Emissions stoker boilers

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. 

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.

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... 

Formation of Airborne Emissions. Airborne emissions are formed from combustion of waste fuels as a function of certain physical and chemical reactions and mechanisms. In grate-fired systems, particulate emissions result from particles being swept through the furnace and boiler in the gaseous combustion products, and from incomplete oxidation of the soHd particles, with consequent char carryover. If pile burning is used, eg, the mass bum units employed for unprocessed MSW, typically only 20—25% of the unbumed soHds and inerts exit the combustion system as flyash. If spreader-stoker technologies are employed, between 75 and 90% of the unbumed soHds and inerts may exit the combustion system in the form of flyash. 

Hahn JL, Von Dem Fange HP, Velzy CO. 1986. Effect of boiler operation and RDF feedstock on emissions of dioxins and furans from an RDF fired spreader-stoker system in Albany, NY. Chemosphere 15 1239-1246. 

Most boilers in China are of the stoker type, which consume about one-third of total coal production, at an average efficiency of about 60% with high pollutant emissions. To raise efficiency and to lower pollutant emissions, recent R D efforts have been focused on CFFBs with low steam capacity, possessing the following features ... 

The emissions of two different commercially available types of wood chip combustion systems were analysed one boiler with a power output of 70 kW with a stoker type burner, the other boiler with a power output of 600 kW and a moving grate burner. Both systems had a computer controlled feed rate and combustion air supply. 

Densified fuel pellets may be used as boiler fuel in place of coal in stoker-fired furnaces. Direct substitution of fuel pellets was demonstrated in two informally reported instances with no furnace adjustments. The particulate emission from each of these installations were monitored with the emission rate results shown in Figure 2. As a comparison, the North Carolina Administrative Code allowable particulate emission rate schedule (a function of heating rate) is also shown. The environmentally acceptable particulate emission contributes to the economic justification of the conversion of small-to-medium size steam generators to the use of pelleted biomass fuels. Two such installations are now using pelleted fuels on a routine basis, and a third has concluded the economic benefit by entering into a contract agreement to purchase pelleted fuel. 

Figure 2. Stoker fired boiler emissions grate burning pelleted fuels...

Johnson, N. 2002. Fundamentals of stoker fired boiler design and operation. Proceedings. CIBO Emission Controls Technology Conference. July 15-17, Salt Lake City, UT. 

Cofiring TDF at Jennison Station required only minor changes in boiler operations. A noticeable amount of bead metal, and some steel belt material melted into nuggets that stuck on or in-between the stoker grate keys. Although the build up of metal was visible, it did not interfere with the ability to efficiently fire the blend. The ESP performance was not changed there were no incidents of black smoke or increased emissions from coal/TDF blends. No reports of odor from tires burning occurred. 

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