NO_x Emissions in Laboratory Scale Pressurized Fluidized Bed Combustor

The Role of NO Reduction by Char

Abstract

A laboratory scale pressurized fluidized bed combustion (PFBC) system was designed and constructed to clear the details of combustion of coal in PFBC. This PFBC system allows visual observation of hot fluid-ized bed by using a transparent electric furnace. To simulate the emission characteristics of coal combustion of the full scale PFBCs by this lab-scale PFBC, a new similarity rule was introduced. Volumetric heat input and mean gas residence time through fluidized bed in lab-scale PFBC were adjusted to the values matching those of full scale PFBCs. Motion of bubbles and burning of coal particles and volatiles were recognized by visual observations. Volatiles burned mostly in the bottom region of the fluidized bed. On the other hand, char particles burned uniformly in the bed. Ranges of conversion of fuel-N to NO agreed with those obtained in a full scale PFBC. Thus, it is verified that new similarity rule explained above is useful to simulate the combustion behaviors in full scale PFBC by a laboratory scale small PFBC.
Effects of operating pressure and O₂ partial pressure in flue gas on NO emission were examined. Linear relationship between O₂ partial pressure and NO emission was observed. NO emission was decreased rapidly with increasing in operating pressure at constant O₂ partial pressure. This may be caused by an enhance-ment of NO reduction by char particles because of increase in number density of char particle in the bed at elevated pressure. In order to confirm this assumption, initial combustion rate of char in the pressurized fluidized bed condition was measured. Initial combustion rate of char increased with 0.6th power of operating pressure. This means that char number density is increased in elevated pressure condition. One of the major reasons to achieve the low NOx emission in PFBC condition, should be the reduction of NOx by char particles through the bed.