AND PULVERIZED COAL FURNACE

The cyclone furnace is a water-cooled horizontal cylinder (5 to 10 ft in diameter) into which coal is introduced.

As the coal moves from front to rear, combustion air is introduced tangentially at high velocity and about 35-in water gage pressure. This causes a whirling or centrifugal action, with the solid fuel particles moving to the periphery of the combustion chamber where their movement is retarded by molten slag that covers its walls. Although the finer fuel particles burn in suspension, the cyclone method of combustion is primarily a surface-burning process. The solid fuel particles in the liquid ash coating on the walls are scrubbed by the incoming air stream, providing intimate coal and air mixing. Combustion in the cyclone furnace is complete and has practically no carbon loss.

The cyclone furnace is water-cooled as an adjunct to the boiler circulation system. It is attached to the steam generating unit, which may have either of two types of secondary furnaces: 1) the water screen type, in which a water screen of tubes divides the furnace into lower and upper sections; and 2) the open furnace type. In the water screen furnace, the fly-ash loading of the flue gases will be about 10 per cent-of the total ash fired. In the open furnace, the loading will be about 15 per cent. This refuse may be collected and returned for reinjection.

Some stations find the cyclone furnace advantageous and there are definite sales trends in its favour. As it is not necessary to pulverize the coal, a considerable saving is obtained in both initial investment and also in-operating expense.

This furnace has been proved to be suitable for a wide range of coals and for firing gas or oil either in combination with coal or as stand-by or substitute fuel. The cyclone fur­nace is also capable of burning waste or by-product fuels such as wood, chars, and cokes.

Goal may be fired as-a finely powdered fuel that is injected into the furnace. The coal is pulverized to a fineness of 70 per cent or more through a 200 mesh sieve. It is then transported by hot primary air (which also dries the coal) to the furnace.

The majority of all central steam generators operating at 200,000-lb steam per hr and over are fired by pulverized coal. The number of pulverizers are determined by pulverizer capacity and stand-by requirements. Larger installations have two, three or four pulverizers. Provision for three pulverizers, one for each row of burners plus one for stand-by is not unusual. Pulverized coal-fired boilers may be either the dry bottom or slag-tap type. Vertical, horizontal, opposed, or tangential firing methods may be employed.

The size of the unit, its pressure and temperature, available space, fuel characteristics, ash-fusion temperature, type of burner, and ash removal method determine the volume of the furnace, the extent of water cooling, and the ultimate design of the entire steam-generating unit.

Pulverized coal furnaces are usually convertible to firing with oil or gas. Units near oil refineries may utilize fluid coke.

Pulverized coal firing removes a limitation on the amount of fuel that can reburned in a boiler (with stoker firing there is a definite limit).

The type and multiplicity of burners, their arrangement and the flame shape will determine the furnace width and depth dimensions. The furnace height is a function of the required furnace volume. The exit temperature of the gases should be below the ash-fusion temperature of the lowerest quality fuel to be used. Thus, coal with a large percentage of low-fusion ash will require larger waterwall surfaces, which in turn make a larger furnace volume necessary. Superheater requirements may govern exit temperatures.

All pulverized coal-fired furnaces constructed to-day are partially or completely water-cooled. If tangential firing is used, the furnace must be completely water cooled, because there is considerable flame impingement. It is desirable to eliminate, as much as possible, blasting of flames against the furnace walls. Molten ash particles stick to and dissolve most refractories. Heat and high sulphur content may induce a slow attack or tube wastage of the water-cooled walls. Flame length varies with coal particle size (the length is shortened by uniformly fine pulverization), the percentage and composition of the volatile constituent, turbulence, furnace temperature, and excess air. With proper mixing, the flame length may be as short as 10 ft. Helical or U-shaped paths[6] may be provided for long flames, the furnace shape being adapted to the available space.

In a wet-bottom (slag-tap) furnace, 40 to 60 per cent of the total fired ash leaves with the combustion gases, and in a dry-bottom furnace, 80 to 90 percent. An individual burn­er may be reduced to about 35 per cent of its maximum continuous rating. With 15 to 22 per cent excess air, the unburned combustible is under 1 per cent. The excess air requirements will vary from 10 to 30 per cent. As the percentage of ash increases, the amount of excess air must also increase if the combustible loss is to be held to a constant minimum. For optimum efficiency, the combustible loss is balanced against the dry gas loss.