Torrefaction

“White” wood pellets are the current state-of- the-art biomass power plant fuel. But they have characteristics that are very different from coal, resulting in higher power plant infrastructure costs, higher plant operating costs, higher transportation costs and limited co-firing ratios.

Torrefaction is a process that turns biomass into bio-coal, which has been described as the “next generation” form of biomass for use in power stations. The goal of torrefaction is to achieve a high-quality fuel without the problems associated with conventional biomass.

The torrefaction process, which can be done in a variety of ways (eg, rotary drum reactor, multiple hearth reactor, moving bed reactor, screw conveyor reactor, torbed reactor, oscillating belt reactor, turbo dryer, microwave reactor) is a “mild” form of thermal conversion (pyrolysis) and devolatilisation, done at temperatures typically in the range 250-300°C in the absence of oxygen.

Broadly speaking, the process is not unlike that used for roasting coffee beans.

During torrefaction, biomass is heated steadily to a temperature generally in the range 240- 300°C. This heating process is typically carried out in a reactor under atmospheric pressure in the absence of oxygen.

Benefits of bio-coal

Bio-coal has characteristics similar to coal which make it an excellent coal replacement, with significant advantages over white wood pellets: higher bulk density; higher energy density; and improved grindability. It is also hydrophobic, with lower moisture content, and can be stored outside.

It can be produced at a cost on par with white wood pellets, while shipping costs are estimated to be around 35% less compared with white wood pellets.

Overall, bio-coal offers significant financial and operational benefits at power plants, with unlimited co-firing ratios, up to 100%. This is why bio-coal and torrefaction have received so much attention over the years.

But, if torrefaction promises such a vastly superior biomass energy feedstock that costs less to use than white wood pellets, then why hasn’t it taken off?

This is an issue addressed in a paper by Thomas Causer of Advanced Torrefaction Systems LLC: “Countless press releases and announcements have heralded supposed torrefaction successes only to be followed by deafening silence, with the exception of occasional news reports of fires, explosions and failed ventures. Many millions of dollars have been spent without a single example of large-scale, ongoing commercial success. Investors have become disillusioned, and many potential customers have all but given up on the idea that torrefaction will ever be commercialised. This inevitably leads to the question, ‘With torrefied biomass holding such promise, what is holding it back from commercial success?’ ”

Causer notes that it is not high capital costs, “which are approximately the same as the capital costs associated with white wood pellets on an energy basis” and it’s not high operating costs, which are “approximately the same as those associated with white wood pellets (again, on an energy basis)”. And it’s not feedstock costs: “The torrefaction process can economically use the same feedstocks as used in the manufacture of white wood pellets, plus additional feedstocks such as forest slash that are generally not usable by white wood pellet manufacturers, and these lower-quality feedstocks are generally available at a lower cost.”

So, what is the problem? It centres around one root technological issue, Causer argues, and the failure to properly address it results in concerns about process stability, system reliability and safety.

The root of the problem

In a more or less self-sustaining process, the torrefaction reactor is typically heated by the combustion of gases released during the torrefaction process itself (torrefaction gas, or torrgas). The heat generated by combustion of the torrefaction gas is used for both torrefaction and drying of the biomass.

The devolatilisation that occurs during the process yields torrefaction gases that include both carbon dioxide and carbon monoxide, in addition to a variety of condensible components such as acetic acid, formic acid, methanol, lactic acid, furfural, hydroxyacetone and water. These torrefaction gases contain a tremendous amount of energy, Causer explains, and “for
an economically viable torrefaction system, it is critically important that this energy is used efficiently in the system.” But “these gases are also highly reactive and can rapidly polymerise into heavy molecular weight bio-oils.”

Current torrefaction gas handling systems are usually based on traditional thermal oxidation technologies — burning the concentrated torrefaction gases to produce heat in the form of a hot flue gas. “Although this traditional approach seems like an obvious choice, actual operations have revealed major shortcomings, predominantly because the oxygen content of that flue gas is too high to be used in direct contact with the biomass. Process design efforts undertaken to overcome this shortcoming result in unacceptably high concentrations of volatile gases in the system, and that is the root cause triggering the problems preventing commercial- scale torrefaction”, Causer argues.

Because of the high reactivity and concentration of the torrefaction gases, “it is difficult to handle them safely and effectively” and “to make matters worse, the gas pressure can fluctuate, resulting in unstable flow and subsequent process control issues.” This is the case whether a directly heated or indirectly heated torrefaction reactor is employed.

Traditional thermal oxidation technologies must adjust for these conditions by using excess combustion air, Causer points out, and that produces an oxygen-rich flue gas that cannot be used in direct contact with the biomass. “This severely limits the efficient use of this energy source inside the torrefaction reactor, and in other system applications such as cooling.”

Directly heated reactor designs attempt to deal with this situation by continuously recycling the concentrated torrefaction gases, via a heat exchanger (as outlined several years ago by ECN of the Netherlands), while indirectly heated reactor designs allow the gases to concentrate inside the reactor and basically reside there for an extended period before being expelled from the reactor simply by the buildup of internal gas pressure.

“As a result, in both cases”, concludes Causer, “these volatile gases are present in high concentrations. This situation leads
to process stability issues and can result in condensation of the gases into pyrolysis oils and tars, which then build up relatively quickly on equipment, in piping, and on process monitoring instrumentation. Formation of these pyrolysis oils and tars not only creates a dangerous environment, but it also disrupts operations requiring frequent shutdowns for cleaning. Even minor leaks of concentrated torrefaction gases create an unacceptable workplace environment and safety hazard. In addition, torrefaction gases continue to diffuse from the solids upon exiting the reactor. This situation can create both environmental and safety issues.”

Furthermore, Causer notes: “Torrefaction involves heating biomass to well beyond its auto- ignition temperature. If the torrefied biomass is exposed to air prior to cooldown, it will burst into flames, potentially causing catastrophic loss.”

“In all cases, process interruptions can and often do occur”, he points out. “A commercially viable process must be capable of operating 24/7, without interruption, for months at a time. Achieving uninterrupted operations without effective handling of the torrefaction gases is not possible.”

Above: Current torrefaction process technology

Above: Torrefaction process with catalytic oxidation technology

The solution – catalytic oxidation

What is needed to effectively handle torrefaction gases? The logical solution, says Thomas Causer, is to adopt a torrefaction system design that uses a very large and continuously available quantity of inert gas. The torrefaction reactor must be continuously flushed (purged) with a hot inert gas stream at torrefaction temperature, diluting the torrefaction gases and quickly removing them from the reactor, away from the torrefied solids.

A secondary benefit of this purge and dilution strategy, Causer notes, is a dramatic reduction in the possibility that the volatile gases will condense onto the surface of the torrefied solids at the point in the process where the solids are about to exit the reactor. “Condensation onto torrefied biomass has been observed by multiple technology developers and is detrimental to product quality and environmental compliance downstream of the reactor.”

But where can large, and continuously available, quantities of inert gas be obtained from?

Purchasing or generating (and heating) that much inert gas is likely to be cost prohibitive, Causer observes, necessitating an alternative approach.

The solution is a torrefaction gas handling system that uses an oxidation catalyst instead of a thermal oxidiser, such as Advanced Torrefaction Systems’ TorreCat technology.

Oxidation catalysts have a long and successful history in industrial and environmental applications, Causer points out. “They very effectively oxidise a wide range of volatile organic compounds such as those contained in the torrefaction gases, and in the process can create a large volume of hot, essentially inert flue gas as a no-cost byproduct of the oxidation. Use of this free inert gas throughout the torrefaction system will address and resolve the process stability, system reliability, and safety issues currently preventing commercial-scale torrefaction.”

Indeed, Advanced Torrefaction Systems goes as far as to argue that “torrefaction will not become commercially viable without catalytic oxidation technology.”

For further information contact: Dan Herren, Advanced Torrefaction Systems, St Louis, Missouri, USA dherren@atscat.com, +1 314 650 1186