Fluidized Bed Flue Gas Desulfurization Research Articles

Desulfurization characteristics of slaked lime and regulation optimization of circulating fluidized bed flue gas desulfurization process—A combined experimental and numerical simulation study

Circulating fluidized bed flue gas desulfurization (CFB-FGD) process has been widely applied in recent years. However, high cost caused by the use of high-quality slaked lime and difficult operation due to the complex flow field are two issues which have received great attention. Accordingly, a laboratory-scale fluidized bed reactor was constructed to investigate the effects of physical properties and external conditions on desulfurization performance of slaked lime, and the conclusions were tried out in an industrial-scale CFB-FGD tower. After that, a numerical model of the tower was established based on computational particle fluid dynamics (CPFD) and two-film theory. After comparison and validation with actual operation data, the effects of operating parameters on gas–solid distribution and desulfurization characteristics were investigated. The results of experiments and industrial trials showed that the use of slaked lime with a calcium hydroxide content of approximately 80% and particle size greater than 40 μm could significantly reduce the cost of desulfurizer. Simulation results showed that the flow field in the desulfurization tower was skewed under the influence of circulating ash. We obtained optimal operating conditions of 7.5 kg·s−1 for the atomized water flow, 70 kg·s−1 for circulating ash flow, and 0.56 kg·s−1 for slaked lime flow, with desulfurization efficiency reaching 98.19% and the exit flue gas meeting the ultraclean emission and safety requirements. All parameters selected in the simulation were based on engineering examples and had certain application reference significance.

CFD analysis of the spray droplet parameters effect on gas-droplet flow and evaporation characteristics in circulating fluidized bed desulfurization tower

The circulating fluidized bed flue gas desulfurization (CFB-FGD) process is a promising semi-dry flue gas desulfurization technology. The gas-droplet flow characteristics and droplets evaporation characteristics are critical to improving the desulfurization efficiency of the FGD tower. The effects of droplet parameters, including spray droplet diameter, spray droplet velocity, spray droplet mass flow rate and spray angle are investigated. Two evaluation indices, the coefficient of variation of droplet concentration (CV) and droplet mass-weighted life, are adopted to evaluate mixing degree of gas-droplet and droplet evaporation time, respectively. Results show that as the spray droplet diameter increases from 40 µm to 80 µm, the amount of water droplets trapped by tower wall increases from 3% to 21%. Increasing the spray droplet velocity and spray droplet mass flow rate could effectively increase the turbulent kinetic energy, which could improve the turbulent mixing ability. Moreover, droplet with smaller diameter and high velocity are beneficial to improving the temperature distribution uniformity. When the spray droplet mass flow rate exceeds 2.7 kg/s, the temperature in the tower would be lower than the dew point temperature, resulting in corrosion of the tower wall. Furthermore, the evaporation rate of droplets decreases with the increase of droplet diameter, spray droplet velocity and spray mass flow rate, while increases with the increase in the spray angle. This study would provide a better reference for the optimum operating scheme in practical applications.

Numerical simulation study on gas-solid flow characteristics and SO2 removal characteristics in circulating fluidized bed desulfurization tower

Circulating fluidized bed flue gas desulfurization technology is widely used in the world. The computational particle fluid dynamics (CPFD) numerical simulation method was used to study the gas-solid two-phase flow and desulfurization characteristics in the circulating fluidized bed desulfurization tower. The CPFD method is based on the MP-PIC method, which uses the stochastic particle method for the particle phase and the Euler method for the fluid phase to solve the dense particle flow. The established model was firstly verified by the experimental results of gas-solid flow field and desulfurization efficiency. Then, the effects of flue gas parameters on the flow field in the desulfurization tower and the effects of flue gas parameters, SO2 inlet concentration, and Ca/S on desulfurization efficiency were numerically simulated. An effective optimization scheme was also proposed to improve the desulfurization efficiency. The research results showed that the CPFD simulation results were in good agreement with the experimental results, which proved the reliability of the method. The particle motion state in the bed of the desulfurization tower presents a core-annular flow. As the flue gas inlet flow increased, the distribution of flue gas and particles has a serious deviation, and the desulfurization efficiency also decreased. The desulfurization efficiency will decrease with the increased of SO2 inlet concentration and increased with the increased of Ca/S. According to the distribution of SO2 concentration in the bed, three characteristic desulfurization zones can be proposed, namely, the initial mixing process zone, the high-efficiency desulfurization process zone, and the low-efficiency desulfurization process zone. Installing a baffle at the bottom of the desulfurization tower can optimize the distribution of gas-solid two-phase flow in the bed and the desulfurization efficiency.

Numerical simulation and optimization of semi-dry flue gas desulfurization in a CFB based on the two-film theory using response surface methodology

Semi-dry circulating fluidized bed flue gas desulfurization(CFB-FGD) is a major industrial FGD technology. Assuming that only one desulfurizer particle is wrapped in each droplet for mass transfer and dissolution, this study proposed a two-film theory-based simplified desulfurization model, with which CFB desulfurization was performed. After model verification, this study simulated the hydrodynamics and desulfurization reaction characteristics and explored the effects of inlet SO2 concentration(A), gas velocity(B), circulating particle mass flux(C), and jet water volumetric flux(D) on the desulfurization efficiency. Simulation results coupling with response surface methodology (RSM) analysis could not only quantitatively reflect the effect degree of a single variable but also apply to the interaction between parameters, thus determining the optimal parameters. RSM results showed that D and CD were the independent and interaction parameters, respectively, that most significantly affected desulfurization efficiency. The simulation under the optimal operating parameters showed that the desulfurization efficiency was greatly improved, reaching 99.69%.

Hydrodynamics of activated char in a novel multistage circulating fluidized bed for dry desulfurization

A novel multistage fluidized bed was proposed to improve the desulfurization efficiency of activated char in a circulating fluidized bed–flue gas desulfurization process. The hydrodynamics of gas–solid flow in the multistage fluidized bed was experimentally studied. Results showed that the overall pressure drop of the bed had a good linear relationship with the ratio of the superficial gas velocity and the solid circulation rate. Compared with the conventional fluidized bed for desulfurization, a relatively uniform upward flow over a cross section was observed in the middle conveying section of the multistage fluidized bed. Intensive gas–solid interaction in the expanding section of the multistage fluidized bed was confirmed. In addition, a uniform suspension upward flow with minimal solid backmixing at the upper portion of the expanding section was observed under limited operating conditions. Finally, the gas–solid flow structure in the expanding section of the multistage fluidized bed was depicted.

Comparative life cycle environmental assessment of flue gas desulphurization technologies in China

This study is a comparative life cycle assessment (LCA) for two widely used flue gas desulphurization (FGD) technologies in China, namely, the circulating fluidized bed flue gas desulfurization (CFB-FGD) and the wet limestone flue gas desulfurization (WFGD). The present study identifies and quantifies the energy consumptions, resource depletions and environmental impacts of CFB-FGD and WFGD technologies throughout the whole life cycle, including production of desulfurizer, production of FGD system manufacturing materials, transport of FGD system manufacturing materials, manufacturing of FGD system, transport of FGD system and FGD system operation. Life cycle assessment (LCA) methodology provides a quantitative basis for assessing potential improvements in the environmental performance of FGD technologies. The functional unit of the study is 7000 h, the annual operating time of FGD systems. The assessment criteria include five energy consumption categories (electricity, coal, diesel, fuel oil and crude oil), three resource consumption categories (coal, crude oil and water), and six environmental impact categories (global warming, acidification, nutrient enrichment, photochemical ozone formation, bulk waste and slag & ashes). Finally, the total energy consumptions, the resource depletion indexes (RDIs) and the environmental impact loadings (EILs) of FGD technologies were produced for the overall assessment. Overall, the WFGD technology is deemed environmentally benign and energy saving in a relative sense based on the LCA in this study. Concerning environmental impacts, bulk waste is the most critical for both types of FGD technologies. Further observations also help identify some sensitive parameters, including mixed solid waste and carbon monoxide (CO) across the CFB-FGD technology, and mixed solid waste and sulfur dioxide (SO

Population balance for CFB–FGD systems

Abstract Population balance is the basis of stable and effective operation of circulating fluidized bed flue gas desulfurization systems (CFB–FGD systems). The population balance model parameters were calibrated by comparing the calculated results for a CFB–FGD system with experimental data from a bench-scale CFB–FGD system. The relative error was less than 5.1%. The influences of the model parameters, reactor structures, operating conditions, and adhesive carrier particles on the population balance of the CFB–FGD system were then analyzed using the calibrated population balance model. The results indicated that particle segregation and the superficial gas velocity strongly influenced the particle size distribution. The cyclone separation efficiency and the selected particle draining coefficient also impacted the particle size distribution and the bottom ash ratio. The calcium to sulfur ratio and the bed temperature mainly influenced the bottom ash ratio and the desulfurization efficiency of the system. The anti-abrasion characteristics of the rapidly hydrated sorbent influenced the particle size distribution, the bottom ash ratio, and the desulfurization efficiency of the system. The population balance in the CFB–FGD system was more easily achieved when circulating ash from a CFB boiler was used as adhesive carrier particles instead of coal fly ash. This research provides guidance for stable, effective operation of CFB–FGD systems to promote industrial applications of CFB–FGD technology.

Adhesion, Abrasion, and Desulfurization Characteristics of Rapidly Hydrated Sorbent with Cheap and Easily Obtained Adhesive Carrier Particles

Adhesive carrier particles have important technical and economic influence on circulating fluidized bed flue gas desulfurization (CFB-FGD) systems that use rapidly hydrated sorbent. Several cheap and easily obtained materials with rough surfaces and porous structures were used as adhesive carrier particles to prepare rapidly hydrated sorbent: the fly ash from the first electrical field in the electrostatic precipitator (ESP) of a circulating fluidized bed (CFB) boiler, the fly ash from a chain boiler, and river sand. Scanning electron microscopy (SEM), particle size distribution (PSD) analysis, surface and pore analysis, abrasion tests, and thermal gravimetric analysis (TGA) tests were used to examine the influence of the adhesive carrier particles’ surface and pore characteristics on the adhesion, abrasion, and desulfurization characteristics of the rapidly hydrated sorbent. Experimental results showed that the CFBB ESP ash, chain boiler ash, and river sand all improved the abrasion and desulfurization c...

Characteristics of fly ash from the dry flue gas desulfurization system for iron ore sintering plants

The characteristics of fly ash from the flue gas desulfurization (FGD) system are important for its reuse and are mainly depend on the desulfurization process. The physical and chemical properties of DSF ash, which refers to fly ash from the dry FGD system for the iron ore sintering process, were investigated. Its mineralogical contents were determined by X-ray diffraction and thermogravimetry analysis, and its micro-morphology was studied by scanning electric micrograph analysis. The results show that DSF ash has a higher CaO and SO3 content, and the main sulfur form is sulfite, with only a part of it oxidized to sulfate. The major minerals present in DSF ash are hannebachite, anhydrite, calcite and portlandite; a minor constituent is calcium chloride. The particles of DSF ash are irregular, fragmentary and small, and hannebachite grows on their surfaces. Particle size is affected by the FGD process, and the ash size from the maximized emission reduction of the sintering–FGD process is lower than that from the circulating fluidized bed–FGD process. The particle size distribution of DSF ash follows the Rosin–Rammler–Bennet equation.

Experimental Study on the Reuse of Spent Rapidly Hydrated Sorbent for Circulating Fluidized Bed Flue Gas Desulfurization

Rapidly hydrated sorbent, prepared by rapidly hydrating adhesive carrier particles and lime, is a highly effective sorbent for moderate temperature circulating fluidized bed flue gas desulfurization (CFB-FGD) process. The residence time of fine calcium-containing particles in CFB reactors increases by adhering on the surface of larger adhesive carrier particles, which contributes to higher sorbent calcium conversion ratio. The circulation ash of CFB boilers (α-adhesive carrier particles) and the spent sorbent (β and γ-adhesive carrier particles) were used as adhesive carrier particles for producing the rapidly hydrated sorbent. Particle physical characteristic analysis, abrasion characteristics in fluidized bed and desulfurization characteristics in TGA and CFB-FGD systems were investigated for various types of rapidly hydrated sorbent (α, β, and γ-sorbent). The adhesion ability of γ-sorbent was 50.1% higher than that of α-sorbent. The abrasion ratio of β and γ-sorbent was 16.7% lower than that of α-sorbent. The desulfurization abilities of the three sorbent in TGA were almost same. The desulfurization efficiency in the CFB-FGD system was up to 95% at the bed temperature of 750 °C for the β-sorbent.

Experimental and model investigation on the mass balance of a dry circulating fluidized bed for flue gas desulfurization system

A moderate temperature dry circulating fluidized bed flue gas desulfurization (CFB-FGD) process was developed using rapidly hydrated sorbent. This technique has the advantages of low cost, no water consumption, and a valuable dry product CaSO4. To keep the system operation stable, a mass balance model, based on cell model considering flow state, particle abrasion, particle residence time, particle segregation and desulfurization processes, was built to predict the system state and optimize the operating condition. Experimental studies were conducted on a pilot-scale CFB-FGD system with rapidly hydrated sorbent made from CFB circulating ash and lime (circulating ash sorbent) or coal fly ash and lime (coal fly ash sorbent). Calculated results were compared with experimental results and the relative error was less than 10%. The results indicated that feed sorbent mass, feed sorbent size, superficial gas velocity, particle abrasion coefficient and cyclone efficiency had significant influence on the mass balance of CFB system. The circulating ash sorbent was better than the coal fly ash sorbent, for providing higher desulfurization efficiency and being better for the CFB-FGD system to achieve mass balance.

Effect of Internal Structure on Flue Gas Desulfurization with Rapidly Hydrated Sorbent in a Circulating Fluidized Bed at Moderate Temperatures

A moderate-temperature desulfurization process at 600−800 °C was studied in a pilot-scale circulating fluidized bed flue gas desulfurization (CFB-FGD) experimental facility with the addition of the internal structure. The rapidly hydrated sorbent and the desulfurization products were analyzed to clarify the influence mechanism of the internal structure on the moderate-temperature desulfurization process. The results show that the desulfurization efficiency with the internal structure at 600−800 °C was 74−93% for the calcium to sulfur (Ca/S) molar ratio of 2.0, which was higher than the desulfurization efficiency without the internal structure of 67−83%. As for the calcium-containing compositions, the desulfurization products included about 70% cyclone recirculation sample and about 30% bag filter sample. The cyclone recirculation sample was mainly composed of CaSO4 and CaO. The bag filter sample not only contained high contents of CaSO4 and CaO but also contained a considerable amount of CaCO3 and even so...

Hydrodynamics of gas–solid flow in the circulating fluidized bed reactor for dry flue gas desulfurization

Process design and scale-up require a fundamental understanding of the hydrodynamics of gas–solid flow in the circulating fluidized bed flue gas desulfurization (CFB-FGD) reactor although the CFB system has been widely used in flue gas desulfurization and flue gas cleaning processes. The hydrodynamics in the CFB-FGD reactor model was investigated by pressure measurements and specially designed sampling probe based on three dimensionless groups for practicable similarity of industrial CFB-FGD process. The results show that the pressure drop in the venturi section is predominant as high as 60% of the total pressure drop and the total pressure drop significantly increases with the increasing external solid circulating rates at the same superficial gas velocity. Moreover, the measurements of radial solid mass fluxes show that the flow pattern in the CFB-FGD reactor is a typical core–annulus flow and this flow structure prevails until the top of the reactor. Reflux ratios are used to quantitatively evaluate the internal solid reflux in the reactor and the values in the low section of the reactor are much higher than those in the upper section.

Effect of Operating Parameters and Reactor Structure on Moderate Temperature Dry Desulfurization

A moderate temperature dry desulfurization process at 600-800 degrees C was studied in a pilot-scale circulating fluidized bed flue gas desulfurization (CFB-FGD) experimental facility. The desulfurization efficiency was investigated for various operating parameters, such as bed temperature, CO2 concentration, and solids concentration. In addition, structural improvements in key parts of the CFB-FGD system, i.e., the cyclone separator and the distributor, were made to improve the desulfurization efficiency and flow resistance. The experimental results show that the desulfurization efficiency increased rapidly with increasing temperature above 600 degrees C due to enhanced gas diffusion and the shift of the equilibrium for the carbonate reaction. The sorbent sulfated gradually after quick carbonation of the sorbent with a long particle residence time necessary to realize a high desulfurization ratio. A reduced solids concentration in the bed reduced the particle residence time and the desulfurization efficiency. A single-stage cyclone separator produced no improvement in the desulfurization efficiency compared with a two-stage cyclone separator. Compared with a wind cap distributor, a large hole distributor reduced the flow resistance which reduced the desulfurization efficiency due to the reduced bed pressure drop and worsened bed fluidization. The desulfurization efficiency can be improved by increasing the collection efficiency of fine particles to prolong their residence time and by improving the solids concentration distribution to increase the gas-solid contact surface area.