The Quest for a definitive Description of Torrefied Wood

In the World today, there are actually no more than 7 or 8 machines making torrefied wood. All of these only exist at a laboratory or pilot plant scale, the largest one produces enough material annually to run a power station for about 3 minutes.

Needless to say - this isn't exactly a statistically significant amount of anything. As power Stations require much higher volumes of this to prove ANYTHING - (and it currently isn't available) we have decided to take matters into our own hands, and prove it Scientifically! (What a concept!)

In spite of the above, there is an ENORMOUS difference between the production of these various reactors.
Some product is relatively low calorific value (21 to 22Gj/tonne) while other product is extremely high calorific value (33 to 34 Gj/tonne)
Some has an HGI (Hardgrove Grindability Index) of 35; while others has an HGI of close to 50
Some have fixed carbon content of 25%; others has 70%

So, how do we know who is right? And who is wrong?

The answer lies in the ultimate definition of what Torrefied wood (Bio-Coal) is. This, is determined by many factors - the predominant one being the demands of the Utility Companies that will utilise the product.

Our definition of torrefied wood is as follows:

"Torrefied wood is completely desiccated biomass, with devolitilised hemicellulose, which has not yet reached the point of “char”. That is to say – that pyrolysis, in any form, has not yet commenced. When the critical surface moisture content of the particle is reached, the evaporation is assumed to take place inside the particle in the moving front between dry and moist regions. In the next stage the surface temperature of the particle never exceeds the pyrolysis temperature. In this case, it means that the drying isotherm reaches the centre of the particle and vanishes before the pyrolysis isotherm appears at the particle surface."

In the above context - the "drying isotherm" is meant to be the torrefaction temperature (from surface to core) that initiates and completes the devolitilization of hemicellulose.

There - NOW you know.

ACTUALLY - that is only part of the equation. (You didn't think this was that SIMPLE - did you?)

ANYBODY can make charcoal - it's been done for 1,000 years or more. ANYBODY can also dry wood. This is a well understood process. Torrefaction is a mystical point, somewhere in between the two extremes. The real challenge lies with the fact that there really is no IDEAL, and many of the desirable characteristics are diametrically opposed to each other.

Let me explain . . . . . .

Everybody knows that the higher the Calorific Value - the better the fuel. RIGHT? (Wrong) Certainly - a high CV is a plus, but it is also a minus. On the plus side - there is increased grindability (easier to pulverise); better particle shape; and better combustion characteristics with increased CV. HOWEVER - this comes at a price. That price is measured in a marked DECREASE in both Durability and ease of pelletisation.

I was recently in Taiwan, and met the most diligent and enthusiastic University student I have ever known. His remit was to pelletise a quantity of "torrefied wood" that had been purchased from a company in the USA. It took me about 10 seconds to figure out that it was charcoal. As it happens, I had the Managing Director of the International Charcoal Cooperative Association sitting beside me to validate my hypothesis. (He said it was charcoal too - but not very high quality) ANYWAY - this tenacious student had ACTUALLY figured out how to turn this charcoal into a pellet. (No mean feat as there was likely ZERO lignin left in the product!) There was however, one tiny problem. If you dropped the pellet from more than 100 mm (that's about 4" in old money) it shattered into its constituent components - and left a lovely little pile of dust on the table. (Very reminiscent of a Rorschach Test)

While the product did have ONE desirable characteristic - it is fair to say that it failed on others.

Durability is the second key feature of torrefied pellets. Because they undergo a significant amount of handling during their life (Conveyors onto trains; belly-dumps into storage hoppers; screw conveyors into ships; grapples to take them off of ships, etc.) there is the small matter of product durability to consider. The only way to maximise this durability, is to ensure that the product still maintains all of its Lignin (which is the "glue" that binds the pellets together). In order to do that - you need to control very closely the CV. So - there's limiting factor #1.

Limiting factor #2 is dust evolution. In the course of handling and transporting and grinding (in a power plant) the material is subjected to a lot of structural impacts. The one way it rebels is to break down into very small particles. On one hand - this is a good thing - as small particles are VERY desirable for PCI boilers. On the other hand - small particles also have a propensity to Explode, if mixed in just the right concentrations with air. (The ratio is 49 g/m3 - if anyone cares) So - that's a bad thing. (Every Utility I have spoken to says that explosions and fires in their Power Stations are their very LEAST favourite events!) AGAIN - it's a fine balancing act. IF the pellet is made of fine particles - it has a much higher density and durability (little particles tend to stick together more tenaciously than bigger particles) but it also has a much higher Risk of explosion. Bigger particles are MUCH less likely to explode - but compromise the integrity of the pellet - so are evolved more easily during handling.

Hmmmmmmm . . . . . . Bit of a "catch 22", don't you think?

ACTUALLY - there is an optimum balance between CV; Grindability; Durability; Combustibility and Safety.

While we have identified in theoretically, it's time for us to get up out of our comfy chairs and start actually doing some work for a living.

To that end, today - we sat down as a group - and worked up a testing regime, that would absolutely, positively, unequivocally define where this "ideal" is. The method is simple - Make samples of various CV's; pelletise them; pulverise them, combust them, and see what happens! Thankfully - we don't have to do this any longer in a microwave oven or Toaster oven.

The lab has a few bits of kit that are very useful for this sort of analysis. For example, we have a "Differential Recording Bomb Calorimeter". (Pretty cool eh?) This handy-dandy machine has the ability to let us see the unseen. We simply take a sample of the product, load it into the device; turn on the heater, then go have a nice cup of cappuccino while it does its thing. (Have I mentioned that there is a great Coffee Shop at the lab?) Anyway - the machine increases the temperature of the sample to a sweltering 1400 degrees C. All the while, the sensors are measuring various parameters of the process, and dutifully recording them on a lovely chart for us to review. When we return, it's simply a matter of looking at that chart, and wondering what the heck these squiggly lines mean. Actually - it is a very useful tool that tracks the phase-changes and kinetics of the reaction. Because we can couple this with a nifty gas analyser (which will tell you - from a breath sample, which winery that glass of wine you had for dinner last night came from)we know EXACTLY what's going on inside there. (WITHOUT actually looking at anything!) Isn't science wonderful?

Now that we know how our fuel is going to react, we can then start looking at the 4 other parameters, and "fine tune" our system - to come up with an optimum. We have a very good idea where that is (after all - we're not the FIRST people on this planet to research this topic) but we need to be able to PROVE it to Mr. Utility Company - as they're the ones with all the ££££'s to spend.

So - is it Bio-Coal? or is it Charcoal? We are now at a point where we are able to tell you.

Now, don't get me wrong - but this is NOT the end of the story. Once we have optimised the characteristics of the fuel pellets - we then have to make sure that they play nicely with coal. After all - nobody is talking about burning 100% torrefied wood. They all want to mix it with another fuel (the EVIL fossil Coal) and that presents a WHOLE different set of challenges. THANKFULLY - we have a little bit of kit for that too.

The machine is called a "Rotating Anode X-Ray Diffusion Chromatograph". (Again - pretty coll name eh? And I'm the ONLY kid on my block with one of these baby's!)

What this bit of kit does is to blast the living daylights out of a sample of Coal/Torrefied wood with X-rays, while it's inside an oven that is roasting the living daylights out of it (up to TWO THOUSAND degrees Celsius!)In the end, it produces yet another set of squiggly lines, that show us the atomic and molecular signatures of all the product that want to get the hell out of there. It also has the ability to analyse the really tough bits that are left (mostly minerals and other inorganic compounds.

Power stations have this odd requirement that they want to KNOW how the product is going to behave in their boilers. (Personally - I don't see what all the fuss is about ) However - I keep hearing about alkaloid deposition, sintering, agglomeration, slagging and other such "nasty stuff" that apparently is second on their list of least favourite events (right after fires and explosions) So - we make sure that WHEN we burn our wonderful, clean, green torrefied wood with their nasty, dirty fossil coal - it won't hurt their precious boilers. Early results indicate that co-combustion of TW actually has a catalytic effect on the coal, and improves it's combustion efficiency. This is much like wine makes cheese taste better and vice-versa. Hopefully - we will ultimately understand WHY.

In the end - the answers are there - just WAITING for us to discover them. The ultimate goal, is to come up with a recipe, that is the optimised iteration of renewable fuel. One that has the Durability; Grindability; Calorific Value, and Density. So far . . . so good. This research will undoubtedly put us at the leading edge of this industry - and will set the standard for all others to follow.

Wish us luck!

RW