Studies in Advanced Oxy-combustion Technologies
In 2013, approximately 87 percent of the total world energy came from combustion sources. While combustion is of critical significance, it poses serious issues. The rapid increase in energy consumption, primarily from increased fossil fuel use, has raised strong concerns over the current energy infrastructure, the emissions of particulate matter, CO, SO2, and NOx, as well as global warming due to the emission of CO2. Improving combustion efficiency and reducing combustion emissions are essential. This dissertation focuses on two areas: (1) Computational fluid dynamics simulations of a novel burner design for a new oxy-fuel technology with relatively high efficiency and low emissions, and (2) numerical studies of flame structure and soot inception, interpreted in the carbon-to-oxygen atom ratio space for laminar diffusion flames. Part I. Oxy-fuel combustion is considered a promising technology for carbon capture, utilization, and storage (CCUS). One of the primary limitations on full-scale implementation of this technology is the significant increase in the cost of electricity due to a large reduction in plant efficiency and high capital costs. The fact that the CO2 captured must ultimately be pressurized for geo-sequestration or Enhanced Oil Recovery (EOR) enables pressurized oxy-combustion to be implemented at no net pumping cost because the energy to pump oxygen is comparable to that to pump CO2. At higher pressure the latent heat of condensation of the moisture in the flue gas can be utilized in the Rankine cycle, increasing the plant efficiency. A new pressurized oxy-combustion technology, namely staged, pressurized oxy-combustion (SPOC) has been developed in which the flue gas recycle is minimized by means of fuel-staged combustion. As determined through ASPEN Plus modeling, this process increases the net plant efficiency by more than 5 percentage points, compared to first-generation oxy-combustion plants. In the SPOC process, pulverized coal is combusted at high-pressure with negligible recycle. A unique burner and boiler have been designed via computational fluid dynamics (CFD) to effectively and safely burn coal under SPOC conditions. CFD is used to model the process and to determine the effects of operating conditions on the radiative and convective heat transfer in the boiler. It is shown through the simulations that a manageable wall heat flux can be achieved even with very high local gas temperatures. The system is also designed to minimize particle deposition to avoid slagging, fouling, and corrosion, and simulations of ash deposition indicate negligible deposition on the furnace wall. Radiation behavior is also studied to demonstrate radiative trapping effects. It is demonstrated, through both analytical and numerical studies, that the system pressurize is a critical tool to obtain an optically thick medium capable of trapping heat inside the furnace. It is further shown that for a sufficiently large optical thickness radiative trapping can occur, and this, combined with the diffusive-convective profiles of the temperature and absorption coefficient, allow us manage the wall heat flux. An average-temperature method is developed to approximate the heat flux and to study the dynamic relations of temperature and the absorption coefficient. The effects of ash particle size on radiative trapping are systematically studied. It is concluded that the wall heat flux is controlled by particle size as well as particle number concentration, in other words, by particle porosity and fragmentation. Ultimately, burners and boilers are designed to minimize the boiler heat transfer surface area, ash deposition, and fire-side corrosion for the SPOC system. Part II. Understanding the structure of diffusion flames is often complicated by the dependence of flame structure on the boundary conditions, such as composition, temperature, and flow field (e.g., strain rate in a counterflow flame.) The utility of interpreting flame results in the carbon-to-oxygen atom ratio (C/O ratio) space, as opposed to physical space or mixture fraction space, is evaluated. Flame and soot zone structures of counterflow diffusion flames are studied for C2¬H4 and C3H8 and interpreted in C/O ratio space as a function of the stoichiometric mixture fraction (Zst). The Burke-Schumann results expressed in C/O ratio space demonstrate how a clear and direct understanding of how structure is affected by Zst can be realized. In C/O ratio space, unlike physical or mixture fraction space, the flame location is independent of the stoichiometric mixture fraction. Numerical results with detailed chemical kinetics also indicate that C/O ratio space is a fundamental variable in the sense that, for a given fuel, the location of the flame zones and critical reactions is invariant with Zst and strain rate. Two zones are clearly observed, the radical pool zone and the soot precursor zone which is located on the fuel side of the flame. The onset threshold of soot precursors (C6H5 and C6H6) on the high temperature side of the soot precursor zone is characterized by the depletion of radicals. The role of the hydrogen radical in flame structure and soot inception is demonstrated by studying its production and consumption channels in C/O ratio space, as are the roles of C2H2 in soot precursor depletion and boundary coincidence. The kinetic ratio is used to study the characteristics of key chemical reactions and to identify regions of equilibrium for these reactions. Finally, a modified C/O ratio ((C/O)*) is given to interpret the physical meaning of C/O ratio. The numerical results in this work indicate and explain the advantages of applying C/O ratio space in the analysis of flame structure and soot precursor chemistry.
computational fluid dynamics, fuel-staged combustion, oxy-combustion, pressurized combustion, radiative trapping, recycle minimization, Engineering
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