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1997

DAVID N. FRENC H, INC ., METALLURGISTSONE LANCASTER ROADNORTH BOROUGH, MASSACHUSETTS 01532TEL: (508) 393-3635 VOL. XIV, No.

A VIEW FROM THE PENTHOUSE: USEFUL INFORMATION FOR THE WORLD OF BOILERSW-NOX BURNERS

In the last several years, low-NOXers have been developed andy installed to meet the EPAts for reduction of oxidesnitrogen, or NOX emissions. TheNOX lumps together all of theinto onecollection. When mixed withalongoxides (sometimesrred to as SOX) form acids duringion, which is the acid-rainlem. These nitrogen oxides comeom two sources; that is, nitrogenwithin the fuel and theof nitrogen oxide byof nitrogen and oxygen incombustion air. The formation ofe oxides starts at flamees above about 2600F,e the kinetics are favorable.

t the fuel nitrogen, but theof the gases within theflame temperature, thethe reaction between nitrogend oxygen proceeds, and the highergen oxide or NOXtion within the combustion

The route that has been taken toce flame temperatures, and thusthe amount of NOX is by off-taged combustion., the complete combustion ofmixed with less air,

on to carbon dioxide and hydrogenwater vapor. With the primaryfuel ratio less than 1, theed tor vapor in the first stage; andf the fuel remains as elementalwhile a portion is burned to

carbon monoxide. Secondary air isadded around the primary air-fuelflame to burn the elemental carbon carbon monoxide and carbon monoxideto carbon dioxide. Finally, tertiaor over-fire air is added to assurean excess of oxygen and lead tocomplete combustion; so the flue gaat the exit is water vapor and carbdioxide. The full available heatfrom the fuel is obtained as thefinal combustion products are fullyoxidized carbon dioxide and water.The problem with this stagedcombustion scheme is that all burneadjustments need to be nearly perfecto assure complete combustion withithe furnace. Usually that is not tcase; and the flue gas containsappreciable quantities of carbonmonoxide, hydrogen sulfide, andunburned or elemental carbon. Andlong lazy flames reach into the topof the furnace and the pendantsuperheaters and reheaters. Ashdeposits from waterwalls and fly-as

deposits on the superheaters andreheaters invariably containelemental carbon, proof that, atleast on the tube surface, reducingconditions exist. Under thesereducing conditions, sulfides form place of or as part of the oxidescale on the steel surface. Sulfurprints of the tube cross-section wilbe positive for metallic (usuallyiron) sulfides, further proof ofreducing environments.A sulfur print is a replica of tsulfide distribution which appears a black or dark brown streak onphotographic print paper, see Figure1; and note the sulfides are on thefire side only. What oxides do formtend to be porous, and the mixture ooxides and sulfides is inherentlyless protective against furtherwastage.

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When the ash deposit also containsspecies that are liquid at theoperating temperature, porous oxidesand sulfides are more readilydissolved in these low melting-pointsalts. The protective scale re-formsby oxidation or sulfidation, and thecycle continues. Metal wastageoccurs by reaction of the steel withoxygen and/or sulfur with concurrentloss of the metal. The result is athinner wall thickness. Transport ofoxygen, sulfur, and carbon to acleaned steel surface is alsofacilitated by the liquid ashconstituents.

Sulfides have been found within thecorrosion debris on the surface ofwaterwall tubes as well as onsuperheater and reheater componentsCoal fineness or lack thereof mayexacerbate the problems. All of thcoal particles need to be smallenough to assure complete combustioin the time available during theshort residence within the flame.The usual coal-fineness limits are70% through a 200 mesh and 98%through 50 mesh. However, the 2%that remains on a 50-mesh screen isstill a considerable amount of coalIf the plant burns 100 tons/hour, 2is still 2 tons of partially burnedcoal that will find its way eitherinto the bottom or the fly ash.Either way, carbon becomes part ofall deposits. When incorporated inash deposits on the superheater andreheater, the elemental carbon willdiffuse into the surface of thesteel, carburize the surface, andreduce corrosion resistance. CarboFigure 1 contents up to 0.8% and higher havebeen measured on superheater ashIn the high-temperature deposits adjacent to the steel.superheater and reheater, reducing Depending on the particular alloy,promote carburization of carbon contents up to 0.47% have betube surface. Carburization of a measured in 304H. Reduced corrosio

304H stainless steel or the chromium- resistance translates into more rapm ferritic steels of T-11, wastage. If there are low melting-and T-9 leads to chromium- point species of potassium andformation. These carbides sodium-iron-trisulfate, thethe chromium content of the combination of liquid ash andoy and thus reduce the corrosion unburned coal can lead to exceeding. The effect is more rapid deterioration.in the higher-chromium Thus, it may be necessary torials. rethink the coal-finenessCarburization effects have been requirements in low-NOX burners toin the T-11, T-22, T-5, T-9 assure complete combustion and redugrades and 304, 321 and 347 the amount of unburned coal withinc stainless steels. the flue-gas atmosphere.

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