Axial Concentration Profiles and NO Flue Gas in a Pilot-Scale Bubbling Fluidized Bed Coal Combustor

Atmospheric bubbling fluidized bed coal combustion of a bituminous coal and anthracite with particle diameters in the range 500-4000 ím was investigated in a pilot-plant facility. The experiments were conducted at steady-state conditions using three excess air levels (10, 25, and 50%) and bed temperatures in the 750-900 °C range. Combustion air was staged, with primary air accounting for 100, 80, and 60% of total combustion air. For both types of coal, high NO concentrations were found inside the bed. In general, the NO concentration decreased monotonically along the freeboard and toward the exit flue; however, during combustion with high air staging and low to moderate excess air, a significant additional NO formation occurred near the secondary air injection point. The results show that the bed temperature increase does not affect the NO flue gas concentration significantly. There is a positive correlation between excess air and the NO flue gas concentration. The air staging operation is very effective in lowering the NO flue gas, but there is a limit for the first stage stoichiometry below which the NO flue gas starts rising again. This effect could be related with the coal rank

Kinetics and mechanisms of NO{sub x}: Char reduction. Quarterly technical progress report, 31 January 1995--30 April 1995

This project is concerned with the mechanism of reduction of both NO and N{sub 2}O by carbons. It was recognized some years ago that NO formed during fluidized bed coal combustion can be heterogeneously reduced in-situ by the carbonaceous solid intermediates of combustion. This has been recently supplemented by the knowledge that heterogeneous reaction with carbon can also play an important role in reducing emissions of N{sub 2}, but that the NO-carbon reactions might also contribute to formation of N{sub 2}. The precise role of carbon in N{sub 2} reduction and formation has yet to be established. Interest in the N{sub 2} and N{sub 2}O-char reactions has been significant in connection with both combustor modeling, as well as in design of post-combustion NO{sub x} control strategies. In our studies, a DuPont thermogravimetric analyzer (TGA) is used for the char reactivity studies. The temperature and mass are recorded as function of time, using a Macintosh computer and software for simultaneous apparatus control and data acquisition. Specific surface areas of char samples were determined by the N{sub 2} BET method at 77 K. A standard flow-type adsorption device (Quantasorb) was used for the measurements. Prior to surface area analysis, all samples were outgassed in a flow of nitrogen at 573 K for 3 hours. The carbonaceous solids used were resin char, graphite, coconut char and a Wyodak coal char. As was noted in the last report, carbons derived from different original materials show quite similar behaviors, in terms of the trends, but there are significant differences in actual reaction rates. It was shown that the spread of the reaction rate data from different studies, when expressed on a mass of carbon reactant- or surface area-basis, was almost the same

Kinetics and mechanisms of NOx - char reduction. Quarterly technical progress report, 1 August, 1996--31 October, 1996

The emission of nitrogen oxides from combustion of coal remains a problem of considerable interest, whether the concern is with acid rain, stratospheric ozone chemistry, or {open_quotes}greenhouse{close_quotes} gases. Whereas earlier the concern was focused mainly on NO (as a primary combustion product) and to a lesser extent NO{sub 2}, in recent years the emissions of N{sub 2}O have also captured considerable attention, particularly in the context of fluidized bed combustion, in which the problem appears to be most acute. The research community has only recently begun to take solid hold on the N{sub 2}O problem. This is in part because earlier estimates of the importance of N{sub 2}O in combustion processes were clouded by artifacts in sampling which have now been resolved. This project is concerned with the mechanism of reduction of both NO and N{sub 2}O by carbons

Kinetics and mechanisms of NO{sub x}: Char reduction

This report conveys recent progress on two issues. The first concerns the prediction on char-NO reactivity in general. The work has been performed upon a literature review, supplemented by our own recent results. The second part has to do with more detailed issues related to the mechanism of the reaction. Specifically, it is concerned with how oxide desorption can affect the observed kinetics. The report is divided into two stand-alone sections, on each of the above two topics. This material recently served as a basis for two manuscripts prepared for presentation at an American Chemical Society National meeting

Kinetics and mechanisms of NO{sub x} - char reduction. Quarterly technical progress report, 1 May, 1996--31 July, 1996

The emission of nitrogen oxides from combustion of coal remains a problem of considerable interest, whether the concern is with acid rain, stratospheric ozone chemistry, or {open_quotes}greenhouse{close_quotes} gases. Whereas earlier the concern was focused mainly on NO (as a primary combustion product) and to a lesser extent NO{sub 2}, in recent years the emissions of N{sub 2}O have also captured considerable attention, particularly in the context of fluidized bed combustion, in which the problem appears to be most acute. The research community has only recently begun to take solid hold on the N{sub 2}O problem. This is in part because earlier estimates of the importance of N{sub 2}O in combustion processes were clouded by artifacts in sampling which have now been resolved. This project is concerned with the mechanism of reduction of both NO and N{sub 2}O by carbons

Kinetics and mechanisms of NOx - char reduction. Quarterly technical progress report, August 1, 1995--October 31, 1995

The emission of nitrogen oxides from combustion of coal remains a problem of considerable interest, whether the concern is with acid rain, stratospheric ozone chemistry, or {open_quotes}greenhouse{close_quotes} gases. Whereas earlier the concern was focused mainly on NO (as a primary combustion product) and to a lesser extent NO{sub 2} (since it is mainly a secondary product of combustion), in recent years the emissions of N{sub 2}O have also captured considerable attention, particularly in the context of fluidized bed combustion, in which the problem appears to be most acute. The research community has only recently begun to take solid hold on the N{sub 2}O problem. This is in part because earlier estimates of the importance of N{sub 2}O in combustion processes were clouded by artifacts in sampling which have now been resolved. This project is concerned with the mechanism of reduction of both NO and N{sub 2}O by carbons. It was recognized some years ago that NO formed during fluidized bed coal combustion can be heterogeneously reduced in-situ by the carbonaceous solid intermediates of combustions. This has been recently supplemented by the knowledge that heterogeneous reaction with carbon can also play an important role in reducing emissions of N{sub 2}O, but that the NO-carbon reactions might also contribute to formation of N{sub 2}O. The precise role of carbon in N{sub 2}O reduction and formation has yet to be established, since in one case the authors of a recent study were compelled to comment that the basic knowledge of N{sub 2}O formation and reduction still has to be improved. The same can be said of the NO-carbon system

STUDY OF ACTIVATION OF COAL CHAR

This is the final report on a project whose aim is to explore in a fundamental manner the factors that influence the development of porosity in coal chars during the process of activation. It is known that choices of starting coal, activating agent and conditions can strongly influence the nature of an activated carbon produced from a coal. This project has been concerned mainly with the process of physical activation, which in fact involves the gasification of a char produced from a coal by oxidizing gases. This is of interest for two reasons. One is that there is commercial interest in production of activated carbons from coal, and therefore, in the conditions that can best be used in producing these materials. Much is already known about this, but there is a great deal that is in the realm of ''trade secret'' or just ''industry lore''. The second reason for interest in these processes is that they shed light on how porosity develops during any gasification process involving oxidizing gases. This has implications for understanding the kinetics and the role that ''surface area'' may play in determining kinetics. In earlier reports from this project, several conclusions had been reached upon which the present results rest. There is an often-cited difference in use of nitrogen and carbon dioxide as molecular probes of carbon porosity when using vapor adsorption techniques. Carbon dioxide is often ''preferred'' since it is argued that it offers greater access to fine microporosity, due to the higher temperature of carbon dioxide as opposed to nitrogen measurements. The early phases of this work revealed that the extreme differences are observed only in chars which are not much activated, and that by a few weight percent burnoff, the difference was negligible, provided a consistent theoretical equation was used in calculating uptake or ''surface area''. In another phase of this study, it was noted in a preliminary way how the use of different oxidizing environments would lead to very different porosity development in the same char. There did not seem to be a link to the overall inherent reactivity of the gas-char combination to the pattern of porosity development. In another portion of this study, it was observed that the expected pattern of porosity development could be seen, as a function of whether the process was carried out in a pure chemical kinetic control regime (Zone I) or in a partially mass transfer control regime (Zone II). This portion of the study was useful in suggesting that the unburned carbon from many practical pulverized coal combustion processes had actually emerged from a Zone II environment. This confirms other published hypotheses, and strongly suggests that the material does not survive the boiler environment because it was produced in a purely oxygen mass transfer limited zone (so-called Zone III) or because it was simply so unreactive that it could not burn up in the allotted time (a pure Zone I argument). Moreover, it is believed that the very rapid initial opening of porosity that is revealed by the rapid disappearance of nitrogen and carbon dioxide accessible porosity may be associated with a very thin surface layer of pyrolytically-formed carbon that effectively blocks the bulk char structure from nitrogen. Once removed by low extent of burn-off this phenomenon disappears. Finally, the project turned to comparing the relative influences of the starting coal and the oxidizing environment on the nature of porosity that was developed. Once again, the Argonne Premium coal suite served as a source of chars that would be representative of the broad range of coals found an utilized in the US. The conclusion is that the starting coal has a profound influence upon the ability of an oxidizing agent to develop porosity in the char. This is the single most important factor. Beyond this, however, there was a surprise to the extent that the ordering of porosity development did not follow a simply predictable pattern related to the reactivity of the activating agents. Oxygen is a very effective activating agent, if operation can be kept under control under so-called Zone I conditions. Its effectiveness is comparable to that of the more widely-employed steam