Lignite is the lowest rank of coals. Lignites are brown in color and have an earthy, crumbly texture. They look more like dirt, than what people normally think of when they think of coal. In the U.S. rank classification, lignites are defined based on calorific value. Lignites have calorific (heating) values less than 8,300 Btu/lb (ASTM, 2014). In some other countries, lignites are defined based on their moisture content. In other classifications, vitrinite reflectance (%Ro) may be used to define lignites. Vitrinite reflectance in lignites is generally %Ro less than 0.4 (UNECE, 1998; Alpern and DeSousa, 2002; ISO, 2005).
Physical and Chemical Changes (Lignite Rank)
The dominant processes during the lignite stage of coalification are physical compaction, dewatering (sometimes called dehydration) and biogeochemical gelification. Biochemical gelification is the biological and chemical breakdown of carbon molecules in the lignite (Stach et al., 1992; Teichmüller, 1989). These processes are part of the broader phases of diagenesis, and eodiagenesis in coalification (see figure).
Moisture content decreases dramatically from peat through lignite rank, through expulsion of water and through changing coal chemistry (the loss of certain chemical functional groups) resulting from biochemical gelification (Barnes et al., 1984; Levine, 1993; Mukhopadhyay, and Hatcher, 1993). At the peat to lignite transition, lignite commonly has moisture contents of 60%, but moisture decreases to approximately 20% in subbituminous coals (Teichmüller and Teichmüller, 1982; UNECE, 1998).
Biogenically-sourced methane (CH4) and carbon dioxide (CO2) are also expulsed from lignite during the chemical and physical changes that accompany gelification (Stach et al., 1982; Levine, 1993).
An important physical change in coal during the lignite rank of coalification is the formation of organized sets of fractures or joints, called cleats. Cleats are what give coal its common blocky appearance. They are also pathways for moisture and gas movement and subsequent mineralization (Stach et al., 1992, Laubach et al., 1998).