Sulfation Of KCl Research Articles

Sulfation of Gaseous KCl by H2SO4

The sulfation of gaseous KCl by H2SO4 in the presence of O2 and H2O was investigated in a laboratory laminar flow reactor at atmospheric pressure and temperatures ranging from 873 to 1523 K. The degree of sulfation was characterized by analysis of the elemental composition (Cl, S, and K) of the submicrometer particles captured on a filter downstream of the reactor. Furthermore, concentrations of HCl and SO2 in the outlet were measured. The experimental results were interpreted in terms of a detailed chemical kinetic model for the K/S/Cl chemistry. The degree of KCl sulfation depends on the reaction temperature and the reactant concentrations. The results show that H2SO4 is effective in sulfating KCl to K2SO4 at temperatures in the range 1073–1273 K, in particular at high H2SO4/KCl ratios. At temperatures of 1373 K and above, the sulfation efficiency decreases rapidly because alkali sulfate formation is no longer thermodynamically favored. The kinetic analysis shows that reaction is initiated by decomposition of sulfuric acid: H2SO4 + H2O ⇄ SO3 + H2O + H2O. Then KCl reacts directly with SO3 to form the intermediate KSO3Cl, which is further converted via KHSO4 to gaseous K2SO4. As the gas is cooled downstream of the isothermal zone, potassium sulfate nucleates homogeneously to form an aerosol, promoting further sulfation in the gas phase. The simulations are in satisfactory agreement with the experimental results, predicting correctly the temperature window for the process. However, the model overpredicts the SO2 yield at high temperature, indicating that the K/S interactions under these conditions are not fully understood. The results are important, both for validation of models of K/Cl/S interactions in combustion and for characterization of the sulfation of KCl by sulfuric acid to evaluate the potential of the sulfur recirculation process and identify the optimum process conditions.

The chemical coupling between moist CO oxidation and gas-phase potassium sulfation

In the present work, the chemical coupling between moist CO oxidation and transformation of gaseous potassium salts (KCl or KOH) in the presence and absence of SO2 was investigated experimentally and through chemical kinetic modeling. The experiments were performed in a laminar flow quartz reactor at temperatures ranging from 873 to 1473 K. The experimental results showed that both KCl and KOH inhibited CO oxidation, but addition of SO2 reduced the inhibiting effect by sulfating the potassium. The degree of sulfation of KCl and KOH by SO2 was evaluated by EDX analysis of the aerosols collected downstream in a filter. Also, the consumption of SO2 and, for KCl, the formation of HCl were indications of the level of sulfation. The results indicated that KCl was only sulfated to a small degree, consistent with the observation that addition of SO2 had little effect on the inhibition of CO oxidation by KCl. Contrary to this, the captured KOH particles were fully sulfated according to the EDX results; however, most of the KOH was captured on the quartz reactor surface, forming potassium silicates.The experimental results were interpreted in terms of a chemical kinetic model. Thermodynamic data for key potassium intermediates were re-evaluated by ab initio methods and the mechanism was updated according to recent results. The modeling predictions were in qualitative agreement with the experimental results for the effect of K/Cl/S on moist CO oxidation, but the degree of sulfation was strongly overpredicted for KCl. Analysis of the calculations indicates that sulfation pathways in the model involving KOSO3 contribute to the overprediction, but both the thermodynamic properties and rate constants in the model involve significant uncertainties and more work is required to resolve the discrepancy.

Assessment of the effect of alkali chemistry on post-flame aerosol formation during oxy-combustion of biomass

The oxy-combustion of biomass enables negative CO2 emissions by combining subsequent CO2 capture technology. The potassium sulfation process significantly affects deposition and corrosion in heat transfer surfaces during biomass combustion. In the present work, a detailed aerosol dynamics model coupling with the detailed gas-reaction chemistry of K-S-Cl is proposed to investigate the effect of alkali chemistry on the evolution of post-flame aerosol during oxy-combustion of biomass. According to the modelling results, the mass-based particle size distributions are generally unimodal. Changing the environment from the air (N2 as a balance gas) to oxy (CO2 as balance gas) has a slight effect on the particle size distribution; yet, a slight left-shift particle size distribution was observed. The difference is mainly explained by the more substantial diffusion capacity of KCl(g) and K2SO4(g) in N2 than that in CO2, indicating a bit higher heterogeneous condensation of KCl(g) and K2SO4(g) in a CO2-based atmosphere. Further, the modelling results revealed that oxy-combustion significantly affects the evolution of aerosol and sulfation of KCl regardless of flue gas recirculation strategy. The wet oxy-combustion case has the largest particle size of PM1.0, that is due to the higher concentration of water and SO2, which increased KCl sulfation with value of ∼ 92%. The increased K2SO4 concentration in the flue gas causes earlier onset nucleation and prolonged the residence time for particle growth. Further ROP analysis results indicate that the reaction pathway for the sulfation of KCl via SO3 and K2SO4 condensation significantly influences the overall sulfation.

Sulfur capture and release by ilmenite used as oxygen carrier in biomass combustor

Oxygen-Carrier-Aided Combustion (OCAC) is a promising technology for the combustion of solid heterogeneous fuels such as biomass and waste. However, a frequently encountered problem with biomass and waste is their high alkali contents, which can lead to the build-up of deposits on heat transfer surfaces, leading to corrosion and increased maintenance costs.The addition of elementary S is one of the options used in combustion processes to mitigate corrosion. Sulfation of KCl reduces the severity of corrosion on heat transfer surfaces due to the transformation of KCl into K2SO4, which is considered to be less corrosive. Even though many studies have been performed on the influence of sulfur on the corrosion process, there is to date little knowledge about the interaction of S with ilmenite used as an oxygen carrier (OC). Therefore, there is a need to understand the influence of S on both the properties of the ilmenite used as an OC and the morphologic development of the ilmenite particles. This study investigates the interactions between S and ilmenite as bed material, with the focus on the roles of ash layers in the capture and release of the added S under different combustion conditions.The present study focuses on bed material that was exposed in the Chalmers 12-MWth Circulating Fluidized Bed (CFB) combustor run with biomass as fuel and with the addition of elemental S. Calcined ilmenite without ash elements was used as a reference material and the collected bed material samples were tested for their abilities to capture S in a laboratory fluidized bed batch reactor run at 950 °C. Controlled SO2 exposures in an oxidizing environment were carried out, and the reversibility of the mechanisms for uptake and release of S was tested. Understanding the uptake and release of S components by ilmenite is of importance not only for the handling of bed materials, but also for developing new protocols for corrosion prevention in OCAC.In the present study, the ilmenite-sulfur interactions were followed using gas analysis, solid material characterization (SEM-EDX), and thermodynamic calculations (FactSage). The results suggest that SO2 is captured under oxidative conditions, preferentially by bed materials with developed ash layers, and released under reducing conditions. Mixed-sulfate-phase K2Ca2(SO4)3, K2SO4 and CaSO4 are found to be the main formed phases that can participate in S uptake and release.

Modeling post-flame sulfation of KCl and KOH in bio-dust combustion with full and simplified mechanisms

The gas-phase interaction between alkali volatiles and sulphur oxides has important implications for deposition and corrosion in combustion of biomass. In the present study, gas-phase transformation of KOH and KCl in the post-flame zone in bio-dust combustion has been studied by detailed chemical kinetic modeling for a woody and a herbaceous biomass, respectively. For both biomasses K > Cl on a molar basis, and KOH is the major alkali species at high temperature. The modeling indicates that KOH is readily converted to K2SO4 in the presence of SO2 at temperatures below 1500 K. Below 1100 K, gaseous K2SO4 nucleates homogeneously, promoting further sulfation. For the woody biomass, where K < Cl + 2S, the sulfation process is kinetically limited due to the competition with chlorination. For the herbaceous biomass, with K > Cl + 2S, the excess KOH facilitates internal equilibration among the alkali species. For both biomasses, the KCl concentration remains constant during sulfation, because any KCl consumed is rapidly replenished by the reaction KOH + HCl ⇄ KCl + H2O. The consequence is that the sulfation process under the investigated conditions does not help to remove the main corrosive agent, KCl. For use in CFD, a skeletal model for the gas-phase sulfation of KCl and KOH has been developed, based on systematic reduction of the detailed chemical kinetic model. However, for fuels with excess K compared to Cl + 2S, a simple equilibrium calculation may yield satisfactory results.

Experimental study of the kinetics of sulfation of alkali chloride deposits

The composition of ash deposits formed in biomass combustion systems can be significantly modified by the sulfation of alkali salts. In order to assess the efficiency of this process, the conversion of alkali chlorides into alkali sulfates occurring in deposits at surface temperatures representative of those in biomass boilers (350–650°C) was quantified by means of specific short-term experiments with realistic deposits. The influence of operating conditions (reaction time, surface temperature and sulfur concentration) was characterized and the results were used to derive a kinetic expression for the in-deposit sulfation of KCl. This kinetics was compared with those proposed by other authors for solid NaCl and molten KCl. Contrary to previous results for solid NaCl, but in agreement with observations for solid KCl and calculations for molten KCl, the proposed kinetics predicts a relevant effect of sulfation on the properties of deposits formed in biomass-firing boilers even in the short-term scale.

Modeling the Use of Sulfate Additives for Potassium Chloride Destruction in Biomass Combustion

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 temperature-dependent 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.

Modeling of ferric sulfate decomposition and sulfation of potassium chloride during grate‐firing of biomass

Ferric sulfate is used as an additive in biomass combustion to convert the released potassium chloride to the less harmful potassium sulfate. The decomposition of ferric sulfate is studied in a fast heating rate thermogravimetric analyzer and a volumetric reaction model is proposed to describe the process. The yields of sulfur oxides from ferric sulfate decomposition under boiler conditions are investigated experimentally, revealing a distribution of approximately 40% SO3 and 60% SO2. The ferric sulfate decomposition model is combined with a detailed kinetic model of gas‐phase KCl sulfation and a model of K2SO4 condensation to simulate the sulfation of KCl by ferric sulfate addition. The simulation results show good agreements with experiments conducted in a biomass grate‐firing reactor. The results indicate that the SO3 released from ferric sulfate decomposition is the main contributor to KCl sulfation and that the effectiveness of ferric sulfate addition is sensitive to the applied temperature conditions. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4314–4324, 2013

Sulfation of Condensed Potassium Chloride by SO2

The interaction between alkali chloride and sulfur oxides has important implications for deposition and corrosion in combustion of biomass. In the present study, the sulfation of particulate KCl (90–125 μm) by SO2 was studied in a fixed bed reactor in the temperature range 673–1023 K and with reactant concentrations of 500–3000 ppm SO2, 1–20% O2, and 4–15% H2O. The degree of sulfation was monitored by measuring the formation of HCl. Analysis of the solid residue confirmed that the reaction proceeds according to a shrinking core model and showed the formation of an eutectic at higher temperatures. On the basis of the experimental results, a rate expression for the sulfation reaction was derived. The model compared well with literature data for sulfation of KCl and NaCl, and the results indicate that it may be applied at even higher SO2 concentrations and temperatures than those of the present study. Simulations of sulfation of KCl particles with different size indicate that only for very small KCl particle...

Experimental Evaluation and Field Application of a Salt Method for SO3 Measurement in Flue Gases

In this study, an SO3 measurement technique was evaluated and developed. In the method, a salt is used to capture gaseous SO3/H2SO4. Various salts were tested to evaluate the suitability to measure SO3/H2SO4 without interference from SO2. Salts tested include NaCl, KCl, K2CO3, and CaCl2. The salts were tightly packed into a Teflon tube, and the gas was fed through the salt tube with subsequent reaction between SO3/H2SO4 and the salt with formation of sulfates of the respective salt. After the measurement, the salt was dissolved in water, and the solution was analyzed for sulfate ions. The SO3/H2SO4 concentration in the flue gas could then be determined because the gas volume flowing through the salt was measured together with the amount of sulfate bound in the salt. The method was tested in laboratory conditions, in a 100 kWth test unit during air-firing and oxy-fuel combustion, and in an industrial boiler. A first attempt to continuously measure SO3/H2SO4 indirectly with an FTIR, by measuring the release of HCl in the sulfation of KCl, was also made. The conversion of SO3 to H2SO4 in flue gas conditions is discussed. It was found that at the measurement conditions almost all SO3 is present as H2SO4. Therefore, the laboratory study was made with gaseous H2SO4 instead of SO3. The laboratory tests showed that all salts captured all H2SO4. The best selectivity toward H2SO4 was shown for NaCl and KCl; no significant amount of SO2 was captured in these salts. An in situ implementation of the salt method using KCl as salt was used during heavy oil combustion in a Kraft recovery boiler. The salt method showed to be an accurate, inexpensive, and easy way to measure SO3/H2SO4 in flue gases.

Post-flame gas-phase sulfation of potassium chloride

The sulfation of KCl during biomass combustion has implications for operation and emissions: it reduces the rates of deposition and corrosion, it increases the formation of aerosols, and it leads to higher concentrations of HCl and lower concentrations of SO2 in the gas phase. Rigorously homogeneous systems are required to characterize the gas-phase formation of alkali sulfates. We have measured the temperature and gas-phase concentrations of KCl and HCl, and detected the presence of aerosols in the post-flame region of a range of hydrocarbon flames seeded with KCl, with and without the addition of SO2. Dilution of the flame products with different amounts of N2 ensured post-flame temperatures in the range 950–1400K. In the absence of SO2, KCl levels were constant in the post-flame zone and no aerosols were formed, even at the lowest temperatures. In the presence of SO2, KCl was consumed and HCl and aerosols formed, most pronounced in flames with the lowest post-flame temperatures. This shows that KCl is sulfated in the gas phase to K2SO4, and this is followed by homogeneous nucleation of K2SO4 to form aerosols. Predictions from a kinetic model of the S/Cl/K chemistry agreed well with the experimental results. The model showed that the rate-limiting steps in the sulfation process are the oxidation of SO2 to SO3 and the homogeneous nucleation of K2SO4.

Lab-scale Investigation of Deposit-induced Chlorine Corrosion of Superheater Materials under Simulated Biomass-firing Conditions. Part 1: Exposure at 560 °C†

Deposit-induced chlorine corrosion was studied under well-controlled laboratory conditions, simulating the conditions in straw-fired boilers and boilers cofiring coal and straw. This was done by exposing pieces of superheater tube (TP 347H FG) covered with synthetic deposits of known Cl content to gas mixtures simulating straw-firing and cofiring of coal and straw, at 560 °C (1040 °F), for 3 days. The corroded specimens, and the reacted deposits, were studied in detail using a scanning electron microscope to determine the corrosion rate, investigate the chemistry and morphology of the corrosion attack, and study the sulfation behavior. Besides the gas compositions, various parameters were studied systematically. Most specimens suffered some internal attack, mostly by selective corrosion and in some cases by grain boundary attack. In all experiments with KCl and KCl−SiO2 deposits, the corrosion products consisted of an oxide scale, containing oxides of Cr and Fe, and on top of that a characteristic mixed layer of iron oxide threads in a potassium sulfate matrix. However, the thickness and shape of this layer was found to be strongly dependent on the experimental conditions. An increase of the percentage of KCl in the deposit resulted in a more uniform and deeper internal corrosion attack. The presence of HCl in the flue gas did not seem to be essential for chlorine-induced corrosion to occur, when a deposit containing KCl was present, but it enhanced the corrosion rate. The degree of sulfation of KCl in the deposits after exposure was quantified by wet chemical analysis and was shown to be dependent on many parameters, including the SO2 concentration in the gas flow, the concentration of KCl in the deposit, and the SiO2 and KCl particle sizes in the deposit. No simple relation was observed between the degree of sulfation in the deposit and the depth of internal attack or the thickness of the oxide or mixed layer. Whereas SiO2 particles were found to be chemically inert with respect to the flue gas and the corrosion attack, CaO particles reacted with HCl from the flue gas, and the resulting CaCl2 played an important role in the corrosion mechanism. As a result, the corrosion rate was strongly enhanced when CaO was present in the deposit, instead of SiO2.

Mechanism and modeling of the formation of gaseous alkali sulfates

The formation of gaseous alkali sulfates is known to yield aerosols and may also contribute to deposition and corrosion in the combustion of solid fuels such as biomass. In the present work a model for the gaseous sulfation of alkali hydroxide ( AOH) and alkali chloride ( ACl) is developed. It relies on a detailed chemical kinetic model for the high-temperature gas-phase interaction between alkali metals, the O/H radical pool, and chlorine/sulfur species. Thermodynamic properties of a number of potassium and sodium species have been estimated from Gaussian 3 ab initio calculations. Particular attention is paid to alkali hydrogen sulfates and alkali oxysulfur chlorides as potential gas-phase precursors of A 2 SO 4. Rate constants have been drawn from the literature, from analogy with known reactions, or from QRRK theory. A detailed reaction mechanism for sulfation is proposed. The alkali transformations proceed by a number of molecule–molecule reactions, which can be expected to exhibit ionic behavior. The sulfation is initiated by oxidation of SO 2 to SO 3. Sulfur trioxide subsequently recombines rapidly with AOH or ACl to form an alkali hydrogen sulfate, AHSO 4, or an alkali oxysulfur chloride, ASO 3Cl. According to the present work, both of these complexes are sufficiently stable in the gas phase to act as precursors for alkali sulfate. Alkali hydrogen sulfate and alkali oxysulfur chloride are subsequently converted to alkali sulfate in fast shuffle reactions such as ASO 3Cl + H 2O→ AHSO 4 + HCl and AHSO 4+ ACl → A 2 SO 4 + HCl. Modeling predictions compare favorably with the experimental results of K. Iisa et al. [Energy Fuels 13 (1999) 1184–1190] on the gas-phase sulfation of potassium chloride at 1373 K. They investigated the degree of sulfation of KCl in a gaseous mixture of SO 2 + O 2 + H 2O + N 2 as a function of reaction time and gas composition in an entrained flow reactor. The modeling predictions are not sensitive to the estimated properties in the alkali subset (thermodynamic data, rate constants) within the assigned error limits; the predicted degree of sulfation is influenced mainly by the rate of oxidation of SO 2 and the production of chain carriers in the system.

Evolution of HCl from KCl, CaCl2 and NaCl in an SO2-O2-H2O Atmosphere under a Simulated Condition of Municipal Wastes Incineration.

The HCl evolution behavior in the process of sulfation of KCl, CaCl2 and NaCl was studied by using a lab-scale gas flow-type tubular reactor, in simulated conditions of municipal waste incinerations, under mixed gas of SO2: 0.3–1.3 vol%, O2: 2.5–15 vol%, H2O: 5–20 vol%, in the temperature range of 623–1123 K. The sulfation rates of KCl, CaCl2 and NaCl changed distinguishably with the temperature above 923 K, for each inorganic chloride of KCl, CaCl2 and NaCl. Comparing the time-changes of the concentration of HCl released from KCl, CaCl2 and NaCl, the rate of HCl evolution from CaCl2 was the highest; the maximum concentration of HCl discharged from CaCl2 was more than 5 times higher than those from NaCl and KCl at 923 K. The reaction kinetic parameters like the rate constant and the reaction orders with respect to SO2, O2 and H2O partial pressure for the sulfation of NaCl, KCl and CaCl2 were determined. It was suggested that the mechanism of formation of HCl by sulfation of KCl, CaCl2 and NaCl was different above and below the temperature range of 850–900 K. A higher sulfation rate of KCl, CaCl2 and NaCl might be attributable to a partial melting and/or volatilization of these inorganic chlorides at the temperatures higher than the temperature range of 850–900 K. The effect of SO2 partial pressure on the sulfation rate was 2–4 times larger for NaCl than for KCl and CaCl2. For the sulfation of KCl, the reaction order with respect to the O2 partial pressure was almost a half of that of NaCl. On the other hand, the rate of sulfation of CaCl2 was almost independent on the O2 partial pressure. The dependence of H2O partial pressure on the sulfation rates of KCl, CaCl2 and NaCl was almost the same in lower temperatures of 623–923 K. However, the reaction order with respect to H2O at 923–1123 K became 2–4 times larger than that in the lower temperature regions.

Sulfation of Potassium Chloride at Combustion Conditions

The sulfation of KCl was studied in the gas and molten phase in a laminar entrained-flow reactor. The experiments were performed at 900−1100 °C with residence times of 0.24−1.22 s. Small, 65−125 mi...