1.6.1 Fast Pyro l y s i s
An important challenge to the use of fast pyrolysis bio-oil for production of liquid transportation fuels is its high oxygen content. To make the bio-oil more compatible with the existing transportation fuels infrastructure, much of the oxygen needs to be removed.
The high oxygen content issue is particularly acute for the fraction of the bio-oil that comes from the cellulose and hemicelluloses fractions of
lignocelluloses. The bio-oil fraction from lignin is significantly lower in oxygen content.
In previous studies, the bio-oil was hydrotreated at high pressures (2000 – 2500 psi) and low space velocities (0.1 – 0.2 LHSV). The resulting hydrotreated oil was then cracked in a fluid catalytic cracker, or hydrocracker, to produce gasoline. At these high pressures and low space velocities, hydrodeoxygenation predominates. However, large quantities of hydrogen are required during hydrodeoxygenation due to the high level of oxygen in the bio-oil. Alternative strategies that require the use of less hydrogen to remove oxygen would be quite attractive. It is possible that these alternative strategies would work better with a certain product distribution in the bio-oil. Processing to upgrade bio-oil is intimately linked with how the fast pyrolysis unit is operated. Therefore, research is needed to establish the bio- oil characteristics that would be most desirable to obtain from the fast pyrolysis reactor system. Another serious problem for fast pyrolysis processing is the high acid number of the bio-oils, which will cause corrosion in standard refinery units. Although the bio-oils can probably be processed using 317 stainless steel cladding, this material is not standard in refinery units making it difficult to introduce bio-oil into the existing refinery infrastructure. Therefore pyrolysis bio-oils require pre-processing to reduce the acid number before processing in typical refinery units.
Research is needed to understand how to accomplish this pre-processing in an efficient manner. There is an opportunity to introduce chemical transformations during this pre- processing that would not only reduce the acid number, but also decrease the oxygen content,
area than the fast pyrolysis area. Some work has been performed with solvents other than water, which would potentially allow operation at somewhat lower pressures. However, liquefaction systems using alternative solvents create a myriad of new problems, so none are currently in
development.
As with fast pyrolysis, the chemistry in biomass liquefaction is complex. So, thus far, investigators have attempted to draw only empirical
relationships between reaction conditions and biomass composition. In addition, significantly less work has been reported on analyzing the chemical composition of liquefaction bio-oil than fast
pyrolysis bio-oil. Therefore, further advancement in this process area needs to be supported by a more fundamental understanding of the chemical
reactions that occur during the liquefaction process. There is also an opportunity to modify this chemistry by introducing a catalyst into the process.
1.7 RECOMMENDAT I O N S
Research needed to advance the technology of selective thermal processing of lignocellulosic biomass can be grouped into six topics: overarching technical needs, plant characteristics, feedstock preprocessing, deconstruction selectivity, bio-oil recovery (fast pyrolysis), and alternative
deconstruction approaches.
1.7.1 Ove r a rching Technical Needs
•Development of more detailed thermal deconstruction microkinetic models that can account for a broad range of chemical reactions. These can be used to provide a basis for
choosing which reactions should be enhanced to increase selectivity for desired chemical thereby addressing two bio-oil challenges.
Due to the complexity of the biomass pyrolysis reaction system, the underlying chemistry is not well understood. Therefore, the correlations that have been developed between bio-oil composition and pyrolysis and condensation conditions are empirical. The lack of basic understanding of the reaction system limits the ability to draw general relationships between biomass composition, pyrolysis conditions, condensation conditions, and bio-oil composition. Therefore, research is needed to develop better foundational chemistry knowledge about the fast pyrolysis reaction, which can drive the process to more desirable product compositions.
As mentioned above, the alkaline cations in the lignocellulose feedstocks serve as a catalyst for reactions in the pyrolysis reactor. While this result is understood empirically, little work has been done on the intentional manipulation of these alkaline cations. Instead, research has largely focused on examining how their naturally varying levels in native lignocellulose result in different bio-oil properties or on their removal in a pre- processing step. There is a need for a more systematic understanding of the role of different alkaline cations, as well as the alkaline cation concentration, on the pyrolysis reaction. Additionally, the intentional introduction of different catalytic moieties could be used to alter the product bio-oil composition.
1.6.2 Liquefaction
Due to its lower oxygen content, bio-oil from liquefaction has more desirable properties than bio-oil from fast pyrolysis. However, the high pressure conditions required in the liquefaction process makes the capital requirement significantly higher than for a fast pyrolysis process. Since this limitation is intrinsic to the process, there is significantly less work ongoing in the liquefaction