Upgrading Biogas Produced from Biogas Pilot Plant through Absorption
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Date
2014-06
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Addis Ababa University
Abstract
Biofuels derived from biomass are considered as good alternative to petroleum fuels. Biogas is produced by anaerobic digestion of various organic substances such as food waste, agricultural waste, municipal solid waste (MSW), cow dung (CD) etc., which offers low cost and low emissions than any other secondary fuels. It can be a supplemented to liquefied petroleum gas (LPG) and compressed natural gas (CNG), if it is used in compressed form in cylinders. Ethiopia is among the developing countries where biogas upgrading system doesn’t include in biogas production plants. The use of unrefined biogas has several social and environmental impacts such as GHG emissions, the incombustible part of biogas, CO2, lowers its calorific value, the presence of hydrogen sulphide forms sulphuric acid with water, which is highly corrosive, rendering the biogas unusable. Furthermore, the use of biogas has remained near to the generating plants and compression requirement is high. This paper examines the effect of raw biogas flow rate, water flow rate and water head on methane enrichment of biogas; using water scrubbing which is based on the physical effect of dissolving gases in liquids. In a scrubber, CO2 as well as the H2S, dissolves into the water while CH4 does not because of their difference in solubility. This makes it a very simple process. There is a lot of potentials in terms of increased energy per unit volume, wider application and ease of handling if biogas is upgraded to >95% CH4. Raw biogas methane content in the studied pilot biogas plant ranged from 45 % to 56 %, carbon dioxide from 39 % to 50 % and trace amount of hydrogen sulphide. Oxygen content in all the measured raw biogas was < 2%. A counter-current absorption process upgraded the pilot plant gas near to 80 % methane content. The carbon dioxide content of the product gas ranged from 17 to 35 %. Hydrogen sulphide was removed from the raw pilot plant gas with over 90 % efficiency with the upgrading system. Removal of contaminants (CO2 and H2S) from biogas through upgrading was conducted at a water head of (80, 110 and 140 cm), water flow rate of (8, 16 and 25 l/min) and gas flow rate of (1, 1.5 and 2 l/min). In all cases, the increase in water head from 80 to 140 cm and the increase in water and gas flow rates increased the methane concentration. The highest methane content (80 %) was obtained at the highest water head (140 cm) and water flow (25 l/min) and when the gas flow was maintained at 1.5 l/min. The resulted CH4 enriched product gas implies that carbon dioxide and hydrogen sulphide gases have been trapped in the absorbent water with higher removal efficiency (60 and >90 % respectively).
Keywords: biogas; methane; upgrading: Anaerobic digestion: enrichment: biofuel
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Keywords
Biogas, Methane, Upgrading, Anaerobic digestion, Enrichment, Biofuel