Lignin from Eucalyptus spp. Kraft Black Liquor as Biofuel - American

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Lignin from Eucalyptus spp. kraft black liquor as biofuel Andrés Dieste, Leonardo Clavijo, Ana Inés Torres, Stéphan Barbe, Ignacio Oyarbide, Leonardo Bruno, and Norberto Francisco Cassella Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02086 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 3, 2016

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Lignin from Eucalyptus spp. kraft black liquor as biofuel Andrés Dieste*1 Leonardo Clavijo1 Ana I. Torres1 Stéphan Barbe2 Ignacio Oyarbide1 Leonardo Bruno1 Francisco Cassella1 1

Instituto de Ingeniería Química

Facultad de Ingeniería Universidad de la República Montevideo, Uruguay 2

Chemical Engineering

Faculty of Applied Natural Sciences Technische Hochschule Koeln Leverkusen, Germany

ACS Paragon Plus Environment

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KEYWORDS: Eucalyptus spp.; kraft pulping; lignin; biofuel; Uruguay

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 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 content. The results of the economic analysis showed that a production of 3,400 t of lignin per year could be produced at a cost of 692 U$S/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.

INTRODUCTION. Presently there are no fossil fuels available in Uruguay, neither petroleum nor coal. Due to the low cost of wood against other fuel alternatives, the local industry relays heavily in Eucalyptus spp. plantations for thermal energy (Table 1); in 2015 the use of wood as fuel compromised 2,7


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million cubic meter, 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 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 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 in 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, LignoBoost 10 and LignoForce 11; in addition, there is also a commercial operation running to recover lignin from black liquor at large scale, using the LignoBoost process: Domtar, US 12. The lignin could be used as an internal biofuel at the mill or sold as substitute for other fuels. Furthermore, it could be marketed as a technical


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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 to 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. 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 cost-competitive biofuel, produced in a small-scale plant (up to 3,400 ton/year), under the assumption that there would be no significant impact on the operation of the recovery boiler.

MATERIAL 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


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has a removable perforated plate, leaving 42% of the 0.023 m2 surface free, that supports the filtering mesh (Scheme 1). Two precipitations were performed, each using approximately 30 kg of black liquor from the same batch. The black liquor was heated up to 80 °C, with regular pneumatic agitation. At this point, 140 g of H2SO4 (diluted 50% v/v) was added per kg 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 one hour, 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 re-suspended in water, acidified with H2SO4 (50% v/v) to reach pH=2 to avoid the solubilization of lignin (40 L water to 1 kg solid), agitated and let to sediment. Such excess of water was used to allow 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) were determined at pH=2, performing four runs, two for each precipitated slurry. Total lignin content, measured as the sum of acid soluble and insoluble lignin (Klason lignin), was determined as an indication of purity 9. Ash content was determined by calcination at 700°C for 2 h, following Gosselink et al. (2003)20. The carbohydrate content of the lignin was determined by 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 H2SO4 4mM at a flow rate of 0.6 mL/min; the standards xylose, glucose, arabinose,


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acetic acid, furfural and 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 done 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 done externally by the Laboratorio Tecnológico de Uruguay (LATU); and O was determined by difference. The content of Na and K in lignin ash was determined by atomic absorption spectrometry 22. The higher heating value (HHV) of lignin was determined in a calorimetric pump (Parr, USA) and the lower heating value (LHV) was obtained by correction with 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) was determined according to the methodology presented by Ripperger et al. 24 (Eq. 1). =

∙∙ ∙∆

∙

∙ + ∙∆

Eq. 1

where: t: time (s) A: filter area (m2) αm: average cake resistance relative to dry mass (m/kg) β: resistance of the filter medium (1/m) η: viscosity (Pa.s) m: mass of the dry cake (kg) VF: volume of filtrate (m3) Km: concentration factor, proportionality mass of cake/volume of filtrate (kg/m)


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At constant pressure difference, the plotting of (t/VF) vs. VF allows 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) .

=

∙∙ ∙∆

∙ +

∙ ∙∆

Eq. 2

RESULTS AND DISCUSION. 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 39% (m/m) 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 98% and 96% total lignin, respectively; additionally, the ash content for run 1 and 2 was 3% and 1%, respectively. The physical and chemical properties of lignin obtained in run 1 were thoroughly analyzed (Table 2), and it 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 it has been reported by other researchers 3,25,26

. Regarding its chemical composition, it presents a level of sulfur comparable to the fuel oil

refined in Uruguay from imported petroleum 27, and it has a low content of K or Na (Table 2). Scale-up calculations


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It was observed that the filtration of the precipitated lignin was easily done, where the filtering of the re-suspended 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 re-suspended slurry at 2.5 bar (Eq. 1 and Eq. 2). According to these results, 650 kg dry solids of lignin could be obtained by the filtration of the re-suspended 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). Due to the specific cake resistance, the filtration of acid-washed lignin is the bottle-neck 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 by using compressed air. Such an equipment, known as Nutsche filters, are standardized products that usually combine filtration and vacuum drying that enables 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 large-scale 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 U$S 660,000. 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 to produce low quantities of lignin, adjusted to specific customers demand for niche markets.


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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, USA) heat exchangers; and a double drum dryer. Experimentally, 40 L of water for kg of lignin were used to acid-wash the lignin in the pilot plant to allow the observation of the sedimentation of the solid,; however, the escalation model considered the relation presented by Loufti et al.: 4.2 kg water/kg 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 3,400 ton/year, which could substitute 5% of the thermal energy obtained from 145,000 ton of fuelwood, as reportedly was used in 2012 by the local industrial sector of Cellulose, Paper and Wood 30. This size is comparable to the 4,000 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 U$S/ton 32. It was assumed that the plant employed three full-time operators and one quarter of the time of one supervisor per shift, and labor cost for operators and the supervisor was taken at 5 U$S/h and 15 U$S/h, respectively. Electricity was considered at 92 U$S/MWh 33 (Table 4). The reported price for lignin ranges between 600 and 800 U$S/ton 34, therefore, at 676 U$S/ton, the cost of production obtained seems high. At a production rate of 10 ton/day, H2SO4 represents more than one third of the total cost. The cost of H2SO4 to lower the pH during


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washing was included in the operating cost because 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 U$S/t 36. Due to the

high cost of CO2, the alternative of using CO2 produced at site using combustion gases could be analyzed 34. At 26 U$S/GJ, the obtained lignin is more expensive than other fuel alternatives, and much more expensive than air-dried wood (Table 1). Under the local conditions, wood as biofuel is more economically attractive than lignin, or any other available fuel. Major changes in the energetic matrix of Uruguay must occur before other sources of thermal energy than wood are considered (Table 1). Therefore, the technology should be improved to lower the production cost. Further research should consider the following aspects: 1) the substitution of H2SO4 for CO2 produced on site as the acidifying agent, as proposed in existing patents 10,11; 2) the improvement of the recovery efficiency to diminish the capital cost of the process. Two more considerations: 1) in Uruguay the market price of electric energy obtained by biomass is declining, due to the increase of energy produced by wind farms; at the end of 2015 the reference price of that source was 71 U$S/MWh 37; 2) to compete locally as biofuel, lignin should at least have a lower price than fuel-oil, 13 U$S/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 LignoForce39. Differently from both processes, this work presents the use of H2SO4 as acidifying


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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 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 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 than biofuel. CONCLUSIONS. 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 single-vessel pilot plant with a daily production capacity of half of ton of lignin, such as the one modelled in this work, could be the adequate equipment to design a recovery process that complies with detailed specifications. At 26 U$S/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


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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.


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Scheme 1.

Representation of the pilot plant


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Table 1. Cost of available fuel for Uruguayan conditions (LHV) U$S/ton

U$S/GJ

60

5

Fuel oil2

561

13

Gas oil2

1,049

23

Fuel Air-dried wood1

Natural gas from Argentina3

24

1

Eucalyptus spp. wood dried for at least five months (30% wb.) sold at the plant.

2

ANCAP 2016 (tax included)

3

ANCAP 42


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Table 2.

Physical and chemical properties of lignin obtained in the pilot plant by acid

precipitation (run 1) Lignin property (unit)

Value

LHV (MJ/kg)

26.2*

Bulk density (freely settled dry lignin) (kg/m)

804

Particle density (kg/m)

1010

Acid-insoluble lignin (mass fraction %, dry basis)

86.7*

Acid-soluble lignin (mass fraction %, dry basis)

7.4*

Carbohydrate content (mass fraction %, dry basis)

0.7*

Elemental analysis (mass fraction %, dry basis) C

60.4

H

5.3

N

0.2

S

3.5

O

28.5**

Ash content (mass fraction %)

2.1*

Na (ash mass fraction %, dry basis)

13.7

K (ash mass fraction %, dry basis)

2.0

* Average of two repetitions ** Obtained by difference


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Table 3. Cake and filter medium resistance, adjustment to linearity of t/VF vs. VF, and acidsoluble and –insoluble lignin content for acid-washed precipitated slurry.

Run

Filter Cake medium resistance resistance (αm) (β) m/kg

R

2

1/m

Acid-insoluble lignin

Acid-soluble lignin

Mass fraction %, dry basis

Mass fraction %, dry basis

1

2x1012

1x1013 0.88

80.6

7.7

2

7x1011

1x1013 0.97

92.7

7.2

3

1x1012

8x1012 0.85

85.4

7.2

4

4x1012

9x1012 0.98

92.2

7.5


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Table 4.

Costs for the installation and operation of a plant for the recovery of 10 ton/day of

lignin from black liquor simulated in Aspen Plus V8.6

Cost component Capital cost

Cost (U$S/ton) 92

Operating cost*

257

H2SO4 (98% v/v)

263

Electricity Total

64 676

* Excluding H2SO4 used for precipitation and electricity


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AUTHOR INFORMATION Corresponding Author *Andrés Dieste Instituto de Ingeniería Química Facultad de Ingeniería Universidad de la República Julio Herrera y Reissig 565 Montevideo 11300 Uruguay E-mail: [email protected]

FUNDING SOURCES The research was founded by the Fondo Sectorial de Energía de la Agencia Nacional de Investigación e Innovación of Uruguay (FSE_1_2013_1_10726). ACKNOWLEDGMENT 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 Energía de la Agencia Nacional de Investigación e Innovación (ANII) (FSE_1_2013_1_10726). The research group also acknowledges ANII for financially support the visit of Dr. Stéphan Barbe. Additionally, the research group would like to thank the collaboration of UPM and FANAPEL for the provision of raw materials and logistic support, and Mr. R. Pérez Veiga from Dupla SA for his technical assistance during the filtration experiments.


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Lignin from Eucalyptus spp. Kraft Black Liquor as Biofuel - American

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