Burning cement clinker under reducing conditions in a rotary kiln -: one way of lowering the NOx emissions?

It has been a long held tradition in the cement industry that Portland cement clinker which is produced from rate! meal with CaCO3, as the main constituent should be burnt in an oxidizing atmosphere. It is known both from laboratory investigations and from the operation of cement kilns that the clinker quality deteriorates on changing to a sub-stoichiometric oxygen supply at the kiln flame and in the sintering rone. The reduction of Fe2O3 in the calcium aluminoferrite clinker mineral was recognized as the cause of the reduction in quality. The formation of dicalcium silicate can also be impeded in a CO atmosphere at temperatures up to 1100 degrees C. One disadvantage of the oxidizing process regime is that nitrogen oxides are produced at the high temperatures in the sintering zone of up to 2000 degrees C and in the presence of oxygen, and then have to be reduced to nitrogen with ammonia or carbon monoxide in the exhaust gas from the rotary kiln in order to satisfy environmental protection requirements. On the Ether hand, the gypsum sulphuric acid process, which produces cement clinker and SO2-containing kiln gases from raw meal with CaSO4, as the main constituent, can provide a clinker which, in spite of the reducing combustion conditions due to the nature of the process, is virtually identical in chemical and mineralogical composition with the clinker produced from CaCO3 in an oxidizing atmosphere, and also provides equivalent cement qualities. In the exhaust gas from the cement kilns using the gypsum sulphuric acid process the specific NO,emissions are only 150 mg/m(3) at an oxygen content around 0.5 vol. %. This article explains the chemical and technical aspects of clinker production in the gypsum sulphuric acid process, and also describes and comments on the current understanding of burning cement clinker under reducing conditions with the aid of selected information from the literature. In order to reduce the NO, emissions during clinker burning it is recommended that use should be made of the experience gained with the gypsum sulphuric acid process and that the application of the reducing mode of operation should be tested in, the classical cement production process.