The construction of a fixed-dome biogas plant is inclusive of several stages. This article highlights fixed-dome construction in-depth, the construction of an expansion chamber and the steps to be followed until its completion.
1.0 Dome Construction
Constructing a dome is quite an involving process. This is because it requires the utmost concentration and accuracy. This process is very technical. A qualified biogas technician is highly recommended in installing it. Dome construction involves the following: fixing a gas pipe, building and closing the dome, and lastly, dome plastering.
The steps in fixation of the dome gas pipe are as follows:
- Thread one end of a piece of galvanized pipe of 1-1.5 inches and a length of 41 cm and mount a T-Joint. It is advisable to grease the thread in advance to prevent corrosion.
- Wield a disc and some scrap pieces of iron bar 4 cm on the other end of the galvanized pipe.
- Join the other components of the testing unit to the galvanized pipe.
- Provide temporary support below the testing unit.
- Position the pipe with the testing unit side above on top of the digester.
- Reinforce the mixture with concrete in the ratio of 1:3:3 including 2% of waterproof cement.
- Lastly, place the main gas valve on the test unit.
NOTE: Plastering the inside of the digester takes place before fixing the dome gas pipe.
The disc prevents the gas from escaping along the pipe wall. Iron bars act as anchors and give additional strength to the pipe fixed in the brickwork. Under normal operation, the T-joints should remain sealed and closed with plugs. All threads are joined with seal/thread tapes, which as a rule of thumb should be placed in approximately eight turns.
The test unit chamber is where activities such as gas leakage testing, measuring of gas pressure in the digester, and gas flow/consumption rate are normally conducted. From this place, pipes are laid and connected directly to the consumption point. Dome construction should start where the wall ends. Prepare a formwork to support the bricks on the outlet to create space for the passage of bio-slurry.
Sealing of the dome involves the following:
a) To begin with, prepare a radius stick of the dome radius and fix the center pipe. The opposite end should have a nail fixed at 90 degrees to the stick.
b) Lay mortar on the wall and shape to assume the dome shape according to the radius stick. Secondly, lay the bricks on the mortar bed one by one making sure each brick is at a right angle to the stick. Thirdly, close the brickwork. Finally, install the dome gas pipe and support the pipe.
c) Position the sand in a moist, spherical shape with a vertical height of about 8 mm and place it on the wooden plank. Cover the sand with cement paper. Most importantly, you should use wet sand to form a nice curvature with plywood. The curvature should correspond with the shape of the arc.
d) Place a galvanized iron rod on top of the sand such that its center coincides with the center of the dome. After that, place the bricks around the rod. This partly holds them in place and fills the empty space.
e) Pour masonry mortar around the bricks to dress them to take the shape of the dome. Allow the mortar to dry for 4 days. Remove the scaffold and the wooden plank.
f) Lastly, build a turret immediately around the gas pipe to protect it after the fourth day.
Closing the dome
To close the dome, build a scaffold inside the dome reaching the very top of the dome.
Cut wooden planks into a circular shape and join them together. Thereafter, place the circular-shaped wooded plank beneath the dome’s opening.
Plastering the dome
The purpose of plastering is to make the dome gas-tight. Remove all the framework inside the dome and scrub the dome with a hard brush and clean water. This is to get rid of all mortar pieces and loose particles left behind on the brickwork.
Plastering of the dome takes place in the following stages:
Stage 1: Plain cement-water flushing: Mix one part of cement and 3.5 parts of water thoroughly to form a mixture. After that, apply the mixture with a brush on the inside of the dome. The purpose of this layer is to foster the bonding between the total dome masonry surface and the next plaster layer.
Stage 2: Cement punning: This is to get a smooth and even surface for the next stage. Mix the mortar from one part of cement and three parts of sand to a thickness of about 6 – 10 mm.
Stage 3: Cement sand punning: Mix one part of cement with one part of sand to form mortar. Apply the mortar with a trowel. The layer of mortar is about 3-4 mm thick.
Stage 4: Mixing of cement with acrylic paint: Mix one part of acrylic paint with 12 parts of cement to form a uniform mixture. Apply the mixture on the surface of the dome to 3 mm thickness.
Stage 5: Application of cement and acrylic paint mixture: Make the last layer with one part of paint and mix with two parts of cement. Add the appropriate amount of water to obtain a viscosity. This should make it smearable with a brush and thick enough to give it a solid coating layer.
2.0 Mixing Chamber Construction
An Overview: The mixing chamber is where the mixing of water and bio-slurry takes place on entry to form a fine paste of the bio-slurry. This fine paste proceeds onto the biogas dome for further fermentation.
The figure below illustrates a mixing chamber.
The mixing chamber’s purpose is to mix the entering water and bio-slurry to mix to perfection to make the required paste. Place the foundation of the inlet pit on a well-rammed, hard, and leveled surface. A circular shape is ideal in terms of cost and operation (Bensah & Ahiekpor, 2021).
Steps in construction of a mixing chamber:
a) Construct the rectangular base of the mixing chamber, preferably on a ramped surface. The floor of the mixing chamber should be at least 10 cm above the outlet overflow level from the reference line.
b) The circular mixing chamber measurements are 600mm diameter and 600 mm high.
c) After bricklaying is complete and after at least one day of waiting, plaster both inside and outside of the chambers with cement mortar (1 part of cement to 4 parts of sand).
3.0 Expansion Chamber Construction
An Overview: An expansion chamber is a square-shaped compartment in a biogas plant. It helps to contain the excess bio-slurry in the biogas that is displaced from the dome.
Function: The expansion chamber serves to control the pressure in the dome at any point in time and also as an additional digestion chamber in some cases. It is constructed with concrete bricks just as the biodigester. The base is cast with concrete, after which the center is located. The laying of bricks follows the same protocol as brickwork for the dome.
 The inside of an expansion chamber |
 Mixing chamber under construction |
The chamber should be constructed in such a way that the cover is above ground level. This is to ensure that mudslide does not cover the expansion during rains. Cover the manhole with a concrete slab of about 60 x 60 cm. The manhole in the expansion chamber should be tightly fitted. Excess manure from the expansion chamber should be contained by the manhole. This greatly avoids contamination. The bio-slurry should not be allowed to flow onto the farmer’s field or into a storage tank, or into natural water bodies.
Conclusion
A properly constructed biogas plant with a thoroughly constructed dome and expansion chamber has the potential to provide top-notch biogas production with a long service period. The materials for construction should be in immaculate condition. This project should be undertaken by highly-trained personnel. In culmination, this guide is a source of good reference in construction. This is to encourage high levels of proficiency.
References
Edem Bensah, Edward Antwi, Julius Ahiekpor, (May 2021). Guide For The Design And Construction Of Fixed-Dome Biodigester. Retrieved from https://www.researchgate.net/publication/351818542
Otávio Bravim da Silva, Lucas Silva Carvalho, Gabriela Carneiro de Almeida, Juliana Davies de Oliveira, Talita Souza Carmo and Nádia Skorupa Parachin, (2017 February). Biogas – Turning Waste into Clean Energy. Retrieved from: https://www.intechopen.com/chapters/51487
This article is an excerpt from the book “The Blue Flame: Biogas Training Blueprint”
Suggested Citation in APA
Wanjohi, A.M. (2022). The Blue Flame: Biogas Training Blueprint. Nairobi: Kenya Projects Organization
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