**3. Chemical pulping and pulp bleaching**

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**. 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

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.

> **of L/W**

**Wall thickness (um)**

**(%) Avg. Range Avg. Range**

72 3.8E)

**Lumen diameter (um)**

65 — — — 26.7-

33.1 (E) 16.6 (L)

**Internodes Nodes Leaves**

**2 W/D Non-fiber** 

1.5

0.23 (E) 1.05 (L) **cell content** 

**Sample Ash Extractives Pentosanes Lignin Nitrate-EtOH** 

10.65 5.36 23.15 — 44.56 25.56 22.34 40.40

**Benzene-EtOH**

Klason lignin (%) 19.66 19.98 14.18 Holocellulose (%) 70.27 67.97 60.95 Ash (%) 6.27 8.69 12.06

**water**

**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].

Wheat straw (Hebei)

**Species Fiber length** 

226 Global Wheat Production

**(mm)**

Birch (*Betula alba*) 1.21 1.01–1.47 18.7 14.7–

*Pinus massoniana* 3.61 2.33–5.06 50.0 36.3–

**Hot water**

**cellulose Cold** 

8.7(L)

**1%NaOH**

kind of wheat straw from Hebei province, China, are illustrated in **Table 2** [10].

**Width (um) Rate** 

22.0

65.7

Wheat straw 1.32 1.03–1.60 12.9 9.3–15.7 102 5.2 2.5 4.16 37.9

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 from hydrolysis [12, 13].

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 bleaching operations.

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

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 maximum filtration area of each drum being up to 120 m2 , and with a conical chamber structure 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 water consumption can be reduced to 40m3 /t pulp or less in Xinya Paper Group, contrasting to normal water consumption of more than 100 m3 /t pulp by traditional process [4].

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 of CEH (chlorination—alkali extraction—hypochlorite) was not applied until the 1980s of the twentieth century [14]. All of the above bleaching sequences contain elemental chlorine chemicals with generation of AOX and other organic toxicities in these bleaching effluents. Due to poor bleaching selectivity upon residue lignin, there were a large amount of carbohydrates degraded while bleaching, resulting a large water consumption and high chemical dosage, to produce bleached pulps with low brightness and weak physic strength as well as poor drainability of water. Following more and more strict regulations to effluent discharge, elemental chlorine chemicals were forbidden to be used for pulp bleaching, and the bleaching techniques were developed toward more environmentally friendly processes, such as ECF (elemental chlorine-free) and TCF (totally chlorine-free) bleaching sequences. The first midconsistency and shortened TCF bleaching sequence in the world, namely, OQPo (oxygen delignification—chelating metal ions—peroxide bleaching assisted with oxygen) sequence, was successfully commercially operated with the capacity of 150 t/d by Xianhe Co., Henan, China (**Figure 4**) in 2008 [15]. By this bleaching sequence, bleached wheat straw pulps (BWSPs) with brightness of more than 80%ISO, pulp viscosity of 653 ml/g and breaking length of more than 7000 m (**Tables 5** and **6**) and less amount bleaching effluent generated about 30 m3 /t pulps, decreased by more than 60% of a traditional CEH bleaching process for wheat straw pulps [15]. Later, a bleached wheat straw pulp production line by TCF sequence with an annual capacity of 200 t/d was installed in Baiyuan Paper Co., Henan, China. Also, enzymes have been considered by many pulp scientists to assist bleaching operation with a decrement of chlorine-contained bleaching chemicals, providing an optional measure for bleached pulp quality improvement and chlorine-contained bleaching chemical dosage decrement toward an environmentally friendly clean production concept.


**4. Paper products from wheat straw pulps**

**Grammage (g/m2 )**

**Beating degree (o**

**SR)**

**Tensile (N.m/g)**

**Figure 4.** A short OQPo bleaching sequence for production of bleached wheat straw pulps [15].

**Breaking length (km)**

58.3 **44.3 71.23 7.16 4.83 4.68 24**

**Table 5.** Physical strength properties of bleached wheat straw pulps by an OQPo sequence [15].

**Wheat straw pulps Brightness (%ISO) Viscosity (mL/g)**

Screened brown pulp 34.7 965 Pulp after oxygen stage 48.0 850 Pulp after Po stage 81.6 653

**Table 6.** Viscosity and brightness of wheat straw pulps from different stages [15].

**Tear (mN. m2 /g)**

**Burst (kPa. m2 /g)**

Wheat Straw Pulping for Paper and Paperboard Production

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

229

**Fold endurance times (1.5 kgf tension)**

As previous discussed, bleached wheat straw pulps can be characterized with good strength properties, reasonable brightness, good printability, etc., which can be used to manufacture a wide range of paper and paperboard grades, such as light-weighted printing paper [16], letterpress printing paper, typing paper, writing paper, aluminum foil base paper, cylinder polished paper, tissue and sanitary papers, white board, coated ivory board, and almost all grades of paper and paperboard grades [17–22]. Practically, to improve pulp stock wet-web

**Table 4.** Advanced technical specifications of wheat straw pulping in Quanlin paper group [15].

**Figure 4.** A short OQPo bleaching sequence for production of bleached wheat straw pulps [15].

of CEH (chlorination—alkali extraction—hypochlorite) was not applied until the 1980s of the twentieth century [14]. All of the above bleaching sequences contain elemental chlorine chemicals with generation of AOX and other organic toxicities in these bleaching effluents. Due to poor bleaching selectivity upon residue lignin, there were a large amount of carbohydrates degraded while bleaching, resulting a large water consumption and high chemical dosage, to produce bleached pulps with low brightness and weak physic strength as well as poor drainability of water. Following more and more strict regulations to effluent discharge, elemental chlorine chemicals were forbidden to be used for pulp bleaching, and the bleaching techniques were developed toward more environmentally friendly processes, such as ECF (elemental chlorine-free) and TCF (totally chlorine-free) bleaching sequences. The first midconsistency and shortened TCF bleaching sequence in the world, namely, OQPo (oxygen delignification—chelating metal ions—peroxide bleaching assisted with oxygen) sequence, was successfully commercially operated with the capacity of 150 t/d by Xianhe Co., Henan, China (**Figure 4**) in 2008 [15]. By this bleaching sequence, bleached wheat straw pulps (BWSPs) with brightness of more than 80%ISO, pulp viscosity of 653 ml/g and breaking length of more than

7000 m (**Tables 5** and **6**) and less amount bleaching effluent generated about 30 m3

ronmentally friendly clean production concept.

6. Black liquor properties at the exit of the black liquor extraction stage:

8. Mid-stage water discharge (i) Water discharge amount (m3

228 Global Wheat Production

decreased by more than 60% of a traditional CEH bleaching process for wheat straw pulps [15]. Later, a bleached wheat straw pulp production line by TCF sequence with an annual capacity of 200 t/d was installed in Baiyuan Paper Co., Henan, China. Also, enzymes have been considered by many pulp scientists to assist bleaching operation with a decrement of chlorine-contained bleaching chemicals, providing an optional measure for bleached pulp quality improvement and chlorine-contained bleaching chemical dosage decrement toward an envi-

**Items Technical specification**

/t o.d. pulp) <50

1. Biomass lose rate at wheat straw chip preparation stage (% w/w) 23–28 2. Caustic soda dosage in cooking on raw materials saved (% w/w) 2.5 3. Bleached pulp yield increased on raw materials (% w/w) 5.0 4. Pulp brightness after oxygen delignification (% ISO) 50–55 5. Tensile strength of the unbleached wheat straw pulp (N.m/g) 58.8

(i) Extraction efficiency (%) >90 (ii) Viscosity of black liquor (cp) (100°C, 50% solid consistency) <500 7. Black liquor properties at the exit of normal vacuum evaporation system (%w/w) 60–62

(ii) COD of treated effluent (mg/L) <60

**Table 4.** Advanced technical specifications of wheat straw pulping in Quanlin paper group [15].

/t pulps,


**Table 5.** Physical strength properties of bleached wheat straw pulps by an OQPo sequence [15].


**Table 6.** Viscosity and brightness of wheat straw pulps from different stages [15].
