Petre I. Miu

Bulk density and compaction behavior of knife mill chopped switchgrass, wheat straw, and corn stover.

Bulk density of comminuted biomass significantly increased by vibration during handling and transportation, and by normal pressure during storage. Compaction characteristics affecting the bulk density of switchgrass, wheat straw, and corn stover chopped in a knife mill at different operating conditions and using four different classifying screens were studied. Mean loose-filled bulk densities were 67.5+/-18.4 kg/m(3) for switchgrass, 36.1+/-8.6 kg/m(3) for wheat straw, and 52.1+/-10.8 kg/m(3) for corn stover. Mean tapped bulk densities were 81.8+/-26.2 kg/m(3) for switchgrass, 42.8+/-11.7 kg/m(3) for wheat straw, and 58.9+/-13.4 kg/m(3) for corn stover. Percentage changes in compressibility due to variation in particle size obtained from a knife mill ranged from 64.3 to 173.6 for chopped switchgrass, 22.2-51.5 for chopped wheat straw and 42.1-117.7 for chopped corn stover within the tested consolidation pressure range of 5-120 kPa. Pressure and volume relationship of chopped biomass during compression with application of normal pressure can be characterized by the Walker model and Kawakita and Ludde model. Parameter of Walker model was correlated to the compressibility with Pearson correlation coefficient greater than 0.9. Relationship between volume reduction in chopped biomass with respect to number of tappings studied using Sones model indicated that infinite compressibility was highest for chopped switchgrass followed by chopped wheat straw and corn stover. Degree of difficulty in packing measured using the parameters of Sones model indicated that the chopped wheat straw particles compacted very rapidly by tapping compared to chopped switchgrass and corn stover. These results are very useful for solving obstacles in handling bulk biomass supply logistics issues for a biorefinery.

Direct measures of mechanical energy for knife mill size reduction of switchgrass, wheat straw, and corn stover.

Lengthy straw/stalk of biomass may not be directly fed into grinders such as hammer mills and disc refiners. Hence, biomass needs to be preprocessed using coarse grinders like a knife mill to allow for efficient feeding in refiner mills without bridging and choking. Size reduction mechanical energy was directly measured for switchgrass (Panicum virgatum L.), wheat straw (Triticum aestivum L.), and corn stover (Zea mays L.) in an instrumented knife mill. Direct power inputs were determined for different knife mill screen openings from 12.7 to 50.8 mm, rotor speeds between 250 and 500 rpm, and mass feed rates from 1 to 11 kg/min. Overall accuracy of power measurement was calculated to be +/-0.003 kW. Total specific energy (kWh/Mg) was defined as size reduction energy to operate mill with biomass. Effective specific energy was defined as the energy that can be assumed to reach the biomass. The difference is parasitic or no-load energy of mill. Total specific energy for switchgrass, wheat straw, and corn stover chopping increased with knife mill speed, whereas, effective specific energy decreased marginally for switchgrass and increased for wheat straw and corn stover. Total and effective specific energy decreased with an increase in screen size for all the crops studied. Total specific energy decreased with increase in mass feed rate, but effective specific energy increased for switchgrass and wheat straw, and decreased for corn stover at increased feed rate. For knife mill screen size of 25.4 mm and optimum speed of 250 rpm, optimum feed rates were 7.6, 5.8, and 4.5 kg/min for switchgrass, wheat straw, and corn stover, respectively, and the corresponding total specific energies were 7.57, 10.53, and 8.87 kWh/Mg and effective specific energies were 1.27, 1.50, and 0.24 kWh/Mg for switchgrass, wheat straw, and corn stover, respectively. Energy utilization ratios were calculated as 16.8%, 14.3%, and 2.8% for switchgrass, wheat straw, and corn stover, respectively. These data will be useful for preparing the feed material for subsequent fine grinding operations and designing new mills.

Biomass Pre-Processing Size Reduction with Instrumented Mills

Rotary size reduction equipment were instrumented to identify important operating parameters with the aim of minimizing energy consumption while reducing the size of lignocellulosic biomass. Size reduction equipment included a knife mill, hammer mill, and disc mill. Monitored parameters included input power and particle size distributions.. Operating parameters included angular velocity, mass rate, screen size, and biomass selection. Results indicated optimum operating conditions with minimal energy input. Geometric mean dimensions of biomass particles varied from 5x for particle length to 0.3x for particle width when comparing actual to ASABE sieve results – highlighting the need for better consensus standards on particle size reporting.

Analysis of Biomass Comminution and Separation Processes in Rotary Equipment - A Review

Biomass utilization for bio-energy, bio-fuels or bio-based products requires reduction of the material size within specific ranges depending on feedstock specie, handling and further processing / conversion processes. Biomass size reduction is a mechanical treatment process that refers to either cutting or comminuting processes that significantly change the particles size, shape and bulk density of organic material. Rotary equipment such as knife mill, hammer mill and disc mill perform the above-mentioned processes in various ways, as function of physical-mechanical properties of the material, tools geometry, feeding parameters and dynamics of particles separation. This paper is intended as a comprehensive review of rotary machines design and process investigation that is closely related to material properties as well as requirements for further mechanical or bio-chemical processing.

Linear Knife Grid Application for Biomass Size Reduction

Most of the commonly used biomass size reduction devices use rotary action. An alternative mode of size reduction is by linear action of cutting element directly on the biomass material. A linear action knife grid model device was developed to as a prototype to solve issues related to scale-up and determine cutting characteristics of selected biomass materials. Major components of the prototype were ram, feed block, knife grid, knife holder block, product block, and bottom tray. Tool steel hardened knifes were arranged in a square grid pattern and their spacing was adjustable. The whole assembly were mounted and tested on a universal testing machine. Dry corn stalks and switchgrass were selected as the test materials. Cutting experiments were conducted at various grid spacing, fill depths, and refill runs. The output variables measured were maximum force, peak stress, energy, material mat thickness, and mass of products after each refill runs. New surface generated by the cuts will be evaluated theoretically using the circle packing theory. Ultimate cutting stress and net energy requirements increased with increasing fill depth and decreasing knife grid spacing. A final product size of 50 mm required 1.69±0.81 kWh/t for corn stalks and 0.49±0.04 for switchgrass, while a product size of 101 mm required 0.48±0.08 kWh/t for corn stalks and 0.14±0.02 kWh/t for switchgrass. Mean values of specific energy based on new surface generated were estimated as 83.78±24.5 and 63.34±12.3 kN/m for corn stalks and 21.50±1.9 and 15.53±1.7 kN/m for switchgrass for product size of 50 and 101 mm, respectively. Corn stalks required 3.3 to 3.4, and 3.9 to 4.1 times higher cutting energy than switchgrass based on mass and new surface area generated energy basis, respectively. Scaling up is highly feasible for large product sizes because the determined specific energy values are small; and they are well below the reported values.

Comminution Energy Consumption of Biomass in Knife Mill and its Particle Size Characterization

Current research is driven by the need to reduce the cost of ethanol production from biomass. Preprocessing research is focused on developing processes that would result in reduced bioconversion time, minimize enzymes usage, and/or maximize ethanol yields. Size reduction is an important preprocessing unit operation of biomass, which utilizes major portion of input energy. It changes the particle size, shape, and bulk density, and increases the total surface area of biomass and number of contact points for chemical reaction. Objectives of the present study were to chop switchgrass, wheat straw, and corn stover in knife mill and analyze the particle size distribution. Direct power inputs were determined for different knife mill screen openings from 12.7 to 50.8 mm, rotor speeds between 250 and 500 rpm, and mass feed rates from 1 to 11 kg/min. During the experiment, data were collected for determining effective and total specific energy for chopping. The chopped samples were analyzed for particle size distribution using ASABE sieve analyzer. For knife mill screen size of 25.4 mm a speed of 250 rpm gave the optimum performance. The optimum feed rates at these conditions were 7.6, 5.8, and 4.5 kg/min for switchgrass, wheat straw, and corn stover, respectively. The corresponding total specific energies were 27.3, 37.9, and 31.9 MJ/Mg and effective specific energies were 4.6, 5.4, and 0.9 MJ/Mg for switchgrass, wheat straw, and corn stover, respectively. Mathematical equations adequately fitted the total specific energy consumption and particle size distribution data. knife mill chopping of switchgrass/wheat straw/corn stover resulted in ‘well-graded’ ‘strongly fine-skewed mesokurtic’/’fine-skewed mesokurtic’/’fine-skewed mesokurtic’ particles with reduced size screens (12.7 to 25.4 mm) and ‘well-graded’ ‘fine-skewed mesokurtic’/’strongly fine-skewed mesokurtic’/’fine-skewed mesokurtic’ particles with increased size screen (50.8 mm).

Simulation des Gesamt-Mähdreschers

Mit Simulationswerkzeugen lasst sich der Versuchsaufwand in der Mahdrescherentwicklung reduzieren. In diesem Beitrag wird ein modular aufgebautes, mathematisches Mahdrescher-Gesamtmodell vorgestellt und ein Anwendungsbeispiel fur die Gutflusssimulation gezeigt. Die Simulationsergebnisse sind realistisch. Deshalb stellt das Simulationswerkzeug ein zeitsparendes Hilfsmittel in der Mahdrescherentwicklung dar.