Experts say the technology is especially suitable for low - rank coals like lignites and sub - bituminous coal , which produce less heat when burned due to their high ash content , and are highly polluting . " There are , of course , major concerns about water pollution or land subsidence caused by " cavities created when the coal seams are drilled and burned out , " Winning writes . Advocates say water can be protected by managing pressures in the coal seam , and subsidence can be avoided by choosing a site with strong rock layers . And what about the main concern about coal projects ?

Generate a question about the above context.
Why is the narrator concerned about using coal ?