Possibly carbon black: see
https://en.wikipedia.org/wiki/Carbon_black
Never heard of soot being taken away from power stations. Nearly all of milled coal ends up as ash which is /was usually pumped to massive land fill lagoons. Soot did I believe, collect on electrostatic precipitators prior to going up the stack and the rapping gear would rattle the wires etc and presumably the soot would fall into hoppers? Perhaps this is what you are referring to? Any ex CEGB on here please?
Some of my older tyres have the start of cracking and a garage said it may be down to sunlight? My day to day ZT never suffers from this as tyres replaced every couple of years through use.