Characterization of Powder River Basin coal pyrolysis with cost-effective and environmentally-friendly composite Nasingle bondFe catalysts in a thermogravimetric analyzer and a fixed-bed reactor

https://doi.org/10.1016/j.ijhydene.2018.02.102Get rights and content

Highlights

  • Catalytic pyrolysis kinetic characteristics of PRB coal were studied.

  • Model free and master-plots methods were used.

  • Composite Nasingle bondFe catalysts can lower activation energy.

  • Pyrolysis mechanism of PRB coal with Nasingle bondFe catalysts was proposed.

Abstract

The pyrolysis characteristics of PRB coal with use of Nasingle bondFe composite catalysts were investigated in a thermogravimetric analyzer and a fixed bed reactor. Model-free methods developed by Friedman (FR), Kissinger-Akahira-Sunose (KAS), Flynn-Wall-Ozawa (FWO) and Vyazovkin (VA) were compared and applied to determine the pyrolysis kinetic parameters. A Master-plot method was used to determine the reaction order and pre-exponential factors. Raw coal with the addition of 4% FeCO3 achieved the highest effect on coal conversion; specifically, 4% Na2CO3 and 1% Na2CO3–3% FeCO3 showed higher effect than 3% Na2CO3–1% FeCO3 and 2% Na2CO3–2% FeCO3. The averaged E values of raw coal with 4% FeCO3 catalyst decreased by 10% compared with that of raw coal. Compared with the individual use of 4% Na2CO3, the addition of FeCO3 can be effective in decreasing the E values of the raw coal. The nonlinear VA method appears to be superior in determining activation energies of coal pyrolysis at considered conversion range. Further, the An (random nucleation and nuclei growth model) appears to be the appropriate reaction model for raw coal pyrolysis with and without the use of Nasingle bondFe composite catalysts. From the pyrolysis in the fixed bed reactor, XRD and FTIR tests for coal chars produced by PRB coal pyrolysis with use of catalysts were conducted. By combining the gas evolution and XRD/FTIR results, a reaction mechanism is proposed for coal pyrolysis with composite Na2CO3single bondFeCO3 catalysts.

Introduction

The rising interest in clean coal conversion and utilization technology is expanding research efforts in coal science and engineering worldwide. As a key step in coal thermochemical conversion technology, gasification and combustion processes, pyrolysis and its applications is a point of focus for such researches. By means of pyrolysis, coal can be converted into useful energy holder (bio-oil), adsorbent bio-char and various useful-chemicals such as benzene, toluene, naphthalene, phenol, creosote oil, and so on [1], [2]. However, conventional coal pyrolysis techniques are often plagued with problems, such as low efficiency of coal conversion, high contents of oxygen and heavy component (boiling point higher than 360 °C) in the coal oil product, difficulties in separating coal particle and coal oil. Thus, coal catalytic pyrolysis has become exceedingly desirable technology to improve coal pyrolysis conversion and orientation of pyrolysis products, especially developing a cost-effective and environmentally-friendly catalyst.

Previously, catalysts used in the pyrolysis of coal mainly include alkali and alkaline earth metallic compounds, transition metallic compounds, natural minerals, or mineral matters in coal [3], [4], [5], [6]. It has been demonstrated that alkali metal carbonates, such as Na2CO3 and K2CO3, can improve the gas yield and reduce tar and char yields during coal pyrolysis. Moreover, some results indicate that the addition of Na2CO3 can improve the quality of pyrolysis oil produced by oil sludge and biomass. Also, the oxygen content of the bio-oil from biomass pyrolysis is decreased from 47.5 wt.% to 16.4 wt.% with Na2CO3 [4], [7], [8]. These results suggest that Na2CO3 is a potentially superior catalyst for producing chemicals by coal pyrolysis and gasification. Meanwhile, iron-based catalysts are also widely used in coal pyrolysis and gasification due to its low price, favorable environmental attributes, and its better catalytic activity for hydrogenation reactions. Studies have shown that Fe2CO3 catalyst can be decomposed into iron oxides and then be reduced to iron and iron carbide above 719 °C during coal pyrolysis [9], [10]. Fe2CO3 could help decompose most of the hydrocarbons at 900 °C, producing only a small amount of CH4 [11], which is beneficial to coal-to-liquid utilization processes.

Proper use of composite catalysts has the potential to improve reaction rates and production rates of desirable products when compared with the use of individual catalysts. By using the composite Na2CO3single bondFeCO3 catalysts, one may expect some advantages such as changes in the selectivity of H2/CO and tar yields, increased conversion rate or general improvements of the overall efficiency of pyrolysis and gasification. With these aims, the knowledge of pyrolysis kinetics of coal with the use of composite Na2CO3single bondFeCO3 catalysts deserves a careful study. This will be critical to the goal of developing new coal conversion technologies and to the design of proper and efficient reactors.

Thermogravimetric analysis (TGA) is one of the most common techniques used to investigate the characteristics of decomposition and kinetic parameters during pyrolysis of solid samples such as coal, biomass, plastic, and so on [12], [13]. Historically, various methods were applied in evaluating devolatilization kinetic process of solid fuels using non-isothermal TG analysis, especially some popular model-free methods developed by Friedman (FR) [14], Flynn-Wall-Ozawa (FWO) [15], [16], Kissinger-Akahira-Sunose (KAS) [17], [18] and Vyazovkin (V) [19], [20]. These methods are based on the assumption that the reaction rate at a constant extent of conversion only depends on the temperature. Hence, the activation energy can be evaluated without the need of a reaction model. Moreover, many methods including master plot methods, Popescu and Satave [21], [22], [23] methods have been developed to establish the kinetic model of thermal decomposition of solid fuels without assuming of kinetic model. All methods were adopted in this study to calculate kinetic parameters of coal pyrolysis with and without catalysts.

In our present work, Powder River Basin (PRB) coal has been analyzed by proximate analysis, ultimate analysis, FTIR analysis, X-ray diffraction analysis, and solid state 13C NMR analysis to obtain its physical and chemical characteristics. In addition, the pyrolysis behavior of coal with and without composite Na2CO3single bondFeCO3 catalysts were studied using a thermogravimetric analyzer and a fixed-bed reactor. The aim is to investigate effects of catalysts and heating rate on the pyrolysis process of coal and to study pyrolysis kinetic characteristics of coal with and without catalysts. The activation energies were estimated by model-free methods, which includes KAS, FWO, FR, and VA. In addition, From the pyrolysis in the fixed bed reactor, XRD and FTIR tests for coal chars produced by PRB coal pyrolysis with use of catalysts were conducted. The catalytic mechanism of PRB coal pyrolysis with Na2CO3single bondFeCO3 composite catalysts was proposed.

Section snippets

Sample preparation

Raw coal used in this work is from the Wyoming Powder River Basin and is provided by Wyodak Resources Development Corp. The proximate analysis was measured according to ASTM D5142 and D5016 [24]. The ultimate analysis (C, H, N, and S) of raw coal was performed using the elemental analyzer (Vario EL cube, Germany) and the oxygen (O) content was calculated by difference. The results of proximate and ultimate analysis of the coal are shown in Table 1. The ash of raw coal was prepared by air

Physical and chemical characteristics of raw coal with and without catalysts

13C NMR was used to analyze the different carbon types in the raw coal structure. Generally, the solid-state 13C NMR spectrum of coal can be divided into two main chemical regions: aromatic carbon (90–220 ppm) and aliphatic carbon (0–90 ppm) functional groups, respectively [39]. Different carbon functional groups can be represented by different chemical shifts, which are assigned into different carbon types, while the relative size of different peak areas indicates the relative contents of

Conclusions

Four model-free methods were applied to study the pyrolysis kinetics of a Powder River Basin coal with a Nasingle bondFe composite catalyst. The results show that the nonlinear VA method appears to have the most reasonable E values in the range of α considered. Further, the An (random-nucleation and nuclei growth model) appears to be the appropriate reaction model for the raw coal pyrolysis with and without the use of Na2CO3single bondFeCO3 composite catalyst. As an effective catalyst for coal gasification, Na2CO3

Acknowledgements

The authors gratefully acknowledge the Department of Energy (DE-FE0023999) and the Idaho National Laboratory Directed Research and Development Program under DOE Idaho Operations Office Contract DE-AC07-05ID14517 for supporting this work.

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