Cement Energy and Environment

Such high levels of recirculating sulphate often result in the clinker becoming dusty with low particle size grading as the viscosity of the flux in the kiln is reduced. The technical manager was able to confirm that the kilns had indeed become increasingly dusty and that the problems were worse on the kiln with no alkali bypass installed. This provided corroborative evidence that the problem of the high free lime clinker was related to recirculating volatile species. But what could be causing the very high sulphate recirculation? This is where the 1/20 2 j term on the right hand side of the equation for the breakdown of calcium sulphate is important. This is an equilibrium reaction and if there is a deficiency of oxygen then the reaction is driven to the right, ie promoting the breakdown of calcium sulphate into free lime, s~lphur dioxide and oxygen. Was there any deficiency of oxygen in the kiln? The technical manager reported that the kiln inlet oxygen was around eight per cent, so apparently there was no oxygen deficiency. However, there could still be local reducing conditions due to coarse grinding of the coal or poor mixing of the coal and combustion air in the burner. Coarse ground coal wou ld fall from the flame into the charge of clinker, creating local reducing conditions. Poor mixing of the coal and combustion air would arise from insufficient momentum of the primary air delivered through the main burner of the kiln , where a minimum of 1ON/MW of energy delivered by the burner is required to obtain good mixing of the coal and combustion air. Strong suspicions were that the kiln burner was the underlying culprit. These suspicions ultimately proved to be correct. The burner in question was a burner where the momentum is controlled by the MAS setting of the burner. In the belief that refractory life would be extended the MAS number had been set much lower than recommended. The result was insufficient momentum of the primary air, poor mixing of the coal and combustion air, in turn leading the heavy sulphur recirculation, the presence of calcium sulphate in the clinker and the breakdown of that calcium sulphate too late in the process for the free CaO liberated to be combined into the clinker. Increasing the MAS number of the burner to the recommended level solved the problem! The Technical Forum and the moderator are always pleased when we can assist cement companies to resolve any technical issues they are facing . Subsequent correspondence and enquiries have revealed that problems similar to that being experienced by the technical manager on his Middle Eastern cement factory are by no means rare. Lack of oxygen In the first instance, a cement factory high in the Andes was experiencing problems of low clinker output and the formation of large clinker balls in the kiln. There had been no change of fuels (the kiln was fired with natural gas) or change of the burner. However, the problem here was the difficulty of aspirating the kiln to provide sufficient oxygen for the combustion of the natural gas, combined with the variable sulphur content of the raw materials. The density of air falls with increasing altitude, making it increasingly difficult to draw sufficient combustion air and oxygen for burning the fuels into the kiln. The 1/20 2 j term on the right hand side of the equation for the breakdown of calcium sulphate again becomes important. General lack of oxygen in the kiln means heavy sulphate recirculation. This adds to the flux in the kiln and can lead to the formation of large clinker balls. (It can also lead to the formation of rings in the back of a kiln fitted with preheater, but that problem was not encountered in this case). The solution to this issue will be much more intractable than simply increasing the momentum of the main burner. More air must be drawn into the kiln, or the oxygen content of that air will need to be increased, perhaps by oxygen enrichment. That would be an expansive solution. As a second example, a cement factory in West Africa was suffering from excess ring formation and dust return from "The chemistry doesn 't change dependent on where the cement kiln is operating. Knowledge of cement kiln chemist1y is critical whether you are operating in Western Europe, the Middle East, Southeast Asia, Latin America, West Africa or anywhere else for that maller." the cooler. This was coupled with free-lime swinging from 2- 13 per cent. Attempts were made to adjust the flame and a massive build-up was found in 5