Prime Movers

Depending on the site requirements, the prime mover may be a steam turbine, reciprocating engine or gas turbine. The prime mover drives the electricity generator and waste heat is recovered. The basic elements are all well established items of equipment, of proven performance and reliability.

Prime Movers

Cogeneration units are generally classified by the type of prime mover (i.e. drive system), generator and fuel used. The following sections examine the main types.

Steam Turbines

Steam turbines have been used as prime movers for industrial cogeneration systems for many years. High-pressure steam raised in a conventional boiler is expanded within the turbine to produce mechanical energy, which may then be used to drive an electric generator. This system generates less electrical energy per unit of fuel than a gas turbine or reciprocating engine-driven cogeneration system, although its overall efficiency may be higher, achieving up to 84% (based on fuel gross calorific value).

For viable power generation, steam input must be at a high pressure and temperature. The plant is capital intensive because a highpressure boiler is required to produce the motive steam. At existing sites, where steam systems are supplied by low-pressure boilers, it will be necessary to replace these boilers with highpressure plant, possibly retaining the original equipment as stand-by.

Steam cycles typically produce a large amount of heat compared with the electrical output, resulting in a high cost installation in terms of Euro/kWe.

Gas Turbines

The gas turbine has become the most widely used prime mover for large-scale cogeneration in recent years, typically generating 1-100 MWe. A gas turbine based system is much easier to install on an existing site than high-pressure boiler plant and a steam turbine. On many sites plot space is at a premium, a factor weighing heavily in favour of gas turbines. This, together with reduced capital cost and the improved reliability of modern machines, often makes gas turbines the optimum choice.

The fuel is burnt in a pressurised combustion chamber using combustion air supplied by a compressor that is integral with the gas turbine. The very hot (900ºC- 1200oC) pressurised gases are used to turn a series of fan blades, and the shaft on which they are mounted, to produce mechanical energy. Residual energy in the form of a high flow of hot exhaust gases can be used to meet, wholly or partly, the thermal demand of the site.

The available mechanical energy can be applied in the following ways:

A gas turbine operates under exacting conditions of high speed and high temperature. High-premium fuels are therefore most often used, particularly natural gas. Distillate oils such as gas oil are also suitable, and sets capable of using both are often installed to take advantage of cheaper interruptible gas tariffs.

Waste gases are exhausted from the turbine at 450oC to 550oC, making the gas turbine particularly suitable for high-grade heat supply. The usable heat to power ratio ranges from 1.5:1 to 3:1 depending on the characteristics of the particular gas turbine. Supplementary firing may be used to increase exhaust gas temperatures to 1,000oC or more, raising the overall heat to power ratio to as much as 10:1.

Supplementary firing is highly efficient as no additional combustion air is required to burn extra fuel. Efficiencies of 95% or more are typical for the fuel burned in supplementary firing systems.

Exhaust gases can be used in either of the following ways:

Gas turbines are available in a wide power output range from 250 kWe to over 200 MWe, although sets smaller than 1 MWe have so far been generally uneconomic due to their comparatively low electrical efficiency and consequent high cost per kWe output. This is starting to change. The turbine is typically mounted on the same sub-base as its generator, with a stepdown gearbox between the two to reduce the high shaft speed of the turbine to a speed suitable for the generator. A gas turbo-generator is extremely noisy and generally housed in an acoustic enclosure for noise attenuation.

Reciprocating Engines

The reciprocating engines used in cogeneration are internal combustion engines operating on the same familiar principles as their petrol and diesel engine automotive counterparts. Although conceptually the system differs very little from that of gas turbines, there are important differences. Reciprocating engines give a higher electrical efficiency, but it is more difficult to use the thermal energy they produce, since it is generally at lower temperatures and is dispersed between exhaust gases and cooling systems.

The usable heat:power ratio range is normally in the range 0.5:1 to 2:1. However, as the exhaust contains large amounts of excess air, supplementary firing is feasible, raising the ratio to a maximum of 5:1.

Compression-ignition ('diesel') engines

Compression-ignition ('diesel') engines for large-scale cogeneration are predominantly four-stroke direct-injection machines fitted with turbochargers and intercoolers. Diesel engines will accept gas oil, HFO and natural gas. The latter is in reality a dual-fuel mode, as a small quantity of gas oil (about 5% of the total heat input) has to be injected with the gas to ensure ignition; as the engine can also run on gas oil only it is suited to interruptible gas supplies. Cooling systems are more complex than on spark-ignition engines and temperatures are often lower, typically 85oC maximum, thereby limiting the scope for heat recovery. Exhaust excess air levels are high and supplementary firing is practicable. Compression-ignition engines run at speeds of between 500 and 1500 rev/min. In general, engines up to about 500 kWe (and sometimes up to 2 MWe) are derivatives of the original automotive diesels, operating on gas oil and running at the upper end of their speed range. Engines from 500 kWe to 20 MWe evolved from marine diesels and are dual-fuel or residual fuel oil machines running at medium to low speed.

Spark-ignition engines

Spark-ignition engines are derivatives of their diesel engine equivalents and have their same parameter equivalents as 90°C cooling water.

Traditionally, shaft efficiency has been lower than for compression ignition engines. The output of a spark-ignition engine is a little smaller, typically 83% of the diesel engines.

They are suited to smaller, simpler cogeneration installations, often with cooling and exhaust heat recovery cascaded together with a waste heat boiler providing medium or low temperature hot water to site.

Spark-ignition engines operate on clean gaseous fuels, natural gas being the most popular. Biogas and similar recovered gases are also used but, because of their lower calorific value, output is reduced for a given engine size. Spark-ignition engines give up less heat to the exhaust gases than diesel engines. The large lean-burn engines have typically 12% oxygen in exhaust gases, and this can be used with supplementary firing. The following are among the most common applications for the thermal energy produced by reciprocating engines:

Reciprocating machines by their nature have more moving parts, some of which wear more rapidly than those in purely rotating machines, and have running as well as shutdown maintenance requirements. Nevertheless, typical availability is about 90-96%.

Gas engines are operated under two distinct air/fuel ration regimes that have a market effect upon environmental performance:

NOx emissions can be reduced markedly by operating with large excess of combustion air (lean-burn). However, this has an adverse effect upon the engine’s power output.

Stoichiometric engines tend to be smaller (typically <300 kWe) than their lean-burn counterparts and are based upon standard vehicle engine blocks with adapted cylinder heads and spark ignition systems.

Cogeneration diesel plants HFO systems have been built in those places where gas is not available. This includes many islands and developing countries. In places where gas availability will arrive later, the plants can use HFO at the beginning and later switch to gas, or use HFO in winter and gas in summer.

Combined Cycles

Some large systems utilise a combination of gas turbine and steam turbine, with the hot exhaust gases from the gas turbine being used to produce the steam for the steam turbine. This is called a combined cycle.

Gas turbine combined cycle (CCGT) systems have been adopted by public utility companies where supplies of natural gas are plentiful: power stations of up to 1,800 MWe have been constructed. In cogeneration applications of the CCGT, exhaust or pass-out steam from the steam turbine is used for process or other heating duties. The main advantage of CCGT cogeneration is its greater overall efficiency in the production of electricity.

Waste heat recovery units

The heat recovery boiler is an essential component of the cogeneration installation. It recovers the heat from the exhaust gases of gas turbines or reciprocating engines. The simplest one is a heat exchanger through which the exhaust gases pass and the heat is transferred to the boiler feedwater to raise steam. The cooled gases then pass on the exhaust pipe or chimney and are discharged into the atmosphere. In this case, the composition or constituents of the exhaust gases from the prime mover are not changed. The exhaust gases discharged, contain significant quantities of heat, but not all can be recovered in a boiler. One typical feature of the exhaust heat boiler (or waste heat recovery unit) is that the typical size is bigger than a conventional fuel-burning unit. This is for two main reasons:

• The lower exhaust gas temperatures require a greater heat transfer area in the boiler;
• There are practical limitations on the flow restriction.

Excessive flow resistance in the exhaust gas stream must be avoided as this can adversely affect operation of the turbine or engine.

Exhaust heat boilers are not, therefore,‘offthe- shelf’ items: they need to be designed for the particular exhaust conditions of the specified turbine or engine. The usual procedure is to provide the boiler supplier with details of the exhaust gas flow from which the heat is to be recovered, and with the temperature and pressure conditions of the required heat output. The boiler supplier will then be able to advise on the quantity of heat that can be recovered, and the temperature at which the exhaust gas will be discharged from the boiler.

The choice of prime mover is based on a number of factors and even with similar energy requirements, no two sites are the same.

The critical factor is the Heat to Power ratio of site demand. Where the electrical power requirement is relatively high as a proportion of total energy this tends to favour engines. Conversely where heat demand is typically more than 3 or 4 times electrical demand the turbine begins to have an advantage. Another key factor is the quality of heat required by the customer site. Some industrial processes have little use for low grade heat – the hot water produced in enginesbased schemes. Where high temperature steam is the primary heat requirement then the turbine is clearly superior.

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© Irish CHP Association. Last Updated: Wed 04 March 2009.