Overcoming Barriers in Induction Heating for the Electrification of Chemical Reactors: Development of In-situ Power and Temperature Measurements, Modelling and Scale-up Analysis

: (Alternative Format Thesis)

Abstract

There is a clear and pressing need to move away from fossil fuels as the main source of heating for chemical conversion processes, an industry with associated carbon emissions of 0.65 Gte CO2 equivalent per annum. Renewable electricity is a promising heat source for replacing fossil fuels in the chemicals industry and there are a number of different electrical heating technologies, each at a different state of maturity. These include induction heating, in which electrically conductive or magnetic susceptor materials generate heat when placed in an alternating magnetic field.

Induction-heated chemical reactions have been demonstrated at the lab scale for a wide variety of chemical reactions and susceptor materials, which heat either by magnetic hysteresis or induced eddy currents. This thesis aims to develop tools and models that will help transform induction heating from a lab-scale technology to one that is viable for chemical manufacturing at an industrial scale.

Most temperature instruments are unsuitable for use in applied magnetic fields. Induced eddy currents can destroy instrument wires and measurement circuits and induction heating of the instrument tip gives inaccurate temperature measurement and can exceed the temperature limits of their construction materials. This thesis demonstrates an induction-tolerant thermocouple capable of measuring temperature up to 600 °C in an induction heated reactor at applied field strengths of 20 kA·m-1 at a frequency of 400 kHz.

Characterisating the heating performance of a bed of magnetic material is a key requirement for tuning and optimising the performance of the heating material in an industrial reactor. Heating power is a function of material properties, frequency, applied field strength and temperature. It also depends on the size distribution, shape and agglomeration of the magnetic particles. The heating material needs to generate sufficient power across a wide range of temperatures. In an endothermic reactor it needs to heat the reactor from ambient temperature to reaction conditions as well as supplying the reaction energy at the target reaction temperature.

A novel magnetometry method is developed for measuring magnetic hysteresis curves in-situ and in real-time. This is demonstrated on nano-powder samples of magnetite and maghemite up to kA·m-1 and 400 kHz up to the Curie temperature of each material. The data show good agreement with the Rayleigh law of hysteresis at low applied field strengths and exhibit the characteristic fall in magnetisation expected as the samples approach the Curie temperature, 585 °C for magnetite. These experiments are carried out using a pulse heating method to overcome any thermal lag effect between the temperature measured by the thermocouple and the temperature of the rapidly heating sample bed.

The author outlines a new model of magnetic hysteresis for major and minor hysteresis curves called the LangArc model. The parameters of the LangArc model are shown to relate to the key features of the major magnetic hysteresis curve and it reduces to Rayleigh’s law for lower applied field strengths. This model can be used for optimising the heating performance of the material. The author showcases an innovative method in which characterising the magnetic properties of a material as a function of temperature then allows for the instantaneous temperature of the sample to be determined from in-situ magnetometry measurements. This method has no thermal lag, which is critical for the safe control of induction heated systems, where the temperature can rise rapidly. Temperature rise rates of 30 °C·s-1 were determined using this technique.

This thesis contains a new method for measuring the heating power supplied to an induction heated reactor bed called reflected impedance. It uses the magnitude and phase of the current and voltage supplied to the induction heater work coil to characterise both the power and inductance of the heated bed. This can be used for both magnetic materials, which heat through hysteresis, and electrically conductive materials, which heat through eddy currents. Previous methods to measure the heating power in beds heated by eddy currents have been restricted to a heat balance across the reactor bed, a method that is subject to significant inaccuracy. The author demonstrates that both magnetometry measurements and the resonant frequency of the system can be used to validate the reflected impedance measurements under isothermal conditions, providing high confidence in these power measurement techniques.

These developments in modelling and instrumentation are applied to derive equations for the thermal efficiency of an induction heated chemical reactor as a function of the reactor size. Induction heating at lab-scale typically has an efficiency of less than 10% of the electrical energy supplied to the work coil converted to useful heat in the reactor bed. The remainder of this energy is lost as heat in the work coil. The novel efficiency model predicts that the efficiency of induction heated reactors increases with size and is applied to a case study of ethanol dehydration to ethylene over a zeolite catalyst. Real-world constraints are imposed on the reactor design, such as removing heat from the work coil and applying voltage limits to the resonant tank circuit used to generate the magnetic field. This analysis shows that the voltage rating placed a significant limit on the maximum efficiency.

For a maximum circuit voltage of 11 kV heated using 97 nm magnetite powder or 5mm radius of insulated, non-magnetic stainless steel balls, the case study yields an industrially viable reactor with 0.2 m radius and 0.8 m length; a heating material volume fraction of less than 8%; an applied field strength of 10 kA·m-1; and a resonant frequency of circa 8 kHz. The cacluated effiency is approximately 65% for both cases. This is comparable with the efficiency of using hydrogen derived by water electrolysis as a replacement to natural gas for a chemical process heating fuel. Above this reactor size, the resonant frequency falls due to the larger inductance of a bigger reactor, resulting in a rapid drop in energy efficiency. The author proposes that radiofrequency alternators, such as the Bethenod-Latour or Alexanderson alternators, are possible alternatives to resonant tank circuits. These would allow the reactor to be operated at frequencies in excess of the resonant frequency, further increasing the efficiency of these reactor systems.

The tools and models developed in this thesis allow for more detailed characterisation of heating materials and provide a theoretical basis for their optimisation in a flowing chemical reactor. It shows that magnetite or maghemite nano-powders are viable for induction heating chemical reactions. These techniques should be applied to a wider variety of magnetic and eddy current heating media, such as exchange spring magnets, to provide an optimised heating material that is stable in long term catalyst studies. Furthermore, the instruments developed in this thesis are vital for temperature and reaction control in industrial induction heated reactors.

Date of Award	13 Sept 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Alfred Hill (Supervisor), Simon Bending (Supervisor) & Asel Sartbaeva (Supervisor)