g of H2S/m3(Lapp et al., 1978). Biogas is best dehydrated by absorption in ethylene or triethylene glycol solutions. The advantages of glycols are high hygroscopicity, good stability to heat and chemical decompositions, low vapor pressures, and commercial availability at moderate cost (Ashare et al., 1978). In a typical glycol dehydration process, water vapor is continuously absorbed by a concentrated glycol solution in a countercurrent pack or bubbly tray column. The glycol is regenerated by inert gas stripping under heat. Energy Requirements Table IV summarizes the energy requirements for systems fermenting beef cattle manure operating at 55 O C , 5 days retention time, and 80 g of VS/L influent concentration (Chen and Hashimoto, 1980). Table IV shows that the heating requirement to maintain the fermenter at 55 "C comprises the major portion of energy consumption, ranging from 39.7% to 37.0% of the gross energy production at an assumed ambient temperature of 10 "C. Of this heating energy requirement, 87.4%to 93.9% is used to heat the influent slurry (Chen and Hashimoto, 1980). This indicates the absolute necessity of recovering the effluent heat energy for heating the influent. The heating
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 4, 1980
Table IV. Energy Production and Requirement for Anaerobic Systems Fermenting Beef Cattle Manure at 5 5 O Ca plant size, Mg TS/day parameter volume of fermenter liquor, m3 gross methane energy production, GJ/day heating energy required,b GJ/day heating energy required,b w/50% effluent heat recovery, GJ/day pumping energy required,c GJ/day mixing energy required,d GJ/day CO, scrubbing, GJ/day CH, compression, GJ/day
1 53.1
10 531
100 1000 5310 53100
7.85
78.5
785
3.12
29.79 293.0 2907
1.757
16.15 156.5 1543
7850
0.0345 0.283 0.524 3.44 0.551
5.51
55.1
551
0.2142 2.142 21.42 205 0.0900 0.900 9.00 90.0
Influent concentration 80 g of VS/L, 5 days retention time, 7~ = 3.96 m 3 of CH,/m3-fermenter.day. Ambient and process slurry temperature, 1 0 "C. 1 0 h of pumping, 300 m effective length. 24 h of mixing. a
,
energy requirement is reduced from 37.0% to 19.7% of the gross energy production for a 1000 Mg TS/day plant if 50% of the effluent heat is recovered. The next major energy consumption occurs in mixing. It amounts to 7.3% of the total methane energy production. Our laboratory and pilot studies showed that the methane production did not vary with the duration of daily mixing of the fermenter liquor (Hashimoto et al., 1979c), indicating that the minimum mixing requirement for fermentation systems may be based on the slurry and effluent handling aspects. The energy consumption of mixing can be cut at least one half by reducing the mixing time. The least energy was consumed in pumping. Pumping energy depends less on the plant size because larger pipes can be used for handling larger volumes of the slurry. Energy consumed per volume pumped decreases as pipe size increases. The total energy consumption, including C 0 2scrubbing and methane gas compression, but excluding heating energy, accounts for 11.3% of the gross methane energy production for the 1Mg TS/day plant and 10.8% for the 1000 Mg TS/day plant. Methane Production Costs Since the capital and operational cost of the system is generally proportional to a 0.7 power of the fermenter volume (Hashimoto and Chen, 1980), a relative cost of the system as compared to fermenter system having a volume of 1000 m3 can be defined as relative cost = (~olume/1000)~." (3) Figure 6 compares the net thermal energy production (gross thermal energy production minus gross heat energy requirement) per relative cost of the fermenter operating a t 35 and 55 "C as a function of retention time. Figure 6 shows that conducting a fermentation at 55 "C does not produce a significant economic gain over 35 "C when an influent concentration such as 60 g of VS/L is added because the increase in the methane energy production at 55 "C is offset by the extra heat required. It has been demonstrated that a successful operation at 80 g of VS/L influent concentration at 55 "C is possible (Hashimoto et al., 1978). There is, however, an indication that a fermenter at 35 OC with 80 g of VS/L influent concentration is inhibited (K = 1.7) (Hashimoto et al., 1980). Figure 6
20
28
RETENTION TIME (days)
Figure 6. Comparing the net thermal energy production per relative cost of the fermenter operating at 35 and 55 "C and influent volatile solids concentrations of 60 and 80 g/L. Table V. Costs for Producing Biogas (Option A ) O and Methane (Option B)b from Anaerobic Fermentation of Beef Cattle Manure at 55 "C plant size, Mg TS/day parameter 1 10 100 1000 capital costs, $1000 option A option B fixed costs, $1000/year option A option B utility costs, $1000/year option A option B labor costs, $1000/year total annual costs, $1 000/year option A option B production cost, $/GJ option A option B
70.7 89.6
354 449
17.0 21.5
85.0 525 107.7 665
2.24 2.86 11.8
21.9 28.2 22.9
31.0 36.2
130.1 782 158.8 985
13.94 5.72 16.28 6.98
2185 13490 2769 17090 3240 4100
213 2126 276 2760 44.3 85.9 5450 6942
3.41 2.61 4.29 2.74
Biogas is compressed to 860 kPa 1 2 h of mixing fermenter liquor and influent slurry. & CO, is scrubbed and methane is compressed to 860 kPa, 1 2 h of mixing fermenter liquor and influent slurry.
shows that the fermentation system with an influent concentration of 80 g of VS/L and retention time at about 5 days has the highest net thermal energy production per relative cost. In the following calculations, COzwas scrubbed from the biogas and the methane gas was compressed to 862 kPa (125 psi). Fifty percent of the effluent heat was recovered to heat the influent. The electricity cost was 5&/kWhand the water cost was llt/m3. The influent volatile solids was 80 g/L. The volumetric methane production was 3.96 m3 of CH4/m3 fermenter-day at 5 days retention time (Hashimoto et al., 1980). The capital cost ranged from $1685/m3 for a fermenter volume of 53.2 m3 to $321/m3 for a fermenter volume of 13300 m3. The total capital costa were estimated by using engineering and inspection fees, contingency, escalation and start-up costs of 14,10,18, and 10% of installed equipment costa, respectively (Hashimoto and Chen, 1980). The annual fixed cost was taken to be 24% of the capital cost, assuming an interest rate of 14% and a 20-year straight-line depreciation of the total capital cost and 3,1,5, and 3% of the installed equipment cost for taxes, insurance and repair and maintenance, respectively. Table V shows that the annual cost of thjs system ranged from $36 100 for 1Mg TS/day plant to $6 942 OOO for lo00 Mg TS/day plant, the maintenance and utility cost ranged from $2860 for 1Mg TS/day plant to $2755000 for 1000
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 4, 1980 477
Mg TS/day plant. The methane production cost decreased from $16.28/GJ for l Mg TS/day plant to $2.74/GJ for lo00 Mg TS/day plant. This production cost includes COz scrubbing and gas compression costs. Without COz scrubbing, the methane production would reduce the cost to $13.94/GJ for 1Mg TS/day plant and to $2.61/GJ for lo00 Mg TS/day plant. Considering that the current natural gas price is about $3.00/GJ, the 300 Mg TS/day plant would prdduce methane at a competitive price a t current natural gas prices. It is apparent that at current natural gas prices, only large plant size will produce methane from livestock residues economically. However, Hashimoto and Chen (1980) have discussed the different options of using biogas and have shown that the effluent from fermenting beef cattle residue has a feed value of at least $60 per dry ton if the effluent is used as a protein supplement in livestock feed. With the refeed credit of the effluent, the anaerobic fermentation of beef cattle residue will become economically feasible at a plant size between 3 to 6 Mg TS/day (Hashimoto and Chen, 1980). Literature Cited American Public Health Assoclatlon, “Standard Methods for the Examination of Water and Wastewater”. 14th ed.;American Public Health Assoclatlon, Inc.; New York, 1975. Anderson, L. L. U . S . Bur. Mines Inf. Circ. 1972, No. 8549. Andrews, J. F.; Pearson, E. A. Int. J. Air Water Pol/ut. 1985, 9 , 439. Ashare, E.; Augensteln, D. C.; Young, J. C.; Hossan, R. J.; Duret, G. L., Engineering Report COO-2991-19, Dynatech RID Co., Cambridge, MA, 1978. Bois, S. 0. Water, Air, So// Polkit. 1977, 7 , 147. Bryant, M. P. Am. J. Clin. Nutr. 1974, 27, 1313. Bryant, M. P.; Tzeng, S. F.; Robinson. I. M.; Joyner. Jr. A. E., A&. Chem. Ser 1971, No. 105. Bryant, M. P.; Varel, V. H.; Froblsh, R. A.; Isaacson, H. R. “Semlnar on Microbial Energy Conversion”, H. G. Schlegel, Ed.; E. Goltz KQ Gottlngen, Germany, 1976. Bryant, M. P.; Campbell, L. L.; Reddy, C. A.; Crablll, M. R. Appl. Environ. Microbbi. 33. 1162. Buhr, H. 0.; Andrews, J. F. Water Res. 1977, 11, 129. Burford, J. L.; Varanl, F. T.; Schellenbach, S.; Turnocllff, W. F.; Shelley, D.; Pace, B., Final Report to Four Corners Regional Commission, Grant No. 672-366-002, BIo-Gas of Colorado, Inc., 1977. Chen, Y. R.; Hashlmoto, A. G. Bloechnol. Bloeng. Symp. No. 8, 269-282. Chen, Y. R.; Varel, V. H.; Hashlmoto, A. 0. “Proceedings, Second Symposium on Biotechnology In Energy Production and Conservatlon”, Gatllnburg, TN, Oct 3-5. 1979. Chen, Y. R.; Hashlmoto, A. G. “Proceedlngs of the Fourth International Symposium of Llvestock Wastes”, Amarillo, TX, Apr 15-17. 1980. Chlu, S. Y.; Fan, L. T.; Kao, I. C.; Erlckson, L. E. Bbtechnol. Bloeng. 14, 179. Chlu, S. Y.; Erlckson, L. E.; Fan, L. T.; Kao, I. C. Bbtechnol. Bioeng. 1972b, 14, 207. Contols, D. C. J. Gen. Microbiol. 1959, 21, 40. Converse, J. C.; Evans, G. W.; Verhoven, C. R.; Gibbon, W.; Glbbon, M., Paper No. 77-0451. ASAE, St. Joseph, MI, 1977. Cooney, C. L.; Wlse, D. L. Biotechnol. Bloeng. 1975, 17, 1119. Copplnger, E.; Hermanson, R. E.; Baylon, D., Paper No. 78-4566, ASAE, St. Joseph, MI, 1978. Eckenfelder, W. W., Jr., In ”Advances In Blologlcal Waste Treatment”, Eckenfeider, W. W., and McCabe, J., Ed.; Permagon Press: New York, 1963. Flscher, J. R.; Iannottl, E. L.; Slevers, D. M.; Fulhage, C. D.; Meador, N. F., In “Proceedlngs, Great Plains Seminar on Methane Production from Livestock Manure”, Sweeten, J. M., Ed.; Texas Agr. Exp. Sta.,College Statlon, TX, 1978.
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Received for review May 5 , 1980 Accepted August 1, 1980 This paper was presented a t the Symposium of Chemicals from Cellulosic Material held at the 179th National Meeting of the American Chemical Society, Mar 23-28,1980, at Houston, Texas, Division of Industrial and Engineering Chemistry.
