1. Pollutant Formation in Combustion Systems

2. Pollutant Formation in Combustion Systems
NOx Formation
Thermal NOx
Prompt NOx
Fuel NOx
Carbon Monoxide (CO)
Volatile Organic Compounds (VOC)
Polycyclic Aromatic Hydrocarbons (PAH), Soot and Sub­micron Particulates (solid-phase pollutants)
Sulphur Compounds

3. NOx formation What is NOX?
NOx = Oxides of Nitrogen which are produced by combustion:
Nitric Oxide (NO)
Nitrogen Dioxide (NO2)
Nitrous Oxide (N2O)
Highly reactive due to lone electron at N atom
Not particularly toxic
Major precursor of photochemical smog (NO  NO2)
It is produced by most of combustion systems

4. Nitrogen Dioxide (NO2)
Brown, poisonous gas
Emissions of NO2 from most combustion < 10% of NOX
Adverse health effects include: lung irritation, bronchitis, pneumonia and a lowering respiratory resistance
Ambient limit = 120ppb [NEPC, 1998]
Significant direct emissions of NO2 occur from processes involving premixed flames:
- Indoor gas appliances (20‑100% of NOx)
- Gas turbines  yellow/brown plumes

5. Nitrous Oxide (N2O)
Relatively inert
Uses: Dental anaesthetic
Strong absorber of infrared radiation (~300 x CO2)
Stability = long atmospheric residence times (~150 yrs)
Hence, potentially significant greenhouse gas
Long life‑time also allows its transportation into stratosphere and participates in ozone depletion
Only significant from low-temperature processes (eg. Fluidised bed combustion)

6. chemical reactions
combustion activities

8. Sources of Nitrogen
Formation of NOX requires a source of nitrogen
Two sources of nitrogen:
a. Molecular nitrogen from air (1/2 N2 (from air) +1/2 O2 → NO)
Thermal or Zeldovich Mechanism.
Prompt‑Fenimore Mechanism (HC + N2).
Other minor mechanisms
b. Nitrogen chemically bound within fuel
Fuel NOx,
Most of NOx in the form of NO

9. Thermal NO
One of the most important issues for combustion engineers is: 'What are my NOx emissions?'
In most cases, unusually high NOx emissions are due to NO formed by the Thermal (Zeldovich) mechanism
Thermal NO mechanism involves the attack of molecular nitrogen (N2) and atomic nitrogen (N) by oxygen (O2) and oxygen‑containing radicals (O, OH). This can occurs in oxygen rich mixture.
First identified by Zeldovich (1946) and extended by Fenimore and Jones (1957)
Described by the following reactions:
N2 + O → NO + N (R.1)
N + O2  NO+O (R.2)
N + OH  NO + H (R.3)

10. Westenberg (1971) invoked the steady‑state approximation and determined that the maximum NO formation rate is given by:
Hence, [NO] depends on:
Temperature (the higher the temp, strongly the higher the NO formed)  high temp environment
O2 concentration (the higher the oxygen conc, the higher the NO formed)  oxygen-rich environment
Residence time

11. Prompt NO Fenimore (1971) observed an additional formation of
NO which could not be explained by the thermal
mechanism
NO formed close to the burner (hence ‑ "prompt' NO)
Effect is not observed under very fuel‑lean conditions or in systems with H2 or CO as fuel
Mechanism involves the attack of N2 by hydrocarbon fuel fragments, mainly CH radicals and C‑atoms.
The Prompt‑Fenimore mechanism is initiated mainly by R.4 (the formation of HCN) with a lesser contribution from R.5:
CH + N2 → HCN + N (R.4)
C + N2 → CN + N (R.5)

12. HCN is subsequently oxidised to NO (see diagram)
Prompt mechanism dominates for hydrocarbon combustion in fuel‑rich, in both premixed and diffusion flames

13. Minor Mechanisms
1. N2O‑Intermediate Mechanism:
It occurs under fuel‑lean, low‑temperature conditions
Minor source of NO in most practical combustors
Mechanism is given by:
O + N2 + M → N2O + M (R.6)
O + N2O → NO + NO (R.7)
H + N2O → NO + NH (R.8)

14. 2. NNH-Intermediate Mechanism:
It is observed under laboratory conditions
H2 and CH4 (high H) fuel‑rich, laminar premixed flames
Yet to be definitely observed in practical combustors
Mechanism is given by:
H + N2 → NNH (R.9)
NNH + O → NO + NH (R.10)

15. Fuel‑NO 1. The Nature of Fuel‑Nitrogen:
Nitrogen in coal (and oil) originates in the plant material from which the fuel is formed
Plants contain nitrogen in the form of proteins, amino acids, alkaloids, chlorophyll and porphyrins
These were transformed, during the coalification process, into polycyclic aromatic compounds with pyridinic, pyrrolic or other functional groups
Nitrogen content of coals typically vary between 1‑2.5 wt% and is largely independent of rank

16. 2. Coal Combustion and the Release of Fuel‑Nitrogen:
First stage of coal combustion is rapid devolatilisation of the coal
Devolatilisation: volatile components such as light hydrocarbons and tars are released and then oxidised in the gas~phase at very short timescales (< 10 ms)
The solid product of coal pyrolysis is the char which is oxidised at much slower timescales (~ 1s)
Fuel‑nitrogen is released during both pyrolysis and char combustion but in very different ways
Partitioning of fuel‑nitrogen between the volatiles and char depends on pyrolysis conditions (normally approx. equal)

17. 3. Nitrogen released with the volatiles:
Nitrogen contained within the volatiles is released as, or rapidly converted to compounds such HCN, NH3 or HNCO
These simple nitrogenous species then react in the gas phase to form either NO, N2O or, under fuel‑rich conditions to N2 (see next slide)
Relative amounts of NO, N2O and N2 depends strongly on the local O2 concentration during pyrolysis and temperature

18. NOX formation is reduced if volatiles are released under fuel‑rich conditions
Pulverised‑fuel combustors ‑ NOX control techniques include: low‑NOX burners, flue gas recirculation (fuel-lean) and air staging
NO emissions are reduced in these techniques by changes in stoichiometry and/or temperature near the burner thus reducing both volatile‑NO and thermal‑NO (important to take into account in combustion)

19. 4. Char combustion and the Fate of Char‑Nitrogen:
The fate of char‑nitrogen is still leading edge research
Primary product of char‑N oxidation is NO
However, both HCN and HNCO are observed at low temperatures (Ashman et al., 2000)
NO may be reduced to N2 by direct reaction with the char surface or by a char‑catalysed reaction with CO
Effective techniques aimed specifically at controlling char‑NO are not available
5. In principle, NOx is reduced if it is reacted with C or CxHy  concept of reburn

20. Summary of NOx Formation

22. References
Ashman, P1, Haynes, B.S., Nicholls, P.M. and Nelson, P.F. (2000). Interactions of gaseous NO with char during the low temperature oxidation of coal chars. 28th Symp. (Int.) on Combustion,
Bowman, C.T. (1992). Conrol of combustion- generated nitrogen oxide emissions: technology driven by regulation, 24th Symp. (Int.) on Combustion, The Combustion Institute, pp 859‑878.
Fenimore, C.P. (1971). Formation of nitric oxide in premixed hdrocarbon fames. 13th Symp. (Int.) on Combustion, The Combustion Institute, pp 373‑379.
Fenimore. C.P. and Jones, G.W. (1957). The water catalysed oxidation of carbon monoxide by oxygen at high temperatures. J. Phys. Chem, 61, pp 651‑654.

23. Pershing, D. W. and Wendt, LO. L. (1976)
Pershing, D.W. and Wendt, LO.L. (1976). Pulverized coal combustion: the influence of flame temperature and coal composition on thermal and fuel NOx, 16th Symp. (Int.) on Combustion, The Combustion Institute, pp 389‑399.
Westenberg, A.A. (1971). Kinetics of NO and CO in lean premixed hydrocarbon air flames, Combust. Sci. Technol., 4, pp 59‑64.
Zeldovich, Ya.B. (1946). The Oxidation of nitrogen in combustion explosions. Acta Physiochimica, USSR, 21, pp 577‑628.

24. Further Reading
Turns, S.R. (1996). "An Introduction to combustion concepts and applications", McGraw‑Hill, Chapter 15 and pp143‑146 (1st Edition)
Borman, G.L. and Ragland, KM. (1998). "Combustion Engineering", WCB/McGraw‑Hill, pp 125‑138
Wendt, LO.L. (1995). Combust. Sci. Tech., 108, pp 323‑344
van der Lans, R.P., Glarborg, P. and Darn‑Johansen, K. (1997). Prog. Energy Combust. Sci., 23, pp 349‑377

25. CO (carbon monoxide) formation
All hydrocarbon fuels are converted to CO2 via CO
CH4 → CH3 → CH2O → CHO → CO → CO2
Conversion of CO to CO2 occurs largely downstream of the flame‑zone and is an equilibrium process which occurs relatively slowly
Emission of CO as a stable product is minimised if CO → CO2 is allowed to proceed to completion
There are 3 processes leading to incomplete combustion:
Insufficient Oxygen (low O2)
Aerodynamic Quenching (low temp)
Impingement Quenching (low temp)

26. 1. Insufficient Oxygen
In this case, there is simply not enough O2 present in the downstream gases to fully oxidise the CO to CO2
Usually as a result of poor design or operation and, in flames, often related to inadequate mixing of secondary air
2. Aerodynamic Quenching
Caused by a rapid decrease in the temperature of the postflame gases, either by expansion of the gases or by mixing with ambient air

27. Rapid decrease in temperature results in kinetic "freezing" of the CO at the equilibrium condition
Aerodynamic quenching is a serious problern for spark-ignition engine when too much air used (not really for open‑flame systems)
3. Impingement Quenching
Quenching occurs as a result of the hot postflame gases impinging on a relatively cold surface
Again, a problem in internal combustion engines

28. CO Toxicity

29. Balance between CO and NO
We have seen that CO is largely a product of insufficient combustion (low O2, rapid cooling, etc)
In comparison, the formation of NO which is enhanced by conditions which could be termed as over‑aggressive combustion (high O2, high temp, etc)
There is a fine balance between the formation of NO and CO such that control measures which are effective against one species often result in an increase in the other species
Fortunately, the two mechanisms are decoupled somewhat since NO formation occurs primarily in the near‑burner zone and CO consumption occurs in the postflame gases.

31. Volatile Organic Compounds (VOC’s)
Volatile organic compounds (VOC's) are also products of incomplete combustion
VOC consist of various unburned fuel fragments, aliphatic and aromatic hydrocarbons and partially oxidised fuel fragments
The formation and destruction of VOC's occurs much closer to the flame front (higher temp zone) than does CO and hence greater extents of incomplete combustion cause the emission of VOC's
VOC's play a key role in photochemical smog cycles (reactions in atmosphere triggered by photon of the sunray)

32. PAH's, Soot & PM10 (solid phase pollutants)
What are PAH's, Soot & PM10?
PAH's (polycyclic aromatic hydrocarbons) are clusters of aromatic rings up to about 4 rings in size.
Soot is formed when, under appropriate conditions, PAH's grow in size until they reach approximately 20‑50 nm
PM10 is a general term given to all air‑borne particulates which have a size < 10 microns

33. PAH's (polycyclic aromatic hydrocarbons):
PAH's are formed in, and may be emitted from, combustion processes under very fuel‑rich conditions
The main cause of concern with PAH's is that many species are known mutagens, co‑carcinogens, or carcinogens
While PAH's are oxidised quite rapidly in the urban atmosphere, they may be stabilised considerably by adsorption to the surface of particulate (e.g. soot)
PAH's then become quite long-lived and are capable of penetrating deep into the human lungs

34. Soot:
The health effects of soot are serious and are related to the transport of fine particles within the lungs
PM10 is the environmental standard used to quantify airborne particulates from all sources
Soot particles also contribute to visible pollution and haze within the urban environment
Soot formation is encouraged within some combustion applications on the basis of the significantly enhanced radiative heat transfer obtained from sooting flames

35. PAH & Soot Formation:
PAH's and soot are formed mainly in diffusion flames and in diesel engines (also from I.C. engines with lubricant oil leakage)
A precursor species (believed to be acetylene) is formed under fuel‑rich conditions
Acetylene and other hydrocarbon fragments combine through pyrolytic (without oxygen) reactions to form progressively larger ring structures (PAH's)
Without competition with oxidation reactions, which break ring structures, the clusters continue to increase in size

36. Small particles of a critical size are formed by the growth of these aromatic clusters by both chemical and physical means, i.e. coagulation
At this point, the molecules are identifiable as primary soot particles
The soot particles continue to grow by both chemical (surface growth) and physical (particle agglomeration) processes whilst they remain within the bath of precursor species, clusters and adolescent soot particles

37. Sulphur Compounds
All of the sulphur contained within various fuels is emitted from combustion processes as either SO2 or SO3
The ultimate fate of oxides of sulphur in the environment is transformation to H2SO4 (acid rain)
Only two possible abatement procedures:
remove S before combustion (from the fuel)
remove SOX after combustion (from the flue gases)
Most common technique is the reaction of SO2 in the flue gas with limestone (CaCO3) or lime (CaO)
A slurry of limestone or lime is sprayed in an aqueous absorption tower through which the flue gas is passed

38. References
Muzio, L.J. and Quartucy, G.C. (1997). Implementing NOx control: Research to Application, Prog. Energy Cornbust. Sci., 23, pp 233‑266.
Wendt, LO.L. (1995). Mechanisms governing the formation and destruction of NOx and other nitrogenous species in low NOx Coal Combustion Systems, Combust. Sci. Tech., 108, pp 323‑344
Further Reading
Turns, S.R. (1996). "An Introduction to combustion concepts and applications", McGraw‑Hill, chapter 15, pp 291‑293 and ppl43‑146 (1st Edition)

39. Borman, G. L. and Ragland, KM. (1998)
Borman, G.L. and Ragland, KM. (1998). "Combustion Engineering", WBIMcGraw‑Hill, pp 125‑138 and pp 413‑421
Wendt, J.0.L. (1995). Combust. Sci. Tech., 108, pp 323‑344
van der Lans, R.P., Glarborg, P. and Darn‑Johansen, K. (1997). Prog. Energy Combust. Sci., 23, pp 349‑377