Optimising waste energy efficiency

Paul Boughton

Johan van der Kamp and Hans van Essen report on how reheaters were crucial to highly-efficient incinerator.

The Afval Energie Bedrijf (AEB) in Amsterdam, The Netherlands, has been processing waste for almost a century. The company aims for maximum environmental efficiency, in the sense that waste does not simply remain waste, but is converted into energy and basic materials. The waste is incinerated and the energy released is used to generate electricity, while the remainder of the energy released becomes usable heat. As many valuable materials as possible are reclaimed from the ash residue.

To produce electricity from the available combustion energy, the well-known cycle of steam boiler, steam turbine (connected to the generator), condenser and feed water pump is used. It is also commonly known that the higher the pressure and temperature of the steam supplied to the turbine and/or the lower the pressure and temperature of the steam leaving the turbine, the higher the conversion efficiency is of fuel into electricity.

In a steam turbine not only the pressure of the steam decreases, the temperature also drops increasingly, creating 'wet' steam; an increasing amount of condensate forms in the steam. If the water content in the steam becomes too high, it will damage the turbine.

As a result, the condensate in the steam ultimately determines the achievable outlet pressure, and thus to a large extent, the shaft power.

When the steam entering the turbine is superheated steam (dry steam), the moment that condensate formation starts is delayed, and the steam pressure at the outlet of the turbine may be reduced. The result is a higher shaft power.

Moisture content

The design goal is not that the moisture content of the exhaust steam determines the pressure at the turbine outlet, but the temperature of the condenser cooling medium, usually the outside air or cooling water.

There is sufficient cooling water at AEB, so in principle a very deep vacuum could be achieved at the outlet of the turbine. But then extreme superheating of the boiler steam would be required.

However, the many corrosive constituents in the boiler's combustion gases limit this temperature, since the already very expensive materials would corrode too quickly.

To break this circle of limited conversion rate, AEB opted for a two-stage turbine, with reheating the steam between the two stages. The steam - moderately superheated - flows through the first (high-pressure) stage of the steam turbine, up to a location where, approximately, condensation would start.

Here, a connection has been created in the turbine housing for the steam to exit. This steam is then extremely superheated in a re-heater and will enter the second (low-pressure) stage of the steam turbine as superheated steam. A very deep vacuum can now be achieved in this second stage. Using a two-stage turbine with reheating increases the efficiency of the system by more than 30 per cent, which means additional electricity production of over 30 per cent from the same amount of fuel!

The reheater

The high-pressure steam from the boiler (1) has a pressure of 130 bars. This means an evaporation temperature of 330°C. The steam flowing to the high-pressure stage (2) of the turbine is superheated by more than 100°C. After this high-pressure stage, the steam has a pressure of 14 bars, at the corresponding temperature of more than 190°C.

The steam first flows to the separators. Here any water droplets formed in the turbine are collected and discharged. Then the dry steam goes to the superheater (position 6 in Fig 2), where the temperature is increased by approximately 130°C, from 190°C to 320°C. A portion of the high-pressure steam is used as a heating medium. The superheated low-pressure steam flows through the red pipes to the low-pressure stage of the turbine (4). In addition to the separator and the reheater, both reheater units consist of a condensate vessel for the high-pressure condensate formed. These vessels are necessary for controlling the units. The low-pressure steam from the turbine is condensed in the condenser (5). The high-pressure stage of the turbine and the low-pressure stage transfer their mechanical energy to the generator (3), where it is converted into electrical energy.

The reheater units were so crucial to realising a properly working, highly-efficient installation, that AEB decided to purchase the installations themselves directly. When Bronswerk received the request for the reheaters, it proposed supplying the complete units, including separator, condensate vessel and the connecting piping. The proper operation of the separator is crucial to the proper operation of the reheater. To prevent having the warranty spread over two suppliers, it was wise to keep everything in one hand. AEB gave Bronswerk the opportunity to supply the complete installations.

Combination of techniques

The proposal by Bronswerk Heat Transfer contained a combination of techniques that together result in a compact and robust unit. The High Pressure Compact Header, which had just been developed by Bronswerk, was provided for the high-pressure steam. To avoid vibrations due to high flow rates of the low-pressure steam in the condenser, a 'no-tubes-in-windows' type was used, while low-finned tubes provide good heat transfer to the low-pressure steam.

For a company aspiring to be a market leader, it is necessary to introduce a breakthrough development to the market every now and then. The High Pressure Compact Header referred to here is just such a Bronswerk development. It is also essential to know the discipline to the finest detail. This makes it possible to use the right combination of existing and new technologies. This project shows that Bronswerk also masters these aspects.

AEB was so pleased with the design of the reheater unit that it also awarded Bronswerk the dump condenser. Ultimately, AEB now has an incinerator with uniquely high efficiency.

For more information at www.engineerlive.com/ipe

Johan van der Kamp and Hans van Essen are with Bronswerk Heat Transfer BV, Nijkerk, The Netherlands. www.bronswerk.com

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