OCR Text |
Show Flame emissivity plays a double role in this: if emissivity increases, the direct contribution from the flame increases (other things such as flame temperature being equal) but there will be an increase in the fraction of wall-to-load radiation that is absorbed by the flame. If the two factors of increased flame contribution and reduced wall contribution exactly balanced, there would be no change in thermal efficiency in a hot wall furnace as the flame emissivity changed. In general, the two factors do not balance but the offset is often sufficient in a hot wall furnace to make flame emissivity only a second order factor in controlling thermal efficiency (13). In experiments with firing COM in a very similar furnace (12), as the coal concentration increased the thermal efficiency initially rose as the emissivity increased and then fell again as the radiative flux from the roof was reduced due to the increased interception by the blacker flame. In other experiments in the same furnace firing oil-water emulsions, thermal efficiency fell with increasing water addition but POC and, evidently, flame temperatures rose. From this it appeared that the effect of the water was to reduce the flame emissivity/absorbtivity. These are all factors to be considered in evaluating the results given here. (i) The most common source of evident thermal efficiency differences is gross error of measurement, but this cannot be the case here. Although a total check on measurement accuracy by determining the energy * Losure on the furnace was not possible because wall losses were not independently measured, nevertheless, the comparison of the wall losses obtained by difference (Sec. 4.3) show expected trends and values, with losses |