Exporting lignin or power from heat-integrated kraft pulp mills: A techno-economic comparison using model mills

摘要

Lignin separation and increased power generation are two alternatives for utilising a steam surplus at a kraft pulp mill. Lignin can be separated from the black liquor by using CO_2 in a precipitation process. The resulting lignin can be sold as a biofuel. In this study, lignin separation and increased power generation have been compared by using computer models representing typical Scandinavian mills (327,000 ADt/a). In the studied mills, the steam surplus was up to 437 GWh/a (4.82 GJ/ADt) which enabled either increased power generation by 126 GWh/a (+65%) or extraction of 500 GWh/a lignin. A consequence of extracting this amount of lignin was that the power generation decreased by 59 GWh/a (-30%), which means that the profitability of lignin separation was dependent on the electricity price. Moreover, the profitability was mainly dependent on the lignin price and the cost for CO_2 to precipitate the lignin. For lignin separation to give the same annual earnings as increased power generation, the ratio between the electricity price and lignin price had to be 1.9-2.3. For ratios below 1.9, lignin separation was economically preferable, while ratios above 2.3 meant that increased power generation was preferable. It was noticeable that the profitability under all studied conditions was considerably lower for lignin separation than for power generation at high electricity prices. Lignin prices around 15 ?/MWh implied that the electricity price had to be below 33 ?/MWh for lignin separation to give higher annual earnings than increased power generation. For a high electricity price (55 ?/MWh, e.g. including policy instruments), the lignin price had to be above 23.8 ?/MWh for lignin separation to give higher annual earnings. These lignin prices are significantly higher than equivalent biofuel prices today. However, the lignin price would probably be higher than 25 ?/MWh if lignin were used e.g. as raw material for specialty chemicals or as a replacement for fossil fuels.