OCR Text |
Show Up to about 100°C of air temperature, the increase of NOx concentration is rather low but over 200°C or 300°C, the rate shows a sharp increase. For boilers, however, the influence is rather low compared with metal heating furnaces of 1,300°C furnace temperature. Accordingly, NOx concentration can be reduced by lowing air temperature, but the practice of this method is difficult in most cases because the boiler efficiency is reduced (for NOx discharging, the effect is not so high as the reduction of NOx concentration because the thermal consumption is increased) and improvement of facilities is required*; 3-5. Two stage combustion With two stage combustion method, the air for combustion is supplied in two stages as shown in Fig-11; at the burner, the combustion is made with logical air quantity or less, and the remaining air is used for complete combustion. That ±s, in the 1st stage, generation of NOx is suppressed by preventing high temperature with low O2 combustion, thus the maximum temperature is reduced by expanding the combustion area for NOx suppression. .._T -- »*«• ^i Fig.,11 Principle of Off-stoichiometric combustion Burner r y Boiler U^ ibr Z^ Forces) blower For two stage combustion, the air volume ratio between the 1st stage Qi and 2nd stage Q2/ and the distance between the burner ports of the 1st stage and 2nd stage are the important factors. If the ratio Q2/Q1 or the distance from the 1st stage port to the 2nd stage port (also called NOx port) becomes close to zero, the effect of two stage combustion is naturally reduced to zero. On the other hand, if the absolute value of the 1st stage air ratio is too much getting close to ordinary combustion, the effect is also reduced to zero arrger the value of Q2/(Qi + Q2)/ higher the effect of NO 8 - 16 . Air |