Article published in the APEGBC's members journal Innovation, November
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
in the town of Quesnel in BCs 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
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
Onsite Verification and Problem Resolution
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
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.
Effluent Heat Exchanger Cleaning
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 1-2oC, providing a significant safety
margin for the environmental effluent temperature permit while reducing
freshwater usage by 9%.
Boiler Feedwater Preheating
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.
Glycol Heat Exchanger Utilization
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.
Waste Steam Utilization
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.
Reboiler Modifications and Retubing
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
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.
Chris Connaghan P.Eng., Project Manager at Duncan
Industrial Engineering, managed the computer simulation modelling, Pinch
analysis subcontract and preliminary identification of energy and
effluent reduction projects for the Quesnel River Pulp improvements.
Mike Van Aert P.Eng., Engineering Supervisor for
Quesnel River Pulp Company, selected and implemented many of the energy
projects identified in the mill study.
Gaetan Noel M. eng, Associate Principal and Technical
Specialist for Pragmathic Inc, completed the Pinch analysis portion of
the project and identified energy projects and cost estimates.