Microwave-induced Catalytic Decomposition And Reforming Of Methane For Hydrogen Production By Carbon Materials

As a series of intensified greenhouse effect such as the global climate warming, the polar ice and snow melting, etc, the development of new clean energy source and low carbon economy has become the consensus of all countries in the world. Since2000, CO2emission have gone through a sharp increase in China along with the high-speed development of economy and society. For the responsibility of the international community, we must take into account the protection of the ecological environment in the economic development of our country, especially to reduce CO2emissions. Hydrogen, as a clean fuel, is considered to be an ideal energy in the future, because its combustion product is only H2O with no pollution to the environment. In the known methods of hydrogen production, hydrogen production of fossil fuels is relatively mature and widely used. Because CH4has high hydrogen to carbon ratio, so the hydrogen production with natural gas (main component is CH4) is the most economic and reasonable method. Since the natural gas has the lowest carbon emissions in all fossil energy, it is an important strategy to improve the consumption ratio of natural gas in our country’s energy sector. Based on the above analysis, it is very realistic to carry out the technology research of hydrogen production with CH4in our country at the present stage.In this paper, the effects of carbon-based catalysts for methane cracking and reforming for hydrogen production with a multi-mode microwave were studied. Firstly, the catalytic effects of the activated carbon (AC) and the pyrolysis residue of sewage sludge (PRSS) for CH4cracking were studied. The experiments were conducted in microwave oven and electric furnace, respectively. The mechanisms of the decomposition reaction were also studied by FTIR, XRD, SEM, EDS and BET analyzer. Secondly, the mechanisms of the gasification of the AC by CO2were also conducted. The mechanisms of the gasification process were studied by FTIR, XRD, SEM, EDS and BET analyzer. Finally, the catalytic effects of AC for CO2reforming CH4, and the mechanisms of reforming reaction were also studied by FTIR, XRD, SEM, EDS and BET analyzer. Through the work of this paper, the following main conclusions and results were obtained:(1) The results of the CH4cracking experiments indicated that the catalytic effects of the AC and PRSS on the CH4conversion show the same trend under the different experimental condistions. The higher temperature, resulted in the higher CH4conversion. When the microwave power raised from40%TOP (total output power) to70%TOP, the initial CH4conversion over AC and PRSS raised from20.6%and63.2%to79.6%and63.2%, respectively; The smaller the space velocity, the more beneficial to the improvement of the CH4conversion. When the space velocity was0.1L/(g-h), the initial CH4conversion over AC reaches100%. When the space velocity was0.3L/(g-h), the initial CH4conversion over PRSS reaches99%. The CH4conversion decreases rapidly with time, and then gradually stabilized.(2) The results of the sustainability experiments of CH4cracking indicated that the reasons of the better catalytic effect of PRSS is not the physical and chemical properties of the catalyst itself, but the PRSS is easier to produce "hot spot effect" under the condition of microwave radiation.(3) The results of the CO2gasification experiments indicated that the catalytic effects of the fresh AC and the deactivated AC on CO2conversion show the same trend under the different experimental condistions. The higher temperature resulted in the higher CO2conversion. When the microwave power raised from40%TOP to70%TOP, the initial CO2conversion over fresh AC and deactivated AC raised from48.7%and40.3%to86.8%and80.8%, respectively. The smaller the space velocity, the more beneficial to the improvement of the CO2conversion. When the space velocity reduced from0.7L/(g-h) to0.1L/(g·h), the initial conversion of CO2raised from51.1%to80.8%. The CO2conversion increases gradually with time. Under the same microwave power, CO2conversion over the fresh AC is significantly greater than that over the deactivated AC;(4) The results of the CH4reforming experiments indicated that the effects of experimental conditions on the CH4and CO2conversion show the same trend. The higher temperature, the smaller the space velocity and the higher CO2/CH4mole ratio, resulted in the higher CH4and CO2conversion. And the higher temperature, the smaller the space velocity and the smaller CO2/CH4mole ratio, resulted in the higher H2/CO ratio. When the microwave power raised from40%TOP to90%TOP, initial conversion of CH4and CO2raised from82.0%and49.1%to94.5%and49.1%, respectively, and the H2/CO ratio of initial value raised from0.51to2.49. When the space velocity reduced from0.7L/(g-h) to0.1L/(g-h), the initial conversion of CH4and CO2raised from84.5%and81.3%to97.0%and81.3%, respectively, and the H2/CO ratio of initial value raised from1.05to2.37. When CO2/CH4mole ratio increased from0.5/1to2/1, CH4and CO2initial conversion raised from81.8%and89.8%to95.3%and89.8%, respectively, and the H2/CO ratio of initial value droped from2.76to0.89.(5) The results of BET indicated that carbon deposition blocked the pore channel of the AC and PRSS in the CH4cracking reaction, especially the micro-pore’s blockage which resulted in the significantly reduction of the surface area and pore volume. The surface area and pore volume of fresh AC is greater than those of the PRSS. After CO2gasification reaction, the total surface area and pore volume of deactivated AC had more recovery, and the pore size of AC was destroyed in the gasification process and some new pore was formed. There was still some deposition carbon that was deposited on surface of AC in the CH4reforming reaction. Because of the existence of CO2gasification, reaction, the reduction extents of the specific surface area and pore size of AC were less than that in the CH4cracking reaction.(6) The results of FTIR indicated that the catalytic activity of surface functional groups of the AC was more beneficial for the CH4cracking reaction than PRSS. In the variety of functional groups, only the oxygen-containing functional groups had catalytic activity. After CO2gasification reaction, the oxygen-containing functional groups of the deactivated AC samples did not obtain certain recovery. The oxygen-containing functional groups on the surface of the AC also showed good activity for CH4reforming reaction.(7) The results of XRD indicated that the deposited carbon generated form CH4cracking process is graphite like carbon. The CO2gasification process consumed part of the deposited carbon. The deposited carbon generated from CH4reforming process is also graphite like carbon.(8) The results of SEM-EDS indicated that the original surface of both AC and PRSS are predominantly black. After the CH4cracking process, the number of particles with bright color on their surface increased. Only the surface of the deactivated PRSS under high microwave power appeared many bright color balls. EDS results indicated that the main elements of the bright color balls are Al, Si and O. Therefore, the balls may be mainly composed of Al2O3,SiO2or aluminum silicate. After CO2gasification, the particles of the AC surface became smaller, with obvious cracks on the surface. This may be due to the reaction between CO2and AC. In the CH4reforming reaction, part of deposited carbon was deposited on the surface of AC, and the bright color ball is different from those appeared on the surface of deactivated PRSS after CH4cracking reaction.