Nitrogen oxide chemistry in combustion processes

Mikko Hupa

Background - NOx

• Nitrogen oxides, ”NOX” = NO + NO2

• In combustion flue gases >95% NO and <5% NO2

• NO2 formed via NO

• In the atmosphere NO reacts to NO2, and further to

compounds such as HNO3 and nitrates (acidification)

• NO2 + hydrocarbons → photochemical ”smog”

• In some special cases also emissions of N2O

Background – NOx (ii)

• In some special cases also emissions of N2O

• N2O stable in the atmosphere – not part of the

acidification

• However, N2O is green-house gas and upper

atmosphere ozone destructor

• Nitrogen oxide chemistry in combustion complicated

– and fascinating!

Emissions are expressed in a variety of units – be aware!

100 ppm (3 % O2)

= 72 ppm (8 % O2)

= 83 ppm (8 % O2, dry gas)

= 237 mg NO2/m3 (3 % O2, dry)

= 71 mg NO2/MJ (LHV)

= 0.104 gr/DSCF (3 % O2, dry)

= 0.00116 gr/BTU (LHV)

100 ppm (3 % O2)

= 72 ppm (8 % O2)

= 83 ppm (8 % O2, dry gas)

= 237 mg NO2/m3 (3 % O2, dry)

= 71 mg NO2/MJ (LHV)

= 0.104 gr/DSCF (3 % O2, dry)

= 0.00116 gr/BTU (LHV)

Background -Nitrogen oxide emissions from various combustion

processes without reduction measures

(Hupa et al.)

Nitrogen oxide chemistry in combustion processes – Terms and concepts

• Chemical equilibrium and NO

• Thermal-NO (Zeldowich NO) and kinetics

• Fuel-NO

• Volatile-NO – HCN, NH3

• Char-NO

• Staged Combustion: Low Nox Technologies

• NO formation tendency characterization

• (NO chemistry in fluidised bed combustion)

Nitrogen species in a typical flue gas under chemical equilibrium conditions

Equilib

rium

concentr

ation

Temperature Temperature

Nitrogen species in a typical flue gas under chemical equilibrium conditions

• Most abundant ones

λ=1.1: N2, NO, NO2, N2O

λ=0.9: N2,HCN, NH3

• Equilibrium conc. of NOx decreases with temp.:

data from flue gas with λ=1.1:

T=1800 K 1200 ppm

T=500 K <1ppm

NO – Chemical equilibrium vs. Kinetics

Schematic view of NO formation in a boiler

Time

NO

concentr

ation

Equilibrium concentration (λ=1.1)

Typical measured concentration

Temperature

Furnace Superheaters

1.E-22

1.E-20

1.E-18

1.E-16

1.E-14

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

1.E+00

1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01

Mo

le f

racti

on

(-)

Time (s)

O2

CO

CO2

H2O

NO

NO2

N2O

H

O

OH

H2

HO2

H2O2

HCO

CO Vs. NO kinetics 1000 °C

Kinetic calculation, plug flow reactor

1.E-22

1.E-20

1.E-18

1.E-16

1.E-14

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

1.E+00

1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01

Mo

le f

racti

on

(-)

Time (s)

O2

CO

CO2

H2O

NO

Equilibrium NO

Equilibrium CO

1000 °C CO Vs. NO kinetics

Kinetic calculation, plug flow reactor

1.E-22

1.E-20

1.E-18

1.E-16

1.E-14

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

1.E+00

1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01

Mo

le f

rac

tio

n (

-)

Time (s)

O2

CO

CO2

H2O

NO

Equilibrium CO

Equilbrium NO

1500 °C CO Vs. NO kinetics

Kinetic calculation, plug flow reactor

1.E-22

1.E-20

1.E-18

1.E-16

1.E-14

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

1.E+00

1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01

Mo

le f

racti

on

(-)

Time (s)

O2

CO

CO2

H2O

NO

Equilibrium CO

Equilibrium NO

2000 °C CO Vs. NO kinetics

Kinetic calculation, plug flow reactor

NO – Chemical equilibrium vs. Kinetics

Schematic view of NO formation in a boiler

Time

NO

concentr

ation

Equilibrium concentration (λ=1.1)

Typical measured concentration

Temperature

Furnace Superheaters

Nitrogen chemistry in boilers

• At equilibrium: high temperatures -> high NO, low temperatures ->

no NO

• Formation of NO determined by kinetics – not equilibrium

• Decomposition at lower temperatures kinetically ”frozen”

• Great variations in NO emissions. Which factors influence NO-

formation kinetics?

• Major research activities to understand mechanisms of NO formation

since 1970’s

• NO emissions brought detailed chemistry into combustion engineering

Thermal-NO

• NO formed from N2 in combustion air

• Direct reaction N2 + O2 → 2NO

very slow even at high temperatures (N2

• Zeldowich (1946): atomic oxygen needed to react with N2:

O + N2 → NO + N (1)

• The N atom reacts further to yield a second NO

N + O2 → NO + O (2)

• d(NO)/dt = k1 · (O) · (N2)

Estimated rates of thermal-NO formation

atm/s

10-3

10-4

10-5

10-6

ppm/s

1000

100

10

1

1700 1800 1900 2000 2100 K

Temperature

Thermal NOx - Summary

• Molecular nitrogen in air reacts to form NO

• Reaction kinetically possible with atomic

oxygen

• Significant only if T > 1400 C

• Formation steeply dependent on temperature

• Main contributor to NO formation in gasolin

engines and gas combustion systems

Fuel-NO:

Gradual Exploration of the Mechanisms

Fuel NOx – Fuel Oil with Organic Nitrogen Addition

Air factor λ

NO

concentr

ation (λ=

1.0

)

Oil + pyridine (0.5% N)

Pure Oil (<0.01% N)

Based on Martin and Berkan, Air pollution and its control, AICHE symp. Series 68, Nr 126, 45, 1972

Fuel-NO

• NO formed from N in fuel

• Fuel-N organically bound - reactive

• Until the 1970’s fuel-N was not

considered a source of NO

Amines

Pyridine Pyrrole Quinoline

(1 (2

(3

1) Calculated as CaO 2) Calculated as Na 3) Not organic-N, but molecular-N, N2

NO formation – Fuel Oil Containing N

N2 NO

Nfuel Nvol

Ntar HCN NH3

Thermal

NO

N2

N2

NO formation – Fuel Oil Containing N

N2 NO

Nfuel Nvol

Ntar HCN NH3

Thermal

NO

N2

N2

Which one is more important?

Air factor λ

NO

concentr

ation

Thermal-NO

Fuel-NO

Air

Ar+CO2+O2

When burning heavy fuel oil or pulverized coal. Based on - Pershing et al., Influence of design variables on the production of thermal and fuel NO from residual oil and coal combustion, 66th annual AICHE Meeting, Philadelphia, 1973 - Pershing and Wendt, Effect of coal composition on thermal and fuel NOx production from pulverized coal combustion, The Combustion Institute Control States Section Spring Meeting, Columbus, Ohio, USA, 1976 - Wendt and Pershing, Combust. Sci. Technol. 16, 11, 1977 - Wendt, Prog. Energy Combust. Sci. 6, 201, 1980

Thermal-NO vs. Fuel-NO in Oil Firing

When burning pulverized coal. Based on Wendt and Pershing, Combust. Sci. Technol. 16, 11, 1977

NO

concentr

ation (λ=

1.0

)

Air factor λ

NO from volatiles

NO from char

Thermal-NO vs. Fuel-NO in Coal Firing

NO formation

N2 NO

Nfuel

Nvol

Nchar

HCN, NH3

Thermal

NO

NO

N2

N2

N2

Schematic view of NO emission

distribution by source

0

NO

x-e

mis

sio

ns,

mg N

O2/M

J

100

200

300

400

1000 1600 1200 1400

500

Thermal-NO

Fuel-NO

Prompt-NO

(Volatiles)

(Char)

(Combustion

air)

oC

Fuel NOx – How to Control?

• Traditional efficient combustion: Maximize the

three T’s: Temperature, Turbulence, Time

• Gives low CO and CxHy

• ...but effective conversion of Fuel-N to NO!

• ”Low NOx” concept: staged combustion to

convert Fuel-N to N2

• A variety of novel burner and furnace

technologies with Low-NOx

Conventional Burner for Solid Fuels

Fuel powder

+

Transport air

Combustion air

Char

combustion

Devolatilization

and

gas combustion Ash

Low-NOx Burner for Solid Fuels

Fuel powder

+

Transport air

Primary air

Primary

zone

l < 1

Secondary air

Secondary air

Secondary

zone,

“Burn-out

zone”

l > 1

How to Characterize Fuels with Respect to their NO Formation Tendency?

N2 NO

Nfuel

Nvol

Nchar

HCN, NH3

Thermal

NO

NO

N2

N2

N2

Experimental procedure #1

• 2-step burning of fuel in 5 vol-% O2 +Ar

- Pyrolysis: Ar through bed + secondary O2

- Char ox: Ar + O2 through bed

• Release curves integrated to

obtain cumulative CO2 and NO (NO I + NO II)

European bark

Tbed=850°C

Volatile NO and Char NO – Laboratory Tests (NO results given as weight-% N per original fuel)

Fuel-N

Vol NO (NO I)

Total NO (NO I + NO II)

Fuel-N

Vol-NO (NO I)

Total NO (NO I + NO II)

Volatile NO and Char NO – Laboratory Tests (NO results given as weight-% N per original fuel)

Fuel NOx Summary

• Fuel-N very reactive: 20-60 % conversion to NO

• Not very sensitive to temperature (cf thermal NO)

• Main contributor to NO in solid fuel combustion

• Different fuels have different fuel N bahavior

• Staged air combustion reduces fuel NO

Special Case:

NO chemistry in FBC

Nitrogen Oxides with Coal and Wood (Leckner et al.)

Coal Wood

Volatile, % db 40 80

N(fuel), % db 1.6 0.14

CFBC NO, ppm 50 60

NO Profiles in CFBC (Leckner at al.)

0 2 4 6 8 10 12

Reactor height, m

0

100

200

300

400

500

NO

x p

pm

Secondary air

Wood

NO Profiles in CFBC (Leckner et al.)

0 2 4 6 8 10 12

Reactor height, m

0

100

200

300

400

500 Secondary air

Wood

Coal

NO

x p

pm

NO formation for solid fuels

N2 NO

Nfuel

Nvol

Nchar

HCN, NH3

Thermal

NO

NO

N2

N2

N2

NO formation for solid fuels in CFBC

( N2 NO )

Nfuel

Nvol

Nchar

HCN, NH3

Thermal

NO

NO

N2

N2

N2

N2

NO Profiles in CFBC (Leckner et al.)

Modelling Attempts (Goel et al.)

0 2 4 6 8 10 12

Reactor height, m

0

100

200

300

400

500 Secondary air

Wood

Coal

NO

x p

pm

17th FBC Conference, May 21, 2003, Jacksonville, Fl.

Hu

pa

& K

ilpin

en

(1

99

5)

Fuel-N Conversion in CFBC, Coal

Nfuel

Nchar1

Nvol 2

NH33

NH35

HCN4

N2 9 N2

6

NO10 NO 8

N2O7

N2 15

NO13

N2O14

N2 16

N2O12

N2 11

NO Nchar NH3 HCN N2O

17th FBC Conference, May 21, 2003, Jacksonville, Fl.

Hu

pa

& K

ilpin

en

(1

99

5)

Fuel-N Conversion in CFBC, Wood

NO Nchar NH3 HCN N2O

Nfuel

Nchar1

Nvol 2

HCN4

NH35

NH33

N2 6

N2O7

NO 8 N2

9 NO10

N2 11

N2O12 NO13

N2O14

N2 16 N2

15

NO Emission with Wood-Coal (Leckner et al.)

Nitric oxide ppm

(6% O2)

0

50

100

150

Wood 50% (Energy)

Coal

NO Emission with Wood-Coal (Leckner et al.)

Nitric oxide ppm

(6% O2)

0

50

100

150

Wood 50% (Energy)

Coal

NO Emission with W&C (Leckner et al.)

Nitric oxide ppm

(6% O2)

0

50

100

150

Wood 50% (Energy)

Coal

NO Emission with W&C (Leckner et al.)

and Modelling Attempts (Engblom et al.)

Nitric oxide ppm

(6% O2)

0

50

100

150

Measured

Case 2

Case 1

Wood 50% (Energy)

Coal

Nitrogen oxide chemistry in combustion processes – Terms and concepts

• Chemical equilibrium and NO

• Thermal-NO (Zeldowich NO) and kinetics

• Fuel-NO

• Volatile-NO – HCN, NH3

• Char-NO

• Staged Combustion: Low Nox Technologies

• NO formation tendency characterization

• (NO chemistry in fluidised bed combustion)