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Текст 9. Steam boilers: Grate furnace boilers
Steam boilers can be classified by their combustion method, by their appli- cation 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 perspec- tive 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 begin- ning 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 New furnace technology (especially fluidized bed technology) has practically superseded the use of grate furnaces in unit sizes over 5 MW.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 coni- cal. The grate can consist of a conveyor chain that transports the fuel forward. Al- ternatively 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.