Coal conversion pyrolysis

Occidental Petroleum Coal Conversion Process. Garrett R D Co. (now the Occidental Research Co.) developed the Oxy Coal Conversion process based on mathematical simulation for heating coal particles in the pyrolysis unit. It was estimated that coal particles of 100-mm diameter could be heated throughout their volumes to decomposition temperature (450—540°C) within 0.1 s. A large pilot faciUty was constmcted at LaVeme, California, in 1971. This unit was reported to operate successfully at feed rates up to 136 kg/h (3.2 t/d). 

Goal Processing to Synthetic Fuels and Other Products. The primary approaches to coal processing or coal conversion are thermal decomposition, including pyrolysis or carbonization (5,6), gasification (6), and Hquefaction by hydrogenation (6). The hydrogenation of coal is not currently practiced commercially. 

It is possible to produce some liquid hydrocarbons from most coals during conversion (pyrolysis and hydrogenation/ catalytic and via solvent refining)/ but the yield and hydrogen consumption required to achieve this yield can vary widely from coal to coal. The weight of data in the literature indicate that the liquid hydrocarbons are derived from the so-called reactive maceralS/ i.e. the vitrinites and exinites present (7 8 1 9). Thusf for coals of the same rank the yield of liquids during conversion would be expected to vary with the vitrinite plus exinite contents. This leads to the general question of effect of rank on the response of a vitrinite and on the yield of liquid products and/ in the context of Australian bituminous coals, where semi-fusinite is usually abundant/ of the role of this maceral in conversion. 

Pyrolysis. In this context it is relevant to consider initially the effect of hydrogen contents on tar yields during pyrolysis (carbonization). This is particularly so, since, in all coal conversion processes little happens until the coal is at a temperature above that where active thermal decomposition normally sets in. In other words, all coal conversion processes may be regarded as pyrolysis under a variety of conditions which determine the nature of the primary decomposition and the reactions which follow. 

Pyrolysis. Conversion of coal by pyrolysis involves heating coal to a temperature of 500 to 700°C. Gases and liquids are evolved from the coal at these temperatures, leaving char which has a lower H/C ratio than the original coal ... 

At the present time, few, if any, details of chemical reaction mechanisms in coal conversion are known with certainty. This situation is particularly distressing in the areas of coal liquefaction and pyrolysis where chemical kinetics may strongly influence process efficiency and product quality. To improve this situation, in recent years a number of research groups have been performing chemical studies of coal and "model" compound reactions. 

Coal hydropyrolysis is defined as pyrolysis under hydrogen pressure and involves the thermal decomposition of coal macerals followed by evolution and cracking of volatiles in the hydrogen. It is generally agreed that the presence of hydrogen during the pyrolysis increases overall coal conversion. (, )... 

Likewise, Orr et al.29,30 have explored the possible use of tyre pyrolysis oil as a solvent for coal liquefaction. The potential of this alternative was demonstrated by the fact that coal-TPO mixtures were transformed with higher conversion than when coal was reacted directly with ground waste rubber tyres. It is proposed that the polyaromatic compounds present in the TPO favour coal dissolution during liquefaction. Treatment of coal-TPO mixtures (50/50%) at 430 °C under 68 atm of cold-hydrogen pressure in the presence of a Mo catalyst led to a high coal conversion in just 10 min of reaction. From electron probe microanalysis of the coal particles after the reaction, the authors conclude that TPO favours the catalyst dispersion and its contact with coal, which results in enhanced coal conversion. 

Pyrolysis is the first step of aU coal conversion processes and its understanding is essential for effective use of coal. The pyrolysis of coal consists of two series of reactions first is the primary decomposition that consists of the formation of radical fragments and their stabilization and the other is the secondary gas phase reaction of gaseous components produced by primary decomposition. The product yield during the pyrolysis depends on coal type and operating conditions of pyrolysis (Howard, 1981a,b Gavalas, 1982 Kawamura et al., 1993). 

C. Huang, J. Y. Zhang, J. Chen, and C. G. Zheng, Quantum chemistry study on the pyrolysis of thiophene functionalities in coal. Coal Conversion 28(20), 33-35 (2005). 

In a strict sense, coal pyrolysis means heating of coal in an inert atmosphere. When heated, coal undergoes thermal decomposition, which results in the evolution of gases and tars. In all coal conversion and utilization processes (including gasification, combustion, and liquefaction), some form of coal pyrolysis occurs. Because coal pyrolysis by itself is an important coal utilization process for making coke and oil, a vast amount of literature exists on this topic. By carefully varying the... 

The pyrolysis takes place for all coal conversion reactions when coal is heated above the "pyrolysis temperature. The behavior of coal dxaring pyrolysis is governed by coal type and experimental conditions such as particle size, heating rate, reaction temperature and pressure, and species of gas (inert, H2,... 

Liquid Fuels. Liquid fuels can be obtained as by-products of low temperature carbonization by pyrolysis, solvent refining, or extraction and gasification followed by catalytic conversion of either the coal or the products from the coal. A continuing iaterest ia Hquid fuels has produced activity ia each of these areas (44—46). However, because cmde oil prices have historically remained below the price at which synthetic fuels can be produced, commercialization awaits an economic reversal. 

The conversion of coal into hquid materials can be accomphshed by pyrolysis or by direct liquefaction— heating coal in the presence of a hydrogen source. Neither of these routes is yet economically feasible. 

The first part of this paper has shown that Australian black and brown coals differ significantly in a number of respects from coals of similar ranks from North America and elsewhere in the northern hemisphere. The rest of the paper than proceeded to indicate the progress being made to determine how the characteristics of Australian coals influence their conversion to volatile and liquid products during pyrolysis and hydrogenation. 

Conversion of coal to benzene or hexane soluble form has been shown to consist of a series of very fast reactions followed by slower reactions (2 3). The fast initial reactions have been proposed to involve only the thermal disruption of the coal structure to produce free radical fragments. Solvents which are present interact with these fragments to stabilize them through hydrogen donation. In fact, Wiser showed that there exists a strong similarity between coal pyrolysis and liquefaction (5). Recent studies by Petrakis have shown that suspensions of coals in various solvents when heated to 450°C produce large quantities of free radicals (. 1 molar solutions ) even when subsequently measured at room temperature. The radical concentration was significantly lower in H-donor solvents (Tetralin) then in non-donor solvents (naphthalene) (6). 

Conversion from coal to natural gas. Sasol 1 was designed as a coal-to-liquids facility. A natural gas pipeline was constructed and commissioned in 2004. This allowed the Sasol 1 facility to be converted to a gas-to-liquids plant. Although it implied that the associated coal tar refinery would become redundant, the decision was made by Sasol to keep the coal-to-chemicals units at Sasol 1 in operation by supplying coal pyrolysis products from its larger CTL facility in Secunda. 

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