In fuel technology and combustion engineering, the “calorific value” of a fuel is defined as the heat of which thermochemical process?

Introduction / Context:
In energy engineering, the calorific value (also called heating value) is a key fuel-quality metric used for boiler ratings, burner design, and fuel economy calculations. It expresses how much heat is released when a specified amount of fuel is completely burned and the products are cooled to a reference state (for higher heating value) or left with water as vapor (for lower heating value).

Given Data / Assumptions:

• Standard definition from thermochemistry.
• Complete combustion to stable products (e.g., CO2, H2O, SO2, N2).
• Reference conditions are implied (usually 25°C, 1 atm).

Concept / Approach:
The calorific value equals the magnitude of the enthalpy change for complete combustion of the fuel. Two practical versions are used: HHV (higher heating value) assumes the water formed condenses to liquid; LHV (lower heating value) assumes water remains as vapor. Both are specific cases of heat of combustion, not heats of formation, vaporisation, or a generic “reaction.”

Step-by-Step Solution:
Identify the defining process for calorific value → complete combustion.Relate to enthalpy change → calorific value = absolute value of ΔH_comb (per unit mass, volume, or mole).Conclude that the correct thermochemical term is the heat of combustion.

Verification / Alternative check:
Fuel specification sheets list HHV and LHV; both are measured by bomb calorimetry (HHV) or computed from combustion stoichiometry and water phase assumptions—confirming they are heats of combustion.

Why Other Options Are Wrong:
Formation: heat of formation pertains to forming a compound from its elements, not burning a fuel.Reaction: too vague; calorific value refers specifically to combustion.Vaporisation: relates to phase change, not fuel energy content.

Common Pitfalls:
Confusing HHV vs LHV (water phase treatment); mixing units (kJ/kg, kJ/m^3, kJ/mol).

Final Answer:
combustion