DEPOSITION MECHANISMS OF HIGH CHLORINE COAL AS A FUNCTION OF STOICHIOMETRY AND TUBE TEMPERTURE

摘要

An increase in the use of low rank and moderate-chlorine coals, combined with combustion systems that utilize reducing conditions for NO_x control, has created an increase in corrosion problems in many commercial boilers. Under oxidizing conditions, sulfur has been shown to form alkali sulfates rather than the more corrosive alkali chlorides that occur under reducing conditions. Equilibrium calculations support these initial findings. This paper further investigates the corrosive effects of alkali metals, chlorine, and sulfur compounds due to ash deposition on simulated boiler tubes using a laminar flow, premixed reactor. Using scanning electron microscopy, the compounds deposited on the surface of tubes in cross-flow and parallel to the flow and their corrosive effects are reported as functions of stoichiometry and tube temperature for a chlorine-rich coal and a blend of the chlorine-rich coal and an alkali-rich coal. Stoichiometry was investigated at stoichiometric ratios of 0.7 and 1.2 to examine the results of reducing, stoichiometric, and oxidizing combustion on the ash deposition and resultant corrosion. The results showed chlorine condensation to occur under reducing and oxidizing conditions at low tube temperatures of approximately 90℃, intermittently at 240℃, and not at all at 300℃ and higher. The data indicate lower dew point temperatures for chlorine than are calculated from equilibrium assuming all of the chlorine and alkali are available. Under oxidizing conditions, sulfur is seen within deposits when the sample deposition time is above 3 hours while none is seen at or below 30 minutes. This suggests that the reaction of sulfur with chlorine is heterogeneous at the probe surface.