313 Turbine Power Production

Gases enter the turbine at 1800 to 2200°F, under continuous full load via a nozzle assembly that restricts, accelerates, and directs the flow of gas into the turbine wheel. As the superheated air-fuel mixture is sprayed into the rotor, the gas expansion process continues as it passes the turbine blades, creating the rotational force. Figure 3.3 shows a typical turbine performance map.

INLET AIR TEMPERATURE, "C ("F) 1

FIGURE 3.3

Performance map for industrial gas turbines (courtesy of Solar Turbines, Inc., with permission).

INLET AIR TEMPERATURE, "C ("F) 1

FIGURE 3.3

Performance map for industrial gas turbines (courtesy of Solar Turbines, Inc., with permission).

3.1.4 Ancillary Equipment

A turnkey turbine generator installation will also require a controls package, fuel supply system, electrical system, and attendant power-switching and safety protection features such as grounding, circuit breakers, and transfer switches. The balance-of-plant equipment and labor required to install a turbine generator set can generally be estimated at 30% of the total turnkey cost per kilowatt output installed.

3.2 Recuperated Brayton Cycle

Turbine efficiency can be enhanced with recuperation. Figure 3.4 shows the recuperator location in the basic Brayton (GT) cycle and how the recuperator uses the hot exhaust to preheat compressed air before it enters the combustor. The main obstacle to the use of recuperated turbines is the size and cost of the recuperator. Several manufacturers have developed relatively compact and highly effective recuperators, and the market potential for microturbines (which generally must have recuperators) is further encouraging this development. The recuperated turbine offers an attractive route for the user of smaller gas turbines with higher efficiency and better part load characteristics than the combined cycle machine, but the cost is high. Recuperation is less suitable for high pressure-ratio machines, however, which already have higher simple-cycle efficiencies and in which system maximum operating temperature may be set by the temperature tolerance of the recuperator. For this application to be effective, the turbine exhaust temperature must exceed the temperature at the compressor exit. In very high pressure ratio designs, this may not be possible.

Recuperators do contribute to pressure loss (lower pressure ratio entering the turbine), however. A 90% efficient recuperator may cause a 2% pressure loss on the air side and up to a 4% loss on the exhaust gas side, with an additional 1% loss estimated for the additional piping. Thus, the final system would have a better heat rate but lower net power (electrical) output.

3.3 Modified Gas Turbine Cycle

The combined-cycle GT is becoming increasingly popular due to its high efficiency. The exhaust air-fuel mixture exchanges energy with water in the boiler to produce steam for the steam turbine. The steam enters the steam turbine and expands to produce shaft work, which is converted into additional electric energy in the generator. Finally, the outlet flow from the turbine is

AIR INLET

AIR INLET

RECUPERATOR

EXHAUST

RECUPERATOR

COMPRESSOR

TURBINE

COMPRESSOR

COMBUSTOR

TURBINE

002-317M

FIGURE 3.4

Recuperated GT cycle.

condensed and returned to the boiler. However, GT installations below 10 MW are generally not combined-cycle, due to the scaling inefficiencies of the steam turbine. Figure 3.5 shows the combined cycle schematically as well as how GT exhaust heat is used to produce steam used in a "bottoming" Rankine power cycle.

There is scope for a range of small, high-performance steam turbines for combined cycle duty, but the major obstacle to their use is the complexity involved. Highly efficient steam turbines require condensers, vacuum pumps, cooling water, and water treatment plants before they can operate. The amount of supervision required is disproportionately high compared with the GT components, but the situation may not be unacceptable if the user has sufficient on-site technical staff (e.g., a large manufacturer or processing plant).

Steam turbines may also be used in a backpressure mode, taking high quality steam from a heat recovery boiler on the gas turbine exhaust to generate power exhausting low-pressure process steam for industrial use or space heating. The possibilities for utilizing GT exhaust heat range from producing steam alone (for process use) to producing electric power. In the first case, 80% or more of the heat in the primary fuel may be utilized, and in the second case perhaps only 45% will be utilized, but the output will be all electrical power with a high market value.

Alternatives for improving the efficiency of an open simple-cycle include the following:

• Intercooling the air after compression

• Reheating the exhaust gases after combustion

• Recovering part of the energy lost to exhaust gases

GAS TURBINE STEAM TURBINE

118-011M

FIGURE 3.5

Schematic diagram of combined cycle.

GAS TURBINE STEAM TURBINE

118-011M

FIGURE 3.5

Schematic diagram of combined cycle.

A GT with a heat exchanger recaptures some of the energy in the exhaust gas, preheating the air entering the combustor. This cycle is typically used with low pressure-ratio turbines.

3.4 Turbine Performance

Factors influencing the performance of GTs include operating temperatures at the compressor and turbine inlets, pressure ratio, and aerodynamic efficiencies of the compressor and turbine sections. Aside from the compressor inlet temperature, all of these tend to be size related. Thus, large GTs — above 30 MW output — may achieve efficiencies up to 40% without exhaust heat recuperation, while typical efficiencies in the 5 MW range may be closer to 35%, falling to as low as 15 to 17% in the unrecuperated microturbine range (25 to 100 kW). The reasons for this variation are summarized below.

The turbine inlet temperature (TIT) together with the pressure ratio (PR) define the amount of energy that can be extracted from the hot gas leaving the combustor; hence, they control the power output of the machine and also its efficiency. In large turbines, it is feasible to use TIT levels that are substantially above the temperatures that can be tolerated by metal components such as turbine blades because blades can be cooled by air introduced into internal passages via the blade roots. This becomes impractical in small blades with thin sections due to manufacturing difficulties and the risk of partial or complete blockage of the cooling passages by dust or oxidation products in the coolant. One alternative is to use a more refractory material; ceramics are currently being developed and tested. Figure 3.6 summarizes the evolution of materials for GT applications.

Pressure loss must be minimized because of its deleterious effect on cycle performance; however, some pressure loss is necessary to promote the turbulence and fuel/air mixing required for efficient combustion. Apart from these requirements, the system should provide stable and smooth combustion, rapid and reliable ignition, freedom from carbon deposits and minimum smoke, oxides of nitrogen (NOx), carbon monoxide (CO), unburned hydrocarbons (UHC), and sulfur oxides (SOx). The combustion system has to be capable of fulfilling the above requirements in most cases not only at the design point, but also over a wide range of part-load conditions. The suitability of the use of non-standard fuels must be determined with due regard to meeting these requirements.

The pressure ratio that can be achieved in a compressor is proportional to the square of the rotor tip speed. Tip speeds in the order of 800 feet per second can be generated quite easily on large rotors, but microturbines require rotational speeds up to100,000 rpm. Mechanical complexity usually rules out the use of multiple compressor stages in very small machines. Multi-stage axial flow compressors are made for industrial compressors when a high pressure ratio is desired, but expense may become an issue. The same effect can be achieved more economically with a series of radial-stage compressors.

The aerodynamic efficiency of turbo machines is limited by gas friction turbulence losses at blade tips and the hub attachment and gas-air leakage between the rotor and its casing. The compressor is even more sensitive to these issues. All of these are more difficult to control as the components become smaller and the penalties become especially severe at very small sizes. Above 1 MW output, compressor efficiency ranges from 85 to 90%.

1427 (2600)

1204 (2200)

1093 (2000)

760 (1400)

Oxide Ceramics

Cooling

Convections

Conventionally Cast Superalloys Forged

"TBCsN

Fim\ Silicon-Based y Ceramics

(3rd Generation) Single Crystal

MSX-4 (2nd Generation)

1950 1960 1970 1980 1990 2000 2010 YEAR OF IMPLEMENTATION

FIGURE 3.6

Evolution of cooling and materials in GT combustors.

Since the GT is an ambient-air breathing turbine, its performance will be changed by anything affecting the mass flow of air to the compressor, most obviously changes from the reference conditions of 59°F and 14.7 psia. Correction for altitude or barometric pressure is simple because less-dense air reduces mass flow and output proportionately; heat rate and other cycle parameters are not affected. A typical temperature correction curve is presented in Figure 3.7. Similarly, humid air, being less dense than dry air, will also have an effect on output and heat rate. This humidity effect has taken on more significance recently because of the increasing size of gas turbines and the utilization of humidity to bias water and steam injection for NOx control.

Inserting air filtration, silencing, evaporative coolers, chillers, and exhaust heat recovery devices in the inlet and exhaust systems, respectively, causes pressure drops in the system. The effects of these pressure drops are unique to each turbine design. Fuel type will also impact gas turbine performance. Gaseous fuels with heating values lower than natural gas have a significant impact on performance. As the heating value (Btu/lb) drops, the mass flow of fuel must increase to provide the necessary heat input (Btu/hr). At the same time, more air is required to combust the lower heating value fuel. Therefore, there are several associated side effects that must be considered in the GT designs that are likely to burn lower heating value fuels.

Generally, it is not possible to control the factors affecting the GT performance, since most are determined by site location and plant configuration. In the event that additional output is needed, there are several possibilities which may be considered to enhance performance. These include technologies to achieve inlet air cooling, steam and water injection, and peak performance ratings. All turbomachinery experiences loss of performance with time, and for GTs, performance losses can be classified as recoverable or non-recoverable losses. Recoverable loss is usually associated with compressor

FIGURE 3.7

Example temperature correction curve.

FIGURE 3.7

Example temperature correction curve.

fouling and can be rectified by water washing. Non-recoverable loss is due primarily to increased turbine and compressor clearances and changes in surface finish and airfoil contour. Power output deterioration at the 25,000 hour operating point of a typical turbine could vary from 3 to 5% and heat rate within 1% of the "new and clean" situation. These mechanisms of component efficiency losses can only be recovered through replacement of affected parts at recommended inspection intervals. It is often extremely difficult to quantify the specific performance degradation, but one generalization that holds true is the fact that turbines located in dry, hot climates will degrade less than those in humid climates.

3.5 Future Developments

Recent technical developments in GT technology have concentrated largely on addressing the needs of a prospective mass market for small high performance turbochargers for automobiles and, at the other end of the scale, for achieving higher efficiencies in base-loaded control electric power generators. This former effort has relied extensively on the use of ceramic materials to allow high operating temperatures in uncooled turbines, while the latter effort has concentrated on more advanced metals and cooling systems with an emphasis on combined cycle efficiencies.

By reducing the requirement for cooling air, the use of ceramics in the hot gas path of a turbine improves both output and efficiency because air extracted for cooling does not contribute to power generation. Ceramic components are also relatively free from hot gas attack and do not distort so that aerodynamic efficiency is retained over extended service. However, ceramics are inherently brittle and are subject to failure due to stress generated by temperature gradients. Over the long term, ceramics are subject to morphological changes and, in some cases, to oxidation and chemical attack. While these effects are relatively minor over the life of a GT installation (and a small fraction of that time is spent at full load), they become very important under industrial power turbine conditions which call for outputs of 20,000 kW or more, much of it at full power rating.

While there is no firm limit to the size of turbine that can benefit from ceramic technology, the problems of making large ceramic components and their cost are fundamental to these materials. Another factor is the technical competition from metals. Aircraft turbines and their industrial aeroderiva-tives have demonstrated the use of internally-cooled turbulent blades and other components, and techniques for casting even small metal blades with cooling passages are well established. Such blades can operate with gas temperatures higher than those that can be tolerated by today's ceramics, although the cooling process involves performance penalties. On large turbines, steam cooling has been used to minimize these penalties, and this approach may filter down to turbines in the industrial or distributed generation sizes; but the possibility of restrictions developing in cooling passages will inevitably call for caution in adopting internal cooling in small components. There still remain other avenues for efficiency improvement. Many of these fall in the category of design detail — they include seals, including self-compensating designs that can deal with problems posed by thermal expansion, and aerodynamic refinements.

3.6 Controls

Controls and safety devices represent a fairly high proportion of the total cost of a gas turbine that is coupled directly or through gearing to an alternator running at synchronous speed. The electrical connection to a power supply grid will require control of speed and voltage. GT generator systems require the following, typically provided by programmable logic controllers (PLC):

• Startup and shutdown sequencing and protection

• Vibration monitoring

• Fuel or steam governors

• Surge control

• Alarm annunciation

• Fire and gas monitoring

• DC conversion/rectification

Several basic starting systems are available for GTs:

• Electric motor supplied from batteries

• Compressed air or gas system

• Small reciprocating engine (fired with diesel liquid or gas fuels)

In larger turbine systems, it is common to start the turbine by using the generator as a synchronous starting motor to crank the turbine up to its self-sustaining speed.

3.7 Costs

The capital cost of combustion turbines is ultimately to be determined by manufacturers' quotes prepared in response to engineers' specifications.

However, for planning purposes, past projects can serve as a guide. For example, the Gas Turbine World Handbook lists budget prices for sample systems which can be adjusted for site using the Means Mechanical System Cost reports. The former publication also provides a comprehensive list of the key performance characteristics of all turbines by major manufacturers. Figure 3.8 shows the average costs of turbines as a function of size. The prices quoted are as of 1995. Techniques described in Chapter 8 can be used to adjust costs to the present dollar value.

3.8 Fuels

A standard liquid fuel system for industrial GTs typically accepts liquid fuels ranging between kerosene and diesel fuel (JP-5, kerosene, No. 1 diesel, Grade 1 and 2 fuel oils, and No. 2 diesel fuel). An alternative liquid fuel system for industrial GTs using high vapor pressure and low viscosity fuels, such as natural gas liquids (NGL) and liquefied petroleum gas (LPG), gasoline, and naphthas, is typically used in pipeline applications where a high-pressure fuel supply is available. A throttle valve rather than a bypass valve is used to control the fuel flow pressure and maintain a higher pressure drop

FIGURE 3.8

Average costs of GTs (courtesy of EPRI, with permission).

FIGURE 3.8

Average costs of GTs (courtesy of EPRI, with permission).

system. In this system, the fuel orifices are located adjacent to the fuel injectors so that high vapor pressure fuels can be kept at high pressure until the point of actual fuel injection, thus avoiding two-phase flow in any part of the fuel injection system.

The development of dual fuel injectors with dry, low-NOx combustors by some manufacturers has been a major factor in combustion system versatility. Requirements arise in industrial GTs for burning natural gas as the standard fuel, but with provision for standby operation burning liquid fuel. It has become increasingly more important for GT combustion systems to become more versatile in the use of different fuels. Most industrial gas turbines are designed to operate on both standard natural gas and liquid fuel distillates. With minor modifications to the fuel control system, conventional combustion systems can operate on a wide range of fuels, including NGL, LPG, gaseous fuels rich in hydrogen, and gaseous fuels with a medium heating value, such as landfill or bio-derived gases.

The LHV of a gas is used to classify individual fuels into several distinct classes. These classes require different handling and control systems and, for more radical fuels, redesigned combustion systems.

Gaseous fuels are normally classified by using the Wobbe Index, a standard that accounts for variation in fuel gas density and heating value. The Wobbe Index is used to indicate the changes required to the fuel system so that fuels with different heating values can be accommodated. This index relates relative heat input to a combustion system of fixed geometry at a constant fuel supply pressure and can be calculated using the following formula:

where:

LHV = lower heating value of the fuel in MJ/nm3 (Btu/scf)

S.G. = fuel specific gravity

If two fuels have the same Wobbe Index, direct substitution is possible and no change to the fuel system is required. The normal design criterion is that gases having a Wobbe Index within ±10% can be substituted without making adjustments to the fuel control system or injector orifices. This volume ratio is a significant design parameter, and when the fuel injector controlling orifices have to be changed, the gas Wobbe Index should be inversely proportional to the effective controlling area of the injector orifices. For example, a typical landfill gas Wobbe Index is one-third the value of standard pipeline quality natural gas. The designer must enlarge the controlling orifices on the injectors to three times their previous area. This allows the fuel flow rate of the landfill gas to have an equivalent pressure drop across the injector at full-load condition. This will provide stable, high efficiency combustion with the desired turbine inlet temperature distribution for long combustor and blade life. As fuel heating values decrease below standard levels, the torch igniter and the combustion system may require standard natural gas or liquid fuel for start-up or shutdown, as well as possible restrictions on turbine transient load operation.

Standard fuel gas systems can handle gas with lower heating values down to about 23.6 MJ/nm3 (600 Btu/scf) through minor modifications to fuel injector orifices and control system components. Alternate fuel gas systems use multiple fuel control components, manifolds (or single large manifold), and fuel injectors in parallel to further extend the handling of fuel heating value change to 11.8 MJ/nm3 (300 Btu/scf).

In order to provide greater flexibility for alternative gaseous fuels, a dual fuel system is generally recommended. A standard dual fuel system consists of a standard gas fuel and liquid fuel system and requires the use of dual fuel injectors. Start-up and shutdown with a standard backup fuel of consistent heat content, such as natural gas or distillate liquid fuel, is more reliable and has the added advantage of separating the GT start-up operation from the plant's fuel generating operation. Although it is desirable to have natural gas or distillate liquid fuel for start-up, it is not always required. In fact, for landfill gas applications, the GT typically starts and operates on the landfill gas.

If an alternative gaseous fuel is used for GT start-up, the extent of modification to fuel handling, control, and injection components to provide a dual gas fuel system is a function of the difference in Wobbe Indices between the two gases. A dual gas fuel system involving large variations in heating value, such as 19.7 MJ/nm3 (500 Btu/scf), medium Btu gas, and a 35.4 MJ/nm3 (900 Btu/scf) high Btu gas, requires two different gas manifolds and sets of

GAS FUEL

GAS FUEL

FIGURE 3.9

Typical dual fuel injector (courtesy of Solar Turbines, Inc., with permission).

FIGURE 3.9

Typical dual fuel injector (courtesy of Solar Turbines, Inc., with permission).

metering orifices. These are needed to maintain gas injector pressure drop necessary for fuel distribution during start-up and stable operation under load. Figure 3.9 depicts a dual fuel metering system.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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