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

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 (H2 + 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 CO2 with carbon to form CO; the water−gas shift reaction, where CO and water react to form H2 and CO2; and the carbon−hydrogen reaction, which generates methane from the reaction of H2 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 H2 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 H2 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). ■ INTRODUCTION The reaction between superheated steam and carbon to produce synthesis gas (reaction 1) is part of the general category of gasification reactions used to obtain hydrogen from coal and other carbon-rich sources. Gasification reactions typically occur at temperatures ≥700 °C depending on the carbon source, while industrial processes, such as coal gasification, run at much higher temperatures (>1000 °C). These high temperatures are required to drive the endothermic components of the primary reactions and to obtain useful reaction velocities. + ⇌ + Δ = + H C H O CO H 131 kJ/mol 2 2 (1) + ⇌ + Δ = − H CO H O CO H 41 kJ/mol 2 2 2 (2) + ⇌ Δ = + H C CO 2CO 172 kJ/mol 2 (3) + ⇌ Δ = − H C 2H CH 75 kJ/mol 2 4 (4) Along with production of synthesis gas (reaction 1), the reactions between carbon and high-temperature steam consist of a complex set of equilibria, which produce not only hydrogen and carbon monoxide but also carbon dioxide through the water−gas shift (WGS) reaction (reaction 2), carbon monoxide through the disproportionation of carbon and carbon dioxide (Boudouard reaction 3), and methane through the reaction of carbon and hydrogen (reaction 4). The complexity of the equilibria, along with the primary steam−carbon reaction (reaction 1) being very endothermic, means that the composition of the gases produced in gasification will depend critically on the temperature and pressure of the reaction. Because of the industrial importance of these reactions in the production of hydrogen for direct use as a clean alternative fuel or for the production of hydrocarbons through the Fischer− Trøpsch process, the development of less energy-intensive methods for driving these reactions is desirable. Recently, we have shown that for the carbon−carbon dioxide (Boudouard) Received: February 3, 2014 Revised: March 24, 2014 Published: April 8, 2014 Article