Explosions 9

Question 1

What is the problem with working out an oxygen balance with ANFO?

Answer 1

• Difficult to calculate an oxygen balance for ANFO because fuel oil is a diverse mixture of hydrocarbons
• 3NH4NO3 + “CH2” –> 7H2O + CO2 + 3N2
• Gives an oxygen balance of 0

Question 2

What is the approximate ideal composition by mass for ANFO?

Answer 2

3NH4NO3 + “CH2” –> 7H2O + CO2 + 3N2
Total idealised MW = 3(80.05) + 1(14.03) = 254.18 g/mol
% weight of NH4NO3 = (240.15/254.18)x100 = 94%
% weight of fuel oil = (14.03/254.18) x 100 = 6%

Question 3

What does the approximate ideal composition by mass of ANFO tell us?

Answer 3

• Need a large composition of ammonium nitrate to give complete oxidation
• Composition will be less sensitive because there isnt much fuel
• Might want to increase the fuel oil and sacrifice the oxidation to make it more sensitive

Question 4

What is the true oxygen balance of ANFO?

Answer 4

• Slightly negative due to the fuel oil being a mixture
• Composed primarily of C(n)H(2n+2)
• The detonation product will also contain some CO

Question 5

What is a fuel-air explosion?

Answer 5

• A finely divided (aerosolised) fuel can explode
• The oxidiser is O2 from the air so the KW rules dont apply
• Doesn’t have to accurately detonate, as long as it is close it will use the oxygen in the air to send a shockwave out
• Fuel is ususally ethylene oxide and propylene oxide
• Combustion: C3H6O + 4O2 –> 3CO2 + 3H2O

Question 5

What lessons were learnt from the Buncefield Explosion?

Answer 5

• ‘High integrity’ containment for fuels are required
• Incorporate a secondary containment system (another layer around storage tanks)
• Better detection systems/alarms for flammable gases now used
• Investigate possible ignition sources further from containment site