Oxidation rates of carbon and nitrogen in char residues from solid fuels

Computational fluid dynamics (CFD) modeling is an important tool in designing new combustion systems. By using CFD modeling, entire combustion systems can be modeled and the emissions and the performance can be predicted. CFD modeling can also be used to develop new and better combustion systems from an economical and environmental point of view. In CFD modeling of solid fuel combustion, the combustible fuel is generally treated as single fuel particles. One of the limitations with the CFD modeling concerns the sub-models describing the combustion of single fuel particles. Available models in the scientific literature are in many cases not suitable as submodels for CFD modeling since they depend on a large number of input parameters and are computationally heavy. In this thesis CFD-applicable models are developed for the combustion of single fuel particles. The single particle models can be used to improve the combustion performance in various combustion devices or develop completely new technologies. The investigated fields are oxidation of carbon (C) and nitrogen (N) in char residues from solid fuels. Modeled char-C oxidation rates are compared to experimental oxidation rates for a large number of pulverized solid fuel chars under relevant combustion conditions. The experiments have been performed in an isothermal plug flow reactor operating at 1123-1673 K and 3-15 vol.% O2. In the single particle model, the char oxidation is based on apparent kinetics and depends on three fuel specific parameters: apparent pre-exponential factor, apparent activation energy, and apparent reaction order. The single particle model can be incorporated as a sub-model into a CFD code. The results show that the modeled char oxidation rates are in good agreement with experimental char oxidation rates up to around 70% of burnout. Moreover, the results show that the activation energy and the reaction order can be assumed to be constant for a large number of bituminous coal chars under conditions limited by the combined effects of chemical kinetics and pore diffusion. Based on this, a new model based on only one fuel specific parameter is developed (Paper III). The results also show that reaction orders of bituminous coal chars and anthracite chars differ under similar conditions (Paper I and Paper II); reaction orders of bituminous coal chars were found to be one, while reaction orders of anthracite chars were determined to be zero. This difference in reaction orders has not previously been observed in the literature and should be considered in future char oxidation models. One of the most frequently used comprehensive char oxidation models could not explain the difference in the reaction orders. In the thesis (Paper II), a modification to the model is suggested in order to explain the difference in reaction orders between anthracite chars and bituminous coal chars. Two single particle models are also developed for the NO formation and reduction during the oxidation of single biomass char particles. In the models the char-N is assumed to be oxidized to NO and the NO is partly reduced inside the particle. The first model (Paper IV) is based on the concentration gradients of NO inside and outside the particle and the second model is simplified to such an extent that it is based on apparent kinetics and can be incorporated as a sub-model into a CFD code (Paper V). Modeled NO release rates from both models were in good agreement with experimental measurements from a single particle reactor of quartz glass operating at 1173-1323 K and 3-19 vol.% O2. In the future, the models can be used to reduce NO emissions in new combustion systems

Including time in a travel demand model using dynamic discrete choice

Activity based travel demand models are based on the idea that travel is derived from the demand to participate in different activities. Predicting travel demand should therefore include the prediction of demand for activity participation. Time-space constraints, such as working hours, restricts when and where different activities can be conducted, and plays an important role in determining how people choose to travel. Travelling is seen as a possibly costly link between different activities, that also implicitly leads to missed opportunities for activity participation. With a microeconomic foundation, activity based models can further be used for appraisal and for accessibility measures. However, most models up to date lack some dynamic consistency that, e.g., might make it hard to capture the trade-off between activity decisions at different times of the day. In this paper, we show how dynamic discrete choice theory can be used to formulate a travel demand model which includes choice of departure time for all trips, as well as number of trips, location, purpose and mode of transport. We estimate the model on travel diaries and show that the it is able to reproduce the distribution of, e.g., number of trips per day, departure times and travel time distributions

Dissolution of bioactive glass S53P4 in a three-reactor cascade in continuous flow conditions

This study expands the knowledge of bioactive glass S53P4 dissolution by implementing a cascade reactor to a continuous dissolution setup. Three reactors were coupled in a series to study the effects of released ions on S53P4 reactions in each reactor. The pH and ion concentrations were measured in Tris-buffer and simulated body fluid flowing through the cascade reactor for five days. The reaction layer formed on the particles in each reactor were also analysed. In Tris, the dissolved Si decreased from 100% to 40% and 26% in the consecutive reactors after five days. In SBF, Si decreased from 64% to 11% and 8%. Thus, the ions released and decrease of available hydrogen ions for ion exchange influenced the dissolution behaviour of S53P4. The results partly explain the differences in the reaction degree between individual bioactive glass particles used as a bone graft in the same defect site

AIR-FIRED AND OXY-FIRED COMBUSTION: RATE OF CHAR OXIDATION BY O2, CO2 AND H2O

Char conversion rates and mechanisms are investigated at oxy-fired and air-fired combustion conditions. Experimental char conversion data was obtained from the IFRF Isothermal Plug Flow Reactor within the RELCOM project. Char conversion rates were computed at the investigated conditions using a detailed single particle model. The model takes into account development of internal surface area, diffusion of gaseous species inside the particle, homogeneous chemistry outside the particle, the Stefan-flow, temperature gradients and diffusion inside and outside the particle and Langmuir-Hinshelwood type surface mechanisms. The modeled char burnout is in good agreement to the experimental char burnout. The results show that char oxidation can be more rapid at conventional combustion conditions than at oxy-combustion conditions, although the oxygen concentration is that same in both cases. This is surprising, considering that a significant part of the char is consumed by CO2 and H2O. This is mainly explained by additional consumption of O2 in the boundary layer of the particle, by decreased diffusion rates of O2 and by an increased coverage of occupied carbon sites in the oxy-combustion case. The results suggests that more than 50% of the char is consumed by gasification reactions both in conventional combustion and in oxy-combustion of pulverized coal

Mobility constraints and accessibility to work: Application to Stockholm

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