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Furnace


The furnace is the section of the incineration plant where the combustion takes place.
 

Contents

1 Combustion Process
2 Types of Furnaces
   2.1 Firing Grate
   2.2 Fluidized Bed
3 Residues

Combustion process

The incoming waste enters the combustion chamber and advances experiencing different temperatures. The first, and lowest, temperature occurs at the upper part of the furnace, on this stage the waste is dried by radiation or convection at temperatures over 100°C. As the waste moves further the temperature increases reaching eventually the 250°C. At this point "under reduced atmospheric pressure and under further addition of heat” (Bilitewski et al., 1997) pyrolysis takes place, expelling volatile components. At the third section of the grate, the complete oxidation takes place at temperatures between 600°C and 1300°C and with an air/fuel ratio (λ) value between 1.5 and 2.0. The main combustion products are heat, carbon dioxide, water and ashes. The last stage in the combustion chamber is called afterburning. Its function is to reduce the carbon monoxide and unburned substances by circulating flue gas at a minimum of 850°C for at least two seconds resident time (European Union, 2008).

The speed of the transition among the mentioned phases is determined by the waste composition and its heat value. The fuel triangle (Figure 1) shows graphically the required composition of waste needed to be combusted without the need of auxiliary fuel. European waste is reported to have a mean composition of 35 percent combustibles, 30 percent ashes and 35 percent water, resulting this in a heating value of approximately 6.0 MJ/kg (Bilitewski et al., 1997).
 
The Fuel Triangle
Figure 1: The fuel triangle showing European waste range and mean values (Habeck-Tropfke, 1985)

Types of Furnaces


Firing Grate Incinerator

The most common grate firing systems utilized nowadays are shown in Figure 2. The traveling grate consists of a series of rigid and movable grates which move the waste on its natural process direction, the retention time in this type of system is not given by the weight of the waste, but by the speed of the grates. The counter direction push over grate works similar to the traveling grate, but this system adds grates pushing also on the opposite direction, resulting in a better mixture and better stoking than traveling grate. The state-of-the-art in grates is the reverse acting grate, which consists of fixed and moving grates, sloped towards the slag dumping end, pushing the waste back up. "This upward motion in combination with the gravity-assisted downward motion results in excellent stoking, because a continuous flow of embers from the combustion chamber is moved to the forward edge of the grate” (Bilitewski et al., 1997). Finally, the rotating drum grate consists of six side-by-side individually-controlled drums sloped at 30° angle towards the discharger, resulting in good mixing and stoking (Bilitewski et al., 1997).
 
Grate_Firing_Systems
Figure 2: Grate firing systems possible designs
 
A very important target of the incineration grate is a good and enough distribution of the incineration air into the furnace to guarantee complete combustion. For that reason, a primary air blower introduces air through the burning grates into the fuel layer. Most of the waste incinerators also utilize a secondary air, air/steam or air/cool-cleaned flue gas mixture input to enhance combustion. The second input is generally added above the waste bed (Figure 2). The recirculation of the air and cool-cleaned flue gas mixture as secondary air is "reported to reduce heat losses with the flue-gas and to increase the process energy efficiency by around 0.75 – 2 percent” (European Commission, 2006). As well as reducing "the nitrogen oxide content in the flue gas by another 20 percent” (Bilitewski et al., 1997).
 
 

Fluidized Bed Incinerator

An alternative to grate firing systems are the fluidized bed incinerator. On this type of waste incinerators the waste has to be processed prior to burning into a refuse-derived fuel, free of noncombustible materials. Once the waste is homogenized, it is fed into the fluid-bed chamber which burn wastes in a turbulent bed of heated inert materials, such as sand or other noncombustible substances (Rhyner et al., 1995). Fluidized bed systems operate in a temperature range between 750 and 850°C and, because they can operate with only 30-40 percent excess air, are more energy recovery efficient than grate furnaces (Bontoux, 1999).

Residues

Slag Removal Discharger
Ram slag remover (Image: Martin GmbH)

The ashes and metals resulting from the combustion process are collected, and separated in the bottom ash discharger. The ashes are cooled down with water to prevent them of reaching its melting point between 950 and 1000°C, which "causes the slag to assume a paste-like consistency” (Bilitewski et al., 1997) as well as to reduce the emission of dust. At the exit, the water used for cooling is separated from the grate ash and may be re-circulated (European Commission, 2006). The resulting ashes can be used in the building sector as fill for noise protection walls, as surface layers on parking lots and as frost protection layers in roads, among others building-related uses. The recovered metal fraction is recycled.

See Also


Waste Incineration Plant Scheme
Bunker > Feeding Unit > Furnace > Boiler > Energy Generation > Flue Gas Cleaning

References:

  • Bilitewski, B.; Härdtle, G.; Marek, K., 1997: Waste Management. Springer, Berlin, ISBN: 3-540-59210-5
  • Bontoux, L.; 1999: The Incineration of Waste in Europe: Issues and Perspectives. European Commission Joint Research Centre.
  • European Commission, 2006: Integrated Pollution Prevention and Control Reference Document on the Best Available Techniques for Waste Incineration
  • European Union, 2008: Directive 2008/98/EC of the European Parliament and of the Council of 19 November 2008 on waste and repealing certain Directives.
  • Habeck-Tropfke, H., 1985: Müll- und Abfalltechnik. Dusseldörf: Werner Verlag.
  • Rhyner, C.R., Schwartz, L.J., Wenger, R.B., Kohrell, M.G. 1995: Waste Management and Resource Recovery. CRC Press. ISBN: 0873715721, 9780873715720. 524 pp

Created by Prof. Dr.-Ing. Peter Quicker (Lehr- und Forschungsgebiet Technologie der Energierohstoffe an der RWTH Aachen), (), last modified by ()




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