Laboratory scale pulverized char feederR. A. Kriegbaum, R. A. Vogt, and N. M. Laurendeau Citation: Review of Scientific Instruments 48, 464 (1977); doi: 10.1063/1.1135051 View online: http://dx.doi.org/10.1063/1.1135051 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/48/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Compare pilot-scale and industry-scale models of pulverized coal combustion in an ironmaking blast furnace AIP Conf. Proc. 1547, 564 (2013); 10.1063/1.4816909 Scaling supernova hydrodynamics to the laboratory Phys. Plasmas 6, 2065 (1999); 10.1063/1.873497 Pulverized solid injection system. Application to laboratory burners and pyrometric temperature measurements Rev. Sci. Instrum. 66, 4041 (1995); 10.1063/1.1146440 A practical pulverized coal feeder for benchscale combustion requiring low feed rates Rev. Sci. Instrum. 62, 480 (1991); 10.1063/1.1142091 Laboratory Powder Feeder Rev. Sci. Instrum. 34, 588 (1963); 10.1063/1.1718515
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Laboratory scale pulverized char feeder R. A. Kriegbaum, R. A. Vogt, and N. M. Laurendeau
The Combustion Laboratory, School of Mechanical Engineering, Purdue University, W. Lafayette, Indiana 479v.
(Received 4 October 1976; in final form, 15 November 1976)
A laboratory scale mechanical pulverized char feeder that is competitive with, yet inherently simpler than, fluidized bed designs has been developed. Performance evaluation of the feeder using 100% through 140-mesh char indicates a feed range of 7-18 g/min with short-term flow stability within 3% throughout 90 min of continuous operation. Independent control over the char and carrier gas flowrates allows for precise adjustment of the char/air feed ratio, the tested range being 0.07-0.10 by mass. The prototype incorporates a carrier gas preheater (673 K maximum) and is designed to feed at atmospheric pressure.
A reliable means for supplying a controlled flow of pulverized char entrained in air or other carrier gas has been developed to facilitate laboratory studies of char combustion, pyrolysis, and gasification. The design utilizes a mechanical screw-type fine particle feeder to supply pulverized char at flowrates ranging from 7 to 18 g/min, accurate to within ±3% and stable for up to 90 min of continuous operation. The fine particle feeder is controlled independently of the air supply, thus allowing precise adjustment of the char/air ratio, a typical mass ratio being 0.10. The separation of the char and air supply systems also allows the use of air preheat (673 K) prior to char entrainment. The feeder described herein is designed to supply a furnace operating at atmospheric pressure.
diffuser mating the 6.35 mm feed tube to the 12.7 mm output line; thus, a low pressure region is established at the base of the funnel. By proper adjustment of the pressure vessel environment a zero (or slightly negative) pressure gradient is maintained across the funnel to ensure complete entrainment of the char falling into the air line from the feed barrel. The pressure vessel encloses the entire char feed system to avoid the develop-
FEEDER DESCRIPTION
The feeder design (Figs. 1 and 2) consists of two basic components, the char feeder and the air system. The char feeder is a modified version of the multiple-orifice fine particle feeder developed by Lucas and Smith (1974).1 The design consists of a series of five, staged conical hoppers that supply char to the feed screw (6.35 mm drill bit). The larger top hopper serves as a head tank and the lower four hoppers function as flow restricting orifices thus providing a constant head of char at the screw. The agitator rod (6.35 mm drill rod wrapped with wire spirals) is mounted slightly off the centerline ofthe hopper system so as to receive the upward thrusts from the rotating screw. The section of the screw upon which the agitator rides is modified to simulate a twowinged cam. The action of the agitator rod induces flow throughout the lower four hoppers. A low speed (1.7 rpm) stirring mechanism promotes flow in the large hopper and maintains a level profile across the top of the char. Screw speed regulates the char feed rate within the limits imposed by the size of the screw and the bore of the feed barrel.
The air system is composed of a collection funnel, an air supply line, and a pressure vessel. The collection funnel is attached to the air line just upstream of a
464 Rev. Sci. Instrum., Vol. 48, No.4, April 1977
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FIG. 1. Pulverized char feeder, front view.
Copyright © 1977 American Institute of Physics 464
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ment of pressure gradients across the char feeder. The location of the preheater downstream of the pressure vessel air supply minimizes the effect of air preheat on the pressure vessel environment. The total air flowrate through the feeder is monitored upstream of where the pressure vessel taps into the air line. The section of the air line containing the collection funnel is removable and an access port is designed into the front wall of the pressure vessel to facilitate sampling of the char feeder.
The pressure vessel and the majority of the char feeder are constructed of Plexiglas to allow maximum visibility. Bakelite components are used to interface the air line and the collection funnel with the pressure vessel and the feed barrel, respectively. This design allows a maximum air preheat of 673 K and maximum vessel pressure of 117 kPa. The wholesale material cost for the Plexiglas feeder (without preheater and gas flowmeter) is estimated at $400.
FEEDER OPERATION
The feeder startup procedure is as follows: (1) fill the hopper system with dried, pulverized char, (2) remove the collection funnel from the air line, (3) set the screw speed to obtain the desired char flowrate, (4) monitor the char flow for 10 min taking 30-sec samples, (5) stop the char flow, (6) replace the collection funnel, (7) seal the pressure vessel, (8) set the air supply to the desired flowrate, (9) set the vessel pressure, and (10) resume char flow. Shutdown is achieved by the reverse of steps 10-44 inclusive. The 1O-min char flow tests preceding and following the feeder operation are taken as representative samples of the char flow.
The feeder is presently limited to a maximum of 90 min continuous operation. If the hopper system is empty prior to filling with char, the char feeder alone should be run continuously at a moderate feed rate for
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FIG. 2. Screw feed section, side view.
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465 Rev. Sci. Instrum., Vol. 48, No.4, April 1917
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FIG. 3. Calibration of feeder with pulverized char (100% through 140-mesh).
4 h (refilling every hour) to allow the char to settle in the . system and the flowrate to stabilize. Such a settling
period is not required for partial refilling of the system following operation. A dust cloud issuing from the top of the collection funnel upon initiation of char flow into the air line (step 10) is a fair indication of insufficient vessel pressure. Typical operating pressures range from 107 to 110 kPa.
FEEDER PERFORMANCE
The results presented herein are based upon feeder operation using pulverized char sized to 100% through 140-mesh. The calibration curve (Fig. 3) indicates the screw speeds required for the various char feed rates. Each data point represents the average rate obtained from 10 min (minimum) of continuous flow sampled at 30-sec intervals. The data scatter indicates that minor speed adjustments are necessary if feed rates are to be precisely set.
The flow characteristics ofthe char feeder were determined using a test version of the feeder operating procedure. During the startup and shutdown operations a 20-min "no-flow" period was substituted for steps 6-9 inclusive to simulate the normal time lapse between the 1O-min flow tests and operating period. The operating period was simulated by 2 h of continuous char flow monitored at 30-sec intervals. Figures 4 and 5 show the results of one such test. Plotted as cumulative flow vs operating time (Fig. ,4), the test data typically indicate constant flowrates. Flow fluctuations become evident if the feed rate is determined for each 30-sec sample (Fig. 5). The standard deviation of the feed rates calculated at 30-sec intervals typically ranged within 3% of their mean value. Flow was usually most erratic during the final 30 min of the 2-h tests; therefore, the 90-min operation limit was imposed. The two 10-min test periods generally typified the 2-h runs, the combined average flowrate for the two 1O-min tests being within
Pulverized char feeder 465
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FIG. 4. Flow stability test data for char (100% through 140-mesh) fed at 15.78 glmin (average), sampled over 30-sec intervals.
±3% of the 2-h average and the standard deviation of the test data being similar to that ofthe operating period.
The effect of sample time on flow fluctuation (Fig. 6) was investigated by sampling during a 2-h test at 15-, 30-, 60-, and 120-sec intervals progressively. The increase in flow instability noted for sample times less than 30 sec is attributed to measurement errors incurred during the weighing of the samples. A brief evaluation of the effect of feed barrel size on char feeder performance indicated that for a 6.35 mm screw, maximum
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flowrates are attained with a barrel bored to an inside diameter of about 8.7 mm. Experiments showed that vibrating the feeder does not improve the stability of the char flow.
The feeder prototype is presently operable as an integral component in an atmospheric pressure continuous flow reactor system. Typical char/air mixtures have ranged from 0.07 to 0.10 mass ratio with char feed rates from 10 to 18 g/min. Use of the prototype to feed pulverized coke and coal has met with reasonable success. Since the feeder is volumetric by nature, the mass feed rate range clearly depends on the bulk density of the feed stock. Preliminary analysis of coke and coal flow stabilities indicates that average flowrates are controllable within 7%; moreover, standard deviations of 30-sec flow samples generally fall within 5% of the mean flowrate. Presently, the feeder has proven reliable for only 70% of the runs using pulverized coke or coal (100% through 140-mesh) as feed stock. Flow problems encountered are accredited to the higher propensity of coke and coal to agglomerate.
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FIG. 5. Feed rate fluctuation for 30-sec samples of char (100% through 140-mesh) flowing at 15.78 glmin (average).
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Rev. Sci. Instrum., Vol. 48, No.4, April 1977 Pulverized char feeder 466
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COMPARISON WITH FLUIDIZED BEDS
When compared to fluidized bed systems designed for a similar coal feed range, the mechanical feeder described herein falls drastically short in flow duration, compares admirably in flow stability, and greatly exceeds the fluidized bed design in simplicity and versatility. Fluidized beds2 have been reported operable at coal feed rates from 8 to 17 g min- 1 for up to 10 h of continuous operation with short-term flow fluctuations of less than ±5%. This mechanical design is reliable for'only 90 min of continuous flow at 7-18 g of char/min and exhibits char flow fluctuations such that the standard deviation of the feed rate over 30-sec intervals is within 3% of the mean (within 5% for coke and coal).
Fluidized bed feeders 2 are relatively complex instruments requiring precise pressure regulation downstream of the bed to control the coal/gas feed rate. Appropriate precautions must be incorporated into the design to prevent coal blocking problems, since such feed systems are primarily composed of small bore (1 mm) tubing. The coal feed rate is dependent upon the operation of the gas system and thus the bed produces specific coal/ gas ratios. Typical methods used to determine the rate and stability of the coal flow include the use of strain gauges to measure feeder weight loss, cyclone and/or filtering systems to collect timed samples, or the con-
467 Rev. Sci. Instrum., Vol. 48, No.4, April 1977
tinuous analysis of combustion product gases from furnaces supplied by the feeder. A series of two flowmeters is normally required to monitor the gas flowrate through the feed line.
Comparatively, the mechanical feeder provides flowrates independent of downstream pressure. The output line is large bore (12.7 mm) tubing which is considerably less prone to clogging. The char feed rate is controlled independently of the air flow, allowing for a variety of char/air ratios. The feeder was designed to provide simple access for collecting timed samples of the char feed, thus facilitating calibration and flow stability analyses. Only one flowmeter is required to monitor the air flowrate and the feeder system is readily adaptable to air preheat.
ACKNOWLEDGMENT
We gratefully acknowledge our support under NSF Grant GK-42141: Nitric Oxide Formation in Pulverized Coal Flames.
1 H. G. Lucas and N. S. Smith, Jr., Multiple-Orifice Feeder for Fine Particles, U.S. Bureau of Mines Report 7944 (Washington, DC, 1974).
2 R. J. Hamor and I. W. Smith, Fuel SO, 394 (1971).
Pulverized char feeder 467
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