Cement production is an energy-intensive process, which traditionally has been dependent on fossil fuels. However, the use of alternative fuels, i.e., selected waste, biomass and by-products with recoverable calorific value, is constantly increasing. Combustion of these fuels is more challenging compared to fossil fuels, due to lack of experience and different chemical and physical properties. When complete oxidation of fuels in the calciner and main burner is not achieved, they burn in direct contact with the bed material of the rotary kiln, causing local reducing conditions and increasing the internal circulation of S, Cl, Na, and K. Compounds containing these elements, such as alkali salts, evaporate when exposed to high temperatures and subsequently condense in colder parts of the plant. The transformation of the volatile inorganic species at different locations in the cement plant is important, because a high internal circulation affects the process stability and operation through formation of build-ups and blockages, ring formation, and shell corrosion, resulting in reduced clinker production, higher heat consumption, and kiln or plant stops. The present review describes the internal cycles of inorganic elements that are established within the cement plant and their dependence on process parameters. Special focus is given to the sulfur cycle. This cycle is intensified by CaSO 4 decomposition, making it sensitive to local reducing conditions.

:The combustion of alternative fuels in direct contact with the bed material of the rotary kiln may cause local reducing conditions and subsequently decomposition of sulfates from cement raw materials, increasing the SO 2 concentration in the gas phase. The decomposition of sulfates 2 increases the sulfur circulation and may be problematic because high sulfur circulation can cause sticky material build-ups, affecting the process operation of the cement kiln system. The SO 2 release from cement raw materials during combustion of pine wood and tire rubber has been studied experimentally in a high temperature rotary drum, focusing on the influence of fuel particle size and volatile content. The SO 2 release increased with decreasing fuel particle size and with increasing fuel volatile content. Furthermore, CO, H 2 and CH 4 , which are the main reducing gases released during fuel devolatilization, were introduced in different concentrations under the bed material. A threshold concentration for each reducing gas, below which no SO 2 release occurs, was found. Introduction of the same molar amount of gas in different concentrations during different time periods showed that a higher reducing gas concentration during a short period (representing fuel devolatilization) released a higher total SO 2 amount compared to a lower concentration during a long period (representing fuel char oxidation).