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Performance efficiency of locally available low-cost adsorbents in purification of biogas for high grade applications
Abstract
Despite having several benefits, the uptake of biogas as a green fuel in rural areas has remained low. One of the technical challenges responsible for slow biogas adoption rate in rural households is biogas contamination. Biogas requires treatment before use, this includes; purification and upgrading. Purification involves the removal of impurities whereas upgrading aims to convert the biogas to a higher fuel standard by increasing its low calorific value. The aimof this study was to develop and test the performance of a cost-effective and affordable system for purification of biogas using low-cost absorbents, thus upgrading biogas to a sustainable clean cooking fuel. The study employed an experimental research design. A pilot domestic scale biogas cleaning system was set up at the Jomo Kenyatta University of Agriculture and Technology (JKUAT) IEET workshop. The study sought to determine the potential and efficiency of locally available and low-cost adsorbents in purification of raw biogas. The raw biogas was passed through the cleaning units, each packed with individual adsorbent. Analysis ofthe biogas for methane (CH4) composition was done before and after each cleaning unit using digital biogas analyzer model GASTiger 2000, supplied by Yantai Stark Instrument Company limited. The locally available low-cost adsorbents used in the study were; red soil, charcoal, steel wool, clay soil, iron shavings and wood ash. This study also tested the effect of the various physical interfaces of the adsorbents, where a cleaning unit packed with two or more of the adsorbents was used. The study shows that adsorption by clay soil and charcoal gave highest methane concentrations at 67.48±2.33% and 67.20±1.08% respectively from 58.47±2.12% in raw biogas. Cleaning by the other adsorbents; red soil, wood ash, steel-wool and iron shavings improved methane concentrations to 61.76±0.71%, 60.75±2.47%, 58.97±1.92% and 58.56±1.96% respectively. Combination of the various adsorbents into one cleaning unit further improved methane levels to 69.83±1.57%. These results, despite exhibiting relatively low adsorption of the impurities in raw biogas, positively confirmed that indeed the low-cost adsorbent materials have a potential in biogas purification. Modification of these low-cost adsorbent materials, such as use of activated charcoal, would be one of the ways to improve their performance. However, this study specifically sought to explore the potential of these low-cost adsorbents in purification of raw biogas, in their natural state. This study recommends further exploration of other relatively less technical methods such as water scrubbing that could be coupled with the use of natural low-cost adsorbents to improve the purification of raw biogas to near, if not equivalent to, natural gas (that is in the regions of 97-100% CH4 concentration). Thus, for a possible adoption as an alternative means of cleaning and upgrading raw biogas to a clean cooking fuel.
