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thermo 07 combustion


									Chapter 1: Introduction to combustion 1.1 Why study combustion? future, the main source of the world’s energy. It is therefore a fundamental driver of almost all human activity.

• combustion of fossil fuels is currently, and should remain for the foreseeable

• it is also now generally accepted that the combustion of fossil fuels is the major
cause of global warming mainly via the release of carbon dioxide CO2 into the atmosphere. Given that fossil fuel combustion is currently essential to the world economy, the more benign use of this resource is of increasing concern.

• world atmospheric temperature rise over the last 140 years:
Temperature (deg.) 15.4 15.2 15.0 14.8 14.6 14.4 14.2 1860 1880 1990 1920 1940 1960 1980 2000

• whilst different parties have different views on this highly political issue, this
lecturer considers that adaptation and mitigation of global warming will be the major problems faced by humanity on the 21st century: “Climate change is a far greater threat to the world than international terrorism” “Climate change is the most severe problem we are facing today” Prof. Sir David King, UK Chief Scientist [2]

• the importance of combustion can be seen by considering the world total energy
consumption of energy by fuel (mtoe) from 1971-2000 [3]:

• note that 1 mtoe (mega tonne of energy) = 4.19x104 TJ. • the combustion of coal, oil, and gas all convert previously bound carbon to CO2
and release it into the atmosphere.

• furthermore, the total predicted energy supply (TPES in mtoe) suggests that the
combustion of fossil fuels is going to increase significantly [3]:


Basic definitions

• ‘combustion’ reactions are one form of chemical reaction
1. combustion is defined as the oxidation of a ‘fuel’, with large amounts of released energy 2. the oxidiser is in most cases air (or more specifically, O2 in air) because of its abundance. 3. a ‘fuel’ is any material that can be burned to release energy. Hydrocarbon fuels of the form CxHy are the most common.

• for example, many hydrocarbon fuels are mixtures of many different
hydrocarbons although they mainly consist of the following:

gasoline ~ octane, C8H18 diesel ~ dodecane, C12H26 methanol = methyl alcohol, CH3OH LNG (liquefied natural gas) ~ methane, CH4 LPG (liquefied petroleum gas) ~ propane, C3H8 1.2.1 Composition of air

• on a molar (or volume) basis, dry air is composed of:
20.9% oxygen O2 78.1% nitrogen N2 0.9% CO2, Ar, He, Ne, H2, and others

• a good approximation of this by molar or volume is: 21% oxygen, 79% nitrogen • thus, each mole of oxygen is accompanied 0.79/0.21 = 3.76 moles of nitrogen. • at ordinary combustion temperatures, N2 is inert, but nonetheless greatly affects
the combustion process because its abundance, and hence its enthalpy change, plays a large part in determining the reaction temperatures. 1. this, in turn, affects the combustion chemistry, as we shall see later. 2. also, at higher temperatures, N2 does react, forming species such as oxides of nitrogen (NOx), which are a significant pollutant. 1.2.2 Stoichiometry and air/fuel ratios

• the amounts of fuel and air taking part in a combustion process are often
expressed as the ‘air to fuel’ ratio:


mair mfuel


• where: mx = nx M rx = mass of fuel or air ( kg ) , nx = number of moles of x ( mol )
and M rx = molar mass of x ( kg / mol )

• a combustion process is ‘complete’ if all the fuel is burnt. The minimum amount
of air needed to do this is called the ‘stoichiometric’ or ‘theoretical’ air.

• e.g. the stoiciometric combustion of methane:

CH 4 + 2(O2 + 3.76 N 2 ) → CO2 + 2 H 2O + 7.52 N 2


• equivalent, and widely used, terms to the AFR are the fuel/air ratio FAR , the
equivalence ratio φ and the ‘lambda’ ratio λ : 1 FAR φ= = λ FARstoich (1.3)


Types of flame combustion reaction. 1. it is typically a very localised region that features non-equilibrium chemistry and complex fluid motion. 2. it can be visible or invisible to the naked eye. Indeed many of the photons emitted from the flame are outside the visible spectrum, and are often in the UV or IR bands.

• the ‘flame’ is the transition region between the reactants and products of the

• flames can normally be classified as:

• ‘nonpremixed’ flames include ‘diffusion’ flames that we will discuss later • for example, ‘laminar premixed flames’ features the laminar premixing and
combustion of fuel and oxidiser:

• turbulent premixed flames feature premixed flame fronts burning and propagating
into a turbulent flow:

• the regimes of turbulent premixed combustion depend strongly on the conditions
during combustion, which is discussed later. 1.4 Environmental considerations as engineers, we must rely on the information from others as to the impact on people and the environment of devices that we design and build.

• the effect of combustion generated air pollution is highly complex. Furthermore,

• in highly simplified form, the complexity of the issue can be summarised:
major effects 1. long term environmental change 2. health hazards for people, animals and plants 3. material degradation 1. anthropological activity (ie. man made) 2. natural organic emissions 3. stratospheric activity 1. atmospheric chemistry (liquid, solid gaseous) 2. dispersion and transport (meteorology) 1. 2. 3. 4. source modeling formation and distraction modeling control strategy measurements


formation mechanisms

control methods

social implications

1. 2. 3. 4. 5.

impact on trade energy use patterns transportation planning employment inter-country and inter-continental interactions

1.4.1 Classification of pollutants

• pollutants are typically classified by their origin:
1. primary pollutants: emitted directly into the atmosphere from some device eg. HC (hydrocarbons), CO, NOX, SOX, particulates, Pb, asbestos, etc. 2. secondary pollutants: generated within the atmosphere from primary sources by chemical or photochemical reactions 1.5 Global warming Oceanic and Atmospheric Administration) [4]: Global warming “is the result of heat absorption by certain gases in the atmosphere (called greenhouse gases because they effectively 'trap' heat in the lower atmosphere) and re-radiation downward of some of that heat. Water vapor is the most abundant greenhouse gas, followed by carbon dioxide and other trace gases. Without a natural greenhouse effect, the temperature of the Earth would be about zero degrees F (-18°C) instead of its present 57°F (14°C). So, the concern is not with the fact that we have a greenhouse effect, but whether human activities are leading to an enhancement of the greenhouse effect.”

• Definition of global warming and greenhouse gases from the NOAA (National

• in order to slow down the potential growth of global warming, the Kyoto Protocol
was adopted by most countries around the World.

• The goal of the protocol, according to the 1992 United Nations Framework
Convention on Climate Change (UNFCC), is “to stabilise concentrations of greenhouse gases at levels to avoid dangerous anthropogenic (human induced) interference with the climate”. More recent analysis suggests that we may not be achieving this.

• sources of CO2 equivalent emissions (i.e. CO2 and other pollutants scaled by their
global warming potential) in Australia [4]:

• CO2 production (tonnes) per unit GDP (US dollars, 1995 values) [6]:
2.50 tonnes CO2 / $US (thousands 1995) United States China Australia





0.00 1980


1990 year



• for developed countries, CO2 generation is strongly linked with economic activity
(ie. it is roughly a constant relationship). The challenge is therefore to stop global warming without stopping the world economy! 1.6 CO2 emissions from different fossil fuels CO2. The heat generation is therefore directly related to CO2

• basic chemistry dictates that the combustion of carbon based fuels produces • the appropriate measure of CO2 production is therefore kg(CO2) per Joule of
energy produced: Coal Black 105 Oil Brown 115 75 Gas 55

tonnes / MJ

• of the carbon based fuels, coal is the worst performer and natural gas is the best.

• given the varying use of these fuels (see earlier), the world CO2 emissions are
( Mt of CO2) [3]:

• coal is really bad!
1.7 Smog formation pollutants and aerosols, some of which are photochemically produced.

• ‘smog’ or photochemical air pollution consists of a complex mixture of gaseous • among the gaseous compounds are ozone O3, nitrogen dioxide NO2, and
peroxyacyl nitrate. The most commonly found species in this mixture is peroxyactelynitrate H3C(CO)OONO2 or ‘PAN’.

• the three compounds O3, NO2 and PAN are usually grouped together and called
‘photochemical oxidant’.

• HC emissions by themselves are harmless, but in the presence of NO and U.V
radiation they produce oxygenated hydrocarbons, PAN, O3, which finally decompose to NO2.

• these photochemical reactions are complicated. For example, consider a
‘simplified’ schematic of reactions involving oxides of nitrogen in the atmosphere:

• however, the objectives for the designer of a combustion system are clear:
reduce the emissions of NOX, unburnt HC’s, CO, SOX, etc and, increasingly, CO2. References 1. Reports to the Nation: Our Changing Climate, in University Corporation for Atmospheric Research (UCAR) and the National Oceanic and Atmospheric Administration (NOAA). 1997: Boulder, Colorado. p. 20. 2. 3. “Key world energy statistics from the IEA (International Energy Acency)” 4. 5. AGO, A.G.O., National greenhouse gas inventory 6. US Department of Energy (DOE),

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