2.1 Low-pressure boiler plant
Fig. 2-1: Example of a low-pressure boiler plant
The low-pressure boiler plant described here is installed in hundreds of boiler houses all over Europe. The components illustrated in the diagram are explained below in brief. The details of their functions will be discussed in the next few chapters.
Steam boiler
Shell boilers (or fire tube boilers) are normally used for steam pressures up to 25 bar and capacities up to 25 t/h (Fig. 2-2). This boiler type has a cylindrical outer shell containing one or two furnace tubes (or combustion chambers), including a burner, reversal chambers and fire tubes. The fire tubes and the reversal chambers are surrounded by boiler. The burner is connected to the front of the corrugated furnace tube, where the combustion process takes place. The generated steam exits the boiler at a saturation temperature corresponding to the steam pressure.
The steam can be superheated if necessary. The superheater required for this purpose is located in the front reversal chamber. It heats the steam above the saturation temperature while maintaining a constant pressure.
Fig. 2-2: Three-pass fire tube boiler
DIN EN 12953 describes the product standards for shell-type boiler plants.
Deaerator
The purpose of the deaerator is to remove oxygen and carbon dioxide from the feedwater. By largely eliminating these gases from the feedwater, corrosion can be prevented in the boiler as well the downstream steam and condensate system. A pressure of 1.2 bar (T = 105 °C) is typically maintained in the deaerator by steam injection. Condensate returned from the plant is directed to the deaerator, any losses are made up by softened water. The make-up water is sprayed into the dome section of the deaerator together with atmospheric condensate, thereby releasing the gases dissolved in the water, such as oxygen and carbon dioxide. These gases are discharged into the atmosphere with a small amount of steam. Refer to Chapter 3.14 Deaerator
Economiser
The economiser (feedwater preheater) is usually installed in the chimney and used to recover residual heat from flue gases. This is well worth the effort if we remember that flue gases exit from a boiler with 10 bar steam pressure at a temperature of approx. 220 °C. The residual heat from the flue gases can be used to heat feedwater, which exits the deaerator at 105 °C, to a temperature of 130 °C before it enters the steam boiler. Between 4 and 5 % of the gas consumption can be saved with a feedwater preheater.
Continuous feedwater level control
In order to ensure that the correct amount of feedwater is supplied via the economiser for a particular flue gas flow rate, the boiler should be equipped with a continuous water level control system.
Flue gas condenser
Flue gas condensers are installed in the chimney downstream of the economiser. In addition to the sensible heat, they also recover the energy from vapours by cooling them to below their saturation temperature, releasing the enthalpy of evaporation. This heat is ideal for preheating the feedwater supplied by the water treatment plant. Flue gas condensers require a sufficient amount of cold water to work efficiently
Returned condensate
The condensate that is formed during the heat exchange process on the plant, but has not come into contact with the atmosphere (or other contaminants) does not need to be atomised in the deaerator, though it is usually sparged into it below the water level.
Condensate vessel
Condensate that has come into contact with the atmosphere and therefore absorbed oxygen – and possibly other atmospheric gases as well – is generally supplied to the deaerator via a condensate collection vessel. Cold, softened feedwater is likewise supplied to this vessel. The atmospheric condensate and the softened feedwater are sprayed in the steam space of the deaerator and heated to a temperature of approx. 105 °C.
Softener
The softener removes hardness ions such as calcium and magnesium from the feedwater. Hardness is the cause of scale on the flame and fire tubes. Scale, in turn, reduces the rate of heat transfer and these deposits can ultimately lead to overheating of the fire tube material, serious leakages, and even explosion.
Continuous (or TDS) blowdown valve
Impurities like chloride and sulphate get into the boiler via the feedwater. The steam that exits from the boiler, on the other hand, is clean. The concentration of impurities in the boiler water increases as a result of the evaporation process: the water is said to be "concentrated". To prevent concentration from exceeding an acceptable level, an amount of water is drained from the boiler by means of the blowdown valve. The blowdown process can be controlled automatically on the basis of conductivity.
Blowdown flash vessel
Blowdown water has a energy content which is lost if it is dumped into the sewage system is high. Moreover, the sewage system could be damaged owing to the high discharge temperature, it may also be illegal The pressurised blowndown water should be directed to a flash vessel, and the (clean) flash steam used in the deaerator. Refer to Chapter 3.2 Bottom or Intermittent and continuous blowdown valves.
The heat in the condensate may also be used, via a heat exchanger, to heat the incoming feedwater, see Fig. 2-1.
Bottom blowdown valve
Chemical sludge and dirt particles can accumulate on the bottom of a fire tube boiler. The bottom blowdown valve is normally opened fully once a day for a short time to allow this dirt to be drained off. It is not worth recovering the heat from this drainage. The flash steam that is produced during the manual blowdown process is discharged into the atmosphere. The remaining blown down water is discharged into a blowdown vessel before overflowing into the sewage system. Manual blowdown valves must not be used while a water tube boiler is operating because they will interfere with the circulation of the boiler water. The same applies if the blowdown valve is connected to a fire tube boiler with water tubes in the reversal chamber.
2.2 Combined heat and power boiler house
Fig. 2-3: Steam generator with combined heat and power
Water tube boiler
The water in a water tube boiler is contained in the tubes and combustion takes place in a furnace comprised of tube banks and tube walls. High-pressure steam boilers nearly always have a superheater because the steam is used to generate electricity using a turbine and alternator. Water tube boilers can have the following layouts:
- Natural circulation boiler: Water is pumped into the deaerator by means of a feed pump. The water circulates by convection between the top and bottom drums.
- Forced circulation boiler: The water is circulated by a pump.
- Once-through boiler: The water is pumped through the piping system in one direction.
The steam boiler in Fig. 2-3 shows a high-pressure boiler with forced circulation. A circulating pump connected to the drum (steam / water vessel) on the suction side circulates the water over the evaporator heating surfaces. The amount of water circulated through the evaporator heating surfaces is about six times as high as the generated quantity of steam. The steam bubbles that are formed in the evaporator are separated from the water in the drum, e.g. by means of cyclone separators. The steam collects in the drum over the surface of the water, which is continuously circulated to the evaporator. The economiser is mounted in the steam boiler, as indicated in the diagram. This is understandable in view of the fact that the flue gas temperature ahead of the economiser is relatively high (owing to the high steam pressure). In a 100 bar boiler it would be 350 ºC.
In order to extract as much heat as possible from the flue gases, the economiser installed here should be considerably larger than with a low-pressure boiler. In the same way as with low-pressure boilers (Fig. 2-1), an economiser is used to reduce the temperature of the flue gases to around 125 ºC
Fig. 2-4 shows a corner tube boiler with natural circulation. The furnace is comprised of water tubes that are welded on both sides with steel strips approx. 15 mm apart to obtain a gas-tight wall, also known as a membrane wall.
The tubes are connected to collection drums at the top and bottom. The tubes inside the membrane walls are sometimes referred to as risers, while the lower collection vessels (or drums) are fed from the drum via downcomers. The downcomer tubes are outside the heated part of the boiler. The upper collection vessels are connected to the drum.
The natural circulation in the riser tubes is maintained due to convection. The steam is separated from the boiler water in the drum. The saturated steam that flows from the drum to the superheater should not contain any water. The water and steam are separated by means of vertical baffles inside the drum. Other types of water tube boiler have cyclone separators mounted in the drums to dry the steam. The saturated steam is further heated above the saturation temperature in the superheater (refer to Chapter 1.0 Heat Engineering Concepts).
Fig. 2-4: Corner tube boiler
DIN EN 12952 describes the product standards for water tube boiler plants.
Back pressure turbine plants
Fig. 2-3 shows a back pressure turbine with a generator as part of a plant.
High-pressure superheated steam is generated in the water tube boiler. The superheated steam is used to drive the turbine generator. After the uncondensed steam has been utilised in the back pressure turbine to generate electricity, it can also be used for other processes. The turbine exhaust steam is usually still slightly superheated.
Steam turbo-alternator
This book can only provide a rough outline of the operating principle and applications. The superheated steam from the boiler is fed directly to the turbine. It is expanded with a nozzle so that pressure energy is converted to kinetic or velocity energy. The kinetic energy contained is transferred to the turbine blades, which in turn transmit their energy to the turbine wheel, through the shaft alternator.
Fig. 2-5: Principal of a steam turbine
Not all the energy contained in the steam can be transferred to the turbine via a single turbine wheel, so several turbine wheels are mounted in series. Stationary blades are inserted in the casing to guide the steam exiting from one turbine wheel to the next turbine wheel. As the steam flows through the turbine, the pressure drops and the specific volume of the steam increases. For this reason, the turbine wheel diameters will increase from the inlet to the exhaust end of the turbine.
2.3 Turbine types
There are two main types of turbine that are used for different purposes (refer to Fig. 2-6).
Back pressure turbines
After the steam has driven the turbine, it is reused as process steam. The pressure of the exiting steam is controlled to suit to the requirements of the particular process. This principle is referred to as combined heat and power. In addition to steam for process applications, the generator also generates electricity. The average overall efficiency is 85 %. In some applications, steam is extracted at various points on the turbine housing for higher pressure applications.
Condensing turbines
The exiting steam condenses in a condenser under vacuum. As a result of the vacuum condenser, these turbines work with a larger enthalpy difference (hence improved Rankine efficiency) between the inlet and exhaust than back pressure turbines and are capable of generating more electricity per tonne of steam. Condensing turbines are normally used in power stations. Power stations with condensing turbines have an overall efficiency of between 40 and 50 %.
Fig. 2-6: Turbine types
Demineralised Water
Demineralised water is the only water suitable for high-pressure steam boilers. The steam pressures and temperatures occurring in this type of boiler are so high that a simple softening plant cannot prepare water of good enough quality. In addition to hardness constituents such as calcium and magnesium, various other impurities like sulphates, silicates, chlorides and sodium have to be removed. The carbon dioxide (CO2) that is bound to water is likewise removed in the demineralisation process by means of a CO2 scavenger.
2.4 CHP Plant with a Gas Turbine
To round off the most popular solutions for boiler houses, Fig. 2-8 shows a combined heat and power plant with a gas turbine, a waste heat boiler with auxiliary firing and a back pressure turbine.
A gas turbine plant comprises three main components (refer to Fig. 2-7):
- A compressor to compress the combustion air.
- One or more combustion chambers in which combustion air and fuel (natural gas, refinery gas, heating oil) are intensively mixed and ignited.
- A gas turbine in which hot flue gases expand as they exit the combustion chamber at 900 to 1200 ºC.
Fig. 2-7: Principle of a gas turbine
The compressor and the turbine are mounted on a common shaft; and they drive an alternator which generates electricity.
The flue gases exit the turbine at a temperature of approx. 500 ºC, then enter a waste heat boiler. Superheated steam is generated in this boiler with the residual heat from the flue gases. Gas turbines require a large amount of excess air (typically 4 times the required amount) owing to the temperature limitation of the turbine blades. In theory, stoichiometric combustion of 1 m3 of natural gas requires 8.5 m3 of combustion air. 4 x 8.5 m3 = 34 m3 of combustion air have to be supplied to a gas turbine per m3 of natural gas
At the end of the combustion process, the flue gases still contains sufficient oxygen to allow it to be used for further firing in the waste heat boiler.
The waste heat boiler generally consists of a high-pressure boiler with an economiser and a superheater. All waste heat boilers are designed with a forced circulation system. The superheated steam that is generated is fed to a back pressure turbine, which is connected to an alternator to generate additional electricity, and the pass-out (or exhaust) steam from the turbine may be used on the process.
Fig. 2-8: Example of a combined heat and power plant with gas turbine