Experimental study on suppression efficiency of soot from biomass pellets under acoustic and electric fields

As a type of renewable energy source, solid biofuels have received lots of attention. However, the combustion of solid biofuels releases large amount of soot, which greatly inhibit their application. Development of effective strategy for suppressing the soot emission from solid biofuel combustion is of great importance. In this work, a novel approach integrating acoustic and electric fields is proposed to effectively suppress soot emissions during redwood pellet combustion. The effects of experimental parameters, i.e., the vertical height between the electrodes (EH), applied voltage (E), current (I), acoustic frequency (f), acoustic pressure (A) and electric field intensity on soot suppression are systematically investigated. Results show that application of an electric field causes the flame tip to spread outward and reduces flame height, while the acoustic and electric fields further suppress flame height and enhance stability. Under a constant EH, flame temperature increases with voltage; with constant voltage, it first rises, then falls, and stabilizes as EH increases. The flame centerline temperature is higher, and upper-region temperature fluctuations are reduced under combined fields. Maximum suppression reaches 77 % under either field alone and up to 100 % under optimal acoustic and electric fields. The acoustic field does not alter the current-voltage characteristics. The acoustic and electric fields generate transverse acoustic oscillations and longitudinal ionic wind, which alter the trajectories of fuel and soot particles, enhance fuel-air mixing, and promote soot re-oxidation.