Biomass fast pyrolysis thermally decomposes the feedstock using high heating rates and intermediate temperature (typically 400–600°C) under an inert atmosphere. Under these conditions, the polymeric cellulose, hemi-cellulose and lignin fractions of the biomass fragment to form volatile bio-oil products, in addition to char and permanent gas. The bio-oil product is an unstable mixture of low-energy oxygenated hydrocarbons, which must be upgraded prior to use as a transportation fuel.

… low bio-oil yields, carbon loss to solids and permanent gas, as well as low bio-oil heating value are major obstacles, which must be minimized in order to make the fast pyrolysis process viable. Moreover, a substantial majority of hydrocarbons formed from CFP are aromatic in nature, while traditional gasoline typically contains about 25 wt% aromatics.

… Raw biomass is rich in oxygen, some of which is removed as water, CO and CO₂ after pyrolysis alone. The bio-oil may then be upgraded over an acid catalyst to form a more stable blend of aromatic hydrocarbons and low levels of oxygenates. However, due to the hydrogen depleted environment during pyrolysis, even the most deoxygenated bio-oil has an atomic H:C ratio of about 1, which more closely resembles the H:C ratio of coal, rather than a typical petroleum blend. Hydrodeoxygenation (HDO) must be performed on the upgraded bio-oil to improve higher heating value (HHV), remove remaining oxygen heteroatoms and bring the total H:C ratio closer to 1.75.

… substantial work is still necessary to maximize HDO catalyst effectiveness, reduce process energy intensity and limit overall hydrogen consumption. In this work, we explore the feasibility of catalytic fast hydropyrolysis (CFHP) as a means to circumvent this cycle, and produce high returns of renewable alkanes and aromatics from lignocellulosic biomass in a single- or two- step catalytic process.

—Gamliel et al.

CFHP is the catalytic pyrolysis of biomass at a high heating rate under a pressurized hydrogen atmosphere. The hydrogen in the CFHP reactor increases carbon efficiency by shifting the reaction pathway from decarbonylation and decarboxylation to dehydration, thus removing oxygen in the form of water. Inclusion of a ZSM-5 catalyst in the CFHP reactor assists in pyrolysis vapor deoxygenation and char reduction, while the presence of a transition metal, such as Ni or Ru may assist in hydrogenation and HDO reactions.

To understand the factors that drive CFHP product selectivity towards true drop-in fuel compounds—i.e., a blend of aliphatic and aromatic hydrocarbons—the researchers performed CFHP with anisole, a common bio-oil model compound, at a range of operating conditions. They proposed and tested two process modifications to overcome the thermodynamic barrier for hydrogenation in the context of CFHP vapor upgrading: reduction of the heating rate; and CFHP followed by second-stage hydroprocessing (CFHP-SH).

Lowering the heating rate resulted in increased alkanes selectivity, but low overall bio-oil yield and high solid yield. CFHP-SH produced the very high alkane yield noted above, with the gasoline-range HHV and aromaticity levels.

Resources

• David P. Gamliel, George M. Bollas, Julia A. Valla (2017) “Two-stage catalytic fast hydropyrolysis of biomass for the production of drop-in biofuel” Fuel 10.1016/j.fuel.2017.12.017