Guo, F
1999
Kinetics of NO Reduction by Char: Effects of Coal Rank
Guo, F. and Hecker, W.C.
Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1999/pp.3085-3092
The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and postcombustion clean-up processes to control NOx emissions from combustion. The reaction is also important in understanding the formation and reduction of NO during coal combustion. In this study, the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah-Zap [NDL], Dietz, Utah Blind Canyon [UBC], Pittsburgh #8, and Pocahontas #3) were investigated in a packed-bed reactor at temperatures between 723 and 1173 K. Graphite and coconut char were also studied.
The low-rank chars were found to be significantly more reactive than the high-rank chars (NDL> Dietz>> coconut ~ Pittsburgh #8 ~ UBC ~ Pocahontas #3 >> graphite) with the T50 (temperature required for 50% NO conversion) varying from 870 K for NDL to 1100 K for graphite for a given set of conditions. For all chars studied, the reaction was found to be first order with respect to NO partial pressure and to exhibit an activation energy 0(EA) shift from 100-160 kJ/mol at low temperatures to 190-250 kJ/mol at high temperatures. The shift to distinctly different and higher EA's at higher temperature is opposite to what would be expected if a reaction is shifting from chemical rate control to mass transfer control and suggests different mechanisms or rate-determining steps at high and low temperatures. Although all chars exhibited the shift in EA, the shift temperature and the EA within each temperature regime tended to increase with increasing rank. Also, the relative reactivity of the chars depends not only on organic char surface area but also on inorganic content, specifically, CaO surface area.
1998
Kinetics of NO Reduction by Char: Effects of Coal Rank
Guo, F. and Hecker, W.C.
Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, accepted.
The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and post-combustion clean-up processes to control NOx emissions from combustion. The reaction is also important in understanding the formation and reduction of NO during coal combustion. In this study the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah-Zap (NDL), Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3) were investigated in a packed bed reactor at temperatures between 723 and 1173 K. Graphite and coconut char were also studied.
The low rank chars were found to be significantly more reactive than the high rank chars (NDL > Dietz >> Coconut ~ Pitts ~ UBC ~ Pocah >> Graphite) with the T50 (temperature required for 50% NO conversion) varying from 870 K for NDL to 1100 K for graphite for a given set of conditions. For all chars studied the reaction was found to be first order with respect to NO partial pressure and to exhibit an activation energy (EA) shift from 100-160 kJ/mol at low temperatures to 190-250 kJ/mol at high temperatures. The shift to distinctly different and higher EA's at higher temperature is opposite to what would be expected if a reaction is shifting from chemical rate control to mass transfer control, and suggests different mechanisms or rate determining steps at high and low temperatures. While all chars exhibited the shift in EA, the values of the shift temperature and the EA within each temperature regime tended to increase with increasing rank. Also, the reactivity of chars seemingly depends not only on organic char surface area, but also on mineral content, specifically CaO surface area.
The data also indicate that the rate of the heterogeneous reduction of NO by char is significant and comparable with the rate of homogeneous reburning of NO by CHI, and thus, the heterogeneous reaction should be included in models of NO formation during coal combustion.
1997
NO Reduction by Char: Effects of Coal Rank, Burnout Level, and Burnout Conditions
Guo, F. and Hecker, W.C.
Carbon 97:392(1997). Presented at the 23rd Biennial Conference on Carbon, Penn State University, July 16, 1997. Funded in part by ACERC.
The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and post-combustion clean-up processes to control NOx Emission from coal Combustion. The reaction is also very important in understanding the formation and reduction of NO during coal combustion. However, the heterogeneous kinetics and mechanism of the NO-char reaction are still poorly understood. Many questions regarding the mechanism and kinetics of the reaction, regarding the effects of char surface area, mineral matter, coal rank, burnout, and flue gases on the reaction remain. In this work, we explore the effects of coal rank, burnout level, and burnout conditions on the kinetics of the NO-char reaction.
The Kinetics of NO Reduction by Char: The Effects of Coal Rank, Burnout Level, and Burnout Conditions
Guo, F. and Hecker, W.C.
Western States Section/ The Combustion Institute, Livermore, California, Paper #97S-068 (1997). (Also presented at the Western State Section/The Combustion Institute, Livermore, California, April 15, 1997.)
The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and post-combustion clean-up processes to control NOx emissions from combustion. The reaction is also very important in understanding the formation and reduction of NO during coal combustion. In this study the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah-Zap, Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3) were investigated in a packed bed reactor at temperatures between 723 and 1173 K. Graphite and coconut char were also studied.
The reaction is first order with respect to NO partial pressure and exhibits an activation energy shift from 20-40 kcal/mol at low temperature to 45-60 kcal/mol at high temperatures for all chars studied. The low-temperature activation energy increases as coal rank increases. The shift to a distinctly different and higher activation energy at higher temperature is opposite to what would be expected if a reaction is shifting from chemical rate control to mass transfer control, and suggests different mechanisms or rate determining steps at high and low temperatures. The low rank chars are generally more reactive than the high rank chars. Also, the reactivity of char seemingly depends not only on organic char surface area, but also on mineral content and specifically CaO surface area. The experimental data also indicate that for NO formation in coal combustion the heterogeneous reduction of NO by char is significant, compared with homogenous reburning of NO by Chi.
The trend of variation of rate constant with char burnout significantly depends on char burnout conditions. The rate constant consistently decreases burnout when the chars are burnt out to different levels in a drop tube reactor at 1800 K and 3-5% O2. However, the rate constant increases as char burnout increases (up to 90%) when the char burnout levels result from reacting the char with 3000 ppm NO in a packed bed at 723-1173K. For the latter case, the relationship of rate constant (based on maf char mass) and char burnout is approximately linear with roughly the same slope between 20 and 80% burnout for all coal chars. The activation energy to the reaction is apparently independent of both char burnout level and burnout conditions.
1996
The Effects of CaO and Burnout on the Kinetics of NO Reduction by Beulah Zap Char
Guo, F. and Hecker, W.C.
ASC Division of Fuel Chemistry, (in press), 1996. (Also presented at the Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pennsylvania, 1996.) Funded by ACERC.
The heterogeneous reaction of NO with char is important in understanding the formation and reduction of NOx from coal combustion processes. The kinetics of NO reduction by North Dakota lignite char (NDL, its acid-washed char (NDW), and its calcium-reloaded char (NCa) were investigated in a packed bed reactor at temperatures from 723 to 1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content of the char increases. They also indicate clearly that the reaction is first order with respect to NO pressure and that there is a sharp increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher, but decrease form 60-45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases.
Using a series of six NDL chars, the effect of Char burnout level on the reaction of NO with char was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and CaO surface area (determined by CO2 uptake at 573K) decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the importance that CaO sites play in reduction process. The correlation of rate with CaO surface area is quantitative and also holds for 3 char types in the high temperature regime.
Nitric Oxide Reduction by CaO-Containing Coal Char: Correlation of Kinetics with CaO Surface Area
Guo, F. and Hecker, W.C.
Proceedings at the AIChE Annual Meeting, Chicago, Illinois, November 12, 1996. Funded by ACERC.
The heterogeneous reaction of NO with coal char has potential as a flue gas clean-up process. It appears that the inorganic constituents in coal, especially CaO and K2O, increases the rate at which NO is reduced by carbon. The kinetics of NO reduction by North Dakota Beulah Zap lignite char (NDL, its acid washed char (NDW), and its calcium-reloaded char (NcA) were investigated in a packed bed reactor at temperatures form 723 to 1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content increases. They also indicate that the reaction is first order with respect to NO partial pressure and show an increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher, but decrease form 60-45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases.
The effect of char burnout level on the reaction of No with char was also studied using tow methods. In one method, a series of NDL chars with different burnout levels was made by oxidation with 3% O2 in a drop tube reactor (DTR) at about 1800 K. In the other method, NDL parent char was continuously reacted with NO in a packed bed reactor at 948 K until a 90% burnout level was achieved. Different kinetic behaviors of the NO/char reaction for the two cases were observed. For the DTR chars (high temperature), the reaction rate constant and CaO surface area decreased as char burnout area, they became essentially independent of burnout level, which suggests the important part that CaO sites play in the reduction process. The transition temperatures and apparent activation energies were found to be independent of char burnout. For the char made by continuous reaction with NO (low temperature), however, the reaction rate constant increased with increasing burnout level up to a burnout level of 65%, then, quickly dropped off. Like the high temperature char, a shift in activation energy with temperature was also found for the lower burnout level chars, but the shift disappeared for the high burnout level chars (>35%).
Effects of CaO Catalysis on the Kinetics of NO Reduction by Beulah Zap Char
Guo, F. and Hecker, W.C.
ACS Division of Fuel Chemistry, (in press), 1996. (Also presented at the 211th ACS National Meeting, Division of Fuel Chemistry Symposium on Gasification Mechanisms, New Orleans, Louisiana, March 24-28, 1996.) Funded by ACERC.
The reduction of NO emissions from combustion processes has become increasingly important in protecting the world's environment. It has been shown that Selective Cataytic Reduction (SCR) with ammonia is an effective commercial technique to remove NOx from combustion flue gas. However, the implementation of this technique is limited by high investment and operating costs, "ammonia slip," and SOx poisoning which motivate the search for alternatives. Carbon (activated carbon or char) is a promising reducing agent for NOx reduction with many potential advantages, such as low cost, easy availability, high efficiency, simplicity of process, and no secondary pollution. Moreover, the heterogeneous reaction of NO with char is very important for the understanding of the formation of NOx from coal combustion processes. The reaction may significantly destroy the NOx formed earlier in coal combustion, which partially contributes to low NO emission from fluidized bed combustion. Therefore, the reaction of NO with char is receiving significant attention in the literature. Previous investigations on the reaction of NO with char involve the kinetics and mechanism, the effects of char surface area, the effects of feed gas composition, and the catalytic effects of metals. The reaction of NO with char has generally been reported to be first order with respect to NO partial pressure, bur reaction orders between 0.42 and 0.73 have also been reported. A sharp shift in the activation energy has been observed in the temperature range of 873-973 K, which suggests a complex reaction mechanism. Several mechanisms have been proposed. However, questions concerning N2 formation, the surface complexes, the nature of active surface sites, and the effects of minerals in char ash are still not well understood. In most previous studies, chars were taken to be pure carbon, thus the effects of the ash in chars and its composition on kinetics and mechanism of the reduction reactions are not well known. Although the catalytic effects of certain metals of metal oxides on the reactions have been investigates, little is known about their effects on the kinetics of the reactions. Therefore the objectives of this study are to investigate the kinetics of the reaction of NO with Beulah Zap chars to study the effects of CaO on kinetics.
Effects of CaO and Burnout on the Kinetics of NO Reduction by Beulah Zap Char
Guo, F. and Hecker, W.C.
Twenty-Sixth Symposium (international) on Combustion/The Combustion Institute, 2251-257(1996). Funded in part by ACERC.
The heterogeneous reaction of NO with char is important in understanding the formation and reduction of NOX from coal combustion processes. The kinetics of NO reduction by North Dakota lignite char (NDL), its acid-washed char (NDW), and its calcium-reloaded char (NCa) were investigated in a packed-bed reactor at temperatures from 723-1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content of the char increases. The also indicate clearly that the reaction is first order with respect to NO pressure and that there is a sharp increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher, but decrease from 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases.
Using a series of six NDL chars, the effect of char burnout level on the reaction NO with char was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and the CaO surface area (determined by CO2 uptake at 573 K) decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the importance that CaO sites play in the reduction process. The correlation of rate with CaO surface area is quantitative and also holds for the tree char types (NDL, NDW, and NCa) in the low-temperature regime. It does not hold for the three char types in the high-temperature regime.
Effects of Burnout Level and CaO Catalysis on the Kinetics of Nitric Oxide Reduction by Beulah Zap Char
Guo, F. and Hecker, W.C.
Presented at the Eighteenth Symposium of the Rocky Mountain Fuels Society, Albuquerque, New Mexico, February 29, 1996. Funded by ACERC.
The heterogeneous reaction of NO with chars is very important in understanding the formation and reduction of NOx in coal combustion processes. The reaction also has potential as the basis for and inexpensive, flexible approach to reduce NO emissions in post-combustion clean-up processes. Thus, the reaction is receiving increased attention in the literature; nevertheless, the kinetics of the reaction and the factors that influence them are not yet well understood.
North Dakota Beulah Zap lignite char (NDL), a portion of the NDL washed with HCI to remove mineral matter (NDW), and a portion of NDW reloaded with calcium oxide (NCa) were prepared previously in our lab and used for this study. Kinetic experiments were carried out in a 10 mm ID packed-bed reactor at temperatures from 723 to 1073 K, at an inlet NO concentration of 3130 ppm, and at flow rates of 100 to 500 ml/min. Compositions of inlet outlet gases were measured by GC and chemiluminescence NOx analyzer. Nitrogen and oxygen mass balances were determined for each run and always within ±5%.
Experimental results show that the NO/char reaction rate varies with char type, increasing significantly as the CaO content increases. They also indicate that the reaction is first order with respect to NO partial pressure and show an increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher but decrease form 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases. The shift in activation energy suggests a complicated reaction mechanism.
The effect of char burnout level on the reaction of NO with char was also studied using two methods. In one method a series of NDL chars with different burnout levels was made by oxidation with 3% O2 in a drop tube reactor (DTR) at about 1800 K. In the other method, NDL parent char was continuously reacted with NO in a packed bed reactor at 984 K until a 90% burnout level was achieved. Different kinetic behaviors of the NO/char reaction for two cases were observed. For the DRT chars (high temperature), the reaction rate constant with CaO surface are decreased as char burnout level increased from 0 to 80%. When the reaction rates were normalized by CaO surface area, they became essentially independent of burnout level, which suggests the important part that CaO sites play in the reduction process. The transition temperatures and apparent activation energies were found to be independent of char burnout. For the char made by continuous reaction with NO (low temperature), however, the reaction rate constant increased with increasing burnout level up to a burnout level of 65%, the quickly dropped off. Like the high temperature char, a shift in activation energy with temperature was also found for the lower burnout level chars, but the shift disappeared for the high burnout level chars (>35%).
Guo, F
1999
Kinetics of NO Reduction by Char: Effects of Coal Rank
Guo, F. and Hecker, W.C.
Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1999/pp.3085-3092
The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and postcombustion clean-up processes to control NOx emissions from combustion. The reaction is also important in understanding the formation and reduction of NO during coal combustion. In this study, the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah-Zap [NDL], Dietz, Utah Blind Canyon [UBC], Pittsburgh #8, and Pocahontas #3) were investigated in a packed-bed reactor at temperatures between 723 and 1173 K. Graphite and coconut char were also studied.
The low-rank chars were found to be significantly more reactive than the high-rank chars (NDL> Dietz>> coconut ~ Pittsburgh #8 ~ UBC ~ Pocahontas #3 >> graphite) with the T50 (temperature required for 50% NO conversion) varying from 870 K for NDL to 1100 K for graphite for a given set of conditions. For all chars studied, the reaction was found to be first order with respect to NO partial pressure and to exhibit an activation energy 0(EA) shift from 100-160 kJ/mol at low temperatures to 190-250 kJ/mol at high temperatures. The shift to distinctly different and higher EA's at higher temperature is opposite to what would be expected if a reaction is shifting from chemical rate control to mass transfer control and suggests different mechanisms or rate-determining steps at high and low temperatures. Although all chars exhibited the shift in EA, the shift temperature and the EA within each temperature regime tended to increase with increasing rank. Also, the relative reactivity of the chars depends not only on organic char surface area but also on inorganic content, specifically, CaO surface area.
1998
Kinetics of NO Reduction by Char: Effects of Coal Rank
Guo, F. and Hecker, W.C.
Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, accepted.
The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and post-combustion clean-up processes to control NOx emissions from combustion. The reaction is also important in understanding the formation and reduction of NO during coal combustion. In this study the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah-Zap (NDL), Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3) were investigated in a packed bed reactor at temperatures between 723 and 1173 K. Graphite and coconut char were also studied.
The low rank chars were found to be significantly more reactive than the high rank chars (NDL > Dietz >> Coconut ~ Pitts ~ UBC ~ Pocah >> Graphite) with the T50 (temperature required for 50% NO conversion) varying from 870 K for NDL to 1100 K for graphite for a given set of conditions. For all chars studied the reaction was found to be first order with respect to NO partial pressure and to exhibit an activation energy (EA) shift from 100-160 kJ/mol at low temperatures to 190-250 kJ/mol at high temperatures. The shift to distinctly different and higher EA's at higher temperature is opposite to what would be expected if a reaction is shifting from chemical rate control to mass transfer control, and suggests different mechanisms or rate determining steps at high and low temperatures. While all chars exhibited the shift in EA, the values of the shift temperature and the EA within each temperature regime tended to increase with increasing rank. Also, the reactivity of chars seemingly depends not only on organic char surface area, but also on mineral content, specifically CaO surface area.
The data also indicate that the rate of the heterogeneous reduction of NO by char is significant and comparable with the rate of homogeneous reburning of NO by CHI, and thus, the heterogeneous reaction should be included in models of NO formation during coal combustion.
1997
NO Reduction by Char: Effects of Coal Rank, Burnout Level, and Burnout Conditions
Guo, F. and Hecker, W.C.
Carbon 97:392(1997). Presented at the 23rd Biennial Conference on Carbon, Penn State University, July 16, 1997. Funded in part by ACERC.
The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and post-combustion clean-up processes to control NOx Emission from coal Combustion. The reaction is also very important in understanding the formation and reduction of NO during coal combustion. However, the heterogeneous kinetics and mechanism of the NO-char reaction are still poorly understood. Many questions regarding the mechanism and kinetics of the reaction, regarding the effects of char surface area, mineral matter, coal rank, burnout, and flue gases on the reaction remain. In this work, we explore the effects of coal rank, burnout level, and burnout conditions on the kinetics of the NO-char reaction.
The Kinetics of NO Reduction by Char: The Effects of Coal Rank, Burnout Level, and Burnout Conditions
Guo, F. and Hecker, W.C.
Western States Section/ The Combustion Institute, Livermore, California, Paper #97S-068 (1997). (Also presented at the Western State Section/The Combustion Institute, Livermore, California, April 15, 1997.)
The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and post-combustion clean-up processes to control NOx emissions from combustion. The reaction is also very important in understanding the formation and reduction of NO during coal combustion. In this study the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah-Zap, Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3) were investigated in a packed bed reactor at temperatures between 723 and 1173 K. Graphite and coconut char were also studied.
The reaction is first order with respect to NO partial pressure and exhibits an activation energy shift from 20-40 kcal/mol at low temperature to 45-60 kcal/mol at high temperatures for all chars studied. The low-temperature activation energy increases as coal rank increases. The shift to a distinctly different and higher activation energy at higher temperature is opposite to what would be expected if a reaction is shifting from chemical rate control to mass transfer control, and suggests different mechanisms or rate determining steps at high and low temperatures. The low rank chars are generally more reactive than the high rank chars. Also, the reactivity of char seemingly depends not only on organic char surface area, but also on mineral content and specifically CaO surface area. The experimental data also indicate that for NO formation in coal combustion the heterogeneous reduction of NO by char is significant, compared with homogenous reburning of NO by Chi.
The trend of variation of rate constant with char burnout significantly depends on char burnout conditions. The rate constant consistently decreases burnout when the chars are burnt out to different levels in a drop tube reactor at 1800 K and 3-5% O2. However, the rate constant increases as char burnout increases (up to 90%) when the char burnout levels result from reacting the char with 3000 ppm NO in a packed bed at 723-1173K. For the latter case, the relationship of rate constant (based on maf char mass) and char burnout is approximately linear with roughly the same slope between 20 and 80% burnout for all coal chars. The activation energy to the reaction is apparently independent of both char burnout level and burnout conditions.
1996
The Effects of CaO and Burnout on the Kinetics of NO Reduction by Beulah Zap Char
Guo, F. and Hecker, W.C.
ASC Division of Fuel Chemistry, (in press), 1996. (Also presented at the Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pennsylvania, 1996.) Funded by ACERC.
The heterogeneous reaction of NO with char is important in understanding the formation and reduction of NOx from coal combustion processes. The kinetics of NO reduction by North Dakota lignite char (NDL, its acid-washed char (NDW), and its calcium-reloaded char (NCa) were investigated in a packed bed reactor at temperatures from 723 to 1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content of the char increases. They also indicate clearly that the reaction is first order with respect to NO pressure and that there is a sharp increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher, but decrease form 60-45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases.
Using a series of six NDL chars, the effect of Char burnout level on the reaction of NO with char was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and CaO surface area (determined by CO2 uptake at 573K) decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the importance that CaO sites play in reduction process. The correlation of rate with CaO surface area is quantitative and also holds for 3 char types in the high temperature regime.
Nitric Oxide Reduction by CaO-Containing Coal Char: Correlation of Kinetics with CaO Surface Area
Guo, F. and Hecker, W.C.
Proceedings at the AIChE Annual Meeting, Chicago, Illinois, November 12, 1996. Funded by ACERC.
The heterogeneous reaction of NO with coal char has potential as a flue gas clean-up process. It appears that the inorganic constituents in coal, especially CaO and K2O, increases the rate at which NO is reduced by carbon. The kinetics of NO reduction by North Dakota Beulah Zap lignite char (NDL, its acid washed char (NDW), and its calcium-reloaded char (NcA) were investigated in a packed bed reactor at temperatures form 723 to 1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content increases. They also indicate that the reaction is first order with respect to NO partial pressure and show an increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher, but decrease form 60-45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases.
The effect of char burnout level on the reaction of No with char was also studied using tow methods. In one method, a series of NDL chars with different burnout levels was made by oxidation with 3% O2 in a drop tube reactor (DTR) at about 1800 K. In the other method, NDL parent char was continuously reacted with NO in a packed bed reactor at 948 K until a 90% burnout level was achieved. Different kinetic behaviors of the NO/char reaction for the two cases were observed. For the DTR chars (high temperature), the reaction rate constant and CaO surface area decreased as char burnout area, they became essentially independent of burnout level, which suggests the important part that CaO sites play in the reduction process. The transition temperatures and apparent activation energies were found to be independent of char burnout. For the char made by continuous reaction with NO (low temperature), however, the reaction rate constant increased with increasing burnout level up to a burnout level of 65%, then, quickly dropped off. Like the high temperature char, a shift in activation energy with temperature was also found for the lower burnout level chars, but the shift disappeared for the high burnout level chars (>35%).
Effects of CaO Catalysis on the Kinetics of NO Reduction by Beulah Zap Char
Guo, F. and Hecker, W.C.
ACS Division of Fuel Chemistry, (in press), 1996. (Also presented at the 211th ACS National Meeting, Division of Fuel Chemistry Symposium on Gasification Mechanisms, New Orleans, Louisiana, March 24-28, 1996.) Funded by ACERC.
The reduction of NO emissions from combustion processes has become increasingly important in protecting the world's environment. It has been shown that Selective Cataytic Reduction (SCR) with ammonia is an effective commercial technique to remove NOx from combustion flue gas. However, the implementation of this technique is limited by high investment and operating costs, "ammonia slip," and SOx poisoning which motivate the search for alternatives. Carbon (activated carbon or char) is a promising reducing agent for NOx reduction with many potential advantages, such as low cost, easy availability, high efficiency, simplicity of process, and no secondary pollution. Moreover, the heterogeneous reaction of NO with char is very important for the understanding of the formation of NOx from coal combustion processes. The reaction may significantly destroy the NOx formed earlier in coal combustion, which partially contributes to low NO emission from fluidized bed combustion. Therefore, the reaction of NO with char is receiving significant attention in the literature. Previous investigations on the reaction of NO with char involve the kinetics and mechanism, the effects of char surface area, the effects of feed gas composition, and the catalytic effects of metals. The reaction of NO with char has generally been reported to be first order with respect to NO partial pressure, bur reaction orders between 0.42 and 0.73 have also been reported. A sharp shift in the activation energy has been observed in the temperature range of 873-973 K, which suggests a complex reaction mechanism. Several mechanisms have been proposed. However, questions concerning N2 formation, the surface complexes, the nature of active surface sites, and the effects of minerals in char ash are still not well understood. In most previous studies, chars were taken to be pure carbon, thus the effects of the ash in chars and its composition on kinetics and mechanism of the reduction reactions are not well known. Although the catalytic effects of certain metals of metal oxides on the reactions have been investigates, little is known about their effects on the kinetics of the reactions. Therefore the objectives of this study are to investigate the kinetics of the reaction of NO with Beulah Zap chars to study the effects of CaO on kinetics.
Effects of CaO and Burnout on the Kinetics of NO Reduction by Beulah Zap Char
Guo, F. and Hecker, W.C.
Twenty-Sixth Symposium (international) on Combustion/The Combustion Institute, 2251-257(1996). Funded in part by ACERC.
The heterogeneous reaction of NO with char is important in understanding the formation and reduction of NOX from coal combustion processes. The kinetics of NO reduction by North Dakota lignite char (NDL), its acid-washed char (NDW), and its calcium-reloaded char (NCa) were investigated in a packed-bed reactor at temperatures from 723-1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content of the char increases. The also indicate clearly that the reaction is first order with respect to NO pressure and that there is a sharp increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher, but decrease from 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases.
Using a series of six NDL chars, the effect of char burnout level on the reaction NO with char was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and the CaO surface area (determined by CO2 uptake at 573 K) decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the importance that CaO sites play in the reduction process. The correlation of rate with CaO surface area is quantitative and also holds for the tree char types (NDL, NDW, and NCa) in the low-temperature regime. It does not hold for the three char types in the high-temperature regime.
Effects of Burnout Level and CaO Catalysis on the Kinetics of Nitric Oxide Reduction by Beulah Zap Char
Guo, F. and Hecker, W.C.
Presented at the Eighteenth Symposium of the Rocky Mountain Fuels Society, Albuquerque, New Mexico, February 29, 1996. Funded by ACERC.
The heterogeneous reaction of NO with chars is very important in understanding the formation and reduction of NOx in coal combustion processes. The reaction also has potential as the basis for and inexpensive, flexible approach to reduce NO emissions in post-combustion clean-up processes. Thus, the reaction is receiving increased attention in the literature; nevertheless, the kinetics of the reaction and the factors that influence them are not yet well understood.
North Dakota Beulah Zap lignite char (NDL), a portion of the NDL washed with HCI to remove mineral matter (NDW), and a portion of NDW reloaded with calcium oxide (NCa) were prepared previously in our lab and used for this study. Kinetic experiments were carried out in a 10 mm ID packed-bed reactor at temperatures from 723 to 1073 K, at an inlet NO concentration of 3130 ppm, and at flow rates of 100 to 500 ml/min. Compositions of inlet outlet gases were measured by GC and chemiluminescence NOx analyzer. Nitrogen and oxygen mass balances were determined for each run and always within ±5%.
Experimental results show that the NO/char reaction rate varies with char type, increasing significantly as the CaO content increases. They also indicate that the reaction is first order with respect to NO partial pressure and show an increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher but decrease form 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases. The shift in activation energy suggests a complicated reaction mechanism.
The effect of char burnout level on the reaction of NO with char was also studied using two methods. In one method a series of NDL chars with different burnout levels was made by oxidation with 3% O2 in a drop tube reactor (DTR) at about 1800 K. In the other method, NDL parent char was continuously reacted with NO in a packed bed reactor at 984 K until a 90% burnout level was achieved. Different kinetic behaviors of the NO/char reaction for two cases were observed. For the DRT chars (high temperature), the reaction rate constant with CaO surface are decreased as char burnout level increased from 0 to 80%. When the reaction rates were normalized by CaO surface area, they became essentially independent of burnout level, which suggests the important part that CaO sites play in the reduction process. The transition temperatures and apparent activation energies were found to be independent of char burnout. For the char made by continuous reaction with NO (low temperature), however, the reaction rate constant increased with increasing burnout level up to a burnout level of 65%, the quickly dropped off. Like the high temperature char, a shift in activation energy with temperature was also found for the lower burnout level chars, but the shift disappeared for the high burnout level chars (>35%).