Appendix C: Air Quality

Combustion Turbines

Technical Description

Combustion turbines (or CTs, also called gas turbines) are the same technology used in jet engines. In the basic CT design, air enters a compressor, which packs large amounts of air into a combustor at high pressure. In the combustor, fuel is added to the air and burned, releasing heat energy and producing a high-temperature, high-pressure exhaust gas. This gas is expanded through a turbine, which powers a generator and the compressor.

Natural gas or distillate oils are the primary fuels used in combustion turbines. Gasified fuels, such as the syngas derived from coal, are also potential fuel candidates. The heat rate (or efficiency) for gas turbines is about the same as steam turbine generators. However, CT thermal efficiency is improving as the technology improves and CTs gain the flexibility of conversion to combined-cycle operation.

The inefficiency of a combustion turbine can be seen in the high temperatures of the gases discharged from the turbine. There is significant available energy in the exhaust gases, which can be recovered through a heat recovery process. One way to take advantage of this available energy is to use steam injection (which also has the benefit of reducing NOx emissions). In a steam-injected turbine, hot exhaust gases are recirculated to heat pressurized water into superheated steam. The steam is then injected into the combustor of the turbine and mixes with compressed inlet air. The additional inlet steam helps drive the turbine.

CT efficiencies can also be improved by using multi-stage compressors with inter-cooling between stages and by operation at higher turbine inlet temperatures. Currently, turbines achieve temperatures around 1111 degrees Celcius (ºC) (2000 degrees Fahrenheit (ºF)) , but improvement in heat-tolerant materials can increase this limit to more than 1278º C (2300ºF).

The high thermal energy in the turbine exhaust makes CTs ideal in cogeneration applications where high-grade process heat is used in addition to electricity. Another way to take advantage of the energy in the exhaust gases is to use the combustion turbine as the "topping cycle" in a combined cycle plant.

Combustion turbine technology is proven and widely used. Simple-cycle CT (SCT) designs are basic, reliable, and relatively easy to site. They can be installed with minimum site renovation and preparation because they are compact and generally do not require additional equipment, such as cooling towers or elaborate fuel processing subsystems.

A combined cycle combustion turbine (CCCT) combines a combustion turbine with a steam cycle plant to generate power very efficiently. Electricity is first generated from the combustion turbine. The exhaust gases from the CT then become the heat source for raising water to steam in a steam cycle system. The combustion turbine cycle is referred to as the "topping cycle," and the steam turbine cycle as the "bottoming cycle."

Combined cycle plants are designed to maximize the thermal efficiency of a power plant by using the available energy in the combustion turbine's high-temperature exhaust gases. The key to the combined cycle is the heat recovery steam generator system, which takes the place of the steam cycle boiler. Typical steam conditions in a heat recovery steam generator are 500 to 556ºC (900 to 1000ºF) and 70,307 to 105,461 grams per square centimeter (1000 to 1500 pounds per square inch). Instead of rejecting heat to the environment at gas turbine temperatures of more than 555ºC (1000ºF), the combined cycle eliminates heat at the steam cycle condenser temperature, which is the temperature of available cooling water--approximately 28 to 39ºC (50 to 70ºF).

Operating Characteristics and Capacity Contribution

Combustion turbines can be operated to meet both peak and energy loads. CTs can quickly respond to load demand changes; however, maximum efficiencies are obtained when operating at design capabilities. Because of high fuel costs, CTs tend to be used at a constant rate for a limited period of time. CTs can be quickly fired up and have proved effective in meeting short-term peak loads and load fluctuations due to extreme weather conditions.

CT availability factors run 80 to 90 percent. Simple CTs operate at heat rates of 11,000 to 12,000 Btu/kWh. Combined cycle applications operate at heat rates of 7500 to 8500 Btu/kWh.

Combustion turbines offer very good dispatchability, which provides many options in how CTs may be operated. These options include: (1) baseload-type operations where the plant is running most of the year; (2) daily peaking, where the plant is ramped up during the day to meet peak loads, but ramped down at night, thereby reducing problems of returning energy to the Northwest hydro system; or (3) seasonal or short-term peaking, where the plant is running for a period of prolonged heavy loads (e.g., during a cold snap) or during periods of low streamflow. Each of these options would result in different amounts of energy and capacity being provided. Under baseload-type conditions, the annual energy would be produced; however, under the other options, the resulting energy would be less than the annual amount. Capacity would be provided under any of the options, but ability to react to daily or weekly load fluctuations may be greater under the second option. If it is economic and if nonfirm energy is available, CTs could be displaced under any of these options.

If operated for capacity, a combustion turbine would meet peak loads but provide less total energy throughout the year. For example, at an expected capacity factor of 50 percent, a CT could provide extra capacity in several modes. One mode would be to operate it at 50 percent per day, running at maximum during the day and much lower at night. Another mode would be to use a CT to recharge the hydro system when it is drawn down to meet prolonged heavy loads (e.g., during a cold snap). The CT would be kept idle perhaps half of the weeks of the winter, but turned on for maximum, flat operation during cold weather, allowing the reservoirs to refill and increase their capacity effectiveness by increasing the head at each reservoir.

Costs

Cost estimates shown in Table C-1 are based on documentation contained in a July 1988 report, Development of Combustion Turbine Capital and Operation Cost, prepared for BPA by Fluor Daniel, Inc. The cost of power resulting from using nonfirm energy with CTs is dependent on the amount of nonfirm energy available, the value of nonfirm energy, and the cost and availability of fuel to operate such CTs.

Table C-1:

Costs - Combustion Turbines (1988$)

(Source: Resource Programs Final Environmental Impact Statement, Bonneville Power Administration, April 1993)

Capital Cost ($/kW)
Simple Cycle	660a
Combined Cycle	747a
O&M Cost
Fixed ($/kW-yr)
Simple Cycle	3.06
Combined Cycle	7.51
Variable (mills/kWh)
Simple Cycle	b
Combined Cycle	b
Real Levelized Costs (mills/kWh)	c
Nominal Levelized Costs (mills/kWh)	c

a These capital cost estimates include a $120/kW transmission adder, which

reflects siting on the east side of the Cascades.

b The variable costs have been loaded into the fixed costs.

c Combustion turbine cost depends on how they are used. When displaced by

nonfirm hydro power, combined cycle CTs have a cost of 26 to 34 mills/kWh (real).

Environmental Effects and Mitigation

The primary environmental effects of CTs are shown in Figure C-1. CTs that use natural gas are relatively clean burning. Only NOx emissions tend to be a problem because of the high combustion temperatures, but significantly less so than in coal combustion. NOx can be controlled with either water or steam injection into the CT combustor, eliminating up to 80 percent of the NOx. Water use and visible steam plumes in this case become an environmental concern, but water use can be minimized by re-using the condensed exhaust steam for steam injection.

Figure C-1: Environmental Effects and Mitigation - Combustion Turbines (This figure not available in electronic format)

If oil fuels are used, there is some sulfur dioxide pollution. SOx exhaust gas can be mitigated with scrubbers, which add to the cost of CTs. As in all combustion technologies, significant amounts of CO2, a "greenhouse" gas, and waste heat are produced. Simple-cycle CTs reject waste heat directly to the atmosphere, so cooling water is not required.

Because CTs are often sited close to where gas transportation and transmission lines meet, effects on urban environments need to be considered. CT noise can be a problem. Noise levels of unsilenced CTs can run 65 to 70 decibels at 366 meters (1200 feet) from an operating turbine. Silencing packages can reduce this to 51 decibels at 122 meters (400 feet).

Environmental impacts for combined cycle plants are the combined impacts of waste heat boiler plants and combustion turbines. For the amount of fuel combusted, though, plant efficiencies are proportionately higher, and, therefore, the environmental impacts are proportionately less.

Examples of potential environmental impacts for the gas-fired combustion turbine fuel cycle are shown in

Table C-2: Potential Annual Environmental Impacts Per Average Megawatt Per Year of Energy Generation for the Natural Gas-Fired Combined Cycle Combustion Turbine Fuel Cycle

(This table is reproduced from the Resource Programs Final Environmental Impact

Statement, Bonneville Power Administration, April 1993)

Potential Impacts	On-Shore Gas Extraction	Transportation	Generation
Air Pollutants Sulfur Oxides (tons) Oxides of Nitrogen (tons) Particulates (tons) Carbon Dioxide (tons) Carbon Monoxide	0.95 0.056 0.0013	0.0004 tons 0.266 tons	0.03d 5.81d 0.03d 3,904.95d 2.23e
Water Quality Impacts Consumption (acre-ft) Discharge Biological Oxygen Demand (tons) Chemical Oxygen Demand (tons) Oil and Grease (tons) Chromium (tons) Zinc (tons)	0.0058 acre-ft drilling mud 0.0011 0.0074 0.0228 0.00006 0.00002	·	3.4f 0.0081 0.651
Total Dissolved Solids (tons)	0.305	·	1.06
Total Suspended Solids (tons)	·	·	1.14
Ammonia (tons)	·	·	0.00012
Chloride (tons)	0.057	·	·
Sulfate (tons)	0.046	·	·
Thermal Discharge	·	·	28,800
Land Effectsb Acreage Requirements	.025 Permanent .032 Temporary	4.18	0.15 per MW capacity corrected for capacity
Waste Streams Solid Wastes (tons)	2.24 (Drill Cuttings)	·	undetermined
Employmentb Construction (employee-years)	.029	0.45	1.4 (per MW capacity)
Operations (employees per year)	.003	0.013 employees	0.1 (per MW capacity)
Occupational Safety and Healthc	·	·	·
O&M Injuries	7.7 x 10-8 to 2.174 x 10-6	1.06 x 10-7 to 1.7 x 10-7	3.4 x 10-6 to 6.34 x 10-5
O&M Deaths	9 x 10-10 to 2.23 x 10-8	3 x 10-10 to 3 x 10-9	2.5 x 10-8 to 1.1 x 10-6
Construction Injuries	·	·	6.8 x 10-6 to 9.88 x 10-5
Construction Deaths	·	·	2.23 x 10-8 to 4 x 10-7

a Unless otherwise indicated, these generic estimates are adapted from: U.S. DOE. 1983. Energy Technology Characterizations

Handbook, Environmental Pollution and Control Factors. DOE/EP-0093. Washington, DC.

b See sources and calculations in Appendix F to this EIS. Sixty-five percent capacity factor assumed.

c Adapted from Arthur D. Little. 1985. Analysis of Routine Occupational Risks Associated with Selected Electrical Energy

Systems. EA-4020. Electric Power Research Institute, Palo Alto, California.

d From BPA's emission estimates for environmental costs and planning.

e Adapted from Northwest Power Planning Council. 1991. Northwest Conservation and Electric Power Plan, Volume II-Part II.

f Flow rate requirements taken from Fluor Daniel, Inc. 1988. Development of Combustion Turbine Capital and Operating Costs.

DOE/BP-63056-1. Bonneville Power Administration, Portland, Oregon.

Supply Forecast

The quantity of combustion turbines installed is not inherently limited. Constraints that are typically discussed include ability to site and availability of fuel supply. These constraints will not impose an impediment for the first several hundred megawatts. For this EIS, 1680 MW of CCCT capacity (1394 aMW energy) is considered to be available to the region, of which 1260 MW capacity and 1046 aMW energy would be available to BPA. It is possible to initially install simple cycle CTs that are configured for conversion to combined cycle units.