THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS
June 2008
Volume 23, Number 3
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This biogas pretreatment system in Dublin, California, U.S.A., uses austenitic stainless
steel for its corrosion resistance.
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These micro-turbines in West Bengal, India use nickel alloys for minimal maintenance
and reliability.
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Nickel alloys play an important role in biogas projects
By Carroll McCormick
Nickel Magazine, June 2008 -- India is the world’s fifth-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 bacterial degradation of organic matter) harnesses
these greenhouse gases and reduces demand for fossil fuel-fired generating plants.
To generate even more biogas energy, Capstone Turbine Corporation of Chatsworth, California, U.S.A., plans
to install more micro-turbines in India this year, building on lessons learned from its first installation,
in Purulia, West Bengal, in 2006.
“We need four to five [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.
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 independently of the power grid. “When the grid is down,
we connect to the 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 considering extending, by thousands of hours,
the replacement schedule for the combustor and turbine head. Such an extension is desirable in field
conditions where long, trouble-free operation of equipment is required. “Clients tell us that they are
operating well past the original replacement deadlines,” says Hynes.
Austenitic stainless steels are the most cost-effective material for the systems that clean and compress
the corrosive biogas prior to combustion in micro-turbines.
Biogas is mostly methane and carbon dioxide, with hydrogen sulphide present as a contaminant. When
hydrogen sulphide is mixed with water (with which biogas is always 100% saturated), 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 specializes
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 simplified description of this process, the gas first passes through filters that remove the hydrogen
sulphide. These tanks will typically be constructed of stainless steel plate 4.8 or 6.4 millimetres thick and
are made to withstand negative pressure.
The gas then passes through a compression system, where it is raised to pressures as high as 8.4 kilograms
per square centimetre (kg/cm2). The piping used in this process is typically stainless steel. Next
it passes through heat exchangers to reduce the temperature to about 4°C, forcing more water out of the gas.
The gas is then re-heated to 27°C, reducing the relative humidity to 25%.
Subsequently, the gas passes into stainless steel vessels where siloxane, a chemical used in lubricants
and personal care products, is filtered out. Since siloxane turns into a glass-like substance at high
temperatures it must be removed before the gas is burned. After this step, the biogas is delivered to an
end-use device, such as a turbine, internal combustion engine, or fuel cell.
Carroll McCormick is a Montreal-based freelance writer.
Photos: Unison Solutions Inc. and istockphoto.com
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