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
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.
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:
calcines the limestone, and
drives off surface and bound moisture,
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 Reclaim from piles Transfer from silo Transfer from storage Primary crushing Grinding Pyroprocessing Grinding
Secondary crushing Transfer to silo Clinker cooling Transfer to storage
Stockpiling 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
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-
electricity-
4.21 mmBtu per ton (4.63 mmBtu per tonne).
144 kwh per ton or 0.48 mmBtu (158 kwh per tonne or 0.53 mmBtu per tonne).
no change in energy consumption in finish milling.
Heat contents.
-
coal: 22.7 mmBtu per ton-
petroleum coke: 28.2 mmBtu per ton-
natural gas: 1000 ~ t u per ft31995 Portland cement production from domestic clinker: 75 million tons
.
(Total U.S. Portland cement production minus cement ground from imported clinker).
C K D 5.28 million tons (4.8 million tonnes) per year.
Reductions of inputs and emissions are directly proportional to the percentage of limestone added.
Assumptions
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)
Limes tone Calcining loss 0.570
Oag50
t l-ooo
1.520
T ,
1.216 Clay
0.304
d
Raw mealGypswn
Clinker
0.050 I
Cement
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 consisting of 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.
0.050
1.152 Raw meal
-
0.900 ClinkerZlay 0.288
Material flow with 5% limestone addition to cement (tons)
r y
Ah P 1.000 Cement
lLimestone
Calcining loss 0.5409
I
0.050 ICONCLUSION
Resources conserved per l,OOO,OOO tons of cement at 5% limestone addition are;
0 limestone 0.014 x 1 x 106 = 14,000 tons
0 clay 0.016 x 1 x 106 = 16,000 tons
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
0 0 0
9,700 tons of coal, or 7,800 tons of coke, or
220 million
fF
of natural gas.Electricity
Based on a sample of 10 plants, the use of electricity by process step can be approximated as follows:
w
% -Quarry 5 7.20
Raw mix preparation 17 24.48
Pyroprocess 29 41.76
Total 100 144.00
Finish milling
49 2a36
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.
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 94.293 lbs 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
1.
2.
3.
4 5.
U.
.
Cem h Economic Research Department, Portland Cement Association, Skokie, Illinois, 1996.Unpublished results from a Portland Cement Association survey, 1994.
John R. Richards, Compilation of Cement Industry Air F w * sions Data for 1989 to 1996, SP 125, Portland Cement Association,, Skokie, Illinois, 1996.
3
mDilation f Air Pollut r -42 'f h E i i.
U.S. EPA, Office of AirQuality Planning and Standards, 1995.
*
rometric Retrieval A1 C1 ific '0 a n E ' iFactors J a istinp 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.
.
, L I , a ,*- '-TAc- * -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 0.9841 05 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
- -
Emissions lbs per ton of cement 2.5515 x 1 . 6 ~ 0.95 = 3.8783Emissions lbs per ton of raw meal
