OBILEKE, KECHRIST

Design and fabrication of a biogas digester systemOBILEKE, KECHRISTSupervisor:Prof. N.S. MamphweliCo-supervisors: Prof. E.L. MeyerProf. G. Makaka

Aim/purpose of the study To develop a mathematical equation for the design of a biogasdigester system. To fabricate the biogas digester chamber using high densitypolyethylene (HDPE) plastic.2

Introduction Biogas digester is any structure that converts organic material(waste) into energy in the absence of oxygen. Various materials and geometric configuration have been usedfor the design of biogas digester system. Examples of the geometric configuration are horizontal,spherical, cylindrical and dome shape Examples of common materials used are brick, cements, fiberglass for the dome shape and metal (stainless steel and mildsteel). Biogas is a good source of renewable energy, composing of 5070% of methane and 30-50% of carbon dioxide with othertraces of gases.3

Problem statement The fabrication/construction of biogas digesters are usuallyundertaken using bricks but has some limitations. Such limitations are cracks in the brick structure after a shortperiod of operation due to tensile stresses. Effect of corrosion that mostly occurs in the biogas digesters builtfrom metals results in the failure of the digesters. Previous designs and constructions are quite expensive, comparedto the present design.4

Previous designs of biogas digesters5

Previous designs of biogas digesters6

Design specification of the biogasdigesterThe desired biogas digester should meet the followingspecifications; Absence of water or gas leakages; Operating temperature of 35 C (mesophilic condition); High corrosion resistance; Easy to maintain; Low cost and affordable7

Design of the digester chamber0.29 0.115 1.35Shape of the biogas digester: cylindrical,hemisphere and frustumDesign equation for the volume of the digesterchamber is given as;VD VN VG VS1.86 0.77000.77000.7700VN π r 2 hVS 2π r331* area of base * height388.14 1.300.025VG 0.0251 1 volume of large pyramid 12 (0.39 ) 0.675 20.7859 10.943776.35m 323 h 20.01591 1 volume of small pyramid 12 (0.36 ) 0.65 20.0159 9.3674412m 33 2 VD 0.0075 0.6441 1.576VD 2.2 m 38

Inlet and outlet chamber designIn term of pressure:VN 0.0120 m3dA P Vinlet 0.48 m3Voutlet 1.8 m3VG 0.6441 m3MgSinθ110 mmP inlet dzMass of the slurryP outlet A outlet P inlet FA inlet F Cos θA inlet 0.8830 P inlet P outlet 513179160 F128 CVS 1.567 m3FFAFIntroducing cos 𝜃𝜃 dw 300180MgPThe inlet and outlet chamber is rectangularprism in shape. Hence, its volume iscalculated as:Vinlet L W H 0.77 0.68 0.81 0.41m 3Pinlet 1.720 10 -9Pressure in the outlet tank was proved tobe greater than the inlet chamber which isless than 1, assuming all forces are equal.Voutlet L W H 0.96 0.96 1.78 1.64m 39

Design of the inlet pipe Since the inlet pipe is of cylindrical in shape, the bursting pressure was calculated usingthe equation:2STtmPb Dmwhere; Pb bursting pressure in psiST tensile strength of the pipe (52 Mpa)tm minimum wall thickness of the pipe (2.2 mm)Dm mean diameter (110 mm). Calculated bursting pressure 2.08 psi 0.143 bar The volume of the slurry moving through the inlet pipe to the digester per unit time canbe calculated using the volumetric flow rate equation below: V Avwhere; V π r 2vr radius of the pipe, v mean velocity of the slurry moving through the pipe and A cross sectionarea of the pipe.10

Calculation of operating pressure Volume and density are the two variables contributing to the pressure of theslurry inside the digester. Pressure of the slurry is given as SP ρ g hSP 0.00137585 Nmm-2The maximum shear stress on the digester was adapted from Gere and Timoshenko[1997] given by τ Prmax2t The shear stress 1τ max δ y2 Therefore, gas pressure expected in the digester is given as: G P PT - SP Expected operating gas pressureG P 0.24477415 Nm m -2 2.4477 bars Hence, the pressure of the system should not exceed 2 bars.11

Pressure design The biogas digester was designed to withstand pressuresubjected during operation. Let the design pressure be 10% above the operating pressure(2.4475 bar, previously calculated) Design pressure 10% of the operating pressure 2.4478bars. That is 0.24475 2.4475 2.70 bars. Hence, the calculated design pressure is 2.70bars 0.27N/mm2.12

2-D view of the biogas digester13

Fabrication of the digester chamber The process of rotational moulding wasemployed for the fabrication of the digesterchamber made of high density polyethylene(HDPE) plastic. Fabrication specificationThe digester chamber fabricated with HDPEhas the following specifications; Volume of the digester 2.2 m3 Height of the digester 1.7 m Thickness of the HDPE plastic material 5mm14

Above ground biogas digester15

Underground biogas digester16

Conclusion The choice of material used in the design of biogas digester havean impact on its life span . The biogas digester is capable of generating 1.57 cubic meter ofbiogas which is equivalent to 11 kwh of energy The design and fabrication of the biogas digester is of loweffective compare to the usual designs The present study has the potential to create employment for bothskilled and unskilled labour.17

Acknowledgement The author would like to acknowledge the Energy and WaterSector Education and Training Authority (EWSETA) for thefinancing the study. The staff and students of Fort Hare Institute of Technology(FHIT) for their contributions.18

Thank youTHANK YOU FOR LISTENINGThink renewable energy: BIOMASS is the solution19

The choice of material used in the design of biogas digester have an impact on its life span . The biogas digester is capable of generating 1.57 cubic meter of biogas which is equivalent to 11 kwh of energy The design and fabrication of the biogas digester is of low effective compare to the usual designs