OCR Text |
Show suppression but it is limited since the combustion becomes incomplete to produce soot, which is left unburnt even in the 2nd stage. m With gas, the air ratio of 2nd stage can be increased to about 40 to 50%, but with oil 30% is the maximum. With heavy oil, in particular, Over 25% is very hard because of the deposition of carbon particles. With gas combustion, two stage combustion is easier than for oil combustion but the examples of application to boilers are less than that for heating furnaces as shown in Fig-12 and 13. The problem for -the practice of two stage combustion includes the followings; (1) Installation of secondary air port(NOx port) on the body of boiler (2) Improvement of furnace wall or 'water cooling wall of boiler (3) Mounting of air duct, damper and control equipment (problem at starting) (4) Balance between the lowering of air ration at 1st stage . and generation of soot (5) Longer flame load reduction(rate down) Off-Stoichiometric combustion With Off-stoichiometric combustion, burners of combustion rich and air rich are arranged, and the unburnt gas from the fuel rich is mixed with the extra O2 from the burner of air rich for combustion, thus the complete combustion is ensured as a whole. This method can be regarded as a kind of two stage combustion and the principle is as shown in Fig-11 and Fig-12. Fig. 11 Principle of <J£f- Stoichiomecric combustion Fuel . excess / 4 Air excess t 1 1 1 / "\ \ / s o. z Xx X X2 Fig.12 Off-Stoichiometric combustion air rich air fuol rich 8 - 17 - |