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H2S Filters Compression System Siloxanes Filters

Biogas pretreatment systems such as this one in Dublin, California, U.S.A. use austenitic stainless steel for its corrosion resistance.

nickel, vol. 23, no. 3, june 2008

©ISTOCKPHOTO.COM/ROgIERvDE UNISON SOLUTIONS INC. ©ISTOCKPHOTO.COM/gARy gOLDEN

©ISTOCKPHOTO.COM/FRANK vAN DEN BERgH

India’s Biogas Boom

Nickel alloys play an important role in biogas projects

India is the world’s fth-largest and second-fastest-growing producer of greenhouse gas emissions. Generating electrical power by burning biogas (a mixture of methane and carbon dioxide produced by the bac- terial degradation of organic matter) harnesses these greenhouse gases and reduces demand for fossil fuel- red generating plants.

To generate even more biogas energy, Capstone Turbine Corporation of Chatsworth, California, U.S.A., plans to

dairy and keep it running,” explains Hynes.

The micro-turbines use nickel alloys N06002, N07713, N07718,

S30100 and S34700 in components such as the combustion chamber, spinning turbine, main rotor shaft, and recuperator housing, all of which run continuously with minimal maintenance.

So well are the nickel alloys performing that the company is consid- ering extending, by thousands of hours, the re-

install more micro-turbines in India this year, building on lessons learned from its rst in- stallation, in Purulia, West Bengal, in 2006.

“We need four to ve [biogas] projects in a new region to understand the hurdles to development; then the market starts to grow,” says Tony Hynes, vice-president and general manager of Capstone’s operations in Europe, the Middle East, Asia and India.

“Austenitic stainless steels are the most cost-effective materi- al for the systems that clean and compress the corrosive biogas prior to combustion.”

placement schedule for the combustor and turbine head. Such an extension is desirable in eld condi- tions where long, trouble-free operation of equip- ment is required. “Clients tell us that they are oper- ating well past the original replacement deadlines,” says Hynes.

Austenitic stainless steels are the most cost-ef- fective material for the systems that clean and compress the corrosive biogas prior to combustion in micro-turbines.

The Purulia installation is at a dairy farm

and consists of two 30-kilowatt micro-turbine

systems, one of which is for backup. These can run either linked or inde- pendently of the power grid. “When the grid is down, we connect to the

Biogas is mostly methane and carbon dioxide, with hydrogen sulphide present as a contaminant. When hydrogen sul- phide is mixed with water (with which biogas is always 100% saturat- ed), it becomes a weak acid (hydrosulphuric acid). Carbon dioxide

forms a mild carbonic acid in the presence of water.

“Biogas is very corrosive, which is why we use a lot of stainless

steel,” explains Adam Brotherton, manager of engineering for Unison Solutions Inc., a company based in Dubuque, Iowa, U.S.A., that spe- cializes in biogas conditioning and distributed generation. “It has good corrosion resistance to those acids. Predominantly we use S30400 and S30403, though sometimes customers request S31600 and S31603, as these are more resistant to corrosion. The S30403 might last 40-50 years whereas the S31603 can last 100 years.”

A typical compression and cleaning system takes the gas drawn off the top of the sludge in a digester, where manure has decayed for 20-30 days, and compresses and cleans it before it is burned.

In a simpli ed description of this process, the gas rst passes through lters that remove the hydrogen sulphide. These tanks will typically be constructed of stainless steel plate 4.8 or 6.4 millimetres thick and are



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