course title: heat and combustion technology … ntnu course title: heat and combustion technology...

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Course title: Heat and Combustion Technology(Combustion and Emissions)

Course number: TEP 4170

Dr. Md. Nurun NabiSenior Postdoc ResearcherEnergy and Process Engineering DeptNTNU, NORWAY

Dr. Md. Nurun NabiProfessor

Mechanical Engineering DeptRUET, BANGLADESH

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Text books

Title Author ISBN

1. An Introduction to Combustion S R Turns 978-007-126072-52. IC Engine Fundamentals J B Heywood 0-07-100499-83. IC Engines V Ganesan 0-07-462122-X

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LECTURE CONTENT

Overview

Introduction to diesel and gasoline combustion

Pollutants from combustion

Quantification of emissions

Emissions from premixed combustion

Emissions from nonpremixed combustion

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PROCESS

Fuel is injected into compressed air Fuel evaporates and mixes with the hot air Auto-ignition with the rapid burning of the fuel-air (Premixed) This is followed by Diffusion burning

CHARACTERISTICS OF DIESEL COMBUSTION

Heterogeneous Turbulent Diffusion flame

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DIESEL COMBUSTION PROCESS

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(p-5

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Diffusion

Premixed

1 & 2 A rich fuel mixture with no premixed oxygen produces a yellow sooty diffusion flame

3 & 4 Oxygen premixed flame produces no soot and the flame color is produced by molecular radical band emission.

Premixed flame is a flame in which the oxidizer mixes with the fuel before it reaches the flame front. This creates a thin flame front as all of the reactants are readily available.

Diffusion flame is a flame in which the oxidizer mixes with the fuel by diffusion. Flame speed is limited by the rate of diffusion. Diffusion flames tend to burn slower and to produce more soot than premixed flames. There may not be sufficient oxidizer for the reaction to go to completion.

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Premixed and diffusion flames

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http://upload.wikimedia.org/wikipedia/commons/b/b0/Bunsen_burner_flame_types_.jpg

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FUEL INJECTION

PURPOSES OF FUEL SPRAY

Atomization

Fuel distribution

Fuel-air mixing

TYPICAL INJECTOR

Currently high injection pressure to reduce emission

Single or multiple holes with very small hole diameter

Injection timing is set before TDC for best torque

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Lean A/F mixture

Rich A/F ratio

Spray plume from injector

Stoichiometric A/F

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EXAMPLE OF INJECTION PROCESS

(Cummins six-cylinder engine, rated at 160 kW, 2500 rpm)

Typical injection duration 22

Typical BSFC 220 g/kWh

Typical fuel deliver per cylinder per injection

- 0.078 gm (100 mm3)

Average fuel flow rate during injection

- 68 mm3/ms

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AUTO IGNITION PROCESS

PHYSICAL PROCESSES (Physical delay)

Droplet atomization

Evaporation

CHEMICAL PROCESSES (Chemical delay)

Chemical reaction

To reduce ignition cetane improver is added to diesel fuel

EHN (ethyl hexyl nitrate), DTBP (Di tertiary butyl peroxide)

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Diesel knockN

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TDC

Injection point

Normal combustion

Diesel knock

I. delay

Compression pressure

Cyl

inde

r pre

ssur

e

Crank angle degree

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Gasoline combustion

A/F mixture is compressed

Ignition occurs with a spark plug

Homogeneous combustion

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100 80 60 40 20 TDC 20 40 60 800

10

20

30C

ylin

der p

ress

ure

(bar

)

B

I II III

A

C

Crank angle (degree)

A Start of spark B Start of ignitionC Maximum pressureI Ignition lagII Flame propagationIII After burning

Stages of gasoline combustion

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Normal combustion Slight knock Heavy knock

CA

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Combustion Chamber

Crank Shaft

Piston

Connecting Rod

TDC

BDC

Gasket

VC

VS Stroke

Stroke

Bore

Crank Radius (crank throw)

Crank Radius

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Details of an engine

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TDCatpistonaboveVolumeBDCatpistonaboveVolume

cr

cVsVcV

cr

Bore

VS

TDC

BDC

Ve = k.Vs rc = compression ratioVc = clearance volumeVs = displacement or swept volume = d2l/4l = stroked = bore diameterVe = capacity of the enginek = number of cylinders

Stroke

Compression ratio and capacity of an engine

VC

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Power and mean effective pressure

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Torque and brake power

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b = (BP*100)/(mf * CV)b = (BP*100)/(vf * CV)i= (IP*100)/(mf * CV)i= (IP*100)/(vf * CV)

i : indicated thermal efficiency (%)b : brake thermal efficiency (%)m : mechanical efficiency (%)mf : fuel mass flow rate (kg/sec)v : volumetric efficiency (%)Vf : fuel volume flow rate (m3/sec)CV: calorific value of fuel CV is kJ/kg for mf in kg/secCV is kJ/m3 for vf in m3/sec

m = (BP*100)/IP = (Pb l a N k/) / (Pi l a N k/)

m = (BP*100)/IP = (Pb *100)/Pi

Efficiencies of an engine

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IPrateflowfuelisfc

BPrateflowfuelbsfc

Powerrateflowfuelsfc

sfc: specific fuel consumption (kg/kWh)bsfc: brake specific fuel consumption (kg/kWh)isfc: indicated specific fuel consumption (kg/kWh)

Specific fuel consumption

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Problem

During a trial (60 minutes) on a single cylinder oil engine having cylinder diameter 300 mm, stroke 450 mm and the working on the four stroke cycle. The following observations were made:

Total fuel used=9.6 liters; calorific value of fuel=45000 kJ/kg. Total number of revolutions=12624; indicated mean effective pressure (imep)=6.9 bar. Net load on the brake=1575N; diameter of the brake wheel drum=1.82 m; density of the fuel=0.8 g/cm3. Determine (i) indicated power; (ii) brake power; (iii) mechanical efficiency and (iv) BSFC

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LECTURE CONTENT

Overview






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Overview

Control of pollutants is a major factor in the design of modern combustion systems.

PollutantsParticulate matter (PM) (soot, fly ash, aerosols, etc), SOx (SO2 and SO3 ), NOx (NO and NO2 , N2 O, unburnt hydrocarbons, CO and CO2

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Concern (International Treaty/US regulation

Combustion generated or related species

Local/regional air quality (national ambient air quality standards)

Criteria pollutants: particles (PM10)a, O3 , NO2 , SO2 , CO, lead

Air toxics/hazardous air pollutants(1990 clean air act amendments)

189 substances: selected aliphatic, aromatic hydrocarbons; selected halogenated hydrocarbons; various oxygenated organics; metals and other compounds

Greenhouse effect/global warming(Kyoto protocol, 1997)

CO2 , CH4 , N2 O, water vapor, tropospheric and stratospheric O3 , C (carbon soot), sulfatesb

Stratospheric O3 destruction(Montreal protocol, 1987)

CH4 , N2 O, CH3 Cl, CH3 Br, stratospheric H2 O, atmospheric O3

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bSulfates (SO2 /SO42-) act as anti-greenhouse gases.

Table 15.1 Combustion generated or related air pollution concerns

Overview

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Effect of pollutantsPrimary pollutants (emitted directly from sources) Secondary pollutants (formed via reactions with primary pollutants)

Effects- Reduced visibility; resulting from carbon based PM, sulfates, nitrates, organic compounds and NO2.

- Increased fog formation and precipitation; resulting from high SO2 that form H2 SO4

- Altered temperature and wind distributions- Acid rain (regional climate for HNO3 )

- Global climate (greenhouse gases: water vapor, CO2 , CH4 , O3 , CFCs, N2 O)

Harm to vegetation- SO2 , peroxyacetal nitrate (PAN), C2 H4 destroy chlorophyll and disrupt photosynthesis

NO2 + H2 O →

HNO2 + HNO3HNO2 →

HNO3 + 2NO + H2 O 4NO + 3O2 + 2H2 O →

4HNO3

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O(g)NO(g)Sunlight(g)2NO

NO2 (g) + O2 + hydrocarbons CH3 CO-OO-NO2 (g) (PAN) Sunlight

Effect of pollutants

Soiling and deterioration of materialsPM soils clothing, building, reducing aesthetic quality, acid and alkaline particle those containing sulfur corrode paint, electrical contacts and textiles.

Increase sickness and mortality- Aggravate pre-existing respiratory ailments- Carbon based particles contain adsorbed carcinogens- Effect of CO to health is well documented - Secondary pollutants (O3 , organic nitrates, oxygenated hydrocarbons, aerosol formed primary by the reaction of NO and different hydrocarbons) create photochemical smog in presence of sunlight.N2 (g) + O2 (g) = 2NO(g); 2NO(g) + O2 (g) = 2NO2 (g); O + O2 = O3 ; NO(g) + O3 (g) = NO2 (g) + O2 (g)

Stratosphere- Catalytic destruction of stratospheric ozone by NO+O3 = NO2 + O2 (Increased UV

radiation on the earth)

Photochemical smog is a mixture of pollutants which includes particulates, nitrogen oxides, ozone, peroxyacetyl nitrate (PAN), unreacted hydrocarbons, etc. The smog often has a brown haze due to the presence of nitrogen dioxide. It causes painful eyes.

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LECTURE CONTENT

Overview






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Quantification of emissionsEmissions indices: Emission index for species i is the ratio of the mass of species i to the mass of fuel burned

MWFiMWx

2XCOXCOiX

iEI (15.2)

Xs = mole fractions; MW = molecular weightx = number of moles of carbon in a mole of fuel

mi = mass of species imF = mass of fuel

burntFmemittedim

iEI (15.1)

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Example 15.1

A SI engine is running on a dynamometer test stand and the following measurements of exhaust products are made: CO2 = 12.47%, CO = 0.12%, O2 = 2.3%, C6 H14 (equivalent) = 367 ppm, NO = 76 ppm. All concentrations are by volume on a dry basis. The engine is fuelled by isooctane. Determine the emission index of the unburned hydrocarbons expressed as equivalent hexane.

Solution: We know

MWF

MWix2XCOXCO

XiEIi

EIi = (367 *10-6)/(0.1247+0.0012) * (8*86)/(114)

EIC6H14 = 0.0176 kg/kg or 17.6 g/kg

MWF

MWix14H6XC2XCOXCO

XiEIi

Considering unburned hydrocarbons

EIC6H14 =(367 *10-6)/(0.1247+0.0012+6*367*10-6) * (8*86)/(114)

EIC6H14 = 0.0173 kg/kg or 17.3 g/kg (considering UBHC)

Xi = 367 ppm = 367 *10-6

XCO = 0.12% = 0.0012XCO2 = 12.47% = 0.1247x = 8MWi = C6H14 = 72+14=86 g/molFuel: C8H18

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Mass specific emissions are expressed as g/kWh

(15.11) (g/kWh)15.1) (from

outputpowerbrake pollutantofrateflowmass(MSE) emissionspecificMass

WiEIFm

i(MSE)


EIi = mi emitted / mF burnt

(MJ/kg) combustion of heat fuel Δhc

(g/kg)indexemissioniEI(kW)outputpowerW

(kg/hr)ratemassfuelF.

m

(g/MJ)Δhc

iEIfuel in Energy

iEI

(15.12)

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Equivalence ratio () is the ratio of actual fuel air to the stoichiometric fuel air.

Excess air ratio or excess air factor () is the ratio of actual air fuel to the stoichiometric air fuel.

)2.Mand1.MeqsFrom(

)2.M(st)F/A(

mixture)F/A(

)1.M(st)A/F(

mixture)A/F(

1

=1=

stoichiometric mixture

1(1) rich mixture

1(1) lean mixture

Definition of equivalence ratio and excess air factor/ratio

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Quantification of emissionsNmix,dry = x+b+3.76a (A)Nmix,wet = x+b+3.76a+y/2 (B)

(B)/(A) we have Nmix,wet / Nmix,dry = (x+b+3.76a+y/2) / (x+b+3.76a)=1+y/2(x+b+3.76a) (C)From equation 15.3 (CxHy + aO2 + 3.76aN2 →

xCO2 + (y/2)H2 O + bO2 + 3.76aN2 )2a = 2x+y/2+2b (balancing O atoms)b = a-x-y/4

Putting the value of b in equation (C) we have,

Nmix,wet / Nmix,dry = 1 + y/2(4.76a-y/4) (15.5)

From equation 15.4b [Xi ,wet = Ni / (x + y/2 + b + 3.76a)]

XO2,wet = NO2 /(x+b+3.76a+y/2) = b/(x+b+3.76a+y/2) = (a-x-y/4)/(x+b+3.76a+y/2)

a = [x+ (1+XO2,wet )y/4] / (1-4.76XO2,wet ) (15.6a)Similarly,

a = [x+ (1-XO2,dry )y/4] / (1-4.76XO2,dry ) (15.6b)

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Xi,dry = Ni / Nmix,dry (15.4a)Xi,wet = Ni / Nmix,wet (15.4b)

(15.4b) / (15.4a)

Xi,wet / Xi,dry = Nmix,dry /Nmix,wet

Xi,wet = Xi,dry Nmix,dry / Nmix,wet

Similarly, (15.7)

Xi,dry = Xi,wet Nmix,wet / Nmix,dry

Nmix,wet = x+y/2+b+3.76a (from 1) = x+y/2+(a-x-y/4)+3.76a = 4.76a + y/4

Nmix,wet = 4.76[x+(1+XO2,wet )y/4 / 1-4.76XO2,wet )] + y/4 (from 15.6a) (15.9a)

Similarly,

Nmix,dry = 4.76[x+ (1-XO2,dry )y/4 / 1-4.76XO2,dry )] - y/4 (from 15.6b) (15.9b)

(substituting the value of a)

(putting the value of b)

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In premixed combustion, fuel and oxidizer are mixed at the molecular level prior to ignition, that is the fuel and oxidizer are intimately mixed before they enter the combustion device. Reaction then takes place in a combustion zone that separates unburnt reactants and burnt combustion products. Combustion occurs as a flame front propagating into the unburnt reactants.

Concept of premixed combustion

Air + Fuel

Air

Fuel

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Figure: Nature of premixed emissions

NOx

HC

CO

S

Em

issi

ons

Rich Lean

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The primary pollutants from premixed combustion are oxides of nitrogen, carbon monoxide, unburned and partially burned hydrocarbon and soot. Oxides of sulfur are very low or zero for premixed combustion.

There are a number of nitrogen oxides, but only three of these are of interest for combustion processes

Nitrogen monoxide or nitric oxide, NO

Nitrogen dioxide, NO2

Di-nitrogen oxide or nitrous oxide or laughing gas, N2 OThe first two, NO and NO2 are collectively referred to as NOx.

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(N.12)HNOOHN(N.11)2HNHNH

(N.10)CONHHNCO(N.9)HNCOOHCN

(N.8) NCN2NC (N.7)NHCN2NCH

2. N2 O mechanism is important for fuel lean (1), low temperature condition

mixture the in species the ofbody third otherany M

(N.6) NONOO2NO

(N.5) NHNOO2NH

(N.4) MO2NM2NO


Nitric oxide (NO) emission (Three mechanisms)

(N.3)HNOOHN(N.2)ONO2ON(N.1)NNO2NO

3k

2k

1k

1. Zeldovich mechanism (Thermal mechanism, Zeldovich)

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Emissions from premixed combustionNO formed from fuel N2 is less important for premixed combustion as most fuels used in premixed combustion has little or no N2 .

NO formation depends on (i) reaction time; (ii) gas temperature and (iii) oxygen in the mixture of products.

Figure Effect of excess oxygen on NOx emission

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From equations N.1, N.2 and N.3 we may write

But for quasi steady state of N atom the term d/dt[N] becomes zero. Eqs R.1 and R.2 becomes

)13.15(2NO2dNOd

1t k

)2.R(OHN2ON22NOdNd

)1.R(OHN2ON22NOdNOd

31t

31t

kkk

kkk

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Emissions from premixed combustionNOx reduction technique

Reducing peak temperature can reduce NOx. FGR (flue gas recirculation) or EGR (exhaust gas recirculation) can reduce peak temperature. FGR/EGR circulates a part of the exhaust gas to the combustion chamber

Effect of FGR/EGRincreases heat capacity of burnt gases

dilutes flue gas

i.e. reduces the peak temperature

Figure 15.3 Effect of EGR on NOx emissions

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Figure 15.4 Correlation of NO reduction with diluent heat capacity

Emissions from premixed combustionNOx reduction technique

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NOx reduction technique

Figure 15.4a Effect of injection timing on NOx emission(Source: SAE paper number 2005-01-3677)

Injection timing retardation: lower peak temperature lower NOx emission.

Figure 15.4a shows the effect of injection timing on NOx emission. Injection timing is retarded by 4. Base fuel is used as standard diesel fuel. The result is compared with PME (pongamia methyl ester, a biodiesel). 4

retarding injection timing with 100% PME shows lower NOx emission compared to that of diesel fuel.

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Unburned hydrocarbons

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Figure 15.7 Schematic representation of unburned hydrocarbon emission mechanism

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Premixed combustion emission control

Catalytic treatment

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Catalytic treatment of premixed emissions

Figure 15.8 Pellet bed type catalytic converter

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Catalytic treatment of premixed emissions

Figure 15.9 Monolith catalytic converter

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Conversion efficiency of a three-way catalyst

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Particulate matter (PM)

PM = DS (dry soot) + SOF (soluble organic fraction)

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In nonpremixed combustion, fuel and oxidizer enter the reaction zone in distinct streams. Examples of nonpremixed combustion include methane combustion, pulverized coal furnaces.

Concept of nonpremixed combustion

FuelAir

Air

Fuel

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NOx emissions from nonpremixed combustion

Figure 15.2 NOx emission as functions of air-fuel and equivalence ratio for various spark timings

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Effect of FGR on NOx emission

Figure 15.14 Effect of FGR on NOx emission from staged fuel burners using ambient or preheated air

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Figure 15.15 Low NOx burner employing fuel staging (lean rich combustion)


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Figure 15.19 illustrates the temperature dependence

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course title: heat and combustion technology … ntnu course title: heat and combustion technology...

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