What do Poskim say regarding using chemical cooking technology on Shabbos where a combination of iron, magnesium and salt with water can heat up food?
I just saw by "coincidence" as I randomly opened up the new book The Shabbos kitchen (expanded edition), that R Simcha Bunim Cohen asks your question. He writes (p. 13)
There is a dispute among contemporary poskim if [heating water with chemicals thereby causing a chemical reaction] is considered cooking and if it is forbidden on Shabbos or not. One should consult a halachic authority for a final ruling.
Since he is himself a halachic authority, you can imagine he would only authorize it in emergency cases.
He writes in the footnote this might be a melacha of make bepatish. He suggests there it might be permitted in emergency cases such as in the army. He refers to a Kovetz Ohr Yechezkel 32 for details on each side of the issue.
That would be worse than electricity because chemical cooking creates multiple new chemicals due to the chemical reactions. One of the largest issues with cooking is that you are creating something new. That's why only reheating is allowed and why you can't even cook soup for shabbos morning according to many poskim because it would have matured into a different dish than when it started.