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Methane reforming with carbon dioxide in a directly irradiated particle receiver seeded with carbon black is presented in this study. Carbon particles were entrained in the reacting gases and acted as heat transfer and reaction surface. The reactions were not catalyzed by a metal catalyst. The molar ratio between the entrained carbon particles and the working gases (Ar, CO2 and CH4) was 4–7 mmol carbon/mol gas. The temperature of the reforming experiments varied from 900°C to 1450°C with CO2/CH4 ratios of 2–6. Experimental results show that methane reacts at lower temperatures than expected for its thermal decomposition; this indicates that the decomposing reaction is enhanced by the presence of the carbon black particles. At 1170°C 90% of the methane reacted in the receiver during a residence time of 0.3 s. The reaction between carbon dioxide and carbon black is faster than is documented in the literature, but the reaction rate does not seem to change if only carbon dioxide and carbon black are present in the receiver, compared to experiments where methane is also part of the gas mixture. The experimental results indicate that a high solar flux, i.e., about 2500 kW/m2 or higher, significantly accelerates the reaction rate of methane decomposition. Total or partial blockage of the solar radiation reduced the yield by about 50%, compared to tests when the receiver was exposed to the full solar radiation flux, at the same operating temperature.

Dry methane reforming with carbon dioxide in a directly irradiated particle receiver seeded with carbon black is presented in this study. Carbon particles were entrained in the reacting gases and acted as heat transfer and reaction surface. The reactions were not catalyzed by a metal catalyst. The molar ratio between the entrained carbon particles and the working gases (Ar, CO2, and CH4) was 4–7 mmol carbon/mol gas. The temperature of the reforming experiments varied from 750°C to 1450°C with CO2/CH4 ratios varying from 1:1 to 1:6. Experimental results show that methane reacts at lower temperatures than expected for its thermal decomposition; this indicates that the decomposing reaction is enhanced by the presence of the carbon black particles. At 1170°C 90% of the methane reacted in the receiver during a residence time of 0.3 s. The reaction between carbon dioxide and carbon black is faster than is documented in the literature, but the reaction rate does not seem to change if only carbon dioxide and carbon black are present in the receiver, compared with experiments where methane is also part of the gas mixture. The experimental results indicate that a high solar flux, i.e., about 2500 kW/m2 or higher, significantly accelerates the reaction rate of methane decomposition. Total or partial blockage of the solar radiation reduced the yield by about 50%, compared with tests when the receiver was exposed to the full solar radiation flux, at the same operating temperature.

This experimental study shows the behavior of a directly irradiated, high temperature, solar receiver seeded with a low concentration of carbon black particles as the radiation absorbing media in the presence of air or nitrogen as the working fluid. Experiments were conducted in the presence of highly concentrated solar energy with an energy flux of up to 3MW∕m2 at the aperture of the receiver. 99.9% of the particles had an equivalent diameter of <5μm, but the remaining larger agglomerates accounted for 51% of the overall projected surface area. The molar ratio of carbon to gas in the fluid entering the receiver was 0.004–0.008. The heat transfer from the solar radiation to the working gas was accomplished almost exclusively via the particles. The receiver behavior during steady-state operation was evaluated. The receiver gas exit temperatures achieved during the experiments were between 1000 and 1550°C. When nitrogen was used as working gas, its exit temperature exceeded the average wall temperature, whereas when air was used, its exit temperature was lower than the average wall temperature. The air flow may have been heated to some extent by the receiver walls, whereas in the case of nitrogen, the particle-to-gas heat transfer was dominant throughout the receiver. When the gas exit temperature was above 1200°C, the particle seeded nitrogen flow absorbed 12–20% more energy than particle seeded air flow under the same operating conditions (insolation, particle load, flow rate, close proximity in time). The air tests reached high exit temperatures despite the reduction of particle concentration due to combustion. This indicates that heat transfer mainly occurs over a relatively short time period after the particle seeded flow enters the cavity close to the receiver aperture, before significant particle burning takes place. The energy due to carbon combustion was 3–5% of total energy absorbed in the high temperature air experiments. The carbon particles’ oxidation rate in the presence of molecular oxygen was found to be significantly lower than values documented in the literature for high temperature carbon black combustion in air. The high solar flux, which promotes very high radiation→particle→gas heat transfer rate, might account for this retardation.

In high-temperature solar-thermal systems the conversion of solar to thermal energy requires a radiation absorbing surface to transfer the radiative solar energy to the working fluid. The present study focuses on the generation of a moving radiation absorber using particles suspended in the working fluid. Three methods of particle entrainment in a gas were investigated. Elutriating fine particles from a spouted bed was found to be the preferred method. The diameter range of the entrained carbon black particles was 0.030-25μm, with 99.7% of the particles having an equivalent diameter less than 1μm, and 48% of the projected surface area was due to agglomerated particles with average equivalent diameter >5μm. The moving radiation absorber was tested in a solar receiver using nitrogen as a working fluid. The inner wall temperatures in the receiver cavity were below the gas exit temperature, which shows that the bulk heat transfer from the incoming solar radiation to the gas takes place via the moving radiation absorbing particles.