Wheat is a common kind of crops, wheat flour can be processed into bread, steamed bun, cookies, noodles, etc. Wheat also applies to beer production and biomass fuel making. Wheat is rich in starch, protein, fat, minerals, calcium, iron, thiamine, riboflavin, niacin and vitamin A, etc. As one of three major grains, wheat is mostly used to eat, and only one-sixth of wheat is used as feed.
Wheat straw is the by-product of wheat. As we all know, wheat straw can be used to straw pellets production. Just like wood fiber pulp, bamboo, sugarcane bagasse, reed, jute and other plant fibers, wheat straw is also applied to paper pulp making. The biological structure of wheat straw is uneven.
The composition of wheat straw
The nodes, sheaths, leaves, and Stachys takes 49% weight of stalk, the stalk itself only takes a half of weight. Each part of the straw stalk has its own special fiber morphology and chemical composition. The table1 and table2 show that sheath, leaves, and Stachys contain more ash, extractive, pentosane content than wheat straw stalks. Especially the ash content is 5% higher than stalk, the extractive of these parts is 10% higher than stalk, while the holocellulose content is lower than stalks. Compared to stalk, the sheath, leaves, and Stachys have thicker and shorter fiber cell, we can get that these parts have lower pulp yield, higher alkali consumption, and they are also harmful to wheat straw pulp and papermaking. so that, the preparation of the material is the key step for obtaining high-quality paper pulp, low ash content, and low viscosity black liquor in the wheat straw pulp processing.
| chemical component | caudex | central stem | stem tip | sheath | leaf | stachys | whole straw |
|---|---|---|---|---|---|---|---|
| Ash | 5.93 | 6.19 | 7.19 | 11.34 | 12.06 | 9.82 | 8.28 |
| 1%NaOH extraction | 36.42 | 37.19 | 43.6 | 47.28 | 52.97 | 49.42 | 42.59 |
| pentosan | 23.28 | 23.20 | 24.66 | 25.56 | 19.21 | 26.03 | 24.04 |
| holocellulose | 71.24 | 69.92 | 69.06 | 69.86 | 60.95 | 64.52 | 68.12 |
| Total lignin | 22.96 | 21.71 | 20.41 | 18.39 | 17.48 | 19.50 | 20.75 |
table1: components of wheat straw
| Part of wheat straw | length | length-width ratio | ratio of cell wall to cavity |
|---|---|---|---|
| caudex | 1.85 | 103 | 1.67 |
| central stem | 1.69 | 104 | 1.61 |
| stem tip | 1.29 | 93 | 1.11 |
| sheath | 1.36 | 90 | 1.19 |
| leaf | 1.05 | 73 | 0.95 |
| stachys | 0.80 | 42 | 0.66 |
| whole straw | 0.82 | 37 | 0.83 |
table2: fiber morphology of each part of wheat straw
Besides that, straw fiber has a higher content of non-fibrous cell than wood fiber, it is the primary cause of bad drainability and low strength of straw pulp. Meanwhile, in the straw pulp production, delignification rate of the parenchymal cell is low.
Wheat straw pulp market
Wood fiber is the main material for paper and pulp making. Due to the resource constraints and other reasons, non-wood fibers is another resource of papermaking fiber material. The non-wood fiber mainly refers to the therophyte, including bamboo fiber, bagasse fiber, reed fiber and etc. Take the Chinese market as the example, straw is one of the main grain, annual production reaches 80 million tons, can process 30 million tons wheat straw pulp.
Wheat straw pulp plays an indispensable role in the paper and pulp industry. There are some restrictive factors that limit the development of wheat straw pulp. Firstly, the collection and transportation of raw materials; secondly, the straw pulp mill usually has a small scale, poor technology, and paper pulper machines, which result in low-quality pulp and market competition. Last but not least, serious environmental pollution caused by wheat straw pulp production. With the development of straw pulping technology, the straw pulp will have a bright future in the industry.
Reasons for difficulty in straw pulp extraction
The bottleneck of straw pulp processing is the black liquor extraction. Reasons for difficulty in straw pulp extraction as follows:
- Straw pulp has strong hydrophily and bad drainability.
- Black liquor has a high content of ash and carbohydrate and high viscosity, and it is hard to evaporate, burn, and causticize.
- Black liquor extraction equipment and processing technology have limitations.
Tips for wheat straw pulp making
CNBM is a Chinese paper and pulper machines manufacturer, we not only provide a complete solution for wood pulp, non-wood pulp, and waste paper pulp, also supply single pulp making equipment as you need, such as pulp digester, vacuum drum washer, blow tank, twin roll press, single screw press.
For the unique property of wheat straw fiber, we sum up three basic principles for wheat straw pulp processing.
The importance of material preparation
As we mentioned above, due to the fiber morphology of wheat straw, material preparation of straw pulp decide the pulp quality. There are dry-method and wet-method or dry-wet method. The dry-method is suitable for small and medium scale pulp mill, and wet method requires the large investment and high water consumption, applies to large-scale pulp mills.
Screening and dust removal are also necessary for material preparation. Screening removes the crops, grass-blade, straw knots and dust from the wheat straw, decrease the chemicals consumption and improve the straw pulp quality.
Materials preparation functions as follows:
- It is the most cost-effective way to reduce the silicon content and viscosity of black liquor.
- Good for reducing the chemicals consumption and content of parenchyma cells.
- Good for drainability of wheat straw pulp.
- Increase the pulp yield and bleaching properties and strength of straw pulp.
Displacement cooking
According to the real applications, the displacement cooking is more applicable in the straw pulp processing. Common pulp cooking methods include RDH, Super Batch cooking system. Why choose displacement cooking?
- Cold blow technology and high-temperature black liquor circulation. The heat energy recovery can reduce the energy consumption.
- The lignin and carbohydrate produced from the straw fiber can be displaced outside the digester in the primary cooking, which reduces the alkali consumption of later cooking.
- Pulp cooking has good evenness.
- Improve the yield and strength of straw pulp, and reduce the pollution.
- Decrease the dilution factors and water consumption, high black liquor.
- The obtained pulp is easy to bleach and the pulp has a lower hardness.
Related pulping equipment
To apply for the characters of wheat straw pulp, improve the extraction equipment and pulp process. Continuous pulp digester has replaced the rotary spherical digester. The vacuum drum washer has wide application in the wheat straw pulp production, the extraction rate of black liquor reach 85%-90%. Ensure the vacuum drum washer is operated under the condition of the perfect vacuum. As for the bleaching process, traditional CEH bleaching is replaced with hydrogen peroxide bleaching, chlorine dioxide bleaching and etc.