TECHNOLOGY TO TURN THE CLIMATE TIDE

White pellets are made by compressing sawdust while torrefaction is a dry roasting process. It batch-heats biomass gradually in the absence of oxygen to create char that is pressed into pellets, on occasions using chemical binders.

Valmet’s BioTrac™ system primes biomass for transformation with a hydrothermal pre-treatment process. Fire-damaged deadwood, fed on a continuous loop into a pressurized reactor, is penetrated uniformly with 18 bar, 240°C steam – causing plant cell wall hemicellulose to hydrolyze and its lignin to soften.

A sudden discharge from the reactor to atmospheric pressure vaporizes moisture in cells, causing them to explode and transform the structure of wood fibre into a chemically-altered and uniform, fine granular powder. This is then compressed into compact, energy-dense, water resistant pellets using the biomass’ resident energy-rich lignin as a natural binder.

White wood pellets disintegrate on contact with water while torrefied pellets offer partial water-resistance due to their more brittle form than steam-treated black pellets. Torrefied pellets are prone to crumble easily, and more so after contact with water, creating quantities of volatile, harzardous dust when dry again.

Steam-treated black pellets are bound tightly by their higher retension of lignin, a natural polymer which makes plant life water-resistant. The offtake from steam explosion can be pelleted with much greater bulk density than torrefied or white pellets as a result. And, once pelleted, their lignin glaze renders them highly water resistant – and readily storable outdoors.

Most importantly, they pack much more energy per unit shipped – greatly reducing logistics, handling and storage costs for downstream users. And steam-treated pellets burn with up to 30 percent more volumetric energy than white pellets, and up to 20 percent more than torrefied ones. [2]

Valmet’s BioTrac™ system produces a viscous and honogenous granular powder that is easily controlled before it is pelleted into transportable biofuel, and after when as it is ground for firing in furnaces. In pellet form it is robust and resilient, creating minimal dust and fragments.

As a result, steam-treated black pellets are an encouragingly safe biofuel to process at all points of the supply chain. Generating stations switching to them from burning coal need not make major upgrades to storage, handling or processing infrastructure.

Torrefied and white pellets present more dust and explosion risks in proximity to station furnaces and generators – often necessitating complex and expensive safety systems that include dry silo storage, conveyor modifications, and multiple humidifier, dust extraction and fire detection units. [2]

Why is Valmet BioTrac system a superior industrial process?

Torrefaction relies on batch processing, so new biomass entered into a chamber can disrupt thermal conditions in the system’s primary kiln. Even in batches, the torrefaction process can struggle to consistently heat biomass throughout – resulting in less homogeneity between pellets and more inconsistent energy release.

Valmet’s BioTrac™ system is set apart from competing biofuel technologies by its continuous operational design. Its proprietary plug screw feeder allows a constant inflow of biomass into a stable steam explosion reactor environment.

Its constant outflow produces much greater quantities of a highly homogenous, standardized product because it has all undergone exactly the same treatment. Industrial-scale end users can rely on it for better boiler stability, and consistent amounts of energy production. [1]

How does Valmet BioTrac system deliver superior pellet consistency?

Torrefaction’s primary drawbacks stem from difficulties with heat transfer and subsequent product uniformity. Maintaining the same temperature at the core of a wood chip as on its surface is a slow and delicate task.

When biomass is pushed through torrefaction at industrial speeds, kilns struggle to maintain necessary stable isothermal conditions. Heat fluctuations lead to non-homogenous pellets. Over-torrefied biomass can be too brittle to pelletize, while under-torrefied biofuel has little water resistance.

But, as torrefaction wrestles with commercial maturity, Valmet has developed BioTrac™ steam explosion to provide the consistency in product attributes and supply that generating stations demand. And, in doing so, it has created the first truly viable ‘drop in’ replacement for coal. [3]