Contents lists available at ScienceDirect Biomass and Bioenergy journal homepage: www.elsevier.com/locate/biombioe Research paper Experimental investigation on biomass shrinking and swelling behaviour: Particles pyrolysis and wood logs combustion Gianluca Caposciutti a,∗ , Hernán Almuina-Villar b , Alba Dieguez-Alonso b , Thomas Gruber c , Joachim Kelz c , Umberto Desideri a , Christoph Hochenauer c,d , Robert Scharler c,d , Andrés Anca-Couce d a Department of Energy, Systems, Territory and Constructions Engineering, University of Pisa, Italy b Chair for Energy Process Engineering and Conversion Technologies for Renewable Energies, Institute of Energy Engineering, Technische Universität Berlin, Germany c BIOENERGY 2020+ GmbH, Graz, Austria d Institute of Thermal Engineering, Graz University of Technology, Austria ARTICLE INFO Keywords: Biomass Thermochemical conversion Particle shrinking/swelling Image analysis ABSTRACT Biomass is a suitable energy source to reduce the carbon footprint and increase the use of renewable energy. However, the biomass exploitation is still slowed by many technical issues. In most practical applications, such as gasifcation or combustion devices, it is important to predict the fuel physical behavior in order to determine the emissions and heat release profle as well as for modeling and design purposes. Within this paper, the study of the dimensional evolution of a biomass fuel (beech wood) in pyrolysis and combustion processes were carried out with the use of the image analysis tool. Sizes from 15 mm to 300 mm characteristic length range were employed in the experiments and the collected data were related to the mass loss and temperature evolution of the biomass particle. It was found that for all the fuel sizes employed a similar volume reduction (60%–66%) was obtained at the end of pyrolysis. However, for the small particles with minor intra-particle gradients shrinkage took place mainly at the end of conversion, while for bigger particles the size variation patter was more linear. Furthermore, swelling was detected in the pyrolysis experiments, and it was higher for a bigger particle size, while cracking and fragmentation phenomena was observed for large wood logs combustion in the stove. 1. Introduction It is well known that renewable energies are a way to produce cleaner and more sustainable energy. Biomass represents an attractive renewable energy resource for production of heat and power which can be easily planned and controlled. In particular, thermochemical con- version processes, such as combustion or gasifcation, are widely used, especially when working with lignocellulosic biomass as feedstock. Woody biomass is widely used as a sustainable fuel, and agricultural waste or energy crops can be as well employed, but they are more challenging fuels [1,2]. Despite many advantages, the large variety of biomass composition and structure causes important technical issues in its employment. In order to avoid the technical difculties and increase in expenses of biomass grinding, many industrial conversion processes use woody biomass in the cm-scale, i.e. wood chips or wood logs. Basic control parameters of biomass combustion such as the air excess and dis- placement have to be fnely tuned with the biomass characteristics to control the emissions and the thermal behavior of biomass boilers [3–5]. Combustion stages behavior is infuenced to a great extent by the fuel properties [6]. Recent studies have highlighted the signifcant impact of the fuel size, especially when working with relatively large particles, on the reaction rate and product composition of pyrolysis [7–9]. For this reason, the study of the biomass dimensional evolution at particle level during the conversion process has a key role in order to develop models with more accurate predictions of emissions [10–15]. Furthermore, in terms of reactor design and optimization, both size distribution and particle size are important parameters to be con- sidered. Therefore, fuel size evolution at the particle level has been studied in diferent conversion devices, mainly for pyrolysis and com- bustion processes. The most relevant studies will be afterwards dis- cussed, and a summary is presented in Table 1. For small particles (< 1 mm) and high heating rates (> 10 4 K/s), studies can be found regarding the morphological evolution of a single particle. Holmgren et al [16]. used a drop tube reactor with pine stem wood and wheat straw under pyrolysis conditions to characterize shape https://doi.org/10.1016/j.biombioe.2019.01.044 Received 12 August 2018; Received in revised form 18 January 2019; Accepted 29 January 2019 ∗ Corresponding author. E-mail address: gianluca.caposciutti@ing.unipi.it (G. Caposciutti). Biomass and Bioenergy 123 (2019) 1–13 0961-9534/ © 2019 Elsevier Ltd. All rights reserved. T