1. Two systems for "composition"

(A) Elemental analysis (industrial standard: ultimate analysis)
Gives mass fractions of elements in fuel: $C,H,O,N,S$, plus ash $A$ and moisture $W$. Used for air supply, flue gas volume, heat balance, and emissions accounting.

(B) Industrial analysis (proximate analysis, core for solid fuels)

• Moisture $W$
• Volatile matter $V$
• Ash $A$
• Fixed carbon $FC$
  Relationship: $FC = 100$ (on the same basis).

2. Common "property" indicators (thermotechnical view)

• Heating value: higher $Q_{HHV}$, lower $Q_{LHV}$ (MJ/kg or kJ/kg)
• Ignition/burnout: ignition temperature, burn rate, burnout time, slagging tendency
• Physical properties: density $\rho$, viscosity $\nu$ (key for heavy oil), volatility, flash point, pour point
• Impurities/corrosiveness: $S$ (sulfur), $Cl$, alkali metals, vanadium (heavy oil), ash softening temperature
• Combustion limits (gas): explosion limits, flame propagation speed

3. Basis for expressing components (must be unified first)

Common bases:

• As-received (ar): original moisture and ash
• Air-dried (ad): external moisture removed (lab common)
• Dry (d): no moisture
• Dry ash-free (daf): no moisture and no ash (for comparing combustible portion)

Typical conversions (moisture as example; mass fraction):

• From as-received to dry: $X_d = \dfrac{X_{ar}}{1-W_{ar}}$
• From dry to dry ash-free: $X_{daf} = \dfrac{X_d}{1-A_d}$