OCR Text |
Show yielding low-molecular-weight compounds such as HCN and NH3 [10-14] . These compounds 'react depending on the prevailing thermochemical and .aerodynamic environment and critically influence the final NOx emissions. A simplified NOx formation path is shown in Figure 2. The pyrolysis process, including the concurrent evolution of volatile nitrogen species, is not instantaneous. Therefore, there will be simultaneous evolution and reaction until pyrolysis is complete. It may be estimated that pyrolysis under controlled conditions at typical flame temperatures is complete within 50 ms for a range of coals of practical interest [15, 16]. 2.1 Influence of Fluid Dynamics on NOx Formation Consideration of the behaviour of coal bound nitrogen species during pyrolysis and subsequent gas phase reactions gives a clear impression of the species which are important for NOx formation. However, this knowledge is not sufficient for a full understanding of the process in practical flames, because the majority of pulverised coal flames are dominated by turbulent mixing phenomena. The phenomenological framework of turbulent mixing processes make the treatment of turbulent combustion extremely complex. Although momentum, enthalpy and chemical species transport occur on a global scale, the chemistry can only proceed after molecular diffusion has taken place between the reactants. Therefore, the fuel-oxidant reactions rates are related to the turbulence dissipation mechanisms, where the size of the reaction zone is related more closely to the microscale of turbulence than it is to the macroscale or mixing length. However, to allow the molecular process to proceed, global macromixing must first take place. In swirling pulverised coal flames, this infers macromixing between pyrolysis products, combustion air and recirculated combustion products within the characteristic internal recirculation zone. Consequently, the turbulent flame structure consists of macro-sized regions composed of fuel, oxidant or combustion products and of smaller interfacial zones with dimensions of the microscale in which combustion occurs. It is within this complex turbulent flame structure that the conversion of the nitrogen species occur concurrently with the combustion reactions. It may be estimated that the typical chemical reaction time scale for NO formation from HCN precursors is in the range of 0.1 to 0.5 ms; whereas, the typical integral time scale of turbulent micromixing is around 10 ms [17, 18]. 3 |