Combustion and pollutant formation in the overthrow region of spreader stokers

A considerable amount of research has been performed on pulverized coal in recent years. There are, however, a number of alternatives to direct combustion of a powdered coal. One method which has become popular is the coal stoker. In stoker combustion relatively large particles (up to 1.5 inches in diameter) are burned, primarily in large flat beds. The purpose of this work was to examine the combustion and pollutant formation characteristics of a single type of coal stoker: the spreader stoker. Specifically, the overthrow or feed region of the spreader stoker was studied. The experimental portion of the study consisted of the injection of various sizes of coal into a vertical tube furnace designed to simulate the environment within a spreader stoker. Experimental results were in the form of the flue gas composition measurements, which were subsequently converted to burn rate, and nitrogen conversion (to NO). It was found that particles greater than 0.10 inch undergo very little combustion in the overthrow region. Further, of the particles which did react (less than 0.10 inch), a relatively high nitrogen conversion of 35%-40% was noted. The process was modelled mathematically by a system of three transient, coupled second-order partial differential equations. The results (predictions) of the model were consistent with experimental observations with respect to burn rate - particle size functionality. No attempt was made to model the gas phase kinetics of NO formation.