Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Fundamentals of Combustion

Dr. Zayed Al-Hamamre

Lec 3: Chemical Thermodynamics

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Content

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-3Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-4Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-5Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Theoretical and Excess Air Combustion reactions are usually run with more air than is

needed to supply oxygen in stoichiometric proportion to the fuel.

This has the effect of increasing the conversion of the valuable reactant at the expense of the cost of the excess reactant and additional pumping costs.

Theoretical Oxygen: The moles (batch) or molar flow rate (continuous) of O2 needed for complete combustion of all the fuel fed to the reactor, assuming that all carbon in the fuel is oxidized to CO2 and all the hydrogen is oxidized to H2O

Theoretical Air: The quantity of air that contains the theoretical oxygen.

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Excess Air: The amount by which the air fed to the reactor exceeds the theoretical air

If 50% excess air is supplied

ExampleOne hundred mol/h of butane (C4H10 ) and 5000 mol/h of air are fed into a combustion reactor. Calculate the percent excess air.

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

The theoretical air from the feed rate of fuel and the stoichiometric equation

The stoichiometric equation for complete combustion of butane:

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example

Ethane is burned with 50% excess air. The percentage conversion of the ethane is 90%; of the ethane burned. 25% reacts to form CO and the balance reacts to form CO2. Calculate the molar composition of the stack gas on a dry basis and the mole ratio of water to dry stack gas.

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Basis: 100 mol C2H6 Fed

Assumption: nitrogen is inert-that is, Thus, neglect the trace amounts of Nox that might form

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Degree-of-Freedom Analysis

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

25% Conversion to CO

75% Conversion to CO2

0.75 CO2

Another approach:

CO2

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Atomic Hydrogen Balance:

Atomic Oxygen Balance:

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

The analysis of the stack gas is now complete

The mole ratio of water to dry stack gas is

Quiz:Find the composition in wet base

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

ExampleA hydrocarbon gas is burned with air. The dry-basis product gas composition is 1.5 mole% CO, 6.0% CO2, 8.2% O2, and 84.3% N2. There is no atomic oxygen in the fuel. Calculate the ratio of hydrogento carbon in the fuel gas and speculate on what the fuel might be. Then calculate the percent excess air fed to the reactor.

Basis: 100 mol Product Gas

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Degree-of-Freedom Analysis

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

The fuel composition described by the formula

To find the Percent Excess Air

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-20Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-21Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-22Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-23Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-24Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-25Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-26Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-27Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-28Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-29Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-30Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-31Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-32Process Heat Transfer

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Summary and Examples

25 oC and 1 atm

Example

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

If there are no gaseous reactants or products, then

Heat of reaction

Example

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

The standard heat of formation of an elemental species (e.g., O2) is zero.If A is the stoichiometric coefficient of the i th species participating in a reaction (+ for

products, - for reactants) and is the standard heat of formation of this species,then the standard heat of the reaction is

Formation reaction and heat of formation

Example

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Heat of combustionThe standard heat of combustion of a substance, , is the heat of the combustion

of that substance with oxygen to yield specified products [e.g., CO2(g) and H2O(l)],with both reactants and products at 25oC and 1 atm (the arbitrary but conventionalreference state).

standard heats of combustion for a substances are tabulated.The given values are based on the following assumptions:

(a) All carbon in the fuel forms CO2(g),(b) All hydrogen forms H2O(l),(c) All sulfur forms SO2(g), and(d) All nitrogen forms N2(g).

Standard heats of reactions that involve only combustible substances and combustionproducts can be calculated from tabulated standard heats of combustion.

For CO2, H2O and SO2, the standard heats of combustion is equal to zero.

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Heat of combustion

Example

For combustion reaction,

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Energy balance on reactive processesExample

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

1. Use the heat of reaction method for the energy balance.2. Choosing as references the reactant and product species in the states for which the

heat of reaction is given

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Or one step or heat the liquid water from 25C to l00C, vaporize it, heat the vapor from l00C to 300C,

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Thus, 19,700 kW of heat must be transferred from the reactor to maintain the producttemperature at 300C.

If less heat were transferred, more of the heat of reaction would go into the reactionmixture and the outlet temperature would increase.

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Two or multiple reactions system or Single reaction with unknown r

material balance is not required

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

to calculate then

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Processes with Unknown Outlet Conditions: Adiabatic Reactors

Example

Quiz: Find the composition of the output stream

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Use either the Heat of Reaction Method or Heat of Formation Method

As references for enthalpy calculations

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example cont.

Quiz

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

The higher heating value (or total heating value or gross heating value) isWith H2O(l) as a combustion product,.

The lower heating value (or net heating value) is the value based on H2O (v) as a product.

Since is always negative, the heating value is positive.To calculate a lower heating value of a fuel from a higher heating value or vice versa,

you must determine the moles of water produced when one mole of the fuel is burned.o If this quantity is designated n, then

Fuels and Their Properties

Where the heat of vaporization of water at 25oC is

If a fuel contains a mixture of combustible substances, its heating value (lower or higher) is

The heating valueThe heating value of a combustible material is the negative of the standard heat of

combustion.

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Fuels and Their Properties

xi is the mass fractions of the fuel components if the heating values are expressed in units of (energy)/(mass), or xi mole fractions if the dimensions of the heating values are (energy)/(mole).

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.Example

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Adiabatic Flame TemperatureWhen a fuel is burned,

o A considerable amount of energy is released. o Some of this energy is transferred as heat through the reactor walls, o The remainder raises the temperature of the reaction products; o The less heat transferred, the higher the product temperature. o The highest achievable temperature is reached if the reactor is adiabatic and all

of the energy released by the combustion goes to raise the temperature of the combustion products (adiabatic flame temperature, Tad).

In adiabatic reactor ,

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888 1-68Process Heat Transfer

Adiabatic flame temperaturedepends on air/fuel ratio

Tad: Increases for preheated air, or for O2instead of air.

Tad: decreases, if cooled exhaust gas ismixed with reactants (recycling)

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Adiabatic Flame Temperature

Example

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Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

Example Cont.