Philippine Agricultural Engineering Standard: Biogas Plant PDF

PHILIPPINE AGRICULTURAL ENGINEERING STANDARD PAES 413:2001 Agricultural Structures - Biogas Plant Foreword The formulation of this national standard was initiated by the Agricultural Machinery Testing and Evaluation Center (AMTEC) under the project entitled “Enhanci...

PHILIPPINE AGRICULTURAL ENGINEERING STANDARD PAES 413:2001 Agricultural Structures - Biogas Plant Foreword The formulation of this national standard was initiated by the Agricultural Machinery Testing and Evaluation Center (AMTEC) under the project entitled “Enhancing the Implementation of the AFMA Through Improved Agricultural Engineering Standards” which was funded by the Bureau of Agricultural Research (BAR) of the Department of Agriculture (DA). This standard has been technically prepared in accordance with PNS 01-4:1998 (ISO/IEC Directives Part 3:1997 – Rules for the Structure and Drafting of International Standards. It specifies the general requirements for the construction of biogas plant. The word “shall” is used to indicate requirements strictly to be followed in order to conform to the standard and from which no deviation is permitted. The word “should” is used to indicate that among several possibilities one is recommended as particularly suitable, without mentioning or excluding others, or that a certain course of action is preferred but not necessarily required. In the preparation of this standard, the following references were considered: Aguilar, Francisco X., How to Install a Polyethylene Biogas Plant. Biogas Generation, Noncon Energy Report of the NCED, EUMB, Department of Energy. Biogas Plants, Gobar Gas Company, Nepal. Biogas Technology, UNDP-ESCAP-FAO-CHINA Biogas Training Course. The Asian- Pacific Regional Biogas Research-Training Center, Chengdu, China, 1983. Chinese Biogas Digestion, Selective Dissemination of Science and Appropriate Technology Information in the Philippines. Science and Appropriate technology Information Services, PCATT-SATIS Technology Information Network, Batangas City, Philippines. ISAT – AT information: Biogas. Environment: A system approach to biogas technology, June 1997. Rodriguez, Lylian and T. R. Preston, Biodigester installation manual. University of Tropical Agriculture Foundation Finca Ecologica, University of Agriculture and Forestry, Thu Duc, Ho Chi Minh City, Vietnam. Rokai Pig Farm Demonstration Biogas Plant, Kaunas, Lithuania, 2000. Tambong, Arthur It., Biogas Plant Design, April 1992. D-92  PHILIPPINE AGRICULTURAL ENGINEERING STANDARD PAES 413:2001 Agricultural Structures - Biogas Plant 1 Scope This standard specifies the minimum requirements for the design and construction of a biogas plant utilizing animal wastes. 2 Reference The following normative document contains provisions which through reference in this text constitute provisions of this National Standard: PAES 414:2002 Agricultural Structures - Waste Management Structures 3 Definition For the purpose of this standard, the following definitions shall apply: 3.1 biogas plant plant used to process animal wastes or manure to produce biogas and sludge consisting of an inlet/mixing tank, digester, gas chamber and outlet/sludge tank 3.2 integrated plant biogas plant where the digester and gas chamber form one unit 3.3 split-type plant digester and gas chamber form separate units 3.4 multi-digester plant plant with series of digesters 3.5 floating type plant consisting of digester and a moving, floating gasholder that either float directly in the fermenting slurry or in a separate water jacket 3.6 fixed type closed digester with an immovable, rigid gas chamber and a displacement pit D-93 PAES 413:2001 3.7 balloon type plant consisting of a heat-sealed plastic or rubber bag (balloon), combining digester and gasholder 3.8 collecting tank holding tank chamber where manure and water are collected, stored and separated from heavy and non- biodegradable materials before feeding them into the digester 3.9 inlet pipe serves as conveyor of the manure-water mixture or slurry from the mixing tank to the digester 3.10 digester biodigester bio-reactor anaerobic reactor any water and air tight container designed for the process of anaerobic microbiological degradation of organic matter into which the slurry is introduced for digestion and methanization 3.11 baffle board division in the digester that prevent the slurry from premature exit into the sludge/outlet tank 3.12 stirrer mixer agitator mechanical device inside the digester used to stir the slurry 3.13 gas chamber space inside or outside the digester for the collection and storage of biogas 3.14 gasholder retainer cantilever beam that holds the gasholder/movable cover in position at the desired biogas pressure 3.15 outlet pipe serves as conveyor where the effluent or the slurry is forced out 3.16 D-94  PAES 413:2001 backfill layer of compacted soil and gravel to support the digester wall 3.17 loading rate amount of slurry fed per unit volume of digester capacity per day 3.18 substrate organic material used to produce biogas 3.19 seeding adding or introducing anaerobic bacteria to the digester 3.20 slurry mixture of manure and water 3.21 freeboard difference in height between the digester wall and the filling line 3.22 filling line level of slurry when the digesters is at full load 3.23 retention time average period that a given quantity of slurry is retained in the digester for digestion 3.24 toxic materials materials that inhibit the normal growth of pathogens in the digester such as mineral ions, heavy metals and detergents 3.25 methanization digestion various processes that take place among the methanogens, non-methanogens and substrates fed into the digester as inputs 3.26 methanogens anaerobic bacteria that act upon organic materials and in the process, produce biogas 3.27 D-95 PAES 413:2001 mesophilic temperature rage temperature range of 20 oC – 40 oC where mesophilic bacteria operates 3.28 gas production rate amount of biogas produced per day per cubic meter of slurry 3.29 biogas mixture of gas (composed of 50 to 70 percent methane and 30 to 40 percent carbon dioxide) produced by methanogenic bacteria 3.30 scum layer of floating material (mainly fibrous) on the slurry 3.31 sludge settled portion or precipitate of the slurry; a mud-like, semi-solid mass 3.32 effluent residue that comes out at the outlet after the substrate is digested/processed inside the digester 4 Classification 4.1 According to plant set-up 4.1.1 Integrated 4.1.2 Split-type 4.2 According to number of digester 4.2.1 Single-digester 4.2.2 Multi-digester 4.3 According to gas chamber design 4.3.1 Floating type 4.3.2 Fixed type 4.3.3 Balloon type D-96  PAES 413:2001 4.4 According to feeding method 4.4.1 Continuous-Feed 4.4.1.1 High-Rate Mixed 4.4.1.2 Intermittently Mixed 4.4.1.3 Unmixed 4.4.2 Batch-Feed, Mixed or Unmixed 4.4.3 Hybrid 4.5 According to buried position 4.5.1 Ground digester 4.5.2 Semi-buried digester 4.5.3 Underground digester 4.6 According to geometrical shapes 4.6.1 Rectangular type 4.6.2 Cylindrical dome type 4.6.3 Square type 4.6.4 Ellipsoidal type 4.6.5 Spherical type 4.6.6 Octagonal type 4.6.7 Dome top type 4.6.8 Inverted dome type 4.6.9 Rectangular dome type 4.6.10 Cylindrical tube type D-97 PAES 413:2001 5 Location 5.1 Biogas plant should be located at a site with good drainage. 5.2 It should be located as near as possible to the animal pen and should be lower than elevation of animal pen canal. 5.3 The utilization of biogas should be near. 5.4 Soil foundation should be stable and away from tree roots intrusion. 6 Size of biogas components 6.1 Collecting tank volume 6.1.1 For continuous-fed biogas plat, the size of the tank for collecting and separating manure from heavy and non-biodegradable materials should not exceed to the total slurry volume for 10 days. Table 1 shows the estimated daily quantity of animal manure. 6.1.2 The slurry volume is the volume occupied by the manure and water at a ratio of 1:1 (1 kg of manure: 1 L of water). Table 1 - Estimated daily quantity of available manure Manure available Animals kg/day/animal Pigs Porker, 3-8 months old, mixed ages 2.20 18-36 kg 2.55 36-55 kg 5.22 55-73 kg 6.67 73-91 kg 8.00 Cow Feedlot animal 14.0 Breeding animal 13.0 Work animal 7.50 Buffalo Breeding animal 14.0 Work animal 8.00 Horse Breeding animal 13.50 Work animal 7.75 Chicken Layer, 6 months or older 0.075 Broiler, day-old to 8 weeks 0.025 6.1.3 For batch-fed, the slurry input rate shall be multiplied by the interval of slurry charging. D-98  PAES 413:2001 6.1.4 Calculation of optimum dimension should follow the same procedure used for digester tank. 6.2 Inlet pipe The minimum diameter of the inlet pipe shall be 0.2 m. 6.3 Digester volume 6.3.1 Slurry volume The digester tank capacity is calculated from the daily slurry volume multiplied by the retention time (Table 2). 6.3.2 Retention time Table 2 – Retention time for animal manure for mesophilic temperature range Retention time Substrate days Liquid pig manure 15 – 25 Liquid cow/carabao manure 20 – 30 Liquid chicken manure 20 – 40 Animal manure mixed with plant material 50 – 80 6.3.3 Optimum cross-section of a digester plant 6.3.3.1 Floating type 6.3.3.1.1 Table 3 shows the summary of recommended ratios for different cross-section of a floating type. Table 3 – Optimum height/length ratios of digesters and tanks (freeboard excluded) for volume up to 70 m3 and wall thickness of up to 25 cm Height/Length Ratio, r (Height/Diameter or Height/side) Horizontal Cross-section Floating (Separate Floating Type (Integrated) Gasholder) and Fixed Plants and Open Tanks Type Plants Circular 0.500 1.00 Square 0.500 1.00 Rectangulara L = 1.2W 0.455 0.91 L = 1.4W 0.420 0.84 L = 1.6W 0.385 0.77 a Coefficient of W is the desired length/width proportion, p D-99 PAES 413:2001 Table 3 (continued) Height/Length Ratio, r (Height/Diameter or Height/side) Horizontal Cross-section Floating (Separate Floating Type (Integrated) Gasholder) and Fixed Plants and Open Tanks Type Plants Rectangulara L = 1.8W 0.360 0.72 L = 2.0W 0.340 0.68 L = 2.5W 0.295 0.59 L = 3.0W 0.260 0.52 L = 3.5W 0.235 0.47 L = 4.0W 0.215 0.43 L = 5.0W 0.185 0.37 a Coefficient of W is the desired length/width proportion, p 6.3.3.1.2 If baffle board is provided, it shall be located midway between the inlet and outlet pipes and extends from wall to wall. The height should range from 25% - 50% of the height of the filling line. The height of the filling line should be calculated by subtracting the freeboard from the digester height. If there is no freeboard, filling line is equal to digester height. 6.3.3.2 Fixed type 6.3.3.2.1 The digester and gas chamber is integrated into one tank. The total height of the plant depends on the sum of the digester and gas chamber volume. Eighty percent of the total digester volume is occupied by the slurry. 6.3.3.2.2 For optimum dimension ratio, refer to Table 3. 6.3.3.3 Balloon type The digester volume is 80% of the total digester volume is occupied by the slurry. 6.4 Gas chamber volume 6.4.1 The volume of gas production potential should be calculated by multiplying the amount of manure by its gas production rate. Annex B (Table B.4) shows the gas yield for various manure at different retention time. 6.4.2 For floating type, the effective gas chamber volume should depend on the gas production and gas consumption. It should be calculated by getting the product of accumulation rate of biogas and the longest duration when all non-continuous devices are simultaneously idle. The product should be multiplied by 1.3 to account for the 30% fluctuation in biogas production. Biogas accumulation rate is the difference between the biogas production potential less the biogas consumption of each devices. D-100  PAES 413:2001 6.4.3 For fixed type and balloon type, about 20% of the total digester volume is occupied by the gas generated. 6.4.4 For cost minimization, effective gas volume should also be calculated by getting the daily biogas production less the daily biogas consumption. 6.4.5 Annex J shows the type and biogas requirements of various appliances. 6.5 Outlet pipe The minimum diameter of the outlet pipe shall be 0.2 m. 6.6 Outlet tank volume 6.6.1 For floating and balloon type, the minimum of volume of the outlet tank shall be equal to the daily slurry input of the digester. 6.6.2 For fixed type, the volume of the outlet tank shall be 1/3 of digester volume occupied by the slurry. 6.6.3 Calculation of optimum dimension should follow the same procedure used for digester tank. 7 Functional requirement 7.1 Collecting tank 7.1.1 Concrete channels shall be provided from the source of substrate to the collecting tank with a minimum slope of 2%. 7.1.2 The tank should be concreted and a sluice gate should be provided to control or allow the proper mixture of water and manure. 7.1.3 The floor of the mixing tank should be inclined from 8.5% - 17.5% toward the inlet pipe and it should be elevated at least 0.2 m from the filling line. 7.1.4 A cover made of G.I. sheet shall be provided. 7.2 Inlet pipe 7.2.1 Concrete pipe (prefabricated RC pipe) should be used and it should be inclined 58% with digester wall. 7.2.2 Lower end of inlet pipe should be positioned below the gasholder retainer for floating type. If there is no retainer, the lower end should be located 100 mm from the floor of the digester. D-101 PAES 413:2001 7.2.3 For balloon type, the inlet pipe shall be directly connected to the plastic skin of the balloon. The pipe should be inserted to one half of its length in the interior of the plastic tube and the plastic tube shall be folded around it and shall be secured around the pipe. 7.2.4 The inlet pipe shall be sealed. 7.3 Digester 7.3.1 Fixed and Floating type 7.3.1.1 Digesters should be made of ferrocement, metal, adobe, bricks, reinforced concrete, or reinforced CHB. 7.3.1.2 For reinforced digester, reinforcement shall be a minimum of 10 mm diameter RSB spaced at 0.15 m (both the curved and the horizontal bars) and the curved bars shall be anchored at the top beam. All reinforcement bars shall be secured and tied together with GI wire. 7.3.1.3 More steel reinforcements shall be used for larger digester volume. 7.3.1.4 The concrete walls of the digester shall be reinforced with G.I. chicken wire mesh before plastering with class A mortar mixed with sealing compound or water-proofing compound. Plaster shall be applied in three layers (13 mm, 6 mm, and 6 mm thick). Each layer shall be applied continuously and should be finished within one day. All corners of the digester shall be curved. 7.3.1.5 Floors, beams and foundation shall withstand the maximum load and shall be made of reinforced concrete. 7.3.1.6 Access to the digester should be through the manhole or through the outlet chamber. If a manhole is used as the access inside the digester, it should be constructed in the center of the dome and it should be tightly sealed. Manhole cover should be 0.65 m in diameter and 0.125 m thick. 7.3.2 Balloon type 7.3.2.1 A trench should be dug out from the ground to protect the digester from any damage (from wild and domestic animals) and to help to maintain an appropriate environment for the production of biogas. 7.3.2.2 The sides and floor of the trench should be smooth with no protruding stones or roots, which could damage the plastic film. 7.3.2.3 The floor shall have a slope of about 2.5% from the inlet to the outlet 7.3.2.4 Balloon should is made of red mud plastic, natural polyethylene plastic tube, heat- sealed plastic or rubber balloon where the upper portion serves as the gas storage. In setting the balloon digester, it should be made of two layers of snugly fitted plastic. D-102  PAES 413:2001 7.3.2.5 The plastic tube shall be at least 200 microns. 7.4 Agitator/Stirrer 7.4.1 Natural agitation is recommended for small, low-cost and simple biogas plant. 7.4.1.1 The continuous feeding of a biogas plant can impart motion to the rest of the slurry (Figure 1). Figure 1 – Mixing substrate through inherent flow in fixed-dome plants 7.4.1.2 Other kind of agitation occur as biogas is formed in the sludge layer at the bottom layer, the gas forces the sludge particles to rise to the surface, where they are released and the then particles will fall back to the sludge layer. 7.4.1.3 Natural agitation occurs when the sludge is heated. The hotter slurry will tend to rise within the body of the cooler slurry. 7.4.2 Mechanical agitation should be provided for larger biogas, floating and balloon plants several times a day to: to avoid and destroy swimming and sinking layers, to improve the activity of bacteria trough release of biogas and provision of fresh nutrients, to mix fresh and fermenting substrate in order to inoculate the former, and to arrive at an even distribution of temperature thus providing uniform conditions inside the digester. 7.4.2.1 Agitation should be performed as much as necessary but as little as possible. Annex F shows the various methods of mixing the substrate. 7.5 Gas chamber 7.5.1 Floating-drum plant 7.5.1.1 The gas drum should consist of 2.5 mm mild steel sheets for the sides and 2 mm sheets for the top. It should have a welded-in brace, which break up the surface scum. The drum should be protected against corrosion with suitable coating (oil paints, synthetic paints and bitumen paints). D-103 PAES 413:2001 7.5.1.2 If the floating-drum is made of 20 mm wire mesh-reinforced concrete or fiber cement, it shall have gas-tight internal coating. PVC drums should not be used. 7.5.1.3 The gas drum should have a sloping roof (16.5% slope) and it should be provided with guide frame. 7.5.1.4 The sidewall of the gas drum shall be as high as the wall above the support ledge. Guide frame shall be provided to prevent the floating drum to come into contact with the outer wall and to allow the gas drum to be removed for repair. 7.5.1.5 The depth of liquid jacket should be about 95% of the height of the gasholder and it shall not be less than 300 mm. 7.5.1.6 If the gasholder is too light or too heavy to maintain the desired gas pressure, external weight shall be provided. 7.5.1.7 U-tube manometer should be provided to indicate the relative amount of gas in storage within the biogas plant. 7.5.2 Fixed dome plant 7.5.2.1 The concrete dome shall be reinforced with screen before plastering with class A mortar mixed with sealing compound. Plaster shall be applied in three layers (13 mm, 6 mm, and 6 mm thick). It should be applied continuously and should be finished within one day. 7.5.2.2 The gas chamber shall be capable of withstanding an internal pressure of 0.15 bar. 7.5.2.3 The top part of a fixed-dome plant shall be painted with a gas-tight layer (‘water proofer’, latex or synthetic paints). 7.5.2.4 A weak-ring in the masonry of the digester should be provided to reduce the risk of cracking of the gas chamber. This should be placed between the lower (water-proof) and the upper (gas-proof) part of the hemispherical structure. 7.6 Gas Outlet pipe 7.6.1 Gas outlet pipe shall be provided and it shall be connected to outgoing biogas valve. Ball valves or cock valves should be used. It should also be installed at all gas appliances as shutoff devices. 7.6.2 Sealed T-joints should be connected before and after the main valve to test the digester and the piping system for their gas-tightness separately. 7.6.3 Gas escape valve prepared from three pieces of PVC pipes should be provided, one arm of the T-joint is connected from the gas outlet and the other arm links to the pipe which goes to the kitchen. The T-joint is inserted in the bottle and water is added to a depth of 40mm - 50 mm above the lower point of the T. Sides of the bottle should punched with small holes with a height equal to the desired level of pressure to be maintained. D-104  PAES 413:2001 7.6.4 Piping system connects the biogas plant to the gas appliances or to the gas reservoir. Gas reservoir should be made of plastic or steel. 7.6.5 Piping system should be made of G.I. pipes or PVC pipes. 7.6.6 PVC pipes are susceptible to UV radiation and can easily be damaged; hence, PVC pipes should be placed underground. If the site is located in an area with high intensity of sunlight, G.I. pipe should be used. 7.6.7 PVC should be laid at least 0.25 m deep underground. It should be placed in a sand bed and be covered with sand or fine earth. 7.6.8 Table 4, shows the recommended pipe diameters depending on the flow-rate of biogas through the pipe and the distance between biogas digester and gas appliances. Table 4 - Pipe diameter for different pipe lengths and flow-rate (maximum pressure loss