Södra Cell has a highly efficient energy system. Its pulp mills supply electricity, district heating and biofuel, making Södra the largest supplier of bioenergy in the country. As pulp mills continue to make more effective use of energy they are reducing their fossil fuel consumption, contributing to relatively low CO2 emissions.
Ninety-five per cent of the energy Södra Cell uses in its production process is renewable. Some 80 per cent of the energy comes from the black liquor combusted in the recovery boiler that contains the cooking chemicals and released lignin that make up around half the wood.
Methanol, turpentine and tall oil are recovered from the cooking process in addition to pulp. Methanol is usually burned in a burner that combusts NCG, Non Condensable Gases, with a high sulphur content.
Bark is combusted in the bark boiler when the process requires extra heat. The surplus of bark is first dried and then sold as biofuel on the open market.
Biofuel is used in the lime kilns at Mönsterås (bark powder) and Värö (today gasified bark and from 2014 saw dust powder). Biologically-based tall oil and tar oil are also used.
The small amount of fossil fuel consists primarily of heating oil burnt in the lime kiln and for starting and stopping the recovery boiler.
Södra Cell supplied the following by-products (biofuels) in 2013:
- Bark and bark pellets 164 GWh
- Tall oil 410 GWh
- Turpentine 10 GWh
Mönsterås and Värö supply the market with dried bark. Tall oil is produced from the soap formed during the cooking process. Södra Cell supplies tall oil for biodiesel production. Tall oil is also used by the cosmetic and other industries. The residual product, tall oil pitch, is combusted as a substitute to fossil oil. Turpentine is recovered from gases formed during cooking and emptying of the digester. Turpentine is used in the paint industry and applications such as perfume manufacture.
The process of minimising energy consumption has been underway for many years in Södra Cell, both in heat and electricity consumption. This means that Södra Cell pulp mills are well within BAT (Best Available Technique) levels as shown in the table.
Södra Cell is participating in the PFE programme sanctioned by the energy authorities. The programme aims to make electricity consumption more efficient, and the goal of reaching 2 % higher efficiency compared with the level of 2008 will be achieved by Södra Cell.
A new seven-stage evaporation plant with optimal energy efficiency was installed in Värö in autumn 2009. The investment reduced energy consumption during evaporation by some 30 %, which means that normally only the recovery boiler is needed. The surplus steam resulting from the investments is used for drying bark for sale on the open market and from 2014 also for drying sawdust in order to replace oil as fuel in the lime kiln.
At Mönsterås, flue-gases are used to dry bark that is combusted as powder in the lime kiln as a replacement for fossil heating oil. Surplus energy is also used to dry bark for sale on the open market.
In Mörrum it may be possible to recover lignin from black liquor if production is increased in future. Lignin fuel has a high thermal value and can be used as an oil substitute in lime kilns and/or as a coal substitute in power stations. Lignin extraction would also reduce the surplus energy during the summer half-year.
All the Södra Cell mills have an energy surplus that is used for producing electricity as well as district heating to the surroundings. Thanks to the investments in new turbines all the mills are net suppliers of green electricity to the grid.
Back pressure/condensing turbines
Approximately 1,300 GWh of electricity was produced in 2015, 99 per cent of which is green electricity. This makes Södra Cell one of Sweden’s biggest producers of biobased green electricity.
Description of back pressure and condensing turbines
As well as producing electricity, a back pressure turbine produces high temperature steam after the turbine (4 bar(a), 160ºC) that can be used to heat the process. Steam can be tapped from the turbine in several levels at different pressures and temperatures. The more steam that can go through the entire turbine, the more electricity is produced. The figure shows the principle for a recovery plant in a pulp mill with a back pressure turbine.
A pulp mill the size of Mönsterås generally has a large energy surplus, i.e. more energy is produced from the recovery boiler when the digester chemicals are recovered than is used in the process and in the back pressure turbines. This means there is scope to maximise electricity production in a condensing turbine. The steam temperature in a condensing turbine is kept at a minimum after the turbine by having a very low pressure (0.06 bar(a), 35 ºC), as the steam is not used for process heating. The lower pressure and temperature after the turbine, the more electricity it produces. The steam after the turbine is condensed by a water circuit which in turn is cooled in a cooling tower.
The diagram shows how heat in waste water from the bleach plant is exchanged to district heating. The temperature loss is not more than 2 ºC over the twenty kilometre pipe. Waste water then continues via the cooling tower to the biological treatment plant.
In 2015, 346 GWh were supplied to local communities, which is equivalent to some 33,000 m3 of fuel oil or 95,000 tonnes of carbon dioxide.
Södra has a strong environmental profile, with a number of key elements:
- Traceability of all wood and certification of forest operations according to PEFC and FSC.
- Highly-closed loop in the process water systems of the pulp mills, facilitated by the TCF and ECF bleaching methods.
- Modern effluent treatment and other eco-adapted process systems, maintaining most emission levels well within BAT (Best Available Technology) standards.
- Open reporting and third party certified systems in all environmental areas.
- Highly efficient energy system. Södra Cell is today Sweden’s largest bioenergy supplier.
- Very low emissions of greenhouse gases. Taking into account carbon sequestration in the forest, district heating and electricity production, Södra Cell products provide you with a considerable carbon reduction to carry forward in the value chain.
Södra operates in a part of the world where conditions for forestry and the forest industry are favourable from a water perspective. Our industry doesn’t cause water shortages, and we’ve made major investments in order to keep the quality of the water so high that continued use of the water is not impeded. For forest owners and managers, taking forest water environment into consideration is a part of their day-to-day work.
Water flows are complex systems and an international standard to set out how the amount of water used to produce goods and products should be calculated and presented is currently under development. The present proposal includes two options which can be used for reporting:
Either a “water inventory”, with actual figures for a raw material, or a more extensive “water footprint” for an end product. Södra has made a “water inventory” which reports on water use for its main products as a starting point. The water inventory does not only include fresh water and waste water, but also evaporation and water in raw materials, products and waste.
Forestry and water management
The effects of forestry on water quality and access to water are complex and extremely difficult to quantify. The forest has many positive effects on the water system, such as water transport from the coast to inland, keeping soil moist and reducing erosion.
Södra promotes greater consideration for water in forest management through the Forest Water campaign. One feature of the campaign is the Water Management target included in the Green Forest Management Plan. This provides a basis for the considerations required during forest management and harvesting operations.
The production process
Södra has a broad product portfolio, comprising both paper and dissolving pulp. The raw materials used are spruce and pine, as well as hardwood (mainly birch). Södra has unique knowledge in how forest raw materials should be sorted for the right end-product properties. Paper pulp products are divided into four main categories, dissolving pulp into two.