Energy is released from the incineration and leaves the furnace as flue gases at a temperature of approximately 1,000-1,200˚C. The flue gases are cooled through a boiler, where the energy released from incineration is initially recovered as hot water or steam.
The steam boiler is divided into one to three open radiation passes and a convection part. The radiation part includes the evaporator where saturated steam is produced. After passing through the radiation part, the flue gases enter the convection heating surfaces. There, they first transfer heat to super heaters and then to the economizer, and after that they go through to the flue gas cleaning system.
The superheater aims to increase the temperature of the condensed steam that comes from the evaporator. The temperature of the flue gases that head to the superheater does not exceed 630°C. The heating surface of the superheater is divided into three sections with intermediate water injection between the stages (de-superheaters) in order to control the steam temperature (max 400°C).The last section of the economizer aims to preheat the feed water prior to entering the boiler steam drum, and also to decrease the gas temperature to 160-220°C.
The radiation part of the boiler requires a space of up to 30 to 40 meters in height. The convection part of the boiler can be arranged either horizontally or vertically. The horizontal arrangement takes up approximately 20 meters more space than the vertical arrangement in the longitudinal direction. The arrangement of the convection section can significantly affect building costs and should be determined as early as possible.
The waste-fired plant cannot be designed with steam parameters similar to those of traditional power plants fired with coal, gas, or oil. This is because waste differs from fossil fuel, particularly in terms of the content of chlorine, which - combined with sulphur - may lead to high-temperature corrosion, even at relatively low temperatures. The risk of corrosion and erosion can be reduced by observing a number of specific design criteria and by designing the boiler for moderate steam parameters (pressure and temperature).
The primary features to minimise fouling and corrosion are as follows:
• optimised combustion chamber dimensions and low gas velocity to reduce ash entrainment;
• long gas residence time in the radiation passes before entering the convective pass;
• horizontal flue gas flows in the convective pass, which have higher thermal efficiency;
• on-line cleaning systems in the convective pass are made of mechanical rapping devices rather than soot blowers; and
• wide tube spacing in the horizontal pass including free space for additional heating surface.
Some combustion processes may, furthermore, carry a risk of CO corrosion. The corrosive nature of the flue gas from waste incineration usually limits the steam parameters to a maximum temperature of approximately 400 °C and a pressure of approximately 40 bar. The temperature of the water returning to the boiler (feed water) is maintained at a minimum of 125 to 130 °C to limit the risk of low temperature corrosion in the coldest part of the boiler.
Hoppers are provided under the vertical and horizontal passes to collect boiler ash. Collected dust is discharged by mechanical conveyors into the ash silos. Boiler ash is considered to be part of the produced fly ash.
The energy recovery from a steam-producing boiler is conventionally known as the Rankine process. The Rankine process allows energy outputs in the form of power, steam, and combinations of power, steam, and hot water.
The energy from the hot flue gases is recovered through the boiler and passes on to the internal steam circuit. The steam energy may be converted to power by a turbine and a generator set. The superheated and highly pressurized steam of the boiler is expanded in the steam turbine, which transforms the energy content of the steam to kinetic energy, which is further transformed to electrical energy by the generator. The excess heat of the low-pressure steam is converted to hot water within the heat exchanger (condenser) and either conveyed to a district heating network or cooled away.
Condensate from the condenser is returned to the feed water tank and the deaerator, in order to have suitable heating and degassing. The feed water tank and the deaerator are made of a cylindrical horizontal storage tank. It is fitted with baffles and steam distribution nozzles in the bottom and a dome in the upper part of the tank equipped with spray nozzles. To compensate for continuous boiler blowdown, demineralised water is transferred to each feed water tank through make-up water pumps. The boiler water is supplied to the boiler from the feed water tank and the deaerator by high pressure feed water pumps. Chemicals are used to raise the pH level to prevent boiler corrosion and scaling. A deoxidizer which decreases dissolved oxygen is issued in order to prevent corrosion of the boiler.