Abstract
The presented work explores the structural properties, gasification reactivity, and syngas production of Greek lignite fuel (LG) and ex-situ produced chars during CO2 gasification. Three different slow pyrolysis protocols were employed for char production involving torrefaction at 300 °C (LG300), mild-carbonization at 500 °C (LG500), and carbonization at 800 °C (LG800). Physicochemical characterization studies, including proximate and ultimate analysis, X-ray Diffraction (XRD), and Raman spectroscopy, revealed that the thermal treatment under inert atmospheres leads to chars with increased fixed carbon content and less ordered surface structures. The CO2 gasification reactivity of pristine LG and as-produced chars was examined by thermogravimetric (TG) analysis and in batch mode gasification tests under both isothermal and non-isothermal conditions. The key parameters affecting the devolatilization and gasification steps in the overall process toward CO-rich gas mixtures were thoroughly explored. The gasification performance of the examined fuels in terms of carbon conversion, instant CO production rate, and syngas generation revealed an opposite reactivity order during each stage. TG analysis demonstrated that raw lignite (LG) was more reactive during the thermal devolatilization phase at low and intermediate temperatures (da/dtmax,devol. = 0.022 min−1). By contrast, LG800 exhibited superior gasification reactivity at high temperatures (da/dtmax,gas. = 0.1 min−1). The latter is additionally corroborated by the enhanced CO formation of LG800 samples under both non-isothermal (5.2 mmol) and isothermal (28 mmol) conditions, compared to 4.1 mmol and 13.8 mmol over the LG sample, respectively. The pronounced CO2 gasification performance of LG800 was attributed to its higher fixed carbon content and disordered surface structure compared to LG, LG300, and LG500 samples.
Highlights
The world coal proved reserves were estimated at 1070 billion tons at the end of2019, with the global coal production reaching ca. 8.1 bt in 2019, resulting in the largest reserves-to-production ratio of 132 years among the fossil fuels [1,2]
Grounded lignite samples were slowly pyrolyzed under nitrogen flow at 300, 500, and 800 ◦ C, resulting in three different chars, i.e., LG300, LG500, LG800
The increasing trend of the aromaticity factor is reflected in the increasing number of aromatic carbon atoms in lignite chars, as confirmed by the X-ray Diffraction (XRD) analysis presented below
Summary
The world coal proved reserves were estimated at 1070 billion tons (bt) at the end of2019, with the global coal production reaching ca. 8.1 bt in 2019, resulting in the largest reserves-to-production ratio of 132 years among the fossil fuels [1,2]. In 2019, the European Commission announced the European Green Deal to direct the EU economy down a pathway focused on climate neutrality by 2050 [5]. This ambitious target will require a fundamental transformation of the energy system. The estimated number of direct coal mining and power generation jobs at risk under various coal phaseout scenarios ranges from 54,000 to 112,000, without considering the losses in jobs indirectly related to the coal sector [6,7]. Alternative strategies to diversify local economies are required to ensure that the transition will be fair and leave no one behind [6,7]
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