(475b) Origins of Char during Fast-Hydropyrolysis of Biomass to Fuels

Fast-pyrolysis based technologies for biomass upgrading to fuels are considered promising due to the higher yields of liquid fuel products versus other thermochemical processes [1]. However, fast pyrolysis produces a carbon rich residue, bio-char, accounting for 25-40 wt.% of the product stream depending on biomass type [2]. To make this technology more efficient and economical, potential char contributors need to be identified and strategies to mitigate char production must be developed.

In this work, a combination of biomass pretreatments and mass spectrometric analysis coupled with lab-scale reactors have been used to clarify the char contributions during biomass fast pyrolysis from key components such as lignin, hemicellulose and inorganics. Cellulose is found to be a minor char contributor; theoretical calculations have shown that dehydration is the dominant mechanism leading to the formation of the small amount of char that is formed. Studies with lignin model compounds and delignified biomass have revealed that lignin is major char contributor. In case of hemicelluloses, the results are complicated by the presence of inorganics. Biomass pretreatments coupled with characterization using mass spectrometry, have enabled synthesis of a set of inorganic-free, hemicellulose-rich and hemicellulose-lean samples to test for the char contributions from hemicellulose fast-pyrolysis. These materials have shown that removal of hemicellulose from the biomass reduces the fast-pyrolysis char production. The extent of char reduction for hemicellulose extracted samples is lower than for the delignified biomass samples suggesting that lignin is the stronger char contributor. Using complementary mass spectrometry and Py-GC/MS experiments to probe the role of inorganics in altering the pyrolysis product distribution showed that divalent cations (Mg, Ca) and anions (SO₄^2-) lead to higher abundances of lower molecular weight products and catalyze dehydration and ring-opening reactions on stable pyrolysis products. Inorganic-doped samples were also found to produce more char than the un-doped samples.

Using this knowledge of char sources resulting from fast-hydropyrolysis, we can optimize bio-refining process sequencing using a combination of pre-treatments and fast-pyrolysis and thus achieve higher carbon efficiency. For example, calculations have shown that the overall carbon recovery to C₆₊ products of a pyrolysis-based bio-refining can be improved by up to 28% through initial removal of lignin via the catalytic depolymerization processes to reduce carbon losses to char.

References:

[1] A. V. Bridgewater, Biomass and Bioenergy. 38 (2012) 68-94

[2] D. Carpenter et al., Green Chemistry. 16 (2014) 384-406