Power Generation through Cogeneration Process Engr. Dr. Muhammad Nawaz Iqbal

Some of the early electrical generation plants used cogeneration. Industries that generated their own power used exhaust steam for process heating before central stations provided electricity. Large office and apartment complexes, hotels, and retail establishments frequently produced their own power and heated their structures with waste steam. These combined heat power activities persisted for many years after grid electricity became available because the early purchased power was so expensive.
Typically, high-pressure steam is fed into steam turbines in thermal power plants. This steam then exits the turbine at a condenser that operates at a temperature and pressure just a few millimetres above the surrounding air. Before it condenses, this steam essentially has very little useful energy. With the un-extracted steam continuing through the turbine to a condenser, steam turbines for cogeneration are intended for the extraction of some steam at lower pressures after it has gone through a number of turbine stages. In this instance, the steam extraction results in a mechanical power loss in the turbine’s later stages.
The most effective usage of combined heat and power is when heat is available on-site or relatively nearby. Longer heat transfer distances result in a reduction in overall efficiency. While electricity can be delivered through a relatively simple wire and over much greater distances for the same energy loss, it requires expensive, inefficient, poorly insulated pipelines. A Stirling engine or a reciprocating engine may be used in smaller cogeneration units. The radiator and exhaust are used to remove heat. Because small gas and diesel engines are less expensive than small gas or oil-fired steam-electric plants, the systems are popular in modest capacities. Some cogeneration facilities use biomass, as well as commercial and municipal solid waste, as fuel. Waste gas is sometimes used in combined heat and power plants to generate both heat and electricity. Waste gases include sewage gas, landfill gas, gas from coal mines, gas from animal waste, and gas from flammable industrial waste. For added technical and environmental performance, some cogeneration plants combine gas and solar photovoltaic generation. These hybrid systems can be scaled down to the level of a building or even a single residence.
The theoretical efficiency restrictions of the Carnot cycle or subset Rankine cycle for steam turbine power plants or the Brayton cycle for gas turbine plus steam turbine plants apply to every heat engine. When a turbine exhausts its low temperature and pressure steam to a condenser, the latent heat of steam vaporization that is lost is primarily responsible for the efficiency loss in the steam power production process.
Cogeneration is a more effective use of fuel or heat because it makes use of otherwise wasted heat from the production of electricity. Combined heat and electricity plants recover thermal energy for heating that would otherwise be squandered. District heating using combined heat and power is another name for this. One example of decentralized energy is small combined heat power plants. Absorption refrigerators can also utilize by-product heat that is at moderate temperatures (100–180 °C) to provide cooling.
Cogeneration plants are frequently used in the industry for thermal production processes for process water, cooling, steam production, or CO2 fertilization. They are also frequently found in district heating systems of cities and central heating systems of larger buildings such as hospitals, hotels etc.
Compared to utilities, industrial cogeneration units typically operate at much lower boiler pressures. Condensate that is recycled to cogeneration plants may be contaminated. Industries typically need to treat proportionately more boiler make up water since boiler feed water from cogeneration units has significantly lower return rates than 100% condensing power plants. The feed water for boilers must be totally de-mineralized and oxygen-free, and the higher the pressure, the more important it is that the feed water be as pure as possible.n