Letter: Letter

From Jim Ring

Some misleading information on the relative safety of hydrogen and other
fuels appeared in your feature. It is really very difficult to ignite a mixture
of petrol or gasoline vapour and air. At most ratios these two gases will not
ignite. A near-empty fuel tank in a car contains a considerable quantity of
oxygen, and no amount of sparking will cause ignition. When a mixture of petrol
vapour and air burns, it does so relatively slowly, so that an explosion hardly
ever occurs.

But most ratios of hydrogen and air can be ignited with a spark. The rate of
burning is extremely high, as well, so an explosion often results.

The claim that to work as a fuel, hydrogen requires more oxygen than
gasoline, assumes complete combustion. This is true, but there’s more to the
story. Assuming that gasoline is octane, 1 tonne of it requires about 3.5 tonnes
of oxygen to combust. Compare this with the 8 tonnes of oxygen needed to combust
1 tonne of hydrogen. However, if you accompany that 1 tonne of hydrogen with 3.5
tonnes of oxygen and a spark, the resulting explosion will destroy a
considerable amount of property. Anybody unfortunate enough to be in the
vicinity will not survive to reflect that most of the hydrogen remains
unconverted to water vapour.

I have survived one or two very minor mishaps with hydrogen. It is very
dangerous stuff indeed.

Nelson, New Zealand