Abstract
The primary function of a cement clinker cooler must always be the adequate cooling of the clinker product; however from the point of view of fuel economy, such coolers are also designed to utilise the sensible heat of the clinker to preheat the secondary air necessary for combustion. In modern, large capacity plant, the efficiency of heat recuperation has assumed a growing importance because of two related economic factors. The first is cutting fuel costs via the well understood benefits of heat recuperation; the second factor is the gradual improvement in fuel economy of cement kilns in general. As kilns become more finely tuned, the air required for combustion is reduced in direct proportion to the fuel usage, i.e. less air is available to cool a unit quantity of clinker. Hence, in order to avoid the situation where more air is supplied to the cooler than is required for complete combustion of the fuel, more efficient heat recuperation is needed. The Fuels and Energy Research Group at the University of Surrey (FERGUS), first became aware of these kiln/cooler problems in the late 1960's, during the course of their pioneer work on the aerodynamics of rotary cement kilns. Consequently during the last decade, FERGUS has actively investigated the limitations of grate cooler systems, in general. In the initial stages of the research, a perspex water model was constructed at the University, based on some of the largest kilns in the world, those at. Northfleet Works of Blue Circle Industries. These coolers were experiencing "air sliding" of the clinker bed, causing red hot clinker to be dumped onto the rubber belt conveyors. Using suitable flow tracing media under controlled, scaled flow conditions, the aerodynamics of the Fuller grate cooler were studied, thus revealing that the air flow balance through the chambers at the hotter end of the cooler was more significant in its effect on the clinker bed than the balance at the colder end. A solution to this problem could then be achieved by using a three step grate. Water modelling, although giving quick visual solutions to aerodynamics problems, is only qualitative. In order to quantify results, various models of rotary kiln cooler systems have been constructed, in particular a 1/24th scale model of the No. 2 kiln of Rugby Cement's South Ferriby Works. This kiln had for some time suffered from excessive wear rates in the burning zone refractory lining, an expensive problem in terms of down-time and rebricking. Air model pitot static measurements indicated that the air jet (representing the fuel/primary air stream) was exhibiting a peculiar "corkscrewing" effect, which could possibly be the cause of impingement on the refractory lining, leading to premature failure of the refractory coating. Correct kiln firing practice is to promote recirculation of combustion products; these gases, being cooler than the flame, protect the brickwork from overheating. It was already understood that the primary cause of flame impingement was a lack of recirculation due to insufficient burner momentum flux, the solution being a costly change in the firing system. However, the air model results were later verified by full scale plant trials and a solution was proposed to eliminate the "corkscrewing" effect by means of a wedge- shaped bluff body welded along the jet axis. This study has quantified the macro aerodynamic effects of grate cooler systems, through the media of air and water modelling. Considerable information has also been obtained on the dynamic behaviour and heat transfer mechanisms of a particular polysize bed of clinker particles. From this information, putative predictions can be made regarding the general behaviour of clinker in beds. Recommendations have been made for further studies in this area. (Abstract shortened by ProQuest.).