Fuels are any material that store potential energy in forms, which upon burning in oxygen liberates heat energy.
Calorific value of fuel is the total quantity of heat liberated when a unit mass or volume of fuel is completely burnt.
Higher or gross calorific value (HCV) in the total amount of heat produced when a unit mass/volume of fuel has been burnt completely and the products of combustion have been cooled to room temperature (15°C or 60°F).
Lower or net calorific value (LCV) is the heat produced when unit mass (volume) of the fuel is burnt completely and the products are permitted to escape.
LCV = HCV – Latent heat of water formed
Natural or primary fuels are found in nature such as wood, peat, coal, natural gas, petroleum.
Artificial or secondary fuels are prepared from primary fuels charcoal, coal gas, coke, kerosene oil, diesel oil, petrol, etc.
Fuels are further classified as
- Solid Fuels
- Liquid Fuels
- Gaseous Fuels
Characteristics of solid fuels
- Ash is high.
- Low thermal efficiency
- Form clinker
- Low calorific value and require large excess air.
- Cost of handling high
- Cannot be used in IC engines.
Characteristics of liquid fuels
- High calorific value
- No dust ash and clinker
- Clean fuels
- Less furnace air
- Less furnace space
- Used in IC engines
Characteristics of Gaseous fuels
- Have high heat content
- No ash or smoke
- Very large storage tanks are required
An ideal fuel should have the following properties:
1. It should possess high calorific value.
2. It should have proper ignition temperature. The ignition temperature of the fuel should neither be too low nor too high.
3. It should not produce poisonous products during combustion. In other words, it should not cause pollution o combustion.
4. It should have moderate rate of combustion.
5. Combustion should be easily controllable i.e., combustion of fuel should be easy to start or stop as and when required.
6. It should not leave behind much ash on combustion.
7. It should be easily available in plenty.
8. It should have low moisture content.
9. It should be cheap.
10. It should be easy to handle and transport.
CRUDE OIL
Crude oil is not used directly as a fuel but as a feedstuff for the petrochemical factories to
produce commercial fuels, synthetic rubbers, plastics, and additional chemicals. Oil
refineries were originally placed near the oil fields, in part because natural gas, which could
not then be economically transported long distances, was available to fuel the highly energy-
intensive refining process, but since 1950, for strategic reasons crude oil was transported by
tankers and oleoducts to local refineries.
Bioethanol and ETBE
Bioethanol is bio-fuel substitute of gasoline; i.e. it is ethanol obtained from biomass (not
from fossil fuels), and used as a gasoline blend. Pure bioethanol (E100-fuel) is by far the most
produced biofuel, mainly in Brazil and USA. More widespread practice has been to add up to
20% to gasoline by volume (E20-fuel or gasohol) to avoid the Fuel properties 4 need of
engine modifications. Nearly pure bioethanol is used for new ‘versatile fuel vehicles’ (E80-
fuel only has 20% gasoline, mainly as a denaturaliser). Anhydrous ethanol (<0.6% water) is
required for gasoline mixtures, whereas for use-alone up to 10% water can be accepted.
DIESEL, KEROSENE, AND JET FUEL
Diesel fuel is any liquid fuel used in diesel engines, originally obtained from crude-oil
distillation(petrodiesel), but alternatives are increasingly being developed for partial or total
substitution of petrodiesel, such as biodiesel (from vegetal oils), and synthetic diesel
(usually from a gas fuel coming from coal reforming or biomass, also named gas to liquid
fuels, GTL). In all cases, diesel nowadays must be free of sulfur.
Kerosene is a crude-oil distillate similar to petrodiesel but with a wider-fraction distillation
(seePetroleum fuels). Jet fuel is kerosene-based, with special additives (<1%). Rocket
propellant RP-1 (also named Refined Petroleum) is a refined jet fuel, free of sulfur and with
shorter and branched carbon-chains more resistant to thermal breakdown; it is used in
rocketry usually with liquid oxygen as the oxidiser (RP1/LOX bipropellant). The tendency to
change to biofuels or GTL fuels is also applicable here. Contrary to its etymology, present-day
kerosene and derivatives are less waxy than diesel (i.e. less lubricant). Diesel and kerosene
should not be taken as fully interchangeable fuels at present, because kerosene has no
cetane-number specification and thus it may have large ignition delays (producing lots
ofunburnt emissions and engine rough-running by high-pressure peaks); besides, kerosene
has less lubricity, and diesel-fuel less cold-start ability.
Biodiesel
Biodiesel is a biomass-derived fuel, safer, cleaner, renewable, non-toxic and biodegradable
direct substitute of petroleum diesel in compression-ignition engines, but more expensive.
Biodiesel is a monoalkyl-ester mixture obtained from natural oils, currently produced by a
process called transesterification, where a new or used oil (sunflower, colza, soybean, or
even animal fat) is first filtered, then pre-processed with alkali to remove free fatty acids,
then mixed with an alcohol (usually methanol) and a catalyst (usually sodium or potassium
hydroxide); the oil’s triglycerides react to form esters and glycerol, Fig. 1, which are then
separated from each other and purified. Usually 10% methanol (non-renewable) is added,
and some 10% glycerol forms. Colza is also known as rape (RME=rape methyl ester, and
REE=rape ethyl ester). Biodiesel surrogates are longer-chain hydrocarbons than petrodiesel:
C13H28, C14H30, or C15H32
FUELOIL
Types. There are two basic types of fueloil: Distillate fueloil (lighter, thinner, better for cold-
start) and Residual fueloil (heavier, thicker, more powerful, better lubrication). Often, some
distillate is added to residual fueloil to get a desired viscosity. They are only used for
industrial and marine applications because, although fueloil is cheaper than diesel oil, it is
more difficult to handle (must be settled, pre-heated and filtered, and leave a sludge at the
bottom of the tanks). Notice that, sometimes, particularly in the USA, the term ‘fuel oil’ also
includes diesel and kerosene.
NATURAL GAS, BIOGAS, LPG AND METHANE HYDRATES
Biogas is a flammable gaseous mixture, composed mainly of methane and carbon dioxide,
obtained by anaerobic fermentation of condensed biomass (manure or sewage). The
production may range from 20..70 m3 of biogas per cubic metre of manure, lasting 10..30 days
within a digestor (depending on the temperature, that is 20.40 ºC), where biomass is first
hydrolysed by some bacteria in absence of oxygen, yielding monomers that are made to
ferment by other bacteria, yielding alcohol that later turns to acetic acid and finally
decomposes to methane plus carbon dioxide, the later step being the controlling stage.
LPG (liquefied petroleum gas) are petroleum derivative mixtures (gaseous at ambient
temperature, but handled as liquids at their vapour pressure, 200..900 kPa), mainly
constituted by propane, n-butane, isobutane, propylene, and butylenes.
All gaseous fuels are odourless (except those containing traces of H2S), and odour markers
(sulfurcontaining chemicals, as thiols or mercaptans, e.g. ethanethiol, CH3CH2SH) are
introduced for safetybecause its detection threshold for human smell is 0.4 ppm in volume).
Methane hydrates are solid icy-balls (of some centimetres in size) found trapped under high
pressure (>30 MPa) and chilling temperatures (0..5 ºC) in plant-covered moist places like the
continental sediments on the sea floor and permafrost soil on high-latitude lands. They
might be the major source of natural gas in the future; presently they are a nuisance in high
-pressure gasoducts, where they may block valves. Strictly speaking, they are not hydrates
(chemical compounds of a definite formula), but clathrates, i.e. an unstable network (they
tend to the liquid state) of host polar molecules like water, characterized by Hbonds and
regular open cavities, stabilised to a solid state by incorporating small guest non-polar
molecules of appropriate size (to which they are not bonded; only van-der-Waals forces act
to stabilise the network). Besides methane, carbon dioxide, hydrogen sulphide, and larger
hydrocarbons such as ethane and propane, can stabilize the water lattices and form
“hydrates”; smaller molecules like nitrogen,oxygen or hydrogen are much more difficult to
stabilise in water.
Methane hydrates (approx. CH4·6H2O) fizzle and evaporate quickly when depressurised,
yielding some 150 times its volume of methane. Since this methane comes from very large-
time biomass decomposition, the problem of global warming remains: it yields CO2 on
burning, and released CH4 losses are worse: 20 times more relative greenhouse effect that
CO2. Hydrates soils are prone to accidental landslides, particularly during exploitation, what
constitutes a high risk to extraction platforms.
HYDROGEN
In the long term, hydrogen-energy appears as the final solution to face the energy-
environment dilemma of scarcity and pollution, not only for the much-pursued nuclear-
fusion power stations (using hydrogen isotopes), but for the using of hydrogen as an
intermediate energy carrier (like electricity), cleanly produced from water and solar energy,
and cleanly converted back to water, to drive fuel cells engines and clean combustors.
Safety
Hydrogen is a dangerous flammable gas, with the same self-ignition temperature as methane
(850 K), but much wider flammability limits (in air, 4..75% instead of 5..15%), smaller energy
for ignition (15 times less), smaller quenching distance (0.6 mm instead of 2 mm), nearly
invisible non-premixed flame, and more prone to detonation (in air, detonability limits are
18..59% instead of 6..14%). But, in relative terms to other fuels, hydrogen is not so much
dangerous (some claim it is safer), its main advantage being its extreme lightness, which, in
ventilated spaces, makes H2 leaks and flames to vertically escape quickly, minimising
possible horizontal spreads (most ignition sources and valuables accumulate horizontally).
Hydrogen is not toxic itself, and burns with not toxic fumes (most deaths caused by fire are
actually due to deadly fumes and gases).
Biomass
Here, biomass is synonymous of vegetable matter used as fuel (biofuel), either grown for
that purpose, or recovered from other industries waste (forestry, farming, food industry…)
urban and animal waste migh be included too, but its importance is marginal. Municipal solid
waste (MSW) has great organic content and can be used as a fuel in incineration power
plants, with HHV=7..12 MJ/kg, but dioxin emission is a problem. It excludes organic material
which has been transformed by geological processes into coal, petroleum, or natural gas
(fossil fuels).
Biomass is a renewable fuel, and, to a first approximation, carbon neutral, in the sense that
the CO2 released in biofuel combustion was previously captured from the environment
during biomass growth, although in October 2007, Nobel Laureate Paul Crutzen published
findings that the release of Nitrous Oxide (N2O) from rapeseed oil, and corn (maize),
contribute more to global warming than the fossil fuels they replace.
- The traditional biomass through the ages has been wood. Besides the biofuel production
here discussed, biomass is also used as a fertiliser (compost), paper industry and other
chemical stuff, building (e.g. straw in adobe and roofs, timber), etc.
- Biomass can be directly burned in furnaces and boilers, but the preferred way to easy
handling and transportation, and to minimise pollution, is by transforming raw biomass into
gas (known as biogas or syngas), liquid (which may range from alcohols to tars), and solid
(char, pellets…). At present, liquid biofuels (bioethanol and biodiesel) are mixed with oil
derivatives (gasoline and diesel) in a 5%..20% biofuel fraction.