62nd Annual Report on Research 2017

We have presented a simple way of obtaining hydrogen gas from various ranks of coal, coke, and graphite using nanosecond laser pulses under different conditions such as water, air and argon atmosphere. It was observed that 532 nm laser pulses were more effective than 1064 nm pulses in gas generation and both were nonlinearly correlated with respect to the laser energy density. Gas chromatography measurements indicate that mainly hydrogen and carbon monoxide were generated. The hydrogen to carbon monoxide ratio shows that the highest efficiency rank was anthracite coal, with an average ratio of 1.4 due to its high fixed-carbon content and relatively high hydrocarbon amount. Graphite was used as a pure carbon source to study the possible reactions of gas yielded during the irradiation process. In addition, theoretical simulations using a standard finite difference method supported experimental observations. The possible mechanisms of gas generation were explained with chemical reactions.

1. Experimental

Different ranks of coal, namely anthracite, bituminous, lignite, and coke were used. In a 20 mL vial with a small magnetic stirring bar, the powder samples were dispersed in distilled water at a ratio of 30 mg: 9.5 mL. The mixture was sonicated with ultrasonic cleaner for 5 minutes to get the uniform powder-water mixture. Vials were sealed with aluminum seal caps, there was no leak even at a pressure higher than that one created by gas generation. Then the water-coal mixture was irradiated with an unfocused beam of 5 ns laser pulses from a Q-Switched Nd: YAG laser (Continuum Surelite SL II-10) at 10 Hz frequency for 45 minutes, with magnetic stirring (Sci-Basics, MHS-800) 1500 rpm. The laser energy density was adjusted in the range of 90-700 mJ/cm². The mixture was irradiated with both 1064 and 532 nm laser pulses; a second harmonic crystal (Continuum SL SHG T-2) was used in order to obtain the 532 nm laser pulses. Gas components generated from these irradiations were analyzed by gas chromatography Results and Discussions

1. Gas amounts, components, and possible reactions

For the further investigation, the generated gases after irradiating samples with 1064 and 532 nm wavelength laser pulses in the energies between 90-700 mJ/cm² were analyzed by gas chromatography (Supplementary file, Table S1-S4). Table 3 shows the percentage of generated gas components (~78% nitrogen and ~21% oxygen gas already exist in air) and the remaining air after irradiating coal-water mixture samples in an air atmosphere with 532 and 1064 nm laser pulses at 350 mJ/cm² energy.

Table 3. Generated gas concentrations (bold) and remaining air from each rank of coal-water mixture in air after irradiating by 1064 nm (532 nm) laser pulses at the energy of 350 mJ/cm².

Hydrogen and carbon monoxide were the main gases generated (~80% of the generated gas). The ratio of hydrogen to carbon monoxide was calculated to find the efficiency of each rank at hydrogen generation. On average, gas generated from anthracite produced nearly 1.4 times more hydrogen than carbon monoxide, while this ratio is less than 0.8 for other ranks. The highest ratio of hydrogen to carbon monoxide while irradiating anthracite coal-water mixture was obtained at 350 mJ/cm² in 1064 nm irradiation, (ratio of 1.98) and at 180 mJ/cm² in 532 nm irradiation (ratio of 2.63). Thus, the highest ratio is increased from 1.98 to 2.63 as well as the amount and the rate of the gas.

Figure: gas generation from different ranks of coal under 1064 and 532 nm irradiations

It is known that the optical absorption of the coal is higher at 532 nm compared to 1064 nm and surface temperature generated by a nanosecond laser is related to the materials absorption coefficient, to the reflectance of the material and to the laser wavelength. The increased efficiency of gas generation can possibly be related to the change in temperature and an additional induced electric field generated by green laser pulses. Therefore, further analysis was carried out with 532 nm laser pulses to see the effects more clearly. To understand the mechanism behind hydrogen gas generation, the following experiments under different conditions have been performed.

• Graphite under air and argon atmosphere without water
• Graphite under air and argon atmosphere with water
• Coal samples under air and argon atmosphere without water
• Coal samples under air and argon atmosphere with water

2. Conclusions

Hydrogen was generated from a mixture of carbon-water and graphite-water by irradiating with nanosecond laser pulses through dehydrogenation and combustion reactions. The possibility of each reaction was discussed for different conditions. There appear to be several reactions occurring at different rates depending on the experimental conditions. Graphite did not generate hydrogen efficiently due to the lack of hydrocarbons in its structure (99.99% carbon). The amount of gas generated by 532 nm laser pulses was higher than the one generated by 1064 nm pulses due to the higher absorption rate. Anthracite was shown to be the most effective rank of coal at generating hydrogen as a clean fuel source by nanosecond pulsed laser irradiation of coal powders in water. This result can be related to the high fixed carbon and hydrocarbon contents due to the fact that each of these processes triggers certain reactions which are responsible for gas generation. This hydrogen production method differs from conventional gasification methods by virtue of its simplicity. In this process carbon monoxide by-product is generated, thus further improvements are needed to increase the efficiency of hydrogen generation relative to CO. Produced hydrogen via such a method might be employed to power fuel cell devices for portable applications such as emergency generators and laptop computers.