Chapter 10

Combustion

Gaseous Fuel

Liquid Fuel

Ariziel Ruth D. Marquez

COMBUSTION

• The rapid reaction of a fuel with oxygen.

• Important chemical reaction despite the fact that combustion products are relatively worthless compared to the fuels burned to obtain them.

• Exothermic reaction that releases tremendous amount of heat.

COMBUSTION CHEMISTRY

FUEL + Oxygen gas Combustion products

• When fuel is burned:

▫ C is converted to CO2 and/or CO

C + O2 CO2

C + 1/2O2 CO

▫ H2 is converted to H2O

H2 + 1/2O2 H2O

▫ S is converted to SO2

S + O2 SO2

COMPLETE COMBUSTION

• All of the combustible components of a fuel are gasified.

▫ All the carbon to CO2

▫ All the hydrogen to H2O

▫ All the sulfur to SO2

INCOMPLETE COMBUSTION

• Not all fuels are burned.

• Presence of CO, H2 and soot in the exhaust gas.

• Presence of combustibles in the refuse

• Represents a loss of heat.

• Should have been given-off for additional power use.

FUEL

• Gaseous fuel

▫ Natural hydrocarbon gases

▫ Gases manufactured solely for fuel use

▫ Gases obtained as by-product of some industries

▫ Analysis of gaseous

Percentage by volume (% vol)

Percentage by mole (% n)

FUEL

• Liquid fuel ▫ Light oils – suitable for internal combustion

engines and jet engines Lighter and more volatile fractions from distilling or

cracking petroleum oils (gasoline, diesel) Alcohol Benzole

▫ Heavy oils or Furnace oils – heaviest grades of

natural petroleum oils and lubricating oils from which more valuable fractions have been removed

FUEL

• Liquid fuel

▫ Analysis of liquid fuel

Percentage by mass or weight (% wt)

Need to convert to molal units

Individual chemical analysis is rarely known

Elemental percentage by weight (C, H, S, O)

Kinds of Hydrogen (H) in liquid fuel

▫ Combined Hydrogen – equivalent to oxygen in the complex compounds of the fuel treated as combined oxygen in the proportion of water (COMBINED WATER)

▫ Net Hydrogen – the hydrogen that uses oxygen from air

AIR

• The source of oxygen for its obvious economic reasons.

▫ Free and available anytime

• Molar composition

▫ 78.03% N2, 20.99% O2

▫ 0.94% Ar, 0.03% CO, 0.1% H2, He, etc.

To simplify calculations

79% N2 and 21% O2

COMBUSTION PRODUCTS

• Stack gas or Flue gas – product gas that leaves a combustion chamber

• Analysis of flue gas

▫ Wet basis analysis – mole fraction or percent of the gas including water

▫ Dry basis analysis – mole fraction or percent of the same gas excluding water

ORSAT analysis – uses selective liquid absorbent in series and measure the decrease in volume after each absorption.

CO2 – caustic solution CO – Cu2Cl2 solution

O2 – pyrogallol solution

Wet and Dry Basis Analysis

1. A stack gas contains 60.0% mole N2, 15.0% CO2, 10.0% O2 and the balance H2O. Calculate the molar composition of the gas on a dry basis.

2. An Orsat analysis yields the following dry basis composition: 65% N2, 14% CO2, 11% CO, 10% O2. A humidity measurement shows that the mole fraction of H2O in the stack gas is 0.0700. Calculate the stack gas composition on a wet basis.

Wet and Dry Basis Analysis

3. A gas contains 5% wt C3H8, 5% C4H10, 16% O2, 38% N2, and the balance water. Calculate the molar composition of this gas on both wet and dry basis, and the ratio of mole H2O to mole dry gas.

Theoretical Air

• Theoretical oxygen – moles of oxygen needed for the complete combustion of all the fuel to the reactor.

Fuel is always the limiting reactant

• Theoretical air – quantity of air that contains the theoretical oxygen.

Does not depend on how much is actually burned

Excess Air

• Excess air – the amount by which the air is fed to the reactor exceeds the theoretical air

Does not depend on the O2 consumed in the reactor or whether the combustion is complete or incomplete

100_'%

_

__x

n

nnairsx

airtheo

airtheofedair

100_'%

2

22

_

__

2 xn

nnOsx

Otheo

OtheofedO

Theoretical air calculations

1. Methane burns in the reactions: CH4 + 2O2 CO2 +2H2O

CH4 + 3/2O2 CO +2H2O One hundred moles of methane per hour are fed to the

reactor. Find the following: a. What is the theoretical oxygen flow rate if complete

combustion occurs in the reactor?

b. What is the theoretical oxygen flow rate assuming that 70% of methane reacts to form CO?

c. What is the theoretical air flow rate? d. If 100% excess air is supplied, what is the flow rate of air

entering the reactor?

e. If the actual flow rate of air is such that 300 mole/h of oxygen enters the reactor, what is the % excess air?

Theoretical air calculations

2. Determine the theoretical moles of dry air required for the combustion of one hundred moles of refinery gases containing 6% H2S, 5% H2, 57% C3H8, 2% CO2, and 30% C4H10.

3. A furnace is fired with petroleum oil containing 80% C, 13% H, 3% S, 1% N, and 3% O. Determine the theoretical moles of air required for the combustion of one kilogram of oil.

Material Balance Calculations

• Calculations based on fuel analysis 1. A synthesis gas analyzing 6.4% CO2, 0.2% O2,

40.0% CO, and 50.8% H2 (the balance is N2), is burned with 40% dry excess air. What is the composition of the flue gas.

2. A natural gas consisting entirely of methane is burned with an oxygen-enriched air of composition 40% O2 and 60% N2. The Orsat analysis of the product gas as reported by the laboratory is CO2: 20.2%, O2: 4.1%, and N2: 75%. Can the reported analysis be correct?

Material Balance Calculations

• Calculations based on flue gas analysis 1. The combustion products from an industrial

furnace using a hydrocarbon fuel and dry air enters the stack at normal barometric pressure and 375oF and have the following Orsat analysis: 12.2% CO2, 3.1% O2, 1.2% CO, and 82.5% N2. Determine the following:

a. The percent excess air. b. The volume of the gases entering the stack,

expressed as cubic feet per pound of carbon burnt in the furnace.

c. The atomic ratio of hydrogen to carbon in the fuel.