Attachment I Typical design calculation for an agricultural biogas plant

Transcription

1 407 Attachment I Typical design calculation for an agricultural biogas plant The size of an agricultural plant should be suitable for the number of domestic animals and the area available for cultivating co - ferments. From these values, the daily biogas yield can be calculated. Biogas from Waste and Renewable Resources. An Introduction. Dieter Deublein and Angelika Steinhauser Copyright 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN:

2 408 Attachment I Typical design calculation for an agricultural biogas plant Table A.1 Products and biogas yields from an average substrate. Description Basic substrate Biomass Feedback from the residue storage tank Unit Liquid manure from cattle Muck from cattle Liquid manure from pigs Muck from pigs Liquid manure from poultry Muck from poultry Silo maize Grass Silage Total Animals in GVE * GVE Liquid manure /GVE.day m /(GVE.d) Area for cultivation (A) ha/a Yield per hectare Mg/ha/a Yield per day ( ṀG ) Mg/d DM-content % Yield of DM per day kg DM/d odm in DM % Yield of odm (DM BR ) kg odm/d Spec. yield of biogas m /(kgodm.d) Yield of biogas ( VBR ) m /d * One animal unit (GVE) corresponds to the liquid manure from one full - grown cow, 5 calves, 6 beef cattle, or 250 hens.

3 Silo for maize and/or grass 409 Based on the daily rate of biogas and some additional assumptions, the equipment of a complete biogas plant can be designed as follows: Preparation tank The preparation tank shall be a vertical cylindrical container of concrete. In the preparation tank, only the daily produced liquid manure shall be stored for t PT = 10 days in order to carry out cleaning and maintenance work at the bioreactor. The density of the liquid manure ρ G can be set equal to the density of water ρ G = ρ W. A factor f VPT = 1.25 shall be assumed to take into consideration the volume for air and fixtures. The relationship between height and diameter of the tank shall be H PT /D PT 2. Volume: VPT = Ṁ G tpt/ ρg f VPT = 5Mg/d 10d/1000 kg/m = m, Height: H PT = 6.8 m, Diameter: D PT =.4 m, Pump of the preparation tank In the preparation tank a centrifugal pump with a wide chamber and a submerged motor shall be installed. The pump of the preparation tank shall be able to deliver VVP = 10 m / h liquid manure to the bioreactor or to pump the complete volume of the bioreactor (V BR = 41 m as calculated below) within t BRI = 5 h (given below). Its efficiency is assumed to be η VP = 0.5. Its pressure head shall be 1 bar. Throughput: ( VVP) 1 = 10m / h or V ( VP) 2 = VBR/ tpt = 41m / 5h= 86m / h Pressure head: PVP = 1 bar, Capacity of the motor: actual: PVP = ( V VP) 1 P VP/ η VP 0,6 kw; nominal: P = ( ) P / η,0kw VP VP VP VP Silo for maize and/or grass The complete harvest ( ṀS = 750 Mg/a) of maize (density ρ S = 0.7 Mg/m ) of one year shall be stored in a concrete silo, which is accessible to traffic. The dimensions can be freely chosen. The gras is dried and stored on fields. Volume: VS= Ṁ S/ρ S= 750Mg/a/0,7Mg/m = 1100 m Breadth: BS = 10 m, Height: H S =.5 m, Length: L S = 2 m

4 410 Attachment I Typical design calculation for an agricultural biogas plant Silo conveyors Two screw conveyors in series with nominal capacity V SC = 1 m / h each and motor capacity P SC = 5 kw each are driven between silo and preparation tank twice a day for t SC = 1 h/d. Total power consumption of the two screw conveyors: (P SC )tot = 2 P SC 2 t SC /24 h = 2 5 kw 2 1 h/24 h = 0.8 kw Bioreactor A vertical cylindrical tank of concrete shall be used as bioreactor. The residence time of the substrate in the bioreactor shall be t BR = 0 days. A factor f VBR = 1.25 has to be chosen to take into consideration the volume for air and fixtures in the bioreactor. The relation between height and diameter of the bioreactor shall be H BR /DBR 1/2. The completely filled bioreactor shall be emptied within t BRl = 5 h at a flow rate of v BRl = 0.5 m/s. Two propeller-agitators (diameter D BRR = 0.5 m, Newton number Ne BRR = 0.5, revolution n BRR = 150 rpm) shall be installed for intermittent mixing and breaking off the floating layer with a working period of t BRR = 5 min/h. Both agitators are equipped with submerged motors and their height shall be adjustable by chains. Volume: VBR = Ṁ G/ ρg t BR f VBR = 11. 5Mg/d/1000 kg/m 0 d 1.25 = 41m Height: HBR = 5.5 m, Diameter: DBR = 10 m, The volume load of this medium - sized bioreactor is then B BR = DMBR /VBR = 1571 kg odm/d/41 m =.64 kg odm/m d. The average volume load for small plants is B BR = 1.5 kg odm/m d and of large plants is B BR = 5 kg odm/m d. Diameter of the discharge pipe: DBRI = ( VBR/ tbrl/ vbrl 4/ π) = ( 41m / 5h/ 0.5m/s 4/ π) 0. m Capacity per agitator drive: P BRR = 1. NeBRR ρ G n BRR D 5 BRR = kg/m (150 π /0) 0.55 m 5 = 78.7 kw 80 kw Power consumption of both agitators: (P BRR )tot = 2 P BRR t BRR = 2 80 kw 5 min/h = 1.4 kw

5 Heating (pipes) 411 Heating (pipes) For fermentation, mesophilic temperatures shall be chosen at ϑ BR = 50 C. The lowest outside temperature in winter is ϑ A = 20 C (humid soil)). The substrate with its specific heat capacity of c SU = 4.2 kj/kg C has thus to be heated from 20 C to 50 C, i.e. ca.. ϑ SU = 0 C. The bioreactor walls shall be insulated with a layer (s BR = 0.1 m thick) of polystyrene. The heat transmission coefficient of polystyrene is λ BR = 0.05 W/m K. Although the substrate surface does not reach the ceiling of the bioreactor, the complete wall shall be taken into consideration when calculating the heat losses; heat losses through the ceiling are negligibly low, because the ceiling is in contact with gas and/or air inside and outside. The heat transfer coefficients inside at the wet bioreactor wall shall be assumed to be ( α BR ) i = 4000 W/m2 C for agitated liquid and outside to ( α BR )a = 400 W/m 2 C for humid soil; Then the k - factor can be calculated: k BR α BR i s BR λ BR α BR a 2 = 1/(( ) + / + ( ) ) = 1/( 1/ / / 400) = 05. W/m C The maximum temperature difference between substrate and environment is ϑ BR = ϑ BR ϑ A = (50 C) ( 20 C) = 70 C The heating medium (warm water) shall cool down from ϑ HE = 70 C to ϑ HA = 60 C and the temperature difference can be calculated ϑ = ϑ ϑ = 10 H HE HA C The flow rate of the heating medium in the heating pipe shall be v H = 1 m/s. The heat transfer coefficient inside and outside the heating pipes shall be asumed to be the same ( α H ) i = ( α H )a = 400 W/m2 C for slowly flowing liquid; The heating pipe is well heat - conducting and therefore negligible in the calculation; then the k - factor for the heating pipe wall can be calculated 2 k = 1/( 1/(( α ) + ( α ) ) = 1/( 1/ / 400) = 200W/m C H H i H a The average temperature difference between heating medium and substrate in the bioreactor is ϑ = ( ϑ + ϑ )/2 ϑ = 15 BH HE ΗΑ BR C Heat for heating the substrate: QSU = ṀG csu ϑsu = 11. 5Mg/d 4.2 kj/kg C 0K = 17 kw Surface area of the bioreactor, which conducts heat: A BR = π D 2 BR /4 + π D BR H BR = 250 m 2

6 412 Attachment I Typical design calculation for an agricultural biogas plant Heat losses of the reactor: Q BR = k BR A BR ϑ BR = 0.5 W/m2. C 250 m2 70 C = 8.8 kw Necessary heat: Q V = Q SU + Q BR = 17 kw kw = 25.8 kw Necessary heating liquid V w, for heat supply to the bioreactor: V w = Q V /( c w ρ w ϑ H ) = kw/ (4.2kJ/kg. C 1000 kg/m 10 C) = 6.14m / h Diameter of the heating pipe: DHR = ( V w/ vh 4/ π = ( 614. m / h/1m/s 4/ π) = m Length of the heating pipe: L HR = Q V /(kh ϑ BH π 2 D HR ) = 25.8 kw/(200 W/m. C 15 C π 0.05 m = 55 m The result of the calculation is, that a pipe with three windings of a diameter of D W = 8 m is sufficient. Because sinking layers and a disturbance of the heat transfer must be taken into consideration, a higher number of windings should be chosen. Aeration Aeration with an air flow rate VL referring to the biogas flow rate VBR of V V L/ BR= 004. is sufficient for desulfurization. Oil - free compressed air shall be blown in at a pressure of p k2 = 6 bar. The velocity of the air in the air pipe shall be v L = 2 m/s. Blown in air: V V V V L = L/ BR BR = m / d = 16. Nm / h Diameter of the air pipe: D = ( V / v 4/ π) = ( 1. 6Nm / h/2m/s 4/ π) 0.02 m L L L Compressor with pressure vessel of volume V K = 0.05 m Volume rate of the compressor: V K = 17Nm. / h Pressure head from p K1 = 1 bar to p K2 = 6 bar (there is to be assumed friction in the pipe) Capacity of the compressor: P K = 0.5 kw Gasholder A low- pressure gasholder of plastic foil shall be used The relationship of the volume of the bioreactor to the volume of the gasholder shall be (V BR /VGS ) = 1 : 2. The ratio V BR /V GS is the usual value (between 1 : 1 and 1 : ).

7 Volume of the gasholder: V GS = V BR /(V BR /VGS ) = 41/(1/2) = 862 m. The gasholder enables the storage of the biogas production of nearly 1 day. Engine The biogas- energy content shall be Ė spec = 6 kwh/m. The biogas plant shall be equipped with an ignition oil Diesel engine in the CHP. 9% by weight of ignition oil shall be added to the biogas in a ratio of M M OIL/ BR = The energy content of the ignition oil is E OILspec = 10 kwh/kg. Efficiency of the engine: Electrical efficiency: h el = 0% Thermal efficiency: h th = 50% Consumption of ignition oil assuming a density of the biogas of d * = 1.11 kg/m : MBR = VBR δ* = 976m /d 1.11kg/m = 108kg/d M M M M OIL = BR ( O / BR) = 108kg/d 0.09 = kg/d Yield of energy: Etot = Espec V BR + EOILspec M OIL = (6kWh/m 976m / d + 10 kwh/kg 97.6 kg/ d)/24 h/d = 284 kw Eel = Etot ηel = = kw E = E = = kw th tot η th Nominal capacity of the engine with a reserve of 0%: E= 111 kw Residue storage tank Residue storage tank 41 A vertical cylindrical tank of concrete shall be used as residue storage tank. The residence time of all residue in the storage tank shall be t E = 100 d, according to the period in which the soil is frozen, when the residue could not penetrate into the soil, would possibly flow into rivers, and would deteriorate the water quality. It has to be taken into consideration that some water ( VE = 25. m/d ) from the residue tank is fed back into the bioreactor. A factor f VB = 1.1 is chosen to take into consideration the volume for air and fixtures in the storage tank. The storage tank shall have the same height as the bioreactor. Volume: VE= ( M G/ ρ V E) te f VE= ( 11. 5Mg/d/1Mg/m 2. 5m / d) 100d 1.1 = 990m.

8 414 Attachment I Typical design calculation for an agricultural biogas plant Height: Diameter: H E = 5.5 m D E = 15 m Power and heat consumption of the entire plant Table A.2 Calculated energy consumption of the plant. Energy consumer Abbreviation Energy Agitators (2 pieces) (P BRR ) tot 1.4 kw Pump P VP 0.6 kw Screw conveyors for maize and grass (P SC ) tot 0.8 kw Air compressor P K 0.5 kw Total power consumption E Ee l 15. kw Heat losses via the bioreactor wall Q BR 8.8 kw Heat for heating the substrate Q SU 17.0 kw Total heat consumption Q V 25.8 kw