MEMORANDUM

SUBJECT:	Cost and Impacts of Wasting Cement Kiln Dust or Replacing Fly
Ash to Reduce Mercury Emissions

FROM:	Mike Laney, RTI

		David Green, RTI

		Keith Barnett, EPA

TO:		Docket Number OAR-2002-0051

DATE:		June 11, 2006	(Revised December 5, 2006)

A. Wasting Cement Kiln Dust

The mercury that enters cement kilns in coal and feed materials is
vaporized at high kiln temperatures and is either emitted in cement kiln
exhaust gases or absorbed on to particulate matter (cement kiln dust
[CKD]) and collected by particulate matter control devices. Little or no
mercury is retained in the clinker. A large percentage of the CKD that
is generated is recycled back into the kiln. Both European and U.S.
investigators suggest that periodically removing, or wasting, a portion
of the CKD is effective in removing mercury from the kiln and control
device system and reducing mercury emissions (VDZ; Schreiber, et. al.,
2005).

According to the Portland Cement Association (PCA), the industry has
been increasing the amount of CKD recycled and reducing the amount that
is sent to landfills.  The PCA claims that in 2003 more than 75 percent,
or 8 million tons per year, of CKD was recycled directly back into the
cement kiln as raw material (PCA 2005). Based on these figures, the
total amount of CKD generated by the industry is 10.7 million tons/yr.
The USGS (2006) reported US cement production in 2003 to be 81,882,000
metric tons or 90,259,000 tons. Thus, the industry is producing
approximately 12 tons of CKD per 100 tons of cement.  Other
investigators report 9 tons of CKD/100 tons of clinker (University of
Maine, 2006). 

Evidence suggests that cement kiln systems have the capacity to retain a
portion of the mercury in the CKD. In a study of the mercury content of
cement kiln raw materials, investigators identified CKD as having the
highest mercury concentration of all of the raw materials analyzed
(Hills and Stevenson, 2006). The authors found the mercury concentration
in 19 samples to range from 5 – 24,560 ppb, with a mean of 1,530 ppb
and

a median value of 53 ppb.  EPA (1998) reported the results of analyses
of 57 samples for mercury where the mean concentration was 0.33 ppm and
a range of 0.003 – 2.9 ppm.

Another study, conducted to demonstrate that cement kiln systems have an
inherent ability to control mercury emissions, supported the findings
that mercury is retained by CKD that is collected by control devices as
well as being collected by the raw materials in in-line raw mills
(Schreiber, et. al., 2005). Based on emission tests conducted on
hazardous waste-burning kilns, as well as mercury concentrations
measured in other input and output streams, mass balances and system
removal efficiencies (SRE) for mercury were calculated for different
kiln types with the following results:

Kiln type	Average fractional closure (Hg mass out/Hg mass in)	Average
mercury System Removal Efficiency (%)

All kilns	0.407	77.3

All wet kilns	0.582	71.1

All dry kilns	0.192	84.9

Dry kilns – raw mill on	0.061	96.0

Dry kilns – raw mill off	0.346	72.0



The authors concluded that the very poor mass balance closure data
suggested that these studies were not reliable.  Based on other
regulatory compliance testing, primarily on hazardous waste burning
kilns, they observed that when high rates of mercury entered the kiln
system, SREs greater than 90 percent were found, while at low input
rates, SREs varied significantly.

As the CKD is fed back to the kiln, the mercury is cycled back through
the kiln system and increases as additional mercury from fuel and new
feed materials enter the kiln. Similarly, mercury collected on the
material in the raw mill while the raw mill is operating, is also cycled
through the kiln. Eventually the level of mercury in the kiln reaches a
steady state. With the addition of mercury above this steady state
level, mercury is revaporized and ultimately emitted to the atmosphere.
To prevent mercury from being re-emitted, a portion of CKD could be
wasted rather than recycled back to the kiln.  If some or all of this
material could be mixed with the clinker in the finish mill, it would
also reduce emissions.

Using the information on the mercury concentration of CKD, an estimate
can be made of the amount of mercury emissions that would be potentially
avoided by not recycling CKD (see Table 1). Using the average mercury
concentrations of 0.33 ppm (EPA 1998) and 1,530 ppb (Hills and
Stevenson, 2006), an estimate of the range of mercury emitted annually
from the recycling of CKD can be made. Based on mercury concentrations
ranging from 0.33 to 1.53 ppm and a recycling rate for CKD of 75
percent, mercury emissions would range from 2.65 to 12.28 ton/yr. 

Table 1. Mercury Emitted from the Recycling of Cement Kiln Dust

Mercury concentration

(ppm)	CKD generated

(Mtons/yr)	Total Hg generated

(tons/yr)	CKD recycled

(%)	Hg emitted

(tons/yr)	Hg emitted per ton of CKD recycled

(lb/ton)

0.33	10.7	3.5	75	2.65	0.0007

1.53	10.7	16.37	75	12.28	0.0031



On a model plant basis, a 600,000 ton/yr kiln would produce 72,000 tons
of CKD/yr (600000 x .12 ton CKD/ton clinker). At concentrations ranging
from 0.33 ppm to 1.53 ppm mercury, the amount of mercury in the 72,000
tons of CKD would range from  47.5 lb (0.33 ppm x 72000 ton CKD x 2000
lb/ton) to 220.3 lb. The amount of mercury emitted from the CKD would
depend on the amount recycled. Mercury emissions on a model plant basis
assuming 50 percent of the CKD is recycled are presented in Table 2.

Table 2. Mercury Emissions from a Model Kiln

Model kiln

(tons clinker/yr)	CKD generated

/ton clinker)	CKD

(tons/yr)	CKD recycled (%)	Mercury concentration

(ppm)	Mercury emissions

(tons/yr)

600,000	0.12	72,000	50	0.33	0.012

600,000	0.12	72,000	50	1.53	0.055

 

The cost of a requirement to waste CKD rather than recycle can be
estimated by assuming that wasted CKD has a value equal to the cement
final product. The price of cement in 2004 was approximately $80/metric
ton, or $73/ton (USGS, 2006). Disposal of wasted CKD would be an
additional cost. Disposal cost for CKD are influenced by a variety of
factors including whether the waste is disposed of on-site or off-site,
and if disposed of off-site, the distance to the landfill. Prices can be
as low as $5/ ton of CKD to $20 - $40/ton. Assuming a disposal cost of
$30/ton and that the 600,000 ton/yr model plant wasted one-half of the
CKD it produced, or 36,000 tons/yr, the cost for wasting the CKD would
be as follows:

Annual Cost	= (36,000 tons CKD x $73/ton) + (36,000 tons CKD x $30/ton)

to Waste CKD

		= $3.7 million

At a mercury concentration of 0.33 ppm, the emission reduction from
wasting 36,000 tons/yr of CKD would be 0.012 tons/yr of mercury (0.33
ppm x 36,000), for a cost effectiveness of $308 million/ton of mercury
reduction. The cost effectiveness assuming CKD mercury concentrations
ranging from 0.33 to 1.53 ppm for a 600,000 ton/yr clinker production
where 50 percent of the CKD is recycled is summarized in Table 3.

Table 3. Cost Effectiveness for a Requirement to Waste Cement Kiln Dust
for a 600,000 Ton/Yr Kiln

Mercury concentration

(ppm)	Reduction in mercury emissions

(tons/yr)	CKD recycled

(tons/yr)	Annual cost

($M/yr)	Cost effectiveness

($M/ton mercury reduction)

0.33	0.012	36,000	3.7	308

1.53	0.055	36,000	3.7	67



The degree to which wasting CKD would reduce mercury emissions has not
been studied sufficiently to accurately quantify the environmental or
economic impacts associated with this practice. The degree to which
wasting a portion of CKD would reduce mercury emissions is likely to be
highly variable among plants and kilns and would likely vary according
to a number of kiln specific process conditions including the make up of
the feed and fuel materials. The impacts would be further complicated by
the fact that there are advantages associated with recycling CKD
including:

Reduced need for limestone and other raw materials

Reduced energy consumption because the CKD is already calcined

Less waste going to landfills

Reduced CO2 emissions due to reduced fuel consumption and calcination
(0.5 -1 ton of CO2 is emitted for each ton of cement produced)

Other beneficial uses of CKD include soil benefaction and soil
stabilization.  In addition, under some circumstances, CKD can be
blended with clinker to become part of the product.

B. Fly Ash Ban

Because of the presence of mercury in raw materials and coal, portland
cement plants are a source of mercury emissions. One of the materials
used in the production of some cement is flyash, which can replace other
raw materials, such as shale. As a coal combustion product, flyash can
also contain mercury. 

According to the PCA (July 2005), over 3 million short tons of flyash
were used in kilns to produce cement. In 2005, 39 portland cement plants
were using fly ash as a raw material in the manufacture of clinker, and
3 plants were blending fly ash into one or more cement products. As
electric utility boiler begin to reduce their mercury emissions, there
is concern that mercury emissions from portland cement plants may
increase over current levels. One option for reducing mercury emissions
is restricting the use of mercury-containing flyash in kilns to produce
portland cement.

Air Quality

One portland cement facility reported a mercury content of their flyash
at 0.5ppm. The quantity of flyash consumed annually was 245,272 tons.
Assuming all of the mercury is the flyash is emitted, the facility’s
annual mercury emissions from the flyash would be 245.3 lb/yr (0.5 x
245272/1000000 x 2000). The median mercury concentration reported by PCA
(2006) for shale was 0.022 ppm. If an equivalent amount of shale was
consumed in place of the flyash, the amount of mercury emitted annually
would be 10.8 lb/yr (0.022 x 245272/1000000 x 2000). The reduction in
mercury emissions by replacing all flyash with shale would be 234.5
lb/yr.

On a nationwide basis, replacing 3 million tons/yr of flyash with shale
would result in a reduction in mercury emissions of 2,868 lb/yr (3000000
x [0.5 – 0.022]/1000000 x 2000).

Cost

One portland cement facility reports receiving $15 for each ton of 
flyash they accept. (EPA 2006) At 245,272 tons of flyash per year, the
value of the flyash disposal is $3,679,080. At a cost of $50/ton of
shale (EPA 2006), purchasing the equivalent amount of shale to replace
the flyash would be $12,263,600/yr. The total cost to the facility of
replacing the flyash with shale would be $15,942,680yr.

On a nationwide basis, replacing 3 million tons/yr of flyash with shale
would result in an additional cost of $195,000,000 ([15 + 50] x
3000000).

Limitation of Impact Estimates

It should be recognized that: 

1) the value of the flyash disposal varies with the transport distance
between the flyash supply and the kiln (so that cement plants further
from boilers might incur lower costs in switching to shale);

2) The Hg concentration of the flyash varies with the the Hg content of
the coal as well as the type of emission control system associated with
the boiler (so that substituting shale for flyash generated in a boiler
burning low mercury coal might result in lower mercury emission
reductions and  substituting shale for flyash collected by some types of
ESPs might result in lower mercury emission reductions than substituting
shale for flyash collected by some types of fabric filters.

3) The mercury content of shale also varies, which will impact mercury
reductions.

4) Other materials than shale can be substituted for flyash.

5) The cost of shale to a cement plant is dependent on distance to a
shale supply, and some cement plants may have shale available as a
byproduct of a contiguous limestone quarry.

 

References

Hills, L. M. and Stevenson, R.W. Mercury and Lead Content in Raw
Materials, R&D Serial No. 2888, Portland Cement Association , Skokie,
IL, 2006,45 pages.

PCA 2005. Report on Sustainable Manufacturing: Solid Waste Production.  
HYPERLINK "http://www.cement.org/smreport05/sec_page3_2.htm" 
http://www.cement.org/smreport05/sec_page3_2.htm .

PCA, July 2005. Portland Cement Association Sustainable Manufacturing
Fact Sheet. Power Plant Byproducts.

PCA 2006. Mercury and Lead Content in Raw Materials. PCA R&D Serial No.
2888.

Schreiber, R., Kellett, C., Joshi, N, Inherent Mercury Controls Within
the Portland Cement Kiln System, R&D Serial No. 2841, Portland Cement
Association , Skokie, IL, 2005, 24 pages.

University of Maine. Beneficial Use of Solid Waste in Maine. March 12,
2006.   HYPERLINK
"http://useit.umaine.edu/materials/ckd/current_disposal.htm" 
http://useit.umaine.edu/materials/ckd/current_disposal.htm .

U.S. EPA 2006. Communication between K. Barnett, EPA and B. Gasiorowski,
Lafarge North America Inc. Costs for replacing fly ash with shale.

U.S. EPA, Office of Solid Waste. Technical Background Document on Ground
Water Controls at CKD Landfills (Draft). June 1998. p. 1-8.

U.S. Geological Survey. Cement. Mineral Commodity Summaries, January
2006.

Verein Deutscher Zementwerke e.V. (German Cement Works Association). Hg
reduction.   HYPERLINK "http://vdz-online.de/403.html" 
http://vdz-online.de/403.html 

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