Page 204 - Advances in Eco-Fuels for a Sustainable Environment

168 Advances in Eco-Fuels for a Sustainable Environment

6.6 Biogas purification using chemical scrubbing

Biogas composition with carbon dioxide and moisture can be removed by passing
through a chamber containing limestone in a solid form. Next, biogas is passed to
the chamber containing potassium hydroxide prepared at the level of 40% concentra-
tion. Due to the exothermic reaction, a high amount of heat will be formed. The heat
formed will be dissipated gradually. Potassium hydroxide reacts with carbon dioxide
and forms potassium carbonate (K 2 CO 3 ) and water. Then the biogas is passed to the
third chamber, having steel wool or iron fillings (chips available from lathe operation).
This will result in reduced H 2 S level and moisture content [58].
Biogas purification using an aqueous solution of caustic soda and lime is one of the
efficient techniques. The raw biogas containing the composition of carbon dioxide and
hydrogen sulfide is first passed to the chamber containing iron fillings, where the
hydrogen sulfide and moisture get removed. Next, biogas is passed to the chamber
containing sodium hydroxide, where a chemical reaction takes place that results in
sodium carbonate and water. Finally, a chamber with silica gel is used to remove mois-
ture from the biogas [59].

6.7 Classification of anaerobic digestion system

Based on the feedstock mode (batch or continuous), the moisture content of the sub-
strate (wet and dry) and temperature condition digester systems are classified.

6.7.1 Batch and continuous system
In the batch process, the feedstock is fed into the digester only once (with or without
inoculum) and completely sealed over a specified period. After the biogas formation,
the digested substrate is emptied from the digester, and the new feedstock is filled to
restart the procedure again. In this type, the production of gas varies from time to time.
At the start and the end, the biogas formation will be low compared to the middle part;
the biogas production is high. So, to avoid this problem, many batch reactors can be
operated in parallel. In a continuous process, the digester is filled continuously to get
constant and continuous gas production and a mechanical agitator is used to mix the
content continuously [60, 61].

6.7.2 Wet and dry systems

The process can be termed as wet or dry according to the substrate solid concentration.
If the solid concentration is less than 20%–40%, then it is termed a dry digestion sys-
tem [62]. The wet system processes the substrate with less than 15%. The dry system
processes thick substrate and slurry and hence needs more energy input to the process,
but in a wet system, the substrate can be easily transported through pumps with less
energy input. Also, the exchange of material is smooth, and the communication