| OCR Text |
Show The overall rate of combustion is dependent on the slowest of the above stages. In most industrial combustion systems, the mixing is slow while the other steps are very fast. The rate and completeness of the combustion process is therefore controlled by the rate and completeness of fuel/air mixing. Insufficient mixing produces unburnt C O in the flue gases, wasting fuel. For good combustion, it is necessary to ensure that adequate air is supplied and that the burner mixes the fuel and air streams effectively and efficiently, hence the combustion is controlled by the rate and completeness of the fuel/air mixing. One common phrase is:- IF IT'S MIXED, IT'S BURNT For kiln burners, fuel/air mixing occurs as a result of jet entrainment. Figure 3 shows a schematic of a fuel jet issuing from a conventional jet burner nozzle in a rotary kiln. M o m e n t u m exchange occurs between the boundary of the jet (which is normally fuel and primary transport air) and its surroundings, causing the surrounding secondary air to be locally accelerated to the jet velocity. The accelerated air is then pulled into the jet thus expanding it. This process is momentum controlled and continues until the velocity of the jet is the same as that of its surroundings. The greater the momentum of the jet, the more rapidly the surrounding secondary air is entrained into the fuel. If the jet has momentum in excess of that required for the complete entrainment of the secondary air, then recirculation will occur. A moderate degree of recirculation is a positive indication that fuel/air mixing is complete, while its absence is a clear indication that not all of the secondary air has been entrained into the fuel jet up to the point at which the fuel jet impinges on the kiln refractory wall. In the latter case, the production of significant levels of carbon monoxide is normal, as hot reducing gases will then be in direct contact with the coating and refractory, tending to "wash" away the coating and causing subsequent brick failure. The recirculating gases from a high momentum flame, however, provide a 'cushion' of cooler neutral gases which prevents this direct impingement of the flame on the coating and refractory. Since the secondary air must be entrained into the primary air and fuel jet, the secondary air flow patterns and temperature have a huge effect on the fuel/air mixing. The aerodynamics are determined by the design of the cooler and secondary air inlet system (hood). As a result the design of these appurtenances significantly affects the combustion in the kiln. Any effective modeling of the combustion process must take these factors fully into account. 6 |
