Pulping and fractionation

Pulping is a process that extracts fibrous material from biomass, most commonly as a precursor for paper making. The process is often combined with fractionation processes to separate and valorise lignin. Pulping and fractionation processes separate the fibrous cellulose and lignin from the other components and impurities in the biomass. Main processes are mechanical, chemical, and a combination of mechanical and chemical pulping in a hybrid pulping process. Mechanical pulping relies on physical separation methods without added chemicals. However, water can be added to reduce the damage to the fibres from friction. Chemical pulping uses chemicals to remove the lignin from the pulp, resulting in a higher quality pulp. Hybrid technologies use chemicals to soften the lignin before a physical separation results in a pulp that still contains a substantial amount of the lignin. Finally, biological pulping uses biotechnology for the pulping process.

Feedstock

Origin and composition

Pulping is performed on feedstock with a high fibre content. Before the pulping process, any material that is low in fibrous material should be removed. For example, wood undergoes debarking before the pulping process. Next, the biomass should be sized, for example by chipping. The most common feedstock for pulping is woody biomass. Examples of non-woody biomass are sisal, rice straw, cotton linen, sugarcane bagasse, pineapple, and straw.

Process and technologies

Chemical pulping

The chemicals in chemical pulping allow for a near complete removal of the lignin from the biomass. This results in high quality pulps, which can be used for printing and writing paper. However, the yield of chemical pulping is generally lower than other methods, resulting in more expensive pulps.

Dissolving pulp and organosolv

Dissolving pulp, and more specifically organosolv processes, is a typical example of the combination of pulping and fractionation. Dissolving pulp production entails a hydrolysis step before the pulping process, which is commonly sulfate or sulfite pulping. Most common method is to apply steam to the biomass, which hydrolyses and removes the hemicellulose and dissolves the organic acids. Organosolv methods, where organic solvents are introduced, can be used on biomass types that are not suitable for dissolving pulp technologies.

Cold soda pulping

Cold soda pulping uses room temperature sodium hydroxide (20 to 30 °C) before a disk refining. The cold soda uses a fast impregnation of the biomass, which reduces the losses in lignin and polyose, resulting in high yields (85 – 92%). It can be combined with the Kraft process to recover the sodium hydroxide. The resulting pulp has a low brightness, but can be bleached with peroxide-hypochlorite.

Sulphate pulping (Kraft)

The Kraft process is the most common pulping process used globally. It uses the chemicals sodium hydroxide and sodium sulfide at elevated temperatures (155 – 180 °C) and a steam pressure of 800 kPa to break down the lignin in the biomass. The lignin breaks down into hydroxyl and hydrosulfide ions, which dissolve in the liquor. Part of the hemicellulose and cellulose is also broken down by the treatment. The used chemicals are known as black liquor, which contains lignin, hemicellulose and extractives (oils, resins, and terpenes). The chemicals can be recovered and replenished with sodium salt, resulting in a cost-effective process.

Sulphite pulping

Sulfite pulping is similar to sulfate pulping, where both methods cook the biomass with chemicals to cleave the lignin bonds. In sulfite pulping a bisulfite of ammonium, calcium, magnesium, or sodium is used together with sulfur dioxide. Unlike Kraft pulping, this process is sensitive to extractives, which makes the process unsuitable for hardwood species. Moreover, chemical recovery is nearly impossible. The resulting pulp is brighter, easier to bleach and refine compared to Kraft pulp.

Hybrid pulping

In hybrid pulping, the lignin is softened by chemicals, such as Sodium Sulfite and alkaline salts, before a mechanical pulping step. This results in stiff fibres which are commonly used in corrugated board, roll cores, and containers.

Chemi-thermo-mechanical pulping (CTMP)

In chemi-thermo-mechanical pulping, the biomass is pre-treated by steam and chemicals. The steam and chemicals soften the lignin, which reduces the mechanical energy required for the pulping. The pulp is obtained in high yields (85-95%) and has a high strength, suitable for high-grade printing paper.

Neutral Sulfite Semi Chemical Pulping (NSSC)

The Neutral Sulfite Semi Chemical Pulping (NSSC) technology uses a combination of chemical pulping and refining. First the biomass is impregnated with sodium sulfite at 160 to 190 °C to remove lignin. Anthraquinone can be added to increase the rate of delignification. The sulfite is usually combined with a buffer solution to negate the effect of released organic acids. A second step in the process is a disk refining. Up to 15 to 20% of the lignin remains in the NSSC pulp, which is often used in unbleached products, where strength and stiffness are required.

Mechanical pulping

Mechanical pulping is inexpensive and results in the highest yields. However, mechanical pulp also results in paper with a large number of imperfections. Technological advances are improving the quality of mechanical pulps, while maintaining the low cost and high yields.

Refiner

Refiner mechanical pulping (RMP)

In a refiner mechanical pulping process, the biomass is ground between rotating metal discs or plates. In a first step, the biomass is defibrated into separate individual fibres. In a second step, the fibres are loosened. The RMP pulp is stronger, freer, bulkier, and darker compared to traditional SGW pulp.

Thermomechanical pulping (TMP)

In this refiner process the biomass is preheated by impregnation of steam under pressure. The high temperature (115-155 °C) softens the lignin and helps in fibre separation. The refining takes place in two steps, the first at elevated pressure and temperature, around the glass transition temperature of lignin (140 °C), the second at atmospheric pressure and temperature. The resulting yields are high (>93%) and the pulp is characterised by its high strength.

Groundwood

Pressure groundwood (PGW)

Information still missing.

Stone groundwood (SGW)

Stone groundwood pulping is the oldest mechanical pulping method, where the biomass is pressed against a rotating grindstone. The grindstone breaks apart the biomass into thin fibres and fragments, which are washed away with a water stream. The friction results in an increased temperature, which helps the process. The product stream is scanned to remove the larger particles, then the water is removed to thicken the pulp. The process has high yields (about 95%), because most lignin remains in the product.

Thermal groundwood (TGW)

Information still missing.

Biological pulping

Biological pulping takes advantage of natural methods to break down fibrous materials. For example, white-rot fungi can be used to soften and remove lignin.

Product

The resulting product of the pulping process, called pulp, can be further processed into many paper and board products. Depending on the qualities of the pulp, different products are made. Mechanical pulps, which are low quality pulps, are suitable for low-quality paper, such as newspaper, catalogues, paper towels, tissues, and sanitary papers. High quality pulps from chemical pulping are used for printing and writing paper. Finally, the hybrid pulping processes give pulps with stiff fibres and are commonly used in corrugated board, roll cores, and containers.

The resulting products from fractionation processes are commonly lignin or lignin derivatives.

Technology providers

Technology comparison

Company name	Country	Technology category	Technology name	TRL	Capacity [kg/h]	Pressure [bar]	Reagent	Temperature [°C]	Yield [%]	Feedstock: Food waste	Feedstock: Garden & park waste
Company 1	[Country HQ location]	[Technology category (if different sub-categories are defined this has to be specified here, the available categories can be found on each technology page under the chapter Process and technologies)]	[Technology name (the "branded name" or the usual naming from company side)]	[4-9]	[numeric value]					●	●
Company 2	[Country HQ location]	[(if different sub-categories are defined this has to be specified here, the available categories can be found on each technology page under the chapter Process and technologies)]	[Technology name (the "branded name" or the usual naming from company side)]	[4-9]	[numeric value]					●	●

Valmet

Valmet is a developer and supplier of process technologies, automation and services for the pulp, paper and energy industries. The company has over 200 years of industrial history and was reborn through the demerger of the pulp, paper and power businesses from Metso Group. Valmet offers tailored technology solutions for softwood and hardwood kraft pulp production, as well as various mechanical pulping technologies.

Open access pilot and demo facility providers

Pilots4U Database (Pulping)

Pilots4U Database (Organosolv pulping)

Pilots4U Database (Sulphate/Sulphite pulping)

Patents

Currently no patents have been identified.

References

Technology
Technology details
Name:	Pulping and fractionation
Category:	Conversion (Other processes and technologies)
Feedstock:	Woody biomass
Product:	Pulp

Pulping provider

General information
Company:	Valmet
Country:	Finland
Contact:
Webpage:	https://www.valmet.com/
Technology and process details
Technology name:	Chemical and mechanical pulping	Technology category:	Conversion (Other processes and technologies)
TRL:	9	Capacity:	kg·h-1
Pressure:	bar	Reagent:
Temperature:	°C	Other:
Feedstock and product details
Feedstock:	Hardwoods, softwoods, bamboo	Product:	Pulp