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
Gossman Consulting, Inc.
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
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.
Potentially Problematic Alternative Raw Materials
PCBs that decomposed into dioxins
Organic precursors or dioxins
Secondary aluminum smelting dross
Dioxins or organic precursors
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
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.
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.
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.