Rising raw material prices, increasing waste disposal costs and expanding legislation are the major drivers behind the rise of sustainable technologies. Producers around the world are forced to evaluate their production processes and to search for alternative technologies with lower environmental impact. A comprehensive technology mapping can help producers to compare sustainable technologies and to select viable alternatives.
Here we present a case study on the paper pulp industry. This industry was historically one of the largest polluters, but has recently been making the shift towards sustainable technologies.
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Introduction: The paper pulping process
There are different methods to produce paper pulp. The most common way of pulping is a chemical method where the molecular “fiber glues” (i.e. lignin and hemi-cellulose) are dissolved. A paper pulping process typically consists of various steps.
- First, there is a preconditioning step which starts with shredding of the wood fibers (a).
- This process is followed by impregnation of the shredded biomass with pulping chemicals (b).
- The next step, the actual pulping process, is completed in a large cooking vessel (c).
- In this so-called digester, paper pulp is produced due to a selective chemical destruction of the gluing compounds by cooking for several hours. In the last step, the paper pulp is separated from the residual chemical mixture, called black liquor (d).
- Black liquor contains the pulping chemicals together with the partly degraded lignin and hemi-cellulose (causing the dark color). Half of the original wood biomass ends up in the black liquor. By drying and burning the black liquor, part of the heat requirements of the process are provided for, furthermore most of the added pulping chemicals are recycled to the process (e).
- In parallel, the paper pulp slurry is dried to sell to a papermaking plant (f).
Throughout, raw materials like wood & water are heavily consumed and large streams of polluted water & air are generated. Various process factors contribute to the high final environmental impact.
Green technology alternatives in the paper pulp industry
For each of these processing steps there are several sustainable technologies that can help to offset the environmental impact of the process. Here, we present a brief mapping of the most important green technologies per processing step.
1. Alternative pulping chemicals
The mechanism of wood pulping is based on the action of pulping chemicals. These chemicals have various disadvantages: they cause a foul smell and end up partially in the wastewater & exhaust gases. More fundamentally, these chemicals degrade lignin and hemi-cellulose, so that only 50% of the initial biomass is effectively used to make paper.
A biotechnology-based process, called biopulping, has been developed to eliminate the use of these environmentally harsh compounds. Biopulping is based on enzymes which selectively degrade and dissolve the wood gluing compounds. The mechanism is much like the conventional chemical process but without the need for high cooking temperatures and without the use of chemicals.
Organosolv pulping is another technology that is able to overcome these problems. Organic solvents at high pressure and temperature are used to selectively dissolve lignin and hemi-cellulose, leaving only the pulp. After fractionation, the solvents are stripped and internally recycled in the process. The hemicellulose and lignin-rich side streams can be recovered for other bio-based applications.
2. Replace the Digester
The digesters used in paper pulping are only viable at large scales due to high investment costs. Temporary switching from wood to (more sustainable) non-wood fibers such as hemp or kenaf in these digesters is highly cumbersome and undesirable from an economical point of view.
The digester can however be completely replaced by small scale high-shear extruders. These run at much lower throughput rates, are flexible towards the type of biomass and have a lower investment cost. These properties make the extruder technology suitable for farm-scale pulping of non-wood biomasses, such as forest and crop residues. Furthermore the combined chemical and mechanical actions of extruders decrease consumption of energy, water & chemicals.
3. Black liquor alternative
All chemical pulping processes produce black liquor as the major sidestream. Black Liquor is basically a soup of pulping chemicals and soluble organics. This waste stream is already internally recycled in the current process by evaporating the water and burning the liquor in a furnace. As such, most of the pulping chemicals are recycled. Still, the process produces large amounts of sulfur-rich flue gasses and spills large amounts of heat through cooking out the water.
Instead of burning black liquor in a furnace, the black liquor can be sent to a gasification process. Next to recycling the chemicals, this process produces both heat and syngas. Syngas is a mixture of hydrogen gas, methane and carbon monoxide that can be used as energy source or as building blocks for organic synthesis.
The above mentioned organosolv pulping is even better at resolving the issues with black liquor, since it completely eliminates the generation of black liquor.
4. Less Water
The paper industry is a large consumer of fresh water. In parallel with the high consumption rate of fresh water, waste water is produced in large quantities.
A significant part of the water is lost through vaporization. Cooling these humid hot air streams can be used to recapture part of the water vapor through condensation. The energy that is contained in the waste heat can be validated by an Organic Rankine Cycle. This technology is a variation of the conventional Rankine Cycle, used in most electricity plants, with a low-temperature cooking organic fluid as working fluid instead of water.
Systematic innovation of production processes starts with the exploration of the existing technology landscape. By making this clear draft of all technologies in the playing field, process innovation can be more effective. After mapping the different sustainable pulping technologies, process engineers can decide if the investment in a sustainable alternative and the cost savings associated with it are worth to consider.
Interested in mapping the sustainable technology alternatives for you production process?
- Bajpai, P. (2012). Biopulping. In Biotechnology for Pulp and Paper Processing (pp. 67–92). Boston, MA: Springer US. doi:10.1007/978-1-4614-1409-4
- Sridach, W. (2010). The environmentally benign pulping process of non-wood fibers. Suranaree Journal of Science & Technology, 17(2), 105–123
- Moscicki, L. (2011). Extrusion-Cooking Techniques. (L. Moscicki, Ed.). Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA
- Ziviani, D., Beyene, A., & Venturini, M. (2014). Advances and challenges in ORC systems modeling for low grade thermal energy recovery. Applied Energy, 121, 79–95. doi:10.1016/j.apenergy.2014.01.074