Biomass and Bioenergy 139 (2020) 105631 Available online 19 June 2020 0961-9534/© 2020 Elsevier Ltd. All rights reserved. Compaction of chopped material in a mini silo Aleksander Lisowski a, * , Joanna W� ojcik a , Jacek Klonowski a , Michał Sypuła a , Jarosław Chlebowski a , Krzysztof Kostyra a , Tomasz Nowakowski a , Adam Stru _ zyk a , Adam � Swiętochowski a , Magdalena Dąbrowska a , Leszek Mieszkalski b , Michał Piątek a a Department of Biosystems Engineering, Institute of Mechanical Engineering, Warsaw University of Life Sciences, Nowoursynowska 166, 02-787, Warsaw, Poland b Department of Production Engineering, Institute of Mechanical Engineering, Warsaw University of Life Sciences, Nowoursynowska 166, 02-787, Warsaw, Poland A R T I C L E INFO Keywords: Chopped material Cyclic compaction Compaction parameters Density model ABSTRACT The aim of the study was to explain the effect of pressure and compaction time, number of layers and compaction cycles of biomass from six energy plant species intended for silage on the density of mini silos as well as energy consumption and compaction indicators. A mathematical model was developed to predict the silage density against changed process factors. Chopped biomass was compacted in four layers using three cycles at 17–63 kPa pressure and 6–10 s per cycle. The greatest changes in compacted density were achieved in the frst cycle of the frst layer. At subsequent stages, the recompression curves were steeper, and more stable and higher densities were obtained. For giant knotweed the required silage dry matter density of 225 kg m 3 was achieved for plants at the physiological maturity stage, with a lower moisture content of 23.6%, than that obtained for plants harvested in June. The silage density was greater for deciduous plants (Virginia mallow and Jerusalem artichoke, but not giant knotweed) than that for grasses (miscanthus, Spartina pectinata, and big bluestem); this result was due to the lower moisture and to differences in the structure of the shoots. Silage density describes the model well in terms of pressure, number of layers, compaction time, particle size and dry matter. 1. Introduction Obtaining high silage density is a necessary requirement for good process fermentation; at the same time, it reduces forage loss by decreasing oxidation losses, which are associated with a reduction in porosity [1]. Proper compaction of material in tower silos with a height of not less than 6 m is achieved by gravity [2]. Based on various study results from the literature, it was found that in tower silos of lower height and in horizontal silos, it is necessary to actively compact the plants material to the minimum dry matter (DM) density of 225 kg m 3 [3]. Other researchers indicate, that grass and maize silages may have lower DM densities of 150 kg m 3 and 180 kg m 3 , respectively [4]. In tower silos, compaction is carried out by special devices with rotary motion, and in horizontal silos most often by the use of a wheeled tractor with additional weights. After evenly spreading a new layer of forage about 15 cm of thickness, the tractor compacts the material by passing the track next to the track. Layer thickness, tractor weight, and tractor multiplicity are practically selected depending on the type of material and its properties. By increasing the silage density, the effciency of using the silo capacity for silage storage increases, reducing the number of silos required; however, achieving these benefts is associated with greater energy consumption. It is necessary to use heavier equipment for packing of forage and thinner one-time compacted layers [5]. The preparation of the silo will be more expensive, but considering that high feed value is preserved, the fnal economic balance can be positive. Silage density depends on the characteristics of the plant material and on technological and technical factors. The technological factors affecting silage density include the type and maturity of the plant, the time of day at which the plants are harvested, the particle size and distribution, the dry matter content of the forage, the rate of forage supply (t⋅h 1 ), and number of tractor passages, the compression time, the compacted layer thickness; and technical factors include the tractor weight, tire pressure, the silo depth and the maximum silo height [6–9]. Materials from various plants are used for silage: maize, sunfower, alfalfa, grasses and other special crops that must be harvested at the right maturity to ensure the best silage quality. It was found that with a delay in sunfower harvesting from the state of maturity of the beginning of anthesis (fowering) through one-third milk line to black line, the silage density increased as a result of DM concentration [10]. Under * Corresponding author. E-mail address: aleksander_lisowski@sggw.pl (A. Lisowski). Contents lists available at ScienceDirect Biomass and Bioenergy journal homepage: http://www.elsevier.com/locate/biombioe https://doi.org/10.1016/j.biombioe.2020.105631 Received 10 December 2018; Received in revised form 26 May 2020; Accepted 2 June 2020