Evaluation of the NOx formation in a rotary kiln test facility
Examensarbete för masterexamen
Sustainable energy systems (MPSES), MSc
The rotary kiln process for iron ore pelletizing is one of the main methods to upgrade crude iron ore. The mining company LKAB runs four Grate-Kiln production sites in northern Sweden, where a grate and a rotary kiln are combined to thermally indurate the iron ore pellets. The high temperature needed for the process is provided by combustion of coal with a high amount of extremely preheated air, what creates an atmosphere inside the furnace of which the present theoretical understanding is low. So far, the high amount of excess air (λ = 5-6) made standard NOx mitigation strategies in the rotary kiln unsuitable. Environmental issues and need for fuel flexibility has enticed LKAB to carry out experimental campaigns in a test facility to characterize the combustion process. The results of the experimental campaign of 2013 and previous campaigns are reviewed in the present work. The measurement results were evaluated through gas-phase chemistry modelling with a detailed chemical reaction scheme. The evaluation of the 2013 experimental campaign suggests measurement problems of the temperature and the combustion behaviour inside the test furnace. Gas and oil flames showed to combust almost instantaneously within the first centimetres after the burner. Biomass and coal combusted significantly slower, but also had the highest reaction intensity close to the burner inlet. Measured exhaust NOx levels could not be achieved in the model with the measured temperature and modelling results proposed peak temperatures more than 500°C above the measured temperature inside the kiln for oil and gas combustion. For oil and gas it was found that thermal-NOx is the major contributor to the NOx formation inside the pilot scale kiln. The lower NOx emissions for coal and biomass were explained by lower temperatures inside the kiln and the relation fuel-N and thermal-N. Modelling fuel bound nitrogen for the solid fuels showed that the NOx is formed there in similar amounts via both the fuel-NO and thermal-NO formation route. By comparing the high NOx levels of the experimental to the lower levels in the full scale plant, it was concluded that the NOx formation differs between them, as in the experimental one significantly higher temperature prevail for all fuels. This showed that this facility does not resemble the actual kiln adequately.
Energi , Energiteknik , Energy , Energy Engineering