Bituminous coal I guess? Anthrasite burns down to powder I thought? You are right though it works great.
I think either type coal ash would work. I burned hard coal in my house. Come to think of it, the neighbor burned soft coal in his shop. That ash worked well at getting unstuck. So yes. Bituminous or anthracite wil do the trick.
I think wood ashes would work. I just know that coal is super for providing traction. As Dave_in_Az said it is abrasive.
I can vouch for this. I come from the Eastern PA coal region (anthracite) and I can tell you with 100% certainty that the ashes are one of, if not the best, anti skid material you can get.
personally, i prefer speedi-dri (oil dry) over kitty litter, and i can get a 40 pound bag for about $10. some stores have a 100 pound bag, for about $20-$25. all automotive stores have oil-dry.....just depends on what they stock. then i have 2- 5 gallon pails that i keep filled. one will be in my truck, the other in my car truck, to replace as the one in the trunk becomes empty, then i refill the other, i always have a supply of speedi-dri (oil dry) kitty litter is good as well, as long as you do not buy the "clumping" type, but kitty litter can be more costly, i have found i even use oil dry at home, when the weather is dry, i sweep up the stuff, and store it in a big container down in my basement, for the next use. kitty litter can also be recycled as well, but if you do happen to have a cat, he/she may not like the odors the recycled litter has. (as it can be mixed with road salts from your city/town, when they go by your house.