2 SIMPLE-CYCLE GAS TURBINE

2.1 AVAILABLE SIZE RANGE

Currently available sizes of simple-cycle gas turbines range from 80 to 134,000 shaft horse power (SHP) for a portable-type machine and from 12,000 to 50,000 SHP for a regenerative unit.

2.2 SPACE REQUIREMENT

The space requirement for a gas turbine can be estimated by the power density, which is the SHP/ft'. The power density is an important parameter in that historically gas turbines have been used in applications requiring high output in a relatively small volume, e.g., for aircraft engines. For ICES applications, power density will be less important than thermodynamic considerations. For simple-cycle units up to 10,000 SHP, the power density varies greatly from 132 SHP/ft³ for aircraft-type engines to less than 1 SHP/ ft³ for industrial types. Above 10,000 SHP, the power density is nearly constant at about 5 SHP/ft³.

2.3 WEIGHT REQUIREMENT

The weight requirement of a gas turbine can be estimated by the specific capacity, which is the SHP/lb. Weight will not be a primary consideration in ICES applications. Data from 13 manufacturers on specific capacity versus rated capacity for simple-cycle units are widely scattered and range from 0.08 to 5.5 SHP/lb for units less than 10,000 SHP. Above 10,000 SHP, the specific capacity is nearly constant at about 0.6 SHP/lb.

2.4 MATERIALS BALANCE

2.4.1 Primary Material Inputs

A gas turbine has two primary material inputs - working fluid and fuel. Conventional practice³ assumes air as the working fluid throughout the cycle. In actual operation, products of combustion are mixed with air while passing through combustion chambers and turbines. Figure 2.1 shows the specific heat, C_p, of the working fluid as a function of the exhaust temperature. ⁴

2.4.2 Incidental Material Inputs

Although lubricating oil is required by all gas turbines, the oil consumption is very low because most single-shaft gas turbines have only two bearings. To allow for thermal expansion, one bearing (a journal-thrust bearing) is fixed, while the other (a journal bearing) is free to move slightly. Oil may also be consumed by the reduction gearing, if any.

2.5 ENERGY BALANCE AND PERFORMANCE

2.5.1 Primary Energy Inputs

Fuel is the primary energy input into any gas turbine cycle. Various fuels have been used to power gas turbines⁵. Most production gas turbines run on natural gas or diesel oil and have the necessary equipment to convert from one to the other as needed. Many gas turbines, particularly those in pipeline service, burn crude oil. Prototype installations using other distillates, residual oil, or solid fuels through fluidized-bed combustors⁶ also are in service around the world.

The low heat value (LHV) and the high heat value (HHV) of several gas turbine fuels are listed in Table 2.1.

The type of fuel being utilized by a gas turbine can be changed by adjustment of the combustor type and pretreatment methods. For high-Btu liquid fuels (distillates, aviation fuels, diesel) the combustors are simple, and any pretreatment necessary is for environmental considerations only. However, for heavy liquids, such as crude oil⁸ and residual oils,⁹ the pretreatment involves heating to reduce viscosity and introduction of chemical additives to control deposits and corrosion. Because most gaseous fuels are supplied at low pressure, equipment to inject this fuel becomes more complex and bulky. With low-Btu gaseous fuels, the compression of the volume of gas needed for proper combustion becomes a significant portion of the parasitic losses.

The rate of fuel consumption of any gas turbine cycle is directly related to thermal efficiency of the system. The thermal efficiency, nth of gas turbines can be defined as the ratio of actual or net work output of the turbine to the heat generated from combustion of the fuel, and is determined by Eq. 2.1, as follows:

A detailed derivation of Eq. 2.2 is presented in Appendix A. Full-load efficiency of the simple-cycle gas turbine can be approximated by the above equation. The result should be slightly higher than actual turbine efficiency, because combustion efficiency and pressure losses in the combustor were not included in Eq. 2.2.

2.5.2 Primary Energy Outputs

Table 2.2 shows continuous-duty, full-load performance data at standard conditions for typical, simple-cycle gas turbine units. This table has

been included to show a representative sample (10 manufacturers) of data available from the 69 manufacturers listed in Ref. 20.

Figure 2.2 shows data from 18 manufacturers for simple-cycle thermal efficiency vs full-load, continuous-duty capacity. The fitted curve applies up to 40,000 SHP, above which the thermal efficiency is virtually constant as capacity increases. The data shown in Fig. 2.2 are represented by the following expression of thermal efficiency vs turbine capacity (X, SHP):

• Part-Load Performance. Generalization of Gas turbine performance at off-design conditions is difficult because performance depends on specific component characteristics usually obtained empirically from test data.

Figure 2.3 shows part-load thermal efficiency of an industrial size (13,000 hp) simple-cycle gas turbine.¹⁰ The data in Fig. 2.3 can be represented by the following equation:

Expressions such as Eq. 2.4 are not generalized equations. For each application, specific information about each individual machine should be known. Not all industrial gas turbines have the same off-design characteristics. For instance, an advanced, high-pressure ratio gas turbine of the size shown in Fig. 2.3 can have higher efficiencies at part-load conditions.

• Heat recovery at part load. This section outlines the method of calculating recoverable heat from a typical simple-cycle gas turbine.

The lower curve (a) in Fig. 2.4 shows thermal efficiency vs percent load for a typical, simple-cycle small gas turbine unit whose full-load thermal efficiency is 24.3%.

The upper curve (c) of Fig. 2.4 shows the sum of thermal and recoverable heat efficiency vs percent load.

The middle curve (b) in Fig. 2.4 shows recoverable heat efficiency vs percent load. The recoverable heat efficiency is defined as the ratio of heat recovered to fuel input. For boilers or other combustion processes with only ~10% excess air, exhaust temperature usually is limited to a minimum of approximately 325°F, where the SO₃ begins to condense as sulfuric acid. Because a yes turbine uses about 400% excess air, its exhaust gas temperature can be reduced to about 150°F. The exact temperature minimum would be a function of the sulfur content of the fuel and the amount of excess air present during combustion. Using data from Fig. 2.5, exhaust temperature vs percent load, the recoverable heat can be calculated from the expression:

2.5.3 Auxiliary Energy Inputs

Gas turbines require auxiliary energy inputs for startup power. Startup power is provided by compressed air or by battery. Control power usually is provided by batteries.

2.5.4 Incidental Energy Outputs

For simple-cycle gas turbines, minor heat rejections occur through lube oil cooling and effects of radiation and convection. Most manufacturers do not even mention these losses because they are so small. Mechanical losses to the lube oil are less than 2%.¹¹ One manufacturer supplies nominal 1,400 Btu/min lube oil cooling equipment which is 0.15% of fuel input for a 6,556 SHP unit at worst possible conditions. Radiation and convection losses are controlled by design and can be reduced further by insulation. One manufacturer gives losses of 0.02% of fuel input at design conditions with insulation. Thus, the lube oil cooling and radiative-convective losses are considered too small for economical recovery.

2.6 ENVIRONMENTAL EFFECTS

2.6.1 Thermal Discharge

One of the greatest advantages of the gas turbine cycle is that there is no thermal discharge to rivers or ponds because cooling water is not required. However, there is thermal discharge directly to the atmosphere through the exhaust gases with temperatures as high as 1200°F or as low as 800°F.

2.6.2 Noise Attenuation

Most manufacturers provide a noise-attenuating enclosure as standard or optional equipment. Figure 2.6 shows sound level vs distance for both opened and closed for ends of the gas turbine unit with the enclosure doors a large (20,000 SHP) gas turbine installed in an existing power plant. Recorded data indicate that the noise level was no higher than at other parts of the plant. ¹² Furthermore, the sound level was not affected by part-load operation.¹²

2.6.3 Aesthetics

Most manufacturers supply an enclosure of some type. The enclosure is primarily for noise attenuation for smaller units because these units normally are installed inside an existing building or do not need aesthetic treatment because of their location (offshore platforms, desert pipelines, etc). Larger units, intended for electrical generating plants, do not generally come with enclosures, because large gas turbines are installed for repowering or to supplement an existing plant.

Exhaust stacks are no taller than necessary to direct the flow of hot gases away from personnel or structures.

2.6.4 Effluent Discharge

The exhaust products considered to be pollutants are:¹²

(1) unburned hydrocarbons (UHC),
(2) carbon monoxide (CO),
(3) particulate matter (PM),
(4) smoke,
(5) nitrogen oxides (NO_X), and
(6) sulfur oxides (SO_X).

The first two are caused primarily by incomplete combustion. Gas turbines have an inherent advantage in this respect because they have a continuous combustion process. Only during startup and low-load conditions are UHC and CO produced in significant quantities.¹³

Smoke and particulate matter come from several sources. Carbon particles left from incomplete combustion can stick together and form a particle large enough to be visible as smoke. Dust and other matter may be drawn into the compressor intake and pass through as particulates. If the fuel is low quality, it may contain ash-producing contaminants, and the ash will be exhausted as particulates. However, with the proper fuel and the efficient combustion process of the gas turbine, smoke and PM can be held to acceptable levels. ¹⁴

Small amounts of atmospheric and fuel-borne nitrogen are oxidized to form NO_X. The rate of formation depends on the combustion temperature and increases rapidly above 3,200 °F. A typical production model gas turbine will produce one to three grams NO_X per horsepower-hour. ¹⁵

Sulfur oxides, produced from sulfur in the fuel, can be easily controlled by the proper choice of fuel and pretreatment methods.

Table 2.3 shows the emission factors published by the EPA for fuel oil combustion from power plants. No flue gas cleaning equipment was utilized to control emissions from the fuel oil combustion equipment. These emission factors believed to be representative of what might be expected from a properly maintained and operated gas turbine.

2.7 OPERATING REQUIREMENTS

2.7.1 Operating Ranges

Most gas turbines are capable of operating from O to 100% peak load rating. Peak rating, 110-115% of full-load continuous rating, may be sustained for short periods only.

2.7.2 Operating Procedures

All manufacturers provide some type of starting mechanism. Smaller units usually are battery-powered; whereas, larger units can have hydraulic systems, externally-powered electric starters, small gas turbines, or diesel engines to provide starting power.

The startup procedure generally is quite simple and proceeds as follows:

(1) The rotor is spun up to a certain speed.
(2) Fuel is introduced and ignited.
(3) The turbine is warmed up.
(4) Load is applied.

The time to full load varies from a few seconds to 30 min.

Several methods of controlling output are used, and each method affects part-load performance differently. Most units are controlled by varying the fuel flowrate. This variation allows the turbine inlet temperature to fluctuate and affects the thermal efficiency similarly. Some units are controlled by varying the air flowrate or pressure ratio with variable-pitch compressor blades or a variable-speed turbine shaft.

Each method of control has advantages and disadvantages, depending on the application and part-load performance desired.

Shutdown is essentially the reverse of startup, except that the hot rotor must be turned slowly to prevent damage from uneven cooling. A small motor usually is provided for this purpose.

2.8 MAINTENANCE AND RELIABILITY

2.8.1 Maintenance Requirements

Three maintenance procedures are performed regularly:

(1) compressor cleaning,
(2) inspection, and
(3) major overhaul.

Compressors ingest large amounts of dust, insects, moisture, etc. and must be cleaned periodically. The biweekly procedure consists of admitting an abrasive material to the compressor intake while the gas turbine is running, and the most common materials used are pecan or walnut shells.¹⁵ An inspection consists of shutdown of the turbine; removal, cleaning, and inspection of the combustor nozzles; and inspection of the turbine and compressor blading. Major overhaul is similar, with replacement of parts, where necessary. Heavy industrial units experience major overhauls between 8,000 and 10,000 hours; whereas, aircraft-derivative turbines undergo modular maintenance between 3,000 and 5,000 hours."' Aircraft derivative turbines require more frequent maintenance because generally they are used for rough peak-load duties and burn liquid fuel.

2.8.2 Expected Equipment Life

Gas turbines have a life expectancy of between 5 and 20 years. The useful life of a particular gas turbine depends on whether it is used continuously or as a standby unit and on the quality of maintenance it receives.

2.8.3 Reliability

Assuming a gas turbine unit can be repaired (or replaced) the reliability can be expressed as the unit availability. The availability of gas turbines is roughly equal to that of diesel engines (about 0.95).¹⁸

Some specific data are:

(1) five units in the first year of operation had an average of 83% availability;¹⁹
(2) two 20MW units had an average availability of 97%;²⁰
(3) one hundred users report average availability of 93% with a high of 98%.²¹

 

2.9 COST CONSIDERATIONS

2.9.1 FOB Capital Cost

Figure 2.7 shows capital cost (in 1976$)* vs capacity for typical simple-cycle gas turbine units of a single manufacturer. The shaded area shows the relative position of all manufacturers. The curve shown in Fig. 2.7 is represented by the following expression of cost vs gas turbine capacity (Q, SHP):

2.9.2 Delivery Cost and Time

Most manufacturers quote a delivery cost of 1-5% of capital cost depending on distance and method of transportation.

Delivery times range from one week for small, prepackaged units to one year for very large machines.

2.9.3 Installation Cost and Time

Installation cost is based on the degree of modularization which generally depends on the size of the unit.* Small units of less than 10,000 SHP usually come as a complete package and, assuming installation in an existing building, can be placed and connected in less than two weeks. Larger units--which come as several components or subassemblies--require up to one man-year to install.

2.9.4 Operating and Maintenance Costs

An important point concerning operating and maintenance costs is that gas turbines are available in two basic types. The aircraft-derivative type engine consists of a free power turbine added on to a turbojet gas generator. The industrial or heavy-duty types are specifically designed for stationary power plant applications. The operating and maintenance costs are much different for the two types as shown in Table 2.4.²³

A recent study¹⁷ indicates that industrial maintenance costs are much closer to aircraft derivative costs than the apparent 2 to 1 maintenance cost ratio shown in Table 2.4. The same study concludes that, when advanced industrial and advanced aircraft (modular industrial turbine) are compared, the aircraft is less expensive to operate.

2.10 STATUS OF DEVELOPMENT

Many large companies recently have opened multimillion dollar research facilities^20,23 dedicated to gas-turbine technologies. The greatest efforts are directed toward improving thermal efficiency. Because the theoretical limit of efficiency is a function of maximum temperature, research, being conducted to increase allowable turbine inlet temperature, is proceeding in two directions. The first-stage turbine blades are cooled by compressor air which is admitted to the hollow center of the blade and passed into the flow stream through a porous-mesh blade surface material. 24 Currently, this allows up to 2,500°F turbine inlet temperatures and thermal efficiencies of 38%. Allowable temperatures may be further increased by use of ceramics. If the combustor, nozzles, and turbine blades were made of a high-temperature ceramic, the theoretical stoichiometric temperature limits could be attained. However, this would cause other problems, such as emissions.

Fuel is also an important area of development. The design of a gas turbine allows it to burn almost any fuel, as mentioned in Sect. 3. The advantages of burning solid waste, crude and residual oils, pulverized coal, and high-sulfur fuels are many, so research in this area could be valuable in view of our present energy problems.

Another area of development for gas turbines is maintenance. Many components of industrial gas turbines are going through an evaluation of design and therefore, are subject to improvements in maintenance intervals, procedures, and control. The classical, aircraft-engine approach is to schedule maintenance at very short time intervals to protect passengers and planes. However, because industrial-type gas turbine reliability is not as critical, maintenance is subject to other constraints.