Catalytic upgrading of biomass pyrolysis volatiles to bio-fuel under pre-plasma enhanced catalysis (PPEC) system
Introduction
The increased energy demand and environmental issues with fossil fuels around the world especially for China (the world's largest greenhouse gas emitter) has motivated us to develop the friendly renewable energy. In recent years, attention has shifted to the waste lignocellulosic biomass, as one of the most important renewable source for liquid fuel and chemicals synthesis [1,2]. Biomass has been carbon neutral to fulfill the global reduction in CO2 emission, and it is abundant and readily available to meet the alternative fuel demands. The production of biomass derived bio-oils via pyrolysis of different biomass feedstock's has been extensively investigated, typically for woody biomass [3,4]. Furthermore, in order to obtain the high production of hydrocarbons from biomass, catalyst employed in the pyrolysis conversion process is mandatory. Catalysts such as zeolites (FCC, H-Y, H-β, HZSM-5, etc.) [[5], [6], [7], [8]], alkalines (Na2CO3, K2CO3, CaO, MgO, etc.) [[9], [10], [11]], and gasification catalysts (dolomite, nickel base, charcoal, etc.) [12,13] in biomass pyrolysis influence the products. Among them, HZSM-5 zeolite is efficient in lowering oxygen content, displaying a great capacity of deoxygenation and hydrocarbon production due to its special acidity and pore structure. For examples, Du et al. [6] employed three zeolites (H-Y, H-β and HZSM-5) to catalytic pyrolysis of biomass, and the results showed that HZSM-5 was the most effective in aromatic hydrocarbon production. Wang et al. [8] compared the catalytic performance of HZSM-5, H-β and USY and HZSM-5 obtained the highest deoxygenation efficiency. Recently, some new catalysis materials such as proton-form zeolites (FER, MOR) [14], high-silica zeolites [15], medium-pore framework zeolites [16,17] and faujasite zeolites [18] have been developed to upgrade the bio-oil, and achieve further progress. However, HZSM-5 still has attracted widespread attention so far because of its higher cost-effectiveness ratio, and mostly focus on the development and application of bifunctional catalysts based on HZSM-5, while the modified elements involve mang types [[19], [20], [21], [22]]. The bifunctional catalyst of Ti/HZSM-5 has been proven to increase the selectivity of desired hydrocarbon (including light aliphatic and aromatics) production in the previous studies [23,24].
Meanwhile, some disadvantages and limitations have been accounted for traditional catalysis aspects, including high temperature for catalysis, high coking rate for catalysts, and low selectivity for expected products [25,26]. Plasma as the fourth state of matter (containing energetic electrons and a variety of chemically active species) can be divided into thermal plasma and non-thermal plasma [27]. Plasma, especially for non-thermal plasma, that is a promising technique to break through the conversional limitations and it has been investigated and applied in several fields, such as NO decomposition [28], CO2 reduction [29], CH4 coupling [30], degradation of heavy organics and volatile organics [31,32]. Chen et al. [28] studied the removal of NO by using non-thermal technology and the interactions between plasma and catalyst increased the conversion rate by 40%. Zhu et al. [29] found that the participation of non-thermal plasma significantly enhanced the catalyst activity at lower temperature during the CO2 reduction, that is, effectively reduced the catalyst activation temperature, and the conversion rate increased by 10%. Scapinello et al. [30] developed a new method for non-oxidative methane reforming via plasma-assisted technology and it showed obvious advantages. Rahimpour et al. [32] studied co-cracking of methane and n-hexadecane through combining non-thermal plasma and heterogeneous calysis, and the energy efficiency was improved and the highest hydrocarbon production were obtained, besides, the catalyst deactivation was improved to different degrees. Non-thermal plasma can be produced and maintained by using electrical discharge which can be realized in several methods based on two aspects: the applied voltage types and the reactor specification. Corona discharge, microwave discharge, gliding arc, mospheric-pressure glow discharge, spark discharge and DBD are different types of discharge to generate non-thermal plasma which have been proven to be effective ways for dissociation and cracking of reactants [[28], [29], [30], [31], [32], [33], [34]]. And DBD manner is usually considered more suitable for most situations because of its simple design and operation, capacity to induce reactions at conventional temperature and pressure conditions [[32], [33], [34]].
The plasma-catalyst combination can be arranged in two configurations including in-plasma (one-stage) and pre-plasma or post-plasma (two-stage). For in-plasma configuration, the catalyst places directly into the discharge zone, in which the reaction efficiency can be improved but the process was more complicated due to the simultaneous occurrence of plasma discharge and catalysis reactions including interactions [33]. Recently, most attention is paid to in-plasma catalysis [32,35,36] and this pattern had been firstly applied to the upgrading of bio-oil in previous study and some progress had been made, but some adverse effects existed including the catalyst used in plasma zone suffers from increasing coking and the optimal working temperatures for the plasma and catalyst are difficult to realize simultaneous control [37]. While post-plasma operation is aimed to complete the reformation process and to destroy unwanted by-products generated in the catalytic process [38], and this configuration is not appropriate for the bio-oil upgrading. Pre-plasma configuration has been studied in other fields, especially for the catalytic process requiring pre-activation or dissoration of reactants [27,39], it can simultaneously realize plasma and catalyst working at the optimal temperatures respectively, but it is ignored in the bio-oil upgrading so far.
Therefore, to fill the research gap, the pre-plasma technology by employing pulsed DBD manner was firstly introduced for bio-fuel preparation in this study. The proposed PPEC system integrates pre-plasma with post-catalysis. Comprehensively study of using this system is essential and the specific objectives of this paper as follows: (i) to investigate and optimize operation conditions, including catalytic temperature, amount of catalyst (catalytic height) and discharge power, and a new target parameter was proposed in this stage; (ii) to study the catalytic performance of bifunctional catalyst Ti/HZSM-5 under the optimal conditions and try to explore the conversion mechanism; (iii) to evaluate the catalytic stability through catalyst characterizations. This study will lay the experimental and theoretical foundations for the efficient preparation of hydrocarbon bio-fuel from biomass.
Section snippets
Biomass
The woody biomass of camphorwood sawdust collected from Zhenjiang, Jiangsu Province, China. The biomass was sieved for small particle sample in the 0.1–0.3 mm range. The sample was dried at 105 °C for 2 h to remove external moisture. The ultimate analysis of dried sample was carried out in an elemental analyzer (FLASH 1112A, Italy), the proximate analysis was performed according to ASTM D-2974 using a thermo-gravimetric analyzer (Thermo TGA/DSC 1, Swiss), and the results are listed in Table 1.
Catalyst characterizations
The XRD spectra of catalysts are shown in Fig. 2(a). There are no characteristic diffraction peaks of other crystals besides the HZSM-5 MFI diffraction peaks (2θ = 7.96°, 8.83°, 23.18°, 23.99°, 24.45° (JCPDS card: PDF 44-0003)) for Ti/HZSM-5, which indicated that Ti ions were dispersed and no aggregation occurred. Due to the partial occlusion of Ti components, the diffraction intensity decreased slightly. The FT-IR spectra of catalysts are shown in Fig. 2(b). The absorption peaks located in
Conclusions
Bio-fuels was prepared from biomass volatiles by pre-plasma enhanced catalysis. Process parameters of catalytic temperature, catalytic height and discharge power had significant effects on the CEI proposed firstly to evaluate the upgrading effect. There are significant interactions between catalytic temperature and catalytic height, and between catalytic temperature and discharge power. Optimal conditions for the maximum CEI of 1.32 were attained at catalytic temperature of 455 °C, catalytic
Acknowledgements
This work is currently supported by the National Natural Science Foundation of China (No. 11604288), the Natural Science Foundation of Jiangsu Province (No. BK20150425), the Foundation of Jiangsu Province Key Laboratory (QK17007) and the scientific research project for the introduction talent of Yancheng Institute of Technology (XJ201708).
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