The two oxides of nitrogen related to combustion are nitric oxide (NO) and nitrogen dioxide (NOJ. NOx refers to either one of these gases, which play a major role in the formation of smog and ozone.
NO and NO2 are considered toxic, with NO2 being one of the most toxic of commonly found atmospheric gases. NO2 contributes to the formation of acid rain and several airborne carcinogens.
NOx formed during combustion using nitrogen and oxygen in the combustion air is referred to as "thermal NOx :'Several important variables influencing thermal
Flame temperature is the most important variable. The higher the flame temperature, the higher the NOx concentration. Air preheat and furnace temperature are related to flame temperature and, as a consequence, have a major influence on NOx generation. As excess air is increased above stoichiometric, the amount of NOx produced begins increasing. The amount of NOx produced will start decreasing when excess air continues to increase because the suppressed combustion temperature begins having a major influence. For practical and economic reasons, it is generally not feasible or desirable to reduce NOx by using excess air.
There are a variety of methods for controlling thermal NOx . Among them are:
. Burner design
. Exhaust or Flue Gas Recirculation (EGR or FGR) . Chemical additives (e.g., ammonia) . Catalyst aided
Burner manufacturers utilize designs such as low NOx baffles and air staging as well as flue gas recirculation. Flue gas recirculation may be achieved by inducing products of combustion (POC) into the flame from the furnace or by using POC from the exhaust system to mix with air or fuel which reduces flame temperature. Oxygen, which controls the reaction rate, is also diluted, reducing the probability that the available oxygen will enter Into the NOx generating reactions.
Air staging controls flame temperature and the chemical environment to reduce NOx formation.
Bloom's low NOx burner product line spans the input range commonly used in industrial applications. It includes direct fired and radiant tube fired burners which can be designed for cold air or hot air produced by recuperators or with regenerators.
The heart of the direct fired burners is the well proven baffle concept. With regenerative applications, efficiencies and emissions levels which reduce NOx , CO and CO2 below the levels expected even from modern cold air burners are achieved.
In radiant tube designs, Bloom uses unique engineering approaches to provide exhaust gas recirculation to the burner with only insignificant or very small losses in thermal efficiency.