Research On Directional Conversion Of Gaseous Products From Biomass Pyrolysis Into Syngas Production Under Microwave Irradiation

Biomass is an important energy resource for people's survival, and it is highly likely to be one of the important components of the future sustainable energy system. For a long time, biomass resources were mostly used for direct combustion, with a low level in utilization of biomass. In view of serious situation of energy supply and demand, converting biomass resources into alternative fuels in a large-scale is considered to be one of the effective measures to alleviate energy crisis.As one kind of thermochemical transform mode, pyrolysis can produce gas, liquid and solid products from biomass energy. However, it was undeniable that a certain amount of CO2, CH4, water vapor and tar fractions were still contained in gas products derived from biomass pyrolysis. The further conversion of these products into syngas production is of great importance for directional conversion of biomass into syngas, and can impose a positive impact on reducing greenhouse gases emission. Based on the characteristics of microwave heating labeled by immediacy, integrity, selectivity and high-efficiency, more importance has been attached to microwave pyrolysis of biomass for obtaining alternative energy. And numerous studies indicated that the content of syngas from microwave pyrolysis was always higher than that from conventional pyrolysis under the same conditions. Since catalytic reactions performed under microwave irradiation were characterized by lower reaction temperature, higher reactant conversions as well as more excellent product selectivity, it is essential to perform experimental study on CO2reforming of CH4and tar catalytic conversion by means of microwave heating. In addition, the combined reforming of CH4with CO2and H2O was also launched by microwave heating in order to investigate the effect of water vapor on the reforming of CH4with CO2. Char obtained from biomass pyrolysis was served as the catalyst in this paper, which was favorable for reducing the cost of metal catalysts involved in conventional reaction and improving comprehensive value of the products.Based on the above analysis, simulation study on CH4reforming with CO2and H2O was performed by software of Aspen Plus at first. And then reforming of CH4with CO2, combined reforming of CH4with CO2and H2O, as well as catalytic conversion of tar were conducted over biomass char, in order to explore reaction characteristics of directional conversion of gas products obtained from biomass pyrolysis into syngas production. Finally, an economic analysis on sysgas production from biomass by microwave conversion was reviewed.In order to opitimize the selection of operating parameters, simulation study on CH4reforming with CO2and H2O was performed by Aspen Plus software in this paper. The results revealed that CH4and CO2conversions were strengthened at higher temperatures. Increasing molar ratio of CO2and CH4enhanced CH4conversion, and simultaneously restricted CO2conversion. And CO2and CH4conversions were always higher at lower pressures than those at higher pressures. With the increase of temperature, the ratio of H2and CO decreased sharply up to a certain temperature, beyond which the ratio varied very slightly. At the same temperature, adding more CO2to CH4led to a decrease in the ratio of H2and CO. Moreover, higher pressures contributed to the lower ratio of H2and CO. In accordance with thermodynamic simulation, water production was lowered with the increase of temperature, at temperatures lower than973K. At temperatures above973K, the formation of water was restricted at a lower ratio of CO2and CH4(CO2/CH4≤1), and a slow increase in water production with increasing temperature was achieved when the CO2/CH4ratio was higher than1. For CO2/CH4ratio equal to0.5, carbon formation was inhibited with increasing temperature up to1173K, and afterwards a considerable and nearly constant amount of carbon was remained. As the CO2/CH4ratio was higher than1, moles of carbon reduced rapidly with the increase of temperature, and no carbon formation was achieved at a certain temperature. Furthermore, no carbon was produced at lower temperature with the increase of CO2/CH4ratio (≥1). In addition, higher pressures led to augment the quantity of water and carbon formation. With the increase of H2O/CH4ratio, the conversion of CH4decreased first and then increased within the lower temperature range (≤1073K), and CH4conversion was improved slightly at higher temperatures (≥1173K). And increasing H2O/CH4ratio reduced CO2conversion and restricted carbon formation. After a certain value of H2O/CH4ratio was reached, a further increase in H2O/CH4ratio can cause a gentle improvement in H2/CO ratio.On the basis of simulation study, reforming of CH4with CO2was carried out to study the effect of different char and operating parameters on the reforming reaction, and the following conclusions were reached. The char prepared by corn stalk exhibited better catalytic performance, owing to more favorable absorbing microwave properties, higher surface area and more metal elements contained in the ash. It was found that the catalytic activity of char obtained from materials by picking deash was more liable to be weakened. The char modified by Na2CO3and K2CO3improved the conversion of CO2, but inhibited the conversion of CH4. Compared with that of original char, catalytic activity of the char modified by MgO, CaO and NiO was improved. Biomass char with a size range between0.25and0.83mm was favorable for the reforming reaction. A promotion in the conversion of reactant gases into syngas production can be achieved through increasing microwave power, enhancing molar ratio of CO2and CH4or reducing volumetric hourly space velocity. Increasing microwave power led to an increase in the ratio of H2and CO, while enhancing molar ratio of CO2and CH4or reducing volumetric hourly space velocity resulted in a decrease in the ratio of H2and CO. During the reforming reaction, higher conversions of CO2and CH4were observed at the initial stage, followed by a continuous reduction in both CO2and CH4conversions at different degrees. During the process of90-minute reaction, the conversion of CO2and CH4decreased respectively by10.4%and23.8%at the end. Compared with the reforming reaction, a more evident drop in CH4conversion was observed in CH4decomposition reaction. The previous CO2conversion in CO2gasification reaction was higher than that in the reforming reaction, while it was lower than that in the reforming reaction at later period. The kinetic model which agreed with experimental datas in our work was determined, and activation energy and preexponential factor were calculated to be29172.2J/mol and2.37×10-4, signifying that microwave heating was favorable for the reforming of CH4with CO2via lowering activation energy.The combined reforming of CH4with CO2and H2O was launched by means of microwave heating, with the aim of searching after the influences of water vapor on CO2reforming of CH4. The results indicated that the conversions of CH4and CO2were enhanced when a certain amount of water vapor was introduced, with an increase in the ratio of H2and CO in outlet gases. Enhancing microwave power, increasing molar ratio of CO2and CH4or reducing volumetric hourly space velocity was demonstrated to be beneficial for the conversion of reactant gases, and changes in these parameters were more liable to affect CH4conversion. The ratio of H2and CO was improved with increasing microwave power, and a decrease in the ratio of H2and CO was reached by enhancing molar ratio of CO2and CH4or reducing volumetric hourly space velocity. Different from that in the reaction of CH4reforming with CO2, syngas content in the combined reforming reaction was lowered with the increase of molar ratio of CO2and CH4. Higher conversions of CH4and CO2were emerged at the initial stage, accompanied by a sustained decrease in CH4and CO2conversions. The decrease in both CH4and CO2conversions was separately marked by15.6%and12.7%when the90-minute reaction was terminated, along with the ratio of H2and CO reduced to0.88. CH4conversion in the combined reforming reaction was significantly higher than that in the reforming reaction, and the difference was enhanced gradually with heating time, up to12.4%after90minutes. CO2conversion in the combined reforming reaction was higher than that in the reforming reaction at the beginning, and then it was almost equal to that in the reforming reaction in the latter. The ratio of H2and CO obtained in the combined reforming reaction was0.93, which was higher than that achieved in the reforming reaction.Using toluene and benzene as model compounds of biomass tar, an experimental study on tar catalytic conversion was initiated over biomass char, assisted by microwave heating. The results revealed that the cracking conversions of toluene and benzene were strengthened with increasing microwave power, especially for toluene conversion. H2and CH4was the main component of gaseous products obtained from toluene cracking, and enhancing microwave power brought about an increase in H2content. The content of H2was reached up to91.3vol%at microwave power of640W, and no obvious change was found thereafter. Gaseous products produced by benzene cracking was very abundant in H2, amounted to96.0vol%, and the content of H2was almost unaffected by the variation of microwave power. Toluene and benzene conversions, together with H2yield, were repressed with the increase of volumetric hourly space velocity in cracking reaction. Moreover, enhancing volumetric hourly space velocity brought about a slight decrease in H2content in toluene cracking reaction, while it was difficult to influence the gas composition in benzene cracking reaction. With the increase of CO2introduced into the reactor, toluene and benzene conversions increased at first and then decreased, with the same trend in syngas content in outlet gases. The highest conversions of toluene and benzene can be achieved, using a molar ratio of CO2and carrier gas (φCO2/φcarrier gas) equal to0.4and0.1, respectively. Enhancing molar ratio of CO2and carrier gas resulted in a decrease in H2and CH4content, as well as an increase in CO content, thereby reducing the ratio of H2and CO. During the90-minute reaction, the conversion of toluene cracking reduced continuously until the end of reaction, and it was observable that toluene conversion in the reforming reaction decreased very rapidly as the reaction proceeded and leveled off after approximately40min. Toluene conversion in the cracking and reforming reaction decreased by47.2%and38.3%at the end, respectively. Syngas content in toluene cracking reaction ultimately reduced from86.0vol%to65.3vol%, while syngas content in toluene reforming reaction became relatively constant at94.0vol%. The initial conversion in benzene cracking reaction fell quickly and reached a stable level of42.5%after the reaction performed40minutes continuously. Although an obvious decreasing tendency in benzene conversion was notable in the reforming reaction, benzene conversion in the reforming reaction was always higher than that in the cracking reaction.Finally, an economic analysis on a pilot-scale device which was applied to convert biomass into syngas production under microwave heating was estimated, and a technical route on biomass recycling utilization was put forward. The results indicated that energy conversion efficiency of the system reached up to52.76%when power consumption was assumed to be2.3kW-h/(kg straw). And the cost of syngas production was evaluated to be2805$/ton, under the conditions that system handling capacity, syngas yield as well as the price for material acquisition were2ton/h,52.5wt%and300￥/ton, respectively. The most influencing factors on the cost of syngas production was the price for material acquisition and power consumption, followed by handling capacity and annual operating days of this system. Considering high additional value of syngas and char, a technical route for multi-production of biomass utilization was proposed on the basis of biomass conversion into syngas production under microwave irradiation.