CHE112P Lecture 3

Chapter 10

Combustion

Gaseous Fuel

Liquid Fuel

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

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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

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.

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

n

n

air

s

x

100

_

'

%

x

n

n

n

O

s

x

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% H

S, 5%

H

, 57% C

H

, 2% CO

, and 30% C

H

.

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.