Consequences of lignin precipitation in the pulp and paper industry

Artikel i övrig tidskrift, 2006

We evaluated the overall economic and energy consequences of the lignin precipitation process proposed by the Swedish national Eco-Cyclic Pulp Mill research program. The study compared four different bleached kraft pulp model mills, representing greenfield mills with different steam demands and varying surpluses of thermal energy that can be precipitated as lignin. The results suggest that 37%–50% of the lignin in the black liquor must be removed if the thermal energy surplus generated by the model mills is to be exported as lignin. From an energy savings perspective, the most important consequence is that the power balance changes significantly when precipitating lignin, in some cases even leading to a power deficiency in the mill. Other important consequences include an increased live steam demand for evaporation and a decreased load in the recovery boiler. We concluded that it is reasonable from an energy perspective to precipitate lignin in the evaluated model mills. At the given economic conditions, the model mills with lignin precipitation have similar total costs as the model mill without lignin precipitation, if the lignin price is approximately US$ 12/MWh. At today’s pricing of biomass fuel, this means that it must be possible to sell the lignin at a higher price than bark to make lignin precipitation interesting from an economic perspective. The lignin price at which the model mills with lignin precipitation have similar total costs as the model mill without lignin precipitation changes with changing economic conditions; the most important parameters are the electricity price and the cost for carbon dioxide (CO2). If the cost for CO2 is eliminated, the lignin price necessary decreases to below US$ 9/MWh, and the possibility to use flue gases instead of pure CO2 for the precipitation must therefore be studied further.