Chemistry - Could blue flames arise within the hull of a burning ship?

Solution 1:

In a Bunsen burner, the gas (methane or propane) may burn under two practical regimes : reductive or oxidizing conditions. In the reductive conditions, the flame is yellow and unstable, because the amount of oxygen is low with respect to the stoichiometric ratio. In the oxidative conditions, the flame is noisy, strong and stable, and it is due to an excess of oxygen in the mixed gases before the combustion. The oxidative flame is nearly colorless, but it has a characteristic blue cone at its base. This blue cone has a temperature of about $900°$C, and its spectrum has been analyzed by Hertzfeld in $1935$ : it shows a line spectrum typical of the diatomic "molecule" $\ce{C_2}$, with plenty of vibrational and even rotational sublevels. Maybe that was the origin of the blue flame described by Joseph Conrad. Why not ?

Solution 2:

The observed blue flame and explosion (the latter following per the quote, "Finally, the gases in the hull explode and blow up the deck") are likely the products of a hydrogen and air explosion.

There are two potential sources for the hydrogen gas here. First, the coal fire heats the mass of coal releasing a gas mix, to quote Wikipedia on Coal Gas on the particulars:

Coal gas is a flammable gaseous fuel made from coal and supplied to the user via a piped distribution system. It is produced when coal is heated strongly in the absence of air.

And further:

Coal gas contains a variety of calorific gases including hydrogen, carbon monoxide, methane, ethylene and volatile hydrocarbons together with small quantities of non-calorific gases such as carbon dioxide and nitrogen.

The second possible source of $\ce{H2}$ is from the failed attempt to flood the fire producing a large quantity of steam in the presence of hot iron, to quote an educational source:

If you pass steam over hot iron the steam reacts with the iron to produce a black, magnetic oxide of iron called triiron tetroxide, $\ce{(Fe_3O_4)}$.

$\ce{3 Fe(s) + 4H2O (g) -> Fe3O4(s) + 4 H2 (g)(1)}$

The hydrogen produced in the reaction is swept away by the stream of steam.

So, the hydrogen is formed and is trapped in pockets in the hull where it may eventually mix with oxygen from air and finally, a hot splitter causes an explosive ignition forming a striking blue flame.

[EDIT] On the explanation of the 'reported' (albeit, not always actual) color of a hydrogen flame, I quote a source:

There are also several text-books on chemistry which assert that hydrogen burns with a characteristic faint blue flame. It is easy to prove, however, that the flame of pure hydrogen has no blue tinge whatever. The blueness so frequently associated with the flame of hydrogen is really due to the presence of sulphur as is shown in a little paper I published in the Philosophical Magazine for November 1865.*

Now, in the current context, in the presence of burning coal, likely containing a significant sulfur impurity, a pronounced more unusual blue appearance apparently was reported, which would be in agreement with the hydrogen gas impurity argument, as is, the mention of a possible very high-temperature flame.

Others believing that this was a more normal occurrence (associated with, for example, a flame "with characteristic blue cone at its base") are likely incorrect. Further, not accounting for the chemistry as to how such a quantity of hydrogen gas was likely amassed, and its potential implications for future ship's fire, is a significant, perhaps even egregious, omission. More profoundly, it lies at the very explanation for the occurrence of the reported explosion and any resulting loss of human lives.