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Moving forward on black pellets

Neatly tucked away in a workshop in Sundsvall, Sweden is a pilot reactor for producing steam exploded materials. A seemingly inconspicuous piece of kit, it belongs to a Valmet Fibre Technology R&D facility, a division of Finland-headed global forest industry and energy technology major Valmet Oyj. With its novel approach, the company may have cracked the “chicken and egg” market impasse.

As a location, Sundsvall has a long tradition within the wood fibre, pulp and paper processing technologies – Sunds, SCA, Defibrator became Sunds Defibrator, a mechanical defibering process that was acquired by what became Valmet after the Metso demerger in 2013.

Steam exploded or steam treated “black” pellets are a renewable biomass fuel that proponents argue are safer and a more cost-effective fuel than conventional “white” biomass pellets. Like their torrefied black pellet counterparts, steam exploded black pellets pack more energy per kg, have strong hydrophobic properties, no self-heating or off-gassing, give rise to lower amounts of dust yet grind and burn more like coal.

According to Valmet, its black pellets can replace coal up to 100 percent in smaller installations and up to 70 percent in larger plants without defining the size specifics. As a major global supplier of industrial-scale multi-fuel boiler systems, the company undoubtedly knows a thing or two about the performance of different fuels in its boilers.

However, unlike its equally well known and respected Austria-headed colleague Andritz AG – who incidentally chose the torrefaction route – Valmet is perhaps more of a dark horse in the wood pelleting industry simply because its current technology portfolio does not include pelleting equipment.

That said the company does have everything else one could want in an industrial-sized pellet plant such as material handling, debarkers, size-reduction, belt-dryers and conveying as well as the technology to produce steam exploded material.

Leveraging on differences

Other technology developers, like Norway-based Arbaflame and US-based Zilkha Biomass, have developed steam treatment technologies and both have been busy with offering their solutions to presumptive investors while supplying product to the power utility market. Herein already lie two significant differences: company size and the lack of demonstration facilities.

A publically listed company, Valmet is financially strong and large enough to operate as a complete turnkey engineering, procurement, and construction (EPC) contractor. Indeed the company is well versed in working with industrial process projects such as pulp and paper mills or biomass power plants in the multi-million EUR scale around the globe.

Secondly, both Arbaflame and Zilkha operate a black pellet production facility and have a least one commercial off-take agreement in place whereas Valmet does not even have a demonstration plant, just the pilot unit in Sundsvall.

And although the company says it offers revamps of existing white pellet plants, this lack of an own demonstration plant may be proving a sticking point, at least as far as the wood pellet industry is concerned.

An effort to address this while leveraging on these differences came about in February 2014 when Valmet and Zilkha signed a five-year collaborative agreement to “bring steam exploded black pellets to the market” by developing a “joint global offering”, based on Zilkha’s solution. According to Valmet, the partnership has given rise to a number of potential black pellet plant projects.

However, the company also notes that different regions present different challenges and that the technology may not be suited to all. Furthermore, company presentations at various conferences in recent years suggest that it might not be the pellet industry itself that sees the first Valmet installations.

Established concept

Valmet may seem a late entrant, however, it is worth pointing out where the company is coming from. In short, the steam explosion process itself can be traced back to William Mason and the masonite process – a fibreboard product that was commercialised back in the 1930’s.

Tucked away in a workshop at Valmet’s Fibre Technology R&D centre in Sundsvall, Sweden is the pilot reactor in which all the black magic happens.

Thus the concept whereby wood fibre is treated with steam under pressure and then rapidly depressurized is not new. Variants of the process are used in the wood-based panel industry, an industry the company serves too. It is this process technology that the company has isolated and adapted for the production of steam-exploded material for black pellets.

Continuous process

The material in the form of woodchips or micro-chips is pre-dried before being fed into the pressurized reactor from a buffer silo using a plug screw – a common technology used in the pulp industry to feed material into a pressurized vessel.

With a plug screw, the material itself forms the seal against pressure loss as it enters the reactor. Once inside, only steam is added, explained Peter Björklund, Senior Process Engineer at Valmet.

The material is moved along inside the reactor, the residence time dependent on the type of biomass and particle size, before being discharged from the reactor through a blow valve into a so-called blow line.

It is here the actual steam explosion phenomenon takes place – as the steam saturated material moves from high pressure to atmospheric pressure via the blow valve, friction and the sudden drop in pressure causes the material to instantaneously disintegrate or “explode” into small particles, almost powder size.

This also means that there is no need for further treatment between steam explosion and the pelletizing step.

Bark and integrated CHP plants

The single most important factor in the biomass pellet fuel value chain is the cost of the raw material – access to abundant low-cost raw material will greatly improve the economics of any pellet project. The steam explosion process creates an opportunity for upgrading low-grade, low-cost materials into a premium product.

One such raw material is bark from the wood processing industry such as sawmills and pulp mills and as these industries become more energy efficient, not all the bark is needed for internal use. In addition, low-grade residual heat for the bark pre-drying, and process steam are also readily available at these industries.

Our pilot unit in Sundsvall produced black pellets from bark in order to verify the product quality parameters. The pilot runs have confirmed the suitability of softwood bark for production of black pellets, which soon may offer a promising choice for the heat and power industries, said Peter Björklund.

Another possibility Björklund highlights is integrating black pellet with a packaged combined heat and power (CHP) production system that provides energy for the production with excess power to the grid.

This is very site and project specific but can be very attractive in terms of overall feasibility especially with feed-in tariffs, Björklund said quickly adding that there was not much point in making a high-grade fuel to be used on site.

Instead, the CHP plant would use a low-grade wet fuel such as waste or bark whereas the woodchips would be upgraded into black pellets to be sold externally along with the excess power.

Peter Björklund, Senior Process Engineer at Valmet discussing the benefits of its steam treatment tech at WSED in Austria.

For some reason, inland Japan using “unused wood” springs to mind – time will tell if Valmet’s approach has helped break the impasse.

This article was first published in Pellets Special 4 2018. Note that as a magazine subscriber you get access to the e-magazine and articles like this before the print edition reaches your desk! 

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