By Ian Amos, Siemens Industrial Turbomachinery Ltd.
Gas turbine power output can range widely from a few kilowatts up to large utility-sized units of 300 MW. “Industrial” gas turbines are widely used for generating on-site heat and power in a diverse range of applications. The market segments for these units are very diverse and include:
- Process industries (for example; food and drink, pulp and paper, ceramics, chemicals and pharmaceuticals, textiles and petrochemicals)
- Utilities and municipalities ( including district heating)
- Buildings and institutions (hospitals, airports )
- Independent power producers (IPPs) and contract energy managers (CEMs).
This article will explore some of the market drivers for cogeneration, then will explore how the parameters of the gas turbine cycle can influence simple cycle and cogeneration efficiencies. A description of some of the ways that gas turbine based cycles can be configured to provide a flexible balance of heat and power also will be covered.
Industrial Cogeneration Drivers
Prime drivers for installing an independent power generation capability are to realize fuel savings and to provide security of supply.
Fuel costs can be a large percentage of the overall costs of production in many process industries. With large increases in energy costs over the last few years this fraction has increased further. Demands for companies to be both commercially competitive and to respond to increasing environmental pressures for reduced energy usage are powerful drivers. These drivers force companies to consider alternative ways of providing the power needed by their business.
Gas turbines are well suited for cogeneration schemes due to the good quality of thermal energy in the exhaust. In simple cycle operation, the heat contained in the exhaust gases of the gas turbine is lost to the atmosphere. With typical industrial turbine exhaust temperatures of about 500 C, this limits plant efficiency to about 35 percent. This would usually make the generation of electrical power on its own uneconomic, assuming there was an available grid connection. Using the exhaust energy to offset fuel required to provide the heat requirements will typically raise overall thermal efficiency to more than 80 percent. In the case of an application with use for a low temperature heat source (such as hot water) the overall efficiency can be 90 percent.
The impact of fuel savings is illustrated in Table 1, which compares the annual fuel costs for a facility requiring 15MWe of electrical power and 32MWth of heat. Fuel prices are based on average 2006 Q4 prices for a Mid-Atlantic industrial user.
Another major driver is to ensure power supply security for critical applications. Many process industries need to ensure continuous operation as shutdown and startup times and costs can be considerable. Lost production and wastage of materials in production can prove expensive. Facilities such as hospitals should be able to continue to function even in the event of grid black-out to supply necessary services to the community.
In many cases, such as the example shown in Table 1, a connection to the grid will be maintained. In normal operation this can be used to export surplus power or to import power in the case of a shortfall. When the gas turbine is unavailable, such as during scheduled servicing, the full demand for electricity can be imported and heat raised locally as in the central generator model. In cases when the grid fails, a properly designed power management system will isolate the local system from the grid and allow it to operate in island mode with no interruption.
The majority of installed gas turbines will run on natural gas, but they can be supplied with dual-fuel capability, which means they can run on either gas fuel or liquid fuel. Fuel change can be initiated automatically when falling gas pressure is sensed and the fuel switched to liquid while under load. On re-establishing the gas supply, the turbine can then be switched back to the primary fuel.
These various features all contribute toward increasing overall system reliability and robustness.
Gas Turbine Performance
In the case of a gas turbine operating in a cogeneration configuration, the thermal energy in the exhaust is recovered in a heat recovery system (or it can be used for direct drying in some applications). In the majority of installations, the heat recovery system will be a steam generator, raising either saturated or superheated steam for the facility to use in a manufacturing process or for heating. The exhaust gas temperature from the stack after thermal energy recovery will now be much lower than in the simple cycle case (typically reduced from about 500 C to 140 C). The reduction in the energy lost to the atmosphere is directly related to the increase in overall thermal efficiency.
The exhaust mass flow and exhaust temperature parameters determine the realizable heat content of the exhaust and are established during the concept design of the gas turbine. The influence of key cycle parameters (such as pressure and temperature) on simple cycle and combined cycle efficiencies are significant and are discussed next in more detail.
Pressure ratio is one of the most basic parameters in gas turbine design. For a simple cycle gas turbine, the efficiency of the gas turbine is a strong function of the pressure ratio, as illustrated in Figure 1. With successive product generations, pressure ratios have increased.
The latest generation aero engines can reach pressure ratios of 50:1. Industrialized versions of these units for land-based duties (aero-derivatives) tend to be at lower pressure ratios but can still operate at pressure ratios of about 30:1 using the technology that has been developed for their airborne relatives. This high pressure will give aero-derivative gas turbines a high simple cycle efficiency in most cases.
Gas turbines with an industrial heritage have traditionally operated at lower pressure ratios, typically ranging between 10:1 and 15:1, although the latest generation of products are in excess of this. One advantage with a lower pressure is that an expensive fuel gas compressor can either be reduced in size and cost or eliminated completely. In addition, pressure casings and sealing arrangements will be less complex, leading to a less costly design.
The firing temperature is the temperature of the gases after combustion. In real cycles (as opposed to ideal cycles with no losses and reversible flow processes) firing temperature has some impact on the simple cycle efficiency. The main effect, however, is on the specific power of the engine; the kilowatts generated for each unit of mass flow passing through the core. This effect is shown in Figure 1. Increases in firing temperature will get more power out of the same engine frame, so this has been the most common method for engine uprate. However, higher internal gas temperatures put additional burden on the cooling systems, which will often need to be upgraded to maintain component life. If the turbine is not resized during an uprate, a higher firing temperature will increase the cycle pressure ratio and give some additional improvement in cycle efficiency.
Exhaust gas temperature is essentially a result of the pressure ratio and firing temperature. A higher pressure ratio will tend to depress the exhaust temperature for a given firing temperature. The relationship of the parameters over the range for practical gas turbines is shown in Figure 2. A large variation exists in exhaust temperatures for industrial gas turbines, although the general trend over the years has been for this to increase. Typical values will vary between 400 C and 600 C.
Simple Cycle and Combined Cycle Efficiency
In practice, the achievable firing temperature is limited by the requirements for high component life and the availability of cost effective materials and cooling technology. It was seen earlier that high efficiency for simple cycle gas turbines require high pressure ratios. This gives relatively low exhaust temperatures, which can be shown to decrease the cycle’s overall thermal efficiency when the useful heat production is included. In short, a tradeoff exists between simple cycle efficiency and cogeneration efficiency.
Table 2 compares two different gas turbine models and, in particular, shows how the turbine with the higher simple cycle efficiency has the lower cogeneration total efficiency. It is clear that in the selection process for the prime mover for a particular application a choice based on the brochure figures for the simple cycle is inappropriate. An assessment based on the true site heat and power requirements is needed to make a better judgement.
Site requirements for steam and power are not static. Heat or electrical loads can vary over a short timescale due to production schedules or can vary over long timescales such as summer or winter district heating requirements. Therefore, it is essential that the gas turbine cogeneration plant has flexibility in operation.
The exhaust gas of gas turbines still contains up to 15 percent of oxygen, so it is possible to burn more fuel to raise the gas temperature before entering the boiler. By using supplementary firing, the exhaust temperature can be raised from about 500 C to 850 C and the steam output increased by a factor of two. Higher temperatures are possible, but have a significant effect on the costs of the heat recovery steam generator (HRSG).
The efficiency of conversion of the energy content of the fuel into steam is close to 100 percent in many cases. This is because the stack mass flow is only increased by a small amount (the mass of the fuel input) and the stack temperature could be unchanged (or even lower due to the thermodynamics of the HRSG cycle); this means there is little, if any, additional heat loss from the system. Supplementary firing increases the heat/power ratio.
The boiler can be modified to enable steam to be raised even if the gas turbine is shut down. A general relationship between steam-raising capability and engine size is shown in Figure 3.
There could be times when the required steam is less than the amount that can be raised by the HRSG. In some cases, the surplus steam can be injected back into the gas turbine after the compressor to increase the mass flow through the turbine and increase power output. Enhancements of 50 percent more power are possible with steam/fuel injection levels of up to 10:1.
A consequence of steam injection is that it significantly increases the pressure level of the gas turbine, which can take it outside compressor aerodynamic or mechanical design limits. This means that steam injection is not universally available on all gas turbines and there are likely to be restrictions on the steam/fuel ratio. The water needs to be treated to meet a stringent specification and is a consumable item in this process as it is lost to atmosphere. Water recovery technologies are available, but are not always economic. The use of steam injection decreases the overall plant steam/power ratio.
If the site already has a steam plant or if a requirement exists for additional flexibility, the gas turbine can be integrated with a steam cycle. Steam is generated at a higher pressure and a proportion can be extracted from the steam cycle and diverted to use in the process via a control valve. The steam flow is varied according to the process needs. Power can continue to be generated during gas turbine downtime through air-firing of the boilers or use of separate package boilers that feed steam to the steam turbines.
This arrangement provides full flexibility to vary the heat/power ratio and reduces the problem of the volume of consumable water used in the steam injection case. However, it involves a significantly higher capital cost.
Author: Ian Amos is a Product Strategy Manager with Siemens Industrial Turbomachinery Ltd. He has been in the gas turbine industry for over 22 years with a background in research and development specializing in turbine design.