Roger Hayter wrote:
I don't think fire is an issue except with some Lithium cells.
Mercury, which is quite insidiously toxic and can leak into groundwater
used to be used in some button cells,[1] but I don't think it is any
more. But it is largely metal toxicity which is seen as a risk when
batteries are disposed of in landfill or incinerated. I must admit I
am pretty ignorant about which current batteries present a serious risk.

There's at least three fire risks:

Cells that are unstable and liable to catch fire in the wrong environmental
conditions like heat or pressure - eg lithium ion cells (Galaxy Note 7 etc)

Cells composed of material that catches fire on exposure to air. eg
https://www.youtube.com/watch?v=tBg4ximDrsk

Cells that get hot if shorted - eg alkalines - and whatever they might short
through catching fire.

I could well imagine that waste compaction etc could cause one or more of
these risks.

Theo