FLAME TRANSFORMATIONS AND BURNER SLAGGING IN A 2.5-MW FURNACE FIRING PULVERIZED COAL .2. SLAGGING

Results are reported of a 3 day slagging trial with a 2.5 MW semi-indu strial-scale furnace in which the near-burner conditions of a wall-fir ed boiler were simulated. The fuel was a pyrite-rich pulverized coal. Slag deposit probes were placed near the burner wall, simulating the r efractory burner quarl with surrounding wall tubing. The side of the p robe facing the incoming fuel stream was covered with a molten deposit similar to that on the burner quarl of the furnace and to deposits in the near-burner region of full-sized furnaces. Slag was formed on the refractory material of the probes and not on the water-cooled tubes s urrounding it, suggesting that slagging in the near-burner region in r eal boilers may also be initiated on the refractory quarl rather than on the surrounding water-wall. The ash deposits were analysed in-depth by SEM and EPMA. The nature of the deposits was related to the change s undergone by the minerals in the flame before deposition. It was ded uced that deposition of molten pyrrhotite (the first flame product of pyrite) together with silicates resulted in an iron silicate deposit w ith a very low melting point, accounting for the molten character of t he slag. The deposit adhered less strongly to silicon carbide as the p robe material than to alumina. The reverse side of the probe, facing d ownstream, was covered with a loose powder of discrete iron oxide and silicate particles, consistent with the presence of fully oxidized sol id pyrite particles in the furnace at this position.