This invention relates to a process for thermal upgrading of low rank coal, for instance brown coal and other carbonaceous material, for instance peat, lignite, wood or carbonaceous waste. In this specification such other carbonaceous material will be deemed to be included in the term "coal".
Low rank coal, in the least favorable case can contain up to 90%w water, but more generally has a water content of 40 to 70%. The water is present in various forms, namely, chemically bound, gel, and absorbed water. This water needs to be removed from the coal not only for efficient transport, but also in order to increase its calorific value (and thus its worth) and to improve its combustion properties. This water can be removed either partially by thermal drying, in which the surface and absorbed water is evaporated, or completely by thermal upgrading. In general, water removed by thermal drying will over a period of time be reabsorbed, but water removed by thermal upgrading will not, due to chemical changes which take place in the coal.
Thermal upgrading of coal can be divided into two distinct stages, namely, dewatering, and decarboxylation (removal of oxygen-containing groups). Dewatering generally takes place at about 200.degree. C. and results in a shedding of an important part of the bound water. On separating the water from the coal, there is not a tendency for it to be reabsorbed Decarboxylation takes place at a rather higher temperature, particularly above 300.degree. C. and results in a further structural change in the coal. The coal significantly becomes hydrophobic due to the removal of the oxygen-rich polar groups which are responsible for the hydrophilic properties of the coal. For instance, confirmation of the structural change of the coal is found from petrographic analysis, from which it is seen that the vitrinitic reflectance for a typical brown coal as a result of treatment above 300.degree. C. has improved from ca. 0.35 before treatment to 0.70, which is equivalent to that of a subbituminous coal.
While for some types of coal thermal drying can be used with advantage, where the coal has a high water content it is less efficient in that the latent heat of vaporization of the water has to be supplied. Furthermore, there is practically no change in structure of the coal, so that the resulting dried product remains hydrophilic, and in the absence of special precautions will thus reabsorb much water in a humid environment. And, the calorific value may not be sufficiently increased to command a price which can justify the expense of transport over any considerable distance. For example, prior processes for dewatering wet, solid carbonaceous fuel materials have been described in patents such as the following: U.K. Pat. No. 844,556 describes removing water and bitumen from oil shale or peat or the like, by heating the moist material in oil, first at a temperature at which the water evaporates, then at a temperature at which the bitumen forms and evaporates. U.S. Pat. No. 3,552,031 describes heating brown coal as received from the mines at from about 100.degree. to 300.degree. C. at a pressure which prevents water vaporization but is less than 5000 psi, mechanically separating a fines-containing aqueous liquid which is subsequently separated into waste liquid and a sludge which is recycled into the coal to be treated. U.S. Pat. No. 3,992,784 by E. Verschuur, B. P. Ter Meulen, T. V. Herwijnen and J. Boom describes heating a slurry of brown coal and water at a temperature above 150.degree. C. and a pressure which prevents the vaporization of water, mechanically separating this slurry into coal, water and wet fines and recycling at least some of the water to preheat the incoming coal.