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.

By Robert Rapier. You can find me on TwitterLinkedIn, or Facebook.