Research paper Multiscale structure characterization of sawdust-waste water sludge extrudates dried in a pilot-scale fixed bed Jie Li a, * , Erwan Plougonven a , Laurent Fraikin a , Thierry Salmon a , Dominique Toye a , Emmanuel Nistajakis b , Ang  elique L  eonard a a Laboratory of Chemical Engineering, Department of Applied Chemistry, University of Li ege, 4000 Li ege, Belgium b Industrie du Bois Vielsalm & Cie SA, Zoning Industriel de Burtonville, 6690 Vielsalm, Belgium article info Article history: Received 14 May 2015 Received in revised form 16 June 2015 Accepted 18 June 2015 Available online xxx Keywords: Waste water sludge Sawdust Convective drying X-ray tomography Mercury porosimetry abstract Convective drying of waste water sludge and sawdust-sludge mixtures in a pilot-scale fixed bed was studied. Drying was performed in a cross-flow convective dryer using 500 g of wet material extruded through a disk with circular dies 12 mm in diameter. The structure of the bed mainly shows volume shrinkage and crack formation during drying. Several characterization techniques were used over a wide range of scales from nm up to mm. The overall bed structure was imaged with X-ray macrotomography, at a resolution of 0.36 mm per pixel. Single extrudates were scanned with X-ray microtomography, at a resolution of 41 mm per pixel. Pore structure of the dried samples were characterized by mercury porosimetry (7.5 nm < d p < 150 mm). Results show significant structural changes on all scales with increasing amounts of sawdust: shrinkage decreases, crack formation increases, and the pores become larger. This confirms the benefits of sawdust addition for sludge drying applications. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction As more waste water sludge is being produced year after year, its efficient disposal becomes increasingly important. The vast ma- jority of sludge is disposed of through gasification and combustion in furnaces, but to do so water content must be lowered by drying, in order to reach a calorific value adapted to such thermochemical conversion process [1]. The amount of energy needed for drying such soft and pasty material is significant, therefore new ap- proaches are sought to speed up the process. Adding sawdust to the sludge is a way of reinforcing the structure and help reduce drying time, thus lowering the amount of energy necessary for drying process [2]. On the basis of a previous study dealing with back mixing [3], where it was found that expansion of the sludge bed enhanced heat and mass transfer, the addition of sawdust in the sludge is a promising approach to achieve such expansion. The choice of sawdust is justified in that it is useful for gasification, and it is produced in large amount by the forest industry and also needs safe disposal solutions [4]. Typically, sawdust is used in the manufacture of compressed biofuels or wood boards [5], but new applications should be explored. A mixing machine can be added to the typical industrial sludge drying setup which consists of a sludge extruder and a belt dryer. The investment cost will slightly increase because of the addition of the mixing machine, but the overall cost may be considerably reduced with the reduction in drying time. Shrinkage and crack formation are important phenomena in sludge drying: as the sample loses its moisture it exhibits volu- metric reduction before the internal stresses exceed the tensile strength and leads to cracking [6,7]. Such structural changes during the drying process affect the paths of heat and mass transfer, which in turn affects the drying characteristics of the material [8]. This inter-coupled process governing the drying mechanism needs to be better understood in order to identify the final sludge properties, and this requires more thorough characterization of the structural changes that occur during drying. X-ray tomography is a non-destructive 3D imaging technique that can provide such structural information. It works by per- forming a series of X-ray radiograms of the sample from different viewing angles, and algorithmically reconstructs a 3D X-ray atten- uation map [9e11]. This imaging technique has many advantages, and it has applications in many research fields [12e16]. On the study of sludge drying, L eonard et al. [17e19] were the first to use X-ray microtomography for the quantification of volume, Abbreviation: WWTP, waste water treatment plant. * Corresponding author. E-mail address: jeff_lee@foxmail.com (J. Li). Contents lists available at ScienceDirect Biomass and Bioenergy journal homepage: http://www.elsevier.com/locate/biombioe http://dx.doi.org/10.1016/j.biombioe.2015.06.009 0961-9534/© 2015 Elsevier Ltd. All rights reserved. Biomass and Bioenergy 81 (2015) 98e107