Article published in the APEGBC's members journal Innovation, November 02 issue

Quesnel River Pulp Improvements:
Towards a More Sustainable Operation

Chris Connaghan PEng
Duncan Industrial Engineering

Mike Van Aert PEng
Quesnel River Pulp Company

Gaetan Noel M. eng
Pragmathic Inc

Located in the town of Quesnel in BC’s central interior, Quesnel River Pulp Company — a large and complex industrial operation — produces about 1,000 tonnes per day of thermomechanical pulp. With 11 refiners totalling 118,000 horsepower, the mill is a major user of electrical power in BC. Only about 10% of the power is used to refine the pulp, with the balance generating heat in the form of dirty steam. A reboiler — supplemented by a gas-fired package boiler and glycol boiler — converts this dirty steam into clean steam that is used for other pulping processes in the plant.

During the past several years the amount of clean steam generated from the reboiler had decreased, along with the amount of energy recovered from various process and effluent heat exchangers. This required more steam from the gas-fired package boiler at significant additional cost, also resulting in increased generation of greenhouse gases. In addition, a temperature increase in the effluent discharged to the Fraser River necessitated the addition of cold fresh well water to maintain effluent temperature below that permitted, with those wells working at maximum capacity.

Using two leading edge technologies — computer simulation modeling with extensive onsite verification, in combination with Pinch analysis — Nanaimo and Prince George-based Duncan Industrial Engineering conducted a study for Quesnel River Pulp to identify several projects to reduce mill effluent temperature and volume. Energy conservation projects were also identified and implemented that allowed both the gas-fired package boiler and glycol boilers to be shut down, resulting in major reductions in energy costs and greenhouse gas emissions.

Systems Model Development

With three production lines, Quesnel River Pulp comprises a large and complicated industrial complex that involves the interaction of various systems: stock, white water, fresh and hot water, heat recovery and exchangers, and effluent handling. A change in heat recovery in any one of these systems will affect operations throughout the rest of the mill. Because of this complexity, until recently it was not only very difficult to analyze these systems to precisely define the problem but also challenging to determine the optimum solution.

As a first step, Duncan Industrial used CadsimPlus (available from Burnaby-based Aurel Systems) to model all the systems in the mill. This leading edge dynamic software, which has been developed and refined over the past 15 years, has module libraries developed specifically for the pulp and paper industry. It allows the generation of equipment drawings similar to mill process and control diagrams while at the same time generating an overall heat and material balance, making the simulation easier to interpret by mill operators and supervisory personnel.

A single computer simulation was developed for the entire mill including the three production lines. All major equipment including refiners with electric power input, package steam boiler, glycol boiler, heat exchangers and process equipment was modeled. Freshwater supply and effluent treatment systems were included. The simulation included stock tonnage, consistency, flow and temperature for all process and equipment streams, as well as steam loads and natural gas fuel requirements for the steam and glycol boilers and heat recovery equipment.

Once the computer simulation was developed, it was extensively verified during a two month onsite visit to ensure that it accurately matched mill results. With an accurate and verified process simulation of the entire mill in place, the various problems could be clearly and concisely defined, and resolution scenarios evaluated, using the model. The overall impact of one change or even numerous changes on the entire mill could be accurately predicted, and the extensive verification of the model allowed a high level of confidence in the predicted results.

The millwide process simulation was coupled with Pinch analysis, a new thermodynamic concept that identifies all process cold streams in a mill that require heating, and all hot effluent streams that can supply heat. SodeXpro/Pragmathic of Quebec was subcontracted to develop the Pinch analysis.

Using Pinch analysis, data was extracted from the verified computer simulation mass and energy balance. Hot and cold composite curves, developed from the extracted data, were used to define the Pinch temperature. Using a very systematic approach, heat exchangers are used to match the cold streams that require heating versus the available hot effluent streams. Pinch analysis allows the theoretical optimum energy performance of a complex mill or system to be defined.

As a result of the computer simulation and Pinch analysis, over a dozen energy conservation opportunities were identified along with savings and order of magnitude cost estimates. After evaluating and selecting the desired projects, Quesnel River Pulp retained Duncan Industrial Engineering, Utility and Recovery Engineering, Johnstone Boiler and Tank, and Claus Engineering for the project implementation, which included the following.

Three shell and tube heat exchangers are used to reclaim heat from the effluent. These exchangers are very large (three to four feet in diameter and over 30 feet in length) with an exchanger area of about 6,500 square feet each. The computer simulation confirmed that the heat transfer coefficient had fallen to about 25% of the original design due to fouling over the years.

The exchangers underwent chemical cleaning to improve the heat transfer rate to the point that fresh water for effluent cooling was no longer required. This improvement also served to transfer heat from the effluent into the mill process, reducing steam requirements. The effluent temperature, with the cold fresh water shut off, was reduced by 1-2^oC, providing a significant safety margin for the environmental effluent temperature permit while reducing freshwater usage by 9%.

Demineralized fresh water is used as makeup to the package steam boiler. A new spiral heat exchanger was installed to preheat the makeup using hot effluent. This significantly reduced the steam required at the boiler deaerator, in turn reducing steam requirements from the package boiler along with its gas fuel requirements and greenhouse gas generation.

Originally, energy in the dirty waste steam from pulp cyclones was designed to be recovered using glycol heat exchangers. The hot glycol was then to be used to preheat the air supply to the gas-fired flash dryers used for drying pulp production. A number of mechanical problems had resulted in much of this dirty steam being vented, requiring the gas-fired glycol boiler to make up the difference.

The computer simulation and Pinch analysis clearly defined the amount of energy available and the savings that could be achieved by resolving these mechanical problems. This information provided the impetus for the mill to address these problems, allowing the glycol boiler to be shut down and reducing natural gas consumption and greenhouse gas generation.

Excess waste steam from Production Line 3 secondary and reject refiners was being vented to the atmosphere, while fresh clean steam from the package boiler was required for the Production Lines 1 and 2 atmospheric presteaming bin. The combination computer simulation/Pinch analysis identified both streams. A simple piping run utilized this waste steam to displace package boiler steam and again save natural gas fuel and reduce greenhouse gases.

The reboiler converts waste dirty steam generated from the chip refiners to clean steam that is utilized throughout the pulping process in the rest of the plant. Both the steam package boiler and the glycol boiler supplement the reboiler.

During 14 years of operation, the reboiler (a very large vertical falling film heat exchanger) had been cleaned many times on the tube side. However, the shell side was neither accessible for pressure washing nor practical for chemical cleaning. The simulation found that the heat transfer coefficient and clean steam generation had dropped significantly from the original design.

Based on this information, the mill worked with several engineering firms to rebuild the reboiler, which included the addition of flanges and a port at the top head for inspection and cleaning. The tube sheet pattern was changed from triangular to square to provide access between the tubes and allow the critical water distribution areas to be cleaned. The single centre water distribution pipe was replaced with outside piping and four inlets to improve water distribution and increase the area available for tubes. The reboiler was also lengthened to provide a 15% increase in surface area.

The net result of these modifications was a 40% increase in clean steam production from the reboiler, thereby significantly reducing the additional steam required from the gas-fired package and glycol boilers. The package and glycol boilers are now shut off during normal production in summer and winter conditions, saving 275,000 GJ of gas annually.

The Quesnel River Pulp study demonstrated that environmental improvements can be attained for an industrial complex while at the same time reducing energy consumption and improving competitiveness.

Once all projects are completed, the mill expects to save 430,000 GJ of gas annually; at a nominal $5/GJ, this will reduce operating costs by $2.1 million (capital and expense costs to achieve these savings will be approximately $1.3 million). Using less natural gas will reduce greenhouse gas emissions by 21,500 tonnes annually.

The combined use of leading edge technologies in computer simulation modeling and Pinch analysis has contributed significantly to the long term sustainable operation of the mill while simultaneously reducing its environmental impact. It represents a high level engineering approach that can potentially can be applied for the benefit of the pulp and paper industry across Canada.