Article pubs.acs.org/EF
Lignin from Eucalyptus spp. Kraft Black Liquor as Biofuel Andrés Dieste,*,† Leonardo Clavijo,† Ana I. Torres,† Stéphan Barbe,‡ Ignacio Oyarbide,† Leonardo Bruno,† and Francisco Cassella† †
Energy Fuels 2016.30:10494-10498. Downloaded from pubs.acs.org by UNIV OF READING on 08/19/18. For personal use only.
Instituto de Ingeniería Química, Facultad de Ingeniería, Universidad de la República, Julio Herrera y Reissig 565, 11300 Montevideo, Uruguay ‡ Chemical Engineering, Faculty of Applied Natural Sciences, Technische Hochschule Koeln, Kaiser-Wilhelm-Allee, Gebäude E39, 51373 Leverkusen, Germany ABSTRACT: Pulp mills located in Uruguay process Eucalyptus spp. wood from plantations. Black liquor is burnt in the recovery boiler, generating an excess of energy that is converted to electricity and sold to the grid. Lignin, the main component of black liquor, is a natural polymer, abundant, and readily obtained by acid precipitation. A recovery process of lignin from black liquor in a pulp mill produces a biofuel to be used within at the plant or to be marketed locally, diversifying the energy offer of Uruguay, a country with no fossil fuels. In the present contribution, technical-grade lignin (average of 94% total lignin) was obtained experimentally in a pilot plant by acid precipitation (H2SO4), sedimentation, filtration, and washing of the slurry and assessed as fuel: it presented a high caloric value (26 MJ/kg), low carbohydrate content, and low K and Na contents. The results of the economic analysis showed that a production of 3400 tons of lignin per year could be produced at a cost of 692 US$/ton. At a small production scale, the production costs of the operation discourage the use of kraft lignin as biofuel and clearly direct the possible application of this polymer to the production of a technical chemical.
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expected to generate 66 million tons of lignin.3 Modern pulp mills satisfy their thermal energy demands and, therefore, are able to sell some of the excess energy as electricity.4 The recovery of lignin from the black liquor lowers the heat load of the recovery boiler and, therefore, allows us to increase the pulp production; this is particularly interesting if the boiler has reached its capacity limit.2,5,6 The standard technology for the large-scale recovery of lignin consists of lowering the pH value of the black liquor by means of adding an acidifying agent, such as mineral acid or CO2. This, in turn, leads to the protonation of the phenolic groups of the lignin and a reduction of its solubility. It forms agglomerates and precipitates in a slurry that can be mechanically separated from the mother liquor, e.g., via filtration, and washed to achieve the specified purity.6−10 There are at least two patents that described variations of the process, LignoBoost10 and LignoForce;11 in addition, there is also a commercial operation running to recover lignin from black liquor on a large scale, using the LignoBoost process, Domtar, Montreal, Québec, Canada.12 The lignin could be used as an internal biofuel at the mill or sold as a substitute for other fuels. Furthermore, it could be marketed as a technical chemical for diverse applications, creating a source of revenue for the mill; however, the vegetal origin of lignin and the recovery process determine the properties of the lignin obtained.3,13−15 The amount of lignin that is actually used for high-value products is still limited, estimated between 1 and 2% of the total lignin present in the black liquor of the global pulp industry.15 The higher value applications of lignin, such as carbon fibers,16 activated carbon, and adhesives,17 require highly purified lignin, with limited dispersion in molecular size.2
INTRODUCTION Presently, there are no fossil fuels available in Uruguay, neither petroleum nor coal. As a result of the low cost of wood against other fuel alternatives, the local industry relays heavily in Eucalyptus spp. plantations for thermal energy (Table 1); in Table 1. Cost of Available Fuel for Uruguayan Conditions (LHV) fuel
US$/ton
US$/GJ
air-dried wooda fuel oilb gas oilb natural gas from Argentinac
60 561 1049
5 13 23 24
a
Eucalyptus spp. wood dried for at least 5 months (30%, wb) sold at the plant. bAdministración Nacional de Combustibles, Alcoholes y Portland (ANCAP) 2016 (tax included). cANCAP.42
2015, the use of wood as a fuel comprised 2.7 million m3, 20% of the total forestry output.1 There is a strong interest to diversify the energy matrix with new fuels, and lignin offers an interesting alternative. Cellulose, hemicellulose, and lignin are the structural components of the cell wall of vascular plants forming wood. Lignin may be regarded as a cross-linked phenolic polymer. During kraft pulp production, wood is heated in an alkaline medium to dissolve the lignin and separate the cellulose, generating a solution, known as black liquor, which is concentrated and burnt to produce energy in the recovery boiler. The inorganic chemicals present in the black liquor are recovered and reused in the digestion of wood. It is estimated that 55 million tons of lignin are produced by the pulp industry,2 while the industry of biofuels produced by the fermentation of cellulose obtained from lignocellulosic stock is © 2016 American Chemical Society
Received: August 18, 2016 Revised: October 31, 2016 Published: November 2, 2016 10494
DOI: 10.1021/acs.energyfuels.6b02086 Energy Fuels 2016, 30, 10494−10498
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for the observation of the sedimentation of lignin agglomerates. Once again, the supernatant was pumped out, and the lignin-rich slurry was filtrated at room temperature with a pressure difference of 2.5 bar. The average specific resistance of the filter cake (α) and the filter medium (β) was determined at pH 2, performing four runs, two for each precipitated slurry. The total lignin content, measured as the sum of acid-soluble and -insoluble lignin (Klason lignin), was determined as an indication of purity.9 The ash content was determined by calcination at 700 °C for 2 h, following the study by Gosselink et al.20 The carbohydrate content of the lignin was determined by high-performance liquid chromatography (HPLC, Shimadzu Prominence, Japan) equipped with a refractive index detector after hydrolysis with 2% (v/v) H2SO4 at 127 °C for 1 h, following standard NREL/TP-510-42623.21 The separation column used was a Aminex HPX-87H, heated up to 35 °C, and the mobile phase was 4 mM H2SO4 at a flow rate of 0.6 mL/min; the standards xylose, glucose, arabinose, acetic acid, furfural, and 5hydroxymethylfurfural (HMF) were used to construct the calibration curves.21 The column used, Aminex HPX-87H, does not properly separate mannose and galactose from xylose. Nevertheless, when the carbohydrate determination in Eucalyptus spp. wood was performed using column Aminex HPX-87P, which does separate xylose from mannose and galactose, it was found that the amount of mannose and galactose was negligible. Elemental analysis of C, H, N, and S was performed externally by the Laboratorio Tecnológico de Uruguay (LATU), and O was determined by difference. The contents of Na and K in lignin ash were determined by atomic absorption spectrometry.22 The higher heating value (HHV) of lignin was determined in a calorimetric pump (Parr, Moline, IL), and the lower heating value (LHV) was obtained by correction with the hydrogen content.23 The filtration area as a function of time, the filter resistance of the cake (αm), and the resistance of the filter medium (β) were determined according to the methodology presented by Ripperger et al.24 (eq 1)
Arguably, such specialized lignin could be obtained by a tailored recovery from black liquor, varying temperature, pH, agitation, acidulation source, filtration, and washing.7,18,19 A single-vessel plant could be the adequate equipment to produce such lignin at small scale. The objective of this study was to evaluate lignin as a costcompetitive biofuel, produced in a small-scale plant (up to 3400 ton/year), under the assumption that there would be no significant impact on the operation of the recovery boiler.
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MATERIALS AND METHODS
Lignin was obtained by acid precipitation of fresh kraft black liquor from Eucalyptus spp., kindly provided by the Fray Bentos mill using a pilot plant designed and built for this purpose. It consists of one reactor and one decanter, both of 50 L capacity, constructed with 316L stainless steel to resist 8 bar pressure. The plant was operated inside a fume extraction cabinet. The reactor is a cylinder with torispherical ends, equipped with an electric heater (5 kW). The vessels are connected to a compressed air source to move the fluids within the plant and to agitate the solutions, by means of a ring sparger located at the bottom of the reactor. The decanter is constructed by welding a cylinder with a torispherical lid to a 45° cone. The bottom of the cone has a removable perforated plate, leaving 42% of the 0.023 m2 surface free, that supports the filtering mesh (Scheme 1).
Scheme 1. Representation of the Pilot Plant
t=
αmηK m 2 βη VF + VF 2 A ΔP 2A ΔP
(1)
where t is the time (s), A is the filter area (m ), αm is the average cake resistance relative to dry mass (m/kg), β is the resistance of the filter medium (m−1), η is the viscosity (Pa s), m is the mass of the dry cake (kg), VF is the volume of filtrate (m3), and Km is the concentration factor, proportionality mass of cake/volume of filtrate (kg/m). At a constant pressure difference, the plotting of t/VF versus VF allows for the calculation by a minimum square technique of the resistance of the filter cake, relative to dry mass, and the resistance of the filter medium, taken from the slope and the intersection, respectively (eq 2) (Table 2). 2
Two precipitations were performed, each using approximately 30 kg of black liquor from the same batch. The black liquor was heated to 80 °C, with regular pneumatic agitation. At this point, 140 g of H2SO4 (diluted 50%, v/v) was added per kilogram of dry solid content of black liquor using a peristaltic pump, with constant agitation, at a rate of 135 mL min−1, to reach pH 9. After 1 h, the mixture was transferred to the decanter. The next day, the supernatant was removed using a pump, and the precipitated slurry was filtered through a mesh installed at the bottom of the decanter, using compressed air to create a pressure difference of 2.5 bar. The resulting filter cake was resuspended in water, acidified with H2SO4 (50%, v/v) to reach pH 2 to avoid the solubilization of lignin (40 L of water to 1 kg of solid), agitated, and left to sediment. Such excess of water was used to allow
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α ηK βη t = m2 m VF + VF AΔP 2A ΔP
(2)
RESULTS AND DISCUSSION Experimental Results. The black liquor used for precipitation presented a dry solid content of 32% (m/m) from which 32% (m/m) was lignin. After the precipitation and filtration, the dry content of the filter cake of runs 1 and 2 was
Table 2. Cake and Filter Medium Resistance, Adjustment to Linearity of t/VF versus VF, and Acid-Soluble and -Insoluble Lignin Content for Acid-Washed Precipitated Slurry run 1 2 3 4
cake resistance, αm (m/kg) 2 7 1 4
× × × ×
1012 1011 1012 1012
filter medium resistance, β (m−1) 1 1 8 9
× × × ×
1013 1013 1012 1012
R2
acid-insoluble lignin (mass fraction %, dry basis)
acid-soluble lignin (mass fraction %, dry basis)
0.88 0.97 0.85 0.98
80.6 92.7 85.4 92.2
7.7 7.2 7.2 7.5
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pressed air. Such equipment, known as Nutsche filters, are standardized products that usually combine filtration and vacuum drying that enable the dewatering of the solid after the filtration operation. Nutsche filters are readily available in the market from suppliers of industrial equipment. However, the filtration area constrains the use of this equipment at largescale operations.28 After a market research, the maximum available standard filter area and vessel height for a Nutsche filter was 15 m2 and 1 m, respectively, with an estimated market cost of 660 000 US$. Considering those dimensions, the maximum αm and β obtained experimentally, and a production cycle of 22.5 h (considering all operations), a single-vessel plant could produce 455 kg/day of dry lignin. Such equipment could be used as a pilot plant to test different process conditions or produce low quantities of lignin, adjusted to specific customers demand for niche markets. A simulation model process based on the experimental results was built using Aspen Plus V8.6. A continuously operating plant was considered, equipped with one reactor, mechanically agitated, and jacketed, with a residence time of 16.5 h, a plate and frame filter, two generic Tubular Exchanger Manufacturer Association (TEMA, Tarrytown, NY) heat exchangers, and a double-drum dryer. Experimentally, 40 L of water for 1 kg of lignin was used to acid wash the lignin in the pilot plant to allow for the observation of the sedimentation of the solid; however, the escalation model considered the relation presented by Loufti et al.: 4.2 kg of water/kg of precipitated lignin.6 In addition, the lignin yield, calculated as the fraction of the lignin present in the black liquor that is recovered at the end of the process, was taken as 70%.6 Aspen Process Economic Analyzer V8 was used to model the production costs of the plant, considering the following settings: (1) cost base for the first quarter of 2013, (2) project duration of 10 years, (3) salvage value of 20%, and (4) location factor in Uruguay at 1.12.29 The maximum production capacity of the plant was considered at 3400 ton/year, which could substitute 5% of the thermal energy obtained from 145 000 ton of fuelwood, as was reportedly used in 2012 by the local industrial sector of Cellulose, Paper, and Wood.30 This size is comparable to the 4000 ton/year of lignin presently produced by the LignoBoost demonstration plant located at the Nordic Paper mill, in Sweden.31 The cost of H2SO4, the acidifying agent proposed, was taken at 250 US$/ton.32 It was assumed that the plant employed three full-time operators and one-quarter of the time of one supervisor per shift, and the labor cost for operators and the supervisor was taken at 5 and 15 US$/h, respectively. Electricity was considered at 92 US$/MWh33 (Table 4). The reported price for lignin ranges between 600 and 800 US$/ton;34 therefore, at 676 US$/ton, the cost of production
39 and 56% (m/m), respectively. During the precipitation step, the formation of agglomerates with different sizes was observed. Large agglomerates have a higher sedimentation velocity and built a first sediment layer with a coarse porosity. The next sediment layers consisted of much finer particles. The first layer (coarse porosity) played a major role for both filtration steps (after precipitation and after acid washing) by acting as a filtration aid. The lignin obtained was a brown powder with a slight sulfide odor. The two precipitations recovered 1100 and 1200 g of lignin, with 94 and 96% total lignin, respectively; additionally, the ash content for runs 1 and 2 was 3 and 1%, respectively. The physical and chemical properties of lignin obtained in run 1 were thoroughly analyzed (Table 3), and it Table 3. Physical and Chemical Properties of Lignin Obtained in the Pilot Plant by Acid Precipitation (Run 1) lignin property (unit) LHV (MJ/kg) bulk density (freely settled dry lignin) (kg/m) particle density (kg/m) acid-insoluble lignin (mass fraction %, dry basis) acid-soluble lignin (mass fraction %, dry basis) carbohydrate content (mass fraction %, dry basis) elemental analysis (mass fraction %, dry basis) C H N S O ash content (mass fraction %) Na (ash mass fraction %, dry basis) K (ash mass fraction %, dry basis) a
value 26.2a 804 1010 86.7a 7.4a 0.7a 60.4 5.3 0.2 3.5 28.5b 2.1a 13.7 2.0
Average of two repetitions. bObtained by difference.
presented a chemical composition similar to other descriptions reported in the literature.2,19,25 The analysis showed that it had acceptable properties to be used as a fuel, as reported by other researchers.3,25,26 With regard to its chemical composition, it presents a level of sulfur comparable to the fuel oil refined in Uruguay from imported petroleum27 and has a low content of K or Na (Table 3). Scale-up Calculations. It was observed that the filtration of the precipitated lignin was easily performed, where the filtering of the resuspended slurry required considerable more time. Therefore, the filtration area as a function of time, the filter resistance of the cake (αm), and the resistance of the filter medium (β) was determined for the resuspended slurry at 2.5 bar (eqs 1 and 2). According to these results, 650 kg of dry solids of lignin could be obtained by the filtration of the resuspended slurry, performed in 5 h, and considering a viscosity of 0.36 mPa s (water at 80 °C), a filtration area of 15 m2, and an efficiency of 70% for the acidic washing step (eq 1). As a result of the specific cake resistance, the filtration of acid-washed lignin is the bottleneck of the process. Notwithstanding, the whole process of recovering lignin from black liquor, including precipitation, sedimentation, filtration, and acidic washing, could be performed in a single vessel, mechanically or pneumatically agitated, with a porous bottom. The suspended solid is retained by filtration, while the mother liquor (precipitation) or the washing water (acid washing) would pass through. The necessary pressure difference could be achieved using com-
Table 4. Costs for the Installation and Operation of a Plant for the Recovery of 10 Tons/Day of Lignin from Black Liquor Simulated in Aspen Plus V8.6
a
10496
cost component
cost (US$/ton)
capital cost operating costa H2SO4 (98%, v/v) electricity total
92 257 263 64 676
Excluding H2SO4 used for precipitation and electricity. DOI: 10.1021/acs.energyfuels.6b02086 Energy Fuels 2016, 30, 10494−10498
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the adequate equipment to design a recovery process that complies with detailed specifications. At 26 US$/GJ, lignin recovered from Eucalyptus spp. kraft black liquor is a more expensive biofuel than local wood or some other alternatives obtained from imported oil or natural gas, which directs the application of lignin to specialty chemicals of higher value. However, the refinement of the technology to precipitate lignin from black liquor, especially the use of CO2 produced on site from biomass combustion or from the lime kiln of pulp mills as well as the benefits from larger scale fabrication could produce an industrial biofuel worth researching under the Uruguayan conditions. The results of this study open the discussion of lignin as an alternative biofuel in an expensive fossil fuel economic scenario, such as the one present in Uruguay.
obtained seems high. At a production rate of 10 tons/day, H2SO4 represents more than one-third of the total cost. The cost of H2SO4 to lower the pH during washing was included in the operating cost because it is the method used in the washing stage by other technologies proposed to extract lignin. Therefore, other sources of acidulation for precipitation, such as CO2,2,6,10,11 or a different process altogether, such a membrane filtration,8,35 should be considered. In Uruguay, the cost of industrial CO2 is 550−600 US$/ton.36 As a result of the high cost of CO2, the alternative of using CO2 produced on site using combustion gases could be analyzed.34 At 26 US $/GJ, the obtained lignin is more expensive than other fuel alternatives and much more expensive than air-dried wood (Table 1). Under the local conditions, wood as biofuel is more economically attractive than lignin or any other available fuel. Major changes in the energetic matrix of Uruguay must occur before other sources of thermal energy than wood are considered (Table 1). Therefore, the technology should be improved to lower the production cost. Further research should consider the following aspects: (1) the substitution of H2SO4 for CO2 produced on site as the acidifying agent, as proposed in existing patents,10,11 and (2) the improvement of the recovery efficiency to diminish the capital cost of the process. Two more considerations: (1) In Uruguay, the market price of electric energy obtained by biomass is declining as a result of the increase of energy produced by wind farms; at the end of 2015, the reference price of that source was 71 US$/MWh.37 (2) To compete locally as biofuel, lignin should at least have a lower price than fuel oil, 13 US$/GJ (Table 1), which does not seem feasible. Therefore, considering the decreasing price of biomass electricity, it is reasonable to think that, in the future, the Uruguayan pulp industry would consider the production of lignin to valorize the total output of the plant. In terms of purity and ash content, the lignin produced by the process described in this study is comparable to that obtained by LignoBoost38 or LignoForce.39 Different from both processes, this work presents the use of H2SO4 as an acidifying agent of black liquor, which use, instead of CO2, could be argued for a small plant under the local conditions. In addition, the lignin characterized is recovered from black liquor produced by the digestion of Eucalyptus spp., which is the most common wood used for the production of cellulose in the Southern Hemisphere. However, the main contribution of this work is that a slim and flexible pilot plant could be used to recover lignin at a small scale, performing all operations in a single vessel, obtaining approximately 0.5 ton/day. Such could be the adequate equipment to test different processes to recover lignin to be customized and sold for higher value applications different from biofuel.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: andresdieste@fing.edu.uy. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This paper was written with the results obtained from the project “Extraction of Lignin from Black Liquor as Fuel”, financed by the Fondo Sectorial de Energiá de la Agencia Nacional de Investigació n e Inno vació n (ANI I, FSE_1_2013_1_10726). The research group also acknowledges ANII for financially supporting the visit of Dr. Stéphan Barbe. Additionally, the research group thanks the collaboration of UPM and Fábrica Nacional de Papel SA (FANAPEL) for the provision of raw materials and logistic support and R. Pérez Veiga from Dupla SA for his technical assistance during the filtration experiments.
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REFERENCES
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CONCLUSION There is a strong incentive to build larger plants to recover lignin from black liquor, because there is a smaller capital cost per unit of product.40 Additionally, lignin has not yet achieved the status of a commodity to be in the applications described in the literature,2,15,41 leaving a large polymer resource reduced to the production of energy. One reason for this is the large variability that lignin presents, affected both by the vegetal origin of the raw material and the recovery process. A singlevessel pilot plant with a daily production capacity of half of a ton of lignin, such as the one modeled in this work, could be 10497
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(33) Unidad Reguladora de Servicios de Energiá y Agua (URSEA). Tarifas Energiá Eléctrica; URSEA: Montevideo, Uruguay, 2016; www. ursea.gub.uy. (34) Benali, M.; Périn-Levasseur, Z.; Savulescu, L.; Kouisni, L.; Jemaa, N.; Kudra, T.; Paleologou, M. Implementation of lignin-based biorefinery into a Canadian softwood kraft pulp mill: Optimal resources integration and economic viability assessment. Biomass Bioenergy 2014, 67, 473−482. (35) Wallberg, O.; Jönsson, A. S.; Wimmerstedt, R. Fractionation and concentration of kraft black liquor lignin with ultrafiltration. Desalination 2003, 154, 187−199. (36) Praxair Uruguay. Praxair Uruguay: Montevideo, Uruguay, 2016. (37) Alvarez, M.; Di Chiara, L.; Palacio, F.; Soubes, P.; Chaer, R.; Gurin, M.; Groposo, V.; Gaggero, G.; Pedrana, M.; Rodrigo, H. Programación Estacional (PES) Noviembre−Abril 2016; Administración d e l M e r c a d o E l é c t r i c o : M o n t e v i d e o , U r u g u a y , 2 0 1 5 ; DNC201511051445, pp 31. (38) Tomani, P. The LignoBoost process. Cellul. Chem. Technol. 2009, 44 (1−3), 53−58. (39) Kouisni, L.; Holt-Hindle, P.; Maki, K.; Paleologou, M. The LignoForce system: A new process for the production of high-quality lignin from black liquor. J. Sci. Technol. For. Prod. Processes 2012, 2 (4), 6−10. (40) Towler, G.; Sinnott, R. Capital cost estimating. In Chemical Engineering Design, 2nd ed.; Towler, G., Sinnott, R., Eds.; ButterworthHeinemann: Boston, MA, 2013; Chapter 7, pp 307−354. (41) Cazacu, G.; Capraru, M.; Popa, V. Advances concerning lignin utilization in new materials. Advances in Natural Polymers; Springer: Berlin, Germany, 2013; Vol. 18, Chapter 8, pp 255−312, DOI: 10.1007/978-3-642-20940-6_8. (42) Marroig, R. Natural gas in Uruguay. Infrastruct. Mark. 2015.
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