Density Functional Theory And Experimental Studies On The Reaction Mechanisms Of Thermal Plasma Pyrolysis

Due to its unique advantages of high temperature, high density of enthalpy and active species, thermal plasma has been to be an effective and clean way to convert various raw materials, such as coal, biomass, heavy oil and liquefied petroleum gas (LPG) into acetylene, hydrogen, carbon monoxide and other light hydrocarbons.Acetylene is one of the most important and basic raw materials in chemical industry. The most common way to produce acetylene at present is based on a reaction between calcium carbide and water. However, this reaction consumes high energy and is environmentally unfriendly. Simple and efficient approaches for acetylene production are desired. It has been proved by many researches that thermal plasma pyrolysis is a promising technology for acetylene production, and there have been some pilot tests on this new technology reported. However, the pyrolysis mechanism in plasma is still unclear yet because of the special reaction conditions (super high temperature and very short residence time) in plasma reactor.As a quantum chemical method, density functional theory (DFT) can be used to build reaction networks and simulate the complex reaction processes in gas/liquid/solid phase. Enthalpies and activation energies for reactions can be calculated and then analyzed. Thus the most possible reaction pathways can be gained, and the possible products and distribution can be predicted. Based on the previous researches on the acetylene production via thermal plasma, in this work we chose LPG, coal and biomass as raw materials, and did a study on the pyrolysis mechanisms of these raw materials in hydrogen plasma using density functional theory. Besides, we did some experiments on small experimental set-up to verify our theoretical study. Our work includes:(1) C3H8and C4H10, the main component of LPG, were chosen as the model compounds to investigate the reaction mechanism of LPG pyrolysis in hydrogen plasma. Over60possible reactions have been considered. And after DFT analysis we brought up a main reaction pathway. According to our prediction, the main products of LPG pyrolysis in hydrogen plasma would be C2H2, H2, CH4and some other light hydrocarbons including C2H4, and C2H6. C2H2mainly comes from the pyrolysis of C2H4and C3H6, so its percentage in product will increase as the reaction temperature rises. And extra hydrogen gas is produced through hydrogen abstraction reactions.(2) Coal pyrolysis in hydrogen plasma proceeds through two steps. Firstly volatiles such as aliphatic hydrocarbons, aromatic hydrocarbons and cycloalkanes vaporize from the coal, and then volatiles decompose into acetylene, hydrogen and other chemicals. Based on this two-step mechanism, CH4, c-C6H12and C6H6were chosen as the model compound of aliphatic hydrocarbons, cycloalkanes and aromatic hydrocarbons in volatiles, respectively. Many possible reaction pathways have been calculated and analyzed. It can be concluded that the main products of coal pyrolysis in hydrogen plasma are hydrogen and acetylene. Hydrogen abstraction reactions are the main routes of hydrogen formation. CH4, c-C6H12and C6H6make their contribution to acetylene formation through the thermal coupling of methyl radicals, the decomposition of cyclohexyl radicals and the decomposition of phenyl radicals, respectively. Apparently, volatiles are a key factor during coal pyrolysis in thermal plasma since their decomposition leads to the formation of products.(3) Biomass is mainly composed of cellulose, hemicellulose and lignin. In order to investigate the reaction mechanism of biomass pyrolysis in hydrogen plasma,(3-D-glucopyranose and phenethyl phenyl ether (PPE) are chosen as the model compound of cellulose and lignin, respectively, and xylose, O-acetyl xylose and4-O-MeGlc are used to present hemicellulose because of the complicated chemical structure of hemicellulose. According to our calculation, the main products of biomass pyrolysis in hydrogen plasma would be CO, H2, C2H2and CO2. Cellulose, hemicellulose and lignin all contribute to the formation of syngas (CO+H2), and hemicellulose and lignin will give higher hydrocarbons yield, while CO2can only come from the primary pyrolysis of hemicellulose. Apparently, three components of biomass make their contribution to product formation and distribution through different ways, which means the composition of biomass has effect on its pyrolysis in hydrogen plasma.(4) We verify that the active hydrogen atoms (H·) in hydrogen plasma can participate in pyrolysis reactions though calculation, and this phenomenon have been found in previous experimental studies. Calculation indicates that H· can remarkably lower the energy needed for dehydrogenation process, which makes dehydrogenation proceed much more easily.(5) According to our calculation results, we chose C3H8-C4H10mixed gas, toluene and two biomasses (rice husk and corncob) as raw materials, and carried out a series of pyrolysis experiments in hydrogen plasma. Experimental results indicate that, the composition of C3H8-C4H10mixed gas has no effect on product distribution; when reaction temperature is too high, the dehydrogenation of tulolene is enhanced which will aggravate coking; the contents of lignin and hemicellulose have important effect on product composition. In a word, experimental results of thermal plasma pyrolysis are in good agreement with our prediction based on DFT study.