An experimental study on the primary fragmentation and attrition of limestones in a fluidized bed

In this paper, an experimental study on the primary fragmentation and attrition of 5 limestones in a fluidized bed was conducted. The intensity of fragmentation and attrition were measured in the same apparatus but at different fluidizing velocities. It was found that the averaged size of the particles decreased by about 10-20% during the fragmentation process. The important factors for particle comminution include limestone types, heating rate, calcination condition and ambient CO₂ concentration. Fragmentation mainly occurred in the first a few minutes in the fluidized bed and it was more intense than that in the muffle furnace at the same temperature. The original size effect was ambiguous, depending on the limestone type. The comminution caused by attrition mainly occurred during calcination process rather than sulphation process. The sulphation process was fragmentation and attrition resisted. The attrition rate of sulphate was similar to that of lime in trend, decaying exponentially with time, but was one-magnitude-order smaller than that of lime. Present experimental results indicate that fragmentation mechanism of the limestone is dominated by CO₂ release instead of thermal stress.     

Sulphation of limestones in a fluidized bed combustor: The relationship between particle attrition and microstructure

Sulphation of two limestones in a fluidized bed combustor has been investigated. One limestone (coarse‐grained) was characterized by a significant population of relatively large pores after calcination; the other (fine‐grained) presented a finer and fairly unimodal pore size distribution. Differences in the microstructure were reflected by different thickness of the sulphate shell formed upon sulphation and ultimate calcium conversion degree. Particle attrition/fragmentation were fairly small under moderately bubbling fluidization conditions. Fragmentation upon impact was significant. The fine‐grained limestone, characterized by a thinner sulphate shell, was more susceptible to fragmentation than the other. Particle fragmentation discloses unreacted CaO enabling secondary sulphation of exhausted particles.     

Model of fragmentation of limestone particles during thermal shock and calcination in fluidised beds

Fragmentation of limestone due to thermal shock and calcination in a fluidised bed was studied through experiments and modelling. The time for heating was estimated by model calculations and the time for calcination by measurements. Fragmentation due to thermal shock was carried out by experiments in a CO₂ atmosphere in order to prevent the effect of calcination. It was found to be much less than fragmentation due to calcination. Average particle sizes before and after fragmentation are presented for several types of limestone. The effects of particle size and gas composition on the primary fragmentation were studied through experiments. Increasing the fluidisation velocity increased the tendency to fragment. The evolution of the particle size distribution (PSD) of limestone particles due to thermal shock and during calcination (or simultaneous calcination and sulphation) were calculated using a population balance model. Fragmentation due to thermal shock is treated as an instantaneous process. The fragmentation frequency during calcination is presented as exponentially decaying over time. In addition to the final PSD, this model also predicts the PSD during the calcination process. The fragmentation was practically found to end after 10 min. Furthermore, a population balance method to calculate the particle size distribution and amount of limestone in fluidised beds in dynamic and steady state, when feeding history is known, is presented.     

Calcium-based sorbents behaviour during sulphation at oxy-fuel fluidised bed combustion conditions

Sulphur capture by calcium-based sorbents is a process highly dependent on the temperature and CO₂ concentration. In oxy-fuel combustion in fluidised beds (FB), CO₂ concentration in the flue gas may be enriched up to 95%. Under so high CO₂ concentration, different from that in conventional coal combustion with air, the calcination and sulphation behaviour of the sorbent must be defined to determine the optimum operating temperature in the FB combustors.In this work, the SO₂ retention capacity of two different limestones was tested by thermogravimetric analysis at typical oxy-fuel conditions in FB combustors. The effect of the main operating variables affecting calcination and sulphation reactions, like CO₂ and SO₂ concentrations, temperature, and sorbent particle size, was analysed.It was observed a clear difference in the sulphation conversion reached by the sorbent whether the sulphation takes place under indirect or direct sulphation, being much higher under indirect sulphation. But, in spite of this difference, for a given condition and temperature, the CO₂ concentration did not affect to the sulphation conversion, being its major effect to delay the CaCO₃ decomposition to a higher temperature.For the typical operating conditions and sorbent particle sizes used in oxy-fuel FB combustors, the maximum sorbent sulphation conversions were reached at temperatures of about 900 °C. At these conditions, limestone sulphation took place in two steps. The first one was controlled by diffusion through porous system of the particles until pore plugging, and the second controlled by the diffusion through product layer. As a consequence, the maximum sulphation conversion increased with decreasing the particle size and increasing the SO₂ concentration.     

Attrition of limestones by impact loading in fluidized beds: The influence of reaction conditions

The extent of attrition associated with impact loading was studied for five different limestones pre-processed in fluidized bed under different reaction conditions. The experimental procedure was based on the measurement of the amount and the particle size distribution of the debris generated upon impact of sorbent samples against a target at velocities between 10 and 45 m/s. The effect of calcination, sulfation and calcination/re-carbonation on impact damage was assessed. Fragmentation by impact loading of the limestones was significant and increased with the impact velocity. Lime samples displayed the largest propensity to undergo impact damage, followed by sulfated, re-carbonated and raw limestones. Fragmentation of the sulfated samples followed a pattern typical of the failure of brittle materials. On the other hand, the behaviour of lime samples better conformed to a disintegration failure mode, with extensive generation of very fine fragments. Raw limestone and re-carbonated lime samples followed either of the two patterns depending on the sorbent nature. The extent of particle fragmentation increased after multiple impacts, but the incremental amount of fragments generated upon one impact decreased with the number of successive impacts.     

Characterization of the reactivity of limestones with SO2 in a fluidized bed reactor

The reaction between SO₂ and calcined limestone particles has been studied in a fluidized bed combustor. Measurements of sorbent reactivity with SO₂ were made for small batches of limestone injected into the combustor. Simultaneous continuous combustion of bituminous coal provided conditions like those of a boiler for study of the sulphation reaction. A semi-empirical rate model of the CaO-SO₂ reaction has been developed. External mass transfer of SO₂, diffusion within the particles and chemical reaction are taken into account. The limestone reactivity with SO₂ is characterized by two parameters which are dependent on the temperature and sorbent particle size. A model for predicting the limestone requirements in a fluidized bed boiler has been developed. Parameters from the batch experiments are included. The predictions for sulfur retention agree with the experimental results. In addition, effects of operating conditions (gas velocity, recycle, limestone particle size) on the retention of SO₂ were simulated using the model.     

Flue gas desulfurization under simulated oxyfiring fluidized bed combustion conditions: The influence of limestone attrition and fragmentation

Flue gas desulfurization by means of limestone injection under simulated fluidized bed oxyfiring conditions was investigated, with a particular focus on particle attrition and fragmentation phenomena. An experimental protocol was applied, based on the use of complementary techniques that had been previously developed for the characterization of attrition of sorbents in air-blown atmospheric fluidized bed combustors. The extent and pattern of limestone attrition by surface wear in the dense phase of a fluidized bed were assessed in bench scale fluidized bed experiments under simulated oxyfiring conditions. Sorbent samples generated during the oxyfiring tests were further characterized from the standpoint of fragmentation upon high velocity impact by means of a particle impactor. The experimental results were compared with those previously obtained with the same limestone under air-blown atmospheric fluidized bed combustion conditions. The profound differences in the attrition and fragmentation extents and patterns associated with oxyfiring as compared to air-blown atmospheric combustion and the role played by the different attrition/fragmentation paths were highlighted. In particular, it was noted that attrition could effectively enhance particle sulfation under oxyfiring conditions by continuously disclosing unconverted calcium to the sulfur-bearing atmosphere.     

Reinvestigation of hydration/reactivation characteristics of two long-term sulphated limestones which previously showed uniformly sulphating behaviour

Two Canadian limestones, Calpo and Luscar, were fully sulphated, and the residues were hydrated with liquid water and steam and then re-sulphated with synthetic flue gas in a thermogravimetric analyzer (TGA). Both sulphated limestones were previously classified as showing uniform sulphation patterns, and it was expected that they would not demonstrate significant reactivation by hydration. However, the current work demonstrates that both spent sorbents can be reactivated by steam hydration, while one of them, Luscar limestone, can also be reactivated by hydration with liquid water, whereas water hydration is less effective with Calpo limestone. Long-term sulphation was employed on the fresh limestones to ensure that the sorbents were fully sulphated to levels typical of full-scale units during the first sulphation. No evidence was found that the SO₂ concentration for first sulphation influenced the degree of reactivation, indicating that these sulphation times are sufficient. Total calcium utilization after re-sulphation was markedly improved – up to 90% by hydration. Possible explanations for the failure to reactivate these limestones by previous workers may well be that they chose unsuitable hydration conditions and/or that there are wide variations in limestone properties between different batches, even from the same supplier. It is also evident that it may be premature to categorize the sulphation patterns of a given limestone on the basis of limited tests.     

Modeling of sulphur retention in atmospheric fluidized bed combustors. Sensitivity analysis and simulation

For the design, simulation and optimization of sulphur retention in atmospheric fluidized bed coal combustors, a mathematical model is needed that would be able to predict the behaviour of the combustor in a wide range of operating conditions. In this work, a sensitivity analysis of the sulphur retention predictions of the different hypotheses, equations and parameters, which define the different submodels and phenomena occurring in the combustor, has been carried out. It has been found that the hypotheses related to the gas flow, devolatilization type and sulphur distribution in the pyrolysis products imply an important division among models. The greatest effect on sulphur retention predictions is exercised by the parameters defining the fines elutriation and sorbent sulphation capacity. However, those corresponding to the bed hydrodynamics (minimum fluidization velocity and bed expansion) do not have a significant effect on the sulphur retention predictions. The sulphur retentions obtained in the combustion of high sulphur lignites with eight different limestones were used for model validation. A good fit of the experimental sulphur retentions was found, without using any adjustable parameter. Finally, a simulation of the process was made. The great effect of the bed height, air velocity and the particle size distribution of the limestone must be pointed out, as well as the effect of its reactivity through the maximum conversion attainable by each particle size.     

Assessment of ettringite from hydrated FBC residues as a sorbent for fluidized bed desulphurization

The performance of synthetic ettringite as a sorbent in fluidized bed desulphurization has been assessed and compared with that of a commercial limestone. Experiments have been carried out in a bench scale fluidized bed reactor under simulated desulphurizing (steadily oxidizing) combustion conditions. Sorbent performance has been characterized in terms of desulphurization rate, maximum sulphur uptake and attrition propensity. Fluidized bed sulphation experiments have been complemented by microstructural characterization of solid samples, accomplished via X-ray diffraction analysis, scanning electron microscopy and sulphur mapping of cross-sections of particles embedded in epoxy resin.

Experimental results show that both the rate and the maximum extent of sulphur uptake by ettringite significantly exceed those of the limestone. Maximum degree of free calcium utilization is 0.58 for ettringite compared with 0.27 for the limestone. Sulphation tests also indicate that attrition propensity of ettringite is larger than that correspondingly observed for the limestone. Microstructural characterization indicates that sulphation of ettringite takes place evenly throughout the particle cross-section, whereas sulphation of limestone mostly conforms to a core-shell pattern.

Along a parallel pathway, the rate and yield of ettringite formation by hydration of fly ash from a utility fluidized bed boiler have been assessed. Formation of ettringite in these experiments appears to be quantitative upon curing of ash at 70 °C for times up to 4 days.     

原料粒径对石灰石热分解反应动力学影响

利用热同步分析仪,在纯二氧化碳气氛、升温速率为10~30 K/min的条件下,对4种不同粒径的石灰石进行热分解特性研究,并采用改进的双外推法求解石灰石的热分解反应动力学参数。数据分析结果表明：石灰石粒径与其分解所需的活化能成正比例关系,粒径越小所需要的活化能越小,反之越大;升温速率越快,对应的分解反应温度越高,达到同一转化率所需要的时间越短,说明反应进程越快;纯二氧化碳气氛下,4种不同粒径的石灰石热分解均遵循随机成核和随后生长模型,不同粒径的石灰石对应不同的反应级数,反应级数变化范围为1/2~2;不同升温速率下石灰石热分解反应速率不同,粒径范围为38~250 μm的石灰石热分解反应速率受界面化学反应控制。     

Limestone fragmentation and attrition during fluidized bed oxyfiring

Attrition/fragmentation of limestone under simulated fluidized bed oxyfiring conditions was investigated by means of an experimental protocol that had been previously developed for characterization of attrition/fragmentation of sorbents in air-blown atmospheric fluidized bed combustors. The protocol was based on the use of different and mutually complementary techniques. The extent and pattern of attrition by surface wear in the dense phase of a fluidized bed were assessed in experiments carried out with a bench scale fluidized bed combustor under simulated oxyfiring conditions. Sorbent samples generated during simulated oxyfiring tests were further characterized from the standpoint of fragmentation upon high velocity impact by means of a purposely designed particle impactor. Results showed that under calcination-hindered conditions attrition and fragmentation patterns are much different from those occurring under air-blown atmospheric combustion conditions. Noteworthy, attrition/fragmentation enhanced particle sulfation by continuously regenerating the exposed particle surface.     

A method of evaluating limestone reactivity with SO2 under fluidized bed combustion conditions

The reactivity of limestone with sulphur dioxide has been evaluated during conditions similar to those existing in a fluidized bed combustor using a fixed-bed quartz reactor. The reactivity of up to 11 particle sizes of 2 limestones were evaluated, and an exponential decay function was found to best describe the rate behaviour versus conversion of CaO to CaSO₄. The two constants in the exponential decay approximation could both be expressed as functions of particle size. Subsequently, the limestone reactivity as a function of both size and conversion could be reasonably well described by a total of 4 to 6 constants. An analytical sulphur capture model for fluidized bed boilers (FBB) that incorporates this type of reactivity function is proposed.     

Absorption of HCl by limestone in hot flue gases. Part III: simultaneous absorption with SO2

In this work, the simultaneous absorption of HCl and SO₂ with limestone was studied using thermogravimetry. The objective was to investigate the effects of the temperature and gas atmosphere on the absorption results. Special emphasis was placed on elucidating the formation of the molten phases while these gases were being absorbed. Partially reacted absorbent particles were examined using scanning electron microscopy and energy dispersive spectrometry to study the reaction patterns inside them. This research showed that the chlorination of both calcined and uncalcined limestone was considerably suppressed when simultaneous sulphation occurred. Furthermore, the experimental results indicated that limestone sulphation was significantly enhanced by the presence of HCl. The results for both the suppressed chlorination and enhanced sulphation were similar under all experimental conditions, i.e. for all the temperatures and gas atmospheres studied. Molten phases were observed to have been formed in the samples at both 850 and 650 °C, and our thermodynamic modelling results for the CaCO₃–CaSO₄–CaCl₂ system indicated that the system has a eutectic point at approximately 580 °C. The formation of molten product phases could explain the enhancement seen in SO₂ absorption, although it cannot explain the suppressed chlorination during the simultaneous absorption of HCl and SO₂. The results of this study suggested that the calcium chloride initially formed is consumed in a subsequent reaction with SO₂.     

Hydration products of FBC wastes as SO2 sorbents: comparison between ettringite and calcium hydroxide

Fluidized bed combustion (FBC) enables the in situ capture of SO₂, but generates large amounts of wastes whose composition and physico-chemical properties make both landfilling and reuse in traditional fields of application (e.g., cement and concrete industries) problematic. Reactivation by water hydration of the desulphurizing ability of these residues is considered a viable mean for their recycling: besides Ca(OH)₂, this process can generate other hydration products, such as ettringite. This paper is devoted to a comparison between the behaviour of Ca(OH)₂ and ettringite as SO₂ sorbents. To this end, synthetic preparations (in the particle size range 0.4–0.6 mm) of the two materials were dehydrated and then sulphated in a lab-scale fluidized bed reactor. Sulphation tests were carried out at 850 °C by fluidizing the bed with an SO₂–N₂–O₂ mixture (1800 ppm SO₂) at 0.8 m/s. Calcium conversion degree and fines elutriation rate were evaluated as a function of sulphation time. The propensity of the sorbents to undergo fragmentation was also estimated by particle sizing of in-bed exhausted sorbent particles, with the aid of laser granulometry. Mercury intrusion porosimetry of samples was directed to the assessment of the influence of sorbent dehydration and subsequent sulphation on pore size distribution and porosimetric texture. X-ray diffraction and differential thermal analysis on the synthetic sorbents complemented the characterization. Results showed that dehydration/thermal decomposition brought about a significant increase of the overall porosity for both sorbents, more extensive than it is commonly observed with calcined commercial limestones. Upon sulphation, the two sorbents showed satisfactory degrees of calcium conversion, larger than those usually observed with limestones. Sulphation resulted into a decrease of particle voidage (that of the Ca(OH)₂-based sorbent was negligible after the process). Ettringite was more prone to attrition/fragmentation than calcium hydroxide. Results are discussed with a focus on differences between calcium hydroxide and ettringite and on key-parameters affecting the performance of the two materials as sorbents.     

H2O和SO2对CFB内石灰石同时煅烧/硫化反应中煅烧动力学的协同作用

采用自制恒温热重分析仪,研究了CFB工况下石灰石同时煅烧/硫化反应中H₂O和SO₂对石灰石煅烧动力学和孔结构的协同作用。煅烧环境中的H₂O能够促进石灰石的分解,但SO₂会减慢石灰石分解速度,且测试发现SO₂使煅烧后颗粒的孔容积下降,分解反应的效率因子减小。基于此提出SO₂减缓煅烧反应的机理：高温下,石灰石颗粒外层首先分解并生成多孔CaO层,其中的孔隙作为内部CaCO₃分解产生CO₂的外扩散通道,当煅烧气氛中含有SO₂时,颗粒的CaO层与SO₂反应生成CaSO₄,堵塞了CaO中的孔隙,增加了CO₂扩散的阻力,从而减缓了其分解速度。当石灰石在含有15% H₂O和0.3% SO₂的环境中分解时,其分解速度比不含二者的环境下快,而比含15% H₂O但不含SO₂的环境下慢,说明H₂O和SO₂对改变石灰石分解的速度有协同效应,但15% H₂O的作用比0.3% SO₂的作用更大。对效率因子的计算表明,该现象可能由于石灰石煅烧反应的速度控制步骤中本征反应速度的影响比扩散阻力的作用更大,而H₂O能够直接加速煅烧反应的本征速度。温度、粒径等均能够影响石灰石同时煅烧/硫化反应的中的煅烧速度。H₂O还能够促进CaO的烧结,并且H₂O和SO₂在降低石灰石煅烧产物的孔面积和孔容积上具有叠加效应。     

A simplified model of SO2 capture by limestone in 71 MWe pressurized fluidized bed combustor

A mathematical model of SO₂ capture by uncalcined limestone particles with solid attrition under pressurized fluidized bed combustion conditions was developed based on the shrinking unreacted-core model. Since the thickness of the product layer is sufficiently much smaller than the particle size, a flat surface model was employed. The difference in SO₂ capture behavior between continuous solid attrition and intermittent attrition was investigated. The reaction rate for intermittent solid attrition was found to be lower than that for continuous attrition mode under low SO₂ concentration conditions. A simple mathematical expression to calculate reaction rate of SO₂ capture per unit external surface area of limestone is proposed.The present simplified mathematical model of SO₂ capture by single limestone particle under periodical attrition conditions was applied to the analysis of a large-scale pressurized fluidized bed combustor. By giving the period of attrition as a parameter, the experimental results agreed well with the model results. From the vertical concentration profile of SO₂ concentration, the emission of SO₂ was found to be governed by the balance between SO₂ formation rate from char and SO₂ capture by limestone at the upper surface of the dense bed. A simplified expression to estimate SO₂ emission from pressurized fluidized bed combustors was proposed.     

The effect of water on the sulphation of limestone

A series of tests was conducted in a thermogravimetric analyzer (TGA) to study the sulphation behaviour of limestone in the presence of water over the temperature range of 800-850 °C. Four different Canadian limestones, all with a particle size range of 75-425 μm, were sulphated using a synthetic flue gas with a composition of 15% CO₂, 3% O₂, 0% or 10% H₂O, 1750 ppm SO₂ and the balance N₂. Water was shown to have a significant promotional effect on sulphation, especially in the diffusion-controlled stage. However, the effect of water during the kinetic-controlled stage appeared to be much less pronounced. Based on these results, it is proposed that the presence of water leads to the transient formation of Ca(OH)₂ as an intermediate, which in turn reacts with SO₂ at a faster rate than CaO does. Alternatively stated, it appears that H₂O acts as catalyst for the sulphation reaction of CaO.     

Enhancement of Ca‐Based Sorbent Multicyclic Behavior in Ca Looping Process for CO2 Separation

The Ca‐based sorbent looping cycle represents an innovative way of CO₂ capture for power plants. However, the CO₂ capture capacity of the Ca‐based sorbent decays sharply with calcination/carbonation cycle number increasing. In order to improve the CO₂ capture capacity of the sorbent in the Ca looping cycle, limestone was modified with acetic acid solution. The cyclic carbonation behaviors of the modified and original limestones were investigated in a twin fixed‐bed reactor system. The modified limestone possesses better cyclic carbonation kinetics than the original limestone at each cycle. The modified limestone carbonated at 640–660 °C achieves the optimum carbonation conversion. The acetic acid modification improves the long‐term performance of limestone, resulting in directly measured conversion as high as 0.4 after 100 cycles, while the original limestone remains at a conversion of less than 0.1 at the same reaction conditions. Both the pore volume and pore area distributions of the calcines derived from the modified limestone are better than those derived from the original limestone. The CO₂ partial pressure for carbonation has greater effect on conversion of the original limestone than on that of the modified sorbent because of the difference in their pore structure characteristics. The carbonation conversion of the original limestone decreases with the increase in particle size, while the change in particle size of the modified sorbent has no clear effect on cyclic carbonation behavior.     

Reactivity of calcined limestone and dolomite with sulfur dioxide

Under the conditions prevailing in in-situ desulfurization of fluidized bed combustion two types of limestone and two types of dolomite were tested for their reactivity with sulfur dioxide. Experimental variables were sorbent size, sulfur dioxide concentration and reaction temperature. In general reactivity increased with temperature with activation energy ranging 15.9 to 19.5 kcal/mol for initial reaction rate. Reaction order with respect to sulfur dioxide was close to one and reactivity decreased with increase of sorbent size. With continuous feeding of coal and sorbent sulfur dioxide concentration in the effluent gas decreased with increase of Ca/S ratio.