The invention relates to a method for the production of cement clinker from raw cement meal which is preheated in at least one heat exchanger string, through which the exhaust gas from a rotary tubular kiln flows, and is burnt in the sintering zone of the rotary tubular kiln to form cement clinker which is cooled in a following cooler, comprising the removal of a partial hot flow (bypass gas flow) of the rotary kiln exhaust gas, said partial flow being laden with dust loads and/or gaseous/vaporous harmful substances inclined to cause cakings, comprising the cooling of the bypass gas flow in a mixing chamber and comprising the following separation of dust containing harmful substances from the cooled bypass gas flow. The invention relates, moreover, to a plant for carrying out the method.
In the production of cement clinker from raw cement meal, it is known that many batch materials, such as raw cement meals, but also many fossil fuels used, contain secondary constituents, such as, for example, alkali compounds, chlorine, sulfur compounds, heavy metals, etc., which, in the region of the sintering zone of the rotary tubular kiln, evaporate, for example, as alkali chloride compounds and alkali sulfate compounds, condense/crystallize again in the preheater region of the cement clinker production line and thus build up circulations, with the result that both the quality of the cement clinker may be adversely influenced and the combustion process itself may be disturbed considerably.
To suppress such circulations in a cement clinker production line and to reduce the content of circulation-forming materials in the clinker production process, it is known, for example from the pamphlet “Drehrohrofenanlagen” [“Rotary tubular kiln plants”], No 8-100d of K H D Humboldt Wedag A G, pages 10/11, of May 1984, by means of what is known as bypass gas removal, to branch off part of the hot dust-laden kiln exhaust gases containing the volatile compounds out of the lower region of the rotary kiln exhaust gas riser line or directly out of the rotary kiln entry chamber, to cool them in a mixing chamber by the introduction of external air, to cause the vaporous harmful substances contained in the bypass gas flow to condense on the entrained solid particles and then to clean the cooled bypass gas flow by the separation of the dust containing harmful substances in a specific dust separator. In order to ensure that the volumes of the bypass gas flow that are to be treated and its dedusting devices do not become too large, it is also known not only to mix external air as a cooling medium into the bypass gas flow, having a temperature of, for example, 1150° C., but also to inject water which is intended to assist the shock cooling of the bypass gas flow.
Furthermore, DE-C-27 24 372 discloses a cement clinker production line comprising the removal of a bypass gas flow which is cooled in a mixing chamber, apart from injected water, not in this case by means of fresh air, but by means of a partial flow of the production exhaust gas or system exhaust gas which has already been cleaned in an electric separator. This partial exhaust gas flow already cleaned in the system filter, however, is laden with dust again in the mixing chamber of the bypass gas flow, at least this partial exhaust gas flow then having to be cleaned a second time in the separate bypass gas flow dust filter, so that the known cement clinker production comprising bypass gas removal takes up relatively large filter volumes, along with the associated high investment and operating costs.
In cement clinker production, there are increasing bypass problems, because, in cement clinker production, both Western industrial nations and emerging and developing countries increasingly use chlorine-laden and sulfur-laden waste fuels and residual materials as what are known as secondary fuels. Many operators of cement clinker production lines therefore attempt to lower the circulation level of volatile components (in particular, chlorine and sulfur) to a tolerable level by locking out the system filter dust. However, they shy away from installing a separate bypass system which incurs considerable investment and operating costs for additional dedusting devices, mostly electrostatic dust separators, dust transports and fans. To be precise, as a rule, the mixed bypass gas flows occurring as a result of the supply of large bypass cooling air quantities are so great that they cannot be treated in already existing electrostatic dust separators in addition to the already existing quantities of system exhaust gases.