CO2-assisted catalytic pyrolysis of digestate with steel slag
Introduction
Primary energy consumption worldwide has reached up to 535 EJ yr−1 [1]. Based on the world’s population (7.6 billion in 2018), energy consumption per capita is approximately equivalent to 70 GJ yr−1 [1]. This value (70 GJ yr−1) implies that ∼5.5 L of oil per capita is required daily [1]. Considering energy injustice [2], the amount of oil per capita in the developed countries may be much higher than ∼5.5 L. Indeed, fossil fuels contribute ∼80% of the global primary energy [3]. Despite the numerous socio-economic benefits from fossil fuel use [4], their massive consumption has inevitably resulted in perturbation of the natural carbon cycle [5]. In short, CO2 emissions from fossil fuel use are exceeding the Earth’s full capacity to assimilate carbons, which poses global-scale environmental problems, such as global warming [6]. To abate the detrimental consequences from the carbon imbalance induced by the surplus carbon input (CO2), considerable research regarding renewable energy has been completed [[7], [8], [9], [10], [11]]. In particular, biogas (i.e. a mixture of CO2 and CH4) production from organic wastes via the anaerobic digestion (AD) process has been widely practiced because of its carbon neutrality and technical completeness relative to other renewable energy technologies [12,13]. As such, many countries have commercialized the AD process [12]; the total number of AD plants has noticeably increased [14]. In the European Union, the number of biogas plants dramatically increased from 10,433 in 2010 to 17,240 in 2014 [15].
The AD process is the fermentation process that converts organic wastes into biogas via four consecutive biological reactions (i.e. hydrolysis, acidogenesis, acetogenesis, and methanogenesis) [16]. Various carbon substrates (livestock manure, food wastes, food waste leachate, sewage sludge, etc.) are used during the AD process [[17], [18], [19]]. In addition, the final digestion residue (i.e. digestate) has been used as a fertilizer in agricultural practices [20]. However, there many environmental concerns (odor control, transportation cost, pathogen, heavy metal (loid) contamination, etc.) remain in the further use of digestate as a fertilizer [21]. In detail, direct use of digestate as a fertilizer is problematic in terms of ammonia (NH3) emission and odor control [22]. Recognition of offensive odorants and the resulting nuisance have also been considered as among the most serious issues triggering public complaints [23]. Indeed, odors remain one of the top three complaints to air quality regulators and government bodies in different countries [24]. Moreover, 70% of the N source in digestate is emitted in the form of NH3 when used as a fertilizer [25], degrading air quality as NH3 plays a critical role in the formation of particulate matter (PM) via complex photo-induced reactions [26]. NH3 is among the well-known greenhouse gases [27]. In addition, heavy metal (loid)s, organic pollutants, and pathogens in digestate can be potential hazards in the direct use of digestate as a fertilizer [21,28]. Alternatively, digestate is also incinerated [29]; however, air pollution controls during incineration are difficult to implement because of the large amount of volatile matter (VM) in the digestate [30]. Thus, it is desirable to develop an environmentally benign technology for digestate disposal. Preferably, energy recovery during digestate disposal would be desirable [31].
As such, digestate pyrolysis offers an effective means of reducing its volume while recovering energy [[32], [33], [34]]. Pyrolysis is among the proven fuel processing technologies that reallocate carbons in carbonaceous materials into three pyrogenic products: syngas (H2 and CO), pyrolytic oil, and char [35,36]. Nevertheless, controlling the carbon distribution to modify the compositional matrix of three pyrogenic products is challenging in that it is sensitive to operational parameters [37,38] and the carbonaceous material physico-chemical properties [37,39]. To seek a new means for manipulating the carbon distribution, we used CO2 as reactive gas medium. In a systematic investigation into the possible use of CO2 to manipulate the carbon distribution during digestate pyrolysis, the thermolysis of digestate in CO2 was thermo-gravimetrically characterized. To elucidate the effectiveness of CO2 during digestate pyrolysis, all the pyrogenic products from lab-scale pyrolysis were characterized using online gas chromatography (GC) for gaseous products, GC/mass spectrometry for liquid products, and inductively coupled plasma-optical emission spectrometry (ICP-OES) and field emission-scanning electron microscope/energy dispersive X-ray spectroscope (FE-SEM/EDX) for char. For conducting the lab-scale non-catalytic and catalytic pyrolysis of digestate, one-stage and two-stage pyrolysis reactor setups [40] were used to more carefully elucidate the effects of CO2 and steel slag catalyst on the pyrolysis. All experimental results were referenced to those from the inert condition (i.e., N2) to elucidate and/or evaluate the effectiveness of CO2 and steel slag catalyst. Lastly, to enhance the CO2 effectiveness during digestate pyrolysis, the identified roles of CO2 were further evaluated by employing steel slag as a catalyst.
Section snippets
Sample preparation and chemical agents
Digestate from the AD process was collected from the Jungnang Water Reclamation Center (37.557650, 127.065363) in Korea. The collected digestate sample was dried at 80 °C for 2 d. The ultimate analysis of the dried digestate sample was conducted using an organic elemental analyzer (Vario MACRO cube, Elementar Analysensysteme GmbH, Germany). Percentages of 32.09 wt% of C, 5.25 wt% of H, 4.46 wt% of N, 17.01 wt% of O, and 1.51 wt% of S were determined via the ultimate analysis of digestate. Steel
Thermo-gravimetric analysis of digestate in CO2
A series of TGA tests was conducted to characterize the digestate thermolysis providing fundamental information regarding the mass change of the analyte as a function of the thermolytic temperature. Note that a digestate TGA test in an inert condition (N2) was performed as a reference to discern any differences in the thermolytic behavior induced by CO2. Thus, the digestate mass decay as a function of the thermolytic temperature in the CO2 in reference to N2 is shown in Fig. 1a. In addition,
Conclusions
This study investigated the role of CO2 as a reactive gas medium during digestate pyrolysis. All experimental observations confirmed that CO2 roles occurred via a gas phase reaction (i.e. homogeneous reactions between VM evolved from digestate pyrolysis and CO2). The main CO2 roles during digestate pyrolysis resulted in enhanced CO generation while suppressing dehydrogenation. Such an observation also suggests shifting the carbon distribution from pyrolytic oil to pyrolytic gas was only
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2018R1A2B2001121).
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These authors contributed equally to this study.