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Research Papers

Dry Methane Reforming Without a Metal Catalyst in a Directly Irradiated Solar Particle Reactor

[+] Author and Article Information
Hanna Helena Klein, Jacob Karni

Department of Environmental Science and Energy Research, Weizmann Institute, Israel

Rachamim Rubin

Solar Energy Facility, Weizmann Institute, Israel

J. Sol. Energy Eng 131(2), 021001 (Mar 11, 2009) (14 pages) doi:10.1115/1.3090823 History: Received November 24, 2007; Revised August 11, 2008; Published March 11, 2009

Abstract

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.

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Figures

Figure 1

A schematic of the solar receiver and the solar receiver used in the experiments

Figure 2

Optical system for the solar experiments

Figure 3

Gas exit temperature as a function of time for solar experiment case 12 (Table 1)

Figure 4

Relative power absorbed in CO2/N2 pairs listed as a function of the average gas exit temperature of the pair at near steady-state conditions. Data from Table 1.

Figure 5

SEM images with magnification of 2000. Samples collected on a filter after passing the receiver: (above) working gas CO2, Tg max=1330°C; (below) working gas N2, Tg max=1400°C. The scale is 10 μm.

Figure 6

k2[Ct]∗1018 as a function of the reciprocal temperature for different carbon compounds; results (1) from Ref. 27 and (2) from Ref. 28

Figure 7

Experimentally determined carbon black consumption as a function of the exit gas temperature (data listed in Table 2). The calculated values from (1) Ref. 27 and (2) Ref. 28 are included.

Figure 8

Example of reforming in the solar receiver. 35 SLPM Ar and 40 SLPM CO2 is heated. When 16 SLPM CH4 is added to the receiver, the reforming reaction starts and the temperature sinks until steady state is reached. CH4 amounted to 18 vol % of total flow in to the reactor (No. 6, Table 3).

Figure 9

Dry methane reforming in a solar receiver. Methane amounted to 15–17 vol % of gas entering the receiver. Molar ratio of CO2/CH4 was 2–5. Circular data points indicate that they were measured when a cloud was blocking the solar radiation. The data are listed in Table 3.

Figure 10

Methane conversion as a function of gas exit temperature and CO2/CH4 ratio. Data are listed in Table  34.

Figure 11

Theoretically calculated reaction time for 90% CH4 conversion in noncatalytic methane reforming of carbon dioxide (29). The residence time in the receiver is marked in the graph.

Figure 12

mol % of hydrogen and carbon monoxide in the dried exhaust gas after the neutral gas, argon, has been discounted. The ratio of CO2/CH4 in the inlet flow was 1/1. The data are presented in Table 5.

Figure 13

Arrhenius plot with experimental values listed in Table 4 and Eq. 13; EA=293 kJ/mol, k0=1.25×1010

Figure 14

Methane conversion as a function of gas exit temperature. Experimental results from experiments listed in Table 4 with a CO2/CH4 ratio of 1/1 is compared with calculations using the correlation developed by fitting the experimental data (1) and calculations using the correlation developed by Trommer (35) (2) and Dahl (36) (3).

Figure 15

% CO exiting the receiver as a function of gas exit temperature. The measured mol % of CO in the exhaust gas is plotted together with calculated CO generation due to the rWGSR (Eq. 21) with EA and k0 values from Refs. 39 (1) and (41) (2). The estimated mol % value of CO from the rWGSR using the data for methane consumption and hydrogen generation is seen in (3).

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