FLUE GASES POLLUTANT CHARACTERIZATION BY MULTISPECTRAL EXTINCTION MEASUREMENT

ABSTRACT
We present preliminary experimental results relevant to tests carried out on an innovative instrument properly designed for real time measurement of both particle size distribution and concentration in particle laden flows. The system performs on line, continuous, in situ measurements. The instrument, which is based on a multi-wavelength (300nm – 900nm) extinction technique, provides measurements over long optical paths (10m), in the 0.1μm - 3μm diameter range. Precise concentration measurement are guaranteed also in the presence of changes in the particle size distribution due to power load modification and fuel type variations. Preliminary tests have been performed on a thermoelectric fossil fired power plant in Northern Italy. The measured concentrations are in good agreement with the data provided by reference techniques. It is worth noticing that the measured extinction spectrum can also be used to get information about chemical species present in the flue gases. This indicates the possibility of designing a single instrument able to perform both particle and pollutant gases monitoring.

1 INTRODUCTION
Particulate pollution can be produced by many sources, most of them related to combustion processes (automotive, heating, power production, chemical industries, etc.). Particle emissions from thermoelectric power plants are
continuously monitored with instruments (opacimeters) directly mounted on the flue gases duct. It is well known that monitoring systems like the commonly used opacimeters can provide information about the amount of solid emissions only if a particulate distribution is assumed, or, in an equivalent way, if a calibration procedure requiring sampling and analysis of the particulate matter is performed.

An optical technique capable of overcoming the above mentioned limitations and recovering both the size distribution and concentration of particulate, is based on the spectral extinction method [1]. This technique is based on the
measurement of the extinction coefficient spectrum (i.e. the extinction coefficient measured as a function of the illuminating wavelength) due to particles suspended in flue gases, the size distribution and particle concentration being
determined via a properly developed inversion algorithm. Examples of applications of such techniques can be found in the literature [2, 9].

The measurement of the extinction coefficient can be carried out by shining a white light beam and detecting the spectrum of the transmitted radiation after propagation. By comparing this spectrum with the one obtained in absence of particulate, it is possible to determine the spectral extinction coefficient due to the particles suspended along the optical path.

In this work we present a new instrument capable of carrying out semi-automated almost real-time (~ 10s delay) spectral extinction measurements over optical paths of a few meters.