RE: Berkshire Energy Laboratory - Weekly Update

To: Noa

From: Tom

Date: November 7, 2008

RE: Berkshire Energy Laboratory - Weekly Update

Summary

·  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:

o  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.

o  Microwave Gasification: Feedstock slurry is heated using microwave radiation. High energy requirements to generate microwaves. Still have issues with tar formation. Experimental.

o  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.

·  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.

·  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.

·  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).

·  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.

·  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).