چكيده به لاتين
Gasification of heavy fuel oil (Mazut) spary is studied in this dissertation. Considering the importance of spray quality in combustion or gasification processes, the Mazut spray has been investigated. The heavy fuel spray was studied empirically and in this direction, a better understanding of the characteristics of the heavy fuel oil was obtained. An injector was designed and fabricated for an entrained flow gasifier. Experiments were conducted to identify the effective factors and how each one affects the spray characteristics. These factors include fluid viscosity, surface tension and density which depend on injection temperature and pressure. The most important characteristics of the spray are the breakup length, the spray angle, the most unstable wavelength, and the diameter distribution of the droplets. At low temperatures due to the high viscosity, the atomization process is extremely difficult. The results show that the SMD is more sensitive to temperature variations than pressure variations, and with increasing temperature from 100 °C to 110 °C, there is a significant decrease in SMD.
Mazut spray was also studied numerically. Experimental measurements on a pressure-swirl injector were used to validate numerical study in different injection pressures. A numerical study was performed using Gerris code. To help improve the spray characteristics for heavy fuel oil spray, an idea was developed to introduce a new injector, pulsed pressure-swirl injector, combining a pressure-swirl injector with an ultrasonic pulse jet injector. The numerical simulation showed that using a pressure-swirl injector and generating pulses of 40 kHz can reduce the 78% breakup length and 17% SMD compared to no-pulse case. The use of pulsed pressure-swirl injectors reduces the average diameter of the droplets in a way that unlike air assist injectors does not change the air-to-fuel ratio. Therefore, the use of this type of injector can be very effective in entrained flow gasifiers, which need to be able to control this ratio.
The experiments and the numerical spray simulations lead to the determination of the droplets diameter distribution, which is presented as functions such as the Rosin-Rammler distribution function. To simulate an entrained flow gasifier, if the droplet distribution function is available, a large part of the calculations, which is via the Lagrangian approach and very time-consuming, is eliminated.
Numerical simulation of the chemical combustion and gasification chamber is done by developing an open source code of OpenFOAM. Physical and thermodynamic properties of Mazut were defined to simulate the chamber using an Eulerian-Lagrangian method. Fuel Spray droplets are followed in the Lagrangian phase and after evaporation, enter to gas phase and react in a high temperature environment. A parametric study was conducted to investigate the effect of the equivalence ratio on temperature and distribution of different species.
The results show that, due to the reduction of air to fuel ratio, the percentage of CO and H2 components is increased. For CO, the trend is increasing-decreasing and at Er = 0.4 is maximum. The syngas efficiency and total efficiency (including heat) was maximum at Er = 0.4. In all cases, a small percentage of CH4 is observed in the output products. In this chapter, the effect of changing the model for evaporation of droplets in the combustion chamber was studied and no significant effect observed on this relationship.
Supercritical water gasification of heavy fuel oil was studied. Due to the complexity of this computational fluid dynamics simulation and unavailability of chemical kinetics, in this case, the thermodynamic approach was performed. A parametric study was conducted to define the sensitivity of various parameters on the performance of a supercritical water gasifier feeding with heavy fuel oil (Mazut). The parameters include the mass fraction of the feedstock, gasifier pressure and temperature. The study was carried out in two cases, one at a constant gasifier pressure (P = 22.1 MPa) and the other at a constant gasifier temperature (T = 600 °C). The results are compared varying the other two parameters. The results showed that the feedstock yields a maximum value of about 81% when the mass fraction of the feed is close to 30% and the gasifier temperature is close to 500 °C. No significant change in specie concentration or efficiency by change in gasifier pressure.
