Optimzation of efficiency of a biogas digester for small- scale electricity generation

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Daka, Kamzalaba Agripper
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The University of Zambia
Reliance on oil importation, electricity on the national grid, and other conventional fuels has a direct effect on the National Energy security and has become one of the most challenging problems that require to be tackled as the fossil sources are rapidly diminishing and electricity supply unreliable; together with deforestation. A glance beyond these sources is critical for long term energy security and sustainability as there are several uncertainties about these sources coupled with the greater environmental dangers encountered from fuel utilization. Thus, this research focused on the concept of an improved conversion of biodegradable organic matter to produce biogas through anaerobic digestion for the sole purpose of smallscale electricity generation. This is timely especially in view of the current energy (electricity) crisis being experienced in the country. The objective of this study was to develop models which predict the quantities of biogas and the corresponding feedstock required to generate electricity ranging from 50 kW to 500 kW. This was accomplished by; firstly, identifying the factors that affect the rate of biogas production and electricity generation. These factors were: temperature T, hydraulic retention time HRT, substrate loading rate LR, feedstock characteristics, specific fuel (biogas) consumption of the biogas engine, and number of hours per day of operation by biogas engine etc. and then the feedstock materials readily available in Zambia were also identified i.e. sludge water – water treatment plant, cow dung, swine manure, chicken droppings, vegetable and fruit waste, catering waste, grass silage, and corn silage. Using MATlab computer software, and MS Excel, models were developed and; then the findings were validated using data from a selected farm in Chongwe and from authentic literature. The generic model for predicting biogas production after demand for electricity was established was: where fc is biogas consumption rate and Ep, electricity demand. Under the Zambian climatic conditions, the optimum temperature for better efficiency of biomethanation was determined as 24.7oC; from the developed model (rates of gas production vs average monthly temperatures) with the knowledge of the feedstock characteristics (e.g. Specific gas yield). Also, to generate 300 kW of electricity (for example) would require 21,173.60 tonnes/year of swine manure or 33,190.20 tonnes/year of cow dung or 17,797.64 tonnes/year of chicken droppings or 10,505.02 tonnes/year of grass silage or 8,860.30 tonnes/year of corn silage or 17,056.07 tonnes/year of vegetable and fruit waste or 81,869.16 tonnes/year of sludge waste-water treatment plant and 11,163.98 tonnes/year of catering waste. v The digester sizing was determined with respect to 50 to 500 kW of electric power range. The plotted graphs which were to scale could effectively be used to estimate digester volumes for all the eight selected feedstock materials. A relationship between digester volume and feedstock characteristics showed that the digester volume is inversely proportional to the product of feedstock density and specific biogas yield.
Biogas--Zambia , Electricity generation--Zambia