Understanding NOx

What is NOx?

NOx is a catch-all term commonly used in heavy industries to refer to two oxides of Nitrogen, typically in the following proportions: Nitric oxide (NO) at 95% and Nitrogen dioxide (NO₂) at 5%. The proportion of NO₂ can often be less (97:3%), so each case must be established individually. There are also other oxides, such as nitrous oxide (N₂O), better known as laughing gas, which was a favourite of 19th Century dentists and doctors treating tuberculosis, among various other oxide forms.

NOx directly reacts with water to form nitric acid in the atmosphere, leading to 'acid rain', which harms soil health and kills trees and other plants that provide us with food. It is also implicated in the formation of 'photochemical smog' through ozone (O₃₎ production in the troposphere (0 – 14 km altitude). The Environmental Protection Agency, a US government agency, describes it quite simply as:

Volatile organic compounds – (VOC) + NOx + heat + sunlight = Ozone (= smog)

Smog affects people with respiratory weakness (e.g. asthma) and has been linked to the premature deaths of over 20 000 people per annum in the EU alone.

How does it appear in the kiln system

There are three accepted production processes of NOx in a cement kiln system:

• Thermal NOx: conversion from N₂ in the air at high temperature.
• Fuel NOx: N in fuel oxidises (combustion).
• Prompt NOx: carbon radicals in the flame oxidise N₂ – negligible for cement kilns using modern burners.

There are three basic chemical equations to describe these reactions:

• N₂ + O → NO + N
• N + O2 → NO + O
• N + OH → NO + H

Thermal NOx

Formation depends heavily on temperature, residence time at that temperature, the availability of oxygen, and the partial pressures of N₂ and O₂ in the host gas. The kinematics of the chemical reactions were investigated by Mr Yakov Borisovich Zeldovich, who is responsible for the 'Zeldovich mechanism.' This work allowed for the characterisation of the effects of temperature and residence time on NOx formation. Incidentally, Russia even issued a postage stamp in his honour due to his scientific accomplishments.

Fuel NOx

Any fuel that contains nitrogen as a basic constituent will oxidise this nitrogen. This conversion involves various intermediate reactions involving OH and carbon ions. Again, temperature is important, but if the fuel is properly burned, then 100% of the nitrogen is necessarily oxidised.

Prompt NOx

Prompt NOx can be ignored as it is of practically negligible relevance to the cement production process. Whatever the mechanism, the resulting gas appears at the kiln exhaust emission point and can be measured, usually within a continuous emissions monitoring system (CEMS) at the chimney.

The chemiluminescence method currently gives the preferred and practical solution with good sensitivity for low-range NOx measurement. This method entails generating a chemical reaction and measuring the resulting light, then correlating the light measured to a concentration of NO with the help of a calibration gas. NO₂ is not directly measured in this case; it must either be converted to NO using a converter module or inferred using previous proportions testing. Much of this depends on the producer’s or government’s requirements in the country of operation.

Avoiding NOx

Thermal NOx

Maintaining stability in the process is essential for avoiding or minimising peak flame temperatures and their duration. Kiln stability also allows for fine-tuning other levers, such as oxygen levels.

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Read the article online at: https://www.worldcement.com/special-reports/23012025/understanding-nox/