Moisture and the die

Pressing works only in a moisture window. Too dry and the meal will not flow or bond, and in a long die it can jam or scorch. Too wet and steam pockets swell the pellets so they crack and crumble. Research on pelleting reports that wood presses best at roughly 5 to 10 percent moisture while straws and grasses prefer about 10 to 20 percent.

The ratio of hole length to hole diameter sets how hard the die squeezes. Wood dies often run near 6:1, while straw can need 8:1 or more. The quality flag and durability percentage here are a simplified teaching score built on those two windows, not a lab test, and the mill capacity limit (about 1,600 kg/h at 8:1, falling for longer holes) is an assumption.

The heat content math

One published study measured hemp biomass at about 18.1 MJ/kg gross and 16.6 MJ/kg net, on a dry basis. The readout corrects the dry net value for the pellet's moisture with the standard net calorific value relation used in solid biofuel testing: q(as received) = q(dry) x (1 minus M/100) minus 0.02443 x M, where M is moisture in percent and 0.02443 MJ/kg is the energy to evaporate each percent of water. It then converts MJ per kg to kWh per tonne (multiply by 1000 and divide by 3.6).

This is a teaching model: the cooler is assumed to remove about a quarter of the meal's moisture, output counts only durable pellets after fines are screened out, and real residue varies with ash and soil content.