Optimizing Kraft Pulp Mill Material Flows to Reduce Fossil Fuel Use, 2010 TAPPI/PAPTAC International Chemical Recovery Conference

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Kraft pulp mills are presently facing hard times due to the global economic recession. Inefficient mills have been shut down and the rest are struggling to make ends meet. Therefore, high availability and cost efficiency are vitally important for the survival of the pulp mill. To be able to reach high cost efficiency, detailed knowledge of every enthalpy and material flow is needed. Changes in wood consumption or in total yield, for instance, have significant effects on the costs of produced pulp.

Modern kraft pulp mills have high energy self sufficiency and the surplus power is sold to the energy grid. Depending on the amount and price of electricity, cash flow from this by-product can be significant. In addition to this, green electricity produced with biofuels is free of CO2 taxes. The total use of fossil fuels in Botnia mills in 2008 was only 2% of the total energy demand, mainly in the form of natural gas or heavy fuel oil. Most of these were used in the lime kiln. Reduction of the fossil fuels at the pulp mill necessitates detailed knowledge of material flows within the mill.

An Excel^® spreadsheet simulation model was developed to calculate all raw materials, fuel, and energy flows for a kraft pulp mill. The model can be applied to both softwood and hardwood mills. A unique feature in the model is the ability to model also the raw material flows, including wood and bark. With the model it is possible make mill-wide material balances, to predict material and energy flows and to evaluate the effect of different process parameters on the efficiency of the mill.