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.