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Combustion Gas Turbine-Driven Electric Generation Systems

Stationary combustion gas turbine engines have been prominent in large-scale power cogeneration over the past two decades. Recent advances in emissions control, production cost reductions, and ease of operation have allowed combustion gas turbines to achieve further market penetration in the medium- and large-scale commercial peaking and cogeneration industries, where they have become cost-competitive with conventional steam power plants. They have also had significant market penetration in capacities ranging down to 1 MW

Commercially available gas turbine engine electric generation systems range from 30 kW to more than 100,000 kW, though capacities of below 400 kW are still uncommon. Designs and configurations include:

• Aeroderivative or industrial design.

• Single-shaft or multi-shaft design; alternative compressor and combustor types.

• Fuels used: natural gas, LPG, alcohol, kerosene (jet fuel), coal-derived gas, refinery gas, landfill gas, naphtha, distillate oils, crude oils, and residual oils.

• Emissions control applications, i.e., wet injection, dry low-NOX (DLN) combustors, and selective catalytic reduction (SCR) systems.

• Capacity enhancements based on injection of steam or water.

Other distinctions include number of stages, pressure ratios, operating pressures, emissions controls, coolant systems, combustion sequences, and materials used. Cycle enhancement techniques include intercooling, reheat, and regeneration (recuperation).

Combustion Gas Turbine Characteristics

Gas turbines may be characterized by the following qualities:

• High power density resulting from high rotating speeds, materials derived from flight technology, and absence of reciprocating parts.

• High mass flow and exhaust temperatures that allow for generation of large quantities of steam or process heat.

• High level of availability and low frequency and cost of maintenance. W hile this is not always true, it is commonly found to be the case in most capacity ranges. Gas turbine electric generation shows simple-, combined- and cogeneration-cycle efficiencies that are equal to or higher than conventional coal power plants. Large- and medium-capacity units in heat recovery applications exceed the thermal efficiency of the best available utility power generation cycle. Smaller gas turbine systems offer lower simple-cycle thermal efficiencies than reciprocating engines of similar capacity, but offer a greater proportion of recoverable heat.

Combustion gas turbines in a wide range of capacities are available as factory-packaged power generation systems with emissions and sound attenuation options, guaranteed performance, and factory testing. The packaged system offers relatively low installation costs and can minimize space requirements. Figure 26-19 shows the assembly of a small-capacity packaged Kawasaki gas turbine generator set. Larger cogeneration systems with HRSGs can be delivered to the site as separate standardized assembled components.

Commonly, gas turbine electric generation systems are applied in facilities that feature large baseload, high- or intermediate-pressure steam loads, where thermal loads are relatively large in proportion to electric loads. In these applications, somewhat lower simple-cycle thermal fuel efficiency is offset by increased heat recovery. However, their high power density, high level of reliability, and the potential to recover high-temperature heat also make gas turbines attractive for a wide range of other electric generation applications, including stand-by power service, small capacity

Fig. 26-19 Assembly of Small-Capacity Packaged Gas Turbine Generator Set. Source: Kawasaki Heavy Industries, LTD

cogeneration, and stand-alone prime power service in remote locations.

Operation, Maintenance, and Reliability

When applied properly, maintained well, supplied with clean fuel and air, and operated and monitored properly, gas turbine electric generation systems offer higher reliability and significantly lower life-cycle maintenance costs than most available alternatives.

Preventative maintenance procedures typically include a shutdown every 3 to 6 months for detailed inspection and routine maintenance. Monthly inspections that do not require shutdowns are also required to check fittings, filters, oil levels, and instrumentation and to perform cleaning.

200psia 400psia 600psia 800 psia 1000psia '1200

Fig. 26-20 Typical Heat Recovery Performance Curves for FT8 with Single-Pressure HRSG. Source: United Technologies Turbo Power Div.

200psia 400psia 600psia 800 psia 1000psia '1200

Fig. 26-20 Typical Heat Recovery Performance Curves for FT8 with Single-Pressure HRSG. Source: United Technologies Turbo Power Div.

Control of fuel quality and control of airborne particles with high-performance primary air filters are critical routine procedures. Time between overhaul (TBO) may range from 15,000 to 100,000 running hours, depending on the quality of applied equipment, type of duty cycle, load characteristics, and the quality of the preventative maintenance program.

Gas turbine generation systems typically offer high on-line availability. Availability factors of 95% and higher are common. In most electric generation applications, this is critical to the realization of savings and/or fulfillment of power sale obligations. Key elements of high availability include routine maintenance, provisioning and inventory maintenance of spare parts, and careful planning for major overhauls.

Use of Rejected Heat

Gas turbine systems provide most or all of their recoverable waste heat in exhaust gas at between 850 and 1,100°F (450 and 600°C). This valuable, high-temperature heat can be used in a wide range of applications, including direct contact process heating and indirect heating/steam production. Watertube boilers, including once-through designs, are used almost exclusively. Depending on the turbine's simple-cycle thermal efficiency and the temperature and pressure of the steam being generated, a heat-recovered unfired gas turbine system will typically produce 4 to 7 lbm (1.8 to 3.2 kg) of steam per kWh generated.

The thermal output of gas turbine systems can be easily augmented through the use of supplemental firing. This refers to the use of a duct burner or a register burner in the hot gas duct upstream (i.e., ahead) of the HRSG to raise the temperature of the exhaust gas. This process increases steam mass flow (the amount of steam generated) and may improve steam conditions (temperature and pressure) as well. With duct firing, the exhaust temperature can be elevated to as high as 1,800°F (982°C) and temperatures exceeding 2,200°F (1,478°C) may be accomplished by register burners in boilers equipped with radiant heat transfer. Figure 26-20 shows the typical heat recovery performance curves for the FT8 gas turbine with a single-pressure level HRSG.

A regenerator, or recuperator, is a heat exchanger that transfers heat from turbine exhaust gas to compressor discharge air prior to fuel combustion, displacing a portion of fuel heat input that would otherwise be required. Unit capacity (as a result of heat exchanger pressure drops) is usually decreased somewhat, but cycle thermal fuel efficiency is increased. Reduced exhaust temperatures limit the quality and quantity potential for

Steam

Driven *

Chiller Chilled

M Water

Fig. 26-21 Integration of Steam Driven Absorption Chiller with a Gas Turbine Cogeneration System. Source: British Gas

Steam

Driven *

Chiller Chilled

M Water

Fig. 26-21 Integration of Steam Driven Absorption Chiller with a Gas Turbine Cogeneration System. Source: British Gas exhaust gas heat recovery.

Recovered heat can be effectively used to drive either single- or double-stage absorption chillers. Usually, this is accomplished with heat recovery-generated steam, though turbine exhaust can also be used directly with a custom-designed heat recovery absorption chiller/heater to produce chilled and hot water. Figure 26-21 illustrates the integration of a steam-driven absorption chiller with a gas turbine cogeneration system.

Gas Turbine Application Examples

Figures 26-22 to 26-27 provide examples of gas turbine electric power generation applications.

• Figure 26-22 shows a 1,050 kW packaged gas turbine generator set. This unit features a Kawasaki gas turbine and a Kato generator, with an off-skid-mounted electrohydraulic starting system. Exhaust heat is used to produce steam to serve thermal load requirements at a university campus.

• Figure 26-23 shows a skid-mounted Kawasaki M7A-01 gas turbine generator set designed for cogenera-tion-cycle operation. With operation on natural gas and steam injection used for NOX control, at a rate of 150% of fuel flow, the rated baseload capacity of this generator set under ISO conditions is 5,480 kW. The twelve-stage axial compressor, which achieves a pressure ratio of 12.7:1, and the four-stage axial turbine both operate at 14,000 rpm. Inclusive of assumed inlet and outlet losses and a generator efficiency of 96%, the rated simple-cycle electric generation efficiency is 28.2% (LHV basis). The module is configured with a 250 kW ac motor with torque converter for starting and a single-stage parallel shaft gear unit with a gearbox shaft output speed of 1,800 rpm for 60 Hz and 1,500 rpm for 50 Hz electric service. The brushless synchronous generator has a maximum capacity of 7,059 kVA with a PF of 0.85. A 160 hp (120 kW) single-stage screw compressor is used to deliver natural gas to the combustor at a pressure of 256 psig (18.7 bar).

This unit can be packaged for cogeneration applications in a prefabricated, self-standing, steel, sound-attenuating enclosure that achieves a sound emission level of 85 dba at 3.3 ft (1 m). While the gas turbine itself only weighs about 10,000 lbm (4,500 kg), the entire generator set package weighs about 160,000 lbm (73,000 kg), with dimensions of 36 ft (11 m) long by 12.5 ft (3.8 m) wide by 24 ft (7.4 m) high, including the intake air filter.

• Figure 26-24 is a system flow chart of a cogeneration

Fig. 26-22 1,050 kW Packaged Gas Turbine-Driven Cogeneration System Applied at a University. Source: United States Turbine Corp.
Fig. 26-23 Skid-Mounted Gas Turbine Generator Set Designed for Cogeneration Duty. Source: Kawasaki Heavy Industries, LTD.
Fig. 26-24 System Flow Chart of a Cogeneration Application Featuring a Dual-Fuel-Fired Gas Turbine. Source: Kawasaki Heavy Industries, LTD

application featuring a dual-fuel-fired M7A-01 gas turbine. The relatively high exhaust temperature of about 1,000°F (540°C) allows for a high rate of heat recovery (about 49% of the thermal fuel energy input) from the 2 million ft3/h (60,000 m3/h) exhaust gas flow. About 30,000 lbm/h (13,000 kg/h) of saturated steam can be generated at a pressure of 240 psig (17.6 bar), with a feedwater (economizer inlet) temperature of 140°F (60°C). Steam flow paths are shown for steam injection and process use. About 14% of the steam output is used for NOx emission abatement, with the rest available for process use. The system flow chart shows both natural gas and oil fuel delivery systems. The cooling circuit, served by a cooling tower, and the feedwater circuit are also shown.

Figure 26-25 is a model of an ABB 83 MW dual-fuel gas turbine electric-generation system module. This heavy-duty industrial generator set features a silo-type dry low-NOx combustor which, when equipped with 36 EV burners, can reportedly achieve NOx emissions levels of 9 ppmvd (corrected to 15% O2) with

Fig. 26-25 Model of an 83 MW Heavy-Duty Industrial Gas Turbine Electric Generation System Module. Source: ABB

100 psig Steam

Natural Gas 17.9 MMBtu/hr

1,109 kW

T-1500 Gas Turbine

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