The experiment was short - lived, but it proved the point that ammonia — plus a small amount of coal gas to help combustion — could be used as a transport fuel. (newscientist.com)
There are other routes for coal gas to escape, including fractures or undocumented structures from legacy mines and abandoned wells. (dotearth.blogs.nytimes.com)
So far we have used coal gas and now oil. (earthtimes.org)