OCR Text |
Show the deposit growth appears to be a combination of friable as well as molten materials, and is the result of continuous accumulation of molten droplets. These deposits receive heat by radiation from both the flame and the hot gases passing over them. Typically, firetube boilers operate over a wide turndown range. At full firing rate, a bright, yellow, stable flame is observed. As the combustion proceeds, the flame is observed to deteriorate due to: 1. A reduction in flow area caused by ash deposits in the firetube combustion chamber or on the back door creates a back pressure which in turn reduces the combustion air flow through the diffuser and, therefore, results in a very lazy flame. This may lead to complete flame failure. 2. The agglomeration of coal particles blocks the fuel passage (nozzle) and reduces the fuel flow rate. This ultimately results in a thin flame which generates insufficient heat to maintain a stable flame. At reduced firing conditions, flame shape and brightness are similar to the flame observed at full rated capacity. But, it appears that the radiative character of the flame is diminished due to the reduced concentration of black clouds per unit volume of the flame and reduced flame temperature. Thus, this situation reduces the heat transfer rate which affects the boiler efficiency. Boiler operation was limited to approximately 10 hours due to deposit build up. Results of these studies indicate that molten deposit build up is a major limitation to use of COM in existing firetube boilers. 15-11 |