Semidry Flue Gas Desulfurization Processes Research Articles

Multiple field synergy mechanism of the desulfurization process in the intensified spouted beds

The semi-dry flue gas desulfurization process in the conventional spouted bed (CSB), integral multi-jet spouted bed (IMJSFB), spouted bed with longitudinal vortex generator (SB with LVG) and swirl blade nozzle (SB with SBN) was simulated using the multi fluid model. Meanwhile, the multiphase reaction system of intensified spouted beds was explored based on the field synergy theory. The analysis of the synergy angle among three fields of the gas phase velocity, temperature, and pressure in the spouted bed shows that adding intensification structures can effectively enhance the gas heat transfer and reduce the gas drag and pressure drop. The intensified spouted beds also had better synergy among the multiphase field of velocity, velocity gradient, and temperature gradient. The desulfurization efficiencies in the CSB, SB with LVG, IMJSFB, and SB with SBN were 77.75 %, 80.85 %, 87.97 %, and 88.81 %, respectively, and the promotion effects in descending order of SBN > multi-jets > LVG.

Effect of swirling spouting intensification structure on semi-dry desulfurization process in 3D spouted beds

The swirling blade nozzle structure was introduced at the inlet of flue gas in the spouted bed to enhance interphase mixing and improve the desulfurization efficiency. The computational fluid dynamics (CFD) and experimental methods were used to study the gas–solid two-phase flow, water vaporization and semi-dry flue gas desulfurization processes in a spouted bed with an integral swirling blade nozzle (SB with an ISBN). It is found that the introduction of the swirling blade can significantly intensify heat and mass transfer process in spouted beds, the gas–liquid-solid temperature in the SB with an ISBN is higher than that of the conventional SB. During the desulfurization process, the desulfurization efficiency of spouted bed with an ISBN is about 85.42 %, while that of conventional SB is 76.14 %, and the desulfurization efficiency is increased by 12.2 %.

Effects of NOX and O2 on the Reaction of Ca(OH)2 with CO2 at Low Temperatures

The effects of NOX and O2 on the reaction of Ca(OH)2 with CO2 were studied under low-temperature and humid conditions prevailing in dry or semidry flue gas desulfurization processes. Without the simultaneous presence of NOX and O2, the extent of the carbonation of Ca(OH)2 was low and not affected by the concentration of CO2, NOX, or O2. With the simultaneous presence of NOX and O2, the carbonation of Ca(OH)2 was greatly enhanced, and the CO2 capture increased by more than 2 times at a relative humidity higher than 40%; in addition, the relative humidity and NOX concentration had positive effects, the temperature and O2 concentration had negligible effects, but the CO2 concentration had a negative effect. The simultaneous presence of NOX and O2 enhanced the formation of nitric and nitrous acids and salts of calcium nitrate and nitrite. The deliquescence of these salts collected a great quantity of water and thus enhanced the reactions of Ca(OH)2 with the reactive gases. Ca(OH)2 could be completely converted when reacted with the mixture of CO2, NOX, and O2 at a relative humidity ≥ 50% and a NOX concentration ≥ 300 ppm; CaCO3 formed would further react with NOX and O2 to form Ca(NO3)2 when Ca(OH)2 was depleted. The highest CO2 capture obtained was 0.49 g of CO2/g of sorbent (reacted at 60 °C, 70% relative humidity (RH), 12.6% CO2, 2400 ppm NOX, and 5% O2 for 1 h). The major products for the reactions of NO2 with Ca(OH)2 and CaCO3 were calcium nitrite and nitrate, respectively. The capture of CO2 in practice is preferably carried out before removing NOX and after removing SO2 from the flue gas. This work is useful to the development of technologies for CO2 capture using solid wastes containing CaO/Ca(OH)2.

Numerical Analysis of Semi-dry Flue Gas Desulfurization Process in the 3D Spouted Bed

This paper presents a numerical study of gas-solid two-phase flow and water vaporization in a 3D powder-particle spouted bed (PPSB) by means of the combined approach of the Eulerian-Eulerian two-fluid model (TFM) and kinetic theory of granular flows (KTGF). The simulation shows that the gas temperature in the injection area is higher than water and particles, and the three-phase temperatures in other areas are similar. The water vaporization rate is higher at the junction of the injection area and the annulus area, lower in the center of the injection area, and lowest in the annulus area. By increasing gas velocity and liquid temperature, reducing gas humidity, promoting water vaporization.

Eulerian-Eulerian Numerical Study of the Flue Gas Desulfurization Process in a Semidry Spouted Bed Reactor.

The Eulerian–Eulerian two-fluid model (TFM) in conjunction with kinetic theory of granular flows (KTGF) was used for analyzing water vaporization and the semidry flue gas desulfurization process in a two-dimensional powder–particle spouted bed (PPSB). In an environment with high-temperature gas, desulfurization slurry is wrapped on the surface of moving particles and evaporated, along with the application of the user defined function (UDF) method to accomplish water heat and mass transfer by considering evaporation in the simulation process. The simulation results revealed that the best mass- and heat-transfer effect of each phase can be found in the outer annulus and the near spout region, both of which are also the main areas where water vaporization occurs. The rate of desulfurization products decreases with the increase in inlet gas temperature as the water vaporization rate increases. The volume fraction of desulfurization reaction products decreases with the increase in inlet flue gas temperature. Compared with other working conditions, the highest desulfurization efficiency reaches 84% when the inlet flue gas temperature is 480 K. The change of the desulfurization product rate with the radial distance is the same under different superficial gas velocities, with the peak desulfurization efficiency appearing in the annulus. The optimal operating parameter for the desulfurization process is available in PPSB, and the desulfurization efficiency and gas handling capacity reach the best result when the superficial gas velocity equals 1.2 Ums.

Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed

Computational fluid dynamics (CFD) combined with the two-fluid model (TFM) was used for simulation of water vaporization and semi-dry flue gas desulfurization process in a two dimensional powder-particle spouted bed (PPSB), on the basis of gas-solid two-phase flow, the mathematical and physical models of water vaporization process and flue gas desulfurization reaction process have been established through reasonable hypothesis and simplification of the system. The numerical method was used to simulate the desulfurization reaction process and the heat and mass transfer in the powder-particle spouted bed. Simulation results indicate that water vaporization rate was high in spout and annular regions. The main area where flue gas desulfurization reaction occurs was annular area, as a result, the maximum value of desulfurization product rate appears in the annulus. Under the same condition, the desulfurization efficiency of simulation value is 75.75% when the value of slurry water content equals 40 kg-H2O/kg-dry_sorbent, which is close to but greater than the experimental value (75.03%). The desulfurization efficiency of spouted bed increases first and then decreases with the increase of water content of desulfurization slurry, and the optimum slurry water content for desulfurization process in powder-particle spouted bed was obtained by numerical simulation, which was 40 kg-H2O/kg-dry_sorbent.

Influence of the Lime Slurry Droplet Spectrum on the Efficiency of Semi‐Dry Flue Gas Desulfurization

AbstractA numerical model based on computational fluid dynamics is presented to predict the efficiency of a semi‐dry flue gas desulfurization process. The Euler‐Lagrange particle tracking method is used to predict both evaporation and mass transfer between the flue gas and the lime slurry droplets. The overall desulfurization process is split into two periods, i.e. the constant‐rate and the falling‐rate period, during which the fundamental chemical reaction between sulfur dioxide and calcium hydroxide is modeled. The absorption efficiency is calculated for different spray characteristics, taking into account different size spectra of the lime slurry droplets characterized by different standard deviations for a constant mass mean diameter d50 of 25 μm. The results are compared with published experiments and show excellent agreement as well as a significant role of the spray characteristics in the FGD efficiency.

A modeling and experimental study of flue gas desulfurization in a dense phase tower

We used a dense phase tower as the reactor in a novel semi-dry flue gas desulfurization process to achieve a high desulfurization efficiency of over 95% when the Ca/S molar ratio reaches 1.3. Pilot-scale experiments were conducted for choosing the parameters of the full-scale reactor. Results show that with an increase in the flue gas flow rate the rate of the pressure drop in the dense phase tower also increases, however, the rate of the temperature drop decreases in the non-load hot gas. We chose a water flow rate of 0.6 kg/min to minimize the approach to adiabatic saturation temperature difference and maximize the desulfurization efficiency. To study the flue gas characteristics under different processing parameters, we simulated the desulfurization process in the reactor. The simulated data matched very well with the experimental data. We also found that with an increase in the Ca/S molar ratio, the differences between the simulation and experimental data tend to decrease; conversely, an increase in the flue gas flow rate increases the difference; this may be associated with the surface reactions caused by collision, coalescence and fragmentation between the dispersed phases.

Experimental investigation and model improvement on the atomization performance of single-hole Y-jet nozzle with high liquid flow rate

Y-jet nozzle, as an efficient multi-hole internal-mixing twin-fluid atomizer, has been widely used for liquid fuel spray in many industrial processes. However, single-hole Y-jet nozzle with high liquid flow rate is indispensable in some confined situations due to a small spray cone angle. In this paper, the atomization performance of single-hole Y-jet nozzles with high liquid mass flow rates ranging from 400 to 1500 kg/h for practical semidry flue gas desulfurization processes was investigated by the laser particle size analyzer, and the effects of spray water pressure, atomizing air pressure and air to liquid mass flow ratio on the liquid mass flow rate and the droplet size distribution were analyzed. Moreover, the secondary atomization model was modified on the basis of previous random atomization model of Y-jet nozzle. The predicted results agreed well with the experimental ones, and the improved atomization model of Y-jet nozzle was well validated to design the nozzle geometry and to predict the droplet size distributions for single-hole Y-jet nozzle with high liquid mass flow rate.

Reactivities of NaOH Enhanced Iron Blast Furnace Slag/Hydrated Lime Sorbents toward SO2 at Low Temperatures: Effects of the Presence of CO2, O2, and NOx

The effects of the addition of NaOH on the reactivities toward SO2 of iron blast furnace slag/hydrated lime (BFS/HL) sorbents were studied without and with the presence of flue gas components, CO2, O2, and NOx, under the conditions prevailing in the bag filters of dry and semidry flue gas desulfurization processes. The reactivities of sorbents were markedly enhanced by the addition of NaOH when CO2, O2, and NOx were not present in the gas mixture. The enhancement effect mainly resulted from the deliquescent property of NaOH. With the presence of CO2, O2, and NOx at their typical concentrations in the flue gas, the sorbent reactivity was greatly increased due to the oxidation of bisulfite and sulfite ions to sulfate ions as well as the formation of deliquescent salts, calcium nitrite and nitrate, being enhanced by the presence of NO2 during the sulfation of a BFS/HL sorbent. The addition of NaOH in this case promoted the formation of sulfates but became less important in raising the sorbent reactivities for sorbents except BFS. Without the presence of CO2/O2/NOx, the BFS/HL sorbents with ratios ≤70/30 had higher reactivities than HL, but with the presence of CO2/O2/NOx, HL had the highest value of SO2 capture among the BFS/HL and BFS/HL/NaOH sorbents tested. Placing the SO2 removal unit before the NOx removal unit in practical operations can take advantage of the enhancement effect of CO2/O2/NOx on increasing the Ca utilization and SO2 capture of the sorbent.

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.

Study on Multiphase Flow and Mixing in Semidry Flue Gas Desulfurization with a Multifluid Alkaline Spray Generator Using Particle Image Velocimetry

Particle image velocimetry (PIV) technique was used to measure the velocity fields of gas-droplet-solid multiphase flow in the experimental setup of a novel semidry flue gas desulfurization process with a multifluid alkaline spray generator. The flow structure, mixing characteristic, and interphase interaction of gas-droplet-solid multiphase flow were investigated both in the confined alkaline spray generator and in the duct bent pipe section. The results show that sorbent particles in the confined alkaline spray generator are entrained into the spray core zone by a high-speed spray jet and most of the sorbent particles can be effectively humidified by spray water fine droplets to form aqueous lime slurry droplets. Moreover, a minimum amount of air stream in the generator is necessary to achieve higher collision humidification efficiency between sorbent particles and spray water droplets and to prevent the possible deposition of fine droplets on the wall. The appropriate penetration length of the slurry droplets from the generator can make uniform mixing between the formed slurry droplets and main air stream in the duct bent pipe section, which is beneficial to improving sulfur dioxide removal efficiency and to preventing the deposition of droplets on the wall.

The effect of hydrogen peroxide solution on SO 2 removal in the semidry flue gas desulfurization process

The present study attempts to use hydrogen peroxide solution to humidify Ca(OH) 2 particles to enhance the absorption of SO 2 to achieve higher removal efficiency and to solve the valuable reuse of the reaction product in the semidry flue gas desulfurization (FGD) process. Experiments were carried out to examine the effect of various operating parameters including hydrogen peroxide solution concentration, Ca/S molar ratio and approach to adiabatic saturation temperature on SO 2 removal efficiency in a laboratory scale spray reactor. The product samples were analyzed to obtain semi-quantitative measures of mineralogical composition by X-ray diffraction (XRD) with reference intensity ratio ( RIR) method and the morphology of the samples was examined by scanning electron microscope (SEM). Compared with spraying water to humidify Ca(OH) 2, SO 2 removal efficiency was improved significantly by spraying hydrogen peroxide solution of 1–3 wt.% to humidify Ca(OH) 2 because hydrogen peroxide solution enhanced the dissolution and absorption rate of SO 2. Moreover, XRD and SEM analyses show that the desulfurization products contain less amount of unreacted Ca(OH) 2 and more amount of stable calcium sulfate with increasing hydrogen peroxide solution concentration. Thus, the process mechanism of the enhanced absorption of SO 2 by spraying hydrogen peroxide solution to humidify Ca(OH) 2 was elucidated on the basis of the experimental results.

Modeling of SO 2 removal in fabric filter

Fabric filters are involved in most semi-dry flue gas desulfurization process and represent ability of SO 2 removal. SO 2 removal efficiency in fabric filter after a semi-dry scrubber is investigated. Experimental results showed that SO 2 inlet concentration has little effect on SO 2 removal efficiency, SO 2 removal efficiency increases as flue gas inlet temperature increases and relative humidity affects SO 2 removal efficiency significantly. The kinetic model based on shrinking core theory has been presented. It is found that, in the beginning, when calcium hydroxide conversion ratio is less than 0.3, SO 2 removal process is mainly controlled by chemical reaction (Model-2); and when calcium hydroxide conversion ratio is greater than 0.3, SO 2 diffusion through product layer is rate limiting (Model-3). The experimental results in fabric filter are successfully correlated by Model-3.

A novel semidry flue gas desulfurization process with the magnetically fluidized bed reactor

The magnetically fluidized bed (MFB) was used as the reactor in a novel semidry flue gas desulfurization (FGD) process to achieve high desulfurization efficiency. Experiments in a laboratory-scale apparatus were conducted to reveal the effects of approach to adiabatic saturation temperature, Ca/S molar ratio and applied magnetic field intensity on SO 2 removal. Results showed that SO 2 removal efficiency can be obviously enhanced by decreasing approach to adiabatic saturation temperature, increasing Ca/S molar ratio, or increasing applied magnetic field intensity. At a magnetic field intensity of 300 Oe and a Ca/S molar ratio of 1.0, the desulfurization efficiency (excluding desulfurization efficiency in the fabric filter) was over 80%, while spent sorbent appeared in the form of dry powder. With the SEM, XRD and EDX research, it can be found that the increase of DC magnetic field intensity can make the surface morphology on the surface of the ferromagnetic particles loose and enhance the oxidation of S(IV), hence reducing the liquid phase mass transfer resistance of the slurry droplets and increasing desulfurization reaction rate, respectively. Therefore, the desulfurization efficiency increased obviously with the increase of DC field intensity.

Kinetics of the Reaction of Hydrated Lime 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 hydrated lime at low temperatures were studied using a differential fixed-bed reactor. When O2 and NOx were not present together the reaction kinetics was about the same as that under gas mixtures containing SO2, H2O, and N2 only. When both O2 and NOx were present, sulfation of hydrated lime was greatly enhanced, forming a large amount of calcium sulfate in addition to calcium sulfite. Sulfation of hydrated lime was well described by the surface coverage model, despite the gas-phase conditions being different. Relative humidity is the major factor affecting the reaction, and its effect was more marked when both O2 and NOx were present. The kinetic model equations obtained in this work can be used to describe the sulfation of hydrated lime in the low-temperature dry and semidry flue gas desulfurization processes with or without an upstream NOx removal unit.

Characterization of Blast Furnace Slag−Ca(OH)2 Sorbents for Flue Gas Desulfurization

A series of sorbents for flue gas SO2 scrubbing have been prepared from hydrated lime (HL) and blast furnace slag (BFS) with the factorial experiment design method. The sorbents were characterized and tested for SO, reactivity in a differential fixed-bed reactor. It was found that, due to the formation of calcium silicate hydrates (CSHs), the reactivities of the sorbents prepared were higher than that of Ca(OH)(2) alone. The present sorbent gave better utilization of the Ca originated from the Ca(OH)(2) precursor and provided additional utilization capacity of the Ca derived from BFS. The present sorbents were mesoporous, and their specific surface areas were linearly correlated with their mesopore volumes. When the weight ratio of BFS/HL varied between 1/9 and 3/7, the SO2 capture of the sorbents prepared was independent of specific surface area. When the weight ratio of BFS/HL varied between 7/3 and 9/1, the SO, capture was seen to increase linearly with specific surface area. Among the four process variables studied for the sorbent prepared in this work, the weight ratio of BFS/HL was found to be the most significant for the specific surface area of the sorbents. The optimum BSF/HL ratio looked to be around 9/1. The second and the third process variables studied were hydration time and the ratio of water/solid, respectively. The optimum hydration time was about 10 h. while the optimum water/solid ratio was 25/1. The last process variable studied was slurrying temperature. Slurrying temperature (T) had little effect on the specific surface area of the BFS/HL sorbent in the range 50-80 degrees C. This is useful for preparing high-performance silica-enhanced desulfurization sorbents for dry and semidry flue gas desulfurization processes, taking advantage of the waste BFS.

Effect of Near-Wall Air Curtain on the Wall Deposition of Droplets in a Semidry Flue Gas Desulfurization Reactor

The wall deposition of droplets is an important issue affecting the desulfurization efficiency and operating stability of semidry flue gas desulfurization (FGD) reactors. Various near-wall air velocities, near-wall air flow inlet heights, and spray characteristics were analyzed numerically to investigate their effect on the gas-liquid flow and droplet deposition characteristics. The analytical results show that the near-wall air curtain effectively reduces the wall deposition of droplets in the semidry FGD reactor. The droplet deposition ratio decreased rapidly with increasing near-wall air velocity due to the increased gas flow rates and the altered gas velocity distribution. The near-wall air flow inlet height had an optimum value due to the rapid decline of the near-wall air momentum along the reactor height. The optimum distance between the near-wall air inlet height and the droplet injection height was 1.2 times that of the droplet vertical movement distance before deposition based on the linear droplet movement. For commonly used spray characteristics in the semidry FGD process, i.e., droplet diameters of 50-150 microm, spray angles of 10-70 degrees and droplet initial velocities of 20-100 m/s, the droplet deposition ratio with the addition of the near-wall air curtain varied slightly with the droplet diameter and the spray angle but increased rapidly with the initial droplet velocity. Therefore, for the semidry FGD processes, the near-wall air curtain is an effective method to reduce the wall deposition of droplets for various droplet diameters and spray angles while the initial droplet velocity should be carefully controlled to reduce the wall deposition of droplets and improve the operating stability.

Effects of Flue Gas Components on the Reaction of Ca(OH)2 with SO2

A differential fixed-bed reactor was employed to study the effects of the flue gas components, H2O, CO2, NOX, and O2, on the reaction between Ca(OH)2 and SO2 under conditions similar to those in the bag filters of a spray-drying flue gas desulfurization (FGD) system. The presence of CO2 with SO2 in the gas phase enhanced the sulfation of Ca(OH)2 only when NOX was also present. When either NOX (mainly NO) or O2 was present with SO2, the enhancement effect was slight, but became great when both NOX and O2 were present, and was even greater when CO2 was also present. The great enhancement effect exerted by the presence of NOX/O2 resulted from the rise in the NO2 concentration, which enhanced the oxidation of HSO3- and SO32- to SO42- in the water layer adsorbed on Ca(OH)2 surface and the formation of deliquescent salts of calcium nitrite and nitrate. The enhancement effect due to the presence of NOX/O2 was more pronounced when the relative humidity was above that at which the salts deliquesced; the extent of sulfation was more than twice that obtained when SO2 alone was present. The presence of H2O, CO2, NOX, and O2 in the flue gas is beneficial to the SO2 capture in the low-temperature dry and semidry FGD processes. The presence of NOX/O2 also enhanced CO2 removal when SO2 was absent.

Study on a Novel Semidry Flue Gas Desulfurization with Multifluid Alkaline Spray Generator

The advantages and disadvantages of the typical semidry flue gas desulfurization (FGD) processes are analyzed, and a novel semidry FGD process with multifluid alkaline spray generator is first proposed to improve the colliding contact efficiency between sorbent particles and spray water droplets, and to form a large amount of aqueous lime slurry. The experimental results show that the colliding contact efficiency between lime particles and water droplets in the prefix alkaline spray generator may reach about 70%, which is significantly higher than the colliding contact efficiency of 25% in duct sorbent injection. The SO2 removal efficiency can reach 64.5% when the Ca/S molar ratio is 1.5, the approach to the saturation temperature is 10.3 °C, and the flue gas residence time is 2.25 s. It is higher than that of in-duct sorbent injection under similar conditions, and the sorbent utilization is improved to 43%. Therefore, the FGD process with a prefix alkaline spray generator can greatly improve SO2 removal efficiency and sorbent utilization and it will be a new, simple and efficient semidry FGD process for industrial application in the future.