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Annual plant pulp making

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annual plant

Annual plant pulp solution

Raw materials: straw, wheatgrass, reed, bamboo, cotton stalk, salix, bagasse, etc.

Uses: Box board, corrugated, sanitary paper, kraft paper, etc.

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Annual plant refers to a non-woody plant that completes its life cycle (germination, growth, flowering, fruiting, death) within one year. Many annual plant can be used as raw material of paper pulping making, such as straw, wheatgrass, reed, bamboo, cotton stalk, salix, bagasse, soybean stalk, corn stalk, hemp, mulberry stalk and miscellaneous wood crotch, etc.

Why choose annual plant for paper pulping plant?

There are two main reasons to answer this question, Firstly, wood pulp is the most common on the market, but wood-based materials require a long life cycle, and large wood material consumption is harmful to our environment, annual plants need shorter life cycle, it is more suitable for paper pulp making. Secondly, wood materials can’t be planted in a large scale, and paper pulping plants need large consumption of raw materials, annual plants can meet this production demand. In a word, compared to wood pulp, annual plant pulp is more fit the new concept of green pulping.

compariosn of wood and non-wood fiber materials

How to make pulp from annual plant?

The physical structure and chemical composition of annual plants are significantly different from wood. Annual plants have higher cold and hot water extraction and lower lignin content, with correspondingly simpler structures. There is no need for high-temperature, high-pressure, or high-alkaline cooking. Instead, a low-temperature, low-pressure, low-alkaline approach should be taken, along with anaerobic fermentation to increase biogas production and achieve standard discharge of waste liquid. The details are described as follows:

  • Dry and wet feed preparation: the raw materials should be cleaned before pulp cooking.
  • Pulp cooking and pulp bleaching: both pulp cooking and pulp bleaching processes are completed in the same digester without an washing process between two sections. After pulp cooking and bleaching, there is a normal pulp washing process and pulp screening process.
  • Recycling: a small amount of the washed waste liquid is reused for pulp cooking, while the majority enters anaerobic, aerobic, and chemical treatment systems. The treated wastewater becomes recycled water for pulp production or meets the standards for discharge.
  • Gas purification: The biogas produced through anaerobic fermentation undergoes purification to become a fuel similar to natural gas.
  • Heat and electricity production: The recovered biogas is used for heat and electricity production.

Note: this complete technology is known as green pulping technology, consisting of two divisions that are indispensable. It is similar to the sulfate pulping method, which includes cooking, bleaching, and recycling. These processes have relative independence but are inseparable. There are many new pulping technologies available, but without supporting waste liquid treatment technology, they cannot be considered green pulping technology.

common annual plant pulping solution

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Related paper pulp machine for annual plant pulp making

pulping processing for annual plant pulp making

Dry and wet feed preparation

Non-wood fiber raw materials contain many impurities that are not suitable for pulp cooking. These impurities were not given much attention in the past alkaline pulping of grass fiber. However, they actually affect subsequent processes such as pulp cooking, pulp bleaching, pulp screening, and recycling, causing a series of serious issues. In China, there is mechanical impurity removal and wet preparation. In foreign countries, wet preparation is more common, and the equipment is similar to hydrapulper. The water used in wet preparation can be reused after simple sedimentation, and the chemical oxygen demand (COD) can reach around 5000ml/g, mostly in the form of water extractives that can be treated biochemically. After treatment, a portion of the water is reused, while the rest enters the biochemical treatment system to recover the COD. After dry and wet preparation, the cleanliness of the raw materials is significantly improved. After removing 20% to 30% of the water extractives, the moisture content of the raw materials increases, providing conditions for better impregnation with chemicals.

Decomposition and bleaching of grass materials

In traditional paper pulping methods, cooking and bleaching are carried out in two separate stages. After pulp cooking, there are four stages of washing, and pulp bleaching is also divided into multiple stages, with washing steps in between. In green pulping, pulp cooking and pulp bleaching are often combined into one stage. Some methods simultaneously add cooking and bleaching agents to the grass, which not only breaks down the plant tissue to separate the fibers but also bleaches the pulp to achieve a brightness exceeding 70% ISO. Pulp cooking uses a penetrating agent to accelerate the wetting of chemicals, which facilitates the accelerated separation of fibers. Pulp bleaching involves the use of strong oxidizing agents to destroy color-forming genes in the plant tissue. This improves the brightness of the pulp and prevents it from becoming darker, achieving further brightness improvement. Other methods involve the use of ultraviolet or microwave treatment. Combining cooking and bleaching simplifies the process flow significantly.

Methane gas recovery

For wood cooking waste liquid, the alkali can be recovered, but for non-wood fiber cooking, there is little or no inorganic substance to be recovered. The organic matter consists mainly of cold and hot water extractives. As mentioned earlier in the wet preparation process, most of the water extractives have already been removed. The removal of lignin during pulp cooking is not the main purpose. The removed organic matter can be treated biochemically. Therefore, the purpose of pulp washing is not extraction efficiency. Expensive extraction equipment is not necessary; it is sufficient to wash the pulp thoroughly. The washed waste liquid is an excellent wetting agent that can be partially reused for cooking. The majority of it enters the wastewater treatment system for reuse in production. The obtained biogas is purified to increase the methane purity, and its calorific value is 25.92 kJ/Nm3. Approximately 0.5 cubic meters of biogas can be produced per kilogram of COD, which is cheaper for power generation than coal. Purchased electricity costs 0.41 yuan/kWh, while self-produced biogas electricity costs 0.12 yuan/kWh. Although the waste liquid does not undergo alkali recovery, biogas power generation is still considered biomass power generation, achieving the same goal through different means. If the biogas is used for residential gas supply, the biogas from a semi-chemical pulp plant with an annual production of 100,000 tons can provide gas for 80,000 people’s daily use.

Utilization of wastewater

After undergoing the aforementioned treatment processes, the wastewater can be used for production. Some factories have very little remaining treated water and only discharge it once every two weeks. Others store the treated water and use it for spring irrigation, which is highly appreciated by farmers.