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Environmental Processes
Nanostructure of Gasification Charcoal (Biochar) Jacob W. Martin, Leonard Nyadong, Caterina Ducati, Merilyn Manley-Harris, Alan G. Marshall, and Markus Kraft Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06861 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019
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Environmental Science & Technology
Nanostructure of Gasification Charcoal (Biochar) Jacob W. Martin,†,‡ Leonard Nyadong,¶ Caterina Ducati,§ Merilyn Manley-Harris,k Alan G. Marshall,⊥ and Markus Kraft∗,†,‡,# †Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, West Site, CB3 0AS Cambridge, UK ‡Cambridge Centre for Advanced Research and Education in Singapore (CARES), CREATE Tower, 1 Create Way, Singapore 138602 ¶Phillips 66 Research Center, Highway 60 & 123, Bartlesville, OK 74003-6607, United States §Department of Materials Science and Metallurgy, University of Cambridge, Philippa Fawcett Drive, West Site, CB3 0FS Cambridge, UK kSchool of Science, University of Waikato, Hillcrest, Hamilton 3216, New Zealand ⊥National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310-4005, United States #School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459 E-mail:
[email protected] Phone: +44 (0)1223 762784
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Abstract
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In this work, we investigate the molecular composition and nanostructure of gasifi-
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cation charcoal (biochar) by comparing it with heat-treated fullerene arc-soot. Using
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ultrahigh resolution Fourier transform ion-cyclotron resonance and laser desorption
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ionisation time of flight mass spectrometry, Raman spectroscopy and high resolution
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transmission electron microscopy we analysed charcoal of low tar content obtained
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from gasification. Mass spectrometry revealed no magic number fullerenes such as C60
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or C70 in the charcoal. The positive molecular ion m/z 701, previously considered a
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graphitic part of the nanostructure, was found to be a breakdown product of pyrolysis
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and not part of the nanostructure. A higher mass distribution of ions similar to that
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found in thermally treated fullerene soot indicates that they share a nanostructure.
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Recent insights into the formation of all carbon fullerenes reveals that conditions in
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charcoal formation are not optimal for fullerenes to form, but instead curved carbon
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structures coalesce into fulleroid-like structures. Microscopy and spectroscopy sup-
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port such a stacked, fulleroid-like nanostructure, which was explored using reactive
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molecular dynamics simulations.
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Introduction
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Charcoal is formed from the thermal transformation of a carbon-rich substance, often woody
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biomass, into a solid carbon product. It has been produced in fires and kilns for millennia
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and has been used for creating art, smelting ores to produce metals and as a smokeless
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fuel. 1 Recently, advanced applications have been developed for charcoal such as electrode
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materials for lithium-ion batteries, 2 activated carbons 3 and supercapacitors. 4 Environmental
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concerns have also prompted research into charcoal as a carbon capture technology. Storing
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atmospheric carbon is now possible considering that biomass, as it grows, photosynthetically
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captures carbon dioxide. Subsequent thermal treatment traps some of that carbon in a stable
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charcoal that will not break down. As this charcoal is also known to enhance crop yields
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(allowing costs to be offset), many researchers are suggesting addition of charcoal in order to
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improve soils while storing carbon. 5,6 Charcoal produced for application to the soil is often
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called biochar.
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One means of offsetting biochar’s cost is by producing energy through a process called
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gasification. 7 The gasification process injects a restricted supply of oxygen into a sealed vessel
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that oxidises some of the carbon in the charcoal to produce high temperatures (1000 ◦ C).
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The surface of the hot carbon is then able to perform the water-gas shift reaction and steam
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reforming, reducing carbon dioxide and water to carbon monoxide and hydrogen. The hot
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carbon is also able to crack unburnt hydrocarbons (tar) 8,9 leading to a clean syngas fuel that
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can power a turbine or internal combustion engine. The high temperatures reached inside a
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gasifier produces a high quality charcoal with low tar content (low hydrogen content), which
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is necessary for the long-term stability of the material. 5,10 The nanostructural understanding
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of gasification charcoal is therefore important for determining the reactivity of the char for
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producing syngas and also for considering the long-term stability of such a material for
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carbon capture and storage. Understanding of the material properties could also stimulate
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interest into new applications for carbon nanostructured materials produced from natural
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biomass precursors.
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Concentrating on woody biomass and considering how it thermally transforms into char-
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coal in the absence of air (pyrolysis) a dynamic molecular structure has previously been
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proposed. 11 In this scheme, initially the crystalline and non-crystalline saccharide precur-
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sors dehydrate, decarboxylate and depolymerise forming pyrogenic amorphous carbon. 12
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Analysis of hydrothermally treated saccharide suggests a pyrogenic structure containing six-
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membered aromatic rings and species such as hydroxymethylfurfural. 13,14 From 400–700 ◦ C
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ring condensation leads to the formation of stacked aromatic domains. 15 These small lay-
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ered turbostratic (graphitic without ideal ABA stacking) disordered regions are completely
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developed by 700 ◦ C and have been recently imaged in charcoal. 16 At the temperature the
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material is nearly entirely composed of carbon (95 wt%). Thermal treatment to 1000 ◦ C
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further decreases the amount of oxygen to 1-3%, removes any remaining hydrogen (1500 ◦ C) such as glassy carbon. 21 A recent review of the nanostructure of
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non-graphitising carbon, focusing on charcoal in particular, has also highlighted the role of
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oxygen in inhibiting planarisation of the structure 22 and experimental results have suggested
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the mechanism for pentagon integration is the loss of oxygenated fragments along the zig-zag
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edge of aromatic species. 23
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Laser desorption time of flight mass spectrometry (LDI-TOF MS) is one of the many
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techniques that has been used to study the structure of these non-graphitising carbons. In a
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study of graphite, diamond, glassy carbon, carbon nanotubes and diamond-like thin layers,
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Sedo et al. observed only in glassy carbon some high intensity, positive ions in the range
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m/z 400-800, the highest being m/z 701. 24 The ions were discarded as an artefact by the
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authors but were later observed by Bourke and co-workers in biomass and sugar-derived
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charcoals prepared with flash pyrolysis at 950 ◦ C. 25 Sedo et al. also found that by increasing
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the laser power a distribution of more massive ions was produced. However, the absence of
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the m/z 720 ion (C60 fullerene) in charcoal and also of large even-numbered carbon cages in
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glassy carbon called into question a fullerene-like nanostructure.
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In this article, we aim to understand how the mass spectrometry of charcoal, indicating
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no fullerenes, can be understood in light of the microscopy and spectroscopy, suggesting
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curved fullerene-like fragments, by comparing a gasification charcoal with thermally treated
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fullerene-soot, which we have previously studied. 26
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Materials and methods
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Production of charcoal
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Untreated pinus radiata wood sourced from Carter Holt Harvey Wood Products, New
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Zealand (NZ), was cut into (30 x 30 x 10 mm) blocks that were then carbonised with the
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use of a custom built downdraft gasifier based on the microlab class gasifier with Fluidyne
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Gasification Ltd, NZ. Figure 1 shows the reaction segment of the gasifier. Wood flowed under
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gravity and heated from below was dehydrated and then thermally pyrolysed into charcoal.
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Nozzles were then used to inject air and the charcoal was oxidised. Once the oxygen had been
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consumed, reduction of CO2 and H2 O occurred on the surface of the hot carbon. Charcoal
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then dropped through the grate and was collected in the lower chamber.
Figure 1: Photograph of the Microlab class gasifier (left). Schematic of the gasifier throat showing the different reaction zones (right). Used with permission from Fluidyne Gasification Ltd, NZ.
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Positive pressure was supplied to the air inlet connected to the nozzles and the gasifier
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was ignited and run for 20 minutes. The reactor was then starved of oxygen and cooled
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for a period of two hours. Charcoal was taken from the interface between the pyrolysis
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and oxidation zones, which had yet to undergo significant oxidation and activation but
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had attained high temperatures. These zones are well separated in the downdraft gasifier we
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have employed. Charcoal from this region is not significantly oxidised but does not have soot
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present on its surface, which was determined by a visible change in the charcoal’s appearance.
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Before analysis of the charcoal by LDI-TOF MS the carbon was stored in evacuated glassware
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in order to prevent contamination from plasticisers and other environmental species.
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Ultimate analysis of the C, H, N, S and ash percentage weights (%wt) was determined
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by use of the ASTM D 3176 standard (Table 1). Oxygen %wt was determined through
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difference and does not include oxygen contained in the ash oxides. The Campbell Micro
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Analytical Laboratory at the University of Otago conducted the analysis of the oven dried
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charcoal. Table 1: Ultimate analysis of gasification charcoal (%wt) C H O N Ash 90.80
