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This document has been republished with the permission of Gossman Consulting Inc.

A Review of Potential Solutions to Control Dioxin (PCDD/PCDF) Emissions from Cement Kilns

Presented at the April 2003 A&WMA Hazardous Waste Combustors Specialty Conference

David Gossman

Gossman Consulting, Inc.

INTRODUCTION

Cement kilns first focused on the need to control dioxin emissions as part of the BIF regulations over a decade ago.  More recently the Hazardous Waste Combustor (HWC) MACT and Portland Cement (PC) MACT regulations have imposed strict limits and testing requirements on all cement plants in the United States.  Numerous plants both in the U.S. and around the world have faced the “surprise” of needing to address dioxin emission measurements that were higher than expected – sometimes dramatically higher.  Potential control strategies based on actual experiences are reviewed in this paper.

QUALITY CONTROL FIRST

Before looking at control strategies the first issue that must be addressed when the “surprise” of higher than expected (or permitted) dioxin emission measurements occur is to….check the data!  There have been numerous circumstances where a poorly designed test, an inexperienced stack test crew and/or calculation errors have resulted in erroneously high dioxin emission results1.  GCI continues to see errors in about 30% of the reports we review.  These types of errors can materially impact results and the net impact on corporate credibility and community relations can be enormous.  Preventative measures including a thorough QA/QC program and careful selection of stack test crews and analytical laboratories are clearly warranted.

RAW MATERIALS

Despite rather clear Clean Air Act language to the contrary and EPA’s contention that temperature control alone is sufficient to control PCDD/PCDF emissions, changes in raw materials have remedied dioxin emission problems at some facilities.  Over a decade ago it was clear that naturally occurring dioxins and organic precursors found in some limestone, shale and clay were the cause of elevated dioxin emissions from some cement plants2.  Finding alternatives to these raw materials became an important strategy for BIF regulated cement kilns.  More recently some alternate sources of raw materials have been found to contain organic precursors or dioxins themselves and have resulted in elevated dioxin emissions from cement plants.  Table 1 provides examples.


Table 1

Potentially Problematic Alternative Raw Materials

Raw Materials

Potential Problem

Smelter sand

PCBs that decomposed into dioxins

River water

Organic precursors or dioxins

Secondary aluminum smelting dross

Dioxins

Mill scale

Dioxins or organic precursors

TEMPERATURE CONTROL

Temperature control at the inlet of the APCD is the methodology advocated by EPA to control dioxin emissions from cement kilns.  In many cases it can work.  There is an inverse exponential relationship between dioxin emissions and APCD inlet temperatures3.  The critical point on the curve is frequently kiln specific, most likely because of gas residence times and temperature measurement location issues.

For older kilns improvements in design and maintenance of chain sections can lower kiln exit temperature and have the added benefit of improving energy efficiency.  Additional temperature control techniques include water spray systems and air quenching.  The down side with many of the temperature lowering techniques is that additional draft fan capacity may be required.  A novel, proprietary technique for wet kilns developed by Continental Cement Company, LLC involves a slurry spray.

ALTERNATE CONTROL OPTIONS

There are a number of alternate control options that may be considered, however these may be expensive or problematic to implement.  Gore manufactures a Remedia Catalytic Filter System.  The Gore bags for baghouses actually destroy dioxins as the exhaust gasses are simultaneously cleaned of particulate.  This system has been tested on cement plants and shown to reduce 90% or more any dioxins present4.  This is a relatively expensive option but may, nonetheless, be worth exploring if there are no other suitable options.

For alternative raw materials that make up a relatively small fraction of the kiln feed, yet contain dioxin precursors or even dioxins themselves, feeding the raw materials to the feed shelf in a preheater/precalciner kiln allow the organics to be destroyed and still allow functional use of the alternate raw material.  This however may limit the volume of the raw material that can be utilized since these materials will not be ground and uniformly distributed into the raw feed.

In preheater/precalciner kilns with inline raw mills the development of recycling loads of volatile metals is well known4.  This same phenomenon with organic precursors can lead to dioxin emissions an order of magnitude greater when a raw mill is off versus operational.  Careful design of operating conditions for stack testing and ongoing operations, as well as potential modification for how CKD is recycled back into the system, should minimize this phenomenon.

CONCLUSION

Clearly the understanding of dioxin emissions and control strategies for cement kilns has advanced considerably in the last ten years.  The EPA approach, temperature control, is not the only methodology available and cannot be counted on to provide operationally acceptable and economically feasible results in every circumstance, yet even in this area innovation potential still exists.  An important part of any ongoing dioxin emission control strategy is effective testing of alternative raw materials.  Gone are the days of bringing in a truck full to “just try out”.  That does not mean that a new stack test is required for every new alternative raw material – despite what a regulator might say.  An effective alternative raw material testing program, included as part of a facility’s Operations & Maintenance plan, can eliminate or significantly curtail the need for new stack testing for each change in raw materials.

REFERENCES

1.   Gossman, D.  Quality Control Review of Mercury and PCDD/PCDF Emissions Data From Cement Kilns Used for MACT Evaluations.  A&WMA Conference.  Nashville, TN.  June, 1996

2.    Constans, D.  Sources of PCDDs/PCDFs in Cement Kiln Emissions.  A&WMA Conference.  Nashville, TN.  June, 1996.

3.    W.L. Gore & Associates, Inc. 101 Lewisville Road – 21921, P.O. Box 1100 Elkton, MD 21922

4.   Gossman, D.  Factors Influencing Emission Levels of PCDD/PCDFs from Cement Kilns.  A&WMA Conference.  Kansas City, MO.  March, 2001.