• combustione
• energia
• modellazione CFD
• ossicombustione

The supply of energy has been dominated by fossil fuels for decades and still nowadays, almost 80% of the world’s energy is produced from fossil based fuels. Coal is still one of the important resources meeting the demand for power generation and about 45% of the global energy demands was met by coal in the past decade.
The power generation industry uses a combination of fossil fuels, renewable sources, hydro and nuclear power to generate electricity worldwide. The fuel mix has changed due to prices, technological developments and policies. Thus the magnitude of future coal demand growth hinges critically on the actions that governments take to address these issues, taking into account their aspirations for energy security, affordability and improved access to modern energy.
Energy production from fossil fuel combustion results in the emission of greenhouse gases, the dominant contributor being CO2. Coal is causing serious environmental pollution and it is the major carbon dioxide emissions cause.
Nonetheless projections into the future energy mix still see coal, which in some country is currently used to meet the energy demand up to 90% of total.
In order to minimise the emissions of Co2 new technologies have been developed. One of the most important is oxy-fuel combustion, part of post combustion carbon capture. Post-combustion capture (PCC) refers to the separation of CO2 from flue gas derived from combusting fossil fuels – coal, natural gas, or oil – in air. In the case of coal-based power, as shown, coal is combusted in air and the liberated heat is converted to electricity by steam-driven turbines connected to generators. The combustion results in a flue gas mixture consisting of N2, CO2, H2O, O2, and a host of compounds such as SOx, NOx, and heavy metals amongst others.
Conventional pulverized coal-fired boilers, i.e., currently being used in power industry, use air for combustion in which the nitrogen from the air (approximately 79% by volume) dilutes the CO2 concentration in the flue gas. During oxy-fuel combustion, a combination of oxygen (typically of greater than 95% purity) and recycled flue gas is used for combustion of the fuel. A gas consisting mainly of CO2 and water is generated with a concentration of CO2 ready for sequestration. The recycled flue gas is used to control flame temperature and make up the volume of the missing N2 to ensure there is enough gas to carry the heat through the boiler.
The utilisation of computational fluid dynamic (CFD) to describe the combustion has become a powerful tool in order to design the combustion equipment and get the insight of the combustion process. The utilization of simulation allows to save economic and time resources compared to the experimental research.
Although CFD approaches have been used in some studies to better understand the flowfield and combustion processes in oxy- coal combustion, several problems remain that need to be resolved to achieve a higher predictive accuracy of combustion characteristics in a CO2-rich environment.

This thesis work has been focusing on some of the major challenges of CFD study of coal simulation and oxy combustion. The project has been carried out in the period from March to August 2015, at the University of Sheffield (UK).
The project consists in CFD simulation of coal combustion in a semi-industrial furnace. The solver was Ansys fluent 15.0. The followed methodology is Verification & Validation, as the results provided by the solver have been validated through experimental results provided by the Energy 2050 Group, part of the University of Sheffield. The experimental results have been collected from the PACT (Pilot scale Advanced Capture Technology) coal burner, developed by Doosan-Babcock, with a nominal power of 250 kWth. The burner has the capability for coal and biomass combustion, in both air and oxy environment, and the system is capable of providing flue gas concentration, as well as in-flame radial and axial measurements. The oxy-combustion system is once-through, therefore the recirculation of the combustion flue gases (CO2, H2O, NOx,) is simulated by tanks, giving the possibility to investigate different combustion environments.
In the present study only air-coal axial and radial results are used for validation, while the oxy-combustion results were only partially available (only flue gases concentration, not in-flame measurements). The used fuel was El-Cerrejon, a low-ash bituminous coal.
The main focus of the project is on two of the major areas that are critical in CFD combustion modelling, turbulence and pollution production. In particular NOx production is of particular importance in coal combustion, moreover it is necessary to investigate on the processes affect NOx production during combustion, and investigate on the differences of these processes in air and oxy environment. In conclusion the subjects treated and the results regard three major areas:
1. Assess the difference in NOx production during air and oxy combustion, trying to study the differences from both experimental and modelling point of view;
2. Evaluate the effect of different turbulence models (realisable k-ε, Reynolds Stress Model) on the furnace fluid dynamic in both air and oxy combustion;
3. Evaluate the effect of the reburning process on the NO production and in particular assess the effect of modelling parameters, like the reburning equivalent fuel, through a sensitivity analysis.