Lignite, often referre to as brown coal, or Rosebud coal by Northern Pacific Railroad, is the lowest rank of coal and used almost exclusively as fuel for steam-electric power generation. It is brownish-black and has a high inherent moisture content, sometimes as high as 66%, and very high ash (50%) content compared with bituminous coal. It is also a heterogeneous mixture of compounds for which no single structural formula will suffice.