Increased capacity in kraft pulp mills: Lignin separation and reduced steam demand compared with recovery boiler upgrade

For increased production in pulp mills, the recovery boiler is often a bottleneck. Two different approaches to debottleneck the recovery boiler have been investigated in two model mills that represent typical Scandinavian pulp mills. As a reference approach, the recovery boiler was upgraded. In connection with this, the turbine system was also upgraded to enable increased electricity production. As an alternative to this conventional approach, the load of the boiler was kept constant by extracting lignin from the black liquor in proportion to the production increase. To keep the steam balance, the specific steam consumption at the mill was reduced through steam-saving measures to such an extent that the amount of steam produced from burning the lignin-lean black liquor was sufficient. Consequently, the extracted lignin was not needed in-house and could be sold as a biofuel. The net profit of implementing lignin separation was calculated for different conditions. The result was that, depending on the conditions, the lignin price had to be 2-17 Euro/MWh for lignin separation to be equally profitable as boiler upgrade. With an assumed lignin price of about 15 Euro/MWh, the conclusion is that lignin separation should be an economically attractive alternative for debottle-necking the recovery boiler. With a high power price and a long-term investment strategy, however, upgrading the recovery boiler and the turbine system can be more profitable.