The substitution of alternative for fossil fuels in cement production has increased significantly in the last decade. Of these new alternative fuels, solid state fuels presently account for the largest part, and in particular, meat and bone meal, plastics and tyre derived fuels (TDF) accounted for the most significant alternative fuel energy contributors in the German cement industry. Solid alternative fuels are typically high in volatile content and they may differ significantly in physical and chemical properties compared to traditional solid fossil fuels. From the process point of view, considering a modern kiln system for cement production, the use of alternative fuels mainly influences 1) kiln process stability (may accelerate build up of blockages preventing gas and/or solids flow), 2) cement clinker quality, 3) emissions, and 4) decreased production capacity. Kiln process stability in particular is influenced by insufficient carbon burnout in the calciner system, which results in reducing conditions in the material inlet of the rotary kiln and consequently an increased tendency to form deposits induced by sticky eutectic melts. Clinker quality is mainly affected by minor components from the fuel ashes or from carbon dropping into the material charge of the rotary kiln. As regards the presently most used solid alternative fuels, phosphorous from meat and bone meal or zinc from TDF are the main components to consider with respect to clinker chemistry. The emissions seem not to have been affected by the alternative fuels used up until now. However, caution should be taken with regard to emissions of CO when using alternative fuels. As alternative solid fuels are typically high in volatile content, the devolatilization stage in the combustion process is responsible for a large part of the fuel heating value. In addition, the devolatilization time of alternative fuels cannot be neglected in kiln system process analyses, as these fuels are typically in the cm-size with devolatilization times in the order of minutes. The devolatilization characteristics of large particles of tyre rubber were investigated in two experimental setups with the emphasis being on devolatilization rates and times, and the results were analysed using mathematical modelling. During devolatilization, the large TDF particles formed a crackling char layer, which was seen to be removed depending on whether external mechanical interaction was present or not. Both pathways were investigated experimentally and a significantly shorter devolatilization time was observed in the situation where the char layer was removed. In addition, the experiments showed a significant effect of particle size on devolatilization time, where increased particle size increased the devolatilization time. Model analyses demonstrated that the overall devolatilization kinetics of large particles of tyre rubber is mainly controlled by heat transfer and intrinsic pyrolysis kinetics, whereas mass transfer has negligible influence. The models developed are used to predict devolatilization conversion times for tyre rubber as a function of relevant parameters. The devolatilization rates of other alternative fuels are also expected to be controlled by conversion pathway, heat transfer and intrinsic kinetics. The char combustion stage has a decisive influence on the fuel carbon burnout in cement kiln systems. The oxidation kinetics of a char from TDF was investigated experimentally and by mathematical modelling. Experiments were performed in a fixed bed reactor under well - iii - defined conditions, where small particles (102-212μm) of TDF were combusted at 750-850°C at up to 10 vol.% O2. The effluent of the reactor was analysed for CO and CO2, and used to derive conversion against time. The experimental data demonstrated that mass transfer was important within the investigated temperature range of 750-850°C, and a mathematical model for intra-particle diffusion and reaction was developed in order to analyse the data. A reaction expression for the intrinsic kinetics of TDF char oxidation was proposed and comparison with literature data showed fair agreement. For larger TDF char particles with realistic sizes up to 7 mm, it was demonstrated that they are converted according to a shrinking particle mechanism, and based on this observation a model was developed in order to explain the controlling factors for TDF char combustion under conditions relevant to cement kiln systems. It was demonstrated that external mass transfer was the rate limiting parameter, as the kinetics are sufficiently faster than external mass transfer. The intrinsic kinetics of other typical alternative fuels is demonstrated to be comparable or faster than intrinsic TDF char oxidation kinetics. Consequently, the char oxidation stage for large particles of other alternative fuels is also expected to be controlled by mass transfer, under conditions relevant to cement kiln systems. A comparison between the mechanisms behind the devolatilization and char combustion stages for large alternative solid fuel particles indicates that the devolatilization kinetics are mainly controlled by heat transfer and intrinsic kinetics, whereas char oxidation kinetics are mainly controlled by mass transfer, under conditions relevant to cement kiln systems. Measurements at two industrial HOTDISC’s using TDF indicate that up to about 75% fuel conversion takes place before discharge into the subsequent calciner. Model analyses of the measurements, using the previously developed sub-models for devolatilization and char oxidation rates, explain that devolatilization takes place in the HOTDISC whereas char oxidation takes place both in the HOTDISC and in the calciner system.
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Jensen, Lars Skaarup, Frandsen, Flemming, Glarborg, Peter, Dam-Johansen, Kim
Technical University of Denmark, Department of Chemical and Biochemical Engineering, 2007