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Two-stage catalytic fast hydropyrolysis of biomass produces biofuel with heating value, aromaticity close to gasoline

A team at the University of Connecticut Storrs reports on a two-stage catalytic fast hydropyrolysis process (CFHP) that produces a drop-oin biofuel with heating value and aromaticity close to that of gasoline. Their paper is published in the journal Fuel.

The two-stage CFHP followed by hydroprocessing produced nearly 18 mol% carbon yield to alkanes, and a total bio-oil carbon yield of 25 mol%. The approximate bio-oil higher heating value (HHV) and aromaticity were 43.4 MJ/kg and 28 wt%, respectively—both within the range of gasoline.

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

Comments

Roger Pham

The Hydrogen component of this Catalytic Fast Hydro-Pyrolysis can come from electrolysis of water using Grid-Excess Solar and Wind electricity, thereby providing financial incentive for continual growth of Solar and Wind energy until well past 100% power grid penetration.

SJC

"..increases carbon efficiency by shifting the reaction.."
Most excellent.

JMartin

Roger P: and the product can be stored for seasonal needs, not just short-term load balancing.

SJC

Good combination of biomass and solar, I like it.

Engineer-Poet

The carbon yield is good compared to previous processes, but pathetic when compared to what is required to actually replace petroleum fuels given the existing biomass supply.  1 billion tpy of dry biomass at 45% carbon and 25% yield turns into 131 million tons of (CH2)n.  At a density of 0.749 this is 175 billion liters, 46 billion gallons.

In 2016, the USA produced 9.995 million barrels of motor gasoline per day.  That is 154 billion gallons for the year.

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