Browsing by Author "Assefa, Gizachew"

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    Optimum Production of Biogas from Bio-Municipal Solid Wastes Using Two Stages Anaerobic Digester
    (2011-06) Assefa, Gizachew; Assefa, Birhanu (PhD)
    Various aspects of Anaerobic Digestion (AD) have been became a promising technology for the management of Municipal Solid Waste. Thus, the objective of this study was to optimize the production of biogas from these wastes by using two-stage anaerobic digester. To achieve this, batch anaerobic digestion experiments were conducted to determine the optimum methane produced from the digestion and co-digestion of Organic Fraction of Municipal Solid Waste (OFMSW) and Cow Manure (CM) in different mix operated under ambient condition. Moreover, this study presents several runs of experiment and analysis with the aim of substrate mix and the duration of overall stage of digestion process while optimizing the biogas generation with higher methane content. The physical and chemical characteristic of the sample done by analytical method, volume of biogas produced and methane composition of different mix of the feed stock were determined by water displacement and digital gas (SR2-BIO gas analyzer), respectively. The highest specific gas production observed was 353 L/Kg VS added for the digestions of 1:2 mix of OFMSW: CM. It was observed that the independent variable – mix ratio significantly affect (p-value 4.18*10-9) the dependent variable – amount of methane produced since the p-value is less than the chosen α-level usually 0.05. The calculated volume of methane in the biogas produced per Kg of volatile solid removed were 510L, 355L, 375L, 435L and 600L for OFMSW (100%), 2:1 mix of OFMSW: CM, CM (100%), 1:1 and 1:2 mix of OFMSW: CM, respectively. In general From the digestion of 1:2 mix of OFMSW: CM: 0.47 m3/d/m3 biogas with 51.5% methane content was produced; 0.45 m3/d/m3, 0.35 m3/d/m3, 0.26 m3/d/m3, and 0.23 m3/d/m3, biogas with 31.4%, 42.9%, 20.3% and 33.7 % methane composition is produced from 1:1 mix of OFMSW: CM, CM alone, OFMSW alone and 2:1 1 mix of OFMSW: CM respectively. The mix ratio significantly affects the biogas production (p-value<<0.05).
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    Upgrading Biogas Produced from Biogas Pilot Plant through Absorption
    (Addis Ababa University, 2014-06) Assefa, Gizachew; Assefa, Berhanu(PhD)
    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