8/7/2019 Penthouse Spring 2006

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DAVID N. FRENCH METALLURGISTSPh 502.955.9847 Fax 502.957.5441 www rlavidnfrench.com

All fossil fuels contain both hydro en and carbon invarious amounts and compounds. ~ a tu r a fas is neady puremethane. CHI 75% carbon, and 25% hy%rogenby welght.Fuel oils usually have less carbon and more hydrogen. Coalsare widely different in their com ositions but all contain car-bon and other hydrocarbons. AR ive off heat enerp whenburned,and behave in a similar fapfiion when burne with alrIn a boller.The hydrogen, Hz, component burns first and com-pletely In air or oxygen, 02, to water vapor, H20:2H2 + 02 = 2H20 EQ 1

this gives off considerable heat energ The carbon C, be-haves in a different fashion. There are L o omm,on,&idp~ ofcarbon carbon monoxlde, CO, and carbon dloxlde, CO2.De ending,on the relativeamount of air, (oxygen) one or bothof Wese oxides of carbon will form. thus:2C + 02 = 2CO EQ 2C + 02=C02 EQ 3Combinations of hydrogen and carbon in fuel, e.g. CH4,will burn to water vapor and carbon monoxlde or carbon dlox-ide: CH4+ 202 = C02 + 2H20 EQ 42CH4 + 302 = 2C0 + 4H20 EQ 5

There are three oxides of iron3e0, Fe203 and FeaOd.The temperature and oxygen concentration will determinewhich oxrde will form when steel is oxidized in a boiler. Con-versely, the manufacturin of steel from iron ore, mainlyFez03 plus impur~t~es,t a k with the reduction of iron oxideby carbon or carbon monoxlde:3C + Fez03 = 2Fe + 3C 0 EQ 83C + 2Fe203 = 4Fe + 3C02 EQ 9 or:

When methane is burned with inadequate oxy en inEQ 4 for complete combust~ono carbon dloxlde an%watervapor,. conditions are said to be "off stoichiomekic" or"reducln The ratlon of COlCOzcan be large enough todestroy protective oxide scale on the external surface offurnace tubes. "Reduc~ng ondition corros~on"s the nameglven to this attack on the boller.This is a simple, single fuel system with ure methaneas fuel. Most real situahons involve impure f u e l that containmeasurable amounts of sulfur. Sulfur bums In alr to formsulfur dioxide and sulfur trioxide:S + 0 2 = SO2 EQ 112s + 302 = 2S03 EQ 12

What is important in EQ's 4 and 5 is the ratio of oxygento methane, 211 In EQ 4 and 3R in EQ 5. When lack ox enoccurs, the carbon won't burn at all and comes out oyyheflame as soot or carbon black:CH4+02=C +Hz0 EQ 6

These ases alone do enou h dama e, but underreducin con&ions, the sulfur is in We form 09 hydro en sul-fide. HZ When reducing c?nditiovs exist, alqn a !urnacewall, some of h e protective lron oxlde scale will %e replacedby sulfides.CO+2H2S= Fez03= 2FeS + 2H20 +C02 EQ 13

The oxy en methane ration here is 111. EQ 6 is thebasis for man%ciuring of carbon black used as a pigment inpaints and automobile tires. Unbumed carbon ives a candlek ight. Carbon particles are heated to a glow % combu,stionof the hydrogen in the wax. Prove it to yoursel?bbyholdlng aclean plece or glass or steel well above the burnlng candlefor a few minutes.

Sulfide scales are not as dense, hard or protective asoxide scales. They are porous, fragile, and easily abraded,and come,off with ash and slag removal. Corrosion depositsthat contaln sulfides are clear evldence of "reducing condl-tion corrosion" has occurred. Sulfides are also the easlestcompound to detect, slnce they leave a sulfur print.

Boiler steels develop oxidation and corrosion resistanceby the formation of an lron oxrde scale In air or oxygen:4Fe + 302 = 2Fe20~ EQ 7Once this oxide forms, the steel underneath is pro-tected from further oxldatlon.

The ap earance of this form of corrosion will show arather smootl! surface and may have severe wastage, seeFigure 1. The wall thickness on the fireside is 0.191" and onthe cold side is 0.286" two years after low NOx burners wereinstalled. If the corrosion deposits are still intact they will con-tain sulfides. Stress is not a factor in this morphology. Theremay or may not be a liquid phase present.

Another form of "reducing condition corrosion" results indeep, fin er like circumferential penetrations into the tubewall, see$igure 2 There is also the metal loss as the wallthickness has been reduced compared to the cold side of thewater wall tube. The wall thickness, excludin the grooves is0.149" while on the cold side it is 0.269". #us most of {hecorrosion wastage is confined to the deep grooves. Corrosiondeposits also contain sulfides. There is as o a llquld phaseresent. and an axial stress necessarv. Thus. this is a form ofborrosion fatigue."Reducing Condition Corrosion" has three conditionrequirements: reducin conditions variable axial stress, andliquid phase in the as\ de osit keducing conditions attackthe iron oxide, and form sulldes: as has been discussed. Theaxial stress comes from sharp tem erature spikes that followsoot blower action, The liquid witRin the ash weakens theash, promoting slag falls, and also causes tern eraturespikes. Chordal thermocouple measurement recorls showtemperature increases up to 200F. As the ash reforms, thetem erature returns to normal until the next soot blower cycleor sLg all occurs.The temperature increase may crack or craze the oxidefilm especially when it is already weakened by the reducingcoditions. These cracks from an axial stress are found goinacross or circumferential to the tube surface. With repeate!cycles, the cracks grow into the tube wall as shown in Figure2. lnvarlabl , the ash deposlts contam free carbon from un-burned coayand are reducing.Correction qr revention of "reducing conditioncorrosion'' is easier sald tRan done Adequate a r to producecomplete combustion of the CO2 with little (a trace) or no COand carbon free ash de osits, is required, excess oxygen inthe flue is not enough. 8 n average eve thing may look likean oxidizing atmosphere, but local conxtions can be quitedifferent. Burners may be out of alignment, fuel and air maynot be pro erly mixed, staged combustion to reduce NOxproductiqn lo meet environmental requirements may lead toa fuel rlch condltlon along some zones of the furnace ,etc.All of these factors and more lead to localized reducing con-ditions, resultant tube wastage, and finally tube failures.

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