METAL EQUILIBRATION AND PROCESS CAPTURE EFFICIENCIES IN CEMENT KILNS

Presented at the AWMA International Specialty Conference on Waste Combustion in Boilers and Industrial Furnaces March, 1993

ABSTRACT

The BIF Compliance Tests recently performed by cement plants burning HWF have provided a wealth of technical information on the behavior of metals in cement kilns. Two key aspects of metal behavior in cement kilns are equilibration periods and process capture efficiencies. Data from a large number of compliance tests are presented and analyzed to compare and evaluate trends in the data. Because the BIF compliance tests provide data on emissions, input rate and equilibration during unusually high injection rates of selected metals; these tests provide invaluable insight into the behavior and fate of metals in cement kilns. The compiled data should be useful in preparing future test plans and permit applications.

INTRODUCTION

Recently completed BIF compliance tests performed by cement plants provide technical information on the behavior of metals in cement kilns heretofore unavailable. Key aspects of the behavior of metals are equilibration times and system removal efficiencies (SREs). An examination of the data indicates that equilibration times are frequently faster than previously anticipated and SREs are higher than expected, especially for arsenic and chromium.

EQUILIBRATION TIMES

The BIF regulations require that cement kilns which recycle kiln dust be verified to be in equilibrium with regards to metal emissions prior to metal emission stack testing. The method of choice for verifying equilibrium was to analyze kiln dust samples for total metals on an hourly basis prior to testing. Data from a few kilns in Europe had suggested equilibration times could be quite large. However, those kilns recycle 100% of their dust, thus preventing certain volatile metals from escaping the system except through the stack. No cement kilns currently utilizing HWF in the U.S. recycle all of their kiln dust. The wasted kiln dust thus allows kilns to quickly come to equilibrium. Industry experience now indicates that kilns that do not recycle kiln dust come to equilibrium quite quickly, generally in two to six hours.

Table I provides a typical set of data obtained during the equilibration period. Another example data set (different) is provided in Figure 1. Both sets of data are for wet process cement kilns that insufflate dust. They show a quick (2 hour) rise in metal concentration followed by a gradual rise and then relatively small fluctuations which likely represent typical kiln cycling and lab data errors. Some fluctuations may have also been caused by changes in metal spiking feed rates.

Table I. CKD Metal Determinations During Kiln Stabilization
 

Date	Time	As	Be	Cd	Cr	Pb
4/21/92	8:43PM	0.667	0.891	2.17	39.2	165
4/21/92	9:37PM	12.7	4	59	131	728
4/21/92	10:36PM	14.3	6.79	54.9	138	640
4/21/92	11:33PM	4.93	6.9	89.6	112	911
4/22/92	12:15AM	15.5	6.38	96.6	137	1050
4/22/92	1:15AM	18.3	6.72	92.2	188	1390
4/22/92	2:15AM	5.38	6.64	88.8	153	1180
4/22/92	3:12AM	6.6	6.38	85.6	191	1220
4/22/92	4:13AM	17.8	8.32	93.3	233	1420
4/22/92	5:12AM	16.8	6.8	80.6	195	1090
4/22/92	6:10AM	12.3	8.02	99.2	232	1430
4/22/92	7:13AM	16.7	8.56	103	208	1410
4/22/92	8:37AM	16.4	7.75	96.8	190	1350

 

Where available, metal equilibrium times during the compliance tests are reported in Table II.Some of this data was not available for preparing this table, either because it was not in the compliance report, or was placed in appendices not obtained from EPA.Where available, reasonable estimates of equilibrium times were made based on tabular or graphic data.Large fluctuations in metal input rates during equilibrium periods makes some of these estimates fairly crude.

Additional verification that metal equilibration has been reached can be performed by reviewing the individual metal emission run data for spiked metals to verify that there was no upward trend during the testing period.Table III provides an example set of data with no clear upward or downward trend.

Generally speaking, most kilns utilizing HWF will be at equilibrium within 12 hours.However, this time period cannot be assumed and is largely dependent on how long recycled dust takes to return to the kiln.A careful process evaluation should be performed prior to testing to estimate the equilibration time in advance.
 
 

Figure 1. Example Equilibration Graph
 

Table II. Metal Equilibrium for Specific Metals fromRecently Completed Compliance Tests(Hours)
 

								Equilibrium Time
		As	Be	Cd	Cr	Pb	Sb	Planned	Actual
Continental Cement Co.	Hannibal, MO	4	10	7	10	10		12	10
Essroc Materials	Logansport, IN	No data found						NR	4 *
Giant Cement Co.	Harleyville, SC (Kiln 4)	8		5	6	5		48
	Harleyville, SC (Kiln 5)	11		11	8	8		48	24
Heartland Cement Co.	Independence, KS	10		10	10	10		12	10
Holnam Inc.	Clarksville, MO	4	10	4	8	8		12	10
	Holly Hill, SC (Kiln 1)	3	3	6	7	7	4	12	7
	Holly Hill, SC (Kiln 2)	4	2	4	2	6	2	12	6
Lafarge Corp.	Alpena, MI					14			14
	Demopolis, AL					30			30
	Fredonia, KS (Kiln 2)					30			30
	Paulding, OH (Kiln 2)					12.5			12.5
Lone Star Industries	Cape Girardo, MO	7		7	4	9		12	9
	Greencastle, IN	11	11	11	5	11		12	11
Medusa Cement Co.	Wampum, PA (Kiln 2)			5	6	4			6
River Cement Co.	Festus, MO	2	10	10	10	8		12	10
Texas Industries	Midlothian, TX							2

* Reported without substantiating data
 
 

Table III.Example metal emission data.
 

	MMTL-101  (g/hr)	MMTL-102  (g/hr)	MMTL-103  (g/hr)
Arsenic	0.473	0.544	0.822
Beryllium	0.132	0.133	0.239
Cadmium	16.304	17.050	47.001
Chromium	23.400	5.166	7.845
Lead	110.663	125.315	106.821

 

SYSTEM REMOVAL EFFICIENCIES

Limited data on cement kiln SREs has been presented in the literature.¹Speculation on metal volatility in cement kilns based on metal balances or physiochemical data has also been presented.^2,3

Metal SREs calculated from data presented in recently completed BIF compliance testing are provided in Table IV.SREs are calculated from worst case metal emission rates and maximum total metal input rate limits which were calculated from data obtained during the compliance tests.Most kilns had 99% or better SREs even on metals such as lead and cadmium.Average SREs with ranges indicating one standard deviation are provided in Figure 2. 

Figure 2.SREs in Cement Kilns - BIF Compliance Test.
 

CONCLUSION

Equilibration periods of less than 12 hours are common for cement kilns that recycle some but not all of their cement kiln dust.Nonetheless, a close examination of the process with special attention to dust recycle time is warranted.

The high SREs for BIF metals as well as factors such as allowable emission rates and the relative prominence of certain metals in HWF suggest that the EPA needs to revisit whether or not certain BIF metals require the level of monitoring and control currently in place.Perhaps guidance for the Part B permit writes could meet this need since testing under a Part B can be significantly different from Interim Status requirements.

Table IV. Metal SRE's from Recently Completed Compliance Tests
 

Company-Plant-Kiln	As	Be	Cd	Cr	Pb	Sb	Ba	Hg	Ag	Tl
Ash Grove Cement Co.  Chanute, KS
Kiln 1	99.9819%		99.5544%	99.9912%	99.7217%
Kiln 2	99.9855%		99.3535%	99.9874%	99.5629%
Foreman, AR
Kiln 1	99.9569%	99.6006%	99.0787%	99.9926%	99.9882%
Kiln 2	99.9889%	99.9821%	99.2865%	99.9869%	99.2797%
Kiln 3	99.9502%	97.6464%	99.9436%	99.9975%	99.9236%
Louisville, NE
Kiln 1	99.9461%	99.8605%	99.1241%	99.9836%	99.0446%
Kiln 2	99.9103%	99.9653%	99.4873%	99.9920%	99.4982%
Continental Cement Co.
Hannibal, MO	99.9935%	99.9694%	99.4335%	99.9576%	99.3954%
Essroc Materials
Logansport, IN	99.9282%	99.8252%	98.3995%	99.9834%	99.0229%	99.8720%
Giant Cement Co.  Harleyville, SC
Kiln 4	99.6932%	96.1837%	99.8650%	99.9914%
Kiln 5	99.8931%	99.0307%	99.9565%	99.9919%	99.9699%
Heartland Cement Co.
Independence, KS	99.9705%		99.9506%	99.9800%	99.9426%
Holnam Inc.
Clarksville, MO	99.9710%	99.9876%	99.6154%	99.9978%	99.5966%
Holly Hill, SC
Kiln 1	99.9770%	99.9406%	98.8431%	99.9912%	98.9483%	99.9906%
Kiln 2	99.9905%	99.9870%	99.8684%	99.9969%	99.8058%	99.9971%
Keystone Cement Co.  Bath, PA
Kiln 1	99.9702%	99.9856%	98.6820%	99.1281%	99.2980%
Kiln 2	99.9749%	99.9884%	99.7093%	99.8635%	99.5117%
Kosmos Cement Co.  Louisville. KY
Condition 2	99.9809%	99.9442%	99.7202%	99.9535%	99.9571%
Lafarge Corp.
Alpena, MI	99.9955%	99.9899%	99.9603%	99.9801%	99.9877%
Demopolis, AL	99.9939%	99.9953%	99.9453%	99.9898%	99.9600%
Fredonia, KS
Kiln 1	99.9884%	99.9933%	99.8438%	99.9863%	99.8825%
Kiln 2	99.9333%	99.9803%	99.1338%	99.9597%	99.2646%
Paulding, OH	99.9857%		99.4422%	99.9826%	99.1388%
Lone Star Industries, Inc.
Cape Girardeau, MO	99.9486%	99.9547%	99.7821%	99.9503%	99.3278%
Greencastle, IN	99.9882%	99.9844%	98.9167%	99.9692%	99.5690%
Medusa Cement Co.  Wampum, PA
Kiln 1	99.9491%	99.9005%	99.9039%	99.9987%	95.8215%	99.4328%	99.9315%	99.9888%
Kiln 2	99.9401%	99.8995%	99.9112%	99.9986%	97.1556%	99.4710%	99.9588%	99.9898%
Kiln 3	99.9572%	99.9783%	99.9768%	99.9705%	98.7530%	99.8619%	99.9927%	99.6376%
Natl. Cement of CA
Lebec, CA	99.9963%	99.9907%	99.9057%	99.9865%	99.9674%	99.9979%	99.9993%		99.9964%	99.9729%
North Texas Cement
Midlothian, TX	99.9944%	99.9898%	99.8762%	99.9869%	99.8128%
River Cement Co.
Festus, MO	99.9162%	99.8530%	99.5479%	99.9731%	98.7470%	99.9848%
Southdown Inc.  Fairborn, OH
Condition 2	99.9894%	99.9874%	99.4539%	99.9661%	99.9374%
Knoxville, TN
Condition 1	99.9946%	99.9798%	99.9785%	99.9878%	99.9936%
Texas Industries
Midlothian, TX	99.3968%	98.9010%	99.9144%	99.9895%	99.8646%	99.9864%	99.9298%		99.9863%	99.9295%

  References

1. T. Tompkin, M. Von Seebach, "The Behavior of Metals in Cement Kilns", Paper presented at Rock Product's 26th International Cement Seminar, New Orleans, LA, 1990.

2. D. G. Gossman, M. Black et al., "The Fate of Trace Metals in the Wet Process Cement Kiln", in Waste Combustion in Boilers and Industrial Furnaces", SP-72, Air & Waste Management Association, Pittsburgh, 1990, pp 70-93.

3. J. Chadbourne, Ph.D., "Behavior of Toxic Metals in Cement Kilns", Paper presented at Portland Cement Association, Emerging Technologies in Resource Recovery and Emission Reduction in the Cement Industry, Dallas, 1990.