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Rethinking Refining

Mechanical pulping produces a higher yield than its chemical counterpart, but the equipment used in the process, particularly the high consistency (HC) refiners, is very energy-intensive, which is a challenge at a time of high and increasing...

April 1, 2013  By Pulp & Paper Canada

Mechanical pulping produces a higher yield than its chemical counterpart, but the equipment used in the process, particularly the high consistency (HC) refiners, is very energy-intensive, which is a challenge at a time of high and increasing energy prices.
Case in point: The NewPage Port Hawkesbury thermomechanical pulp mill in Nova Scotia, before its shutdown in 2011, used to spend about $100 million per year on electricity, according to court documents presented with regard to the mill’s bankruptcy. In addition to the TMP line, NewPage Port Hawkesbury operated one newsprint machine and one supercalendered paper machine.
In western Canada, researchers have been investigating ways to reduce energy consumption in mechanical pulping. At the University of British Columbia (UBC), The Pulp and Paper Centre has been running a research project on this topic since 2007.
The purpose of the project is to help mechanical pulp mills make the transition to more energy-efficient low consistency (LC) refining and to reduce their reliance on HC refining. (Some conventional mechanical pulp mills use only HC refining. In other mills, both HC and LC refiners are used, with the former preceding the latter in the refining process.)
According to project participant BC Hydro, pilot plant trials and mill energy efficiency feasibility studies in the first phase of the project (2007 to the end of 2012) showed that LC refining was between 5 and 12% more efficient than HC refining for treating high freeness pulp, which is used to make board, tissue and towels.
Norman Wild, senior energy management engineer, Power Smart Engineering – Industrial, says BC Hydro is looking forward to getting on with Phase Two, which will “advance the technology potential” of low freeness pulp.
“Low consistency refining showed great promise in the higher freeness grades (in Phase One),” Wild said. “One TMP mill in BC was able to replace high consistency refining with low consistency refining as a direct result of the UBC research.”
The potential of low consistency refining is not only recognized in British Columbia. Alberta Newsprint Company, a TMP/newsprint mill located in Whitecourt, Alta., incorporated a low-consistency refiner to its reject refining line, and was able to eliminate reject screening. The move saved the mill 3,100 MWh of electrical power per year.
Goal: eliminate one HC refiner
Phase Two of the research project at UBC’s Pulp and Paper Centre, which is set to begin in September 2013, will expand previous work aimed at implementing multiple stages of LC refining and eliminating one of the two HC refiners used in mechanical pulping.
Dr. James Olson, director of the Pulp and Paper Centre, says LC refining is the primary method for providing significant energy savings in mechanical pulp production, because it is more efficient than HC refining.
“LC refining is also the primary means of improving the strength and smoothness of paper,” Olson explains. “Paper [made by LC refining] can be more than twice as strong.”
The UBC project addresses a serious challenge facing the Canadian pulp and paper industry. The manufacture of mechanical pulps and wood-containing papers is one of the few sectors of the pulp and paper industry where Canadian companies are better placed than their competitors. Their advantage lies in Canada’s investment in clean, hydroelectric infrastructure and in our slow growing northern wood species which have superior properties and which support the manufacture of wide range of high-value pulp and paper grades.
The process of mechanical pulping uses a great deal of electricity, due mainly to the low efficiency of mechanical refiners. In British Columbia, for example, mechanical pulp mills consume approximately 10% of the electrical energy produced in the province.
High energy costs, particularly in Alberta, where the cost of electricity is set in an open market and is therefore unpredictable, jeopardize the global competitiveness of Canadian mechanical pulp mills. The survival of the industry in this country depends on the ability of mills to reduce energy consumption per tonne of product produced.
Olson says the Pulp and Paper Centre’s research on mechanical pulping benefits the Canadian pulp and paper industry in a number of important ways. “Energy savings speak to both improving the competitiveness of the industry, which protects and adds to jobs in rural communities, and to improving the environmental footprint of those operations.”
He says the energy reduction project has demonstrated a reduction in energy use of nearly 1000 KWh/t, an almost 30% reduction.
“And we can still produce pulp of the same quality,” he said. “Our partners are starting to implement some of these new technologies at their mills.”
A research program that was undertaken by Catalyst Paper Inc. at its now-defunct Elk Falls Mill in Campbell River, B.C., was the starting point for the UBC pilot plant, says Gord Harrison, mechanical pulping energy manager in Catalyst’s Port Alberni Division.
“The objective of the research was to determine the no-load energy, which is the amount of energy required to turn equipment over without putting work into refining the wood fibre, which is consumed by LC refiners,” he said.
BC Hydro’s Markus Zeller says his organization was one of the initiators of the Pulp and Paper Centre’s research program on energy reduction in mechanical pulping.
“In Phase One of the project we were the first funding partner and we were able to help get the mills and the other partners on board,” Zeller said. “We expect to play the same role in Phase Two.”  Zeller is BC Hydro’s Power Smart Technology Innovation Manager for Industrial.
BC Hydro is also conducting mill trials with model predictive control (MPC). The objective of the demonstration is to combine two technologies (continuous pulp quality monitoring and MPC) to determine if it can improve mainline refining control. BC Hydro wants to show power savings with reduced product variability and improved refiner operating targets.
“We did a demonstration project with an on-line fiber analyzer, but results have been intermittent and somewhat inconclusive,” said Zeller. “Our challenge with MPC is measurement and verification of energy savings. The lack of mill staff and resources makes any demonstration project challenging.”
LC refining shakes up the traditional process
At Alberta Newsprint Company (ANC), low consistency refining is contributing to energy savings by allowing the elimination of rejects screening. Surendra Singh, ANC’s director of energy, pulp and utilities, explains that screening removes undeveloped fibre (“rejects”) from pulp before the pulp is used to produce paper.
“There was a time when screening was thought to be a necessary step in paper making,” Singh says. “However, with the proper refining, including low consistency refining, the amount of undeveloped fibre in the pulp has been reduced to an acceptable level without screening.”
Singh says ANC has benefited in two ways since it discontinued screening for rejects in July 2012.
“First, it helped us to eliminate the capacity limitation in the reject refining system,” he said. “And secondly, it enabled us to remove six electrical drives from the refining process, which saves us 3,100 MWh of electrical energy per year. The performance of our paper in press rooms, in term of percent break, has been better than ever.”
According to Singh, ANC consumes 830 GWh/y of electrical power, or 3.2 MWh/t.

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