Nonwood fibers are normally associated with developing countries, but these raw materials could have a role to play within Europe

by Leena Paavilainen

European prospects for using nonwood fibers

The share of nonwood fibers used as a proportion of the total fiber used for papermaking around the world stands at 6.5% and is expected to increase to 7.5% by the year 2010. While this may appear relatively small at first glance, nonwood fibers play an important role in the global pulp and paper business. For example, nonwood fiber pulp plays an important part in balancing regional fiber deficits in China and India.

China currently produces 77% of the world's nonwood pulp, and plant-based fibers will continue to be an important raw material source for papermaking in the country long into the future. One of the main reasons for this is that China and other countries in Asia are among those leading the growth of papermaking fiber consumption. At the same time, China's wood or recycled fiber supply is not able to satisfy the demand for papermaking fiber. As a result, there is also an increase in the demand for nonwood raw materials, although it has to be said that the consumption of recovered paper is growing even faster than that of nonwood fiber.

There is a threat that without investments from outside the nonwood industry, the capacity growth envisaged cannot be achieved. This means that Indonesian market pulp would be used more and more in China, affecting global market pulp trade flows. European papermakers, for example, would not benefit from tougher competition tightening the hardwood pulp market. Taking this cue from Asia along with a number of other fiber supply scenarios has prompted renewed interest in developing a nonwood fiber resource for papermaking in Europe.

Besides the narrow issue of raw material availability, there are other driving forces behind the growing interest in nonwood pulping. In Europe, the overproduction of food and the large subsidies delivered to the agriculture sector have resulted in a need to find alternative uses for grain fields. This will become even more critical with the expansion of the European Union beyond the year 2000.

Nonwood fibers provided the bulk of raw material supplies for papermakers over many centuries, and they still play an important role

Environmentalists claim that nonwood papermaking is more sustainable and environmentally friendly than wood-based paper production. In addition to most papermakers, many consumers will already be aware that they campaign vociferously against the use of primary wood fibers in papermaking.

Without taking any stand on the pros and cons of their arguments, it is an undeniable fact that both Europe and America share a large, untapped papermaking fiber resource - namely, straw.

It should also be kept in mind that fibers harvested annually have a high biomass production potential. Some nonwood plants even give more pulp per hectare than wood. From a marketing perspective, the environmental perceptions related with papers containing nonwood pulp are very positive. However, this image could quickly be dimmed by the environmental problems of many small nonwood mills.

Class fiber

Nonwood fibers can be divided according to their origin into agricultural by-products, industrial crops and naturally growing plants. Agricultural by-products are characterized by a low raw material price and moderate quality. High quality pulp can be produced from industrial crops, but the raw material is expensive. The raw material costs of natural plants, though, are competitive with wood. Not only that, but the pulp quality can be excellent. One of the main problems for the moment is limited availability which restricts widespread use of such raw materials in papermaking.

An even more important classification involves dividing nonwood raw materials according to the fiber length. In broad terms, this means long fibers can be considered as a softwood-type material, and short fibers can be used instead of hardwood sources in different end products. Some plants, like hemp, kenaf and jute, contain both types of fiber. By separating the bast and core fibers, it is possible to produce a high quality long fiber and a lower quality short fiber from these plants.

Nonwood pulp and paper production is based on integrated mill operations. The production and consumption of nonwood pulp was 16.8 million tons in 1995-1996, while the market pulp output was an estimated 0.5 million tons. It is a prerequisite for utilization forecasts in this area that nonwood pulping technology be further developed to an adequate level of sophistication and to an environmental standard equivalent to modern wood pulping technology.

Looking ahead

Since the growth of paper and board consumption is greatest in developing countries, it should be no surprise to learn that the demand for nonwood fibers will experience a still higher growth in the future. China will remain the largest producer in years to come, although its share of worldwide nonwood pulp production is expected to decrease from the current 76% level to 72% in 2010.

It is forecast that the production of nonwood pulp (based on straw and new industrial crops) will double by 2010 in Eastern European countries from the current level of less than 400,000 tons/yr. This is one way in which the countries can cope with increasing competition and at the same time take full advantage of agricultural residues. In western Europe, current levels of nonwood pulp production are expected to increase by 300% by 2010. It is also predicted that small-scale nonwood pulp production, circa 30,000 tons/yr, will be underway in the Nordic countries by 2010.

End-uses

In China, agricultural by-products, industrial crops and naturally growing plants are used in nearly all paper and board grades. Elsewhere in the world where other raw materials are dominant, nonwood fibers are used in a more limited range of products, often to achieve a particular quality niche or an availability edge.

The most common grades for inclusion are woodfree papers, fluting, cartonboard and numerous specialties. The bulk of the end-uses focus on uncoated woodfree papers, liner and fluting. In Europe, the main end-uses are in printing/writing papers and corrugating materials. But specialty products are also produced, especially in the eastern part of Europe.

Globally, nonwood pulp production is based primarily on straw (46%), bagasse (14%) and bamboo (6%). Agricultural by-products account for 73% of the world's nonwood pulp capacity, while natural plants such as reed and bamboo account for 18% and the remainder consists mainly of industrial crops.

Among agricultural by-products, most of the straw pulp production capacity is located in China. Bagasse pulp production capacity is distributed between China, India, Colombia, Mexico and Argentina. Industrial crops - including kenaf, hemp, flax and jute - account for a smaller portion of the production capacity, at just 4.4 million tons and are based for the most part in China. The major part of the production capacity based on naturally growing plants, particularly bamboo and reeds, is also located in China. There are, however, some major mills based on bamboo in India.

In the future, the main nonwood fiber sources will be agricultural by-products such as straw and bagasse, or naturally growing plants like reed. Reed will continue to be an important raw material for printing/writing papers in China. No significant increase can be foreseen in the production of bamboo pulp because of the various other uses open to people using the fiber.

Specialty pulps, produced from cotton linters, esparto, abaca, flax, hemp and sisal, account for the main part of nonwood market pulp. In Europe, pulp capacity is based mainly on straw. Industrial crops are the second most important raw material source in the European nonwood pulp industry, especially in the form of cotton linters. Political, economic and environmental issues favor growing special plants. More advanced utilization techniques and logistics of reed canary grass, miscanthus, sorghum, kenaf and hemp are also being developed in Europe.

Major producers

The number of nonwood mills is high and the industry is characterized as mainly local or regional. Most of the mills are small, with a capacity less than 20,000 air dry tons/yr, and as such they are uncompetitive on the global market. There are only 11 mills in the world producing 100,000 air dry tons/yr or more of nonwood pulp. Integrating a nonwood fiber line into a mill cooking wood brings a scale benefit, such as has been witnessed at the Phoenix mill in Thailand. In fact, most nonwood pulp production is integrated into wood-based pulp and paper mills.

The capacity of most mills in Europe is less than 5,000 tons/yr, but there are also a few larger-scale operations in Europe from a global perspective. The largest European mill is owned by Saica and located in Zaragoza, Spain. The mill produces mainly testliner and fluting and the annual straw pulp capacity is 120,000 tons. In neighboring France, most nonwood mills produce uncoated woodfree or cigarette papers. France's largest nonwood mill, Papeteries de Mauduit, has a production capacity of 16,000 tons/yr. Mauduit produces primarily cigarette papers, but it also produces lightweight uncoated woodfree papers from rags and hemp. Across from France, Germany has a 40,000 ton/yr mill in Glückstadt, Schleswig-Holstein. The mill is owned by Buckeye Cellulose and produces bleached linter pulp.

In eastern Europe, nonwood pulp production is normally based on straw, while in Russia rags are used. The largest eastern European mill is located at Dunaujvaros in Hungary. It produces uncoated woodfree papers from 30,000 tons/yr of straw.

Fiber properties

The papermaking properties of a certain fiber are determined by the morphological, chemical and fine structural properties of the plant. Cooking and bleaching have to be carried out so that fiber strength is preserved (the formation of weak points in fiber wall is avoided) and the hemicellulose content is kept on an optimal level for good bonding and high stiffness.

The chemical and morphological properties vary between different parts of the plant (stem, leaf, sheath) within the stem and between species and varieties. Different growing conditions help account for this, but the variations are also dependent on the development stage of the plant at the time of the harvest.

Nonwood plants are a very heterogeneous group. The variation of fiber characteristics between the crops is considerable. In comparison to wood pulps, nonwood pulps produced from specific plants are more heterogeneous. They contain many types of fibers and a high fines content is typical for certain nonwood pulps, particularly grass pulps. Nonwood pulps can be divided into short and long-fiber pulps. In papermaking terms, short nonwood fibers behave like hardwood fibers, while long nonwood fibers can be used to replace softwood fibers in the paper furnish.

Nonwood crops such as flax, hemp, kenaf and jute contain both long bast fibers and short core fibers. Bast fibers - abaca, sisal and sabai fibers - are longer than or as long as softwood fibers in general. They are more slender than softwood fibers and have a lower coarseness value. This, together with the fact that long nonwood fibers are also strong, gives them excellent reinforcement ability.

Reed, straw, bagasse and some bamboo and esparto fibers are short. Their length as well as cell wall thickness (coarseness) is close to that of hardwood fibers. It should be remembered that short nonwood raw materials contain so-called non-fibrous cells (fines). A high fines content together with a high hemicellulose content causes difficulties in water removal from pulp.

Chemical considerations

A characteristic of nonwood raw materials is their low lignin content, which makes the fibers easy to pulp. Long and short-fiber nonwood crops have major differences in chemical composition, which affects both the processibility of the raw material and the quality of the pulp.

Long nonwood fibers such as flax and hemp have a low lignin content or a part of the fibers is free of lignin. The cellulose content is high and the hemicellulose content low compared to other nonwood raw materials. Cotton fibers are almost pure cellulose. The silica and ash content is low. Due to the low lignin content and a favorable plant structure, many nonwood fibers require a less drastic treatment than wood chips. As a result, semichemical and chemimechanical nonwood pulps have been produced with good results.

Agricultural by-products have a high ash and silica content. The silica content varies between species and in different parts of the plant. It is especially high in leaves and affected by the soil type and irrigation water used. Natural plants have a somewhat lower silica content. The high silica and fines content must be taken into consideration when designing nonwood pulp and paper mills. Agricultural by-products and natural plants in general have a lower lignin content than wood, but their hemicellulose content is higher. An exception is bamboo, which cannot be cooked without sulfur due to its higher lignin content. Some naturally growing plants can even have a higher cellulose content than hardwood.

Nonwood potential

Nonwood pulps can be divided into long fiber specialty pulps and short fiber pulps used in bulk products. Nonwood pulps are often characterized by inferior quality - especially short fiber products. However, if proper techniques and equipment are used, the properties and the quality of nonwood pulps are such that they can be used instead of wood pulp in the furnish.

The role of long fibers is to give the paper the required strength. Bast fibers have interesting quality characteristics and are in some respects superior to all types of wood fibers. Long, strong, nonwood fibers produce pulp with high tear strength. Low coarseness, on the other hand, guarantees good surface properties. Due to the low hemicellulose content, the energy needed to reach a certain tensile strength (bonding) level is higher than with softwood pulp. Some long nonwood fibers, including abaca, cotton linters, flax, hemp and sisal, have superior properties in comparison with the best softwood market pulps. For instance, these fibers exhibit excellent reinforcement ability (calculated from fiber length, coarseness and strength) in highly demanding, low basis weight products.

If nonwood fiber production is to take off in Europe, straw looks set to form the bulk of the material used in the pulp and paper industry

In addition to providing excellent strength and performance, bast fibers also give paper certain special properties. For example, abaca fibers are very strong and recognized for their high heat resistance. Sisal pulp boasts high absorbency and cotton has a high chemical purity.

The role of short fiber pulp is to deliver good surface and printability properties to paper, as well as adding stiffness to board. The adequate strength for runnability is adjusted by adding long fiber pulp. Traditionally, short nonwood fibers have been used for board. However, short and slender nonwood fibers can potentially produce printing/writing grade pulp. A good example is esparto pulp, which has excellent printing properties.

Straw poll

Straw is a short fiber resource and has a good bonding ability, attributable to the high hemicellulose and fines content. On the other hand, dewatering properties are poor. The short fibers lead to a lower tear strength than that of birch pulp. The high number of particles per unit weight guarantees a reasonable light scattering ability.

Quality-competitiveness among short fiber nonwood pulps is characterized by the number of fibers per unit weight. Short fiber nonwood pulps - reed canary grass, bagasse and wheat straw pulp - are comparable with good quality, hardwood market pulp. Bamboo pulp, on the other hand, is at the same level as tropical hardwood or southern mixed hardwood pulps in terms of any quality comparison.

The high fines content together with a high hemicellulose content in pulp might limit runnability on high speed machines if straw is used without pretreatment, the portion of wood pulp is below 30% or double-felted pre-drying is used. The fines might also cause quality (linting) problems in printing. Consequently, proper raw material pretreatment should be taken into account in process design.

In the end-use

Most paper and board grades can be produced either by blending different nonwood fibers or by replacing a wood pulp component in the furnish with a specific nonwood pulp. Long fiber raw materials (eg kenaf and sabai) produce pulp with strength properties comparable with typical softwood reinforcement pulp fibers. Some fibers, like abaca, hemp, flax and cotton, have properties that are superior to wood fibers. These fibers are long, slender (low coarseness) and extremely strong, giving both good surface and strength properties to the end product. Such fibers are used in specialty papers, such as security, industrial and thin papers. They would also be excellent reinforcement fibers in terms of quality for low grammage coated mechanical printing papers, if the raw material price were competitive with wood.

Traditionally, short nonwood fibers such as straw have been used in the middle layer of board as the addition of straw pulp to corrugating medium improves stiffness. However, straw, bagasse, bamboo and reeds produce pulp with good printing properties and adequate strength. These pulps provide a realistic alternative to hardwood pulps in woodfree printing/writing papers, cartonboard and, in some cases, in tissue and newsprint.

Preparation of bamboo on site

So-called "ecopapers" are an interesting feature. Cartiera Favini in Italy is using alga and maize flours as organic fillers to improve the biodegradability of their printing/writing and packing ecopapers. The mill's tree-free grade is made from agro-residues without adding any wood pulp.

The overall quality of straw-based printing/writing paper leaves a lot to be desired. On the other hand, there are mills producing high quality paper with consistent quality. Thus, the quality of straw raw material is not the only reason for quality variations in printing/writing paper. The development of a quality control system plays a key role in fulfilling the customer expectations.

In the bag

Bagasse is used in practically all paper grades. The quality of bagasse raw material originating from different sugar mills varies mainly because of variations in the pith content. This leads to difficulties in controlling the quality of pulp. Depithing is very important, because pith tends to stick on press rolls and drying cylinders.

Reed is a longer fiber, but otherwise resembles straw pulp. It should be noted, however, that reed pulp exhibits better dewatering characteristics than straw pulp, and also provides better light scattering properties.

Esparto fibers are the correct fibers to use for high quality, low grammage printing/ writing papers that require high bulk and good porosity (ink absorption), surface smoothness and dimensional stability. A small amount of softwood pulp is added to the furnish in most instances. High cost and limited availability restrict the use of esparto.

Some bamboo species may have quite long fibers with strength characteristics close to those of softwood fibers, especially tear strength. They can be used in a wide range of end products. The coarseness of bamboo fibers limits their use in coated grades. Runnability problems on the paper machine and linting and picking problems in the press room can be avoided by adding some long fiber pulp (about 30%) and using proper surface sizing. Short nonwood fibers are often slender. This results in pulp with a high number of fibers per unit weight, guaranteeing good printing properties for the paper.

Class action

As we have seen, nonwood fibers can be classified according to fiber length into short, hardwood-type and long, softwood-type fibers. Another way to classify nonwood raw materials is by origin; agricultural by-products, industrial crops and naturally growing plants. The first classification determines the papermaking potential of nonwood fibers, while the raw material costs and availability relate to the latter classification. By using proper techniques and equipment, the properties and quality of nonwood pulps could be such that they could be used to replace wood pulp in almost all paper and board grades. The role of proper raw material pre-treatment is especially important in meeting quality requirements.

Checking bamboo stalks

Fibers from industrial crops are excellent not only for specialty products, but they can also be used as reinforcement fibers in highly demanding (woodcontaining) printing papers. Unfortunately, high prices limit their wider use.

The special properties of certain naturally growing plants should be utilized in high quality specialty and niche products. Raw material availability is a limiting factor though. Agricultural by-products, especially straw, are already the main nonwood papermaking fiber source and this is likely to be the case in the future as well. Straw pulp can compete with hardwood pulp in terms of quality and straw is also available at reasonable cost in regions that suffer from a shortage of wood.

The most common nonwood plant is sugar cane waste (bagasse)

Apart from the problem of ensuring sufficient supplies of wood fiber with specific properties, there are many political, environmental and economic issues that could lead to better utilization of existing nonwood fiber sources, especially straw. But to make more efficient use of nonwood raw materials in papermaking in the future will require not only investment, but the adoption of new technology. Optimization of fiber production and processing will also be required, but before all else, the industry will have to find a new way of thinking about the use nonwood fibers.

Leena Paavilainen is vice president of research and development at Jaakko Pöyry in Finland