Basics of Combustion Training on Technologies for Converting Waste Agricultural Biomass into Energy Organized by United Nations Environment Programme (UNEP.

Basics of Combustion

Training onTechnologies for Converting Waste Agricultural Biomass into Energy

Organized by

United Nations Environment Programme (UNEP DTIE IETC)23-25 September, 2013

San Jose, Costa Rica

Surya Prakash ChandakSenior Programme Officer

International environmental Technology CentreDivision of Technology, Industry and Economics

Osaka, Japan

• Combustion Generation of heat through rapid chemical reactions of

fuels is known as combustion

• Products of Combustion- CO2

- H2O

- NO2

- SO2

- CO, - HCs, - NOX, SOX, ….

BASICS OF COMBUSTION

Complete Combustion

Incomplete Combustion

Main parameters for proper combustion

- Temperature: To initiate and sustain combustion

- Turbulence: For proper mixing of fuel and air

- Time: Sufficient for complete combustion

BASICS OF COMBUSTION

3T’s : Time, Temperature, Turbulence

• Combustion Flame of different fuels

BASICS OF COMBUSTION

• Combustion Reactions During combustion, molecules undergo chemical

reactions.

The reactant atoms are rearranged to form new combinations (oxidized).

The chemical reaction can be presented by reaction equations.

However, reaction equations represent initial and final results and do not indicate the actual path of the reaction, which may involve many intermediate steps and intermediate products.

This approach is similar to thermodynamics system analysis, where only end states and not path mechanism are used.

BASICS OF COMBUSTION

• Combustion Reactions

Types of combustion reactions:

- Exothermic: Heat is released

- Endothermic: Heat is absorbed

BASICS OF COMBUSTION

• Combustion Reactions

BASICS OF COMBUSTION

ExothermicEndothermicC + 4H + 4O

Break two “O=O” bonds

+ 988 kJ/molC + 4H + 2O2

Break four “C-H” bonds

+ 1644 kJ/mol

CH4 + 2O2 (Reactants)

Form two “C=O” bonds

-1598 kJ/mol

CO2 + 4H + 2O

Form four “O-H” bonds

-1836 kJ/mol

CO2 + 2H2O (Products)

Net energy change-802 kJ/molExothermic – gives off heat energy

+2000

+1000

0

-1000

+3000

• Combustion Reactions Some fundamental reactions of combustion:

C + O2 CO2 + 33.8 MJ/kg-C 2H2 + O2 2H2O + 121.0 MJ/kg-H S + O2 SO2 + 9.3 MJ/kg-S 2C + O2 2CO + 10.2 MJ/kg-C

Note: Above equations are in accordance with conservation of mass. For example consider the first reaction:- 1 kmol C + 1 kmol O2 1 kmol CO2, or- 12 kg C + 32 kg O2 44 kg CO2, or - 0 vol. C + 1 vol. O2 1 vol. CO2.

BASICS OF COMBUSTION

• Combustion Reactions In fuels, the combustion reactions are more complex than

above: In general, air is used in combustion than pure oxygen Fuels consists of many elements such as C, H, N, S, O In addition to complete combustions, fuels undergo incomplete

combustions too. Heat generation during combustion:

- Combustion reactions together with enthalpies of components could be used to predict the net heat generation.

- This needs identification of all the combustion products.

BASICS OF COMBUSTION

• Composition of Air On a molar (or volume) basis, dry air is composed of:

– 20.9% oxygen O2

– 78.1% nitrogen N2

– 0.9% CO2, Ar, He, Ne, H2, and others

A good approximation of this by molar or volume is: 21% oxygen, 79% nitrogen

Thus, each mole of oxygen is accompanied 0.79/0.21 = 3.76 moles of nitrogen

BASICS OF COMBUSTION

• Composition of Air At ordinary combustion temperatures, N2 is inert, but

nonetheless greatly affects the combustion process because its abundance, and hence its enthalpy change, plays a large part in determining the reaction temperatures.

- This, in turn, affects the combustion chemistry.

- Also, at higher temperatures, N2 does react, forming species such as oxides of nitrogen (NOx), which are a significant pollutant.

BASICS OF COMBUSTION

• Stoichiometry and Air/Fuel Ratios Oxidation all the elements or components in a fuel is known

as complete combustion or “Stoichiometric Combustion”.

The amounts of fuel and air taking part in a combustion process are often expressed as the ‘air to fuel’ ratio:

Minimum amount of air (or oxygen) required to have a complete combustion is represented by Stoichiometric Ratio AFRstoich.

For a fuel CxHyOz

BASICS OF COMBUSTION

.

1612

2432.34Stoich zyx

zyxAFR

.fuel

air

m

mAFR

• Stoichiometry and Air/Fuel Ratios Eg: Combustion of Methane

CH4 + 2(O2 + 79/21N2 ) CO2 + 2H2O + 158/21N2

Therefore, AFRStoich = (232 + 22879/21)/(12 + 41) = 17.16

BASICS OF COMBUSTION

Fuel Phase AFRStoichVery light fuel oil liquid 14.27

Light fuel oil liquid 14.06

Medium heavy fuel oil liquid 13.79

Heavy fuel oil liquid 13.46

Generic Biomass solid 5.88

Coal A solid 6.97

LPG (90 P : 10 B) gas 15.55

Carbon solid 11.44

• Stoichiometry and Air/Fuel Ratios In order to obtain complete combustion, supply of excess

amount of air (or oxygen) is required in practice.

The amount of excess air required depends on the properties of the fuel and the technology of the combustion device.

Amount of excess air is usually represented by the equivalence ratio, φ, or the ‘lambda’ ratio λ:

BASICS OF COMBUSTION

• Stoichiometry and Air/Fuel Ratios Eg:

BASICS OF COMBUSTION

Fuel Type of Furnace or BurnersExcess air %

by weight

Pulverized Coal

Completely water-cooled furnace for slag-tap or dry-ash-removal

Partially water cooled furnace for dry-ash-removal

15 – 2015 - 40

Crushed coal Cyclone furnace – pressure or suction 10 - 15

Coal

Spreader stroker Water-cooled vibrating grate stroker Chain-grate and traveling grate strokers Underfeed stroker

30 – 6030 – 6015 – 5020 - 50

Fuel oil Oil burners, register type Multi-fuel burners and flat-flame

5 – 1010 - 20

Acid sludge Cone and flat-plate-type burners, steam-atomized 10 - 15

Natural coke ovens and refinery gas

Register-type burners Multi-fuel burners

5 – 107 - 12

Blast furnace gas Intertube nozzle-type burners 15 - 18

Wood Dutch oven and Hofft-type 35 – 50

Bagasse All furnaces 25 - 35

Black liquor Recovery furnace for kraft and soda-pulping processes 5 - 7

• Combustion Reactions of Fuels Complete combustion of hydrocarbons:

Incomplete combustion of hydrocarbons :

BASICS OF COMBUSTION

Heat.N4

2176.3OH

2CON76.3O

4

21OCH 22222xy

xyyxy

Heat.N76.3OHCO

ONOCHHCON76.3OOHC

222

2X4222zyx

p

srp

• Estimation of Heating Values Eg: Methane:

CH4 + 2(O2 + 79/21N2 ) CO2 + 2H2O + 158/21N2

EnthalpiesCH4 : -4.667 MJ/kg; O2 : 0.0; N2 : 0.0

CO2 : -8.942 MJ/kg; H2O : -13.423 MJ/kg (Gas) / -15.866 MJ/kg (Liquid)

(i) Net Calorafic Value NCV = - (Hproducts – Hreactants)/mass of CH4

= - [{-8.94244 + -13.423218} – {-4.66716}]/16 = 50.125 MJ/kg

(ii) Gross Calorafic Value

GCV = - (Hproducts – Hreactants)/mass of CH4

= - [{-8.94244 + -15.866218} – {-4.66716}]/16 = 55.622 MJ/kg

Note: NCV = GCV – (Mwater/Mmethane)hfg = 55.622 – (36/16)2.443 = 50.125 MJ/kg.

BASICS OF COMBUSTION