Reduction of NOx Emissions from Cement Kiln/Calciner through the Use of the NOxOUT Process

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Table 1 - Tested KilnlCalciner Conditions Fuel to Heat Input to Preheater Uncontrolled calclner calciner oxygen (%) NOx (pph) 1 gas 5% 1.8-2.1 415-455 2* gas 9% 2-2.2 420-520 3* gas 5% 2-2.2 420-520 4 gas 5% 2.2 500-550 5 gas 5% 2.2-2.4 350-400 6 gas 5% 2.4-2.7 450-530 7 gas 5% 2.3-2.5 450-500 8 coal 9% 2.3-2.7 480-550 9 coal 5% 2.6-2.9 340-500 10 coal 9% 2.0-2.2 410-426 • Raw mill was off and sodium bicarbonate injection was on from several operating conditions showed NOx reduc­tion was lower by 20% at NSR 1.5 when oxygen increased from 2% to 2.6%. Even at this reduced effi­ciency, the reduction was still high: -50% at NSR 1. NOxOUT-A injection through three different eleva­tions in the calciner showed similar reductions, indi­cating that process requirements are satisfied as long as the reagent is injected in the calciner. Baseline NOx varied from 350 pph for condition 5 to 550 pph for condition 4. This variation in the baseline had negligible impact on NOx reduction which remained at -60% at NSR 1. The NOxOUT Process, like any other chemical reac­tion, requires good mixing between the reagent and NOx and appropriate temperature and residence time. In this kilnlcalciner application, the reagent was well mixed with the flue gas due to highly turbulent flows in the calciner and cyclone preheater and an appropriate placement and use ofNFT injectors. Gas temperature of about 1530°F is at the lower end of the process temperature window. However, high CO concentration from coal or gas oxidation inside the calciner shifted the required process temperature to a lower temperature.6 The calciner temperatures, therefore, became more optimum as the result of elevated CO concentration. The available reaction time at this temperature exceeded 1 second in the calciner and another 1-2 seconds in cyclone #5. This reaction time of 2-3 seconds is higher than reaction times of 0.5 to 1 second in other applications. As the available reaction time increases, the required temperature shifts to a lower temperature and NOx reduction efficiency increases.6 This combination of long residence time, good mixing, and an appropri­ate reaction temperature (a result of elevated CO and long residence time) gave excellent process performance. A typical heat input to the calciner was about 5% of the total heat input to the kilnlcalciner. Increasing this heat input had a negligible effect on NOx reduc­tion when gas was injected into the calciner. Chang­ing the calciner fuel from gas to coal or decreasing the coal feedrate decreased the reductions. This decrease was likely caused by an increase in oxygen concentration that accompanied coal injection. When coal is injected into the calciner, air carries pulver­ized coal from a mill to the calciner. This additional air increased preheater oxygen from 2 to 2.5% for gas to 2.4 to 3.3% for coal. High oxygen preheater condi­tion with gas as the calciner fuel was virtually the same as the coal fired cases with high O2, Similarly, improvement in reduction with increasing coal feedrate is because of reduced preheater oxygen. As indicated in Table 1, oxygen was the highest at 2.8- 3.3% under 5% coal and 2.6-2.7% with 9% coal feedrate. BY-PRODUCT EMISSIONS SNCR processes can release unreacted ammonia, known as "ammonia slip", and increase CO above baseline levels. A part of the NOxOUT Process tech­nology is a method and know-how to minimize by­product emissions. During this test, NOxOUT-A in­jection increased concentrations of CO and S02 in stack gas. This increase was related to oxygen con­centration at the preheater exit and can be minimized by injecting through the bottom ports, operating the calciner at oxygen greater than 2.3%, or by limiting the NOxOUT-Aflowrate to less than NSR of 0.7. CO The CO emissions exiting the preheater were higher than from the kiln. At the kiln exit, CO was typically less than 0.02% while the preheater exit gas contained between 0.08 and 0.1 %. This CO increase in preheater is the result of gas or coal firing in the calciner in addition to oxidation of organics in the raw mix. The temperature in the calciner is 1500 to 1550°F and oxygen concentration is typically less than 1%. Such condition delays combustion of gas or coal resulting in an elevated CO emission. In Figure 6, CO at the stack corrected to 10% oxygen is plotted against preheater exit oxygen concentra­tion for several chemical injection rates (NSR) and without injection. The best fit curves through each set of data are also presented in the figure. The stack CO emission increased with decreasing preheater exit oxygen and with increasing NSR. This CO increase was negligible at preheater oxygen concen­tration greater than 2.4% and at NSR less than 0.7. When the reagent was injected through the bottom