Anaerobic digestion on a diary farm: Overview

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<ul><li><p>Energy in Agriculture, 4 (1985) 347--363 347 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands </p><p>ANAEROBIC DIGESTION ON A DAIRY FARM: OVERVIEW </p><p>L.P. WALKER, R.A. PELLERIN, M.G. HEISLER, G.S. FARMER and L.A. HILLS </p><p>Department of Agricultural Engineering, College of Agriculture and Life Sciences, Cornell University, Riley Robb Hall, Ithaca, NY 14853 (U.S.A.) </p><p>(Accepted 24 July 1985) </p><p>ABSTRACT </p><p>Walker, L.P., PeUerin, R.A., Heisler, M.G., Farmer, G.S. and Hills, L.A., 1985. Anaerobic digestion on a dairy farm: overview. Energy Agric., 4: 347--363. </p><p>This paper explores the design, implementation and performance of an on-farm plug flow anaerobic digestion system. Capital costs for the digester and justifica- tions for certain design decisions are presented. Seasonal variation in the total and volatile solids concentrations, ammonia and organic nitrogen contents, and pH were documented. Biogas outflow of 400--495 m3/day was also documented for an 180-cow herd. </p><p>INTRODUCTION </p><p>In the fall of 1980, Cornell University researchers embarked on an am- bitious undertaking: the design, implementation and demonstration of an integrated energy farm system. The primary goal was to reduce fossil fuels and fossil fuel-based inputs into the farm system by: (a) substituting energy efficient processes and practices for energy-intensive ones; and by (b) using solar-based energy sources -- wind, active solar and biomass. In addition, the sponsors of the project required that the system be designed for, and implemented on, a private farm. Although conducting this project on a private farm placed limits on the logistics of conducting research, it provided a unique opportunity for assessing the concept of an integrated farm system. </p><p>During the 1st year of the project, a variety of energy conservation practices and alternative energy sources were identified and evaluated (Walker et al., 1984). One solar-based energy source assessed was biogas produced from animal waste. This assessment suggested that a plug flow digester, working under mesophilic conditions, could be a reliable and low- cost energy source for the farm. As a result, a plug flow digester was de- signed and constructed on the Millbrook Farm. </p><p>This project was sponsored by the U.S. Department of Energy, New York State Energy Research and Development Authority, New York State Electric and Gas, Agway and Cornell University. </p><p>0167-5826/85/$03.30 1985 Elsevier Science Publishers B.V. </p></li><li><p>32.3</p><p> (~</p><p>06) </p><p>317 </p><p>317 ,t</p><p>O4 </p><p>( 10</p><p>4~</p><p>32, </p><p>/ (IO</p><p>8) </p><p>34.1</p><p> [I</p><p>12) </p><p>MAN</p><p>URE </p><p>HOPP</p><p>ERS </p><p>0o</p><p> I YO</p><p>UNGS</p><p>TOCK</p><p> BAR</p><p>N </p><p>~ </p><p>MAI</p><p>N D</p><p>AIRY</p><p> BARN</p><p> 1~</p><p>2~1 </p><p>D~</p><p>N </p><p>PIPE</p><p>-~I~</p><p> II ~</p><p>S, La</p><p> ~A</p><p>,N P</p><p>,PE-</p><p>'--,</p><p>~N</p><p>UR</p><p>E H</p><p>OP,</p><p>~R </p><p>3 3,</p><p> ,_</p><p>_ .~</p><p> ~</p><p> / </p><p>(~</p><p>~(</p><p> ,04~</p><p>---3"7 </p><p>MET</p><p>HAN</p><p>e REA</p><p>c.ICO</p><p>R ~D</p><p> RA</p><p>IN ~</p><p> "~</p><p>,.= </p><p>(,02</p><p>)'--</p><p>-305</p><p> 0</p><p>0~</p><p> (,0</p><p>4)~</p><p> -- ~</p><p> 3.</p><p>5 (9</p><p>8 (,o2</p><p>~...</p><p> ~</p><p>."</p><p> --</p><p>~ </p><p>;~:'</p><p>CA</p><p>TTLJ</p><p>E </p><p>l=A</p><p>.q_~</p><p>/ ('</p><p>~ </p><p>~ </p><p>~ </p><p>3z.3</p><p> / </p><p>32.9</p><p> \ </p><p>~3</p><p>os</p><p> (,o</p><p>o~</p><p> D</p><p>RAIN</p><p>AGE P</p><p>IPE -</p><p>'~L~</p><p> '. ~</p><p>'--M</p><p>ANURE-~</p><p> PIPE</p><p> --- </p><p>---- -</p><p>- ~</p><p>'~</p><p> - </p><p>-- </p><p>-~. </p><p>(to </p><p>Io,Q</p><p>oo.) </p><p>~-~ ~</p><p>'~ </p><p>'~'~</p><p> ""</p><p>\- </p><p>(92) </p><p>28</p><p>0 </p><p>311 3</p><p>1.7</p><p> M</p><p>AC</p><p>HIN</p><p>E </p><p>(92)</p><p> z~z)</p><p>. z9 3 </p><p>~o o</p><p> 3o</p><p>.s (~</p><p>oz)0</p><p>o4) </p><p>SHED</p><p> (9</p><p>6}(9</p><p>6) (</p><p>~00</p><p>) </p><p>O0</p><p>Fig</p><p>. 1</p><p>. S</p><p>ite</p><p> lay</p><p>ou</p><p>t at </p><p>the</p><p> sta</p><p>rt o</p><p>f th</p><p>e p</p><p>roje</p><p>ct. </p></li><li><p>349 </p><p>The objectives of this paper are: (a) to identify the site characteristics and farm management constraints which influenced the initial digester design; (b) to discuss subsequent system modifications; and (c) to present some of the preliminary results on the digester performance. </p><p>PROCESS DESIGN </p><p>Site description and constraints </p><p>At the start of the project, the farm consisted of approximately 200 head of dairy cattle -- 120 milking cows and associated youngstock. Total farm areage was 154 ha, with an additional 30 ha leased by the farm owners. </p><p>Figure 1 shows the layout of the farm. The dairy barn is a pole-clear span structure with a concrete block foundation. It is a freestall facility with drive-through feed bunkers and alleys. Manure was scraped from the alleyways of the dairy barn using a front-end loader. The youngstock barn is of similar construction, with feeding and manure handling in the same fashion as the dairy barn. </p><p>Figure 2 summarizes total farm fossil fuel and electricity usage at the time the project was started. Gasoline was the largest single fuel consumed on the farm. Planting, harvesting, waste management, feeding and miscel- laneous tasks consumed 800 GJ of gasoline. </p><p>Plug flow digester design variables and alternatives </p><p>Jewell et al. (1978, 1980) conducted the most comprehensive assessment of the plug flow digester concept. In this study, full-scale plug flow and completely mixed digesters were operated simultaneously. Under all full- scale tests, the plug flow digester out performed the completely mixed digester in terms of solids conversion efficiency and volumetric gas pro- duction rates (Jewell et al., 1980). The study found the plug flow digester achieved between 13 and 23% greater volatile solids destruction than the completely mixed unit, and produced net energy even during the coldest periods of the year. </p><p>Optimization study </p><p>Optimal retention time and insulation thicknesses for the walls, floor and surface of the digester were determined from an optimization study (Hills and Walker, 1982). The optimization involved the development of models of the digester energetics and economics. The variables optimized were hydraulic retention time, and insulation thickness for the walls, floor and surface of the digester. Other variables and parameters such as length- to-widt ratio, total and volatile solids concentration of the manure, daily averages of the soil and ambient temperatures, and a dollar value for the gas were also used in the model. </p></li><li><p>700 </p><p>600 </p><p>500 </p><p>400 </p><p>300 </p><p>200 </p><p>100 </p><p>0 </p><p>DISTRIBUT ION 9O0 </p><p>800 t </p><p>OF- FUEL USE 1980 </p><p>ON </p><p>Electricity I </p><p>Gasoline </p><p>350 </p><p>Diesel </p><p>Fuel Type </p><p>Fig. 2. Farm energy usage at the start of the project </p><p>FAR M </p><p>Fuel Oil </p><p>Results from the optimization study indicated a hydraulic retention time of 25 days, and insulation thicknesses of 5 cm for the walls, floor and top of the digester would be the optimum assuming that biogas was worth $2.40/GJ. Further synthesis of the results from the optimization study and more engineering and economic analyses were done to assess the impact of an expanding herd on the digester design and operation. From this assessment, the dicision was made to design the digester with a retention time of 24 days for a herd with 180 milking cows and 120 asso- ciated dry cows and youngstock. The thicknesses of the polystrene insula- tion obtained from this assessment were 5 cm for the floor and walls, and 7.5 cm for the manure surface. </p><p>Digester construction </p><p>A perspective of the digester constructed is presented in Fig. 3. Digester dimensions are 22.6 m X 6.1 m X 2.4 m. Wall heights of 2.4 m were selected because this is the standard height of concrete walls fro basements. Manure entered the digester through two 0.8-m diameter corrugated steel pipes located in the northwest and northeast corners of the digester. The manure exits the digester through three PVC pipes located in the south end of the digester. These effluent pipes are connected to a manhole, which is con- nected to the manure storage facility (Fig. 3). To retard heat loss from the surface of the manure, insulation was placed in the digester above the manure surface (see Fig. 3). </p></li><li><p>t i </p><p>~H</p><p>OR</p><p>T H</p><p>EA</p><p>TIN</p><p>G </p><p>3R</p><p>ID </p><p>see d</p><p>eta</p><p>il) </p><p>"/FL</p><p>UEN</p><p>T </p><p>IAN</p><p>HO</p><p>LE</p><p>- </p><p>IO c</p><p>m F</p><p>OO</p><p>TE</p><p>R </p><p>~"</p><p>~ </p><p>4")</p><p> D</p><p>RA</p><p>IN </p><p>5 c</p><p>m R</p><p>IGID</p><p> IN</p><p>SU</p><p>LATIO</p><p>N -</p><p>- </p><p>2")</p><p>MA</p><p>NU</p><p>RE </p><p>FRO</p><p>M </p><p>YOU</p><p>NG</p><p>STO</p><p>C K</p><p> B</p><p>AR</p><p>N </p><p>:LE</p><p>XIB</p><p>LE</p><p> C</p><p>OV</p><p>ER</p><p> sh</p><p>ow</p><p>n in</p><p>flo</p><p>led</p><p>) </p><p>3 c</p><p>m R</p><p>IGID</p><p> IN</p><p>SU</p><p>LATIO</p><p>N </p><p>3")</p><p> (w</p><p>ith</p><p> wood fr</p><p>om</p><p>e) </p><p>4A</p><p>NU</p><p>RE F</p><p>RO</p><p>M </p><p>~IA</p><p>IN B</p><p>AR</p><p>N </p><p>VIA</p><p>NH</p><p>OLE</p><p>TO E</p><p>AR</p><p>TH</p><p>EN</p><p>.~_..</p><p>__.~</p><p>....</p><p>...I</p><p>--~</p><p> LO</p><p>NG</p><p> HEA</p><p>TIN</p><p>G G</p><p>RID</p><p>--</p><p> ,5</p><p> cm</p><p> RIG</p><p>ID IN</p><p>SU</p><p>LATIO</p><p>N </p><p>(2")</p><p>Fig</p><p>. 3</p><p>. D</p><p>ige</p><p>ste</p><p>r pe</p><p>rsp</p><p>ect</p><p>ive</p><p>, co</p><p> 91 </p></li><li><p>352 </p><p>The digester's flexible cover was initially a 0.9 mm thick hypalon material. A neoprene gasket was placed between the concrete and the flexible cover. A mechanical seal was selected because of the ease with which it could be installed and, more importantly, the ease with which it could be removed to make repairs on the digester. A silicone gel was applied between the flexible cover and neoprene gasket. </p><p>There are two heat exchangers in the digester, as shown in Fig. 4. An influent heat exchanger was designed to heat the incoming manure to 35C. The second or maintenance heat exchanger spans the length of the digester, as shown in Fig. 4. The two heat exchangers share a common return. Heat exchangers were also installed in each of the three hoppers to preheat incoming manure during winter. </p><p>Fig. 4. Digester heat exchange design. </p><p>Two categoreis of instrumentation were included in the system. The first is for the day-to-day management of the digester. In this system thermo- couples were interfaced with three temperature controllers which control the water circulators for the influent and maintenance heat exchangers. A similar arrangement was established for controlling the temperature of the hoppers. Gas production was measured with a positive displacement gas flow meter. </p><p>The second type of instrumentation monitored research variables and parameters. A microcomputer interfaced with an analog-to-digital converter, and a channel scanner read 27 thermocouples at different locations in the digester. In addition it read the gas flow meters, pressure transducer and weather station. </p><p>In addition to the on-line data acquisition system, manure samples were collected periodically and assessed for total and volatile solids, organic nitrogen and ammonia, and pH. Carbon dioxide was measured periodically using a fyrite tester. This method of measuring CO2 concentration in the gas was checked against a gas chromatograph for accuracy and found to be within 1 to 2 percentage points. </p></li><li><p>353 </p><p>System cos ts </p><p>As the project proceeded, a detailed data base of the costs incurred for the various subsystems was compiled. Capital costs for the digester are summarized in Table 1. The total capital cost of $68 500 reflects the total investment for the digester system. Not reflected in the cost are the R &amp; D costs associated with the design of the digester, nor the cost of contract and construction management. </p><p>The major operating costs for the digester system are propane for digester start-up, the cost of seeding the digester when performance is poor or during start-up, cost for maintaining the instrumentation and testing chem- icals, and labor cost. Annual propane usage will vary depending on the type of problems encountered with the digester over a given year. It is part of the cost of digester start-up and can be a very significant cost if problems with gas production occur during the winter months for extended periods. On the average, annual operating costs will be between $300 and $1000. </p><p>Using a herd size of 180 milking cows, the capital cost per cow was $380 per cow. The cost per volume of digester is $204/m 3. </p><p>TABLE 1 </p><p>Capital cost for digester </p><p>Category Cost (US$) </p><p>Digester and hoppers construction 44 300 Digester cover 2 400 Gas transport 400 Digester heat exchange system 4 100 Second digester heat exchange system 2 700 Hopper heat exchange 1 700 Back-up heating system 3 200 Digester instrumentation 2 200 Utility building 7 000 </p><p>Total 68 500 </p><p>SYSTEM OPERATION, PERFORMANCE AND MODIFICATIONS </p><p>All major construction was completed by December 1981. Several modifi- cations have been made to the systems. These modifications are discussed as the system performance is examined in this section. </p><p>Digester s ta rt-up </p><p>Heating of the digester was initiated in January 1982. Two 151-liter propane fueled hot water heaters, each rated at 11.7 kW, were installed, but not as the primary heating system. The waste heat from the cogeneration </p></li><li><p>354 </p><p>system, fueled by biogas, is the primary heating system for the digester. The two heaters lacked the capacity to keep the digester at optimal tem- perature during this first Winter. </p><p>Digester cover failure </p><p>The first problem encountered with the digester was failure of the digester cover. In February 1982, two 5 cm long tears approximately 30 cm apart along one seam of the cover were discovered. One week after the cover was repaired additional tears along the adjacent seam occurred. A series of tests indicated the cover failure was caused by material failure. The cover was replaced with a 1.14-mm PVC-reinforced material with light gray cover and seams running perpendicular to the digester. Since its instal- lation, no problems have been encountered with the cover. </p><p>Quantity of manure and retention time </p><p>Since the herd was allowed to graze on pasture land during late spring, summer and early fall, the quantity of manure available for anaerobic digestion, and the retention time varied throughout the year. Volumetric measurements of the available manure were made several times. From these measurements hydraulic retention times were calculated. These re- sults are presented in Table 2. </p><p>TABLE 2 </p><p>Manure available for anaerobic digestion </p><p>Time frame Available manure (m3/day) </p><p>Hydraulic retention (days) </p><p>Summer 1983 7.1 35.5 Winter 1983/84 10.3 24.5 Fall 1984 10.4 24.2 </p><p>Characteristics of substrate </p><p>Tabulated in Table 3 are the monthly averages for the total and volatile solids concentrations for the manure entering the digester from the dairy barn. A relatively high total solids (TS) concentration of 16.8% or higher was not unusual during the summer months. The volatile solids concentra- tion followed a similar pattern (see Table 3). Dally average volatile solids (VS) concentration for the last two years ranged from a high of 14.6% to a low of 11.2%. The total and volatile solids concentrations varied by as much as 3 percentage points between summer and winter. This variation </p></li><li><p>355 </p><p>can be attributed to a number of factors, such as changes in feed ration and evaporation rate. Total and volatile solids concentrations for the young- stock barn are presented in Table 4. The youngstock barn exhibited the same seasonal variation observed in the main barn manure. Total and volatile solids from bothe the dairy barn and the youngstock barn are relatively high when compared to operating values reported by other investigators (Converse et al., 1977; Jewell et al., 1978, 1980; Bartlett et al., 1980; Schellenbach, 1982). </p><p>The ammonia and organic nitrogen concentrations for the manure from the dairy barn and the youngstock are also presented in Tables 3 and 4, </p><p>TABLE 3 </p><p>Monthly averages for dairy barn manure </p><p>Month Year pH TS VS (mg N per g manure) (%) (%) </p><p>NI-13--N Org--N TKN </p><p>May 82 6.97 14.00 11.15 1.22 3.29 4.51 June 82 7.35 15.63 12.39 1.31 4.20 5.50 July 82 August 82 15.02 12.96 1.31 3.73 5.04 September 82 7.44 14.05 11.89 1.27 3.54 4.80 October 82 8.09 13.30 11.46 1.63 3.88 5.51 November 82 8.05 12.81 11.09 1.38 3.60 4.99 December 82 8.15 13.13 11.44 1.38 3.57 4.95 January 83 8.16 12...</p></li></ul>