NOx Emission

NOx Emission

The causes of high NOx emission from internal combustion is normally due to the following variable factors:

- Oxygen concentration and mixing pattern

- Nitrogen content of fuel

- Combustion temperature

These are influenced by:

1) Combustion condition

2) Type of Burner

3) Type of fuel

Note: Formation of thermal NOx depends on combustion temperature. Above 1,538 oC (2,800 oF), NOx formation rises exponentially with increasing temperature. (Stultz & Kitto 1992)

You see, in the Malaysian climatic condition the ambient temperature is relatively high all-year round. Our fuel are normally exposed to prolonged heat from transport to transit storage. This increases the gas pressure inside the enclosed container and definitely affects the physical dynamic of Nitrogen in the hydrocarbon fuel e.g diesel. (Hydrocarbon fuel is mainly made up of carbon, hydrogen, oxygen, Nitrogen and other radicals)

Okay, not withstanding those elements, we have to look into the boiler operation itself. We need the steam. To generate the operationally required steam pressure, fuel are being introduced at certain flowrate. When the steam pressure drops, we normally have to introduce more fuel and adjust the fuel-air mixture. I believe you can imagine the other variable factors on operational control that influences the factors that leads to high NOx emission as discussed in the first paragraph. Bearing this in mind, let us now proceed to the following consideration as to 'How to overcome this problem'.

Combustion Control - mechanism

a) Reducing peak temperature in the combustion zone (chamber).

b) Reducing the gas residence in the high-temperature zone

c) Reducing Oxygen concentration in the combustion zone

This is achieved by applying several techniques in the following areas;

1) Process Modification :

i ) Low - NOx burners

ii) Reburning

iii) Staged Combustion

iv) Flue gas recirculation

v ) Reduced air preheat and reduced firing rates

vi) Water or steam injection

2) Modification in Operating Conditions

Low - excess - air firing (LEA)

3) Flue Gas Treatment

i ) selective catalytic Reduction (SCR)

ii) Selective noncatalytic Reduction (SNCR)

The above is an overview on the available remediation methods for such problem where some costs for process modification may incur. Now, lets observe these available options one-by-one and see if it can be applicable to you.

1) (i) Low - NOx Burner

- applicable for new and existing power plants.

- for existing plant - system is retrofitable.

- limits the formation of nitrogen oxide by controlling the mixing of fuel and air.

- can reduce NOx emission by 40-60%

- not suitable for cyclone-fired boilers.

1) (ii) Reburning Technology

- Applicable for cyclone furnaces

- A secondary combustion zone concept

- Operates wtihin substoichiometry

1) (iii) Staged Combustion (Off Stoichiometric Combustion)

-   Burns fuel in two or more steps.

- By firing some of the burners with fuel-rich and the rest fuel-lean; (i.e by taking some of the burners out of service and allowing them only to admit air into the furnace, or by firing all the burners fuel-rich in the primary combustion zone and admitting the remaining air over the top of the flame zone. (Cooper and Alley, 1986)

      - Can reduce 20-50% NOx emission.

1) (iv) Flue Gas Recirculation (FGR)

- Rerouting some of the flue gasses back to the combustion Chamber (furnace)

- by using the flue gas from the economizer outlet, both the furnace air temperature and the furnace oxygen concentration can be reduced.

- very expensive!, but reliable.

- applicable for oil & gas fired boilers

- reduces NOx by 20-50%

1) (v) Reduced air preheat and reduced firing rates.

- lowers peak temperature in combustion zone (chamber)

- leads to higher emission of smoke and carbon Monoxides!

Thus, requiring its emission control ! - and leads to reduced operational flexibility.

1) (vi) Water or Steam Injection

- reduces flame temperature and thus thermal NOx.

- Effective for gas turbines

- reduces NOx emission by about 80%

- The energy penalties ;

a) For Gas Turbine - 1%

b) For Utility Boiler - 10%

c) For Diesel Fired Units - 25-35%

2) Low - Excess - air Firing (LEA)

- Simple and effective

- Excess air is the amount of air in excess of what is theoretically needed to achieve 100% combustion.

( Note : From studies elsewhere, it was shown that reducing excess air from an average of 20% to an average of 14% can reduce emission of NOx by an average of 19% (- cooper & Alley 1986)

3) (i) Selective catalytic Reduction

- more effective than combustion control.

- higher cost

- comman in Japan and in OECD Countries (Organization for Economic Co-operation and Development)

- uses ammonia as 'reducing agent' to convert to NOx to Nitrogen in the presence of a catalyst in a 'converter' situated upstream of the air heater.

- almost similar to our car's 'Catalytic converter' lah!

- Catalysts : Usually mixture of titanium dioxide, vanadium pentoxide and tungsten dioxide (Bounicore and Davis, 1992)

- Can remove 60-90% NOx

- Expensive (RM200-350/kW)

- potential ammonia slipstream in emission!

- also concern over anhydrous ammonia storage!

3) (ii) Selective noncatalytic reduction (SNCR)

- uses ammonia or urea based compounds!

- still development stage

- expected to reduce NOx emission by 30-70%

- lower capital cost

Conclusion

The most cost-effective methods of reducing emissions of NOx are the use of low - NOx burners and the use of low nitrogen fuels such as Natural Gas.

Please observe the following table illustration that summarizes the NOx reduction rates that are normally achieved through combustion modification and flue gas treatment system.

NOx reduction Tecnique Coal Oil Gas
Combustion modification
Low-excess- air firing	10 - 30	10 - 30	10 - 30
Staged Combustion	20 - 50	20 - 50	20 - 50
Flue gas recirculation	n.a.	20 - 50	20 - 50
Water/stream injection	n.a.	10 - 50	n.a.
Low-Nox burners	30 - 40	30 - 40	30 - 40
Flue gas treatment
Selective Catalytic reduction	60 - 90	60 - 90	60 - 90
Selective noncatalytic reduction	n.a	30 - 70	30 - 70
n.a Not applicable

NOx removal Efficiencies for Combustion Modifications and Flue Gas Treatment (percentage reduction in NOx)

Auxiliary Information

Removing Nitrogen from fuel 1?

- called Fuel Denitrogenation

- by mixing the fuel with Hydrogen Gas, heating the mixture and using a catalyst to cause nitrogen in the fuel and gaseous hydrogen to unite.

- produces ammonia and cleaner fuel.

References

1) Stultz & Kitto 1992

2) Cooper and Alley, 1986

3) Bounicore and Davis, 1992

Kindly pass your kind words around and see if we can meet in the training. It's always not the same!

Except, of course, our "Fish and Mother - in - Law " Story !

Cheerio.