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ECN publication
Silica fines from included quartz in pulverized-coal combustion
Published by: Publication date:
ECN 1995
ECN report number: Document type:
ECN-RX--95-021 Other
Number of pages: Full text:
18 Download PDF  

Recently adherent silica has been found on furnace tubing in the form ofa very fine dust, causing a novel 'quartz problem'. The white silica reflects the furnace/flame radiation and reduces the amount of heat absorbed in the water-walls of the furnace with the result that steam production is reduced. Since less heat is absorbed in the furnace, the temperatures at the furnace exit are too high and extra cooling has to be applied there. Both factors result in a decrease in boiler efficiency. For this reason the formation of reflective silica layers has to be understood and prevented. This was the reason for a study into the origin of the fine silica-dust. The investigation was performed in the reproducible environment of a bench scale burner-furnace facility using a commercial quartz-rich bituminous coal. It was found in the present study that the origin of the fine silica dust were micron-sized inclusions of this mineral inside coal particles. For this reason much attention was given to the proper choice and preparation of the coal. Since the mentioned operational problems were connected with the staged combustion of coal, the effect of this technique on the formation of the silica fines was studied. The heart of the system is a methane-burner which consists of a small primary burner in which the particles are carried along surrounded by a large secondary burner. A furnace on top of the burner provides a residence time of the particles of 300 ms at temperatures and gas-conditions simulating the conditions in the flame and the lower furnace. The gas mixture in both burners had 5% excess oxygen to simulate a conventional oxidizing coal-flame. Staged combustion was simulated by firing the primary burner with excess gas thus generating a mildly reducing primary flow. With a small pipe on top of the primary burner the mixing-in of oxygen-rich secondary air is delayed. The characteristic time for mixing in of the secondary air is 30 ms, comparable to the typical values in low-NOx burners. 3 figs., 1 tab., 10 refs.

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