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\nboiler is often a bottleneck. Two different approaches to\ndebottleneck the recovery boiler have been investigated in two\nmodel mills that represent typical Scandinavian pulp mills. As a\nreference approach, the recovery boiler was upgraded. In\nconnection with this, the turbine system was also upgraded to\nenable increased electricity production. As an alternative to this\nconventional approach, the load of the boiler was kept constant\nby extracting lignin from the black liquor in proportion to the\nproduction increase. To keep the steam balance, the specific\nsteam consumption at the mill was reduced through steamsaving\nmeasures to such an extent that the amount of steam\nproduced from burning the lignin-lean black liquor was sufficient.\nConsequently, the extracted lignin was not needed in-house\nand could be sold as a biofuel. The net profit of implementing\nlignin separation was calculated for different conditions. The\nresult was that, depending on the conditions, the lignin price\nhad to be 2-17 €/MWh for lignin separation to be equally\nprofitable as boiler upgrade. With an assumed lignin price of\nabout 15 €/MWh, the conclusion is that lignin separation\nshould be an economically attractive alternative for debottlenecking\nthe recovery boiler. With a high power price and a\nlong-term investment strategy, however, upgrading the recovery\nboiler and the turbine system can be more profitable.