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The present study focuses on the axial-mixing characteristics and flow behavior of fluidized particles, which so far have received scant attention compared to the bubbling and flow behavior of flui...

The bubble-chord lengths, l[sub v]'s, and pressure fluctuations were measured in three-phase fluidized beds by focusing on the effects of gas flow rate, liquid flow rate, and particle size on them. The pressure fluctuations have been analyzed by resorting to fractal analysis. Specifically, the rescaled range analysis has been applied to time series of the pressure fluctuations, thus giving rise to the Hurst exponent, H. It has been revealed that as the gas flow rate in a bed increases, l[sub v] and the standard deviation of its distribution increase but H decreases, and that the opposite is true as the liquid flow rate increases. The Hurst exponent, H, has been correlated in terms of the operational variables and the geometrical features of the beds, and l[sub v] has been correlated as a function of H and the radial position in the bed.

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.