| With the depletion of fossil fuels and environmental pollution caused by the combustion process,the development and utilization of new energy sources are urgent.Hydrogen energy,as a kind of new energy,has drawn more and more attention due to its advantages of high combustion calorific value and no environmental pollution during the combustion process.Microwave discharge plasma reforming of ethanol into hydrogen is a new type of hydrogen production technology developed in recent decades.Due to the high plasma density,large hydrogen production and mild reaction conditions,the technology has attracted wide attention from domestic and foreign scholars.However,in view of the difficult issues that complicated structure and operation for the direct-coupling microwave discharge in gas phase,meanwhile,the coaxial microwave discharge in liquid cannot stably maintain at high power,a reactor of direct coupling microwave discharge in liquid is designed and while a plasma can be ignited successfully by this reactor.The reactor which combines the advantages of both microwave discharge in gas and liquid can perform microwave high-power output to achieve large capacity of hydrogen production,meanwhile operation of the device is relatively simple.On this basis,the microwave matching characteristics,discharge characteristics,hydrogen production effect,the impact of the suspension electrode on hydrogen production and hydrogen production mechanism were studied respectively.The main conclusions are as follows:1.Matching characteristics of microwave discharge reactor:the network analyzer was used to investigate the influence of liquid properties and electrode position on Standing wave ratio,SWR.The experimental results show that the relative position of the electrode tip in the cavity and the ethanol volume fraction and water temperature have significant effects on SWR.Theoretically proves that it is necessary to re-adjust the transmission line matching to achieve the maximum efficiency energy output.2.Characteristics of microwave discharge in liquid:behavior of the plasma was observed by a high-speed video camera,the results show that the plasma generated by microwave discharge in liquid depends on the electrodes and bubbles,time scale of transient evolution for discharge is on the order of?10 0 s.At the same time,the rotational temperature of C2 and the line pattern of H atom of Balmer system?H? and H??in the plasma were studied by using emission spectroscopy.The results show that,the rotational temperature of C2 in plasma reaches the order of several kelvin;the temperature increases with the increase of microwave power and it rises first and decreases as the ethanol volume fraction increases and reach the maximum when the volume fraction of ethanol is 70%.The rotational temperature does not change significantly with the internal pressure of the reactor.The contour of H atomic emission spectra is close to Gaussian linear in theory,and the line widths of Ha and Hp spectra decrease in turn and accord with the law of Doppler broadening,which indicates that thermal effect of microwave discharge in liquid is significant.3.Characteristics of hydrogen production by microwave discharge in ethanol solution:optimized analysis of the reactor parameters and solution parameters found that there is the best ratio between ethanol and water,when the volume fraction of ethanol is about 70%,the hydrogen production and total gas production reach the maximum;In the mixed gas,hydrogen and carbon monoxide are the main the gas products,accounting for 81.5?84.7%of the total volume of the gas mixture.Increasing the microwave power can appropriately increase the percentage of hydrogen in the mixed gas.When the microwave power increases from 900 W to 1500 W,the volume percentage of hydrogen increases from 54.4%to 58.1%.Under the best reaction conditions,hydrogen production,hydrogen content and hydrogen production efficiency reached 13.53 NL/min,58.1%and 48.33 g?H2?/kWh,respectively.4.Suspension electrode to enhance hydrogen production:the addition of suspension electrode can increase the residence time of the bubbles in the plasma region,which increase the breakdown frequency of the bubbles and further increase the hydrogen production.The relative distance between the suspension electrode and the electrode tip and the size of the suspension electrode are both impact on bubbles escape and then effect on hydrogen production,and the type of suspension electrode has no significant effect on hydrogen production.5.Mechanism analysis of hydrogen production:the main active particles produced by microwave discharge in ethanol aqueous solution are · OH,·C2,·CH2,·CH,·C and ·H;the main products of the gas phase are hydrogen,methane,acetylene,carbon monoxide,ethylene and carbon dioxide.It is found that the corresponding spectral and mass spectral response values of the intermediate active particles and the gas phase products all significantly changed with the change of the ethanol volume fraction.According to the analysis of the correlation between the intermediate products and the products of the gas phase,it is concluded that the main hydrogen generation pathways are the dimerization reaction between ·H and the direct cracking of CH2O. |
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