FTIR analysis of transient CO2 emissions in a circulating fluidized bed boiler

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Raines, Thomas
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Robert C. Brown
Committee Member
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Mechanical Engineering
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Fourier transform infrared (FTIR) spectroscopy was used to qualitatively and quantitatively analyze the gaseous products of combustion in an industrial-scale fluidized bed boiler. The analyses were used to determine characteristics of the coal fed into the boiler. The method was non-intrusive and performed under normal operating combustion conditions;An FTIR operates on the principle that an infrared beam passed through a gas will be absorbed by that gas in an amount proportional to the gas concentration and at wave lengths related to the gas species present;For short time frames (less than about 1/3 of the char time constant), both devolatilization and char burnout can be expressed as exponential decays of the CO2 emissions resulting from introducing a batch of coal into the boiler;The FTIR system sampled combustion products from an Ahlstrom Pyropower circulating fluidized bed boiler at the Iowa State University power plant. The gas sample was drawn through the FTIR gas cell from the boiler cyclone outlet by a vacuum pump. To determine the coal properties, batches of coal were introduced into the boiler through a specially designed hopper installed on the end of the coal drag conveyor. These simulated pulses yielded sharp spikes in CO2 emissions with rapid exponential decay during the volatiles release and combustion followed by a slower exponential decay in CO2 to background levels during char burnout;Calculation of time constants was performed in both time and frequency domains with similar results. Frequency domain analysis was based on the assumption of first order processes for the devolatilization and char phases, and was found to provide results consistent with time domain parameters. Plotting decibels of the square of the fast Fourier transform (FFT) of the data (10*log10(FFT2)) vs. frequency resulted in the time constants of interest being visible as reciprocal cornering frequencies;Combustion dynamics in the operating boiler were found to be far more complex than previous research had indicated, with chemical kinetics being the limiting mechanism for char combustion. The ability to characterize coal combustion characteristics in an on-line boiler was demonstrated.

Mon Jan 01 00:00:00 UTC 1996