Best Management Practices (BMPs) for Integrated Size Reduction and Separation
(Note: “Grinder” is a generic term used here to refer to any equipment for size reduction, and is inclusive of choppers, knife mills, hammer mills, disk mills, and other mechanisms.)
1. Biomass selection and moisture content affect its weakest mode of failure (ie. natural fracture) mechanisms. Use size reduction with shear cutting for biomass that primarily fails by tensile fracture. Use either shear or impact loading for biomass materials that fail with brittle fracture. Assess basic biomass properties to reduce trial and error approach in equipment selection.
2. Select size reduction equipment to deliver the mode of failure that most efficiently matches the weakest mode of failure of the selected biomass and moisture content. Most size reduction equipment apply a combination of failure (shear, tensile, impact) – but one mode usually dominants and may be estimated from the mechanics of the biomass-engaging tool. This implies that a single grinder design will not work equally well for all biomass types and moisture conditions – to maintain efficient size reduction. For example, equipment that works well for wood products may not work that well for energy grass crops, and vice-versa.
3. Configure size reduction equipment for appropriate input and output particle sizes determined by size reduction efficiency and particle separation, if the latter is desired. Size reduction efficiency often depends on the resident time of biomass in the grinder, especially rotary grinders equipped with classifying screens. Increased resident time due to additional internal circulation of biomass creates losses due to frictional rubbing between biomass and grinder internal surfaces. Reduce the ratio of input to output particle sizes to reduce frictional losses. Input and output sizes affect particle separation by physical means to purify chemistries associated with plant anatomical components. Ensure that output particle sizes retain enough physical traits of the plant anatomical to accommodate physical separation.
4. Reduce strain rate (rpm) to reduce size reduction energy. Grinder energy is required to accelerate biomass, to frictionally rub biomass against internal grinder surfaces and the biomass mat, and to overcome parasitic grinder energy losses. Increased strain rate greatly increases grinder energy input, yet there is insignificant increase in maximum mass throughput rate through grinders equipped with classifying screens.
5. Use linear knife grid technology to cut up packaged biomass in bales to minimize energy. Linear knife grids use input energy typically about 5 kW-h/ton, whereas rotary grinders use from 25 to 100 kW-h/ton.
6. Feed the maximum mass throughput rate to minimize size reduction energy expended per ton. Parasitic energy to operate an empty grinder is also present when operating a mill at part load operation. Determine the maximum feed rate that is sustainable with minimum problems in plugging of biomass flow paths through the grinder. Adjust the selection of biomass moisture and input particle size to maximize mass throughput rate.
7. Pick size reduction equipment to reduce particle “fines” or “dust.” Fine particles can be problematic in terms of air quality, worker exposure, energy input, and grind quality. Greater than 10 percent mass fraction of size reduced biomass passing 1 millimeter sieve is an indicator that “fines” will be a problem.
8. Understand that actual particle sizes represented by sieves differ, often substantially, from sieve-calculated values. For grass-like materials (long and slender) with original cross-sections intact, actual geometric mean lengths were five times longer than sieve-calculated geometric means.
9. Use estimated surface area to quantify external surface area. Note that gas-based particle surface area measures (for example the B.E.T. method) are not valid for biomass external surface area, since the gas permeates internal pores. Use image analysis to estimate two-dimensional geometric shapes of a high number of representative particles. Estimate third-dimensions and then calculate surface volume from external measures. Note that spheres are rarely created from biomass. As a check compare calculated surface areas before and after size reduction. Wrong assumptions of particle shape are often disclosed by observing opposite-than-expected trends in surface area before and after size reduction (after-grind surface area should be greater than before-grind surface area).
10. Identify anatomical separation physics relative to desired separation. For example, use terminal velocity of desired components, or select sieve sizes. Generally, the physical basis for separation should be greater than ten percent. Absolute separation of entire samples may require greater than ten percent difference. Expect to use multiple stages of separation to improve separation efficiency.
11. Simultaneously design biomass feedstock supply systems, size reduction equipment, and separation processes. Else, the efficacy of size reduction and separation may be severely compromised because of the supply of biomass is not within the required specification. Moisture content, soil particles, foreign matter, and initial chop length may limit the degree of success of subsequent processing.