Control of Nitrogen Oxides

Forms of nitrogen Nitrogen forms different oxides. NO and NO 2 are principal air pollution interests (NOx). N 2 O N 2 O 3 N 2 O 5 N 2 O 4 N 2 O 2

NO X and SO X : similarities NOXSOx Causes acid rainHNO 3 H 2 SO 4 Contribute to PM10 and PM Are they regulated pollutants?Yes Respiratory irritants?++ Major sources are combustion++

Emissions of Nitrogen oxides Relatively higher contribution

Pros and cons NOXSOx Acid rainHNO3H2SO4 Contribution to PM10 and PM Are they regulated pollutants?Yes Respiratory irritantion++ Major sources are combustion++ Motor vehicles are major emitters+- Removal of Sulfur from fuels solves SO X problemyes Removal of nitrogen from fuels solves NO X problemNo Manipulation of time, temperature and oxygen content reduce formation of oxides YesNo Low solubility salt formation is a solution for pollutant removalNoYes Absorption is a proper technique for pollution preventionNoYes

Reactions of Nitrogen oxides Concentration (ppm)

NO+HC+O 2 +sunlight NO 2 +O 3 NO 2 +h  O+NO (2) O+O 2 +M  O 3 +M (3) NO+O 3  NO 2 +O 2 ozone is consumed In the presence of VOC VOC+2NO+O 2  H 2 O+RCHO+2NO 2 (1) NO is converted to NO 2 without consuming ozone

NO and NO 2 equilibrium N 2 +O 2 2NO They are reversible reactions

Increase with T Decrease with T N 2 +O 2  2NONO+1/2O 2  NO 2

Conclusions If the only mechanism is the chemical equilibrium, we should have less than a ppb of NO and NO 2. However concentrations of NO and NO 2 exceed this values in urban atmospheres… So equilibrium does not explain alone the observed concentrations. The equilibrium concentration of NO increase rapidly with temperature. At low temperatures equilibrium concentration of NO 2 is much higher than that of NO. Flames and lightning strikes are major sources of NO

Thermal, Prompt and Fuel NOx Thermal NOx: forms by heating of N 2 and O 2 in flames Prompt NOx: N 2, O 2 plus some hydrocarbon species in fuel. Fuel NOx: Conversion of nitrogen in fuel into NOx

Contribution of Thermal, Prompt and Fuel NOx

THERMAL NO (Zeldovich mechanism) N 2  2N O 2  2O H 2 O  H + OH O+ N 2  NO+N N+ O 2  NO+O N 2 +O 2  2NO

Reaction Rate is fast. Equilibrium is reached in about 0.3 s. Equilibrium conc. of NO is higher Equilibrum conc of NO is low. Reaction rate is slow. Even at 30 th second it does not reach equilbrium

Remarks: Zeldovich mechanism In order to decrease NO Reduce T Reduce O 2 At high temperature flames, Zeldovich mechanism predictions are high in accuracy. At lower temperatures it predicts much lower NO concentrations.

In automobile engines and large coal fired furnaces, thermal NOX forms.

Prompt NO Forms due to carbon bearing radicals from the fuels CH+ N 2  HCN +N N+O 2  NO+O NO in low temperature flames are prompt NO and weekly depend on T. Prompt NOx formation cannot be prevented in spite of the temperature and oxygen amount adjustment.

FUEL NO Fuels contain little nitrogen. NO due to fuel nitrogen depend on NO/O 2 ratio. Lowering O 2 lowers the fraction of N converted to NO

Control of NOx emissions Modify the process Post flame treatment

Combustion Modification NO increase by Increase in T Increased time at high T High oxygen content at high T Reduction of air nitrogen is a way but expensive (instead of air, pure oxygen can e used) and not practical

Overfire air, Staging (Two-stage or off-stochiometric combustion) The oxygen amount is reduced in the first flame zone by using fuel-rich mix (results in reduction of NOx). Unburnt fuel exists. Air is supplied again by forming a second combustion zone. Thus, the unburnt fuel in the first stage burns and the CO formed in the first combusiton is oxidized to CO 2. In the second combusiton zone, the flame temperature is low since the amount of fuel is quite low. Hence, NOX formation is minimized in both the first (less O2) and the second combusiton zone (low T).

(Low-Excess Air Firing) Reducing the amount of excess air causes less NOx formation since the amount of oxygen in the flame zone also decreses. But CO emissions may increase.  It is achieved with very low investments but it requires very careful operation and maintenance. Efficiency around% 0-25, and in advanced systems %15-55

Flue Gas Recirculation Part of the flue gas is recycled back to the combustion air. Therefore, the oxygen in the combustion air is diluted (reduced O2) The nitrogen present in the recycled air also serves as a heat sink and reduces the flame temperature (reduced T).

Flue Gas Recirculation

Reducing the air preheat In many industries, the temperature of the flue gas is used for the pre-heating of the combustion air. But this causes the increasing of the flame temperature. (Unheated air has a higher capacity for absorbing the heat released during combustion) Reducing the amount of pre-heating reduces the NOx formation by lowering the flame temperature.

Reducing the firing rate Reducing both the air and the fuel amount would not change the theoretical flame temperature. But since there is heat loss from the walls and similar effects in the combustion chamber, reducing the fuel and air amount reduces the flame temperature.

Water/steam injection The injection of water or steam into the combustion chamber creates a heat sink and reduces the flame temperature. This measure can achieve NOx reductions reaching 50% in systems burning natural gas. But, the reducing medium created by the breakdown of steam to hydrogen and oxygen may create a more serious problem. 

Burners out of Service (BOOS) In multi-burner furnaces, feeding of the fuel to some burners may be stopped and the fuel is distributed to other burners. But the air is distributed to all burners. This achieves the previously mentioned staged firing (The oxygen is reduced in the first combustion zone, the flame temperature is low in the second combustion zone because of the small amount of fuel).

Reburn In order to create a second combustion zone after the primary flame zone, extra hydrocarbon is added to the outer part of the primary flame zone. The hydrocarbon radicals formed in this operation react with the NOx. In order to complete the combustion, overfire air is added after this second combustion zone. Research has shown that NOx reductions of % percent can be achieved with this technique in coal-fired plants.

Low-NOx Burners It is, principally, an aplication of the previously mentioned techniques (staged firing and recombustion) at the burner with a certain burner design. There are two approaches: staging of the air or staging of the fuel

Low-NOx Burners (Staging of air) The same principle as in staged air combustion technique

Low-NOx Burners (Staging of fuel) In this design, contrary to the previous one, air/fuel ratio is high in the primary flame zone. Therefore there is high NOx and high flame temperature. In the second combustion zone, the remaining fuel is introduced. Since the oxygen is low and the temperature is high in this case, NOx is converted back to oxygen an nitrogen because of kinetic reasons.

Low-NOx Burners (Staging of fuel) Since the flame will be physically longer, the hitting of the flame on furnace walls may cause problems like corrosion (aşınma  )

Flue gas treatment

Selective Noncatalytic Reduction (SNCR) The principle is the reduction of NOx to nitrogen and water by using NH 2 -X (mostly amonnia, NH 3 ) or chemicals like urea % NOx reduction is achieved

Selective Noncatalytic Reduction (SNCR) It is critical to operate at the mentioned temperature range. In lower temperatures, amonnia is oxidized and causes NOx formation! 

Selective Noncatalytic Reduction (SNCR)

Selective Catalytic Reduction (SCR) By the use of a catalyst bed together with ammonia, reduction is enhanced and also the reaction efficiency is increased at lower temperatures % NOx reduction is achieved NO 2 removal is also achieved

Selective Catalytic Reduction (SCR)

Some amount of amonnia may escape from the catalyst bed and be emitted from the stack. NH 3 is among hazardous air pollutants.  Catalysts which are effective and work at lower temperatures are more expensive.  In the case of sulfur-containing fuel, there is the problem of SO 2 →SO 3 conversion. Some special catalysts may be needed to prevent this conversion.  If particulate emissions are also high, catalyst fouling is possible because of dust loading. Some special measures/designs to reduce this effect are needed. 

Low-temperature oxidation followed by absorption NOx, is oxidized to N 2 O 5. The solubility of N 2 O 5 in water is higher. Removal efficiencies reaching 99% have been observed

Low-temperature oxidation followed by absorption Ozone is used as the oxidizing agent. N 2 O 5, forms nitric acid in the wet scrubber column. Caustic (NaOH) is used for the neutralization of the nitric acid. If there is also SO 2 problem, coastic has a dual benefit.

Low-temperature oxidation followed by absorption While reducing the gas emissions, the nitrate amount in the wastewater may be increased. 

Absorption If no ozone oxidation is done, NO/NO 2 molar ratio should be 1:1. Then strong aqueous alkaline solutions like NaOH and MgOH can be used Neutralization can be done by using H 2 SO 4, too. NO + NO 2 +2H 2 SO 4 → 2NOHSO 4 + H 2 0 (needs elevated temperatures since H2O drives the reaction to the left otherwise)

Catalytic absorption A single catalyst achieves both NOx and CO removal. First; NO, CO and unburnt HC, are oxidized to NO 2 and CO 2 NO2 is absorbed in the catalyst coated with potassium carbonate. NOx emissions as low as 2 ppm can be observed when this technique is used together with other NOx control techniques

Catalytic absorption When potassium carbonate is consumed, it is regenerated with dilute hydrogen and carbon dioxide. During this process, the absorbed nitrogen is released as molecular nitrogen, N 2. Since the regeneration must be done in oxygen-free medium, the gas flow in the column under regeneration is stopped.

Catalytic absorption There are not problems like the storage or emission of ammonia and similar chemicals

Corona-Induced Plasma A non-thermal plasma may be created with Corona discharge. Te plasma creates radicals which oxidize NO to NO 2 and N 2 O 5. Since these NOx species are soluble, as mentioned earlier, they can be stripped by absorption. A classical electrostatic precipitator can be used for creating the Corona discharge.

Heating and Cooling times N 2 +O 2  2NO The concentration of NO smostly depend on heating and cooling rates of flames

Ultra Low-NOx Burners Apart from staging of air/fuel, obtaining flue gas recycling inside the furnace can create extra staging effect according to the principle explianed earlier. This recycle effect is achieved by the feeding of the gas or the liquid with high pressure.

Ultra Low-NOx Burners This design can be enhanced by the additon of inserts (barriers which will increae the contact time) efficient combustion can be achieved with lower oxygen amount and lower temperature. Since the combustion efficiency is increaed, HC and CO emissions resulting from low oxygen can be reduced.