Modeling the Use of Sulfate Additives for Potassium Chloride

Oct 24, 2013 - Metso Power Oy, Post Office Box 109, FI-33101 Tampere, Finland. •S Supporting Information. ABSTRACT: Potassium chloride, KCl, formed ...
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Modeling the Use of Sulfate Additives for Potassium Chloride Destruction in Biomass Combustion Hao Wu,*,† Morten Nedergaard Pedersen,† Jacob Boll Jespersen,† Martti Aho,‡ Juha Roppo,§ Flemming Jappe Frandsen,† and Peter Glarborg† †

Department of Chemical and Biochemical Engineering, Technical University of Denmark, Søltofts Plads, Building 229, DK-2800 Kongens Lyngby, Denmark ‡ VTT Technical Research Centre of Finland, Post Office Box 1603, FI-40101 Jyväskylä, Finland § Metso Power Oy, Post Office Box 109, FI-33101 Tampere, Finland S Supporting Information *

ABSTRACT: Potassium chloride, KCl, formed from biomass combustion may lead to ash deposition and corrosion problems in boilers. Sulfates are effective additives for converting KCl to the less harmful K2SO4 and HCl. In the present study, the rate constants for decomposition of ammonium sulfate and aluminum sulfate were obtained from experiments in a fast heating rate thermogravimetric analyzer. The yields of SO2 and SO3 from the decomposition were investigated in a tube reactor at 600−900 °C, revealing a constant distribution of about 15% SO2 and 85% SO3 from aluminum sulfate decomposition and a temperaturedependent distribution of SO2 and SO3 from ammonium sulfate decomposition. On the basis of these data as well as earlier results, a detailed chemical kinetic model for sulfation of KCl by a range of sulfate additives was established. Modeling results were compared to biomass combustion experiments in a bubbling fluidized-bed reactor using ammonium sulfate, aluminum sulfate, and ferric sulfate as additives. The simulation results for ammonium sulfate and ferric sulfate addition compared favorably to the experiments. The predictions for aluminum sulfate addition were only partly in agreement with the experimental results, implying a need for further investigations. Predictions for the effectiveness of the sulfur-based additives indicate that ferric sulfate and ammonium sulfate have similar effectiveness at temperatures ranging from approximately 850 to 900 °C, whereas ferric sulfate is more efficient at higher temperatures and ammonium sulfate is more effective at lower temperatures.

1. INTRODUCTION Ash-related issues, such as fouling and corrosion, are critical challenges in the use of biomass energy through combustion technologies.1−7 Alkali chlorides, predominantly KCl, generated during biomass combustion can induce severe ash deposition in the heat-exchange surfaces of the boiler8−11 and, subsequently, lead to a fast corrosion of the superheater tubes.12−14 Sulfurbased additives, including elemental sulfur,15−18 SO2,19−22 ammonium sulfate,17,23−27 ferric sulfate,15,18,28,29 aluminum sulfate,28 and sulfuric acid,30 have been injected to biomassfired boilers or waste incineration plants to minimize the metalchloride-induced problems. These sulfur-based additives convert KCl to K2SO4 and HCl according to the global reactions of eqs 1 and 2 and could thereby reduce deposit formation24−26 and the corrosion potential of the deposits18,29,31−33 on superheater tubes.

under grate-firing conditions, the effectiveness of ferric sulfate is higher than that of elemental sulfur.15 The general higher effectiveness of these sulfates compared to element sulfur or SO2 is believed to be related to SO3 produced from sulfate decomposition.15,17,28 Modeling predictions with detailed gasphase reaction mechanisms of KCl sulfation indicate that SO3 reacts with KCl significantly faster than SO2,34−36 and under biomass combustion conditions, the oxidation of SO2 to SO3 is limited.35,37 Besides, operational conditions also have a pronounced impact on the effectiveness of sulfur-based additives. Ferric sulfate shows a low effectiveness when injected at a position with higher flue gas temperatures,15 which was attributed to an increased dissociation of SO3 to SO2.38 The experimental and simulation work of Kassman et al. suggested that the concentrations of O2 and volatile combustibles could influence the conversion of SO3 to SO2, thus affecting the effectiveness of ammonium sulfate on KCl sulfation.23 Despite the recognized importance of SO3 production and temperature conditions on the effectiveness of sulfate additives, fundamental studies on the decomposition rate and product distribution of these sulfates under boiler conditions are scarce,

2KCl(g, l, s) + SO2 + 0.5O2 + H 2O → K 2SO4 (g, s, l) + 2HCl

(1)

2KCl(g, l, s) + SO3 + H 2O → K 2SO4 (g, s, l) + 2HCl (2)

Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies

Sulfation of KCl by sulfur-based additives in biomass-fired boilers is influenced by both the type of additives used and the operational conditions. It has been found that ammonium sulfate is much more effective than elemental sulfur in sulfation of KCl under fluidized-bed combustion conditions,16,17 and © 2013 American Chemical Society

Received: July 31, 2013 Revised: October 24, 2013 Published: October 24, 2013 199

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be released as SO3. The temperature range chosen for ammonium sulfate and aluminum sulfate studied was 600−900 °C, where the conversion of SO3 to SO2 is expected to be negligible in the absence of free radicals.37 To ensure the liability of the results, a number of repetition experiments (3−5 times) were performed under the same operational conditions. It should be noted that solid sulfates were used in the laboratoryscale tube reactor, which differs from boiler applications, where the sulfates were typically injected as spray form.15,17,18,25,26,28,29 In the present study, it is assumed that the yields of SO2 and SO3 are similar, independent to the form of added sulfates (powder or spray). We believe this is a reasonable assumption because the droplets would first undergo a drying process in the boiler, which would produce solid sulfate. Validation of this assumption through experiments is desirable but outside the scope of this work. 2.3. BFB Pilot Reactor. Pilot-scale experiments were conducted in the 20 kW BFB reactor at the VTT Technical Research Centre of Finland. Part of the results from the experiments has been published in an early work,28 while novel results for ammonium sulfate addition are presented in this work. The fuels used in the experiments were a mixture of 60% bark and 40% refuse-derived fuel (RDF) and a mixture of 80% bark and 20% RDF (all mixture fractions on an energy basis). The RDF was comprised mainly of packaging waste from households and industry. The properties of the fuel mixtures are given in Table 1. The additives used in the experiments were ammonium sulfate, aluminum sulfate, and ferric sulfate aqueous solutions.

limiting the optimal use of these additives in biomass combustion. In an earlier work, we have studied the decomposition of ferric sulfate experimentally and proposed a model to describe the sulfation of KCl by ferric sulfate addition during grate firing of biomass.39 In the present work, we aim to study the decomposition behavior of ammonium sulfate and aluminum sulfate under boiler temperature conditions and to develop a model to describe the sulfation of KCl by these sulfates. Following the approach of our earlier work,39 the decomposition of ammonium sulfate and aluminum sulfate was studied in a fast heating rate thermogravimetric analyzer (TGA) and a laboratory-scale tube reactor, respectively. The model developed involved a description of sulfate decomposition, a detailed gas-phase kinetic model for KCl sulfation based on Hindiyarti et al.,35 and a simplified model for homogeneous and heterogeneous condensation of K2SO4.36 Modeling predictions were compared to data from biomass combustion experiments in a pilot-scale bubbling fluidized-bed (BFB) reactor, where ammonium sulfate, aluminum sulfate, and ferric sulfate were injected to the post-combustion zone. Moreover, the effectiveness of different sulfates as well as that of SO2 and SO3 was compared through simulation.

2. EXPERIMENTAL SECTION

Table 1. Properties of the Fuel Mixtures28

2.1. Fast Heating Rate TGA. The thermal decomposition of ammonium sulfate and aluminum sulfate was studied in a fast heating rate Netzsch STA 449 F1 Jupiter TGA. The experiments were carried out in N2 atmosphere, using pure sulfates from Sigma-Aldrich. The ammonium sulfate was in a powdery form and with a purity above 99%. The aluminum sulfate was prepared from aluminum sulfate hydrate with a purity above 98%, which was dehydrated at 400 °C for 1 h under N2 atmosphere and then manually milled to powders in a mortar. In the TGA, the sulfate was heated rapidly to a predefined end temperature and then kept isothermally. The mass loss during the isothermal period was recorded and used to derive the kinetic parameters of sulfate decomposition. To minimize the uncertainties caused by the small mass of residual sample, the kinetic parameters were determined on the basis of a conversion up to 90%. For the different sulfates, the end temperatures and heating rate applied in the experiments varied. For aluminum sulfate, the end temperatures were in a range of 750−800 °C and the heating rate was 500 °C/min. For ammonium sulfate, because of its fast decomposition rate, the end temperatures were chosen to be between 200 and 350 °C. The heating rate was reduced accordingly to 50−200 °C/min, to minimize the effect of temperature overshooting, which could lead to a complete conversion of ammonium sulfate before the isothermal period was achieved. 2.2. Laboratory-Scale Tube Reactor. The yields of SO2 and SO3 from the thermal decomposition of ammonium sulfate and aluminum sulfate were studied in a laboratory-scale tube reactor described in detail in our earlier work.39−41 The reactor consists of an electrically heated horizontal ceramic tube with a length of 1200 mm and an inner diameter of 50 mm. During the experiments, a thin layer of the pure and dehydrated sulfate was placed in an alumina boat and inserted into the middle of the reactor, where the temperature and gas flow conditions (5 NL/min N2) were well-controlled. SO2 released from sulfate decomposition was analyzed continuously by an infrared (IR)based gas analyzer (Fischer-Rosemount NGA 2000 analyzer) with an accuracy of ±2.7%. The experiments lasted until the concentration of SO2 in the flue gas was below 1 ppm, which implied that a full conversion of the sulfate was achieved. This assumption was supported by the fact that the experimental mass loss of the sulfate fitted well with the theoretical mass loss of complete sulfate decomposition. On the basis of the analyzed SO2 concentrations over time, a fractional conversion of sulfur in the sulfate to SO2 under the given temperature condition was calculated. The remaining sulfur in sulfate is assumed to

properties

60% bark/RDF

moisture (wt %, ar) volatiles (wt %, db) ash at 850 °C (wt %, db) C (wt %, db) H (wt %, db) N (wt %, db) S (wt %, db) Cl (wt %, db) K (wt %, db) Na (wt %, db) Ca (wt %, db) LHV (MJ/kg, db)

30.0 76.6 5.0 52.0 6.7 0.43 0.08 0.237 0.20 0.19 1.21 22.9

80% bark/RDF 30.0 75.8 3.8 51.3 6.3 0.35 0.06 0.112 0.17 0.11 1.07 19.7

A schematic diagram of the BFB reactor is shown in Figure 1. The fuel mixture was fed into the bed with a screw-type feeder. The ratio between the primary (fluidizing), secondary, and tertiary air was 50:30:20. The fuel feeding rate was kept at 2.48 kg/h (dry basis), and the total air feeding rate was 278 NL/min. The overall excess air ratio was about 1.25. The measured (by thermocouples) temperature distribution as a function of the residence time in the reactor is shown in Figure 2. It can be seen that the highest temperatures were measured close to the secondary and tertiary air inlets. The sulfate additive was sprayed co-currently to the upper part of the freeboard through a nozzle (see the detailed injection position in Figure 1). The residence time corresponding to the additive injection position was 5.8 s, as illustrated in Figure 2. The drop in the gas temperature because of vaporization was 0−70 °C, depending upon the distance from the spraying point. However, the temperature profile (shown in Figure 2) was always constant because the mass flow of solution was kept constant in all of the tests. The mean droplet size of the spray was about 10 μm. The amount of additive injected was determined on the basis of the fuel Cl content and the fuel feeding rate. The molar ratio of Sadditive/Clfuel was chosen to be 0.225 and 0.75, respectively, for the different sulfates. The aerosols generated in the reactor were collected by a lowpressure impactor at flue gas temperatures of about 650 °C (see the sampling position in Figure 1). The compositions of the aerosols ( ferric sulfate > aluminum sulfate. This tendency is consistent with the tendency of their decomposition temperatures given in the literature, which are 235, 480, and 770 °C for ammonium sulfate, ferric sulfate, and aluminum sulfate, respectively.42 The yields of SO2 and SO3 from the decomposition of ammonium sulfate and aluminum sulfate in the laboratory-scale tube reactor are shown in Figure 5, along with the results of

Fe2(SO4 )3 → Fe2O3 + 1.2SO3 + 1.8SO2 + 0.9O2

(8)

3.2. Detailed Chemical Kinetic Model. A detailed chemical kinetic model is applied to describe the gas-phase 202

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To take into account the impact of the decomposition temperature on the production of SO2/SO3 (see Figure 5), two parallel reactions are introduced to describe the decomposition of ammonium sulfate in CHEMKIN.

reactions of the potassium, chlorine, and sulfur species. It was based on the reaction mechanism proposed by Hindiyarti et al.,35 with updated thermodynamic properties of the species KO2. The detailed kinetic model in CHEMKIN format is available in Supporting Information II. 3.3. Condensation of Sulfate. For the homogeneous and heterogeneous condensation of K2SO4, a first-order reaction as used by Li et al. is chosen to describe the processes.36 K 2SO4 (g) → K 2SO4 (s)

(10)

3.4. Simulation of the BFB Reactor. To model the BFB experiments, the region from the sulfate injection location to the impactor sampling location (see Figure 2) was simulated as an ideal plug-flow reactor. The simulation was carried out with CHEMKIN 4.1.1, using the plug-flow reactor model. The temperature profile used in the simulation was predefined on the basis of the profile shown in Figure 2. The amount and composition of the inlet flue gas were estimated according to the experiments without additive injection. The flue gas was assumed to contain only N2, O2, CO2, H2O, SO2, HCl, and KCl. The concentrations of N2, O2, CO2, and H2O were obtained by assuming that the fuel was fully converted in the reactor. The concentration of KCl was derived on the basis of the amount of Cl in the aerosols collected during the experiments without additive injection. Although Cl collected in the aerosols contained both KCl and NaCl, in the simulation, we assumed that Cl found in the aerosols was only comprised of KCl. This assumption is believed to be reasonable because the molar ratio of K/Cl found in the aerosols was about 0.9, suggesting that KCl was dominated over NaCl. Besides, the sulfation rate of NaCl would be similar to that of KCl under boiler conditions. The HCl concentration in the flue gas was calculated by performing mass balance calculation on Cl based on the amount of input fuel Cl and Cl collected as aerosols. Similarly, the SO2 concentration was obtained from the mass balance calculation of S, according to the input fuel S and S collected as aerosols. For the experiments with additive injection, a pseudogaseous species, SO3*, was defined to simulate the sulfate additive in CHEMKIN. This pseudo-species SO3* only undergoes irreversible reaction to produce SO2, SO3, and O2 (other gas/solid residues from sulfate decomposition, such as NH3, Fe2O3 , and Al2O3 , were not considered in the simulation). For aluminum sulfate, the following reaction is used in the simulation in CHEMKIN: SO3* → 0.85SO3 + 0.15SO2 + 0.075O2

(13)

SO3* → SO2 + 0.5O2

(14)

The rate constants for the equations above are estimated from the kinetic parameters of ammonium sulfate decomposition (Table 2) and the yields of SO2 at different decomposition temperatures (Figure 5). The detailed rate constants used in CHEMKIN and the procedures for obtaining them are given in Supporting Information I. The initial concentration of SO3* was calculated on the basis of the Sadditive/Clfuel ratio used in the experiments. For example, the initial concentration of SO3* in the experiments with 60% bark/40% RDF was 43 and 143 ppmv for Sadditive/Clfuel ratios of 0.225 and 0.75, respectively. The detailed inlet gas compositions used in the simulations are given in Table 3.

(9)

The rate constant for the reaction above, which approximates the condensation rate predicted from the aerosol theory,43 is kcondensation = 1 × 10−61 exp(150000/T )

SO3* → SO3

Table 3. Compositions of the Inlet Gas Used in the Simulation for the Experiments with 60% Bark/40% RDF and 80% Bark/20% RDF species

60% bark/RDF

80% bark/RDF

H2O (%) O2 (%) CO2 (%) SO2 (ppmv) HCl (ppmv) KCl (ppmv) SO3* (ppmv) N2 (%)

16.31 3.96 12.33 68 120 70 0/43/143 balance

15.75 4.65 12.18 50 63 27 0/20/67 balance

From the simulation results, the mass flow of Cl as aerosols was obtained by assuming that all KCl and K2Cl2 found at the outlet of the reactor (i.e., the aerosol sampling location) would be collected as aerosols. 3.5. Simulation for Comparison of Sulfur-Based Additives. To compare the effectiveness of the different sulfur-based additives (sulfates, SO3 and SO2), simulations were carried out using the temperature profile shown in Figure 6, which was estimated on the basis of the cooling rate of the BFB reactor. The simulation was conducted with different sulfur-

(11)

The activation energy and pre-exponential factor for the elementary reaction above are identical to the kinetic parameters of aluminum sulfate decomposition in Table 2. For ferric sulfate, the reaction used in the simulation is SO3* → 0.4SO3 + 0.6SO2 + 0.3O2

(12) Figure 6. Temperature profile used for comparing the effectiveness of different sulfur-based additives (sulfates, SO3, and SO2). The solid symbols denote the different additive injection temperatures/positions used in the simulation.

Similarly, the activation energy and pre-exponential factor for the equation above are the same as the kinetic parameters of ferric sulfate decomposition in Table 2. 203

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based additives and different additive injection temperatures, as illustrated in Figure 6. Because the same outlet temperature (650 °C) and the same cooling rate were applied in the simulation, the residence time varied with different injection temperatures. For instance, the residence time corresponding to an injection temperature of 1100 and 800 °C, was 6 and 2 s, respectively. The flue gas composition used in the simulations was the same as that used for simulating the BFB experiments with a mixture of 60% bark and 40% RDF, and the molar ratio of Sadditive/Clfuel was kept at 0.184, which is the stoichiometric ratio (Sadditive/KCl = 0.5) needed to fully sulfate the KCl in the flue gas. It should be mentioned that the objective of the simulation is to compare the effectiveness of different sulfates under simplified conditions. It is not aimed to compare the results directly to boiler results because the temperature versus residence time profile can be very different in different boilers depending upon its type and configuration.

Figure 8. Concentration profiles of the major S and Cl species during the simulation of ammonium sulfate addition under a Sadditive/Clfuel ratio of 0.75. The fuel was a mixture of 60% bark and 40% RDF.

as SO3*) is completed almost instantaneously (∼30 ms) after injection, forming both SO2 and SO3. The concentration of SO2 becomes constant after the ammonium sulfate is completely decomposed, implying a negligible conversion of SO3 to SO2. Under the residence time of 0−1.5 s, the sulfation of KCl appears to be thermodynamically constrained, reflected both by the small amount of K2SO4 formed and the simultaneous presence of reactive SO3, KHSO4, and KCl. Under the residence time of about 1.5−2 s, a rapid conversion of KCl is observed, which is accompanied by the formation of a considerable amount of K2SO4 (s). This is mainly due to the occurrence of homogeneous and heterogeneous condensation of K2SO4 in the temperature range of 790−750 °C, which removes gaseous K2SO4 and promotes the following reactions of SO3, KHSO4, and KCl:

4. RESULTS AND DISCUSSION 4.1. Ammonium Sulfate Addition in the BFB Reactor. Figure 7 compares the experimental and simulated mass flow of

Figure 7. Comparison of the experimental and simulated mass flow of Cl (mg/Nm3) in the aerosols during ammonium sulfate addition under different molar ratios of Sadditive/Clfuel. The fuel was a mixture of 60% bark and 40% RDF.

KCl + SO3(+ M) = (KO2 )S(O)Cl( +M)

(15)

(KO2 )S(O)Cl + H 2O = KHSO4 + HCl

(16)

KHSO4 + KCl = K 2SO4 + HCl

(17)

4.2. Ferric Sulfate Addition in the BFB Reactor. The experimental and simulated mass flows of Cl (mg/Nm3) in the aerosols during ferric sulfate addition in the BFB experiments with 60% bark and 40% RDF are shown in Figure 9. The simulation results are in good agreement with the experimental results. Both the experimental and simulation results suggest an almost complete sulfation of KCl for a Sadditive/Clfuel ratio of 0.75. Shifting the temperature profile used in the simulation

Cl (mg/Nm3) in the aerosols during ammonium sulfate addition in the BFB experiments with 60% bark and 40% RDF. It can be seen that the model captures the experimental results reasonably well. The mass flow of Cl in the aerosols is slightly underpredicted by the model, implying a possible overprediction of the effectiveness of ammonium sulfate. This could be related to an overestimation of the release of SO3 from ammonium sulfate decomposition, which may be induced by the relative large experimental uncertainties shown in Figure 5. On the other hand, the uncertainties in the temperature profile during the BFB experiments may also influence the simulation results. In the BFB experiments, the temperature in the reactor was measured by simple thermocouples (not suction pyrometers). Consequently, the measured temperature may be slightly lower than the actual flue gas temperature. As illustrated in Figure 7, if the temperature profile is shifted by 20 °C to higher values, less KCl is sulfated by the added ammonium sulfate, resulting in the simulation results being closer to the experimental results. The distribution of the S and Cl species in the simulation zone during ammonium sulfate addition (Sadditive/Clfuel = 0.75) is shown in Figure 8. It is seen that, under the conditions of the BFB boiler, the decomposition of ammonium sulfate (denoted

Figure 9. Comparison of the experimental and simulated mass flows of Cl (mg/Nm3) in the aerosols during ferric sulfate addition under different molar ratios of Sadditive/Clfuel. The fuel was a mixture of 60% bark and 40% RDF. 204

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(Figure 2) by ±20 °C does not have a pronounced impact on the effectiveness of ferric sulfate. However, for a Sadditive/Clfuel ratio of 0.225, slightly more KCl is sulfated when the temperature profile is shifted by 20 °C to higher values, because of a more complete decomposition of ferric sulfate at higher injection temperatures. The distribution of the S and Cl species from the simulation of ferric sulfate is shown in Figure 10. It is seen that the

Figure 10. Concentration profiles of the major S and Cl species during the simulation of ferric sulfate addition under a Sadditive/Clfuel ratio of 0.75. The fuel was a mixture of 60% bark and 40% RDF.

Figure 11. Comparison of the experimental and simulated mass flows of Cl (mg/Nm3) in the aerosols during aluminum sulfate addition under different molar ratios of Sadditive/Clfuel: (a) results for a mixture of 60% bark and 40% RDF and (b) results for a mixture of 80% bark and 20% RDF.

decomposition of ferric sulfate (denoted as SO3*) is much slower than that of ammonium sulfate. Approximately 15% of the ferric sulfate remains unconverted at the outlet of the reactor. However, because of the high Sadditive/Clfuel molar ratio (0.75) and the significant release of SO3 from ferric sulfate decomposition, KCl in the flue gas is still fully converted to K2SO4. The conversion of KCl takes place primarily in the temperature range of 790−750 °C, similar to that of ammonium sulfate addition. 4.3. Aluminum Sulfate Addition in the BFB Reactor. Figure 11 presents a comparison of the experimental and simulated mass flows of Cl (mg/Nm3) in the aerosols during aluminum sulfate addition. For the experiments with a mixture of 60% bark and 40% RDF (Figure 11a), the model significantly underpredicts the effect of aluminum sulfate addition, resulting in a much higher mass flow of Cl in the aerosols than that obtained experimentally. The simulation results indicate that only about 6−7% of the aluminum sulfate is decomposed under the temperature conditions (see Figure 2) of the BFB reactor, which is significantly lower than the conversion of ferric sulfate (∼85%) under the same conditions. This difference is primarily due to the different rate constants applied in the sulfate decomposition model, which are listed in Table 2. When the temperature profile increases by 20 °C, although the effectiveness of aluminum sulfate is increased, the simulation results still differ significantly from the experimental results. On the other hand, for the experiments with a mixture of 80% bark and 20% RDF, the experimental and simulation results show a reasonable agreement (Figure 11b), particularly considering that the actual flue gas temperatures might be slightly higher than the measured (by thermocouples) flue gas temperatures. The discrepancies between the experimental results of panels a and b of Figure 11 imply that there is a need to conduct more experiments to better understand the effect of aluminum sulfate addition on KCl sulfation and to validate the model proposed

in this work. These further investigations will be a subject of future work. 4.4. Comparison of Sulfur-Based Additives. Figure 12 shows a comparison of the simulated effectiveness of ferric

Figure 12. Comparison of the simulated effectiveness of ferric sulfate, ammonium sulfate, SO3, and SO2 under different injection temperature conditions based on the profile in Figure 6. The gas compositions were based on the BFB experiments with 60% bark and 40% RDF. The Sadditive/Clfuel molar ratio was kept at 0.184, which is the stoichiometric ratio needed for a complete sulfation of KCl by the added sulfur-based additive.

sulfate, ammonium sulfate, SO3, and SO2 under different injection temperature conditions based on the temperature profile of Figure 6. The results of aluminum sulfate are not presented because of the existing discrepancy between the simulation and experimental results in the BFB reactor. From the simulation results of Figure 12, it is seen that that the highest effectiveness for ferric sulfate addition is achieved with injection temperatures between 900 and 1000 °C. For a 205

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900 °C, the decomposition of aluminum sulfate produces about 15% SO2 and 85% SO3, whereas the yield of SO3 from ammonium sulfate decomposition decreases with an increasing decomposition temperature. Following the approach of our earlier work on modeling ferric sulfate addition,39 models are developed to describe the sulfation of KCl by aluminum sulfate and ammonium sulfate addition. The simulation results are compared to the biomass combustion experiments conducted in a BFB reactor, where aluminum sulfate, ammonium sulfate, and ferric sulfate are used as additives. The model captures the BFB results for ammonium sulfate and ferric sulfate addition well. For aluminum sulfate, only part of the BFB experiments is predicted satisfactorily, implying a need of conducting further experiments to understand the effect of this additive on KCl sulfation. A comparison of the effectiveness of ammonium sulfate, ferric sulfate, SO3, and SO2 through simulation suggests that the effectiveness of different sulfur-based additives differs and can be influenced significantly by the injection temperatures. The simulation results indicate that ferric sulfate and ammonium sulfate have comparable effectiveness at temperatures ranging from approximately 850 to 900 °C, whereas ferric sulfate is more efficient at higher temperatures and ammonium sulfate is more effective at lower temperatures. Overall, we believe that the model developed in this study gives a basis to estimate the potential of different sulfates at power-plant furnaces. The model can be helpful to find cases to which this problem solution concept can be applied.

Sadditive/Clfuel molar ratio of 0.184, the maximum conversion of KCl to K2SO4 is about 40−45%, because the decomposition of ferric sulfate produces about 60% SO2, which reacts with KCl slowly. When the injection temperature is higher than 1000 °C, the effectiveness of ferric sulfate is decreased because of a more pronounced dissociation of SO3 to SO2, particularly at an injection temperature of 1100 °C, where the effectiveness of ferric sulfate becomes almost identical to that of SO2. However, when the injection temperature is lower than 900 °C, the amount of ferric sulfate decomposed is limited under the residence time conditions of Figure 6, which also limits the effectiveness considerably. According to the simulation results of Figure 12, the effectiveness of ammonium sulfate appears to increase with a decreasing injection temperature, primarily because more SO3 is released when the decomposition of ammonium sulfate takes place at lower temperatures. Under a Sadditive/Clfuel molar ratio of 0.184, the highest conversion of KCl (∼65%) is achieved in the simulation when the ammonium sulfate is injected at 700 °C. The high simulated effectiveness of ammonium sulfate is mainly related to its fast decomposition rate and the significant release of SO3 (see Figure 5) at low injection temperatures. The simulated effectiveness of SO3 injection is also shown in Figure 12. At high injection temperatures (e.g., 1100 °C), the effectiveness of SO3 is similar to that of SO2, because of the rapid dissociation rate of SO3 to SO2. The effectiveness of SO3 increases with a decreasing injection temperature. When SO3 is injected below 900 °C, a complete sulfation of the KCl is achieved for a Sadditive/Clfuel molar ratio of 0.184. In comparison to SO3, the effectiveness of SO2 injection is generally low and seems to decrease with decreasing injection temperatures, which is partly attributed to a less pronounced conversion of SO2 to SO3 and/or generation of radicals at low injection temperatures.35 In general, the simulation results suggest that the effectiveness of different sulfur-based additives differs significantly and their desirable injection temperature varies considerably. It is observed that ferric sulfate and ammonium sulfate have comparable effectiveness at temperatures ranging from approximately 850 to 900 °C. The simulation also indicates that ferric sulfate is more efficient at higher temperatures and ammonium sulfate is more effective at lower temperatures. It should be noted that the results of Figure 12 are based on simulation, which have not been validated through rigorous experiments and cannot be applied directly to complicated fullscale applications. On the other hand, the results of our model have been compared to the experimental results from the BFB reactor, where the power of different sulfates has been studied under fixed conditions. Although the temperature versus residence time profile differs between different power plant furnaces and can be greatly influenced by the boiler load, we believe that the results obtained give a basis to estimate the potential of different sulfates at power plant furnaces. This can help greatly to find cases to which this problem solution concept can be applied.

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ASSOCIATED CONTENT

S Supporting Information *

I, Implementation of the ammonium sulfate decomposition model in CHEMKIN simulation; II, detailed kinetic model used in CHEMKIN simulation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from European Union (EU) Contract 23946 “Demonstration of a 16 MW High Energy Efficient Corn Stover Biomass Power Plant”, funding from the Danish Strategic Research Council (GREEN), and funding from TEKES and support from Metso Power Oy to the contribution of VTT are acknowledged.

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REFERENCES

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5. CONCLUSION Decomposition of aluminum sulfate and ammonium sulfate is studied in a fast heating rate TGA and a laboratory-scale tube reactor. The results reveal that the decomposition of aluminum sulfate and ammonium sulfate can be described well by a volumetric reaction model. In the temperature range of 600− 206

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dx.doi.org/10.1021/ef4015108 | Energy Fuels 2014, 28, 199−207