G3 Properties of Fuel Gases 2 — Questions and Answers
Question 1: What is the ignition temperature of natural gas?
- 250°C (482°F)
- 400°C (752°F)
- 593°C (1,100°F) (Correct answer)
- 815°C (1,500°F)
Correct answer: 593°C (1,100°F)
Natural gas ignites at approximately 593°C (1,100°F).
This relatively high ignition temperature compared to propane (493°C) provides a margin of safety.
Question 2: How does propane's specific gravity differ from natural gas?
- Propane is lighter; natural gas is heavier
- Propane is heavier than air (SG ~1.52); natural gas is lighter (SG ~0.60) (Correct answer)
- Both have the same specific gravity
- Natural gas is heavier than propane
Correct answer: Propane is heavier than air (SG ~1.52); natural gas is lighter (SG ~0.60)
Propane sinks (SG ~1.52); natural gas rises (SG ~0.60).
This critically affects gas detector placement and emergency procedures.
Question 3: What is propane's heating value compared to natural gas?
- Propane has lower heating value
- Propane has approximately 2.5 times the value per cubic foot (Correct answer)
- Both are identical
- Natural gas has twice propane's value
Correct answer: Propane has approximately 2.5 times the value per cubic foot
Propane delivers about 2,520 BTU/ft³ versus natural gas at about 1,000 BTU/ft³.
Converting appliances between fuels requires orifice and valve setting changes.
Question 4: What is the flammable range of propane in air?
- 2.1% to 9.5% (Correct answer)
- 5% to 15%
- 1% to 5%
- 10% to 20%
Correct answer: 2.1% to 9.5%
Propane's flammable range is approximately 2.1% to 9.5%.
Combined with propane's heavier-than-air density, this makes propane leaks particularly dangerous.
Question 5: What determines flame speed and why does it matter?
- Flame colour; affects aesthetics
- Rate a flame front travels through gas-air mixture; affects burner design (Correct answer)
- Time to maximum temperature; affects pipe sizing
- Rate of gas flow; affects noise
Correct answer: Rate a flame front travels through gas-air mixture; affects burner design
Flame speed affects burner port sizing and flame stability.
Proper port sizing ensures gas velocity matches flame speed for stable operation.
Question 6: What is the chemical equation for complete methane combustion?
- CH4 + O2 → CO + H2O
- CH4 + 2O2 → CO2 + 2H2O (Correct answer)
- CH4 + 3O2 → CO2 + 3H2O
- 2CH4 + O2 → 2CO2 + H2O
Correct answer: CH4 + 2O2 → CO2 + 2H2O
Complete combustion: CH4 + 2O2 → CO2 + 2H2O.
This helps calculate air requirements and identify incomplete combustion when CO appears instead of CO2.
G3 Properties of Fuel Gases 2
What is the ignition temperature of natural gas?