Taking the heat
Hot air recovery saves New Brunswick mill $40,000.
A UNB student’s energy project demonstrates the value of a strong connection between Canadian industry and academia.
Rapidly rising power costs are driving home the importance of energy efficiency in pulp mills.
Although AV Nacakwic was the first mill with prehydrolysis kraft conversion capabilities in Canada, it struggled to find energy savings without heat recovery technologies in place.
A graduate student from the University of New Brunswick delivered a solution that’s saving the mill $40,000 a year, demonstrating the benefit of industry connecting with post-secondary institutions.
Today, the mill in tiny Nackawic, a town of about 1,000 in central New Brunswick (also home to the world’s largest axe), employs more than 350 people, but it has endured a bumpy ride.
Constructed in 1967, it manufactured photographic grade kraft pulp until 2004, when the St. Anne Nackawic Pulp Co. Ltd. declared bankruptcy, owing more than $100 million to its creditors.
The company blamed high energy, material and labour costs for the closure, which put 450 people out of work. Digital cameras also cut into demand for photographic paper.
It reopened after an announced partnership between Tembec and India’s Aditya Birla Group in 2006 under the AV Nackawic banner, but Tembec subsequently backed out.
The New Brunswick government awarded the company a five-year loan of $10 million in 2009 to protect 300 jobs, which followed $10 million to convert the plant to produce pulp for rayon.
The mill now produces more than 175,000 tonnes of northern bleached hardwood kraft woodpulp used in coated paper, specialty printing paper, packaging and tissue produced from aspen, birch and maple fibres.
In 2010, AV Nackawic was awarded $2.37 million through the government of Canada’s $1 billion Pulp and Paper Green Transformation Program (PPGTP) to make upgrades that would reduce the mill’s water and energy consumption by recycling filtrate in the bleach plant process and reducing the amount of cutting chemicals used.
The company also wanted to enhance its heat recovery systems but it lacked the resources needed to complete the project, so it went looking for a solution, and found Rohan Bandekar. He’s a graduate student from the department of chemical engineering at the University of New Brunswick and a product of the Mitacs Accelerate program, which supports research and development projects by partnering industry with academia.
Born in Dharwad, Karnataka, India in 1984, Bandekar came to North America to get his Master’s degree in chemical engineering at Wayne State University and he graduated in 2008.
After a year-long job search, he finally received an offer. Dr. Yonghao Ni at the University of New Brunswick invited him to work on a research project with Corner Brook Pulp and Paper Ltd., while he completed a second Master’s degree.
His research with Ni resulted in an opportunity between Mitacs and AV Nackawic, and he has never looked back.
Through the Accelerate program, companies ante up $7,500 for the internship, and Mitacs matches the funding dollar-for-dollar, with $10,000 of the funding going to the intern as a stipend, and the remaining $5,000 supporting other costs associated with the research project.
“The Black Hole”
“The project had been something I had been half working on, but it was a detailed project that was going to take some time,” said Carolyn Drost, senior process engineer at AV Nackawic. “[Bandekar] recognized the inefficiency in the process.”
The blow heat recovery system is popularly known as “the Black Hole,” where the pulp and paper industry has utilized the energy of blow vapors from the digesters to produce process hot water for washing and bleaching operations.
The system also reduces the vapour volume of the gases for non-condensable gases (NCG) incineration.
“It’s a challenge to extract maximum performance because of the system’s dual functionality,” says Bandekar.
But his solution addresses performance issues caused by that dual functionality. Now the system recovers hot blow gases as they are released from the digester while also reducing the vapour volume of the non-condensable gases for NCG incineration.
Increased NCG volume required additional cooling of blow gases to further reduce the total volume incinerated.
Another issue lay in the inability of the process to supply constant temperature hot water to the bleach plant operation.
“Understanding the thermodynamics of the blow heat recovery system avoids errors in calculating the amount of energy needed and prevents vapour release and energy loss to the atmosphere,” says Bandekar. “A heat balance across the blow heat system revealed that the secondary condenser was not cooling the NCG to the required temperature.”
Further investigation revealed the cooling water flow to the secondary condenser was insufficient to condense the NCG. Gases exiting the secondary condenser are cooled to the set point of 84 degrees C and the gases from the tertiary condenser are also cooled to the required temperature of 40 degrees C.
“The condenser’s control loop was re-tuned to supply enough cooling water to the secondary condenser, allowing us to reduce our steam use,” he says.
Thanks to Bandekar, the mill has converted its cooking process from a kraft to a dissolving pulp process.
The project was completed in just eight months under the supervision of Drost and is saving energy.
The connection of the mill also paid off for Bandekar personally. He now works for AV Nackawic full-time as a junior process engineer.
This article appears in the May/June edition of PLANT.
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