Heat recovery steam generator

A waste heat boiler ( AHK) (including heat recovery steam generator, heat recovery steam generator english, short HRSG ) is a steam boiler, which uses the hot exhaust gas from an upstream process for steam generation ( rarely also for hot water ). In this way, the heat from the process, which would otherwise be wasted into the atmosphere, recovered and it improves the energy efficiency of the plant.

Applications

The steam produced 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 present at a low temperature level and a suitable heat absorber is present.

The most common by far is the form of the waste heat boiler. Behind a gas turbine or an internal combustion engine in a gas - and -steam combined cycle power plant or a cogeneration plant Through combined heat and power and the waste heat utilization efficiency is thereby increased particularly effective.

Construction

Smoke -sided design

A waste heat boiler differs in its construction from a conventionally fired steam generator especially the flue gas side in the arrangement of heating surfaces. Since a heat recovery steam generator usually has no furnace and enters the exhaust gas with a relatively low temperature in the boiler, no or very little consideration to the burn or ash removal must be taken. All heating surfaces are Berührungs-/Konvektionsheizflächen arranged in a simple manner in the flue gas stream. The flue gas flows through the waste heat boiler is normally in a single train, and there are boiler with horizontal and vertical main flow direction.

Waste heat boilers are normally operated without induced draft fan. From the upstream process or set then the flue gas side, a slight excess pressure, which discharges the exhaust gases to the atmosphere through the chimney.

Since the suitable for use in gas turbine fuels burn relatively low emissions, waste heat boiler downstream of gas turbines have not normally exceed a flue gas cleaning ( desulfurization, dust, ...) At most, a catalytic denitrification of NOx reduction is provided. Due to the low sulfur content of the fuels available and the consequent low dew point temperature waste heat boilers are able to drive low exhaust gas temperatures (< 100 ° C), without having problems with sulfuric acid corrosion of the " cold end ".

Water-side structure

In contrast to conventionally -fired steam generators is the waste heat boiler, in particular in gas and steam turbines behind gas turbines, often run as a multi- autoclave. In this case, a plurality ( usually up to three) are steam generators, each consisting of superheater, evaporator and economizer connected in series on the flue gas side in counter-current heat exchanger operating at different pressure levels. More in pressure vessels of the high pressure is often provided with a reheater which is in turn often combined with the intermediate pressure superheater.

The series connection of the pressure stages is necessary in order to achieve a low exhaust gas temperature, and thus a good efficiency of the boiler. The exhaust gas temperature decrease is much more complex in a waste heat boiler downstream of a gas turbine as in a steam boiler behind a "normal" firing as the " Zwick point " (english pinch point, the point of least terminal temperature difference, that is the temperature difference between the flue gas side and water side) at the hot end of the heat exchanger is located. This is an effect of the heat capacity flow ratio, which is significantly higher in a gas turbine as exhaust gas mass flow and temperature are not primarily determined by the combustion process but by Joule. While a furnace for reasons of economy will normally tries to keep the air ratio (lambda ) as possible just above 1, the air ratios are higher by a multiple gas turbines.

In the evaporator, all types are possible: natural circulation boiler, forced circulation boiler, once-through boiler. The latter allows particularly low wall thicknesses and thus faster load gradient, that is, higher flexibility, which is particularly favorable for gas - and - steam power plants as these are usually used as a means of load power plant with control tasks.

Literature ( sources)

• Rolf Kehlhofer: gas turbine power plants, combined cycle power plants, heating plants and industrial power plants. Resch, Munich 1992, ISBN 3-88585-094- X ( set of books energy. Band 7).
• Walter Bitterly Sabine Ausmeier, Ulrich Lohmann: Gas turbines and gas turbine plants. Representation and calculation. Teubner, Stuttgart 2002, ISBN 3-519-00384-8.
• Richard Zahoransky: energy technology. Systems for energy conversion. Kompaktwissen for study and practice 3rd edition. Vieweg, Wiesbaden 2007, ISBN 978-3-8348-0215-6.
• Richard Dolezal: 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 generation. 4th edition. Springer, Berlin 2010, ISBN 978-3-642-01424-6.

• Steam boiler
• Energy saving