Sensitivity analysis of a model for direct reduction in swelling coal char-hematite composite pellets

This paper describes a study of the optimisation of the modelling of the direct reduction process in swelling coal char-hematite composite pellets. Approximations of important physical parameters such as heats of reaction, specific heat and thermal conductivity of the reducing mixture have been developed. Without introducing significant errors, the computation time can be halved. The effect of the determination of pellet size and of the activation energies of the reducing reactions on the modelling results has also been investigated

Thermal Decomposition Kinetics of Woods with an Emphasis on Torrefaction

The pyrolysis kinetics of Norwegian spruce and birch wood was studied to obtain information on the kinetics of torrefaction. Thermogravimetry (TGA) was employed with nine different heating programs, including linear, stepwise, modulated and constant reaction rate (CRR) experiments. The 18 experiments on the 2 feedstocks were evaluated simultaneously via the method of least-squares. Part of the kinetic parameters could be assumed common for both woods without a considerable worsening of the fit quality. This process results in better defined parameters and emphasizes the similarities between the woods. Three pseudo-components were assumed. Two of them were described by distributed activation energy models (DAEMs), while the decomposition of the cellulose pseudo-component was described by a self-accelerating kinetics. In another approach, the three pseudo-components were described by n-order reactions. Both approaches resulted in nearly the same fit quality, but the physical meaning of the model, based on three n-order reactions, was found to be problematic. The reliability of the models was tested by checking how well the experiments with higher heating rates can be described by the kinetic parameters obtained from the evaluation of a narrower subset of 10 experiments with slower heating. A table of data was calculated that may provide guidance about the extent of devolatilization at various temperature residence time values during wood torrefaction

Kinetic Behavior of Torrefied Biomass in an Oxidative Environment

The combustion of four torrefied wood samples and their feedstocks (birch and spruce) was studied at slow heating programs, under well-defined conditions by thermogravimetry (TGA). Particularly low sample masses were employed to avoid the self-heating of the samples due to the huge reaction heat of the combustion. Linear, modulated and constant-reaction rate (CRR) temperature programs were employed in the TGA experiments in gas flows of 5 and 20% O2. In this way the kinetics was based on a wide range of experimental conditions. The ratio of the highest and lowest peak maxima was around 50 in the experiments used for the kinetic evaluation. A recent kinetic model of Várhegyi et al. [Energy & Fuels 2012, 26, 1323-1335] was employed with modifications. This model consists of two devolatilization reactions and a successive char burn-off reaction. The cellulose decomposition in the presence of oxygen has a self-accelerating (autocatalytic) kinetics. The decomposition of the non-cellulosic parts of the biomass was described by a distributed activation model. The char burn-off was approximated by power-law (n-order) kinetics. Each of these reactions has its own dependence on the oxygen concentration that was expressed by power-law kinetics, too. The complexity of the applied model reflects the complexity of the studied materials. The model contained 15 unknown parameters for a given biomass. Part of these parameters could be assumed common for the six samples without a substantial worsening of the fit quality. This approach increased the average experimental information for an unknown parameter by a factor of 2 and revealed the similarities in the behavior of the different samples

Modelling the Reduction of an Iron Ore-Coal Composite Pellet with Conduction and Convection in an Axisymmetric Temperature Field

A mathematical model of the coal-based direct reduction process of iron ore in a pellet composed of coal and iron ore mixture is investigated using a finite-control volume method. Heat transfer by conduction in the solid, convection by gaseous media inside, and radiation from the surroundings of the pellet are included in the model. The pellet is assumed to be spherical initially and the temperature around the pellet is taken to be symmetric about an axis passing through the centre. The parameters of the process, such as thermal conductivity, specific heats, and heats of the reaction, are all temperature dependent. The shrinkage/swelling of the pellet is also considered. We find that the effect of convection on the temperature and on the overall average reduction is small. However, the effect on the local concentration of the reaction components is significant. We predict that a uniform surrounding temperature field around the pellet yields a better average reduction

Optimization of coal pyrolysis modeling

Coal pyrolysis is complex, involving a large number of chemical reactions. The most accurate and up-to-date approach to modeling coal pyrolysis is to adopt the Distributed Activation Energy Model (DAEM), also called the Multiple Reaction Model (MRM). This can be very computationally expensive, since it involves a complicated multiple integration. A novel method of optimizing the mathematical modeling based on the DAEM has been developed. It has been shown that for a given accuracy, the method involves significantly less computational time than standard methods. Another advantage of the new method is that it allows errors to be estimated. Copyright (C) 2000 The Combustion Institute

Approximate modelling of coal pyrolysis

A new approach to the modelling of the evolution of different chemical species from coal during its thermal decomposition under variable heating rate regimes was developed. The approach, which can be based on experimental data or the multiple-reaction model (MRM) with distributed activation energies, uses an Nth order reaction model where the pre-exponential factor and the apparent activation energy are functions of the rate of heating. A comparison of the predictions of the new model with the MRM was carried out. The new approach significantly cuts down the computation time with almost no loss of accuracy. The functional group pyrolysis model was examined and the parameters for the new Nth order approximation obtained considerably simplifying pyrolysis modelling. In addition, the article shows how to obtain estimates for certain parameters, which characterize chemical processes modelled by the MRM