Bypassing challenges

Many bypass systems have been installed in cement plants worldwide during the past two decades, often triggered by the local raw material situation and also by an increased utilisation of alternative fuels or raw materials. With the cement industry’s experience in finding new solutions to optimise its processes, the main technical questions such as removal efficiency, gas and dust treatment have been solved satisfactorily.

The introduction of preheater kilns in the cement industry has certainly contributed to significant energy savings in clinker production. However, compared to long wet or long dry kiln processing this fundamental change of technology and production equipment had strong effects on various process parameters, especially on the formation of internal cycles. As soon as the solidified salts of these chlorine, sulphur and alkali compounds, which are highly volatile, reach the sintering zone, they partially evaporate and are transported with the combustion gases back to the kiln inlet. At the temperature level which prevails in the calciner and lower cyclone stages of the preheater, the condensation points of the vaporised salts are reached so that liquid and solid phases start to form. Depending on the overall intake of these components into the kiln, build-ups can develop (see Figure 1) which can have significant and disturbing impacts on the production process, even causing unplanned kiln stops.

Nowadays, as well as in the past, the major input streams of chlorine and sulphur are geogenic and originate from the raw materials used. In most cases, the raw material limestone represents the main source of the chlorine, alkali and sulphur input into the kiln system. In addition, some of the alternative fuels, eg RDF, animal meal or sewage sludge, can also contribute to the chloride intake. Typical concentrations are shown in Table 1.

The situation regarding sulphur is different. Its content in various alternative fuels is often lower than, for example, in coal or petcoke. Depending on the specific situation in a kiln, a change in fuel usage can in some cases enhance the inner cycle formation and can therefore put additional stress on the kiln system.
While a change in raw materials and fuels is in most cases not feasible, and for economic reasons, bypass systems have been developed worldwide as a state-of-the-art technology to decrease the chlorine load in the kiln systems. From its early stages onwards this measure has allowed considerably higher plant availability. Meanwhile, bypass systems have been significantly further developed with respect to optimum energy and removal efficiencies as well as emission levels.