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Cooling systems for gas-fired CHP power generators (part 2/2)
Heat from the gas mixture cooling system in a cogeneration system
For proper operation of an internal combustion engine and for it to achieve the desired power, an internal combustion engine needs a properly prepared gas mixture, i.e. with the right chemical composition, temperature and pressure of the gas mixture. A properly prepared mixture ensures proper combustion (with low NOx emissions and with a non-stick process). For this purpose, after preparing the air-gas mixture (mixing the engine supply gas in the correct proportion with air), it is compressed in a turbocharger. As a result of the compression of the mixture in the turbocharger, it heats up to relatively high temperatures, 200°C and more, making it necessary to cool it down before using it for the combustion process. Thermal image no. 3 shows the “cold” side of the turbocharger during full-load operation. Depending on the engine design and construction principles, the mixture fed into the combustion chambers should usually be around 50°C. This makes it necessary to cool it down. Thermal image no. 3 shows the mixture collector where the cooled mixture flows.
Image no. 3.
To maximise the thermal energy of the gas mixture, the mixture coolers are two-part/stage (they have a high-temperature part and a low-temperature part, this is conducted via two separate coolant circuits). Depending on the design of the engine, the mixture cooler may have two cooling systems combined in a single mixture cooler component, as is the case in Caterpillar CG-132 and CG-170 series engines and the following units: MWM TCG 2016, MWM TCG 3016, MWM TCG 2020, or with an explicit separation and use of separate mixture cooler components, as is the case the in Caterpillar G3512E, Caterpillar G3520E series engines. Image no. 4 shows an engine under overhaul with brand new mixture coolers installed (they are grey, above the cylinder heads).
Image no. 4
Removing heat from the gas mixture with two cooling systems is intended to increase the overall efficiency of the cooling system of a gas-fired CHP unit. This is because the high temperature of the gas mixture can transfer its heat to the engine block cooling system and it affects the heat balance of the entire cooling system. The engine cooling system usually operates in the temperature range (about 85-100°C), which prevents the mixture from cooling down to the required temperature (about 50°C) before being submitted to the combustion process. Therefore, a second system is used to cool the mixture to the required temperature. It is separated from the engine block cooling system due to its significantly lower operating temperature. In this system, the temperatures at the exit of the mixture cooler reach about 50-60°C. Due to the low temperature of the medium and the relatively small thermal output of this system, the heat from it is lost, that is, dissipated into the atmosphere by a cooler outside the power plant room of the gas CHP units. Thermal image No. 5 shows the mixture cooler in the foreground, with the engine block emergency cooler in the background.
Image no. 5
Under favourable conditions, the heat from this system can be used to increase the temperature of the air feeding the boiler burners or to heat CNG gas before feeding it to the engine power system. Due to the very frequent substitution of boiler operation with gas CHP unit operation and the occasional use of CNG gas to power gas CHP units, virtually no heat from the low-temperature gas mixture cooling system is used.
Cooling of exhaust system gases in cogeneration systems
The location of the CHP system operation most often determines how the heat from exhaust gases is used. In cogeneration systems installed in municipal factory CHP plants, usually, the heat of the exhaust gases is used to produce heat to supply municipal or factory district heating systems. In turn, depending on the specifics of the production plant and its technological processes, in some cases, it is desirable to generate process steam or high-temperature process heat carried by thermal oils. Image no. 6 shows a steam boiler/exchanger mounted above a gas-fired cogeneration unit. Sketch No. 7 shows a drawing of a natural gas-fired cogeneration unit with steam production in a boiler/exchanger powered by exhaust gases, shown in image no. 6.
Image no. 6
Sketch no. 7.
In summary, the heat from gas-fired CHP units using internal combustion engines is extracted from the following elements/components:
– engine block with cylinder heads,
– engine oil cooling,
– gas mixture cooling system,
– cooling of exhaust system gases.
Written by: Piotr ROMAŃSKI
CAT Power Generators Sets Expert
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