Global rising energy costs are focusing efforts by many industrial and municipal entities to find new sources of energy. Generating electricity from Biogas is of course nothing new however the cost of cleaning up certain Biogas streams with excessive levels of Hydrogen Sulphide (H2S) has previously proven to be financially unsustainable. Rising energy costs combined with improvements and cost reductions in desulphurisation systems have increased the number of potential biogas streams that can be efficiently and cost effectively treated to provide the end user with a lucrative additional revenue stream from their anaerobic digestion process.
DMT a Dutch based manufacture of odour control and Biogas desulphurisation systems, is also offering the DMT Sulfurex® process in the UK. The first UK system is installed in the summer of 2006 at the Mauri plant in Hull, Yorkshire. This system is designed to reduce the level of incoming Biogas from 20,000 ppm down to 135 ppm prior to it being fed into the CHP plant. The DMT Sulfurex® process is a multiple stage scrubbing process based on the selective absorption of H2S in a solution of sodium hydroxide. Numerous DMT Sulfurex® systems are operational throughout Europe. Despite the success of the system it has been significantly improved in recent years, the key developments are as follows:
• Improved selective absorption of H2S
• Reduced consumption of sodium hydroxide
• Treatment of spent lye
• Regeneration of spent lye
• Biogas pre-treatment with activated sludge or waste water.
Pre-treatment of Biogas using activated sludge can significantly reduce the consumption of sodium hydroxide and thereby lower operational costs. Selective absorption reduces the amount of CO2 absorbed and consequently reduces the volume of sodium hydroxide required. The second stage scrubbing process of the DMT Sulfurex® system in which the spent lye can be used to further improve the rate of absorption. The DMT Sulfurex® system is capable of reducing the H2S levels, up to 20,000 ppm in the incoming Biogas
stream by more than 99%.
Further developments also include the potential for desulphurisation without using any chemicals at all, this system is known as the DMT Biosulfurex® process and utilises a purely biological approach. When a limited amount of Oxygen is dosed into the Biogas flow “Thiothrix & Thiobacillus” bacteria oxidise H2S to elemental sulphur and sulphate, dependant on process conditions, or sulphuric acid. In order for the bacteria to oxidise effectively Oxygen, nutrients, and a contact surface are required. Oxygen is provided by a compressed air supply, nutrients from activated sludge and the contact surface from the media within the scrubber itself.
This paper will describe the operation and performance of both the DMT Sulfurex® and the DMT Biosulfurex® systems, detailing system efficiencies, capital costs, running costs and payback periods.