Real time, non-intrusive measurement of particle emissivity and gas temperature in coal-fired power plants
We present a novel, remote technique for measuring in situ and in real time (every 2 s) the spectral emissivity of particles at λ = 3.95 µm, in optically thick combustion environments. The novelty lies in the use of spectral information in the mid-IR (the blackbody emission profile of the 4.3 µm CO2 band and the gray emission profile of particles between 3.8 and 4.1 µm) to determine the physical and brightness temperatures of the gas-particle medium, from which particle emissivity can be calculated. The retrieved particle emissivity at 3.95 µm is a reasonable average of total particle emissivity between 1 and 15 µm. Thus, CFD researchers who work with radiation sub-models may use this technique to obtain in situ emissivities at different locations, with a portable, rugged and inexpensive device. A small prototype was built with off-the-shelf components: standard light collection optics, a grating spectrometer and a linear-array pyroelectric detector. The particle emissivity is calculated from the asymptotic solution of the radiative transfer equation for optically thick media with isotropic scatterers. Results from a proof-of-concept test at a full-scale, coal-fired boiler 10 m above the top row of burners showed an average particle emissivity of 0.41 and an average gas temperature of 1533 K. Intrinsic and prototype error as well as the impact of temperature gradients in the line of sight of the instrument are discussed.