**2. Fiber anatomic structure, chemical composition and pulping properties of wheat straw**

Wheat is a kind of annual vascular-bundled herbal arthrophyte (**Figures 1** and **2**) [5], containing spike head, straw stalk (nodes and internodes), and root parts. Countering its mass percentage of each part, there are proximate ranges of internodes (68% w/w), leaves-sheaths (20% w/w), leaves-blades (6% w/w), nodes and fines (4% w/w), and grain and debris (2% w/w). Practically, only internode parts are acceptable for pulping [6]. The physical and optical properties of the stubble pulp were better than those of whole wheat straw pulp [7]. Therefore, at a commercial pulping process line, about 25–30% w/w loss of biomass, unsuitable parts for pulping, should be separated during wheat straw chip preparation (**Figure 3**), for purposes of chemical saving and pulp quality improvement as well as silica content decrements of black liquor from cooking [6, 8].

Papermaking is the process to form a fiber mat by a mesh net from fiber suspension stock through draining water. Fibers are the key element of pulp furnishes for papermaking. With contrast to wood fibers, wheat straw fibers are characterized as shorter in average fiber length and narrow in width, and there are more non-fiber cell contents. After proper pulping, cleaning, and screening, pulps from wheat straw can be used for manufacturing various paper product grades, such as linerboard, corrugated medium, writing and printing papers, etc.

**Figure 1.** Baled wheat straw.

**1. Preface**

224 Global Wheat Production

chemical softening, etc.

**properties of wheat straw**

Wheat cultivation area is the world's largest agricultural lands, wheat being the most prolific and most widely distributed food crops. At present, the global sum of wheat plant area is of more than 2.2 million ha, with the annual wheat grain output of 730 million tons, accounting for onethird of the world's total food production [1]. Wheat straw is a good fiber material, produced annually in huge quantities worldwide in a much shorter growing cycle than wood. The utilization of no-woody fiber materials to produce cellulosic pulps is the most economically justified solution fitting with the EU's environmental directives, which aim to reduce the consumption of wood fiber in paper and board products and replace it with other plant biomasses [2]. From the production point of view, the world's largest wheat production country is China, followed by India, the United States, and Russia, and these four countries produce 45% of the total world wheat production. Therefore, there are more than 650 million tons of wheat straws available annually [3, 4]. How to utilize these lignocellulosic biomass resources efficiently has been becoming a very urgent issue worldwide [3]. Practically, some countries start to produce electricity by using biomass fuel boilers, and some parts of the world feed straws back to agricultural land by cutting, but most of those resources have not been properly utilized. There are some countries making paper and paperboard products from wheat straw, such as China, Spain, and so on.

The term "pulping," in technical processes or methods for production of fibers from cellulosic raw materials, might be classified mainly as chemical pulping, chemi-mechanical pulping, and mechanical pulping processes. The principle of pulping is to separate cellular fibers from plant tissues by chemical cooking to remove lignin or mechanical separation combined with

Wheat is a kind of annual vascular-bundled herbal arthrophyte (**Figures 1** and **2**) [5], containing spike head, straw stalk (nodes and internodes), and root parts. Countering its mass percentage of each part, there are proximate ranges of internodes (68% w/w), leaves-sheaths (20% w/w), leaves-blades (6% w/w), nodes and fines (4% w/w), and grain and debris (2% w/w). Practically, only internode parts are acceptable for pulping [6]. The physical and optical properties of the stubble pulp were better than those of whole wheat straw pulp [7]. Therefore, at a commercial pulping process line, about 25–30% w/w loss of biomass, unsuitable parts for pulping, should be separated during wheat straw chip preparation (**Figure 3**), for purposes of chemical saving and pulp quality improvement as well as

Papermaking is the process to form a fiber mat by a mesh net from fiber suspension stock through draining water. Fibers are the key element of pulp furnishes for papermaking. With contrast to wood fibers, wheat straw fibers are characterized as shorter in average fiber length and narrow in width, and there are more non-fiber cell contents. After proper pulping, cleaning, and screening, pulps from wheat straw can be used for manufacturing various paper product grades, such as linerboard, corrugated medium, writing and printing papers, etc.

**2. Fiber anatomic structure, chemical composition and pulping** 

silica content decrements of black liquor from cooking [6, 8].

**Figure 2.** Cross section of wheat straw [5]. (1) Outer epidermal cells, (2) fibrous tissue band, (3) vessel, (4) bundle sheath, (5) vascular bundle, (6) xylem, (7) phloem, (8) parenchyma cell, (9) internal epidermal membrane.

**Figure 3.** Two types of pretreatment processes for wheat straw chip preparation [6, 8].

In some case, it also can be used in cigarette paper furnish with less than 30% proportion to adjust paper's air permeability. Fiber morphology data of wheat straw are listed in **Table 1**.

**3. Chemical pulping and pulp bleaching**

from hydrolysis [12, 13].

ing operations.

Soda and soda-anthraquinone (AQ) cooking, as sulfur-free processes, are more suitable in producing pulp fibers from some no-woody raw materials [12]. Most common operated chemical pulping processes are soda or soda-AQ cooking, using caustic soda or caustic soda-AQ solution as cooking liquor to dissolve and remove lignin from wheat straw. It was proven that AQ can improve selectivity of delignification by caustic soda to prevent carbohydrates

Since chemical pulping process was employed for wheat straw pulping, some disadvantages occurred during commercial practices, i.e., higher steam consumption, shortage of heat recovery, changeable in pulp qualities, lower pulp yield, poor pulp drainability, high viscosity of black liquor, and so on, which caused some challenges as high production cost, limited application of pulps, difficulties on chemical recovery, etc. Some advanced techniques have been improved and invented in the past decades [14], summarized as follows: (1) the wheat straw chip preparation processes combined with dry method and wet method were developed to remove nonfibrous components selectively and efficiently; (2) rapid cooking process at lower temperature combined with mechanical fiber dissociation to enhance uniform cooking effects and improve pulp qualities; and (3) elemental chlorine-free and total chlorine-free bleaching processes invented to eliminate generation of AOX and other bio-toxicities from pulp bleach-

The first installation of continuous cooking system with an annual capacity of 100,000 metric tons of wheat straw pulps was operated at Quanlin Paper Group, Shandong, China. This successful practice scales up an individual pulp production line capacity from non-wood materials in the world and improves significantly technical specifications of wheat straw pulping.

A new type of pulp washer series, i.e., serialized ZXV-type pulp washers, has been innovated by Wenrui Machinery Co. Ltd., Shandong, China. The new pulp washers, with the maxi-

designed and plane distributing valves being applied to improve water flow turnover and keep higher vacuum degree in sucking chambers, resulted in a good pulp cleanness and high extraction rate of black liquor; on the other hand, dispersing press, agitating device applied to improve washing efficiency [3]. The proposed acceptable washing-screening process is the sequence with press extraction—replacement washing—closure screening; it has been proven by many commercial operations that the new concept of the combined countercurrent washing sequence improves black liquor extraction rate significantly, resulting a remarkable water-saving effect. For instance, the black liquor extraction rate reaches up to 94.6%. The

The bleaching techniques had experienced a long-term development in China's wheat straw pulping practices. Early, the single-stage low consistency bleaching with hypochlorite was commonly applied in many non-wood pulping lines, and a three-stage bleaching sequence

, and with a conical chamber structure

Wheat Straw Pulping for Paper and Paperboard Production

http://dx.doi.org/10.5772/intechopen.77274

227

/t pulp or less in Xinya Paper Group, contrasting

/t pulp by traditional process [4].

The detailed technical specifications are shown in **Table 4** [8].

mum filtration area of each drum being up to 120 m2

water consumption can be reduced to 40m3

to normal water consumption of more than 100 m3

With contrast to wood materials, there are less lignin and more hemicellulose contents in wheat straw [9] and extremely high content of extractives by 1% caustic soda solution, but its biological organizations are bulky and loosened, which is helpful for pulping process. In fact, there were a series of pulping processes, successfully employed by wheat straw pulping practices, such as soda pulping (soda), kraft pulping (KP), and neutral or alkaline sulfite pulping (NS or AS) for production of brown pulps and bleached pulps. Chemical compositions of one kind of wheat straw from Hebei province, China, are illustrated in **Table 2** [10].

Zhan et al. [11] analyzed chemical components of differential parts from wheat straw, and results show significant differences on chemical compositions from each part, listed in **Table 3**. Internodes of wheat straw contain higher hemicellulose which is the key element of pulps.


**Table 1.** Fiber morphology datum of wheat straw and wood [10].


**Table 2.** Chemical composition of wheat straw (%, wt/wt) [10].


**Table 3.** Chemical compositions of wheat straw parts (% wt/wt) [30].
