Utilization of Organosolv Waste Waters as Liquid Phase for

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Utilization of organosolv waste waters as liquid phase for hydrothermal carbonization of chaff Barbara Weiner, Harald Wedwitschka, Juergen Poerschmann, and Frank-Dieter Kopinke ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01665 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 6, 2016

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ACS Sustainable Chemistry & Engineering

Utilization of organosolv waste waters as liquid phase for hydrothermal carbonization of chaff Barbara Weinera*, Harald Wedwitschkab, Juergen Poerschmanna, Frank-Dieter Kopinkea AUTHOR ADDRESS a

Helmholtz-Center for Environmental Research - UFZ, Department of Environmental

Engineering, Permoserstr. 15, D-04318 Leipzig, Germany b

DBFZ-Deutsches Biomasseforschungszentrum, Department of Biochemical Conversion,

Torgauer Straße 116, D-04347 Leipzig, Germany *Email: [email protected] KEYWORDS hydrothermal carbonization – organosolv process – anaerobic digestion – process water – industrial waste water treatment.

ABSTRACT

Organosolv waste water was used as liquid source during the hydrothermal carbonization (HTC) of chaff at 200°C for 4 h and compared to a control HTC of chaff in water. The produced hydrochars had a carbon content of 58 wt% and a higher heating value (HHV) of 23 MJ kg-1

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independently of the employed liquid source. Concentrations of inorganic elements were analyzed as well as organic acids in the process water. The use of organosolv waste water had no negative effects on the HTC process and hydrochar properties. Subsequently, the HTC process waters were used for biomethane production. Performing HTC in organosolv water led to a doubling of the biomethane yield as compared to water as liquid source. The use of this industrial waste water in combination with the solid agricultural waste chaff has the potential of increasing the sustainability of the HTC process by disposal of two waste stream sources at the same time as well as by reducing the amount of contaminated waters that would eventually have to be treated.

INTRODUCTION In hydrothermal carbonization (HTC) biomass is heated in an aqueous suspension under subcritical conditions at temperatures of 180-250°C for several hours in a closed system.1 A solid carbonaceous material, the hydrochar, is produced, but significant amounts of the organic carbon are dissolved in the process water (PW).2 Due to a variety of chemical reactions occurring during carbonization, such as dehydration and decarboxylation, hydrochars have higher carbon contents than their feedstocks and, therefore, increased higher heating values (HHV). Thus, hydrochars can be used as alternative fuel to replace coal-derived energy sources,3 but they have also been tested as biochars for soil amendment1 and sustainable carbon materials.4 A large variety of biogenic feedstock materials has been employed for HTC, among those carbohydrates and lignocellulosic materials,5,6 but also waste biomasses such as municipal solid waste7,8 and sewage sludge9.


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Because of its high dissolved organic carbon (DOC) content beyond 10 g L-1 and high chemical oxygen demand (COD), the PW needs to be treated in a waste water treatment plant prior to discharge. This can be costly as large amounts of liquids are recovered from HTC. Solutions for reducing PW treatment costs could be a subsequent post-treatment by wet oxidation,10 recirculation of PW and its reuse in the HTC process,11,12,13 or valorization by anaerobic digestion to produce biogas. The anaerobic digestion of PWs has been described by several groups for a variety of HTC-substrates, such as corn silage,14 agricultural residues,15 olive mill waste water,16 brewer’s spent grains,17 paper,13 orange pomace,18 and sewage sludge19. As an alternative liquid source for HTC, industrial and municipal waste waters could be employed. An additional benefit would be the disposal of waste waters, which could result in economic savings due to reduction in disposal costs. The use of waste streams, that contain organic carbon material, may have positive impacts on hydrochar production as seen with process water recirculation.11 Lu et al. were the first to test this concept by using model additives, such as various concentrations of inorganic acids, base, salt and acetic acid, in the employed water during HTC of cellulose.20 The addition of these additives resulted in little influence on the HTC process, the carbonized products and yields of hydrochars. Secondly, municipal waste streams, precisely landfill leachate and activated sludge, were investigated as liquid source in HTC of municipal solid waste.8 Similarly to the test system, only minimal impacts were seen on the carbonization process: The only statistically significant influence was obtained on the energy content of the solid products. An alternative industrial waste water source could also be organosolv waste waters. The organosolv process is a biorefinery process for the sustainable use of lignocellulosic, non-food renewable biomass to produce value added products for industry, namely cellulose but also pure


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lignin for chemicals and materials usage.21 In order to maximize the valorization of the complete lignocellulosic precursor, ideally all (waste) fractions should be utilized within the biorefinery process. The organosolv process at the Frauenhofer CBP in Leuna, Germany, uses a mixture of ethanol and water to fractionate beech wood under a maximum pressure of 35 bar and a maximum temperature of 200ºC into solid cellulose and dissolved hemicellulose as well as lignin.22,23 The solid cellulose residue is disintegrated and washed, dewatered, and enzymatically treated to form carbohydrates, which could be further converted to fine chemicals. From the liquid pulping liquor lignin is precipitated by either removal of ethanol by distillation or dilution by addition of water, filtered off, washed and dried. The used ethanol is entirely recovered by distillation from the filtrate and the hemicellulose derived C5-sugars remain as an aqueous solution. In an economic assessment of the organosolv process it was pointed out that this aqueous fraction also needs to be valorized for an integrated material utilization concept for lignocellulose in order to avoid waste effluent streams with high treatment costs.22 Some of the chemical constituents of this organosolv water stream are residual ethanol, acetic acid, xylose and other C5-sugars, oligopentoses and other low molecular weight organic molecules.22 Among those hydroxyacetone, furfural, 5-methylfurfural, 5-hydroxymethylfurfural, 2-ethylbutanoic acid, phenol, eugenol, syringol, vanillin, syringylaldehyde, and coniferaldehyde have been identified.24 But higher molecular weight organic compounds are also likely to be present. For further valorization, it is challenging to separate individual compounds from the complex mixture. Therefore, treating and utilizing the complete organosolv liquid effluent without any separation in the HTC process could be a promising way.


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The objective of this project was to study the impact of using organosolv waste water as alternative liquid source for the HTC of chaff. Chaff is a dry waste material obtained from grain threshing with dry matter contents of 88 wt%, hence, water has to be added anyways for HTC purposes. Chaff obtained from cereal mills can be used for livestock feed provided the material quality is sufficient. However, the material is often contaminated with mycotoxins and must be disposed. Chaff can further be used for energy production by combustion but has limited use for biogas production. In biogas processes mycotoxin contaminated substrates can cause severe process disturbance. Hydrothermal treatment of mycotoxin contaminated biomass is an option for substrate detoxification. It is expected to destroy mycotoxins in accordance to experiments on organic pollutant degradation under HTC conditions.25 Thus, two waste streams would undergo a valorization combining the production of hydrochars with the treatment of waste effluents. The impact of the organosolv water source on the produced hydrochar and process water quality was investigated via carbon balance, energy and mass yields, HHV and distribution of inorganic minerals and nutrients. Furthermore, the potential for the subsequent production of biomethane from HTC-PWs compared to native organosolv water was assessed.

METHODS Experimental. Chaff was obtained as a residue material from a flour mill in Thuringia, Germany, and stored at 4ºC until use. Organosolv water was produced in the lignocellulose biorefinery plant at Frauenhofer CBP in Leuna, Germany, by fractionation of debarked beech wood (Fagus sylvatica) chips as described in

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and stored at -18ºC until use. The hydrothermal

process was performed in an autoclave with a capacity of 200 mL (Roth, Karlsruhe, Germany)


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filled with i) 150 mL of organosolv water (1 wt% dry mass), ii) 150 mL of distilled water and 35 g of chaff (88 wt% dry mass) adjusted with H2SO4 to pH 2, and iii) 150 mL of organosolv water and 35 g of chaff, respectively. The autoclave was heated to 200°C for 4 h. After cooling to room temperature, the mixture was filtered under vacuum through medium filter paper (Schleicher & Schuell 589/2, Germany). The solids were washed with distilled water (110 mL) in order to completely remove the PW for carbon balance analysis of the liquid and solid phase. The solids were dried at 105°C for 24 h. Experiments were performed in duplicate, values are reported as averages over these experiments. Error bars in figures correspond to estimated mean deviations among individual experiments. Process water analysis. The process waters and native organosolv water were filtered through 0.45 µm syringe filters (regenerated cellulose; Lab Logistics Group, U.S.) prior to DOC analysis. DOC was analyzed using a total organic carbon analyzer TOC 600 (Shimadzu, Germany). Total nitrogen (TN) content was measured on a total nitrogen measuring unit TNM-1 coupled to a TOC-VCSN analyzer (Shimadzu, Germany). The COD was determined using test kits (LCK014, Hach Lange, Germany), the total-P with phosphate test kits including a hydrolysis step (LCK349). The pH was measured with a pH meter (MP 225, Mettler Toledo, Gieβen, Germany) and the electrical conductivity on a multimeter (MultiLine P4, WTW, Weilheim, Germany). Ion chromatography was performed on a Dionex ion chromatograph (DX600) equipped with an anion suppressor (ASRS300), conductivity detector (CD20) and two IonPac AS18 AnionExchange columns (4 x 250 mm) in series using a flow rate of 0.8 mL min-1 and gradient elution with the following program: 2 mM KOH from 0 – 11 min, increase to 18 mM KOH over 6 min, hold at 18 mM KOH for 1 min, followed by an increase to 35 mM KOH over 12 min and held


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for 10 min. Prior to analysis, the samples were filtered through 0.2 µm syringe filters (PTFE; VWR international, U.S.). Hydrochar analysis. Elemental analysis (C, H, N) of hydrochars was performed on an automatic analyzer (CHN 932, LECO Instruments, Moenchengladbach, Germany). The ash content was determined by a burning process at 900°C. The oxygen content was calculated from the difference between the ash-free dry mass and the combined masses of the measured elements. HHVs were calculated according to Channiwala and Parikh.26 The corresponding equation, HHV improvement and energy yield calculations can be found in the Supporting Information. X-ray fluorescence measurements were performed using a WDXRF-spectrometer S4 PIONEER (Bruker-AXS, Bremen, Germany), equipped with a 4 kW-Rh X-ray tube (75 lm Be window) and a 60 kV generator. Biochemical methane potential test. The biochemical methane potential (BMP) test was conducted on laboratory scale with the AMPTS2-device (Bioprocesscontrol, Lund, Sweden) in accordance to the method guideline.27 The incubation took place in glass culture bottles at mesophilic conditions (38°C) to determine the specific methane yield of organosolv water and HTC process waters. The samples consisted of 380 g inoculum and 20-40 g sample volume, each in 3 replications. The inoculum was taken from the research biogas plant of the DBFZ from a continuous digestion of cattle manure and maize silage. To monitor the inoculum performance, each test run included triplicate control cultures containing inoculum and microcrystalline cellulose as reference substrate. The required inoculum performance was achieved when the inoculum converted at least 70% of the reference cellulose to biogas during the incubation period as defined by the American Society for Testing and Materials.28 The methane yields were standardized (273.15 K, 0.1013 MPa). The BMP test was terminated after 12 days when the


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daily biogas production did not exceed 1% of the total accumulated biogas volume (see VDI 4630).27

RESULTS AND DISCUSSION Carbon and mass balance Carbon balance and mass and energy yields of HTC of chaff in organosolv water were compared to a control experiment with chaff in distilled water adjusted with H2SO4 to pH 2 in order to exclude effects of different pH values. Additionally, organosolv water itself was treated hydrothermally. The carbon distribution between solid and liquid phase with regard to carbon input of chaff is shown in Figure 1a. 74% of the chaff input-C was deposited in the hydrochars independently of the liquid source. Both process waters contained roughly 22% of the chaff carbon input. In all HTC experiments only minor amounts of gases were formed. HTC of organosolv water alone yielded low amounts of solids, only 8% of the input-C carbonized. When the combined carbon input of chaff and DOC in the organosolv water was considered (Figure 1b), 67% of the input-C was found in the solid phase, 22% in the liquid and 10% as gas or loss. The higher gas formation in the presence of organosolv water as compared to pure water could be due to an additional release of CO2 from the dissolved organics in the organosolv water. The carbon content of the hydrochars from HTC of chaff in organosolv or pure water was both 58 wt% (Table 1). The organic elemental composition remained constant irrespectively of the liquid source. Similarly, the HHV was 23 MJ per kg for both HTC chars. The impact of the


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organosolv water on the mass yield of solids was insignificant because of the small input mass on a dry matter basis of the organsolv solids (DM 1 wt%) as compared to the solid chaff (DM 88 wt%). Thus, no additional carbonization of dissolved organic matter (DOM) from the organosolv water contributed to the yields of hydrochars. Solid energy contents were unaffected by carbonization in the presence of organosolv matter. These results are comparable to the results obtained by Li et al. when landfill leachate and activated sludge were used as liquid source.8 It indicates that no or very little additional carbonization of DOM from the liquid source occurred. Contrary, Stemann et al. reported an increase of the carbon content of the hydrochars during HTC of wood with recirculated process water due to carbonization reactions of dissolved organics11, and Blöhse et al.12 as well as Weiner et al.13 reported an increase in solid mass yields when reusing PW for HTC of biogenic waste and paper, respectively. In all of these PW recirculation experiments, the DOC content was much higher than in the above described HTC with landfill leachate and activated sludge and the herein conducted experiments with organosolv water. It suggests that alternative liquid sources with high concentrations of organics could have a positive impact on yields or energy content of solids. The findings herein showed that both waste streams could be combined to obtain hydrochars with increased carbon contents and heating values as compared to the input-material. The use of the organosolv waste stream as liquid source did not have any negative impacts on the hydrochar characteristics. Distribution of inorganic matter


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Table 2 summarizes the inorganic cations as well as S, P and Si in native chaff and organosolv water and the produced hydrochars based on a dry matter basis determined by x-ray fluorescence analysis. The most abundant hetero-element in the organosolv water was sulfur. The sulfur species constituted not only of sulfate ions, which were determined by IC in concentrations of 403 mg L-1, but likely also of organosulfur compounds. The sulfur components are known to be problematic substrates for biogas production as toxic and odor intensive H2S is formed during anaerobic digestion.24 Most abundant in chaff was Si, but also K and Na. P amounted to 1.8 g kg1

of dry mass. During HTC, Al and Cu were nearly quantitatively transferred into hydrochars

when carbonizing chaff in the presence of water or organosolv effluent. The recovery of P in the hydrochars was relatively low under the given process conditions with 22% in HTC of chaff and only 16% in HTC of chaff with organosolv effluent, respectively. Thus, the largest fraction of P was transferred into the PW in the form of phosphate as determined by IC and total-P tests (Tables 3 and 4). Sulfur was retained to 66% in hydrochars of chaff and to 37% in hydrochars of chaff with organosolv water. The lower recoveries for S in the latter were mainly due to the comparably high amounts of sulfur in organosolv waters. Most cations such as Mg2+, Ca2+ and Na+ are present in the solid as well in the aqueous phase, while K+ prefers clearly the dissolved state which is in conformity with the existing literature.29 Phytotoxic elements, such as Zn and Cu, were present in low concentrations in the hydrochars. The concentrations were below the legal limits for application as solid fertilizers, thus, the hydrochars could potentially be used in agriculture as biochars. The ash content in the solids was roughly 5 wt% for all hydrochars, thus the produced hydrochars might be suitable alternative fuel sources. Of the original mineral content 87% were recovered in the hydrochar when chaff was carbonized in water and 79% in organosolv water. The residual inorganic minerals were likely dissolved in the PW.


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Liquid phase analysis The characteristics of all PWs and organosolv water are shown in Tables 3 and 4. The DOC content of the original organosolv effluent was around 10 g L-1 and only slightly lower for the hydrothermally treated organosolv PW (9 g L-1) indicating that carbonization of dissolved organic species does not significantly take place. Correspondingly, the CODs of the two aqueous phases – before and after the HTC treatment – are identical. The COD and DOC content of the PW increased when using organosolv as liquid source in HTC of chaff, contrary to the results observed by Li et al. in which the initial liquid stream source did not influence COD and TOC of the produced PW.8 For HTC of chaff in organosolv water, a predicted CODpred was calculated according to Li et al. via equation 1,8

= + ∗

!"#$ % & '(#))

(1)

where CODHTC chaff was the COD of the PW after HTC of chaff in water and CODHTC organosolv the COD of PW in the control HTC of organosolv effluent, Mliq

HTC organosolv

was the mass of the

organosolv liquid input and Mliq HTC chaff the mass of the input water for the HTC of chaff, both in g, respectively. When comparing the measured COD (69.6 g L-1) to the predicted COD (83.9 g L1

), it became evident that the COD was over-estimated by equ. 1, suggesting that some of the

organosolv organics were released as gases or might have carbonized to solids (Figure 1). A COD to DOC ratio of 3.0 suggests the presence of more reduced species derived from the organosolv waters, in the control experiment with water it was 2.3 illustrating the presence of more oxidized species. Liquid mass recoveries slightly above 100% showed the formation of water due to dehydration of solids during HTC. The pH of 2 was unchanged after hydrothermal treatment of organosolv water alone and was around 3 when chaff was used, indicating a net


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consumption of acidity by carbonization reactions. The conductivity of chaff containing PW was higher than that of organosolv effluent alone which is in line with the dissolution of electrolytes and minerals as shown in Table 2 as well as the production of organic acids (Table 4). Among the organic acids produced from HTC of chaff, acetic acid amounts to the highest concentrations, followed by lactic and formic acid. The differences among the liquid sources for HTC of chaff were relatively low. Only the concentration of acetate was slightly higher than calculated by addition of the two liquid sources which could be favorable for biological degradation of the corresponding PW organics. Phosphate was found to be transferred from chaff to a large extent into the PW independently of the liquid source. Phosphorus was present in the form of phosphate as determined by total-P analysis (Table 3) and IC analysis of phosphate (Table 4). Thus, using the PW as fertilizer might be a potential pathway to return phosphorus to soil. Biogas production The use of organosolv water for biomethane production (BMP) had already been tested.24 However, it was reported that these liquors could pose potential problems for biogas production. Among those were the low pH of 2, a deficit in macro- and micronutrients which needed to be externally supplied, and an inhibition through phenols and furfurals. Methane potential tests had shown process instability also due to organic acid accumulation. The BMP tests carried out in this study did not show inhibitory effects caused by the substrate. Continuous digestion trials could be carried out to determine the long time process stability. Hydrothermal treatment increased the methane yield from organosolv water (Figure 2). This could be due to conversion, and thus removal, of furfurals and polymeric compounds that are known to inhibit biomethane production. The PW from HTC of organosolv effluent alone


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showed the highest biogas production. When using organosolv effluent as liquid source for the carbonization of chaff, biomethane formation was more than doubled compared to PW from HTC of chaff alone. Chaff PW seems to contain many hardly biodegradable components. The biogas potentials in relation to the content of reduced organic matter (per gCOD) and the processed amount of water (per mL of water) are shown in Table 5. For reasons of comparison, residual PWs from biomass processing industries such as distillers spent grain gave methane yields of ~22 mL mL-1, potato juice of ~30 mL mL-1, and raw glycerol of ~147 mL mL-1.30 Conclusions In order to minimize the production of organically contaminated water from clean fresh waters, organosolv waste water could be used as liquid source without changing the properties of the produced hydrochars. The use of organosolv water as alternative liquid source in HTC has the potential of increasing the sustainability of the organosolv process as well as the HTC process. The PW from the hydrothermally treated organosolv waste stream could be used for biogas production, which yields higher amounts of methane than native organosolv waters. An alternative utilization could be the application of the PW from chaff in organosolv water as fertilizer as nutrients, especially phosphorus, were transferred to the liquid phase. However, it needs to be assured that no toxic contaminants remain in the PW. Hydrochars obtained through carbonization of chaff showed an increase in HHV and would be suitable as fuel and potentially also as biochars, as the amount of phytotoxic minerals was below the legal limits for agricultural applications. Further HTC experiments under variation of the process conditions should be conducted in order to make a sustainability and life cycle assessment for quantification of the benefits for using organosolv water as the alternative liquid source. Practical limitations of this


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approach are the necessary proximity of the solid waste source for example chaff, the organosolv facility and HTC plant in order to minimize transportation costs.


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Figure 1. Carbon distribution between solid and liquid phases as percent of carbon input: a) as percentage of solid carbon input, for HTC organosolv: input organosolv-C, HTC chaff and HTC chaff+organosolv: input chaff-C; b) as percent of total carbon input for HTC chaff+organosolv as the sum of chaff-C + organosolv-C. HTC conditions: 200°C for 4 h, pHstart = 2 .


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250 Specific standard volume methane [mLN g-1 COD]

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200

150

100

HTC organosolv 50

HTC chaff+organosolv organosolv HTC chaff

0 0

2

4

6 time [d]

8

10

12

Figure 2. Specific standard volume of biomethane production by anaerobic digestion of PWs and native organosolv water.


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Table 1. Mass balances and composition of input materials and solid hydrochars. Source

C

H

N

O

ash

solids mass HHV recovery [MJ kg-1]

energy yield [%]

wt% Organosolv water

45.72

6.28

0.12

41.70

6.2

-

-

-

Chaff

45.39

6.30

1.13

44.09

3.1

-

18.6

-

HTC organosolv

66.19

4.55

Utilization of Organosolv Waste Waters as Liquid Phase for

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