Influence of Input Biomass Conditions and Operational Parameters on Comminution of Short-Rotation Forestry Poplar and Corn Stover Using Neural Networks

Abstract

Two kinds of biomass, short-rotation forestry (SRF) poplar and corn stover, were milled with a hammer mill at any combination of several levels of moisture content at mill inlet, w H 2 O = 7−30%, feeding chip size, 20−50 mm, and under varied operational milling conditions, such as the opening sizes of the internal screen, d target = 2−5 mm, and the angular velocity of the hammers, 2000−3000 rpm, to determine their influence on the milling energy consumption and the physical properties of the milled product (moisture content, particle size, bulk density, and angle of repose). The use of neural networks allows one to obtain three-dimensional predictive surfaces for each output variable, which describe tendencies and behavior as a function of the milling conditions. The moisture content at the mill inlet and opening sizes of the screen were found to be the key variables in the process. Specific energy requirements per oven dry tonne of about 28 kWh for poplar and 22 kWh for corn stover were obtained with almost dry biomass (w H 2 O = 7%) and d target = 5 mm, but this energy consumption increases 10-fold under opposite conditions, in which screen blocking was promoted by the high moisture content and the small opening sizes. Under these conditions (w H 2 O ≈ 30% and d target = 2 mm), particle drying can remove half of the initial moisture because of the strong increase in residence time of the particle in the mill chamber but also barely 1% under the opposite milling conditions, with low moisture and bigger opening sizes. Additionally, a bulk density increase between 11 and 98% for poplar and between 89 and 360% for corn stover was registered in relation to the previous chipped form and as a function of the milling conditions. With regard to the angle of repose, a higher moisture content and particle size made the handling behavior worse for both biomasses.

Hammer mill operating and biomass physical conditions effects on particle size distribution of solid pulverized biofuels

“…Raw materials at different conditions of input particle size and moisture content allow evaluating their effects on milling process. A complete description about the physical conditions of each biomass fraction has been reported on Gil et al [13]. [33]) with apertures of (0.1, 0.25, 0.5, 1, 2 and 5) mm were assembled on a vibrating screen machine (Orto Arlesa, vibro model).…”

“…Input biomass rate was regulated by means of the linear velocity of the feeding conveyor belt as a function of the current electric consumption of the mill, avoiding mill clogging (more details of the hammer mill and experimental facility can be found in Gil et al [40,13]). Tests were carried out on stationary continuous regime in which around 15-20 kg of biomass were milled.…”

“…Mani et al [8] and Bitra et al [9] tested switchgrass, corn stover and wheat straw with hammer mill at several operational conditions. In addition, [9] also studied the knife mill varying the rejection screen size from 12.7 to 50.8 mm, the feed rate from 120 to 660 kg h −1 and the operating speed from 250 to 500 rpm [10][11][12].Gil et al [13] showed also the influences of input biomass conditions of poplar and corn stover and operating factors on milling energy consumption, on the drying effect of biomass during milling and on other handling behavior indicators such as bulk density and angle of repose of the ground particles.…”

Manufacturing an Integrated Hammer Mill Unit for Grinding Rice Straw Intended for Multiple Applications

“…(Tavares, 2004). Gil et al (2013) studied the influence of several operational parameters and the biomass kinds on milling energy consumption and milled product size distribution. They tested wheat straw and corn stover under the variable milling knives and screen size from 12.7 to 50.8mm, with feed rate from120 to 660 kg/ h and operating drum speed between 250 to 500 rpm.The bio mass has moisture content (>20%) increased the specific energy requirements by 50%.…”

On the challenges facing the handling of solid biomass feedstocks

“…Due to biomass products having extremely low energy density per unit mass, pre‐treatment processes such as drying, steaming, binding, torrefaction, chopping, shredding, and grinding (comminution) the feedstock are often necessary to optimise the energy content and bulk density . Biomass input conditions and physical particle properties have been found to influence the energy requirement for comminution . Densification of biomass is needed for transporting large volumes thereby reducing operational costs and making trade more economically feasible.…”