(477d) Induction-Heated Molten Salt Reactor for Methane Pyrolysis

As the world shifts away from fossil sources as a chemical feedstock and fuel, much attention has been paid towards clean energy infrastructure based on the hydrogen economy. Hydrogen stands as one of the most promising green fuel and chemical substitutes for several reasons: 1) hydrogen has a high heat of combustion; 2) upon combustion, the primary byproduct is water and no carbon dioxide is produced; 3) hydrogen has a high energy density; and 4) hydrogen is an essential feedstock for many chemicals such as ammonia and plastics. Amongst the several clean energy concepts for hydrogen production that are being explored, methane pyrolysis has become a focal point for study because it produces hydrogen from a carbonaceous feedstock and the solid carbon byproduct is easy to sequester. Methane pyrolysis is also economically attractive, as it is a scalable thermochemical process and the solid carbon byproduct can potentially be utilized to further enhance its economics.

Methane pyrolysis is a challenging process to implement at scale because it is a highly endothermic reaction and requires exceptionally high temperatures on the order of 1000 °C and above. In addition, methane pyrolysis poses challenges when performed in the presence of solid catalyst materials, as carbon generated from the pyrolysis process can coat the catalyst surface and cause deactivation. Along these lines, molten salt reactors for methane pyrolysis have emerged to tackle this very issue. Carbon generated from pyrolysis in a molten salt reactor either dissolves into the liquid or stays at the gas-liquid interface, which prevents catalyst deactivation. The solid carbon can be retrieved after removing residual salt, and since the salts used are water soluble, the residual salt contamination can be removed by flushing, which improves carbon product quality. There is nonetheless the challenge of heating molten salt pyrolysis reactors using green energy sources, such as renewable electricity, to very high temperatures.

We present an inductively heated molten salt reactor that can heat chloride-based molten salts to temperatures above 1000 °C. In our reactor concept, alternative magnetic fields generated using a helical magnetic coil directly couple to the molten salts themselves, producing volumetric eddy currents within the molten salts that dissipate as heat. The concept takes advantage of the fact that molten salts are ionic liquids and are electrically conductive, and they can therefore serve as a volumetric heating susceptor. Our concept in wireless molten salt heating has multiple advantages compared to alternative concepts. First, it circumvents the need for immersion heaters, which run into challenges pertaining to chemical and temperature compatibility with the caustic, high temperature molten salt bath. Second, it can be tailored to supply heat to the molten salt bath in a volumetric fashion, eliminating heat transfer bottlenecks in the methane pyrolysis reaction. Third, it is scalable and can readily adapt to meter-scale reactors and megawatt power levels. Fourth, it is relatively safe to operate.

To experimentally realize our inductively heated molten salt reactor concept, we implement a 9 cm-diameter reactor system containing a 1:1 sodium chloride and potassium chloride mixture as the salt bed. Heating is performed using a combination of resistively heated wires and magnetic induction coils surrounding a molten salt reactor. The resistive heating jacket, made from kanthal wires, is used to initially heat the solid salt to a molten state, and it is required because solid salts are not electrically conductive. Once the salts reach a molten state, the resistive heater is turned off and the 23 cm-diameter, 30 cm-long helical copper induction coil is activated and continues to deliver power to the salt. The heater and coil are carefully designed to minimize parasitic induction heating of the resistive heater.

Co-design of the power electronics with the molten salt bed is essential to ensuring that the induction heating process is volumetric and efficient. Along these lines, we specify the induction frequency to be 2.5 MHz, from which the skin depth (i.e., length scale specifying magnetic field penetration into the molten salt) is approximately the reactor radius. AC impedance measurements of the molten salts are used to quantify the AC resistivity of the molten salts at different temperatures and confirm that our skin depth condition is maintained. Coupling efficiency, which is defined as power delivered to the molten salt divided by total power delivered to the molten salt, resistive heater and helical coil, is measured using large signal analysis to be 92% and is consistent with theoretical calculations. Finite element multiphysics simulation tools coupled with experimentally measured temperature profiling confirm that the inductively heated molten salt bed supports a volumetrically uniform heating profile.