Command over consumption

Pulp & Paper, Aug 2005 by Shaw, Monica

Rising energy costs are a battle for North American mills, but advanced process control has helped Augusta Newsprint fight back with lower consumption at the 100% purchased electricity mill

Cost control is an overriding concern these days as mill margins are pinched by rising energy and raw material prices, but competitive pressures dictate that gaining command over consumption cannot impact end product quality. Precise control of processes, however, offers some hope of curtailing expenses.

At Augusta Newsprint in Augusta, Ga., advanced quality control for the mill's thermomechanical (TMP) process has helped the mill reduce energy costs, as well as the use of costly kraft pulp, while still improving the quality of its sheet. The reduction in energy use was especially critical for the mill, which runs off of 100% purchased electricity from Georgia Power and is the utility's largest single customer.

The necessity for installing advanced TMP control became apparent in July 2001, when Augusta Newsprint Co., in conjunction with the U.S. Department of Energy (DOE), performed a mill-wide energy efficiency assessment that showed improved TMP efficiency could cut costs significantly. According to TMP plant supervisor Michael Alford, the mill could see the handwriting on the wall.

"At 100% purchased energy, we knew we were vulnerable since electricity mirrors the cost of fuel oil, and the TMP plant accounts for 71% of total mill energy use," Alford describes. "We were also using 5% purchased kraft in our sheet that was impacting the bottom line, and we wanted to wean off that."

In December 2001, Augusta began installing Metso Automation's PacSim Advanced Quality Control (AQC) system, which was partially funded by the DOE's Office of Industrial Technology. The system, helped along by other process adjustments, is accomplishing its goals while providing a platform for further fine-tuning of the TMP process and cost reductions.

TMP plant idiosyncrasies

Augusta Newsprint, a joint venture between Abitibi-Consolidated and Woodbridge Co. Ltd., produces 440,000 mtpy of newsprint on two paper machines. The TMP plant produces around 1,000 mtpd, and there is a 450-mtpd recycled plant to support sheet requirements of up to 45% old newspapers and magazines (see sidebar, p. 36).

Augusta's TMP plant is comprised of four main lines, each with a primary and secondary 50-in. Andritz (Sprout Waldron) Twin 50-CP refiner driven by a 12,000-hp motor. There is also a screen room that features two stages of 0.006-in. slotted screens and three 12,000-hp, Twin-50 reject refiners. The screen room is also equipped with five Bird screens with 0.070-in. hole baskets.

Some of the idiosyncrasies of the TMP plant factor into recent process improvements at the mill. For one, the refiners predate gap control. Also, the Metso (formerly Sunds Defibrator) slotted screens are variable speed, allowing the mill to adjust the rotors.

In addition, due to cost considerations when the TMP plant was constructed in 1983, the four main TMP lines feed into only one 214,000-gal latency chest, meaning the mill loses some of the individual knowledge associated with each line.

"One thing we needed to do was control specific energy on each line, but having one latency chest prevents you from knowing exactly how much mass is going through each refiner line," says Alford. "You can't just divide what comes out of the chest by four, because each line is different."

Installations and innovations

To correct the efficiency issues in the TMP plant and support the use of less kraft pulp, Augusta Newsprint purchased a PacSim Advanced Quality Control (AQC) system from Metso Automation for its refiner lines after research led by RNP superintendent Chris West and paper mill manager Jim Herrmann. The system incorporates a model predictive, multivariable controller (MPC) that interfaces with the mill's existing Honeywell distributed control system (DCS) and provides algorithms that suggest process changes to control quality based on software sensors. The DOE absorbed about 50% of the AQC's $600,000 purchase price, while the mill spent another $100,000 upgrading instrumentation so the MPC would have reliable information.

In December 2001, installation began for the AQC, which includes control systems for the mainline refiners, reject refiners, and screen room. These systems help control specific energy and blowline consistency to maintain freeness targets. Sensors are used to predict freeness, consistency, and reject rate, and historical data was used to create the predictive models. Figure 1 shows the information infrastructure for the AQC.

Sensors for freeness were installed on all refiners, as well as those for blowline consistency. Internal calculations are used to predict refiner freeness, and the mill controls to a freeness target as opposed to a specific energy target since there is only one latency chest.

Figure 2 provides a schematic of the installed AQC. The system gathers data such as motor load and dilution flow from the DCS and predicts freeness. Then, it sends adjusted settings for motor load back through the DCS.