Johan van der Kamp and Hans van Essen of Bronswerk Heat Transfer discuss the energy-efficiency solutions in waste energy that they implemented at waste management company Afval Energie Bedrijf in the Netherlands.
The Afval Energie Bedrijf (AEB) in Amsterdam 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, and 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.
Super heat: a closer look at the different steam types and pressure variations
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. The design goal is that not 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 expensive materials would corrode too quickly.To break this circle of limitedconversion 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 reheater 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%; this means an increase of 30% electricity production from the same amount of fuel.
The reheater: a look inside the innovative heat-transfer solution
The high-pressure steam from the boiler has a pressure of 130bar. This means an evaporation temperature of 330°C. The steam flowing to the high-pressure stage of the turbine is superheated by more than 100°C. After this high-pressure stage, the steam has a pressure of 14bar, 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, 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. 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. The high-pressure stage of the turbine and the low-pressure stage transfer their mechanical energy to the generator, where it is converted into electrical energy.
A must-have item: how Bronswerk and AEB worked together
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.
The proposal by Bronswerk Heat Transfer contained a surprising 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.
Looking to the future: the need for a successful company to devise a breakthrough product
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.