Fluidized Bed Combustion Conditions Research Articles

Particle evolution mechanism of wide size distribution limestone under fluidized bed combustion conditions

The dynamic evolution of particles in limestone with a wide size distribution is significant for in-furnace desulfurization in circulating fluidized beds (CFB). In this study, air/SO2 mixtures were selected to simulate the flue gas combustion atmosphere, and the evolution on particle size of limestone with a wide size distribution was investigated using a self-built fluidized bed, with a focus on the evolution mechanism. The results show that the change in the cumulative particle size distribution (PSD) of limestone with a wide size distribution exhibited three stages as a function of reaction time. The cumulative PSD decreased continuously and reached 79.3% after 20 min during the first stage. The cumulative PSD was unchanged from 30 to 120 min in the second stage. The cumulative PSD increased from 81.9% to 88.3% in the range of 120–240 min during the third stage. The effects of attrition, heat stress, calcination, and sulfation on the changes in particle size were quantified and correlated with time. A novel method is proposed to predict the evolution mechanism of limestone with a wide size distribution. Calcination dominated in the time range of 0–60 min, and the predominant effect transformed from calcination to attrition at 240 min. However, the contribution of calcination was not negligible. The evolution of limestone with a wide size distribution can be promoted by the interactions of different effects.

Experimental study and modeling of oxy-char combustion in a pressurized fluidized bed combustor

Pressurized oxy-coal combustion has received wide attention recently due to its potential of high efficiency and low cost in CO 2 capture. However, there is still a significant knowledge gap on the effects of pressure on the oxy-coal and oxy-char combustion behaviors. In this study, oxy-char combustion experiments were carried out in a pressurized fluidized bed combustor, and the effects of pressure and oxygen concentration on the burnout time, reactivity and temperature of the char particles were investigated. The experimental results showed that the burnout time of the char particles decreased significantly with an increase in pressure from 0.1 MPa to 0.5 MPa. Besides, an increase in either the combustion pressure (within the range of 0.1 – 0.5 MPa) or the oxygen concentration (within the range of 10 – 30% O 2 in CO 2 ) led to increases in the reactivity and temperature of the char particles, while the promotional effects of the pressure increase were weaker at the higher pressures. In order to better interpret the experimental results, a combustion model considering reaction kinetics, mass and heat transfer, and energy balance was developed and validated by the experimental results. The model simulation results showed that an increase in pressure would reduce the burnout time of char particles only when the combustion was kinetically controlled or combined kinetic-diffusion controlled. Once the combustion became diffusion-controlled, a further increase in pressure would have an insignificant effect on the char burnout time. Although the values of kinetic constants were different when different kinetic models, the global power-law model and Langmuir-Hinshelwood model, were adopted, the simulation results of the burnout time of the char particle under the pressurized fluidized bed combustion conditions were almost the same since the char combustion was mainly diffusion-controlled. The elevated combustion pressure accelerated both the char combustion rate and the char-CO 2 gasification rate, while the carbon consumption ratio by gasification increased with pressure. Moreover, the enhanced gasification reaction had a great impact on the particle temperature especially under the higher oxygen concentration combustion conditions.

Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O2/N2 and O2/CO2 atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O2/CO2 (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O2/N2 (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O2/N2 condition or the O2/CO2 condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.

Release and migration characteristics of sodium and potassium in high alkali coal under oxy-fuel fluidized bed combustion condition

Zhundong coal is promising to replace inferior coal in fossil-fueled power plants, considering its high abundance, low contents of sulfur and nitrogen. However, the relative high contents of alkalis, especially sodium and potassium, damage the operational economy and safety badly since the release of alkali metals in flue gas can lead to fly ash-related problems such as slagging, ash accumulation and corrosion. Although, oxy-fuel circulating fluidized bed (CFB) combustion is beneficial to reduce the release of sodium and potassium by lowering the operation temperature, the ash formation characteristics of Zhundong coal under the oxy-fuel CFB is still scarce. Our results showed that the residual amounts of sodium and potassium in the ash samples, both tend to decrease with increasing temperature no matter under air combustion or oxy-fuel combustion. However, the sodium content in the cinder obtained under the oxy-fuel combustion atmosphere was higher than that under the air combustion atmosphere, and the insoluble sodium, mainly existed in the form of aluminosilicates, was the main form. In addition, the increase of steam content in both atmospheres reduced the sodium content in the cinder while the residual amount of potassium increased. Density functional theory calculations verified the transformation behavior between sodium and potassium and the inhibiting effect of steam. This study elucidates the release and migration characteristics of sodium and potassium in Zhundong coal both experimentally and theoretically and will benefit the popularity of Zhundong coal.

Direct desulfurization of limestones under oxy-circulating fluidized bed combustion conditions

We compared the direct desulfurization behavior of four different domestic limestone samples (CaO fraction 40–55%, average particle size 37.5 μm) using a thermogravimetric analyzer (TGA) for experiments conducted under oxy-circulating fluidized bed (CFB) combustion conditions (temperature 600–850 °C, CO2 60–95%). In addition, morphological variations of the limestones were analyzed during indirect and direct desulfurization using a scanning electron microscope (SEM). The data derived from the TGA experiments were applied to a gas–solid reaction rate model, and reaction kinetics at different temperature and carbon dioxide (CO2) ratios were analyzed; we suggested the most suitable kinetic models for direct desulfurization. The direct desulfurization reactions of limestones increased in rate with increasing temperature. The activation energy of 50–120 kJ/mol was calculated using various gas–solid reaction models. By measuring the Brunauer-Emmett-Teller (BET) specific surface areas of the limestone samples, we explained the differences in the final conversion rates of the limestone samples.

Re-carbonation of Calcined Limestone Under Oxy-Circulating Fluidized Bed Combustion Conditions

Re-carbonation of Calcined Limestone Under Oxy-Circulating Fluidized Bed Combustion Conditions

Study on the limestone sulfation behavior under oxy-fuel circulating fluidized bed combustion condition

In order to investigate the behavior of limestone sulfation under oxy-fuel circulating fluidized bed (CFB) combustion condition, experiments were conducted in a 50 kW oxy-fuel CFB system under the O2/CO2 and air combustion conditions. A small cage, containing limestone particles, was dipped into the dense zone of the CFB combustor during the experiments. The calcination of limestone, pore structure of the product layer, and calcium conversion were studied. It was found that the increasing of temperature would promote the calcination of limestone and the high concentration of CO2 would inhibit calcination of limestone. The formation process of the product layer was completely different between the direct and indirect sulfation, while it was almost the same during the indirect sulfation under the oxy-fuel and air combustion. However, both the temperature and gas compositions played important roles in determining the pore structures of the product layer during the limestone indirect sulfation process. Under the O2/CO2 combustion condition, the calcium conversion of indirect sulfation was always higher than that of direct sulfation. The highest final calcium conversion after 60 min was found at 900 °C under the O2/CO2 combustion condition.

The characteristics of recycled NO reduction over char during oxy-fuel fluidized bed combustion

To explain the reduction process of recycled NO over char during oxy-fuel fluidized bed combustion, the characteristics associated with the reduction of recycled NO were investigated in the present study. The experimental results were obtained using a bubbling fluidized bed. The effects of typical impurities within the recycled flue gas on the reduction of recycled NO were also studied. It was determined that significant reduction of the recycled NO over char occurred under oxy-fuel fluidized bed combustion conditions. The apparent activation energy of the recycled NO reduction reaction was much lower when combustion occurred. The competition between the NO formation from char nitrogen and the reduction of recycled NO over char determined the final NO concentration of the flue gas. The recycled NO concentration had little influence on the reduction ratios during oxy-fuel fluidized bed combustion when NO formation from char nitrogen was considered. Recycled SO2 can enhance the reduction of recycled NO over char during oxy-fuel combustion while the low concentration of recycled CO had a little effect on the process of recycled NO reduction. The reduction ratios of recycled NO increased as the recycled SO2 concentration increased. Moreover, significant capture of recycled SO2 was observed during oxy-fuel fluidized bed combustion.

Fundamental Study of Indirect vs Direct Sulfation under Fluidized Bed Conditions

Direct dry sorbent injection is a relatively simple and low-cost process for the capture of SO2 emissions, particularly for in-bed capture during fluidized bed combustion. SO2 capture by limestone may be accomplished by either direct or indirect sulfation, depending on gas-phase environment and operating conditions. In this paper, a combination of thermogravimetric analysis (TGA) and a bench-scale bubbling fluidized bed reactor were used to study sulfation, calcination and carbonation behavior of limestone at different CO2 concentrations and various temperatures to identify the relative contributions of indirect and direct sulfation reactions under air- or oxy-fired fluidized bed combustion conditions. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy analyses were also used to help identify product layer microstructure and composition. A significant temperature dependence on the sulfation of limestone was seen under oxy-fired conditions (when direct sulfation dominates); however, limited temperature dependence was seen for air-fired conditions (where indirect sulfation dominates). A distinct CaSO4 product layer is seen in SEM images for the air-firing condition that can significantly inhibit gas transport into internal pores. No such distinct product layer was seen for oxy-firing conditions. For the size range of limestone used in this study (600 to 1000 mu m), indirect sulfation (dominant under air-fired conditions) appeared to be strongly limited by diffusion, whereas direct sulfation (dominant under oxy-fired conditions) was mainly under kinetic control or a mixture of kinetic and diffusion control.

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.

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 CO 2 concentration. In oxy-fuel combustion in fluidised beds (FB), CO 2 concentration in the flue gas may be enriched up to 95%. Under so high CO 2 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 SO 2 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 CO 2 and SO 2 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 CO 2 concentration did not affect to the sulphation conversion, being its major effect to delay the CaCO 3 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 SO 2 concentration.

Influence of Water Vapor on the Direct Sulfation of Limestone under Simulated Oxy-fuel Fluidized-Bed Combustion (FBC) Conditions

Oxy-fuel combustion of fossil fuel is one of the most promising methods for producing electricity, together with a stream of concentrated CO2 ready for sequestration. Oxy-fuel fluidized-bed combustion (FBC) can also use limestone as a sorbent for in situ capture of sulfur dioxide. However, although a limited number of studies have been performed on sulfation of limestone under oxy-fuel combustion conditions, there are still a number of important but unanswered questions. Here, the effect of water vapor on the sulfation of limestone was studied, because it has not been examined in detail in previous sulfation studies and past studies on direct sulfation of limestone in FBC either did not explore the influence of H2O or did so under unrealistic conditions for oxy-fuel FBC. The purpose of this study is to identify the effect of water vapor on direct sulfation of limestone under simulated oxy-fuel circulating FBC (CFBC) conditions. Direct sulfation of three limestones was conducted in a thermogravimetric anal...

Devolatilization and ash comminution of two different sewage sludges under fluidized bed combustion conditions

Two different wet sewage sludges have been characterized under fluidized bed combustion conditions with reference to their devolatilization behavior and ash comminution with the aid of different and complementary experimental protocols. Analysis of the devolatilization process allowed to determine the size of fuel particle able to achieve effective lateral spreading of the volatile matter across the cross-section of medium-scale combustors. Primary fragmentation and primary ash particle characterization pointed out the formation of a significant amount of relatively large fragments. The mechanical properties of these fragments have been characterized by means of elutriation/abrasion tests using both quartz and sludge ash beds.

The influence of temperature on limestone sulfation and attrition under fluidized bed combustion conditions

The influence of temperature on attrition of two limestones during desulfurization in a fluidized bed reactor was investigated. Differences in the microstructure of the two limestones were reflected by a different thickness of the sulfate shell formed upon sulfation and by a different value of the ultimate calcium conversion degree. Particle attrition and fragmentation were fairly small under moderately bubbling fluidization conditions for both limestones. An increase of temperature from 850 °C to 900 °C led to an increase of the attrition rate, most likely because of a particle weakening effect caused by a faster CO 2 evolution during calcination. This weakening effect, however, was not sufficiently strong to enhance particle fragmentation in the bed. The progress of sulfation, associated to the build-up of a hard sulfate shell around the particles, led in any case to a decrease of the extent of attrition. Sulfation at 900 °C was less effective than at 850 °C, and this was shown to be related to the porosimetric features of the different samples.

Formula omitted] looping cycles with CaO-based sorbent pretreated in [formula omitted] at high temperature

In this study, pretreatment of CaO-based sorbent in a CO 2 atmosphere at high temperature is investigated for its effect on CO 2 capture. Three limestones from three widely different geographical locations are used for the tests: Kelly Rock (Canada), La Blanca (Spain), and Katowice (Poland). The particle sizes used are typically as employed in fluidized bed conversion systems. Pretreatment was done in a tube furnace at different temperatures and for different durations. The pretreated samples are characterized by nitrogen physisorption tests, scanning electron microscopy (SEM), and carbonation/calcination conversion measurements in a thermogravimetric analyzer (TGA). The results obtained showed significant decrease of sorbent surface area after pretreatment and the presence of smooth CaO grains was typical of the sorbent particle surface morphology. The pore surface area of pretreated sorbent samples increased after CO 2 cycling, with a peak in pore volume distributions at 50 nm, and SEM images showed the reappearance of smaller CaO grains. In the case of Kelly Rock and Katowice samples, this led to an increase in CO 2 capture activity, up to 45% after 20 cycles. After that, conversions decreased but still remained 5–10% above those for the original (no pretreatment) samples. This beneficial effect means that particles of larger size, typical of fluidized bed combustion (FBC) systems, can be suitably pretreated for use in longer series of CO 2 capture cycles. An additional expected advantage of pretreating sorbent in this manner is reduced elutriation at any given FBC condition. Attempts to pretreat La Blanca failed, as they did when using N 2 , and it is believed that this is explained by the high Na content of this limestone.

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.

The Fate of Trace Elements during the Co-Combustion of Wood-Bark with Waste

An experimental study has been made into the behavior of trace elements during the combustion of spruce wood-bark with three auxiliary fuels, agricultural waste, pulp sludge, and plastic waste, under simulated fluidized bed combustion conditions. A novel suspension-firing reactor has been used to achieve these conditions, with operation at 800 and at 900 °C, with an air/fuel ratio of 1.2:1 throughout. Experimental results have been interpreted using a thermodynamic equilibrium model. The behavior of seven trace elements (Cd, Cr, Mn, Ni, Pb, Se, Zn) has been studied using inductively coupled plasma ionization-mass spectrometry. In addition, an atomic absorption-based technique has been used to study the behavior of Hg. The experimental data indicate significant emissions of all the elements except Ni and Se, which was present in the fuels at low concentration below the lower limit of quantification. Whereas the majority of fuel Hg is likely to be emitted from the combustor, irrespective of fuel blend employed, the level of emissions of Cd, Cr, and Mn may be affected significantly by the addition of auxiliary fuel. The use of wood-bark as a base fuel for boilers is pertinent to paper mills and timber producers where large quantities of bark are generated as a byproduct.

A Novel Particle Trap Impactor for Use with the Gas-Quenching Probe Sampling System

A novel particle trap impactor has been developed for use with the gas-quenching probe in order to exclude solid particles from entering into the probe during sampling of gaseous metallic species in fluidized bed combustion conditions. The impactor must be small in size (Øimpactor ≤ Øprobe = 45 mm) but capable of collecting a relatively large amount of particles at elevated temperatures. As the first step, the impactor was designed, constructed, and tested at room temperature for KCI aerosol particles. The impactor with a nozzle of 0.95 mm in diameter, in combination with the orifice-to-jet diameter ratio of 1.5 and the ratio of the jet-to-plate spacing to jet diameter at 1.4 yielded a sharp cutoff curve with a maximum collection efficiency of about 0.9 and a √Stk50 value of about 0.22. In addition, the collection efficiency of the impactor was compared with the particle removal efficiency of a filter of the same type as the filter previously used with the gas-quenching probe. The difference from the comparison is very small, indicating that the impactor can be used to replace the filter to prevent fly ash particles from entering the gas-quenching probe in fluidized bed combustion conditions.

A Study on the Reactivation of Five Fly Ashes from Commercial Circulating Fluidized Bed (CFB) Boilers

Five different fly ashes and their carbon-free derivatives from commercial-scale circulating fluidized-bed combustion (CFBC) boilers were hydrated with liquid water or steam, to determine whether hydration would improve sorbent utilization in the samples under fluidized-bed combustion conditions. After hydration by water, for one fly ash (FA1) and two carbon-free fly ashes (FA2-A and FA3-A), the capacity of the ashes for taking up SO2 showed limited improvement; however, hydration was not effective in reactivating the remaining samples. Generally, for these fly ashes, reactivation by hydration with either liquid water or steam seemed to be less promising than that for bed ashes, which have been shown to exhibit significant improvement in sulfur capture during resulfation. Hydration, whether by steam or liquid water, is not recommended for fly ash, which has a very limited residence time in the boiler, because of its small particle size. This paper recommends alternative strategies.

Catalytic reduction of nitrous oxide by hydrocarbons over a Fe-zeolite monolith under fluidised bed combustion conditions

Fluidised bed combustion (FBC) is an important source of nitrous oxide emissions. The catalytic reduction of nitrous oxide with different hydrocarbons was carried out over the catalytic system Fe-ZSM-5 supported on ceramic monoliths, under conditions representative of the off-gases of FBC. The selective catalytic reduction (SCR) of nitrous oxide by propane and propene is activated in the presence of oxygen, whereas its conversion is not affected by the increase of oxygen concentration in the feed gas. The SCR of nitrous oxide by methane is inhibited in the presence of oxygen and the increment of oxygen content in the feed gas leads to a decrease in the catalyst activity and selectivity. Water strongly inhibits the nitrous oxide catalytic reduction independently of the type of hydrocarbon. This effect is more pronounced as water concentration in the process gas increases. Propane resulted to be the most appropriate hydrocarbon of those easily available for the SCR of nitrous oxide over the catalytic system. Methane is little active, whereas propane is slightly more active and selective than propene and results in lower hydrocarbon residual emissions, at oxygen and water levels similar to those found in real combustion off-gases. Furthermore, with respect to methane, propane allows lower reaction temperatures in addition to higher reduction efficiency. Moreover, the emission of unreacted hydrocarbon is higher in the case of methane, whose emission is unwanted, because methane is a greenhouse gas itself.