Page 206 - Advances in Eco-Fuels for a Sustainable Environment

170 Advances in Eco-Fuels for a Sustainable Environment

6.8.2 Fixed dome digester

A fixed-dome plant is made up of a closed, dome-shaped digester with an immovable
fixed gas-holder. The gas produced from the digester is stored between the slurry
and the roof of the digester. Here, the pressure of the biogas produced is not
constant. The cost of the fixed dome digester is less compared to the floating dome
digester [66].

6.8.3 Low cost polyethylene tube digester
Low-cost polyethylene tube digester used in many countries such as Bolivia, Ecuador,
Colombia, Peru, Mexico, the United States. Two layers of a tubular polyethylene film
of 300 μm are folded at each end around a 6 inches pipe of drainage out of vinyl
polychloride (PVC) and is rolled up with a rubber bracelet of recycled tire tubes.
A hermetic isolated tank is obtained with this system. One end acts as an inlet for
the substrate and the other end acts as an outlet for the digestrate. The capacity of
the gas holder resembles one-fourth of the total capacity of the reaction tube.
To overcome the lower gas flow rate, two 200-μm tubular polyethylene can be
used [67].

6.8.4 Balloon plants
A balloon plant consists of a rubber bag or heat-sealed plastic joining the digester and
gas-holder. The gas will be stored in the upper part of the balloon. The inlet portion
and outlet portion are connected directly to the casing of the balloon. By applying
weights on the balloon gas, pressure can be increased if it exceeds the limit; a safety
valve should be provided or otherwise it may damage the skin. The material has to be
ultraviolet-resistant, reinforced plastic, and red mud plastic. The useful lifespan is
about 2–5 years. Advantages are standardized prefabrication at low cost, and difficult
substrates such as water hyacinths can be used. The high temperatures of the digester
in the warm climates, shallow installation suitable for areas with high groundwater
table and compared to other types of digesters maintenance cost is significant. Some
disadvantages of this plant are that the scum cannot be removed during operation, it
requires a gas pump when gas pressure is low, the plastic balloon life span is very
short, and has little scope for the creation of local employment [68].

6.8.5 Horizontal plants
The horizontal installations of biogas are generally selected when the total volume of
3
3
15m (working volume of 12m ) with shallow installation and the section of the lower
part of the digester have a round form to avoid mud precipitations. Advantages are
shallow construction provides a large slurry space. Drawbacks are leakage of gas
and elimination of scum [68].