The effects of oxygen enrichment on clinker cement and concrete quality
The Effects of Oxygen Enrichment on Clinker, Cement and Concrete Quality Frederick Hommel St. Lawrence Cement, Catskill, NY ABSTRACT From 1/19-2/27/2000 the Catskill plant with the assistance of Air Products personal ran a trial test onenriching the kiln combustion air with pure oxygen to improve production and quality. This paper gives anoverview of this test and describes how this effected the clinker, cement and concrete quality. INTRODUCTIONEnriching combustion air with oxygen to improve clinker production was first suggested back in 19031.During the 1940’s commercial attempts at enriching combustion air in blast and rotary kilns were tested inGermany and the USSR1. They found that upon the addition of oxygen that the flame shape became shorterand brighter2. Full commercial implementation of this technology in these countries was not enacted do toproblems with refractory life 2. Commercial attempts in wet kiln were conducted in the early 6o’s by UnionCarbide at Southwestern Cement’s Victorville, CA wet kiln. They found that a 1 to 2.3% oxygen enrichmentlevel increase clinker production by 5% and specific fuel consumption decreased by 7%. Increasing clinkerproduction beyond 5% was limited by their clinker cooler capacity. After modifying their clinker cooler theywere able to increase the oxygen enrichment to 4% which increased production rates to 35% and decreasedspecific fuel consumption by 15% 2. In addition to the production increases and decreases in fuelconsumption it was indicated that cement clinker quality improved. Particularly they found that the free CaOcontent decreased and the concentration of C3S increased2. The use of oxygen enrichment of the kilncombustion air since the 60’s has been limited do to the high cost of oxygen. However in resent years thevalue of clinker has increased while the cost of oxygen has remained stagnate, making the use of oxygenenrichment again a possibility. The possibility of increasing clinker production and improving cementquality by oxygen enrichment was the main reason for trying this technology at the Catskill Plant.St. Lawrence Cement owns the Catskill Plant that is located on the banks of the majestic Hudson Riverapproximately 100 miles north of New York City. The Catskill plant contains a single long wet kiln with anannual clinker production of 520000 metric tons. The kiln is 550 feet long 18x17 feet in diameter with an F LSchmidt grate cooler. The fuel is a mixture of coal and petroleum coke and is blown into the kiln by thedirect-fired method. Currently the pet coke substitution rate for coal is around 12%. Wasted kiln dust isrecycled back into the kiln by a dust scoop system located just after the chain section. The burningprocess, raw and finish grinding is semi-automated with the operation being handled from a central controlroom.From 1/19-2/27/2000 the Catskill Plant conducted a trial test to oxygen enrich the kiln combustion gas. Thegoal of this test was to see at least a 10% increase in production and a 10% decrease in specific energyconsumption with improved cement quality via a oxygen enrichment in the range of 1 to 4%. The plan was toincrease feed and speed while the volume of kiln gases decreased. Data showing how this process wouldchange clinker, cement and concrete quality was very limited and so would be also investigated. AirProducts provided the engineering and equipment needed to run the test. They worked side-by-side withour operators in the control room to develope operating ranges for the kiln operating parameters. For thetrial period we were to pay Air Products $0.15/100 scf oxygen and $6.00/ton for clinker which exceeded ournormal clinker production. INSTALLATIONThe outside installation consisted of a 9000-gallon tank of liquid oxygen with 8 ambient vaporizers locatednear the kiln (see Figure 1). Oxygen from this tank went to a controlling regulator which feed oxygen into aflow-controlling device located on the kiln deck. From this regulator, oxygen was injected into a stainless
steel lance that was located approximately 12 inches behind the nose of the coal pipe and was positionedtowards the load quadrant of the kiln (see Figure 2). Heat generated by the injection of oxygen wouldradiate towards the load while the main body of the flame would act as a shield protecting the refractory andit’s coating from the intense flame generated by the oxygen enrichment. A safety interlock system was inplace to guarantee that if there were a loss in fuel the oxygen enrichment would be shutoff automatically. Standard Oxygen Enriched Firehood Flame Section of Flame Burner Fuel Clinke Oxygen r Lance Oxygen Load Clinker Load Front View Side ViewFigure 1. View of a Typical Liquid Oxygen Figure 2. Diagram of oxygen enrichment inStorage System in the rotary kiln QUALITY OBSERVATIONSOxygen enrichment did effect the clinker quality. Clinker density increased and became harder to grind withincreasing oxygen. Alites in general became larger and more rounded with increasing oxygen and some ofthe more rounded alites had a slight fringe around them (Graph 1). The large alites tended to be less reactiveto 1% Nital etching solution and turned brown, however there was areas of very small crystals which werevery reactive and changed to dark brown to blue. Alites remained bright under plane-polarized light and thebirefringence improved slightly at 25000 scfh but decreased at higher oxygen levels. Belites tended todecrease in size and appear to be slightly more ragged with increasing oxygen, however they were veryreactive to the 1% Nital solution and turned blue in this etching solution (see Figures3-7 & Table#1). Mostof the belites had a fine twin lamellae structure and had a color of clear to pale yellow under transmittedlight. Under plane polarized light the belites with no oxygen enrichment had a higher percentage of higherorder color. Many clinkers throughout the test had very large nests of belites. The matrix surrounding thealite and belite crystals remained well differentiated for the entire test, however at times the distancebetween crystals were very narrow especially at the higher oxygen levels.
Figure 3 Clinker taken on 1-8-2000, no oxygen Figure 4 clinker taken on 1-20-2000, oxygenenrichment, alites and belites have sharp edges enrichment rates at 25000 scfh, some crystals are slightly larger, both alites and belite continue to have sharp edging
Figure 5 clinker taken on 2-1-2000, oxygen figure 6 clinker taken on 2-3-2000, oxygenenrichment rate at 30000 scfh, alites are larger and enrichment rate at 350000 scfh, alites are large andmore rounded, belites are large and some have rounded, belites are large with some ragged edges,ragged edges liquid phase poor
Figure 7 clinker taken on 2-24-2000, oxygen enrichment rate at 40000 scfh, alites are very large, rounded and some have a belite fringe, belites are smaller and some have ragged edges Clinker Crystal Size Polished Surface Method 80 70 60 Alite Length Microns 50 Alite Width 40 Belite Width 30 20 10 0 10000 20000 30000 40000 50000 O2 Enrichment Rate scfh Graph 17 and 28 day cement strengths improved and setting times lengthen until the oxygen level exceeded 30000scfh. After exceeding the 30000 scfh the 7 and 28-day cement strengths decreased and the setting timeshorten slightly. Water demand as measured by the Normal consistency test and cement cube flow stayedabout the same throughout the test (see Table2 & Graph2). Graph 2 Cement Strengths 7000 6000 5000 PSI 4000 3000 2000 1000 O2 Enrichment Rate scfh 0 10000 20000 30000 40000 1 day cement strength 3 day cement strength 7 day cement strength 28 day cement strength
Concrete tests were fewer in number than the cement testing however they showed similar results to 28-daycement strengths. The 28-day concrete strengths improved with increasing oxygen enrichment until theoxygen level exceeded 30000 scfh. At 30000 scfh the 28-day strength decline. 7-day concrete strength didnot improve but stayed the same (see graph3). Water demand as measured by the slump increased slightlyhigher upon the addition of oxygen, but then remained the same as oxygen increased. Concrete Strengths 7000 6500 6000 5500 PSI 5000 4500 4000 3500 0 10000 20000 30000 40000 O2 Enrichment Rate scfh 7 day conc. Strength 28 day conc. Strength Graph 3 BRICK & COATING OBSERVATIONSDuring the test maintaining a good coating was not a problem. After the trial test was completed the kilnwas shutdown for our annual maintenance overhaul. The brick in the burning zone did not show signs ofglazing which might be expected if excessive heat was produced. The brick however showed rounding ofcorners which is more indicative of material wear. The kiln was shutdown during the early part of the test doto a coal mill fire. At the time of that shutdown a ring was found at 106 ft with no coating between 80 and 100ft. The Chemistry of the ring showed that it was regular coating with slightly higher SO3 and K2O. At theend of the test there was no ring and the coating was very good to the 90ft. Normally at a major shutdownthere is a mud ring at the kiln feed inlet but this time there was none. KILN OPERATION OBSERVATIONWith the addition of the oxygen we expected to see the flame shorten and become brighter like an acetylenetorch, however with our kiln that did not appear. Only the flame nearest to the oxygen lance increased inbrightness. Kiln burning zone temperatures and secondary air did increase with additional oxygen butmaterial and backend temperatures remained constant. The kiln draft decreased and the kiln amps increasedas oxygen levels increased. Kiln operators adjusted the speed of the kiln by monitoring the heat profile, asthe kiln got hotter they increased the speed. We did experience some persistence problems with maintainingconstant fuel in the kiln. Wet coal gave us problems in the coal mill and so we experienced several timeswhen the fuel and oxygen enrichment had to be shutdown. To maintain the heat profile, kiln operatorstended to over burn the kiln. Another problem experienced during the test was that the oxygen lance wouldwarp if it were not removed quickly enough when the oxygen was discontinued. ENVIRONMENTAL OBSERVATIONSWhen the trial test was started it was thought that the NOx levels might increase. However after looking atthe Nox readings the conclusion is that there was no change (see graph 4).
O2 trial at Catskill plant ppm NOx 1000 900 800 700 ppm NOx 600 500 400 300 200 100 0 1/4/00 1/6/00 1/8/00 1/10/00 1/12/00 1/14/00 1/16/00 1/18/00 1/20/00 1/22/00 1/24/00 1/26/00 1/28/00 1/30/00 2/1/00 2/3/00 2/5/00 2/7/00 2/9/00 2/11/00 2/13/00 2/15/00 2/17/00 2/19/00 2/21/00 2/23/00 2/25/00 2/27/00 ppm NOx STDEV ppm NOx O2 SCFH*f Graph 4 SUMMARY OF TESTThe clinker microstructure and visual inspection of the kiln indicate that the flame did not get shorter butremained a long flame. The clinker was over burned, but with increasing oxygen the kiln refractory waseasier to coat. The kiln refractory was not damaged by excessive heat caused by the injecting of oxygen inthe combustion gases. Coal mill problems increased instability within the kiln and frequent changes inoxygen enrichment also increased variations in clinker microstructure. Clinker and cement quality improvedwith some oxygen enrichment but decreased after a certain point. This exact point needs to be determinedwhen the kiln is operating steadily and the tendency to over burn is minimized. Oxygen enrichment had noover all effect on raising NOx emission levels. Clinker production increased about 9% and the heatconsumption decreased about 5% by the end of the test. Lowering the heat consumption was hampered doto a very high kiln feed moisture caused by poor weather conditions and lack of chain in the kiln. It isprojected that enriching the combustion gases with oxygen will increase clinker production by 9% and savethe plant $1.7 million per year.REFERENCES1. “Use of Oxygen in Cement Clinker Burning”, Zement Kalk Gips, Vol. 4, 140-145, 1967.2. “Oxygen Enrichment of Combustion Air in Rotary Kilns”, Regional Fall Meeting General Technical Committee, Portland Cement Association, Miami Beach, 1961.