Pulverized-coal boiler plants with the ring (annular) furnace chamber for major power generating units

Design features of pulverized-coal boiler plants

Annular furnace chamber is the octagon vertical water-cooled prism, inside which an octagon water-cooled insert is installed all along its whole height coaxially (in alignment). Lateral dimension of this insert for different fuels takes 50-60% of the lateral dimension of the outer chamber. Walls of the inner and outer chambers are made of the gas-proof welded panels.

In this context, the rotating flame is a sort of nipped in the annular space between the inner and outer screens. Conditions of mixing, igniting and burn-up of fuel, and conditions of heat exchange in such a furnace chamber are significantly different from conditions usually existing in traditional tangentially fired furnace chambers, where there is practically no active gas flow in the central (paraxial) area. The conducted comparison tests of the cylindrical and polyhedral annular chambers showed that at octahedral cross-section of the annular furnace chamber, its aerodynamics is close to the flow in cylindrical annular chamber.

Figure 2 shows constructions of boiler plants with annular furnace for the 800 MW power generating units combusting brown and black coals. T-shaped construction with the two convectional gas passes (heating surfaces of superheater and economizer are placed in them) that symmetrically go from the furnace is approved for both boiler plants.

—равнительные размеры разных моделей котлов

Figure 1 – Comparative dimensions of different models of boiler plants.

 отлы с кольцевой топкой

Figure 2 – Boiler plants with annular furnace chamber.

Annular furnace for both boiler plants is the octagon outer chamber, which regulation diameter is 27.5-28 m, and inside which the hollow octagon chamber with passage diameter of 13.75-14 m is installed in alignment.

Screens of the inner and outer chambers are made in the form of vertical all-welded gas-proof panels produced of the tubes 32 x 6 mm in diameter (art. 12 x 1 millifarads) with the step of 48 mm.

The upright prominent construction supporter (pillar), lift, and maintenance floor are installed in the inner chamber.

The furnace is equipped with thirty-two straight-flow burners that are put in four tiers with one burner application installed at every outer end of the furnace. The burners’ axles are directed by tangential scheme to provide rotary flow in horizontal section.

Specifically adjusted burners are used in the annular furnace in order to provide possibility to change the direction of burner’s jets input to the furnace. This allows adjusting location of rotating flame quite easily excluding active contact with its inner and outer chambers.

To create additional conditions focused on diminishing NOx emission, some part of secondary air is delivered to furnace separately from the burners, at their level of location, and also - to the area, which is located higher than the burners. In doing so, and in order to diminish swirling of gases before they are directed to convectional gas passes, over-fire air (OFA) is arranged by tangential scheme, where it is directed opposite to rotation of the main flow of flue gases.

Annular furnace chamber allows the following:

  • To reduce boiler plant’s height by 25-30%;
  • To reduce steel intensity, and consequently to decrease the cost of heating surface of the boiler plant with annular furnace chamber by 20%;
  • To improve the reliability of operation of furnace water-wall panels through a more uniform heating along the furnace perimeter;
  • To reduce fouling and slagging in furnace through lowering the temperature level in the furnace;
  • To reduce NOx emissions through lowering the temperature, stimulation of mixture formation, and internal recirculation;
  • To lighten the design of building and support-suspension structures of boiler plant.

Calculations, made according to the FLUENT software, allow learning velocity and temperature distribution (see Figure 3) in horizontal section of the furnace.

–аспределение скорости и температуры в горизонтальном сечении топки

Figure 3 - Velocity and temperature distribution in horizontal section of the furnace.

The scheme of steam-water tract of the boiler plant with annular furnace chamber actually coincides with steam-water tract of the traditional boiler plant of the T-shape design. The primary steam tract is designed in the form of the two (in accordance with the number of gas passes) immiscible parallel flows with independent control systems. And in doing so, each flow is divided in its turn into two sub-flows that are periodically mixed in order to exclude heat expansion.

Secondary steam tract consists of four independent adjustable flows. The scheme of engagement of heating surfaces, their construction, and usage of full-opening built-in separators allows providing boiler plant’s operation on flexible parameters.

A peculiarity of this boiler plant’s construction is that the upright prominent construction supporter (pillar), lift, and maintenance floors are installed in the center of inner chamber, which makes isolating the framework and the whole suspension system of the boiler plant considerably easier.

Trials and experience of operating the boiler plant have showed efficiency and practicability of annular furnace chambers for application in large boiler units, in case when different fuels are combusted. In 2001, RAO UES of Russia decided to recommend the boiler plant with annular furnace chamber for industrial application.

Major parameters of boiler plants

є Boiler plant for brown coals Boiler plant for brown coals Boiler plant for black coals
1 Capacity of boiler plant, t/h 2 650 2 450
2 Pressure of primary steam behind the boiler plant, MPa 25 25
3 Temperature of primary steam at the outlet of the boiler plant, °C 545 545
4 Temperature of feed water, °C 275 275
5 Secondary steam consumption, t/h 2 186 2 020
6 Pressure of secondary steam at the outlet of boiler plant, MPa 3,5 3,5
7 Temperature of secondary steam at the outlet of the boiler plant, °C 545 545
8 Coefficient of efficiency of the boiler plant, % 91,0 92,6

Temperature of gases in the furnace, °C

in the end of active combustion area

at the outlet of furnace

1 174

1062 1054

Fuel specifications

Name Symbol Brown coal Black coal
Humidity, % W` 33 15,5
Ash content, % A` 4,7 6,6
Heat conduction, kJ/kg Q` 15 655 23 750
Output of volatiles, % V` 48