Simulations of Evaporation Plants in Kraft Pulp Mills: Including Lignin Extraction and Use of Excess Heat

Doctoral thesis, 2009

In this thesis, evaporation plants at kraft pulp mills are simulated with the purpose of making them more energy efficient. The work is important since energy saving is assumed to be one of the major solutions for handling the world's energy demand in the future. Pulp and paper mills can do much in this respect; they represent almost 50% of the energy consumption in Swedish industry. In kraft pulp mills, the greatest energy demand is usually in the evaporation plant. To simulate the plants, an existing tool was developed and used; important parameters for the simulations are the amount of water evaporated, the number of evaporation effects, and the solids content profile of the evaporation train. The evaporation plants are assumed to be situated in a model mill resembling typical Scandinavian market pulp mills. To assure realistic assumptions, the project was conducted in cooperation with industrial representatives in a national research programme. Previous research shows that great energy savings can be obtained in evaporation plants by reusing excess heat, provided that excess heat can be made available in the mill. Evaporation plants that reuse excess heat are called process-integrated (PI) plants. The energy surplus resulting from the savings could be exported from the mill to replace fossil fuels. For example, the surplus could be extracted as the energy-rich component lignin. As an excerpt of the results, 26% of the live steam could be saved in the evaporation plant by employing a 7-effect PI plant (1.0 GJ/ADt of excess heat) instead of a modern 7-effect conventional plant. The additional profit for PI plants was 0.3–1.5 €/ADt in comparison with conventional plants (for the conditions in Paper 6). With predictably higher energy prices in the future, the profits from energy-saving measures could increase further. As an example of the results for lignin extraction, an evaporation plant with 190 kg/ADt lignin extraction (LE) requires 12% more live steam than a plant without LE. Should the viscosity of lignin-lean black liquor be as low as recent experiments indicate, the investment cost for a plant with LE may be only 5% higher than that for a plant without LE. An overall conclusion from the cooperative work is that LE may be economically interesting for pulp mills, at least in connection with increased pulp production. However, the results depend greatly on the electricity and lignin prices.