Energy requirements to produce fine powders of raw and torrefied wood at pilot scale, and characterization of their flowability

Abstract

This work aims at comparing different preparation chains to produce wood powders suitable for further gasification in an entrained flow reactor. Three wood powders, with particle size below 1 mm, have been produced at pilot scale from resinous wood chips: a powder of raw wood and a powder of torrefied wood both ground in a knife mill, and a powder of raw wood ground in a vibration mill. The respective requirements both in energy and in feedstock material resource have been determined for each chain of production. The production of the raw wood powder requires 0.83 MWh per ton of dry powder (tdp). The additional grinding step with the vibrating mill adds 0.63 MWhel.tdp−1. The production of the torrefied wood powder requires 3.56 MWh.tdp−1. Moreover, the wet wood chips requirements vary from 2.5 tons for the chains of production without torrefaction to 4.4 tons for the chain including a torrefaction step. In the latter, the step with the highest energy demand is the gas-cleaning step in an instrumented post-combustion reactor. Heat recovery from the combustion gases could supply energy to both the drying and torrefaction steps. It would reduce by half the total energy cost of the chain, down to 1.66 MWh.tdp−1. The resource requirements would be reduced down to 3.2 tons. The morphology and flowability of the powders have been investigated and compared. Torrefaction or vibration milling significantly improve the ability of the wood powder to flow both without any stress or when consolidated by a vertical stress.