4. Combustion Properties of Liquid and Gaseous Fuels
Types of fuel
Flash point is rarely used for solids, and has no meaning for gases.
Gases
Flammable materials in gaseous form will burn when mixed with the proper amount of air and
properly ignited. Flammable range of mixture with air, vapor density, and igniton energy and
temperature are important propertes of gases.
Liquids and their vapors
Vapors from liquids are the materials that support the fame over a liquid fuel. Vapor pressure, fash
point, and boiling point are important propertes of liquids. Igniton temperature, range of fammable
concentratons of vapor/air mixtures, and vapor densites are important propertes of vapors. Liquids
themselves do not undergo combuston.
Solids
Solids burn by direct combinaton of oxygen with their surface (smoldering combustonn as volatied
materials, or as fuels that pyrolyie to form combustble gases and vapors and leave a
noncombustble solid residue. Reactve metals and carbonaceous fuels burn only at their surface, as a
glowing fre. Nonpyrolyiable solids have no fash point, vapor density and single igniton
temperature. Propertes that control their fammability are density, conductvity, and thermal
capacity. Porosity and meltng point will infuence their performance as fuels under some conditons.
Chemical propertes of a solid fuel will determine the nature of volatle pyrolysis products and the
rate at which they are generated.
Physical Properties of Fuels
Vapor pressure
If evaporaton of a liquid occurs in a sealed system, an equilibrium state will be reached in which the
vapor reaches saturaton, with no further evaporaton. The pressure exerted by this vapor is called
the saturated vapor pressure. This is a direct measure of the volatlity of a liquid and is determined
by:
Its molecular weight; the lower the molecular weight, the higher the vapor pressure;
Its chemical structure; a branched alkane will have a lower flash lpoint᐀ than an alkane;
Temperature; the higher the temperature, the more liquid will evaporate, and the higher the
vapor pressure (logarithmicn;
Contour of the surface of the liquid; higher for a convex surface than for a fat surface (so, an
aerosol of small droplets is easier to ignite than an entre pool of the same liquidn;
o Siie; a large total surface and small siie of droplets, allows them to absorb heat more
quickly. Adding, the small droplets evaporate faster.
The vapor pressure has a great infuence on the
fammability of a liquid fuel.
When the vapor pressure of a liquid reaches 760
mmHg, the liquid s at its boiling point; boiling
temperature.
Flammability (explosive) Limits
Mixtures of fammable gases/vapors with air will
combust only in partcular ranges of
, concentraton. A concentraton below its lower explosive limit (LEL) (or lower flammabnln᐀a llnmn᐀ (LFLnn
cannot be ignited. At concentratons above its upper explosive limit (UEL), the fuel/air mixture is too
rich to ignite.
Closed Systems
Within a closed system, if the mixture will not explode, it will not ignite; the explosive range and
fammability range of a gas/vapor are almost the same. However, both are determined under
different conditons (constant pressure versus constant volumen. Flammability limits are temperature
dependent: the higher the inital temperature, the wider the range.
Open Systems
In an open system, other variables control the ignitability. If the mixture is lower than the LEL, efforts
to ignite it may raise the temperature and increase the amount of material volatliied sufciently to
cause igniton. When the concentraton is above the UEL, prolonged applicaton of a fame may add
so much heat, that the fammability limit is affected and start a fre. Because of the inital overly rich
mixture, the fre will develop as a rolling fire. Above is only possible in an open system.
The maximum siie of a fre is controlled by the area of the pool (acceleratng liquidn exposed,
multplied by the kW/areaffactor (determined empiricaln.
Flash Point
Characteristics of Flash Point
The flasa point of a material is the lowest temperature at which it produces a fammable vapor. The
vapor pressure of the fuel is then equal to that fuel’s lower limit of fammability. A liquid fuel must be
able to generate a vapor in sufcient quantty to reach that lower limit in air before it can burn.
However, the vapor will not ignite spontaneously at this temperature.
Determining Flash Point
The fash point is determined by placing a small sample of the fuel in the cup of a testng apparatus
and heatng or cooling it to the lowest temperature at which an arc or small pilot fame will cause a
little fash to occur over the surface of the liquid. This can be done with an open or closed cup. The
openfcup fash points would be a few degrees (<10%n higher than the closedfcup values.
Class I: fashpoint <38 oC (flammablel llnuinssn;
Class II: fashpoint 38f60 oC (cimbistiblel llnuinssn;
Class III: fashpoint >60 oC (combustble liquidsn.
Flame Point/Fire Point
The flamel lpoint᐀ or fire point is the lowest temperature at which a liquid produces a vapor that can
sustain a contnuous fame. It is usually a few degrees above the fash point. The values of fash point
and fame point temperatures of fuels depend on (1n the siie of the igniton source, (2n how long it is
held over the source each tme, (3n the rate of heatng of the liquid, and (4n the degree of air
movement over the fuel.
There is a big difference between fame point of alcohols determined with fame igniters, and
electrical arc igniton. This is probably due to the infrared absorpton characteristcs of alcohol fuels.
Ignition temperature
The ai᐀ingtntiit l᐀elmpoelra᐀irel (AITn is the temperature at which a fuel will ignite on its own. This is
determined by injectng a small amount of fuel in a hot environment. If the AIT is reached, the fuel
will burst into fame. For hydrocarbons, the longer the carbon backbone, the lower the AIT. Catalytc
oxidaton of fuels can occur at temperatures lower than the AIT.
Types of fuel
Flash point is rarely used for solids, and has no meaning for gases.
Gases
Flammable materials in gaseous form will burn when mixed with the proper amount of air and
properly ignited. Flammable range of mixture with air, vapor density, and igniton energy and
temperature are important propertes of gases.
Liquids and their vapors
Vapors from liquids are the materials that support the fame over a liquid fuel. Vapor pressure, fash
point, and boiling point are important propertes of liquids. Igniton temperature, range of fammable
concentratons of vapor/air mixtures, and vapor densites are important propertes of vapors. Liquids
themselves do not undergo combuston.
Solids
Solids burn by direct combinaton of oxygen with their surface (smoldering combustonn as volatied
materials, or as fuels that pyrolyie to form combustble gases and vapors and leave a
noncombustble solid residue. Reactve metals and carbonaceous fuels burn only at their surface, as a
glowing fre. Nonpyrolyiable solids have no fash point, vapor density and single igniton
temperature. Propertes that control their fammability are density, conductvity, and thermal
capacity. Porosity and meltng point will infuence their performance as fuels under some conditons.
Chemical propertes of a solid fuel will determine the nature of volatle pyrolysis products and the
rate at which they are generated.
Physical Properties of Fuels
Vapor pressure
If evaporaton of a liquid occurs in a sealed system, an equilibrium state will be reached in which the
vapor reaches saturaton, with no further evaporaton. The pressure exerted by this vapor is called
the saturated vapor pressure. This is a direct measure of the volatlity of a liquid and is determined
by:
Its molecular weight; the lower the molecular weight, the higher the vapor pressure;
Its chemical structure; a branched alkane will have a lower flash lpoint᐀ than an alkane;
Temperature; the higher the temperature, the more liquid will evaporate, and the higher the
vapor pressure (logarithmicn;
Contour of the surface of the liquid; higher for a convex surface than for a fat surface (so, an
aerosol of small droplets is easier to ignite than an entre pool of the same liquidn;
o Siie; a large total surface and small siie of droplets, allows them to absorb heat more
quickly. Adding, the small droplets evaporate faster.
The vapor pressure has a great infuence on the
fammability of a liquid fuel.
When the vapor pressure of a liquid reaches 760
mmHg, the liquid s at its boiling point; boiling
temperature.
Flammability (explosive) Limits
Mixtures of fammable gases/vapors with air will
combust only in partcular ranges of
, concentraton. A concentraton below its lower explosive limit (LEL) (or lower flammabnln᐀a llnmn᐀ (LFLnn
cannot be ignited. At concentratons above its upper explosive limit (UEL), the fuel/air mixture is too
rich to ignite.
Closed Systems
Within a closed system, if the mixture will not explode, it will not ignite; the explosive range and
fammability range of a gas/vapor are almost the same. However, both are determined under
different conditons (constant pressure versus constant volumen. Flammability limits are temperature
dependent: the higher the inital temperature, the wider the range.
Open Systems
In an open system, other variables control the ignitability. If the mixture is lower than the LEL, efforts
to ignite it may raise the temperature and increase the amount of material volatliied sufciently to
cause igniton. When the concentraton is above the UEL, prolonged applicaton of a fame may add
so much heat, that the fammability limit is affected and start a fre. Because of the inital overly rich
mixture, the fre will develop as a rolling fire. Above is only possible in an open system.
The maximum siie of a fre is controlled by the area of the pool (acceleratng liquidn exposed,
multplied by the kW/areaffactor (determined empiricaln.
Flash Point
Characteristics of Flash Point
The flasa point of a material is the lowest temperature at which it produces a fammable vapor. The
vapor pressure of the fuel is then equal to that fuel’s lower limit of fammability. A liquid fuel must be
able to generate a vapor in sufcient quantty to reach that lower limit in air before it can burn.
However, the vapor will not ignite spontaneously at this temperature.
Determining Flash Point
The fash point is determined by placing a small sample of the fuel in the cup of a testng apparatus
and heatng or cooling it to the lowest temperature at which an arc or small pilot fame will cause a
little fash to occur over the surface of the liquid. This can be done with an open or closed cup. The
openfcup fash points would be a few degrees (<10%n higher than the closedfcup values.
Class I: fashpoint <38 oC (flammablel llnuinssn;
Class II: fashpoint 38f60 oC (cimbistiblel llnuinssn;
Class III: fashpoint >60 oC (combustble liquidsn.
Flame Point/Fire Point
The flamel lpoint᐀ or fire point is the lowest temperature at which a liquid produces a vapor that can
sustain a contnuous fame. It is usually a few degrees above the fash point. The values of fash point
and fame point temperatures of fuels depend on (1n the siie of the igniton source, (2n how long it is
held over the source each tme, (3n the rate of heatng of the liquid, and (4n the degree of air
movement over the fuel.
There is a big difference between fame point of alcohols determined with fame igniters, and
electrical arc igniton. This is probably due to the infrared absorpton characteristcs of alcohol fuels.
Ignition temperature
The ai᐀ingtntiit l᐀elmpoelra᐀irel (AITn is the temperature at which a fuel will ignite on its own. This is
determined by injectng a small amount of fuel in a hot environment. If the AIT is reached, the fuel
will burst into fame. For hydrocarbons, the longer the carbon backbone, the lower the AIT. Catalytc
oxidaton of fuels can occur at temperatures lower than the AIT.
