StExCell: novel steam-explosion-based biorefinery concept for dissolving pulp production

2.2.1 Steam explosion

SE experiment was performed in a pilot reactor system (constructed by the workshop of School of Chemical Engineering, Aalto University Finland) consisting of a 10-L reactor vessel heated by an oil-jacket and connected to a 10-L steam generator (for chip steaming and direct heating) and a 10-L collection tank (for rapid pressure relief, i.e. explosion). A representative steam explosion experiment is illustrated in Figure 2. The reactor system was controlled by the eLabs program from eLabs Oy Engineering, Rauma, Finland. The SE was conducted either uncatalyzed (henceforth referred to as SE) or catalyzed by oxalic acid (henceforth referred to as CSE). A reference vapor-phase prehydrolysis (PH) was also conducted in the same reactor system. The experimental conditions are summarized in Table 1 and 2. Chronologically, the experiments with 5 and 10 min (CSE 1–4, SE 1–3 and SE 7) were conducted as mapping trials to preliminarily investigate the extent of hemicellulose removal and cellulose degradation, based on which the remaining experiments (SE 4–6) were performed to optimize the reaction time.

Figure 2:

A representative temperature-intensity (P-factor) profiles of steam explosion and reference prehydrolysis experiments with a P-factor of 1,250. The isothermal time at maximum reaction time for steam explosion (210 °C) was 7 min and for prehydrolysis (170 °C) was 115 min.

The intensity of the pretreatment (steam explosion or pre-hydrolysis) is expressed by P-factor, which unify the prehydrolysis time and temperature as a single variable in a time-integral computed Arrhenius-type equation using experimentally determined average activation energy and frequency factor for the cleavage of glycosidic bonds of the carbohydrate material in biomass (Sixta 2006a).

In uncatalyzed SE experiment, the frozen birch chips were loaded into the reactor at least 12 h before the reaction for thawing. In oxalic-acid-catalyzed SE, the chips were submerged in 7 L of 1.25 wt.% oxalic acid solution for 3 days with occasional mixing to ensure a homogenous distribution of oxalic acid on the wood. The wood chip suspension was then dewatered by centrifugation to remove the free liquid. The oxalic acid in the entrapped liquid within the chips corresponded to 1 wt.% of the wood chip dry mass (determined by mass balance of a preliminary submerge-dewatering trial, assuming negligible Donnan effect). The acidic wood chips were then loaded into the reactor.

The biomass was heated to reaction temperature with the combination of oil-jacket heating and steam, fed from the steam generator to a liquid-to-wood ratio (L:W) of 1.5 and held for the pre-defined isothermal reaction time before releasing the steam to the receiving tank which was connected via a cooling coil to a bottle at atmospheric pressure for collecting the condensate. Due to inherent hardware limitation, the pressure took 4–6 s to drop from maximum pressure (28 bar at 230 °C and 19 bar at 210 °C) to 4 bar and another 10–20 s to atmospheric pressure, which is slower than a typical industrial steam exploder. However, such pressure relief should still induce sufficient stress to the biomass cell-wall, resulting in more accessible structure for subsequent treatments.

Immediately after the reactor pressure reached atmospheric level, 5 L of ice-cold water was pumped into the reactor from the top, effectively serving as the first washing stage, as well as quenching the reaction. When the reactor temperature reached 70 °C, the raw washing filtrate was drained from the bottom valve and the reactor was opened. The solid residue, henceforth referred to as “steam-exploded wood”, was simultaneously washed and defibrated by disintegrating into an excessive amount of boiling water in a stainless-steel mixer. The washing suspension was then centrifuged to collect the washed steam-exploded wood, which was subsequently homogenized in a dough mixer (Metos Karhu RN20) and stored in sealed plastic bags at 4 °C. The collected centrifuged liquid was combined with the raw washing filtrate and the condensate, henceforth referred to as “SE-filtrate”, and stored at −18 °C.

2.2.2 Total chlorine free purification of steam-exploded wood

Steam-exploded wood produced from experiment SE 4 (uncatalyzed SE at 210 °C for 7 min, equivalent to a P-factor of about 1,250) was further purified by E−, OO-, Z- and P- stages to produce a fully bleached dissolving pulp. The E−, OO- and P-stages were conducted at medium consistency (10 %) in the 2.5-L autoclaves heated in an air-bath reactor (Haato-tuote 16140–538). The Z-stage was conducted at high consistency (>30 %) in a rotary glass reactor connected to the Wedeco GSO30 generator which produced ozone from pure oxygen.

After each purification stage, the pulp was washed with excessive hot water in plastic bucket by manual mixing. Following the W2 and W3 stages (post-E and post-O washing, Figure 1), the pulp was screened in a table-top screener (G.A. Serlachius A.B., model 16140–567) with slotted//0.35 mm and//0.17 mm mesh-opening, respectively, for reject removal. The washing suspension was then centrifuged to collect the filtrates (named according to the stages, Figure 1), which was stored at −18 °C, and the washed pulp, which was subsequently homogenized in a kitchen blender (Bosch Multitalent 8) and stored in sealed plastic bags at 4 °C.

The oxygen delignification conditions were adapted from the laboratory knowhows and previous works conducted at Department of Bioproducts and Biosystems, Aalto University (Granatier et al. 2023; Jafari 2015). The caustic extraction and ozone treatment stages were first optimized (Supporting Material 1), from which the selected conditions were applied on a newly produced steam explosion pulp batch with the SE 4 recipe (which is the source of minor deviations in the pulp yield and properties between Table 4, Table 5, Table S1 and Table S2).

2.3.1 Pulp samples

The carbohydrate and lignin contents of the pulp samples were analyzed in accordance with the 2-step sulfuric acid hydrolysis method described in the NREL/TP-510-42618 standard. The hydrolyzed suspension was then filtered through a Robu^® glass crucible (porosity 4) to gravimetrically determine the acid insoluble (Klason) lignin fraction. The acid soluble lignin content of filtrate was quantified by measuring the absorbance at 25 °C at the wavelength of 205 nm (Shimadzu UV-2550 spectrophotometer) with an extinction coefficient of 110 L/(g.cm) (Swan 1965). The monosaccharide content in the filtrate was determined by high-performance anion-exchange chromatography (HPAEC) in a Dionex™ ICS-3000 device. The HPAEC system was equipped with an amperometry cell detector and a CarboPac™ PA20 column (3.0 mm × 150 mm). The column and detector were at 22 °C. The eluent was water with a flowrate of 0.37 mL/min. The samples were filtered through a 0.22 μm syringe filter before the analysis. From the amount of neutral monosaccharides, the cellulose and hemicelluloses content in wood and pulp samples was estimated with the Janson formula (Janson 1970).

The pulp samples were analyzed for kappa number and intrinsic viscosity in accordance with the SCAN-C 1:00 and SCAN-CM 15:88 standards, respectively. The intrinsic viscosity of lignin-containing samples is reported as lignin-free intrinsic viscosity, assuming a negligible contribution of lignin to the viscosity of the pulp solution in cupriethylenediamine, calculated by dividing the intrinsic viscosity value obtained from the standard measurement by (100 %-lignin content of the pulp). For validation of the correction calculation, selected lignin-rich pulp samples were bleached with sodium hypochlorite in acetic acid (Wise method) (Wise and D’ Addieco 1946) for the selective removal of lignin before intrinsic viscosity measurement and the values obtained from Wise-treated pulp and untreated pulp with lignin-free correction were reasonably comparable.

The molar mass distribution (MMD) of the pulp samples was determined by size exclusion chromatography (SEC). Prior to the analyses, the samples were subjected to the water/acetone/N,N-dimethylacetamide (DMAc) solvent exchange sequence. The pulp in DMAc was dissolved in 90 g/L LiCl containing DMAc at room temperature and under constant gentle stirring. The samples were then diluted to 9 g/L LiCl/DMAc, filtered with 0.2 μm syringe filters, and fed to a Dionex Ultimate 3000 HPLC system equipped with four PLgel MIXED-A 7.5 × 300 mm columns and a Viscotek/Malvern SEC/MALS 20 multi-angle light-scattering (MALS) detector. The eluent was 9 g/L LiCl/DMAc with a flowrate of 0.75 mL/min. A narrow (low polydispersity index, PDI) polystyrene standard (Mw = 96[thin space (1/6-em)]000 g mol⁻¹, PDI = 1.04) was used to obtain the detector constants for MALS and DRI, whereas a broad polystyrene sample (Mw = 248[thin space (1/6-em)]000 g mol⁻¹, PDI = 1.73) was applied to test the calibration of the detectors. A ∂n/∂c value of 0.136 mL/g was reported for cellulose dissolved in 9 g/L LiCl/DMAc (Potthast et al. 2015). The distribution of the cumulative weight fractions over the entire molar mass range was also evaluated to calculate the weight fractions with degree of polymerization (DP) <100 and DP > 2000.

2.3.2 Liquid samples

The carbohydrate content in the liquid samples were analyzed in accordance with the method described in the NREL/TP-510-42623 standard, by the same HPAEC system described in Section 2.3.1. The monomeric sugar content was determined by direct injection in the HPAEC, while the total sugars content was determined by HPAEC after hydrolysis in 4 % H₂SO₄ at 121 ± 1 °C for 60 min. The lignin content in the liquid samples was determined by UV-Vis spectrophotometry (Shimadzu UV-2550) at 25 °C by dilution in 50 wt.% ethanol and measuring the absorption at a wavelength of 205 nm, with an extinction coefficient of 110 L/(g.cm) (Swan 1965).

The content of furanic compounds (furfural and hydroxymethylfurfural) and organic acids (formic acid, acetic and levulinic acid) in the liquid samples was determined by high-performance liquid chromatography (HPLC) in a Dionex UltiMate 3,000 system equipped with a UV diode array detector and a HyperREZ XP Carbohydrate Ca+ column (7.7 × 300 mm). The UV detection wavelength was 210 nm and 280 nm for organic acids and furanic compounds, respectively. The column and detectors were at 70 °C and 55 °C, respectively. The eluent was 0.0025 mol/L H₂SO₄ with a flowrate of 0.5 mL/min. The samples were filtered through a 0.22 μm syringe filter before the analysis.

2.4.1 Ionic liquid and cellulosic spinning solution preparation

The [DBNH][OAc] (1,5-diazabicyclo[4.3.0]non-5-ene acetate) was prepared by neutralization of DBN with an equimolar amount of acetic acid (OAc, glacial, 100 %, Merck, Germany) at 75 °C under stirring. The mixture was subsequently stirred for another 30 min to ensure complete conversion.

A 13 wt.% cellulosic spinning solution (dope) was prepared by dissolving the pulp in [DBNH][OAc] in a vertical kneader operating at 75 °C under a vacuum of 10–20 mbar for a duration of 2 h. Subsequently, the dope was filtered using a pre-heated hydraulic press filtration system with a 5–6 µm mesh metal filter fleece to remove the undissolved cellulose fibers at 150–200 bar.

2.4.2 Rheology measurement of the spinning dope

The viscoelastic properties of the cellulose dope were assessed using an Anton Paar MCR 302 rheometer employing a 25 mm plate-plate geometry and 1 mm measuring gap. A dynamic frequency sweep was conducted from 100 to 0.01 s⁻¹ to acquire data on the complex viscosity (η*), as well as the dynamic moduli (G′, G″). The frequency sweeps were performed over a temperature range of 50–90 °C, with intervals of 5 °C. The zero-shear viscosity, η°, was determined by fitting the complex viscosity data to the Cross-viscosity model, assuming that the Cox-Mertz rule is valid.

2.4.3 Fiber spinning

2.4.4 Testing of regenerated fibers

The mechanical properties of the cellulose fibers were determined by using a Favigraph automatic single-fiber tester (Textechno H. Stein GmbH & Cpo, Germany) based on the ISO 5079:2020 standard (20 mm gauge length, 0.06 cN/tex pretension, 20 mm/min test speed). The fibers were conditioned overnight (20 ± 2 °C, 65 ± 2 % relative humidity) before the testing. For each sample, 10−20 fibers were tested at 20 °C and 65 % relative humidity.

2.4.5 Fiber finishing, yarn spinning and fabric knitting

The air-dried fibers produced from the multi-filament spinning were immersed in an aqueous finishing solution (consisting of Afilan CVS and Leomin PN with a mass ratio of 80:20) at 50 °C for 5 min to reduce the fiber friction and static properties. The targeted amount of spin finish was 0.25 % based on the mass of dry fibers and the sample to liquid ratio was 1:20. The excess water was removed by hand pressing to reach fourfold of its oven-dry weight and was then air-dried. Afterward, the spin-finished fibers were opened with a fiber opener (Trash Analyzer 281C Mesdan Lab, Mesdan S.p.A. Italy).

Yarn spinning was performed as described by Michud et al. (Michud et al. 2016). Firstly, the staple fibres were treated with target amount of 0.25 % m/m_fibre of lubricant; 80/20 % of Afilan CVS (ARCHROMA, Switzerland) and Leomin PN (ARCHROMA, Switzerland), respectively. The fibres were submerged into a solution (liquor ratio of 1:20) comprising Afilan CVS (0.677 g L⁻¹) and Leomin PN (0.233 g L⁻¹) and heated at 45 °C for 5 min (manually stirred). Then, the fibres were pressed to reach 300 % pick-up and air dried. Next the fibres were manually opened and left overnight in a conditioned room (65 % relative humidity; 20 °C). The opened fibres were carded (carding machine 337A, Mesdan SpA, Italy) and rolled into sliver using Stiro lab 3,371 (Mesdan SpA, Italy) for roving, stretching and doubling. The sliver was drafted twice, and in the second drafting the sliver was doubled by folding in half. Finally, the doubled sliver was again folded and roving was produced. The roving was then ring spun (Ring Lab 82BA; SER.MA.TES srl, Italy) into a 20-tex yarn (Nm 50, Ne 30). The yarn had Z torsion and the number of twists per meter (TPM) was 700. The calculated twist multiplier expressed in Ne was 3.4. The linear density of the spun yarn was determined from 10-m skeins (n = 10; expressed in tex).

The yarn was plied (without any torsion) for knitting with the jersey pattern by the Lab-knitter 297E (Mesdan SpA, Italy).

3.2.1 Caustic extraction

Caustic extraction of the steam-exploded wood SE 4 at 80 °C and with a moderately low NaOH charge of 100 kg/t of steam-exploded wood (corresponding to a concentration of ca. 11 g NaOH/L) could remove 85 % of the residual lignin, 50 % of the residual hemicelluloses and <5 % of cellulose (the lower molecular weight fraction) from the steam-exploded wood, yielding a pulp with a kappa number about 45 and a lignin-free viscosity over 800 mL/g (Table 4), which is potentially suitable for subsequent oxygen delignification and TCF bleaching. Increasing the alkali charge to 200 kg NaOH/t of steam-exploded wood only slightly improved the hemicellulose extraction without any visible enhancement on delignification. In comparison with steam explosion, caustic extraction with 100 kg NaOH/t on prehydrolyzed wood (170 °C, P-factor 1,250) yielded significantly more rejects (6.1 % vs 1–2 %, Table 4) and a pulp with 50 % more lignin and twice the hemicellulose content. The enhanced porosity and accessibility of the steam-exploded wood evidently facilitated the removal of hydrolyzed component during mild caustic extraction, thus resulting in a substantial decrease in the reject proportion (Table 4). The cellulose yield loss in prehydrolysis-caustic extraction was similar (about 5 %), but the remaining cellulose exhibited a considerably higher DP (intrinsic viscosity of about 1,300 mL/g).

Martín-Sampedro et. al (2014) conducted single- or double-staged steam treatments without explosion (prehydrolysis) and mild steam explosion, all at T_max = 183 °C, on Eucalyptus globulus, followed by kraft pulping. Even though the cellulose fraction in both pulps (indicated by the intrinsic viscosity) was relatively comparable, due to improved structural accessibility compared to the equivalent non-exploded steam treated-kraft pulp, the delignification on steam-exploded-kraft pulp was progressively enhanced with increasing treatment intensity (Martin-Sampedro et al. 2014). The current study employed a harsher pretreatment (P-factor 1,250 for SE4 and PH experiments versus P-factor 350 for 13 min at 183 °C) and a more violent explosion (19 bar at 210 °C versus 10 bar at 183 °C) so that a proper pulp could be produced by applying a mild alkali extraction after steam explosion. Without a strong nucleophile (Na₂S) and higher temperature (160 °C) of a kraft cook, the pretreatment efficiency of steam explosion and a standard vapor-phase prehydrolysis was distinguishably amplified.

The pulp produced from the caustic extraction with 100 kg NaOH/t of steam-exploded wood, denoted as SE-E₁₀₀, was selected for the subsequence oxygen delignification and TCF bleaching.

3.2.2 Oxygen delignification and total-chlorine-free bleaching

The optimization of the TCF sequence is demonstrated in detail in Supporting Material 1. The rejects collected in the post-oxygen screening (slotted//0.17 mm screen type) was less than 0.1 % on original wood. As expected from the underperformance of prehydrolysis against mild steam explosion with comparable treatment intensity (Table 4), continuation to oxygen delignification after caustic extraction followed similar trend. The PH-E-OO pulp was less degraded (intrinsic viscosity ∼1,050 mL/g versus ∼695 mL/g), less delignified (kappa number ∼14.5 versus ∼7.0) and darker (ISO-brightness ∼37 % versus ∼ 57 %) compared to the equivalent SE-E-OO pulp (Table S1 of Supporting Material 1). Due to such large differences in pulp quality after oxygen delignification, no further bleaching trial was conducted with the PH-E₁₀₀-OO pulp and the StExCell process was subsequently benchmarked with a Visbatch^® kraft pulping process followed by oxygen delignification and Z-P bleaching (Sixta 2006a) (Table 5).

Oxygen delignification of SE-E₁₀₀ pulp was effective with >80 % κ reduction (Table 5) and an intrinsic viscosity loss of 3.8 mL/g per % brightness gain (Figure 3), which can be attributed to the enhanced accessibility of the pulp produced from steam explosion and presumably a low content of condensed lignin structure. Due to the higher residual lignin content after oxygen delignification (κ = 7.6 of SE–E–OO pulp versus κ = 2.6 of PHK-O pulp), higher chemical charges were required to produce a fully bleached StExCell pulp (Table 5). However, both pathways (PHK–O–Z–P and SE–E–OO–Z–P) arrived at the same pulp specification target of 3.6 % residual xylan, 90 % brightness and approximately 420 mL/g intrinsic viscosity.

Figure 3:

Selectivity of total chlorine free purification of pulp produced from uncatalyzed steam explosion (210 °C, 7 min, equivalent to a P-factor of about 1,250) of Betula pendula wood chips, in comparison to a reference Visbatch^® PHK pulp (adapted from Sixta 2006a, chapter 4.2.7.2.3). (a) Intrinsic viscosity drop against brightness gain and (b) yield loss against brightness gain. The blue square is the steam explosion route while the red circle is the PHK one. The xylan content of bleached pulps produced from both pathways was 3.6 % on pulp.

The yield of bleached pulp was significantly higher in steam explosion than in PHK treatment (Figure 3, Table 5), which is because alkaline hydrolysis and secondary peeling reactions are insignificant in mild caustic extraction, whereas carbohydrate losses are much more pronounced in a more concentrated alkali environment with a significantly higher treatment temperature, such as in kraft pulping (Sixta 2006a).

Despite similar intrinsic viscosity, the molar mass distribution of the bleached steam-exploded pulp was broader than that of the TCF-bleached Visbatch^® PHK pulp and the commercial ECF-bleached PHK pulp, with PDI of 4.4, 3.0 and 2.8, respectively (Figure 4), indicating a higher proportion of degraded (lower molecular weight), or so-called beta-cellulose. However, the bleached Visbatch^® pulp were not analyzed by the same SEC system and in the same sequence as the bleached StexCell and bleached commercial PHK pulp, therefore, direct comparison should be assessed with caution.

Figure 4:

Molar mass distribution of the bleached steam-exploded pulp (SE–E–OO–Z–P), in reference to the bleached Visbatch^® PHK pulp (adapted from Sixta 2006a, chapter 4.2.7.2.3) and a commercial elemental chlorine free-bleached pulp. All the pulps are produced from Betula pendula.

In traditional regenerated cellulosic fibers production by the viscose process, bleached SE pulp would result in lower fiber yield due to the removal of lower Mw carbohydrate fractions during steeping in concentrated NaOH. However, such yield loss does not occur in direct dissolution spinning technologies such as lyocell (NMMO or ionic liquid), but the broader MMD would impair, to certain extent, the mechanical strength of fibers (which is further discussed in Section 3.3).

3.3.1 Rheology of the cellulosic spinning solution

Oscillation measurements revealed the viscoelastic properties of the cellulose solutions in a specific temperature range. A dynamic frequency sweep was performed to measure the time-dependent behavior of the spinning dopes in the angular frequency range of 0.01–100 s⁻¹. In practice, the parameters complex viscosity, complex moduli (G’ = G’) and angular frequency at the cross-over point (COP) proved to be relevant for assessing the viscoelastic properties of the spinning solution.

The complex viscosity of the StExCell dope is higher over the entire temperature range than that of the reference dope, which explains the 5 °C higher spinning temperature of the StExCell dope in order to operate at comparable complex viscosities (23,250 Pa s at 70 °C for StExCell dope versus 22,400 Pa s at 65 °C for the reference dope). The angular frequency at COP is characteristic of the mass-weighted molar mass of the solute. Although the intrinsic viscosity of the SE–E–OO–Z–P pulp (425 mL/g) is slightly lower than that of the reference pulp (434 mL/g), the angular frequency is slightly higher at COP over the entire temperature range, which can be explained by the higher proportion of long molecules (DP > 2000) of SE–E–OO–Z–P pulp (12.8 wt.%) than that of the reference pulp (8.0 wt.%), as demonstrated in Figure 4 and Table S2 of Supporting Material 1. The rheological behavior of polymer solutions is disproportionately determined by the long-chain molecules. Therefore, an increase of spinning temperature by 5 °C would match the angular frequency at COP of the StExCell dope to that of the reference dope.

The complex moduli at COP, G’ = G’ of the StExCell and reference dopes exhibited considerable difference, which can be attributed to the significantly different PDI of the cellulose MMD. The cellulose molecules of the SE–E–OO–Z–P pulp are significantly more widely distributed than those of the reference pulp (Figure 4). At ca. 2,500 Pa, the complex moduli at COP of the StExCell spinning solution was just within the easily spinnable range (Michud et al. 2015). Further optimization shall aim to reduce the PDI of the SE–E–OO–Z–P pulp by avoiding excess degradation of long chains during steam-explosion in order to reach the range of 3,000 Pa for the dynamic modulus in the COP.

3.3.2 Production of Ioncell^® fibers

The bleached steam-exploded pulp (SE–E–OO–Z–P) exhibited relatively good spinnability during dry-jet wet spinning. The Ioncell^® fibers were successfully produced and subsequently converted into yarns (Figure 6). As the Ioncell^® spinning of StExCell pulp was not the focus of this study, but served as a proof-of-concept, only a few spinning trials were conducted, whereby a stable draw ratio of 6 was achieved. The fibers were therefore slightly thicker than standard fibers with a titer of 1.3–1.7 dtex and also exhibited a slightly lower fiber tenacity than the Ioncell^® fibres produced from standard PHK pulp, which were, however, still higher than commercial fibers such as lyocell and viscose fibers (Table 6). In a continuation research, the spinning parameters shall be optimized in order to obtain fibers with a standard titer and a slightly higher fiber strength.

Figure 6:

Ioncell^® fibers (left) and yarns (right) produced from the bleached steam-exploded pulp (SE–E–OO–Z–P).

3.4.1 Chemical usages and chemical recovery cycle

The extremity of the steam explosion treatments resulted in a more accessible fiber structure, thus facilitating a successful subsequent caustic extraction with a relatively low alkali charge. With a higher kappa number, the steam explosion-caustic extraction pulp consumed almost three-fold of bleaching chemical to reach similar dissolving pulp quality. However, due to the lack of a concentrated alkali stage, the overall chemical consumption of StExCell concept is only half of that of a TCF PHK process (Table 7), more notably, no sulfur-containing chemical (such as Na₂S) or pulping catalyst (such as anthraquinone) were employed.

Even though the chemical recovery of the proposed StExCell process has not been systematically investigated and is therefore not the subject of this article, the lack of a sulphur-based delignification agent and the significantly lower total alkali requirement suggest a simpler and smaller chemical recovery system with significantly lower investment costs.

For example, NaOH recovery could be accomplished with a small soda boiler instead of a capital-intensive kraft recovery boiler system. A fraction of the NaOH can be hypothetically recycled directly into the circuit by ultrafiltration from the E-filtrate and O-filtrate, so that the soda boiler required to recover the remaining NaOH can be even smaller.

3.4.2 Mass balances and potential valorization of the extracted wood components

The yield loss in StExCell process mainly occurred in the steam explosion and caustic extraction stages (Table 5). The identification, quantification, isolation and valorization of those extracted biomass components shall naturally be focused on the filtrates from those two stages. Similar to any other pulping methods, the bleaching filtrates (Z- and P-) are generally too diluted to be recycled beyond treatment in the wastewater treatment plant.

The valorization of the extracted wood components in the SE-filtrate and the E-filtrate is the subject of further research. However, some treatment pathways are speculated and discussed in this chapter.

Unlike kraft pulping, where the concentrated alkali environment and high temperature promote the decompositions of carbohydrates through peeling reaction, alkaline hydrolysis and retro-aldol fragmentation to various hydroxy carboxylic acids and a wide variety of C2 and C3 fragments (Sjöström 1993; Sixta 2006a), which are unrecoverable and combusted in the recovery boiler, the degraded and extracted carbohydrates in StExCell were relatively well detected with over 80 % of the original wood hemicelluloses collected as furfural, monomeric and oligomeric sugars in the SE-filtrate (Figure 7). A small fraction of wood lignin was removed into the SE-filtrate, most probably as low Mw and sticky lignin, similar to that of prehydrolysis stage (Leschinsky et al. 2008a,b), and should be removed before valorizing the extracted carbohydrate-based components, for example by activated carbon adsorption (Gütsch and Sixta 2011). The remaining carbohydrates (mainly xylose and xylan) can be isolated by quantitative hydrolysis to monomeric forms followed by crystallization (Granatier et al. 2023) or catalytic conversion into furfural, which can be distilled for purification (Karinen et al. 2011; Metkar et al. 2015).

Figure 7:

Extracted (calculated from the composition of the pulp) and recoverable (quantified by the analyses of the liquids) fractions of the steam explosion and caustic extraction stages.

Most of the extracted lignin was collected in the E-filtrate, which can be valorized as sulfur-free lignin precipitate via the ultrafiltration route and as energy recovered by a soda boiler as discussed in Section 3.4.1.