Biorefinery lignosulfonates as a dispersant for coal water slurry
© The Author(s). 2016
Received: 27 October 2015
Accepted: 11 May 2016
Published: 2 June 2016
Valorization of lignin from biofuel production is the key to developing biorefinery technologies for sustainable and economic utilization of lignocellulosic biomass. Here we present isolating lignosulfonate from the spent liquors of Sulfite Pretreatment to Overcome the Recalcitrance of Lignocelluloses (SPORL)-pretreated lodgepole pine and Douglas-fir forest residue as a dispersant for coal water slurry. The two SPORL pretreatments were conducted at a pilot scale and resulted in very high ethanol yield from the pretreated biomass. Therefore, demonstrating the commercial utility of these lignosulfonates has practical significance.
The two isolated biorefinery lignosulfonates (LSs), Na-LS and Ca-LS, both had a molecular weight of approximately 9000 Da. Fundamental lignin properties such as chemical structure, functional groups were analyzed. The two LSs showed slightly better to equal performance in modifying CWS rheology than a commercial dispersant naphthalene sulfonate formaldehyde condensate (FDN), despite they were less sulfonated than FDN.
The practical importance of this study is that the pilot-scale pretreatments that produced the two LSs also produced excellent bioethanol yields at high titer without detoxification and washing. This suggests SPORL pretreatment is a promising technology for economic bioconversion of under-utilized woody biomass.
The concept of biorefinery is to mimic petroleum refinery to produce multi-products such fuels, chemicals, polymers from a lignocellulosic feedstock to diversify product portfolio, avoid market saturation, and maximize resource utilization. The sugar platform as a major lignocellulosic biomass conversion pathway relies on the conversion of carbohydrates to sugars for subsequent processing to fuels and chemicals. While it is very attractive because sugars are flexible building blocks for producing a variety of chemicals and products , valorization of the lignin fraction is the key to commercial success because lignin is the second most abundant fraction in lignocelluloses of approximately 15–30 % . Current technologies for the sugar platform rely on a pretreatment (or fractionation) step followed by enzymatic saccharification of the pretreated solids . Depending on the pretreatment process employed, lignin is often fractionated into a soluble fraction in the pretreatment spent liquor and a fraction retained in solids. The current utilization of these two lignin fractions—biorefinery lignin—remains as a low value boiler fuel as practiced in pulp mills, despite substantial research and development efforts have been made in bioconversion of lignocelluloses .
Here, we demonstrate a biorefinery lignin, i.e., the water soluble lignin fraction from Sulfite Pretreatment to Overcome the Recalcitrance of Lignocelluloses (SPORL)  of softwoods—lignosulfonate (LS), as a dispersant of coal water slurry (CWS) without further processing. Coal is an important energy source. Approximately 39 % of the electricity was produced from coal in the U.S. (US Energy Information Administration). CWS was developed in the 1920s in Russia. Due to the shortage of oil supply in the 70s, CWS technologies was further developed as an alternative to liquid fuel in a variety of applications. CWS is a clean technology compared with coal itself which can alleviate many concerns of coal combustion [6, 7]. For example, it can produce high combustion efficiency, low discharge of ash, and lower NOx and SOx air emissions [8, 9]. Typical CWS contains 60–75 % small suspended coal particles in 25–40 % water, and 1 % chemical dispersants. CWS can be directly burned without dewatering . Dispersants play an important role to reduce CWS viscosity and stabilize rheological properties for good atomization and efficient combustion [11, 12]. To meet the potential demands for CWS, several dispersants such as naphthalene sulfonate formaldehyde condensate , sulfonated acetone-formaldehyde , carboxylate type copolymer , cardanol formaldehyde sulfonate , sodium polystyrene sulfonate , sodium dodecyl benzenesulfonate  have been studied. However, lignin based dispersants attracted great attention [18, 19].
The practical significance of this study is the existence of a mature commercial market for CWS dispersant and the valorization of LS from wood biorefinery as a co-product. Furthermore, with the gradual closing of sulfite pulp mills in the last 40 years around the world, there is a shortage of commercial LS products. Some regions rely on a low quality LS derived from sulfonation of kraft lignin—from kraft pulping  to meet market demand. Therefore, LS from SPORL can be a commercially and economically viable co-product for biorefinery.
Results and discussion
FT-IR and 1H-NMR spectra of LSs
Assignments of lignin IR and 1H-NMR spectral bands
IR wavenumber (cm−1)
OH stretching in phenolic and aliphatic structures
C–H vibration in –CH3 and –CH2–
C–H vibration in CH3O–
Conjugated carbonyl groups
Aromatic skeleton expansion vibration
Asymmetric stretching vibration of SO 3 2−
Assignment S=O stretching vibration
C–O–C stretch stretching vibration
1H-NMR chemical shifts (ppm)
H in carboxylic acid
The aromatic proton of the guaiacyl units
aromatic protons of syringyl units
Hα, Hβ, Hγ in β-O-4′, β-5′ and β-β’structure
H in methoxyls
H in phenolic hydroxyl group
H in aromatic acetates
H in aliphatic acetates
The band at 3420 cm−1 relates to the aromatic and aliphatic OH groups in lignin . The peak at 1421 cm−1 confirms the presence of COO-group . The bands at 1190 and 1037 cm−1 are from asymmetric and S=O stretching vibration of SO 3 2− , respectively.
The two LSs were also analyzed by 1H-NMR spectroscopy (Fig. 1b). Chemical shift assignments are listed in Table 1. The regions of the 7.52–6.80 and 6.80–6.50 ppm are detected in the aromatic proton of the guaiacyl units and syringyl units , respectively. The signals at 6.00–4.00 ppm are Hα, Hβ and Hγ in β-O-4′, β-5′ and β-β′ structure . The signals between 3.32 and 3.10 ppm correspond to H in phenolic hydroxyl group . The signals at 2.3–2.1 and 2.1–1.8 ppm are owing to aromatic and aliphatic acetates , respectively.
Dispersant molecular weight and function groups
Functional group contents and molecular weights of three CWS dispersants
Functional group content (mmol/g)
M w /M n
1.44 ± 0.06
1.84 ± 0.07
2.31 ± 0.10
9300 ± 104
7735 ± 85
1.20 ± 0.03
1.19 ± 0.07
1.65 ± 0.08
2.55 ± 0.09
8870 ± 123
7625 ± 64
1.17 ± 0.04
2.24 ± 0.04
8100 ± 76
7700 ± 46
1.05 ± 0.03
FDN had much higher sulfonic acid group content than the two biorefinery LSs, and almost two times of that of Ca-LS (Table 2). However, the two biorefinery LSs also contained phenolic hydroxyl and carboxyl groups. Though Na-LS and Ca-LS were not substantially different, Na-LS was slightly more sulfonated with slightly higher phenolic hydroxyl content and lower carboxyl group content, in agreement with FTIR (Fig. 1a) and 1H-NMR (Fig. 1b) analyses. The uncertainty analysis based on measured quantities in Eq. (1) showed a relative error of propagation of 2 % while the measured relative errors reported in Table 2 were 2–6 %.
Adsorption of dispersants by coal particles
Parameters for predicting dispersant adsorption (at 1.0 wt% dosage) by coal using the Redlich-Peterson model
B × 105 (1/mg)
2.068 ± 0.657
67.2 ± 17.3
1.330 ± 0.343
1.061 ± 0.341
6.21 ± 2.87
1.587 ± 0.639
0.934 ± 0.187
2.76 ± 1.13
1.699 ± 0.592
The two Redlich-Peterson isotherm constants A and B were in the similar ranges for Ca-LS and FDN when fitting errors were taken into consideration (Table 3). This can be clearly seen form Fig. 3. However, Na-LS showed slightly more absorption than FDN despite FDN had a higher sulfonic acid group content (Table 2).
Zeta potential of coal particles in CWS
Viscosity-reducing capacity of dispersants
High value utilization of biorefinery lignin with minimal processing is critical to improve the commercial viability of biofuel production. This study demonstrated two biorefinery LSs directly isolated from the spent liquors of SPORL pretreatment of softwoods as dispersant for coal water slurry. Both biorefinery LSs showed slightly better or equal performance in modifying the rheological properties of CWS compared with a commercial dispersant FDN. Since, the SPORL conditions under which the two biorefinery LSs produced also prodcued excellent sugar and biofuel yields at high titer without detoxification and solids washing, this study further supported the commercial viability of SPORL.
The feedstock and pretreatment conditions used for the production of the two biorefinery LSs along with LS and ethanol yields
Lodgepole pine wood
Douglas-fir forest residue
T and time
165 °C for 60 min
145 °C for 4 h
Chemical loadings on wood
2.2 wt% H2SO4
2.4 wt% free SO2
8.0 wt% NaHSO3
6.5 wt% Ca(HSO3)2
Liquor to wood ratio
Fermentation total solids
Ethanol yield and titer
288 (L/tonne); 52.2 g/L
284 (L/tonne); 41.9 g/L
Excellent ethanol yield at high titer without detoxification in fermentation were achieved from both pretreatments, indicating the LSs produced from these two pretreatments are representative of biorefinery LSs. Separation of LS from the SPORL spent liquors were performed using an in-house pilot plant ultrafiltration (UF) system equipped with single-tube modules each with a separate permeate outlet . The liquors were first centrifuged at 4000 rpm for 20 min to remove solids. Two membranes ES404 and FP200 (Xylem PCI Membranes, Kostrzyn, Poland) that had cut-off molecular weight of 4 and 200 kDa, respectively, were used to remove low molecular weight impurities such as sugars and sugar degradation products (furans and organic acids, etc.) and very fine particular matters.
Naphthalene sulfonate formaldehyde condensate (FDN), a commercial dispersant for CWS from Zhanjiang additive company (Guangdong province, China), was used for comparison study.
Proximate and ultimate analyses of the coal sample on air dried basis
Proximate analysis (wt%)
7.23 ± 0.11
8.02 ± 0.08
35.04 ± 0.16
Ultimate analysis (wt%)
81.35 ± 0.13
4.72 ± 0.04
11.66 ± 0.08
0.88 ± 0.08
0.51 ± 0.04
Fourier transform infrared and hydrogen nuclear magnetic resonance spectra
The two LS samples were analyzed by Fourier transform infrared (FTIR) analysis using a Nicolet 380 FT-IR spectrometer (Thermo Scientific Nicolet, Waltham, MA, USA), as well as by hydrogen nuclear magnetic resonance ( 1H-NMR) spectroscopy using a Bruker DRX-500 spectrometer (Bruker Co., Ettlingen, Germany) at 25 °C. Sample preparation for these analyses was described previously .
LS molecular weight
The molecular weight distributions of the dispersants were determined by aqueous gel-permeation chromatography (GPC) using Ultrahydrogel 120 and Ultrahydrogel 250 columns and UV detection at 280 nm (Waters 2487, Waters Co., MA, and USA). Sodium nitrate was used as mobile phase at a flow rate of 0.50 mL/min. Sodium polystyrene sulfonates with different molecular weights were used as standards for calibration. The uncertainty in calibration was less than 0.05 %.
LS functional group contents
where S is sulfonic acid groups content (mmol/g), C NaOH is the molar concentration of NaOH (mmol/L), V NaOH is the volume (L) of NaOH solution used, m is the mass of the LS sample (g). The pH change was 0.78 through titration.
The p-hydroxybenzoic acid was used as the internal standard and the tetrabutyl aqueous ammonia standard solution was used as the titrant to measure carboxyl group content .
Phenolic hydroxyl content was measured using FC-reagent method . Dried LS of 50 mg was dissolved in 100 mL distilled water in a flask. An aliquot of 15 mL of the LS solution was mixed thoroughly with 1.5 mL of the FC-reagent and then added 5 mL of 20 % (w/v) Na2CO3 solution and adjusted the volume to 25 mL with distilled water. The mixture was kept stirring for 2 h at 30 °C. Absorption measurements at 760 nm were carried out by a spectrophotometer (UV-2450, Shimadzu, Kyoto, Japan). Vanillin solutions were used for calibration.
Determination of adsorption isotherms
The amount of dispersants adsorbed onto CWS was measured by the residual mass fraction method. Firstly, dispersant solutions with different concentrations between 0.2 and 1.2 g/L were added into CWS with coal powder consistency of 10 wt%. Each mixture was mixed on a shaking bed at 200 rpm for 5 h at 25 °C. The mixture was then centrifuged at 10,000 rpm for 10 min. The content of the dispersant in separated solution was measured by a UV spectrophotometer (UV-2450, Shimadzu Corp., Tokyo, Japan) at 280 nm. The amount of dispersant adsorbed was determined through calibration.
Determination of Zeta potential of coal particles
The zeta potential of coal particles was measured using a ZetaPALS analyzer (Brookhaven Instruments, Holtsville, NY, USA). Coal aqueous solutions of 0.2 wt% with different concentrations of dispersant were prepared. After shaking at 200 rpm for 5 h at 25 °C, five replicate samples were taken and analyzed. The averages were reported.
CWS rheological property
The prepared CWS was allowed to stand for 5 min. Measurements of rheological properties were performed by a rotational rheometer (RV I, Haake Corp., Karlsruhe, Germany) with a Z43 measure cup and a Z41 rotor at 25 °C. The shear rate was first ramped up from 0 to 200 s−1 in 3 min and then ramped down in 3 min. All measurements were taken at a shear rate of 100 s−1 during ramping up period. The measured viscosity value was the apparent viscosity.
FG and JYZ conducted pretreatments for lignosulfonates. YQ purified the lignosulfonates and drafted the manuscript. XL conducted viscosity measurements. WX conducted FTIR measurements. DY and JYZ designed study plan and edited manuscripts. DY conducted NMR measurements. All authors read and approved the final manuscript.
This work was supported by a USDA Small Business Innovative Research (SBIR) Phase II project (Contract Number: 2010-33610-21589) to Biopulping International, a USDA National Institute of Food and Agriculture (NIFA) competitive Grant (No. 2011-68005-30416) through the Northwest Advanced Renewables Alliance (NARA), the International Science and Technology Cooperation Program of China (ISTCP): 2013DFA41670, the National Natural Science Foundation of China 21436004, and the Chinese Scholarship Council. The funding from these programs made the visiting appointments of Qin and Gu at the USDA Forest Products Laboratory (FPL) possible.
This work is conducted on official government time of Zhu while Qin and Gu are visiting scientists at the USDA Forest Products Lab.
Zhu is a co-inventor the SPORL process.
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