Alternative Technologies for Biofuels Production in Kraft Pulp Mills—Potential and Prospects

Is Kraft Pulping the Future of Biorefineries? A Perspective on the Sustainability of Lignocellulosic Product Development

By reflecting on the history and environmental impact of conventional biorefining, such as kraft pulping, we aim to explore important questions about how natural polymers can be more sustainably sourced to develop bio-products and reduce reliance on plastics. Since the Industrial Revolution, chemical pulping processes have enabled the mass production of cellulosic products from woody biomass. Kraft pulping, which dominates within modern pulp and paper mills, has significantly contributed to environmental pollution and carbon emissions due to sulfurous byproducts and its high water and energy consumption. While chemical pulping technologies have advanced over time, with improvements aimed at enhancing sustainability and economic feasibility, conventional biorefineries still face challenges related to biomass conversion efficiency and environmental impact. For example, efforts to fully utilize wood resources, such as isolating lignin from black liquor, have made limited progress. This perspective provides a thoughtful examination of the growth of chemical pulping, particularly the kraft process, in the production of consumer goods and its environmental consequences. It also presents key insights into the bottlenecks in developing truly sustainable biomass conversion technologies and explores potential alternatives to traditional chemical pulping.

Direct conversion of lignin-rich black liquor to activated carbon for supercapacitor electrodes

The escalating industrialization trend underscores the imperative for sustainable waste management practices. The present investigation explores a sustainable methodology for managing the waste generated from the kraft process by directly converting it into activated carbon (BLAC) through a cost-effective hydrothermal-assisted activation method. The research involved a comparative analysis of BLAC with acid-washed black liquor lignin-derived activated carbon (ABLAC) and commercial lignin-derived activated carbon (SALAC). The analysis revealed that BLAC possesses a well-developed micro and mesoporous structure, yielding a significantly higher surface area of 2277.2 m2/g as compared to ABLAC (1260 m2/g) and SALAC (1558.4 m2/g). The presence of inherent alkali in the black liquor is the main factor influencing the surface area of the BLAC. Furthermore, it demonstrated impressive electrochemical performance, showing a specific capacitance value of 871.4 F/g at 1 A/g current density, positioning it as a formidable electrode material for supercapacitor applications. The proposed direct conversion strategy will eliminate the need for high-temperature pre‑carbonization and additional lignin extraction, reducing chemical usage and presenting a greener approach.

Biodegradable, UV-blocking, and antioxidant films from lignocellulosic fibers of spent coffee grounds

Plastics are strong, flexible, and inexpensive and hence desirable for packaging. However, as they biodegrade very slowly, their waste remains a global burden and pollution, warranting a search for safer alternatives. Towards this end, residual fibers from biowaste, such as spent coffee grounds (SCGs), stand out for creating biodegradable packaging materials. Herein, lignocellulosic fibers from SCG were extracted, and various amounts (0.6, 0.8, 1.0, and 1.2 g) were solubilized using 68 % ZnCl2 and crosslinked with salt (CaCl2) amounts 0.1, 0.2, 0.3 and 0.4 g and prepared biodegradable films. The films were characterized for their color, thickness, moisture content, tensile strength, elongation at break, water vapor permeability, transmittance of electromagnetic radiation, biodegradability, and antioxidant properties. The results reveal that the films possess the highest tensile strength of 26.8 MPa. The tensile strengths are positively correlated to salt and SCG extract amounts. The percentage of elongation decreased with an increase in the calcium ions but increased with SCG residue increment. The films biodegraded in the soil, and most lost >80 % of their initial weight in 45 and 100 days, respectively, at 30 % and 12 % soil moisture. Biodegradability and water vapor permeability decreased with an increase in salt content. Films also showed antioxidant properties and blocked UV and IR radiation significantly. Overall, this research involving green and recyclable chemicals in preparation of SCG residue fibers is a promising, economical, and sustainable route to produce strong biodegradable films to replace petrochemical plastics and thus is an attractive contribution to the circular bioeconomy.

A Comparative Evaluation of Combustion Characteristics of Araucaria cunninghamii, Intsia bijuga and Pometia pinnata for Bio-Energy Source

Burning woody biomass for energy is gaining attention due to the environmental issues associated with fossil fuels and carbon emissions. The carbon released from burning wood is absorbed by plants and, hence, offsets pollution. The purpose of this study was to investigate the combustion characteristics (heat calorific values and ash contents) of three timbers: Araucaria cunninghamii, Instia bijuga, and Pometia pinnata to recommend for fuelwood. The test samples were sawdust particles (treatment) and solid woods (control) extracted from the heartwoods. The sawdust particles were oven dried, sieved, and pelletized into pellets using a hand-held pelletizing device, thus, forming a cylindrical dimension (volume 1178.57 mm3, oven-dry density 0.0008 g/mm3). Meanwhile, the solid woods were cubed and oven dried (volume 1000.00 mm3, oven-dry density 0.001 g/mm3). Prior to combustion in a semi-automatic bomb calorimeter, 90 test specimens (15 replicates per treatment and control per species) were conditioned to 14% moisture content (at a temperature of 105 °C) and weighed to a constant (unit) mass (1.0 g). The heat energy outputs and ash residues (of treatments) were analyzed statistically. The results indicated variability in heat energy outputs and ash residues between the test specimens of the three species. Comparatively, the treatment specimens of A. cunninghamii produced a higher calorific value (18.546 kJ/g) than the control (18.376 kJ/g), whilst the treatment specimens of I. bijuga and P. pinnata generated lower heat calorific values (17.124 kJ/g and 18.822 kJ/g, respectively) than the control (18.415 kJ/g and 20.659 kJ/g, respectively). According to ash content analysis, A. cunninghamii generated higher residues (6.3%), followed by P. pinnata (4.5%), and I. bijuga (2.8%). The treatment specimens of the three species could not meet the standard heat energy requirement (20.0 kJ/g) and, thus, were unsuitable for fuelwood. However, the control specimens of P. pinnata generated an equivalent heat energy (20.659 kJ/g) and could be a potential fuelwood.

Forest Dendromass as Energy Feedstock: Diversity of Properties and Composition Depending on Systematic Genus and Organ

Exhaustion of fossil fuel resources, shrinking forest areas, with accompanying deterioration of their quality and striving (also of the society) to make forests perform their ecological function, with simultaneous development and propagation of the biomass conversion technologies—all of this necessitates research of forest biomass diversification. It is a consequence of the fact that its properties and composition depend not only on the genus but also on the plant organ, and they each time determine its usability as a raw biomaterial in a wide range of thermal, physical, or chemical conversion processes. This study reviewed and analysed selected qualitative and quantitative features of forest dendromass, taking into account the genus and a plant organ/morphological part, followed by a group of trees (coniferous and deciduous) and without the latter differentiation. The study involved an analysis of data covering 15 selected qualitative-quantitative features of forest dendromass within three main and nine additional plant organs/morphological parts and 21 genera (5 coniferous and 16 deciduous) typical of the temperate climate.

Improved Wear-Resistant Performance of Epoxy Resin Composites Using Ceramic Particles

This work investigated the effects of using a new fabrication technique to prepare polymer composite on the wear-resistant performance of epoxy resin composites under dry friction conditions. Polymer composite samples with different weight contents of silicon carbide (SiC) particles were manufactured. This paper addresses the wear behavior of the obtained samples. With the suggested technique, the samples were prepared from epoxy/silicon carbide particles using a layer of thin kraft paper to prevent the sedimentation of the ceramic particles and to control the weight content of ceramic in the polymer. Kraft paper was used as a layer in the polymer composite. The hardness, wear resistance, and water absorption capacity of the produced epoxy composite samples prepared using the kraft paper technique were evaluated. The morphology of epoxy composite samples showed a significant improvement in the ceramic distribution and enhancement of interface bonding between ceramic and the polymer. The hardness values of the developed polymer composites were enhanced by up to 42.8%, which was obtained at 18 wt.% SiC particles. Increasing the ceramic content in the epoxy also led to the enhancement of wear resistance compared with pure epoxy. The results of the microstructure study also showed that the kraft paper layers helped in maintaining the distribution of the ceramic particles according to the previously specified content in each layer in the sample. Wear tests showed that the wear rate of the polymer composite decreased with the increase in the ceramic content. This study provides a new recycling method for using old kraft paper in polymer composite manufacturing to improve the distribution of ceramic particles in the polymer matrix.

Removal of wood extractives as pulp (pre-)treatment: a technological review

Wood extractives usually do not exceed five percent of dry wood mass but can be a serious issue for pulping as well as for the pulp itself. They cause contamination and damages to process equipment and negatively influence pulp quality. This paper addresses not only the extractives-related problems but also different solutions for these issues. It is an extensive review of different technologies for removing wood extractives, starting with methods prior to pulping. Several wood yard operations like debarking, knot separation, and wood seasoning are known to significantly decreasing the amount of wood extractives. Biological treatment has also been proven as a feasible method for reducing the extractives content before pulping, but quite hard to handle. During pulping, the extractives reduction efficiency depends on the pulping method. Mechanical pulping removes the accessory compounds of wood just slightly, but chemical pulping, on the other hand, removes them to a large extent. Organosolv pulping even allows almost complete removal of wood extractives. The residual extractives content can be significantly reduced by pulp bleaching. Nevertheless, different extraction-based methods have been developed for removing wood extractives before pulping or bleaching. They range from organic-solvent-based extractions to novel processes like supercritical fluid extractions, ionic liquids extractions, microwave technology, and ultrasonic-assisted extraction. Although these methods deliver promising results and allow utilization of wood extractives in most cases, they suffer from many drawbacks towards an economically viable industrial-scale design, concluding that further research has to be done on these topics.

Graphical abstract

Energy optimization of Multiple Stage Evaporator system using Water Cycle Algorithm

Black liquor, a residual stream from the Kraft recovery process of paper mills is an incipient biomass energy resource which finds prospective biofuel-based industrial applications to ensure process self-sufficiency and sustainability. Black liquor is concentrated using Multiple Stage Evaporator, the utmost energy intensive unit, before using it as biofuel. Pertaining to the contemporary global energy scenario, improvement in energy efficiency of Multiple Stage Evaporator becomes indispensable. The present work investigates the non-linear modeling and simulation-based optimization of Heptads' stage based Multiple Stage Evaporator in backward feed flow configuration integrated with various energy saving strategies. A novel metaheuristic approach, Water Cycle Algorithm has been employed to search the optimum estimates of unknown process variables and therefore, the optimum energy efficiency parameters. The optimization results demonstrate the efficiency of Water Cycle Algorithm in screening the most appropriate operating strategy, i.e., hybrid model of all energy saving strategies (steam-split, feed-split and feed-preheating) with optimum energy efficiency i.e. Steam Economy of 7.092 and Steam Consumption of 1.919 kg/s. Moreover, a comparative analysis of the results with previous literature and real-time plant estimates reveal that the hybrid model offers improvement of 52.84% in Steam Economy and reduction in Steam Consumption by 28.13% when compared to the real plant data.

Modification of Kraft Lignin with Dodecyl Glycidyl Ether

Kraft lignin (KL) is extensively produced in industry but is mainly burned as fuel. To broaden its use, KL was grafted with dodecyl glycidyl ether to alter its thermal properties. The reaction of KL with dodecyl glycidyl ether (DGE) was analyzed using nuclear magnetic resonance (NMR), Fourier infrared spectroscopy (FT-IR) and elemental analysis. Alternatively, KL was methylated to mask its phenolic hydroxy groups to investigate how phenolic hydroxy groups impact the grafting of the alkyl chain of DGE onto lignin (methylated Kraft lignin, MKL). The methylation facilitated the molecular weight enhancement and thermal stability reduction of Kraft lignin via grafting with DGE. The influence of grafting alkyl chains on the structural and thermal properties of KL and MKL was studied using thermogravimetric analysis and differential scanning calorimetry analysis. Our data suggest that, due to their high molecular weights and lower glass transition temperatures, the produced lignin derivatives may be promising feedstocks for composite production.

Impact of Structural Changes on Energy Efficiency of Finnish Pulp and Paper Industry

A key challenge in prevention of global warming is how to increase energy efficiency, to be able to deal with increased fossil CO2 emissions from rising energy usage. Increasing energy efficiency will decrease energy usage and is in a key role in emission mitigation. The focus is the pulp and paper industry, which is energy-intensive. Development of industrial energy efficiency has been studied before but the role of industrial transformation is still mostly unknown. The knowledge must be improved, to be able to predict future developments in the most effective way. In this research, impact of various production unit closures and start-ups on energy efficiency of the Finnish pulp and paper industry were studied utilizing statistical analysis. Results indicate that about 20% of the Finnish pulp and paper industry energy efficiency improvement between 2011 and 2017 is caused by the major structural changes. The rest, 80% of the progress, was mainly due to improved technology and more optimal operational modes. Additional findings suggest that modern mill start-ups have a significantly greater potential to reduce energy consumption than old mill closures.

The effect of alkali on the product distribution from black liquor conversion under supercritical water

Lignin in chemical pulping waste, or black liquor (BL), can be converted into various products via supercritical water gasification (SCWG). However, the inherited alkaline contents from the pulping chemicals may affect the product yields and properties. In this research, the influence of the residual alkali on the product distribution via SCWG of soda BL and kraft BL was evaluated. The SCWG was performed in a batch quartz reactor for 10 min at various temperatures (673, 773 and 873 K) and pressures (250, 300 and 400 bar). The highest hydrogen (H₂) production occurred at 873 K for the soda BL. The water-gas shift reaction with sodium ions played an important part in the H₂ production, while only small amounts of methane and carbon monoxide were detected. Hydrocarbons, carboxylic acids and esters were the dominant substrates in the liquid products, which denoted the potential of this method for bond cleaving of the lignin macromolecule. As a result, BL, which typically contains alkali salt, was an appropriate feedstock for the SCWG reaction to produce renewable fuel. This method not only has a positive influence on the generation of value added products from highly corrosive waste but also helps avoid some technical problems commonly encountered with direct firing in a recovery boiler.

Caffeic acid production by simultaneous saccharification and fermentation of kraft pulp using recombinant Escherichia coli

Caffeic acid (3,4-dihydroxycinnamic acid) serves as a building block for thermoplastics and a precursor for biologically active compounds and was recently produced from glucose by microbial fermentation. To produce caffeic acid from inedible cellulose, separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) reactions were compared using kraft pulp as lignocellulosic feedstock. Here, a tyrosine-overproducing Escherichia coli strain was metabolically engineered to produce caffeic acid from glucose by introducing the genes encoding a 4-hydroxyphenyllactate 3-hydroxylase (hpaBC) from Pseudomonas aeruginosa and tyrosine ammonia lyase (fevV) from Streptomyces sp. WK-5344. Using the resulting recombinant strain, the maximum yield of caffeic acid in SSF (233 mg/L) far exceeded that by SHF (37.9 mg/L). In the SSF with low cellulase loads (≤2.5 filter paper unit/g glucan), caffeic acid production was markedly increased, while almost no glucose accumulation was detected, indicating that the E. coli cells experienced glucose limitation in this culture condition. Caffeic acid yield was also negatively correlated with the glucose concentration in the fermentation medium. In SHF, the formation of by-product acetate and the accumulation of potential fermentation inhibitors increased significantly with kraft pulp hydrolysate than filter paper hydrolysate. The combination of these inhibitors had synergistic effects on caffeic acid fermentation at low concentrations. With lower loads of cellulase in SSF, less potential fermentation inhibitors (furfural, 5-hydroxymethyfurfural, and 4-hydroxylbenzoic acid) accumulated in the medium. These observations suggest that glucose limitation in SSF is crucial for improving caffeic acid yield, owing to reduced by-product formation and fermentation inhibitor accumulation.

Emerging biorefinery technologies for Indian forest industry to reduce GHG emissions

The production of biofuels as alternative energy source over fossil fuels has gained immense interest over the years as it can contribute significantly to reduce the greenhouse gas (GHG) emissions from energy production and utilization. Also with rapidly increasing fuel price and fall in oil wells, the present scenario forces us to look for an alternative source of energy that will help us in the operation of industrial as well as the transportation sector. The pulp mills in India are one of the many options. The pulp mills in India can help us to produce bio-fuels by thermo-chemical/biochemical conversion of black liquor and wood residues. These technologies include extraction of hemi-cellulose from wooden chips and black liquor, lignin from black liquor, methanol from evaporator condensates, biogas production from waste sludge, syngas production from biomass using gasification and bio-oil production from biomass using pyrolysis. The objective of this paper is to overview these emerging bio-refinery technologies that can be implemented in Indian Forest Industry to get bio-fuels, bio-chemicals and bio-energy to reduce GHG emissions.