Portland Cement Association
Research and Development
Information
5420 Old Orchard Road
Skokie, IL U.S.A. 60077-1083 Fax (847) 966-9781 (847) 966-6200
PCA R&D Serial No. 2086
The Reduction of Resource Input
and Emissions Achieved by
Addition of Limestone to
Portland Cement
by M. A. Nisbet
JAN Consultants
Montreal, Canada
0 Portland Cement Association 1996
OBJECTIVE
Analyze the impact of addition of up to 5% limestone to cement expressing the results in terms of
reduction of resource input, energy savings, reduction of emissions and conservation of capital.
SUMMARY OF RESULTS
A comparison was made between the material and energy inputs and emissions per ton of cement under
three conditions:
1) a base case condition without limestone addition,
2) addition of 2.5% limestone to the cement,
3) addition of 5.0% limestone.
The results of the analysis, as shown in Table 1, indicate that the impact of replacing the clinker in
cement with limestone is greatest in the raw meal preparation and pyroprocessing steps where 2.5 and
5% limestone addition rates reduce the amount of kiln feed per ton of cement produced by 2.63 and 5.26%
respectively. This results in a comparable reduction of energy inputs to and emissions from these process
steps.
The quantity of material quarried and crushed decreases 0.98% at 2.5% limestone addition and 1.97% at
5% limestone addition, assuming that the limestone added at the finish milling step comes from the
same quarry. This results in a decrease in electricity use which is relatively minor because electricity
consumption in this process step is about 7 kWh per ton of cement or 5% of total power consumption per
ton. Particulate emissions also decrease by 0.98 and 1.97%. This decrease is more sigruficant since quarry
and crushing emissions can represent over 50% of the particulate emissions from a plant.
It was assumed that power consumption in the finish grinding step would not change, thus the overall
reduction in power use per ton of cement was estimated to be 1.26 and 2.43% respectively at 2.5 and 5%
limestone addition.
Addition of 5% limestone to total domestic cement production is equivalent to an increase of 3.55 million
tons in clinker capacity.
.d
Table 1. Reduction of Inputs and Emissions Resulting from Addition of
Limestone to U.S. Cement Made from Domestic Clinker
Per Million Tons of Cement
Per 75 Million Tons
Note: tons and lbs. have been rounded to the nearest whole number.
Abbreviations:
mmBtu million British thermal units.
kWh
kilowatt hours.
rh4
filterable particulate matter.
so2
sulfur dioxide.
NO,
nitrogen oxides.
co
carbon monoxide.
co2
carbon dioxide.
THC
total hydrocarbons.
CKD
cement kiln dust.
2
-
INTRODUCTION
The raw materials for cement manufacturing, primarily limestone and clay or shale, are quarried,
crushed, p u n d and proportioned so that the resulting mixture, or raw meal, has the desired fineness
and chemical composition. The raw meal is fed to a pyroprocessing system which:
drives off surface and bound moisture,
calcines the limestone, and
forms the hydraulic cement minerals dicalcium silicate, tricalcium silicate, tricalcium
aluminate and tetracalcium aluminoferrite.
The product of the pyroprocessing stage is clinker. Which is ground with the addition of
approximately 5% gypsum to give portland cement.
Cement Manufacturing Process Steps
Quarrying
Primary crushing
Secondary crushing
Stockpiling
Reclaim from piles
Grinding
Transfer to silo
Transfer from silo
Pyroprocessing
Clinker cooling
Transfer to storage
Transfer from storage
Grinding
Transfer to storage
Load out
This analysis deals with the impacts an resource consumption and emissions that would omur i f
limestone were ground into the finished product replacing 2.5 and 5% of the clinker.
The decrease in process inputs and emissions results from reduction of the percentage of clinker in
cement, thus there are direct impacts cm the first three steps of the manufacturing process. However,
since the limestone is added to replace clinker in the finish grinding step, it is assumed that changes in
energy input and emissions from finish milling will be minimal. The possibility that finer grinding
might be required to maintain product performance has not been included in the analysis. The analysis
does not apply to cement ground from imported clinker. The results are expressed as the reduction of
resources consumed and emissions per million tons of cement for each level of limestone addition. The
results are also expressed per 75 million tons of cement which is approximately the quantity of cement
produced from domestic clinker in the US. in 1995. This number is based an total production of 78.8
million tons minus 3.6 million tons ground from imported clinker.
Information on energy inputs is from the U.S. Cement Industry Fact Sheet, Fourteenth Edition (1).The
data used is the average for all plants and does not refer to any specific manufacturing technology. CKD
tonnage is preliminary data from a 1994 PCA survey (2). Data an emissions of particulates and total
hydrocarbons (THC)from the pyroprocess are from John R. Richard's 1996 draft report (3). Emissions of
kiln stack gas such as SO, and NO,, and emissions from other sources in the plant, are made from
emission factors provided by the U.S.Environmental Protection Agency (EPA).Two sets of factors have
been used: emissions from for the kiln and cooler stacks were estimated from AP-42 factors (4);
particulate emissions from other point soums such as cooler vents and other dust collectors, and from
fugitive sources like stockpiles and haul roads were estimated as total particulates using SCC factors
(5). Based on these factors, the emission estimates are probably higher than those actually occurring.
3
Assumptions
Raw mix to clinker ratio: 1.61
Clinker to cement ratio: 0.95:l
Two component raw mix: 80%limestone, 20% clay.
Energy consumption:
- kiln fuel 4.21mmBtu per ton (4.63mmBtu per tonne).
- electricity 144 kwh per ton or 0.48 mmBtu (158 kwh per tonne or
0.53mmBtu per tonne).
- no change in energy consumption in finish milling.
Heat contents.
coal:
22.7 mmBtu per ton
petroleum coke: 28.2mmBtu per ton
natural gas:
1000~ t per
u ft3
-
1995 Portland cement production from domestic clinker: 75 million tons .
(Total U.S. Portland cement production minus cement ground from imported clinker).
C K D 5.28million tons (4.8million tonnes) per year.
Reductions of inputs and emissions are directly proportional to the percentage of
limestone added.
CONSERVATION OF RAW MATERIALS
Using the assumptions of 80%limestone in the raw mix, a raw mix to clinker ratio 1.6:l and clinker to
cement ratio of 0.95:1, the material flow under baseline conditions is shown below. The units are tons of
material per ton of cement.
Material flow without addition of limestone to cement (tons)
Limestone
1.216
Clay
0.304 d
Calcining loss 0.570
1.520
T
, t
l-ooo
Oag50
Clinker
Raw meal
Cement
Gypswn
0.050
I
4
In calculating the impact on mass flows it is assumed that the limestone added to the cement comes from
the same quarry as that used in the raw mix. If 5% limestone is added to the cement, the amount of
clinker used in the final product drops by 0.05/0.95 x 100 = 5.26%, the raw mix drops by the same
amount, as do the materials quarried for the raw mix. However, 0.05 tons of limestone are needed per
ton of cement ground. Thus the total amount of limestone quarried per ton of cement decreases by 0.014
-
tons going from 1.216 to 1.202 tons consistingof 1.152 tons for the raw mix plus 0.05 tons to be added at the
finish milling step. Clay drops from 0.304 to 0.288 tons per ton of cement, a difference of 0.016 tons.
Material flow with 5% limestone addition to cement (tons)
lLimestone
Calcining loss 0.540
9
0.050
1.152
0.900
Zlay
Raw meal
0.288
I
ry
Ah
Clinker
P
1.000
Cement
I
0.050
CONCLUSION
Resources conserved per l,OOO,OOO tons of cement at 5% limestone addition are;
0
0
limestone
clay
0.014 x 1x 106 = 14,000 tons
0.016 x 1x 106 = 16,000tons
ENERGY CONSERVATION
Fuel
The PCA 1994 Labor and Energy Survey gives an average fuel consumption of 4.25 mmBtu per ton
(4.68 mmBtu per tonne) of cement. Of this number 0.045 mmBtu per ton is middle distillates and gasoline
which are used primarily for quarry trucks and other mobile equipment. By subtraction, kiln fuel
becomes 4.21 mmBtu/ton (4.63 mmBtu per tonne) of cement.
In the base case, 0.95 tons of clinker are grarnd with 0.05 tons of gypsum to give 1.0 ton of cement.
Addition of 5% or 0.05 tons of limestone lowers the clinker input to 0.9 tons per ton of cement which is a
reduction of 5.26% in the amount clinker required per ton of cement. This will result in reduction of 4.21 x
0.0526 = 0.22 d t u per ton (0.24 m B t u per tonne) of cement or 0.22 x l x 106 = 220,000 mmBtu per
million tons.
5
Conclusion
Fuel savings per 1,000,000 tons of cement are 220,OOO mmBtu which is equivalent to
9,700 tons of coal, or
7,800tons of coke, or
220 million fF of natural gas.
0
0
0
Electricity
Based on a sample of 10 plants, the use of electricity by process step can be approximated as follows:
w
Quarry
Raw mix preparation
Pyroprocess
Finish milling
Total
% 5
17
29
7.20
24.48
41.76
49
2a36
100
144.00
Since the proportion of electricity used in quarrying is low, the analysis is insensitive to small changes
in quarry throughput. The change in energy in finish grinding 5% Limestone instead of clinker is
expected to be relatively small, thus power savings from both these process steps need not be considered
further.
Average power consumption is 144 kwh per ton (158 kwh per tonne) of which a total of about 46% or
66.2 kwh are used in the raw mix preparation and pyroprocess steps. A savings of 5.26% of 66.2 is
3.5 k W h per ton.
Conclusion
Electric power savings per 1,000,OOO tons of cement are:
3.5 x lx 106 = 3,500,000 kwh
REDUCTION OF AIR EMISSIONS
Emissions vary considerably between manufaduring processes and between plants with similar
processes. As a conservative first approximation, emissions per ton of cement are estimated using
emission factors. The AP-42 factors have been used for the pyroprocess and SCC factors have been used
for the other point and fugitive sources. Emissions are calculated as total filterable material because
the factors do not provide data for all emission sources on PM,,,, meaning particles of 10 microns or less in
diameter.
A sununary of emission estimates in pounds per ton of cement is given in Table 2. Assumptions used in
developing the emission estimates are contained in the attached appendix.
6
No change in emissions from the finish milling step is expected, since replacing clinker with limestone
in finish grinding will not change the mass of material being processed.
As previously pointed out, quarry operations will be less affected by limestone addition than the raw
mix preparation and pyroprocessing steps. Quarried material only drops from 1.52 to 1.49 tons per ton of
cement which is 1.97%. This will result in a proportional drop in particulate emissions.
A greater impact occws in raw mix preparation and pyroprocessing where throughput decreases by
5.26% causing a reduction of 5.26% in particulate and gaseous emissions.
The reductions in emissions are summarized in Table 3,based on the following percent reductions:
quarry: PM reduced by 1.97?/0,
raw mix preparation and pyroprocess: all emissions and CKD reduced by 5.26%,
finish milling: no change.
Table 3. Reduction of Emissions and Residuals Resulting from Addition of 5%
TO,and CKD units are tons
Lbs and tons have been rounded to the nearest whole number.
Carbon Dioxide
Setting carbon dioxide emissions at an average of 1,792.65 lbs per ton of cement and assuming that fuel
combustion and calcination each contribute 50%, then the addition of 5% limestone to cement would
result in a decrease of 5.26% in C 0 2 emissions or 94.293lbs per ton of cement, or 47,147 tons per 1,000,000
tons of cement.
If this reduction in emissions were to be made by improving energy efficiency, which only reduces CO,
from combustion, a gain in fuel efficiency of 10.52% would be needed.
Conclusion
Addition of 5% limestone to cement would reduce carbon dioxide emissions by 47,147 tons per 1,000,000
tons of cement.
7
Cement Kiln Dust
PCA survey data from 1994 indicates that total CKD generation was 5.3 million tons. The rate of
generation based on the survey sample was 145.8 lbs per ton of clinker or 145.8 x 0.95 = 138.5 lbs per ton of
cement. If reduction in CKD is directly proportional to the decrease in clinker in cement, then 5%
limestone addition will reduce CKD generation by 138.5 x 0.0526 = 7.3 lbs per ton of cement. CKD
reduction per million tons becomes 7.3/2000 x l x W = 3,643 tons.
Conclusion
CKD generation will drop by 3,643 tons per 1,OOO,OOO tons of cement.
POTENTIAL GAIN IN CEMENT CAPACITY
In the base case, 0.95 tons of clinker are used to make 1.0 ton of cement. Addition of 5% limestone to
cement means that 0.95 tons of clinker can make 1.05 tons of cement. If limestone were not added, 1.05
tons of cement would require 1.05 x 0.95 = 0.9975 tons of clinker. This potential gain of 5% in clinkering
capacity (0.9975/0.95 x 100 = 105) when applied to 1995 clinker production of 71 million tons, represents
an increase of 3.55 million tons.
REFERENCES
h
Economic Research Department, Portland Cement Association,
1. U. . Cem
Skokie, Illinois, 1996.
2. Unpublished results from a Portland Cement Association survey, 1994.
* sions Data for 1989 to 1996, SP 125,
3. John R. Richards, Compilation of Cement Industry Air F w
Portland Cement Association,, Skokie, Illinois, 1996.
mDilation f Air Pollut
r
-42 'f h E i i . U.S. EPA, Office of Air
4 3
Quality Planning and Standards, 1995.
rometric Retrieval
A1
C1 ific '0
a n E ' i
5.
Factors J aistinp for Criteria Air Pollutants. U.S. EPA, Office of Air Quality Planning and
Standards, 1990.
*
ACKNOWLEDGMENT
The research reported in this paper (PCA R&D Serial No. 2086) was conducted by JAN Consultants,
with the sponsorship of the Portland Cement Association (PCA Project Index No. 95-07a). The contents
of this paper reflect the views of the author, who is responsible for the facts and accuracy of the data
presented. The contents do not necessarily reflect the views of the Portland Cement Association.
8
.
, a ,*,
L
'-TAc-
I
APPENDIX
Estimated Average Emissions
Particulate emissions are lbs of total filterable material
Water spray control efficiency is 50%
Baghouse control efficiency is 99.5%
Haul distance of quarry trucks is 4 miles round trip
Haul truck capacity is 50 tons
Stockpiles contain an average of 1Ooh of the material flow
For weighted averages 29% of capacity is wet, 31% dry, 20% preheater and 20% precal.
Data sources
Kiln stack
Clinker cooler
Other
Particulates and total hydrocarbons; Air Control Techniques (3), 1996
Gaseous emissions :US EPA AP-42 factors (4)
US EPA AP-42 factors (4)
Particulate emissions from point and fugitive sources: SCC factors (5)
Kiln Emission Factors
Ibs per ton
~~~~~~
Total
# per T cement
0.528221 5.51 76 5.51 57 0.984105 1792.65 0.1 35945
9
* -
Quarry and Crushing
Total Particulates Ibs/ton of Quarried material
Raw Meal Preparation
Total particulates Ibs per ton of raw meal
Finish Grinding
Total particulates Ibs per ton of cement
Emissions per ton of material processed in the quarry and crushing, and raw milling steps are converted
to emissions per ton of cement by taking into account that 1.6 tons of quarried material and raw meal are
required per ton of clinker, and 0.95 tons of clinker are required per ton of cement.
Emissions lbs per ton of quarried material
2.5515 x 1 . 6 ~0.95
Emissions lbs per ton of raw meal
0.2287 x 1.6 x 0.95
-
Emissions lbs per ton of cement
=
3.8783
-
Emissions lbs per ton of cement
0.3476
10