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Decomposition of Clathrate Hydrates by High Frequency in-Liquid Plasma

Authors
Keywords
  • Hydrogen Production
  • In-Liquid Plasma
  • Clathrate Hydrates

Abstract

In recent years, 81% of total energy consumption is fossil fuel. Transportation, power generation, agriculture and industry activity is driven by oil and petroleum as a fossil fuel. Nevertheless, Petroleum prices are rising due to oil crisis in 1973, the Gulf War in 1991, and depletion of petroleum reserves. In addition, the environmental issues such as global warming, the Kyoto Protocol, and the emission of greenhouse gases are also a consideration to look for alternative energy fuel. Today, natural gas that is the cleanest burning of all the fuel has been a major commodity of the energy market. Natural gas plays an important role as an energy supply to electricity generation through gas turbine and alternative automotive fuel. In the chemical industry, natural gas is also main feedstocks for the production of ammonia, methanol, and dimethyl ether (DME). Therefore, Natural gas consumption increased from 2,089 billion cubic meters in 1990 to about 3,321 billion cubic meters in 2010. Natural gas is commercially produced from oil fields and natural gas fields, and methane as main content of natural gas bounds in hydrates for future energy needs. Unfortunately, methane emission as a greenhouse gas is more effective to contribute to global warming than CO2 emissions with a high global warming potential (GWP) of 21–25 times more than CO2. Hydrogen is largest application in chemical industry nowadays. Hydrogen has also a great potential as alternative energy. As carbon-free energy, it can replace fuel oil in the future, where 4 - 75% hydrogen in air or mixture of hydrogen-oxygen could be ignited with small energy. The other advantage of using hydrogen as a fuel gas is to reduce harmful pollution, to reduce carbon dioxide (CO2) and methane emissions into atmosphere, and to minimize dependence on fuel oil usage. Methane as the main component of natural gas primarily produces hydrogen using well-known commercial thermal processes. Steam methane reforming is currently favored to produce hydrogen because hydrogen can be produced 70 - 75% dry mass of SMR with byproducts such as 7 – 10% of carbon monoxide, 2 – 6% of carbon dioxide, and 2 – 6% of the unconverted methane. Reaction of this process is endothermic reaction in which heat must be injected into the process during the reaction. Therefore, this process is conventional performed at high pressure and high temperature (700 – 1000 OC) with using catalyst and is followed water-gas shift reaction with the reaction carbon monoxide and steam to produce carbon dioxide and hydrogen. The similar process with steam methane reforming is partial oxidation of methane in a single step reaction in which methane is catalytically reacted with a mixture of steam and oxygen. Partial oxidation with a slightly exothermic reaction, which requires less external heating, only produces the dominant CO as a byproduct. Unfortunately, the process for separating the O2 reactant from the air with high operational costs has made it uncompetitive with SMR. Carbon dioxide, which is resulted as a byproduct from both processes, should be mitigated with further process for environment protection. The mitigation of carbon dioxide emissions from hydrocarbon decomposition for hydrogen production can be engineered by application of methane cracking technology and sequestering carbon dioxide away from the atmosphere. Another reaction to produce hydrogen from methane is methane cracking at high temperature above 700 oC and allows at atmospheric pressure. Water gas shift and CO2 purification in steam methane reforming are not required in this process because it only produces clean carbon as a byproduct of the use of metallurgical industries. The high reactivity of the plasma with the content of reactive radicals, ions and highly energetic electrons is suitable for reforming hydrocarbons to produce hydrogen because the plasma can enhance the chemical reaction rates without a catalyst. The overall reactions in plasma are the same as conventional reforming with the possibility of reducing hydrogen production cost. Gliding arc plasma, dielectric barrier discharge (DBD), microwave plasma/radio frequency plasma, and corona discharge have been applied to facilitate steam methane reforming, partial oxidation and methane cracking usage. Since plasma can be created in the bubble in liquid by irradiation with microwaves and high frequency radio waves. The use of high frequency wave consists of 2.45 GHz microwave and radio frequency wave to generate plasma in liquid is called the in-liquid plasma method. The in-liquid plasma method has been applied to produce hydrogen from waste oil or other such organic liquid, where hydrogen gas and carbon are generated to a purity of 66 to 81%. Meanwhile, the decomposition of n-dodecane (C12H26) by the radio frequency plasma to generate hydrogen also simultaneously produced carbon black. Moreover, approximately 0.7 – 11 % of oxygen-hydrogen ratio was produced by the generation of the radio-frequency plasma in water at atmospheric pressure. The in-liquid plasma method in water was generated by 27.12 MHz radio frequency power source at pressures in the range from 0.1 to 0.40 MPa. Excitation temperature of the plasma slightly decreased from approximately 3700 to 3200 K, and the rotational temperature of OH is increased from approximately 3500 to 5000 K. Likewise, The electron density increases from approximately 3.5 x 1020 to 5.8 x 1021 m-3. In addition, the electron density for 2.45 GHz microwave plasma has no major difference with radio frequency plasma, The electron temperatures for 2.45 GHz microwave plasma are less approximately 1,000K than for 27.12 MHz radio frequency because irradiation of the 2.45 GHz microwave energy absorbed by water to raise the water temperature prior to the generation of plasma. On the other hand, clathrate hydrates, crystalline compounds formed at a suitable pressure and temperature conditions, have a huge potential as an untapped energy source. Therefore, hydrate formation and hydrate decomposition methods are being developed for future energy needs. Hydrate decomposition by depressurization and thermal stimulation is a possible method for economic production of gas. One of thermal stimulation techniques is to introduce microwave (MW) field irradiation. Hydrate technology also has the potential for use in gas storage and gas transportation even at atmospheric pressure. The hydrate-forming molecules are not only those of gases such as methane, CO2 but also that of cyclopentane, a liquid hydrocarbon that can form a hydrate at atmospheric pressure. Therefore, the purpose of this dissertation is to collect gas produced from high frequency plasma decomposition of clathrate hydrates at atmospheric pressure and the gas content identified by gas chromatography. An ordinary microwave (MW) oven is used as the source of 2.45 GHz MW radiation at atmospheric pressure. The plasma decomposition of the hydrates could pave the way for a new utilization of atmospheric pressure plasma. Cyclopentane (CP) hydrate formed at atmospheric pressure was decomposed by plasma in a MW oven generating gas with a content of 65% hydrogen, 12% CO, and 8% CO2. About 7% of the MW input power was consumed to decompose the hydrates. Meanwhile, methane hydrate, formed by injecting methane into 100 grams of shaved ice at a pressure of 7 MPa and reactor temperature of 0 oC, was decomposed by applying 27.12 MHz radio frequency plasma in order to produce hydrogen. The process involved the stimulation of plasma in the methane hydrate with a variable input power at atmospheric pressure. It was observed that production of hydrogen is optimal at a slow rate of CH4 release from the methane hydrate, as analyzed by in light of the steam methane reforming (SMR) and the methane cracking reaction (MCR) processes in accordance with the content of gas production. In comparison with the steam methane reforming (SMR), it was found that methane-cracking reaction (MCR) was dominant in conversion of CH4 into hydrogen. An H2 content of 55% in gas production was obtained from conversion of 40% of CH4 at an input power of 150 W. The results clearly show that hydrogen can be directly produced from methane hydrate by the in-liquid plasma method. Therefore, clathrate hydrates as gas storage and transportation at atmospheric pressure have been a foreseeable source of hydrogen and could contribute hydrogen production from the aspect of practical use.

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