Process-integrated, continuous, separation of lignin from kraft pulp mills which results in a ligninwith low ash content (0.05-1 %) is relatively new technology compared to traditional kraft pulping subprocesses,like recovery boilers, lime kilns, white liquor preparation etc. The LignoBoost technology wasdemonstrated in 2007 and the first commercial full-scale process started in 2013, delivered to Domtarby Valmet. This means that this concept now has been in commercial scale operation for almost 10years. There is also an alternative commercialized concept available today from another supplier,NORAM International. The idea to integrate lignin separation into a kraft pulp mill is today quite provencommercial technology.This paper will discuss different learnings, experiences, from the early development of theLignoBoost process, which includes handling of the separated lignin followed by drying. The dustexplosion risk is relatively high for dry kraft lignin due to a high kst value – so the paper will also discussif there are opportunities to reduce this risk. The paper will also briefly discuss promising productsegments for kraft lignin.

Abstract: Lignin, a cheap renewable natural polymer, can be used as a precursor for the production of carbon fibres, its conversion into which is significantly faster than that of polyacrylonitrile. Lignin can be fractionated in various solvents via dissolution to decrease its polydispersity. Fractions with a higher molecular weight distribution can then be used in solvent-based spinning technologies such as electrospinning. We selected several solvent systems according to the Hansen solubility theory and subsequently tested them for solubility and electro-spinability. The selected solvent systems were then successfully tested for use in the needleless electrospinning process due to their potential for mass production. The solutions used in the electrospinning process needed high concentrations of lignin, which led to a high degree of viscosity. Therefore, we measured the relaxation times and viskosity for selected solutions, a factor that plays a pivotal role in terms of the production of smooth fibres. Finally, these solutions were tested for electrospinning using alternating current. This technology brings a new possibility in mass production of lignin fibres due to its high productivity and ease of use. Such materials can be used in a number of applications such as batteries, supercapacitors or for the production of composite materials. They provide a cheap and renewable natural polymer source which can easily be transformed into a carbon nanofibrous layer. Graphic Abstract: [Figure not available: see fulltext.].

The process of stabilization is essential in the production of carbon fibers from lignins. During stabilization, the initially thermoplastic lignin polymer is converted to a thermoset polymer allowing for high-temperature treatment without a change in shape. In this work, hardwood (HKL) and softwood (SKL) Kraft lignins were stabilized in air at temperatures between 190 and 340 °C before carbonization at 800 °C in a nitrogen atmosphere. Due to the differences in side-chain linkages, functional groups and molar mass, the lignins exhibit different structural changes upon stabilization and hence develop different porosities upon carbonization. Both lignins undergo major crosslinking reactions in the side chains at low temperatures and degradation reactions at high temperatures during stabilization. Crosslinking gives rise to narrow pore size distributions with mainly (sub-) nanometer pores, whereas degradation reactions lead to a more open pore structure with additional mesoporosity (>2 nm). When both types of reactions take place simultaneously, highly accessible (sub-) nanoporosity can be effectively created, which boosts the performance of supercapacitors operating in 6 M KOH(aq). This effect terminates when the crosslinking reactions cease and mainly degradation reactions take place, which occurs in HKL at 340 °C. SKL shows both a lower degree of crosslinking and degradation and hence develops less specific surface area. The optimum performance in an aqueous alkaline supercapacitor is achieved with HKL stabilized at 310 °C. It shows a specific gravimetric capacitance of 164 F g-1 at 0.1 A g-1 and 119 F g-1 at 250 A g-1 with a capacitance retention of more than 90% after 10 000 cycles.

Redox flow batteries represent a remarkable alternative for grid-scale energy storage. They commonly employ carbon felts or carbon papers, which suffer from low activity towards the redox reactions involved, leading to poor performance. Here we propose the use of electrospun freestanding carbon materials derived from lignin as alternative sustainable electrodes for all-vanadium flow batteries. The lignin-derived carbon electrospun mats exhibited a higher activity towards the VO2 +/VO2+ reaction than commercial carbon papers when tested in a three-electrode electrochemical cell (or half-cell), which we attribute to the higher surface area and higher amount of oxygen functional groups at the surface. The electrospun carbon electrodes also showed performance comparable to commercial carbon papers, when tested in a full cell configuration. The modification of the surface chemistry with the addition of phosphorous produced different effect in both samples, which needs further investigation. This work demonstrates for the first time the application of sustainably produced electrospun lignin-derived carbon electrodes in a redox flow cell, with comparable performance to commercial materials and establishes the great potential of biomass-derived carbons in energy devices.

We have discovered a very simple method to address the challenge associated with the low volumetric energy density of free-standing carbon nanofiber electrodes for supercapacitors by electrospinning Kraft lignin in the presence of an oxidizing salt (NaNO₃) and subsequent carbonization in a reducing atmosphere. The presence of the oxidative salt decreases the diameter of the resulting carbon nanofibers doubling their packing density from 0.51 to 1.03 mg cm⁻² and hence doubling the volumetric energy density. At the same time, the oxidative NaNO₃ salt eletrospun and carbonized together with lignin dissolved in NaOH acts as a template to increase the microporosity, thus contributing to a good gravimetric energy density. By simply adjusting the process parameters (amount of oxidizing/reducing agent), the gravimetric and volumetric energy density of the resulting lignin free-standing carbon nanofiber electrodes can be carefully tailored to fit specific power to energy demands. The areal capacitance increased from 147 mF cm⁻² in the absence of NaNO₃ to 350 mF cm⁻² with NaNO₃ translating into a volumetric energy density increase from 949 μW h cm⁻³ without NaNO₃ to 2245 μW h cm⁻³ with NaNO₃. Meanwhile, the gravimetric capacitance also increased from 151 F g⁻¹ without to 192 F g⁻¹ with NaNO₃.

Pure eucalyptus Kraft lignin derived carbon fiber mats were produced based on a model workflow. It covers the preparation and characterization of the lignin precursor and the carbon materials and its testing in the final application (supercapacitor). Sequential solvent extraction was employed to produce a eucalyptus Kraft lignin precursor which could be electrospun into lignin fibers without any additives. The fiber formation from low molecular weight lignin is assigned to strong intermolecular interactions via hydrogen bonding and π-π-stacking between individual lignin macromolecules which gives rise to association complexes in the electrospinning solution. By stabilization in air, carbonization in N₂ and an activation step in CO₂, free-standing microporous carbon fiber mats could be produced. These fiber mats possess mainly basic oxygen functional groups which proved to be beneficial when tested as free-standing electrodes in symmetric supercapacitors. Consequently, the CO₂-activated fiber mats showed a high specific gravimetric capacitance of 155 F/g at 0.1 A/g, excellent rate capability with 113 F/g at 250 A/g and good capacitance retention of 94% after 6000 cycles when tested in 6 M KOH electrolyte. Therefore, we conclude that lignin itself is a promising precursor to produce microporous, oxygen functionalized carbon fibers serving as free-standing electrodes in aqueous supercapacitors.

Sodium ion batteries (SIBs) are emerging as an alternative battery technology to lithium ion batteries because they have the potential of having a similar energy density and the advantage of sodium being more environmentally friendly than lithium. Hard carbon has been shown to be one of the best candidates as anode material for SIBs. However, several challenges need to be solved before commercializing SIBs such as finding cheaper and more efficient precursors to produce hard carbon and increasing the stability of hard carbon electrodes with the electrolyte. Herein, we report a new bio-based free standing electrode made from lignin based electrospun carbon fibers (LCFs) with a high specific capacity of 310 mAh.g−1 and a first coulombic efficiency of 89%. By using high precision coulometry on the LCFs at different carbonization temperatures, it was found that the cycling stability was dependent on the carbonization temperature. The results show that LCFs are a viable and renewable source to be used as anodes in future SIBs. © The Author(s) 2019.

Lignin-based carbon fibers (LCFs) from the renewable resource softwood kraft lignin were synthesized via oxidative thermostabilization of pure melt-spun lignin and carbonization at different temperatures from 1000°C to 1700°C. The resulting LCFs were characterized by tensile testing, scanning electron microscopy (SEM), X-ray diffraction (XRD) and confocal Raman spectroscopy. The microstructure is mainly amorphous carbon with some nanocrystalline domains. The strength and stiffness are inversely proportional to the carbonization temperature, while the LCFs carbonized at 1000°C exhibit a strength of 628 MPa and a stiffness of 37 GPa. Furthermore, the application potential of LCFs was evaluated as negative electrodes in a lithium-ion battery (LIB) by electrochemical cycling at different current rates in a half-cell setup. The capacity drops with the carbonization temperature and the LCFs carbonized at 1000°C have a capacity of 335 mAh g-1. All LCFs showed good cycling stability. Because of the mechanical integrity and conductivity of the LCFs, there is no need to apply current collectors, conductive additives or binders. The advantage is an increased gravimetric energy density compared to graphite, which is the most common negative electrode material. LCFs show a promising multifunctional behavior, including good mechanical integrity, conductivity and an ability to intercalate lithium for LIBs.

Research has been carried out at RISE Bioeconomy into converting lignin to carbon fibres, focusing on melt spinning of kraft lignin to filaments, which were subsequently converted to carbon fibres. New equipment has been installed to improve the properties of the lignin based carbon fibres and now it is possible to produce carbon fibres with equal or even better properties compared with published data on lignin based carbon fibres. For a melt spun softwood kraft lignin that was converted to carbon fibres, the tensile strength had an average value of 952MPa, while the Young's modulus average value was 69GPa. A toy car demonstrator has been produced in a project financed by the Bioinnovation, a research programme by VINNOVA, Formas and the Swedish Energy Agency. The existing roof of the car was replaced with one consisting of a carbon fibre composite, in which the carbon fibres were made of 100% softwood kraft lignin. A lithium ion battery as also installed into the toy car, having a negative electrode containing lignin based carbon fibres