Sulfation Of Sorbents Research Articles

Effect of sulfation on CO2 capture of CaO-based sorbents during calcium looping cycle

Calcium looping cycle is considered as one of the most promising post combustion CO2 capture technologies that can curb CO2 emissions from power plants fired with fossil fuel. However the presence of SO2 and steam in the flue gas leads to the sulfation of CaO-based sorbents, and thus reduces the sorbents’ capacity for CO2 capture. In this study, the effect of sorbent sulfation during both carbonation and calcination stage on the cyclic CO2 capture performance was investigated. The results showed that the reaction time, SO2 concentrations and steam greatly influenced the capacity of CaO-based sorbent. The sulfation degree of the sorbent decreased with the reduction of carbonation and calcination time per cycle. High SO2 concentration present with steam led to a rapid loss in capacity of the sorbent within a few cycles. Furthermore, the synthetic sorbent derived from a sol–gel process had much higher cyclic CO2 capture capacities than natural limestone sorbent whether in conditions without SO2 or with SO2 and steam. Microscopic images showed that after multiple cycles in the presence of SO2 and steam, the porous structure of the synthetic sorbent could be retained while limestone experienced serious pore blockage and agglomeration of grains.

Effect of Support Material on Carbonation and Sulfation of Synthetic CaO-Based Sorbents in Calcium Looping Cycle

Calcium looping cycle for post-combustion CO2 capture has gained increasing attention worldwide. However, CaO-based sorbents derived from natural sources for calcium looping cycle experience rapid loss of capacity during high-temperature cyclic carbonation/calcination reactions. Synthesizing sintering-resistant CaO-based sorbents by adding a support material has been extensively studied as an effective method of combating the problem. The support material in the synthetic sorbents plays an important role in retaining the capacity, and various support materials have been tested in the literature. In practical reactors, sulfur is present and it has been reported that sulfation of sorbents will also reduce the CO2 capture capacity. However, thus far, it is not clear whether/how support material would affect sulfation of sorbents and, thus, CO2 capture. In this paper, four different support materials, such as Ca2MnO4, La2O3, Ca12Al14O33, and MgO, were studied. The cyclic CO2 capture performance of the synthetic sorbents made from CaO and the support materials were investigated in detail, in the presence of SO2 and steam. The results showed that a mass ratio of 20–25% support material would be optimum for synthesizing sorbents with high cyclic CO2 capture capacity, and Ca12Al14O33 and MgO seem to be more effective than Ca2MnO4 and La2O3. The 80:20 wt % CaO/MgO synthetic sorbent achieved the highest CO2 capture capacity under ideal conditions over 100 cycles. However, the CaO/MgO sorbent had a strong affinity to SO2 capture during cyclic reactions, especially in the presence of steam. Under realistic conditions (i.e., both SO2 and steam are present during carbonation), the CaO/MgO sorbent showed the highest cumulative SO2 capture capacity, whereas the CaO/Ca12Al14O33 sorbent obtained the highest CO2 capture capacity after 10 cycles. The smaller average crystallite size of MgO in the sorbent was responsible for the strong SO2 affinity of the CaO/MgO sorbent as well as its stable cyclic CO2 capture abilities under ideal conditions.

Modeling of Limestone Sulfation for Typical Oxy-Fuel Fluidized Bed Combustion Conditions

A model based on the shrinking core model is proposed and evaluated to predict the behavior of limestones during the sulfation reaction under oxy-fuel firing mode in calcining conditions at typical time scales and particle sizes of circulating fluidized bed combustors (CFBCs). This model is chosen due to its simplicity in order to incorporate it in an easy way into available global CFBC models. It is observed that the model predicts properly the evolution of the sorbent sulfation conversion assuming that the first step of the sulfation reaction is controlled by gas diffusion through the porous system of the particle and the second step is controlled by gas diffusion through the CaSO4 product layer. The kinetic parameters needed in the model are determined for two limestones. For that, sulfation reaction tests were carried out in a thermogravimetric analyzer and in a batch fluidized bed reactor.

Kinetics of the Reaction of Iron Blast Furnace Slag/Hydrated Lime Sorbents with SO2 at Low Temperatures: Effects of the Presence of CO2, O2, and NOx

The effects of the presence of CO2, O2, and NOx in the flue gas on the kinetics of the sulfation of blast furnace slag/hydrated lime sorbents at low temperatures were studied using a differential fixed-bed reactor. When O2 and NOx were not present simultaneously, the reaction kinetics was about the same as that under the gas mixtures containing SO2, H2O, and N2 only, being affected mainly by the relative humidity. The sulfation of sorbents can be described by the surface coverage model and the model equations derived for the latter case. When both O2 and NOx were present, the sulfation of sorbents was greatly enhanced, forming a great amount of sulfate in addition to sulfite. The surface coverage model is still valid in this case, but the model equations obtained show a more marked effect of relative humidity and negligible effects of SO2 concentration and temperature on the reaction. The effect of sorbent composition on the reaction kinetics was entirely represented by the effects of the initial specific surface area (Sg0) and the Ca molar content (M−1) of sorbent. The initial conversion rate of sorbent increased linearly with increasing Sg0, and the ultimate conversion increased linearly with increasing Sg0M−1. The model equations obtained in this work are applicable to describe the kinetics of the sulfation of the sorbents in the low-temperature dry and semidry flue gas desulfurization processes either with an upstream NOx removal unit or without.

Sulfation of CaO Particles in a Carbonation/Calcination Loop to Capture CO2

CaO is being proposed as a regenerable sorbent of CO{sub 2} via a carbonation/calcination loop. It is well known that natural sorbents lose their capacity to capture CO{sub 2} with the number of cycles due to textural degradation. In coal combustion systems, reaction with the SO{sub 2} present in flue gases also causes sorbent deactivation. This work investigates the effect of partial sorbent sulfation on the amount of CaO used in systems where both carbonation and sulfation reactions are competing. We have found that SO{sub 2} reacts with the deactivated CaO resulting from repetitive calcination/carbonation reactions. Therefore, the deactivation of CaO as a result of the presence of SO{sub 2} is lower than one would expect if one assumes that SO{sub 2} reacts only with active CaO. This work shows that changes in the texture of the sorbent due to repetitive carbonation/calcination cycles tend to increase the sulfation capacity of the sorbents tested. This suggests that the purge of deactivated CaO obtained from a CO{sub 2} capture loop could be a more effective sorbent of SO{sub 2} than fresh CaO.

Pore Structure Effects on Ca-Based Sorbent Sulfation Capacity at Medium Temperatures: Activated Carbon as Sorbent/Catalyst Support

The reaction between three different Ca-based sorbents and SO2 were studied in a medium temperature range (473–773 K). The largest SO2 capture was found with Ca(OH)2 at 773 K, 126.31 mg SO2•g Ca(OH)2 −1, and the influence of SO2 concentration on the sorbent utilization was observed. Investigations of the internal porous structure of Ca-based sorbents showed that the initial reaction rate was controlled by the surface area, and once the sul-fated products were produced, pore structure dominated. To increase the surface area of Ca-based sorbents available to interact with and retain SO2, one kind of CaO/activated carbon (AC) sorbent/catalyst was prepared to study the effect of AC on the dispersion of Ca-based materials. The results indicated that the Ca-based material dispersed on high-surface-area AC had more capacities for SO2 than unsupported Ca-based sorbents. The initial reaction rates of the reaction between SO2 and Ca-based sorbents and the prepared CaO/AC sorbents/cata-lysts were measured. Results showed that the reaction rate apparently increased with the presence of AC. It was concluded that CaO/AC was the active material in the des-ulfurization reaction. AC acting as the support can play a role to supply O2 to increase the affinity to SO2. Moreover, when AC is acting as a support, the surface oxygen functional group formed on the surface of AC can serve as a new site for SO2 adsorption.

The effect of the porous structure on sorbent sulfation under coal-fired boiler conditions

A study of the behavior of different sorbents (3 limestones, a commercial calcium carbonate and a commercial calcium hydroxide) in the sulfation process has been carried out in a drop-tube, using similar conditions to those existing in sorbent injection in pulverized-coal boilers. The main variables affecting the sulfation process have been analyzed, namely, Ca S molar ratio, SO 2 concentration, temperature, sorbent particle size and reaction time. The work concentrates on the evolution of the porous system in the sorbents and its influence on their sulfation capacity. A direct relation between chemical structure (calcium hydroxide-calcium carbonate) and sorbent reactivity was not found. Sorbents with a widespread pore size distribution with pores above 100 Å showed the highest reactivity and sulfation capacity, maintaining pores of small size (under 80 Å) at all times and operating conditions. In the less reactive sorbents, pores less than 80 Å became blocked during sulfation, being more sensitive to the effect of the different operating variables affecting the sulfation process.

Isotope study on diffusion in CaSO4 formed during sorbent‐flue‐gas reaction

In sorbent-flue-gas reactions, porous CaO sorbent particles are used to capture SO{sub 2} by formation of CaSO{sub 4}. Because of the large molar volume of CaSO{sub 4}, the internal surface area which is originally available for reaction diminishes as CaSO{sub 4} forms. Once the CaSO{sub 4} layer forms, further sorbent sulfation is believed to be controlled by the product layer diffusion process. It has been suggested that the product layer diffusion occurs by gaseous diffusion (Simons and Garman, 1976) and by ionic diffusion (Bhatia and Perlmutter, 1981). In this work, a two-stage sulfation experiment using {sup 32}SO{sub 2} and {sup 34}SO{sub 2} was performed. For the first stage of sulfation, at 1,300 C, 5,000 ppm {sup 32}SO{sub 2}/air mixture was passed into the mullite tube and circulated out through the bubbler continuously. This stage lasted for 14 days. When the first stage was terminated, the tablets were removed from the furnace and examined. At the beginning of the second stage sulfation, 5,000 ppm {sup 32}SO{sub 2}/air mixture was first used during the heating period. As soon as the tube temperature reached 1,300 C, the mechanical pump was turned on and the pressure in the tube was reduced immediately. Upon themore » completion of the evacuation, isotope gas 75%{sup 34}SO{sub 2}-25%{sup 32}SO{sub 2} was introduced into the mullite tube. Appropriate amount of air was also introduced into the tube such that the total SO{sub 2} concentration was roughly 5,000 ppm. The second stage sulfation lasted for three days. The SIMS analysis was performed by Microelectronics Center in North Carolina.« less

Dual SO 2 -NO x Concentration Reduction by Calcium Salts of Carboxylic Acids

The simultaneous removal of SO 2 and NO x by calcium salts of carboxylic acids was evaluated in a laboratory-scale furnace in atmospheres containing 2,000 ppm SO 2 , 1,000 ppm NO, 3% O 2 , and 12% CO 2 at gas temperatures between 550 and 1,250°C (825 and 1,425 K) with a residence times of 4 s in an isothermal zone. The salts studied included calcium formate (CF, Ca(COOH) 2 ), calcium acetate (CA, Ca(CH 2 COOH) 2 ), calcium propionate CP, (Ca(CH 2 CH 2 COOH) 2 ) and calcium benzoate (CB, Ca(C 6 H 4 COOH) 2 ). Maximum reductions in SO 2 concentration due to the formation of CaSO 4 were recorded as follows: 60% for CP, 55% for CF, 35% for CA, and 18% for CB at gas temperatures in the neighborhood of 950°C and at a Ca/S molar ratio near 2.8. With the organic portion of the injected salts acting as secondary fuel for NO x control, maximum NO x concentration reductions were 88% for CP, 65% for CB, 22% for CA and less than 10% for CF at gas temperatures near 950°C at bulk equivalence ratios (actual to stoichiometric organic fuel-to-air ratio) that varied between 0.5 for CF and 2.1 for CB. A recently constructed, narrower furnace increased the gas flow velocities and the particle dispersion in the gas. Therein, at residence times as low as 1 s, CP achieved simultaneous SO 2 and NO x removal of over 80% at gas temperatures ≥950°C. The addition of an oxidizing zone following the carboxylic acid sorbent injection zone affected the overall SO 2 -NO x removal according to the stoichiometry of the sorbent injection zone. When this zone was: (1) fuel-lean, the overall SO 2 removal improved; (2) stoichiometric, the overall SO 2 removal was not affected; and (3) significantly fuel-rich, the overall SO 2 removal worsened. Under most conditions, the overall reduction of NO x was not affected by the oxidizing zone. At temperatures below 800°C, Ca either was retained as CaCO 3 or remained bound to the organic compounds in the sorbents. At temperatures between 950 and 1,150°C, increasing amounts of CaO were found. The composition of the calcined sorbent and the availability of Ca was used to interpret the results of a cenosphere sulfation model. The sorbent sulfation kinetics were found to be bounded by those of pure CaO and pure CaCO 3 .

Interaction between emissions of sulfur dioxide and nitrogen oxides in fluidized bed combustion

NH 3 is an important volatile N-containing compound and a precursor for NO x formation in fluidized bed combustion (FBC). This work focuses on the influence of the sulfation process on the oxidation of NH 3 to NO catalysed by different CaO based sorbents and other bed materials (sand, Al 2O 3). Experiments of simultaneous sorbent sulfation and oxidation of NH 3 catalysed by the sorbents were performed in a laboratory fixed bed reactor in order to study changing catalytic activity. The influence of SO 2 on the oxidation of NH 3 to NO for different bed materials was investigated as well. The results show that desulfurization interferes with the catalytic conversion of NH 3 to NO in two ways. First, CaO based sorbents are active catalysts for the oxidation of NH 3 to NO; this catalytic activity decreases with an increase in the conversion of CaO to CaSO 4. For the limestones, which are subject to pore plugging, the sorbents totally lose their activity for NO formation from NH 3 when the maximum conversion of CaO to CaSO 4 is reached. Second, the catalytic formation rate of NO decreases in the presence of SO 2 for some bed materials. This means that the presence of SO 2 inhibits oxidation of NH 3 to NO. Moreover, it appears that SO 2 inhibits the decomposition of N 2O on the surface of the catalysts as well.

Methods for characterization of sorbents used in fluidized bed boilers

Limestone addition to fluidized bed coal combustors permits the reduction of SO 2 levels emitted into the atmosphere. However, for its industrial use it is necessary to develop simple laboratory methods for the determination of parameters that, inserted in a model, permit the reliable prediction of the behaviour of the sorbent in the boiler. In this paper, the principal methods used for the characterization and determination of these parameters, thermogravimetric and batch fluidized bed, are analysed. Three limestones with different pore size distributions and hardness have been used for this analysis. The effect of the principal process variables affecting sulfur retention, i.e. calcination conditions, sorbent particle size, temperature and SO 2 concentration, has been analysed and compared using the two above mentioned methods. The very different sorbent behaviour, depending on particle size, requires the use of different particle size intervals for the characterization. Moreover, different effects of CO 2 and SO 2 concentration, over the sorbent sulfation behaviour in each method, have been found. The characterization parameters obtained depend on the method and operation conditions used. Therefore, different predictions of limestone requirements in the boiler are obtained. Depending on the type of pore size distribution in the limestones, thermogravimetric analysis cannot be used for sorbent characterization. Batch fluidized bed characterization must be made at an average CO 2 and SO 2 partial pressure, similar to those present during coal combustion.

Interactions between emissions of sulfur dioxide and nitrogen oxides in fluidized bed combustion of coal

Abstract NH3 is an important volatile N-containing compound and a precursor for NOx formation in fluidized bed combustion (FBC). This work focuses on the influence of the sulfation process on the oxidation of NH3 to NO catalysed by different CaO based sorbents and other bed materials (sand, Al2O3). Experiments of simultaneous sorbent sulfation and oxidation of NH3 catalysed by the sorbents were performed in a laboratory fixed bed reactor in order to study changing catalytic activity. The influence of SO2 on the oxidation of NH3 to NO for different bed materials was investigated as well. The results show that desulfurization interferes with the catalytic conversion of NH3 to NO in two ways. First, CaO based sorbents are active catalysts for the oxidation of NH3 to NO; this catalytic activity decreases with an increase in the conversion of CaO to CaSO4. For the limestones, which are subject to pore plugging, the sorbents totally lose their activity for NO formation from NH3 when the maximum conversion of CaO to CaSO4 is reached. Second, the catalytic formation rate of NO decreases in the presence of SO2 for some bed materials. This means that the presence of SO2 inhibits oxidation of NH3 to NO. Moreover, it appears that SO2 inhibits the decomposition of N2O on the surface of the catalysts as well.

The influence of oxygen-stoichiometry on desulfurization during FBC: A simple sure modeling approach

In this paper an extension is presented cf the simple SUlfur REtention SURE model which was published earlier by the authors (Schouten and Van den Bleek, 1987). The present model explains the effect of a decreasing desulfurization efficiency during staged Fluidized Bed Combustion of coal by the relatively fast formation of SO 3 as a gaseous intermediate reactant in the sorbent sulfation reaction. The SURE model provides an analytical expression for the molar (Ca/S) ratio in the reactor feed as a function of the degree of sulfur retention, the maximum sorbent conversion and three dimensionless model parameters. These parameters are functions of fluid bed operating conditions, coal and sorbent properties and the stoichiometric air ratio. It is shown that the oxygen concentration and the decrease in sulfur retention at lower stoichiometric air ratios can readily be described by the model equations. Furthermore, the SURE model is applied as a diagnostic tool for the analysis of plant operational data obtained from the recent literature. It is concluded from an evaluation of SURE model calculations and experimental data that the sorbent residence time is a crucial system parameter. It is demonstrated that the observed neglect of the influence of the sorbent residence time on the steady state character of many retention experimentations gives rise to serious doubt on the reliability and usefulness of these measurements for model verification purposes.

Fluidized-bed coal combustion: In-bed sorbent sulfation model

A model is developed to account for sulfur capture within the sorbent bed of a fluidized-bed coal combustor. The model is expressed in an analytical form that contains two sorbent specific parameters. The sorbent parameters, which describe the kinetics of the sulfation reaction are obtained from thermogravimetric analysis data on the SO 2-sorbent reaction. Various hydrodynamic and mass transfer properties in relation to the bubbling phenomena are taken into consideration in the model. The predictions of the model which are made without arbitrary parameters, compare favorably with the experimental data on sorbent performance from large scale experimental fluidized-bed coal combustors.