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Torrefaction via Radio Waves

During the past five years that I spent in Hawaii, I worked on a number of different projects. The company I worked for invested in energy projects, and our focus was on converting biomass into energy. In my role, I often evaluated companies and technologies to determine the potential technical and economic viability.

I have found over the years that the vast majority of biomass to energy projects aren’t economically viable for one reason or another. I have looked at companies that utilize many different conversion technologies, and most of the time my job consisted of searching for fatal flaws of different approaches. I was the guy who said “No.” That approach saved my employer a lot of money, because none of the companies I said “No” to are thriving today. Most went out of business.

But I didn’t like always being the guy who said “No.” I wanted to put steel in the ground and build something. So I searched for ways to say “Yes”, or at least to turn “No” into “Maybe.”

You don’t always immediately know whether the answer is yes or no, but in the case where a “yes” could make a big impact, sometimes we funded basic research. I can’t talk about most of the things we were involved with due to various nondisclosure agreements, but I was recently given approval to mention a project we funded on radio frequency (RF) heating.

The gist of the idea is that like microwaves, RF waves of the right frequency can efficiently heat objects. If the frequency is right, RF waves can be utilized to make a ceramic cup glow red. The depth of penetration is directly proportional to wavelength, and since RF wavelengths are longer than microwaves, RF can be used to heat thick materials, like a log. Or a big pile of wood.

So why would you want to heat up biomass? To torrefy it. Torrefaction is a mild biomass thermal treatment usually carried out between 200 and 300 degrees C. Torrefaction upgrades the quality of biomass as fuel for combustion and gasification applications. Torrefaction can be referred to as roasting, and in fact the history of torrefaction can be traced to roasting coffee beans for easier grindability. This is also what happens to wood when torrefied. Torrefied wood has the moisture and most of the volatile organic compounds driven off, and the wood becomes brittle in the process. This makes it easier to grind, which enables the creation of pellets that are more energy dense than wood, and that are comparable to coal.

What are the implications? First, significantly more energy can be transported in a container when that material has been torrefied and pelletized, relative to wood pellets. Further, after the biomass has been torrefied it repels water and is much less biodegradable. This enables it to be stored for longer periods of time. Finally, the challenge in burning wood for electricity is that the energy efficiency isn’t great. It takes a lot of energy to grind wood down to a powder for the most efficient burn (which is still lower for wood than for coal). Because of the differences with coal, wood may only be blended in very small quantities in a coal-fired power plant. Torrefied wood, on the other hand, grinds as easily as coal, and can therefore be blended at much higher concentrations in a coal-fired power plant.

So we saw a big opportunity, but a lot of unknowns. There hasn’t been much work done in this area. Would it work? Could we find a frequency that would put the right amount of heat into the wood? Would it be energy efficient?

We worked with a company in Great Barrington, Massuchussetts called JR Technologies. “J” is Jeb Rong, and “R” is Ray Kasevich — two of the foremost RF experts in the world. You can see their biographies here, and a presentation they gave on the technology here (Their contact information is on the first slide should you wish to contact them).

We developed an experimental plan, and following an extensive literature review of the torrefaction process, built a prototype batch reactor.

Ray and Jeb
Ray and Jeb Inspecting the RF Generator and Torrefaction Reactor

In June 2011 we conducted the first set of experiments. (I spent a lot of time in the lab with them there, and was onsite for the initial tests). What we produced looked like torrefied biomass, but one thing I learned is that there are no real standards for what qualifies biomass as torrefied. There is a regime where it’s woody and fibrous (and takes a lot of energy to grind), a regime where it’s torrefied and first becomes brittle, and a regime where it’s charcoal. You want to have it sufficiently in the realm of torrefied wood. Too little torrefaction and it isn’t brittle, and too much and you drive off too much of the initial energy content.

So we secured some samples of torrefied wood, and sent that along with our material to a lab for testing. When we got the results back, we found that the material we produced was essentially the same as the torrefied control sample we sent. We did more testing, and ultimately decided to build a larger, continuous reactor.

What happened next is that my company made a decision to no longer fund the German biomass-to-liquids company Choren. (See What Happened at Choren?) Choren’s process had some synergies with what we were doing with the torrefied biomass, so we ultimately decided to stop funding the torrefaction research as well (which my boss was funding directly out of pocket). This has left development in limbo for the past couple of years.

But I remain in close contact with Ray and Jeb, and they continue to pursue their RF work. Beyond the RF torrefaction, there are a number of applications for RF heating. One of their most interesting uses of RF is for environmental remediation (see the previously-linked presentation for more details). But the RF torrefaction work is still in the “maybe” column today, and I hope to someday further this work with them.

Robert Rapier's picture

Thank Robert for the Post!

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Rick Engebretson's picture
Rick Engebretson on November 25, 2014

I agree, it is a “maybe” approach.

Understanding the electro-magnetic spectrum can be essential in chemical physics. As you know, frequency and absorbtion efficiency and power generation will be determined by what molecules you have, what molecules you want, and how you might produce the process EM energy to get them. Molecular dipole strength, dielectric function, bonding forces, and more variables, were (are?) important topics in the OLD textbook “Physical Chemistry of Macromolecules,” by Tanford. Cellulose certainly is a macromolecule.

I’m not sure chemical engineers and RF engineers speak the same language, then add biochemistry and it’s hard. This is a highly inter-disciplinary technology development. Sounds like you made a great start for some bigger players to take the ball from you (very sad, but very possible).

Bob Meinetz's picture
Bob Meinetz on November 25, 2014

Robert, one of my clients is a consultant for Los Angeles and other municipalities on ways to turn municipal waste into energy. This is something which would interest him, although the variety of materials in municipal waste obviously compounds the problem of what frequency (or frequencies) to use. His experience is similar to yours – making these processes economical is the tough nut to crack.

Keith Pickering's picture
Keith Pickering on November 25, 2014

Robert, can you give us some idea of the energy cost of the RF system, and the energy content of the torrified wood?

Bill Woods's picture
Bill Woods on November 26, 2014

What about pushing biomass all the way to biochar?

Robert Rapier's picture
Robert Rapier on November 26, 2014

You lose too much of the energy content that way. 

Robert Rapier's picture
Robert Rapier on November 26, 2014

Keith, we have done some calculations on that, but were going to do a bit more testing at a larger scale. It’ not all that favorable at the scale we did it at, but we estimated that it’s still a net positive. I told the guys that the way to think of the system is this: If we had to use the torrefied wood to make the electricity to run the machine, could we do it? Is there enough excess power produced to make the effort worthwhile? We are pretty sure there is, but a larger test would have confirmed this one way or the other.

Bill Woods's picture
Bill Woods on November 26, 2014

Well the usual proposed uses of biochar are as a soil amendment and/or for carbon sequestration, so its energy content isn’t a feature.

Robert Rapier's picture
Robert Rapier on November 26, 2014

Yes, but a far less lucrative market than as a coal replacement. 

Roger Faulkner's picture
Roger Faulkner on November 27, 2014
Kudos on passing this along, after your company withdrew. There is no question that it can work, it’sonly a matter whether it is cost effective. The article does not mention that torrified wood = biochar essentially, though other comments do. Introducing air into the still hot torrified wood will lead to biochar and some low-BTU gas. One can also go all the way to gasification. If oxygen rather than air is introduced, one coud get good quality syngas, I think at lower temperatures than would be required for coal (and with far less mercury than coal). Syngas can feed combined cycle gas turbines, which are much more efficient than coal-burning stem-based power plants.
It seems obvious to me that this process will work best with low-moisture content (but not totally dry) biomass, and that the process wants to be done in a pressure vessel (for best energy efficiency). I did not see mention of these factors. Any biomass should work, too, not just wood chips.

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