Chemical absorption involves formation of reversible chemical bonds between the
solute and the solvent. Regeneration of the solvent, therefore, involves breaking of these
bonds and correspondingly, a relatively high energy input. Chemical solvents generally
employ either aqueous solutions of amines, i.e. mono-, di- or tri-ethanolamine or aqueous
solution of alkaline salts, i.e. sodium, potassium and calcium hydroxides.
Biswas et al. [10] reported that by bubbling biogas through 10% aqueous solution of
mono-ethanolamine (MEA), the CO2 content of the biogas was reduced from 40 to
0.5–1.0% by volume. MEA solution can be completely regenerated by boiling for 5 min
and thus can be used again.
Savery et al. [11] suggested that the three agents NaOH, KOH and Ca(OH)2 can be used
in chemical scrubbing of biogas. The absorption of CO2 in alkaline solution is assisted by
agitation. The turbulence in the liquid aids to diffusion of the molecule in the body of
liquid and extends the contact time between the liquid and gas. Another factor governing
the rate of absorption is concentration of the solution. The rate of absorption is most rapid
with NaOH at normality’s of 2.5–3.0.