The review concerns with recent progress on the fuel cell systems utilizing natural gas, petroleum and coal as fuel. Emphasis was made on the fact that it is of essential importance in hydrocarbon fuel cell systems to increase the electrochemical reactivity of the hydrocarbon fuels. In this connection, consideration has been made of, a) the indirect hydrocarbon system with a reformer, b) the electrochemical catalyst, and d) the electrolytes which make possible to operate fuel cell at high temperatures above 500 and around 1, 000°C.
It was mentioned that the indirect hydrocarbon fuel cells in the use of natural gas and coal is likely to be most promising at present with respect to commertial application.

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Since it is very difficult to investigate the chemical structure of coal itself by chemical means, studies of the regenerated humic acid which is the primary decomposition product of coal have been done.
In this paper, the results of experiments carried out for preparing the regenerated humic acids from some kinds of coal by alkaline fusion method were reported. Sodium or potassium hydroxide was used as a reagent of the alkaline fusion, and the reaction temperature was arranged in the range from 175°C to 350°C. The carbon content of coals used in the experiment was between 70% and 80%.
In the case of reaction of Ube Coal having carbon content of 70.0% with sodium hydroxide, the yield of humic acid was maximum, about 55%, at 250°C; and that of residual coal (the unreacted) was minium, about 17%, at 300°C.
Reactivity of potassium hydroxide with coal was greater than sodium hydroxide at the same temperature.
The following results depended on the degree of the carbon content of sample coals were obtained:
(1) C>80%
(a) The yield of humic acid was very low.
(b) The carbon content of residual coal was similar to that of the original coal.
(2) C=77-80%
(a) The yield of humic acid was low with sodium hydroxide at 250°C, while very high with potassium hydroxide at 300°C.
(b) Even in the case of the low yield of humic acid, the carbon content of residual coal was higher than that of the original coal.
(3) C<76%
(a) The yield of humic acid was high.
(b) The carbon content of residual coal was higher than that of the original coal.
It was remarkable that the carbon content of the regenerated humic acid obtained from coal by alkaline fusion method was more than 70%, and different from the humic acid obtained from the other ordinary oxidation methods.

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High vacuum pyrolysis was performed for Japanese original coals and heat treated coals. The molecular weight of resulting distillates was measured by vapour pressure and surface area of residual coal was determined by carbon dioxide according to the theory of Dubinin-Polanyi. The following results were obtained.
1. For Yabari coal, the effect of the pyrolysis temperature was studied. The average molecular weight of distillates is about 400, which remains nearly constant at the temperatures of 350, 400, 450 and 500°C. At each temperature, distillates show an approximately same average empirical formula of C₂₉H₃₁O_1·3. The results indicate that the substances of distillates are scarcely affected by the temperature of high vacuum pyrolysis. An interesting contrast to this result is shown in the yield of distillates which increases with increase in temperature.
The surface area of residual coal increases with increasing temperature of pyrolysis above 400°C, and at 5007deg;C it reaches 167 m²/g, i. e., 50 % increase from the value of original coal.
2. The effect of coal rank was examined with various kinds of coals, at the fixed heating temperature (500°C). The molecular weight of distillates increases with increase in coal rank as shown by Taiheiyo coal (329) and Bibai coal (346), reachs maximum (393) at Yabari coal and decreases on further increase in coal rank. However, Takamatsu coal shows an extremely low value (288). This relation bears clear resemblance to the previously reported relation between the yields of distillates and coal rank. When molecular weight of distillates were plotted against C% of distillates, almost all coals were found to lie on the straight line, including Takamatsu coal. This signifies that molecular weight of distillates increases with increasing C%.
As for the surface area of residual coal, the youngest Tempoku coal shows the highest surface area (about 300 m²/g), and the surface area tends to decrease with increasing degree of coalification. The difference of the surface areas between original coal and residual coal is large for young coals, and it shows a decreasing tendency with increase in degree of coalification.
3. The high vacuum pyrolysis of heat treated coals was performed under the idential condition (500°C) and the effect of heat treatment temperature was investigated. The molecular weight of distillates is generally unaffected when heat treatment temperature is below 400°C, but decrease rapidly at 450°C. This tendency can also be observed for the yields of distillates as reported previously, and combined results suggest that polymerization and decomposition might have been induced to an appreciable extent by the heat treatment at 450°C. The surface area of residual coal tends to increase with increase in heat treatment temperature above 500°C.

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