Microwave-Specific Effects on the Equilibrium Constants and Thermodynamics of the Steam–Carbon and Related Reactions

Abstract

The steam−carbon reaction, which is the essential reaction of the gasification processes of carbon-based feed stocks (e.g., coal and biomass), produces synthesis gas (H 2 + CO), a synthetically flexible, environmentally benign energy source. The reaction is very endothermic, which mandates high temperatures and a large expenditure of energy to drive the reaction. We have found that using microwave irradiation to selectively heat the carbon leads to dramatically different observed thermodynamics for the reaction. From measurement of the equilibrium constants as a function of temperature, the enthalpy of the reaction under microwave radiation was found to become significantly more exothermic, dropping from 144.2 kJ/mol at the median reaction temperature of 880 K to 15.2 kJ/mol under microwave irradiation. The reaction conditions under which the steam− carbon reaction was run, and under which the equilibrium measurements were determined, consisted of three other reactions that came to equilibrium. These reactions were the Boudouard reaction, which is the reaction of CO 2 with carbon to form CO; the water−gas shift reaction, where CO and water react to form H 2 and CO 2 ; and the carbon−hydrogen reaction, which generates methane from the reaction of H 2 with carbon. We determined the equilibrium constants and thermodynamic parameters for all of these reactions. The Boudouard reaction, which is also strongly endothermic, was found to be more exothermic under microwave radiation (180.2 kJ/mol (thermal) and 27.0 kJ/mol (MW)). The water−gas shift reaction became more endothermic (−36.0 kJ/mol (thermal) and −11.4 kJ/mol (MW)). The carbon−hydrogen reaction also underwent an endothermic shift, from −79.7 to −9.1 kJ/mol. From the associated equilibrium expressions and the equilibrium constants for the steam−carbon reaction system, the mole fractions of the system components under thermal and microwave conditions were estimated. The effect of the microwave radiation was to change the position of the equilibrium so that the temperature at which H 2 was at a maximum dropped from 643 °C in the conventional thermal reaction to 213 °C in the microwave. Notwithstanding the predicted temperature shift, there was an observable threshold below which microwaves could not produce products. In our system, the minimum energy at which H 2 appeared was 373 °C (30 W), while the temperature at which equilibrium could be established in a reasonable period of time (100 min) was 491 °C (100 W).

H2-reduction Behavior of FeS-CaO Mixture during Microwave Heating

“…Microwave irradiation enhances the rate of interfacial chemical reaction where the gas diffusion acts as dominant resistance for the reduction reaction from the onset of microwave irradiation. This is likely attributed to the non-thermal (catalytic) effect of the microwaves on speeding up the chemical reactions, as reported previously [8][9][10]19] . On the other hand, such behavior is likely related to the different interaction between particles and microwaves.…”

“…In the present study, the chemical reaction is likely progressed rapidly during microwave irradiation owing to the extraordinary effects of microwave irradiation (thermal and nonthermal) on speeding up the chemical reactions [8][9][10]19] and the overall reaction is controlled by gas diffusion after the onset of microwave irradiation.…”

“…(6) Interfacial chemical reaction (ICR): (7) Mass transfer (MT): (8) Mixed GD, ICR, and MT: (9) Where X (dimensionless), K (dimensionless), De (cm 2 .s -1 ), and kf (cm.s -1 ) are the reduction degree, the equilibrium constant for reduction reaction, the effective diffusion coefficient of gas in micro pores, and the mass transfer coefficient in the gas film, respectively. CAb and CAe (mol.cm -3 ) are the concentration of reducing gas in bulk and at the equilibrium state, respectively.…”

“…In addition, microwave irradiation is an energy-efficient and a rapid-heating method to decrease activation energy of chemical reactions owing to both thermal and non-thermal effects of microwave photons. The non-thermal effect of microwave irradiation on the rate of chemical reactions, has attracted the attention of researchers on microwave irradiation energy in terms of speeding up chemical reactions [8][9][10] . The thermal effect of microwave irradiation has the potential to mitigate CO2 emission and decrease the amount of carbonaceous materials required f or carbothermic reduction of iron resources due to its specific characteristics, such as rapid and selective heating, volumetric heating, and highefficiency heating.…”

Physical Insight to Microwave Special Effects: Nonequilibrium Local Heating and Acceleration of Electron Transfer

“…Microwave special effects have been widely reported for hydrodesulfurization of thiophene (exothermic) and decomposition of hydrogen sulfide (endothermic) on MoS2/Al2O3 3) , enhanced reactions of steam-carbon and water-gas shift reactions 4) , and enhanced reduction of copper oxide under microwave magnetic-field irradiation by coupling of the H-field with the Fermi level electrons of CuO, resulting in cleavage of the Cu _ O bond 5), 6) . Two aspects of microwave special effects were recently discovered supported by carefully designed experiments.…”