Combustion, Fluid Dynamics and Heat Transfer in Oxy-fuel and Air-Fired CFB boilers

Doktorsavhandling, 2014

The studies described in this thesis develop and encode modeling tools for the design and scale-up of oxy-fuel circulating fluidized bed (CFB) boilers across a wide range of operational conditions. The modeling comprises fluid dynamics, combustion, and heat transfer and uses expressions that are proposed in this work or derived from the literature, together with experimental data obtained under both oxy-fuel-fired and air-fired conditions. First, a mathematical model that takes into account both the mixing and combustion kinetics of both heterogeneous and homogeneous reactions is developed for simulating gas and solids combustion under air- and oxy-fuel-fired conditions. Second, a new correlation for gas mixing is proposed based on the multi-phase structure of the gas-solid flow in the furnace. Third, a mathematical model of CFB heat transfer is developed that accounts separately for convective and radiative heat transfer under air-fired and oxy-fuel-fired conditions. Model simulations and validations are carried out for a broad range of operational conditions, involving both oxy-fuel firing and air-firing. The results are in good agreement with the data obtained using laboratory-scale (100-kW) and 4-MW industrial-scale, oxy-fuel-fired units. The model is used subsequently to investigate the design of a utility-scale (300–1100 MW) oxy-fuel-fired CFB boiler. In the laboratory-scale unit, to describe the axial mixing, the furnace is divided into three distinct zones in which both mixing and kinetics control the combustion. However, the rate of mixing is shown to be lower in the dense bed and the upper parts of the furnace, whereas it is higher in the zone above the dense bed due to bubble eruptions and secondary gas injections. For the 4-MW oxy-fuel-fired unit, the modeling and experimental results show that both the peak CO concentration and lateral gas maldistribution increase with increasing O2 concentration at the inlet. The modeling results show that the specific emission of CO [g/MJ] at the furnace outlet decreases with increases in the inlet concentration of O2. The outcomes of modeling heat transfer show good agreement with the experimental results with respect to the levels of heat extraction and temperature in the furnace and the heat extraction required from the external heat exchanger (EHE). The utility-scale simulations reveal that the furnace temperature increases at elevated inlet O2 concentrations. To limit the in-furnace temperature to an acceptable level (i.e., 1273 K to avoid ash melting), the modeling indicates that the external circulating solids flow has to be increased substantially; flow rates of 10, 19, and 31 kg/m2s are required to ensure inlet O2 concentrations of 48%, 56%, and 70%, respectively.