The Spring 2015 edition of Today’s Boiler magazine published Dr. Jianhui Hong’s article “NOx Emissions Reduction Strategies.” In the article Dr. Hong explains the importance of low NOx and provides solutions for complying with low NOx emissions regulations.
He explains that “NOx is a term used to include two important air pollutants: NO (nitric oxide) and NO₂ (nitrogen dioxide). These pollutants are sometimes called mono-nitrogen oxides…NOx is a term used to include two important air pollutants: NO (nitric oxide) and NO₂ (nitrogen dioxide). These pollutants are sometimes called mono-nitrogen oxides.”
NOx gases are harmful in a number of ways. Exposure to NOx gases is harmful to human health by irritating the mucous membranes and penetrating the lungs, “causing oxidizing damage to the tissues.”
When NOx reacts with water or water vapor, “it forms nitrous acid (HNO₂) and nitric acid (HNO₃). These acids in the rain can make ‘acid rain.’ Acid rain can damage plants and man-made structures such as buildings, bridges, and outdoor sculptures.”
In highly populated areas, “NOx emissions from combustion processes are primarily in the form of NO. In the air, NO reacts with oxygen to produce NO₂. In the presence of sunlight, NOx can react with hydrocarbons, especially VOC (volatile organic compounds) in the air to form ground-level ozone, which is an important ingredient of smog. The reddish brown color of the hazes hanging over the skies of some major cities comes from NO₂ gas… It can cause irritation to eyes, noses, throats, and lungs. It can even cause asthma and other chronic lung diseases such as emphysema and chronic bronchitis.” NOx can also combine with moisture in the air which “induces changes in phytoplankton and produces toxic brown or red algal blooms (i.e. “red tides”). The algal blooms can cause the death of other plants and marine animals in the water.
Thermal NOx, Fuel NOx, and Prompt NOx create the conditions for NOx emission.
Dr. Hong presented five solutions for the combustion community to reduce NOx emissions.
- Flue Gas Recirculation “targets the thermal NOx by reducing the peak ﬂame temperature and also oxygen concentration…The use of external FGR increases the requirements for the combustion fan” which “become a signiﬁcant factor in the overall costs of the burners (including ﬁxed costs and operating costs).”
- Steam/water injection “works similarly to external FGR. It targets thermal NOx by reducing peak ﬂame temperature and oxygen concentration.”
- Ultra lean premixing “aims to reduce the ﬂame temperature by staying away from stoichiometric condition. Ultra Lean Premixing, if used alone, has the downside of high oxygen level (up to 9%) in the ﬂue gas, and the loss of fuel efﬁciency due to the very high excess air.”
- Air Staging supplies combustion air in two or more stages. “The general goal is to reduce ﬂame temperature, and create fuel rich conditions in the early stages, before the ﬁnal stage of air is supplied.”
- Fuel Staging supplies fuel in two or more stages. “The general goal again is to reduce peak ﬂame temperature. This technique is often combined with Ultra Lean Premixing to overcome the efﬁciency issue of the latter.”
In conclusion, Dr. Hong cites that although NOx emissions can be controlled by the mentioned techniques, “the most cost-effective methods tend to be combustion modiﬁcations, especially using low NOx and ultra-low NOx burners.”
Dr. Hong’s greatest contribution to the combustion community is his ability to present solutions for common problems in the industry and the stringent regulations placed on emissions.
Read the full article here.