Microturbine Technology: Measuring up

Jim Watts, Ingersoll-Rand Energy Systems, USA

The gap between microturbines and small gas turbines is narrowing, with larger-sized microturbines coming to market. One of these is Ingersoll-Rand’s MT250, a 250 kW unit which began shipment earlier this year.

To this point, the size difference between the new generation of very small gas turbines known as microturbines and ‘traditional’ small gas turbines has been rather large. Whereas a gas turbine producing 1 MW of electrical power would be considered small, microturbines were introduced in capacities of only up to 70 kW. The gap between the two technologies is now starting to be filled by larger sizes of microturbines.

One example is the Ingersoll-Rand (IR) MT250 microturbine that began shipment this spring. It offers the same advantages as its smaller brethren; a comprehensive package for ease of installation, a recuperated engine cycle for high electrical efficiency, very low emissions, low package noise, and very little vibration.

The MT250 package integrates all of the major system components into one enclosure. The base engine includes all turbomachinery, the recuperator, combustor, gearbox, rotating generator, controls, and auxiliaries. A fully integrated fuel gas booster is available in the package for low-pressure gas supply installations. And the package is designed to include an integral Heat Recovery Unit (HRU) for combined heat and power (or cogeneration) applications.

Figure 1. IR’s MT250 microturbine package
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Design basis

The MT250 basic engine design differs from that of IR’s 70 kW microturbine unit, the MT70, in that it is based on the Dresser-Rand KG2 gas turbine engine. The latter is a 1.5-2.2 MW simple cycle engine with over 20 years of reliable service in 54 countries around the world. Over 900 KG2 systems have been shipped and over 14 000 000 hours of operation have occurred in tough marine, industrial, and desert environments. The KG2 is the most compact, proven heavy-duty industrial gas turbine in its power range.

The MT250 design draws from key aspects of the KG2 technology. For example, the core engine design uses the same overhung, back-to-back rotating component configuration with all bearings located on the ‘cold’ side of the machine. The radial compressor uses a vaned diffuser and the radial turbine includes nozzle guide vanes.

The shaft speed is 45 000 r/min and the cold end is connected to a planetary gearbox that drops this to the 1800 r/min design speed of the electric generator. The latter is a rugged, three-phase synchronous generator; a clean, reliable technology used today to produce power for the grid and well understood by electric utilities.

With this generator, the MT250 can operate in either grid-parallel or grid-isolated mode. In grid-parallel operation, the MT250 typically helps to defer facility electrical demand. The grid provides all voltage and frequency regulation and the MT250 supplements the kW power supplied by the grid. In addition, the customer can set the power factor of the MT250 generator to help provide kVARs to the facility.

With its switchgear option, the MT250 can maintain continuous electrical power for segregated loads by providing a closed transition between parallel and isolated states. Therefore, these segregated loads do not see any loss in AC power upon grid failure since the MT250 maintains a smooth transition upon loss and return of grid power.

Key component

The MT250 uses the same breakthrough recuperator technology currently integrated into IR’s 70 kW microturbine and sold by IR to other gas turbine manufacturers. The IR recuperator is the key component that allows the MT250 to provide high electrical efficiencies (around 30 per cent LHV) relative to conventional gas turbine engines. It does this by recovering heat from the exhaust of the core engine, which is then used to preheat air coming into the engine’s combustor.

Figure 2. The MT250 design draws from rugged KG2 technology
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The IR-patented combustor in the MT250 uses a lean, pre-mix configuration to produce the very low emissions offered by the microturbine. This dry, low-NOx technology results in very low NOx and CO emissions without the need for post-exhaust treatment. For example, IR’s 70 kW microturbine using the same technology is certified by the California Air Resource Board to meet its 2003 emissions limits for distributed generation (DG) systems (

Right from its initial launch, the MT250 was able to handle a wide range of gaseous fuels. This includes relatively low Btu-value fuels such as methane recovered from landfill sites, which the MT250 will accept with values down to 13 000 kJ/m3 (350 Btu/ft3). At the high end, the MT250 can use fuels up to 93 100 kJ/m3 (2500 Btu/ft3) such as propane or the gases associated with oil well operations.

Of course, the MT250 is designed for natural gas service as well and can accept a wide range of gas inlet pressures. By incorporating a fuel gas booster in its package, the MT250 can accept fuel delivery pressures as low as 0.02 kg/cm2 (8″WC) standard, with 4″WC as an option. The integral IR fuel gas booster eliminates any concerns or building code difficulties with high pressure gas lines in a facility. Based on IR’s in-house screw compressor technology, the fully sealed compressor design and attention to enclosure layout and ventilation means the MT250 meets the requirements of standards such as NFPA 37 and is thus fully qualified for indoor installation.

Figure 3. The MT250 industrial-quality integrated fuel gas booster
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CHP applications

The MT250 package also can include a fully integrated HRU for combined heat and power customer applications. This hot water heat exchanger is specifically designed to fit within the MT250 enclosure to provide a reduced overall system footprint. It is built into the MT250 exhaust system, is rated for potable water use, and is designed to serve many kinds of applications including domestic hot water, space heating, or process needs. It includes a variable position damper to control recovered heat rates for thermal load handling flexibility and will even accommodate HRU conditions without water flow.

The amount of heat recovered in a particular application depends on water inlet temperatures and flow rates. For example, the MT250 can recover over 340 kW (1 150 000 Btu/hr) of heat for a domestic hot water system supplied by cold water. With typical exit temperatures of 250à‚°C (480à‚°F), microturbine exhaust heat can also be used directly by a customer to supply a particular facility heating process.

By providing new and reliable onsite electric capacity, the MT250 is helping both business and the environment. The continuous operation of onsite microturbine power can meet the energy loads and reliability requirements of facilities such as hotels, schools, hospitals, industrial plants, and multi-family dwellings. The clean-burning microturbine even further helps the environment when operating on landfill gases and other renewable waste fuels and associated gases.

Shipment begins

In February 2004, IR shipped its first MT250 microturbine. The unit will be used by the Sanitation Districts of Los Angeles County Lancaster Water Reclamation Plant. It will burn digester gas to generate electricity and provide hot water for the plant’s on-site use. “This shipment represents the culmination of an intense development effort that has produced a state-of-the-art distributed generation technology,” said Chip Bottone, president of IR Energy Systems. “The vision of a few and the efforts of many have made this achievement possible, he added.

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