Caking coals

The fluidized-bed system (Fig. 3) uses finely sized coal particles and the bed exhibits Hquid-like characteristics when a gas flows upward through the bed. Gas flowing through the coal produces turbulent lifting and separation of particles and the result is an expanded bed having greater coal surface area to promote the chemical reaction. These systems, however, have only a limited abiUty to handle caking coals (see Fluidization). 

Process development on fluidized-bed pyrolysis was also carried out by the ConsoHdation Coal Co., culminating in operation of a 32 t/d pilot plant (35). The CONSOL pyrolysis process incorporated a novel stirred carbonizer as the pyrolysis reactor, which made operation of the system feasible even using strongly agglomerating eastern U.S. biturninous coals. This allowed the process to bypass the normal pre-oxidation step that is often used with caking coals, and resulted in a nearly 50% increase in tar yield. Use of a sweep gas to rapidly remove volatiles from the pyrolysis reactor gave overall tar yields of nearly 25% for a coal that had Eischer assay tar yields of only 15%. 

Tai yields of approximately 0.32 m /1 were quoted. Because the scheme used hydrogen, the pyrolysis Hquids generally exhibited lower heteroatom contents than conventional tars derived from coal pyrolysis ia an iaert atmosphere. Process development proceeded through a 270 t/d semiworks plant which was operated successfully on noncaking coals. OperabiUty for caking coals was difficult, however. 

Coals Used and Sample Preparation. Two coals were selected for this study. The first ( A ) is a British Coal class 500 and represents a caking coal, whereas the second ( B ) is a class 700 which is used as a steam-raising coal within a 1,730 MW coal-fired generating station. An outline of their respective characteristics is given in Table I. 

Full scale synthetic natural gas (SNG) plants based on standard Lurgi gasifiers (250 mm scf/D) can be built now to operate on noncaking or weakly caking coals. The developmental gasifiers which are candidates for demonstration will be ready for commercial deployment by 1988 for the conversion of all types of coals with potential reduction in capital and operating cost of 15 to 20 percent. Advanced processes with additional improvements will not be ready for commercialization until the 1990 s. 

Buffo s experiment with reflectors, 2. caking coal, 7A caloric 1,S. 

A comparative study has been made by optical and electron microscopy of the anisotropic texture of several cokes from caking coals and pitches carbonized near their resolidification temperature. A simple technique made it possible to examine, by both methods, the same area of each sample and to identify the corresponding zones of the two very similar images. The anisotropy observed in polarized light appears in electron microscopy as differences in contrast resulting not from inequalities in electron absorption, but, as revealed by microdiffraction and dark Reid examinations, from diffraction phenomena depending on the general orientation of the carbon layers within each anisotropic area. 

Raw Materials and Their Carbonization. The original materials are vitrain-enriched fractions of three different rank caking coals (Table I) and two pitches of different types (Table II), from which certain mixtures have been prepared. 

---