Текст 8. Main types of a modern boiler

In a modern boiler, there are two main types of boilers when considering the heat transfer means from flue gases to feed water: fire tube boilers and water tube boilers.

In a fire tube boiler the flue gases from the furnace are conducted to flue passages, which consist of several parallel-connected tubes. The tubes run through the boiler vessel, which contains the feedwater. The tubes are thus surrounded by water. The heat from the flue gases is transferred from the tubes to the water in the container, thus the water is heated into steam. An easy way to remember the principle is to say that a fire tube boiler has “fire in the tubes”.

In a water tube boiler, the conditions are the opposite of a fire tube boiler. The water circulates in many parallel-connected tubes. The tubes are situated in the flue gas channel, and are heated by the flue gases, which are led from the furnace through the flue gas passage. In a modern boiler, the tubes, where water circulates, are welded together and form the furnace walls. Therefore the water tubes are directly exposed to radiation and gases from the combustion. Similarly to the fire tube boiler, the water tube boiler received its name from having “water in the tubes”.

A modern utility boiler is usually a water tube boiler, because a fire tube boiler is limited in capacity and only feasible in small systems. The various designs of water tube boilers are discussed further in “Steam/water circulation design”.

Текст 9. Steam boilers: Grate furnace boilers

Steam boilers can be classified by their combustion method, by their application or by their type of steam/water circulation. In this chapter the following boiler types will be presented and briefly described, to give the reader a perspective of the various types and uses of various steam boilers:

1. Grate furnace boilers

2. Cyclone boilers

3. Pulverized coal fired (PCF) boilers

4. Oil and gas fired boilers

5. Heat recovery steam generators (HRSG)

6. Refuse boilers

7. Recovery boilers

8. Packaged boilers

Grate furnace boilers

Grate firing has been the most commonly used firing method for combusting solid fuels in small and medium sized furnaces (15 kW–30 MW) since the beginning of the industrialization. New furnace technology (especially fluidized bed technology) has practically superseded the use of grate furnaces in unit sizes over 5 MW. Waste is usually burned in grate furnaces. There is also still a lot of grate furnace boilers burning boifuels in operation. Since solid fuels are very different there are also many types of grate furnaces. The principle of great firing is still very similar for all grate furnaces (except for household furnaces). Combustion of solid fuels in a grate furnace follows the same phases as any combustion method:

1. Removal of moisture

2. Pyrolysis (thermal decomposition) and combustion of volatile matter

3. Combustion of char

When considering a single fuel particle, these phases occur in sequence. When considering a furnace we have naturally particles in different phases at the same time in different parts of the furnace.

The grate furnace is made up a grate that can be horizontal, sloping or conical. The grate can consist of a conveyor chain that transports the fuel forward. Alternatively some parts of the grate can be mechanically movable or the whole grate can be fixed. In the later case the fuel is transported by its own weight (sloping grate). The fuel is supplied in the furnace from the hopper and moved forward (horizontal grate) or downward (sloping grate) sequentially within the furnace.

The primary combustion air is supplied from underneath the fire bed, by which the air makes efficient contact with the fuel, when blowing through the bed, to dry, ignite and burn it. The secondary (and sometimes tertiary) combustion air is supplied above the bed, in order to burn combustible gases that have been released from the bed. The fuel is subjected to self-sustained burning in the furnace and is discharged as ash. The ash has a relatively high content of combustible matter.

Текст 10. Cyclone firing

The cyclone furnace chambers are mounted outside the main boiler shell, which will have a narrow base, together with an arrangement for slag removal. Primary combustion air carries the particles into the furnace in which the relatively large coal/char particles are retained in the cyclone while the air passes through them, promoting reaction. Secondary air is injected tangentially into the cyclone. This creates a strong swirl, throwing the larger particles towards the furnace walls. Tertiary air enters the centre of the burner, along the cyclone axis, and directly into the central vortex. It is used to control the vortex vacuum, and hence the position of the main combustion zone which is the primary source of radiant heat. An increase in tertiary air moves that zone towards the furnace exit and the main boiler.

Cyclone-fired boilers are used for coals with a low ash fusion temperature, which are difficult to use with a PCF boiler. 80–90% of the ash leaves the bottom of the boiler as a molten slag, thus reducing the load of fly ash passing through the heat transfer sections to the precipitator or fabric filter to just 10–20% of that present. As with PCF boilers, the combustion chamber is close to atmospheric pressure, simplifying the passage of coal and air through the plant.

Cyclone firing can be divided into horizontal and vertical arrangements based on the axis of the cylinder. Cyclone firing can also be dry or molten based on ash behavior in the cyclone. Based on cooling media the cyclones are either water-cooled or air-cooled (a.k.a. air cooled). Cyclone firing has successfully been used to fire brown coal in Germany. Peat has been fired in cyclones at Russia and Finland.

Compared with the flame of a conventional burner, the high intensity, high-velocity cyclonic flames transfer heat more effectively to the boiler’s water-filled tubes, resulting in the unusual combination of a compact boiler size and high efficiency. The worst drawbacks of cyclone firing are a narrow operating range and problems with the removal of ash. The combustion temperature in a cyclone is relatively high compared to other firing methods, which results in a high rate of thermal NOx formation.