Two-dimensional flame temperature and emissivity measurements of pulverized oxy-coal flames

A broadband, RGB, two-color pyrometry technique for measuring the flame temperature and total emissivity of a two-dimensional image of a coal flame has been developed and used on an oxy-coal flame. The method uses a single, relatively inexpensive, RGB, digital camera. The camera software permits the light intensities of the red, green, and blue light collected for each pixel to be recorded separately. The response of each pixel was calibrated for each color using a blackbody radiating cavity and a monochrometer, which enabled an absolute, broadband emission measurement. The image obtained by the camera was processed to produce a temperature and total emissivity for each pixel. Two spectral emissivity models were explored for use in determining the temperature and emissivity: a Gray model and Hottel and Broughton’s soot emissivity model. Significant differences of 7.1% in average temperature and 24.2% in average emissivity were found. While neither model is ideal for the entire coal flame, the Hottel and Broughton model was selected for future image processing because the images and analysis suggested soot was the more dominant emitter for most of the image. Images were obtained in a 150 kWth, pulverized-coal reactor at two different mixture oxidizer ratios of O2/CO2. The addition of CO2 decreased the average flame temperature from 2183 K to 2022 K and reduced the average emissivity from 0.59 to 0.13. The increase in CO2 lowered the temperature by increasing the dilution. The increased flow rate caused increased mixing, which reduced soot formation and thus the emissivity.

► Two emissivity models were investigated for use with pyrometry in coal flames.
► The oxy-fired coal produced flame temperatures in excess of 2400 K.
► The H. B. Model produced an average temperature 9% lower and emissivity 81% lower.
► Addition of CO2 to the secondary air decreased the flame temperature by over 200 K.
► The measured and calculated decrease in flame temperature with CO2 addition were similar.