Study of Hydrogen Production Process Using Low Current, Non-Thermal Plasma Reformation of Hydrocarbons

Abstract

This thesis focuses on the design, development and processes involved for the reformation of hydrocarbons. The method is used for reformation of hydrocarbon in an atmospheric low current, non-thermal plasma system popularly known as ‘Plasmatron’. This gives advantage over the conventional catalyst reforming process in terms of compactness, reduced soot formation, weight and cost. This process can be used for in-situ production of hydrogen for compact applications like automobiles, fuel cell etc. Initial part of the thesis focuses on the entire design and development of the plasmatron system in terms of its chamber, fuel injection system, electrode geometry and power supply. The power system is a pulsed source with a pulse repetition frequency of about 30 kHz. Three electrode materials viz., stainless steel, copper and nickel are used. Here the plasma discharge is generated between the electrodes by giving sufficient amount of power. Through this discharge, different hydrocarbon fuels (liquid as well as gas) are passed through to initiate and enable the reformation process. This entire process occurs at atmospheric pressure. The breakdown characteristics of the developed plasma system have been understood in detail and discussed in chapter 3 of this thesis. The application of non-thermal plasma for fuel conversion and hydrogen production is very effective, because here plasma is not used as a source of energy but as a non-equilibrium generator of radicals, charged and excited particles. These active species can lead to long-chain reactions of fuel conversion and hydrogen production. The energy required for fuel conversion and hydrogen production can be provided mostly by chemical energy of reagents and low-temperature heat in non-thermal plasma. The plasma generated active species just stimulate this process and contribute only a very small fraction of the total process energy. Here as the plasma is in nonequilibrium and high pressure stage, it is difficult to maintain a glow discharge at higher pressure.The idea of using pulsed power supply to avoid the transition to spark regime is also discussed. Attempt has been made to calculate theoretically the ionization rate and electron temperature of the plasma generated here. This has also been experimentally compared using Langmuir probe diagnostic. Discharge voltage has been studied with respect to the input power, voltage and output discharge current for different combination of electrode material namely steel, copper, nickel and combinations of these. A current hysteresis has been observed in case of nickel electrodes and which can be attributed to the magnetic properties of nickel. Third objective of this thesis is to study the non-linearity in the system due to the atmospheric pressure plasma and discussed in chapter 4 of this thesis. An analysis of the random floating potential (current in the ion saturation regime) signals achieved from a Langmuir probe is done with the help of various mathematical tools. Empirical Mode Decomposition (EMD) along with modified Hilbert transform techniques has been introduced to understand the whole phenomena. This method successfully eliminates the noise and also brings out the relevant frequency modes out of the signals. All the floating potential signals achieved from electrodes of different materials have been studied using this method. The effectiveness of this method has been demonstrated by comparing the analysis results with that of the Fast Fourier Transforms and other techniques. Lastly it discusses the surface profiling of the electrode used in the system at various conditions using electron microscopy and image processing techniques. This is important as the life and quality of electrode does impact the final overall efficiency of the system. Comparative studies of electrode surfaces have been done with respect to the raw surface, plasma exposed surface and also with respect to different fuels reformed surface. This has been done for all the three electrode materials as well as for fuels like Liquefied Petroleum Gas (LPG) and Compressed Natural Gas (CNG). Elemental surface study of the electrode surface image has helped in understanding the surface deterioration/erosion and carbon depositions. Lastly it concludes with scope for future work in these kinds of systems. Interesting ideas in terms of the system design like the use of magnetic field has been mentioned.