DEVELOPMENT OF SORBENTS FOR A FLUID BED PROCESS TO CONTROL SOX AND NOX
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Title
DEVELOPMENT OF SORBENTS FOR A FLUID BED PROCESS TO CONTROL SOX AND NOX
ICCI Project ID
00-1/2.2B-2
Investigator
Abbasian
Institution
Illinois Institute of Technology
ICCI Abstract
During the pulverized coal combustion process sulfur in coal is released in the form of sulfur dioxide (SO2) in the flue gas and a small fraction of nitrogen in the form of NO2 and NO, commonly referred to as NOX. The SO2 and NOX emissions are very damaging to the environment because they combine with moisture to form acids which then fall as acid rain. The threat from acid rain is made more of a concern in Illinois where over 90% of the high sulfur coal mined is consumed by electric utilities that are based on pulverized coal combustion, and only a very small fraction of the coal-based power plants in Illinois is currently equipped with Flue Gas Desulfurization (FGD) processes.
Conventional FGD processes include "wet throwaway" processes using lime or limestone-water mixture, and "dry throwaway" processes using lime slurries for spray drying or duct injection. However, there are several disadvantages associated with these processes, which include, among others, generation of large quantities of waste, a tendency towards scaling and plugging, erosion, and negligible NOX removal capability. Therefore, advanced processes that are based on dry regenerable sorbents offer attractive advantages over the conventional FGD processes.
The copper oxide based processes are one of the most promising emerging technologies for SO2 and NOX removal from flue gases. In these processes, SO2 in the flue gas reacts with the reactive component of the sorbent, i.e. CuO, and O2 to form copper sulfate (CuSO4). The sulfated sorbent is regenerated by a reducing gas, such as methane (CH4). decomposing the sulfate to elemental copper (Cu) and a concentrated stream of SO2. In addition to SO2 removal via chemical reaction, the sorbent also serves as a catalyst to remove a significant portion of the NOX in the feed flue gas. This step is accomplished by the controlled addition of ammonia (NH3) to the feed flue gases prior to entering the desulfurization unit.
Various reactor designs have been proposed, including fixed bed, fluidized-bed, and moving bed reactors. fluid bed copper oxide processes offers several advantages over the moving bed processes which include much smaller reactor size, simpler reactor design, and improved gas /solid contacting. Because of the significantly smaller size of the sorbent granules (or powder) in the fluid bed process, higher sorbent conversion can be achieved. Furthermore, because of the small size of the pellets, instead of low capacity supported pellets, higher capacity bulk sorbent granules can be used. Significantly higher capacity of the sorbent granules combined with the higher conversion results in tremendous reduction in sorbent requirement. However, highly reactive and attrition resistant sorbents are required to minimize attrition losses and the sorbent make-up rate.
Development of highly reactive and attrition resistant sorbents will expedite the commercialization of an economically more attractive fluid bed copper oxide flue gas desulfurization process, which will result in increased consumption of Illinois coal and creation of substantial number of new jobs and new source of revenue for the State of Illinois.
In earlier ICCI funded programs, the principal investigator of this project has developed highly reactive and highly attrition resistant sorbent granules for H2S removal in fluid bed using a sol-gel based technique. The modified sol-gel technique was applied to produce reactive copper oxide pellets with significantly improved crush strength for the moving bed copper oxide process.
The overall objective of the this project was to develop highly reactive and attrition resistance copper oxide sorbents for combined SO2 and NOX removal from coal combustion flue gases in a fluid bed process. leading to the development of a more efficient and economically more attractive fluid-bed copper oxide process to control SO2 and NOX .
To achieve the project objectives, a total of twenty-one (21) new sorbent formulations were prepared. The nominal copper content of the sorbents ranged from 5% to 33%. To improve the sorbent reactivity toward SO2, ammonium hydroxide (NH4OH) was added to the gel matrix. The molar ratio of NH4OH:HNO3 was also controlled, as was the dispersion of the NH4OH solution over the gel matrix. The ammonium hydroxide to nitric acid ratios [i.e., (NH4OH) : HNO3 ] were 0,1, and 2. The attrition resistance of these sorbents was determined and compared with that of the commercially available sorbent. The results indicate that the attrition indexes of the new sorbents developed in this project are 5 to 9 times lower than the commercially available baseline sorbent.
The sorbents were evaluated for their SO2 sorption capacities in packed-bed as well as fluidized-bed reactors. The results obtained with the seven (7) leading sorbents developed in this project indicate that all the seven sorbents have higher sulfur capacity compared to the commercially available baseline sorbent. The best result was obtained with ICCI-Cu-10, which contains 15% copper. This sorbent has 92 % higher sulfur capacity and nine (9) times lower attrition index compared to the baseline sorbent.
All the above sorbents were produced from aluminum tri-secondary butoxide (i.e., ALTSB, Al(OC4H9)3) precursor. Two additional formulation were produced using aluminum isopropoxide (i.e., ALISOP, Al(OC3H7)3). The copper content of these sorbents was 15%. The results of initial evaluation of these sorbents indicate that the sorbents produced using ASTLB precursor have significantly higher SO2 sorption capacity, compared to those produced with ALISOP precursor.
The higher effective sulfur capacity exhibited by the ICCI-Cu10 sorbent is believed to be due to higher copper content as well as higher surface area and porosity compared to other sorbents. It should be noted that the lower copper content of the commercially available baseline sorbent is mainly due to the limitation imposed by wet impregnation techniques. Higher copper loading in wet impregnated sorbents generally leads to pore plugging, resulting in lower sorbent reactivity. This was one of the most important features of the sol-gel method by which the copper-based sorbents for this project were prepared.
To determine the effects of operating parameters on sorbent performance, parametric studies were carried out with ICCI-Cu-10 sorbent. The operating variables investigated in this project included sulfation temperature (350°-450°C), regeneration temperature (400°-500°C), space velocity (4000-8000 hr-1), and inlet SO2 concentrations (1200-5000 ppmv). The results of these tests indicate that the effective sulfur capacity of the sorbent significantly improve with increasing sulfation the effective sulfur capacity of the sorbent increase by increasing regeneration temperature in the proceeding cycle. The lower capacity of the sorbent following regeneration at 400°C (compared to the baseline 450 °C) may be attributed to incomplete regeneration, while the improved performance of the sorbent following regeneration at 500°C is believed to be the result of changes in the pore structure of the sorbent. It should be noted that although increasing the regeneration temperature to 500°C, appeared to have beneficial affect on the sorbent performance, it is believed that sorbent exposure to higher temperature can have adverse affect on longterm performance of the sorbent. Therefore, based on the results of the parametric studies, the "optimum" sulfation and regeneration temperatures determined to be 450 oC, which also leads to desirable isothermal operation, eliminating the need for external heating of the sulfated sorbent
To determine the long term durability of the sorbent, a "life-cycle" test series consisting of 25 sulfation/regeneration cycles was conducted with the ICCI-Cu-10 sorbent. The results indicate that the effective sulfur capacity of the sorbent initially increases during the first few cycle, followed by a graduate decrease during the next 10-15 cycles. The effective sorbent capacity of the sorbent appears to stablize at 2% after the 20th cycle.
The catalytic activity of the sorbent ICCI-Cu-10g for reduction of NOX from the flue gases was also determined. The tests were carried out at a temperature of 450 oC and a space velocity of 4000 hr-1, using a simulated flue gas mixture containing 500 ppmv of NOX and 500 ppmv of NH3. These tests were performed using fresh, sulfated (after 2nd and 25th cycles), and regenerated (after 25th cycle) sorbents. The results indicate that the fresh sorbent and regenerated sorbent are capable of reducing 91.4% and 92.6% of the NOX content of the flue gas, respectively, while the sulfated sorbent can reduce more than 99.2 % of the NOX to nitrogen. The results also indicate that the catalytic activity of the sorbent for reduction of NOX is not adversely affected by the repeated cycling.
Given that repeated calibration of the NOX analyzer used in this experiment indicated the instrument was not able to detect the NOx levels below 8 ppmv, it can be concluded that the NOX removal efficiency of the sulfated materials is about 99.2 % (± 0.8 %). Therefore, it is conceivable that the sorbent may be capable of completely reducing NOX content of the flue gases.
Conventional FGD processes include "wet throwaway" processes using lime or limestone-water mixture, and "dry throwaway" processes using lime slurries for spray drying or duct injection. However, there are several disadvantages associated with these processes, which include, among others, generation of large quantities of waste, a tendency towards scaling and plugging, erosion, and negligible NOX removal capability. Therefore, advanced processes that are based on dry regenerable sorbents offer attractive advantages over the conventional FGD processes.
The copper oxide based processes are one of the most promising emerging technologies for SO2 and NOX removal from flue gases. In these processes, SO2 in the flue gas reacts with the reactive component of the sorbent, i.e. CuO, and O2 to form copper sulfate (CuSO4). The sulfated sorbent is regenerated by a reducing gas, such as methane (CH4). decomposing the sulfate to elemental copper (Cu) and a concentrated stream of SO2. In addition to SO2 removal via chemical reaction, the sorbent also serves as a catalyst to remove a significant portion of the NOX in the feed flue gas. This step is accomplished by the controlled addition of ammonia (NH3) to the feed flue gases prior to entering the desulfurization unit.
Various reactor designs have been proposed, including fixed bed, fluidized-bed, and moving bed reactors. fluid bed copper oxide processes offers several advantages over the moving bed processes which include much smaller reactor size, simpler reactor design, and improved gas /solid contacting. Because of the significantly smaller size of the sorbent granules (or powder) in the fluid bed process, higher sorbent conversion can be achieved. Furthermore, because of the small size of the pellets, instead of low capacity supported pellets, higher capacity bulk sorbent granules can be used. Significantly higher capacity of the sorbent granules combined with the higher conversion results in tremendous reduction in sorbent requirement. However, highly reactive and attrition resistant sorbents are required to minimize attrition losses and the sorbent make-up rate.
Development of highly reactive and attrition resistant sorbents will expedite the commercialization of an economically more attractive fluid bed copper oxide flue gas desulfurization process, which will result in increased consumption of Illinois coal and creation of substantial number of new jobs and new source of revenue for the State of Illinois.
In earlier ICCI funded programs, the principal investigator of this project has developed highly reactive and highly attrition resistant sorbent granules for H2S removal in fluid bed using a sol-gel based technique. The modified sol-gel technique was applied to produce reactive copper oxide pellets with significantly improved crush strength for the moving bed copper oxide process.
The overall objective of the this project was to develop highly reactive and attrition resistance copper oxide sorbents for combined SO2 and NOX removal from coal combustion flue gases in a fluid bed process. leading to the development of a more efficient and economically more attractive fluid-bed copper oxide process to control SO2 and NOX .
To achieve the project objectives, a total of twenty-one (21) new sorbent formulations were prepared. The nominal copper content of the sorbents ranged from 5% to 33%. To improve the sorbent reactivity toward SO2, ammonium hydroxide (NH4OH) was added to the gel matrix. The molar ratio of NH4OH:HNO3 was also controlled, as was the dispersion of the NH4OH solution over the gel matrix. The ammonium hydroxide to nitric acid ratios [i.e., (NH4OH) : HNO3 ] were 0,1, and 2. The attrition resistance of these sorbents was determined and compared with that of the commercially available sorbent. The results indicate that the attrition indexes of the new sorbents developed in this project are 5 to 9 times lower than the commercially available baseline sorbent.
The sorbents were evaluated for their SO2 sorption capacities in packed-bed as well as fluidized-bed reactors. The results obtained with the seven (7) leading sorbents developed in this project indicate that all the seven sorbents have higher sulfur capacity compared to the commercially available baseline sorbent. The best result was obtained with ICCI-Cu-10, which contains 15% copper. This sorbent has 92 % higher sulfur capacity and nine (9) times lower attrition index compared to the baseline sorbent.
All the above sorbents were produced from aluminum tri-secondary butoxide (i.e., ALTSB, Al(OC4H9)3) precursor. Two additional formulation were produced using aluminum isopropoxide (i.e., ALISOP, Al(OC3H7)3). The copper content of these sorbents was 15%. The results of initial evaluation of these sorbents indicate that the sorbents produced using ASTLB precursor have significantly higher SO2 sorption capacity, compared to those produced with ALISOP precursor.
The higher effective sulfur capacity exhibited by the ICCI-Cu10 sorbent is believed to be due to higher copper content as well as higher surface area and porosity compared to other sorbents. It should be noted that the lower copper content of the commercially available baseline sorbent is mainly due to the limitation imposed by wet impregnation techniques. Higher copper loading in wet impregnated sorbents generally leads to pore plugging, resulting in lower sorbent reactivity. This was one of the most important features of the sol-gel method by which the copper-based sorbents for this project were prepared.
To determine the effects of operating parameters on sorbent performance, parametric studies were carried out with ICCI-Cu-10 sorbent. The operating variables investigated in this project included sulfation temperature (350°-450°C), regeneration temperature (400°-500°C), space velocity (4000-8000 hr-1), and inlet SO2 concentrations (1200-5000 ppmv). The results of these tests indicate that the effective sulfur capacity of the sorbent significantly improve with increasing sulfation the effective sulfur capacity of the sorbent increase by increasing regeneration temperature in the proceeding cycle. The lower capacity of the sorbent following regeneration at 400°C (compared to the baseline 450 °C) may be attributed to incomplete regeneration, while the improved performance of the sorbent following regeneration at 500°C is believed to be the result of changes in the pore structure of the sorbent. It should be noted that although increasing the regeneration temperature to 500°C, appeared to have beneficial affect on the sorbent performance, it is believed that sorbent exposure to higher temperature can have adverse affect on longterm performance of the sorbent. Therefore, based on the results of the parametric studies, the "optimum" sulfation and regeneration temperatures determined to be 450 oC, which also leads to desirable isothermal operation, eliminating the need for external heating of the sulfated sorbent
To determine the long term durability of the sorbent, a "life-cycle" test series consisting of 25 sulfation/regeneration cycles was conducted with the ICCI-Cu-10 sorbent. The results indicate that the effective sulfur capacity of the sorbent initially increases during the first few cycle, followed by a graduate decrease during the next 10-15 cycles. The effective sorbent capacity of the sorbent appears to stablize at 2% after the 20th cycle.
The catalytic activity of the sorbent ICCI-Cu-10g for reduction of NOX from the flue gases was also determined. The tests were carried out at a temperature of 450 oC and a space velocity of 4000 hr-1, using a simulated flue gas mixture containing 500 ppmv of NOX and 500 ppmv of NH3. These tests were performed using fresh, sulfated (after 2nd and 25th cycles), and regenerated (after 25th cycle) sorbents. The results indicate that the fresh sorbent and regenerated sorbent are capable of reducing 91.4% and 92.6% of the NOX content of the flue gas, respectively, while the sulfated sorbent can reduce more than 99.2 % of the NOX to nitrogen. The results also indicate that the catalytic activity of the sorbent for reduction of NOX is not adversely affected by the repeated cycling.
Given that repeated calibration of the NOX analyzer used in this experiment indicated the instrument was not able to detect the NOx levels below 8 ppmv, it can be concluded that the NOX removal efficiency of the sulfated materials is about 99.2 % (± 0.8 %). Therefore, it is conceivable that the sorbent may be capable of completely reducing NOX content of the flue gases.
Start Date
11/1/2000
End Date
10/31/2001
Year Funded
2000
Citation
Cengiz, P.A., J. Abbasian, R.B. Slimane, K.K. Ho, and N.R. Khalili, "Formulation and Characterization of Improved Regenerable Sorbents for Flue Gas Desulferization." 26th International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, FL, March 6-9, 2001.
Collection
Citation
“DEVELOPMENT OF SORBENTS FOR A FLUID BED PROCESS TO CONTROL SOX AND NOX,” ICCI Reports, accessed May 20, 2024, https://isgswikis.web.illinois.edu/icci_reports/items/show/24.