Chapter 14Chapter 14Chemical reactionsChemical reactions

Any material that can be burned to Any material that can be burned to release thermal energy is called a release thermal energy is called a fuelfuel..

Most familiar fuels consist primarily of Most familiar fuels consist primarily of hydrogen and carbon.hydrogen and carbon.

They are called They are called hydrocarbon fuelshydrocarbon fuels.. Hydrocarbon fuels exist in all phases, Hydrocarbon fuels exist in all phases, some examples being coal, gasoline, an some examples being coal, gasoline, an

natural gas.natural gas. Although liquid hydrocarbon fuels are Although liquid hydrocarbon fuels are

mixtures of many different hydro mixtures of many different hydro carbons, they are usually considered to carbons, they are usually considered to

be a single hydrocarbon for convenience.be a single hydrocarbon for convenience. For example, gasoline is treated as For example, gasoline is treated as

octane, and the diesel fuel as dodecane.octane, and the diesel fuel as dodecane.

A chemical reaction during which a fuel is A chemical reaction during which a fuel is oxidized and a large quantity of energy is oxidized and a large quantity of energy is

released is called released is called combustioncombustion.. The oxidizer most often used in combustion The oxidizer most often used in combustion

processes is air—it is processes is air—it is free and readily free and readily availableavailable..

Pure oxygen is used as an oxidizer only in some Pure oxygen is used as an oxidizer only in some specializedspecialized

On a mole or a volume basis, dry air is On a mole or a volume basis, dry air is composed of 20.9 percent oxygen, 78.1 percent composed of 20.9 percent oxygen, 78.1 percent nitrogen, 0.9 percent argon, and small amounts nitrogen, 0.9 percent argon, and small amounts of carbon dioxide, helium, neon, and hydrogen. of carbon dioxide, helium, neon, and hydrogen.

In the analysis of In the analysis of combustion combustion processesprocesses, the argon in the air is , the argon in the air is

treated as nitrogen, and the gases that treated as nitrogen, and the gases that exist in trace amounts are disregarded.exist in trace amounts are disregarded. Hence, dry air is approximated as 21 Hence, dry air is approximated as 21

percent oxygen and 79 percent percent oxygen and 79 percent nitrogen by mole numbers.nitrogen by mole numbers.

That is each mole of oxygen entering a That is each mole of oxygen entering a combustion chamber will be combustion chamber will be

accompanied by 0.79/0.21 = 3.76 mol accompanied by 0.79/0.21 = 3.76 mol of nitrogen.of nitrogen.

During a combustion process, the During a combustion process, the components that exist before the reaction components that exist before the reaction are called are called reactantsreactants and the components and the components

that exist after the reaction are called that exist after the reaction are called products.products.

Consider, for example, the combustion of 1 Consider, for example, the combustion of 1 kmol of products, carbon with 1 kmol of kmol of products, carbon with 1 kmol of

pure oxygen, forming carbon dioxide:pure oxygen, forming carbon dioxide:

Note that a reactant does not have to react Note that a reactant does not have to react chemically in the combustion chamber. chemically in the combustion chamber.

If carbon is burned with air instead of pure If carbon is burned with air instead of pure oxygen, nitrogen will appear both as a oxygen, nitrogen will appear both as a

reactant and as a product.reactant and as a product.

Bringing a fuel into intimate contact Bringing a fuel into intimate contact with oxygen is not sufficient to start a with oxygen is not sufficient to start a

combustion process.combustion process. The fuel must be brought above its The fuel must be brought above its

ignition temperature to start the ignition temperature to start the combustion.combustion.

The minimum ignition temperatures The minimum ignition temperatures of various substances in atmospheric of various substances in atmospheric

air are approximately 260°C for air are approximately 260°C for gasoline, 400°C for carbon, 580°C for gasoline, 400°C for carbon, 580°C for

hydrogen, 610°C for carbon hydrogen, 610°C for carbon monoxide, and 630°C for methane.monoxide, and 630°C for methane.

The proportions of the fuel and air The proportions of the fuel and air must be in the proper range for must be in the proper range for

combustion to begincombustion to begin. .

Chemical equations are balanced on the basis of Chemical equations are balanced on the basis of the conservation of mass principle (or the mass the conservation of mass principle (or the mass

balance), which can be stated as follows: The total balance), which can be stated as follows: The total mass of each element is conserved during a mass of each element is conserved during a

chemical reaction.chemical reaction. That is, the total mass of each element on the That is, the total mass of each element on the

right-hand side of the reaction equation (the right-hand side of the reaction equation (the products) must be equal to the total mass of that products) must be equal to the total mass of that

element on the left-hand side (the reactants) even element on the left-hand side (the reactants) even though the elements exist in different chemical though the elements exist in different chemical

compounds in the reactants and products.compounds in the reactants and products. Also, the total number of atoms of each element is Also, the total number of atoms of each element is

conserved during a chemical reaction since the conserved during a chemical reaction since the total number of atoms of an element is equal to total number of atoms of an element is equal to

the total mass of the element divided by its atomic the total mass of the element divided by its atomic mass.mass.

A frequently used quantity in the analysis of A frequently used quantity in the analysis of combustion processes to quantify the amounts of combustion processes to quantify the amounts of

fuel and air is the fuel and air is the air—fuel ratio AF defined as:air—fuel ratio AF defined as:

During combustion, nitrogen behaves as an During combustion, nitrogen behaves as an inert gas and does not react with other inert gas and does not react with other

elements, other than forming a very small elements, other than forming a very small amount of nitric oxides. amount of nitric oxides.

Nitrogen usually enters a combustion chamber Nitrogen usually enters a combustion chamber in large quantities at low temperatures and in large quantities at low temperatures and exits at considerably higher temperatures, exits at considerably higher temperatures,

absorbing a large proportion of the chemical absorbing a large proportion of the chemical energy released during combustion. energy released during combustion.

At very high temperatures, such as those At very high temperatures, such as those encountered in internal combustion engines, a encountered in internal combustion engines, a small fraction of nitrogen reacts with oxygen, small fraction of nitrogen reacts with oxygen, forming hazardous gases such as nitric oxide.forming hazardous gases such as nitric oxide.

A combustion process is A combustion process is complete if all the complete if all the carbon in the fuel bums to CO2carbon in the fuel bums to CO2 all the all the

hydrogen bums to HO2 and all the sulfur (if any) hydrogen bums to HO2 and all the sulfur (if any) bums to SO2 .bums to SO2 .

The combustion process is The combustion process is incompleteincomplete if the if the combustion products contain any combustion products contain any unburnedunburned fuelfuel

or components such as C, H ,CO. or OH.or components such as C, H ,CO. or OH.

Insufficient oxygenInsufficient oxygen is an obvious reason for is an obvious reason for incomplete combustionincomplete combustion, but it is not the only , but it is not the only

one.one.

Incomplete combustion occurs due Incomplete combustion occurs due insufficient insufficient mixing in the combustion chambermixing in the combustion chamber during the during the

limited time that the fuel and the oxygen are in limited time that the fuel and the oxygen are in contact.contact.

Another cause of incomplete combustion is Another cause of incomplete combustion is

dissociation,dissociation, which becomes important at which becomes important at high temperatures.high temperatures.

In actual combustion processes, it is common In actual combustion processes, it is common practice to use practice to use more air thanmore air than the stoichiometric the stoichiometric

amount to amount to increase the chances of complete increase the chances of complete combustioncombustion or to or to control the temperature of the control the temperature of the

combustion chambercombustion chamber..

The The amount of air in excess of the stoichiometricamount of air in excess of the stoichiometric amount is called amount is called excess air.excess air.

The The amount of excess airamount of excess air is usually expressed in is usually expressed in terms of the stoichiometric air as terms of the stoichiometric air as percent excess air percent excess air

or percent theoreticalor percent theoretical air. air.

50 percent excess air is equivalent to 150 50 percent excess air is equivalent to 150

percent theoretical air, and 200 percent percent theoretical air, and 200 percent excess air is equivalent to 300 per cent excess air is equivalent to 300 per cent

theoretical air.theoretical air.

The stoichiometric air can be expressed as The stoichiometric air can be expressed as 0 0 per centper cent excess air excess air or or 100 percent 100 percent

theoretical airtheoretical air..

Amounts of air Amounts of air less than the stoichiometricless than the stoichiometric amount are called amount are called deficiency of airdeficiency of air and are and are often expressed as percent deficiency of air. often expressed as percent deficiency of air.

90 percent theoretical air is equivalent to 10 90 percent theoretical air is equivalent to 10 percent deficiency of air. percent deficiency of air.

The amount of air used in combustion processes is The amount of air used in combustion processes is

also expressed in terms of also expressed in terms of the equivalence ratiothe equivalence ratio, , which is the which is the ratio of the actual fuel—air ratioratio of the actual fuel—air ratio to to

the the stoichiometric fuel—air ratiostoichiometric fuel—air ratio..

The minimum amount of air needed for the complete The minimum amount of air needed for the complete combustion of a fuel is called the stoichiometric or combustion of a fuel is called the stoichiometric or

theoretical air. theoretical air.

When a fuel is completely burned with theoretical When a fuel is completely burned with theoretical air, no uncombined oxygen will be present in the air, no uncombined oxygen will be present in the

product gases. product gases.

The theoretical air is also referred to as the The theoretical air is also referred to as the chemically correct amount of air, or 100 percent chemically correct amount of air, or 100 percent

theoretical air.theoretical air.

A combustion process with less than A combustion process with less than

the theoretical air is bound to be the theoretical air is bound to be incomplete. incomplete.

The ideal combustion process during The ideal combustion process during which a fuel is burned completely which a fuel is burned completely

with theoretical air is called the with theoretical air is called the stoichiometric or theoretical stoichiometric or theoretical

combustion of that fuel.combustion of that fuel.

For example, the theoretical For example, the theoretical combustion of methane iscombustion of methane is

ENTHALPY OF FORMATIONENTHALPY OF FORMATIONAND ENTHALPY OF AND ENTHALPY OF

COMBUSTIONCOMBUSTION

• The enthalpy of combustion is obviously a very useful property for analyzing the combustion processes of fuels.

• However, there are so many different fuels and fuel mixtures that it is not practical to list values for all possible cases.

• The enthalpy of combustion is not of much use when the combustion is incomplete.

• A more practical approach would be to have a more fundamental property to represent the chemical energy of an element or a compound at some reference state.

• We assign the enthalpy of formation of all stable elements (such as O, N H and C) a value of zero at the standard reference state of 25°C and I atm.

• Now reconsider the formation of CO (a compound) from its elements C and O at 25°C and 1 atm during a steady-flow process.

• The enthalpy change during this process was determined to be —393,520 kJ/kmol. However, Hreact = 0 since both reactants are elements at the standard reference state, and the products consist of I kmol of CO at the same state.

• Therefore, the enthalpy of formation of CO at the standard reference state is —393,520 kJlkmol (Fig. 14—18).

The process described above involves no work interactions. Therefore, from

the steady-flow energy balance relation, the heat transfer during this process

must be equal to the difference between the enthalpy of the products and theenthalpy of the reactants. That is,

Q =— Hreact = —393,520 kJlkmol(14—5)Since both the reactants and the products are at the same state, the enthalpy change during this process is solely due to the changes in the

chemical com position of the system. This enthalpy change will be different for different re actions, and it would be very desirable to have a

property to represent the

First Law Analysis of Reacting Systems

Steady-Flow Systems

The enthalpy of a component need to be expressed in a form suitable for use for reacting systems.

The enthalpy term is reduced to the enthalpy of formation at the standard reference state.

The enthalpy of a component is expressed in a way that enables the use of enthalpy values from tables regardless of the reference state.

Neglecting changes in kinetic and potential energies, the steady-flow energy balance relation can be expressed more explicitly.

The process described above involves no work interactions. Therefore, from

the steady-flow energy balance relation, the heat transfer during this process

must be equal to the difference between the enthalpy of the products and theenthalpy of the reactants. That is,

Q =— Hreact = —393,520 kJlkmol(14—5)Since both the reactants and the products are at the same state, the enthalpy change during this process is solely due to the changes in the

chemical com position of the system. This enthalpy change will be different for different re actions, and it would be very desirable to have a

property to represent the

The process described above involves no work interactions. Therefore, from

the steady-flow energy balance relation, the heat transfer during this process

must be equal to the difference between the enthalpy of the products and theenthalpy of the reactants. That is,

Q =— Hreact = —393,520 kJlkmol(14—5)Since both the reactants and the products are at the same state, the enthalpy change during this process is solely due to the changes in the

chemical com position of the system. This enthalpy change will be different for different re actions, and it would be very desirable to have a

property to represent the

The process described above involves no work interactions. Therefore, from

the steady-flow energy balance relation, the heat transfer during this process

must be equal to the difference between the enthalpy of the products and theenthalpy of the reactants. That is,

Q =— Hreact = —393,520 kJlkmol(14—5)Since both the reactants and the products are at the same state, the enthalpy change during this process is solely due to the changes in the

chemical com position of the system. This enthalpy change will be different for different re actions, and it would be very desirable to have a

property to represent the

Adiabatic Flame Adiabatic Flame TemperatureTemperature

In combustion chambers, the highest temperature to In combustion chambers, the highest temperature to which a material can be exposed is limited by which a material can be exposed is limited by metallurgical considerations.metallurgical considerations.

Therefore, the adiabatic flame temperature is an Therefore, the adiabatic flame temperature is an

important consideration in the design of combustion important consideration in the design of combustion chambers, gas turbines, and nozzles.chambers, gas turbines, and nozzles.

The maximum temperatures that occur in these The maximum temperatures that occur in these devices are considerably lower than the adiabatic devices are considerably lower than the adiabatic flame temperature, however, since the combustion is flame temperature, however, since the combustion is usually incomplete, some heat loss takes place, and usually incomplete, some heat loss takes place, and some combustion gases dissociate at high some combustion gases dissociate at high temperatures. temperatures.

Adiabatic Flame Adiabatic Flame TemperatureTemperature

The maximum temperature in a combustion The maximum temperature in a combustion chamber can be controlled by adjusting the amount chamber can be controlled by adjusting the amount of excess air, which serves as a coolantof excess air, which serves as a coolant..

Note that the adiabatic flame temperature of a fuel Note that the adiabatic flame temperature of a fuel is not unique. Its value depends on (1) the state of is not unique. Its value depends on (1) the state of the reactants, (2) the degree of completion of the the reactants, (2) the degree of completion of the reaction, and (3) the amount of air used. reaction, and (3) the amount of air used.

For a specified fuel at a specified state burned with For a specified fuel at a specified state burned with air at a specified state, the adiabatic flame air at a specified state, the adiabatic flame temperature attains its maximum value when temperature attains its maximum value when complete combustion occurs with the theoretical complete combustion occurs with the theoretical amount of airamount of air..

The process described above involves no work interactions. Therefore, from

the steady-flow energy balance relation, the heat transfer during this process

must be equal to the difference between the enthalpy of the products and theenthalpy of the reactants. That is,

Q =— Hreact = —393,520 kJlkmol(14—5)Since both the reactants and the products are at the same state, the enthalpy change during this process is solely due to the changes in the

chemical com position of the system. This enthalpy change will be different for different re actions, and it would be very desirable to have a

property to represent the

The process described above involves no work interactions. Therefore, from

the steady-flow energy balance relation, the heat transfer during this process

must be equal to the difference between the enthalpy of the products and theenthalpy of the reactants. That is,

Q =— Hreact = —393,520 kJlkmol(14—5)Since both the reactants and the products are at the same state, the enthalpy change during this process is solely due to the changes in the

chemical com position of the system. This enthalpy change will be different for different re actions, and it would be very desirable to have a

property to represent the

The process described above involves no work interactions. Therefore, from

the steady-flow energy balance relation, the heat transfer during this process

must be equal to the diference between the enthalpy of the products and theenthalpy of the reactants. That is,

Q =— Hreact = —393,520 kJlkmol(14—5)Since both the reactants and the products are at the same state, the enthalpy change during this process is solely due to the changes in the

chemical com position of the system. This enthalpy change will be different for different re actions, and it would be very desirable to have a

property to represent the

The process described above involves no work interactions. Therefore, from

the steady-flow energy balance relation, the heat transfer during this process

must be equal to the difference between the enthalpy of the products and theenthalpy of the reactants. That is,

Q =— Hreact = —393,520 kJlkmol(14—5)Since both the reactants and the products are at the same state, the enthalpy change during this process is solely due to the changes in the

chemical com position of the system. This enthalpy change will be different for different re actions, and it would be very desirable to have a

property to represent the

The process described above involves no work interactions. Therefore, from

the steady-flow energy balance relation, the heat transfer during this process

must be equal to the difference between the enthalpy of the products and theenthalpy of the reactants. That is,

Q =— Hreact = —393,520 kJlkmol(14—5)Since both the reactants and the products are at the same state, the enthalpy change during this process is solely due to the changes in the

chemical com position of the system. This enthalpy change will be different for different re actions, and it would be very desirable to have a

property to represent the

The process described above involves no work interactions. Therefore, from

the steady-flow energy balance relation, the heat transfer during this process

must be equal to the difference between the enthalpy of the products and theenthalpy of the reactants. That is,

Q =— Hreact = —393,520 kJlkmol(14—5)Since both the reactants and the products are at the same state, the enthalpy change during this process is solely due to the changes in the

chemical com position of the system. This enthalpy change will be different for different re actions, and it would be very desirable to have a

property to represent the