Wood waste generates energy at mills
by Irwin Rapoport

For the past 80 years the pulp and paper industry (PPI) in the United States has been using the Kraft system to produce pulp – a process that promotes recycling of every part of the tree to generate nearly all of the electricity and chemicals used in the paper making process.

In effect, nothing goes to waste in the process, and this has been instrumental in allowing domestic pulp mills to generate, on average, a very high percentage of their electricity requirements from the use of wood shavings and other byproducts of the trees that otherwise would be disposed of.

“They use every part of the tree,” said Carlton Carroll, spokesman and press secretary for the American Forest & Paper Association (AF&PA). “There are several ways to make paper. Probably the most well-known is the Kraft process where wood chips are put into a digester, where chemicals are added. The mix is steamed and the chips are converted into pulp, which is later used to make paper.”

When logs enter the mill, they are immediately placed into a debarking machine, with the bark diverted to a burner that generates electricity to power the majority of the manufacturing process. The wood is then converted into uniform-sized wood chips. A scanner detects imperfect chips and diverts them to the boiler.

Nearly 50 percent of the wood chips are fiber, with the other half being lignin. While long-lasting (archival) paper is lignin-free, newsprint, which is meant to have a short lifespan, is a lesser quality paper product that contains lignin.

During the decomposition/digestion process, the lignin is removed by the chemicals in the digestion task. Once the chemicals have completed the task of removing the fiber, the mixture of the chemicals and lignin that is leftover forms a liquid referred to by the PPI as “black liquor,” a byproduct that helps to break down the wood chips into pulp.

The black liquor is later diverted to a recovery boiler where it is burned to generate electricity.

“When that is all burned away,” said Carroll, “what is left are the chemicals that can be reused over and over again.”

Paper mills take advantage of the steam to co-generate electricity via turbines that are placed at the top of digesters.

Nationwide, the forest products industry produces 28.5 million megawatt hours annually, with paper mills producing 27.1 million megawatt hours.

“Paper mills on average produce 75 percent of their power,” said Carroll. “Some facilities are able to produce 100-percent of their power from renewable resources.”

In most cases, however, a small amount of fossil fuel is still required to light the burners because the black liquor is not very combustible.

“Over the years, papermakers have replaced fuel with carbon neutral biomass,” Carroll added.

“It’s beneficial both to the bottom line and because the fuel is carbon neutral – they are not releasing excess carbon into the atmosphere like they would if they were burning coal.”

Pulp mills have filters and scrubbers installed in their smokestacks to minimize emissions and because chemicals are reused, the practice of dumping them into nearby rivers has ceased.

Because paper making requires large amounts of water, the runoff water is put through on-site treatment plants prior to being released into the environment.

Carroll said that the recycling being practiced by the PPI is not well known outside limited circles.

“But I think the public realizes that the trees are harvested in a sustainable way and that when trees are chopped down, they are replenished by the industry,” he said. “The industry can only survive by using natural resources wisely and it knows that and certainly practices that. The use of sustainable forestry and renewable energy sources, like the burning of wood chips, bark and black liquor and the use of co-generation are all based on that philosophy.”

While the PPI is fiercely competitive, via the American Forest & Paper Association’s (AF&PA) Agenda 2020 Technology Alliance, the industry is working together to advance renewable energy production and develop technologies to produce paper products more efficiently.

The 2006 Forest Products Industry Technology Roadmap, issued by Agenda 2020, focused on research and development priorities related to forest bio-refineries, sustainable forest productivity, breakthrough manufacturing processes, wood products, fiber recovery and environmental performance of the industry.

A new roadmap will be published later this year.

“Because of significant changes since 2006 and challenges facing the industry,” stated an Agenda 2020 press release, “(we) initiated a process in 2008, in partnership with the Institute of Paper Science and Technology at Georgia Tech, for the development of an updated technology roadmap. A Strategic Issues Workshop in December 2008 identified priority issues for which new transformational technologies are needed.”

Last April, a Technology Roadmap Workshop identified priority technology objectives and R&D needs in the priority areas, including the need to reduce carbon emissions and energy consumption in manufacturing by:

•Renewable sources for non-steam thermal demand – use biomass to replace fossil energy.

•Reducing energy intensity of fiber preparation.

•Reducing energy required to wash pulp and prepare black liquor for firing into the recovery boiler by 50 percent.

Another goal is to significantly lower fresh water intake in mills and plants by the:

•Separation of reusable water from dilute contaminants (both inorganic and organic).

•Removal of non process elements (NPE) from the chips prior to pulping.

•Improvement of process modeling tools (engineering).

Towards developing new product features, the plan is to:

•Achieve a 20-50 percent improvement in performance/weight ratio.

•Create functional interfaces between inorganic materials and value-added cellulosic materials.

•Understand and exploit self assembly and non-covalent interactions of wood-based materials.

•Develop bio-based coatings and fiber treatments that can replace non-renewable polymer films used in current and future packaging designs (flexible & rigid).

Improving recovery and recycling of waste wood and paper products is also important and this can be achieved by:

•Enabling recycled fibers to have equivalent runnability to virgin fibers.

•Developing filler separation techniques from recycling mill wastes.

The Agenda 2020 technology goals are clear, including advancing the forest bio-refinery. This is being accomplished by:

•The creation of the $2.9 million Value Prior to Pulping (VPP) program, a multi-year research program investigating the extraction of hemicelluloses from wood chips before pulping and converting them to ethanol. VPP is supported by DOE and eight member companies, and involves six universities in a collaborative project.

•An analysis of the benefits of black liquor gasification completed by Princeton University in 2006.

•Seeking opportunities for collaborative programs that target biofuels, bio-refineries integrated with pulp mills and getting value from woody biomass in new ways.

Maximizing recycling at the paper making stage is critical when the PPI asks the public and businesses to place paper products into recycling bins.

“Last year the paper recycling rate hit an all-time high – 57.4 percent,” said Carroll. “We hope to reach our goal of recovering 60 percent of all paper produced in the United States by 2012, which was set this year. Our companies benefit from paper recycling and when we ask people to do their part, we have to show that we are on the same page.”