Gas turbines for cogeneration – efficiency is everything

The favourable characteristics of gas turbines mean that they lie at the heart of manymodern cogeneration systems. Here, Ian Amos explains some of the parameters that affect the performance and operational efficiency of turbines and complete cogeneration plants, including both fundamental design and site-specific considerations.

Gas turbines are available across a wide range of power from a few kilowatts to utility-sized units larger than 300 MW for operation in combined-cycle power plant. This article examines the so-called industrial gas turbine, which covers the range up to 100 MW.

Typical applications of gas turbine cogeneration are in refineries or industries such as pulp and paper, ceramics, food and drink, chemical and pharmaceuticals. Cogeneration plants are also used as an efficient means of supplying power and heat for local district heating schemes.


A gas turbine and HRSG operating at William Grant & Sons Distillers in Scotland, producing 5 MW electrical and 12.5 tonnes/hour 10 bar steam in baseload operation (Siemens)
Click here to enlarge image

Gas turbines can exist in a variety of different forms depending on their ancestry and application. But if the more complex cycles involving intercoolers and recuperators are ignored, then all gas turbines follow the thermodynamic Brayton cycle (see box on the following page) and the responses to the changes in the parameters discussed below are still relevant.

Land-based gas turbines come from either an ‘industrial’ or an ‘aero-derivative’ heritage. Some of the differences in their characteristics are discussed below, though there has been a gradual convergence over time in response to the market requirements.

FUNDAMENTAL PERFORMANCE PARAMETERS

à‚ 

Pressure ratio

Pressure ratio is one of the most basic design parameters for gas turbines. 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. Gas turbine pressure ratios have continued to increase with successive product generations.


Figure 1. Parametric performance chart
Click here to enlarge image

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 developed for their airborne relatives. In most cases, this high pressure gives aero-derivative gas turbines a high simple-cycle efficiency.

Gas turbines with an industrial heritage have traditionally operated at lower pressure ratios, typically ranging from 10:1 to 15:1, although those of the latest generation of products are higher. 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 are less complex, leading to a less costly design.

Firing temperature

The firing temperature is the temperature of the gases after combustion. In real cycles, firing temperature has some impact on the simple cycle efficiency, but the main effect 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 nicely in Figure 1.

Because increases in firing temperature will get more power out of the same engine frame, this has been the most common method of uprating engines. However, higher internal gas temperatures place an additional burden on the cooling systems, which often need to be upgraded to maintain component life. If the turbine is not resized, the higher firing temperature will increase the cycle pressure ratio and give a further improvement in efficiency.

Exhaust temperature

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, whereas a high firing temperature will raise it. The relationship of the parameters is shown in Figure 2.


Figure 2. Impact of pressure ratio and firing temperature on exhaust temperature
Click here to enlarge image

There is a large variation in exhaust temperatures for industrial gas turbines, although the general trend over the years has been for this to increase. Typical values vary between 400à‚°C and 600à‚°C.

Simple-cycle efficiency

Simple-cycle efficiency is simply measured as the ratio of useful power output divided by the heat input (from fuel). High simple-cycle efficiencies are achieved at high pressure ratios and high firing temperatures, although significant improvements can be made by adopting more complex cycles such as intercooled/recuperated arrangements (not discussed here).

The high pressure ratios required for good simple-cycle thermal efficiency usually mean that the exhaust temperature is low. This has an impact on the cogeneration overall thermal efficiency, as discussed below.

COGENERATION CONSIDERATIONS

à‚ 

Cogeneration efficiency

In cogeneration, a proportion of the gas turbine exhaust energy is recovered in a heat recovery system. In some applications, the clean nature of the exhaust gases means that this energy can be suitable for direct drying.

Cogeneration efficiency is calculated by summing the useful electrical and thermal power output and then dividing by the fuel heat input. Well matched gas turbine cogeneration systems will achieve overall thermal efficiencies of more than 80% and, if there is a use for low-grade heat, the efficiency can approach 90%.

The amount of heat extraction is usually limited by the allowable stack temperature, which should be high enough to avoid condensation of combustion products in the exhaust system in order to reduce the risk of corrosion. A typical lower stack temperature limit is around 130à‚°C. Many heat recovery steam generators (HRSGs) have internal thermodynamic restrictions such as the boiler approach temperature and pinch point; these will force a higher stack temperature and correspondingly increase the lost energy. These limitations are dependent on the required properties of the steam and need to be evaluated on a case-by-case basis.

However, gas turbine exhaust temperatures are relatively low in comparison with the temperatures achieved in package boilers. This can restrict the steam conditions and flows that can be achieved. Gas turbines with high exhaust temperatures can generally supply better-quality steam and better total efficiencies.

Cogeneration efficiency is always reduced if the heat that can be raised in the recovery system is not utilized. For this reason, many cogeneration systems are sized around the heat requirement. Any imbalance in the electrical requirement can be adjusted by import or export to the grid, though this is not always practical or possible in a number of markets.

It is important to appreciate that engines with relatively low simple-cycle efficiency can still be very efficient in cogeneration. Table 1 compares two different gas turbine models and, in this particular example, shows how the turbine with the higher simple-cycle efficiency has the lower cogeneration total efficiency.

Click here to enlarge image

It is clear that, in the selection process for the prime mover, 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 in order to make a better judgement.

Impact on gas turbine design

The ideal product will have good simple-cycle efficiencies coupled with a good steam-raising capability. It will be purchased at a low capital cost and be expected to run continuously with very high levels of reliability and availability. Some of these features are achieved only at the expense of another and it is inevitable that compromises need to be made in the design.

As explained above, the ability of a gas turbine core to provide good simple cycle efficiency and good cogeneration efficiency depends on a high pressure ratio and a high exhaust temperature. Figure 1 shows how this requirement forces up the firing temperature; this parameter and the pressure ratio are good indicators of the level of technology.

However, technology comes at a price. For example, the use of advanced high-temperature materials and complex bearing seal arrangements have a direct impact on product cost. Market economics will determine an acceptable level of capital cost for any particular application and this has a strong influence on the core design.

The expense of developing tailored solutions for each sector means that gas turbine original equipment manufacturers (OEMs) need to cover a number of market sectors with the same product platform. Although the core engine may be common, there will be some variation in the package to suit different applications.

OPERATIONAL FLEXIBILITY

Site requirements for steam and power are not static. Heat or electrical loads can vary over a short time scale 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. Some examples of the plant configuration that provide this flexibility are described below.

Supplementary firing

The exhaust gas of gas turbines still contains up to 15% of oxygen, so it is possible to burn more fuel to raise the gas temperature before entering the boiler. By carrying out so-called supplementary firing, the exhaust temperature can be raised from about 500à‚°C to 850à‚°C, and the steam output increased by a factor of two. Higher temperatures are possible, but have significant impact on the costs of the HRSG.

The efficiency of conversion of the energy content of the fuel into steam is very close to 100% 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, along with specific examples for the Siemens industrial turbine range.


Figure 3. Relationship between steam-raising capability and engine size
Click here to enlarge image

null

Steam injection

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% 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 not economic. The use of steam injection decreases the overall plant steam/power ratio.

Combined cycle and ‘combi-cogeneration’

If the site already has a steam plant or there is a requirement 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.

OTHER CYCLES

à‚ 

The gas turbine based trigeneration cycle

Trigeneration involves the simultaneous production of power, heat and cooling from a single fuel source (Figure 4). In this configuration, a proportion of steam is passed through an absorption chiller to provide cool water. The amount of heat and cooling generated can be varied according to facility needs.


Figure 4. The gas turbine-based trigeneration cycle
Click here to enlarge image

null

The gas turbine direct drying

Exhaust gases from a gas turbine are directed into a drying cell or kiln. The exhaust gases are regarded as clean and so minimise contamination.

Alternative schemes using supplementary firing and air to air heat exchangers for indirect drying are also used. Typical applications for direct drying include paper mills and ceramic factories.

SITE PERFORMANCE CONSIDERATIONS

à‚ 

Site ambient conditions

The power output of a gas turbine is a function of the ambient pressure (altitude) and temperature, and needs to be taken into consideration during the specification of the project.

Power enhancement can be achieved by reducing the inlet air temperature. A 10à‚°C drop in the air entering the compressor can increase the power output by up to 10%. Cooling can be achieved by:

  • Chilling coils. These do not require any additional consumable (water) and their use is usually considered at first installation (this makes sense if absorption chillers are already specified as a use for the steam).
  • Evaporative chillers/fogging. These require a source of high-quality water free of contaminants. They are only useful for sites with low humidity. They can often be retrofitted to suitable sites when a power upgrade is required. Although the capital cost is likely to be lower than for chilling coils, operating costs will be higher.

Parasitic losses

Significant fractions of the power output can be absorbed by other auxiliary equipment required to run the plant and need to be included in the assessment. Principal electrical loads such as the turbine enclosure ventilation fan motors and liquid fuel pump motor are part of the gas turbine package; information for these is provided by the turbine OEM.

An important load, which is dependent on the core engine design, is the fuel gas compressor for gas-fired turbines. The size and requirement for this depend on the cycle pressure ratio (as discussed above). In addition to the capital cost of the equipment, there is a large power requirement, which will reduce the net output of the plant. If no high or medium pressure gas pipeline is available, this can be a considerable disadvantage for core engines with a high cycle pressure ratio.

Filtration

The rate of fouling of the gas turbine compressor depends largely on the location of the installation, the working environment and the effectiveness of the filtration system. Hot washes can be carried out on-line while the engine is under load, but do not generally achieve full performance recovery in the same way as a cold wash (Figure 5). However, a cold wash, which could typically be performed monthly, requires the engine to be shut down for a number of hours and so has a direct impact on availability.


Figure 5. Relative power loss following hot and cold washes
Click here to enlarge image

The specification of high-quality self-cleaning filtration systems can have a major impact on the number of washes and can often pay back their initial higher investment in a short period of time due to the extra availability of the plant. High-efficiency particulate air (HEPA) pre-filters can theoretically remove at least 99.97% of airborne particles with a size of 0.3 à‚µm.

With such pre-filters, it is not uncommon for installations to run without any washing (hot or cold) for extended periods of time – up to years. In these situations, the engine performance should be monitored to identify when deterioration occurs so that a wash can be scheduled if required.

Filtration also acts as the first line of defence to prevent contaminants from entering the engine, where they could cause damage to the engine components. Contamination could be anything from free water, which can cause erosion to the compressor blading, or particulates containing salts and sulphur which can combine to corrosively attack the hot turbine components.

CONCLUSION

Gas turbine cogeneration provides an economical and high-quality supply of power and heat. The design parameters of the gas turbine core engine can have a significant influence of the efficiency in simple cycle and cogeneration, but there are a number of ways the overall plant can be configured to allow flexibility in heat and power production to suit many industrial applications.

Ian Amos is Product Strategy Manager, SGT-400, with Siemens Industrial Turbomachinery, Lincoln, UK. e-mail: ian.g.amos@siemens.com

The Brayton cycle

The Brayton cycle consists of three basic processes in which atmospheric air is compressed, heated, and then expanded back to the atmosphere. In a gas turbine, the gas is expanded through a turbine (or series of turbines) which extracts power. About two-thirds of this power is used to drive the compressor and the rest is exported as useful power.


The Brayton cycle and the gas turbine
Click here to enlarge image

à‚ 

No posts to display