Читать книгу Coal-Fired Power Generation Handbook - James Speight G., James G. Speight - Страница 78

Although the concept of drying using superheated steam was conceived more than a century ago, serious interest in superheated steam drying has emerged in the last three decades (Mujumdar, 1990). Many benefits are associated with superheated steam drying, which include (i) the reduced risk of spontaneous combustion, (ii) the increased drying rates, (iii) the better energy efficiency, and (iv) the improved grindability of the coal (Osman et al., 2011).

In the process, the optimum pressure and drying time depend on the size of the coal particles and the resulting moisture content of the dried particle depends on the steam pressure and temperature, the particle size and moisture content of the coal feedstock.

Superheated steam drying requires less energy than hot gas dryer because there is no need to supply coal moisture with latent heat of vaporization. Drying in superheated steam also increases the apparent density of low-rank coal due to shrinkage of the particles on moisture removal. In addition, the decomposition of sulfur functional groups during steam drying process produces cleaner coal with high heating value. Because fire hazards associated with the spontaneous combustion of coal are eliminated in the superheated steam drying process, the target moisture content can be achieved in a short time by using higher steam temperature.