Waste heat steam generator
A waste heat boiler (AHK) (also waste heat steam generator , English Heat Recovery Steam Generator , HRSG for short ) is a steam boiler that uses the hot exhaust gas from an upstream process to generate steam (and more rarely for hot water ). In this way, the waste heat from the process, which would otherwise be lost unused in the atmosphere, is recovered and the energy efficiency of the system is improved .
Applications
The generated steam can be used in a steam turbine to generate electricity , as process or heating steam in an industrial process or for district heating . Hot water instead of steam is used when the waste heat is at a low temperature level and a suitable heat consumer is available.
By far the most common form of waste heat boiler is that behind a gas turbine or an internal combustion engine in a gas and steam combined cycle power plant or a combined heat and power plant . The power-heat coupling and the use of waste heat are particularly effective in increasing the efficiency.
construction
Construction on the flue gas side
A waste heat boiler differs in its construction from a conventionally fired steam generator, especially in the arrangement of the heating surfaces on the flue gas side . Since a waste heat boiler usually does not have a combustion chamber and the exhaust gas enters the boiler at a relatively low temperature, little or no consideration needs to be given to the burnout or ash removal. All heating surfaces are contact / convection heating surfaces that are simply arranged in the flue gas flow. The flue gas normally flows through the waste heat boiler in a single pass , with boilers with a horizontal and vertical main flow direction .
Waste heat boilers are normally operated without an induced draft fan . The upstream process or unit results in a slight overpressure on the flue gas side, which discharges the exhaust gases to the atmosphere via the chimney.
Since the fuels suitable for use in gas turbines burn relatively low in pollutants, waste heat boilers behind gas turbines do not normally have a flue gas cleaning system (desulphurisation, dust removal, ...) At most, catalytic denitrification is provided to reduce NO x . Due to the low sulfur content of common fuels and the resulting low dew point temperature, waste heat boilers are able to operate at low flue gas temperatures (<100 ° C) without problems with sulfuric acid corrosion at the "cold end".
Waterside construction
In contrast to conventionally fired steam generators, waste heat boilers, especially in gas-and-steam systems behind gas turbines, are often designed as multi-pressure boilers . Here, several (usually up to three) steam generators, each consisting of a superheater , evaporator and economizer , are connected in series on the flue gas side in the counterflow heat exchanger , which work at different pressure levels. In multi-pressure boilers, the high pressure is often provided with an intermediate superheater, which in turn is often combined with the medium-pressure superheater.
The series connection of the pressure stages is necessary in order to achieve a low flue gas temperature and thus a good boiler efficiency. The exhaust gas temperature lowering is much more complex in a waste heat boiler downstream of a gas turbine as in a steam boiler behind a "normal" furnace as the "Zwick point" ( English point pinch , the point of least temperature differential , i.e., temperature difference between the exhaust side and water side) at the hot end of the heat exchanger. This is an effect of the heat capacity flow ratio, which is significantly higher in a gas turbine, since the exhaust gas mass flow and temperature are not primarily determined by the combustion, but by the Joule process . While in a furnace, for reasons of economy, attempts are usually made to keep the air ratio (lambda) as close as possible to just above 1, the air ratios in gas turbines are several times higher.
| size | Conventionally fired boiler | Waste heat boiler behind gas turbine |
|---|---|---|
| Combustion air ratio | 1.0-1.8 | 2 to 8 |
| Flue gas temperature upstream of the boiler | approx. 1000 ° C | <700 ° C |
| Flue gas / steam heat capacity flow ratio | <1 | > 1 |
| Point of least degree ("pin point") | cold end of the economizer (boiler outlet) | hot end of the economizer |
All types of construction are possible in the evaporator: natural circulation boiler , forced circulation boiler , once-through boiler . The latter allow particularly small wall thicknesses and thus faster load gradients, that is, greater flexibility, which is particularly beneficial in combined cycle power plants, since these are mostly used as medium- load power plants with control tasks.
literature
- Rolf Kehlhofer, Norbert Kunze, J. Lehmann, K.-H. Schüller: gas turbine power plants, combined cycle power plants, thermal power plants and industrial power plants. Resch, Munich 1992, ISBN 3-88585-094-X ( Energy handbooks. Volume 7).
- Walter Bitterlich, Sabine Ausmeier, Ulrich Lohmann: Gas turbines and gas turbine systems. Presentation and calculation. Teubner, Stuttgart 2002, ISBN 3-519-00384-8 .
- Richard Zahoransky: Energy technology. Energy conversion systems. Compact knowledge for study and practice 3rd edition. Vieweg, Wiesbaden 2007, ISBN 978-3-8348-0215-6 .
- Richard Doležal: Combined gas and steam power plants. Construction and operation. Springer, Berlin 2001, ISBN 3-540-67526-4 .
- Gunter Schaumann, Karl W. Schmitz (ed.): Combined heat and power. 4th edition. Springer, Berlin 2010, ISBN 978-3-642-01424-6 .