Cement and Energy

.:.... EFFICIENCY AND RELIABILITY . I n a cement kiln grate cooler there is a basic conflict between efticiency considerations and reliability considerations. Efficiency of a grate cooler involves two functions : (i) recovery of heat from the dinker entering the cooler, and (ii) cooling of the clinker. In essence, maximising these efficiency considerations amounts to increasi1 .h the residence time of the clinker in the cooler. The cooler also fulfils the key role of transporting the clinker away from the kiln. Reliability of the complete kiln-cooler operation is maximised by transporting the clinker away as quickly as possible, which inevitably involves shortening the residence time of the clinker in tf1e cooler. This conflict is represented graphically in Figure J. It is through assisting the kiln operator in the resolution of this basic conflict that a Spyrometer installed in the grate cooler can be of significant benefit. Efficiency Figure I Efficiency: Reliability conflict solution. Conventional Grate Cooler Control The efficiency/reliabi lity conflict described above is well known in the cement industry and a standard process control response has evolved to solve the problem. Cooling air is blown into the chambers below the clinker g rate, from where it exits through the grate and the bed of clinker while being moved along the grate. This is the means by which the clinker is cooled and the thermal energy is recovered from the clinker. The pressure under the grate of the cooler is monitored and is taken to be directly proportional to the thickness of the bed of clinker on the grate. When the load of clinker entering the cooler increases, the bed lhickens and the pressure under the grate rises. The control response is then to increase the speed of the grate to transport the additional clinker away from the ki ln. The reverse process takes place when the amount of clinker entering the cooler lessens. The clinker bed thins out and the pressure under the grate falls, with the control response being to slow the grate down and retain the clinker on the grate for longer in order to build up the bed depth. Kiln operators are intuitively aware that there are occasions when the control response is inappropriate, and in these situations have to switch the grate speed into manual control. An automatic control loop which does not always make the correct response, and needs to be monitored, can become a hindrance to the kiln operator. When this is a common occurrence the loop can fall into disuse, as there is a frequent need to switch to manual control. The kiln operators then naturally ope.J:ate cautiously and run the cooler with a thin bed to ensure kiln and cooler keep running, thereby satisfying the reliability criteria of the operation. Thermal efficiency of the kiln/cooler system inevitably suffers. Solving the efficiency/reliability conflict remains a problem whether the grate speed is controlled via the automatic loop or manually by the operator. In both cases inferences of the true situation in the cooler need to be 7