Calciner design for lower CO and NOx using MI-CFD analysis to optimize “Hot-Reburn” Conditions

In an effort to reduce production costs, the cement industry is using more and more alternative fuels and raw materials (AFRs) than ever before. Some successful examples show thermal substitution rate (TSR) as high as 90%. Generally, a higher than 20-30 TSR requires detailed analysis of not only combustion aspects of AFRs, but also their impact on emissions, kiln stability and clinker quality. Usually those conditions that provide a more optimum combustion environment both for plants using conventional fuels as well as those using AFRs or a combination of the two mean an increase in NOx emissions. Implementation of traditional low-NOx techniques can have a negative impact on CO/VOC emissions as well as increase deposit build-up potential and cause kiln instabilities. Several alternative NOx reduction methods are available and have been applied within the cement industry. These methods include Selective Non-Catalytic Reduction/Selective Catalytic Reduction (SNCR/SNR) and can provide NOx reduction efficiencies of 40-80% respectively, without significant negative impact on CO/VOC emissions, but at the same time add significant cost to clinker production. A new technique, termed as "Hot-Reburn", which was known only theoretically for the past 30 years, has now been successfully demonstrated within process industry. When applied to a recently built cement plant, the results show an over 70% reduction in NOx emissions for 50% tire-chip AFR substitution rates, without increasing CO emissions, build-up or causing kiln instability issues. The "Hot-Reburn" conditions were created through a detailed study of mineral interactive computational fluid dynamics (MI-CFD) where important parameters in the destruction of CO and NO, i.e. calciner's flow stratification, rapid mixing of fuel volatiles with kiln-generated NOx concentrations, local stoichiometric conditions, residence time and temperatures were optimized and adjusted to each variable's relative significance. Application under optimized conditions resulted in a cost savings of at least one four hundred thousand dollars per year as compared with SNCR for one plant, while maintaining compliance with environmental legislations and AFR permits. The impact of taking the "Hot-Reburn" methodology out of the theoretical and into the practical realm is an exciting advance in the cement industry, and with the aid of MI-CFD analysis may provide both existing and new plants an economical means of NOx and CO emissions compliance, as evidenced by the cost savings referenced above and detailed in concluding remarks. This detailed calculation procedure based on solving fundamental Navier Stokes equations is derived from transport theorem, where all combustion, turbulence and mineral interactions are combined to analyze existing calciner configurations and to identify those calciner-firing arrangements, which would assist in reducing both CO and NOx emissions while not increasing build-up or contributing to kiln instability. The MI-CFD analytical method greatly assists in suggesting calciner design or modification for creation of optimal "Hot-Reburn Zones". A description of the model along with its validation against two plant studies is presented followed by selected case studies highlighting the concept of "Hot-Reburn" and a brief account of study results is summarized in concluding remarks.