Biogas production from saw dust using acti-zyme as digestion catalyst and its upgrading to bio methane using chemical absorption

Abstract

Zimbabwe has one of the largest deposits of timber in Southern Africa (about 500 000 tons per annum) such that most entrepreneurs venture into timber processing. The venture results in generation of a lot of sawdust dumps that are of no economic use if they are left lying around. There is 10000 metric tons that go to waste each year which when bio-converted can result in biogas. This paper therefore presents the potential for biogas production from saw dust bio-catalysed by Acti-zyme a biocatalyst to enhance the digestion hence biogas yield. This paper presents the bio-catalysed digestion of 4000 metric tons of saw dust per year to produce biogas using Acti-zyme. The generated biogas is then upgraded using chemical absorption for removal of carbon dioxide so as to increase the bio methane energy efficiency. Biogas produced was approximately 24 m 3 per day and was upgraded to 97% from 72% bio-methane composition per day using chemical scrubbing technique in an absorption column. The absorption column design considerations are also presented as well as its process control and the hazard analysis. These are essential in the optimal upgrading of the biogas. Purified bio methane has a higher calorific value of 99 % relative to the 77 % of raw biogas. An economic analysis showed that a 2 year payback period with a return on investment of 48% and break even sales at $331 580.00 with the upgraded bio methane being sold at $2.50/kg. The conversion and upgrading of biogas produced by anaerobic digestion using Acti-zyme is technologically, economically and environmentally feasible.