323 Acta Chim. Slov. 1999, 46(3), pp. 323-338 MODELLING AND EXPERIMENTAL MEASUREMENTS OF LIMESTONE DISSOLUTION UNDER ENHANCED WET LIMESTONE FGD PROCESS CONDITIONS* A. Stergaršek, M. Gerbec, R. Kocjančič, P. Frkal "Jožef Stefan " Institute, Department of Inorganic Chemistry and Technology, Jamova 39, 1000 Ljubljana, Slovenia; tel: (+386)(61) 1773900, fax: (+386)(61) 219385, e- mail: marko. gerbec@ijs. si (Received 31.3.1999) Abstract The influence of ammonium sulphate additive to the solution in the wet flue gas desulphurization (FGD) process performance has been experimentally determined and modelled. It was observed that the buffer capacity of the solution could be increased by up to 500 %. The experimental results on the limestone dissolution rate in a pH stated apparatus are reported under various relevant parameters (concentrations of ammonium and chloride ions, pH value and temperature). The results are reported as a linear dissolution rate (um s-1) and are particle size independent. It was found that the performance of the ammonium ion is comparable to that of adipic acid, which is a well-known additive in FGD applications. Both additives increase the buffer capacity of the solution and the limestone dissolution rate. Introduction The wet limestone process is commonly used for the desulphurization of flue gas from coal-fired power stations. In this process, the SO2 from the flue gas is absorbed in a spray tower by limestone/gypsum slurry with a temperature of about 60 °C. The SO2 absorbed is neutralised with limestone (CaCO3) and oxidised to sulphate with sparged air in a make-up tank. The product of the process is gypsum (CaSO4.2H2O). The factors determining the size of the equipment and the consumption of energy used for the Dedicated to the memory of Prof. Dr. Jože Šiftar 324 process operation are the absorption capacity of the slurry sprayed into the scrubber, the rate of oxidation of the sulphite, and the rate of limestone dissolution in the make-up tank. Possible optimisation of the process can normally be performed in two directions: · Improvements in the absorber. It is possible to use a more efficient type of absorber (e.g. a spray absorber with packing, venturi absorber, falling film absorber, etc.), or to improve the local aero- and hydro- dynamical conditions in the absorber [1-4]. However, these can result in other adverse impacts such as creating areas of potential plugging and scaling. · Improvements in the chemistry of the process. It is possible to use additives which improve the absorption (buffer) capacity of the slurry and the limestone dissolution rate. Organic acids (e.g. adipic acid) are industrially-proven additives; ammonium salts, however, represent an innovation proposed by General Electric, Environmental Services, Inc. [5]. In this paper, the modelling and experimental measurements of the influences of various additives on the buffer capacity to absorb slurry and limestone dissolution will be discussed. The effects of ammonium sulphate as an additive on the operation of a pilot plant are reported in [5]. Modelling The objectives of the research and modelling were as follows: · The effect of ammonium sulphate additive on the equilibrium composition of the relevant ions in the absorbing slurry and · The influence of NH4+ ions in the solution on the buffer capacity. Our goal was to model the buffer capacity to absorption slurry at pH value change (DpH = pHinput - pHoutput) in the absorber tower. The slurry is acidified from input pH (in the neutralisation tank) to a certain pH value at the absorber output. The modelled buffer capacity was correlated with the required L/G volume ratio in the absorber at a given constant output SO2 concentration. The following relevant equilibrium reactions were considered in modelling: dissociation of dissolved SO2 (sulphite), sulphate, carbonate, ammonium, water and ammonium 325 carbonate, solubility products of CaCO3 and CaSO4.2H2O, and CO2 gas solubility in water. The relevant chemical reactions and equilibrium equations can be expressed as follows: H2SO3 +H2O « H3O+ +HSO3- HSO3- +H2O « H3O+ +SO32- H2SO4 +H2O « H3O+ + HSO4- HSO4- +H2O « H3O+ +SO42- H2CO3 +H2O« H3O+ +HCO3- HCO3- +H2O« H3O+ +CO32- NH3 +H2O« NH4+ +OH 2H2O« H3O+ +OH NH 4HCO3 <-» NH 4 + HCO3 NH4CO3- «CO32- +NH4+ Ca2+ + SO42- +2H2O «CaSO4 .2H2O Ca2+ +CO32- «CaCO3 H SO K2 = H2SO3 [H3O+ ][HSO3 [H2SO3 ] [H3O+ ][SO32- ] [HSO3- ] [H3O+ ][HSO4- ] K1H2SO4 = [H2SO4] + 2- K2 = H SO 24 [H3O+ ][SO42 - ] [HSO4- ] [H3O+ ][HCO3- ] H2CO 3 I [H2CO3\ H CO = [H3O+ ][C-O32 - ] [HCO3 ] [NH4+ ][OH- ] 4 K = ---------F-----------T NH3 INH 1 Kh O = \H3O+ \\OH \ \HCO3 \\NH4+\ KNH HCO 3 =3----\ K NH CO- [nh4co \Ca2+\[SO4 \ \Ca2+ \\CO3~ \ H, CO p CO2 H2CO3 K= sp, CaSO4 K= sp, CaCO3 (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) It should be noted that under the given conditions (pH < 6), the conversion of NH3 and CO2 to carbamate anion had a negligible effect and was not considered. The equilibrium constants at the given temperature (60 °C) were calculated from thermodynamic data prepared by Brewer [6]. 2 326 The electroneutrality of the solution was also considered. In addition, the chloride anion (Cl-) was considered, as it is subject to accumulation in the absorbing solution due to the presence of HCl in the flue gas. i = n (14) X zici = 0 i=1 In equation 14 zi and ci represent, for the given ion i, the electrical charge (including sign) and the concentration, respectively, n is the number of all ions considered. The activity was considered using activity coefficients according to the adapted Pitzer/Hebye/Hueckel equation [7]. The overall mass balance (concentration) for the carbonate, sulphite and sulphate species was calculated as CT, S4T and S6T, respectively: CT = [H2CO3] + [HCO3~ ] + [CO3~] + [NH4HCO3 ] + [NH4CO3 ] ( 15) S4T = [H2SO3 ] + [HSO3~ ] + [SO3~ ] (16) S6T = [H2SO4] + [HSO4] + [SO42~] (17) The system of 17 equations and 17 variables was solved numerically. A computer program was developed to perform calculations. The calculation procedure was performed in three steps: • First, for a given total NH4+/NH3 concentration and for a given Cl- concentration, the concentrations were calculated considering the equilibrium with both solid phases (limestone and gypsum). The results were used in the second step. • In the second step the solid phases were “removed”. The equilibrium between solution and the solid phase was no longer considered in the calculations. The solution contained equilibrium concentrations of ions and had a pH value of about 7. Then, the solution was acidified by increasing the Cl- concentration, and the new equilibrium concentrations and the pH value were calculated. This was performed until a given pH value (observed in the pilot plant slurry conditioning tank as pH input) was reached. The corresponding Cl- concentration was then stored. The model acidification with Cl- ions was used to simulate a standard analytical procedure for dissolved carbonate determination, which is carried out by the titration with the HCl solution. 327 · In the third step the solution was further acidified by increasing the Cl-concentration until the pH value reached the measured value of absorbing slurry at the output of the spray absorber (pH output). The final Cl- concentration was compared to the stored value from the second step of the calculations and the difference was used as the expression for the buffer capacity (DcCl- in mol L-1) under the given input conditions mentioned in step one. Modelling results The pilot plant operation data were used in the modelling of the absorbing slurry buffer capacity. The pilot plant was connected to unit 5 at the Šoštanj coal-fired power station, Slovenia. The pilot plant data on the chemical composition of absorbing slurry (concentration of NH4+, Cl- ions, total concentration of sulphite, sulphate and carbonate species) and the pH value in the neutralisation tank (absorber input) and at the absorber output were used. Consequently, the data on the required L/G volume ratio in the spray absorber to reach the prescribed emission mass concentration of SO2 at the absorber output of 400 mg m-3 (at normal conditions) were also used. The other data on pilot plant operation using additives are reported in [5]. The input data for model calculations, the required L/G volume ratio in the absorber and the model results are presented in Table 1. The pilot plant had the following process parameters: input flue gas flow rate 12,000 m3 h-1 (at normal conditions), input SO2 mass concentration in flue gas 4,500 mg m-3 (at normal conditions), absorbing slurry temperature 60 °C. The results from Table 1 are presented in Figures 1 and 2. The effect of ammonium concentration on the required L/G ratio in the absorber (pilot plant data) and on model solution buffer capacity (DcCl-) are presented for input pH at 5.8 and 5.3 at zero Cl-concentration (in the pilot plant). It can be observed that the introduction of the ammonium ion does increase the buffer capacity to absorb slurry and that this increase corresponds to a change (decrease) in the required L/G ratio for the same absorber performance. The increased buffer capacity is caused by the increased concentration of the HCO3- ion as calculated in model procedure step one. 328 Table 1 : Pilot plant data used in model calculations and model results expressed as required increased concentration of Cl- anion to change input pH to output pH. pilot plant data model result pH (input) pH (output) cNH4+ (g L ) cCl- (g L-1) L/G (L m-3) D cCl- (mol L-1) 5.80 4.71 0 0 14.5 2.2 . 10-3 5.98 3.95 3 0 11.8 8.8 . 10-3 5.66 3.86 5 0 11.4 7.6 . 10-3 5.66 3.86 8 0 10.4 9.9 . 10-3 5.76 4.14 5 20 13.5 2.8 . 10-3 5.85 4.81 5 40 14.5 2.6 . 10-3 5.77 4.83 8 40 11.8 2.6 . 10-3 5.58 4.30 0 0 15.0 1.6 . 10-3 5.47 3.93 2 0 14.1 3.4 . 10-3 5.17 3.67 5 0 14.0 4.4 . 10-3 5.23 3.34 5 20 18.0 1.7 . 10-3 5.27 3.90 5 40 26.4 1.2 . 10-3 0.020 ¦ calculated buffer capacity -------smoothed line for buffer capacity ^ required L/G ratio in pilot plant ¦ - ¦ smoothed line for required L/G ratio 3 4 5 6 7 mass concentration of NKi+(g/L) 8 -; 16 -: 14 -; 12 -; 10 -: 8 -; 6 -: 4 -; 2 —: 0 10 Figure 1: The effect of ammonium concentration on required L/G ratio in pilot plant and on calculated buffer capacity of absorbing slurry in absorber (at pH 5.8). 329 ¦ calculated buffer capacity ------smoothed line for buffer capacity ^ required L/G ratio in pilot plant ¦ - -smoothed line for required L/G ratio 3 4 5 6 7 mass concentration of NKi+(g/L) 20 18 -- 16 14 3_ o - 12 2 10 f > - 8 6 + 4 2 0 10 Figure 2: plant and The effect of ammonium concentration on the required L/G ratio in pilot on calculated buffer capacity of absorbing slurry in absorber (at pH 5.3). Experimental work In the experimental work the influence of ammonium sulphate on the limestone dissolution rate was determined in a pH-stated apparatus. This technique has already been reported in the literature [8, 9]. The previously reported methods and results [9] have now been expanded to determine the effect of the introduced ammonium ion to the limestone slurry. The ammonium ion increases the buffering capacity of the solution and consequently promotes the dissolution rate of limestone particles. In contrast, the accumulated chloride ions in the solution inhibit the promoting effect of ammonium ions. Thus, the effect of the ammonium and chloride ions concentration in the bulk of the test solution (limestone slurry) was the subject of measurements in pH-stated apparatus under controlled conditions. Tests were carried out under the following experimental conditions: • The apparatus reported in [9] was used. The volume of test solution was 1 L. • Vigorous stirring was applied by 50 mm turbine stirrer at 720 min-1. 330 • In all tests commercial ground limestone from the Kamnik stone quarry (used in industrial FGD plant unit 4 at the Šoštanj power plant, Slovenia) was used. The size distribution of limestone particles is presented in Figure 3. The initial amount of limestone introduced to the apparatus was 2.00 g per liter of solution. • 0.5 mol L-1 HCl solution was used to maintain a constant pH value during the test. • Test solutions with various concentrations of ammonium and chloride ions were prepared in the same apparatus from distilled water, (NH4)2SO4 and CaCl2 (p.a. quality chemicals were used). After the introduction of chemicals, the precipitation of gypsum occurred. The ranges of ammonium and chloride ion mass concentrations in the test solutions were 0 to 20 g L-1 and 0 to 71 g L-1, respectively. • The pH values of 5.0, 5.5 and 5.8 were maintained. • The temperature of the slurry was maintained at 20 °C or 60 °C. • The solution was sparged with air at 120 L h-1 in order to remove the liberated CO2. The experiment was controlled by a computer that performed the pH measurement, and the regulation and control of the amount of acid solution introduced into the apparatus over the time. The result of the experiment was the time dependence of the amount of undissolved limestone. The dimensionless quotient of limestone mass at a given time by initial limestone mass was introduced, and will be referred to as F. m(t) (18) F(t)=------- m (0) A model of limestone dissolution in a pH-stated apparatus [9] was used to correlate the dependence of F against time t and given polydispersed limestone granulation data (Figure 3) to linear dissolution rate under the given experimental conditions (chemical composition in bulk of solution). Linear dissolution rate (kdis in |im s-1) will be used as the final result of the experimental work. The linear dissolution rate was obtained in the following way. The data on polydisperse limestone particle size distribution was arranged to a set of monodisperse particle sizes, considering the particles as spheres. The value of kdis was arbitrarily chosen, and the dissolution of each particle size in a set was calculated in a small time interval. The reduction of particle(s) diameter was considered in the next iteration. Thus, a model dependence of calculated F against time was obtained. By comparison of experimental and model calculated curves, the best-fit value of kdis was found by trial and error for 331 each experiment. An example of the experimental and model calculated curves of F against time is presented in Figure 4. si 80 : L 50 t 40 0.1 10 particle size (|jm) 100 1000 Figure 3: Particle size distribution for commercial limestone (from Kamnik stone pit) used in the experimetal work. 500 1000 1500 2000 time (s) 2500 3000 3500 4000 Figure 4: Example of model calculation of kdis from experimental F against time curve by guessing (trial and error). Value of 0.0059 |im s-1 was used in this example as the best fit (at 60 °C, pH 5.8, 10 g L-1 NH4+ and 7.1 g L-1 Cl-). 332 It was estimated that the maximum overall error of measurement and calculation of kdis was up to ± 30 %. Experimental results The majority of experimental measurements were performed at pH=5.8 and 60 °C. The effect of the ammonium and chloride ions in bulk of test solution will be presented, as well as comparisons with other cations and other ammonium salts. The effect of additives on the limestone dissolution rate will be given as the calculated kdis or as the experimental curve F versus time. The effect of ammonium ion bulk mass concentration on kdis at three pH values (5.8, 5.5, 5.0), at 60 °C and for 7.1 g L-1 Cl- mass concentration is presented in Figure 5. For higher chloride concentration, the effect is presented in Figures 6 and 7 (at 35.5 and 71 g L-1 Cl-, respectively). The effect of temperature on kdis at different ammonium ion concentrations is presented in Figures 8 and 9 (at pH 5.8 and 5.0, respectively, and at 7.1 g L -1 Cl- concentration). Various other additives were also tested. Magnesium, potassium and sodium sulphates were used at mass concentrations equivalent to that of ammonium sulphate (0.28 mol L-1 sulphate mass concentration was used as the reference value). For comparison, no additive at all and the well-known additive adipic acid at mass concentration of 1 g L-1 , were also tested. The comparison is given as F against time curves in Figure 10 (at 60°C and pH 5.8). It can be concluded from the results, that cations like Mg2+, K+ and Na+ do not promote limestone dissolution and only adipic acid has a comparable effect on the ammonium io n. The comparison of ammonium salts (NH4Cl, NH4NO3) as additives to (NH4)2SO4 is presented in Figure 11 (the curve with no ammonia added is also shown). Comparison is given at 60 °C, pH 5.8, 10 g L-1 NH4+ (0.58 mol L-1) and 7.1 g L-1 Cl-. We can conclude that the choice of ammonium salt has no influence. 333 0.1000 0.0100 0.0010 0.0001 : ¦ ¦ -¦ ¦ pH=5,L -¦- pH=5,J 5 ) i -^. ---------- A' A,- - ir-* 0 2 6 8 10 12 14 16 18 20 mass concentration of NKi+(g/L) Figure 5: Comparison of ammonium ion mass concentration effect on kdis at three pH values (at 60 °C and 7.1 g L-1 Cl-). 0.1000 0.0100 -¦ 0.0010 0.0001 ; —i^pH=5,0 -¦- pH=5,8 - : > »---------" ¦¦" """* ____- ir ^ ** ^ ^^*~ 6 8 10 12 14 mass concentration of NKi+(g/L) 16 18 20 Figure 6: Comparison of ammonium ion mass concentration effect on kdis at three pH values (at 60 °C and 35.5 g L-1 Cl-). 334 0.0100 3 0.0010 - 0.0001 - - ^^pH=5,0 ¦ ¦¦ - pH=5,8 - - ..--•'" .-"¦"* ______4 - <>• ---'' 6 8 10 12 14 mass concentration of NHi+(gl/L) 16 18 20 Figure 7: Comparison of ammonium ion mass concentration effect on kdis at three pH values (at 60 °C and 71 g L-1 Cl-). 0.0100 * 0.0010 0.0001 —i—without NH4+ — 5 g/L NH4+ A---10g/LNH4+ -»-20g/LNH4+ 10 20 30 40 temperature (°C) 50 60 70 Figure 8: Comparison of temperature effect on kdis at various ammonium ion mass concentrations (at pH 5.8 and 7.1 g L-1 Cl-). 335 0.020 0.018 0.016 0.014 : _ 0.012 I 0.010 0.008 0.006 0.004 0.002 } 0.000 10 - - » without additive — 5 g/L NH4+ ¦¦¦*¦¦¦ 10 g/L NH4+ -o-20g/LNH4+ AS ty y ' ^- ------ ^ ------ - ; y ' ty' ; y y' y' :--------------- y y-' - -,'' y * y' * ,y' _^-* G ^ ------- - , —-~-~ I I I * —,---,----,---,— _____________________ ,-----------------------------f 20 30 40 temperature (°C) 50 60 70 Figure 9: Comparison of temperature effect on kdis at various ammonium ion concentrations (at pH 5.0 and 7.1 g L-1 Cl-). u. 0.5 without additive -------NH4+(10g/L) .......Mg2+(6.7g/L) ------Na+ (12.9 g/L) ------K+ (21.9 g/L) -------Adipic acid (1 g/L) Adipic acid 5000 10000 15000 20000 25000 time (s) Figure 10: Comparison of Mg , K and Na cations (as sulphates) to NH4 on limestone dissolution curve of F against time. Curves for absence of additives and for adipic acid are also shown (conditions: 60 °C, pH 5.8 and 35.5 g L-1 Cl-). 336 O 2000 4000 6000 8000 10000 12000 14000 tirne (s) Figure 11: Comparison of various ammonium salts effect on limestone dissolution rate (conditions: 60 °C, pH 5.8, 10 g L-1 NH4+, 7.1 g L-1 Cl-). Conclusion Experimental results show that the introduction of ammonium ions significantly increases the buffer capacity of the absorbing slurry. According to the modelling of solution equilibrium, the cause is the increased concentration of the HCO3- ion due to high NH4+ ion concentration. A small contribution (up to 10 % of overall concentration for all CO2 species) also results from the association of NH4+ and HCO3- ions to NH4HCO30 and NH4CO3- in the solution. The calculations of the buffer capacity in the spray absorber showed that the buffer capacity between the input pH value (e.g. 5.8) and the output pH value (3.3 to 4.9) could be up to 500 % higher. However, the required L/G volume ratio in the absorber is only reduced by up to 30 % [5], as a result of other resistances to the mass transfer in the absorber [2]. Experimental measurements of limestone dissolution in the pH stated apparatus show that the introduced ammonium ion significantly promotes the dissolution rate. The 337 linear dissolution rate, kdis, can be increased by up to 30 times. The quantitative influences of all relevant parameters (pH, temperature, ammonium and chloride ion concentrations) are also presented graphically. The results are independent of limestone particle size distribution and are useful for the modelling of given limestone dissolution in industrial scale FGD plant. It is also shown that other cations (Mg2+, K+, Na+) do not influence limestone dissolution. Three ammonium salts ((NH4)2SO4, NH4Cl, NH4NO3) were also compared and it was found that they have the same effect. The effect of the ammonium ion is comparable to the adipic acid as a typical additive in FGD applications. Notation [i] activity of species i, mol L-1 ci concentration of species i, mol L-1 CT total concentration of carbonate species in the solution (mass balance) F dimensionless quotient of undissolved limestone to initial mass K equilibrium constant of chemical reaction kdis linear dissolution rate, mm s-1 KH equilibrium constant according to Henry’s law L/G liquid flow rate to gas flow rate volume ratio, L m-3 m mass, g pH = - log [H3O+] pi concentration of i in gas phase S4T total concentration of sulphite species in the solution (mass balance) S6T total concentration of sulphate species in the solution (mass balance) zi charge of ion I Greek letters S sum operator D difference Subscripts 0 under initial conditions CaCO3 limestone CaSO4 gypsum (CaSO4.2H2O) 338 H2CO3 H2SO3 H2SO4 NH3 NH4CO3- NH4HCO3 sp t Superscripts 1, 2 References [1] M. Gerbec, A. Stergaršek, R. Kocjančič, Comput. chem. eng. 1995, 19 Suppl., S283-S286. [2] A. Stergaršek, R. Kocjančič, M. Gerbec, Chem. Eng Sci. 1996, 51, 5081-5089. [3] M. Gerbec, Ph.D. thesis 1996, University of Ljubljana. [4] M. Gerbec, A. Stergaršek, R. Kocjačič, D. Ban, P. Frkal, CHISA'98: Proceedings of 13th International Congress of Chemical and Process Engineering (CD-ROM), 23-28 August 1998, Prague, Czech Republic. [5] K. Barenthaler, E. Gal, A. Stergaršek, R. Kocjančič, M. Gerbec, Z. Bešlin, P. Frkal, M. Toplak, Proceedings from Slovene Chemical Days 1996, P. Glavič, et.al., Maribor, 19./20. September 1996, 530-535. [6] L. Brewer, In Flue gas Desulfurization (Edited by Hudson, J.L. and Rochele G.T.) 1982, ACS Symposium Series, Washington D.C., 188, 1-39. [7] M. Luckas, K. Lucas and H. Roth, AiChE Journal 1994, 40, No. 11. [8] P.K. Chan, G.T. Rochelle, In Flue gas Desulfurization (Edited by Hudson, J.L. and Rochele G.T.) 1982, ACS Symposium Series, Washington D.C., 188, 75-97. [9] M. Gerbec, A. Stergaršek, R. Kocjančič, Vestnik Slov. Kem. Društva 1990, 37, 279-297. Povzetek Eksperimentalno smo izmerili in modelirali vpliv dodatka amonijevega sulfata v raztopini v mokrem kalcitem postopku za razžve pije vanje dimnih plinov termocentral. Opazili smo, da se je pufe rska kapaciteta poveč ala do 500 %. Podani so eksperimentalni podatki o hitrosti raztapljanja kalcita v pH statirani napravi v odvisnosti od različnih pomembnih parametrov (koncentracije amonijevih in kloridnih ionov, pH vrednost, temperatura). Rezultati so podani kot linearna hitrost raztapljanja kalcita (um s-1), ki je neodvisna od velikosti delcev. Ugotovili smo, da je vpliv amonijevega iona primerljiv z vplivom adipinske kisline, ki je znan dodatek v mokrih razžvepljevalnih napravah. Oba dodatka povečata pufersko kapaciteto raztopine in hitrost raztapljanja kalcita. hydrated CO2 in water sulphurous acid sulphuric acid ammonium non-dissociated ammonium carbonate anion ammonium hydrogen carbonate solubility product time first and second step of chemical reaction, respectively