Fuel Processing Technology, 4 (1981) 9 3 - - 9 9 9 3 Elsevier Scientif ic Publishing Company , Amste rdam -- Printed in The Netherlands

PNEUMATIC CONVEYING CHARACTERISTICS OF KENTUCKY COAL- LIMESTONE MIXTURES

JAMES C. W A T T E R S

University of Louisville, Chemical Engineering Department, Louisville, Kentucky (U.S.A.)

and R O G E R D. C U N N I N G H A M

University o f Kentucky, Institute for Mining and Minerals Research, Lexington, Kentucky (U.S.A.)

(Received March 23rd, 1979; accepted in revised form S e p t e m b e r 11 th, 1 980)

A B S T R A C T

The feasibil i ty of pneumat ica l ly conveying l imestone and 6:1 Kentucky coal-- l imestone mixtures has been demonst ra ted . At t r i t ion of the particles was about 4% when conveyed through a 150-foot line while a decrease in average particle size of about 30% was observed when the material was conveyed through a 450- foo t line. It is deduced that in a com- mercial f luidized bed combus to r the feed conveying lines be no t more than about 200 feet long and that the coal feed bunker be placed downs t ream of the l imestone bunker as the coal was observed to break d o w n much more than the l imestone. In an a tmospher ic f luidized bed combus to r where the f reeboard or de-en t ra inment height is designed based on the terminal veloci ty o f the average-sized particle, it is predic ted that material which has been carried through 450 feet o f feeder pipeline will yield about 33% more carryover of fines than would be observed with the unconveyed material. This excess carryover is reduced to 4% with material which has been coveyed only 150 feet.

I N T R O D U C T I O N

In its a t tempt to become self-sufficient in providing energy for its people and industry, the United States is turning more and more to coal as a prima- ry combustion fuel.

Problems associated with the combustion of coal are mainly centered around the pollution aspects. The fluidized bed combustor has been devel- oped as one means of reducing the sulfur emissions when coal is burned. Crushed coal (< 10 -2 m) is burned at about 750 to 950°C in the presence of crushed limestone (--10 mesh). The limestone reacts with the sulfur dioxide emitted from the combustion of the sulfur in the coal to form a solid product, calcium sulfate, which is subsequently removed as an ash. It has been demonstrated that judicious control of the calcium to sulfur ratio will permit removal of up to 90% of the sulfur dioxide formed, thereby eliminating the need for much expensive equipment involved in stack gas scrubbing [12]. Work is in progress at a number of private companies and

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government sponsored agencies [1, 3--5, 8, 9, 11--13] on several aspects of both Pressurized Fluidized Bed Combustion (PFBC) and Atmospheric Fluidized Bed Combustion (AFBC).

This current research was carried out to determine the conveyability of Kentucky coals and limestones in a pneumatic feed line similar to what might be used in feeding a fluidized bed combustor .

BACKGROUND

All presently operational fluidized bed combustors are pilot-scale. In such cases feeding of the coal and limestone is not a major problem. However, a multimegawatt unit poses some difficulties in getting the solid raw ma- terials into the unit due mainly to the scale of operations.

Two traditional methods have been widely used in feed design for flu- idized beds. These are pneumatic conveying and rotary stoker feeding. To date pneumatic conveying shows the most promise in feeding a combustor since air is an integral part o f the combustor operation. Rotary stokers lead to problems of overbed burning of fines with less control of the sulfur dioxide emissions.

A majority of current research on feeders is concentrated in the area of pressurized bed feeding. Concerns such as Lockheed [8, 9] , Foster--Miller [ 4 ], Ingersoll--Rand [ 5], Acton Corporation [ 1 ] and Petrocarb Inc. [ 13 ], among others, are developing feeder systems for pressurized gasifiers and combustors. Some work on the feeding of atmospheric combustors is being done by the Mitre Corporation [2] , and Pope, Evans an Robbins Inc. [11, 12] in Rivesville, West Virginia, and by Oak Ridge National Laboratories under an ORNL/HUD/MIUS contract [6, 7] . Lackey, in an Oak Ridge report, comments on a feed splitter system which incorporates multiple riffling of the primary coal stream. However, he notes that solids with a surface moisture content o f 6.7% would not f low readily [7] .

EXPERIMENTAL PNEUMATIC FEEDER TEST

Samples of Kentucky number 11 coal and Kentucky limestone were eval- uated for pneumatic conveying properties. Coal was in the 6 mm X 0 size range while limestone was approximately --6 mesh (3 mm x 0). The mixture composit ion was set at six parts of coal to one of limestone. Tests were run on two different samples of the coal--limestone mixture and the results were averaged. One test was run on the limestone alone.

The conveying system consisted of a feed hopper leading to a 150-foot 2-inch diameter line with both vertical and horizontal sections. It contained 4 90°--and 2 45°--bends and six diverters. This line discharged to a weigh bin and the material was then returned to the feed hopper. Longer test runs were achieved by rerouting the materials through this line a number of times. Conveying conditions consisted of air velocities of 20 to 25 meters per sec- ond (4000 to 5000 feet per minute) and conveying pressures of 34 to 48 kPa (5 to 7 psig).

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TEST RESULTS AND DISCUSSION

As a means of evaluating conveyability, the coal, limestone and coal--lime- stone mixtures were analyzed for bulk density, moisture content and par- ticle size distribution. Table 1 lists bulk densities and moisture contents for the original and conveyed materials. An increase in bulk density as the ma- terial is conveyed is indicative of attr i t ion in the original material. Except in the case of the limestone samples the moisture content in all cases was at least 6%. No conveying problems were encountered even at 6.77% moisture content and air conveying velocities as low as 20.3 m/s {4000 FPM). This is at variance with the work of Lackey [7] who stated that coal will not convey at these moisture contents.

TABLE 1

Sample bulk densities and moisture contents

Bulk density (kg/m 3) Moisture

Sample [lb/cu ft ] Content (%)

Original coal 910 [56.8] 5.98 Original limestone 1440 [90.2] 0.15 Coal--limestone, pass 3 1100 [68.4 ] 6.22 Coal--limestone, pass 1 1060 [66.1 ] 6.77 Limestone, pass 1 1590 [99.4] 1.31

Cumulative particle size distributions for the several samples are com- pared in Fig. 1 through 3. Figure 1 is a comparison of the initial coal and limestone size distributions. The coal-limestone mixture was composed of six parts coal to one part limestone by weight and the mixture composition in Fig. 1 was calculated based on the separate coal and limestone properties.

Figure 2 shows the size distributions for the coal--limestone mixture after zero, one and three passes through the system. Due to time and materials limitations samples were "grabbed" rather than obtained from a riffle pro- cedure. Hence some inaccuracies may be expected in the data, but this does not affect the overall conclusions. In comparing the original material with the once- and thrice- through materials a shift to Smaller particle sizes is very apparent. The shift is not very pronounced for the once- through material but for the material which has been passed three times through the system a major change is obvious. It thus appears that conveying of coal--limestone mixtures through a pipelength of 150 feet does not greatly affect the overall size distribution but that if the material is conveyed through 450 feet con- siderable attri t ion of the larger particles occurs. The implications of this ob- servation on the design of AFBC feed systems are discussed later.

Figure 3 is a comparison of the original limestone with once- through lime- stone. While a shift to smaller sizes after conveying is again apparent, the

9 6

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Fig. 1. S c r e e n a n a l y s e s - - Orig inal mater i a l s

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Fig. 2. S c r e e n a n a l y s e s - - C o a l - - l i m e s t o n e

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A g g r e g a t e G r a d a t i o n Chart NO. No.No. No. NO. NO. NO. NO. NO. NO. NO. NO.

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4 8 I0 16 20 50 40 50 80 I00 200 325 Sieve O p e n i n g

Fig. 3. Screen analyses -- Limestone.

overall distribution remains more or less constant. From comparisons of the limestone data with those of the coal--limestone mixture it can be concluded that most of the attri t ion which occurred in conveying the mixture was due to breaking of the coal particles rather than of the limestone. This indicates that the coal feed bunker should be downstream of the limestone bunker in a feed system where two bunkers must be used in series.

RESULTS WITH RESPECT TO AFBC FEEDER DESIGN

If it is assumed for convenience tha t the fluidizing velocity in a fluidized bed combustor may be designed based on the terminal velocity of the most predominant particle size, it is of interest to evaluate and compare mean particle diameters for the original and conveyed samples. A typical pneumatic

conveying system for a commercial fluidized bed combustor will contain a piping run of about 200 to 300 feet. This will include bends, diverters, etc., and clearance runs in the areas of the storage bins and the combustor bed. Thus, the test runs of 150 and 450 feet may be regarded as representative of the shortest and longest distances through which coal might be conveyed from the storage area to the combustor.

Table 2 shows the weight-averaged particle diameters for the several sam- ples tested. It is apparent from these data that minimal attrit ion occurred in a 150-foot line for both limestone alone and the coal--limestone mixture. In both cases the average diameter changed by five percent or less. However, when the mixture was passed through the system three times, traversing a

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TABLE 2

Sample weight averaged particle diameters

Wt. avg. diameter

Sample (ram) [in ]

Coal, original 2.54 [0.10 ] Limestone, original 2.15 [0.085] Limestone, pass 1 2.27 [0.089] Coal/limestone, original 2.48 [0.098 ] Coal/limestone, pass 1 2.39 [0.094 ] Coal/limestone, pass 3 1.72 [0.068 ]

distance of 450 feet, there was a 30% change in average diameter. This in- dicates that coal and limestone mixtures should not be carried much more than about 200 feet in a pneumatic system to avoid considerable attrition and alteration of the particle dynamics.

Based on fluid and solid properties in a typical fluid bed combustor , par- ticle dynamics indicate that the Intermediate Settling Law governs the par- ticle terminal velocity. In that case the terminal velocity is proportional to the particle diameter raised to the 1.14 power [10]. Hence, a decrease of 30% in average particle diameter decreases the average settling velocity of the particles to 66% of the original value. If the de-entrainment height in the bed is deisgned around the average particle size, approximately one third more of the particles will be carried over to the cyclones using material which has been conveyed through 450 feet of piping than would be carried over using the unconveyed material. When the "450 fee t" material is com- pared to the "150 fee t" material the extra carryover is about 31%. How- ever, the extra carryover of the "150 fee t" material over the original ma- terial is only about 4%. Based on the assumption that 10% additional carry- over due to conveying represents the highest acceptable increase, calculations show that 200 feet is the largest length through which coal--limestone mix- tures should be conveyed from storage area to combustor . This represents a decrease in average particle size of 8.5%.

CONCLUSIONS

The results of the conveying test are summarized briefly below. (1) Pneumatic conveying of coal--limestone mixtures of up to at least

6.7% moisture content is feasible. (2) Change in average particle size for conveying of coal--limestone mix-

tures through a 150-foot typical conveying line is 5% or less, i.e. attrition of particles in this range is minimal.

(3) Conveying of coal--limestone mixtures through a 450-foot piping sec- tion gives a decrease in average particle diameter of about 30%, which re- presents considerable attrition.

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(4) F o r a c o m b u s t o r des igned a r o u n d the average par t ic le d i ame te r , ma- ter ial which has been c o n v e y e d t h r o u g h 150 fee t causes an increase in par- t i cu la te ca r ryove r o f a b o u t 4% while mate r ia l which has been c o n v e y e d t h r o u g h 450 fee t causes a m o r e t han 30% ca r ryove r increase.

(5) L i m e s t o n e u n d e r w e n t min ima l par t ic le size change in all runs. Most o f the a t t r i t i on was a t t r i bu t ab l e to coal b r e a k d o w n .

(6) Based on the above po in t s it is suggested t h a t coal or coa l - - l imes tone mix tu r e s should no t be p n e u m a t i c a l l y c o n v e y e d m o r e t h a n a b o u t 200 fee t f r o m s torage area to c o m b u s t o r , this d i s tance to inc lude diverters , bends , etc.

(7) Coal feeding bins should be loca ted d o w n s t r e a m of l imes tone bins {i.e. neare r t he c o m b u s t o r ) as coal is sub jec t to m u c h m o r e a t t r i t i on t h a n l imes tone .

ACKNOWLEDGEMENTS

The au tho r s are gra tefu l to Dr. Lee E. Brecher , D e p u t y Di rec to r o f the Ins t i tu t e fo r Mining and Minerals Research , fo r his in te res t in this work . T h e y also a c k n o w l e d g e the he lp of Semco , Inc . , H o u s t o n , Texas , fo r p e r f o r m i n g the convey ing tes ts and o f Mr. Rick N e w b o l t o f S e m c o in par t icular .

REFERENCES

1 Acton Corp., Cleveland, Ohio, 1977. Acton Mass Flow System Applied to PFBC Feed. Jet Propulsion Laboratory Publication Number 77--55, Caltech, Pasadena, Ca.

2 Branan, J.G. and Rosborough, W.W., 1977. The MITRE Corporation, Material Handling Systems for the Fluidized-Bed Combustion Broiler at Rivesville, Wv. Jet Propulsion Laboratory, Publication No. 77--55, Caltech, Pasadena, Ca.

3 Ducon Fluid Transport Division Of U.S. Filter Corp., Pennsylvania, 1977. Some De- velopments in the Feeding of Coal to Fluidized Bed Combustors. Jet Propulsion Laboratory Publication number 77--55, Caltech, Pasadena, Ca.

4 Foster--Miller Associates, Inc., Waltham, Mass., Development of Coal Feeders for Coal Gasification Operations. ERDA Contract E (49--18)--1793 (1977).

5 Ingersol--Rand Research, Inc., Princeton, NJ, Development of a Continuous Dry Coal Screw Feeder. ERDA Contract E (49--18)--1794) (1977).

6 Lackey, M.E., 1975. Design and Performance Testing of a Coal Feed and Metering System for MIUS Fluidized Bed Combustor. Report to HUD/MIUS.

7 Lackey, M.E., 1977. Design and Performance Testing of a Coal Feed Metering System for the MIUS Fluidized Bed Combustor. Report to ORNL/MIUS/HUD under ERDA, E (49--18)--1742.

8 Lockheed Missiles and Space Company, Inc., 1975. Coal Feeder Development Program. Quarterly Report, September 30, NTIS, No. FE--1792--4.

9 Lockheed Missiles and Space Company, Inc., 1975. Coal Feeder Development Program. Quarterly Report, December 31, NTIS, No. FE--1792--8.

10 McCabe, W.L. and Smith, J.C., 1976. Unit Operations of Chemical Engineering. McGraw--Hill, Ny.

11 Pope, Evans and Robbins, Inc., 1972. Multi-Cell Fluidized Bed Boiler. OCR Reports, 1972--74.

12 Reed, R.R., (Pope, Evans and Robbins, Inc.), 1977. Multiple Feed Point Test Program. Fluidized Bed Combustion Technology Workshop, April 13--15, sponsored by ERDA.

13 Reintjes, H., 1977. The Petrocarb Pneumatic Feeding System. Presented at A.I.Ch.E. 70th Annual Meeting, NY.