Fast pyrolysis of biomass thermally pretreated by torrefaction

Authors: Boateng, Akwasi; Mullen, Charles

Submitted to: Journal of Analytical & Applied Pyrolysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2012
Publication Date: 12/12/2012
Citation: Boateng, A.A., Mullen, C.A. 2013. Fast pyrolysis of biomass thermally pretreated by torrefaction. Journal of Analytical & Applied Pyrolysis. 100, 95-102.

Interpretive Summary: Torrefaction is French for roasting and applies to the process of partially removing moisture and light volatile matter from organic compounds like coffee to enhance its qualities. Recently torrefaction has found applications in pretreating biomass to reduce its moisture content and increase its carbon content rendering its fuel properties similar to coal. Torrefied biomass tends to be brittle and easily pelletized so it can be more easily stored and economically transported than bulky biomass. Aside from its potential use as renewable coal it has also been suggested as a suitable feedstock for pyrolysis, a thermal decomposition in an absence of air, to produce renewable liquid fuel intermediates with lower acidity and thereby stable since torrefaction removes carboxylic acids so they do not end up in the liquids. We tested this hypothesis by producing torrefied biomass from switchgrass and hardwood pellets using a reactor called the biochar experimenter’s kit (BEK). We then used the solid product as feedstock for pyrolysis in a fluid-bed reactor so we could evaluate and compare the properties of the products with that from pyrolysis of the untreated biomass. We found that the liquids removed by the torrefaction process was an aqueous solution of mainly acetic acid and sugar extracts called levoglucosan associated with dehydration of the carbohydrate fraction of the raw biomass. Subsequent pyrolysis in the fluidized bed produced pyrolysis fuel oil with lower acidity and increased energy content compared with pyrolysis of non-torrefied biomass. However, the liquid yield, carbon conversion and energy conversion to the pyrolysis oils decreased with the torrefection pretreatment. The results will be useful to those looking for suitable feedstock for economic production of transportation fuels from pyrolysis technologies.

Technical Abstract: Torrefied biomass samples were produced from hardwood and switchgrass pellets using the biochar experimenter’s kit (BEK) reactor and analyzed for their utility as pretreated feedstock for biofuels production via fast pyrolysis. The energy efficiency for the BEK torrefaction process with propane gas as fuel source was about 50% for hardwood pellets and over 67% for switchgrass but the energy retained in the torrefied product ranged between 72 and 92% of the initial biomass energy content resulting from a high product yield of 70 – 88 wt%. The liquid pyrolysate by-product of the torrefaction process was an aqueous solution of mainly acetic acid and levoglucosan associated with dehydration of the carbohydrate fraction of the raw biomass. Subsequent fast pyrolysis in a fluidized bed led to production of pyrolysis oil with lower acidity and increased energy content compared with pyrolysis of non-torrefied biomass. However, the yield, carbon conversion and energy conversion to these pyrolysis oils was decreased with the torrefection pretreatment which predisposed the biomass for conversion to biochar rather than liquid products.