Experimental investigation of biogas production from feedlot cattle manure

Abstract

The demand for alternative power sources has increased rapidly over the past few years as the cost of electricity is rapidly increasing in South Africa. Biogas can be generated from biomass in an anaerobic digestion process and used to generate electricity and heat as an alternative energy source to fossil fuel generated electricity. This study is focused on the biogas generation from cattle manure. The manure was analysed for weight loss over 40 days and the energy content determined. The biogas volume produced was measured as a function of time until there was no measurable gas formation after 41 days. The biogas was analysed for methane and carbon dioxide content using a gas chromatograph. A techno-economic model was developed in terms of the design of a simple agricultural biogas plant and the economics of the plant. The same manure sample was divided into different sub-samples to be aged to 0, 4, 7, 11, 14, 21, 28, 35 and 40 days respectively and these were analysed using the biochemical methane potential test. The corresponding cumulative biogas yield was 217, 206, 199, 154, 208, 208, 245, 369 and 295 Nml/g.VS respectively. The test results showed that an average of 240 Nml/g.VS of biogas can be produced from cattle manure that is less than 40 days old, with an average methane and carbon dioxide percentage of 63 % and 31 % respectively. Within 3 to 4 days the manure samples generated 80 % of the potential final biogas volume. The design of the biogas plant was based on 7 000 cattle that would produce 58 330 kg manure per day. The average biogas yield of 240 Nm3/ton.VS was assumed, together with the average TS and VS content of 17% and 80% (as a % of TS) respectively as experimentally determined. The designed total power yield of the biogas plant was 555.3 kW for the CHP. The electrical power of 166.6 kW would be produced from a 220 kW engine and the heat energy produced was 277.7 kW. The total electrical and heat power consumption of the biogas plant was designed to be 5.0 kW and 90.7 kW respectively. The economic viability of the biogas plant was based on a proposed REFIT value of US$ 0.0926 (R1.39) per kWh for the sales of electrical energy generated from biogas. The annual capital, consumption and operational costs of the biogas plant was calculated to be US$ 33 200, US$ 16 617 and US$ 16 209 respectively. The total annual income and costs of the biogas plant was US$ 140 133 and US$ 66 026 respectively. This leads to a net annual revenue of US$ 74 107 for the biogas plant. The return on investment was calculated to be 30.6% (>13%) and is attractive from the commercial point of view and will enter the economic potential. The net present value and internal rate of return was calculated to be US$ 542 792 and 30.4% respectively with a payback time of 3.3 years. Thus the biogas plant would therefore be economically viable in South Africa.