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The cement industry aims to use an increased amount of alternative fuels to reduce production costs and CO 2 emissions. In this study three cement plants firing different kinds and percentages of alternative fuel were studied. A specially developed camera setup was used to monitor the flames in the three cement kilns and assess the effect of alternative fuels on the flame. It was found that co-firing with solid recovered fuel (SRF) would delay the ignition point by about 2 meters and lower the intensity and temperature of the kiln flame compared to a fossil fuel flame. This is related to a larger particle size and moisture content of the alternative fuels, which lowers the conversion rate compared to fossil fuels. The consequences can be a lower kiln temperature and cement quality. The longer conversion time may also lead to the possibility of localized reducing conditions in the cement kiln, which can have a negative impact on the clinker quality and process stability. The burner design may alleviate some of the issues encountered with SRF co-firing. At one of the test plants the burner was changed from a design with an annular channel for axial air to a jet design. This proved to be beneficial for an early ignition and improved dispersion of the fuel and led to an increase in cement quality and higher use of SRF.

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The oxidation of pyrite (FeS 2 ) has been investigated in a fixed-bed laboratory reactor to gain knowledge about the SO 2 formation mechanisms and kinetics at conditions relevant to the upper stages of a cyclone preheater tower in a modern dry kiln system for cement production. Experiments were carried out with a high sulfide containing shale (a mass-average diameter equal to 21 µm) and with pure FeS 2 particles (between 32 and 64 µm). Measurable SO 2 formation started at about 350 °C for the shale and at 400 °C for the FeS 2 particles and increased with temperature for both materials. Experiments showed that the conversion of pyritic sulfide to SO 2 was independent of the inlet SO 2 concentration up to at least 925 ppmv and that the conversion decreased as the O 2 concentration was increased from 5% to 20% (v/v). A shrinkingcore reaction mechanism with FeS 2 as the core and with porous FeS as the intermediate product layer is proposed to account for the experimental observations. According to this mechanism, the oxidation of the FeS 2 core to FeS is relatively fast at moderate O 2 levels (e.g., 5%) because it is easy for O 2 and SO 2 to diffuse through the porous product layer of almost non-oxidized FeS. At increased O 2 levels (e.g., 20%), we believe that the FeS layer also starts to oxidize so its porous structure is disrupted whereby the resistance to diffusion through it increases and thus further oxidation of the FeS 2 core is inhibited, leading to lower overall conversions.