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11. Combustion & PollutantsIntroduction:
• Pollutant emission control is a major factor in de-sign of modern combustion devices.
• Control of emissions may sometime involve
acompromise of thermal efficiency (fuel consumption).
11. Combustion & Pollutants 1 AER 1304–ÖLG
• (Pollutants of concern include: Particulate mattersoot, ash, aerosols); oxides of nitrogen; sulphur
oxides; carbon monoxide; unburned hydrocarbons; nitrous oxide, and carbon dioxide.
Local/Regional Air Quality Concerns:
• Combustion generated and regulated pollutantsare:
- Particulate matter; PM10 and PM2.5
11. Combustion & Pollutants 2 AER 1304–ÖLG
- Oxides of nitrogen; NOx (NO and NO2) - Ozone; O3 (air quality standards).
- Carbon monoxide; CO- Lead- Unburned and partially burned hydrocarbons- Sulphur dioxide
Regulated emissions:• Gasoline engines (SI):
- NOx, CO, unburned HC• Diesel engines (CI):
11. Combustion & Pollutants 3 AER 1304–ÖLG
- NOx, CO, unburned HC, Particulate Matter• Gas Turbines (Stationary and aircraft, limited):
- NOx
• Power plants:
- NOx, CO, Particulate Matter, SO2
11. Combustion & Pollutants 4 AER 1304–ÖLG
SI engine 3-Way Catalytic converter Air Toxics/Hazardous Air Pollutants:
• Close to 200 substances are listed as air toxics:
- Selected aliphatic, aromatic, and polycyclic aromatic hydrocarbons
- Selected halogenated hydrocarbons- Various oxygenated organic compounds- Metals and metal compounds- Polycyclic aromatic hydrocarbons with
nitrogen atoms in the structure
11. Combustion & Pollutants 7 AER 1304–ÖLG
- A list of other compoundsGreenhouse Gases tied to Global Warming:
• Kyoto Protocol identifies the following as theGreenhouse gases:
- Carbon dioxide, CO2
- Methane, CH4
- Nitrous oxide, N2O- Particulates, soot, aerosols- Stratospheric H2O
11. Combustion & Pollutants 8 AER 1304–ÖLG
- Tropospheric and stratospheric ozone, O3
- SulphatesStratospheric Ozone Destruction:
• Montreal (1987), London (1990) and Copenhagen(1992) Protocols cap the following:
- Methane, CH4
- Nitrous oxide, N2O- Methyl chloride, CH3Cl- Methyl bromide, CH3Br - Stratospheric H2O
11. Combustion & Pollutants 9 AER 1304–ÖLG
Chlorine loading of earth’s atmosphere• Stratospheric ozone shields earth from
ultravioletradiation.
• Most of this ozone is contained in a layer between20 and 50 km altitude.
• Three mechanisms control the level of ozone con-centration:
11. Combustion & Pollutants 11 AER 1304–ÖLG
- HOx cycle (H, OH, HO2
- NOx cycle (NO, NO2)- ClOx cycle [halomethanes: CFCl3(Freon-11),
CF2Cl2 (Freon-12); and CH3Cl)]
11. Combustion & Pollutants 12 AER 1304–ÖLG
Ozone removal in lower stratosphere NOx
formation in combustion:
11. Combustion & Pollutants 13 AER 1304–ÖLG
- Thermal NO: oxidation of molecular nitrogen in the postflame zone.
- Prompt NO: formation of NO in the flame zone (Fenimore mechanism).
- N2O-intermediate mechanism.- Fuel NO: oxidation of nitrogen-containing
compounds in the fuel.Relative importance of these three are dependent on the operating conditions and fuel. In most practical combustion devices the thermal NO is the main source.
11. Combustion & Pollutants 14 AER 1304–ÖLG
• The basic mechanism for thermal NO production is given by six reactions known as extended Zeldovich mechanism:
k
3r
- The contribution of reaction 3 is small for lean
11. Combustion & Pollutants 15 AER 1304–ÖLG
O + N2 uD1kf1r NO + N (N.1)
k
N + O2 uD2kf2r NO + O (N.2)
k
N + OH uD3k
f NO + H (N.3)
mixtures, but for rich mixtures it should be considered. Forward reaction 1 controls the system, but it is slow at low temperatures (high activation energy). Thus it is effective in post-flame zone where temperature is high and the time is available.
- Concentrations of 1000 to 4000 ppm are typically observed in uncontrolled combustion systems.
- From reactions 1-3, the rate of formation of thermal NO can be calculated:
d[NO]dt = k1f[O][N2] − k1r[NO][N] + k2f[N][O2]
−k2r[NO][O] + k3f[N][OH] − k3r[NO][H] (5.14)
11. Combustion & Pollutants 16 AER 1304–ÖLG
- To calculate the NO formation rate, we need the concentrations of O, N, OH, and H.
- In detailed calculations, these are computed using detailed kinetic mechanisms for the fuel used.
- For very approximate calculations, these may be assumed to be in chemical equilibrium.
- At moderately high temperatures N does not stay at thermodynamic equilibrium. A better approximation could be to assume N to be at steady-state.
11. Combustion & Pollutants 17 AER 1304–ÖLG
- From reactions 1-3, we have d[N]dt = k1f[O][N2] −
k1r[NO][N] − k2f[N][O2]
+k2r[NO][O] − k3f[N][OH] + k3r[NO][H] = 0 k k
k [NO][H]
1r 2f 2 3f
(5.15)- The reaction rate constants, in [m3/ kmol s], for 1-3
are as follows:
11. Combustion & Pollutants 18 AER 1304–ÖLG
OH ][k]+O[kNO ]+[kr3[ ]+NO][Or2]+2O][N[f1=ssN][
10
kkk11fr = 1= 3..88 ·· 10101110
7Texp(exp(exp(−−−425384680,370/T/T)/T) ) (5.16)
10
kk232ffr = 1= 3= 7...818 ·· 10106Texp(exp(−−45020,/T820)/T)
11. Combustion & Pollutants 19 AER 1304–ÖLG
·
k3r = 1.7 · 1011 exp(−24,560/T)
• Nlean combustion (2O-intermediate mechanism Φ <
0.8). This mechanism canis important in verybe
represented by:
11. Combustion & Pollutants 20 AER 1304–ÖLG
O + N O + M (N.4) H + N NO + NH(N.5) O + N NO + NO (N.6)
- This mechanism is important in NO control strategies in lean-premixed gas turbine combustion applications.
• It has been shown that some NO is rapidly pro-duced
in the flame zone long before there would be time to
11. Combustion & Pollutants 21 AER 1304–ÖLG
form NO by the thermal mechanism. This is also
known as the Fenimore mechanism:
- The general scheme is that hydrocarbon radicals form CN and HCN
CH + N HCN + N (N.7) C + N CN + N(N.8)- The conversion of hydrogen cyanide, HCN, to
form NO is as followsHCN + O uD NCO + H (N.9)
NCO + H uD NH + CO (N.10)
11. Combustion & Pollutants 22 AER 1304–ÖLG
NH + H uD N + H2 (N.11)
N + OH uD NO + H (N.3)- For equivalence ratios higher than 1.2,
chemistry becomes more complex and it couples with the thermal mechanism.
NOx emissions from SI engines:
• Nitric oxide forms in the high temperature
burnedgases during the combustion
process. During expansion, as the burned
11. Combustion & Pollutants 23 AER 1304–ÖLG
gas temperature falls, NO freezes out as the
decomposition chemistry becomes
extremely slow.
• The burned gas temperature, and the
amount ofoxygen in the burned gases, are the
primary variables affecting NO formation.
11. Combustion & Pollutants 24 AER 1304–ÖLG
NOx emissions from SI engines (Cont’d):
• Dilution of the unburned mixture with
EGR leadsto lower burned gas temperature
due to increased heat capacity of the
mixture per unit mass of fuel.
11. Combustion & Pollutants 25 AER 1304–ÖLG
• Dilution with air also increases the heat
capacity,but increasing the oxygen content
has a greater impact on NO formation rate.
11. Combustion & Pollutants 26 AER 1304–ÖLG
• it reduces peak cylinder pressures and burned
gasSpark retard reduces NO formation rate
because temperatures.
Unburned HC emissions from SI engines:
• Unburned HC emissions are various compounds ofhydrogen and carbon.
11. Combustion & Pollutants 27 AER 1304–ÖLG
• a lessor extent, oil.They are unburned or partially burned fuel, and to
• About 1000-3000 ppm under normal operatingconditions (before catalyst).
• flow into the engine.This corresponds to about 1 to 2 % of the fuel
CO emissions from SI engines:
11. Combustion & Pollutants 28 AER 1304–ÖLG
• Carbon monoxide (CO) is the incomplete oxida-tion product of the fuel carbon. It is present in
significant amounts in fuel-rich combustion products, and in high-temperature burned gases.
• Effectively determined by fuel-air ratio.
• Although in chemical equilibrium during combus-tion, recombination with oxygen is slow and CO
11. Combustion & Pollutants 29 AER 1304–ÖLG
levels freeze during expansion and exhaust strokes.Unburned HC emissions from CI engines:
• The unburned hydrocarbons in the diesel
exhaustcome from fuel which escapes
combustion because it is:
- too lean to burn due to over-mixing with air
11. Combustion & Pollutants 30 AER 1304–ÖLG
- too rich to burn because it did not mix with enough air
• mass HC which condense on the soot particlesThe lubricating oil contributes high molecular
in the exhaust and contribute to the particulates.
What is Particulate Matter?
• Soot:- Carbonaceous particles produced through
gasphase combustion process
11. Combustion & Pollutants 31 AER 1304–ÖLG
• Coke or cenospheres:- Carbonaceous particles
formed as a result of direct pyrolysis of liquid
hydrocarbon fuels
• Particulate Matter (PM):- Particles that can be collected on the probes
of measuring instruments such as filters- Originate from a variety of sources
Soot formation in combustion:
11. Combustion & Pollutants 32 AER 1304–ÖLG
• Conversion of a hydrocarbon fuel with
moleculescontaining a few carbon atoms into a
carbonaceous agglomerate containing some
millions of carbon atoms in a few milliseconds
•• Transition from a gaseous to solid phaseSmallest
detectable solid particles are about 1.5nm in diameter (about 2000 amu)
11. Combustion & Pollutants 33 AER 1304–ÖLG
• mixed systems soot does not form unless theIt is an
artifact of diffusive combustion. In preequivalence
ratio is richer than 1.7-2.0
11. Combustion & Pollutants 34 AER 1304–ÖLG
Combustion soot Soot/particulates in gas turbine and diesel engines:
• The soot particles form in the extremely fuel-rich zones of the burning fuel spray as the fuel
molecules pyrolyze and break down and then form increasingly higher molecular mass polycyclic aromatics and polyacetylenes.
• These eventually form nuclei for soot particleswhich grow and agglomerate.
11. Combustion & Pollutants 36 AER 1304–ÖLG
• within the combustion chamber (more than 90-95A substantial fraction of the soot formed oxidizes
%).Soot/particulates in gas turbine and diesel engines:
• PM emissions from diesel engines and gas tur-bines
consist of soot particles and volatile organics
(hydrocarbons and sulfates) absorbed into the particles in the exhaust.
11. Combustion & Pollutants 37 AER 1304–ÖLG
• Particles are agglomerates of 5 to 30 nm
diameterprimary soot particles. Aerodynamic
dimensions of agglomerates range from 10 to 1000
nm.
11. Combustion & Pollutants 38 AER 1304–ÖLG