Stoker, coal

RDF-3 is intended for use as a supplement with coal for semisuspension or suspension firing or for use by itself in similar boilers. d-RDF is intended as a supplement with stoker coal or for use by itself in stoker boilers. Several methods and alternatives for producing RDF-3 or d-RDF have been described (25). 

I Stoker Coal—I j by j-irteft)—This size is largely used for small industrial stokers and domestic firing. (lj x finches = 3.2 x 1.9 centimeters.)... 

Stoker, coal-fired Grate heat-release rate, W/m ... 

The fines content is arbitrarily defined as <10 mm material, instead of <6 mm as with coal. This conservative definition is more inclusive of flaky particles in d-RDF. A low fines content is desired for increased bulk density, improved material flow, and reduced dusting. However, as with coal, some fines are necessary to achieve a mix of suspension and on-the-grate combustion. Table II lists an average fines content somewhat lower than the 20 - 40 wt% range typical of stoker coal. Note that these measurements were taken on the as-produced pellets, and any abuse during transport, storage, and feeding will increase the fines in the fuel as fed to the boiler. However, the pellet mill output can be screened, and the screen underflow returned to the mill, as is done with animal feeds to reduce fines. 

At this time, some note should also be made of the various size designations and size limits for coal. For example, run-of-mine coal and large-lump coal are variable in size with no top or bottom limits whereas lump coal varies from a minimum size of 1 in. (2.5 cm) to a variable top size. On the other hand, cobble (egg or stove) coal varies in size from 2 in. (5 cm) to 6 in. (15 cm) whereas nut coal falls within the size limits 3/4 in. (2 cm) to 2 in. (5 cm). The coals designated as prepared stoker coals all fall into the size range 1/16 in. (0.2 cm) to 2 in. (5 cm) but they are further subdivided... 

In the underfeed stoker, coal is fed by a worm feeder into the bottom of a retort (Figure 14.15). The coal rises vertically in the retort and air enters through tuyeres in the sides. The fire is ignited at the top and the flame front moves downward and its speed is matched by the rising flow of coal, fulfilling the requirement of the flame front traveling in the opposite direction of the primary air. 

Stoker Coal [114 x % in (3.2 x 1.9 cm)]. This is used largely for small industrial stokers and... 

Shredded tire chips have been burned in stoker-fired boilers. Uniroyal fired a 15% mixture of tire chips with coal, and both General Motors and B. 

Another furnace that does not require fuel preparation is the stoker boiler, which was used by New York State Electric Gas Corporation (NYSEG) in its TDE tests. At NYSEG, the stoker boiler, which has a 1649°C (3000°E) flame temperature (as does the cyclone boiler), has routinely blended low quaUty coal, and more recently, wood chips with its standard coal to reduce fuel costs and improve combustion efficiency. In the tire-chip tests, NYSEG burned approximately 1100 t of tire chips (smaller than 5x5 cm) mixed with coal and monitored the emissions. The company determined that the emissions were similar to those from burning coal alone. In a second test-bum of 1900 t of TDE, magnetic separation equipment removed metal from the resulting ash, so that it could be recycled as a winter traction agent for roadways. 

The moisture content of peat or brown coal that is briquetted for fuel must be reduced to about 15% for satisfactory briquetting. Mechanical or natural means are used because of the cost of thermal drying. Moisture is sometimes desirable. About 8% is necessary for prevention of combustible loss from a chain-grate stoker. 

Several developments are being pursued to utilize coal directly, ie, automation of controls, coal and ash handling equipment for smaller stoker and pulverized coal-fired units, design of packaged boiler units, and pollution control equipment. In the cement industry coal firing has been used, because the sulfur oxides react with some of the lime to make calcium sulfate in an acceptable amount. 

Sohd fuels are burned in a variety of systems, some of which are similar to those fired by Hquid fuels. In this article the most commonly burned soHd fuel, coal, is discussed. The main coal combustion technologies are fixed-bed, eg, stokers, for the largest particles pulverized-coal for the smallest particles and fluidized-bed for medium size particles (99,100) (see Coal). 

The Atrita pulverizer (Riley Stoker Coij).) is available in several single and duplex types. Capacities vary from 3400 to 25,000 kg/h (7500 to 54,000 Ib/h). The coal is carried in a a pin miU where most of the pulverizing is done, foUowed by a recycUng classifier. 

In the multiple-retori. stoker, rams feed coal to the top of sloping grates between banks of tuyeres. Auxiliary small sloping rams perform... 

The fuel flows at right angles to the air flow. Only a small amount of air is fed at the front of the stoker, to keep the fuel mixture rich, but as the coal moves toward the middle of the furnace, the amount of air is increased, and most of the coal is burned by the time it gets halfway down the length of the grate. Fuel-bed depth varies from 100 to 200 mm (4 to 8 in), depending on the fuel, which can be coke breeze, anthracite, or any noncaking bituminous coal. 

The performance of spreader stokers is affected by changes in coal sizing. The equipment can distribute a wide range of fuel sizes, but it distributes each particle on the basis of size ana weight. Normal size specifications call for 19-mm (H-in) nut and slack with not more than 30 percent less than 6.4 mm i U in). 

A difference between these firing methods may also be manifested in the initial fuel cost. For efficient operation of a spreader-stoker-fired boiler, the coal must consist of a proper mixture of coarse and fine particles. Normally, double-screened coal is purchased because less expensive run-of-mine coal does not provide the optimum balance oFcoarse and fine material. 

An advantage of a stoker-fired furnace is its easy adaptability to firing almost any unsized solid fuels. Bark, bagasse, or refuse can normally be fired on a stoker to supplement the coal with a minimum amount of additional equipment. Thus, such supplementaiy waste fuels may be able to contribute a higher percentage of the total heat input in a stoker-fired furnace than in a PC furnace without expensive equipment modifications. 

Table 27-21 gives typical values or ranges of these criteria for gas, oil, and coal. The furnace release rates are important, for they establish maximum local absorption rates within safe hmits. They also have a bearing on completeness of combustion and therefore on efficiency and particulate emissions. Limiting heat release on grates (in stoker firing) will minimize carbon loss, control smoke, ana avoid excessive fly a. ... 

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