74 75 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Tracking the shape of nutshells during biochar production Related publication: Barr M. R., Jervis R., ZhangY., Bodey A. J., Rau C., Shearing P. R., Brett D. J. L.,Titirici M-M. &Volpe R.Towards amechanistic understanding of particle shrinkage during biomass pyrolysis via synchrotron X-raymicrotomography and in-situ radiography. Sci. Rep. 11 , 2656 (2021). DOI: 10.1038/s41598-020-80228-x Publication keywords: Biomass pyrolysis; Biochar; X-ray imaging; Morphology; Adsorption A charcoal-like product known as ‘biochar’ can be produced from agricultural waste biomass such as nutshells. One conversion method is pyrolysis, a process that involves heating the waste in the absence of oxygen. During pyrolysis, changes in the size and shape (morphology) of particles increase the surface area of the biomass. This surface area controls how biochar binds to (adsorbs) pollutants, speeds up chemical reactions, and stores energy. A lack of understanding of how biomass morphology changes during biochar productionmakes it difficult to tailor biochar properties for specific applications. Facilities at the Diamond Manchester Imaging Branchline (I13-2) enabled a team of researchers to conduct rapid high-resolution X-ray imaging of biomass. This allowed real-time tracking of particle morphology and porosity during pyrolysis. The results showed that the morphology and porosity of different nutshells evolved differently during pyrolysis. However, these differences were less pronounced in biomass pre-soaked with an alkaline solution. Almond shells shrank more but gained less porosity than walnut shells, which have thicker- walled cells on average. The results suggest that the difference is related to how heat penetrates particles of biomass during pyrolysis. Porosity was found to accumulate towards the centre of particles during pyrolysis for the same reason. The ability to customise biochar morphology would benefit its many environmental applications. These include removing pollutants from air, water, and soil; speeding up chemical reactions; and even storing energy. Tracking themorphology of biomass during biochar production is the first step towards achieving this. Lignocellulosic biomass, the primary sources of which are agricultural and forestry waste, is naturally porous and thus has a high surface area to volume ratio.This allows it to adsorb (bind at the surface) a large quantity of molecules, materials, or organisms (adsorbates) relative to less porous materials. Upon heating without addition of an oxidising atmosphere, biomass undergoes a complex transformation that vastly further increases its adsorptive surface area 1 . The process leading to this transformation is known as pyrolysis and the result as biochar. For this reason, biochar is recognised as a universal adsorbent and is used for such applications as air, water, and soil treatment; catalysis; and energy storage. Efficacy of biochar for these adsorption applications relies heavily on char morphology. Beyond simply maximising adsorbent surface area, adsorbates must be able to access the internal surface area of particles, which is dependent on pore and particle morphology. During pyrolysis, both pore and particle morphology evolve—these processes are commonly referred to as particle shrinkage, but in reality this term encompasses two parallel processes: bulk particle shrinkage and porosity gain. Properly accounting for bulk particle shrinkage and porosity gain based on direct observation of morphological evolution during pyrolysis is the first step towards controlling biochar morphology by choice of production conditions. To achieve this, it was necessary to observe biomass particles during pyrolysis ( in-situ ). Imaging only after pyrolysis ( ex-situ ) would neglect the effects of cooling and recovering chars for analysis. This work represents the first in situ X-ray imaging study of biomass pyrolysis. A combination of in situ synchrotron radiography and ex situ synchrotron X-ray microtomography were used to track the evolution of external and internal particle morphology during pyrolysis. Facilities at the Diamond Manchester Imaging Branchline (I13-2) allowed for rapid acquisition of microscopic phase-contrast-enhanced X-ray images, such that both particle morphology and internal biomass microstructure could be tracked in real time during reaction. A polychromatic ‘pink’beam was used for both 2D imaging (radiography) and 3D imaging (tomography) to achieve the short acquisition times required. In-line phase-contrast enhancement ensured that solid-gas interfaces (surface area) within particles were highlighted in acquired images. A novel reactor (Fig. 1) was developed to enable in situ synchrotron radiography of fixed beds of pyrolysing biomass. Biomass was convectively heated by a stream of preheated argon to peak temperatures between 250 and 450 ° C, during which radiographs were acquired in a single plane (Fig. 2). After pyrolysis, tomographic data were acquired for chars and representative raw samples. These were used to reconstruct tomograms (Fig. 3) with the open- source modular pipeline Savu 2 . Images were first normalised, followed by corrections for optical distortions and ring artefacts. Prior to reconstruction, a Paganin filter was applied to enhance phase contrast in images. Segmentation (determining which pixels represent which materials) to enable quantitative analysis of images relied on custom code, which has been made publicly available 3 . In order to study the effects of feedstock morphology on particle shrinkage, two types of nut shells, almond and walnut, were used as pyrolysis feedstocks. These nut shells are quite similar, both botanically and chemically. However, almond shells, which contain highly porous vascular channels, were found to shrink more and to gain less porosity than were walnut shells, which have thicker-walled and more irregularly shaped cells 4 . In addition to pyrolysis of raw biomass, that of biomass treated with sodium hydroxide (known to increase porosity and reduce lignin content) 5 , was observed and this alkaline pretreatment was found to reduce the difference between feedstocks with respect to morphological evolution during pyrolysis. These phenomena were thought to be related to differences in the heat-transfer properties of feedstocks. The rate at which particles equilibrate to the temperature of their environment affects the way in which volume is lost as gas is formed and released from solid particles. Inside particles, during this transient phase, there exists a cooler region in which heat can move within the particle faster than reaction can occur. This region therefore reacts throughout its volume and thus porosity is gained here. Outside this region, towards the surface of particles, reaction occurs more quickly than heat is able to penetrate into the particle, meaning volume is lost only at the surface in this region and thus bulk shrinkage occurs. The faster a particle can equilibrate, the more bulk shrinkage and the less porosity gain it will experience. Structural differences, like those between almond and walnut shells 4 and those caused by alkaline pretreatment 5 , affect this rate of equilibration. Image analysis 3 of tomograms (Fig. 3) revealed that pyrolysis led to a redistribution of pore volume away from particle surfaces, meaning newly formed surface area may be less accessible to adsorbates. The observed concentration of pores, and therefore adsorptive surface area, towards the centre of particles during pyrolysis poses a challenge to optimising their adsorption capacity. Feedstocks with lower initial porosity have more potential for increased surface area upon pyrolysis. However, although surface area increases as pyrolysis proceeds, so too does it become increasingly inaccessible. One solution may be alkaline pretreatment of biomass, which increases feedstock porosity prior to pyrolysis 5 , thus reducing the heat transfer limitations likely to be driving the concentration of pores towards particle centres. Another solution could be milling particles after pyrolysis to expose newly formed surface area when using biochar for adsorption applications. Equally, concentration of porosity towards the centre of particles may in fact prove beneficial for retention of adsorbates, which is critical when using biochar to remove harmful pollutants from the environment. References: 1. Jones K. et al. New Applications of X-ray Tomography in Pyrolysis of Biomass: Biochar Imaging. Energy & Fuels 29 , 1628–1634 (2015). DOI: 10.1021/ef5027604 2. Atwood R. C. et al. A high-throughput system for high-quality tomographic reconstruction of large datasets at diamond light source. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 373 , 20140398 (2015). DOI: 10.1098/rsta.2014.0398 3. Meredith Rose Barr. (2020, April 6). X-Ray Image Analysis Code (Version v1.0.1). Zenodo. http://doi.org/10.5281/zenodo.3742013 4. Queirós C. S. G. P. et al. Characterization of walnut, almond, and pine nut shells regarding chemical composition and extract composition. Biomass Convers. Biorefinery 10 , 175–188 (2020). DOI: 10.1007/s13399-019- 00424-2 5. Brodeur G. et al. Chemical and Physicochemical Pretreatment of Lignocellulosic Biomass: A Review. Enzyme Res. 2011 , 787532 (2011). DOI: 10.4061/2011/787532 Funding acknowledgement: This research was supported by Queen Mary University of London. We acknowledge Diamond for time on beamline I13-2 under Proposal MG21587. This research utilised Queen Mary’s Apocrita HPC facility, supported by QMUL Research-IT. http://doi.org/10.5281/zenodo.438045. Corresponding authors: Meredith Rose Barr, Queen Mary University of London,
[email protected] ; Dr Roberto Volpe, Queen Mary University of London,
[email protected] Imaging andMicroscopy Group Beamline I13 (Branchline I13-2) Figure 1: Schematic of the novel purpose-built reactor used to acquire in situ radiographs. Figure 2: Radiographs and segmentations of a bed of almond shells undergoing pyrolysis. Figure 3: Reconstructed and segmented tomograms and corresponding 3D renderings of raw and pyrolysed almond shells. In 3D renderings, the scale bar corresponds to the frontmost plane.