The Hydrogen Wall: Looking at the Prospects for Drop-In Biofuels

by Jim Lane (Biofuels Digest) Think affordable, available, sustainable carbon is the biggest barrier to the growth of biofuels? Or, access to market via blender pumps? In the case of drop-in biofuels, the biggest challenge might be finding enough hydrogen. You might have heard of the Hydrogen Economy, the Hydrogen Miracle, the Hydrogen Car, or that free hydrogen (H2) is the most abundant molecule in the universe. ...
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Is there a hydrogen barrier to drop-in fuels? As a trio of authors put it, part of the International Energy Agency’s Task 39 Group working on the development of a global biofuels industry, “One challenge that the development of drop-in biofuels shares with the global petroleum industry is the requirement for increasing amounts of molecular hydrogen.”
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As UBC’s Sergios Karatzos and Jack Saddler, along with NREL’s James D. McMillan explain: “Oil refineries use hydrogen to upgrade low-grade crude oil, both to remove problematic sulfur and other heteroatom impurities (hydrotreating) and to “crack” longer carbon chain molecules to shorter chain molecules while also enriching them with hydrogen (hydrocracking). In the case of petroleum, more hydrogen will be needed to upgrade crude oil feedstocks of declining quality (i.e., increasingly heavier and more sour, as in the case of heavy oils being sourced from Venezuela and Canada).”
So, that’s petroleum. What about biofuels?
“Similarly, greater amounts of hydrogen are generally needed to produce more energy dense and highly reduced drop-in biofuels…to remove oxygen from oxygenated lignocellulose intermediates or lipid feedstocks. Non-hydrogen-consuming processes such as catalytic or thermal cracking can also be used to increase the H/C ratio of petroleum feedstocks by removing carbon in the form of tars and char (coke). However, this approach consumes feedstock and reduces yields and so is generally avoided, particularly when prices for crude oil feedstocks are high.”
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First, with biofuels feedstocks, to get the hydrogen/carbon ratio right for drop-ins, you either are blowing off hydrogen and oxygen (reducing yield), or adding hydrogen (adding complexity and cost — for example, where do you get the free hydrogen in the first place?).
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Turns out, drop-in fuels offer a no-brainer market but quite a challenge in economics and technology.
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What about co-processing biofuels and petroleum at refineries, to reduce capex and opex?
“Even at low biofeed blending levels, the disparity in hydrogen requirements can lead to problems in co-processing biofeeds with petroleum feeds. Reported difficulties include hydrogen starvation and excessive pressure drops within hydrotreating units, as well as poor desulfurization (of the petroleum feed fraction) through coking and deactivation of hydrotreating catalysts.
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“Pyrolysis bio-oils contain up to 40% oxygen and need to be extensively upgraded to produce deoxygenated hydrocarbon drop-in biofuel blendstocks. ..."
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“It has been suggested that a two-step hydrotreatment process might be a more cost effective approach to bio-oil upgrading. The first step would stabilize the bio-oil by selectively hydrotreating its most reactive (unstable) organic species and, once stabilized in this manner, a second step would be used to complete hydrotreatment.”
If pyrolysis were integrated in an existing refinery, how could that happen?
“The present analysis concludes that there are two particularly favorable insertion points for pyrolysis oils, namely before either a refinery’s Fluid Catalytic Cracking units or its hydroprocessing units.”
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“The second, more “boutique” insertion point for bio-oils in a refinery is before the refinery’s hydroprocessing unit operations. Hydroprocessing is more sensitive to oxygen and impurities than FCC units and hydroprocessing insertion of pyrolysis oils relies on substantial hydrogen inputs, much costlier catalysts and extensively pre-processed bio-oils.
The good news?
Taking the hydroprocessing route “produces increased yields of higher value middle distillates such as diesel and jet fuels than the FCC insertion strategy.”
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“The other major thermochemical route to drop-in biofuels is through gasification.
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And, drop-in biofuels might not be the best value proposition for the biochemical processing of biomass and sugars. Biochemical platform routes are already well suited to make oxygenated products such as carboxylic acids, alcohols and polyols that can generate substantially higher revenues in the rapidly growing bio-based chemicals markets. Until these value-added markets are saturated, there is little economic incentive for the biochemical platform companies to focus on drop-in fuels. ”
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Cheap hydrogen generally is sourced from petroleum, so that imposes sustainability and logistic constraints. There’s hydrogen in biomass itself — but using it for processing rather than within the finished fuel reduces yield, and causes more stress on feedstock costs.
Two possible solutions?
First, the biorefinery model where a combination of high-margin, low-volume chemicals generate 20% of the volume and 80% of the profits, while low-margin, high-volume fuels generate 20% of the margin and 80% of the volume. Works generally only in markets where the chemical demand is relatively saturated, or easy to do so.
Second, the two-step processing approach. Small-scale pyro units process biomass locally, and do a preliminary upgrade, to make a biocrude out of bio-oil. The resulting product is shipped to refineries, which can more easily integrate it either into a blend of feedstocks, or process it effectively using modified FCC units. READ MORE and MORE (Fleets and Fuels) and MORE (Fleets and Fuels) Link to Task 39 Publications Slide presentation