فهرست مطالب کاظم آتشکاری

Gasification technology is an important part of the clean coal technology. For further development of this technology, the processes inside the gasifier and the interactions of injected gas and solid fuel particles need to be understood. In the current study, a numerical simulation of an entrained flow coal gasifier is investigated using experimental operating conditions. The reactions and kinetic parameters of the gasification process are fitted using coal gasification data obtained from similar published papers. Comparison of the simulation results, experimental data and two other similar studies confirm the accuracy of the developed model. The focus of this study is on the accuracy of the models presented for the devolatilization process and the effect of the oxidizer change from oxygen to air on the gasifier performance. Four devolatilization models are investigated including chemical percolation devolatilization, single rate, Kobayashi and constant rate models. The predicted trends of species changes are similar in different devolatilization models but the amount of produced syngas is somewhat different depending on the accuracy of each model. The Kobayashi and constant rate models predicted the devolatilization rate lower than the other two models. The results obtained from the chemical percolation devolatilization model are more consistent with experimental data compared to other models, but require higher computational times. The use of air oxidizing agents rather than oxygen reduces the species mole concentration of produced syngas and hence reduces the gasifier efficiency.

Cavity receiver in solar tower concentrator usually experiences highly intense radiation. Due to asymmetric concentration of solar rays, non-uniform heat flux distribution occurs on the different parts of the cavity receiver. This non-uniform distribution leads to uneven thermal expansion and stresses in receiver, which affects the reliable operation and reduces life time of receiver parts. Therefore, it is necessary to reduce the non-uniformity of solar flux on the surface of the absorber tubes and different parts of the solar reactor. The aim of this study was to focuses on the distributions of concatenated solar flux over graphite tubes of a 50kW solar reactor, which was previously designed for methane thermal dissociation at the focus of a solar furnace. In this study, the absorbed solar power on the different parts of the reactor is determined by Monte Carlo ray tracing method. Moreover, the effect of aperture size and the absorptivity of receiver parts on the net magnitude and distribution of absorbed power in reactor are investigated. The results prove that the 16cm aperture absorbs the maximum power and leads to even better solar flux distributions. Replacing the absorbing walls by the reflective walls will also result in more power absorbed by the tubes and better uniformity of flux distribution around the tubes.

The use of biomass by means of gasification to produce bio fuels and reducing the environmental impact of fossil fuels has been the focus of many researchers in recent years. In the present study, the computational fluid dynamics method is used to predict the process of gasification inside a downdraft gasifier. Recent studies have shown that although many studies have been carried out by various researchers to maximize the cold gas efficiency in the gasification process, so far, no study has been done to minimize the emission of pollutants as one of the other important design parameters along with the increase of cold gas efficiency. So, in this study, the effect of changing the equivalence ratio as design variable on the gasification efficiency as well as the amount of pollutant produced simultaneously is investigated. Also, in this study, CO/CO2 and H2 /H2O molar ratios are considered as another objective function in selecting the optimal process point. In order to verify the validity of the results, the simulation data was compared with the experimental results and the previous numerical study, and a good agreement was shown between their comparison. The results of this study show that in the ratio of 0.64, the rate of production of nitrogen oxides relative to cold gas efficiency is optimal considering the maximum production of CO/CO2 and H2/H2O molar ratios . This point is the optimal point. Under the working conditions of the gasification process.

In this study, a combination of micro gas turbine, absorption cooling, and organic rankine cycle is selected. The system is modeled applying the laws of thermodynamic, and based on this model, exergy and economic analyses are performed for each component of the system. Four design variables, namely compressor pressure ratio, turbine inlet temperature, turbine efficiency, and compressor efficiency are considered to evaluate the system performance. Two models have been used and compared to assess the effect of chemical exergy on irreversibility of absorption cycle once regardless of chemical exergy effect and then with the consideration of chemical exergy effect on the system analysis. Exergy efficiency and total cost rate are considered as two objective functions of the tri-generation system. Based on these findings, the exergy efficiency, by promoting micro gas turbine cycle to a tri-generation system, increases from 31% up to 38.6%. The findings, based on Pareto front, indicate that exergy efficiency can be increased up to 52%, and the total cost rate can be reduced to 5.2 $/hr.

This paper presents the numerical study and optimization of CHP power plant consisting of gasification process, solid oxide fuel cell and micro gas turbine. Woody biomass is converted to product gas in the gasification part, and in the CHP producing part, the product gas is converted to electric power and heat by use of a solid oxide fuel cell stack, micro gas turbine and heat recovery steam generator. The model used in the gasification process is a modified thermodynamic equilibrium model, and the steady-state intermediate temperature solid oxide fuel cell model developed hear is one-dimensional which allows for monitoring of the temperature gradients along the cell length under different operating conditions. Zero-dimensional models are used for other components. The effects of main cycle parameters, such as; the gasification agent, average current density and the fuel utilization factor on the cycle important outputs; cooled gas efficiency, temperature gradients, the electric and CHP efficiencies, and the total electric power of the plant are investigated. After extensive parametric analysis, multi-objective genetic algorithms (NSGA II) is then used for Pareto based optimization of CHP plant in two steps. The maximum electric power and electric efficiency are 206/81 kW and 46/27% respectively.

Current study aims to numerically investigate the steady state performance of integrated cycle consisting of biomass gasification process and planar type solid oxide fuel cell for different biomass moisture content and amount of air as gasification agent in the gasifier. Single stage lumped gasifier is analyzed by use of a modified thermodynamic equilibrium model, while the steady-state intermediate temperature solid oxide fuel cell model developed hear is one-dimensional which allows for monitoring of the temperature gradients along the cell length under different operating conditions. Complete electrochemical model, species mass balances and energy conservation equation beside the kinetics describing internal methane steam reforming and water-gas shift reactions organize the structure of fuel cell model. Developed model for two main parts of the integrated cycle are validated against the available experimental and numerical dates. Results indicated that the low heating value of the product syngas with 30% moisture content decreases with decreasing of the modified equivalence ratio and increasing of the biomass moisture content from 10 to 50%. Based on the obtained results, the cell electric power mitigates by increasing of the biomass moisture content.

Many researchers have been considered biomass utilization due to reduction of greenhouse gas effects and environmental impact recently. Achieving a system with the best performance for the application of this type of fuel with low calorific value is to be one of the topics of interest to researchers. This study focus on precise modeling of biomass gasification and design a trigeneration system to produce cooling, heating and electricity using this clean source of energy. In the process modeling of biomass gasification a realistic model includes tar content in syngas is developed. A parametric study of trigeneration system to find the objective functions trend and to achieve the best performance parameter is carried out. Results show that two objective functions in the reasonable range have conflict which emphasis to the multi-objective optimization. Also, with draw Pareto front curve, a suitable relation to estimate the trend of objective functions is derived.