Laser velocimetry measurements of particle distributions for flue gas scrubbing

Concern is building in the United States over emissions from large coal-fired boilers which are thought to be linked with what is known as acid rain, a problem in many parts of the U.S. and Canada. However, the cost to retrofit these boilers with emission control devices such as in-duct scrubbers is expensive and not an attractive alternative to owners of older boilers. Research is being conducted at The University of Tennessee Space Institute (UTSI) to experimentally confirm the feasibility of a proposed low-cost system for simultaneous SO₂ and NO_x control. The injection of active SO₂ and NO_x sorbents directly into the flue gas ductwork is the approach that the study is using.

The effectiveness of the gas scrubbing process can be signifi cantly impacted by the gas-droplet surface contact. Thus, particle sizes and velocities can affect the scrubbing process. Also, the scrubbing process can be altered with time if the slurry droplets are wet and stick when they coat the duct walls. The purpose of this research is to quantify the particle velocity distributions and drop let flux distributions at several axial locations in the UTSI flue gas scrubbing facility using a two-component vectorized Laser Velocimeter (LV) or Laser Doppler Velocimeter. These measurements characterize the spray pattern of the nozzles used to inject the slurry sorbent droplets.

Results indicate that a wide spray nozzle (a multi-orifice nozzle) was not appropriate for the in-duct scrubbing experiments because the spray impinged on the wall before adequate moisture evaporation; a conclusion was that a narrow spray nozzle (a single orifice nozzle) should be used. The experimental ductwork included a 180 deg bend before the injection point causing slight flow non-uniformity which moderately affected the spatial distribution of the particles before they had slowed to the gas velocity. This implies that bends in flue ducts should not interfere significantly with gas scrubbing when appropriate flow straighteners are used.