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Factors Influencing Emission Levels of PCDD/PCDFs from Cement Kilns
David Gossman
 
ABSTRACT
As the Hazardous Waste Combustor MACT and Portland Cement MACT rules are implemented throughout the cement industry, extensive testing is providing further clarification of the factors in cement kiln operation and chemistry that impact PCDD/PCDF emissions. It is generally accepted that PCDD/PCDF emissions require time at temperature, a source of chlorine and a source of organic precursors. In cement kiln operations the source of organic precursors has been thought to be the raw material rather than fuel combustion. Additional data gathered in preparation of MACT compliance is used to examine this and related issues.

INTRODUCTION
As early as 19941 the relationship between temperature and PCDD/PCDF emissions from cement kilns was known.The PC MACT and HWC MACT regulations, establishing an APCD inlet temperature limit, reflect that data and additional data gathered since then. Yet, even in that original data set there was evidence of other factors that could maintain low PCDD/PCDF emission rates even when APCD inlet temperatures are high.

THE PUZZLE
While strong evidence correlates APCD inlet temperatures with PCDD/PCDF emissions from cement kilns there are, nevertheless, intriguing anomalies in the data. PCDD/PCDF formation requires three critical factors, time at temperature, a source of chlorine and a source of hydrocarbons. As early as 1993, it was shown that HCl emissions and chlorine input rates did not correlate with PCDD/PCDF emissions 2. It is generally acknowledged that there is always more than enough chlorine in cement process fuels to form PCDD/PCDFs. That same year, 1993, it was determined that the primary source of CO and hydrocarbon emissions from cement plants was not combustion related but rather originated in the plants raw materials3. Could it be that from some plants the temperature at the APCD inlet is irrelevant because low levels of hydrocarbons procuring in the plants raw materials prevent the development of significant PCDD/PCDFs even at elevated APCD inlet temperatures that usually result in emissions above the MACT limit?
 
THE DATA
A search of both old and more recent test reports on dioxin (PCDD/PCDF) emissions from cement plants was performed to obtain any available THC data from the same tests. Table 1 provides this data along with kiln exit/APCD inlet temperatures. (Because some of the data is from non-public sources, plant identities are not provided.)

Table 1. Cement Kiln TEQs, APCD Inlet Temperatures and THC Emissions

DATE

KILN#

TEQ

APCD Inlet Temperature (°F)

THC

(ng/dscm @ 7% O2)

(ppm @ 7% O2)

01/01/92

1

3.593E-01

478

7.4

03/01/92

2

7.313E-02

493

5.8

04/01/92

3

1.832E-01

597

321.0

04/01/92

3

6.864E-01

595

189.3

05/01/92

4

6.792E-01

356

2.6

05/01/92

5

3.405E+00

545

5.0

05/01/92

5

4.703E+00

513

7.3

05/01/92

6

3.252E+00

527

0.1

05/01/92

7

1.999E+01

601

70.0

06/01/92

8

6.975E+01

466

14.8

06/01/92

9

1.246E+00

280

2.1

06/01/92

10

6.405E+01

480

10.9

07/01/92

11

1.007E+00

478

12.5

07/01/92

12

4.563E+00

494

9.9

07/01/92

13

2.376E-01

470

15.0

07/01/92

13

2.410E-02

470

15.0

07/01/92

13

1.488E+00

530

16.0

07/01/92

13

1.068E-02

450

4.0

06/01/95

14

2.861E-01

413

12.8

06/01/95

15

7.743E-01

413

10.1

08/01/95

16

7.167E-03

482

8.7

02/07/96

7

1.397E+00

603

2.0

05/01/96

10

1.818E+01

487

13.4

06/01/96

17

1.433E-03

532

1.4

06/01/96

7

5.760E-01

558

1.7

06/01/96

7

1.039E+00

562

1.6

07/01/96

18

6.169E-01

468

5.0

02/01/00

6

6.190E-03

419

0.1

09/01/00

19

3.800E-03

407

2.6

10/01/00

20

2.981E-02

508

23.8

10/01/00

21

2.981E-02

519

17.7

10/01/00

21

3.885E-03

459

14.8

11/01/00

3

7.903E-02

428

114.9



THE RESULTS
In order to obtain a better understanding for the input of low THC data in the correlation between temperature and TEQ, all data with an APCD temperature of >400°F and THC <10°F is removed in Figure 2.(Figure 1 represents the complete data set for Table 1.)
Simple, polynomial, and multiple regression analysis on both sets of data show no statistically significant relationships among the data.

Figure 1. Cement Kiln Dioxin Emission TEQs vs. APCD Inlet Temperature

image001



Figure 2.APCD Inlet Temperature vs. TEQ When APCD Inlet Temperature
400°F and THC < 10 ppm - Test Average

Image 002

CONCLUSION
While both APCD temperature and hydrocarbon emissions would appear on the surface to be factors that would explain differences in PCDD/PCDF emissions, there are clear indications that there are one or more other factors that dominate the distribution of the data.These may include:
·    Poor data quality
·    Kiln feed alkali levels
·    Kiln sulfur/chlorine/alkali ratios
·    The chemical makeup of organics in the raw fuel
·    Time at temperature
·    Concentration of recirculating loads
·    In line raw mill on/off scenarios
Significant additional data, including plant process chemistries, is likely to be needed to truly clarify factors that input dioxin emissions kiln to kiln. Nevertheless, the data strongly suggests that the strategy for controlling dioxin emissions need not focus solely on APCD inlet temperatures.
 
REFERENCES
1.  2, 3, 7, 8-TCDD Equivalent Emissions from Cement Kilns Burning Hazardous Waste; Robert J. Schreiber, Jr., David Gossman; AWMA Conference April, 1994.
2.     Commercial BIF Compliance Test Results; Gossman Consulting, Inc., 1992
3.  Suitability of Hydrocarbon and Carbon Monoxide Measurement as Combustion Indicators in Cement Kilns; Jim Woodford, Rex Jameson, Carlos Buckelew, Henry Winders, Richard Parker, Marcia Williams, David Gossman; AWMA Conference March 1993.