Low-rank coals, liquefaction

Fuchtenbusch B, Steinbiichel A (1999) Biosynthesis of polyhydroxyalkanoates from low-rank coal liquefaction products by Pseudomonas oleovorans and Rhodococcus ruber. Appl Microbiol Biotechnol 52 91-95... 

W. G. Willson md co-workers, " AppHcation of Liquefaction Process to Low-Rank Coals," paper presented at /0th BiennialEignite Symposium, Grmd Forks, N.D., May 1979. 

In absolute terms, the quantities of reactor solids found in various processes do vary considerably. The rate of accumulation is related to several factors, such as coal characteristics, recycle solvent quality and reactor design. However, it can be stated in general terms that liquefaction of low rank coals (sub-bituminous C and lignites) does result in higher rates of accumulation of solids than do similar operations with bituminous coals. For example, during normal operations of the SRC-I pilot plant at Wilsonville, Ala., it has been found that the amount of solids retained varies from about 0.2-0.5 wt.% (moisture-free) for bituminous coals to 1.0-1.9 wt.% (moisture free) for a subbituminous C coal (Wyodak) (72). Exxon also reports much larger accumulations for lignites and subbituminous coals than those found for bituminous coals (73). 

The H-Coal process produces approximately 3-3.5 barrels of liquid product for each ton of coal.42 Tests showed that the process is best suited for high-volatile bituminous coal the use of low-rank coals significantly reduced throughput and distillate yields. The successful performance of the ebullated bed reactor in the H-Coal process led to its later use in two-stage liquefaction systems. 

Conventional liquefaction processes developed for Eastern bituminous coals might not be the best choice for Western low-rank coals because of the substantial property and structural differences between them. In general, low-rank coals are more susceptible to reaction with H2, CO, or H2S. 

A diverse number of techniques has been employed to characterize the iron sulfides under coal liquefaction conditions. It is observed that the stoichiometry of the iron sulfides is determined by the H2S partial pressure and that greater activity is observed when the surface is rich in vacancies. All the iron sulfides show great affinity toward oxygen-containing compounds. This activity was demonstrated both with low rank coals as well as model compounds. It is proposed that pyrrhotites can act as catalysts for the cleavage of oxygen bonds in coal. 

Coal liquefaction under supercritical (or subcritical) water condition has some advantages over organic solvents. Supercritical water is miscible with H2, CO, aromatics, and oils, which provides a unique, homogeneous reaction medium for coal liquefaction. CTL process with supercritical water is more environment-friendly than SRC-II process and has higher conversion for low-rank coal [31,32]. 

Figure 6. Highly schematized depiction of the orthogonal influences of coal rank and coal type on coal grade (economic value) as reflected simply by the yield of liquids in a coal conversion (liquefaction) process (( ) reactive ingredients are vitrinite and exinite in higher rank coals, vitrinite and exinite give low yields)...

In this paper, a number of low-severity liquefaction regimes are considered. The influence of different H-donor and non-donor solvents on primary conversions without a hydrogen overpressure is discussed in the light of other recent work (10-131. Also, it is demonstrated that oil yields broadly increase with decreasing coal rank in both H-donor extraction and dry catalytic hydrogenation provided that retrogressive reactions are avoided in the initial stages of coal dissolution. 

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