Combined heat and power (CHP) – Cogeneration

CHP involves the use of a steam turbine which is designed not only to drive a generator, but also to produce steam or hot water. It can provide power more efficiently and economically where there is a steady demand for the heat. Where coal is the fuel, CHP units can use either PCC or FBC. They incorporate turbines which are specially designed to provide flexibility in operation, so that they can be run solely to provide power (for example if district heating is not required) or to provide a consistent and secure supply of steam (for example to an industrial plant) while also generating power.

A CHP turbine has extraction points for heating steam as well as for regenerative feedwater heating (which would be included in a straight condensing turbine). The thermodynamic design and thus the structural design of a CHP turbine is influenced not only by the heat output required, but by the ratio of heat output to power output at various load points. The turbine should be arranged so that power generation and the heat circuit can be run independently of each other over as broad an output range as possible. The power loss due to steam extraction for heating purposes should be kept as low as possible over the entire load range.

A number of different arrangements are possible, but a typical unit consists of two separate parts: a high pressure/medium pressure (HP/MP) section, and a low pressure (LP) section. Live high pressure steam from the boiler enters the HP part of the turbine, releasing energy as it expands through various stages of blading. At the end of the HP/MP section it is extracted, and the flow is split. Some goes to the process heat stream, some to the HP feed heater and the remainder passes through the LP section of the turbine. After passing through this, the extracted steam either joins the process heat stream, or goes through the LP feed heaters.

In spite of the energy efficiency advantage of CHP, such plants contribute less than 10% of the power supply in most countries, and in some, the contribution is extremely small.

The biggest constraint preventing the wider use of CHP systems is the need to have an assured and steady demand for the heat produced. They are ideally suited on sites where there is an adjacent chemical works or industrial complex, operating 24 hours a day. Alternatively the heat can be used for district heating schemes. However, these are only needed during the winter months, and the construction of the necessary infrastructure for heat distribution involves high capital cost.

CHP production is most widely used in Scandinavian and east European countries, where there is a long and sometimes severe winter. The units can be large base load plants which can be run in backpressure mode which facilitates maximum steam extraction for heating, or quite small, supplying a small community. If a CHP plant supplies district heat, then it has to be located near a residential area, where there will be greater sensitivity about visual intrusion, resultant traffic movement and other environmental factors than there would be in a remote location. In the large Danish units, maximum district heat production (around 450 MJ/s) reduces the maximum power production from 400 to ~350 MWe. In the summer, the units will only produce 50-80 MJ/s of heat for supplying hot water to the nearby community.

Liberalisation of the electricity markets increases the opportunities for using CHP, since the power generated can more easily be distributed and sold. Thus an industrial user can set up a small CHP plant on his site to supply process heat, and then sell on any surplus power to the local grid. Until recently, this practice was either not allowed, or at least not encouraged in many countries. This kind of development represents the principal growth area in generating capacity in some countries over the next few years. In a number of countries in western Europe, electricity supplies are subsidised, in order to give an economic incentive for this kind of development.

The most commonly used fuel for new CHP installations is gas, not coal. There are, however, supply constraints and price instability in relation to gas supplies in many countries, and coal is still the preferred fuel in some applications. In terms of boiler/plant format and measures taken for emissions control, it makes little difference to the steam generator design whether the plant is simply making electricity, or whether it is also producing heat. The main differences are that CHP plants are often smaller, and they need to incorporate a different design of turbine, to optimise output. With CHP installations, it is not always possible to take advantage of the economies of scale associated with large PCC boilers with their associated flue gas clean-up equipment. Thus, again, gas is often the preferred fuel.