西方国家的大城市（如洛杉矶）治理雾霾，技术上比较容易做到的都做了(煤改气，脱硫，消烟除尘，CO，VOC）， 到了最后阶段(就是现在），就是不断提高脱硝标准。所以说脱硝是雾霾治理之攻坚阶段。鉴于中国雾霾的严重程度和雾霾治理的难度，许多专家学者估计中国要花三十年的时间来治理雾霾。中国的许多大城市，由于人口密度大的关系，脱硝的环保标准可能要定得比美国加州现有法律更严厉， 才能收到“蓝天常有”的效果。 脱硝技术，从现在开始，可以在中国热门三十年，直到雾霾得到有效治理为止。脱硝技术，可以发展出中国未来巨大的环保产业， 是产业升级的大好机会。 你看见￥￥￥￥了吗？

本文是为美国锅炉协会约稿而写的科普（规定不能包含广告），因为限 1500 Words，又要涵盖所有固定源脱硝方法，所以删了又删， 没有办法全面展开， 结果还是稍微超过字数。

移动源的脱硝方法（三元催化转化器）因为篇幅不包括在本文内。高精尖的无焰燃烧也因为篇幅删掉了。

本文从Word 原文cut and paste过来，许多地方英语单词之间的空格丢了，大概是博客的Editor有点问题。

请原谅。

NOx Emissions: Reduction Strategies

By Dr. Jianhui Hong

If you operate a stationary fired equipment (such as a boiler) and you are concerned about compliance to emission regulations, you may find this article useful.  It provides a basic primer on a class of regulated pollutants called NOx and methods for controlling and minimizing their emissions.

1. Whatis NO_x?

NO_x 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. In contrast, the generic term “nitrogen oxides” includes a family of seven different chemical compounds (NO, NO₂, N₂O, N₂O₂,N₂O₃, N₂O₄, N₂O₅).   NO_x should not be confused with N₂O(laughing gas), an analgesic commonly used in dental operations.  NO_x is formed and emitted in nearly all combustion processes.

2.  Why is NOx bad for human health and the environments?

NO and NO₂ are harmful to human health on their own rights.  But they also play some indirect roles in harming human health and the environments.  

2.1 NO and NO₂

NO is a colorless, poisonous,oxidizing gas with an irritating odor.     NO is toxic by inhalation, and symptoms of over-exposure may not become apparent for up to 72 hours.  Exposure to NO gas in low concentrations produces an irritating effect on the mucous membranes of the eyes, nose, throat and lungs, which can include choking, coughing, headache, nausea and fatigue. 

NO₂ is a reddish brown, poisonous gas with a pungent odor. Because it is relatively insoluble in water, there is little irritation to the mucous membranes of the eyes, nose and throat. Therefore, people who inhale even high concentrations may not be aware of their exposure. This allows NO₂ to penetrate well into the lungs, where it causes oxidizing damage to the tissues.  

2.2 Acid rain

NO_x can react with water or water vapor to form 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.

2.3 Smog/Ground levelozone

NOx emissions from combustion processes are primarily in theform of NO. In the air, NO reacts with oxygen to produce NO₂.  In the presence of sunlight, NOx can reactwith hydrocarbons, especially VOC (volatile organic compounds) in the air toform ground-level ozone, which is an important ingredient of smog.  The reddish brown color of the hazes hangingover the skies of some major cities comes from NO₂ gas.  Ground level ozone is also a healthhazard.  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.  

In the air, nitrous acid(HNO₂) and nitric acid (HNO₃) from the combination of NOx and water vapor can react with ammonia to form nitrite and nitrate salt crystals (NH₄NO₂, and NH₄NO₃).  Together with sulfates, sulfites and organic particles, these nitrites and nitrates can make up 90% of Particulate Matter less than 2.5 microns in aerodynamic diameter (PM_2.5).  The PM_2.5 interferes with the transmission of sunlight in the air, and causes visibility issues in the form of a haze that, unlike a fog, does not clear when the air warms up.  

2.4 Water Quality - Nutrient Overload

NOx can form nitrates in the air as we discussed. These nitrates can then come down with rain and snow.  Nutrient overload problems occurin the bodies of water when the availability of nitrites and nitrates become too abundant. This nutrient overload induces changes in phytoplankton, 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.

3. How is NOx formed

NOx can be formed through three different mechanisms: thermal NOx, prompt NOx and fuel NOx.  

3.1 Thermal NOx

Thermal NOx is produced when nitrogen (N₂) and oxygen (O₂) in the air reacts to form NOx.   Elemental nitrogen (N2) is typically a stable and inert gas due to its strong triple bonds, but under the high temperature conditionsof a flame, it can start reacting with oxygen. Dry air is comprised of 21% oxygen, and 78% nitrogen by volume.  Since air is the most commonly used oxidant in combustion, thermal NOx is present in most combustion processes.  Thermal NOx is very sensitive to temperature.  Reducing peak flame temperature (especially below 1300   ̊C or 2370   ̊F) isvery effective in reducing thermal NOx production.  Many of the measures used to reduce peak flame temperature also helps reduce NOx because of oxygen concentration dilution.  

3.2 Fuel NOx

Fuel NOx is produced when the organically-bound nitrogen in some fuels(such as coal, and to a lesser degree fuel oils) react with oxygen in the flameto produce NOx.  The nitrogen in these fuels are often referred to as “fuel bound nitrogen”.  The formation of fuel NOx is very complex, but typically involves formation of HCN and NH₃ as intermediates, and the subsequent oxidation to form NOx. Fuel NO_x can bemost effectively minimized by staged combustion, which implies delayed mixing between the fuel and air.

3.3 Prompt NOx

Prompt NOx is produced when hydrocarbon radicals react with atmosphericnitrogen (N2) to form HCN, which is subsequently oxidized to form NOx.   Prompt NOx is important under fuel rich conditions due to the abundance of hydrocarbon radicals.  Reducing the fuel rich zone is effective in suppressing prompt NOx.

4. Howcan NOx be reduced?

NOx abatement techniques can be dividedinto pre-treatment, combustion modifications and post-treatment.

4.1 Pre-treatment

Pretreatment often means use of better fuel sources, such as switchingbetween types of coals; switching from coal to fuel oils; switching from #6 oilto #4 or #2 oils; switching from fuel oils to Natural gas.  

4.2 Combustion Modifications

4.2.1 FGR

Flue Gas Recirculation (FGR) is a commonly used NOx abatement technique.  It targets the thermal NOx by reducing thepeak flame temperature and also oxygen concentration.  FGR can come in two forms: external FGR andinternal FGR.  Figure 1 illustrates theuse of external FGR for NOx reduction. This is the most common NOx abatement technique.  The use of external FGR increases the requirements for the combustion fan in terms offlow capacity and electricity consumption. Some burner designs require up to 40% FGR by mass, to achieve ultra-low NOx levels, and the combustion fans become a significant factor in the overallcosts of the burners (including fixed costs and operating costs).  Internal FGR is induced often by sophisticated burner designs and in-depth understanding of fluid dynamics around the burner head, see Figure 2.  The state-of-the-art burner designs use no external FGR to achieve 30 ppm NOx, anduse little (<15%) external FGR to achieve <9 ppm NOx, while maintaining3% oxygen (dry volume based) in the flue gas over a wide turndown range (5:1 oreven 10:1).

Figure 1.  External Flue GasRecirculation

Figure 2.  Internal Flue Gas Recirculation

4.2.2 Steam/water injection

Steam/water injection works similarly to external FGR. It targets thermal NOx by reducing peak flame temperature and oxygen concentration.  The downside of this technique is the loss of efficiency compared to FGR due to the increased heat loss through the flue gas.  

4.2.3 Ultra Lean Premixing

The adiabatic flame temperature is a function of the equivalence ratio(normalized fuel/air ratio), and peaks near the point where equivalence ratio is 1 (stoichiometric condition).  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.  Fiber mesh burners use this technique.

4.2.4 Air Staging

In this technique, combustion air is supplied 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.  This technique is very effective against fuel NOx. Under fuel rich conditions, the fuel bound nitrogen can be largely converted to elemental nitrogen instead of NOx.  

4.2.5 Fuel Staging

In this technique, fuel is supplied 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 ofthe latter.  The excess oxygen left behind from the ultra-lean premixed flame is consumed by the later stages ofthe fuel supply, thus avoiding the efficiency loss associated with ultra-leanpremixing.  

4.3 Post combustion treatment

4.3.1SCR

The Selective Catalytic Reduction (SCR) process reduces NOx in the fluegas into nitrogen by injecting a nitrogen–based reagent (ammonia or urea) into the flue gas, and allowing the resulting mixture to flow through a reactor with catalysts.  To be effective the mixture temperature needs to be within the working range of the catalysts. Optimum temperature for SCR varies between 480 and 800 ̊F. SCR allows high degree of NOx reductions up to 90%.  But the benefits come with increased costs interms of catalyst installation and replacement, reagent and electricity.  The catalysts gradually lose activity over time, accompanied by an increase in ammonia slip. When ammonia slip reaches a maximum allowable threshold, at least part of the catalysts needs to be replaced.  




4.3.2 SNCR

The Selective Non-Catalytic Reduction (SNCR) process reduces NOx in theflue gas into nitrogen by injecting a nitrogen–based reagent (ammonia or urea)into the flue gas.  Optimum temperaturefor SNCR varies between 1600 and 2100 ̊F.  SNCR is effective when theinitial NOx level is relatively high (200-400 ppm), and is not effective at lowNOx levels. For boilers firing Natural Gas, SNCR is not effective.  SNCR alone allows NOx reductions up to 50%,and when applied in conjunction with low NOx burners allows NOx reductions upto 75%.  




5. Conclusion

NOx is a regulated air pollutant formed in nearly all combustion processes.  Its emissions canbe controlled by various techniques, but the most cost effective methods tendto be combustion modifications, especially using low NOx and ultra-low NOx burners.