POWER & ENERGY

Small Cogen System Solves Energy Reliability Dilemma at Sonoco Mill

By Martin Lensink

As the industrial power generation industry evolves-particularly in North America-there has been a proliferation of small, onsite industrial power systems. These systems have been developed for very practical reasons-namely ensuring ready availability of reliable, cost-efficient electric power.

Since cogeneration systems produce electricity as well as useful process heat, usually in the form of steam-the paper industry has certainly benefited from the efficiency of operation provided by these systems. One such small system was installed at Sonoco Ltd.'s 100% recycled paperboard mill in Brantford, Ont. The 3.9 MW cogeneration plant was provided as a turnkey design-build project. The mill at Brantford has two major divisions-paper and industrial products. The mill's main function is to manufacture high-strength paperboard used internally to produce converted packaging products such as tubes, cores, cans, and other products. The mill produces 170 mtpd. The paper machine operates continuously for approximately five weeks between washups and maintenance, during which the turbine also operates continuously.

PROBLEM SOLVING. Brantford had two serious problems to solve as it approached the cogeneration projectÑelectrical costs were escalating and as many as 20 power outages were occurring annually. These interruptions were causing complete production stoppages. As a result, a Sonoco project team was established in 1992 to come up with answers to these two problems.

The team analyzed the situation and opted for a cogeneration plant. Such a facility would essentially be an independent power island, freeing the plant from external disturbances. It would not only generate the power needed to run the plant, but it would provide a source of electricity to Sonoco's Trent Valley plant in eastern Ontario when excess Brantford power was wheeled over Ontario HydroÕs grid. This was done through a special arrangement with the local utility.

THE POWER PLANT. At theheart of the new cogeneration system is a standard U.S. Turbine (div. ofRolls-Royce) Model 4000 electric generating system, nominally rated at 3,944 kWeat ISO conditions. It employs an Allison (also a Rolls-Royce subsidiary) 501-KB5S combustion turbine fired by natural gas, which is derived from the proven Allison T56 turbine. Sonoco's Brantford plant generates electricity at 4,160 V stepped up via a transformer to 27.6 kV to complement Ontario Hydro T, and the system has the flexibility to operate independently from the utility.

Steam is generated by an ABCO Industries' waste heat recovery boiler. Steam produced by the cogen installation is used primarily for the paper machine, as well as office heating. Maximum steam generation rate is 55,000 lb/hour when turbine exhaust gas is supplemented with 32 million BTU/hour (LHV) of natural gas by a duct burner. Feedwater to the boiler is preheated by a boiler exhaust gas economizer. Cycle efficiency is as high as 86% (LHV).

During operation, the gas turbine draws filtered air into a 14-stage axial compressor, feeding it to a gas-fired combustion chamber. These hot combustion products drive a 4-stage power turbine and exhaust into a heat recovery boiler.

An Allen Bradley programmable logic controller controls the boilers. According to Ron Harten, Brantford's paper mill superintendent and project manager, the complete cogen plant, on which construction began in early 1993, was online within 10 months. Although the company had expected to be challenged by the problem of noise emanating from the mill-which is sandwiched in a tight space in a residential area of the town-it was able to avoid those problems by careful engineering and installation of noise mitigation equipment.

These solutions reduced the noise levels to 45 dBA, "anentirely comfortable level," says Harten. "Actually, we are the quietest cogeneration plant in the entire province of Ontario."

ECONOMIC BENEFITS. Prime economic advantage of the new installation has been the energy saving obtained from an efficient cogeneration plant. "Between the energy and demand components,"says Harten, "we are now saving more than a million dollars a year. In addition, when you consider the many facets of the problem we formerly encountered-from 10 to 20 power outages a year-the real savings are quite a bit more than that."

Cogeneration plant powered paper mills are among the lowest cost paper manufacturing facilities in North America, he adds, "which is why we were sure that was the right direction to take. The benefits, however, transcend the flexibility and cost-effectiveness of our cogen plant. The facility is built with safety in mind, is environmentally sound, and is most user-friendly.

"Computer controls put a whole new spin on the project, since none of us had used computers before. The whole concept of running a boiler from a computer controlled center is a new concept to master for an older workforce. With the help of training and even home study, the change in operations has been made with great ease. In addition, U.S. Turbine has been prompt in helping us overcome the few problems that have arisen."

MARTIN LENSINK is with U.S. Turbine Corp., St. Catherines, Ont.