Gasification of indole in supercritical water: Nitrogen transformation mechanisms and kinetics
Introduction
In terms of our current climate, coal burning has made significant – and dangerous – contribution to NOx emissions [1], [2]. Acid corrosion caused by NOx from coal burning has led to considerable economic losses as well as threats to the environment, making NOx emission reduction methods a popular research object and an increasingly urgent endeavor.
Supercritical water gasification (SCWG) is a promising technology for coal conversion [3], [4], [5] due to high solubility [6], [7], high reactivity [8], [9] and high diffusivity [10], [11] of supercritical water. Coal gasification in supercritical water produces clean gas with no nitrogen oxide, thus reducing NOx emissions compared to traditional methods [4]. Coal contains a complex set of polycyclic aromatic hydrocarbons with heterocyclic nitrogen [12]. Model compounds are typically used to study its relative characteristics, e.g., desulfuration and gasification [13], [14]. Simple nitrogen-containing polycyclic aromatic hydrocarbons are favored for studying nitrogen transformation mechanisms of coal pyrolysis, combustion and gasification, while pyridine, pyrrole and quinolone are often used as model compounds for investigating nitrogen transfer in coal pyrolysis [15] or supercritical water oxidation [16], [17], [18], [19], [20], [21]. In this study, indole was selected as a model compound for coal because it is very stable and typically found in coal tar [22], [23], in addition to the fact that its structure contains an aromatic ring that more closely resembles coal characteristics than single ring compounds such as pyridine or pyrrole.
There have been several previous studies on indole gasification in supercritical water that provided valuable references for the present study; the literature suggests, for example, that indole conversion in supercritical water is very slow and intermediates yields are very poor at either 350 °C or 460 °C [24]. Guo [25] investigated indole gasification in supercritical water at temperatures from 550 °C to 700 °C in quartz tube reactors to find that the carbon gasification efficiency was only 20% and hydrogen gasification efficiency 79% at 700 °C for 20 min. The same research team later proposed a reaction network and established a kinetic model for indole conversion without catalyst [26], and also investigated the catalytic gasification of indole in supercritical water with metal catalyst in 316 stainless-steel mini batch reactors at 500 °C to find that the highest carbon gasification efficiency was only 13.9 ± 3.7% [27].
Taken together, research results have confirmed that indole is extremely stable and does not readily gasify under low-temperature or non-catalytic conditions. There have been few reports on high-efficiency gasification of indole in supercritical water with alkali catalysis, and few on its gasification pathway or kinetics.
As mentioned above, in this study, we selected the indole as coal model compound to explore the mechanisms of nitrogen transformations for coal gasification in supercritical water. We used K2CO3 as an alkali catalyst, as it is effective for supercritical water gasification [3], [28]. We examined the effects of temperature and residence time on carbon gasification efficiency and nitrogen distributions, then proposed a reaction pathway of indole gasification in supercritical water accordingly and developed a quantitative kinetics model to describe the gaseous products and nitrogen transformation mechanisms of indole. Based on the model, we were able to provide a condition for complete gasification of indole in supercritical water, which is discussed in detail below.
Section snippets
Material
The analytical reagent indole (>99 wt%) used in this paper was provided by Beijing J&K Scientific. The analytical reagent anhydrous potassium carbonate was produced by Tianjin Honghe Chemical Reagent Factory. The chromatographic pure methanol was provided by Merck, German.
Apparatus and experimental procedures
Supercritical water gasification of indole was conducted in an autoclave which was manufactured with Inconel 625 and ran at designed temperature and pressure of 750 °C and 35 MPa with chamber volume of 567 mL. A schematic
Gaseous products
We detected no nitrogen-containing gaseous products in the samples apart from the environmental N2. Gas compositions included H2, CO2, CH4 and CO. H2 was the main composition. These results marked a departure from those reported by Guo [25] in which methane was the maximum composition due to a higher feedstock concentration. The effects of temperature (650°C–750 °C) and reaction time (1 min−30 min) on the molar yields of H2, CO, CH4, CO2, CE and HE were shown in Fig. 2 (a)–(f), respectively.
As
Conclusion
In this study, we examined indole gasification in supercritical water with K2CO3 as catalyst. The products and nitrogen transformation characteristics were investigated at length to reach the following main conclusions.
- (1)
Carbon gasification efficiency of 95.89% was obtained under the conditions of 750 °C, 25 MPa for 30 min. Gaseous products mainly contained H2, CO, CO2 and CH4, while there were no nitrogen-containing products detected in gaseous products.
- (2)
The main liquid intermediate organic
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51527808 and No. 51323011).
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