COMBUSTION

This is a set of lectures on Combustion Basics (there is little here on practical systems), organised in the following topics:

Combustion characteristics. A descriptive presentation of what is combustion, what it is for (applications), how it is done (practical combustors), and what was historically known.

Fuels, Fuel properties, Fuel consumption, and Pyrotechnics. An extensive descriptive presentation, with an historical development review.

Combustor characteristics. A short description, following a block diagram approach, of what to study in a generic combustor: intake, internals, heat and work flows and exhaust.

Environmental effects and hazards in combustion. A descriptive presentation of potential source of damege in combustion applications, with a review of fire safety, pollutant emissions, and generic safety management.

Combustion thermodynamics. A mathematical formulation of equilibrium conditions, based on the extent of reaction and the affinity of reaction, and with emphasis on the enthalpy of reaction (the maximum heat, or heating value), the exergy of reaction (the maximum work) and the exhaust equilibrium composition.

Combustion kinetics. A descriptive presentation of the detailed mechanism of reaction rates, activation energy and its modification by catalysts.

Combustion models . A mathematical formulation of some key combustion problems: combustion at rest, premixed combustion and non-premixed combustion, with emphasis on flame geometry.

Combustion instrumentation. A descriptive presentation of devices and procedures for the setting, control and diagnosis of combustion processes.

Back to index

COMBUSTION CHARACTERISTICS

Combustion characteristics (Introduction) .................................................................................................... 1

Combustion fundamentals (What it is) ..................................................................................................... 1

What it is not ......................................................................................................................................... 3

Thermal free-flame gaseous combustion .............................................................................................. 4

Thermal trapped-flame combustion in porous media ........................................................................... 4

Smouldering (Thermal non-flame combustion of porous media) ......................................................... 5

Catalytic combustion............................................................................................................................. 5

Detonating combustion ......................................................................................................................... 6

Combustion applications (What it is for) .................................................................................................. 6

Heating .................................................................................................................................................. 7

Propulsion and electricity...................................................................................................................... 7

Absorption refrigeration ........................................................................................................................ 7

Chemical transformations ..................................................................................................................... 8

Combustion systems types (How it is done) ............................................................................................. 8

Steady combustion chambers ................................................................................................................ 8

Unsteady combustion chambers............................................................................................................ 9

Catalytic combustors ........................................................................................................................... 10

Porous burners..................................................................................................................................... 10

Fluidised bed combustion ................................................................................................................... 11

Open fires ............................................................................................................................................ 11

Combustion history (What was known).................................................................................................. 11

Fuel history ......................................................................................................................................... 11

History of Combustion theories .......................................................................................................... 12

COMBUSTION CHARACTERISTICS (INTRODUCTION) There is much more to combustion than a fuel in air and an ignition source. To better appreciate the wide

range of involved phenomena, a description of combustion basics (combustion types and processes), and

combustion applications (combustor types and systems), is presented here, before a more rigorous

treatment of the thermodynamics, kinetics and metrology of combustion.

COMBUSTION FUNDAMENTALS (WHAT IT IS)

Everyone knows from infancy what a fire is; humans have always felt a mixture of fear and magical

appeal for fire. Combustion is burning, a self-propagating oxidative chemical reaction producing light,

heat, smoke and gases in a flame front. Combustion is a process and fire is the actual outcome. What does

it mean in more detail?

Combustion means burning (lat. cum urere-ustus = burn), e.g. burning wood in air, natural gas in

air (or CH4/O2/N2 mixtures in general), hydrogen with oxygen (H2/O2 in gaseous or liquid forms,

and not only H2 in O2, but O2 in H2), and more bizarre burnings, such as sodium with chlorine

(Na(s)/Cl2(g)), aluminium powder with water, magnesium powder with carbon dioxide,

nitrocellulose (cellulose is -(C6H10O5)n- with n=300..2000) within any medium, etc. But the

meaning of combustion is usually restricted to easily flammable substances (typical fuels) in

ambient air. Fuels and oxidisers are presented aside.

Self-propagating, means that, once it ignites, it goes on, sustained by the high temperatures and

radicals (active species) produced, until either the fuel or the oxidiser practically runs out, or an

extinguishing agent is applied that prevents fuel-and-air mixing, or cools the system well below

autoignition, or scavenges active species. Notice the two sequential steps in combustion: first there

is an endothermic process of ignition, followed by a much more powerful exothermic process of

runaway oxidation that propagates the process. Any exothermic adiabatic system will show a

thermal runaway at some high enough temperature (autoignition), but for real non-adiabatic

systems, ignition criteria are governed by an interplay between heat-release rate and heat-loss rate.

Materials are termed non-combustible if they cannot be ignited below 1000 K.

Combustion propagation is usually a slow process; very slow indeed for solid and liquid fuels:

It is the visible light of non-premixed flames that has been traditionally identified with combustion

(it is the standard symbol for fire). In fact, it might help to think of the flame as an invisible, very

hot, burning interface, made visible by non-burning incandescent substances passing by or being

created, for instance soot particles in non-premixed flames (their sublimation temperature is

around Tsubl=3900 K), sodium ions in salt-seeded flames (above the salt boiling point Tb=1690 K),

or calcium oxide in limelight (Tb=3100 K). The latter was used in the 19th c. in theatres as the

brightest, most natural-colour artificial light available, being produced by placing a block of lime

against a hydrogen/oxygen jet flame (practically invisible in spite of its 4000 K temperature; lime

melts at 2850 K). The Sun also gives light and heat (at a temperature of 5800 K in its surface) but

by nuclear fusion reactions in the interior (where the temperature may reach 107 K) and not by

chemical combustion.

What it is not

Combustion vs. explosions

Combustion means burning and explosion means bursting, i.e. combustion is a relatively slow chemical

process yielding light and heat, whereas explosion is a sudden mechanical process causing rupture and

noise, due to great pressure forces that may be originated chemically (e.g. from a confined combustion),

thermally (as in boilers, even electrically heated), mechanically (as in a balloon or any other gas-

pressurised vessel), nuclearly, etc. Detonation, the supersonic combustion taking place under some

circumstances in premixed fuel/oxidiser gaseous mixtures and many explosives, is studied aside.

Combustion vs. fuel cells

Fuel cells are electrochemical generators, like batteries but with continuous fuel-and-oxidiser supply.

Reactions inside a fuel cell, although globally equivalent to combustion, are not properly combustion

because they do not self-propagate (reaction in a fuel cell stops as soon as the electrical load is switched

off, it shows no thermal-runaway). A non-premixed burner (e.g. a lighter) may be thought of as

controllable as a fuel cell (as soon as the fuel injection stops, combustion ceases), but it does not simply

starts over if reopened. A controllable-area catalytic combustor, however, more closely resembles a fuel

cell: no need of igniter, simple reaction control, and for small active areas there is no runaway (a big

difference is that fuel cell directly generates electricity and the catalytic combustor just heat).

The igniter in a combustor (a spark or a hot wire) and the electrical connector in a fuel cell, act as

catalysts that provide a gateway for the reaction; in both cases there is an electron-transfer reaction (redox

reaction), the main difference being that the transfer of electrons from fuel to oxidiser is restricted in a

fuel cell by electrode-interface-area and electrolyte-ion-diffusion, with the external electrical connector

required all the time, whereas in normal combustion the electrons transfer is only limited by diffusion in

the bulk, and the igniter is only needed to start the process. Entropy generation, positive in both cases,

tends to zero in a fuel cell at very low intensities, but it is always above a certain finite value in

combustion.

Combustion vs. oxidation

Combustion is a self-propagating oxidative chemical reaction characterised by a thermal runaway; i.e. it

is a quick exothermal oxidation. The same system may undergo slow oxidation or combustion, with the

same initial and final states, but with different paths (e.g. paper turns yellow (and brittle) with the years

because of slow oxidation, but may burn in seconds). Notice also that oxidation may be exothermic or

endothermic, whereas combustion is always very exothermic.

THERMAL FREE-FLAME GASEOUS COMBUSTION

This is the usual case for combustion, that self-propagates as a result of the high temperature (1500..3500

K) developed after initial ignition (e.g. by a spark), due to a more-or-less adiabatic conversion of

chemical-bond energy to internal-thermal energy within a reacting gas mixture (for condensed fuels, the

latent heats for vaporisation and possibly decomposition has to be subtracted).

According to the initial state of mixing of fuel and air, combustion process can be classified in the limit as

premixed and non-premixed (real processes are in between), with a corresponding premixed and non-

premixed flame. In premixed combustion the unburnt gas is already a perfect mixture of fuel and air, and

the burning or flame-propagation speed is only limited by the chemical kinetics of the reactions involved

and heat diffusion forward, whereas in non-premixed combustion there is not a characteristic burning or

flame-propagation speed, the speed being fixed by the flow-rates of fuel and oxidiser that must approach

the flame by diffusion from each side.

Steady free flames demand an astonishing fine balance for heat and mass flows (e.g. think why a candle

flame sits at a precise distance up the wick). If adiabaticity of the initial ignition region is prevented by

nearby heat sinks, as a cold solid wall, this kind of 'thermal' combustion cannot propagate (safety lamps

and quenching grids are based on this fact); e.g. for premixed methane/air stoichiometric mixtures,

thermal free-flame combustion cannot propagate inside a metal tube of less than 2 mm.

Thermal flames are almost always established in a gas phase: in a fuel/air gas mixture or in the fuel

vapours diffusing in air, from liquid or solid fuels. Only refractory fuels like coal to some extent, tantalum

or zirconium, burn at the solid surface; iron and titanium, having intermediate melting points for both the

metals and the oxides, burn at the surface of a molten mixture of the metal and its oxide, whereas

aluminium and magnesium, which have low boiling points, vaporize and then burn in the gas phase.

The presence of condense matter is always a handicap (all condensed fuels burn worse than their

vapours), and this fact is used in fire-fighting. However, flames may be sustained inside liquids in a

suitable gas envelop. In order to maintain a steady underwater flame (e.g. a hydrogen or acetylene

welding torch), it is necessary to form a stable bubble, usually achieved with an additional compressed-air

jet-stream introduced around the tip of the torch, since the exhaust gases cannot maintain it by themselves

(a great deal of skill is required of divers who perform this kind of work).

Thermal free-flame combustion cannot propagate if the air/fuel ratio lies outside of the lower and upper

flammability limits at ambient conditions: e.g. 5% and 15% of fuel by volume of mixture for methane/air

flames, respectively, although increasing temperature widens this range. Neither can flames propagate

also at very low pressures (a fire safety rescue in spacecraft).

THERMAL TRAPPED-FLAME COMBUSTION IN POROUS MEDIA

Flames cannot propagate through small holes in a solid (this is how Davy's safety lamp work), unless the

solid is hot enough (say >1000 K), what can be achieved by holding a lit free flame close to the solid for

some time. For instance, if a premixed methane/air stream is forced through a finite porous medium

(usually solid, but also fluidised), and ignited at the exit, the free-flame formed may travel upstream or

downstream according to the flow speed. If the injected gas speed matches the deflagration speed, the

flame sits steadily at the mouth and, after the porous end gets hot, slowly decreasing gas injection speed

allows the flame to go backwards and penetrate the porous media, which is being heated by the slowly

moving flame front. The process is also known as filtration combustion.

Notice that the flame-front temperature is lower than the adiabatic value when the front moves

backwards, but, if the gas injection speed is increased to force the flame to travel downstream, then its

temperature is above the adiabatic value (it is moving into an already hot solid).

Presently only Al2O3 and ZrO2 can work above 2000 K, SiC and FeCrAl-alloys being used below 2000 K

with the advantage that they have larger thermal conductivities and mechanical resistance. In spite of this

basic materials difficulty, porous-medium burners have several advantages:

Less NOx emissions because of lower temperatures.

More compact because the deflagration speed increases from 0.5 m/s to 4 m/s (they reach 3000

kW/m2 instead of the 300 kW/m2 of normal burners).

Wider range of ignitable compositions (lower limit decreases from 5% to 4% in methane/air

flames, increasing the air ratio from =1.9 to =2.2, allowing for leaner mixtures to be burnt,

yielding lower emissions).

Wider power-modulation range (0.1 to 1 times full load, against 0.5 to 1 for normal burners, so

that start/stop cycles and accumulators are avoided). Porous media combustion was developed

aiming to stabilise premixed flames near their lower stability limit.

SMOULDERING (THERMAL NON-FLAME COMBUSTION OF POROUS MEDIA)

Before, combustion 'in' a porous-media was considered; now combustion 'of' a porous-media-fuel is

analysed. Some porous (solid) fuels may sustain a self-propagating combustion inside their matrix, i.e. an

heterogeneous reaction, at a very low rate and low temperature, taking the oxidiser from the ambient

through its pores, with little or no visible flame, but with change of external texture (which chars) and

smoke emission (sometimes very toxic).

A typical smouldering process takes place in the tip of a lighted cigarette. The porous fuel is inside a

porous cylindrical envelop (of paper in a cigarette, or a full tobacco leaf in a cigar) containing crashed

dried tobacco leaves (they were smoked or chewed by American Indians since ancient times because of

the euphoric action of nicotine). Once the cigarette lighted, if left in still air without drawing, a dim

burning happens with maximum temperatures of some 85050 K at the centre and some 65050 K at the

periphery. During drawing, however, those values go up to 100050 K and 85050 K, respectively.

A big fire-safety problem is that, the burning taking place in the insides, smouldering can progress

undetected for long periods of time (smoke detectors are used, but diffusion is very inefficient. Thus,

under certain circumstances, smouldering may undergo a sudden transition to flaming; that is why some

forced convection of cabin-air and avionics-air is always procured in spacecrafts, where buoyancy forces

are absent, to help an early detection.

CATALYTIC COMBUSTION

Even in the close proximity of cold solids, combustion may be sustained if there is some catalytic

substance that lowers the activation energy and sustains the combustion process at low temperature at the

porous surface; e.g. H2/air react over Pt at room temperature, CH4/air over Pt requires 350 C. The most

developed catalytic combustor uses C4H10/air over Pt-coated ceramic matrix at 500 C. Low-temperature

catalytic combustors work in the 300..600 K range, and high-temperature catalytic combustors work in

the 700..1400 K. They may work with premixed flows and with a non-premixed fuel-flow and ambient

air.

The advantage of power modulation, widening of ignition limits and less emissions, are maintained or

increases here (

achieved instead of the 3000 kW/m2 of thermal porous-medium burners. At intermediate temperatures, an

hybrid regime of catalytic-assisted thermal combustion may be developed, where both heterogeneous and

homogeneous reactions take place, an interesting trade-off solution when the catalyst is too expensive and

very little can be used (in full catalytic mode, if there is insufficient catalyst, part of the fuel slips to the

exhaust (increasing pollution and expense), or even the whole fuel if the catalyst cools down and

deactivates. The influence of the flow rates is important; for instance, if a thin porous solid is doped with

a catalyst and a premixed methane/air stream is forced through, an exothermic oxidation of the fuel takes

place if the temperature is >600 K, releasing much of the lower heating value, what causes the matrix to

reach some 1500 K at the outer surface (the hotter) in a flameless regime with a power of up to 500

kW/m2; but if more gases are fed, small flames detach from the outer surface and at about 1000 kW/m2 a

blue flame at some 1900 K forms immediately close to the outer surface, that only reaches now some

600 K.

DETONATING COMBUSTION

Supersonic combustion, called detonation, is realised when combustion gets coupled to a shock wave

travelling at supersonic speed. Detonating combustion is considered under Explosives in Pyrotechnics,

and under Supersonic combustion in /Combustion kinetics.

COMBUSTION APPLICATIONS (WHAT IT IS FOR)

Combustion is an old technology to humankind, only second to simple mechanical tools made of wood,

bone and stone. Is it just then a subject of historical interest? For instance, from pre-historic times (some

500 000 years ago, at least) to the end of XIX c. the only lighting method was combustion, but today only

electrical lamps are used, so, why spending effort on candescent lighting?

We want to devote an effort to understand combustion processes for several reasons:

1. To understand it (i.e. for its own sake, the general interest to broad our knowledge of the world).

2. To minimise the risks and damages of controlled combustion and uncontrolled fires (safe fire

handling).

3. To advance the efficient use of fossil fuels (quickly exhausted) to satisfy our needs (e.g.

transportation propulsion, electricity generation).

4. To alleviate the problems associated to combustion emissions.

Combustion always gives off heat and gases, most of the times light, and sometimes smoke (a nuisance in

non-premixed flames, and the major personal danger in fires). Combustion is the major energy release

mechanism in the Earth (it is gravitational dissipation in jovian moons). Why? Because the Earth

atmosphere is plenty of free-molecular oxygen (50%wt of the whole ecosphere is oxygen, but mostly in

compounds) and Earth ecosphere is plenty of fuel (all living matter plus fossil fuels). Why? It is nearly a

miracle because at the start of the Earth ages, 4.5109 yr ago, there was no O2 but some H2O that

generated a small amount of O2 by solar-radiation hydrolysis, part of which was transformed to UV-

shield O3 by solar radiation and thus allowed the start and survival of primitive plants. Plant

photosynthesis has generated all O2 in the atmosphere and all living (and fossil) fuels on Earth.

Combustion (controlled and uncontrolled) releases an average of 101012 W in the globe, as compared to

0.51012 W due to nuclear disintegration (controlled and uncontrolled), 31012 W due to geothermal heat

flux and 120 0001012 W due to the solar absorption.

Combustion is key to humankind today as in the past and the foreseeable future; it is worldwide the major

energy-release process (nearly 90% of the world primary energy is to be burnt), it is the major

environmental-pollution process, and fire is one of the most feared natural disasters. Roman Emperor

Augustus established a corps of fire-fighting watchmen already in 24 b.C.; their fire-fighting techniques

are still applied: removing heat by throwing water with a bucket passed from hand to hand, removing the

fuel by cutting it aside with an axe (or, at a much later times, by providing firebreaks), and opening doors

and windows in enclosures to get rid of smoke and let fresh-air in for people sake (unfortunately feeding

the combustion process, too).

Even nowadays 88% of world commercial energy consumption (i.e. excluding photosynthesis and

uncontrolled fires) is by combustion (plus a 5% hydraulic and a 7% nuclear fission, roughly). This

combustion-released energy (as any energy) is used today for:

Heating. Internal chemical energy is first released to internal thermal energy (high temperature),

and then heat transfer to the load takes place.

Propulsion and electricity generation. Mainly by means of a heat engine with a working fluid that

may be an independent one (e.g. steam), or directly the combustion products.

Refrigeration. By an absorption refrigerator driven directly by the combustion products or through

an intermediate fluid (e.g. steam).

Chemical transformations. Mainly as a heating application, but sometimes directly for reduction,

oxidation, incineration, reforming, etc.

Light emission was a major application of combustion up to the XIX c., but nowadays it is a

rarity: candles, rescue signals, fireworks.

There are other unusual applications of combustion, as smoke generation for visualization, gas generation

for fuel-tank inertisation, aromatherapy, etc. Explosives applications are considered aside.

HEATING

Heating for space conditioning or for materials processing (domestic and industrial) has always been the

basic combustion application. Energy efficiency in heating is usually measured as the fraction of the

lower heating value of the fuel that is fed to the system to be heated (the rest is lost by the flue), and it is

close to 1 (that is not a limit: heat pumps can give more).

A special heating application is the cutting of materials. Thick metal plates can easily be cut with a

normal gasoline, propane or oxy-acetylene torch, with additional oxygen to burn the metal (that is a fuel

itself; hint: think of M+O2=MO2, as for carbon). For cutting or piercing thick ceramic objects, a thermal

lance is used, in which, an iron tube act as the fuel that burns (once ignited) with oxygen being supplied

through it (sometimes in the cryogenic liquid state). A small 5 mm-diameter tube, 0.5 m long, may burn

for one minute consuming some 5010-3 kg of oxygen, producing a >5000 K flame that would pierce a

150 mm-thick steel slab in less than 10 s (a 0.5 m deep hole can be pierced in stone or concrete in a few

minutes, anywhere: in the air, under water, or within mud and slurries).

PROPULSION AND ELECTRICITY

Motion and electricity generation, by means of heat engines (vapour and gas cycles), constitutes the

foundations of Thermodynamics. For electricity generation, energy efficiency is measured by the ratio of

electrical energy delivered divided by the lower heating value of the fuel (it typically ranges from 0.2 to

0.5), or by the specific fuel consumption (of the order of 60 g/MJ or 200 g/kWh), and it could be similarly

done for mechanical power, although for cars it is usually done in litres of fuel consumed per 100 km

travelled (or in miles per gallon in the USA; the change is x [mph]=235/x [L/100 km]).

ABSORPTION REFRIGERATION

Absorption-refrigeration machines use heat (usually from steam or from a fuel burning) to separate the

working vapour from a high-pressure liquid mixture. The working vapour condenses in a heat exchanger

with the ambient, and is flashed through a restriction to a low-pressure heat exchanger where the cooling

effect is produced, exiting into a liquid absorber that is pumped and closes the loop. Energy efficiency is

usually measured as the amount of cooling divided by the amount of fuel or steam consumed (in vapour-

compression refrigeration, energy efficiency is measured by the ratio of cooling energy to electrical or

mechanical energy input).

CHEMICAL TRANSFORMATIONS

Some materials processing, e.g. mineral reduction, cooking, distillation, require an exergy supply (other

materials processes are used as an exergy source, as combustion); incineration may have positive or

negative heat balance, depending on the materials. In the typical case of energy consumption, energy

efficiency is usually measured as the amount of material processed divided by the amount of fuel

consumed.

COMBUSTION SYSTEM TYPES (HOW IT IS DONE)

According to the steadiness of the control frontier (real or imaginary), and to how the propagation is

maintained, one may distinguish several type of combustors, traditionally grouped in steady combustion

chambers and unsteady combustors.

STEADY COMBUSTION CHAMBERS

Steady state combustion takes place in industrial heaters, furnaces, boilers, steam turbine boilers, gas

turbines, and domestic appliances: from the cooking range to the gas lighter. Steady refers to the average

process, but it does not mean that the process is locally steady; most practical systems operate in a highly

turbulent regime. The combustion process may approach the non-premixed flame model or the premixed

flame model, according to how much fuel-and-air mixing exists before burning. Walls must be cooled to

avoid materials problems at the high temperatures involved (>2000 K), and, most of the times, excess air

is used to cool the main burnt flow, that is nearly stoichiometric (see Fig. 1a).

Recently, partial recirculation of exhaust gases is being applied (instead of the secondary air in Fig. 1a)

for emission reduction, heat recovery, autoignition and flame stabilisation. Exhaust gas recirculation

(EGR) helps to keep a high uniform temperature inside the combustor, but lower than the hottest region in

normal flames; e.g. a maximum T1800 K with maximum spatial deviations of T100 K, entering the

air/EGR mixture at some 1500 K with as low as 2% O2 and producing a wide dim green flame, whereas,

in normal combustion, air enters at 300 K with 21% O2, producing a bright and long yellow flame with

maximum temperature T2500 K with maximum spatial deviations of T1000 K.

A modern home boiler (Fig. 1b) is a friendly, efficient and environmentally-clean combustion appliance;

it is permanently ready (safe, reliable, no surveillance, no servicing, 20 years life), provides some 20 kW

of space heating or hot water for sanitary use, is fed by natural gas that burns with 3% excess air,

extracting 94% of the maximum heating value (105% in terms of its LHV, due to exhaust condensation),

is small and silent, and only produces 80 mg/m3 of NOx as major pollution (besides the CO2 inherent to

the fuel carbon content).

Fig. 1. Steady combustion chamber layouts: a) industrial burner, b) domestic water heater.

Solid fuels are no longer burnt in chunks over a grate, but gasified (pyrolysed), pulverised (fine dust) or

fluidised (coarse granulate made to levitate by air entrainment). Dry (3 MW.

UNSTEADY COMBUSTION CHAMBERS

Unsteady combustion chambers are used in reciprocating internal combustion engines (ICE); also in the

(non-reciprocating) Wankel engine, not further mentioned here because of lack of use. The combustion

process may approach the non-premixed flame model (for the major part of the fuel burning in a diesel

engine), or the premixed flame model (for the spark-ignition engine and the initial stage for compression-

ignition engines). The period of burning and fluids renovation is so short, that the thermal inertia of the

wall materials makes higher peak temperatures allowable in the gas, contrary to steady combustors.

The two basic ICE realisations are the spark ignition engine (SI, gasoline, or Otto) and the compression

ignition engine (CI, diesel oil, or Diesel). The normal working for a SI-engine is with a premixture of fuel

and air at the stoichiometric ratio, tuning the output power by essentially chocking the entrance duct,

whereas for a CI-engine non-premixed fuel is added to highly-compressed air. SI-fuels must have high

volatilities (or be gaseous) for quick mixing, and have a high resistance to autoignition, to avoid

knocking. CI-fuels must have a low autoignition temperature and short delay.

The best ICE are direct injection (DI) exhaust gas recirculation (EGR) engines, both of Diesel and of Otto

type, the latter with stratified mixture (by injection control) and exhaust catalytic converter. Typical

air/fuel relative ratios are =0.9..1.05 for Otto engines, =1.3..1.8 for normal Diesel engines, and

=1.6..2.1 for large (>500 kW) slow (

It seems that both Otto and Diesel engines are converging towards an hybrid, the new Homogeneous

Charge Compression Ignition engine (HCCI), where fuel injection during the compression stroke creates

a very lean mixture that auto-ignites by premixed compression near the top dead centre, producing a

better combustion for a steady regime (it is difficult to control the time of ignition if unsteady): less soot

and lower NOx emissions (lower temperatures) than a diesel engine, and higher efficiency that an Otto

engine (but unburnt emissions in between).

CATALYTIC COMBUSTORS

Catalytic combustors are porous catalytic solids inside which a low-temperature combustion takes place.

They are not so much developed as the simple thermal combustors above-mentioned (steady and

unsteady). They have the advantage of very complete oxidation (low emissions) and low-temperature

work, but they are expensive and bulky. They may be used for:

Catalytic burners, where heat is generated by burning a main fuel (C4H10, CH4, H2) in a catalytic

porous media. They are mainly used as radiant catalytic heaters for stoves and open-air industrial

heating (e.g. to cure the epoxy used to join pipes in gasoducts and pipelines, to dry wood and other

biomass).

Catalytic-assisted burners, where very lean mixture may be burnt at moderate temperatures,

avoiding the formation of NOx. They are being introduced in gas-turbine engines and plants.

Catalytic converters, where emissions are remediated without interest in heat generation (e.g. CO

and VOC in car exhausts are completely oxidised). This is the most developed application; most

modern cars have a catalytic converter in the exhaust pipe (see Three-way catalytic converter).

Although simple stagnation flows over a catalytic plate are studied in the lab, practical systems consist of

monolithic honeycombs or granular material (porous media of 1 mm typical porous size), because of the

surface effect on heterogeneous reactions. The combustion catalyst per excellence is a platinum doped

refractory matrix.

Catalysts may work at room temperature (as in Pt for H2/O2), but most usually they require some initial

heating (as in Pt for CH4/air), achieved electrically (from the mains, or with a battery), by a conventional

pilot flame, or even by a room-temperature catalytic combustor (as in hydrogen-assisted natural-gas

catalytic combustor). The element preheats the catalyst bed to some 400 K, the temperature at which the

catalytic pad will sustain a chemical reaction with the fuel and ambient air, and the fuel supply is

regulated to operate steadily at some 700 K.

POROUS BURNERS

Porous burners are porous non-catalytic solids inside which a high-temperature combustion takes place

with small flames trapped in the pores. They are in the development stage, showing promise of very high

combustion intensities (3000 kW/m2 of cross-section) and low emissions, because of increase heat

transfer and flame stability. They may work with a premixed flow (fuel and air been forced through one

end), or, more rarely, with a non-premixed fuel flow that meets a back-diffusing air flow at the exit.

Premixed porous burners consist of two sequential stages: the premix fuel/air stream first enters a fine-

pore hot solid matrix (below the flame quenching size, for safety), where it is heated until it enters the

second larger-pore hottest solid matrix, where alveolar flames stabilise themselves for a wide range of

flow rates (e.g. from v=0.2..4 m/s for CH4/air) and air/fuel relative ratios (e.g. from =1..2 for CH4/air).

Depending on the application, a third stage may be directly added to the burner, with a compact heat

exchanger.

The wide power modulation capability of porous burners make them ideal for applications such as home

water heaters, where the power needed to heat sanitary water cannot be less than say 15 kW and is

increasing, whereas the power for space heating is decreasing from typically 20 kW to 5 kW for better-

insulated apartments. Besides, being more compact, porous burners could be installed in wall niches

instead of protruding significantly. This compactness is also an advantage for industrial heaters and

driers, that must be 1 m to 3 m long if conventional (to protect from the flame), but can be shorten to less

than 0.5 m using a porous burner.

FLUIDISED BED COMBUSTION

Fluidised bed combustion is conceptually similar to porous burner combustion, but, instead of a rigid

solid matrix, a high-temperature combustion takes place within a particulate system (the fuel particles

themselves, or sand or other granular refractory), held over a porous surface. When an upward air-stream

(or a premixed fuel/air stream) is established, for small upward speeds the particulates act just as a filter,

but when the drag on the particles overcome its weight, a fluidised regime is establish (at the fluidising

speed), with more or less bubbling or boiling when speed is increased until at a certain value is

reached (the terminal velocity) beyond which the particles are ejected from the bed and carried away with

the stream.

The main advantage of this modern type of combustors is that additives can be easily added and the

porous material easily refurbished. The disadvantages are the difficult start-up, difficult ignition (it is

ignited above the bed), noise, and erosion of the heat-exchanger walls (that is partially or totally

submerged to limit the maximum temperature within the bed). Steady burning is achieved at 1100 K to

1300 K (uniform red-hot bed), but below 1100 K the burning is noisy with violent small bursts, and above

1300 K the particulate may start fusing and clogging. Fluidised bed combustion is currently used in some

coal-fired power plants, where coal-desulfuration is achieved during combustion by adding lime to the

bed. It is also been used to burn high-moisture fuels like agriculture and industrial residues and sludge.

OPEN FIRES

Most practical combustion systems are enclosed inside easily-identifiable walls (combustion chambers,

empty or filled with porous media). By 'open fires' we here just mean unenclosed combustion setups,

from the traditional fireplace to the wild uncontrolled forest fire, passing by the humble candle flame.

Because the information here compiled is basically historical, it can be found under Torches, oil lamps

and candles, in Fuels.

Smouldering is not intentionally used in engineering, but it is very important in uncontrolled fires. Porous

materials, either solid like wood or particulate like dust, may sustain a low-temperature combustion inside

with air penetrating by diffusion. A smoulder temperature may be identified to describe the flammability

behaviour of a flat dust layer on a hot surface (e.g. the lowest temperature of a heated surface capable of

igniting a 5 mm thick dust)

COMBUSTION HISTORY (WHAT WAS KNOWN)

Although the history of combustion is related to the history of fuels, we treated them here separately

because under fuels we focused on empirical findings and under combustion we centre the attention on

theories.

FUEL HISTORY

This theme can be found in Fuels. Briefly, Fuel history has the subheadings there: Biological fuels before

the XX c., Mineral fuels (coal, crude oil and natural gas), and Biological fuels in the XXI c.

HISTORY OF COMBUSTION THEORIES

Fire has always been a cause of panic, but also of immense power. It had a magical appeal: Prometheus

stole it from the gods, according to Greek mythology. Around 500 b.C., Heraclitus of Ephesus held that

fire is the primordial substance of the universe and that all things are in perpetual change (like a vital

flame). Around 450 b.C., Empedocles of Sicily set the theory of the four universal elements: fire, air,

water, and earth, further developed and spread by Aristotle around 350 b.C. (he added aether as the

quintessence, in his Metaphysics).

Coming down to our physics, it was appreciated from the beginning that combustion required a fuel (e.g.

wood, but not stone) and an igniter (by rubbing woods, sparking stones or compressing air, but not just

hitting the wood). However, it was only learnt in the 17th c. that air was also needed; around 1663, Boyle

realised that, under vacuum, sulfur could not be ignited by concentrated light, as usually achieved, but it

was Lavoisier in 1777 the first to realise that oxygen was the key, both to combustion and to animal

respiration. Much earlier than the need of air for combustion was realised, it was common practice to

preserve fire overnight by putting a fire cover (curfew) over it, to avoid air convection and heat loss.

The concept of energy (chemical, thermal, electrical), as an extension of the primitive mechanical idea of

combination of 'head' (potential energy) and 'vis viva' (kinetic energy), was only generally accepted after

mid 19th century. Before that, imaginary fluids were thought to explain the facts. Around 1660, J.J.

Becher and G.E.Stahl introduced the theory of phlogiston as a fluid that flowed away in combustion

processes. Lavoisier, in his paper "Rflexions sur le phlogistique" of 1783, rejected the phlogiston theory,

but set forth the caloric theory, a fluid that flowed away with heat (some authors even advocated for

another fluid for cooling processes, the frigoric). Although already in 1798, Count Rumford rejected the

caloric theory in his essay 'An experimental enquiry concerning the source of the heat which is excited by

friction', it was not until the 1840s that it was finally discarded and replaced by the principle of energy

conservation, by Mayer and Joule.

A good appreciation of the level acquired in combustion theory can be grasped from the titles of Michael

Faraday's famous series of Christmas Lectures for Children, delivered in the mid 19th c. (starting in 1826)

at the Royal Institution. The most important (actually six of them) was published as "The Chemical

History of a Candle" in 1860:

1. Construction of a candle. The flame.

2. Necessity of air for combustion.

3. Products of combustion; water from combustion.

4. The components of water are oxygen and hydrogen.

5. Components of air: oxygen and nitrogen. Product from a candle: carbon dioxide.

6. Human respiration and its analogy to a candle.

A summary of the chronological development of combustion theory is presented in Table 2.

Table 2. Chronology of combustion theories.

The need of air Robert Boyle-1663 found that combustion requires air, besides the fuel.

Phlogiston Proposed by the German physician and chemist Georg Ernst Stahl-1697 and

discarded by Antoine Lavoisier-1783-Rflexions sur le phlogistique, (lat.

phlegma=fire spirit): a substance (with mass) stored in fuels and released in

chemical reactions (the flammability component), aside of the caloric substance (the

heating component). Fuels released phlogiston quickly, whereas metals did it slowly

(rust). Afterwards, Joseph Priestley-1796 tried to come back to phlogiston.

Phlogiston principles:

Phlogiston is a material fluid present in fuels (wood, coal, metals and living

beings), that escapes from them when bodies are heated to burn.

Air is a phlogiston absorber, needed to let it escape from fuels. If air is

trapped inside a bell, fuels cannot yield all their phlogiston. Plants slowly

take phlogiston from the air on sunshine, and may suddenly release it if

burned.

Phlogiston is conserved; e.g. animals exhalate phlogiston by the mouth, is

stored in the air and plants, and returns to animals when they eat plants, or to

air if plants are burnt. The phlogiston released by metals when rusting may be

fed-back from phlogiston-containing substances (metals are obtained from

their ores by calcination with wood or coal). Water dissolves phlogiston, but

does not destroy it.

Phlogiston can be weighted by difference before and after burning the fuel.

But metals release so little phlogiston that it floats in air, making the metal

heavier when rusted.

Question. What was phlogiston: H2 (released by metals when pouring an acid over)

CO2 (exhaled by animals), or O2? Answer: the closest idea is to -O2, i.e. oxygen

affinity.

Caloric Proposed by Lavoisier-1777 and discarded by Mayer-1842, a substance (with mass)

released in thermal processes. Similarly, electricity was a substance (with mass)

released in electrical processes.

Mass

conservation

Proposed by Lavoisier-1777: the overall mass is preserved in any confined chemical

reaction. He also established the taxonomy of chemical compounds, and identified

oxygen-in-the-air as the main oxidiser, and carbon and hydrogen as the main fuel-

constituents.

Fixed

proportions

Proposed by Proust-1799: chemical reactions take a fixed amount of substances

relative to another, and the excess appears un-reacted (quite different to a mixture).

Multiple

proportions

Proposed by Dalton-1803: different chemical reactions between same reactives take

a fixed amounts of substances relative to another, the ratio between which are small

whole numbers. Berzelius-1826 accurately measured the relative masses in chemical

reactions and established chemical-formula notation (the first letter of the Latin

name with the relative factor as a superscript, later changed to subscript, e.g. H2O).

Flame

quenching

Emissions

Sir Humphry Davy published in 1800 his personal trials on the physiological effects

of some gases of importance to combustion (laughing gas, NO, and water gas, i.e.

CO+H2, which nearly poisoned him). Around 1815 he discovered that flames cannot

go through wire meshes and developed the miners safety lamp.

Premixed flame

Flame spectra

Robert Bunsen, Prof. At Heidelberg, developed his premixed burner in 1855,

measuring flame temperatures and flame speeds. Bunsen-1860 repeated Newton's

experiment of light dispersion by a prism, but with the light of flame, and discovered

that it was not a continuum but separated bands, particularly with premixed flames,

and the bands changed with addition of different substances to the flame (the

spectrum was characteristic of the substance). Bohr-1913 proposed the orbital theory

of electrons in the atom, the quantum numbers, and explained the emission spectra

of simple atoms in absence of magnetic fields (quantum mechanics explains

everything up to now).

Thermal model Mallard and Le Chtelier, Profs. at cole de Mines de Paris, studied flame

propagation in 1868 and proposed the first flame structure theory in 1883.

Chapman and Jouguet in 1900 distinguished deflagrations from detonations and

computed detonation speeds.

Thermal Ignition: Semenov's theory of 1928 (uniform temperature), Frank-

Kamenetskii's theory of 1945 (with thermal gradients).

Thermal explosions. Zeldovich theory of 1943. He also proposed in 1943 his thermal

model for the formation mechanism of NO.

Diffusion model Burke and Shumann in 1928 made the first theoretical computation of non-premixed

flame height and shape.

Thermal-

diffusive model

Zeldovich in the 1940s and Sivashiusky in the 1970s analysed thermal-diffusive

instabilities in two-dimensional flames.

Thermo-fluid-

chemistry

theory

Theodore von Krman, the founder of the U.S. Institute of Aeronautical Sciences

(1933), of the Jet Propulsion Laboratory (1944), and a co-founder of the Combustion

Institute, organised and led (under NATO and UN auspices) an international team,

around 1950, to compile and spread multidisciplinary knowledge on combustion

science, producing his famous Aerothermochemistry notes and lectures (T. von

Krmn and G. Milln, Proc. Combust. Instit. 4, 173 (1953)).

Finite rate

chemistry

Asymptotic methods for different chemical and flow time scales: activation energy

and reaction-rate ratio asymptotics. Steady-state and partial-equilibrium

approximations.

CFD codes Simulation of multidimensional reacting fluid flow, including turbulence,

unsteadiness, multiphase flow, acoustic-coupled instabilities, etc.

(Back to Combustion)

FUELS

Fuels .......................................................................................................................................................... 1

What is a fuel ........................................................................................................................................ 1 The oxidiser........................................................................................................................................... 2 What is a fuel used for .......................................................................................................................... 3 What is the problem with fuels? ........................................................................................................... 3 Hydrocarbon nomenclature ................................................................................................................... 4

Fuel types .................................................................................................................................................. 5 By physical state ................................................................................................................................... 5

Solid .................................................................................................................................................. 5 Liquid ................................................................................................................................................ 5 Gas .................................................................................................................................................... 5

By period of natural renovation ............................................................................................................ 5 Fossil fuels ........................................................................................................................................ 5 Renewable fuels ................................................................................................................................ 6

By production stage (resources type) .................................................................................................... 6 Natural or primary fuels .................................................................................................................... 6 Artificial or secondary fuels .............................................................................................................. 6

By marketing ......................................................................................................................................... 7 Non-commercial................................................................................................................................ 7 Commercial ....................................................................................................................................... 7 Petroleum fuels ................................................................................................................................. 7

By application ....................................................................................................................................... 8 For spark ignition engines ................................................................................................................. 8 For compression ignition engines ..................................................................................................... 9 For gas turbine engines ..................................................................................................................... 9 For boilers ......................................................................................................................................... 9 For small portable applications ....................................................................................................... 10 For fuel cells.................................................................................................................................... 10 For pyrotechnics, propellants and explosives ................................................................................. 10

Fuel history ............................................................................................................................................. 11 Biomass fuels before the XX c. .......................................................................................................... 11

The fireplace and the chimney ........................................................................................................ 11 Torches, oil lamps and candles ....................................................................................................... 12 Engines on biological fuels ............................................................................................................. 14

Mineral fuels ....................................................................................................................................... 14 Biological fuels in the XXI c. ............................................................................................................. 16

FUELS

WHAT IS A FUEL A fuel (from Old French feuaile, from feu fire, ultimately from Latin focus fireplace, hearth) is a substance that may be burned in air (or any other oxidant-containing substance), i.e. that so quickly reacts with oxygen that heat and light is emitted in the form of a sustained flame.

Although the chemical reactions of wood in air, animal-fat in air, coal in air, natural gas in air (approximated by CH4/O2/N2), hydrogen in oxygen (H2/O2), silicon in oxygen (Si/O2), sodium in chlorine (Na(s)/Cl2(g)), zirconium powder in carbon dioxide (Zr/CO2), nitrocellulose (-(C6H10O5)n- with n=300..2000) in any medium, etc., are all of the same chemical type (self-propagating highly-exothermic re-dox reactions, i.e. combustion processes), usually 'fuels' and 'combustion' only refer to easily flammable substances in air (the air is the oxidiser needed by a fuel to burn, and it is needed in larger quantities than fuels, so, a first glance on it seems appropriate).

THE OXIDISER Oxygen in the air is the basic oxidant for fuels: nitrogen is basically inert, although it combines endothermically with oxygen at high temperatures to get the unwanted NOx pollutants. The most abundant element on Earth's crust is oxygen, with 47%wt. It is also the most abundant in the hydrosphere, 86%wt. In the atmosphere, it is second to nitrogen, with 23%wt. In the whole ecosphere it is the first with 50%wt, and in the whole planet Earth it has 30%wt, only after Fe, 40%wt). Oxygen is then readily available from Earth's atmosphere; that is why it is the main oxidiser. Silicon follows as most abundant in the crust (28 %wt), but most natural Si-compounds are already fully oxidised, as sand (SiO2) and silicate rocks (like CaO3Si); there is no silane gas (SiH4) free in Nature. Aluminium follows (8 %wt), but again the main Al-compounds are already fully oxidised, as bauxite (Al2O3) and silicates like Al2O3Si. As for the fuels, hydrogen, the most abundant element of the Universe, is scarcely available in the ecosphere, mainly fully oxidised as water in the hydrosphere, where it comprises only a 3%wt, and only 0.9%wt in the whole Earth crust; the ninth most abundant element). Carbon has a poor abundance:

WHAT IS A FUEL USED FOR Fuels are mostly used as convenient energy stores because of their high specific energy release when burnt with omnipresent ambient air (or other specific oxidiser); the same fuel substance may be also used as a feedstock in chemical synthesis (e.g. polymers from petroleum), lubricants, paints (who has never used a coal chunk to draw), and so on, but these uses are minority. Primary fuels (natural fuels) may be difficult to find, and secondary fuels (artificial fuels) may be difficult to manufacture, but, once at hand, fuels are very easy to store, transport, and use, with the only nuisance of safety (uncontrolled combustion) and pollution (toxic emissions during storage and when burnt, dirtiness...). Energy is a basic need to humans. Besides metabolic energy needed by any living being and supplied by the catalytic reaction of food and oxygen (some 100 W for an adult person), descendants of homo habilis need energy to change Nature to better suit their needs: to heat their home, to cook their meals, to bring potable water in (e.g. from low river courses up to their dwellings) and waste waters out, to remove the ground for aeration, to mill the grain, to turn the potter-mill, to throw weapons, for industry, for transportation of goods and people, for telecommunications, etc. Fuels, and energy in general, are used (see Fuel consumption, for more details) for heat generation, for work generation, or for chemical transformations. A common problem to all human needs (except air, in most cases) is that energy is not available at the location and time we desired, and sources must be found (for energy, water, food, minerals), transportation to a better place must be arranged, as well as storage and end-use details. Storage is sometimes the most cumbersome stage, e.g. for food (all food is perishable, particularly meat and fish), and for electrical energy. Specific energy storage values for fuels are presented aside, in comparison with other energy store systems.

WHAT IS THE PROBLEM WITH FUELS? Several:

Fuels are dangerous, because they accumulate a lot of chemical energy that may be accidentally released, causing deathly thermal and chemical effects.

Fuels are pollutant when burnt (and even before; most liquid fuels are cancerous); they are presently the major contribution to environmental pollution, both locally and at a global scale.

Fuels are scarce (fossil sources are being depleted) and the sources are unevenly spread (most petroleum reserves are in the Middle East, causing economic and political instabilities).

Fuels are difficult to handle: coal is very dirty, crude-oil is too viscous, natural gas has very low density, to say the less.

But, as just discussed, fuels are so convenient energy storage systems, that their associated problems are but to be solved. Elaborating on the above problems:

Danger in fuel handling can be controlled and reduced to a relatively very low risk (in comparison with other accepted risks, as in transportation or sports).

Pollution by fuels can be negligible if clean renewable fuels are used; basically a fuel is a C-H-compound that combines with oxygen to yield CO2 (a natural compound of no harm if it does not accumulate, as for biofuels) and H2O (the most life-compatible compound). A short-term palliative to vehicle-engine pollution is to force a dual-fuel system (with dual fuel-reservoirs and engine controls, or even two different engines), using a non-pollutant fuel inside cities, ports and urban areas, and leaving the present more-convenient but more-pollutant fuels to highway and cruising.

Scarcity of fuels is like scarcity of water: what we mean is scarcity of cheap good-quality sources, i.e. that we have to devote a sizeable effort within our limited capabilities. The atoms

that make the fuel and oxidiser are preserved after combustion, and with the addition of some external energy (freely available from the Sun) those atoms can be arranged to form the initial fuel and oxidiser molecules. It may be difficult to think of such a new gadget added to our cars that would regenerate the gasoline and air from the exhaust gases, but think on a fuel-cell car that runs on H2+(1/2)O2=H2O and uses off-line-produced solar electricity (in the garage or at the station) to recharge the tank by electrolysis H2O=H2+(1/2)O2. Thus, the problem is an ancient one: it is hard to become a farmer if you can find suitable wild plants, animals and fuels.

Water-like fuels seem the best to handle, so coal should be liquefied (i.e., converted to liquid, what is presently done by first gasifying it), crude-oil is distilled, and natural gas should be liquefied (as done with coal gases by Fischer-Tropsch process, since the 1920s). Liquid fuels are a must, particularly for vehicle propulsion (in land, water, air and space).

HYDROCARBON NOMENCLATURE Most practical fuels are hydrocarbon mixtures (i.e. organic chemical compounds of carbon and hydrogen atoms, and maybe some additional ones); the main exception are pure carbon (C, graphite, since coal is already a mixture), and pure hydrogen (H2). Some chemistry refreshing is appropriate here to better understand fuels:

Hydrocarbon nomenclature: By type of additional atoms. Besides the ubiquitous carbon, hydrocarbons may be:

hydrogenated, oxygenated, nitrogenated, etc. By type of molecular shape. chain (linear or branched), cyclic (homo- or heterocyclic). By type of carbon bonding (single, double, or triple bond). By mixture fraction specification (all natural fuels are mixtures):

From fractional distillation of raw materials (wood, coal, crude oil). From chemical reforming (e.g. pyrolysis, cracking of heavy molecules, reforming

by synthesis with vapour or air, like syngas). Hydrogenated compounds may be:

Aliphatic (oils and fats, little odour): alkanes (saturated hydrocarbons: paraffins (linear chain), isoparaffins (branched chain), and cycloparaffins or naphtenes), alkenes (ethylenes or olefins), alkynes (acetylenes).

Aromatic (strong smell): 1-ring: benzene, phenol (-OH), toluene (-CH3), aniline (-NH2), nitrobenzene (-NO2), styrene (--CH=CH2)); 2-rings: naphtalenes; 3-rings: anthracenes; 4-rings: tetracenes; etc...

Oxygenated compounds may be:

Carbohydrates: Cn(H2O)m, like sugars, starch and cellulose. Alcohols: R-OH (phenols if R is aromatic; glycols if double OH-group, glycerols if three OH)).

Methanol comes from wood distillation or natural gas reforming. Ethanol comes from biomass fermentation (directly from sugars, or after hydrolysis from starch).

Ethers: R-O-R'. Alcohol derivatives (obtained from alcohols and isobutene, and oil derivative; in the lab, mixing alcohol and sulfuric acid); used as solvents.

Esters: R-COO-R'. From acid+alcoholester+water: R-COOH+R'-OHR-COO-R'+H2O. Volatile fragrant fats (also found on fruits and flowers).

Aldehydes: R-CHO. Dehydrogenated alcohols. They have the divalent carbonyl group =CO. Used in the manufacture of chemicals.

Cetones (or ketones): R-CO-R'. Volatile flammable pungent liquids, miscible with water, used in the manufacture of chemicals and as solvents.

Nitrogenated compounds may be amines (primary amines R-NH2, secondary R-NH-R'...), amides (R-CO-NH2), imines (R- (R'-) C=NH), imides ((R-CO)2-N-R'), azides (RN3), cyanates (ROCN), nitriles (R-CN), nitro-compounds (R-NO2), etc.

FUEL TYPES

BY PHYSICAL STATE

Solid As coal (mineral), charcoal (from wood) and biomass (wood, dung), but also waxes, metals and non-metals (e.g. sulfur ignites easily, producing a pungent blue flame; aluminium particles are used in the rocket boosters for heavy-lift launchers such as the Space Shuttle and Ariane 5).

Liquid As crude-oil derivatives (gasoline, diesel, fueloil), alcohols, ethers, esters, but also LPG at low temperatures. Notice that the usual U.S., Canadian, and New Zealand word for gasoline is simply gas, and that the usual British word is petrol.

Gas As natural gas, oil derivatives (LPG), acetylene, manufacture gas (from coal or oil residue) and biogas (from manure or sewage).

BY PERIOD OF NATURAL RENOVATION

Fossil fuels Fossil fuels (coal, crude-oil and natural gas) were formed slowly (during millions of years, mainly at certain remote epochs, not uniformly; e.g. American oil was formed some 90 million years ago, whereas the rest dates from 150 million years) by high-pressure-decomposition of trapped vegetable matter during extreme global warming. Fossil fuels are found trapped in Earths crust, up to 10 km depth, although large pressure might stabilise them also at higher depths and temperatures (at 300 km it might be 10 GPa and 1000 C). They are then non-renewable at humankind periods, and will eventually be commercially depleted. Notice that 'resources' refers to the total amount in Nature, whereas reserves refers to that portion of resources that can be economically recovered at today's selling prices, using today's technologies and under today's legislation.

Table 2. Estimated reserves and availability of fossil fuels (oil-discovery peaked in 1960s and oil-production is expected to peak around 2007; gas, some 20 years latter in both cases). Commercial reserve-2000 Reserve/Consumption-2000

Coal 10001012 kg 250 yr Crude oil 1001012 kg 40 yr

Natural gas 1501012 kg. 70 yr Notice, by the way, that nuclear fuel reserves are also short-term: the 2109 kg present commercial reserves would last some 50 years, if only 0.5% of natural uranium is profitable used as in present nuclear plants (0.71% U-235 in natural uranium, times some 70% of burning depth of the enriched stuff, typically from 3% down to 2% U-235 in nuclear fuel rods). Two differences, however, apply to commercial reserves of nuclear fuel: first, that new breeder reactors can burn not only the U-235

fraction but the main U-238 fraction and other fissionable ores, and second, that nuclear ores have been only marginally prospected and price increments of several orders of magnitude are tolerable due to the small share of fuel price in nuclear power.

Renewable fuels Renewable fuels (biomass) are formed in a year or a few years basis (synthetic fuels may come from fossil or from renewable sources):

Gaseous: biogas from anaerobic fermentation or gasogen gas from pyrolysis of biomass. Liquid: alcohols, ethers (biopetrol), esters (biodiesel). Solid: wood, charcoal, fuel pellets (from wood or vegetable residues), agriculture residues,

cattle manure, urban waste. In comparison with fossil fuels, particularly with oil and gas, renewable fuels are more disperse, have less energy content, more moisture and ash content, and require more handling effort (but they are renewable).

BY PRODUCTION STAGE (RESOURCES TYPE)

Natural or primary fuels Any commodity can be artificially produced, but it may cost a lot; humankind progress has always been based on finding raw materials that with no cost or little cost could satisfy their needs. The need for energy, to make machines work, to transport people and goods, and so on, has been met in the past and in the present by primary fuels (biofuels in the past and fossil fuels during the last two centuries). Fossil fuels: coal, crude oil (not used unprocessed), natural gas and biomass. They are obtained by

mining (coal) or welling (oil and gas). Some pumping is usually needed. Actually, crude oil is never used as a primary fuel because there is no economy (residual crude-oil products are cheaper) and because it is difficult to handle (being a mixture of very light and very heavy substances, its handling causes cavitation, vapour traps and sticky clogs).

Biofuels (from biomass). They can be directly taken from nature (e.g. wood and fuel crops), or from human activity waste (agriculture residues, industrial residues, animal residues, or domestic waste).

Artificial or secondary fuels Distillates from natural fuels (fossil or biomass, but without chemical reaction): all petroleum

derivatives, plus alcohols used as additives and mixtures, may be obtained by distilling the raw material. However, modern oil-refinery products really come from a combination of physical methods (distillation) and chemical methods (reforming and cracking).

Reformed from natural fuels (fossil or biomass, by chemical reactions with heat, steam or partial air). They are also called synthetic fuels, and may be gaseous, liquid, or solid: hydrogen, acetylene, synthetic gasoline, synthetic oils, synthetic gases (syngas), charcoal, and coke. Synthetic liquid fuels are most promising because of their high energy density; first results date from 1897 when formaldehyde (HCHO, Tb=98 C) was obtained by electrical discharge on a CO/H2 mixture (syngas), but the main milestone is the Fischer-Tropsch process of 1923, where, desulfurated syngas (generated by passing water vapour over hot coal), was made to react in presence of Fe, Ru and Co catalysts, to yield a liquid hydrocarbon blend (containing from methane to heavy waxes) from which gasoline-like and diesel-like fuels were obtained (e.g. 24H2+12CO=C12H24+12H2O); most of South Africa's diesel fuel is currently produced this way (they also use a 50/50 Jet A-1 substitute, half from oil, half from coal). Syngas preparation is the most expensive stage in the process due to materials handling: purification of input coal, removal of sulfur, nitrogen, and ash.

Methanol synthesis is another important process, as a final fuel (CO+2H2=CH3OH), or as an intermediate step to gasoline and diesel (e.g. through dehydration to dimethylether 2CH3OH=CH3OCH3+H2O).

Exotic fuels (not obtained from fossil or biomass fuels): hydrogen from water electrolysis, and non-hydrogen non-carbon fuels, like metals (Si, Al, Mg, Fe), used as intermediate energy stores, since they are first produced from their oxides (with cheap natural energy) and afterwards burnt to form their oxides (providing valuable artificial energy).

BY MARKETING

Non-commercial Some biomass materials (municipal solid waste (MSW), local industrial wastes, dung), rocket propellants (e.g. black powder, hydrazine: N2H4), metals (Fe, Al, Mg, Si), NH3, etc., are considered non-commercial or special fuels. They may be directly used as fuels (burnt with air), as monopropellants (e.g. 2N2H4=2NH3+N2+H2), or as intermediate products; e.g. 2Al+(3/2)O2=Al2O3 is used for aluminothermy, whereas Si+2H2O=SiO2+2H2 is used to produce hydrogen fuel (Al+3HCl=AlCl3+(3/2)H2 is only used in the lab). Ammonia is not used as a fuel because of its high auto-ignition temperature (925 K) and the difficulty of complete burning (there is NH3 in the exhaust), but it might have a future as a hydrogen carrier since liquid NH3 at 20 C (p>854 kPa), with =610 kg/m3, might yield 109 kg/m3 of H2 whereas the most advanced hydrogen storage systems (metal hydrides) only store 25 kg/m3 of H2.

Commercial Coal. It was the main fuel for the Industrial Revolution in the XIX c., and still traded for domestic

use during the first half of the XX c., but today it is only traded to power stations and heavy industries; mainly bituminous coal, but also lignite.

Crude-oil derivatives (see Table 3). They were developed in parallel with the corresponding internal combustion engine during the XX c. Crude oil is not used directly as a fuel, as said before (it only burns in uncontrolled fires). Most crude-oil is now traded in relation to the spot price of certain market crudes, such as West Texas Intermediate, North Sea Brent, Dubai, or Alaskan North Slope. The spot price is the price of an individual cargo of crude traded at a particular location.

Natural gas. It seems to be the main fuel for the immediate future. Old town gas, manufactured from coal or crude-oil, is no longer on the market.

Special commercial fuels, like acetylene (used for cutting and welding).

Petroleum fuels More than 50% of world's primary energy comes nowadays from petroleum, i.e. all vehicle fuels, and small and medium stationary applications fuels are petroleum derivatives, obtained by fractional distillation and reforming. Main commercial fuels and their physical data (that change according to the market requirements) are presented in Table 3.

Table 3. Main commercial fuels derivatives from crude-oil, and their main averaged properties. Boiling

range Tb [K]

Boiling range

Tb [C]

Carbon chain range

Density (liquid at 15 C)

[kg/m3]

Viscosity at 40 C

106 [m2/s]

Flash point

Tflash [C]

Main use

Liquefied petroleum gases (LPG)

Kerosene 450..650 150..350 10..14 780..850 3 40 aircrafts Diesel 500..600 200..300 10..20 820..880 3 40 cars, lorries,

boats, heaters Fuel oil distillate

600..800 300..500 15..30 840..930 10 60 industry, ships

Fuel oil residue

>800 >500 20..40 930.1010 500 100 industry, ships. Must be heated

BY APPLICATION

For spark ignition engines Spark ignition (SI) internal combustion engine (ICE) fuels (Otto fuels) require knock retardation (oxygenated compounds increase the octane number (RON=research-octane-number) and decrease CO and HC emissions). The reference fuel is gasoline (also named petrol, shortened to 'gas' in the USA), but some other alternative fuels exist.

Gasoline (Eurograde-95 in UE, Premium-95 in USA). Propane (or better LPG), at its vapour pressure (around 1 MPa). It has good octane number

(of order 100, increasing with propane content up to RON=112), and yields less emissions than gasoline (less than a half, particularly at small loads). It is fed liquid to direct injection engines, or gaseous for carburated engines. Most propane-fuelled-engines can work indistinctly with propane or gasoline (dual-fuel engines). LPG is a mixture of mainly propane, propylene, butane and butylenes, with composition varying widely from nearly 100% propane in cold countries, to only 20..30% propane in hot countries (e.g. 100% in UK, 50% in the Netherlands, 35% in France, 30% in Spain, 20% in Greece).

Natural gas (mostly as a compressed gas, CNG, at 30 MPa, but sometimes in as a cryogenic liquid, LNG, at some 500 kPa). Little used in cars because of the storage, but more and more used in slave-fleet buses, power plants and cogeneration plants because it is clean at entrance and exhaust, and it has RON=130, what allows for a compression rate of 12 instead of 9, increasing the efficiency up to 40% (the engine must be specially tuned).

Ethanol and bioethanol. Usually not pure (E100-fuel, i.e. pure ethanol, is used in Brazil) but added up to 20% to gasoline (E20-fuel or gasohol) to avoid engine changes, or nearly pure for new 'versatile fuel vehicles' (E80-fuel only has 20% gasoline, mainly as a denaturaliser). Anhydrous ethanol (

Hydrogen is only used in research prototypes as a possible intermediate to future integrated hydrogen systems. RON=130.

For compression ignition engines Compression ignition (CI) internal combustion engine (ICE) fuels (Diesel fuels) require very high injection pressure and low autoignition temperature and delay (as for cetane, C16H34, n-hexadecane, that is why it is measured as 'cetane number', defined as a cetane / methyl-napthalene mixture which has the same ignition delay-time as the test fuel). The reference fuel is named diesel (formerly gasoil), but some other alternative fuels exist.

Diesel oil (gasoil) from crude-oil distillation. Fuel oil (heavy fuel or residual oil) is only used in large marine engines. Natural vegetable oils (sunflower, colza, soybean) are usually not directly used because of

their high viscosity (10..20 times that of diesel) and glycerine waste. Biodiesel (a mono-alkyl-ester mixture, obtained by transesterification of natural oils) can

directly substitute diesel oil in CI engines (a mixture of 30% biodiesel and 70% fossil diesel is on the market), decreasing pollutant emissions. Biodiesel is renewable, non-toxic and biodegradable.

Natural gas can be used as main fuel in a diesel engine if a small amount of diesel fuel is used for compression ignition (dual fuel engine). The gas is usually added by direct injection in the cylinders before ignition (to avoid high injection pressures). Hybrid solutions in which a small amount of natural gas injected in a small pre-chamber and ignited in a spark plug, provide the high temperatures needed for subsequent autoignition of the main natural gas loading, are being developed. The same approach may be used to burn other high-autoignition-temperature fuels (e.g. hydrogen) in autoignition mode (diesel mode).

Notice that a mixture of gasoline and kerosene makes no good for either the SI-engine (less octane-number) or the CI-engine (less lubrication). In case of accidental mixing, the best is to discard the mixture. The use of diesel/bioethanol mixtures in CI-engines is being investigated; up to 10% in volume of anhydrous ethanol (E10-diesel) may be burnt on unmodified CI-engines, significantly reducing particulate-matter emissions (at the expense of some reduction in cetane-number, ignition-speed and lubrication).

For gas turbine engines As they are internal combustion engines (ICE), they cannot work with solid or heavy fuels; besides, although gas turbine engines are in principle more tolerant on fuel type than gasoline and diesel engines, high-performance gas turbine demand high-performance fuels.

Kerosene is used for mobile applications. It has high heating value and low flash point. Aircraft jet engines currently use Jet A-1 fuel, basically 99% kerosene with 1% additives to enhance cold operations and thermal stability. The first jet engine, however, that flew in the Heinkel He 178 on 27 August 1939, used gasoline as fuel.

Natural gas is used for stationary applications.

For boilers For external combustion engines (vapour turbines) and very large heaters.

Pulverised coal Fluidised-bed coal Fuel oil

For small heaters (like domestic water heaters for space heating or for hot water)

Natural gas (if a network is available) LPG Diesel oil

For small portable applications Fuel, which are odourless, smokeless and non-hazardous are required:

For lighters (portable fire sources) Matches (see Pyrotechnics) GLP (see SI-engines). The butane gas lighter dates back to 1933 and it is the most used

today. Gas lighters are refilled by first letting the remaining gas escape (enhanced by warming up), and then connecting it to an upside-down GLP reservoir (enhanced by heating and shaking the reservoir and cooling the lighter). Keep gas lighters below 50 C.

Gasoline (see SI-engines) Waxed-wick fire-lighters

For illumination GLP (see SI-engines) Kerosene (see SI-engines) Waxes (candles) Acetylene (C2H2, ethyne). It is used in chemical synthesis (80%) and welding (20%), and it

is produced from CaC2 and water, or by hydrocarbon cracking, or by partial oxidation of natural gas. CaC2 was first obtained in 1892 while searching for aluminium synthesis, by passing an electric arc through calcinated limestone (CaO) and coal tar. Acetylene can be produced in situ, but it is usually traded in high-pressure bottles, in liquid form, dissolved in acetone (and stabilised within a solid porous material) since pure acetylene may decompose explosively if p>205 kPa.

For heating (cooking, plumbing, cutting) GLP (see SI-engines, above). Most used at movable cooking ranges and stoves. Barbecue pellets or briquettes (charcoal, pressed sawdust). BBQ-hints: (BBQ=bar-by-

queue): place the charcoal in the BBQ-pan and some firelighters in the middle cavities; light it and left it for half-an-hour until the coal is ready to cook (it gets to embers covered by a fine grey ash). After cooking, the fire may be extinguished by air sophocation, and the unburnt fuel left in place ready for a new fire, or get rid of by soaking with water.

Acetylene (see Illumination, above). The oxy-acetylene torch is the common tool for manual cutting and welding, because of its high heating power and high combustion temperature, 3500 K, the maximum of any fuel. A typical workshop bottle of 40 litres, at 1.5 MPa, yields some 6 m3 of acetylene at room conditions (the flow rate should not be higher than 1 m3/h to avoid acetone carry-over).

For fuel cells Hydrogen is the nominal fuel for low temperature fuel cells, but the storage problem is not yet

solved satisfactorily. Since 2004, there are city buses powered by fuel cells operating in several cities (three, with two 75 kW PEM fuel cell stacks each, in Madrid, costing some 1 200 000 each, instead of the 200 000 for a normal bus, roughly).

Reformed hydrogen from other (fossil or natural) fuels (e.g. natural gas, methanol, etc.).

For pyrotechnics, propellants and explosives (See Pyrotechnics.pdf)

FUEL HISTORY Humans must have mastered fire some 500 000 years ago (from the time of Homo Heidelbergensis). For instance, excavations at Torralba (Spain) suggest fire-hunting for elephants, wild cattle, horses, deer and woolly rhinoceroses 400 000 years ago, and the same in Anatolia 200 000 years ago. Of course, wild fires must date from the beginning of terrestrial vegetation (evidence of fire has been found in coal deposits formed 350 million years ago).

BIOMASS FUELS BEFORE THE XX C. Most fuels used nowadays are fossil (remnant of plants that existed in the distant past), but the first fuels used by humans were from biomass (living matter used as a source of energy, notably firewood). Primitive humans must have used fire for lighting, heating, cooking, fighting, communication, religion, etc. But only non-premixed flames with condensed fuels were used up to 1850, where Bunsen mastered the premixed combustion with gaseous fuels in his famous burner. Elementary carbon is not abundant on Earth (

creosote vapours that may inflamate in the chimney (and never burn plastics, painted wood or glossy paper that generate toxic gases). A chimney is a very effective ventilation set-up (much more than a whole in a wall), being able to remove more heat from already warm walls by air convection than the chemical heat supplied by the fuel (that is why walls extended around fireplaces to have comfort within, and not just the front side roasted and a frozen back). For the same reason, it is difficult to approach, say, 900 K, and enclosed chambers (furnaces) are required to reach higher temperatures. A chimney works on the Archimedes' principle: generating a pressure-imbalance draught, p, on a column of hot air of height L: p=gL. Chimneys get coated with soot and, if not cleaned regularly, may catch fire. A chimney cap is always used to keep out the rain, leaves, and bird droppings, to inhibit downdraughts and as spark arrestor. The chimney is a rather recent invention; the first one dated comes from an earthquake in Venice in 1347. B. Franklin in 1745 wrote some guidelines to avoid smoky chimneys, suggesting to narrow the air entrance by covering most of the front with a hanging cloth (or better a plaster wall), and to widen the flue, to keep smoke from coming out into the room. B. Thompson (Count of Rumford) in 1796 suggested to make the fireplace shallower, with widely angled side-walls, so they would radiate better, and with a streamlined flue throat with a sudden expansion to avoid smoke rebuff (it was shown by P.O. Rosin in 1932 that the sudden expansion is counterproductive). Modern fireplaces have a pre-cast iron box with a pre-cast streamlined chimney throat, which is covered with heavy refractory masonry.

Torches, oil lamps and candles Special devices were invented to transport fire (portable burning appliances), mainly for lighting. The first portable fire was the torch, a burning branch plucked from a fire, later enhanced to a reed or tow soaked in molten fat or oil, or naturally impregnated with resin or pitch. Torches date back at least to 50 000 B.C. Much later on, animal fat in a bowl (sea-shell or skull) with a grass wick, a kind of torch with liquid fuel pumped up by capillarity, must have been developed; the first remains were found at Lascaux famous painting cave, dating from around 20 000 B.C. Oil lamps were common in Egypt in 3000 B.C., made of clay and burning seed oil with a cotton wick. Greek lamps from 500 B.C. looked like saucers (later on with a groove) and burned olive oil in a pottery or bronze container (a modern oil lamp made of brass is shown in Fig. 1). Candles, although known before, only were in widespread since 400 B.C. The next light source, the carbon arc lamp, was not developed until the early 1800s (by Sir Humphry Davy), although it was not in widespread use until late in that century, when the incandescent carbon-filament lamp was also developed (by Thomas Edison in the 1880s). Although the carbon arc lamp is assisted by the burning of graphite rods, electric light marked the decline of fuel lamps, which practically disappear from 1900 onwards, taking over first the traditional tungsten-filament incandescent bulb (developed in the 1920s, and being retired in the 2010s because of its low efficiency), and later by the fluorescent gas-discharge lamp (in widespread use since the 1940s, although introduced around 1900).

Fig. 1. A smoky kerosene oil-lamp.

The candle is as a kind of solid-oil lamp, tuned to feedback heat, by radiation to melt the solid fuel, made of frozen oil, semisolid fat, or wax, enhancing safety and handling (a no-spill lamp). The solid fuel melted at some 55 C to 65 C by radiated heat, migrating up the wick by capillary action, where it is pyrolysed at some 900 K (dark zone in the flame), generating gases that diffuse through the inner cold bulb and reach the thin combustion zone, blue at the well-ventilated bottom, yellow at the richer upper part, where products, including soot particles, are formed at some 1500 K (the hottest zone, up to 1700 K, is found off the centre on the edge of the brighter yellow portion), by reaction with outside air, whose flow is smoothly coupled to burning rate through buoyancy convection (notice that if air is artificially supplied, the candle flame flickers a lot and may be extinct, and that, in absence of a force field, as under microgravity, the candle slowly burns with a very precarious spherical blue flame). The candle was known in Egypt (clay candle holders are found dating 3000 B.C.), but candles were not used because of the higher cost, dimmer light and higher pollution than oil lamps. Candles were made by coating a wick (the fuel pump) with pitch or animal fat. Chinese, Japanese and pre-Colombian Americans also used candles. Bee-wax candles were used by wealthy Romans as a rarity: