Catalytic Combustion And The Enhancement Mechanism Of Hydrocarbon And Oxygenated Fuels In Micro Tube

Micro power systems based on combustion of hydrocarbon fuels and oxygenated alternative fuels, such as methane, methanol, ethanol, and dimethyl ether, are energy-dense and environmentally friendly. Therefore, micro combustion of these fuels is considered as the next-generation power source for potable devices. However, the stabilization and efficiency of their combustion in micro scale are still challenged. It is required to obtain the combustion characteristics, understand the correlation mechanism between physico-chemical properties of different fuels and their catalytic combustion characteristics, and propose more effective methods to enhance the micro combustion of these fuels. Hence, experimental and numerical studies on combustion of hydrogen, methane, methanol, ethanol, and dimethyl ether have been conducted in the micro tube in this paper.Specifically, the composite effects of catalyst spatial density and residence time scale on catalytic combustion of methane were investigated; combustion of methane, ethanol, and DME on Pt/ZSM-5 was compared to obtain the correlation mechanism between their physico-chemical properties and their catalytic combustion characteristics, and to understand the mechanisms of catalyst deactivation for different fuels; two kinetic models for DME oxidation on Pt/ZSM-5 catalyst at low temperature were established; the enhancement and its mechanism of the converging-diverging tube were revealed.Concentrating the catalysts enhanced the active sites density and shortened the residence time in the catalytic zone. The packed bed with the length of 20 mm presented a wider stability range for lean combustion and a higher conversion rate compared to the beds with lengths of 40 and 10 mm, because it achieved a good balance between the effects of catalyst special density and residence time.Methanol presented the widest stability range near the lean stability limit followed by ethanol and methane, because the existence of OH weakens the adjacent C-H bonds. As the flow rate increase, the conversion rate of methane increased; the conversion rate of ethanol decreased; the conversion rate of methanol presented an increase-decrease trend. Although the conversion rate of ethanol was higher than that of methanol at stoichiometric ratio, the energy release efficiency of ethanol was lower due to the lower CO2 selectivity.DME was more reactive than ethanol owing to the existence of C-O-C. The CO2 yield and wall temperature for combustion of DME were higher than those for ethanol. The CO2 yield of ethanol dropped rapidly with the growth of Qin when Qin exceeded 10 W, but the significant drop in the CO2 yield of DME did not occur until Qm exceeded 25 W. The differences in combustion characteristics were explained from the aspects of reaction pathways, the bond energies in molecules, and combustion heat.After 48 h of continuous combustion, CO2 yields of DME and ethanol decreased by 18% and 26%, respectively, attributed to the agglomeration of Pt, the formation of carbonaceous deposits on the catalyst, and the oxidation of Pt. Transmission electron microscopy images showed considerable Pt agglomeration after ethanol combustion. Temperature-programmed oxidation results indicated two types of carbonaceous deposits with oxidation temperatures of 498 and 618 K after DME combustion, but only the former type was detected after ethanol combustion. X-ray photoelectron spectra revealed that the mainly increased surface carbonaceous species were C-C/C-H and O-C=O after the combustion of ethanol and DME, respectively.1.23% of Pt was oxidized from Pt0 to Pt2+ and Pt4+ after ethanol combustion.2.66% of Pt was oxidized to Pt4+ from Pt0 and Pr2+ after DME combustion.Power rate law model and Langmuir-Hinshelwood model were established to predict the combustion rate of DME on Pt/ZSM-5 at low temperature. Reaction orders for DME and O2 were 0.28 and 2.30, respectively. Activation energies for the two models were 99.35 and 109.30 kJ/mol. The reaction orders and adsorption constants suggested DME was more strongly absorbed on Pt/ZSM-5 than O2 at low temperatures.Compared to earlier reports on H2 combustion in 2-mm diameter tubes, the stable combustion range was comparatively larger in the converging-diverging tube. When the equivalence ratio varied from 0.6 to 2.2, the inlet velocity that ensured stable combustion ranged between 3.4 m/s and 41.4 m/s. The converging-diverging structure enhanced the heat exchange between the wall and mixture. It led to a sharp change in velocity, particularly near the wall. The region near the wall with a relatively high temperature and low velocity in the diverging section provided more favorable conditions to maintain the flame stability.