Production of bio-oil with reduced polycyclic aromatic hydrocarbons via continuous pyrolysis of biobutanol process derived waste lignin
Introduction
World-wide interests for the plant-based biofuels are largely increased to mitigate the environmental burden related with the environmental contamination and global warming issues (Park et al., 2019; Kim et al., 2018a; Lee et al., 2016). Innovative technologies for the conversion of biomass to biofuel have been developed extensively in the recent decades as securing the economic efficiency of biofuels production is the recent target in the biomass conversion research area.
Typical biofuels, biodiesel and bioethanol extracted from bean and sugar cane, have been commercialized mainly in Europe and South America. Meanwhile, biobutanol is also one of the other potential biofuels (Ho et al., 2014). While bioethanol is hydrophilic and requires additional water prevention system, biobutanol can be used without additional modification on the transportation and storage tank due to its low solubility. Biobutanol also possess higher energy density than ethanol due to its larger carbon content and its similar fuel property with gasoline oil (Lee, 2016; Pfromm et al., 2010). In addition, biobutanol can also be used as the chemical feedstock for the production of ink, bond, semiconductor detergents, cosmetics, food additive, and so on (Mahapatra and Kumar, 2017). However, the high production cost of biobutanol restricted its actual commercialization (Kumar and Gayen, 2011). Therefore, many researchers are attempting to produce biobutanol from the second-generation biomass, such as herbaceous crop residue, perennial grasses, and forest biomass instead of using first-generation biomass, such as corn, wheat, soy grains, sugar cane, and sorghum cane to reduce the overall process cost (Balan, 2014; Cheng et al., 2012). The use of waste biomass such as wood on the production of biobutanol was also considerably investigated and a demo-plant related with this process was constructed in South Korea.
In the current stage, the amount of profit generated from the biorefineries for bio-alcohols production is limited due to the cost burden related not only with the supply and the transportation of the feedstocks, but also related to the plant operation. The large amounts of bioethanol and biobutanol are produced using the concentrated acid saccharification process. The saccharification process is performed at room temperature and atmospheric pressure and allows a high sugar yield of about 90% from various lignocellulosic biomass species. However, the separation of mineral acid and sugars is difficult to scale up, and the use of expensive apparatus with ability to withstand corrosion is costly, which are the major drawbacks of the process. Therefore, the cost-effectiveness of the biobutanol production process has to be enhanced by various kinds of activities, such as widening the product portfolio on its application towards petrochemical industry (de Bruyn et al., 2016).
A large amount of lignin is produced as the by-product from the bio-alcohols production process (Venderbosch and Prins, 2010) because lignocellulosic biomass is separated into aqueous sugar solution containing sugar monomers such as glucan or xylan produced by the decomposition of cellulose and hemicellulose and solid lignin after saccharification process. In this aspect, the desirable conversion of lignin to value-added products is needed to be emphasized (Cotana et al., 2014). Especially, a significant amount of waste lignin is likely to be produced as byproduct once biobutanol production process from waste wood is commercialized in Korea. To increase the economic value of biobutanol process from waste wood, it is imperative to develop innovative technology that can converts waste lignin to value-added products. However, the valorization of biobutanol-derived waste lignin has not yet been investigated while there has been massive research on the biobutanol production.
Lignin pyrolysis is known as an effective process to produce bio-oil with high phenolics content (Ha et al., 2019; Son et al., 2019; Cho et al., 2019; Yang et al., 2019). Although many researchers have reported on the pyrolysis of lignin, the systematic research for the pyrolysis of waste lignin derived from biobutanol process has never been reported. In addition, the most challenging part in the lignin pyrolysis was the agglomeration of lignin at the feeding line due to melting and char foaming inside the reactor. These challenges greatly hindered the continuous operation of the reactor. Therefore, new reactor type should be applied to solve this problem.
International lignin pyrolysis researches using the conventional pyrolysis reactors (e.g., fluidized bed reactor) also reported that char agglomeration and lignin melting hindered the operation of the lignin feeding system and fluidization (Nowakowski et al., 2010a). They suggested the use of less purified lignin and different reactor systems to reduce these problems during the lignin pyrolysis. Recently, Zhou et al. (Zhou et al. (2016)) performed the pyrolysis of lignin using a pyrolysis centrifuge reactor coupled with the fixed bed reactor using a HZSM-5 catalyst. The reactor showed no char related problems owing to the mechanical effect of a fast rotating vane.
The other problem of lignin pyrolysis could derive from the presence of polycyclic-aromatic hydrocarbons (PAHs) in the bio-oil product. PAHs has been regarded as highly toxic materials to human health because it is carcinogenic and also a source of ultrafine dust (Sun et al., 2018). Recently in East Asia countries such as China and Korea, atmospheric pollution owing to particulate matters such as PM-2.5 and PM-10 has become a major concern that urgently required to be dealt with (Hwang et al., 2019). Therefore, the production of bio-oil with lower amount of PAHs can be an alternative contributing to reduced emission of potentially hazardous particulate matters.
In this study, the continuous production of bio-oil with reduced PAHs via the pyrolysis of biobutanol process derived waste lignin was attempted. For this purpose, various types of reactor with different scales (micro, lab and bench scale) and reactor type (fixed bed and new developed rotary kiln type) were used to compare the pyrolysis reaction and pyrolysis product properties. The physico-chemical properties of the biobutanol process derived waste lignin pyrolysis products were analyzed as well.
Section snippets
Material and methods
Chemically treated lignin, derived from the biobutanol produced from wood pellet, was obtained from GS Caltex in powder form. The lignin powders were sieved to the range of 425 μm - 100 μm before being subjected to pyrolysis. The proximate and ultimate analysis was performed using an element analyzer (Flash EA 113, Thermo Electron Co.) by following the ASTM standard (Kim et al., 2018b). TG analysis of lignin was carried out using a TG analyzer (Pyris 1, Perkin-Elmer) by ramping the pyrolysis
Lignin from biobutanol process
Table S3 shows the proximate and ultimate analysis results of lignin sample used in this study. The carbon, hydrogen, oxygen, nitrogen, and sulfur content of lignin were 57.4, 6.1, 35.2, 0.1, 1.2 wt.%, respectively. The presence of sulfur in the lignin sample was originated from its isolation method from wood pellet in which the biomass was hydrolyzed under high concentration of sulfuric acid condition. The proximate analysis result shows that the lignin consists of high volatile matter of
Conclusion
The yields of oil, gas, and char from the pyrolysis of lignin derived from biobutanol process using lab and bench-scale FB reactors were similar; 32.7∼33.9% for oil, 30.9∼28.4 for gas, and 36.4 ∼ 37.7% for char production, respectively. However, the product distributions using bench-scale FB reactor was quite different than using bench scale RK reactor. Compared to FB reactor, RK reactor produced two times larger amount of organic phase oil due to the higher cracking efficiency of RK reactor.
Acknowledgments
This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2017M1A2A2087674). This work was also supported by the C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2015M3D3A1A01064899).
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These authors equally contributed to this study.