Stoker boilers

In stoker boilers, fuel is either dropped or rammed onto a grate. Stoker boilers are identified by the type of feed mechanism and the type of grate. Feed may be by spreader, overfeed, or underfeed. Grates may be travelling, reciprocating, chain, or dump type. 

Approximately 12 stoker boilers are burning TDF supplementally on a commercial basis, all in the pulp and paper industry (see Chapter 5). One industrial stoker boiler at a tractor factory is testing TDF use. Five of these 13 are underfeed stokers, and 8 are spreader stokers. Of the spreader stoked boilers, 2 are reciprocating grates, 

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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). 

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. 

Most electricity from biofuels is generated by direct combustion. Wood fuels are burned in stoker boilers, and mill waste lignin is combusted in special burners. Plants are generally small, being less than 50 MW in capacity. There is considerable interest in combustion of biomass in a process called cofiring, when biomass is added to traditional fuels for electricity production. Cofiring is usually done by adding biomass to coal, but biomass also can be cofired with... 

Table 7 lists the results of test bums at two coal-combusting power plants a western Kentucky power plant, which burned pure Illinois Basin coal (Table 4) and two blends of this coal + TDF (99 wt% coal + 1 wt% TDF 97 wt% coal + 3 wt% TDF) in a cyclone boiler (Hower et al. 2001) and the Purdue University power plant, which used a stoker boiler to combust a comparable Illinois Basin coal as well as a blend containing 95 wt% of this coal and 5 wt% TDF (Table 4 this study). 

Below, we discuss test bums in industrial facilities that combusted both pure coal and coal/TDF blends. These fuels were burned at identical conditions in each combustor a stoker boiler at the University of Iowa (EPA 1997 Schwarzhoff Milster, personal communication 2003), a stoker boiler at Purdue University (this study), and two different cement kilns (Carrasco et al. 1998 Mukheijee et al. 2003). 

Table 8. Comparison of average emissions resulting from combustion of various fuel blends in stoker boilers... 

Fig. 7. Comparison of normalized metal emissions from two stoker boilers. Values represent emissions from combustion of coal/tyre blends normalized to the respective emissions from combustion of pure coal. Purdue University data set is for 5 wt% TDF. Symbols for University of Iowa data set A, 4 wt% TDF , 8 wt% TDF. Data without error bars have standard deviations that are smaller than the symbols. Data from Table 8.

TDF is difficult to bum in suspension because of its size and weight. Some industrial experience exists burning TDF in pulverized, cyclone, and spreader/stoker boilers. One utility tested whole tires in a pulverized boiler. 

Overfeed Stoker Boilers. Coal combusted in overfeed stoker boilers is fed from above onto a traveling or chain grate, and bums on the fuel bed as it progresses through the furnace. Ash falls into a pit at the rear of the stoker.6 The same TDF issues apply as were mentioned under spreader stoker boilers. 

The most common type of boiler configuration to bum hog-fuel is the spreader stoker type, although some overfeed stokers also exist. Spreader stoker boilers can bum fuel with high moisture content, are relatively easy to operate, and have relatively high thermal efficiency. Overfeed stoked boilers have lower particulate emissions relative to spreader stoker boilers because less combustion occurs in suspension.13... 

Varied boiler firing configurations are found in hog fuel boiler applications, including dutch oven, fuel cell, spreader stoker with traveling or vibrating grates, and cyclone stoker types. As stated previously, the spreader stoker is the most widely used of these configurations. Spreader/stoker boilers in the pulp and paper industry often have an air swept spout added to the front of the boiler to feed bark down on top of the coal.14 Wood is puffed at one-... 

Stoker boilers Hydrograte spreader stoker boilers... 

Fig. 1 Measuring sites at the moving stoker boiler of PINDOS ...

Table 2, Proximate and ultimate analyses (%wt) and calorific values of the fuels used during the co-combustion trials, at the moving stoker boiler.

Pellet density is important to assure proper flow through the storage and volumetric feed systems of stoker boilers. The density will also affect burning rate in the boiler. The particle density of coal is of the order of 1.3 g/cm3. Table II indicates a mean pellet density 10 - 20% below that for coal. 

Post-consumer materials provide the final category of opportunity fuels. Representative post-consumer materials include tire-derived fuel (TDF), commonly used in cyclone boilers, stoker boilers, and cement kilns. Other post-consumer materials used as opportunity fuels include waste oil and re-refined oil, wastewater treatment gas, landfill gas, paper-derived fuel (PDF), plastics-derived fuel, refuse-derived fuel (RDF), sewage sludge, wastewater treatntent gas, and selected hazardous wastes burned in industrial boilers and kilns. 

Junge, D.C. 1979. Design Guideline Handbook for Industrial Spreader Stoker Boilers Fired with Wood and Bark Residue Fuels. Oregon State University Press, Corvallis, OR. 

Herbaceous biomass has long been fired in stoker boilers and fluidized bed boilers in California, where the agricultural community was encouraged by the California Energy Commission (CEC) to enter the energy arena. The CEC has had biomass programs for the agricultural community since the early 1980 s. 

Cofiring Tire-Derived Fuel in Stoker Boilers... 

International Paper s Lock Haven Mill currently bums a 15% TDF blend in their stoker boilers using about 9,000 tires daily eliminating about 3.2 million tires from landfill annually. The TDF has reduced the company fuel costs by about 200,000 per year [40]. 

Dry limestone injection in the combustion chamber of Pulverized coal or stoker boilers gives rise to a new kind of fly-ash the characteristics of which may be different as regards properties, landfill disposal and reuse. 

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