Kraft lignin is on the move

Stefan Andersson, Site Manager, and Robert Gustavsson, Project management & Business Development, at LignoDemo show the belt filter where the participated lignin is separated from the rest of the black liquor.

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Lignin is gaining an ever increasing interest from researchers as well as from forest, plastic and energy industries. Having been mainly regarded and used as an energy source by the pulp and paper industry in the past, it is now also seen as a renewable raw material with great potential for an array of products replacing those fossil-based.

Any development starts with an idea and a vision of what the commercialised idea in the form of a product or service could do and which benefits it could bring. In order to have a long enough lifetime it has to fulfil a need and be profitable, the latter a fact which is sometimes overlooked when screening ideas.

However, I will not spend neither my writing time nor the eventual readers’ time on theorizing about how an idea in principle should be developed and fed until it is an established product on the market. There are many people much better suited to do that. My focus this time will instead be on how lignin demonstration plants can support the introduction of different lignin based products.

The lignin rich black liquors from kraft mills, or corresponding liquors from sulphite mills with a sodium or magnesium base enabling chemical recovery, have since the invention of the Tomlinson boiler been used to produce steam to heat different pulping process steps as well as to dry paper or board in integrated mills. Due to different mill situations, lignin is, and will be even more in the future, available in larger quantities for other than own energy purposes in the mills.

In some mills the recovery boiler is a bottle-neck that prevents an increased pulp production. One way to increase the pulp capacity without investing big money in a new recovery boiler is to remove lignin from the black liquor and turn it into products. A successful example of this is Aditya Birla’s mill Domsjö Fabriker in Sweden where its former owners a number of years ago invested in lignin production in order to increase the cellulose production. Today the mill has a capacity of 120,000 tonnes of lignin and is the second biggest lignin supplier in the world having at the same time increased the cellulose production without investing in a new recovery boiler.

The increased energy efficiency in mills is offering another opportunity to utilise lignin for other purposes than production of steam and electricity. This is particularly true for big new mills where economy of scale can make lignin business very interesting. But as always when there are alternative usages of a raw material it is a matter of finding the most profitable usage of it, as an energy source for different biofuels or as raw material for a wide range of products, anything from air fuel, dispersants, glue, binding agents and concrete additives to carbon fibres. The question is of course how to separate the lignin from the rest of the black liquor in an efficient way.

One method to extract lignin from black liquor is the LignoBoost technology which was developed in collaboration between Innventia, today RISE Bioeconomy, in Stockholm and Chalmers University of Technology in Gothenburg together with other partners. The development was done within the framework of three research programmes that began at the end of the 1990s. In 2005 the technology was ready to be verified on a larger scale and in order to test and prove the LignoBoost technology a suitable place in connection to a kraft pulp mill was needed.

Robert Gustavsson and Stefan Andersson next to the chamber press filter where the lignin is washed and dewatered.

The ideal place was found to be Nordic Paper’s kraft pulp and paper mill in Bäckhammar in Sweden because there was already a kraft lignin plant owned by Borregaard connected with the pulp mill’s recovery cycle. This plant did not any longer fit into Borregaard’s business activities and they were willing to sell it. In 2007 the rebuilt plant was opened and is since then owned and run by RISE LignoDemo AB, a subsidiary of RISE Innventia AB.

The LignoBoost demonstration plant, with an annual capacity of 8,000 tonnes of dry lignin, lives in symbiosis with the pulp mill. The black liquor is pumped straight from the evaporation plant and is cooled down to a suitable temperature. Next step is to lower the pH value from a level of pH 13-14 down to pH 10 by adding carbon dioxide. The result is that the lignin is precipitated and thereafter kept in storage tanks for maturation. After maturation the precipitated lignin is carefully pumped to a chamber press filter or to a belt filter where the lignin is separated and the filtrate, mainly consisting of cooking chemicals, is pumped back to the kraft mill’s evaporation plant.

The lignin is heated and treated with sulphuric acid which lowers the pH down to pH 2-3 and then pumped to a second chamber press filter where washing and dewatering is done in a number of steps. The dry lignin is packed in big bags, or if needed delivered in another form, and the filtrate goes back to the evaporation plant.

The product from LignoDemo in Bäckhammar is pieces of kraft lignin with a dry content of about 65 %.

The lignin plant in Bäckhammar is a unique place due to its flexibility. It is busy within two areas, advanced lignin research projects in near industry scale and commercial customer specific projects. The plant has possibilities to produce tailor-made lignin qualities and tests can also be performed with membrane filtration and centrifuges. Normally black liquor from the nearby pulp mill is used but projects can also be run using black liquor or corresponding liquors from other mills when customers want to investigate own opportunities.

The fact that RISE LignoDemo is a unique lignin plant has made it an attractive development and test site. Test runs on behalf of forest industry companies as well as companies from the plastic industry from all over the world have been done. The interest from the plastic industry is due to the fact that they see opportunities to replace plastic by lignin in certain products.

The LignoBoost technology was patented and in 2008 sold to Valmet to be commercialised. In 2013 the first full-scale LignoBoost plant was taken into operation at Domtar’s Plymouth, North Carolina mill. The second one was started at Stora Enso Sunila Mill in Kotka in southern Finland, a mill with a capacity of 375,000 tonnes softwood pulp and 50,000 tonnes dry lignin.

This car model has a roof made of a composite where the carbon fibre is produced of 100 % softwood lignin. It is developed by RISE Innventia, Swerea, Blatraden and KTH, The Royal Institute of Technology. The car is equipped with a battery where lignin based carbon fibre is used as electrode. Photo: Johan Olsson.

Another lignin process which has been commercialised is the LignoForceTM technology which was developed by FPInnovations and scaled up in a demonstration plant at Resolute Forest Products’ Thunder Bay mill in Ontario. This technology is since spring 2016 in full-scale operation at West Fraser’s Hinton NBSK pulp mill in Alberta with a capacity of 30 tonnes per day of lignin.

However, it would be unfair to write a piece about lignin development and totally ignore the fact that there is an existing global lignin market in a size of about 1.1 million tonnes of solid dry lignin. The biggest player is Borregaard LignoTech with a market share of some 40-45 % and with manufacturing facilities in Europe, the USA and South Africa. The company is the pioneer in developing lignin products for different applications and is today the unrivalled market leader of lignin based products, lignosulphonates as well as lignin.

Coil of lignin filament. Photo: Johan Olsson.

A big part of the existing market is lignin as lignosulphonate and hence produced in sulphite mills. The drive behind the extensive lignin research now is how to extract and use kraft lignin from kraft mills black liquors as it is available in potentially high volumes. This is where e.g. the RISE LignoDemo and FPInnovation LignoForceTM demonstration plants are important for future lignin development.