Methane Catalytic Pyrolysis by Microwave and Thermal Heating over Carbon Nanotube-Supported Catalysts: Productivity, Kinetics, and Energy Efficiency

Abstract

Methane catalytic pyrolysis, which is the reaction to produce hydrogen and carbon without emitting CO2, represents an approach for decarbonization using natural gas as an energy resource. The endothermic pyrolysis reaction was carried out under two heating scenarios: convective thermal heating and microwave-driven irradiative heating. The pyrolysis reaction was conducted at 550–600 °C over carbon nanotube (CNT)-supported Ni–Pd and Ni–Cu catalysts. On both catalysts, an enhanced methane conversion rate was observed under microwave irradiation. The enhanced catalytic activity was hypothetically caused by the presence of free electrons in the carbon atoms within the CNT that enabled the CNT support to absorb microwave energy effectively and to be heated efficiently by microwave. The microwave catalytic pyrolysis has shown improvement in kinetics, where the apparent activation energy dropped from 45.5 kJ/mol under conventional convective heating to 24.8 kJ/mol under microwave irradiation. When the methane conversion rate is increased by 37%, the microwave power consumption only changed by 10.8%. The research demonstrated the potential of transforming natural gas into clean hydrogen and value-added carbon in a more energy-efficient way. Process simulation and techno-economic analysis showed that potentially hydrogen minimum selling price of about $1/kg H2 could be achieved.

Improved Efficiency of the Microwave-Enhanced Catalytic Pyrolysis of Methane through Supplemental Thermal Heating

“…The microwave reactor directly heats the catalyst via joule heating of the CNT support and dielectric heating of the metal nanoparticles. This heating mechanism has been shown previously to improve the catalytic action compared to conventional heating by increasing the active site temperature while keeping the gas phase temperatures low . As seen in Figure , the first-pass conversion of methane is approximately 50% at 550 and 600 °C, with a significantly lower conversion rate at 650 °C.…”

“…The hotspot formations cause the temperature-dependent dielectric properties of the catalyst material and the presence of focused microwave irradiation. As the catalyst material is heated, the material’s ability to absorb microwave irradiation changes, typically increasing with increasing temperatures . It can also be seen in Figure that the MW-heated catalyst temperature distribution becomes more significant with increasing setpoint temperatures.…”

“…As the catalyst material is heated, the material's ability to absorb microwave irradiation changes, typically increasing with increasing temperatures. 20 It can also be seen in Figure 4 that the MW-heated catalyst temperature distribution becomes more significant with increasing setpoint temperatures. This was attributed to increased heat losses from the catalyst bed, resulting in higher excess cavity power.…”

“…This heating mechanism has been shown previously to improve the catalytic action compared to conventional heating by increasing the active site temperature while keeping the gas phase temperatures low. 20 As seen in Figure 2, the first-pass conversion of methane is approximately 50% at 550 and 600 °C, with a significantly lower conversion rate at 650 °C. As the reaction temperature increases in microwave heating, the rate of deactivation increases, likely due to sintering or Ostwald ripening the metal nanoparticles into larger and less active particles.…”

“…These advantages help perform methane decomposition as the methane and hydrogen gas phase does not appreciably absorb microwaves in their normal state. Previously, our group studied the effects of microwave heating on MCP and compared it to conventional thermal heating over the Ni–Pd/CNT catalyst . The microwave heating mechanism of the catalyst studied combines the joule heating of the CNT support and dielectric heating of the metal nanoparticles. , The presence of free electrons in the carbon atoms within the CNT enables the CNT support to absorb microwave irradiation effectively and turn it into heat energy.…”