OCR Text |
Show 5 The PAX-R burner m a y said to be an air two-stage combustion structure burner if the auxiliary air and main air are respectively considered primary air and secondary air. However by dividing the gas into two systems of main gas and auxiliary gas, the combustion rating of the auxiliary flame equivalent to the retention flame can made to be merely 3 % of the total combustion rating at maximum combustion and even if local high temperature area necessary for stable combustion is formed, the increase of total N O x density can be limited to a minimum. Furthermore, since auxiliary air is supplied without passing through the regenerator element, temperature increase is small and the increase of N O x emitted from the auxiliary flame formed by this m ay be limited. The auxiliary flame combines as a pilot flame while the auxiliary air combines as the cooling air for the tube and the ultra violet fire detector. Therefore, no special control is required for lowering of N O x from the main burner and piping may be made simple, same as a that of a normal burner with a pilot flame. 2.3.2 Reduction of burner pressure drop With the air two stage combustion burner, exhaust recirculation inside the furnace is promoted by increasing the flow velocity of secondary air and N O x tends to be reduced. Therefore a blower with a large discharge pressure capacity is required. Moreover, with the regenerative burner, it is also necessary to provide a large capacity exhaust fan. To avoid this with the PAX-R burner, the cross section area of the secondary air injection outlet was enlarged to lower flow velocity and reduce burner pressure drop. The burner air pressure is extremely low at not more than 100 Pa with cold air and not more than 500 Pa with pre-heated air of 1000°C. Even with the addition of regenerator element pressure loss, it may be limited to 2000 Pa. The reducing of flow velocity made it difficult to lower NOx but with the PAX-R burner using the above mentioned low N O x method, it was possible to achieve a low N O x level which poses no problem from a practical point while it is thought that an increase in system cost to achieve further reduction of N O x would be prohibitive. |