Green Liquor Research Articles

Monitoring the free lime content in lime mud using zeta potential

The presence of unreacted lime (free lime) in the lime mud can cause many problems in the chemical recovery operation of kraft pulp mills. There are currently no standards for free lime determination. Conventional methods used in mill laboratories require extensive preparations and give inconsistent and sometime erroneous results. A systematic study was performed to determine if zeta potential, a dispersion characteristic of the lime mud and liquor slurry, can be used to indirectly indicate the amount of free lime in the lime mud. Lime mud with different free lime contents was produced in the laboratory by causticizing green liquors with lime at different liming ratios. The results show that zeta potential changes from negative to positive when the free lime in lime mud exceeds a critical value. This unique characteristic enables a rapid detection of free lime.

Conversion of Levulinic Acid from Various Herbaceous Biomass Species Using Hydrochloric Acid and Effects of Particle Size and Delignification

Acid catalyzed hydrothermal conversion of levulinic acid (LA) from various herbaceous materials including rice straw (RS), corn stover (CS), sweet sorghum bagasse (SSB), and Miscanthus (MS) was evaluated. With 1 M HCl, 150 °C, 5 h, 20 g/L solid loading, the yields of LA from untreated RS, CS, SSB and MS based on the glucan content were 60.2, 75.1, 78.5 and 61.7 wt %, respectively. It was also found that the particle size had no significant effect on LA conversion yield with >3 h reaction time. With delignification using simulated green liquor (Na2CO3-Na2S, 20 wt % total titratable alkali (TTA), 40 wt % sulfidity) at 200 °C for 15 min, lignin removal was in the range of 64.8–91.2 wt %. Removal of both lignin and xylan during delignification increased the glucan contents from 33.0–44.3 of untreated biomass to 61.7–68.4 wt % of treated biomass. Delignified biomass resulted in much lower conversion yield (50.4–56.0 wt %) compared to 60.2–78.5 wt % of untreated biomass. Nonetheless, the concentration of LA in the product was enhanced by a factor of ~1.5 with delignification.

Effects of Green Liquor (GL) and Sodium Carbonate (SC) Pretreatment on Structural Characteristics of Wheat Stem Lignin

The content and structure of lignin have been considered as important factors that affect both pretreatment and enzymatic hydrolysis of lignocellulosic biomass. In this work, wheat stems (WS) were pretreated using mild alkali including green liquor (GL) and sodium carbonate (SC). The results indicate that GL pretreatment exhibits better delignification selectivity and higher enzymatic digestibility than SC pretreatment. Analysis of 1H–13C HSQC NMR and FTIR on cellulolytic enzyme lignin (CEL) preparations isolated from untreated and pretreated WS also proves that a certain amount of lignin degrades which leads to a decrease of β-O-4′ linkages. Under mild alkaline conditions, more guaiacyl units in lignin are removed than syringyl units, which results in a higher condensation degree and S/G ratio of CELs isolated from GL- and SC-pretreated stems. Compared with p-coumarate structures, ferulates in lignin are more stable under mild alkaline conditions.

Extraction of hazardous metals from green liquor dregs by ethylenediaminetetraacetic acid

Green liquor dregs are the major inorganic solid side stream of kraft pulp mills which contain environmentally hazardous metals. The presence of hazardous metals in this industrial residue brings statutory limits for its landfilling, although they are not easily mobilized from the solid phase. In this study, the chelating agent ethylenediaminetetraacetic acid (EDTA) is utilized to extract hazardous metals such as Cd, Pb and Zn from green liquor dregs. Furthermore, the influence of EDTA on the removal of Ca as the main mineral nutrient present in the green liquor dregs is studied. The effect of parameters such as EDTA dosage, L/S ratio and contact time on the removal rate of the elements is investigated. In addition, the experimental data are fitted to the Elovich model and the pseudo-first-order model to describe the desorption kinetics. The results show that 59 wt% of Cd, 13 wt% of Co, 62 wt% of Cu, 3 wt% of Mn, 12 wt% of Ni, 43 wt% of Pb, 16 wt% of Zn, and less than 1 wt% of Ca were extracted from green liquor dregs with EDTA dosage of 0.035 gEDTA salt/gdregs and the L/S ratio of 6.25 ml/g. The current study opens up new possibilities to use the green liquor dregs for improving the soil fertility instead of landfilling.

Efficient separation of hazardous trace metals and improvement of the filtration properties of green liquor dregs by a hydrocyclone

Abstract The treatment of green liquor dregs (GLD), the main inorganic solid residues of kraft pulp mills, is a major concern in the industrial scale. In this study, the effect of hydrocyclone classification on the filtration properties of GLD and the separation of hazardous trace metals, such as Cd, Ni, Pb and Zn, rare earth elements, and other trace metals was investigated. The experiments were designed to find the influence of parameters such as the overflow to the underflow outlet diameter ratio and inlet pressure on the separation efficiency of a hydrocyclone. The results showed that a lower total filtration area was required for the separation of underflow and overflow fractions than for the original GLD sludge. Also, hazardous trace metals were effectively separated into finer overflow fractions, enhancing the possibilities to utilize the purified underflows e.g. in fertilizers and soil amendment. By using the diameter ratio of 3.70 and the inlet pressure of 1 bar, 90.1 wt-% of Cd, 70.1 wt-% of Ni and 91.4 wt-% of Zn were separated into the overflow, collecting 30 wt-% of the dregs in this fraction. The concentrations of rare earth elements (REEs) in the underflow solids were lower than the ones in the original sludge. Unlike the trace metals and REEs, Ca was accumulated in coarser particles that were separated by underflow fractions. Therefore, a majority of the dregs, enriched by Ca, was separated by underflow fractions. The possibility of categorizing the underflow fractions in CE-marked fertilizing products was also studied.

Effect of phenolic compounds on bioconversion of glucose to ethanol by yeast Saccharomyces cerevisiae PE‐2

AbstractThe bioconversion of lignocellulosic biomasses to ethanol consists of three main steps: pre‐treatment, enzymatic hydrolysis, and fermentation by microorganisms. Most pre‐treatments induce the formation of substances with a potential inhibitory effect on the microbial metabolism, which may hinder the process. The objective of this study was to investigate the behaviour of Saccharomyces cerevisiae PE‐2 yeast in the presence of four phenolic compounds (vanillin, syringaldehyde, syringic acid, and acetosyringone) resulting from the alkaline pre‐treatment of eucalyptus with green liquor. Eucalyptus is a fast‐growing tree, widely exploited by the paper and pulp industry in Brazil. It was observed that yeast growth and glucose assimilation were affected to an extent directly proportional to the initial amount of vanillin in the medium. Syringaldehyde in a concentration of 1.0 g/L had an adverse effect on the fermentation parameters related to ethanol production, which exhibited a decrease of approximately 50 %. When syringic acid and acetosyringone were used, although the cell growth and glucose consumption profiles were similar to those of the control, they inhibited the conversion at concentrations of 0.1 and 1.0 g/L.

Cellulose Accessibility and Zeta potentials of Sugarcane Bagasse Pretreated by Green Liquor and Ethanol for High Hydrolysis Efficiency

Green liquor (GL) combined with ethanol (GL-ethanol) was selected to pretreat sugarcane bagasse (SCB). The results showed that the maximum lignin removal of 85.2% was achieved at 160 °C and a GL loading of 1.5 mL/g-dry substrate. The glucose yield of pretreated SCB increased with increased pretreatment temperature, and the maximum glucose yield of 97.7% was reached from SCB pretreated at 160 °C. Simons’ stain (SS) showed that the glucose yield was affected by cellulose accessibility instead of lignin content when lignin removal was > 70%. The cellulase adsorption isotherm fitted by the Langmuir model showed that the strength of interaction between the cellulase and substrate of GL-ethanol-100/1.5 (100 °C, 1.5 mL GL/g-dry substrate) was declining with increased pH. The adsorption was pH-dependent, and negatively controlled by the pH value. Electrostatic interactions can account for the pH-dependency of cellulase adsorption.

Sustainable synthesis of uniform silica microspheres from green liquor of wheat straw pulping and application in natural rubber

In order to recycle alkali chemicals from wheat straw alkaline pulping waste liquor effectively, an environmentally friendly route was developed to remove silicate containing in the green liquor of wheat straw pulping. In this route, silica (SiO2) microspheres were synthesized from wheat straw green liquor (WSGL) using carbon dioxide (CO2) as the precipitating reagent and anhydrous ethanol as the dispersant at different stirring rates. The resulting SiO2 microspheres with narrow size distribution had an amorphous structure with a diameter of about 190 nm under the optimal synthesis conditions with a pH value of 8.6, a stirring rate of 100 r min−1, and an ethanol addition amount of 60 ml. In this optimized route, the SiO2 removal rate (SRT) of WSGL was more than 99%. Except CO2, all the chemical reagents used in the designed process had been validly used. The SiO2 microspheres obtained from WSGL could be used as reinforcing agents to strengthen the mechanical properties of natural rubber.

Effect of Cellulose Accessibility and Electrostatic Repulsion on Enzymatic Hydrolysis of Sugarcane Bagasse Pretreated by Green Liquor Combined with Sulfite

Effect of Cellulose Accessibility and Electrostatic Repulsion on Enzymatic Hydrolysis of Sugarcane Bagasse Pretreated by Green Liquor Combined with Sulfite

Level, causes, and risk factors of stillbirth: a population-based case control study from Chandigarh, India

BackgroundGlobally, India ranks first in the absolute number of stillbirths. Hence, the level, causes, and risk factors of stillbirths were estimated to facilitate designing of prevention strategy.MethodsCapture and recapture method was used to identify 301 stillbirths from 1st July 2013 to 31st August 2014 in Chandigarh Union Territory of India. Verbal autopsies (n = 181) were done at household level to identify causes of stillbirths. Risk factors were determined using case-control approach. Women who had a stillbirth in the past 3 months were enrolled as cases (n = 181) and those who had live-birth in same neighbourhood were included as controls (n = 181). Statistical differences in the distribution of characteristics of cases and controls were tested by t test and chi square test respectively for quantitative and categorical variables. In logistic regression models adjusted odds ratios (aOR) and 95% confidence intervals (CIs) were estimated for various risk factors.ResultsStillbirth rate was estimated to be 16/1000 birth. Antepartum causes were more common (68%) than intrapartum causes (32%). Among maternal conditions, hypertension (18.2%) and chorio-amnionitis (13.8%), and among foetal conditions, growth restriction (19.9%) and congenital anomalies (18.8%) were the leading causes. In about half of the stillbirths foetal (48%) and maternal (44.7%) causes were unidentifiable. Risk factors of stillbirths were: higher maternal age (aOR 1.1, 95%CI 1.0–1.2), vaginal delivery (aOR 8.1, 95%CI 2.6–26), induced labour (aOR 2.6, 95%CI 1.5–4.5), green or light brown liquor (aOR 2.0, 95%CI 1.1–3.8), preterm delivery (aOR 6.4, 95%CI 3.7–11) and smaller household size (aOR 1.2, 95% CI 1.1–1.3).ConclusionsStillbirth rate was high in Chandigarh Union Territory of India. Major causes and risk factors amenable to interventions were infections, hypertension, congenital malformations, foetal growth restriction, pre-maturity and household size. Therefore, better maternity ante-natal and intra-natal care is required to achieve a single digit stillbirth rate.

Integrated forest biorefineries: Recovery of acetic acid as a by-product from eucalyptus wood hemicellulosic hydrolysates by solvent extraction

The biorefinery concept associated with pulp mills appears to be one of the best options for the valorisation of lignocellulosic biomass. Hemicelluloses can easily be pre-extracted from lignocellulosic biomass under mild operational conditions in order to obtain a hydrolysate that is rich in hemicelluloses, which can be used to produce bioethanol, as well as pretreated chips for further pulp production. However, the release of fermentable monosaccharides (mainly xylose) from the pre-extracted hemicellulose fraction is accompanied by the formation of decomposition products such as acetic acid, which inhibits fermentation by reducing conversion rates and yields. Due to the high level of acetyl groups in eucalyptus woods, acetic acid should be separated before the fermentation process in order to limit its inhibitory effect. Consequently, several studies have been conducted regarding the removal and recovery of acetic acid not only because of its inhibitory nature but also because it is a value-added compound in the chemical industry.In this work, acetic acid recovery via liquid-liquid extraction from hemicellulosic hydrolysates of Eucalyptus grandis wood chips was evaluated. This study was carried out mainly because there are few reported data on the experimental recovery of acetic acid from pre-extracted hemicelluloses of eucalyptus wood, considering that the use of eucalyptus chips in a biorefinery platform is important due to the high commercial relevance of eucalyptus pulp over the last decades. Hemicellulose pre-extractions were carried out using green liquor with varying alkali charges (1–6% w/w), temperatures (100–160▪) and reaction times (30–150min). The acetic acid extraction was performed using ethyl acetate at different pH values (2.0–4.0) for 36min, in order to study the effects of the main experimental variables affecting the extraction process (hydrolysate-to-solvent volume ratio, pH and the coupling of the extraction stages). Under the best pH conditions, a single-stage extraction at an H/S of 1:4 achieved 82.0% removal of acetic acid, whereas by a three-stage extraction at an H/S of 1:2, up to 88.0% of the initial acetic acid was recovered. After a fractional distillation, an acetic acid yield of almost 30L/t of dry eucalyptus wood chips was achieved, given the pre-extraction of hemicelluloses from E. grandis wood prior to the Kraft pulping process.

Elemental mobility in sulfidic mine tailings reclaimed with paper mill by-products as sealing materials.

Sealing layers made of two alkaline paper mill by-products, fly ash and green liquor dregs, were placed on top of 50-year-old sulfide-containing tailings as a full-scale remediation approach. The performance and effectiveness of the sealing layers with high water content for an oxygen barrier and low hydraulic conductivity for a sealing layer in preventing the formation of acid rock drainage were evaluated 5years after the remediation. The leaching behavior of the covered tailings was studied using batch leaching tests (L/S ratio 10L/kg). The leaching results revealed that, in general, the dregs- and ash-covered tailings released relatively lower concentrations of many elements contained in acid rock drainage compared to those from the uncovered tailings. A change in the chemical composition and mineralogical state of the tailings was observed for the tailings beneath the covers. The increase in pH caused by the alkaline materials promoted metal precipitation. Geochemical modeling using PHREEQC confirmed most of the geochemical changes of the covered tailings. Both the ash and dregs showed potential to function as sealing materials in terms of their geochemical properties. However, mobilization of Zn and Ni from the lower part of the dregs-covered tailings was observed. The same phenomenon was observed for the lower part of the ash-covered tailings. Ash showed advantages over dregs as a cover material; based on geochemical studies, the ash immobilized more elements than the dregs did. Lysimeters were installed below the sealing layers, and infiltrating water chemistry and hydrology were studied to monitor the amount and quality of the leachate percolating through.

Metabolic engineering of Enterobacter aerogenes for 2,3-butanediol production from sugarcane bagasse hydrolysate

The pathway engineering of Enterobacter aerogenes was attempted to improve its production capability of 2,3–butanediol from lignocellulosic biomass. In the medium containing glucose and xylose mixture as carbon sources, the gene deletion of pflB improved 2,3-butanediol carbon yield by 40%, while the deletion of ptsG increased xylose consumption rate significantly, improving the productivity at 12 hr by 70%. The constructed strain, EMY-22-galP, overexpressing glucose transporter (galP) in the triple gene knockout E. aerogenes, ldhA, pflB, and ptsG, provided the highest 2,3-butanediol titer and yield at 12 hr flask cultivation. Sugarcane bagasse was pretreated with green liquor, a solution containing Na2CO3 and Na2SO3 and was hydrolyzed by enzymes. The resulting hydrolysate was used as a carbon source for 2,3-butanediol production. After 72 hr in fermentation, the yield of 0.395g/g sugar was achieved, suggesting an economic production of 2,3-butanediol was possible from lignocellulosic biomass with the metabolically engineered strain.

Enhancement of enzymatic hydrolysis of sugarcane bagasse by pretreatment combined green liquor and sulfite

Green liquor (GL) is readily available in any pulp mills which could be recycled. Alkaline sulfite pretreatment process was developed for non-woody biomass biorefinery. In this study, GL combined with sulfite was effectively utilized to improve the enzymatic hydrolysis of sugarcane bagasse by selectively removing lignin. Compare to conventional alkaline sulfite pretreatment, the solid yield could be higher with low degradation of polysaccharide. With the increment of the pretreatment temperature from 100 to 140°C, the enzymatic hydrolysis efficiency was improved. And a higher glucose yield was achieved under pretreatment condition of 0.4g/g-DS Na2SO3 and 1.5mL/g-DS GL due to the higher degradation of xylan. The highest glucose yield of 96.8% could be reached with 3.36g/(L·h) of initial rate after Kraft GL-sulfite pretreatment at 140°C. The superior performance of Kraft GL allows conversion of cellulose to glucose more significantly than Soda GL. In addition, surface tension test indicated that a part of lignin was dissolved in the pretreatment liquor as lignosulfonate. The results showed that GL combined with sulfite is a promising way for sugarcane bagasse pretreatment.

Recovery of calcium carbonate waste as paper filler in the causticizing process of bamboo kraft pulping

In this paper, calcium carbonate waste was recovered as paper filler from the causticizing process of bamboo kraft pulping. The effects of the causticizing process parameters on the filler qualities were investigated, in terms of the whiteness, tear strength, tensile strength and ash content of the filled paper. To investigate the effect of silica content on the qualities of the resultant calcium carbonate filler, the green liquor was desiliconized with aluminum salt modified bentonite prior to the causticizing process to obtain calcium carbonate of various silicon contents, particle size and brightness, and different crystal morphology. It was found that the ionic strength and silicate concentration were the two key factors in affecting the size and shape of the calcium carbonate particles formed in the causticizing process.

Enhancement of ethanol production from green liquor\u2013ethanol-pretreated sugarcane bagasse by glucose\u2013xylose cofermentation at high solid loadings with mixed Saccharomyces cerevisiae strains

BackgroundEfficient cofermentation of glucose and xylose is necessary for economically feasible bioethanol production from lignocellulosic biomass. Here, we demonstrate pretreatment of sugarcane bagasse (SCB) with green liquor (GL) combined with ethanol (GL–Ethanol) by adding different GL amounts. The common Saccharomyces cerevisiae (CSC) and thermophilic S. cerevisiae (TSC) strains were used and different yeast cell mass ratios (CSC to TSC) were compared. The simultaneous saccharification and cofermentation (SSF/SSCF) process was performed by 5–20% (w/v) dry substrate (DS) solid loadings to determine optimal conditions for the co-consumption of glucose and xylose.ResultsCompared to previous studies that tested fermentation of glucose using only the CSC, we obtained higher ethanol yield and concentration (92.80% and 23.22 g/L) with 1.5 mL GL/g-DS GL–Ethanol-pretreated SCB at 5% (w/v) solid loading and a CSC-to-TSC yeast cell mass ratio of 1:2 (w/w). Using 10% (w/v) solid loading under the same conditions, the ethanol concentration increased to 42.53 g/L but the ethanol yield decreased to 84.99%. In addition, an increase in the solid loading up to a certain point led to an increase in the ethanol concentration from 1.5 mL GL/g-DS-pretreated SCB. The highest ethanol concentration (68.24 g/L) was obtained with 15% (w/v) solid loading, using a CSC-to-TSC yeast cell mass ratio of 1:3 (w/w).ConclusionsGL–Ethanol pretreatment is a promising pretreatment method for improving both glucan and xylan conversion efficiencies of SCB. There was a competitive relationship between the two yeast strains, and the glucose and xylose utilization ability of the TSC was better than that of the CSC. Ethanol concentration was obviously increased at high solid loading, but the yield decreased as a result of an increase in the viscosity and inhibitor levels in the fermentation system. Finally, the SSCF of GL–Ethanol-pretreated SCB with mixed S. cerevisiae strains increased ethanol concentration and was an effective conversion process for ethanol production at high solid loading.

Field-Grown Transgenic Hybrid Poplar with Modified Lignin Biosynthesis to Improve Enzymatic Saccharification Efficiency

Hybrid poplars (Populus nigra L. × Populus maximowiczii A.) were genetically modified through antisense insertion of the 4-coumarate:coenzyme A ligase (4CL) gene. Compositional changes in response to this genetic change were measured in the field after 2 and 3 years of growth. The stem samples were treated with either green liquor or dilute acid pretreatments, representing alkaline and acid pretreatments. The enzymatic saccharification of the untreated and pretreated transgenic poplars were evaluated. After transgenic species were transplanted into the environment, they showed reduced recalcitrance to chemicals (i.e., pretreatments) and enzymes despite their lignin content and S/V ratio being comparable to those of the wild types. Compared to the field-grown poplars, the sugar yield increased up to 103% for untreated transgenic samples and increased 22% for acid- and green liquor-pretreated transgenic samples. This shows that field-grown transgenic hybrid poplars with modified lignin biosynthesis have imp...

Study on the filtration characteristics of green liquor dregs

Separation of dregs from green liquor is a major challenge in kraft pulp mills, and it results in compressible filter cakes with high specific cake resistance. In industrial applications, the use of lime mud as a filter aid is a practical approach to reduce the resistance of the dregs cake. However, the amount of solid waste then increases considerably, because the lime mud has to be disposed with the dregs. In this study, green liquor sludges from two different pulp mills were filtered without filter aids by using laboratory-scale pressure filters. The effect of some variables, such as the permeability of the filter cloth, the temperature of filtration, the filtration pressure, and the storage period on the filterability of the green liquor sludges was investigated. The results provide information for optimizing the process parameters to improve the filterability of the sludge. The lowest cake resistance (2.20·1012m/kg) was obtained at filtration the temperature of 80°C. However, the higher operating temperature did not necessarily have a positive effect on the recovery of sodium (valuable element for the process) from the cake. The results showed that higher amounts of certain elements, such as Na, K and B were recovered from the dregs cake at the filtration temperature of 60°C. It was also observed that the storage period had a negative effect on the filtration properties. In addition, elemental analysis of the filtrate and dregs cake samples showed that harmful metals, such as Al, Cd, Fe, Mn and Zn were mostly insoluble and remained in the dregs.

A study of black liquor and pyrolysis oil co-gasification in pilot scale

The effect of the blend ratio and reactor temperature on the gasification characteristics of pyrolysis oil (PO) and black liquor (BL) blends with up to 20 wt% PO was studied in a pilot-scale entrained-flow gasifier. In addition to unblended BL, three blends with PO/BL ratios of 10/90, 15/85, and 20/80 wt% were gasified at a constant load of 2.75 MWth. The 15/85 PO/BL blend was used to investigate the effect of temperature in the range 1000–1100 °C. The decrease in fuel inorganic content with increasing PO fraction resulted in more dilute green liquor (GL), and a greater portion of the feedstock carbon ended up in syngas as CO. As a consequence, the cold gas efficiency increased by about 5%-units. Carbon conversion was in the range 98.8–99.5% and did not vary systematically with either fuel composition or temperature. Although the measured reactor temperatures increased slightly with increasing PO fraction, both unblended BL and the 15% PO blend exhibited largely similar behavior in response to temperature variations. The results from this study show that blending BL with the more energy-rich PO can increase the cold gas efficiency and improve the process carbon distribution without adversely affecting either carbon conversion or the general process performance.

Acidogenic Digestion of Pre-pulping Extracts for Production of Fuels and Bioproducts Via Carboxylate Platform Processing.

Hemicellulose extracted from wood prior to processing the wood into paper or composite materials can be a resource for the production of biofuels or bioproducts. Mixed microbial cultures are capable of converting biomass into mixed carboxylic acids, which can be purified as products or converted to biofuels or other biochemicals. Mixed cultures are robust conversion systems and do not require added enzymes to hydrolyze biomass to sugars. We produced mixed carboxylic acids using mesophilic and thermophilic fermentation of raw, unconditioned green liquor and hot water hardwood extracts, as well as baseline sugar solutions. Daily samples were taken from the fermentations and analyzed for composition, pH, and gas volume. The extract digestions were capable of hydrolyzing oligomeric hemicellulose without supplemental enzymes and converting all types of released sugars. Lactic acid was prominent in lower pH systems and acetic acid, the main product at more neutral pH. Compared to thermophilic systems, mesophilic fermentations had higher hydrolysis conversion, carbohydrate conversion, acid yields, and selectivity for C3-C7 acids. Carbon balances on the wood extracts closed to within ±9%. Methane production in all cases was essentially zero.