Fibre customer magazine 2020/2021

WHAT IS WOOD MATTER MADE OF?

“The green liquor sludge is the only solid waste produced by a modern pulp mill.”

1. Organic cellulose, hemicellulose and lignin, mainly composed of carbon, oxygen, hydrogen and nitrogen.

2. Inorganic mineral matter such as calcium, potassium, phosphorus and magnesium.

3. Trace amounts of many other elements such as iron, manganese, boron, copper and zinc, and often heavy metals.

Black liquor, the washing waters and sludge carry large- ly the same metals and other inorganic matter that are left in ashes in the recovery boiler. The washing waters also include bleaching chemicals. Given that inorganic matter tends to accumulate in boilers and kilns, it must be removed to prevent the equipment from clogging or corroding. Potassium and chlorides are separated from the recovery boiler’s fly ash and channelled to wastewater treatment. The phosphorus accumulating in the lime kiln’s lime dust makes the dust a good fertilising agent, which is delivered to fertiliser producers. The rest of the inorganic matter accumulates into green liquor sludge at the point where it is causticised into white liquor. Pehu-Lehtonen says that green liquor sludge is the only solid waste produced by a modern pulp mill. “It is not harmful and can be used in the construction of facilities like arenas, but we have yet to find a continuous application for it. Most of it ends up in landfill. We have conducted a lot of investigations with other industrial op- erators and research institutions, and we are now in the middle of a new development project. I believe we will come up with a solution,” she says. GEOPOLYMERS CONTAINING GREEN LIQUOR SLUDGE Professor Mirja Illikainen of the University of Oulu considers green liquor sludge more appealing than many others do. Her research team develops geopolymers – materials many hope will replace cement, known for its massive carbon dioxide emissions. For example, many types of the mining and energy industries’ mineral side streams have been tried as bind- ers for geopolymers. These are also being studied in a two-year research project started in the spring of 2020. The participants include several companies, including Metsä Fibre.

Illikainen says that Finland alone produces millions of tonnes of mineral side streams each year. The green liquor sludge generated by pulp mills does not compete in this league, because its production amounts to only tens of thousands of tonnes. But then its function in geopolymers would also be unique. “What is so interesting about green liquor sludge is its alkalinity. It could serve as the alkali-activator the bind- er needs to react and become stronger. The commercial alkali-activator is currently the most expensive part of a geopolymer and produces most of its carbon dioxide emissions,” says Illikainen. If the green liquor sludge lives up to the researchers’ expectations, it will eventually be tested on the scale of a pilot project at a cement factory. PRODUCT GAS REPLACING FOSSIL FUELS Self-sufficiency in bioenergy is often cited as an example when discussing a pulp mill’s circular economy. In addi- tion to the recovery boiler, steam and electricity is also produced by the bark boiler. The self-sufficiency ratio in electrical energy at Metsä Fibre pulp mills amounts to 177 per cent. The bioenergy produced by the company is also sold to the Finnish grid and for district heating. A lime kiln is the exception to this rule because it needs to reach temperatures of up to a thousand degrees, best achieved with heavy fuel oil or natural gas. However, lime kilns can burn the product gas generated in the gasifica- tion of bark. Thanks to product gas, Äänekoski no longer uses any fossil fuels at all, and Joutseno only in exceptional circumstances. “At Äänekoski, we also produce biopellets and biogas by decomposing the organic sludge generated during waste-water treatment, and in volumes sufficient for sell- ing too,” says Pehu-Lehtonen. •

Kaija Pehu-Lehtonen Senior Vice President, responsible for business development at Metsä Fibre. She has been a Member of the Management team since 2010.

Mirja Illikainen A professor and leader of the Fibre and Particle Engineering Research Unit at the University of Oulu. Her research team focuses on the utilisation of industrial side streams, particularly as binders replacing cement.