Nitrogen Oxides (NOx)

Chapter 12Page 147-168

NOx emissions include:

• Nitric oxide, NO, and Nitrogen dioxide, NO2, are normally categorized as NOx

• Nitrous oxide, N2O, is a green house gas (GHG) and receives special attention

Smog precursors:

• NOx, SO2, particulate matter (PM2.5) and volatile organic compounds (VOC).

smog calphotochemi O VOCs NOozone level Ground

3Sunlight

x

“Developing NOx and SOx Emission Limits” – December 2002, Ontario’s Clean Air Plan for Industry

Broad base of sources with close to 50% from the Electricity sector in 1999

NOx reaction mechanisms:

NO O 21 N

21

22

• highly endothermic with hf = +90.4 kJ/mol

• NO formation favoured by the high temperatures encountered in combustion processes

Zeldovich mechanism (1946):

N NO O N1-

1

k

k

2

O NO O N2-

2

k

k

2

H NO OH N3-k

3k

k+1 = 1.8 108 exp{-38,370/T}k-1 = 3.8 107 exp{-425/T}

k+2 = 1.8 104 T exp{-4680/T}k-2 = 3.8 103 T exp{-20,820/T}

k+3 = 7.1 107 exp{-450/T}k-3 = 1.7 108 exp{-24,560/T}

N NO O N1-

1

k

k

2

k+1 = 1.8 108 exp{-38,370/T}k-1 = 3.8 107 exp{-425/T}

Rate-limiting step in the process

K+1 is highly temperature dependent

Combine Zeldovich mechanism with

H O OH O 24-k

4k

To obtain

]OH[k ]O[k]NO[k

1

]O[k]NO[kk

- ]N[k [O] 2

dtd[NO]

322

1-

22

22-1-

21

]N[ [O]k 2 dt

d[NO]2 1If the initial concentrations of [NO]

and [OH] are low and only the forward reaction rates are significant

Modelling NOx emissions is difficult because of the competition for the [O] species in combustion processes

“Prompt” NO mechanism (1971):

N HCN N CH 2

H NO OH N

O NO O N 2

N CO NO O HCN 2

This scheme occurs at lower temperature, fuel-rich conditions and short residence times

Fuel NOx

Organic, fuel bound nitrogen compounds in solid fuels

C-N bond is much weaker than the N-N bond increasing the likelihood of NOx formation

Example of proposed reaction pathway for fuel-rich hydrocarbon flames

NOx control strategies:

• Reduce peak temperatures• Reduce residence time in

peak temperature zones• Reduce O2 content in

primary flame zone

• Low excess air• Staged combustion• Flue gas recirculation• Reduce air preheat• Reduce firing rates• Water injection

Combustion Modification Modified Operating Conditions

Control strategies:

• Reburning – injection of hydrocarbon fuel downstream of the primary combustion zone to provide a fuel-rich region, converting NO to HCN.

• Post-combustion treatment include selective catalytic reduction (SCR) with ammonia injection, or selective noncatalytic reduction (SNCR) with urea or ammonia-based chemical chemical injection to convert NOx to N2.

SCR process:

4 NO + 4 NH3 + O2 4 N2 + 6 H2O

2 NO2 + 4 NH3 + O2 3 N2 + 6 H2O

SNCR process:

4 NH3 + 6 NO 5 N2 + 6 H2O

CO(NH3)2 + 2 NO ½ O2 2 N2 + CO2 + 2 H2O

Low NOX burners:

Dilute combustion technology

Industrial furnace combustion:• Natural gas is arguably “cleanest” fuel – perhaps not

the cheapest.• Independent injection of fuel and oxidant streams

(“non-premixed”). Industrial furnaces have multi-burner operation.

• Traditional thinking has been that a rapid mixing of fuel and oxidant ensures best operation.

• This approach gives high local temperatures in the flame zone with low HC but high NOx emissions.

• Heat transfer to a load in the furnace (radiatively dominated) must be controlled by adjustment of burners.

• High intensity combustion with rapid mixing of fuel and oxidant• High temperature flame zones with low HC but high NOx• Furnace efficiency increased by preheating the oxidant stream

A conventional burner

Lance Air

FuelGas

Combustion Air

Dilute oxygen combustion:• An extreme case of staged-combustion.• Fuel and oxidant streams supplied as separate

injections to the furnace.• Initial mixing of fuel and oxidant with hot combustion

products within the furnace (fuel-rich/fuel-lean jets).• Lower flame temperature (but same heat release)

and more uniform furnace temperature (good heat transfer).

• Low NOx emissions – “single digit ppm levels”

Strong-jet/Weak-jet Aerodynamics

•Strong jet = oxidant

•Weak jet = fuel

Strong-jet/Weak-jet aerodynamics

CGRI burner

Pilot burner portUV scanner port

Fuel nozzle

Air/oxidant nozzle

• Dilute oxygen combustion operation with staged mixing of fuel and oxidant• No visible flame (“flameless” combustion)• More uniform temperature throughout flame and furnace• Low HC and NOx emissions

Queen’s test facility

Queen’s test facility

2750-362 0 750 1750 54624500 5100

1362z

0

1000

500

3000

3362

y

x

-362

0

B2B1 B3

Water-cooled floor panels

SideView

Plenum Wall

FurnaceExhaust

TopView

CGRI burner in operation at 1100OC

CFD rendering of the fuel flow pattern

CGRI burner performance (1100OC)

Oxygen-enriched combustion:

• Oxidant stream supplied with high concentrations of oxygen.

• Nitrogen “ballast” component in the oxidant stream is reduced – less energy requirements and less NOx reactant.

• Conventional oxy-fuel combustion leads to high efficiency combustion but high temperatures and high NOx levels.

• Higher efficiency combustion leads to lower fuel requirements and corresponding reduction in CO2 emissions.

• Does this work with dilute oxygen combustion???

NOx emissions as a function of oxygen enrichment

2

2

2 2

OO

O O A

m 100

m + m

Firing rate as a function of oxygen-enrichment level required to maintain 1100oC furnace temperature

Is oxygen-enrichment a NOx reduction strategy?

• NOx emissions are reduced at high oxygen-enrichment levels … but …

• Only at quite significant enrichment levels, and• With no air infiltration (a source of N2).

NOx emissions as a function of furnace N2 concentration

Capabilities of oxygen-enriched combustion:

• Dilute oxygen combustion systems can work with oxygen-enriched combustion.

• NOx emissions are comparable to air-oxidant operation and NOx reductions are limited by air infiltration.

• NOx emissions also limited by N2 content of the fuel.• Primary benefit is energy conservation (reduced fuel

consumption) and associated CO2 reduction.

Limitations of oxygen-enrichment:

• This is not a totally new technology !!!• Cost of oxygen – high purity O2 is expensive, lower

purity is more feasible in some situations.• Infrastructure costs – oxygen supply and handling.• Furnace modifications – burners, gas handling, etc.

Final Examination• Tuesday, April 22, 1900h• 3rd Floor Ellis Hall• Open book, open notes• Red or gold calculator

CHEE 481 Tutorial Session• Saturday, April 19, 0900h

• Dupuis Hall 217