The pulp and paper industry is faced with many challenges including market decline for printing and writing paper, high energy cost, and scarcity of quality wood supply. Electric energy consumption and raw material are the main two variables driving the total manufacturing cost of the final product.
Mills are required to achieve high energy efficiency by targeting the production of optimal pulp quality that will result in attaining customer paper specifications. The pulp quality depends mainly on the amount of energy used and on how the energy is delivered to the pulp. Therefore, one of the main challenges of thermomechanical pulping (TMP) is to optimize energy use and produce high-quality pulp. This can be accomplished by reducing the variability of the pulp quality.
In general, variability has two components; unpredictable and predictable. Unpredictable variability is due to random variation of raw material or noise. This type of variability will stay in the process and cannot be eliminated by any means. Predictable variability is introduced by poor TMP operations, poor control, mistuned control loops, or the malfunctioning of equipment. This type of variability can easily be tracked and eliminated.
Manual control compared to automatic control
Once the TMP operations are well understood and the equipment is maintained correctly, installing an automatic control system of pulp quality will adjust the specific energy for furnish and/or production rate change quickly. This would certainly bring more benefits such as stability of operation and energy savings, therefore, cost reduction. The positive results of automatic control can be explained by the fact that the pulp quality is controlled based on a prediction model that is updated by data received from the pulp quality monitoring system at an uneven interval of up to two hours. This compensates for any predicted disturbance as soon as it occurs, whereas a manual control method relies on two or three pulp quality measurements, and may take up to several hours before a decision by the operator is made. In addition, operators are often given a pulp quality margin target instead of a set-point target. Automatic control is based on controlling the pulp quality to a set-point target. Therefore, the specific energy is a control variable manipulated to maintain the pulp quality at the target set-point.
There are four steps to better pulp quality and energy savings:
- Improve manual operation through operator training and TMP supervision.
- Track TMP variability, eliminate the predictable variability, and benchmark the performance of TMP.
- Repair all malfunctioning equipment.
- Install automatic pulp quality control to obtain cost reduction and added benefits.