Copyright Statement FPC International, Inc

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1 Cpyright Statement All rights reserved. All material in this dcument is, unless therwise stated, the prperty f FPC Internatinal, Inc. Cpyright and ther intellectual prperty laws prtect these materials. Reprductin r retransmissin f the materials, in whle r in part, in any manner, withut the prir written cnsent f the cpyright hlder, is a vilatin f cpyright law.

2 SUMMARY REPORT OF THE EFFECTS OF FERROUS PICRATE ON SULFURIC ACID CORROSION, VANADIUM PENTOXIDE, BOILER EFFICIENCY, AND SLUDGE CONTROL BY: EUGENE S. ADAMS, P.E. PRESIDENT ADAMS INDUSTRIAL SALES MOORESVILLE, NC

3 TABLE OF CONTENTS ABSTRACT... INTRODUCTION. METHODOLOGY. Biler Efficiency (5); Acid Instrumentatin (6) RESULTS.. DISCUSSION APPENDIX. Crrsin (5); Test

4 ABSTRACT Serius crrsin, fuling, and acid st prblems frequently ccur in the flue gases f pwer generatin and prcess steam bilers when sulfur and vanadium bearing N. 6 fuel il is burned. With sulfur, this is due t the cndensatin f sulfuric acid (H2S4) n surfaces f the biler, flue, ecnmizer, r stack where the temperature is belw the acid dewpint pint. Vanadium in the fuel xidizes t vanadium pentxide (V25), a hard depsit that attaches itself t all biler surfaces and impedes heat transfer. This reprt summarizes the results f testing with a ferrus picrate catalyst, (FPCS), frmulated t alleviate the crrsin prperties f heavy fuel ils. FPCS is a generic designatin fr the catalyst used during research. The prduct is nw marketed as FPC48. The spectrum f these tests include studies with individual burners at several cmmercial biler sites. Cmparisns are als made between the ferrus picrate catalyst and magnesium xide (MgO) based additives designed t alleviate the same prblems. This reprt will shw cnclusively that FPCS was fund t be effective in the reductin f prblems related t vanadium and sulfur, and that the catalyst als reduces excess air requirements fr prper cmbustin. Therefre, FPCS has the ability t imprve biler efficiency, as well as reduce biler system maintenance. Further, findings shw FPCS t be 5% mre effect in reducing sulfuric acid prblems than magnesium xide-based additives, and at least as effective as magnesium xide-based additives in reducing vanadium pentxide fireside depsits. 3

5 INTRODUCTION The main bjective in biler peratin is t btain maximum efficiency with minimum maintenance. With the reductin in verall il quality, it is becming mre evident that fuel il additives have a significant rle t play in biler peratin in cnjunctin with the develpment f state-f-the-art mechanical equipment. And as time ges n, and il quality deterirates, prblems will increase. Typically, fuel il additives can be brken dwn int subgrups such as: 1) Preburned cnditiners that prevent frmatin in strage tanks. sludge and water 2) Cmbustin catalysts that act directly n the cmbustin prcess t imprve cmbustin efficiency. 3) And slag inhibitrs that act upn prducts f cmbustin t minimize fireside depsits, crrsin and reduce atmspheric pllutin. While sludge and water frmatin in strage tanks are a prblem, cmbustin efficiency and fireside depsits are mre critical. Fuel additives are mainly used t reduce fireside depsits that decrease the rate f heat transfer, resulting in increased stack temperatures and decreased fuel efficiency. Biler failure caused by depsits and crrsin result in lss f prfit, because f reduced prductin while making repairs. The magnitude f this lss is ften under estimated - lss f prductin fr several days may cst as much as prper fuel treatment fr several years. Fireside fuling and crrsin are principally caused by impurities present in residual fuels. The mst cmmnly fund elements are sulfur, vanadium, sdium, nickel and irn; with sulfur, vanadium and sdium being the majr cntributrs. Typically, sulfur is related t lw temperature prblems, such as acid attack in air heaters, ecnmizers and breeching. Sdium and vanadium cause acid and depsits n biler tubes at high temperatures. Included as Appendix 1 is an f "STEAM", by Babcck & Wilc. detail the relatinship between reactins in biler peratins. excerpt frm the 39th editin This excerpt explains in sme sulfur, vanadium and sdium We have entitled this 4

6 The Appendix is entitled "General Infrmatin Regarding, Sulfur, Vanadium, Sdium and Biler Operatin". Additinal infrmatin has been included when further clarificatin is needed, and is indicated by (*). Such additinal data is frm a surce ther than Babcck & Wilcx. METHODOLOGY T insure cnsistent results, all bilers must be perated under identical lad cnditins, typically base laded with manual peratin, if pssible. Once testing starts, n changes are allwed r all test data becmes invalid. Nrmally, bilers were run base-laded at lw fire, high fire and average lad. Biler Efficiency With bilers determine biler was recrded: manually efficiency set, was the necessary infrmatin t gathered. The fllwing data Steam flw & pressure 2 C2 Cmbustibles Stack temperature Fuel type Ambient temperature WB/DB Oil temperature Oil pressure Atmizatin pressure Cntrl settings Fuel additive used Acid Crrsin Again, with biler peratin cnstant, a Land Mdel 2 Acid Dewpint Meter was used t evaluate the effect f the fuel il additive n acid frmatin. An acid film is gd cnductr f electricity. Therefre, if a nncnducting surface bearing tw electrdes is placed in the flue gas, an cndensate frming n the surface will be detected by a current flwing between the electrdes. The Land meter emplys a dewpint detectr with a thermcuple and a pair f electrdes fused int a detecting surface that is air cled frm the rear. This detectr is munted n the end f a stainless steel prbe. The prbe is inserted int the gas stream and the detectr is gradually cled by the flw f cling air which is cntrlled manually at the cntrl unit. A thin film f acid begins surface when the detectr surface is t cndense n the detectr sufficiently cled. This 5

7 creates a current flw acrss the electrdes which is mnitred at the cntrl unit. The air flw t the detectr is then manually adjusted t maintain a cnstant electrde current. At this cnstant current, the rate f cndensatin and evapratin f acid are equal. The temperature at which this takes place is the acid dewpint temperature, (ADT). This temperature is measured by the thermcuple fused t the surface f the detectr. The crrsin ptential f the flue gas can als be assessed by measuring the rate f acid buildup (RBU) at temperatures belw the ADT. By pltting a graph f RBU vs. temperature, the peak rate f acid cndensatin can be equated t a particular temperature. Any metal surfaces at this temperature that are expsed t the flue gases will experience acid cndensatin, and therefre, crrsin. By utilizing the RBU data, the direct effect f a fuel catalyst can be evaluated. Fuel il which is untreated will have a particular RBU prfile ver a temperature range. If this is cmpared t the identical biler peratin with treated fuel, the RBU graph will shw any effect f the fuel treatment with respect t sulfur and vanadium-related emissins. Test Instrumentatin Land Mdel 2 Acid Dewpint Meter Bacharach Cntinuus 2 Analyzer; Mdel CA-1 Bacharach 2-C2 Analyzer; Mdel 1-52 IMC Instruments Mdel 61 Digital Thermmeter Fisher Scientific Sling (Psychrmeter) Cmbustin Instruments Cntinuus CO-2 Analyzer Thermc Instruments Cntinuus C2 Analyzer. FPCS fuel il cmbustin results in preburner system, inhibitr studies. catalyst has cmbustin shwn the fllwing catalyst, and slag 1. As a preburner cnditiner, n sludge has been fund in test site 'I', (see Table i), which has been treating fr mre than ne year. Only the nrmal dirt and sand trash have been trapped by existing strainers, and strainers remained clear f water and sludge buildup. 6

8 2. As cmbustin catalyst, the primary desired effect is t reduce excess xygen (2), and thereby imprve biler efficiency. In Table 1, several test sites shw reductin in excess xygen t a pint where FPCS was cst-effective when cnsidering nly increased biler efficiency. Other sites shwed nly marginal results, due primarily t the high quality f fuel being burned, the type f atmizatin burner, r limited range biler cntrls. 3. As a slag inhibitr fr fireside depsits, the testing reveals majr reductins in the rate f acid buildup, (RBU). Figures 9, 1, 11, and 12 plt results in bth water and firetube bilers. TABLE 1 Biler Efficiency Imprvements Due t FPC-l Biler Biler Efficiency * + Site Fuel #/Hr Rated #/Hr Ave As Fund Baseline With FPC-l Gain Gain A N.6 58, 28, B N.5 Ht il 5% C N.5 21, 7, D N.6 25, 7, E N.6 17,5 1, F N.6 9, 72, N.6 9, 9, G N.5 3,5 1, H N.6 4, 3, I N.5 2,7 7, * As Fund - Operatin befre adjustments r additives. + Baseline - Operatin with adjustment with n additive. OBSERVATIONS 1. The use f FPCS reduces the rate f H2S4 generatin and ptential acid crrsin. 2. If H2S4 generatin was reduced with all ther parameters held cnstant, such as 2, C2, CO, stack temperatures and ambient weather cnditins, then FPCS reduced the frmatin f S2 t S3. 3. Recalling that S2 xidizes t S3 thrugh the catalyzatin f vanadium pentxide and irn xide: 7

9 V S2 + 1/22 S3 Fe23 it can further be cncluded that the vanadium pentxide frmatin has been greatly reduced r even eliminated. At all test sites, using FPC-l t reduce vanadium pentxide depsits, the nrmal six mnths cleaning is nw nce a year. The biler is pened fr insurance inspectin at this yearly inspectin. With FPCS as an xidizing catalyst, which uses the free 2 fr mre cmplete cmbustin, it fllws that V23 and V24 cannt xidize t V25. Frm the abve equatin, it is nted that irn xide (Fe23) als is a catalyst, and therefre the irn in FPCS culd react t help frm S3. Hwever, the irn in the ferrus picrate f FPCS has a chemical valence f +2 cmpared with the valence f +3 fr Fe23. Therefre, the irn cmpunds f FPC-l d nt prmte S3 frmatin. The rate f acid buildup (RBU), related t H2S4, S3, and V25 frmatin, will varying depending upn the biler size, flame temperature and type. Frm previus discussins n atmic xygen in large high-temperature bilers, it wuld be expected t have high RBU and increased S3 frmatin, as shwn in Table 6. Nte als that varius additives are already being used, and the RBU is still high. TABLE 2 Dewpint-Meter Test Results Frm Oil-Fired Unit (ASME Paper 63-WA-18) -Peak Air Percent Cld-end Peak-rate rate, Remarks preheater sulfur average Dewpint temp #,&/min Crrsin lcatin Miscellaneus QO-215 F 249 F 213 F 96 Middle f cld layer Mg(OH), (ld unit) f Middle f cld layer Mg(OH), (ld unit) f Middle f cld layer Mg(OH), (ld unit) QO-215 F Middle f cld layer M(OH)dld unit) Middle f cld layer D mite Middle f cld layer N ne in heater Lime slurry Lime slurry, gas Lime slurry, gas Lime alurry, gas Ductwrk after gas Lime alurry, PI ut N prblem Gas fired cntinuusly Mvable Operatin with cld end ut Cld end f cld int Changeable Cld end f cld int Elements temp swing culd accunt fr it Cld end f int. layer Mg(OH)" int, layer ut Cld end f cld Cmb. firing, lime slurry layer General in cld layer Higher exceiii air than 118 (CI als)

10 DISCUSSION Test results shwing the effect f FPCS n RBU in il-fired package bilers are given in Figure 9, which shws a typical water tube package biler, and Figure 8, a smaller fire tube unit. In bth figures, FPCS substantially reduced the RBU at all temperatures, as cmpared t fuel withut any treatment. The il fired was typical N.6 East Cast il, with 2.1% sulfur and 2-3 ppm vanadium. Observatins frm Figure 9 and 1 relate t the RBU and firing rate respnse time in treating with FPCS, and crrsive tendencies. As expected, with the increase in firing rate, there will be an increase in the mass flw rate f H2S4, since mre fuel is being burned and mre sulfur mved thrugh the biler firebx. Nte: On Figure 1, 7/1/81, 68 #/hr and 6.% 2; even with FPCS, there is a saturatin pint f high excess 2 which nullifies the use f an xidizing catalyst. The actual saturatin pint will vary frm biler t biler, depending upn the firing rate and burners. Next, it has been bserved that the additin f FPCS t a system requires a respnse r cnditining time. Immediately upn intrducing FPCS, imprvements in 2 and the flame can be detected, but the full effect may take ne mnth t develp. Thus far, n crrelatin has been established between final results, as it differs frm biler t biler, regardless f size, lad, existing depsits r burner assembly. Finally, in testing perfrmed by Clark and Childs in Reference N.1, experience indicates that appreciable crrsive tendencies d nt becme apparent until RBU ges ver 1 micramps per minute. Enugh infrmatin is still nt available t be cnclusive regarding 1 micramps as being the abslute breaking pint. Figure 1 indicates that fire tubes have n "appreciable" prblems, but this is nt the case. Hwever, fr larger water tube and utility bilers, the 1 micramp RBU tends t agree with past perfrmance and maintenance recrds. In cmparing FPCS with MgO additives, findings shw FPCS t be 5% mre effective in reducing sulfuric acid prblems, and equal t MgO in reducing vanadium pentxide fireside depsits. This is based n using bth FPCS and MgO at the factry-prescribed mixing ratis. Figures 11 and 12 shw that the RBU is 9

11 lwer using FPCS, and therefre, acid prblems (r cld-end crrsin) have been reduced r eliminated. The vanadium pentxide prblem was cntrlled well by bth FPCS and the MgO additives, as lng as prper mixing ratis were maintained. Anther fact regarding Figures 11 and 12, is the RBU and the type f biler. All watertubes tested t date shw a 5% reductin in RBU frm untreated fuel t that f fuel with a MgO additive. Hwever, in reviewing the firetube biler perfrmance, (Figure 12), the RBU with MgO is actually higher than the untreated fuel; while with FPCS, the RBU is essentially "". FPCS wrks better in the firetube biler due t the higher firebx pressure and the elngated flame pattern, which allws fr mre resnance time fr FPCS t react with the fuel. 1

12 APPENDIX 1

13 Date 125 Fuel: N 6 Oil ()--() % Sulfur ppm Vanadium N Additives m "': LIJ BASELINE BASELINE FPC-3 FPC-3 FPC-3 FPC-3 4, #/hr 4.2%-2-1ppm CO 6, #/hr 5.2% 2- Oppm CO 4, #/hr 3.1% 2- Oppm CO 42, #/hr 3.7% 2- Oppm CO 44, #/hr 3.1% 2- Oppm CO 6, #/hr 5.5% O 2 Om CO r ::J z H "t "'1 c ::; U H 5 ;::J 2S /.'\ El I {.} 1:.) PROBE TEMPERATURE - Degree F CJ Ii Ii CIJ t-->. ". ;:j "Tj - H G') CIl,,* rt e :;d '-r rr::t ;:j tr1 r Ii rr.... >- t-' rt I r <: Ii CIJ I c t-i - r r :3 " r Ii rr c Ii r

14 Date Fuel: N 6 Oil BASELINE 3% Lad 3.1% 2-86ppm CO 2.17 Sulfur BASELINE 5% Lad 4.6% 2 7ppm CO 2-35 ppm Va 8 BASELINE 67Lad ppm CO MgO Baseline 7-83 FPC:-3 3% Lad 2.8% 2-5ppm CO 6- FPC-3 5% Lad 3.4% 2 Oppm CO FPC-3 67Lad 5.7% 2 Oppm CO 125 A (f) c, I U H 1 (J ti ti p. Cfl..... ::3 r 1-'.-... r 75 C/l=!;:: rr n-:t ::3 r ti n-... :i /-' rr I r 5 ti <: Cfl / 2 25 A '\. -, -, \. "." -, -, \. -, " -, '\. -, \ PROBE TEMPERATURE - Degree F.

15 Date BASELINE 5% Lad 2.1% 2 Fuel: N 6 Oil 2.11 Sulfur a r:j BASELINE 2% Lad 1.8% 2 25 ppm Va FPC:-3 5% Lad' 3.4% 2 -. MgO Baseline., FPC.,c.3 5% Lad 2.9% 2 H z H <, 1 75 (J) a, <: I u H 5 L ::J 2S (!l I-" (') Pi Pi Ul W 1-" '" OJ ;j '1j "rj {J),,* n- H 1-' r G1 n:::r ::l c::: r Pi n- 1-'- M - ()>-rj I-' W - 1-"- Pi I r <: n Ul c I - r r3 '" It) Pi n- c Pi It) \!? PROBE TEMPERATURE - Degree F.

16 Fuel: N 6 Oil 2.1.% Sulfur ppm MgO Baseline Date Va BASELINE FPC-3 FPC-3 32 #/hr 5.5% 2- Oppm CO 32 #/hr 4.8% 2- Oppm CO 32 #/hr 4.6% 2- Oppm CO 25 c...1 H!::l Z H a: f) 15 c U H ;:::J ::l 1 () Ii ri CJl r" w ;:3 VI r (f)::;j:: 11' r'" -... r rr;:t ;:3 rli 11' III 1::;l'"rj. Ii I r < 11' CJl C " I r r3 'TI H C) +: _ "- --: = ="'(.) W 28 '" rii III 11' C Ii r PROBE TEMPERATURE - Degree F.

17 -I GENERAL INFORMATION Regarding Sulfur, Vanadium, Sdium and Biler Operatin Fuel-il Ash The ash cntent f residual fuel il seldm exceeds.2%, an exceedingly small amunt cmpared t that in cal. Nevertheless, even this small quantity f ash is capable f causing severe prblems f external depsits and crrsin in bilers. Of the many elements that may appear in il-ash depsites, the mst imprtant are vanadium, sdium and sulfur. Cmpunds f these elements are fund in almst every depsit in bilers fired by residual fuel il and ften cnstitute the majr prtin f these depsits. Origin f Ash As with cal, sme f the ash-frming cnstituents in the crude il had their rigin in animal and vegetable matter frm which the il was derived. The remainder is extraneus material resulting frm cntact f the crude il with rck structures and salt brines, r picked up during refining prcesses, strage and transprtatin. In general, the ash cntent increases with increasing asphaltic cnstituents in which the sulfur acts largely as a bridge between armatic rings. Elemental sulfur and hydrgen sulfide have been identified in crude il, and simpler sulfur cmpunds are fund in the distillates f crude il including thi-esters, disulfides, thiphenes and mercaptans. Vanadium, irn, sdium, nickel and calcium in fuel il were prbably derived frm the rck strata, but sme elements such as vanadium, nickel, zinc and cpper prbably came frm rganic matter frm which the petrleum was derived. Vanadium and nickel especially, are knwn t be present in rgana-metallic cmpunds knwn as prphyrins which are characteristic f certain frms f animal life. Table 1 indicates the amunts f vanadium, nickel and sdium present in residual fuel ils frm varius crudes. TABLE #1 Vanadium, nickel and sdium cntent f residual fuel ils (parts per millin by weight) Surce f Crude Oil Vanadium Nickel Sdium Africa Middle East United States (Cntinued ) 1

18 (Cntinued.. ) TABLE #1 Surce f Crude Oil Vanadium Nickel Sdium Venezuela Crude il as such is nt nrmally used as a fuel, but is further prcessed t yield a wide range f mre valuable prducts. Fr example, in a mdern U.S. refinery, the average prduct yield, as a percentage f ttal thrughput, is: Gasline Lube il fractin Jet fuel Kersine Distillates Residual fuel Virtually all metallic cmpunds and a large part f the sulfur cmpunds are cncentrated in the distillatin residue, as illustrated fr sulfur in Table 2. Where lw-sulfur residual fuel ils are required, they are btained by blending with suitable stcks, ncluding bth heavy distillates and distillatin frm lw-sulfur crudes. This prcedure is als used ccasinally if a residual fuel il must meet specificatins such as vanadium, r ash cntent. TABLE #2 Sulfur cntent in fractins f Kuwait crude il Fractin Crude il Gasline Light naphtha Heavy naphtha Kersine Light gas il Heavy gas il Residual il Distillatin Range, F Tta 1 Su lfur 1 by Weight Surce,Articleby F.E. Hixn, Shell Refiningand MarketingC., Ltd.,"Chemistryand Industry",March26, 1955;page333. 2

19 Release f Ash During Cmbustin Residual fuel il is preheated and atmized t prvide enugh reactive surface s that it will burn cmpletely within the biler furnace. The atmized fuel il burns in tw stages. In the first stage, the vlatile prtin burns and leaves a prus cke residue; and in the secnd stage, the cke residue burns. In general, the rate f cmbustin f the cke residue is inversely prprtinal t the square f its diameter, which in turn is related t the drplet diameter; refer t Figure #1. Thus, small fuel drplets give rise t cke residues that burn very rapidly and the ash-frming cnstituents are expsed t the highest temperatures in the flame envelpe. The ashfrming cnstituents in the larger cke residues frm the larger fuel drplets are heated mre slwly, partly in assciatin with carbn. Release f the ash frm these residues is determined by the rate f xidatin f the carbn. Figure #1 DROP OR PARTICLE DIAMETER, MICRONS COMBUSTION TIME AS FUNCTION OF PARTICLE SIZE AND TYPE OF FUEL 1r r------r_ / / u uj t/) 1 f- L:> cc.1 Te t ra l ine.1 TetraJinc 111 Flarres Drps O.OOlOL lOO------l.OOO---- lo.ooo---- Surce,Reprt by OJ. Bum ett, BSc; Babcck&WilcxCanada Ltd., ''Develpnent& De sigi f lw &cess Air Oil Burners",April2, 197 3

20 During cmbustin, the rganic vanadium cmpunds in the residual fuel il thermally decmpse and xidize in the gas stream t V 2 3, V 2 4 and finally V 2 S ' Althugh cmplete xidatin may nt ccur and there may be sme dissciatin, a large part f the vanadium riginally present in the il exists as vapr phase V?OS in the flue gas. The sdium, usually present as chlride in tne il, vaprizes and reacts with sulfur xides either in the gas stream r after depsitin n tube surfaces. Subsequently, reactins take place between the vanadium and sdium cmpunds with the frmatin f cmplex vanadates having melting pints lwer than thse f the parent cmpunds; fr example: (4) Na 2 S 4 + V 2 S Melting Pints 162SF 127SF 2NaV 3 + S3 i 116SF Excess vanadium r sdium in the ash depsit, abve that necessary fr the frmatin f the sdium vanadates (r vanadyl vanadates), may be present as V 2 S and Na 2 S 4, respectively. The sulfur in residual fuel is prgressively released during cmbustin and is prmptly xidized t sulfur dixide (S2)' A small amunt f sulfur dixide is further xidized t S3 by a small amunt f atmic xygen present in the httest part f the flame. Als, catalytic xidatin f S2 t S3 may ccur as the flue gases pass ver vanadium-rich ash depsits n high-temperature superheater tubes and refractries. *Vanadium Pentxide Frmatin Vanadium, which is present in fuel il as sluble rganic cmplexes, decmpses during cmbustin. The initial prduct f vanadium cmbustin is vanadium trixide" V 2 3, with a melting pint f 3S78 F. If this frm f vanadium remains, it will pass thrugh the biler as a dry dust causing n slagging r crrsin prblems. If further xidatin ccurs, then: 2V V 2 4 (Vanadium Tetrxide-melt. pt. 3S78F) Vanadium tetrxide is als a dust and easily carried ut f the biler. Hwever, further xidatin accelerated by high levels f excess air, will result in the frmatin f vanadium pentxide: 2V V2S Vanadium Pentxide-melt. pt. 1247F) The v a n a d i urn p c n t. x i d,1:1y als crib i nc- w i r h s d i um, nickel.r n d i r n t frm ther lw-melting pint vanadate cmpunds. Lw-melting vanadate depsits are mst severe n the biler surfaces first cntacted by mlten vanadium cmunds, supprts and hangers, the furnace and the first rws f cotectin tubes are usually severely affected by vanadium depsits. The immediate result is increased draft lss and increased stack temperatures. The inevitable result is the tremendus maintenance burden when attempting t remve the hard vanadium slag. 4

21 In additin t the depsits, vanadium cmpunds als create crrsin prblems. Crrsin ccurs when the metal xide cating which nrmally prtects metal surfaces is disslved by the vanadates. Oxidatin f the metal fllws frm the xygen available in the cmbustin gases and then the cycle repeats until the metal surface fails. The refractry burner thrats can als be attacked by vanadium if burners are nt prperly set. The vanadium can diffuse back int the prus refractry and can be absrbed; and since the slag depsits and refractry expand and cntract at different rates, the thrat can crack r spall. Therefre, vanadium is respnsible fr mst ht-sectin slag depsits and crrsin prblems in the biler fireside. *Sulfuric Acid Frmatin Fuel il nrmally cntains sulfur; either in the elemental frm, r as hydrgen sulfide, nercaptan sulfides, disulfides, thiphenes, r plysulfides. This sulfur reacts with xygen during fuel il cmbustin t frm sulfur dixide: S + 2 _ Abut ne (1) t five reacts with available S2 (5) percent f xygen t frm the sulfur dixide then sulfur trixide: V 2 i S2 + 2 S3 Fe 2 3 The amunt f S3 frmed is therefre a functin f the level f excess air present. Furthermre, the frmatin f SO is catalyzed by vanadium pentxide and irn xide, as shwn y the abve equatin. Finally, the S3 cmbines with misture present in the flue gas t frm sulfuric acid when the cmbustin gases are cled belw the acid dewpint: S3 + H 2 H 2 S 4 There are tw details that nw require mre explainatin with respect t S3 frmatin, depending upn the size f the biler. Typically, in a large utility biler the flame temperatures are higher than a small package biler. As a result, a phenmenn ccurs where xygen (2) is brken t "", r atmic xygen, which will adhere t any free atms and therefre frm S3 in the httest part f the flame. This temperature is estimated at 34 F-38F. Hwever, in the smaller gackage bilers, the flame temperatures are nrmally 24 F-32 F, and S3 frmatin will be the direct result f excess 2 cmbined with vanadium r irn catalyzatin. 5

22 Oil-slag Frmatin and Depsits The depsitin f il-ash cnstituents n the furnace walls and superheater surfaces has been a serius prblem in recent years. This depsitin, cupled with crrsin f superheater and reheater tubes by depsits, was largely respnsible fr the break in the trend twards higher steam temperatures that ccurred in the early 196's. Practically all biler installatins are nw designed fr steam temperatures in the 1-11SF range t minimize thse prblems and t avid the higher capital csts f the mre expensive all 6s required in tubes, steam piping and turbine fr 1SO- 11 F steam cnditins. There are many factrs affecting il-ash depsitin n biler heat absrbing surfaces. These factrs may be gruped int the fllwing interrelated categries: 1. Characteristics f the fuel il 2. Design f the biler 3. Operatin f the biler Characteristics f Fuel-il Ash Sdium and vanadium are the mst significant elements in the fuel il because they can frm cmplex cmpunds having lw melting temperatures, 48-12S F, as shwn in Table 3. Such temperatures fall within the range f tube-metal temperatures generally encuntered in furnace and superheater tube banks f many il-fired bilers. Hwever, because f its cmplex chemical cmpsitin, fuel-il ash seldm has a single sharp melting pint, but rather sftens and melts ver a wide temperature range. An ash particle that is in a sticky, semi-mlten state at the tube-surface temperature may adhere t the tube if it is brught int cntact by the gas flw ver the tube. Even a dry ash particle may adhere due t mutual attractin r surface rughness. Such an initial depsit layer will be at a higher temperature than that f the tube surface because f its relatively lw thermal cnductivity. This increased temperature prmtes the frmatin f adherent depsits. Thus, fuling will cntinue until the depsit-surface temperature reaches a level at which all f the ash in the gas stream is in a mlten state s that the surface is merely washed by the liquid withut freezing and cntinued buildup. 6

23 TABLE #3 Melting pints f sme il-ash cnstituents Cmpund Aluminum xide, Al 2 3. Aluminum sulfate, AI2(S4)3 Calcium xide, CaO Calcium sulfate, CaS 4 Ferric xide, Fe 2 3 Ferric sulfate, Fe2(S4)3 Nickel xide, NiO Nickel sulfate, NiS 4 Silicn dixide, Si 2 Sdium sulfate, Na SO Sdium bisulfate, aho Sdium pyrsulfate, Na22 Sdium ferric sulfate, Na3Pe(s4)3 Vanadium trixide, V Vanadium tetrxide, 264 Vanadium pentxide, V 2 5 Sdium metavanadate, a2'v2(nav3) Sdium pyrvanadate, 2Na 'V 2 2 S Sdium rthvanadate, 3Na.V Sdim vanadylvanadates Na O V 'V 5N2 2b4 tl25 Melting Pint, F *Decmpses at a temperature arund the melting pint "< '"' '"' < 75'"' In experimental furnaces, it has been fund that the initial rate f ash buildup was greatest when the sdium-vanadium rati in the fuel il was 1 t 6, but an equilibrium thickness f depsit (1/8 t 1/4 in. thick) was reached in apprximately 1 hurs f peratin. When the fuel il cntained mre refractry cnstituents such as silica, alumina, and irn xide in additin t sdium and vanadium, an equilibrium cnditin was nt reached and the tube banks ultimately plugged with ash depsits. Hwever, these ash depsits were less dense; ie, mre friable than the glassy slags encuntered with a 1 t 6 sdium-vanadium fuel il. Bth the rate f ash buildup and the ultimate thickness f the depsits are als influenced by physical factrs such as the velcity and temperature f the flue gases and particularly the tube-metal temperature. In predicting the behavir f a residual il insfar as slagging and tube-bank fuling are cncerned, several fuel variables are cnsidered including, (1) ash cntent, (2) ash analysis, particularly the sdium and vanajium levels and the cncentratin f majr cnstituents, (3) melting and freezing temperatures f the ash, and (4) the ttal sulfur cntent f the il. Applying this infrmatin in biler design is largely a matter f experience. 7

24 Biler Design Generally speaking, prgressive fuling f furnaces and superheaters shuld nt ccur if the tube-metal temperatures d nt exceed looof. If such truble is encuntered, the slutin can usually be fund in imprving cmbustin cnditins in the furnace and/r mdifying the stblwing prcedures. Studies n bth labratry and field installatins have shwn that the rate f ash depsitin is apprximately prprtinal t the velcity and temperature f the flue gases, and the cncentratin f il-ash cnstituents in the flue gases. The gemetry f the furnace and the spacing f tubes in the cnvectin banks are selected in the design f a biler t minimize the rate f depsitin. It is cmmn practice t use in-line t arrangements with prgressively wider lateral spacings fr tubes lcated in higher gas-temperature znes. This makes bridging f ash depsits between tubes less likely and facilitates cleaning f tube banks by the stblwers. Biler Operatin pr atmizatin f the fuel il results in lnger flames and frequently increases the rate f slag buildup n furnace walls which, in turn, makes it mre difficult t keep the cnvectin sectins f the biler clean. Cmpleting cmbustin befre the gases pass ver the first rw t tubes is especially imprtant. Relatively large carbnaceus particles have a far greater tendancy t impinge n the tubes than d the smaller ash particles. If these larger particles are in a sticky state, they will adhere t the tubes where xidatin will prceed at a slw rate with cnsequent frmatin f ash. Fuling frm this cause is difficult t detect by inspectin during biler utages because the carbnaceus material has usually disappeared cmpletely. It can generally be detected during peratin since flames are uslly lng and smky and sparklers may be carried alng in the fl gases. Regular and thrugh stblwing can have a decisive effect n superheater and reheater fuling. T be fully effective, hwever, stblwing cycles shuld be frequent enugh s that ash depsits cannt build up t a thickness where their surfaces becme semimlten. If this pint is reached, cmplete remval f the ash depsit can be very difficult because the stblwers nw have a dual task t perfrm: (1) t cl the surface f the depsit belw its freezing temperature and (2) t shatter the nw slid mass f slag and break its bnd t the tube surface. In instances f extreme slagging, it is smetimes necessary t relcate stblwers, install additinal stblwers t cntrl depsitin in a critical zne, r t use additives. 8

25 The biler lad cycle can als have a significant effect n the severity f slagging and superheater fuling. A unit that is base-laded fr lng perids is mre apt t have fuling prblems n a brderline fuel il than a unit that takes daily swings in lad. In the latter instance, the 'f urn ace generally remains cleaner due t peridic shedding f slag, with the result that the gas temperatures thrugh the superheater are appreciably lwer. This eases the burden n the stblwers and ges a lng way twards cntrlling ash-depsit frmatin in ilie superheater-reheater tube banks. Overlading the biler, even fr an hur r tw a day, shuld be avided, especially if excess air has t be lwered t the pint where sme f the burners are starved f air. The furnace is apt t becme slagged and ash depsitin creeps int the superheater and reheater tube banks. Oil-Ash Crrsin High-temperature crrsin The sdium-vanadium cmplexes, usually fund in il-ash depsits, are crrsive when mlten. The crrsin mechanism is prbably ne f accelerated xidatin f metal brught abut by xygen transfer t its surface by the cnstituents in the mlten ash, accmpanied by the remval by the ash f the nrmal prtective xide cating n the metal surface. Crrsin can als be caused by sulfate attack, particularly when sdium (r sme ther) chlride is als present in the fuel il, and this may ccur at metal temperatures as lw as 1F. This type f crrsin is mre apt t be encuntered n bilers burning a lw-vanadium fuel il, but cntaining several hundred ppm f sdium chlride. Even when the chlride cntent f the fuel il is negligible, sulfate crrsin may still be severe when reducing r alternating xidizing-reducing cnditins prevail arund the tubes. A measurable crrsin rate can be bserved ver a wide range f metal and gas temperatures, depending n the amunt and cmpsitin f the il-ash depsit. Fig. 2 shws the cmbined gas and metal temperature effects n crrsin fr a specific fuel il cmpsitin f 15 ppm vanadium, 7 ppm sdium and 2.5% sulfur. As the vanadium cncentratin f the fuel il varies, the amunt f crrsin, cmpared t a lso-ppm vanadium fuel, will increase r decrease accrding t the curve shwn in Fig. 3. 9

26 Figure # >- '" :::;: "" I '" 2 s -' 1: DO s,;$ c: 1/ Pj J / / /,# V /,,-v,'" :: / ","' ) 7>\.,i'\'" ';1'(', (jj l3-1 l.---./ V - V/i\nl--- :::...- \\l -I---l f V/' - 12 F Gas Firing Gas Temperature, Effect f gas and metal temperatures n crrsin f 34, 316 and 321 allys in a unit fired with il cntaining lsoppnvanadiun, 7ppn sdiun and 2.5% sulfur. Test duratin 1 hurs E ::> :. '" 1 E a. a. 8.z '" n; 6 4i c: c: 'Vi e 4 5 u * 2 V / / V / V Figure #3 F I- / Vanadium in Fuel, ppm I I I I I I Effect f vanadiun crcentrat:bn n il-ash crrsin. 1

27 The effect f the sdium level in the fuel il is nt quite s clear-cut because cmbustin cnditins and the chlride cntent f the fuel il may be cntrlling. The sdium cntent des, hwever, definitely effect the minimum metal temperature at which crrsin will be significant. At the present time, there des nt appear t be any ally that is immune t il-ash crrsin. In general, the higher the chrmium cntent f the ally, the mre resistant it is t attack. This is the main reasn fr the use f ISCr-SNi allys fr high temperature superheater tubes. High chrmium cntents, greater than 3%, give added crrsin resistance, but at the expense f physical prperties; 25Cr-2Ni has been used as a tube cladding, but even this ally has nt prvided cmplete prtectin. The presence f nickel in high-temperature allys is needed fr strength. High nickel allys may be fairly resistant t il-ash attack under xidizing cnditins, but they are liable t sulfide attack brught abut by lcal reducing cnditins r by the pesence f chlride in the ash depsit. Since it is difficult t avid such cnditins entirely, high-nickel cntent f allys may be f limited value. In any event, the higher material cst must be justified by lnger life, which is nt always predictable. Lw-temperature crrsin In il-fired bilers, the prblem f lw-temperature crrsin resulting frm the frmatin and cndensatin f sulfuric acid frm the flue gases is similar t that previusly described fr cal firing. Oil-fired bilers are mre susceptible t lw-temperature crrsin than are mst cal-fired units fr tw reasns: (1) the vanadium in the il-ash depsits is a gd catalyst fr the cnversin f S2 t S3 and (2) there is a smaller quantity f ash in the flue gases. Ash particles in the flue gas reduce the amunt f S3 vapr in the gas. Since il has cnsiderably less ash than cal, significant differences wuld be expected. Furthermre, cal ash is mre basic than il ash and tends t neutralize any acid depsited; il ash generally lacks this capability. 11

28 Under certain cnditins, il-fired bilers may emit acidic particulates frm their stacks that stain r etch painted surfaces in the neighbrhd f the plant. The acidic psits r smuts are generally caused by metallic surfaces (air heaters, flues and slacks) perating well belw the acid dew pint f the flue gases r by st which has absrbed sulfuric acid vapr in its passage thrugh the biler. Methds that can be used t prevent acid-smut emissin include: 1. Minimize S1 frmatin in the flue gases, 2. Neutralze S3 in flue gases, 3. Maintain all surfaces in cntact with the flue gases abve abut 25 F and, 4. Cmpletely burn fuel il t eliminate st particles Lw-Tmperatur Crrsin and Percnt Sulfur In rder t reduce lw-temperature crrsin, it is necessary t use fuel with a sulfur cntent belw.3%, r prevent the frma- Lin f S3' Cld-end crrsin can exist even with nly 1% sulfur il, since the rale f cnversin f SO L S3 increases as sulfur cntent decreases; as shwn in Figures 4 and 5. Methds f cntrl The methds n cntrl that have been used r prpsed t cntrl fuling and crrsin in il-fired bilers are summarized in Table 3, but in every instance ecnmics gverns their applicability. There is n dubt that reducing the amunt f ash and sulfur entering the furnace is the surest means f cntrl, and that minimizing the effects f the ash cnstituents, nce they have depsited n the tubes, is the least reliable. Since Lhe severity f fuling and crrsin depends nt nly n the fuel-il characteristics, but als n biler design and perating variables, a generalized slutin t these prblems canhnt be prescribed. TABLE #4 Classificatin f methds fr cntrlling fuling and crrsin in il-fired bilers Reduce amunt f fuel ash cnstituents t the furnace Minimize amunts f fuel ash cnstituents reaching heat transfer surfaces Minimize effects f bnding and crrsive cmpunds in ash depsits 12 Fuel Oil Supply <Selectin Blending Purificatin Design <Furnace gemetry Tube bank arrangement Metal temperature Gas temperature Stblwer arrangement eratin Lad cycle Stblwing schedule Cmbustin-Excess air Additives Water washing

29 Figure #4 IVOr , ::(8 Q. Q. I '".. >-6 a: a... >- 4 >- z u "'2 EXCESS AIR 25"1. GAS SAMPLES FRO"-' COM9uSTION CHAMBER (183 GAS S...,PLES FROl.4 COOL CH48ER (6O F),.. I OL- L- L- L- 2 :3 4 5 ruel SULPHUR CONTENT- % WEIGHT Experimental furnace - variatin f SO. cntent f flue ga with sullur cntent 1 fuel. Cmbustin. Aug. U Figure #5.J > ". I 14 EXCESS AIR 25%..s SAMPLES FRQY CQIrrIBUSTION (HAtr.tBER (IBJ,O f) GAS SAMP(LS fro COOL CHA""8(R ( 6O F) g... z 6 Vi a: z u 2 2 :3 4 5 ruel SULPHUR CONTENT-.1. WEIGHT 6 variatins f flue gas t sulfur cntent, same paper, Cmbustin. Aug Rendle. Wilsden and Whittingham 13

30 Fuel il supply Althugh fuel selectin and blending are practiced t sme extent in this cuntry, it is dne t prvide safe and reliable handling and strage at the user's plant rather than t avid fuling difficulties. Since the threshld limits f sdium, su lf ur and vanadium are nl accuralely de Li u d l r c i t h r luluu; <'I l'i1lttlsil111, utilizatin f these means f cntrl cannt be tully explited. Prcesses are available fr bth the desulfurizatin and ashing f fuel ils. Water washing f residual fuel il has been successfully applied t a few marine-type bilers, but it is dubtful that it will be widely used because nly sdium and sediment, mainly rust and sand are remved by the prcess. Use f lsulfur crudes and desulfurized fuel il is expected t increase. Fuel il additives The practice f water washing ut f service and, t a limited extent, in service has been beneficial in vercming sme f the trubles experienced with present il fuels. In additin, cntinued study f the prblem has revealed anther apprach that is effective where the fuel-il ash is mst trublesme. In brief, the methd invlves adding t the fuel r furnace small amunts f materials that change the character f the ash sufficiently t permit its remval by steam r air stblwers r air lances. Additives are effective in reducing the trubles assciated with superheater fuling, high-temperature ash crrsin, and lwtemperature sulfuric acid crrsin. Mst effective are alumina, dlmite and magnesia. Kalin is als a surce f alumina. Analyses f typical superheater depsits frm a trublesme fuel il, befre and after treating it with alumina r dlmite, are shwn in three bar graphs at the left in Fig. 6. The results fr a different il treated with magnesia are shwn in the bar graph at the right. The reductin f fuling and high-temperature crrsin is accmplished basically by prducing a high melting-pint ash depsit that is pwdery r friable and easily remved by stblwers r lances. When the ash is dry, crrsin is cnsiderably reduced. Lw-temperature sulfuric acid crrsin is reduced by the frmatin f refractry sulfates by reactin with the S3 gas in the flue gas stream. By thus remving the S3 gas, the dew pint f the flue gases is sufficiently reduced t prtect the metal surfaces. The sulfate cmpunds frmed are relatively dry and easily remved by the nrmal cleaning equipment. 14

31 Figure # Oil 11 1 (Withut Additive With Alumina Additive Dlmite '\ p!i!2i!l:=t,fez3 Undeter. ;;;;Si2-i== AI,O) F i44! +A MgO Oil #2- Treated s-, 6.D If? C :;:; 5 Vi 8. E u 4 3 MgO,?::;: Ca1 2 1 _ Trublesme cnstituents "High-Vanadium Oil Effect f fuel il additivesn cmpsitinf il ash depsit In general, the amunt f additive used shuld be abut equal t the ash cntent f the fuel il. In sme instances, slightly different prprtins may be required fr best results, especially fr high-temperature crrsin reductin, in which it is generally accepted that the additive shuld be used in weight ratis f 2 r 3 t 1, based n the vanadium cntent f the il. Several methds have been successfully used t intrduce the additive materials int the furnace. The ne in general use cnsists f metering a cntrlled amunt f an additive il slurry int the burner supply line. The additive material shuld be pulverized t 1% thrugh a 325-mesh screen (44 micrns) fr gd dispersin and minimum atmizer wear. Fr a biler fired by a high-pressure return-flw il system, it has been fund advantageus t intrduce the additive ders by blwing them int the furnace at the desired lcatins. The pwder has t be 1/ thrugh a 325-mesh screen fr gd dispersin. 15

32 A third, and mre recent methd, is t intrduce the additive as a water slurry thrugh specially adapted stblwers r lances. This methd ffers the advantage f applying the additive in exactly the lcatin desired, with a pssible reductin in the quantity required. Sme cautin shuld be bserved with this system t prevent pssible thermal shck (quench-cracking) ge t the ht tubes. The presence f chlrides in the water slurry, frm either the water r the additive material, culd pssibly prduce stress-crrsin cracking f austenitic tubing and shuld be cnsidered. The chice f the particular additive material depends n its availability and cst t the individual plant and the methd f applicatin chsen. Fr example, alumina causes greater sprayerplate wear than the ther materials when used in an il slurry. The quantity f depsit frmed is, f curse, an imprtant cnsideratin fr each individual unit frm the aspect f cleaning. A cmparisn f the amunts f depsit frmed with different additives shws that dlmite prduces the grealesl quanlity because f its sulfating ability; alumina and kalin frm the least, and magnesia is intermediate. Hwever, when adequate cleaning facilities are available, the depsits are easily remved, and the quantities frmed shuld nt be a prblem. Excess air cntrl As mentined previusly, the prblems encuntered in the CaIDustin f residual fuels - high-temperature depsits (fuling), hightemperature crrsin, and lw-temperature sulfuric acid crrsinall arise frm the presence f vanadium and sulfur in their highest states f xidatin. By reducing the excess air frm 7% t 1 r 2%, it is pssible t avid the frmatin f fully xidized vanadium and sulfur cmpunds and, thereby, reduce biler fuling and crrsin prblems. In a series f tests n an experimental biler, it was fund that the maximum crrsin rate f type 34 stainless steel superheater ally held at 125 F in 21 F flue gas was reduced mre than 75% (see Fig. 7) when the excess air was reduced frm an average f 7% t a level f 1-2%. Mrever, the ash depsits that frmed n the superheater bank were sft and pwdery, in cntrast t hard, dense depsits that adhered tenaciusly t the tubeswhen the excess air was arund 7%. Als, the rate f ash buildup was nly half as great. Operatin at the 1-2% excess air level practically eliminated lw-temperature crrsin f carbn steel at all metal temperatures abve the water dew pint f the flue gases (Fig.8). Hwevec, much f the beneficial effects f lw excess air cmbustin qre lst if the excess air at the burner fluctuates even fr shrt perids f time t a level f abut 5%. Carbn lss values fr lw excess air were apprximately.5%, which is generally acceptable fr electric utilityand industrialpractice. 16

33 LOri , Figure # I c. e u.4 E r.2! ((Cet'l r. Lw Excess Air Nrmal Excess Air Test N Effect f lw excess air cmbustin n high-temperature 12 Figure # 8 I.. 2.3% Sulfur In Oil il ash crrsin 1 >.8 V> I < '" 6 '"c. e 84 2, f\ \ 6% 8% Excess Air \ \ \ \ " \ YA\ \. ---;,;( II \ \ -.2,.../ \\ 1%1.5% Excess (r, 1 I I%(EXCm(Ir ''I "- D..Q. / u Metal Temperature, F Effect f excess air n lon-temperature crrsin f carbn steel 17

34 A number f large industrial bilers, bth in this cuntry and in Eurpe, have been perating with lw excess air fr several years. As a result, the benefits f reducing lw-temperature crrsin are well established. Hwever, the benefits n hightemperature slagging and crrsin are nt whlly cnclusive. In any event, great care must be exercised t distribute the air and fuel il equally t the burners, and cmbustin cnditins must be cntinuusly mnitred t assure that cmbustin f the fuel is cmplete befre the cmbustin gases enter the cnvectin tube banks. The abve infrmatin is an excerpt (except where nted with *) frm the 39th editin f "STEAM", by Babcck & Wilcx Cmpany; (pages 1-18) 18