COMBUSTION ENGINEERING: Fuels and Classifications

A comprehensive set of Q&A flashcards covering fuels, classifications, analysis methods, hydrocarbon chemistry, fuel properties, and combustion processes.

What is the basic definition of a fuel in combustion engineering?

A material that liberates heat when it reacts with oxygen (rapid oxidation).

How does hydrogen content affect combustion heat and ignition in fuels?

Higher hydrogen content yields more heat and better ignition (more readily ignites).

What is the composition of dry air?

78% nitrogen, 21% oxygen, and 1% argon.

What are the typical products of hydrocarbon combustion in air?

CO2, H2O, N2, and SO2.

What is the general formula for alkanes (paraffins)?

CnH2n+2.

Which alkanes are gases at room temperature?

Alkanes with 1–4 carbon atoms (C1–C4).

Which alkanes are liquids at room temperature?

Alkanes with 5–16 carbon atoms (C5–C16).

Which alkanes are solids at room temperature?

Alkanes with 18 or more carbon atoms (C18+).

Name examples of artificial solid/secondary fuels derived from primary fuels.

Charcoal, coal briquettes, and coke.

What is charcoal and how is it produced?

A lightweight black carbon residue produced by strongly heating wood in minimal oxygen to remove water and volatiles.

What is coal briquette and how is it produced?

A compacted fuel made from coal dust or fine coal pressed into briquettes.

What is coke and how is it produced?

A gray, hard, porous fuel with high carbon content produced by heating coal at high temperature in absence of air (pyrolysis).

Which coal rank has the highest carbon content (typically >85%)?

Anthracite.

What is the carbon content range for bituminous coal?

About 45–85% carbon.

What is the carbon content range for subbituminous coal?

About 35–76% carbon.

What is the carbon content range for lignite (brown coal)?

About 25–35% carbon.

Which coal is often referred to as brown coal and has low carbon content?

Lignite.

What is the major source of liquid fuels?

Petroleum.

Name examples of prepared liquid fuels.

Gasoline, diesel, kerosene, alcohol, bunker fuels (A, B, C), and biofuels.

What is crude oil?

Raw petroleum extracted from the earth and refined into products like gasoline and jet fuel.

What are the advantages of liquid fuels over solid fuels?

Require less space, higher calorific value, easier control, easier transport/handling, and better storage stability.

Give examples of gaseous fuels and a common alternative fuel storage form for vehicles.

Natural gas (CH4); LPG (propane, butane); CNG (gas phase under high pressure) and LNG (liquefied, stored at -162°C).

What are the main components of LPG on vehicles?

Propane (C3H8) and butane (C4H10).

What does proximate analysis of coal measure?

Mass percentages of moisture, ash, volatile matter, and fixed carbon.

What does ultimate analysis of coal measure?

Elemental composition: carbon, hydrogen, oxygen, nitrogen, and sulfur (CHONS).

What is the difference between inherent moisture and free moisture in coal?

Inherent moisture remains in coal; free (surface) moisture can be evaporated easily; total moisture is the sum of both.

What is ash in coal analysis?

Noncombustible residue left after coal is burnt.

What is fixed carbon in proximate analysis?

The portion of coal that remains after volatile matter distills off; essentially carbon with minor H, O, N, S.

What is volatile matter in proximate analysis?

Matter that is released as gases or vapors during heating of coal.

What is coal basis conversion AD (air-dried basis) and how does it differ from TM (as-received) and DB/DAF bases?

AD includes inherent moisture (IM); TM includes total moisture; DB excludes moisture; DAF excludes moisture and ash.

What is meant by HYDROCARBONS CHONS?

Hydrocarbons composed of carbon, hydrogen, oxygen, nitrogen, and sulfur elements (as in coal-derived materials).

Name the three hydrocarbon family groups and their general formulas.

Paraffins (alkanes) CnH2n+2; Olefins (alkenes) CnH2n; Diolefins (dienes) CnH2n−2.

What is the cetane number and what does it indicate?

A measure of ignition quality in diesel engines; higher cetane means easier and quicker ignition.

What is the octane number a measure of?

Resistance to auto-ignition (knock) in gasoline engines; higher octane resists knocking.

What is the relationship between octane and iso-octane in fuel rating?

Octane rating is determined by blending iso-octane (resists knocking) with n-heptane (knocks). A 70/30 mix gives an octane number of 70.

Define cloud point, flash point, and fire point.

Cloud point: temperature where a mixture begins to solidify or separate; Flash point: lowest temperature for the vapors to ignite briefly; Fire point: temperature where vapors will sustain burning after ignition source is removed.

What is API gravity and typical ranges for crude oils?

Specific gravity of oil relative to water; API gravity typically ranges from about 10 to 50 (most crude oils 20–45).

What is viscosity and what is the Saybolt Universal Seconds (SSU) used for?

Viscosity is the internal resistance to flow; SSU is a unit used to measure Saybolt viscosities for liquids.

How does volatility affect ignition delay?

Higher volatility means more rapid evaporation, leading to shorter ignition delay and higher pressure generation.

What are HHV and LHV in heating value?

HHV (higher heating value) includes heat from condensing water; LHV (lower heating value) excludes the latent heat of vaporization of water.

What device is used to measure heating value by combusting a sample?

Bomb calorimeter.

Define atom, element, molecule, and compound.

Atom: smallest unit with chemical identity; Element: pure substance of one type of atom; Molecule: two or more atoms bonded; Compound: two or more different elements bonded.

What is molecular weight (MW) and its units?

Total mass of a compound; common units are kg/kmol or lbm/lbmol.

What do Amagat’s Law and Dalton’s Law state?

Amagat’s Law: total volume of a gas mixture equals the sum of the volumes of each gas at the mixture's T and p. Dalton’s Law: total pressure equals the sum of partial pressures of each gas.

What is a flue gas mixture?

A mixture of combustion gases such as CO2, H2O, and SO2 produced by combustion.

What are reaction products and reactants in a combustion process?

Reactants are the substances initially present; products are the substances formed after combustion.

What are the typical ignition temperatures for gasoline, carbon, hydrogen, carbon monoxide, and methane in air?

Gasoline ≈260°C; carbon ≈400°C; hydrogen ≈580°C; carbon monoxide ≈610°C; methane ≈630°C.

What is incomplete combustion and when does it occur?

Incomplete combustion occurs when there is insufficient oxygen, producing CO and unburned fuel.

What is complete combustion?

A combustion process with sufficient oxygen where CO2 and H2O are the main products.