Oxygen enrichment at the TXI Midlothian cement plant

Abstract

A facility’s decision to use oxygen enrichment is influenced. by a number of site specific factors. Although current trials are underway to prove other benefits of oxygen enrichment, the primary reason to use oxygen enrichment is to increase kiln production and net profit for the operation. The main factors influencing this decision are market demand balance, selling price of cement, cost of oxygen delivered to the plant, plant design, and of course environmental regulations. TXl’s Midlothian Plant began using oxygen enrichment in August 1997. The Midlothian plant is in a unique position of producing several types of clinker while utilizing liquid recycle fuels, TXl’s patented CemStarSM process and oxygen enrichment, all at the same time. This makes precise ongoing evaluation of the effects of any single variable a difficuA job. When TXI originally evaluated oxygen enrichmentl, it showed a clinker production increase. A controlled test would be the best way to validate the magnitude of the production increase from oxygen enrichment. However, because of the production demands of the Texas cement market and the significant effects on clinker production from each of the previously mentioned variables, no specific controlled testing has been conducted at TXl’s Midlothian Cement Plant since the original evaluation of oxygen enrichment. In July, 2000, a power outage at the oxygen supplier’s facility allowed TXI to see the effects of operating without oxygen enrichment. As a resuli of these power curtailments, the oxygen supplier could not provide oxygen to TXI for about 3 days. TXI ran all four kilns without the benefit of oxygen enrichment for about three days. This event lasted from July 18 to July 20. This situation, even though it was a shortterm event, was an indicator of the benefits of oxygen enrichment and was consistent with the original testing results. This paper will compare July 2000 operational data with and without oxygen enrichment. The operational parameters, production rates and stack emissions were all evaluated and the results will be presented. Overall, this event shows that oxygen enrichment does increase clinker production. 0-7803-6501 l / O l / $ l O . O O 02001 IEEE. 289 Introduction TXI operates four long wet kilns at the Midlothian cement facility. The first kiln was built in 1960 and the fourth was completed in 1972. Currently TXl’s Midlothian Cement Plant produces 2 types of clinker and 11 types of cement. TXI has utilized natural gas, fuel oil, coal and coke, and liquid recycle fuel to fire its kilns at different times since operations began in 1960. The Midlothian kilns operate with a coavcoke blend as the primary fuel and liquid recycle fuels and natural gas as supplemental fuels. Typically, the coaVcoke blend makes up about 60% of the kiln heat input with natural gas and liquid recycle fuels making up the remainder of the heat requirement. TXI uses a single source for coal and natural gas so the heating values from these sources of fuel is held fairly constant. Also, TXI manages a state of the art liquid recycle fuels facility. This fact coupled with a tight laboratory control program for managing the quality of the liquid recycle fuels provides consistent fuels to the kiln. Since late 1996, TXl’s patented CemStarSM process has been used on all four kilns at the Midlothian plant. It took several months to implement this process and a few more to fine-tune the system. In addition, the Midlothian plant began using oxygen enrichment again mid-year in 1997. CemStar and oxygen enrichment both increase clinker production. It is difficult to determine which process contributed what proportion to the production increase. Some controlled tests have been conducted to quantify the contribution of CemStar to clinker production but oxygen enrichment was not differentiated as a contributing factor during those tests. Production demands have kept TXI from doing specific control testing to update the exact production contribution from oxygen enrichment. In July of 2000 TXl’s Midlothian kilns were operated without oxygen enrichment for about three days, caused by a power curtailment to the oxygen supplier. This lack of oxygen supply occurred from July 18 through July 20 and afforded TXI an opportunity to evaluate the effects of oxygen enrichment on kiln production rates during this short-term event. Although this episode lasted a short time, some general suppositions can be made about the effects of oxygen enrichment on clinker production at the Midlothian plant. This report will use data from the event to evaluate and present generalized information to support oxygen enrichment as a method to increase clinker production. An evaluation of certain stack emissions data will also be included. Oxvaen Enrichment Justification Originally, TXI used data from a test conducted at the Midlothian plant in 1984 to justify oxygen enrichment on its kilns. The original report showed a 6% production increase, which justified oxygen enrichment. The original evaluation of this process was fairly simple. It focused strictly on production increase and fuel efficiency. However these types of decisions have become more complex because of continuously evolving environmental regulations. As an example of how this evolution has effected decision making in the cement industry, some cement facilities have a permitted minimum kiln exit oxygen limit. Originally, the environmental agencies placed these limits with the intent of ensuring good combustion in the kiln. Now that NOx is an important regulatory issue, the effects of a minimum kiln exit oxygen limit may actually contribute to higher NOx emissions. This means that all process modifiitions must be well thought out and are now much more complex than in previous years. Because of this, TXI continually reevaluates the value and effect of oxygen enrichment. The evaluation and standards for that evaluation have become more complex than simply determining a production increase. Julv 2000 Data The oxygen enrichment system was shut down on all four kilns on July 18, 2000. Figure 1 below is a graph depicting the average daily and monthly clinker production rates for TXl’s four Midlothian kilns during July 2000. To eliminate bias from atypical kiln performance, kiln data from partial production days or where special clinker was being produced were not included in the graphs. July 3 and 4 were two such days. As Figure 1 indicates, the average clinker production rates for the period with oxygen enrichment averaged over 6.5% higher than the periods where oxygen enrichment was not used. Figure 7 shows the Clinker production average for all four kilns as a percentage of the maximum.