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Recent studies of heterogeneous reburning, i.e., reburning involving a coal-derived char, have elucidated its variables, kinetics and mechanisms that are valuable to the development of a highly efficient reburning process. Young lignite chars contain catalysts that not only reduce NO, but they also reduce HCN that is an important intermediate that recycles to NO in the burnout zone. Gaseous CO scavenges the surface oxides that are formed during NO reduction, regenerating the active sites on the char surface. Based on this mechanistic information, cost-effective mixed fuels containing these multiple features has been designed and tested in a simulated reburning apparatus. Remarkably high reduction of NO and HCN has been observed and it is anticipated that mixed fuel will remove 85% of NO in a three-stage reburning process.

Reburning is an emerging three-stage combustion technology designed for the reduction of NO by introducing a small amount of reburning fuel above the primary flame where the majority of NO is chemically reduced to nitrogen. While coal, in general, has not been considered an effective reburning fuel, research at the University of Mississippi suggested that lignite has a reburning efficiency even higher than that of methane. Furthermore, heterogeneous mechanisms are more important than homogeneous mechanisms for char/NO reaction. The objectives of this research are to investigate: (1) implications of pore structure analysis, (2) parameters governing heterogeneous reactions, and (3) estimation of rates of NO reduction and mass transfer limitations. Experiments have been performed in a flow reactor with a simulated fuel gas at a stoichiometric ratio (SR) 1.1. Reburning fuels in this study include chars derived from Pittsburgh No.8 bituminous coal and Mississippi lignite. Chars were produced in N{sub 2} by suspending a sample basket in a tube furnace. Pore structure analyses include BET-N{sub 2}, BET-CO{sub 2}, and DR-CO{sub 2} surface pore size distribution, micropore volume, total pore volume, and average pore radius. These studies suggest that neither BET-N{sub 2} nor DR- CO{sub 2} surface area is a normalization factormore » of chars of different origin. Reaction with NO leads to closures of pores, which may be contributed by formation of surface complexes.« less

The four major tasks conducted during this quarter include: (1) extensive investigation in pore structures of chars before and after reactions with NO , CO,, and 02, (2) effects of pyrolysis time on char reactivities, (3) estimations of rates of NO reduction and mass transfer limitations, and, (4) char reactivities at low feed NO concentrations. Pore structure analyses include BET-N{sub 2}, BET-CO{sub 2}, and DR-CO{sub 2} surface areas, pore size distribution, micropore volume, total pore volume, and average pore radius. These studies suggest that neither BET-N{sub 2} nor DR-CO{sub 2} surface area is a normalization factor of chars of different origin. Parameter study reveals that the effectiveness of heterogeneous reburning strongly depends on variables in three areas: (1) the origin of char, (2) char devolatilization temperature and time, and, (3) the competitions of NO with C0{sub 2} and 02 for the active sites on the char surface. The studies on pore structure and on parameter screening signify the importance of transient kinetics (TK) and temperature programmed desorption (TPD) in the future research. These two techniques all lead to the direct measurements of both stable and reactive surface oxygen complexes, reactive surface area, and the Langmuir-Hinshelwood model which has both importance to both fundamental understanding of reaction mechanisms and to industrial practice. Estimation of rate of NO reduction has been established to include the conversions in the nonisothermal and isothermal regions of the flow reactor, and internal mass transfer limitations. Solving a set of equations simultaneously with MathCad gives frequency factor, activation energy, Thiele modulus, and effectiveness factor.

During this quarter, we have investigated rates and product compositions of NO reduction on chars in gases. N{sub 2} and CO{sub 2} internal surface areas of chars, selected from runs of various pyrolysis and reaction conditions have been measured to assist in interpreting the experimental results. Implications of Langmuir- Hinshelwood mechanisms and mass transfer limitations were examined. Oxidants suppress NO reduction on bituminous coal char more than on lignite char. Observations suggest that NO adsorption and desorption of stable surface oxygen complexes are potentially important rate- limiting steps and may be catalyzed by mineral matter during reburning with lignite char. Relative inert nature of lignite char to CO{sub 2} presence may have potential value in use of fuel system involving both solid and volatile fuels. Lignite char produced at 950 C and zero holding time has higher reactivity than that produced at 1100 C and 5 min holding time. Bituminous coal chars produced at these two conditions, however, have similar reactivity with NO. Internal surface areas of both type chars vary with pyrolysis conditions and gas composition in the subsequent reaction. When oxidants are introduced in the feed, internal surface areas of these two chars vary in opposite directions.

The investigation of this quarter focuses on the rates of NO reactions with chars in various gaseous environments. The results have revealed significant insights into the NO reduction mechanisms on char surface, particularly when the oxidants, O{sub 2} and CO{sub 2}, are introduced into the feed. Indeed, evidences suggest that the formation of stable oxygen complexes is the major cause of differences in NO reactivity on chars of different origins. The oxidants retard the reactivity of char derived fro the bituminous coal more seriously than they affect the char derived from lignite. Furthermore, additions of these oxidants into the reacting stream produce additional yields of CO and CO{sub 2} during NO reaction with lignite char, suggesting gasification of carbon from lignite char. No excess CO and CO{sub 2} were observed when the bituminous coal char was used. These yields of CO and CO{sub 2} also imply that desorption of stable surface oxygen complex is a rate-limiting step which may be catalyzed by the mineral matters during reactions involving lignite char. Surface area evaluated by CO{sub 2} and Dubinin-Radushkevich (D-R) equation is not a normalization factor of char reactivity during reburning. In the absence of oxidants, the bituminous coal char shows drastic increase in activation energy at about 950 {degrees}C indicating transition of desorption to adsorption controlled mechanisms. When CO{sub 2} is introduced, the transition temperature increases. When oxygen is also added, no such transition is observed up to 1100 {degrees}C. Lignite char reactivity increases smoothly over the temperature range 800 to 1100 {degrees}C. It is also observed that char reactivity decreases with increasing pyrolysis temperature which may be caused by closure of pores due to graphitization. The rate of NO reduction on the Pittsburgh {number_sign}8 coal char is then in good accord with that of a West Virginia coal char reported by De Soete (1980).

The progress in this quarter includes four parts. In the first segment, the implications of our data reported in the List quarter are discussed further. BET N{sub 2} surface area does not seems to be the only contributing factor to the remarkable activity of lignite char during reburning, and chars of different origins probably have different controlling steps in the overall surface reaction mechanisms. Unlike NO reduction in the gas phase, oxygen inhibits the heterogeneous mechanisms. The second part of this report justifies the use of our laminar flow reactor system for the measurement of reaction rate. Dispersion model is used in the analysis. An expression relating the rate constant with the experimentally obtainable NO conversion for our flow reactor have been derived. Rates of NO/char reaction for six series of experiments have been measured over the temperature range 800 to 1100{degrees}C. These six series of experiments have been conducted with two different chars, one bituminous coal char and one lignite char, and three different levels of feed NO concentrations, 200, 400 and 1000 ppm. Results from the comparison of char activities suggest that, in the absence of O{sub 2} and CO{sub 2}, the origin of char is not a significant factor for NO reduction. The CO/CO{sub 2} ratio in the products is higher than one under all test conditions, but the ratio increases with increasing feed NO concentrations. Recoveries of oxygen form the lignite char at temperatures above 1050{degrees}C is higher than 1 indicating gasification of organic oxygen in the char. Surface areas of selected chars after devolatilization and after reburning have been analyzed by BET in N{sub 2}. Results indicated char surface area changes after reburning, which is caused either by the higher temperature of reburning or by surface reaction.

This program addresses the mechanisms of NO reduction in pulverized coal flames using reburning. Using both thermodynamic and kinetic simulation combined with both pilot and bench scale experimental work, we propose to better define the conditions that lead to the destruction of NO, without the concomitant increase in other nitrogeneous pollutants. Specifically, the interactions between NO and hydrocarbon constituents in the fuel, and the fate of fuel nitrogen are the focal points of this research. Isotope doped 15 NO is used for tracing the reaction pathways.

This program addresses the mechanisms of NO reduction in pulverized coal flames using reburning. Using both thermodynamic and kinetic simulation combined with both pilot and bench scale experimental work, we propose to better define the conditions that lead to the destruction of NO, without the concomitant increase in other nitrogeneous pollutants. Specifically, the interactions between NO and hydrocarbon constituents in the fuel, and the fate of fuel nitrogen are the focal points of this research. Isotope doped 15 NO is used for tracing the reaction pathways.

The study of stoichiometric constraints under equilibrium conditions provides us with the following information related to reburning as a NO reduction technology: (1) The stoichiometry of air to fuel is the most important stoichiometry of air to fuel is the most important stoichiometric variable for reburning; (2) based on equivalent energy production, the comparative reduction efficiencies of the reburning fuels follow the trend: methane > ethane, ammonia > butane > aromatics, coal > CO; (3) the nitrogen in burning coals causes increases in equilibrium NO; (4) equilibrium TFN is not very sensitive in inlet NO concentration; (5) the heat carried by the flue gas in reburning could push the NO reduction toward equilibrium. If so, reburning has a strong correlation with air staging with air preheat. Further experimental verification is required.

During this quarter we continued to develop improved techniques of using aluminum diaphragms in the chemical shock tube and of analyzing reaction products. Tests with wide ranges of shock pressures have provided a set of baseline data for the proper selection of Al diaphgrams thickness. Our pioneer work on the analysis of trace amounts of NO, NO/sub 2/, NH/sub 3/, and HCN with gas chromatography/mass spectrometry were concentrated on column selection, and the use of a cold trap at the GC inlet. The down-fired furnace was completed, and initial confirmatory testing began. Several new sections of refractory had to be built because of damage sustained in transit from Kansas State University to LSU. Additionally, the on-line analyzers for O/sub 2/, CO, CO/sub 2/, and NO/sub x/ had to be calibrated and tested under suitable furnace conditions. Finally, the furnace was successfully fired with natural gas/air flames doped with ammonia to assess the potential performance of the sampling system and analyzer under conditions comparable to those to be employed during the second year of the program. 2 refs., 5 figs., 1 tab.

The study of stoichiometric constraints under equilibrium conditions provides us with the following information related to reburning as a NO reduction technology: (1) The stoichiometry of air to fuel is the most important stoichiometry of air to fuel is the most important stoichiometric variable for reburning; (2) based on equivalent energy production, the comparative reduction efficiencies of the reburning fuels follow the trend: methane > ethane, ammonia > butane > aromatics, coal > CO; (3) the nitrogen in burning coals causes increases in equilibrium NO; (4) equilibrium TFN is not very sensitive in inlet NO concentration; (5) the heat carried by the flue gas in reburning could push the NO reduction toward equilibrium. If so, reburning has a strong correlation with air staging with air preheat. Further experimental verification is required.