Sustainable Swedish Industry by Improved Excess Heat Recovery

Abstract: Achieving a sustainable Swedish industry will require better utilization of resources already available and already spent. Nationally, vast amounts of heat are released into the atmosphere and not made use of. If only a fraction of this energy could be exploited, the path to net zero emission society by 2045 would be thousands of tons worth of CO2 closer. In Sweden, 25% of excess heat delivered to district heating has its origin in the pulp- and paper industry. The importance of this, is that it can be assumed that available excess heat can be delivered through existing infrastructure as district heating. Research focused on Kraft method of pulp production, an energy intensive chemical pulping process with great potential for excess heat recovery. One of the key steps in Kraft pulp production is the chemical recovery of spent cooking chemicals, known as black liquor. The spent liquor has an initial water content of around 83 w/w% and needs to be concentrated to 15 w/w% water content, for chemicals to be recovered and burnt in the recovery boiler. Today, evaporation is done by multiple effect evaporators at a capacity of thousands of tons black liquor per day, making it one of the world’s most energy intensive processes. Here, Helios Innovations AB novel evaporation technology is studied, and it is concluded that it is possible to evaporate black liquor using temperatures considered waste heat. Pilot and laboratory scale tests conclude that packing materials, used for mass and heat transfer, can withstand relevant conditions for long-time exposure. Tests reveal performance of vapor condensation rate with an average of 15-17% lower compared to pure water. Large-scale design has potential of recovering 386 GWh/a thermal energy, saving up to 67 GWh/a of steam to power existing effects. Electricity savings of up to 42 GWh per year translate to cost savings on electricity be worth up to 14.7 MSEK/a. A Helios retrofit can potentially increase production capacity in evaporators by 6%. Potential energy savings can be translated to mitigated CO2 emissions of 26,700 tons/a.