1. Design and Construction of Rural Household Hydraulic Biogas Digester
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Presented by:
Shi Guozhong
November, 2012

2. AGENDA §1. Brief introduction
§2. The development of biogas digester in China
§3. Structure of hydraulic biogas digester
§4. Advantages of hydraulic biogas digester
§5. Three key points for design of hydraulic
biogas digester
§6. Design
§7. Summary of design parameters
§8. Construction

3. 1. Brief introduction
China is one of the countries that use biogas early in the world. In 1920, Mr.Luo Guorui built a biogas digester called “Chinese Guorui Natural Gas Store”, which was the first Hydraulic Digester in China.

4. Guorui Natural Gas Store

5. 2. The development of biogas digester in China rural areas
Since 1980s, biogas technology have been developed very fast in China rural areas.

6. 2.1 The number of biogas digester in China rural areas
By the end of 2009,there are about 35 million household biogas digesters have
been built and annual biogas production is 10.2 billion m3 in China rural areas.
There are about 4.5 million household biogas digesters in operation in Sichuan Province.

7. Development trend chart of rural household digester (104)

8. 2.2 Distribution of household biogas digester application in China
Rural household digesters are distributed in 31 provinces all over the country.
Over 16 provinces constructed 200,000 digesters each.
Sichuan Province has the biggest amount of digesters as much as 4.5 million.

9. 2.3 Programs and plans
By the end of 2010, there will be 40 million households with biogas digesters and the annual biogas output shall reach 15.5 billion m3 and the dissemination rate shall reach 30%; 4000 large biogas plants shall be built on husbandry farms and 336 million m3 biogas shall be increasingly produced.

10. 2.3 Programs and plans
By the end of 2020, there will be 80 million households with biogas digesters and the dissemination rate shall reach 70%; 8000 large biogas plants shall be built on husbandry farms and 44 billion m3 biogas shall be produced.

11. 2.4 Standards
By the end of June 2008, 89 rural energy standards have been formulated or revised and 69 of them are in operation;
Among all the issued standards, 18 standards cover rural biogas.

12. 2.4 Standards
Rural household biogas: 18 standards including 6 national standards and 12 industrial standards, covering biogas digester drawings, rules for construction, check and acceptance, biogas cookers and related products, integrated utilization of biogas technologies.

13. 3. Structure of hydraulic biogas digester
3.1 What is a biogas digester ?
A sealing digester that can produce biogas through anaerobic digestion by various microbes is a biogas digester.

14. 3.2 Structure of hydraulic biogas digester
1—Inlet;
2—Inlet pipe;
3—Fermentation
chamber;
4—Gas chamber;
5—Movable plug;
6—Biogas guide;
7—Outlet pipe;
8—Hydraulic
chamber ;
9—Overflow pipe.
10—Storage tank.

15. It’s laid of brick. The function of inlet is to feed raw material .
The geometric dimension
is 400x500mm,the depth is about mm.

16. 2—Inlet pipe:
The inlet pipe is made of concrete, inserting aslant into lower part of digester.
It is easy to build, feed and agitate. Diameter of the inlet pipe is usually cm.

17. 3—Fermentation chamber:
Fermentation chamber is the main part of a biogas digester. Raw material is mixed according to the process technology and fed into it. It should be leak-proof for water.

18. 4—Gas chamber:
Gas chamber
must be
leak-proof for biogas. The function of gas chamber is to store biogas.

19. 5—Movable plug:
The movable plug is at the top center of the biogas space. It can be open or close as required.
Usually we fill some stick clay around the edge of the plug to seal it and store some water above the plug .
In order for a person getting in or out, diameter of movable plug is usually 60cm. Movable plug is useful when user wants to ventilate the chamber or feed raw material during starting up or discharge when needed.
When the pressure is high inside, the movable plug can be burst open to protect the biogas digester.

20. 6—Biogas guide:
The gas guide is made of copper, the diameter is usually 8-10 mm.

21. 7—Outlet pipe:
Outlet pipe is round shaped. It is installed aslant at middle or lower part of the digester.
(It’s similar with inlet pipe.)

22. 8—Hydraulic chamber :
It’s laid of brick, the function of hydraulic chamber is to store the liquid (digested effluent) from digester .

23. 9—Overflow pipe:
The overflow pipe is located at the center between hydraulic chamber and storage tank, the diameter is 10-15cm. The overflow pipe must be 20cm underground at least.

24. 10—Storage tank:
The storage tank is a tank which can store the digested effluent from hydraulic chamber , the volume is about 1-2 cubic meters.

25. 3.3 Principle of hydraulic biogas digester
When a biogas digester produce biogas, the gas press the liquid into hydraulic chamber, making liquid level higher.

26. 3.3 Principle of hydraulic biogas digester
On the contrary, when biogas is used, liquid get back into the digester from the hydraulic chamber so the liquid level drops.
In this way, internal and external pressure is balanced.
That’s why we call it hydraulic biogas digester.

27. 4. Advantages of hydraulic biogas digester
1. Good performance, good adaptability for construction material;
2. Simple structure, easy for construction;
3. Low cost;
4. Convenience for use and operation;
5. Long service life: about years.

28. 5. Three key points for design of hydraulic biogas digester
Circle—The shape of digester should be circle, the raw material can be mixed completely.
save building material; intensity of the structure is even, good for service life.
2. Small— The volume of digester should be small ,usually less than 10 cubic meters; there are four kind of volumes: 4, 6, 8,10 cubic meters;
it’s enough for cooking three meals according to different family members.
3. Shallow—The depth of digester should be shallow, usually less than 2.5 meters.
It will be easy for construction (avoid ground water) and convenient for discharge also.

29. Schematic diagram of digester size
H0 =D0/2.5
f1 =D0/5
f2 =D0/8
R0 =D0/2

30. 6. Calculation (1) From volume formulas of ball cap and cylinder,
we get:

31. Put f1, f2, D0 and H0 into the above formulas, we get:

32. 6. Calculation (2) Total volume of the digester is: V=V1+ V2+ V3
= ( ) =

33. 6. Calculation (2)
If we have determined the volume of a biogas digester that will be built, its diameter and other geometric parameters is gotten as:
D0=
=1.3078

34. Example 1:
Assuming a biogas digester with volume of 10 m3 is to be built, its diameter is:
D0=1.3078×2.155
=2.818(m)
Volume of each part is:
V1= ×2.8183=1.85 m3
V2= ×2.8183=1.12 m3
V3= ×2.8183=7.03 m3

35. Example 1: Other geometric parameters: From the above, we get: f1= f2=

36. ρ1=
= D0
= 0.725×2.818
= (m)
ρ2=
=1.0625D0=1.0625×2.818
=2.994 (m)

37. Calculation for surface area:
F=F1+ F2+ F3
=2πρ1f1+2πρ2f2+2πR0H0
=2π(ρ1f1+ρ2f2+ R0H0)

38. Exercise1:
Assuming a biogas digester with volume of 8 m3 is to be built, its geometric parameters are:
D0=? H0 =? f1 =? f2 =? R0 =?
ρ1=? ρ2=?
Volume of each part is:
V1= ? m3
V2=? m3
V3= ? m3

39. Table for dimension of digester

40. 6. Calculation (4)
Calculation for volume of hydraulic chamber (outlet chamber) :
Volume of hydraulic chamber equals to gas production for a half day, i.e.:
Vh=V×rv×
Where,
Vh= Volume of hydraulic chamber (m3)
V = Volume of biogas digester (m3)
rv = Gas production rate (m3/m3/d)

41. Example 2: Calculation for volume of hydraulic chamber If :
V = 8 m3, rv = 0.35 m3/m3/d,
then
Vh= 8×0.35× = 1.40 m3
Namely, volume of hydraulic chamber is 1.40 m3.

42. 6. Calculation (5) Determination of digester volume :
If we have known gas production rate, raw material quantity, biogas consumption per person per day and number of family members, we can calculate the volume of biogas digester by the following formula:
Average biogas consumption per person×number of persons
V =
Gas production rate

43. Example 3: Calculation for volume of hydraulic chamber
Assuming biogas consumption is 0.3m3/per person per day, number of family members is 5 and gas production rate is 0.25m3/ m3/d.
Then : V=
= 6 (m3)

44. 7. Summary of design parameters
1. Maximum working pressure in digester is 7840 Pa (≤800 mm water gauge) ;
2. Maximum working volume is 90% of gross digester volume;

45. 3. Volume of hydraulic chamber is half biogas production per day;
4. Thickness of soil filled on digester top should not be less than 250 mm;

46. 5. Live load above the digester should be 2kpa
(200 kg/m2);
6. Groundwater level should be lower than one meter depth from the ground;
7. Load bearing capacity of foundation requires more than 50 kpa (5 t/m2).

47. 8. Building materials
1. Clay brick—the most popular material for building digester in China countryside; the geometric dimension is 240x115x53mm.
2. Stone blocks—be adopted in mountain areas.
3. FRP—Fiber Reinforced Plastic (Fiber Glass).
4. Concrete—it’s consists of cement , sand, gravel and water. There are 4 intensity grades,that is: C10, C15, C20, C25. C10 and C20 are used for building digester.
5. Plaster(cement mortar)-- it’s consists of cement , sand, and water. The ratio of plaster for laying is 1:3.

48. 9. The composition ratio of concrete (1 m3 concrete)
Intensity
grade
Cement
(kg)
Sand
(m3)
Gravel
Water
C10
200
0.43
0.92
172
C15
250
0.50
C20
315
0.90
182
C25
350
0.45
0.85

49. 10. The estimated quantity of building material
Volume
(m3 ) 4 6 8 10
Brick
(piece)
800
1000
1300
1500
Cement
(ton )
0.8
1.0
1.2
1.3
Sand
1.4
1.5
1.7
Gravel
Working days
(day) 12 15 17 20

50. 11. The prices of local building material and workers’salary
(CNY)
Workers
Salary (CNY/d)
Brick
(1000pieces)
400
Technician
80-90
Cement
(ton )
450
Labour
60-80
Sand
(m3 ) 90
Gravel 60

51. 12. Construction 12.1 Lay out and digging:
Before digging, the digester should be lay out first depending on the digester size and soil quality. For digester construction, firstly a pit should be dug. It is shaped like the larger part of a cone.

52. Table1. Maximum slope for different soil

53. 12.2 The central rod and radius gauges
It is important to set a rod at the center for the cylinder digester construction to keep digester wall circular and vertical regularly. Link the two couples of diagonal stakes at four corners. Hanging a tag at the point of intersection to determine the center of the digester.
Stand a rod at the center, and then fasten the upper part of the rod with threads as shown in Figure a. The central rod should not be moved after it positioned during construction.

54. 12.3 The central rod and radius gauges
The circular shape of the digester is controlled by the radius gauges. Use them to check every block of the digester wall.
After complete one layer of laying work, move the radius gauges up with a height as the thickness of a layer. The gauges must be always kept horizontally.

55. 12.4 Construction of the digester dome
In order to ensure geometric size of the dome, usually we use inflexible wire as radius gauge for laying dome bricks.
If the center of the dome curvature is lower than the digester bottom, a small pit is dug in advance as shown in Figure d. The pit can be filled after the dome is completed.

56. 12.5 Safety during construction
Collapse prevention: During digging the digester, the wall should be of proper slope depending on the soil quality.
Some measurements must be taken to fix the poor quality soil. Heavy article is not allowed to put near digester side.
Water must be drained during rainy season.

57. 12.5 Safety during construction
During dome construction without mould, use flat stones to insert into gaps between bricks tightly. No people are allowed to stand on the dome until the soil become hardened. Heavy articles are not allowed to put on newly built dome to avoid collapse due to over load.

58. Table 2.1- Procedure of painting the sealing layers on digester internal surface

59. Table 2.2

60. Table 2.3

61. Table 2.4

62. Thank you for
your attention!