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Petroleum Engineering Handbook
Larry W. Lake, Editor-in-Chief
Volume III – Facilities and Construction Engineering
Kenneth E. Arnold, Editor
Copyright 2006, Society of Petroleum Engineers
Chapter 8 – Prime Movers
In the energy industry, pumping units, compressors, chillers, and other forms of related equipment are driven by prime movers. The most common prime movers in the oil and gas industry are natural gas turbines and reciprocating engines. Steam turbines can also be used. Steam turbines are not common in field operations and are used primarily in plants; therefore, they are not discussed in this chapter. Electric motors are not considered "prime" movers and are discussed in the chapter on Electrical Systems. Each type of prime mover has unique characteristics that make it appropriate for an application on the basis of site conditions and fuel energy availability.
- 1 Reciprocating Engines
- 2 Gas Turbine Engines
- 2.1 Maximum Cycle Temperature, TRIT
- 2.2 Airflow
- 2.3 Speed Limitations
- 2.4 Temperature Limitations
- 2.5 Rating Point
- 2.6 Site Rating
- 2.7 Inlet Air Temperature
- 2.8 Performance Correction for Elevation
- 2.9 Increasing Turbine Efficiency
- 2.10 Types of Gas Turbines
- 2.11 Air Inlet System
- 2.12 Exhaust Emissions
- 2.13 Turbine Fuels
- 2.14 Exhaust Heat
- 3 General References
- 4 SI Metric Conversion Factors
The availability and economics of the prime mover fuel source and horsepower requirements frequently dictate that reciprocating internal combustion engines be selected to drive energy industry equipment. This section focuses on the reciprocating internal combustion engine and explores the difference between engine speeds, types of engine aspiration, and typical expected exhaust emissions.
The three main types of engine combustion—two stroke, four stroke, and diesel—are discussed in the following paragraphs. All three types of engine combustion convert the chemical potential energy found in the fuel to mechanical kinetic energy. The combustion cycle and fuel type required to complete this task are what distinguish the three engine types from each other.
Two-stroke cycle engines or two-cycle engines complete their combustion cycle in two piston strokes that are accomplished with one revolution of the crankshaft. The two strokes are the power and compression strokes. The two-stroke engine is unique because it does not control the release of exhaust or the admission of an air/fuel mixture into the cylinder with a traditional valve arrangement, one intake and one exhaust.
Two-Stroke CycleAs shown in Fig. 8.1, the process of filling the cylinder with an air/fuel mixture and exhausting the burned gases occurs almost simultaneously near the end of the power stroke. As the piston moves downward during the power stroke, first the exhaust port is uncovered and then the intake port is uncovered. These ports allow the exhaust to escape under its own pressure and then be displaced by the incoming air/fuel mixture. Because some of the air/fuel mixture can be lost in this process, some two-stroke engines are equipped with fuel injection systems that raise their fuel efficiency close to that of a four-stroke engine. The compression stroke compresses the air/fuel mixture that is then ignited by a spark plug. The burning of the air/fuel mixture pushes the piston through the power stroke. As the power stroke reaches its end, the cycle begins again.
Fig. 8.1—Idealized P-V diagram for a two-cycle engine. Numbers signify the beginning of various events in the two-stroke cycle. For example, 1 through 4 are the compression stroke and 4 back to 1 is the combustion/expansion stroke. Exhaust gas is evacuated between 5 and 6 when the exhaust port is open and fuel mixture is drawn in between 6 and 1 when the intake port is open.
Lubricating a two-stroke engine is also unique compared with other engines because the engine does not have a traditional valve arrangement that separates the crankcase from the combustion chamber. Lubrication of the engine is accomplished by an auxiliary lubricator that injects a prescribed amount of oil into critical areas. Another method, commonly used on small two-stroke engines, is to combine a prescribed amount of oil with the fuel so that it lubricates the engine as the air/fuel mixture passes through the crankcase.
Most two-stroke engines are the slow to mid-speed variety (400 to 700 rpm), primarily because of lubricating concerns and because the crankshaft is turning only once per cycle. Some of the largest engines in the world are two-stroke engines. As engine size and power increase, the size and weight of the internal components also increase, which limits their ability to reciprocate quickly. Therefore, the two-stroke engine design works well for large high-power engines with its slower speed and fewer strokes. There are two-stroke engine models rated well in excess of 5,000 brake hp (BHP).
Four-Stroke CycleA four-stroke cycle engine is commonly called a four-cycle engine. Four-cycle engines complete their combustion cycle in four strokes of the piston that are accomplished with two revolutions of the crankshaft, as shown in Fig. 8.2. The four strokes are intake, compression, power, and exhaust. The four-cycle engine controls the release of exhaust and the admission of a fresh charge or air/fuel mixture using a traditional valve arrangement, one intake and one exhaust. These valves are located in the engine head or block and are typically actuated with camshaft(s) and push rods.
The intake stroke moves the piston to create an expansion of the combustion chamber volume. The intake valve is opened during the intake stroke, and an air/fuel mixture is drawn into the cylinder. The compression stroke compresses the air/fuel mixture, which increases the energy potential of the mixture. The compressed air/fuel mixture is then ignited by a spark plug, and the burning of the air/fuel mixture pushes the piston through the power stroke. The exhaust valve then opens, and the piston pushes the exhaust gases out of the cylinder through the exhaust stroke. One revolution of the crankshaft completes the intake and compression strokes. The second revolution of the crankshaft completes the power and exhaust strokes, ending the four-stroke cycle.
A four-cycle engine is lubricated through a pressurized oil system. Typically, oil is stored in an oil pan or sump. An oil pump draws oil from the sump and sends pressurized oil through a filter and then to all engine bearings and moving parts. The oil then returns to the oil sump. In a four-cycle engine, the fuel system and air/fuel mixture are always kept separate from the crankcase and oil.
Most four-cycle engines used in the energy industry are the mid- to high-speed variety (900 to 2000 rpm). Higher speeds are allowed with four-cycle engines because of the pressurized lubrication systems and smaller reciprocating components. The four-cycle design works well for most applications below 20,000 BHP, where speed and fuel efficiency are concerns. Higher speeds are preferred in numerous energy industry applications. For compression applications, higher speeds mean more turns for the compressor and more gas transferred in a given time. For electrical power generation applications, higher speeds mean a more steady production of electricity at any given frequency. Typically, four-cycle engines used in the energy industry are rated up to about 20,000 BHP. The numbers in Fig. 8.2 indicate the beginning for each of the four main events of the four-stroke engine cycle. The intake stroke is designated 1, where volume is increasing and pressure remains low. The rest of the cycle is illustrated as a combination of pressure and volume changes.
The diesel cycle also completes its work in four strokes, making it a four-cycle engine. However, the difference between a diesel and a typical four-cycle engine is the cause of the air/fuel mixture ignition. Diesel engines do not have spark plugs like two and four-cycle engines. The diesel engine uses a high compression ratio and the resulting heat for the ignition of the diesel/air mixture. Natural gas cannot be used in a diesel cycle because ignition temperature is too high.
The intake stroke moves the piston to create an expansion of the combustion chamber volume. The intake valve is opened during the intake stroke, and fresh air is drawn into the cylinder. The compression stroke compresses the fresh air, which increases the kinetic energy of the air, resulting in a high temperature. A fuel injection valve then injects a prescribed amount of diesel fuel into the cylinder. The injected fuel mixes with the hot compressed air in the cylinder. The heat ignites the fuel without a spark, and the burning of the mixture pushes the piston through the power stroke. The exhaust valve then opens, and the piston pushes the exhaust gases out of the cylinder through the exhaust stroke. One revolution of the crankshaft completes the intake and compression strokes. The second revolution of the crankshaft completes the power and exhaust strokes, ending the four-stroke diesel cycle.
Four-stroke diesel engines can have compression ratios beyond 22:1, double that of a nondiesel engine. Air in the cylinder is subjected to this very high compression ratio. The result is a buildup of heat in the compressed volume of air. The heat generated by compressing air to such an extent results in enough heat to ignite the fuel. The temperatures in a cylinder can be in excess of 1,000°F.
A benefit to diesel technology is high mechanical efficiencies. Diesel engines can have an efficiency of > 40%. This level of efficiency is caused by the high temperatures and unique ignition of the diesel/air mixture. Diesel engines are commonly found in very large high-powered, slower-speed applications like seaworthy cargo ships. Diesel engines are also commonly found in all of the same applications that four-cycle non-diesel engines are found. The major differences are the fuel system and availability of that fuel.
Naturally Aspirated vs. Turbocharged
Regardless of the type of engine selected to be a prime mover, one issue that needs to be addressed is whether the engine will be naturally aspirated or turbocharged. Either option has advantages and disadvantages.
Naturally aspirated engines breathe directly from the environment, which means that air enters the cylinder under atmospheric pressure. During the intake stroke, the open area in the combustion chamber expands, resulting in reduced pressure. In a naturally aspirated engine, the atmospheric pressure causes the intake air to flow naturally from high to low pressure and into the combustion chamber. Because a naturally aspirated engine relies on atmospheric pressure, it is more prone to being affected by altitude changes. Power may be lost at higher altitudes because of the less dense air and lower atmospheric pressure.
In a turbocharged engine, the intake air is compressed with a turbine that is driven by exhaust gases. A turbocharged engine breathes the compressed air that is at a higher pressure than atmospheric pressure. Because the intake air is compressed, a more dense air enters the cylinder during the intake stroke. The effect is more power from the same cylinder size because more fuel is needed to match the extra air molecules of the dense compressed air. More fuel and air in the cylinder means more potential energy to compress and burn. Turbocharged engines have a better chance of maintaining power levels at higher altitudes. This stability results because the engine breathes compressed air, leaving the work of the altitude increase to the turbocharger and not the engine.
However, two advantages of the naturally aspirated engines are fewer parts and less complex applications. The naturally aspirated engine does not have a turbocharger and all of the related equipment, including an intercooler, auxiliary water piping, turbocharger lube oil plumbing, waste-gate valves, and bypass valves. Another advantage is the ability to burn a wider range of fuels. Because the firing pressures of naturally aspirated engines are lower than turbocharged engines, naturally aspirated engines are able to burn richer fuels [i.e., those with higher calorific content (Btu/ft3)] and maintain power levels. The lower firing pressures also allow a longer engine life cycle. The longer life cycle and less complex fuel system sometimes make the naturally aspirated engine preferable for remote, minimal-maintenance applications in which ease of maintenance is required.
Deterioration of the atmosphere from gaseous pollutants is an important environmental issue. All engine types produce emissions of some form and are possibly subject to emission regulation. Local, state, and national governments continue to enact stricter exhaust emission requirements to reduce atmospheric deterioration. Two ways to reduce atmospheric deterioration are to limit the power of engines and to require low emissions levels from an engine. Many applications are subject to emission control regulations.
The six main emission pollutants are classified into six different categories:
- NOx (oxides of nitrogen)
- CO (carbon monoxide)
- HC (hydrocarbons)
- SOx (oxides of sulfur)
- CHO (aldehydes)
- PM10 (particulate matter 10 μm and smaller)
NOx consists of nitric oxide (NO) and nitrogen dioxide (NO2) molecules formed when nitrogen (N2) and oxygen (O2) react with each other. This reaction requires a high combustion temperature and the presence of N2 and O2 in the combustion chamber as a fuel is burned. NO2 is harmful to humans and animals because it reduces breathing capacity and the ability of the blood to carry O2. When NOx is exposed to sunlight, it acts as a precursor in the formation of harmful lower atmosphere ozone (O3).
Carbon monoxide (CO) is formed by incomplete combustion of a fuel. Incomplete combustion occurs when there is insufficient O2 to complete the combustion of the fuel molecule or when the combustion is quenched near a cold surface in the combustion chamber. Carbon monoxide is a poisonous gas that causes nausea, headache, and fatigue; in heavy enough concentrations, CO can even cause death. In the upper atmosphere, CO reacts with O3 to produce CO2. This reduces the O3 in the upper atmosphere. Ozone in the upper atmosphere screens harmful sunrays from reaching the Earth’s surface.
The burnable components of any fuel are the hydrocarbons (HC). A small fraction of HCs will pass through the combustion chamber and retain their original form in the exhaust. The nonmethane HCs (any HC other than methane) can react with the NOx in the lower atmosphere, acting as a precursor for the formation of photochemical smog.
Oxides of sulfur are formed when sulfur-containing compounds are oxidized in the combustion chamber. These compounds can be found in the lube oil or in the fuel of an engine. Oxides of sulfur enter the atmosphere and combine with water to form sulfuric acid. These acids return to Earth as acid rain.
Aldehydes (CHO) are formed during the combustion of liquid fuels and lube oil in an engine. Therefore, CHO levels from gas-fueled engines are extremely low compared with liquid-fueled engines like diesel. Aldehydes contribute to photochemical smog and eye irritation.
Particulate matter is also formed during the combustion of liquid fuels and engine lubricants. Particulate matter is often seen as black smoke coming from diesel engines. Some regulatory agencies have labeled particulate matter from diesel engines as a possible carcinogen.
A popular method for reducing and controlling emission levels is to use catalyst reduction and air/fuel ratio control. A catalyst is a substance that promotes a chemical reaction to convert emissions into harmless, naturally occurring compounds without chemically changing itself. A catalyst operates two ways: it either oxidizes (oxidation catalyst) or reduces (reduction catalyst) the emission component. Catalyst reduction can be applied to either a rich-burn or lean-burn engine. The discussion of rich-burn engines vs. lean-burn engines applies primarily to gas-fueled engines. Liquid-fueled engines are generally rich-burn engines.
Rich-burn engines operate at near-stoichiometric combustion at a point where the air/fuel ratio is nearly 16:1. Stoichiometric combustion occurs when there is a correct proportion of O2 and fuel so that they completely react in the combustion process, thus maximizing fuel efficiency and capturing the most power from the fuel. However, operating at near-stoichiometric combustion produces the highest levels of emissions. Today, most rich-burn engines entering the field use some form of catalyst aftertreatment for emissions. Catalyst performance in a rich-burn engine depends on the composition of the exhaust entering the catalytic converter.
Many rich-burn engines with a catalyst use an air/fuel ratio controller. An air/fuel ratio controller generally monitors the amount of O2 in the exhaust stream, compares it with a desired set point, and then changes the air/fuel ratio accordingly. When the air/fuel ratio of an engine is controlled and held at an ideal setting, the chemical reaction occurring inside the catalyst is maximized. Many catalysts can have a reduction efficiency of > 90%.
Lean-burn engines operate well beyond a 16:1 stoichiometric air/fuel ratio. They can have air/fuel ratios of 24:1 up to 32:1. A lean-burn engine with a 32:1 air/fuel ratio has twice as much O2 than is needed for the fuel in the combustion chamber. However, having the excess O2 ensures that fuel molecules will have a better opportunity to react with the needed O2 for complete combustion of the fuel. Lean-burn engines consistently have lower NOx and CO emissions primarily because of the excess O2 in the combustion chamber and lower exhaust temperatures, which inhibit the formation of NOx emissions. Generally, lean-burn engines do not use catalyst aftertreatment for emissions because the emission levels usually are low enough to meet regulations. An air/fuel ratio controller is typically not used with a lean-burn engine because the exhaust composition does not need to be maintained for a catalytic converter. However, if further emission reduction is needed, a selective catalyst can be used with a lean-burn engine. A common selective catalyst system injects ammonia urea (NH3) to react with NOx. The ammonia is consumed in the reaction to leave the emissions of N2 and H2O. These applications need to be considered with caution. A selective catalyst reduction system is expensive compared with a rich-burn oxidizing catalyst. A selective catalyst reduction system also requires on-site storage of hazardous ammonia.
Fig. 8.3 illustrates NOx and CO emission output versus excess air/fuel ratio and excess air ratio for natural gas engines. Note the impact that air/fuel ratios have on rich-burn engine emissions compared with the impact on lean-burn engines. Air/fuel ratio controllers are very important in rich-burn engine applications in which emissions need to be controlled.
Fig. 8.3—Exhaust emissions of natural gas engines. Parts per million volume (PPMV) is a measure of amount of emission indicating how many parts out of 1 million there are of a particular emission when measured on a volume basis. 15% O2 is a reference oxygen level that must accompany PPMV reporting. Oxygen percentage has a direct impact on PPMV number.
Engine Families and Interchangeability
Regardless of the engine type or engine manufacturer, families and parts interchangeability will most likely play a role in a particular product lineup. Engine manufacturers generally group their products into engine families that are designated by engine, bore, and block type. Sometimes, they are separated to service different markets or different horsepower ranges. Within each engine family are parts interchangeability and commonality from both a manufacturing and a marketing standpoint. Engine manufacturers make parts common within a family so that the same part can be used in multiple engine models. Thus, the manufacturer has to have tooling and engineering support for only one part rather than multiple parts within the same family.
Marketing and sales departments use parts commonality and interchangeability as selling points. For example, gas compression sites may have more than one engine on site, and a power generation distributor may have more than one engine in its lease fleet. If the engines are from the same family and have commonality between them, then many parts may be shared between the multiple engines, so a service truck or a parts warehouse needs to stock fewer parts. An on-site emergency parts supply will also be smaller because the few parts kept can support all the engines. Common parts between engines are usually pistons, cylinder liners, connecting rods, bearings, and valve drive components. Sometimes, within a given family, cylinder heads will be the same and interchangeable. Cylinder head interchangeability can be particularly useful if an engine fleet has many different engine sizes but is from the same family. Heads can be taken off one engine that is being upgraded and then, after servicing, put on a different engine.
Engine fuel can be in a liquid state like diesel, gasoline, and jet fuel, or it can be in a gaseous state such as natural gas, propane vapor, and biogas. Liquid fuels allow on-site fuel storage for applications such as road, sea, or air vehicles. Liquid fuels can also be used for applications in which sites are too remote for a gas utility to reach such as remote power generation. Liquid fuels provide the convenience of having fuel storage built into the equipment. A rental fleet may use base tanks under their units to allow easy relocation. Gaseous fuels, such as natural gas, make it convenient to connect to a utility gas supply to fuel large stationary engines. In these applications, fuel demand is too great for on-site fuel storage to be practical. Gaseous fuels such as biogas are a byproduct of trash decomposition or sewage treatment plant processes. Using this byproduct as a gaseous engine fuel captures energy that would normally be flared off and wasted.
Gas Turbine Engines
This section focuses on the gas turbine engine, the differences between types of turbines, and items to consider when they are applied as the prime mover. Gas turbines range in size from microturbines at < 50 hp (37.3 kW) to large industrial turbines of > 250,000 hp (190 kW).
As shown in Figs. 8.4 and 8.5, the "open" Brayton cycle, consisting of adiabatic compression, constant pressure heating, and adiabatic expansion, is the thermodynamic cycle for all gas turbines. The gas turbine is made up of an air compressor, combustor, and a power turbine, which produces the power to drive the air compressor and the output shaft. Air enters the compressor inlet at ambient conditions (Point 1), is compressed (Point 2), and passes through the combustion system, where it is combined with fuel and "fired" to the maximum cycle temperature (Point 3). The heated air is expanded through the gas producer turbine section (between Points 3 and 5), where the energy of the working fluid is extracted to generate power for driving the compressor, and expanded through the power turbine to drive the load (Point 7). The air is then exhausted to the atmosphere. A starting system is used to get the air compressor up to sufficient speed to supply air for combustion with the fuel injected into the combustor. A turbine’s continuous-burning combustion cycle, combined with continuous rotation of the turbine rotor, allows virtually vibration-free operation, as well as fewer moving parts and wear points than other prime movers.
Maximum Cycle Temperature, TRIT
The output power of a gas turbine may be increased by increasing the maximum cycle temperature. The maximum cycle temperature is designated TRIT, which stands for turbine rotor inlet temperature. API 616 defines rated firing temperature as the vendor’s calculated turbine inlet temperature (TIT) immediately upstream of the first-stage turbine rotor for continuous service at rated power output. TRIT is calculated immediately upstream of the first-stage turbine rotor and includes the calculated effects of cooling air and temperature drop across the first-stage stator vanes.
The output power of a gas turbine may also be increased by increasing the mass flow of air through the gas turbine. The geometry of the gas turbine, particularly the compressor, and the speed of the compressor dictate basic air mass flow. An increase in flow requires an increase in speed, which is limited to the maximum continuous running speed of any particular design. At a given speed, an increase in inlet air density increases air mass flow. Inlet air density increases directly with barometric pressure and inversely with ambient temperature.
The main parameters affecting output power are speed and TRIT for any given mechanical/aerodynamic design. Increasing any one of these parameters increases the output power capacity of the gas turbine. Speed and temperature may be dictated by the output power and heat rate desired within the constraints imposed by component life, cost, and technical feasibility.
As the speed of a gas turbine increases, the centrifugal forces on the rotating components increase. These forces increase the stress on the rotating components, particularly the disks, blades, and blade attachment to the disk. Component materials have stress limits that are directly proportional to their speed limits and should not be exceeded. Thus, the maximum continuous speed of the rotating element is a function of rotor geometry, component material properties, and safety design factors. It is the highest allowable speed for continuous operation.
One way to increase output power is to increase the fuel flow and therefore TRIT. As TRIT increases, hot section components operate at higher metal temperatures, which reduces the time between inspection (TBI) of the gas turbine. Because the life of hot section materials is limited by stress at high temperature, there are limitations on the maximum temperatures for a given TBI. Material life decreases rapidly at higher temperatures. TBI is a function of time at TRIT and the rate of TRIT change during transients such as startup. The creep or stress rupture limit is established by the material properties as a function of their stress level and operating temperature.
A rating point can be established for determining gas turbine performance for specified ambient conditions, duct losses, fuel, etc. The International Standards Organization defines its standard conditions as 59°F, 1.013 bar, and 60% relative humidity with no losses. This has become a standard rating point for comparing turbines of various manufacturers and designs.
The site rating is a statement of the basic gas turbine performance under specific site conditions, including ambient temperature, elevation, duct pressure losses, emission controls, fuel composition, auxiliary power takeoff, compressor air extraction, and output power level. For instance, an increase in ambient temperature reduces output power at a rate influenced by gas turbine design.
Inlet Air TemperatureFig. 8.6 relates output power, fuel flow, exhaust temperature, and exhaust flow to inlet air temperature at optimum power turbine speed for an example gas turbine.
Performance Correction for Elevation
Site elevation affects power output, fuel flow, and airflow of all open-cycle gas turbines because of the reduction in inlet air density caused by reduced barometric pressure.
Increasing Turbine EfficiencySimple Cycle. Most of the mechanical energy extracted from the gas stream by the turbine is required to drive the air compressor, with the remainder available to drive a mechanical load. The gas stream energy not extracted by the turbine is rejected to the atmosphere as heat.
Recuperative Cycle. In the recuperative cycle, also called a regenerative cycle, the compressor discharge air is preheated in a heat exchanger or recuperator, the heat source of which is the gas turbine exhaust. The energy transferred from the exhaust reduces the amount of energy that must be added by the fuel. In Fig. 8.7, the fuel savings is represented by the shaded area under 2 to 2′. The three primary designs used in stationary recuperators are the plate fin, shell and tube, and primary surface.
Combined Cycle. Adding a steam bottoming cycle to the Brayton cycle uses the exhaust heat to produce additional horsepower, which can be used in a common load, as shown in Fig. 8.8, or for a separate load. The shaded area represents the additional energy input.
Types of Gas Turbines
Turbine designs can be differentiated by type of duty, combustor types, shaft configuration, and degree of packaging.
Types of Duty.
- Aircraft Turbine Engines. Aircraft turbine engines or jet engines are designed with highly sophisticated construction for light weight specifically for powering aircraft. These designs require maximum horsepower or thrust with minimum weight and maximum fuel efficiency. Aircraft turbines have roller bearings and high firing temperatures requiring exotic metallurgy. They can be operated on a limited variation of fuels. When a jet engine is used in an industrial application, it must be coupled with an independent power turbine to produce shaft power.
- Heavy Industrial Gas Turbine Engines. The basic design parameters for heavy industrial gas turbine engines evolved from industrial steam turbines that have slower speeds, heavy rotors, and larger cases than jet engines to ensure longer life. These gas turbines are capable of burning the widest range of liquid or gas fuels.
- Light Industrial Gas Turbine Engines. The basic design parameters and technology used in aircraft turbines can be combined with some of the design aspects of heavy industrial gas turbines to produce a lighter-weight industrial turbine with a life approaching that of a heavy industrial gas turbine. These engines are called light industrial gas turbine engines.
- Radial or Annular Combustor. This combustor surrounds the gas turbine rotating parts and is integral to the engine casing (Fig. 8.9). Aircraft turbines and light industrial gas turbines use this design.
- Can Combustor. This is a single- or multicombustion system that is separated from the rotating turbine as external combustion cans (Fig. 8.10). Designs using this type of combustor can burn a wider range of fuels.
- Single Shaft. The gas turbine can have either a single-shaft or a two-shaft design. The single-shaft design consists of one shaft connecting the air compressor, gas producer turbine, and power turbine as one rotating element (Fig. 8.4). This design is best suited for constant-speed applications such as driving electric generators for a constant frequency.
- Two Shaft. The two-shaft design has the air compressor and gas producer on one shaft and the power turbine on a second independent shaft. This design provides the speed flexibility needed to cover a wider performance map of the driven equipment more efficiently. This allows the gas producer to operate at the speed necessary to develop the horsepower required by the driven equipment such as centrifugal compressors or pumps. Fig. 8.9 shows a cutaway view of a typical two-shaft gas turbine. Major components include the compressor, combustion system, gas producer turbine, and power turbine. This design includes a two-stage gas producer turbine and a two-stage power turbine.
Degree of Packaging. The norm for most gas turbines used in industry consists of incorporating the gas turbine into a base frame/skid with all the components required for the basic operational unit. This includes such systems as the start system, fuel system, lubrication system, local controls, and in some cases the gear box and driven equipment. Additional operationally required systems such as air inlet filtration/silencing, oil coolers, remote control systems, sound-attenuated enclosures, and exhaust silencers are all generally separate preengineered packaged systems that can be provided and customized by the turbine manufacturer.
Air Inlet System
Inlet Air Filtration. The quality of air entering the gas turbine is a very important design consideration. Turbine efficiency will decrease over time because of deposits building up on the turbine internal flow path and rotating blades. This buildup results in increased maintenance and fuel consumption. Selecting and maintaining the proper inlet air filtration system for the specific site conditions will affect the rate of decrease of efficiency over time.
Pressure Drop. It is critical to minimize the pressure drop of the air passing through the inlet ducting, inlet air filter, and inlet silencer (see below). Pressure loss on the atmospheric air entering the turbine greatly affects the performance of the gas turbine.
Noise Attenuation. The noise produced by a gas turbine is primarily in the higher-frequency ranges, which are not transmitted as far as the lower-frequency noises produced by slower-speed prime movers such as reciprocating engines. Most high-frequency noise produced by the turbine is generated in the air inlet, with a smaller amount coming from the exhaust. The sources of noise and method of attenuation are as follows:
- Air Inlet. The inlet silencer should be specifically designed to the noise profile of the gas turbine and the site requirements. This silencer is installed in the air inlet ducting between the air filter and the turbine air compressor inlet.
- Exhaust. The exhaust silencer should be specifically designed to the noise profile of the gas turbine and the site requirements. The exhaust stack height in conjunction with the silencer is an important consideration. Discharging the hot exhaust gases as high as practical reduces the measurable noise at ground level plus has the added benefit of reducing the chance of recirculation of the hot exhaust back into the air inlet. Pressure loss (backpressure) on the exhaust of the turbine greatly affects the performance of the gas turbine.
- Casing/Gear Box/Driven Equipment. Sound-attenuating enclosure(s) can be installed directly over the equipment such as skid-mounted walk-in enclosures or a building containing the equipment insulated to meet the requirements or both.
- Oil Cooler. The most common method of cooling the oil is the use of air exchanger/fan coolers. These generate fan noise that can be controlled with fan tip speed. The use of shell and tube water coolers can be noise-efficient if the cooling media is available.
Deterioration of the atmosphere by gaseous pollutants is an important environmental issue. The gas turbine by basic cycle design gives a cleaner combustion and produces a lower level of pollutant compared with other prime movers, which is a major advantage. The gas turbine pollutants that typically are regulated are oxides of nitrogen, carbon monoxide, unburned hydrocarbons, particulates, and sulfur dioxide. The solution to some, but not all, of these pollution problems lies within the gas turbine combustor. A brief discussion follows.
Oxides of Nitrogen (NOx). Only two of the seven oxides of nitrogen are regulated: NO and NO2, referred to collectively as NOx. Almost all emission concerns involving prime movers relate to NOx production and NOx controls. The gas turbine is relatively clean compared with other prime movers. For example, gas turbines burning natural gas generally produce 4 to 12 times less NOx per unit of power than reciprocating engines produce. However, NOx is the major factor in permitting gas turbine installations.
Carbon Monoxide (CO). CO is also at a very low level in turbine exhaust because of the excess air in the combustion process. Therefore, it is usually not a problem. However, in some areas where the ambient level of CO is extremely high or when water injection is being used for NOx control in the gas turbine, CO may be a factor in obtaining permits.
Unburned Hydrocarbons (UHC). Unlike reciprocating engines that produce a significant amount of UHC, gas turbines produce a low amount of UHC because the large amount of excess air involved in the gas turbine combustion process completely combusts almost all the hydrocarbons. Consequently, UHC emissions are rarely a significant factor in obtaining environmental permits for gas turbines.
Particulates. No particulate measuring techniques have been perfected that produce meaningful results on gas turbine exhausts. This is rarely a factor in obtaining permits for gas turbines when clean fuels are burned in the gas turbine.
Sulfur Dioxide (SO2). Almost all fuel-burning equipment, including gas turbines, converts all the sulfur contained in the fuel to SO2. This makes SO2 a fuel problem rather than a problem associated with the characteristics of the turbine. The only effective way to control SO2 is by limiting the amount of sulfur contained in the fuel or by removing the SO2 from the exhaust gases by means of a wet scrubbing process.
The need to meet or surpass the emission standards set by federal, state, and local codes has required industrial gas turbine manufacturers to develop cleaner-burning turbines. Dry emission systems have been developed with lean-premix fuel injectors, special combustion technology, and controls for reducing emissions of NOx and CO by creating lower maximum flame temperatures and more complete oxidation of hydrocarbon fuels. All industrial gas turbine manufactures have dry low emission products. The performance varies with the individual product because of differences in combustor design.
These lean-burn systems reduce the formation of NOx and CO to very low levels, thus making it unnecessary to use expensive high-maintenance catalytic converters to eliminate NOx and CO after they are formed. In extreme high-attainment areas, it may be necessary with some gas turbines to use selective catalytic converters to further reduce the level of NOx and CO. The fuel of choice for the gas turbine is clean dry natural gas, which produces the cleanest exhaust.
Gas turbines can operate on a wide variety of fuels. The fuel injection system, combustor, and control systems are designed to handle the fuel of choice.
- Gas. Turbines can operate on almost any combustible gas. Most operate on natural gas, propane vapor, or biogas. The fuel of choice for gas turbines to maximize engine life and create the lowest emissions is sweet natural gas free of sulfur, contaminants, entrained water, and liquid hydrocarbons.
- Liquid. Turbines can operate on almost any combustible liquids. Most operate on fuel oil, diesel, kerosene, or jet fuel. Large heavy industrial gas turbines can operate on all forms of liquid hydrocarbon fuels, including heavy crude oil.
- Dual fuel. Gas turbines can be used in dual-fuel operation in which the primary fuel such as natural gas can be switched to diesel for emergency backup.
Gas turbines have most of the heat loss from the cycle going out the exhaust. This heat can be recovered and used to increase the overall thermal efficiency of the fuel burned. The most common method of exhaust heat use is in the production of steam.
Barr, G.R. and Jones, M.D. 1989. Turbomachinery Development and Solar’s Product Line Evolution, TTS1. DeSoto, Texas: Solar Turbines Inc.
Gas Engine Emission Technology, third edition, form 536, Waukesha Engine. 1993. Addison, Texas: Dresser Inc.
Odom, F.M. 1984. Gas Turbine Generator Unit and Gas Compression System Performance Rating Philosophy, TTS3. DeSoto, Texas: Solar Turbines Inc.
Slow Running 4 Cycle Cylinder, Waukesha Engine Product Training Center, Waukesha Engine. Addison, Texas: Dresser Inc.
SI Metric Conversion Factors
Conversion factor is exact.