Thermal Stability of Acidic Sulfates in Krafi Recovery Bode

Recoverv Boiler Debosits

Thermal stability of acidic sulfates in krafi recovery bode

Honghi Tran, William Poon, and David Barham

and severe tube wastage via corro- ABSTMCR Acidic sulfates, such us NuHS04 and Nu2S20?, sion (3-5). ure suspected in sticky depositformution und tube corrosion tn In previous work (6), it has been shown that forming 'sodium bisul- the generuting bunk and economizer regions of kru$ recovery fate (NaHSO,) directly from sodium boilers. Their stubility wus exumined in uir, moist conditions, sulfate (Na,SO,) or salt cake is both and in the presence ofNa2C03ut various temperutures. The kinetically and thermodynamically to feasible under boiler upset condi- results showed that, in uir, NuHSO, melts und decomposes tions that lead to high SO, levels in solid Nu2S207una? wuter vupor ut ubout 180 O C. Nu2S207is the flue gas. The reaction kinetics relutively stable up to its melting point of 380 C. Molten depend strongly on temperature and so SOJSO, and H,O concentrations in Nu2S207purtiully decomposes to lid Nu2S04,which reucts the gas phase. At 250qfor instance, with the remuining Nu2S207to form u newly identijed liquid NaHSO, forms readily if the compound, 3Nu2S,0;2Na2S04; this compound melts at SO, (SO, + SO,) concentration ex- its react ceeds 150 ppm. 570' C. Solid Nu2S207and complex compound rupidly In recovery boilers, the SO, con- with H20 vupor ut 300' C to reTom liquid NuHSO,, which centration in the flue gas rarely ex- cun be corrosive for the generuting bunk tubes during boiler ceeds 200 ppm. Therefore, the most operution. These ucidic sulfates can coexist with Nu2C03 below likely compound that makes deposits acidic in recovery boilers is their respective melting points. They ure hygroscopic, u bsorbing NaHSO,, since it requires less than wuterfiom moist uir to&rm su&ric mid, which muy cuuse 10 ppm SOxto form as a solid phase tube wustuge during boiler outuges. and less than 200 ppm to be present as a liquid phase (2). While it may be KEYWORDS: Deposits, recovery firnuces, sodium carbonute, possible for solid sodium pyrosulfate (Na,S,O,) to form in recovery Coil- sulfates, temperature, thermal stability, wuter. ers, liquid Na,S,O, cannot be formed due to its requirement of high SO, levels, i.e., levels greater than 2500 ireside deposits on the generati ers burning high-sulfidity black li- ppm (7). For boilers that burn hard- Fing bank and economizer region quors or in those operating at low wood black liquor, the potassium con- tubes of kraft recovery boilers are bed temperatures (1,Z). Such acidic tent in the deposits may be high. occasionally acidic when dissolved in deposits may be responsible for both Therefore, potassium bisulfate water. This is more common in boil- fouling of heat transfer surfaces (1) (KHSO,) and potassium pyrosulfate (K$,O,) may be formed in the deposits, since they require much lower SO, concentrations than the equiva- Tran is a professor and associate director of The Pulp and Paper Centre. Poon is a lent sodium salts (2, 7). graduate student and Barham is a professor at The Pulp and Paper Centre, Once formed, it is not known if Department of Chemcial Engineering and Applied Chemistry, University of Toronto, NaHSO, can remain stable during Toronto, ON M5S 1A4, Canada.

128 May 1994 Tappi Journal ing samples of NaHSO, and Na,S,O, to air with various relative humidities for up to 18 and 21 days, respectively. Details of the experimental procedures can be found in Ref. 8.

Results and discussion Thermal behavior of NaHSO, -20 1 i15 Figure P shows DTWI'GA results for NaHSO,. The four main endo- thermic events on the DTA curve beginning at 180'33, 240"C, 360"C, and about 400°C are each accompanied by a weight loss event on the TGA curve. The first thermal event is the -50k li0 IO0 3b0 4bO 5bO Si0 7bO 8k10 melting of NaHSO,. The third ther- TEMPERATURE, "C mal event is melting the mixture of NaHSO, and Na,S,O, formed by the partial decomposition of NaHSO, (Reaction l),and the fourth is the 1. X-ray diffraction data for the proposed Experimental procedures decomposition of Na,S,O, (Reaction new compound 3Na,S,0;2Na,S04 (CuKa 2). target, Ni filter) Samples of NaHSO, and Na,S,O, were subjected to increasing-tem- 2NaHSO,(l) + Na,S,O,(s) + 2H,O(g) 29, d value, l/lo perature simultaneous DTNTGA (1) deg A x 100 (differential thermal analysis/ thermogravimetric analysis) in a 12.83 6.90 15 Seiko Thermal Analyzer to investi- Na,S,O,(l) + Na,SO,(s) + SO&) (2) 18.35 4.84 18 19.63 4.52 17 gate their thermal stabilities. In Note that while the total decom- 21.30 4.17 38 DTA, the temperature difference position of NaHSO, to Na,SO, should 25.79 3.45 17 between a reactive sample (e.g., give a theoretical weight loss of 27.56 3.24 29 NaHSO,) and a thermally inert stan- 40.8%, the experimental maximum 28.58 3.12 100 dard (e.g., Al,O,) is plotted against value is only about 38%. This is be- 32.17 2.78 13 temperature. Any temperature dif- cause the original NaHSO, contained ference is related to a thermal event, a small amount (about 6 wt%) of such as melting or decomposition, in Na,SO,. The presence of Na,SO, normal boiler operation; thermody- the sample. In TGA, a plot of weight probably also accounts for the sec- namically it cannot. Furthermore, change vs. temperature for the ond thermal event, which is likely to since deposits usually contain some sample can also be used to identify a be a eutectic melting point in the sodium carbonate (Na,CO,), which is thermal event in the sample. ternary system NaHS0,-Na,S,O,- alkaline, it is questionable whether Separate isothermal (constant Na,SO,. Note also that the decompo- NaHSO, and Na,S,O, can be formed temperature) TGA experiments were sition of NaHSO, after it begins to and be stable in such deposits. performed on both compounds in air melt at 180°C is slow, and that the The present study examines the for time periods from ten minutes to decomposition of Na,S,O, also slows thermal and kinetic behavior of both as long as two months, depending on at about 480°C. Both of these obser- NaHSO, and Na,S,O, in air at high the experimental temperature. The vations suggest that the material re- temperatures. The effects of Na,CO, effect of Na,CO, on the stability of solidifies as it decomposes or that a and H,O vapor on the stability of these acidic sulfates was also exam- solid intermediate compound is these acidic sulfates at various tem- ined by subjecting mixtures of formed. peratures are also examined to make NaHSO, and Na,CO, (2:l molar ra- Figures 2-4 show isothermal TGA the study more applicable to actual tio), and Na,S,O, and Na,CO, (1:l results for NaHSO, heated in air. In kraft recovery boiler conditions. molar ratio), to isothermal TGA. The Fig. 2, the decomposition of NaHSO, stability of acidic sulfates in moist to Na,S,O, at 250 and 300°C occurs atmospheres was studied by expos- rapidly with a weight loss of 7.5%,

Vol. 77, No. 5 Tappi Journal 129 2. Isothermal TGA of NaHSO, in air at 250-350°C 3. Isothermal TGA of NaHSO, in air at 400-550°C

0, -1I I Or----- 1 250°C

300°C

-1 5 -350°C

-25 -20 1 -300L' .A 10 20 30 40 50 60 70 -50; 5 10 15 20 25 30 35 4( TIME, days TIME, h

-~ ~

- I 4. Isothermal TGA of NaHSO, in air at 600-800°C 5. Time for total NaHSO, decomposition at various temperatures in

A I 2.5 - 1

I.iooac -500 a- 12345676 TIME, h TEMPERATURE, "C

but the decomposition to Na,SO,, relatively stable compound between Figs. 24,vs. temperature. The peak with a theoretical total weight loss of the Na,S,O, and Na,SO, from the at 550°C again suggests the pres- 40.8%, does not occur in any reason- sequential decompositionof NaHSO, ence of a relatively stable compound able time. This is also shown by the and Na,S,O,. Similar results are containing Na,S,O, and Na,SO,. 350°C curve. The curves in Fig. 3 are shown for the test at 600°C (Fig. 4). somewhat puzzling unless the results Calculations based on weight loss Thermal behavior of Na2S,O7 from Figs. 1 and 2 are considered at show that this formerly unknown Figure 6 shows DTA/TGA results the same time. As the temperature compound is likely to be 3Na,S,O, for Na,S,O, heated in air at an in- is increased from 400°C to' 550"C, the .2Na,SO,. It shows a completely dif- creasing temperature. The endother- long-term stability of the NaHSO, ferent X-ra,y diffraction (XRD) pat- mic peak at 270°C is probably due to' decomposition product increases. tern compared to other acidic sulfates the melting of an NaHSO, - Na,SO, The leveling-off of the 350°C curve (8). The major XRD peaks for the - Na,S,O, mixture. This is because (Fig. 2) and of the 450"C, 500"C, and new compound are given in Table I. the Na,S,O, used was made by dehy- 550°C curves (Fig. 3) at about 18% Figure 5 shows the time neces- drating NaHSO,, which contained weight loss is interesting. It suggests sary to accomplish complete disso- about 6 wt% Na,SO,, at 300°C for 72 the formation of an intermediate, ciation of NaHSO,, estimated from

130 May 1994 Tappi Journd hours in air. The major endotherm at 380"C, accompanied by a weight loss on the TGA curve, is due to melting the Na,S,O, and its partial decomposition to Na,SO,. The Na,SO, formed reacts with the residual Na,S,O, to form a new compound, determined to be 3Na2S,0; 2NazS0,, which forms a eutectic melt with Na,S,O, at about 540"C, and which decomposes rapidly to Na,SO, at about 600°C. The "spikes" on the DTA curve in the range of 540°C to 600°C indicate a continuous melting process. The melting point of the pure compound was determined to -50 I -10 0 100 200 300 400 500 600 700 800' be about 570°C (8). Figure 7 summarizes the behav- TEMPERATURE, "C ior of NaHSO, and Na,S,O, when heated in air. It is a complicated process involving melting, decomposi- 7. Formation and stability of acidic sulfates in air tion, and intermediate compound formation. Both NaHSO, and Na,S,O, do not decompose directly NaHS04 (s) to Na,SO,. They first decompose to intermediate compounds, which then decompose to Na,SO,.

'-. >240°C Thermal behavior of the NdSO, - Na,CO, mixture i Figure 8 shows simultaneous DTA/ 7(s) +""": TGA results for a 2:l molar mixture of NaHSO, and Na,CO,. At 170"C, 380°C the NaHSO, melts, producing the *... endotherm. This is slightly below its NaHS04*Na2S04(I) *a. : Na2S207 (1) normal melting point due to the presence of Na,CO,. The now-molten 380"-570"C NaHSO, reacts with Na,CO, to produce the exotherm caused by the 3Na2S20~2Na2S04(s) heat of neutralization at about 180°C and a simultaneous rapid weight loss on the TGA curve: 570°C Na,S,O,(l) + Na,CO,(s) + 2Na,SO,(s) 3Na2S20y2Na2S04(I) + CO&) (3) : >570eC The resulting Na,SO, reacts with the v ...... remaining NaHSO, to produce solid NaHSO,*Na,SO,, which stops any further reaction. At 27OoC,this compound melts, producing the endotherm and a weight loss. The decomposition continues until the Na,S,O,. produced melts at about 370"C, gmng the endotherm. An in- crease in the rate of weight loss oc-

Vol. 77, No. 5 Tappi Journal 13 1 -I curs as the Na,S,O, reacts with the 8. DTAITGA of an NaHS0,-Na,CO, mixture (2:l molar ratio) in air residual Na,CO,, giving an exotherm at about 380°C. This reaction is es- sentially complete by 450°C. Note that the rate of weight loss decreases at about 400°C. This suggests that a 0 - 15 new reaction chemistry, probably 8 compound formation, occurs at this w Ex0 temperature. '? -5 Figure 9 shows the isothermal a* 0 behavior of the NaHSOI, - Na,CO, I- mixture (2:l molar ratio) at various P* -10 temperatures. At 180"Cj,there is an W initial rapid weight loss to about half 3 the maximum value po,wible. This -1 5 -0 suggests that the compound NaHSO;Na,SO, is formed. At 250"C, a rapid weight loss of about 10% oc- -20 li0 2;O 3b0 4;O 5iO 6dO 7;O 8!i5 curs, then the rate decreases. This again indicates the formation of TEMPERATURE, "C NaHSO;Na,SO,, which is not very stable in air at a temperature close to its 270°C melting point. residue. These results imply that a formed from the reactions between solid reaction product (3Na,S,O, Na,SO,, SO,, and H,O during upset Thermal behavior of the Na2S,0, .2Na,SO,) has formed; this product conditions that lead to high SOxcon- separates the remaining reactants, centrations in the flue gas, will re- - Na,CO, mixture thus preventing complete reaction. vert to solid Na,SO, when the upset Figure 10 shows simultaneous DTA/ ends. Our work shows that this does TGA results for a 1:l molar mixture Effect of moisture not happen. Liquid NaHSO, either of Na,S,O, and Na,CO,. ,4t 380"C, an decomposes to Na2S0,, which reacts endotherm followed immediately by Figures 12 and 13, respectively, show the effects of long-term expo- with the remaining NaHSO, to form a large exotherm coincides with a a more stable compound, NaHSO, rapid weight loss. The endotherm sure to moist air at room temperature for NaHSO, and Na,S,O,. Both .Na,SO,, or to solid Na,S,O, and H,O represents the melting of Na,S,O,, vapor (Reaction 1).Na,S,O, is stable while the exotherm represents the gain weight at relative humidities over 30%. Under any time-relative up to temperatures close to its melt- neutralization reaction between mol- ing point of 380°C. If Na,S,Q, par- ten Na,S,O, and Na,CO, (Reaction humidity condition, the weight in- crease for the Na,S,O, is approxi- tially decomposes to Na,SO,, then 4). The marked decrease in the rate the remaining Na,S,O, forms the of weight loss before the reaction is mately twice that of the NaHSO,. This is due to the initial reaction compound 3Na,S20,.2Na,S0,, which complete suggests that a solid reac- is relatively stable up to its ineltirig tion product is formed. between water and solid Na,S,O,, which produces NaHSO,, which then point of about 570°C. Na,S,O,(l) + Na,CO,(s) + absorbs water: This suggests that while it is ei- 2Na,SO,(s) + Ca,(g) (4) ther difficult or impossible for Na,S,O,(s) + H,0(1) + 2NaHSO,(s) Na,S,O, to form directly from the The results of isothermal TGA of (5) the mixture at temperatures below reaction between Na,SO, and SO, the melting point of Na,S,O, are When the NaHSO, absorbs water, during upset conditions, it can be shown in Fig. 11. While the theoreti- free HSO; ions are produced; these formed indirectly from the decorn- cal maximum weight loss is 13.4%, ions decompose to produce acidic position of any NaHSO, formed dur- neither the result at 300°C nor that conditions: ing the upset (Reaction 1). Once formed, Na,S,O, is stable, even in at 350°C shows this much loss, even HS0,- *H+ + SO:- (6) after five days. Dissolving the resi- air. The formed NaHSO, and due from these experiments in wa- Na,S,O, can even coexist with ter produces CO, gas bubbles, Practical implications NazCO, if they are solid. Both will suggesting the coexistence of an absorb water from moist air with acidic material and NazCO, in the Reports (2, 7, 9) suggest that any greater than 30% relative humidity liquid NaHSO, and/or solid Na,S,O,, at room temperature.

132 May 1994 Tappi Journal 9. Isothermal TGA of an NaHS0,-Na,CO, mixture (2:l molar ratio) 11. Isothermal TGA of an Na,S,O,-Na,CO, mixture (1 :1 molar ratio) air air

0 NaHS04,18O0C

NaHS04 : Na2CO3= 2:l molar ratlo - 180°C

- 250°C I -201) 1 2 3 4 5 0 1 2 3 4 5 6 TIME, days TIME, days

10. DTAiTGA of an Na,S,O,-Na,CO. mixture (1:l molar ratio) in air deposits do not contain sufficient Na,CO,, the resulting liquid NaHSO, will not be neutralized; hence, corrosion of tubes may result. This probably is the main mechanism for tube -6 thinning near the mud drum surface (3, 5). Details of the effect of H,O -5 concentration and temperature on the -4 reversibility of Reaction 1 are pres- -3 1 ently being examined.

-0 Conclusions - -1 The thermal stability of acidic sulfates in air and in the presence of - -3 H,O and Na,CO, was studied. The -15 I I , -4 0 100 200 300 400 500 600 700 800 results are as follows: * If NaHSO, is heated in air, it does TEMPERATURE, "C not decompose directly to Na,SO,. It rather decomposes first to Na,S,O,, which is relatively stable Since it is now clear that both steam at 300"C, it rapidly reverted up to just below its melting point NaHSO, and Na,S,O, can be formed to liquid NaHSO,, which then de- of about 380°C. in recovery boilers, there is reason composed slowly to solid Na,S,O, to believe that corrosion of heat when the steam was removed. This At higher temperatures, Na,S,O, transfer tubes occurs due to the com- implies that under normal operating melts and partially decomposes pounds' presence during regular op- conditions, acidic sulfates may be to Na,SO,, which immediately re- eration. In recent work at the present at the metal surface as solid acts with the remaining Na,S,O, to form a new compound, University of Toronto (IO) on the Na,S,O, or its complex compound with effect of water vapor on the stability Na,SO,. These sulfates would revert 3Na,SZO,-2Na,SO,. This com- of Na,S,O, using an electrical con- to molten NaHSO, when the nearby pound melts at about 570°C and ductivity method, it was shown that sootblowers are activated, increasing decomposes at higher tempera- when solid Na,S,O, was subjected to the H,O content in the flue gas. If the tures to Na,SO,.

Vol. 77, No. 5 Tappi Journal 133 Recovery Boiler Dcposits

- -I 12. Water absorption of NaHSO, at room temperature 13. Water absorption of Na,S,O, at room temperature

Humidity E 6 W” 44a I 0 cr22 W 50

-2- -2- ’ - -2 0 5 10 15 20 0 5 10 15 20

TIME, days TIME, days

If NaHSO, is formed during a tube thinning occurs if deposits 8. Poon, W., “Formation and stability of acidic sulphates in kraft recovery boil- boiler upset, it is unlikely to re- do not contain sufficient Na,CO, ers,’’ M.A.Sc. thesis, University of vert to Na,SO, when the upset is to neut,ralize the resulting liquid Toronto, Toronto, 1992. over, but decomposes to Na,S,O,. NaHSO,. 9. Ingraham, T. R. and Hotz, M.C.B., Cana- dian Metallurgical Quarterly 7(3): This subsequently decomposes, 139(1968). forming the complex compound 10. Mawhinney, D. C., “The effect of water 3Na2S,0,.2Na2S04, with its own Literature cited vapor on the stability of sodium pyrosulphate,” B.A.Sc. thesis, University decomposition product, Na,S04. 1. Tran, H. N., TAPPZ 1991 Kraft Recovery of Toronto, Toronto, 1993. These compounds are stable un- Operations Short Course Proceedings, der normal operating conditions. TAPPI PRESS, Atlanta, p. 181. This work is part of the research on “Recovery 2. Backman, R., Hupa, M., and Hyoty, P., Boiler Fireside Deposits,” supported by ABB Tappi J. 67(12): 61(1984). Combustion Systems, Ahlstrom Corp., Aracruz NaHSO, and all its decomposi- Cawin. and Nin. C., 66(5): 3. I;. W. TamiI. J. Cellulose SA, Babcock & Wilcox Co., Champion tion products, other than Na,SO,, 61(1983). International Corp., Diamond Power Specialty are hygroscopic and form sulfu- 4. Tran. H. N.. Reeve. D. W.. Barham. D.. et Co., E.B. Eddy Forest Products Ltd., ric acid when wet. al., CPPA 1986 Proceedings of the 5th Gotaverken Energy Systems, Jansen Combus- Internalional Conference on Corrosion tion and Boiler Technologies Inc., James River The presence of acidic sulfates in Pulp & Paper Industry, CPPA, Corp., Tampella Power Corp., Union Camp Vancouver, p. 201. Corp., Westvaco Corp., Weyerhaeuser Co., and their complex compounds 5. Thomson, R., Singbeil, D., Guzi, C. E., et Willamette Industries, and the Ontario Minis- combined with each other or with al., TAPPZ 1992 Proceedings of the 7th try of Colleges and Universities through its Na,SO, in kraft recovery boiler International Conference on Corrosion University Research Incentive Fund program. in Pulp & Paper Industry, TAPPI Research grants provided by the Natural Sci- deposits during a long shutdown PRESS, Atlanta, p. 309. ences and Engineering Research Council of or an incomplete waterwash may 6. Poon, W., Barham, D., and Tran H. N., Canada are also gratefully acknowledged. lead to tube wastage due to sulfu- TAPPI 1992 Engineering Conference Proceedings, TAPPI PRESS, Atlanta, p. Received for review June 28,1993 ric acid attack. 349. 7. Coates, A. W., Dear, D. J. A., and Penfold, Accepted Sept. 30,1993. Increasing the H,O content in the D., Jowmal of Institute of Fuel 41(3): flue gas may convert solid Na,S,O, 129(1968). Presented at the TAPPI 1993 Engineering Con- into liquid NaHSO,. Accelerated ference.

134 May 1994 Tappi Journal