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NATURAL GAS COMBUSTION IN A FLUIDIZED BED HEAT-TREATING FURNACE

ABSTRACT Natural gas-fired fluidized beds have been used industrially for low and medium temperature heat-treating systems for quite some time. Attempts at building higher temperature systems for high carbon steel heat treating have been less successful. There has been little academic work done examining the natural gas combustion process in these types of furnaces. Several studies have been conducted on gas combustion in a fluidized bed, but nearly all these studies have been carried out in laboratory-scale systems. It is well known that many phenomenon in fluidized beds do not scale in a predictable fashion, and therefore industrial scale studies in to the natural gas combustion process are warranted. To provide sufficient information to assist in the development of a fluidized bed heat treating furnace capable of heat treating high carbon steel at temperatures in the range of 900°C and higher, the gas concentrations at various levels in the furnace under pre-mixed conditions were examined. Pollutant emissions (NOx and CO) were also examined over a range of operating conditions. It was found that, at temperatures below 750°C, combustion only occurs at the top of the bed, with no evidence of combustion in the bed. Combustion only begins to move into the bed when temperatures reach approximately 750°C, with full combustion occurring in the bed at temperatures above 900°C. When combustion occurs completely in the bed, it occurs in the lower part of the bed. As a result, a zone is formed from the top of the combustion zone to the bed surface, which has a low oxygen concentration. This zone therefore offers a protective environment for the heat treatment of the high carbon steel without significant oxidation and decarburization occurring on the steel surface. The depth of this zone is a function of bed temperature and fluidizing gas velocity. The emissions of NOx and CO are functions of bed temperature and gas velocity in pre-mixed combustion. The mechanisms behind these phenomena are also discussed in this paper.