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LISTENING STUDY Question 43:
What is the applicability of wood pulping mills to agricultural fibers?

LISTENING STUDY: Many respondents say that agricultural fibers cannot be pulped in mills built to pulp wood.

My understanding is that wood handling equipment in pulp mills is generally not compatible with the tooling needed to process ag fibers. - Jeff Lindenthal, President, Green Field Paper Company

In general, agricultural fibers are not suited to wood pulp mills. The agricultural materials are not available in the geographic vicinity of the wood pulp mills and the technologies for wood pulping are generally not suited for nonwood fibers. One exception is bamboo which can be used in wood pulping mills. - Living Tree Paper Company

Utilizing a tree based pulp mill to run ag fibers can be of some concern. Tree-based pulp mills are built to accept small wood chips or pellets instead of large agricultural bales. Some ag fibers would introduce the silica problem to a tree based mill. The chemicals used in most tree based pulp mills are too harsh for most agricultural fibers. When the mill is trying an agricultural fiber for the first time, sacrificing several batches of pulp while adjusting the chemistry is usually necessary. A dedicated nonwood facility provides optimum efficiencies. - Tom Rymsza, President, Vision Paper

Conventional chemical pulp mills are based on sodium chemistry. In the context of a low (zero) pollution pulp mill, the high potassium content of agri-fibers would overwhelm the sodium chemistry. There is also the specific "silica in agri-fiber" problem. In contrast, wood has very low content of potassium and silica.
      See for example, my presentations, "Toxicity, BOD and colour of effluents from the Kraft pulping of bole wood containing high quantities of bark," "Impact of Biomass Potassium on Operation of Effluent-Free Agri-Pulp Mills," "Potassium Pulping of Straw," and "New Direction in Industry Development and Environmental Protection for Nonwood Pulp Mills in Developing Countries." - Al Wong, Founder, Arbokem

The chemical processing (Kraft and sulfur based) is generally overkill for kenaf. - Tom Rymsza, President, Vision Paper

Converting existing mills would be a difficult transition. The tree pulping industry is enormous and is tailored for softwoods and hardwoods. It is generally very difficult to switch in other fibers other than bamboo. - Jeff Mendelsohn, President, New Leaf Paper

Most agricultural fibers will not process in the raw material handling and pulping stages of existing wood pulp mills. This is due to the high bulk volume of plant materials compared to wood chips. Therefore, new equipment has to be installed to receive the raw material, for temporary storage, and the digesters for pulping. Once pulped the material can be processed through existing bleach plants, although probably at reduced rates. - Michael Jackson, Consultant, Tolovana Park, OR

Many of the tree pulp mills may not be adaptable. Bleaching and fiber technologies differ. Converting a mill could render some of the bleaching and fiber processing machinery obsolete. It is possible to "rent" a mill, but changing the entire process without running large quantities is not economical. Internationally there are more kenaf developments; it might be interesting to see if converted mills or ground-up mills dominate overseas. - Russell Clark, Environmentally Preferable Purchasing Program, US EPA

The cellulose content of kenaf, hemp and wheat straw is similar to that in trees. Annual fiber crops generally have lower lignin content and higher hemicellulose content than wood. Cereal straws have relatively high silica and potassium contents. These differences in chemical composition determine the best pulping processes and pose challenges to pulping non-wood fibers.
      The lignin content of kenaf (15-19 %) and of wheat straw (16-21 %) is significantly lower than that of softwoods (26-34%) and hardwoods (23-30%). The lignin content of kenaf and hemp bast fibers also is lower than that of the core fibers. For instance, for kenaf, the lignin content of the core is 17% whereas that of the bast fibers is only 8%. A lower lignin content generally reduces the amount of energy required to pulp the fiber using either mechanical or chemical processes.
      In addition to the amount of lignin, its chemical nature also matters. The chemical structure of the lignin in non-wood plants differs from that of the lignin found in wood; these structural differences affect the ability of pulping chemicals to break down the lignin. Hemp, for example, has a low lignin content (4.3% for bast fiber and 20.8% for core fiber), however, its lignin structure makes it difficult to pulp and bleach the bast fiber using a chemical pulping process.
      [This study] shows the high silica content of wheat straw and other cereal straws. High silica levels pose two problems: silica creates problems in the chemical recovery system, and it affects the paper quality. If the silica ends up in the paper, it makes the sheet more abrasive and thus creates problems on converting equipment. If the silica is removed from the straw fibers during the pulping process, it appears in the black liquor in the form of sodium silicate and/or other complex siliceous compounds. A high silica content may lead to scaling in the evaporator and recovery boiler tubes. - Environmental Defense Fund Paper Task Force, White Paper 13, "Non-Wood Fiber Sources"

Wheat straw pulping processes have to control the viscosity and the levels of silica and potassium in the black liquor sent to the recovery system. Silica can precipitate out of the black liquor and result in scaling of pipes in the evaporators and recovery boiler. High potassium levels in the black liquor affect the properties of the molten metal mixture in the base of the recovery boiler and can lead to the plugging of pipes in the recovery boiler. Straw's high hemicellulose content increases the viscosity of the black liquor and makes it difficult to fire at high solids concentrations. Currently, no good solutions exist for these challenges to recovery. - Environmental Defense Fund Paper Task Force, White Paper 13, "Non-Wood Fiber Sources"

It is clear that for existing mills, which require upwards of one million tons of raw material per year, agricultural based raw materials are problematic. It is not surprising that they do not support alternatives. But if we care about environmental impacts, and global warming and deforestation, we have got to take a longer term view. - Tom Rymsza, President, Vision Paper


LISTENING STUDY: Some claim that there are larger reasons for the incompatibility.

As Andrew Kaldor writes: ". . . a commonly held view today among the pulp industry experts of developed countries is that the production of nonwood fibers is not viable or competitive in their economic environment. The same industries, on the other hand, are prepared to accept a heavy long-term reliance on wood fibers due to a perceived lack of alternatives." - Maureen Smith, The U.S. Paper Industry and Sustainable Production

Conversion is more complex than just the pulp; the existing infrastructure is a part of the larger wood products industry (rayon, etc.). A pulping mill conversion would affect the far-reaching markets of wood products. - Peter A. Nelson, President, AgroTech Communications, Inc.


LISTENING STUDY: Some report successful nonwood pulping in wood pulping mills.

Certain agricultural fibers, such as kenaf, have been successfully pulped with modified tree pulping processes. - International Paper

Most agricultural fibers will not process in the raw material handling and pulping stages of existing wood pulp mills. . . . The Arundo donax reed that I am currently working with is an exception. It can be cut into chips that have the same bulk density as wood chips and therefore can be processed in existing pulp mills without equipment changes as demonstrated during the recent trial at the Samoa, CA, pulp mill. - Michael Jackson, Consultant, Tolovana Park, OR

When I view the use of agricultural fibers in the pulp/paper industry I am looking at a world that already uses nonwood fibers in various applications, such as Buckeye Technologies' (www.bkitech) use of cotton linters; flax fiber in cigarette papers, etc. We will not see a time when the same wood infrastructure supporting various commodity papers just automatically adapts to nonwoods. We will see a time when consumers demand more specialized products that are manufactured from a diverse amount of fibers, in new factories outside of the traditional "typing paper" and newsprint. - Peter A. Nelson, President, AgroTech Communications, Inc.

The differences between wood and agricultural pulp mills are not much different, but the so-called "retooling" of wood pulp mills may be required to pulp nonwood fibers. Any wood pulp mills can be converted to agricultural pulp mills. Wood pulp mills are usually batch process and there are some continuous process mills handling only nonwood fibers. - James S. Han, Research Chemist, USDA Forest Service Forest Products Laboratory

Most agricultural fibers will not process in the raw material handling and pulping stages of existing wood pulp mills. . . . The Arundo donax reed that I am currently working with is an exception. It can be cut into chips that have the same bulk density as wood chips and therefore can be processed in existing pulp mills without equipment changes as demonstrated during the recent trial at the Samoa, CA, pulp mill. - Michael Jackson, Consultant, Tolovana Park, OR

If the mill uses our patented process, it can run on any regular Kraft mill or TCF process. It runs with 1/3 the chemicals or no chemicals in the TCF process. In both, the processing time is 1/3 that of tree processing. It cooks faster and brightens faster. The Arundo donax H cooking factor is 650-800 whereas for trees it is 1600-2000. There is no concern with silica as with ag residues and other alternative fibers. - Ernett Altherimer, Founder and Chairman, Nile Fiber

It is fairly feasible to convert all or part of the supply for a tree pulping mill to nonwoods. Some of the large mills are attempting this in areas where the nonwood can supplement their wood supply. There have been few or no obstacles to doing that. The conversion is somewhat akin to the introduction of recycled content. Adaptations have to be made depending on the fiber, but the process is not fundamentally different. - Richard Denison, Ph.D., Senior Scientist, Environmental Defense

Although non-wood fibers can be pulped with the same processes used for wood, the technology to do so has not been tested on a commercial scale in the United States, with the exception of the . . . kenaf pulping operation at P.H. Glatfelter's mill in Ecusta, North Carolina. . . . In addition, specific technical issues remain to be solved for certain non-wood fibers:

  • As already discussed, for agricultural residues, such as wheat and rice straw, the high silica and potassium content poses technical problems in the recovery of pulping liquors in chemical pulping operations. . . .
  • Generally, pulping equipment and processes have to be adjusted and/or specifically designed for the different fiber types.
    • Fiber handling and storage systems have to modified to handle bales of non-wood fiber rather than pulpwood and wood chips.
    • Continuous digesters seem to be more appropriate for non-woods than the batch digesters used to pulp wood chips in chemical processes. Non-woods such as wheat straw have a low density and are very bulky. Continuous digesters reduce cooking time and improve pulping productivity. However, for very long fibers, batch digesters are used because the long fibers get entangled in continuous digesters.
    • Paper machines with a longer wire are needed for pulps from short-fibered nonwoods that have low drainage rates, such as wheat straw.

- Environmental Defense Fund Paper Task Force, White Paper 13, "Non-Wood Fiber Sources"

We have produced our pulps and papers with existing mills, mostly in the US. While the existing processes can be used to produce good results, we know that designing a mill specifically for nonwood fibers would produce excellent technical properties at an economically advantaged price, and with significantly less environmental impact. - Tom Rymsza, President, Vision Paper


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