8. NOx Generation in Cement Plants, its Impact & Control Measures
What is NOx – NOx is basically nitrogen oxides which forms due to oxidation
of nitrogen when reacted with oxygen.
NOx emission from Kiln stack consists of:
- 95% NO
- 5% NO2
Oxidation of NO to NO2 at lower temperatures, after stack in atmosphere which
further reacts to form compounds - Nitric Acid (HNO3), which is toxic in nature.
Therefore, the limit for NOx is related to NO2 and reported as mg/Nm3 of NO2. The
latest limit 600/800 mg/nm3.
NOx can be generated in kiln systems in two ways, primarily as thermal NOx due
to oxidation of the nitrogen in the air as a result of the high temperature of the
flame, and secondly as fuel NOx due to the oxidation of nitrogen containing
compounds in the Fuels, raw materials, AFRs.
8.1 Type of NOx
a. Thermal NOx – Predominantly
b. Fuel NOX– Predominantly
c. Raw Material NOX – Negligible
NOx emission in Cement plant are predominantly Thermal and Fuel NOx
a. Thermal NOx:
Thermal NOx is generated from nitrogen and oxygen in the air at high temperature
according to the equations; -
N2 + O2 ---> 2 NO
2 NO + O2 ---> 2NO2
Thermal NOx generation dominates at high temperature in the main kiln flame.
Thermal NOx is attributed predominantly to atmospheric nitrogen. It is created by
reaction of nitrogen and oxygen at higher temperature of > 1300 deg C. Combustion
in calciners can be designed to produce a lower concentration of NOx, particularly
by having Fuel NOx generation as the dominant mechanism and/or by having
a less-oxidizing combustion regime than can be tolerated in the kiln burning zone.
When kiln exhaust gases pass through the calciner combustion region, they attain
the local equilibrium and NOx is “washed” down to the lower concentration.
Modern calciner furnace designs may have several points for injection of fuel and
oxygen-bearing gases, and one or more of these may be used in various
proportions to achieve lower NOx emissions.
Thermal NOx formation mainly depends on: - - Theoretical flame temperature –fuel type
- Flame shape –burner type
- Excess air rate