An Attempt to Reduce NO, Emissions from Pulverized Coal Furnaces Robert J. Robertus,’ Kenneth L. Nielsen’ Department of Chemical Engineering, Washington State University, Pullman, Wash. 99 163
Clayton T. Crowe, David 1.Pratt Department of Mechanical Engineering, Washington State University, Pullman, Wash. 99163
An investigation was undertaken to determine the feasibility of a simple furnace alteration for reducing NO emissions from pulverized coal-fired furnaces. In this experiment pulverized coal was fluidized with a methane-air combustion mixture in a tubular or plug flow reactor. Average gas temperatures from about 690-900’F and coal residence times in the reactor from 1.4-2.8 sec were used. I t was hoped that temperatures below 750’F would volatilize a large portion of the nitrogen in the fuel and that this evolved nitrogen would be in the molecular N2 form. If this could be achieved, a significant reduction in total NO emissions from commercial coal-fired boilers would be realized. Results showed that only a small fraction (16%) of the fuel nitrogen volatilized under the most favorable conditions (high temperature and a long residence time). Using inlet gas temperatures below 755’F, only 3% of the fuel nitrogen was volatilized. After coal is initially heated to about 800°F, fuel volatilization proceeds at a rate of 5.5%per sec independently of temperature over a range of 680-1000°F. This rate continues for a t least 2 sec. The final form of the released nitrogen is not known. Almost none was NO,. It is speculated that most was NH3 and N2 with Nz being the most abundant. It was concluded that the technique used in this experiment would not by itself be a viable method to reduce fuel NO emissions from pulverized coal utility boilers. Nitrogen oxide emissions from coal combustion are formed from two distinct sources of nitrogen (1, 2). First is the nitrogen from the combustion air, and second is the small amount of bound nitrogen in the coal itself. Presently there are no economically successful means for removing NO, emissions from high-volume gas streams (3-5). Strategies for controlling nitric oxides from atmospheric nitrogen (termed “thermal NO,”) depend primarily on controlling the furnace temperature (6-12). The presence of nitrogen in fuels complicates the control of NO, emissions. Fuel NO, formed from these nitrogen compounds is quite significant. For oil or gaseous fuels, 25-60% of the fuel nitrogen ends up as NO, under typical furnace conditions (7, 10, 12-14). The amount of NO, formed is not affected greatly by temperature or type of nitrogen compound in the fuel (7, 10, 14). Most coals contain some bound nitrogen. A nitrogen content of 2% by weight is not uncommon. Although some data are available on coal volatilization products a t low heating rates [coal carbonization studies (15, 1 6 ) ] , little information can be found on rapid heating rates (17) with times on the order of either seconds or milliseconds.
Rationale and Objectives In both face-fired and corner-fired pulverized coal-burning furnaces, the pulverized coal is pneumatically conveyed to the furnace nozzles. Normally 15-20% of the total comDeceased.
bustion air is used for pneumatic coal feed. Fortunately, 15-20% flue gas recirculation also appears to be optimal for control of thermal NO, based on results reported by Martin and by Turner for oil-fired furnaces. If the fraction of flue gas which is recirculated for control of thermal NO were used instead of primary air to convey the coal particles, it should be possible to use the pneumatic feed pipes as chemical reactors in which the coal volatilization could occur in a low-oxygen atmosphere. Since oxygen molecules would be scarce, there is less likelihood that the nitrogen atoms would react to form NO. Optimistically, one could hope that the nitrogen atoms would form molecular nitrogen and then be subject to the same control techniques inside the furnace as is ordinary atmospheric nitrogen. The objective of the research was to determine the rate of volatilization of coal as a function of time and temperature in an exhaust gas-conveyed stream of pulverized coal. The rate of nitrogen volatilization specifically was to be ascertained. The real question to be answered was, “Will significant amounts of bound nitrogen be released if pulverized coal is conveyed into the furnace using hot exhaust gases?”
Experimental Plan Recognized Constraints. As indicated previously, it is not known what conditions of temperature and residence times are necessary to complete the volatilization of the coal particles prior to entry into the furnace. In actual operating systems, constraints are placed on both temperature and time. Existing coal pipes allow average residence times from 1-3 sec. Most commercial pulverizers cannot operate above 500’F. Gases may enter at higher temperatures but are cooled as moisture is driven off the coal, and actually exit a t temperatures closer to 150-200’F. The temperature limitation on the pulverizer might require that pneumatic feed at elevated temperatures with recirculated flue gases would have to be accomplished by some other method than simply feeding the pulverizer with the flue gases. It could be necessary to design a system which would entrain the pulverized coal with the flue gases alone. While pulverizer gas inlet temperatures are limited in conventional furnaces (air from the economizer), recirculated flue gases could in principle be drawn off at any temperature desired, so that feed pipe temperatures could be elevated to whatever temperature is desired, subject only to materials limitations and possibility of flashback. Agglomeration of coal at higher temperatures could also be a problem. On the experimental scale, this would not be expected to hinder achievement of the basic objective. It is, however, something which was watched because of its importance in industrial operations. Inspection of the sample probes after each run showed no significant deposits of coal on the surface. Inspection of the coal pipe after several runs revealed almost no agglommeration. Equipment Design. The experimental equipment was designed to simulate conditions which could be achieved in a coal feed pipe on an industrial furnace. A mixture of Volume 9, Number 9, September 1975
859
TO VENTURI
\
A scrubber chamber was located a t the top of the pipe and used water sprays to cool the gas and coal and flush them into a drain. Equally important is the fact that the spray will quench the volatilization reaction. Samples of coal collected from the drain pipe should have the same chemical composition (except for moisture content) as coal leaving the top of the column. Analytical Procedure
Figure 1. Combustion chamber
methane and air was burned in a cylindrical chamber with water-cooled walls, as schematically illustrated in Figure 1. The burner tip was movable so the temperature of the combustion gases could be regulated prior to entering the test pipe section. From the outlet of the heat exchanger, the product gas a t 500-1200°F passed directly to a venturi section where the coal was entrained. The coal used in this study came from Centralia, Wash., and is a low-sulfur subbituminous Cgrade material. The rate of coal feed was easily controlled by varying the speed of rotation of the vibrating feeder’s auger screw. The test pipe shown schematically in Figure 2 was attached to the top of the venturi and consisted of 20 f t of 2-in. 0.d. stainless steel tubing fitted with 10 sampling ports a t 2-ft intervals. The hot gases carrying entrained coal particles traveled upward in the pipe at velocities variable between about 6 and 20 ft/sec. This gave residence times ranging between 1 and 3 sec. Insulation on the test pipe prevented excessive heat losses, although temperatures were as much as 350’F lower at the top than a t the bottom. The gas-sampling ports are fitted with dismount clamps to enable rapid installation of the sampling probes. For sampling, the probes were fitted with a porous stainless steel filter (2-p rating) through which the hot exhaust gases and coal volatiles passed before entering some water vapor traps prior to being analyzed. Thermocouples were also mounted in the sampling ports to provide an indication of local gas temperature.
Some preliminary calculations showed that concentrations of 2000 ppm NH3 or NO could be expected if 20% of the bound nitrogen in the coal formed either NH3 or NO. At these concentrations it was felt that a gas chromatograph with a thermal conductivity detector would be useful for analyzing the coal volatiles. Later it was found that whatever nitrogen compounds were being formed were in concentrations too low to be detected by the gas chromatograph. The detection limit was on the order of 500-1000 ppm for the compounds of interest. The chromatograph was useful in monitoring combustion products from the hot gas generator. For most runs, the 0 2 content was between 3 and 4% by volume and no CO or unburned methane was detected. For part of run G, the 0 2 content dropped to 0.6%; then the CO content was also 0.6%. Some thought was given to possible catalytic decomposition of the gaseous products as they passed through the porous stainless steel probe. Previous and present studies of the kinetics of NO, formation in CH4 flames used a watercooled stainless steel probe. At gas temperatures below 1800OF there was no evidence of catalytic effects. Although this system is not the same, temperatures were significantly lower, so any catalytic action on compounds of interest was neglected. All studies reported herein involved collection of coal samples from the feed hopper and the drain line. The quantitative results in this report are based on elemental C, H, N, and ash analyses of the coal itself from these sources. A chemiluminescent analyzer was used to monitor gaseous NO, emissions. Budget limitations prevented more sophisticated analyses of the coal volatiles. Such information would have been useful but was not essential to determining the overall rate of coal or bound nitrogen volatilization. Results and Discussion
~
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WATER JACKET
1
A summary of operating conditions and calculated findings is given in Table I. The residence times and coal-gas ratios reported are based on conditions when the coal sample was taken. They are compiled from metered flows of methane and air. Calculations were made to see how the evolved gases would affect residence times. When we took 15% of the coal volatilized and assumed it was given off instantaneously at the bottom of the column (this would give the largest “correction” possible), the change in residence times was less than 10%. Since gases are evolved throughout the column, the correction would be even smaller. The percentages of coal and nitrogen volatilized were calculated from analyses of the feed bin and exiting coal. The bin samples were simply scooped out of the coal feeder. The exit samples were collected by filtering the drain line for a period of ab,out 5 min. Only a small portion of each sample was needed for analytical purposes, so the part analyzed may not have been a “true” average of the exit coal over the sampling interval. The data do, however, appear consistent; a fact which lends credibility to the analysis. In all, four samples of the coal feed were analyzed and the results are shown in Table 11. Samples 1 and 2 were
kVENTUR‘
Figure 2. Venturi and test pipe schematic 880
SS TUBE
Environmental Science & Technology
Table I . Coal Volatilization Results Coal Volatilized Total Group
A B
C D E F G H
Bottop temp, F
Avgo temp, F
Residence time, sec
1042 1008 951 886 871 803 800 754
9 00 875 851 769 777 690 686 686
1.43 1.39 1.72 1.38 1.72 2.8 1 2.84 1.41
Coallgas ratio, Ibllb
0.108 0.32 1 0.368 0.249 0.348 0.290 0.378 0.249
from the same drum. Samples 3 and 4 were from the same shipment, but a different drum. Because the calculational procedures were dependent on accurate coal analyses and assumed uniform composition for a given group (especially for nitrogen and ash), the variations shown in Table I1 could effect small deviations in the final results. The results shown in Table I are not meant to be used as data for any sophisticated mathematical model. Since most values are averages in one way or another, they are for the most part not well enough defined for such purposes. Their primary utility is to show trends and provide enough information to arrive a t some decision concerning the feasibility of releasing bound nitrogen in the coal by flue gas recirculation. The results are listed in order of decreasing temperature with residence time and coal-gas ratio being additional variables. Total Volatilization Products. Groups A and B show the effect of the coal-gas ratio on the volatiles evolved with temperature and residence time remaining about the same. Although the temperatures did differ slightly, the effect of increased coal loading seemed small. Groups F and G were run a t different conditions but support the same conclusion. With only one exception, all of the groups showed an increasing rate of volatilization of total products as temperatures were increased. Groups F and G compared with C, D, and E show that the amount of total volatilization increased as residence times were increased. Since groups F and G were run at lower temperatures, the volatilization rates were lower. Therefore, the fact that there was no decrease in the amount of total volatiles evolved (compared to groups C, D, and E) is due to the longer residence times of groups F and G. Nitrogen Volatilization Products. The nitrogen volatilization rate is remarkably constant (taking into account experimental errors) over the inlet temperature range from 800-1042'F. The amount of nitrogen volatilization is roughly proportional to residence time. If residence time is long enough, the volatilization rate should start to decrease. The data show that a t residence times of about 2% sec, the decrease is not great. It is speculated that most of the nitrogen to be evolved changes form while still in the coal a t temperatures just below 800'F and in a very short time. Once changed to this intermediate form, secondary changes can occur which change the intermediate form to its final volatilized state and the nitrogen leaves the coal a t a near constant rate. The secondary changes are time-dependent but temperature-insensitive and can occur a t temperatures below 800°F (at least down to 700OF). This two-step volatilization model is consistent with both this data and other re-
Nitrogen
%
Rate, % lsec
%
Rate, %/sec
21.0 20.0 16.0 15.0 15.5 16.0 15.0 6.1
14.6 14.4 9.3 10.8 9.0 5.7 5.3 4.3
7.5 8.4 10.5 5.6 8.4 15.0 13.0 3.0
5.2 6.0 6.1 4.1 4.9 5.3 4.6 2.1
Table II. Coal Feed Analyses Composition Sample
1 23 4
C
H
N
Ash
61.8 59.09 61.96 60.10
4.59 4.25 4.60 4.37
1.0 0.93 1.07 1.08
13.83 13.63 12.20 14.80
ported volatilization phenomena. For diffusion-limited coal volatilization (the size of coal particles used in this experiment put it in that category), the rate of volatiles evolution a t constant temperature is proportional to the amount of total volatiles (the ones that would evolve given sufficient time) in the coal (18). If the evolved fuel nitrogen were converted to its final form in one step and very quickly (about 0.1 sec), the volatilization rate would be diffusion-limited and decay exponentially with time. The data show a fairly constant volatilization rate over a range of residence times and hence do not support this. If the evolved fuel nitrogen were converted to volatilized form in one step but were time-dependent and occurred over the length of the pipe, a constant rate of nitrogen volatilization could occur over a range of residence times. A one-step process, however, would have a minimum temperature below which no appreciable conversion would occur. Since, in a one-step process, conversion is equated to volatilization, volatiles would not be emitted below a minimum conversion temperature. Groups G and H show that volatilization has no well-defined lower temperature limit. An appreciable amount of volatilization is occurring at 686'F for group G while little is happening a t 754'F for group H. This invalidates all one-step mechanisms. These experimental results can support only a two-step process. In actual volatilization there may be more than two steps involved, but two is the minimum number necessary to describe the process. Results of this experiment are confirmed by other published works. Even for coal heated to 1480'F, some 83% of the nitrogen remains in the coke (19). Kierner ( 1 5 ) lists a table of the percentage of nitrogen remaining in the coke for coal heated to different temperatures for seven minutes. He reports 13% of the nitrogen volatilized a t 1000'F. This is similar to the results obtained for groups F and G, and indicates that most of the nitrogen which will come off probably evolves early and volatilization rates will drop with longer residence times. Kierner's table also shows a few values which seem out of place, similar to groups C and D and this experiment. He attributes this to the uncertainVolume 9, Number 9, September 1975
861
ty of the determination of nitrogen in the coke. In summary, the results seem consistent with previous work done in this area. T o volatilize a major portion of the fuel nitrogen would require temperatures much higher than those used in this experiment. Although this investigation has determined what fraction of bound nitrogen does disappear, the gaseous form of the nitrogen molecules could not be determined. If all or most of the nitrogen had gone to ammonia as originally suspected, the chromatograph would have detected it. If as little as 5% of what had volatilized had gone t o NO, the chemiluminescent analyzer would have picked that up. When spot checks were made, less than 20 ppm NO, above background levels were ever detected. The most likely possibility is that the released nitrogen was reduced to molecular nitrogen. The quantity involved, however, was so small compared to the volume entering with the combustion air that it could not be detected. A second possibility is that the fuel nitrogen formed many small gas molecules [HCN, “3, N2 (CN)2, etc.], each in such small quantities that they cannot be detected by the chromatograph. Work done by others indicates that a 15% reduction in bound nitrogen content of the coal before it enters the furnace does not necessarily mean that NO emissions will be reduced by 15%. Very small amounts (0.1 wt %) of bound nitrogen are nearly 80% converted to NO. When the concentration is increased t o 1% by weight, only about 40% of the nitrogen goes to NO. It is thus possible that even with the maximum 15% volatilization seen in this study, NO, emissions would not be reduced significantly.
Acknowledgment Coal was supplied by Pacific Power and Light Co. of Centralia, Wash. Literature Cited (1) Bagwell, F. A., Rosenthal, K., Breen, B. P., Bell, A. W., “13th International Symposium on Combustion,’’ 391-400, The Combustion Institute, Pittsburgh, Pa, 1974. (2) Barrett, R. E., “Factors Influencing Air Pollution Emissions from Stationary Combustion Sources,” presented at Seminar on
New Developments for Combustion Engineering, Penn State University, July 26-30, 1971. (3) First, M. W., Viles, F. J., J. Air Pollut. Control Assoc., 21 (3), 122-7 (1971). (4) U.S. Department of Health, Education and Welfare, NAPCA Publication No. AP-67, “Control Techniques for Nitrogen Oxide Emissions,” 1970. (5) Weidersum, G, D., Chem. Eng. Prog., 66 (ll),49-55 (1970). (6) Bartok, W., Crawford, A. R., Skopp, A., ibid, 67 (2), 64-72 (1971). (7) Bartok, W. Crawford, A. R., Skopp, A., “Systematic Investigation of Nitrogen Oxide Emissions and Control Methods for Power Plant Boilers,” presented at 70th Annual Meeting AIChE, Atlantic City, N.J., August 1971. (8) Bartok, W., Engleman, V. S., “Basic Kinetic Studies and Modelling of Nitrogen Oxide Formation in Combustion Processes,” ibid.
(9) Blakeslee, C. E., Qurbach, H. E., J. Air Pollut. Control Assoc., 23 (l), 37-42 (1973). (10) Martin, G. B., Berkau, E. E., “Preliminary Evaluation of Flue Gas Recirculation as a Control Method for Thermal and Fuel Related Nitric Oxide Emissions,” presented at Western States Section/Combustion Institute Fall Meeting, University of California, Irvine, Calif., October 1971. (11) Shaw, J. T., J. Inst. ojFuel, 46 (3851,170-8 (1973). (12) Wasser, J. H., Hangebrauck, R. P., Schwartz, A. J., J.Air Pollut. Control Assoc., 18 (5), 332-337 (1968). (13) Martin, G. B., Berkau, E. E., “An Investigation of the Conversion of Various Fuel Nitrogen Compounds to Nitrogen Oxides in Oil Combustion,’’ presented at 70th Annual Meeting AIChE, Atlantic City, N.J., August 1971. (14) Turner, D. W., Andrews, R. L., Siegund, C. W., Combustion, 44 (2), 21-30 (1972). (15) Lowry, H. H., ed., “Chemistry of Coal Utilization,” 2nd printing, Chap. 22, John Wiley and Sons, New York, N.Y., 1947. (16) Selvig, W. A., Ode, W. E., “Low-Temperature Carbonization Assays of North American Coals,” U.S. Bur. Mines Bull. 571, US. Govt. Printing Office, Washington, DC, 1957. (17) Field, M. A., Gill, D. W., Morgan, B. B., Hawksley, P. G. W., “Combustion of Pulverised Coal,” Chap. 4, The British Coal Utilisation Research Association, Leatherhead, England, 1967. (18) Essenhigh, R. H., Howard, 3. B., “Combustion Phenomena in Coal Dusts and the Two-Component Hypothesis of Coal Constitution,’’ Penn State University, 1971. (19) Williams, A. W., “Coal Manual for Industry,” p 15, ConoverMast Pub., New York, N.Y., 1952. Received for review September 26, 1974. Accepted April 28, 1975. Work supported by Western Energy Supply and Transmission group.
Trace Element Behavior in Coal-Fired Power Plant John W. Kaakinen” Department of Chemical Engineering, University of Colorado, Boulder, Colo. 80302
Roger M. Jorden’ Department of Civil and Environmental Engineering, University of Colorado, Boulder, Colo. 80302
Mohammed H. Lawasani and Ronald E. West Department of Chemical Engineering, University of Colorado, Boulder, Colo. 80302
Environmental control agencies and researchers have become increasingly concerned with the mobilization of trace elements to the environment from fossil fuel burning ( I ) . Natusch e t al. ( 2 ) have reported that preferential concentration of the trace elements As, Sb, Cd, Cr, Pb, Ni, Se, T1, and Zn occurs in the smallest particles emitted from coalfired power plants, which most easily pass through conventional particulate control devices. Billings e t al. ( 3 )indicate that about 90% of the mercury in coal burned in a pulverized coal furnace appears as vapor in the flue gas. Bolton et 1 Present address, Water Purification Technology, Inc., Grand Junction, Colo. 862
Environmental Science & Technology
al. ( 4 , 5 ) and Gordon e t al. (6) report several such elements occurring in greater concentrations in fly ash as compared to bottom ash. Thus, the process of coal combustion releases trace elements to the atmosphere as vapors and particles, and these particles have relatively greater concentrations of certain trace elements than the feed coal or the collected fly ash. Since it appears that the use of coal-fired boilers for the generation of electricity will continue to rise in the future, elucidation of the fate of trace elements in coal combustion is needed. The work reported herein was designed to study the character of trace element partitioning within a single pul-
