Spreader Stoker Boilers

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... 

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. 

D. S. Ensor, R. G. Hooper, R. C. Carr, and R. W. Scheck, "Evaluation of a Eabric Eilter on a Spreader Stoker Utility Boiler," Preprint 76-27.6, 69th APCA.JinnualMeeting, Portland, Oregon, June 27—July 1, 1976. 

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. 

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. 

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,... 

The test was in response to problems resulting from TDF burning in a FBC boiler retrofitted from a spreader/stoker design, and located at a Wisconsin power plant.16 Problems during the commercial test indicated that better tramp metal removal was necessary, combustion was not adequate, and that the particulate control device, an electrified filter bed, was not commensurate with the ash levels generated.16... 

Fort Howard Corporation Croon Hay, Ul Currant 2 2 and 1 xl" 30 tons tlre/IOOO tons coal par day No Nona required plant has 0 boilers totals 3 underfeed type that use 2 s2 TDF 2 spreader-stoker type that use 1> 1 TDF 1 cyclone fed with no IDF use now, but use planned. Produce recycled paper coal other base load fuel. References 3 and 4... 

Hortham Statas Powar frtnch Island, Ul Teat in 1902 Unsuccessful electrified filter bed for PM Inadequate because awtal In tires aborted out device alao heat level In boiler too high. Yes 150,000 Ib/hr stean capacity bubbling fluidized bed retrofitted iron spreader/stoker design prlnary fuel la wood waste. References 8 and 9... 

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. 

Another variation being demonstrated is the combustion of RDF in a dedicated boiler as a principal fuel. Normally the boiler is of spreader-stoker design with some consideration given to the use of fossil fuels such as high sulfur coal as a load leveler and steam production stabilizer. 

The block flow diagram in Figure 5 indicates the principal functional sections of the citrate process, corresponding to the unit now operating in Terre Haute. A brief tour through these sections helps in gaining a perspective on the process. The 2000-SCFM gas stream being treated in the unit comes from a coal-fired spreader stoker-type boiler at 700°F with 25,000 lb/hr steam-rated capacity. 

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