Heating value conventions: Given that petroleum fuels typically contain 12–15% hydrogen by mass, the latent heat lost with water vapour makes the gross (HHV) exceed the net (LHV) by approximately how much (kcal per kg of fuel)?

Introduction / Context:
Gross (higher) heating value (HHV) includes the heat recovered if combustion water condenses to liquid; net (lower) heating value (LHV) assumes water remains as vapour and its latent heat is not recovered. The difference depends largely on the fuel hydrogen content because hydrogen combustion produces water.

Given Data / Assumptions:

• Petroleum fuels contain roughly 12–15% hydrogen by weight.
• Water of combustion remains as vapour in LHV.
• Latent heat of vaporisation of water is the dominant contributor to HHV–LHV difference.

Concept / Approach:
Each kilogram of hydrogen yields 9 kg of water when fully oxidised. The latent heat associated with this water (at typical reference conditions) translates to several hundred kcal/kg of fuel. For 12–15% hydrogen fuels, the HHV–LHV gap typically falls around 600–750 kcal/kg, a standard rule-of-thumb in fuel engineering.

Step-by-Step Solution:

Verification / Alternative check:
Fuel property tables show HHV–LHV differences near this range for diesel, kerosene, and gasoline, consistent with their hydrogen content.

Why Other Options Are Wrong:

Common Pitfalls:
Forgetting that HHV–LHV is fuel-dependent; natural gas (higher H fraction) shows a different gap than residual oils.

Final Answer:
600–750