Gasification of diosgenin solid waste for hydrogen production in supercritical water
Introduction
Yellow ginger, scientifically named Discorea zingiberensis C.H. Wright (DZW), is one of the widely-used raw materials in the pharmaceutical industry for diosgenin production [1]. DZW tubers are mainly composed of 3–5% saponin, 45–50% starch and 40–50% cellulose. Saponins can be hydrolyzed into diosgenin and monosaccharide in acidic solution [2]. Therefore, in pharmaceutical industry DZW tubers were hydrolyzed by acid solution to obtain the diosgenin. But one of the major problems in such conversion is the formation of large amount of diosgenin solid waste (DSW). Valorization of the DSW should be taken into account to improve the economic value and reduce the environmental impact. Several traditional methods have been applied to dispose DSW, including compost, combustion and landfill. New methods also have been introduced to use DSW as a raw material to produce carboxymethyl cellulose, activated carbon and organic fertilizer [3]. But these methods may result in inadequate utilization of DSW and environment pollution.
Hydrogen as one of the most promising energy carriers has been projected to play a great role in the future scenario of energy sectors and attracted increasing attention [4], [5]. Hydrogen is produced from different primary energy sources and various production technologies. Currently, 95% of hydrogen is produced from fossil fuels, while only 4% and 1% produced from water using electricity and biomass, respectively [4], [6], [7]. Hydrogen production from biomass and biomass waste has been widely studied and could be the long-term aim of the hydrogen utopia [8], [9]. Many methods have been applied to convert biomass and its waste to hydrogen, such as pyrolysis [10], fermentation [11], hydrothermal gasification [12] and supercritical water gasification (SCWG) [13].
Among those conversion processes, feasibility study of hydrogen production from pyrolysis and water gasification of various biomass feedstock by Hosseini et al. confirm that SCWG of biomass is the most cost-effective thermochemical process [14]. Since Modell and Amin [15] at MIT performed the SCWG of forest products in mid-1970s, significant research has been performed for various kinds of biomass and organic wastes, such as sugarcane bagasse [16], sewage sludge [17], clover grass [18], black liquor [19], [20] and dairy industry waste [20]. The unique advantages of SCWG are mainly attributed to the special physical and chemical properties of supercritical water (SCW), such as low viscosity, high diffusivity and low dielectric constant. These properties make SCW an excellent solvent for both organic substances and gases [21], [22]. Besides, highly moisturized biomass could be employed directly in SCWG without any high-cost drying process [23].
However, to the authors' knowledge, there is little information in the literature on the study of DSW gasification in SCW. The objective of this research is to investigate the potential of DSW to produce hydrogen from SCWG. According to previous studies [24], [25], such lignocellulosic feedstock need a higher temperature to achieve a complete gasification in SCW. Efforts were made to achieve higher gasification efficiency of DSW at lower temperatures by adding catalysts in this study. In the last few decades, a lot of catalysts were reported to be effective in the SCWG process, mainly alkaline catalysts, activated carbon, metal catalysts and metal oxides [26], [27], [28], [29]. Among them, the alkalis were widely reported to be effective catalyst for SCWG of lignocellulosic materials. Yanik et al. [30] investigated the effect of K2CO3 catalyst on the gasification of corncob and sunflower stalk in SCW. They found that H2 production increased significantly in the presence of K2CO3. Schmieder et al. [12] found that K2CO3 enhanced the gasification of straw and wood in SCW. Kruse et al. [31] studied the phyto mass and zoo mass gasification in the presence of K2CO3 and found that K2CO3 increased the gasification efficiency. Therefore, K2CO3 was chosen to catalyze the SCWG of DSW in this study.
Although K2CO3 showed a high catalytic effect on the gasification of DSW, the high price of K2CO3 may increase the cost of the whole SCWG process, especially as the recovery of alkalis from SCWG process is still a challenge. Therefore, exploiting an alternative alkalis source with a low price may be a good choice. Black liquor is a wastewater generated from the pulping process and mainly contains lignin, hemicellulose, and alkalis. In our previous study, black liquor was used as inexpensive additives to catalyze the gasification of coal in SCW [32]. The results showed that both the inherent alkalis and lignin in black liquor played important roles in improving the coal gasification efficiency. The utilization of black liquor instead of commercial alkalis will not only reduce the operation cost, but also realize the co-gasification of those two biomass waste. As a result, black liquor was also tested as the substitute of alkalis to catalyze the SCWG of DSW.
In this work, the potential of DSW for hydrogen production was firstly explored via SCWG in a fluidized-bed reactor. Firstly, the morphology structures and thermal properties of DSW were characterized by SEM and thermogravimetric analyzer (TGA). Secondly, the influence of the main operation parameters (temperature, flow ratio) was investigated through thermodynamic analysis and experimental study. Then, K2CO3 and black liquor were applied to catalyze the gasification of DSW in SCW. Finally, the solid residues were characterized to study the reaction mechanism.
The highlights of this work can be concluded as following: DSW was firstly proved to be a proper feedstock for hydrogen production by SCWG. K2CO3 was found to be an effective catalyst for the SCWG of DSW. Black liquor was firstly used as substitute alkalis source to catalyze the SCWG of biomass waste. K2CO3 was firstly found to migrate into the residue char during SCWG of biomass.
Section snippets
Materials
DSW was obtained from Yonghong Chemical Co., Ltd in Shaanxi Province, China. DSW was ground by a pulverizer to reduce the particle size and sieved into 100 meshes by a screen mesh. Black liquor from soda pulping of wheat straw was obtained from a pulping plant in Shaanxi Province. It contains plenty of alkalis derived from the pulping process and has a high pH value up to 11.3 MJ/kg. The main properties of DSW and black liquor were listed in Table 1. The samples were dried at 105 °C for 12 h in
Results and discussion
The details of the reactions of DSW in SCW are very complicated because of the complex structure and composition of the DSW. A lot of research has been performed to explore the gasification characteristics of lignocellulosic biomass in SCW [18], [38]. The main reaction processes proposed for the lignocellulosic biomass gasification in SCW include hydrolysis, pyrolysis, liquefaction, extraction and gasification. The main reactions that take place in the gasification processing are listed as
Conclusion
In this work, hydrogen production by SCWG of DSW has been studied in a fluidized-bed reactor under different experimental conditions. The thermodynamic analysis showed that the gas yield changed little when temperature was below 450 °C. The experimental study showed that the formation of gaseous products was significantly affected by temperature and flow ratio and DSW was almost completely gasified at 650 °C without catalyst.
The addition of K2CO3 significantly enhanced the steam reforming
Acknowledgment
This work was financially supported by the National Natural Science Foundation of China (Contract No. 51236007), China National Key Research and Development Plan Project (Contract No. 2016YFB0600100), Australian Research Council (Project No. LP 110100337) and Shanxi Science & Technology Coordination & Innovation Project (Contract No. 2015TZC-G-1-10).
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