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Biological pretreatment of wood chips by fungi is a well-known approach prior to mechanical- or chemical pulp production. For this biological approach, a limited number of white-rot fungi with an ability to colonize and selectively degrade lignin are used to pretreat wood chips allowing the remaining cellulose to be processed for further applications. Biopulping is an environmentally friendly technology that can reduce the energy consumption of traditional pulping processes. Fungal pretreatment also reduces the pitch content in the wood chips and improves the pulp quality in terms of brightness, strength, and bleachability. The bleached biopulps are easier to refine compared to pulps produced by conventional methodology. In the last decades, biopulping has been scaled up with pilot trials towards industrial level, with optimization of several intermediate steps and improvement of economic feasibility. Nevertheless, fundamental knowledge on the biochemical mechanisms involved in biopulping is still lacking. Overall, biopulping technology has advanced rapidly during recent decades and pilot mill trials have been implemented. The use of fungi as pretreatment for pulp production is in line with modern circular economy strategies and can be implemented in existing production plants. In this review, we discuss some recent advances in biopulping technology, which can improve mechanical-, chemical-, and organosolv pulping processes along with their mechanisms.

Sodium sulfite (Na2SO3) is known as a selective chemical agent for wood delignification. In this work, Scots pine (Pinus sylvestris) wood was pulped using alkaline sulfite (AS) with the addition of 1,4-dihydro-9,10-dihydroxy anthracene (DDA) [called also soluble anthraquinone (SAQ)] and ethylene glycol (G). The studies showed the possibility of obtaining Kappa number (KN) 24 to 26 alkaline sulphite-DDA pine pulps with total and screened yields higher by 3.4 to 3.9% and 1.4 to 2.6%, respectively, than in the case of kraft pulping. The AS-SAQ pulping process was also characterized by much higher brightness of pulps but worse defibreability of wood than the kraft process. Increasing the amount of Na2SO3 dosed to the wood from 25% to 30-35% (based on wood) and adding G to the cooking liquor increased the delignification of pine wood in the AS-SAQ method to Kappa number of 17 to 20 units (without G) and approximately 14.5 units (with G). Such a modification had a positive effect on the defibreability of wood after pulping and the brightness of pulps but a negative effect on the screened yield of AS-SAQ and AS-SAQG pulps.

The influence of pulping variables on the pulp and black liquor properties for a neutral sulphite semi-chemical pulping system was investigated in a pilot plant pulping setup situated at an industrial paper mill. Eucalyptus chips were used as raw material and the operating variables were Na2SO3 charge (8–18% w/w on oven-dry wood), Na2CO3 charge (0.5–3.0% w/w on oven-dry wood) and maximum cooking temperature (160–180 °C). Response surface methodology was used to parametrize empirical models to find the optimal conditions for maximizing the short-span compression strength index of the pulp. The derived regression models for the black liquor properties and the pulp hypo number had R2-adjusted values above 0.8 and p-values for overall significance below 0.05. The derived regression models for the handsheet strength properties had R2-adjusted values between 0.3 and 0.45 and p-values for overall significance either below 0.05 or between 0.05 and 0.1. The sulphite charge, followed by the carbonate charge, had the most notable effect on the evaluated properties with the effects of temperature being less significant. Optimization of the pilot plant system showed that the short-span compression strength index of the pulp could be maximized to 26.7 N m/g, using a sulphite charge of 9.4% (w/w on oven-dry wood) and a carbonate charge of 1.94% (w/w on oven-dry wood), similar to short-span compression strength indices typically achieved using other pulping processes.

Using wet-storage pretreatment coupled with oxygen-alkali cooking technique can achieve the high efficiency, energy-saving and clean extraction of bagasse fibers. Based on this view, the delignification and carbohydrate degradation behaviors of wet-storage bagasse under different pulping conditions (cooking time, alkali charge, MgSO 4 dosage, and initial pressure of oxygen) were investigated in detail. The results showed that the bagasse could form pulp after wet-storage pretreatment at a lower temperature (90 °C) and lower alkali charge (24%). In addition, the kinetic models of oxygen delignification and two-step degradation of carbohydrate (bulk fiber to individual fiber bundles, and then to degradable cellulose) were established to control the process of oxygen-alkali pulping. The activation energy for oxygen delignification and two-step degradation of carbohydrates was 31.45, 33.17 and 28.07 kJ/mol, respectively, indicating that the screened pulp fibers were more susceptible to degradation during oxygen delignification for bagasse after wet-storage. Therefore, the cooking end-point needs to be accurately controlled to reduce the significant degradation of carbohydrates in the later stage of cooking for oxygen alkali pulping of bagasse after wet-storage pretreatment. Graphical abstract

The pulp industry, one of the oldest and largest chemical industries globally, plays a critical role in biomass utilization. Increasing kraft pulp yield has long perplexed researchers and engineers. Given that methyl mercaptan (MeSH) can be self-produced during the kraft pulping process, revisiting MeSH-assisted yield enhancement technology offers great attraction for pulp industry. MeSH’s strong nucleophilicity enables its consumption through both demethylation and non-demethylation reactions. Investigating the MeSH’s evolution at typical pulping conditions is crucial for achieving higher recovery rate of MeSH. This study found that higher pulping temperatures, increased alkali charge, and larger Na2S ratios improve the recyclable residual MeSH content in black liquors, increasing it from 17 to 36%. Under moderate pulping conditions, the residual MeSH content was approximately 30% of the original MeSH. Dimethyl sulfide (DMS), a valuable demethylation product of lignin or hemicellulose by MeSH, was found in black liquors at levels up to 15% of the original MeSH. Approximately half of MeSH participated in non-demethylation reactions and could not be recycled. X-ray photoelectron spectroscopy (XPS) analysis of separated kraft lignins suggested the generation pathway and chemical structure of new methylsulfur (MeS)-containing lignin. For economic balance and profitability, DMS recycling is proposed as cost compensation, or by adopting partial MeSH-assisted kraft pulping in batch digesting systems.

This work studied the feasibility of potassium carbonate-glycerol deep eutectic solvent (K 2 CO 3 -Gly DES) as a potential green solvent applied in lignocellulose pulping. Cellulose fibers were extracted from rice straw via novel alkaline DES pulping technique using 1:7 molar ratio of K 2 CO 3 -Gly DES. Optimum pulping parameters were determined using the one-factor-at-a-time (OFAT) method. The cellulose fibers were characterized for chemical composition of cellulose, hemicellulose, lignin and extractives. Changes in physical structure, chemical structure, morphological structure, functional groups and crystallinity index (CrI) were investigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results revealed that the optimum pulping temperature at 140 °C, reaction time of 100 min and 1:10 rice straw to DES mass ratio produced the highest cellulose content of 73.8% for unbleached DES treated pulp. Chemical composition analysis and FTIR verified that this alkaline DES pulping method was able to achieve partial removal of hemicellulose and lignin from lignocellulosic matrix. Moreover, XRD result demonstrated that the CrI of cellulose fiber increased from 52.8 to 60.0% after pretreatment. The cellulose fibers had diameters ranging from 3.58 to 5.68 µm. This study proved that the specifically-designed K 2 CO 3 -Gly DES could successfully isolate cellulose from lignocellulosic biomass through alkaline DES pulping. Graphical abstract

An integrated and feasible approach was proposed using the underutilized grass fibre (stem) derived from Napier grass and sugarcane for paper production in this study. To enhance paper strength, pre-hydrolysis and beating techniques have been used to improve the chemical pulps and mechanical pulping process, respectively. Napier grass and sugarcane are promising non-wood sources for pulp production, owing to their high cellulose and low lignin and extractive content. With the additional mild alkaline pre-treatment to the mechanical pulping process, the lignin content was greatly reduced. The results reveal that the mechanical pulping with alkaline pre-treatment may indeed potentially replace the most prevalent pulping process (chemical pulping). As evidenced by the paper strength properties, mechanical pulping is far more suitable for grass-type biomass, particularly Napier grass, which had a folding endurance capability five times greater than chemical pulping. Furthermore, the remaining high hemicellulose content from mechanical pulping contributed to a high pulp yield, while also facilitating the fibrillation on the sugarcane’s laboratory paper handsheet. The findings also demonstrated that the additional beating process from chemical pulping causes the fibres to be drawn toward each other, resulting in a more robust fibre network that contributes to good paper strength. Consequently, this work sheds new light on the development of advanced paper derived from grass fibre.

With an increasing interest for molded pulp product (MPP) in the industry, it is important to fully understand how the manufacturing process is different from papermaking. One specific way to differentiate the processes is to compare their resulting products. As the paper industry uses several wood fibers with various pulping processes, it is interesting to compare some of these fibers, to further progress our understanding of the MPP process. In this study, six different wood fibers were used (as received) and analyzed to obtain the sample with the lowest moisture uptake and highest tensile properties. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and fiber analysis module (MorFi) observations were performed, as well as moisture uptake measurements after sorption and tensile tests. We observed significant differences between the fibers tested. Kraft fibers (bleached softwood kraft pulp (BSKP), bleached hardwood kraft pulp (BHKP), and unbleached softwood kraft pulp (USKP)) showed smoother surfaces and less non-cellulosic molecules, such as hemicellulose, lignin, and pectin, in the SEM images. Bleached chemi-thermomechanial pulp (BCTMP) and recycled pulps (R-NPM and R-CBB) both showed non-cellulosic molecules and rougher surfaces. These results were confirmed with the FTIR analysis. With kraft fibers, MPP mechanical properties were lower than non-kraft fibers. Resulting moisture uptake is in between the recycled fibers (lowest moisture uptake) and BCTMP (highest moisture uptake). The removal of non-cellulosic molecules reduces the mechanical properties of the resulting MPP. The incorporation of non-wood molecules, as found in recycled fibers, also reduces the mechanical properties, as well as moisture uptake, when compared with BCTMP.

Index levels of the process of pulping wheat straw (WS), industrial hemp stalks (HS), and Miscanthus × giganteus (MS) using the nitric acid-alkali (NA−A) method (HNO3/KOH and HNO3/NH4OH two-stage sequential delignification systems) were determined. The research showed, as a rule, a lower total yield of NA−A pulps than kraft pulps from WS, HS, and MS, and a lower level of the tensile index and tear resistance compared to the properties of soda-AQ and kraft pulps presented in other studies, as well as the author’s own studies. Additionally, the levels of nitrogen (in different forms) and potassium in the combined post-pulping filtrate were determined. Their levels were found to be higher than in the fertilizer solution used to fertilize tomato crops grown on soilless substrates in greenhouses. However, application of filtrates from pulping of nonwoody plant straw using NA-A method requires determining the effectiveness of this type of fertilizer solution, its impact on the horticulture and agriculture plants, and the properties of their output including grains, tubers, and fruits.

The objective of this work was to determine the effect of sodium methyl mercaptide (SMM) on the minimization of peeling reactions of southern pine chips in the kraft pulping process. Two methods were evaluated for SMM addition to the pulping process: (1) pre-treatment before pulping or (2) co-addition with white liquor. The effect of SMM charge, pre-treatment temperature and time, and pH of pre-treatment liquor was studied.The experimental results showed about 1.5 to 2.5% (on O.D. (oven dry) wood basis) increase in the pulp yield after pre-treatment with or co-addition of 4.38% SMM (on O.D. wood basis). The use of 4.38% SMM allowed a decrease of the white liquor effective alkali charge (EA, on O.D. wood basis) by 3%. 4.38% SMM charge seemed to be optimum for the pre-treatment. Pre-treatment at lower pH resulted in a significant decrease in yield and an increase in rejects. The increase in pulp yield was mostly due to the increased retention of cellulose and xylan. The retention of galactoglucomannan was negligible. The degree of polymerization of cellulose increased from 5180 to about 6000 due to SMM pre-treatment. About 80% of the cellulose yield increase is due to the suppression of primary peeling. The remainder (0.3–0.4% of the yield increase (on O.D. wood basis) is due to reduced alkaline hydrolysis and subsequent secondary peeling. Graphical abstract