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The flexible friend

Being able to switch between baseload, peak shaving or backup power has made microturbine power systems a popular option and now Capstone has enhanced that flexibility by extending the range of potential gas feeds. The result is a turbine that can make use of methane byproducts from landfill and sewage plants.

Tony Hynes, Capstone Turbine Corp., California, USA

apstone Turbine has reported significant new orders of microturbines from Europe and Asia, reflecting an industry wide trend in growing sales. A healthy portion of these non-US orders are for flare gas fuelled models, including recent bookings from France and South Korea.

Having sold and shipped more than 3000 microturbines worldwide since debuting the technology in 1998, Capstone currently markets a natural gas fuelled 60 kW model for onsite power, heat and absorption chilling applications. But the company’s 30 kW models are claimed to be unique in their fuel flexibility. This product is able to run on natural gas, propane, diesel and kerosene as well as certain flare gases, ranging from low Btu landfill gas to high Btu, high sour content gases at oil extraction sites on- and offshore.

Methane from decaying garbage at landfills and from anaerobic digesters at wastewater treatment plants and livestock farms, is commonly flared into the atmosphere and gases associated with oil extraction are very often flared off as well. Removing the off-gas content to create pipeline quality natural gas would cost more than the economic value of the gas.

Using flare gas as a fuel source for renewable power generation is becoming somewhat more common, particularly at landfills. In such cases, low methane biogas traditionally has been piped to bus- sized gensets able to produce megawatts of power.

Today, however, microturbine technology is enabling the creation of renewable electricity on the kilowatt scale. It is ideally sized to serve the 100-400 kW site power needs of the vast majority of landfill sites around the world, particularly those in areas, including much of the US, where exporting renewable energy to the utility grid is hampered by economics or outdated utility regulation.

“Fortunately, a number of nations in Europe and elsewhere are more progressive,” said Capstone CEO John Tucker. “Methane from landfills, sewage plants and farms is a big piece of the global warming puzzle, since methane has a more than 20 times greater greenhouse effect than carbon dioxide. As a result, some Kyoto Accord nations are incentivizing renewable energy from flare gas sources.”

Wear and tear

Although flare gas fuelled, megawatt class reciprocating engines have been the most commonly deployed technology, there have been significant issues with reliability and emissions.

Reciprocating engines are sensitive to the corrosive hydrogen sulphide (H2S) content in landfill gas, thus rapidly deteriorating the performance of pistons, valves, injectors and more. Siloxanes – a family of chemicals used extensively in consumer products such as cosmetics and shampoo – present in flare gas turn to silica (sand) at combustion temperatures. The damage done by these trace gases skyrockets engine maintenance intervals, costs and downtime.

Flare gas fuelled microturbines from Capstone Turbine are tolerant of hydrogen sulphide levels many times those found in such biogases, and run well with the assistance of gas conditioning processes.

Figure 1. Landfill gas fuelled microturbines, like these recently commissioned in Taiwan, are seeing a rise in deployment worldwide
Click here to enlarge image

“In addition to knocking out siloxanes, you also need to reduce the moisture level and pressurize the gas to more than 75 psig to ensure reliable performance,” said René Flores, Capstone’s leading biogas applications engineer. “Several companies currently market gas treatment skids that do an excellent job of accomplishing this. When the gas is properly processed, the microturbines just keep going and going, as evidenced by several sites that have performed reliably for 10 000 to nearly 20 000 hours.”

Emissions and odours from biogas fuelled generators have also been an issue. The split second combustion of engine gensets cannot match the ultra low emissions and odour destruction of a turbine’s continuous combustion process. Expensive exhaust treatment devices and catalysts used on natural gas fuelled reciprocating engines are quickly fouled by the residual noxious, toxic and corrosive content in the exhaust of landfill gas fuelled reciprocating engines.

Renewably fuelled microturbines generate vastly fewer emissions than conventional engines. Using landfill gas as fuel, Capstone microturbine emissions of NOx and unburned methane are each less than 3 ppmV at 15 per cent O2.

Microturbines accomplish this without any exhaust treatment devices, catalysts or chemicals. The US Environmental Protection Agency’s (EPA’s) Environmental Technology Verification reports also confirm that natural gas fuelled Capstone microturbines have full load NOx and CO emissions of less than 4 ppmV at 15 percent O2.

CHP efficiency

Electrical efficiency of reciprocating engines is a few points higher than that of microturbines, but that is not much of an advantage when the goal is to destroy biogas. For the same electrical output, microturbines flare a small amount more biogas than reciprocating engines, and do it much more effectively and cleanly.

As microturbines are air-cooled, all the heat energy is in the exhaust stream, making them also very well suited to combined heat and power (CHP) applications. There is no dump radiator and no pumps, hoses, coolants, temperature sensors or secondary heat exchangers. All the thermal energy of the microturbine exhausts through a single, simple air-to-water heat exchanger. As a result, Capstone has demonstrated CHP efficiency in excess of 80 per cent delivered to end user loads.

Biogas-fuelled microturbine CHP is ideal at wastewater treatment plants and livestock farms, since the anaerobic digesters must be kept warm to maximize methane production. But what do you do with heat at a landfill?

Figure 2. This array of eight biogas fuelled microturbines northwest of Paris feeds renewable power into the EdF grid
Click here to enlarge image

One application in France by CHP specialist Soffimat uses exhaust from a landfill gas fuelled array of microturbines to heat a nearby greenhouse.

At an application north of Chicago by Alliant Energy’s RMT division, landfill gas from a former Superfund site is piped more than a kilometre to a high school campus. There, an array of 12 Capstone microturbines cleanly converts the gas into electricity and odour free exhaust that heats the school’s pool, buildings and other thermal loads. This application was named Project of the Year by the landfill methane division of the US EPA, and won several other engineering awards.

Another growing application for microturbines is the use of high Btu flare gas from upstream oil production, both on- and offshore. Petroleum associated gases often have very high levels of corrosive hydrogen sulphide. While this can be problematic for most other types of generator, Capstone’s 30 kW microturbines are unique in being able to accept hydrogen sulphide content of up to 70 000 ppm with no ill effect. In one early example, a 30 kW Capstone microturbine accumulated more than 30 000 hours of operation on such ‘sour’ gas at an oil field in Calgary, Canada, while requiring only routine minor maintenance.

Offshore onshore

The comparably minor and infrequent scheduled maintenance needs of microturbines are particularly advantageous on offshore platforms, where a minute of downtime can cost more than the equipment itself. Other than a few air filter changes, a gas fuelled 60 kW Capstone on a platform in the Gulf of Mexico has been spinning almost non stop for more than two years.

Although these platforms are extracting oil, a few drops spilled offshore can result in very costly environmental fines. Understandably the oil leakage found under virtually every conventional genset can be quite a problem offshore. Capstone microturbines provide the advantage of eliminating the need for any oil or lubricant fluids due to their patented air bearing technology.

“From our point of view, our air bearings offer a significant product advantage,” said Tucker. “Our systems have just one moving part: the turbine shaft that rotates at up to 96 000 rpm. Those speeds can prove to be extremely challenging for conventionally lubricated bearings.”

Space is at a premium on offshore platforms and although microturbines have a relatively small footprint (high power density) and require much less space than conventional generators, siting a generator on an offshore platform can be difficult as it must be placed in a designated ‘non hazardous’ area. This is not a problem on larger platforms, such as Wintershall’s unmanned Q4-C in the North Sea where four site gas fuelled 60 kW microturbines ensure reliable operation 24 hours a day. However, many platforms require the generator to be packaged in an enclosure specially rated for ‘class I, division 2’ operation.

Capstone now offers systems in stainless steel packaging that meet both criteria. Recent purchases by several major oil companies will result in deployment of more flare gas fuelled microturbines in the gulfs of Alaska and Mexico as well as the China, North and Mediterranean seas.

“I believe we will see continued growth, particularly in our overseas markets, for flare gas fuelled microturbines,” Tucker said.

“Small capacity solutions like ours are bringing the advantages of renewable energy and greenhouse gas reduction down to a scale that can be used by a much broader customer base.”

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