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MEC551 THERMAL ENGINEERING

5.0 Combustion Processes

Introduction to fuel and combustion

• Any material that can be burned to released thermal energy is called fuel • It consists of hydrogen) and carbon (hydrocarbon fuels CnHm) • Hydrocarbon can exist in all phases a) coal b) gasoline c) natural gas • Most liquid hydrocarbon fuels are a mixture of numerous hydrocarbons and

obtained from crude oil by distillation • Hydrocarbon fuels are usually considered to be a single hydrocarbon a) octane - C8H18

b) dodecane (diesel fuel) – C12H26

c) methyl alcohol (methanol) – CH3OH d) methane – CH4

Alternative fuels

• Vehicles are a major source of air pollutants - nitric oxides - carbon monoxide - hydrocarbons - greenhouse gas carbon dioxide • There is a growing shift in the transportation industry from the traditional petroleum based fuels to the cleaner burning alternative fuels friendlier - natural gas - Liquefied petroleum gas (LPG) - Compressed natural gas (CNG)

• Combustion process is a process where chemical reaction during which a fuel is oxidized and a large quantity of energy is released

• The oxidizer that commonly used in combustion process is air • Pure oxygen, O2 is used as oxidizer in only some application such as cutting

and welding

22 COOC →+

2222 76.376.3 NCONOC +→++

Oxidizer: oxygen

Oxidizer: air

2222283 8.1843)76.3(5 NOHCONOHC ++→++

OHCOOHC 22283 435 +→+

Propane reacts with pure oxygen:

Propane reacts with air:

Therefore the number of moles of air required for complete combustion is 5(1) + 5(3.76) = 23.8 moles

Air-fuel-ratio (AF)

• Ratio of the mass of air to the mass of fuel

massmolar M & moles ofnumber N where ==

==fuel

air

fuel

air

NMNM

mmAF

Mole & mass fraction

where P – pressure, V – volume, N – no of mole, subscript i – individual component, subscript m – mixture and m – mass.

Mole fraction, y:

Mass fraction, mf:

Ideal Gas Law for Mixture

According to Dalton’s law of partial pressure:

where R – universal gas constant

Example : Balancing the combustion equation

One kmol of octane (C8H18) is burned with air that contains 20 kmol of O2, as shown in figure. Assuming the product contain only CO2, H2O, O2 and N2, determine the mole number of each gas in the products and the air fuel ratio for this combustion process (Given; molar mass of air is Mair=28.97 kg/kmol=29kg/kmol)

Combustion chamber

AIR

C8H18 xCO2

yH2O z O2

w N2

Theoretical & Actual combustion process

• Complete combustion- All the carbon in the fuel burns to CO2, hydrogen burns to H2O and all the sulfurs (if any) burnes to SO2.

• Incomplete combustion- combustion product contain any unburned fuel or components such as C, H2, CO, or OH

• Reason for incomplete combustion 1) insufficient oxygen 3) dissociation 2) insufficient mixing

Combustion chamber

AIR

CnHm xCO2

yH2O excess O2

N2

Fuel

Stoichiometric/ Theoretical Air • The minimum amount of air needed for the complete of a fuel

is called stoichiometric or theoritical air • When fuel completely burned with theretical air, no

uncombined oxygen is present in the product gases • Theoretical air is also referred to the chemically correct

amount of air or 100% theoretical air • Combustion process less than theoretical air is incomplete • Ideal combustion process during fuel is burned completely

with theoretical air is called the stoichiometric or theoritical combustion

• Ex: CH4 + 2(O2 + 3.76N2) →CO2 + 2H2O + 7.52N2

Excess Air • In actual combustion processes, it is common practice to use more air

than the stoichimetric amount to increase the chances of complete combustion or to control the temperature of the combustion chamber

• The amount of air in excess of the stoichimetric amount is called excess air

• Amount of excess air usually expressed in terms of stoichiometric air as percent excess air or percent theoretical air

• Ex: 50% excess air→150% theoretical air 200% excess air → 300% theoretical air 0% excess air → 100% theoretical air • Amounts of air less than the stoichiometric amount are called

deficiency of air and often expressed as precent of deficiency of air • Ex: 90% theoretical air→ 10% deficiency of air • Equivalance ratio- ratio of the actual fuel-air ratio to the

stoichiometric fuel air ratio

Analyzing Combustion Products; • Theoretically, to achieve complete combustion requires the supply of

excess air. • Actual cases even the supply of excess air fails to ensure complete

combustion occurs. lt is almost impossible to predict the exact composition of composition products on the basis of mass conservation alone.

• In practice, products of combustion are analyzed using gas analyzers located downstream from the main combustion chamber. These gas analyzers operates using Orsat gas analysis, where chemical compounds are used to absorb certain gas compositions and measures their volumes

Example: Combustion of coal with theoretical air

Coal from Pennsylvania which has an ultimate analysis (by mass) as 84.36% C, 1.89% H2, 4.4% O2, 0.63% N2, 0.89% S and 7.83% ash (non-combustibles) is burned with theoretical amount of air. Disregarding the ash content, determine the mole fractions of the products and the apparent molar mass of the product gases. Also determine the air-fuel-ratio required for this combustion process

Example: Dew point temperature of combustion products

Ethane (C2H6) is burned with 20% excess air during a combustion process. Assuming complete combustion and a total pressure of 100 kPa, determine a) the air fuel ratio b) the dew point temperature of the products

Combustion chamber AIR

C2H6 CO2

H2O excess O2

N2

Fuel

20% excess

Effect of Moisture in Combustion Air; lf dry air is used as combustion air, than its chemical composition can be

written as ath(O2 + 3.76N2). lf the moisture content is significant (high humidity), it should be

considered in the combustion process ath(O2 + 3.76N2) + Nv,air (H2O). where the number of moles of moisture can be determined from

Example: Combustion of gaseous fuel with Moist air

A certain natural gas has the following volumetric analysis: 72% CH4, 9% H2, 14% N2, 2% O2, and 3% CO2. This gas is now burned with the stoichiometric amount of air that enters the combustion chamber at 20°C, 1 atm and 80% relative humidity. Assuming complete combustion and a total pressure of 1 atm, determine the dew point temperatures of the products.

Example: Reverse Combustion Analysis

Octane (C8H18) is burned with dry air. The volumetric analysis of the products on a dry basis is CO2: 10.02% O2: 5.62% CO: 0.88% N2: 83.48% Determine a) the air-fuel ratio b) the percentage of theoretical air used

First Law Analysis of Reacting System

Energy Analysis of Combustion Processes; Thermodynamics point of view, the amount of heat energy

released during a combustion process is the important aspect to study.

Basically the heat energy released from a combustion process comes from the chemical energy contain in the fuel.

In energy analysis, standard reference defined at 25oC and 1atm. The property value at the standard reference state is written as ho, so and uo

Enthalpy of reaction, hg = difference between enthalpy of the products at a specified state and the enthalpy of reactants at the same state in complete combustion.

Enthalpy of combustion, hc = the amount of heat released during a steady flow of combustion process when 1kmol fuel burn completely at specified pressure & temperature.

Hc = HProduct - Hreactant

In energy analysis, standard reference defined at 25oC and

1atm. The property value at the standard reference state is written as ho, so and uo

Enthalpy of formation, hf = the enthalpy of a substance at a specified state due to its chemical composition.

Q = hc = Hprod – Hreactant = ∑(Nphf)p - ∑(Nrhf)r Normally, N2 and O2 consider as stable elements, thus their

enthalpy of formation consider to be zero.

In actual combustion process, the temperatures of the reactants and products are NOT at standard reference state. Then, the heat value of the reactants or products at elevated temperature states can be written by

And enthalpy of combution; Hc = HProduct - Hreactant ; where hT

= the sensible enthalpy at specified state, h = the sensible enthalpy at the standard reference state of 25oC and 1atm.

Example 1) Determine the enthalpy of combustion of liquid octane

(C8H18) at 250C and 1 atm. Assuming the water in the products is in the liquid form.

2) Liquid propane (C3H8) enters a combustion chamber at 250C

at a rate of 0.05kg/min where it is mixed and burned with 50% excess air that enters the combustion chamber at 70C. An analysis of the combustion gases reveals that all the hydrogen in the fuel burns to H2O (gas) but only 90% of the carbon burns to CO2, with remaining 10% forming CO. If the exit temperature of the combustion gases is 1500K, determine;

a) the mass flow rate of air. b) the rate of heat transfer from the combustion chamber

(heat of formation C3H8 = -118910kJ/kmol)