CWC Fact Sheet

Wheat Straw As A Paper Fiber Source

Introduction

This project characterizes Eastern Washington cereal straw, and it evaluates cereal straw pulps and pulp blends with recycled old corrugated containers (OCC) as a furnish for printing and writing grade papers. Successful production of the short fiber straw pulp and its deployment with long fiber pulp would mean that printing and writing papers can be produced totally independent of wood forests. Simultaneously, this production can minimize the impact of fluctuations in OCC supply and cost. Further, using straw for fiber could increase the value of an under-used agricultural residue and minimize crop disease and air pollution problems associated with the common practice of burning excess straw in the fields.

Background

Nonwood fibers have a long history as a raw material for papermaking. Using this raw material declined in Europe and North America during the first half of this century as the amount of inexpensive and readily available wood fiber increased. Currently, China produces about one-half of the world's nonwood pulp while Europe and North America are relatively small players. These two regions consume about 60% of the world pulp and paper production. Only four modern straw- or grass-fiber production sites exist in Europe and none in the United States. However, in some situations, nonwood plants might provide a viable fiber source in these industrialized regions.

Environmental and population growth pressures are contributing to long-range changes in forest land management practices, which reduce harvest of wood for wood products and for pulp and paper manufacture. At the same time, U.S. crop production of cereal grain in the generates tremendous quantities of straw. For example, three million acres of wheat are grown in Washington State each year, which produce about 3 tons of straw per acre. At least a half ton of straw per acre is required to be maintained on the soil surface for erosion control of steeply sloped land. Yet, excess straw often presents problems for subsequent field operations such as no-till seeding. Thus, straw might represent a significant fiber substitution opportunity. Pulp from cereal grain straw may partially substitute for wood fiber in a range of paper and paperboard products.

The utilization of cereal straw as a fiber source in North America has several potential limitations. The small fiber dimensions typical of cereal straw limits both the paper products strength and paper machine operating speeds. The high inorganic content of straw creates potential problems in conventional chemical recovery systems. Blends of straw and recycled paper or wood pulps can provide useful paper properties.

Wheat Straw Characterization

This project examined the chemical and morphological variations within wheat straw for six commercial cultivars commonly grown in Washington State. Madsen, Eltan, Stephens, Lewjain, Cashup, and Rod cultivars were hand harvested from an experimental, irrigated plot in Moses Lake, WA. Four within-field replicates of Madsen were also collected and analyzed.

As expected, the average fiber length of the Moses Lake (irrigated) straw, had weighted average fiber lengths around 0.1 mm longer than straw grown in dry-land conditions. With the Moses Lake samples, the variation within the field was greater than the variation between cultivars; therefore, no variation could be distinguished between cultivars. However, great differences in fiber length distribution were seen within the plant. The leaf and node sections contained more fines and less long fibers than the internodal sections. Pulping of only the internodal sections should reduce the fines content and improve drainage of the pulp.

The leaves, nodes and internodes (stems) of each plant were hand sorted and the chemical compositions were determined. The leaf fraction contained more silica than the internodes and nodes, thus showing a benefit of leaf removal before pulping. Variation was seen between cultivars, with Eltan leaves containing less silica than other leaves. The internodal and nodal sections of Cashup straw contained more silica than the other cultivars. The variations suggest an opportunity to upgrade raw material through selective harvesting, and possible avenues for genetically altering the wheat.

Pulping & Bleaching

Madsen wheat straw was pulped by vapor phase and liquid phase conditions after presteaming of dry, chopped straw. Vapor phase pulping used short impregnation and cooking times with direct steam heating. At optimum conditions, plant stem nodes comprise the major part of the 5% rejects stream, offering a chance to purge nodes and associated fines and silica from the system. Liquid phase pulping used longer times and higher water and chemical charges. The rejects were less than 0.5% as a result of improved impregnation. Total yields of liquid phase pulps were about 1% less than vapor phase at the same kappa.

Bleaching conditions for the vapor phase pulps resulted in 80�2 brightness units. The soda/anthraquinone (AQ) and soda oxygen pulps bleached with about the same effort and similar properties. Since low reject, liquid phase pulps were of most interest, these pulps were bleached to 86+ brightness with an overall bleached yield of about 40%. Unbleached and bleached viscosities were 32 and 20 cP, respectively, and physical properties of pure straw pulps were similar to other reported literature values.

Handsheet Testing

Blends of straw and Crystal (OCC) pulp increase in density with higher proportions of wheat straw. The fiber size distributions of these two pulps are similar, but the latter contains somewhat larger amounts of the longer fiber fraction. As a result, the furnishes that have larger proportions of Crystal pulp also have substantially higher tear strength. Tensile values change only slightly with furnish composition.

Black Liquor Properties

Black liquor viscosities are substantially different from in wood based kraft liquors. In the range of 20 to 40% solids and up to 70�C, straw liquor capillary viscosities exceed wood-based liquors by a factor of 2 to 3. Few sludge deposits were formed in that range of solids content. The black liquor heating values were about 6300 Btu/lb, falling within the range expected for kraft liquors. Most of the metals tested are within expected ranges except for potassium, which is present in straw in high concentrations. Although black liquor silica concentrations (170 ppm) fall well below many literature reports, the steady-state level in an actual mill recovery circuit will probably be considerably higher.

Conclusions

This work demonstrates that Washington State wheat straw could be successfully pulped by soda/AQ chemistry and bleached by the DEoD sequence to fully bleached levels at about 40% yield based on oven dry straw. Paper physical properties in Crystal pulp or wheat straw blends fit the requirements for producing fine printing and writing grade papers.

Report Dated: April 1997
Fact Sheet Update: April 1997