OBILEKE, KECHRIST

Supervisor: Prof. N.S. MamphweliCo-supervisors: Prof. E.L. Meyer

Prof. G. Makaka

Design and fabrication of a biogas digester system

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.

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Introduction

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• 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 50-70% of methane and 30-50% of carbon dioxide with othertraces of gases.

Problem statement

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• 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.

Previous designs of biogas digesters

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Previous designs of biogas digesters

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Design specification of the biogas digester

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The 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 affordable

Design of the digester chamber

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1.35

0.7700 0.7700

0.025 0.025

1.86°

0.7700

88.14°

0.29

0.115

h= 20.0159

1.30

• Shape of the biogas digester: cylindrical,hemisphere and frustum

• Design equation for the volume of the digesterchamber is given as;

hr πV 2N = 3

G r 32V π=

height * base of area * 31 VS =

SGND V V V V ++=

3D

D

m 2.2 V1.576 0.6441 0.0075 V

=

++=

( ) 3m35.943776.107859.20675.039.02112

31 pyramid large of volume =×

×××=

( ) 3m3674412.90159.2065.036.02112

31pyramid small of volume =×

×××=

Inlet and outlet chamber design

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28°C

Mg

VS = 1.567 m3

VG = 0.6441 m3

VN = 0.0120 m3

Vinlet = 0.48 m3 Voutlet = 1.8 m3

MgSinθ

300

110 mm

F

F1

180

dz

P

dw =

Mass of the slurry

dA

In term of pressure:

Introducing cos 𝜃𝜃

AF P→

→

=

outlet

outlet A

FP→

→

=

inlet

inletA

FP→

→

=

inlet

inletA

Cos FP θ→

→

=

outletinlet P

5131791600.8830P

→→

=

inletP = -910 1.720×

Pressure in the outlet tank was proved tobe greater than the inlet chamber which isless than 1, assuming all forces are equal.

The inlet and outlet chamber is rectangularprism in shape. Hence, its volume iscalculated as:

3inlet 0.41m 0.810.680.77 HWL V =××⇒××=

3outlet 1.64m 1.780.960.96 HWL V =××⇒××=

Design of the inlet pipe

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• Since the inlet pipe is of cylindrical in shape, the bursting pressure was calculated usingthe equation:

where; 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 = Av

where;

r = radius of the pipe, v = mean velocity of the slurry moving through the pipe and A = cross sectionarea of the pipe.

m

Tb D

2S P mt=

v2r V π=

Calculation of operating pressure

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• Volume and density are the two variables contributing to the pressure of theslurry inside the digester.

• Pressure of the slurry is given as

The maximum shear stress on the digester was adapted from Gere and Timoshenko[1997] given by

• The shear stress =

• Therefore, gas pressure expected in the digester is given as:

• Expected operating gas pressure

• Hence, the pressure of the system should not exceed 2 bars.

h g SP ρ= -2P Nmm 0.00137585 S =

y 21 max δτ =

2tP r

max =τ

PP S - PT G =

bars 2.4477 m Nm 0.24477415 G -2 P ⇒=

Pressure design

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• 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.

2-D view of the biogas digester

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Fabrication of the digester chamber

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• The process of rotational moulding wasemployed for the fabrication of the digesterchamber made of high density polyethylene(HDPE) plastic.

• Fabrication specification• The digester chamber fabricated with HDPE

has the following specifications;

• Volume of the digester = 2.2 m3

• Height of the digester = 1.7 m

• Thickness of the HDPE plastic material = 5mm

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Above ground biogas digester

Underground biogas digester

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Conclusion

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• 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.

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.

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Thank you

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