Fossil fuels and their characteristics

A fuel is a substance that releases heat energy on combustion. Traditional (conventional) fuels are the carbon-based fuels used for centuries: wood, coal, petroleum products and natural gas.

Classification

Fossil fuels (coal, crude oil, natural gas) were formed over millions of years from buried plant and animal remains. Wood and charcoal are non-fossil but are also traditional fuels.

Solid fuels

• Advantages: cheap, easy to store and transport, no explosion risk in storage.
• Disadvantages: low calorific value, ash and smoke, difficult to control the flame, low thermal efficiency, labour needed for handling.

Liquid fuels

• Advantages: high calorific value (about 42–46 MJ/kg), clean burning with little ash, easy to transport by pipe, easy to control flow and flame, no ash handling.
• Disadvantages: costly, fire hazard, need closed storage tanks, many are imported, give CO and unburnt hydrocarbons if burnt badly.

Gaseous fuels

• Advantages: highest efficiency, complete combustion with little excess air, no ash or smoke, easy control, can be preheated, transported by pipeline.
• Disadvantages: need large storage volume or pressure cylinders, high risk of leakage and explosion, costly equipment.

Refining separates crude petroleum, a mixture of many hydrocarbons, into useful products and then upgrades them to meet quality needs.

Steps of refining

1. Desalting and heating: water and salts are removed; crude is heated to about 350–400 °C in a furnace.
2. Fractional distillation: the hot vapour-liquid mixture enters an atmospheric column. Vapours rise through bubble-cap trays; lighter components with low boiling points condense higher up and heavier components condense lower down.
3. Vacuum distillation of the bottom residue separates heavy gas oils and lubricating oil without cracking them.
4. Conversion and treatment: cracking, reforming, sweetening and blending.

   Gas (C1-C4)   <-- top, 20 C
  ---------------
   Gasoline      <-- 40-200 C
  ---------------
   Naphtha/Kero  <-- 150-250 C
  ---------------
   Diesel/gas oil<-- 250-350 C
  ---------------
   Fuel oil      <-- 350-450 C
  ---------------
  Crude-->[furnace]
   Residue/bitumen<-- bottom

Main fractions

Cracking

Breaks large molecules of heavy oil into smaller, more valuable ones such as gasoline. Thermal cracking uses 450–750 °C and high pressure; catalytic cracking uses a zeolite catalyst at about 500 °C and gives higher octane gasoline.

Reforming

Rearranges the molecules of low-octane naphtha into branched and aromatic compounds with a platinum catalyst, raising the octane number of gasoline.

Octane number

It is the measure of knock resistance of a spark-ignition fuel. It is the percentage by volume of iso-octane (octane number 100) in a mixture with n-heptane (octane number 0) that matches the knocking of the fuel in a standard CFR engine. Higher octane number lets a higher compression ratio be used without knock.

Cetane number

It is the measure of ignition quality of a diesel fuel. It is the percentage by volume of cetane (n-hexadecane, 100) in a mixture with alpha-methylnaphthalene (0) which has the same ignition delay as the fuel in the standard CFR engine. Diesels need 40–55.

Why they differ

In a petrol engine the charge is ignited by a spark, so the fuel must resist spontaneous ignition (knock) and have a high octane number. In a diesel engine the fuel is injected into hot compressed air and must ignite quickly by itself, so a short ignition delay (high cetane number) avoids diesel knock. The two scales run in opposite ways: straight-chain paraffins have high cetane but low octane numbers, while aromatics and branched chains have high octane but low cetane numbers.

Gasoline versus diesel

Diesel has more energy per litre because of its higher density.

Formulas

Dulong's formula (mass fractions):

$$HCV = 33\,800\,C + 144\,000\left(H - \frac{O}{8}\right) + 9\,270\,S\ \ \text{kJ/kg}$$

Here $C = 0.82$, $H = 0.12$, $O = 0.04$, $S = 0.01$.

(a) Higher calorific value

$$\begin{aligned} H - \frac{O}{8} &= 0.12 - 0.005 = 0.115 \\ HCV &= 33\,800(0.82) + 144\,000(0.115) + 9\,270(0.01) \\ &= 27\,716 + 16\,560 + 92.7 \\ &= 44\,369\ \text{kJ/kg} \end{aligned}$$

(b) Lower calorific value

Each kg of hydrogen forms 9 kg of steam, whose latent heat is not recovered.

$$\begin{aligned} LCV &= HCV - 9H\,(2442) \\ &= 44\,369 - 9(0.12)(2442) \\ &= 44\,369 - 2\,637 \\ &= 41\,731\ \text{kJ/kg} \end{aligned}$$

(c) Minimum air

Oxygen needed per kg of fuel is $\frac{8}{3}C + 8\left(H - \frac{O}{8}\right) + S$. Air contains 23 % O₂ by mass, so:

$$\begin{aligned} A_{min} &= \frac{100}{23}\left[\frac{8}{3}C + 8\left(H-\frac{O}{8}\right) + S\right] \\ &= 4.35\,[2.187 + 0.920 + 0.010] \\ &= 4.35 \times 3.117 = 13.56\ \text{kg air/kg fuel} \end{aligned}$$

Here $\tfrac{8}{3}(0.82) = 2.187$ kg of O₂ for carbon (C + O₂ → CO₂ needs 32/12 kg O₂ per kg C), $8(0.115) = 0.920$ for hydrogen, and $1 \times 0.01$ for sulphur.

Answer: HCV = 44 370 kJ/kg (about 44.4 MJ/kg); LCV = 41 730 kJ/kg (about 41.7 MJ/kg); minimum air = 13.56 kg per kg of fuel.