OCR Text |
Show process since it constitutes a well-mixed region of combustion products which stabilises flames. The combustion products transport heat and mass from downstream of the IRZ to fresh fuel in this region of high intensity turbulence. Since the IRZ is fed by combustion products, it is a zone where low oxygen concentrations typically exist. When devolatilisation is promoted in this region, preferential reaction paths from volatile nitrogen species to N2 are encouraged. Penetration of this region by the coal jet is possible when the momentum of the coal particles is sufficient to overcome the local particle drag forces. In Burner A, swirl was generated using an IFRF moveable block swirler. To allow a controlled expansion of the secondary air vortex and regulate the size of the IRZ, the burner quarl had a length to diameter ratio (L/A) of 1.0, a quarl expansion ratio (B/A) of 2.0 and a zero inlet and outlet angle, see Figure 6. The quarl profile was designed to fit a third order polynomial subject to these boundary conditions. Two coal injectors were designed for this burner, see Figure 7. Gun A is an annular coal injector, producing a bluff body blockage ratio (a/A) of 0.6. Gun B splits the coal jet into four independent streams injected obliquely to impinge on each other at the point of maximum recirculating mass flow in the IRZ [19, 20, 26]. The design of Gun B was intended to reduce the initial mixing between the coal jets and the combustion air. Both guns were "manufactured from high temperature stainless steel. 3.2 Burner B - Air Staged Precombustor Burner A schematic of the air staged precombustor burner (ASPB) is given in Figure 8. The burner employed a precombustor in which fuel-rich primary zone was maintained at a relatively high temperature. The burner was designed to use swirling secondary air which created an IRZ and lead to good mixing between the primary and secondary air. The precombustor was fired under sub-stoichiometric conditions with the additional (staged) air being introduced via 12 radially distributed air ports located in a second quarl at the precombustor exit. In this context, the ASPB may be considered a hybrid of internally and externally air staged burner concepts. The burner quarl was designed in a similar manner to the AASB with a length to diameter ratio of 1.0, a BIA ratio of 2.0 and, zero inlet and outlet quarl angles. The quarl profile was designed to control the expansion of the secondary air vortex and prevent flame impingement on the precombustor walls. Coal was injected via an annular coal injector with a blockage ratio (a/A) of 0.6. 6 |