Pollutant Formation in Combustion Systems
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
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
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
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)
chemical reactions combustion activities
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
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)
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
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)
HCN is subsequently oxidised to NO (see diagram) Prompt mechanism dominates for hydrocarbon combustion in fuel‑rich, in both premixed and diffusion flames
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)
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)
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
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)
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
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)
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
Summary of NOx Formation
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.
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.
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
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)
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
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
CO Toxicity
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.
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)
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
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
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
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
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
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
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)
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