Biofuels production 201Engines are designed to run either on pure methane gas or digester gas.
Either type of gas is a suitable fuel for petrol and diesel engines. Diesel engines
to run on dual fuel (biogas/diesel oil) or diesel fuel only are now manufactured
in India. Kerosene- and gas-operated engines can also be modified to use biogas.
Stationary engines located near a large-size biogas plant can be an economical
and practical proposition. It is more efficient to use biogas to generate electricity
rather than direct lighting. However, high cost of engine and generator might be
prohibitive for the farmers or biogas owner. Figure 4.19 is a co -generator run on
biogas produced from digestion of solid wastes. The produced electricity is used in
the plant and also sold to the electricity authority, while the produced heat is used
to dry the digested sludge to make it suitable for land application.
The biogas from digestion of animal manure and vegetable matter normally
do not contain sufficient quantity of H 2 S to require purification before use. For
cooking and lighting, the biogas does not need to be purified. However, if the
gas is to be stored or transported, then H 2 S should be removed to prevent
corrosion of storage bags. CO 2 should also be removed as there is no advantage
in compressing it. Biogas purification is not normally practiced for small-scale
digesters. For large-scale or institutional digesters, there might be economic
reasons favoring biogas purification. Some practical methods of biogas
purification are described below.
CO 2 removal
Since CO 2 is fairly soluble in water, water scrubbing is perhaps the simplest
method of CO 2 removal from biogas. However, this method requires a large
quantity of water for scrubbing CO 2 , as can be estimated from Table 4.23.
Assuming that a biogas has 35 % CO 2 content and CO 2 density is 1.84 kg/m^3 at
1 atmospheric pressure and at 20 °C, the amount of water required is 429 L to
scrub 1 m^3 of this biogas.
CO 2 , being an acid gas, can be absorbed in alkaline solution. The three
common alkaline reagents are NaOH, Ca(OH) 2 and KOH. Two consecutive
alkaline reactions of CO 2 removal in NaOH solution are:
2 NaOH + CO 2 ĺ Na 2 CO 3 + H 2 O (4.17)
Na 2 CO 3 + CO 2 + H 2 Oļ 2 NaHCO 3 ↓ (4.18)