F. Holzleithner’s research while affiliated with TU Wien and other places

An active fluidization thermal energy storage (TES) called “sandTES” is presented. System design, the fundamental features and challenges of fluidization stability such as mass flux uniformity, powder transport and heat transfer, as well as auxiliary power minimization are thoroughly discussed. The tools and methods for evaluating or simulating the behavior of the fluidized bed heat exchanger (HEX) and the dense particle flow within it are explained along with criteria for the selection of storage powders.

Reliable prediction of char conversion, heat release, and particle temperature during heterogeneous char oxidation relies upon quantitative calculation of the CO2/CO production ratio. This ratio depends strongly on the surface temperature, but also on the local partial pressure of oxygen and thus becomes more impor-tant in simulations of oxy-fuel or pressurized combustion systems. Existing semi-empirical intrinsic kinetic models of char combustion have been calibrated against the temperature-dependence of the CO2/CO pro-duction ratio, but have neglected the effect of the local oxygen concentration. In this study we employ steady-state analysis to demonstrate the limitations of the existing 3-step semi-global kinetics models and to show the necessity of using a 5-step model to adequately capture the temperature- and oxygen-dependence of the CO2/CO production ratio. A suitable 5-step heterogeneous reaction mechanism is devel-oped and its rate parameters fit to match CO2/CO production data, global reaction orders, and activation energies reported in the literature. The model predictions are interrogated for a broad range of conditions characteristic of pressurized, oxy-fuel, and conventional high-temperature char combustion, for which essentially no experimental information on the CO2/CO production ratio is available. The results suggest that the CO2/CO production ratio may be considerably lower than that estimated with existing power-law correlations for oxygen partial pressures less than 10 kPa and surface temperatures higher than 1600 K. To assist with implementation of the mechanistic CO2/CO production ratio results, an analytical procedure for calculating the CO2/CO production ratio is presented.

The COREX®-process is the first commercially operating smelting reduction process for the production of pig iron. For this reason, it can be seen as an alternative to the common industrial iron-making route via the blast furnace. For both, the COREX® melter-gasifier (MG) and the blast furnace, good gas-solid contact is essential for the stable and efficient operation of the process. Due to the very harsh conditions inside the MG, the possibilities for measurement of process conditions are very limited. Thus, detailed numerical models are required to gain additional understanding of the physical and chemical processes occurring within the MG.

A review of the experimental investigations of the CO2/CO production ratio during the high temperature oxidation of carbon reveals a wide variation in this critical parameter for determining the char combustion temperature and burning rate. Of the studies that have been performed, the experiment used by Tognotti et al. [5], with small, laser-heated, electrodynamically levitated Spherocarb particles in cool surroundings, appears to be the most promising for giving accurate results. Proper interpretation of the results from Tognotti’s study requires assumptions of kinetically controlled combustion behavior, negligible CO conversion either within the particle pores or in the boundary layer, and a uniform particle temperature. To evaluate whether the Tognotti data in fact fulfill these assumptions, we have employed a detailed model of porous particle combustion to simulate the Tognotti experiments. The model results indicate that particle temperatures were uniform and there was negligible oxidation of CO either within the particle or in the particle boundary layer over the range of particle temperatures that was used to determine the Tognotti CO2/CO production ratio correlations. On the other hand, the model results show that O2 diffusional resistance became important for temperatures greater than 1050 K in the Tognotti experiments. However, because of the low sensitivity of the observed CO2/CO production ratio to the local oxygen concentration, computational analysis also shows that the influence of this Zone II combustion behavior on the measured CO2/CO production ratio is quite minor. Therefore, it appears that the empirical correlation derived by Tognotti et al. [5] to describe the CO2/CO production ratio during high temperature char oxidation is credible, though its temperature range of empirical validation is limited to less than 1250 K.

Active Fluidized Bed Thermal Energy Storage (sandTES) offers a promising alternative to the current state of the art thermal energy storages (TES), such as active TES based on molten salt or passive TES (Regenerators) realised as a porous packing of ceramics. The characteristic of a sandTES system applying sand in an active TES using a fluidized bed heat exchanger (HEX) is explained. The exergetic performance of a sandTES is compared to a passive Regenerator.

Several semi-global intrinsic char kinetics models have been proposed to describe hightemperature char combustion phenomena with greater detail than is captured in traditional apparent kinetics models, typically formulated as power laws. A critical component of such models is prediction of the CO2/CO production ratio, which has a strong impact on both the combustion heat release and the consumption rate of carbon. Some existing intrinsic char kinetics models have been calibrated to describe the temperature-dependence of the CO2/CO production ratio. However, experimental studies have consistently demonstrated an oxygen-dependence of the CO2/CO production ratio, and none of the existing intrinsic char kinetics models have been validated against this oxygen-dependence. The most extensive previous study of the CO2/CO production ratio was that of Tognotti et al., whose data interpretation relied on the assumption of kinetic-limited char oxidation and negligible conversion of CO to CO2 in the particle boundary layer. Our simulation of the Tognotti experiments, using a detailed model of the porous char particle and the surrounding boundary layer, has confirmed the validity of these assumptions. In addition, we have employed steady-state analysis to demonstrate the limitations of the existing 3- step models and to assess more sophisticated models to adequately capture both the temperatureand oxygen-dependence of the CO2/CO production ratio. A suitable 5-step heterogeneous reaction mechanism has been developed and its rate parameters fit to match CO2/CO production data, global reaction orders, and activation energies reported in the literature. Copyright

Good gas-solid contact is essential in a coal bed gasifier such as in a COREX® melter gasifier. The charged particle size distribution and the particle fragmentation behavior inside the slowly moving fixed bed strongly influence the local counter current gas flow and therefore also the rate of heat transfer between gas and coal, the rate of drying, devolatilisation and gasification. COREX® is the first commercially operating smelting reduction process, based on coal instead of coke, as alternative for industrial ironmaking route via the blast furnace. Besides coal the melter gasifier also contains reduced iron ore and additives, which are not subject of this paper. General multiphase models in commercial CFD codes are not directly applicable to the simulation of moving reactive beds considering changes in particle size distribution. A customized approach based on a combination of Eulerian and Lagrangian formulation is used to describe the flow of gas and solids as well as the physical and chemical processes across the moving bed reactor. The solids flow and the gas flow are represented by a set of Eulerian equations. So the flow of solids respectively the flow of a granular material is treated as a continuum with appropriate material properties. The balances for solids and gas flow are interconnected via source terms. The energy balance of the solids flow and the models for fragmentation and devolatilization are implemented by means of a Lagrangian formulation. The solids flow consists of a set of particle sizes including dust. This set of particle sizes changes according to local process parameters which are: solids pressure, shear stress, rate of water evaporation (coal drying), rate of devolatilization, rate of gasification and rate of temperature change. To take this into account a fragmentation model has been developed which solves a conservation equation for each particle size. The local source terms within these equations are connected to the above mentioned local process parameters. Due to the fact that the considered moving bed consists of nonuniformly sized particles the temperature of small particles will be different from the temperature of larger particles. However, the temperature of the particles is important for the rate of drying, devolatilization and gasification. Therefore an energy balance for each particle size is implemented within the presented model. In a first step the model has been used to study the impact of a changing particle size distribution on the gas flow and heat transfer between gas und solids. The effect of fragmentation on the devolatilization process has been simulated too. Next development steps are the integration of models for coal drying and gasification as well as a gas phase reaction model.