Combined heat and power (CHP)
Combined heat and power (CHP), or cogeneration, refers to the simultaneous generation of both heat and electricity. Depending on the circumstances, a distinction can be drawn between power-led and heat-led CHP plants. In almost all cases this means using small-scale packaged CHP units with internal combustion engines coupled to a generator. The engines run at a constant speed so that the directly coupled generator can provide electrical energy that is compatible with system frequency.
In addition to an internal combustion engine and an appropriately matched generator, a CHP module consists of heat exchanger systems for the recovery of thermal energy from exhaust gas, cooling water and lubricating oil circuits, hydraulic systems for heat distribution and electrical switching and control equipment for power distribution and control of the CHP unit. The engines used in such units are either gas spark ignition or pilot ignition gas engines.
Gas spark ignition engines are engines that operate according to the Otto principle and that have been specially developed to run on gas. To minimise nitrogen oxide emissions, the engines are run as lean-burn engines with high excess air levels. In lean-burn mode less fuel can be converted in the engine, which results in a reduction in power. This is compensated for by turbocharging the engine using an exhaust turbocharger. A gas spark ignition engine relies on a minimum concentration of roughly 45% methane in the biogas. If the methane oncentration is lower, the engine shuts down.
Pilot ignition gas engines operate according to the principle of a diesel engine. They are not always specially developed to run on gas and thus have to be modified. The biogas is added to the combustion air via a gas mixer and is ignited by the ignition oil, which is supplied to the combustion chamber by an injection system. The ignition oil concentration accounts for about 2-5% of the supplied fuel power.
The efficiency of a combined heat and power unit is a measure of how efficiently the supplied energy is converted. The overall efficiency is made up of a combination of electrical and thermal efficiency, and normally lies between 80 and 90%. In the ideal case, therefore, 90% of the total rated thermal input can be used for energy conversion. As a rule of thumb for gas spark ignition and pilot ignition gas engines it can be assumed that the electrical and thermal efficiency will each account for 50% of overall efficiency.
To utilise the heat produced during the generation of electricity, it is necessary to extract the heat using heat exchangers. In a CHP unit powered by an internal combustion engine, the heat is produced at various temperatures. The greatest quantity of heat can be obtained from the engine's cooling water system. It can be used to provide heating energy or process energy.
The temperature of the exhaust gas is between about 460 and 550 °C. Stainless-steel exhaust gas heat exchangers, usually in the form of shell-and-tube heat exchangers, are used for extraction of waste heat from the exhaust gas [6-13]. Typically used heat-transfer media include steam at various pressures, hot water and thermal oil.
The plant's own heat requirements can be met very quickly with the waste heat from the CHP unit. As a rule it is only in winter that these requirements are high, whereas in summer the emergency cooler has to dissipate most of the excess heat, unless the heat can be utilised externally. In addition to the heat needed to heat the digester, which amounts to between 20 and 40% of the total heat produced, it is also possible, for example, to heat work spaces or residential premises. CHP units are fully compatible with standard heating systems and can therefore easily be connected to a heating circuit.