LEO LINTU is Officer for Raw Materials and Market Analysis in the FAO Forestry Department's Forest Industries Division.
Some types of agricultural residues were already used as raw material for paper before the introduction of wood, which at the moment dominates the raw material supply together with waste paper. The most important agricultural residues used in the paper industry are straw and bagasse, and for panel products bagasse and flax strives, but there are a number of other residues which are also used. Theoretically, any fibrous plant can tee pulped to provide cellulosic fibres for paper manufacture, but technical and, more often, economic constraints limit the number of species of usable plants.
Before any fibrous raw material - wood, straw, bamboo - can be used for paper manufacture, it has to be converted into pulp by disintegrating the fibres, which is also true in fibreboard production. Pulp and paper manufacturing are therefore very often integrated operations, especially when agricultural residues are used as raw material. Fibreboard production is an integrated operation. The technical suitability of a fibrous raw material for paper is determined by the ease with which it can be converted into pulp and by the properties of the resulting pulp which determine its suitability for the production of various types of paper. Therefore, in the following, the use and the suitability of agricultural residues are considered for the whole integrated operation of pulping and papermaking.
The quantification of the current use can only be done by making rough estimates. The FAO Annual Pulp and Paper Capacity Survey provides an estimate of pulp capacity based on all non-wood fibrous raw materials. Agricultural residues form only a part of it. In 1975, the total world capacity to produce all types of paper pulp was 136.1 million tons, of which the non-wood pulp capacity, to which the agricultural residues belong, was 9.3 million tons or nearly 7 percent. The relative share of the non-wood pulp capacity is expected to increase slightly by 1980, when some 2.6 million tons of new capacity is estimated to be available for pulping the non-wood fibrous materials. The data are not complete for estimating the share of all agricultural residues as part of the non-wood pulp capacity for the world as a whole.
The most notable area (for which no official estimates are available) in which straw and bagasse are used in the production of pulp and paper is China, where the total pulping capacity for these materials is thought to be as high as 4.7 million tons, or about half of the world total.
The overall importance of non-wood fibrous raw materials in the developing countries is about one third of the total paper pulp capacity. Bagasse pulping capacity, alone, represents about 40 percent of the total non-wood pulping capacity in these countries. Its importance is most pronounced in Latin America.
In the developed market economies, the quantity of non-wood fibre pulp capacity is about half a million tons less than in the developing market economies but the total non-wood fibre pulping capacity is only less than two percent of the total paper pulp capacity in these countries. Straw, especially in western Europe, dominates among the non-wood fibrous raw materials.
As the yields of pulp vary considerably depending on the pulping processes and the type of raw materials used, no more than estimates of the orders of magnitude can be made regarding the fibre input in the form of straw and bagasse. The quantities of moisture-free straw and bagasse which were used in the world excluding China in 1975 for paper and paperboard are estimated to have been something like 4 million tons and 3 million tons respectively. These estimates are based on the assumptions that about 3 tons of moisture-free straw and 3.5 tons of moisture-free bagasse were needed for the production of 1 ton of paper pulp and that the average rate of using pulp capacity was around 75 percent.
As regards the panel products the most widely used raw materials are flax strives producing around 600 000 tons/yr of particle board of which 70 percent in Belgium end bagasse producing some 100 000 tons/yr of particle board. The other materials which are used on a limited scale are hemp, maize stalks, cotton stalks, jute sticks and palm fibre.
It would appear that flax accounted for something like 85 percent (in the order of 1.45 million m³) and bagasse for around 12 percent (some 212 000 m³) of world non-wood-based particle board production capacity which it is estimated totalled nearly 1.7 million m³ in 1973. Europe, it will be noted, accounted for no less than about 1.42 million m³ (or 99 percent) of the flax-based capacity at that time.
It would also seem that, in 1973, of world estimated production capacity to manufacture particle board somewhere around 5 percent was based on non-wood fibrous raw materials (about 6 percent in the case of pulp and paper).
In some European countries the relative share of flax and hemp strives based panels of the total panel production has been diminishing due to insufficient supply of these raw materials rather than to consumer resistance. A great number of other plant residues have been considered and found capable of forming a satisfactory panel. Among these are kenaf, abaca, rice husks, wheta straw and groundnut shells.
Based on the data for the total production of cereals and cane sugar, it can be estimated that something like 1 500 million tons of moisture-free straw and close to 60 million tons of moisture-free bagasse became available in 1975.
A ROLL OF PAPERBOARD - not necessarily made from wood
This comparison of the orders of magnitude of the availability and the current use of agricultural residues for paper and panel products manufacture reveals large theoretical potentials. It should, however, be emphasized that the collectable quantities of bagasse and, especially, straw are considerably smaller, due mostly to the economic constraints related to the scattered availability of these materials. Furthermore, there are, of course, other uses such as energy production, soil improvement, cattle fodder, bedding, and so on, which reduce the availability considerably; but even if the other uses are taken into account, the remaining quantities could still theoretically support large paper and panel industries. Why the residues have not been used to their obvious theoretical potential in the paper and panel industries can be explained by the limiting technical and economic factors resulting from the specific nature of these industries, as well as of the agricultural residues available.
Some of the restrictions to the more extensive use of agricultural residues as raw material for paper and panel products result from the specific features of the industry. These industries and particularly the pulp and paper industry are capital intensive and economies of scale may play an important role. This already sets two requirements for the raw material supply: it must be relatively large; and it must be even all year round. Furthermore, for each type of raw material and final product the manufacturing process has to be "tailored" individually and the modifications due to the changes in raw material supply or market requirements are extremely costly. This sets a third requirement for the raw material supply: it should be guaranteed for at least the life time of the mill equipment. Finally, as the industry produces semi-manufactured products in bulk which must be of a homogeneous quality as the main guarantee for the secured markets at the further processing stages, the quality and especially the continued homogeneity of the quality of raw materials are of vital importance.
The raw material supply for each individual mill has to meet all of these four requirements within certain technical and particularly economic limits. It is the specific nature of the agricultural residues that makes it frequently difficult for them to meet all of these requirements. The group of agricultural residues is comprised of a number of different kinds of plant residues which are produced in differing climatic conditions, therefore their individual ability to meet the set requirements naturally varies greatly.
A COTTON GIN IN SUDAN - left over cotton fibres for high quality paper
It is typical of agricultural residues that their sources of supply are often widely dispersed which results in increased collection and transportation costs. This is true especially as regards the residues like straw becoming available after harvesting the main crop. Even if the residue is available in a more concentrated form such as bagasse and flax strives, after industrial processing the quantities becoming available from one place often need to be supplemented with supplies from other processing plants, which can mean considerably increased transportation costs. Another factor affecting the collection and transportation costs is the low density of the residues. Sometimes it also becomes necessary to transport unwanted material which cannot be separated economically except at the pulp or panel plant. Examples of this are the high moisture and pith content of some residues.
Agricultural residues result exclusively from annual plants, which have a harvesting period limited to a few months at most. As the mill operations cannot be interrupted due to the high fixed costs and employment problems, the mills using agricultural residues have to store their raw material. This affects not only the capital and operating costs but sometimes also the quality of the raw material.
Being residues from agricultural production, the main interest naturally lies elsewhere than in residue production. The demand for and the specifications set to the main crop determine the availability of the residues. For example, the development of new cereal varieties might result in a considerably reduced straw production. A change in agricultural policy also might cause a sudden reduction in the wanted residue supply as has been the case with flax strives. These would be economically disastrous to the mills due to the long-term nature of the investment; therefore the guaranteed long-term supply of standard quality raw material is one of the most vital points when planning this type of an industry.
The suitability of pulp for making various types of paper is determined by the characteristics of raw material and the pulping processes used. One of the main characteristics of the raw material in determining its suitability for various papers is the fibre length. Most of the agricultural residues contain short fibres comparable to those obtained from hardwoods, although notable exceptions exist. High short-fibre content restricts their use for paper grades in which the strength is less important, i.e. mainly for cultural papers and low-grade packaging paperboards. The pulping process used depends on the final use of the pulp and on the properties of the raw material. In general several quite different well-known processes are used for pulping agricultural residues, most of which have been adapted from the wood pulping processes. This also means that the technical problems related to pulping of agricultural residues do not as a rule set any limitations to extension of their use.
Specific features of a few of the most important agricultural residues
Short descriptions are given below of the current use, specific features and the technology applied when using bagasse, straw, cotton linters and flax tow as raw materials in the construction material and paper and paperboard industries.
(Much of this information is from Guide for planning pulp and paper enterprises, FAO, Rome, 1973.)
Bagasse is the fibrous residue remaining after the juice is pressed from the sugarcane in a sugar mill. Green bagasse contains about 50 percent moisture, 2-3 percent residual sugar and close to 50 percent fibre. Out of the dry volume of the bagasse fibre, pith constitutes some 35 percent. Although chemically similar to cellulose, pith does not have the fibrous structure required from raw materials for pulping and panel production and must, therefore, be removed. Only the remaining depithed bagasse is suitable for pulping or panel manufacture. It has a fibre length averaging 1.7 mm (maximum 2.6, minimum 0.8 mm), which is shorter than fibre from coniferous trees but somewhat longer than average broadleaved wood fibres. Its chemical composition resembles that of hardwoods especially as regards the lignin content.
Based on the quantity of world raw cane sugar production of some 50 million tons during the 1975-76 harvesting period and by assuming a dry bagasse quantity of 1.15 tons for each sugar ton, the estimated total dry bagasse quantity is 57.5 million tons.
The amount of bagasse available as raw material for paper and panel mills is, however, limited to a great extent since sugar mills utilize bagasse as fuel in their boilers.
Even without changing to alternative fuels, improved thermal efficiency in sugar mills can result in 20 percent of bagasse production being in excess of fuel requirements. This would mean a surplus of some 11 million tons of dry bagasse corresponding to over 3 million tons of paper pulp. Bagasse pulping capacity in 1975 was only around 1 million tons. The capacity to produce particle boards was estimated at 212 000 tons (1973).
One of the limiting factors to increasing the use of bagasse as raw material is, besides the difficulties of using substitute fuels at sugar mills, that sugar mills quite often are small and widely scattered. Bagasse is bulky and, therefore, its transportation for long distances is a significant economic disadvantage. The new sugar mills are, however, larger and their fuel requirements are better planned so that larger and more concentrated quantities of bagasse become available
STACKING STRAW AT A DEMONSTRATION FARM IN TUNISIA - three tons of straw = one ton of paper
The seasonal nature of the cane processing operation in most places means that extensive storage of bagasse has to be arranged to guarantee an uninterrupted supply of raw material to the paper or paper mill, the operation of which can neither be interrupted nor changed to use other raw materials.
Therefore, economic operations in handling, baling, transport and storage of bagasse are essential for a successful paper or panel mill.
A number of distinct processes are used for the pulping of bagasse, depending on the product desired. Chemical pulps made from bagasse are used for making almost all grades of paper and paperboard; however, due to the short fibre length of bagasse pulp, additional long-fibre chemical pulp is often required to improve the strength of the panel produced. Semichemical and chemi-mechanical processes are used in pulping bagasse for corrugating medium, which is the middle layer of corrugated board. Mechanical pulp has not yet been made commercially from bagasse, although this is an area of active investigation. The successful development of a commercially viable mechanical pulping process for bagasse could be a considerable breakthrough in the production of newsprint from bagasse which has not been economically possible so far.
Unlike bagasse, straw is not a uniform group of raw material but consists of a number of cereal straws, rice straw and corn stalks which all have somewhat differing pulping and papermaking properties. Rye straw and wheat straw are considered to be the best of the cereal straws while corn stalks and barley straw, due to a higher proportion of leafy and other extraneous materials, are considered less suitable. Rice straw, which is perhaps the most difficult of all straws to use, has, besides the extremely slow drainage characteristics common to all straws, also a very high silica content, which makes chemical recovery more difficult. Cereal straws and corn stalks have a fibre length averaging about 1.5 mm with a significant content of long fibres. In this respect they resemble bagasse. Chemically, they have a low lignin content cereal straws contain about 17-19 percent, and rice straw about 12 percent lignin, resulting in easy pulping. The cellulose content of European and North American cereal straws ranges from 36 to 42 percent, that of rice straw, 34 to 38 percent. All straws have a high hemi-cellulose content, which makes them particularly suitable for the production of grease-proof and glassine papers.
Straw has also been used in the production of a special type of panel by compressing the shredded straw which results in an extruded panel used in building construction.
On an average about three tons of moisture-free straw are estimated to be required per one ton of paper.
WHEAT - best for straw
The yields of straw per ton of grain vary according to the location and type of the plant. In calculating the amount of straw actually available to a paper mill, it is unlikely that more than 50 percent would be considered surplus to agricultural requirements, and drought, storms, disease and crop failures have to be taken into account. The following yields of straw Per ton of grain estimated in the United States give an indication of the orders of magnitude by plants:
2.3 tons/ton of grain
3.1 tons/ton of grain
1.6 tons/ton of grain
1.5 tons/ton of grain
1.5 tons/ton of grain
In one calculation, a southern European farming area with a radius of 80 km and an annual wheat production of 320 000 tons was considered to be a reliable supply of only 125 000 tons of straw per year for a pulp and paper mill. This is about 0.4 ton of straw per ton of grain.
The harvest season for cereal straws is brief, which means that nearly a year's supply of these bulky materials must be stored at the pulp and paper mill. There is always deterioration in storage although in mild, dry climatic conditions it is feasible to store straw in the open. In humid or tropical regions storage under cover is mandatory. In some areas harvesting of rice straw is carried out twice a year or three times every two years, in which cases storage of a six months' supply at least must be provided. The problems associated with the supply of corn stalks are in general similar to those of cereal straws.
The processes used to pulp straw are generally similar to those used for pulping bagasse. The high bulk of straw leads to relatively low yields per cubic metre of digester capacity. Straw pulps are slow-draining, and paper machine sheet-forming sections are unusually long compared with those forming wood fibre papers. Washers in the pulp mill must also be over-sized.
Straw pulps may be used in almost all grades of paper and paperboard, from the coarsest to the finest. Paperboards and corrugating medium are usually made of 100 percent straw pulp furnish, although some long-fibre supplementation may be required for the best grades. As with hardwood pulps, a small amount of straw pulp is beneficial to the printing quality of the paper. Some has been used in this way in newsprint manufacture.
Cotton linters are the short fibres remaining on the cotton seed after the staple (long) fibres have been removed by ginning. Machines cut the linters from the seed. While linters are too short to be used in textile manufacture, they serve many related purposes, including felting, batting, etc. There are several grades, such as first-cut, second-cut and mill-run. The first-cut fibres are longest and of the best quality. Linters fibre length is 2 to 7 mm.
Cotton linters are purified in nonintegrated pulp mills and sold for papermaking, dissolving pulp, or other uses. For some years it was the principal material for making the highest grades of dissolving pulp, for cellulose esters or for tire cord. The position of cotton as a fibrous raw material for dissolving pulp has deteriorated almost to the vanishing point. From 1945 to 1955, more than 100 000 tons of cotton linters were used annually in the United States for the production of dissolving pulp, but by 1965 the figure was less than 1 000 tons. These were for specialities such as transparent moulding grade cellulose esters and for X-ray film. It is, however, a source of cotton fibre for high-quality papers. Total United States production of purified linters is about 250 000 tons; but papermakers may buy raw fibres and purify them themselves. Also, the purified linters have many other uses, in the highest quality protective and writing papers, saturating papers and in filter papers.
For use in paper pulp or dissolving pulp, linters are purified by relatively simple, conventional soda processes. The cotton fibre is over 90 percent cellulose as received, so the purification processes involve removing waxes and foreign matter. There are three large linters purification plants in the United States, at Hopewell, Virginia, and at Memphis and Chattanooga in Tennessee. There is also a linters purification plant at Reynosa, Mexico. In these plants the baled linters are first cleaned and then subjected to caustic soda cooking under pressure in vertical or in rotating cyclindrical, horizontal digesters. The resulting pulp is washed, bleached and may be sheeted, dried, cut and baled for shipment, or the bulk pulp may be dried, compressed and baled. There is a significant difference in fibre length, since the sheeted pulp is mechanically refined (fibrillated). The average fibre length of bulk purified linters pulp is 2.2 to 3.0 mm, and that of sheeted pulp 1.1 to 2.7 mm. The alpha cellulose content is 98 percent or better. Sheeted pulp is also sold in rolls.
The proportion of cotton in the papermaking furnish may vary from 25 (or less) to 100 percent. Only about 15 percent of cotton content paper contains 100 percent cotton fibres and about 70 percent has 25 percent or less. Banknote papers are usually 75 percent linen fibres and 25 percent cotton fibres.
Flax is grown extensively for making linen and linen-seed oil. The residual material after removal of the long fibres is known as flax strives. The strives contain about 45 percent cellulose (pure linen is more than 80 percent cellulose) and 13 percent lignin. The yield of flax seed straw per area of cultivation is low. Furthermore, the yield of decorticated fibre and acceptable strives per ton of straw is low. The average pulping yield, varying between 25 and 60 percent depending on the cleanliness of the straw, is also considered to be on the low side. All these result in an extremely expensive pulp which can be used only in very high-priced end products. Such are cigarette paper, for which flax pulp is extensively used and other thin papers requiring high strength, such as banknote and airmail papers. It is estimated that something less than 100 000 tons of flax pulp are produced in the world. The use is expected to continue to increase although the very high price of the pulp will keep the quantities at a modest level. Flax strives on the other hand, as has been mentioned earlier, constitute an important raw material in the manufacture of particle board.
FLAX - expensive
To conclude, the use of agriculture: residues in the panel products and paper industries is restricted mainly by economic factors while technical problems can mostly be overcome. The numerous economic restrictions arise from the specific features of the industries and of the agricultural residues themselves and are related specifically to the economics of harvesting, transportation, storage, chemical consumption, fibre yields and quality of products.
Despite the numerous restrictions on the use of agricultural residues, the economic limitations have been overcome in many individual cases, as evidenced by the numerous mills offering products of satisfactory quality to the local markets. However, when determining the feasibility of the use of agricultural residues for such highly capital-intensive industry as paper and paperboard manufacture, it is essential to make a careful study to ascertain the continuous long-term economic availability of the raw materials. This, together with the correct selection of the products to be made, is the best guarantee of success of such a mill.
The use of bagasse for these industries, due to its more concentrated availability, will increase faster than that of other agricultural waste materials. The increase in the use of straw will be restricted by competitive uses and its scattered availability. The importance of other agricultural residues will remain restricted to products of high price and limited demand.
In the case of panel products, the consumption of non-wood fibrous raw materials has increased over the last ten years but at a slower rate than total panel production. It remains a very small component of the total raw materials used amounting to 2-3 percent of the total.