OCR Text |
Show 3 objective is to reach ultra low NOx concentration (e.g. 40-60 ppm) attention also has to be paid to the processes involving the production and destruction of thermal and ·prompt" NOx- For the purpose of this study it bas been sufficient to focus on the fuel nitrogen processes since prompt and thermal NO can be controlled to a large extent by the same methods as fuel NO, e.g. staged combustion. Early ignition of coal flames is important for establishing a high temperamre core to effect the maximum rate of release of fuel nitrogen from coal particles into a sub stoichiometric gas phase, and consequently to lengthen the residence time for fuel nitrogen conversion. The ignition process is controlled by the rate of volatile matter evolution, which increases exponentially with increasing temperature. Gradual addition of oxygen to the evolved volatile matter is also important for burning the fuel as it becomes available during the ignition process. With increasing air to coal mass ratio, one can generate more heat, elevating the local gas and particle temperature. Part of this heat will be recirculated to the flame front by flame radiation and gas recirculation and will enhance the ignition process. If, however, too much oxygen is provided, the mean stoichiometric ratio increases which, in turn, can increase the amount of fuel nitrogen converted to NOx ' Decreasing the air to coal mass ratio reduces the local stoichiometric ratio; the heat generated by combustion is limited by the reduced concentration of oxygen, limiting the local temperature and consequently delaying the release of fuel nitrogen. This decreases the conversion to molecular nitrogen due to insufficient residence time and temperature in the fuel rich zone. It follows, that there exists an optimal air to coal mass ratio, at which the evolution of fuel nitrogen is fast due to high temperature, while the surrounding gas phase is sub-stoichiometric, and favorable for the conversion of fuel nitrogen to molecular nitrogen. This air to coal ratio is undoubtedly coal dependent. In the second mode of operation (pPM) described above, the well stirred mixture of transport and primary air with coal creates a nearly uniform distribution of stoichiometric ratio and temperature, which can be optimized to obtain the highest rate of fuel nitrogen conversion to molecular nitrogen. Both the heat release and volatile evolution are fast, and, consequently, the residence time and temperature in the stratified region of the fuel rich zone is increased resulting in lower NOx concentrations. Parametric Studies The experiments were conducted with standard grind lllinois #6 coal. The Radially Stratified Flame Core (RSFC) burner (Figure 1) was used in the studies. It has three annuli (primary, secondary and tertiary) and independent adjustable swirl generators can be used to set different input conditions such as the amounts of air flow through the primary, secondary and tertiary annuli with variable axial and tangential velocity to obtain optimum radial stratification for a sufficient axial distance or residence time. Experiments were performed to investigate the effect of burner settings and burner air distributions on the NOx formation and carbon conversion. |