Comminution of Dry Lignocellulosic Biomass: Part II. Technologies, Improvement of Milling Performances, and Security Issues

Abstract

Lignocellulosic feedstocks present a growing interest in many industrial processes as they are an ecological alternative to petroleum-based products. Generally, the size of plant raw materials needs to be reduced by milling step(s), to increase density, facilitate transport and storage, and to increase reactivity. However, this unit operation can prove to be important in term of investments, functioning costs, and energy consumption if the process is not fully adapted to the histological structure of the plant material, possibly challenging the profitability of the whole chain of the biomass conversion. In this paper, the different technologies that can be used for the milling of lignocellulosic biomass were reviewed and different avenues are suggested to improve the milling performances thanks to thermal pretreatments. Based on examples on wheat straw milling, the main points to take into consideration in the choice of a milling technologies have been highlighted in regards to the specifications of ground powder. A specific focus on the hazards associated to the milling and the manipulation of fine biomass particles is also realized at the end of the paper from the perspective of industrial applications.

Keywords: atex explosion hazard; cryogenic milling; energy consumption; grinding; milling; plant materials; torrefaction.

Figure 1

Milling chambers of different devices and their main mechanical stresses: (a) impact mill, (b) pin mill, (c) vibratory ball mill and ppposite jet mill used for fine milling (a,b), and ultrafine milling (c,d) of biomass.

Figure 2

The energy consumption for the intermediate the fine and the ultrafine milling of lignocellulosic biomass and the different grinding laws (Rittinger, Bond, and Kick).

Figure 3

Classification of different milling equipment based on the energy consumption, the particle size reduction capacity, and the speed of milling, from different research works [8,16,21,41,42,43,44,45,46].

Figure 4

Influence of ambient and cryogenic milling on the dissociation of tissues in the plant materials.

Figure 5

Process diagram for the milling of wheat straw up to a median particle size below 10 µm suitable for bioethanol production and charge for bio-composite production.