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D A V I D N . F R E N C H , I N C . , M E T A L L U R G I S T SO N E L A N C A S T E R R O A D

NTER 1989 N O R T H B O R O U G H . M A S S A C H U S E T T S 0 1 5 3 2( 61 7 ) 3 9 3 - 3 6 3 5

VOL. V , No.

A VIEW FROM THE PENTHOUSE: USEFUL INFORMATION FOR THE WORLD OF BOILERS

In an ef fo rt t o imprwe a i r quality ande been forced t o change the i r combustionnology. Reductions in emissions of su lfures (S%) have been achieved mainly bylower-sulfur fuel s. Reductions i nrogen oxide ( q l l e v e l s have been achieved

Of course some new power plants

While some nitrogen oxides are presentcomes from the reaction of atmospherictrogen and oxygen a t combustion-flameUnder normal f ir ing condit ions,the flame temperature increases , so doesThus theplest way t o reduce conbustion-formed N4,s to reduce the flame temperature. Thechemes for achieving t h i s a re t o use off-

bustion a i r is divided. Primary a i r isis less than requiredor complete combustion. Secondary a i r i sdded through w el cf ir e a i r ports. Otherhemes may use concentric ducts with theimary a i r port along the cen ter andcondary a i r in a concentric ring displacedr m the cen ter by some distance. The intents to burn the f uel i n stages. The f i r s tag e combustion would burn the hydrogen t or vapor, and carbon t o carbon monoxide.ter in the combustion sequence, the carbonnoxide would be burned completely t o carbonxide. However, the formation of carbonnoxide leads t o reducing atmospheres in theci ni ty of the burners.The advantage t o t h i s combustion techniques t o reduce flame temperatures and thusreduce the formation ofnitrogen oxides. The drawback is to formducing conditions i n the vi ci ni ty of thethis topic wa s discussed in VOL.V, No. 1 of A View from the Penthouse. Whatnot be fully appreciated is the damage

stainless-steel burner components.Burner parts of austenitic stainless steecoal nozzles, gas burner rings, ign itio ntubes, coa l spreaders, oil-burner t ip s anda i r nozzles all operate a t high metaltemperatures, temperatures in theneighborhood of perhaps 15000r 1600F. Thuse of austenitic stainless steel for thesepa rt s pr wi de s adequate oxidation andcorrosion resistance at these elevatedtemperatures. However under reducingconditions, the corrosion of s tai nle ss st eemay proceed a t unexpectedly and unacceptablhigh ra te s due to the carburization of thesalloys.Under fu l ly oxidizing condi tions ofcombustion, the fuel would be burnedcompletely t o water vapor and carbon dioxidA l l fuels contain both hydrogen and carbon.To il l u s t r a t e the prin cip le consider methan(3%. Complete combustion is as shown inEquation 1. CHq + 202 = 2H20 + C02 EnUsually a small amount of ar cess oxygen,perhaps 2% or so, is added t o assure complcombustion to carbon dioxide (C02) and wate(H20). Flame temperatures under these fi r iconditions would be in the neighborhood of3000F or higher, and the combustion would said to be llstoichiometric".Staged combustion would reduce the amounof oxygen available in the primary flame sothat the combustion would be accomplished iseveral steps, as shown in Equations 1, 2, & 4.

CH4 + O2 = 2H20 + C EQ

Note that the final product is s t i l l watervapor and carbon dioxide but the carbondioxide is formed in three ste ps . Theadvantage from a Ng, formation viewpoint istha t flame temperatures are considerably

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isderably reduced. Note al so , however,in two stages of this combustion(C ) and carbon monoxide, both reducing elements, a re formed.Austenitic stai nles s ste els , similar to 304,dily form carbides a t temperatures as low

1400F, but more ty pic ally , 1500F. Therbon t o a st e e l is calledThe reaction of soo t, o rburned carbon, di re ct ly with the surface ofw i l l form a chromiumbide by th e react ion shown in EQuation 5.

is quite complar and hasCr23C6. Carbon

as shown i n Equation 6.

temperature, these microstructures can al soform structures that are s imilar t o pear l i ti n a plain-carbon st ee l, as shown i n Figure3. These microstructures a r e all taken froa coal nozzle made of 304 stainless steel.Other aramples have been seen in gas-burnerrings and stainless-stee l castings for oil-burner service.In order to prevent this form of rapiddegradation t o burner components understaged-combustion conditions , al lo ys moreres ist an t to carburization need t o be used.

ther unburned carbon or carbon monoxide thus Fig. 1. Normal austenitic, stainless-steelmicrostructure is equiaxed grain s. 500x.ese burner components. etched.The formation of these chromium carb ides

thus reduces the corrosionistance. Stainless st ee ls get th ei rres ista nce t o high-temperaturetion by the addit ion of more than 12%um. When the chromium content i sed t o below 12%, he oxidation resistances more ordinary. Thus the formation ofhromium carbides by the reaction of st a inle ss

e surface of the &eel . With the chromium- Fig. 2. Carburiza tion of 304 s ta in le ss leant reduced t o below 12%, he oxidation to the formation of ti ny carbide pa rt icl es.i s a rapid 500x, etched.Burner components ca& f a i ln only a few months ra ther than se vera l

The microstructures tha t result can be quiteFigure 1 shows the normal.as

Here the grain size has grownis essential ly an all-austenite2 shows thea t the sur face t o be composed Fig. 3. Depending on the temperature ofus ten ite with a myriad of fi ne carbide carburization, the microstructure canrt ic le s. Depending on the time and resemble a pea r li t ic st ee l. 500x, etched.