Between 1995 and 1999, the UScement market grew at an aver-age of seven per cent. Inresponse, domestic manufacturers

looked at all available means to increaseplant capacity. During 1996, Southdownreconsidered the potential of oxygenenrichment reviewing, among otherreported data1,2,3,4, the extensive pro-gramme of testing conducted bySouthwestern Portland Cement during the1960s. Southwestern was subsequentlyacquired by Southdown and in 2000,Southdown was acquired by Cemex.

All of the reports described eitherenrichment of the kiln main burner or riserfiring in air-through kiln systems. The general finding was that, although somecapacity increase was possible, the diffi-culty of maintaining an appropriately posi-tioned lance in the kiln, the hazards ofincreasing flame temperature in the kiln andthe cost of liquid oxygen all combined tooutweigh the potential benefits. Despitecontinuing attempts, by the 1990s the useof oxygen was rare if practiced at all.

During Southdown’s review, however, itoccurred to them that the problems associ-

ated with injecting oxygen into the mainburner could be avoided if oxygen was fedto the secondary firing point of a precal-ciner kiln. It was appreciated that, in manycases, a more economic approach would beto upgrade the kiln ID fan. However, thecapital cost, permit requirements, equip-ment lead time, and kiln down-time forinstallation would prevent rapid responseto an immediate need for increased pro-duction while, in many cases, calciner ves-sels are poorly designed and could not eas-ily accommodate an increased draughtwithout a negative impact upon their com-bustion process. Increased draught mayalso result in excessive preheater pressure

drop or duct velocities. Southdownbelieved, therefore, that a real benefitmight be achieved by calciner enrichmentparticularly for short-term satisfaction ofpeak cement demand.

Initial testing was conducted atSouthdown’s Lyons, CO plant during March1997. Lyons has a single line, which wasoriginally a long dry kiln constructed by FLSmidth in 1969. Due to the high level ofkerogens in the raw materials, the plantexperienced unacceptable hydrocarbonemissions. In 1980, Martin Marietta, whothen owned the plant, contracted withFuller Company to convert the kiln to a precalciner system with a single stage pre-

41INTERNATIONAL CEMENT REVIEW / MAY 2001

CEMENT TECHNOLOGY

Oxygenenrichment

In a constant effort to increase plant capacity, Cemex, formerly Southdown,reconsidered data collected in the 1960s regarding the possibilities associ-ated with oxygen enrichment. However, potential hazards and the cost ofliquid oxygen meant this process had limited practical implications.Despite this, Southdown believed that these problems could be overcome,making calciner enrichment effective for increased production, particularlyfor short-term demand. The subsequent testing at the Lyons, CO plant andoperation of oxygen enrichment are outlined in this article.

by Herman Tseng & John W Lohr, Cemex Inc

Figure 1: liquid oxygen supply system Figure 2: oxygen injection points

INTERNATIONAL CEMENT REVIEW / MAY 200142

CEMENT TECHNOLOGY

heater. This unusual configurationallowed the kiln feed to be intro-duced directly at above 850˚C inorder to prevent distillation andemission of un-burned hydrocarbons.The calciner is of IHI’s SF designwith kiln exhaust gas and tertiary airpre-mixed in a swirl chamber. The calciner vessel is small, and it wasconsidered that increasing ID fancapacity would extend combustioninto the cyclone and beyond to theexhaust duct. The kiln is coal firedwith approximately a 50:50 per centfuel split between main burner andcalciner. Benchmark clinker produc-tion was 56tph.

After discussions with several gas sup-pliers, Air Products was selected to supplycryogenic oxygen. The on-site plantincluded storage tanks for the liquid gas,evaporators, and a valve train (Figure 1).

The literature suggested a typical enrich-ment rate was two per cent of base kilndraught or 10 per cent of available oxygen.

Initially the oxygen was injected to thetertiary duct above the burner floor.Subsequently, a number of different locations were tried between the tertiaryrecouperation zone of the clinker coolerand the calciner vessel (Positions 1, 2 and3 in Figure 2). The location of injectionpoint to the tertiary was not found signifi-cantly to vary the effect, and the ultimateposition selected was at the swirl chamberof the calciner (Position 3 and Figure 3);this being a less hostile environment thanexperienced nearer to the cooler take-off.The injector is a simple pipe, more sophis-ticated diffusers were deemed unnecessary.

Injection of oxygen to the tertiary ductcan allow an increased fuel rate to the cal-ciner. In Lyons, however, it was elected todampen the tertiary to maintain a 50:50

per cent fuel split while draught throughthe kiln was increased. Total fuel and feedrates were increased and the degree of precalcination of hot meal to the kilnremained at 70-75 per cent. No significantchange in NOx emissions was detected. The

incremental clinker pro-duced at successivelevels of oxygen addi-tion is shown in Figure4. It is seen that for upto about 2tph or 1.5per cent of total kilndraught (kiln plus tertiary), some 4.5t ofincremental clinker areobtained for eachtonne of oxygen. Abovethis rate, the benefitswill decrease.

Actual results mayvary somewhat with

the fuel efficiency of the kiln system.However, from repeated testing on a number of kilns, Southdown has elected aconservative figure of 3.5t clinker, pertonne oxygen for production reporting.Then, if oxygen costs US$70/t, the incre-mental clinker productioncost will be US$20/t plusvariable costs. The cost ofcryogenic oxygen has beenfound to vary widely withchanges in geographicallocation and fluctuations inlocal supply.

Following the successfultesting at Lyons, it wasdecided that the inability ofthe calciner to accommodateincreased draught combinedwith the high cost of liquidoxygen in Colorado wouldrender an on-site oxygen

generation plant economicallyattractive. After assessing avail-able systems, Lyons awarded BOCa contract to supply a 50tpd(pure oxygen equivalent)Vacuum Swing Adsorption unit.This plant has been in operationsince August 1999 (Figure 5),and it is capable of producingoxygen of 90 per cent purity fora total cost of some US$20/t(pure oxygen equivalent).

During the past four years,Southdown has engaged in theextensive use of oxygen enrich-ment on six kilns, including a

KHD precalciner kiln at the Victorville, CAplant. At this plant oxygen has also beensuccessfully injected to the tertiary duct.

Southdown Inc has filed a patentapplication under the Patent Co-operationTreaty in July 19975. The counterpart ofthis application in the US and other coun-tries is pending, and it is considered thatthe addition of enriched oxygen to calcinerfiring is proprietary to Cemex. _______ r

References:(1) Humphries & McEntee; OxygenEnrichment of Combustion Air in RotaryKilns; PCA R&D Report (1961) M-169. (2) Gaydos; Oxygen Enrichment ofCombustion Air in Rotary Kilns; PCA R&DReport (1965), Vol 7, No 3, p49. (3) Wrampe & Rolseth; The Effect of Oxygenupon the Rotary Kiln’s Production and FuelEfficiency; Theory and Practice; IEEETransactions of Industry Applications,(1976), Vol 1A, No 6, p568.(4) Leger; Oxygen Lancing for Production ofCement Clinker; US Patent #5,572,938,(1996).(5) International Patent ApplicationPCT/US97/1323.

Figure 3: oxygen injection to calciner

Figure 5: VSA oxygen production unit

Figure 4: incremental clinker