A new production plant for biomass ‘black pellets’ could solve some of the problems associated with traditional white pellets, as well as potentially creating a new revenue stream for biomass power plants. Tildy Bayar investigates
Black pellet plant design
Global demand for biomass pellets is expected to nearly double in the next seven years, from 29 million tonnes in 2015 to 53 million tonnes in 2023. Almost 60 per cent of this demand is expected to come from industry in the EU and Asia, with European countries (topped by Germany, Sweden and Latvia) consuming around 74 per cent of the world’s pellets according to biomass trade group Aebiom. In Asia, Japan and South Korea have been tipped as growth markets, with Japanese demand predicted to grow to 5.4 million tonnes by 2030 and South Korean demand potentially reaching 10.2 million tonnes by that year, according to Pöyry Management Consulting.
Biomass pellets are available, abundant and affordable, with major import flows between countries (Vietnam/Malaysia to South Korea) and regions (North America to Europe) showing growth. But transporting pellets still raises challenges. Traditional ‘white pellets’, made from ground wood compressed in a pellet mill, are problematic to transport and store on-site as their high moisture content means they will rot if not kept dry through costly means and logistics.
An alternative is ‘black pellets’, which solve some of the problems associated with white pellets. Black pellets feature higher water resistance and thus can be stored and loaded/unloaded in wet weather conditions. They are also cheaper to ship than white pellets because less protection is needed. And black pellet manufacturers say their products are around 15 per cent more energy-dense per tonne, which can lead to savings of as much as 30 per cent on transport and logistics.
According to Mattias Erixon, Senior Sales Manager at Valmet in Sundsvall, Sweden, black pellets offer a number of advantages over white pellets. “Black pellets contain and retain more energy than white pellets,” he says. “There is no self-heating or off-gassing, and the low amount of dust minimizes the risk of fire hazards. They grind and burn more like coal than white pellets, which saves customers both capital and operational expenses.” Indeed, Valmet says black pellets can replace coal up to 100 per cent in smaller power units and up to 70 per cent in larger units.
In a move designed to reduce fuel transport costs, address emissions concerns and potentially create a new revenue stream for biomass power plants, Valmet has designed a black pellet production plant that can be installed on-site, such as near a power plant for its own use. The plant is a complete facility, from biomass receiving to finished black pellets, the company says. Valmet also offers retrofits of existing white pellet plants to burn black pellets, says Erixon.
Black pellet storage
How it works
Valmet’s black pellets are produced through a process of steam explosion in which the woody biomass feedstock is steam treated and then rapidly depressurized. After pelletizing, the material can be ground into a powder suitable for burning in large-scale power boilers or other systems suitable for pellets.
“The material is dried before it goes into the steam explosion process,” Erixon explains. In the plant, the biomass is fed from a buffer bin into a pressurized reactor using plug screw technology. “The only thing added to the process is steam, and keeping the biomass for a certain period of time in the reactor.
“The system discharges material from the reactor in a continuous infeed/outfeed loop,” he continues. “As the material leaves the reactor it goes from high pressure through a blow valve into a blow line, connecting and then feeding material into a pelletizing step. The disintegration of biomass happens as it discharges from the reactor system as it goes from high to low pressure. A combination of friction in discharge and explosion of steam happens when changing the pressure from high to atmosphere, causing the material to disintegrate.”
Unloading black pellets in France
The biomass material enters the system with a particle size equivalent to normal or micro-sized wood chips, but comes out as small particles, almost a powder. This means that, unlike torrefied black pellets which require additional densification, there is no need for further treatment between steam explosion and pelletizing. “With a normal white pellet product you always need to do size reduction before pelletizing,” Erixon says, “but this is taken care of by the steam explosion process.”
The black pellet plant can be integrated with a packaged combined heat and power (CHP) system which provides the steam for the drying process as well as power for the plant, and often surplus power to sell to the grid.
“It depends on feed-in tariffs how attractive this is,” Erixon notes. “If you only want to provide the steam for the process there are cheaper solutions, but in some cases integrating a CHP and black pellet plant can be very attractive in terms of overall feasibility. You can have anything that produces steam for the steam explosion process and energy for the drying process.
“In Europe a biomass-fired boiler (steam boiler) or CHP producing both heat and power is used. In North America the industry standard is to use natural gas. We may think what we want about producing an environmentally friendly product through burning fossil fuel, but that is the standard: natural gas burners and a steam boiler.”
To run the CHP plant, waste fuel is recommended. Erixon explains: “We wouldn’t go through the process of making black pellets just to burn them on the same site. Our thinking would be that, instead of going through the process of making a product with a high energy value and certain properties, we would normally suggest using low-grade fuel, for example bark from the debarking process. Wet fuel is available as residue from the process and what you can buy on the market.”
Minimizing technology risk
Although steam explosion’s use in producing black pellets is new, Erixon says it is a low-risk proposition. It is “very old technology and was used in the 1920s to make different kinds of products, such as board for furniture.” In the current application “the process conditions are slightly different, the end product is different, but the technology is relatively old.”
However, it is still very new to the power market. Thus far, Valmet’s collaboration partner Zilkha Biomass Energy LLC, based in Selma, Alabama in the US, operates the only commercial-scale steam-exploded black pellet plant. Zilkha “were pioneers in commercializing this pellet product and first with commercial deliveries,” says Erixon. The two firms signed a collaboration agreement in 2014.
Erixon says the design of the black pellet plant has minimized the risks inherent in any new technology by using standard and proven technologies wherever possible. For an example, he cites the use of a debarking chipper and a chip screening and storage system, “products we’re using in all of our other offerings to pulp, paper and energy customers. The next step is the belt dryer, also a proven concept that there are many references on.
“The steam explosion island is new technology, but we are basing that on experience and deliveries to other applications. So we have a huge amount of information that has a background used in designing these commercial-sized systems, and that we have verified in two pilot systems.
“Downstream, we come back to standard technology: the pelletizing equipment, and then the CHP. And that is just talking about technological things. Of course Valmet is a company which has a certain size and is a quite reputable company which can stand behind our promises.
“We have pilot systems we have used to verify the technology in Sweden; the technology we’re using in this process is based on technology we use in our other industries we support or have worked with for many years. So we think it’s a relatively small technology step,” he says. “Although we don’t have a plant running based on this product, we are comfortable with this technology that it is not high risk for our clients to buy this equipment.”
Taking it global
Zilkha has completed several full-scale tests of the black pellet plant with European and Asian coal-fired power plants ranging from 80 MW to 500 MW. In total, Erixon says, in testing and in Zilkha’s commercial delivery, over 100,000 tonnes of black pellets have been burned.
Zilkha and Valmet’s five-year collaboration agreement is focused on developing steam exploded black pellets and bring them to the global market.
The collaboration, Erixon says, was based on the principle of “better together than separately. [Zilkha is] the only company that has the experience of handling large volumes of this new product.”
He now says their plan is going well. “We have projects going on, not in all parts of the world, but certainly here in Europe for black pellet plants,” he says. “Also in North America as well as Southeast Asia, there are several projects we are developing.” But different regions present different challenges, and the technology may not be well-suited for all. “One of the major costs in total project cost is the cost of raw materials, so that is really one of the factors that has the biggest influence on feasibility in any country,” says Erixon.
For a view of the potential market for black pellet plants, he says Valmet and Zilkha “rely on reports on pellet consumption expansion. This product is industrial pellets, not for use in domestic homes for furnaces; the main use is to replace coal in coal-fired power plants. Global consumption of industrial white pellets is increasing; that is the sort of growth potential that we see. Black pellets have different characteristics – that is the market potential.”
He notes that global pellet consumption grew 13 per cent in 2015, with industrial pellet consumption growing 21 per cent. “Black pellets are taking some part of this market,” he says.
“The market potential is there, but at the moment it is a matter of building up the acceptance of a new product with the end user and, in parallel, developing the production of black pellets.
“The issue with a new product is that no one is willing to commit to buying a product until they feel they have a security of supply. No one will build a plant until they have a committed supplier; it’s a little bit chicken and egg.”