New opportunities from lignin

Most pulp mills today are self-sufficient in terms of their steam needs, based on the Kraft black liquor they produce. By extracting the lignin contained in their black liquor, however, they have the potential to boost their own energy output by using it as a solid biofuel or export it off-site for use as a fuel or raw material for other uses. The best solution depends on the characteristics and location of the mill concerned.

This is where Metso’s LignoBoost technology comes in, as it can produce lignin with a high dry solids content from the material in a pulp mill’s evaporators.

LignoBoost is a complete process solution. A unit measures roughly 25 x 20 x 14 metres in size and includes all the equipment necessary for lignin extraction. Green energy produced from the lignin extracted can be exported as process steam or power, or sold as fuel in the form of pellets or powder. Lignin is also a good raw material for making speciality chemicals.

The process

The history of LignoBoost technology has its roots in a joint project started in 1996 by what is now Innventia and Chalmers University of Technology. Pilot-scale trials led to the construction of a 24 t/day demonstration plant in Bäckhammar in Sweden in 2007.

Metso initially became involved by supplying a number of the key components for the process. Recognising its potential, Metso acquired the technology a few years ago and has been working with Innventia to further develop and enhance it. The result is a technology that is now ready for full-scale commercialisation, and the first commercial plant was ordered at the end of 2011.

LignoBoost works in conjunction with evaporation, and the process begins by precipitating lignin from black liquor by lowering its pH with the help of CO₂. The precipitate is then dewatered using a filter press based on Metso’s vertical plate pressure filters, similar to those used in the mining and energy sectors. Conventional filtering and sodium separation problems are overcome by redissolving the lignin in spent wash water and acid. The resulting slurry is dewatered again and washed, with acidified wash water, to produce cakes of virtually pure lignin.

A biofuel

Lignin is an outstanding biofuel with a high heat value (25-26.5 MJ/kg), and can be used as a fuel in a power boiler or to replace oil in a lime kiln, for example. In the case of a lime kiln, using lignin has the potential to save as much as 50 litres of fuel oil per tonne of pulp. For a 200,000 t/a pulp mill, this translates into a savings potential of 10,000 m³ of oil annually.

The Södra Cell Mönsterås pulp mill in Sweden, for example, has successfully burnt lignin as lime kiln fuel, either mixed with bark and pulverised wood or alone. Much larger quantities have also been burned on a trial basis at Fortum’s Värtaverket heat and power plant in Sweden, with lignin providing 8-15% of the plant’s total energy output at times. Estimates indicate that it could be possible to employ up to 30% lignin using optimised equipment.

Lignin can also be blended into liquid fuels, such as fossil fuel oil and tall oil pitch. Although there are limits as to how much lignin can be blended in this way, tests have shown that as much as 45% can be mixed into both these liquid fuels.

A renewable raw material

Using lignin as a fuel is only the first step, however, as lignin can also be refined into products such as low-cost carbon fibre for use by the automotive industry or activated carbon, for reducing emissions of heavy metals. The plastics industry is another huge market where lignin could be used as a base chemical, to replace the fossil inputs used today to produce phenols, for example.

All in all, lignin and LignoBoost offer mills an exciting opportunity to leverage new sources of income – by increasing pulp output on-site, exporting electricity or district heat to customers off-site, or exporting it as a fuel or renewable raw material.

Advanced automation solutions from Metso, such as this one at a paper mill in Germany, enable customers to run more efficient processes and minimise their use of raw materials and energy.