Biorem's biogas desulphurization product is providing very high removal efficiencies at a landfill site in Quebec. Despite the highly variable concentrations and low winter temperatures, the system has achieved an average removal efficiency of 97%, and has maintained outlet nydrogen sulphide concentration below target consistently. As a result, the customer has realized a significant cost reduction over the previously used chemical scrubbing process, and site environment has improved.
The biological desulphurization process offers many benefits over scavenger media and chemical scrubbing processes. No media replacement is required, and only a small amount of nutrients are needed. The operation is simple and inherently safe. Our cost evaluations show that for systems with hydrogen sulphide removal capacity of 150 kg/d, simple payback of biological treatment over scavenger media is less than 2 -nonths. But as the loading rates decease, the simple payback period increases, but the biological process remains viable even as low as 6 kg/d capacity with a simple payback of less than 3 years. For large biogas power generation systems, biological system is a highly viable desulphurization option.
Landfills charged with organic and construction wastes generate biogas throughout their useful life and for a significant period following closure. Generation of biogas results from anaerobic treatment of organic matter and inorganic sulphates. Rates of generation and composition depend on many factors including temperature, gas withdrawal rates, characteristics of waste treated and landfill layout. The gas is withdrawn using wells drilled in the waste material at appropriate intervals. Historically, the primary objective of collection was to contain the biogas within the landfill boundary, and burn it in a flare to prevent odours.
However, the recent trend is to produce power from biogas. Regulatory and environmental pressures, as well as government incentives such as Ontario's Feed in Tariff program, have improved the economics of power generation from biogas despite the recent drop in natural gas prices. Biogas contains many impurities, which if left untreated, would result in high power generator maintenance cost. In particular, siloxanes, organic compounds of silicon, are of great concern as these form silica deposits in engines and turbines and manufacturers of this equipment specify maximum acceptable loading rates. Engine manufacturers also specify the maximum acceptable hydrogen sulphide loading rates. Regardless, hydrogen sulphide concentration should be reduced upstream of the siloxanes removal system to avoid interference.
Conventional methods for biogas desulphurization include reaction with a scavenger media to retain hydrogen sulphide as iron sulphide, or chemical scrubbing using a mixture of caustic and bleach to oxidize it to sulphuric acid. Other methods such as condensation and adsorption have also been used in some instances. Reaction with scavenger media is the most commonly used process due to its simplicity of operation, despite the frequent media change-outs, high pressure drop and high operating cost. However, as the concentration of hydrogen sulphide increases, so does the cost of media replacement and capital. Chemical consumption in chemical scrubbers increases in proportion to hydrogen sulphide concentration.