This paper presents CFD simulations of an industrial scale rotary kiln for cement clinker production. The fuel for the kiln flame is a mixture of pulverized coal and a Refuse Derived Fuel (RDF). Advanced models were developed to appropriately describe the thermal conversion characteristics and aerodynamics of non-spherical RDF particles. The models are based on detailed fuel analyses (e.g. flight and combustion characteristics, physical and chemical fuel properties) of major RDF fractions, like plastic foils, 3D plastic particles, paper & cardboard and textiles. The processes in the clinker within the kiln are approximated using a simplified one-dimensional model that calculates heat and mass exchange with the gas phase and the resulting chemical-mineralogical reactions in the solid bed. Calculation results of the one-dimensional model are compared to measurements obtained from a semi-industrial laboratory rotary kiln. Two cases, one with 100% lignite and one where 50% of the fuel heat input is substituted with RDF, are simulated. Based on the simulation results, the shift of flame shape and fuel conversion as well as the resulting effects on the clinker phase transition are analyzed and discussed. Results show that co-combustion of RDF can lead to lower gas and clinker temperatures in the sintering zone, which can affect the clinker properties.

The current paper presents a simplified approach which allows the CFD simulation of Refuse Derived Fuel (RDF) combustion. The starting point is the subdivision of a real RDF into characteristic fuel fractions by sorting. Each of the fractions was analyzed concerning elemental composition, heating value, proximate analysis as well as size and shape. The flight behavior of the RDF fractions has been characterized in a drop-shaft. A stereoscopic camera system was used to derive drag and lift coefficients. In addition, a single particle combustion reactor has been used to measure the duration of the relevant combustion phases like volatile combustion or char burn-out.A model calculating the particle trajectories based on the measured drag and lift frequency distributions has been developed. For combustion modelling the RDF has been subdivided into devolatilizing and char forming fractions and into fractions which are converting through a melting and decomposition process. For both types of materials combustion models have been formulated. Intra particle temperature gradients are accounted for. A change of particle shape during combustion is considered using sphericity as a model parameter. The models have finally been introduced into FLUENT by user defined functions.Comparison with drop-shaft measurements and a single particle combustion reactor show that the models forinulated can statistically describe the motion and conversion behaviour of RDF with sufficient accuracy. As an example of application, the models were finally used for the CFD simulation of the furnaces of a 612 MW(e) RDF co-fired coal power plant. The results indicate an overall slower reaction rate of RDF compared to coal, resulting in a total conversion of RDF of 83%. (C) 2017 Elsevier Ltd. All rights reserved.

Cement production in rotary kilns requires large amounts of thermal energy, which is provided by combustion of different fuels. Substitution of fossil fuels by refuse derived fuels (RDF) can minimize production costs and reduce CO2 emissions, but often causes displacement of the sintering zone, impacts flame stability and cement quality. The current paper briefly introduces our numerical approach which describes particle motion and combustion characteristics of typical non-spherical RDF particles. By using these models in CFD simulations, a case study is presented. Fuel properties, primary- and secondary air settings and fuel feed location for a generic rotary kiln of industrial scale are varied to show the effects of operational settings on co-firing of RDF. Shift of flame shape and location as well as particle burnout are analyzed. Based on the information generated, optimized operational settings are identified and discussed.

This work presents DEM-CFD simulations of the transient processes occurring in an industrial scale PFRkiln. DEM allows the numerical simulation of the moving and reacting limestone bed in the kiln and is coupled with a 3-dimensional CFD simulation describing the interstitial gas phase.A PFR-kiln consists of two vertical shafts and a connecting crossover channel. The two shafts periodically switch their function at regular intervals of about 15 min. While one shaft calcines the product in parallel flow with gas temperatures above 900 degrees C, the other preheats the stones in counter flow.The model has been applied to an industrial PFR-kiln of 18 m height. A realistic particle size distribution of the limestone with particles ranging from 50 to 90 mm has been set. Methane combustion provides the heat for calcination and is simulated by a two-step mechanism. Simulation results show a nearly uniform temperature distribution in the calcination zone but significant inner particle temperature gradients. The calcination degree depends on the particle location within the kiln and decreases towards the outer kiln walls. Measured and simulated temperatures are compared. Maximum temperature values as well as its characteristic oscillation induced by the periodic kiln operation could be reproduced by the simulation, especially keeping in mind the difficulties of thermocouple measurements under the harsh conditions in industrial reality. (C) 2017 Elsevier Masson SAS. All rights reserved.

In lime shaft kilns the limestone is heated in counterflow with the flue gas from multiple burners, commonly fired by fossil or alternative fuels. After preheating, the limestone enters the calcination zone with gas temperatures above approximately 900°C. To achieve high thermal efficiencies and calcination degrees, a homogeneous gas flow through the packed bed is essential. Spatial pressure differences and energy sinks and sources resulting from the calcination process and the fuel combustion make it even more difficult to predict the actual three-dimensional temperature and flow distribution in the kiln. Furthermore the particle size distribution is important for the system design, because limestone with larger size needs significantly higher residence time in the calcination zone for full conversion.