combustion & pollutants

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


- 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):


- NOx, CO, unburned HC, Particulate Matter• Gas Turbines (Stationary and aircraft, limited):

- NOx

• Power plants:

- NOx, CO, Particulate Matter, SO2


SI engine:


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


- 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


- 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


- Stratospheric ozone, O3


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:


- HOx cycle (H, OH, HO2

- NOx cycle (NO, NO2)- ClOx cycle [halomethanes: CFCl3(Freon-11),

CF2Cl2 (Freon-12); and CH3Cl)]


Ozone removal in lower stratosphere NOx

formation in combustion:


- 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.


• 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


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)


- 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.


- 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:


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)


·

k3r = 1.7 · 1011 exp(−24,560/T)

• Nlean combustion (2O-intermediate mechanism Φ <

0.8). This mechanism canis important in verybe

represented by:


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


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)


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


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.


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.


• Dilution with air also increases the heat

capacity,but increasing the oxygen content

has a greater impact on NO formation rate.


• 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.


• 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:


• 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


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


- 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


• 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:


• 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)


• mixed systems soot does not form unless theIt is an

artifact of diffusive combustion. In preequivalence

ratio is richer than 1.7-2.0


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.


• 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.


• Particles are agglomerates of 5 to 30 nm

diameterprimary soot particles. Aerodynamic

dimensions of agglomerates range from 10 to 1000

nm.


• as mass of matter that can be collected from aFor

regulatory purposes, PM emissions are defined

diluted exhaust stream on a filter kept at 52o C.

Reading Assignment •

Study Chapter 15 in the Textbook


combustion & pollutants

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