To: Noa
From: Tom
Date: November 7, 2008
RE: Berkshire Energy Laboratory - Weekly Update
Summary
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Developed a Technology assessment designed to define the problems, identify the gaps and
recommend studies to address them.
A fundamental law-of-nature issue with biomass to liquid conversions is energy density and
therefore feedstock availability.
The gasification process is the primary area of efficiency loss in both FT and MTG synthesis.
Some work has been done in dissolution of biomass in ionic fluids which is a promising start to a
more efficient and completely different conversion route that could take place entirely in the
liquid phase at less severe (energy intensive) conditions. That’s ‘going the other way’.
We should also consider setting up a visit to NREL.
I’ve made a couple of e-mail and phone inquiries (Prof. Huber at UMass) but no calls back.
Sludge & Biomass to Liquid Fuels - Technology Assessment
The following is my assessment of the current sludge/biomass-to-liquid fuels technology for the
purposes of identifying gaps and impediments to commercialization, and designing experiments to
address them.
Understanding the Problem
We’ve been investigating the synthesis of liquid transportation fuels from sludge/biomass (SBLF)
so as to take advantage of existing fossil fuel infrastructures. Technologies exist to achieve the
conversion but commercial viability is inhibited by feed stock availability, energy density and
thermal efficiency. In other words we need relatively large amounts of the low density
feedstock to convert it to a useful amount of high density fuel in part because the thermal
efficiency is low.
To illustrate we’ve been working with Fischer Tropsch (FT) synthesis and methanol-to-gasoline
(MTG). Both start with gasification of the feed stock and are about 60% efficiency overall.
Comparing feedstocks, sludge has a lower heating value than wood, but it is also denser so at
60% efficiency, approximately the same amount of feed is required for each on a volume basis:
~ 50 ft3/barrel. A typical filling station would consume about 145 barrels per day requiring
about 7000 ft3 of feed. For reference, as previously noted, I estimate that approximately 3,400
ft3 of dewatered sludge is available to us on a daily basis in the Saratoga area (50 mi radius
includes Capital District). I don’t have an estimate for biomass availability but the example
illustrates the point. All of the currently available sewage sludge in the area is insufficient to
supply the needs of an individual filling station.
If thermal efficiencies could be improved to 90%, the required feed would be reduced by
between a quarter and a third. This would be a nice improvement but even under the best
circumstances, SBLF will require significant feed and large scale material handling and reactor
systems. Some of the current research then is directed toward smaller, increased through-put
reactors but the bulk of it is devoted to improving thermal efficiencies. With integrated heat
recovery, both the FT synthesis reaction itself and the MTG process are highly efficient (> 90%).
Methanol synthesis is less efficient, however most of the efficiency is lost in gasification and
much of the research in biomass conversion is directed there as we’ve seen.
Catalytic Processes in General
A catalyst is a substance added to a reaction to lower the overall activation energy and increase
the reaction rate without consuming the catalyst. For a chemical reaction to occur a certain
minimum energy must be applied to activate it. The catalyst reduces the overall activation
energy by reacting with the original reactants in intermediate steps, each with lower activation
energy. The overall reaction is the same, there are just more steps. The term ‘homogeneous’
catalysis means that the catalyst is in the same phase as the reactants and ‘heterogeneous’
means the catalyst is in a different phase such as the metal catalysts used in gasification
reactions.
Catalysts are useful in a number of reactions but they are subject to the same laws of
thermodynamics as every other reaction (conservation of mass/energy, entropy). They can
lower the activation energy but the overall reaction energy is not changed. They are subject to
fouling (which means essentially that they ‘react’ with the product) and degradation in the form
of sintering and disassociation. Essentially in applying catalysis the energy is moved from the
reaction and applied toward catalyst preparation, handling and rejuvenation. Many reactions
are infeasible without a catalyst and it’s often the best approach from economic point of view
but it’s not thermodynamically ‘free’. If the catalyst is inexpensive and fouls/degrades relatively
slowly it’s a good a choice.
Technology Gaps
Identification and documentation of the technology gaps will support the design of experiments
and studies. The relatively low energy density and high feedstock requirements are constraints
that have to be worked around. Technology efforts are directed toward improving efficiency
and lowering capital costs.
1) Gasification Efficiency
Thermal efficiency is ‘energy out/energy in’ and for gasification the ‘energy out’ is the
heating value of the syngas. The ‘energy in’ is the heating value of the feed plus whatever
processing energy is required for the conversion. A highly efficient (ideal) process would
convert essentially all of the carbon and hydrogen in the feed to H2 and CO at near ambient
temperatures and pressure.
Thermal Gasification: The feed has to be dried and chipped (blocked) which consumes
energy (combustion of some feed). Gas is high in tars which require cleaning before FT or
Methanol synthesis. Difficult to control H2/CO ratio.
Catalytic Gasification: A feed slurry has to be prepared and catalyst added. Slurry has to
heated and pressurized consuming large amounts of energy. Carbon efficiency is high but
catalysts foul and degrade and must be replaced and/or regenerated. Products are high in
CO2 and CH4 and require additional, energy intensive steam reforming. No tars though.
Sludge Gasification (general): Same issues as thermal and catalytic gasification. Foul smell
of feed. Additional issues due to contaminants. Sludge has high ash content – energy
wasted raising it to gasification temps.
Other Experimental Methods:
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Steam Hydrogasification: Steam and hydrogen are injected to create a gas rich in
CH4, which is steam reformed to syngas. Steam reforming is energy intensive but
process claims same overall efficiency as thermal gasification of dried wood.
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Microwave Gasification: Feedstock slurry is heated using microwave radiation. High
energy requirements to generate microwaves. Still have issues with tar formation.
Experimental.
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Plasma Gasification: Plasma arc generated by forcing high voltage electricity
through ionized air. Biomass is cleanly gasified but electricity requirements are very
high.
2) Syngas Cleaning
Syngas gas must be of extremely high purity for both the FT and methanol synthesis
reactors. About 60% of the particulates are relatively large (> 60 microns) and can be
cleaned with a mechanical cyclone and finer particles can be cleaned with wet scrubbers.
Though present in only small amounts from thermal gasification processes, gaseous tars are
more damaging to the downstream processes causing permanent catalyst destruction.
Conventional Gas Scrubbing: Use water spray followed by a venture scrubber. Adds
another waste stream (contaminated water). Pressure drop. Cools gas below FT operating
temps – must be reheated.
Hydrocracking: Catalytically break down the tars with steam injection to more H2 and CO.
Catalysts are cheap but steam has to be added – more energy input. More pressure drop.
3) Fischer Tropsch Reactor Size
Slurry Bubble Column Reactors: Current state of the art. Syngas is bubbled up through a
slurry of catalyst and FT product. Product & catalyst are drawn off and separated. A
commercial scale (15,000 BPD) reactor fed on NG is 200 ft high and 33 feet in diameter. A
pilot or demonstration scale reactor (150 BPD) would be about 6 ft in diameter and 20 feet
high. Capital cost is huge.
4) Fischer Tropsch Process Selectivity/Upgrading
Product selectivity to diesel or gasoline is driven by temperature control in the reactor. The
large size of the FT reactors make temperature control extremely difficult and the approach
is generally to make long chain waxes (low temp FT) and employ cracking methods from the
petroleum industry. Additional hardware requirements (distillation columns,
hydrocrackers, etc) are extensive.
5) Methanol Synthesis Efficiency
The nature of the synthesis is such that is requires multiple passes of the syngas through the
reactor. Heat has to be efficiently removed or side products are synthesized. These
multiple passes decrease the overall efficiency of the processes.
Studies/Experiments for Discussion
The following are concepts for studies and experiments which I’ll be detailing out over the next
couple of weeks. More to come.
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Biomass to Hydrocarbons via Low Temperature/Pressure Liquid Routes
It has been shown that biomass can be dissolved in ionic liquids at relatively low
temperatures and pressures (Dissolution of Wood in Ionic Liquids). This study would
attempt to define a chemical sequence that could convert biomass to liquid fuels at less
severe conditions without phase change.
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Chemical Pretreatment of Sludge/Biomass Prior to Catalytic Gasification
The high temperature and pressure requirements of catalytic gasification may be mitigated
by pretreatment of the biomass in a corrosive (acidic) medium that might increase catalyst
contact area.
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Catalytic Gasification that Favors Syngas Production
The nickel and other catalysts used in catalytic gasification at the operating conditions given
favor production of methane and CO2 fuel gas. This study would define catalysts and
operating conditions that produce syngas at H2/CO ratios of 2 (methanol synthesis) and
0.6(FT Synthesis).
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Hydrogen Donors for Thermal And/Or Catalytic Gasification for Methanol Synthesis
Much of the carbon is lost to CO2 in water gas shift and other reactions. Conducting the
gasification process in a hydrogen rich environment could increase the H2/CO ratio to
required levels for methanol synthesis and improve thermal and carbon efficiencies.
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FT Synthesis Reactor Design for Faster Throughput
FT synthesis is a complicated 3-phase process. Gas molecules must dissolve in the liquid
phase before they can contact the catalyst. Mass transport is extremely slow in the flooded
catalyst particle in the slurry bed which explains in part why the reactors are so large. The
Velocys microchannel reactor apparently addresses some of these issues by increasing
throughput and thus pressure through catalyst lined channels. Product selectivity is
improved but suboptimal (from what I can tell from the website – still no calls). This study
would investigate alternative designs.
NREL – National Renewable Energy Laboratory
NREL may be a productive organization to discuss biomass conversion projects with and help to
define some of the engineering gaps. They’ve got extensive experience and offer a number of
ways to work with them.
http://www.nrel.gov/biomass/workingwithus.html#afuf
They are particularly interested in the concept of a ‘biorefinery’ and have something they call
a Thermochemical Users Facility which is basically a large lab that could be used for gasification,
FT or MTG or as a place to train with those processes.
http://www.nrel.gov/docs/fy04osti/33817.pdf
More Economics
The comparison table below is updated with capital estimates for the various plans. Daily
capital payment is a 20 year loan at 8% with quarterly payments, multiplied by 4 and divided by
365. The high capital expense of the Wood-to-Diesel and Wood-to-Gasoline plants changes
things quite a bit. The sludge-to-#2 diesel plants have low thermal efficiency due to the fact that
so much of the product oil is used to dry the feed. They are profitable however because the
capital cost is low and #2 diesel is only slightly less expensive than transportation grade. The
wood-to-methanol thermal efficiency is based on a proposed plant model (heat recuperation,
burn exhaust gas for electricity, etc).