Bioresource Technology 164 (2014) 162–169

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Characterization of biocoals and dissolved organic matter phases obtained upon hydrothermal carbonization of brewer’s spent grain J. Poerschmann a,⇑, B. Weiner a, H. Wedwitschka b, I. Baskyr a, R. Koehler a, F.-D. Kopinke a a b

UFZ-Helmholtz Center for Environmental Research, Department of Environmental Engineering, Permoserstr. 15, D-04318 Leipzig, Germany DBFZ-Deutsches Biomasseforschungszentrum, Department of Biochemical Conversion, Torgauer Straße 116, D-04347 Leipzig, Germany

h i g h l i g h t s  The by-product brewer’s spent grain was subjected to hydrothermal carbonization.  Biocoal yield is high due to high carbohydrate content.  Biocoals reveal high energy densification and high sorption capabilities.  Biogas production from process water gives high yields.  Phenols and fatty acids proved abundant analytes in the process water.

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Article history: Received 13 January 2014 Received in revised form 15 April 2014 Accepted 17 April 2014 Available online 26 April 2014 Keywords: Hydrothermal carbonization Brewer’s spent grain Biocoal Organic carbon balance

a b s t r a c t The wet biomass brewer’s spent grain was subjected to hydrothermal carbonization to produce biocoal. Mass balance considerations indicate for about two thirds of the organic carbon of the input biomass to be transferred into the biocoal. The van Krevelen plot refers to a high degree of defunctionalization with decarboxylation prevailing over dehydration. Calorific data revealed a significant energy densification of biocoals as compared to the input substrate. Sorption coefficients of organic analytes covering a wide range of hydrophobicities and polarities on biocoal were similar to those for dissolved humic acids. Data from GC/MS analysis indicated that phenols and benzenediols along with fatty acids released from bound lipids during the hydrothermal process constituted abundant products. Our findings demonstrate that the brewer’s spent grain by-product is a good feedstock for hydrothermal carbonization to produce biocoal, the latter offering good prospects for energetic and soil-improving application fields. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Brewer’s spent grain (BSG) is the residue left after separation of the wort during the brewing process and is available in large quantities throughout the year. Its worldwide annual output amounts to about 30 million tons, in Germany it is about 0.7 million tons as dry mass (Schuchardt and Vorlop, 2010). The composition of this agroindustrial co-product may vary with barley variety, time of harvest, adjuncts added and brewing technology (Niemi et al., 2012). The main application of BSG has been chiefly tied to animal feeding, favorably combined with green manure or maize silage. Shortcomings of BSG use in animal feeding are associated with the cyclic generation peaking at summer times, as well as with the low degradation rate of raw protein in rumen (Santos et al., 2003). To upgrade BSG use and develop innovative value schemes, ⇑ Corresponding author. Tel.: +49 341 235 1761; fax: +49 341 235 2492. E-mail address: [email protected] (J. Poerschmann). http://dx.doi.org/10.1016/j.biortech.2014.04.052 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

attempts have been made to use it in biotechnological processes, such as cultivation of mushrooms and actinobacteria and as a source of value-added products like ferulic and p-coumaric acids or sugar alcohols (Faulds et al., 2004). The importance of the wet biomass as an industrial feedstock is hampered by its chemical deterioration and its susceptibility to microbial attacks resulting in rapid colonization by bacteria unless it is further dried to a moisture content of 10% or less. A promising, environmentally benign approach which is pursued in the framework of this contribution is to use wet BSG (dry mass 20–25% w/w) as a feedstock for hydrothermal carbonization (HTC). This is a process in which wet biomass is hydrothermally converted in aqueous suspensions at moderate temperatures (180–250 °C) and medium pressure into carbonaceous materials (Baccile et al., 2009). The hot water serves as the solvent and reactant, favoring ion chemistry while concomitantly suppressing free radical reactions. HTC processes have the tremendous advantage that the starting biomass does not need to be dry, thus the need

J. Poerschmann et al. / Bioresource Technology 164 (2014) 162–169

for energy-intensive removal of water is eliminated. A wide array of natural raw biomasses as well as agricultural and municipal waste products including liquid manure, sewage sludge, maize grist, molasses, and sugar beet residues have been subjected to HTC (Funke and Ziegler, 2009). The HTC process is characterized by a combined hydrolysis– deoxygenation–polymerization mechanism (Titirici and Antonietti, 2010). As a result of hydrothermal treatment of biomasses, an aqueous phase is generated next to the water-insoluble biocoal. The biocoal can be used to enhance soil fertility (Steinbeiss et al., 2009), for energetic purposes and/or to produce tailor-made sorbents and catalysts with defined functionalization and pore structure (Titirici and Antonietti, 2010). In contrast to biocoal, the dissolved organic matter (DOM) in the process water is basically an unwanted side-product. Herein, the suitability of BSG as an input substrate to produce valuable biocoal is addressed including the following objectives: (i) To determine the mass balance of the organic carbon (OC)fraction of the biomass at different operating temperatures. In an ideal scenario, the OC-fraction is overwhelmingly converted into biocoal with a minor DOM content and negligible formation of gases (chiefly CO2). (ii) To determine the mass balance of other elements including nitrogen, phosphorous, and heavy metals. (iii) To study the degree of defunctionalization of biomass compartments on hydrothermal treatment. Clearly, high defunctionalization of biomass compartments (cellulose, hemicellulose, lignin, proteins) results in high calorific value of the biocoals in view of a potential energy source. (iv) To study the sorption capability of biocoals in the face of their potential application as sorbent in wastewater treatment. (v) To characterize low molecular weight organic components (MW 6 300 Da) in the process water in the light of prospective obtaining of value-added products (e.g. levulinic acid) and as a source of biogas when applying a cascadic use of the process water rich in DOM. BSG was already used as input substrate for HTC (Heilmann et al., 2011). The focus of that contribution was the mechanism of coal formation and characterization of the biocoal by microscopy rather than addressing the objectives listed above. Our contribution is aimed at supplementing the findings published by Heilmann et al. rather than compete with them. The combined data (meaning: our data and Heilman’s data) are expected to foster the field of utilizing BSG as a substrate for HTC.

2. Methods 2.1. Hydrothermal carbonization of BSG BSG was delivered by Reudnitzer Brewery Leipzig, Germany, and stored at 4 °C until use. The hydrothermal process was performed in an autoclave with a capacity of 200 mL (Roth; Karlsruhe, Germany) filled with 50 g BSG (23.5% w/w dry mass) and 50 mL of distilled water. The dilution proved beneficial for further handling of product fractions resulting from the HTC-process. The HTC parameters were chosen according to findings from previous HTC experiments with a wide array of biomasses (Poerschmann et al., 2013): operating temperature of 200 and 240 °C, reaction time of 14 h, and 80 lg mL1 of citric acid as catalyst. The pH of the slurry obtained after carbonization was 2.8, that of the native slurry was 7.0. The shift indicates the formation of carboxylic acids. The HTC slurry was allowed to pass

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through a filter paper (Whatman filter paper, Grade 595, 150 lm thickness). After phase separation, process waters were evaporated to dryness and biocoals were dried, both at 105 °C (4 h), for elemental analysis, ash determination, and X-ray fluorescence spectroscopy. 2.2. Elemental analysis, X-ray fluorescence, bulk parameters An elemental analyzer LECO model CHN 932 (Moenchengladbach, Germany) was used to determine C, H, N and S in the input substrate, the biocoal and the lyophilized process water. The oxygen concentration was calculated from the difference between the ash-free dry mass and the combined mass of the elements measured. Elemental composition was determined in duplicate. The averaged data were reported herein. BSG compartments were isolated according to a scheme developed for maize plants to quantify rhizodeposition based on Sparling et al. (1982). Briefly, lipids were extracted with a chloroform/methanol solvent mixture (1:1, v/v), soluble carbohydrates with hot water, delignified holocellulose by refluxing the residue with sodium hypochlorite solution, and cellulose by 24% potassium hydroxide solution. 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. Total organic carbon (TOC) in filtered aqueous solutions was analyzed by a TOC analyzer (Shimadzu, Duisburg, Germany). Determination of the Chemical Oxygen Demand (COD) was carried out by a micro-method with chromo-sulfuric acid (cuvette test kit LCK014 from HACH LANGE GmbH, Duesseldorf, Germany). The biochemical oxygen demand (BOD) was measured manometrically with a WTW BOD analyzer OxiTopÒ IS 6 (Weilheim, Germany). The samples were dried at 105 °C prior to analysis. The ash fraction was determined by a burning process at 550 °C. The specific surface area was determined by BET analysis using a Belsorp Mini (Microtrac, Meerbusch, Germany) with multipoint adsorption isotherms of nitrogen at 77 K. 2.3. Exhaustive extraction of HTC matrices Five hundred mg aliquots of the slurry lyophilizate, the DOM lyophilizate resulting from HTC at 200 °C, and the lyophilized BSG substrate were each subjected to exhaustive pressurized solvent extraction. The lyophilizates were spiked with internal standards before freeze drying. Internal standards included [2H2]palmitic acid, [2H4]-lactic acid, [2H6]-phenol, [2H4]-hydroquinone, and [2H10]-phenanthrene in acetone to give a final concentration of 1000 lg g1 each ([2H10]-phenanthrene: 10 lg g1) referred to BSG as dry matter (DM). Standards were purchased from Ehrenstorfer (Augsburg, Germany); all solvents from Aldrich (Munich, Germany). Labeled palmitic acid was used to quantify long-chain monocarboxylic acids, whereas lactic acid served for quantifying short-chain dicarboxylic acids and short-chain hydroxyl acids. Labeled phenol and hydroquinone served for quantification of phenols and benzenediols, respectively. Hydrophobic analytes were calibrated against labeled phenanthrene. The experimental extraction details were the following: extraction device ASE 300 (ThermoFisher, Dreieich, Germany), 11 mL cell cartridges filled with inert Hydromatrix to capacity, benzene/acetone/methanol (2:1:0.5, v/v/v) solvent mixture. Two 10 min cycles at a temperature of 100 °C and 12 MPa pressure were applied to each sample. Solvents were sparged with helium to prevent oxidative cleavage of susceptible analytes including linolenic acid (18:3xccc) during extraction. Clean-up of the combined extracts was performed by SPE using DSC-diol cartridges (spacer bonded 2,3-dihydroxypropoxypropyl; Supelco, Munich, Germany) with chloroform/acetone (1:1, v/v) as eluent. Purified extracts were dried over anhydrous

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sodium sulfate, then concentrated in a rotary evaporator to a final volume of 100 lL. Derivatization of volume-reduced extracts involved a silylation with BSTFA (N,O-bis-(trimethylsilyl) trifluoroacetamide, Sigma Aldrich) to produce TMS (trimethylsilyl) ethers, as well as a combined transmethylation with boron trifluoride (14% w/w in methanol; Sigma Aldrich Munich, Germany) to produce non-polar methyl esters of mono- and dicarboxylic acids followed by silylation with BSTFA. Structural assignments were confirmed by using silyl ethers produced with isotopically labeled BSTFA-d9 (Ehrenstorfer). Saponification of solvent extracts originating from both the lyophilized process water and the BSG substrate was performed with 3 N methanolic sodium hydroxide solution. 2.4. Sorption of organic compounds on HTC coal Sorption coefficients on HTC coal (200 °C) were determined by Solid Phase Microextraction (SPME) at pH  4. This method was elaborated and validated in our institute to overcome shortcomings in the traditional determination of sorption coefficients in both particulate and dissolved sorbents (Kopinke et al., 2002). The basic idea of using the SPME analytical technique to study sorption phenomena is that only the freely dissolved fraction of the target analytes is sampled by the SPME-fiber. By contrast, the analyte fraction bound to both dissolved and solid sorbents is not enriched onto the fiber coating. Experimental details to conduct these experiments are given in the Supplementary information section (S-1 and S-2) as well as in a recent publication (Poerschmann and Schultze-Nobre, 2014). Solutions contained 100 lg mL1 sodium azide to inhibit microbial activity. Likewise, 5 mM calcium chloride was used to maintain relatively constant ionic strength. 2.5. GC/MS analysis A thin-film capillary column (30 m  0.25 mm) coated with 0.25 lm non-polar DB-1 MS stationary phase was used (Agilent Technologies, Waldbronn, Germany). The linear temperature program was as follows: initial temperature 40 °C, 10 °C min1 to a final temperature of 300 °C. Data acquisition was performed using HP 5973B MSD in full scan mode. 2.6. Biochemical methane potential test The biochemical methane potential test was carried out according to guidelines defined in VDI (2006). The potential methane production was obtained from the incubation of triplicate anaerobic batch cultures containing both the substrate and a methanogenic inoculum at a predetermined substrate/inoculum ratio. Incubation took place in glass culture bottles at mesophilic conditions (38 °C) over 30–45 days. The biogas volume was measured using the Eudiometer technology (Janzon et al., 2014). The methane content was determined with the Geotech GA2000 Landfill Gas Analyzer (Warwickshire, UK). The pH was measured at the beginning and at the end of the experiment. The test is stopped when the daily biogas production does not exceed 1% of the total accumulated biogas volume. The methanogenic inoculum was a mixture of sewage sludge from a wastewater treatment plant and of digested effluent from an agricultural biogas plant. To monitor the inoculum performance, each test run included triplicate control cultures containing inoculum and microcrystalline cellulose. 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 Material standard (ASTM, 2007).

3. Results and discussion 3.1. Mass balance of BSG during hydrothermal treatment 3.1.1. Total mass balance and carbon, nitrogen balance Table 1 provides information on physico-chemical characteristics of native BSG, which are comparable to those previously reported for BSG covering different origins (Santos et al., 2003; Mussatto et al., 2008). Table 2 lists data from gravimetric and elemental analysis along with COD and BOD5 data for BSG and the hydrothermally converted products. BSG gave a high yield of black-colored carbonaceous precipitates as a result of the hydrothermal treatment (51.1% and 47.5%, see Table 2). This finding is in line with the high content of carbohydrates in BSG (see Table 1). Carbohydrates are known as good substrates for HTC to form biocoal (Funke and Ziegler, 2009). Table 2 also shows that two thirds of the feedstock-organic carbon (OC) was fixed inside the biocoal network (67.5% – 200 °C, 67.6% – 240 °C). Increased HTC-temperatures resulted in lower coal mass yields but higher OC- content, which is in line with previous findings obtained with other biomasses (Funke and Ziegler, 2011; Parshetti et al., 2013). The high yield and the high carbon content of the biocoal points to a good suitability of BSG to the HTC process and is expected to be beneficial for a prospective energetic application. The high similarity between the OC- fraction retained in the biocoal at both 200 °C and 240 °C favors low operating temperatures of 200 °C in the face of a linear dependence between energy demand (synonymous with costs) and HTC temperature. Compared to the biocoal, a significantly lower fraction of the substrate OC was converted into the (basically undesirable) DOM-product stream (18.9% at 200 °C, 17.3% at 240 °C). The loss of substrate OC (13.6% – 200 °C, 15.1% – 240 °C) was mainly related to gas formation, as well as to experimental shortcomings (e.g. droplet and spill losses, losses by phase separation). Results from headspace analyses (not detailed here) showed that carbon dioxide constituted the most abundant gas with minor abundances of carbon monoxide, methane, ammonia, sulfur dioxide, methanethiol, dimethyl sulfide, and i-butanal (less than 15% combined). Carbon dioxide arises from decarboxylation reactions, it may also originate from the breakdown product formic acid (Lu et al., 2014). As a rough estimate, the combined fraction of gases referred to the substrate dry mass across a multitude of biomasses and HTC process parameters amounts to 9% (Funke and Ziegler, 2009).

Table 1 Chemical characteristics of BSG. Analysis

Result

Dry matter content (DM) (g kg1) pH of suspension Ash (%-w/w DM) Organic carbon (%-w/w DM) Elemental ratio C/N (g g1) COD (g O2 L1) BOD5 (g O2 L1) Cellulose (%-w/w DM) Hemicellulose (%-w/w DM) Soluble sugars (%-w/w DM) Proteins (%-w/w DM)a Total lipids (%-w/w DM)b Klason-lignin (%-w/w DM)c

235 6.9 4.5 51.3 13 262 153 18.5 26.5 1.0 21.5 7.3 19.1

a Protein fraction for the BSG as dry matter was calculated by multiplying the total nitrogen (standard method ASTM D-5291) value by a factor of 6.25. b Determined by Folch-extraction using chloroform/methanol (1:1, v/v). c Determined gravimetrically (after drying at 105 °C) after lipid extraction and carbohydrate removal.

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J. Poerschmann et al. / Bioresource Technology 164 (2014) 162–169 Table 2 Mass balances after hydrothermal treatment of BSG.

BSGEduct 200 °CDOM 200 °Cbiocoal Loss (%) 240 °CDOM 240 °Cbiocoal Loss (%)

COD (g L1)

BOD5 (g L1)

BOD5/ COD

DMMatrix/DMBSG (%, w/w)

262

153

0.58

100

60.8

47.1

0.77

21.2

b

VStand.,biogas/mODM (mL g1)a

479

51.1b

64.2

51.1

0.80

28 18.8b 47.5

654

b

34

OCDM (%, w/w)

HDM (%, w/w)

NDM (%, w/w)

SDM (%, w/w)

ODM (%, w/w)

AshDM (%, w/w)

DBE

51.3

6.71

4.07

0.45

32.9

4.5

45.7 (18.9%)c 67.8 (67.5%)c 13.6 47.2 (17.3%)c 72.9 (67.6%)c 15.1

6.65 (21%) 6.74 (52%) 27 6.48 (18%) 6.41 (45%) 37

8.55 (44%) 4.18 (49%) 7 8.15 (37%) 3.93 (46%) 17

0.80 (37%) 0.35 (39%) 24 0.75 (32%) 0.34 (36%) 32

29.2 (19%) 15.1 (23%) 58 28.9 (16%) 10.5 (15%) 69

8.7 (40%) 5.6 (64%) 0 8.6 (37%) 6.3 (67%) 0

4.67 (2.27)d 2.30 (1.26) 9.14 (9.68) 5.02 (2.78) 11.73 (17.85)

a

Standard volume of biogas referred to organic dry mass. Mass yield as (non-diluted) DM-dry matter (105 °C), DMBSG = 23.5% w/w; DMBSG, DMDOM, and DMBiocoal are each referred to DMBSG (%, w/w). In parentheses: % carbon recovered in the biocoal, i.e. OCDOM referred to OCBSG, OCCoal referred to OCBSG (%, w/w), correspondingly for HDM, NDM, SDM, ODM, AshDM each referred to the element’s content in dry BSG. d In parentheses: DBE/oxygen. b

c

The high recoveries of substrate-nitrogen in biocoals (49% and 46%) provided strong indication that proteins contributed to biocoal formation, e.g. by Maillard reactions. Since HTC biocoal decreases rapidly in the soil (50% in 100 days regardless of soil type) (Malghani et al., 2013), the bioavailability of the released organic nitrogen is expected to be high, which favors the application for soil improvement.

There was a good agreement between gravimetric data on the one hand and spectroscopic data on the other hand concerning the distribution of the ash fraction between process water and biocoal (compare 64% ash recovery in biocoal at 200 °C obtained by incineration in Table 2 with 70% obtained by X-ray fluorescence in Table 3; analogously for 240 °C: 67% versus 75%). 3.2. Extent of carbonization/defunctionalization

3.1.2. Balance of inorganics Balances of individual inorganic cations along with Si, P, and S from the input substrate to HTC matrices were studied by X-ray fluorescence spectroscopy (Table 3). Inorganics are necessary for the metabolic pathways of plants and can occur in the form of salts or bound to organic structures. The enrichment of calcium in the biocoal was consistent with a previous study (Santos et al., 2003). Bivalent calcium constitutes the most abundant cation in the input substrate, with the overwhelming majority being transferred into the biocoal (85% at 200 °C and 92% at 240 °C). As with the fraction of OC, the mass fraction of the mineral matrix which was fixed inside the biocoal amounted to about two thirds (70% at 200 °C, 75% at 240 °C, see Table 3, last column). The residual minerals (30% at 200 °C, 25% at 240 °C) remained dissolved in the process water. The high recovery of phosphorous in the biocoals – consistent with the findings from a previous study (Stemann et al., 2013) – may refer to a potential application as a fertilizer in case of its P-availability. Unfortunately, phytotoxic elements including copper and zinc were overwhelmingly transferred into the biocoal as well, which may hamper the potential application as soil additive. However, the release of (potentially) phytotoxic elements from biocoal-amendments in soil into their soluble form needs to be clarified. As expected, potassium showed high tendency to transfer into the aqueous product stream (the same was expected to occur with monovalent sodium, whose concentrations were however very low; see Table 3).

The loss of oxygen – calculated on an ash-free basis – and hydrogen was each higher than the loss of OC (58% and 69% for oxygen, 27% and 37% for hydrogen, see Table 2). Oxygen and hydrogen contents in either biocoal or DOM decreased with increasing operation temperature due to more defunctionalization (mainly dehydration), and aromatization reactions. The high hydrophobicity of both biocoals became already evident when solvent extraction experiments have been performed at earlier stages of this project: Both biocoals partitioned into organic solvents such as benzene and chloroform, whereas the input BSG substrate proved immiscible with organic solvents as expected. This hydrophobic feature favors a prospective application of the biocoal as a sorbent for wastewater treatment. According to the elemental composition of the educt and the product streams, stoichiometric equations to characterize the HTC process can be derived. Hypothetical molecular formulas are C305H479N22S1O149 for BSG, C520H620N27S1O87 and C152H266N24S1O73 for the biocoal and the DOM formed at 200 °C, respectively, as well as C571H609N26S1O61 and C171H282N25S1O77 for the biocoal and the DOM formed at 240 °C, respectively. Given an operating temperature at 200 °C, the formal stoichiometry is as follows: 50 C305 H479 N22 S1 O149 ! 20 C520 H620 N27 S1 O87 þ 22 C152 H266 N24 S1 O73 þ1356 CO2 þ 105 CO þ 45 CH4 þ 2711 H2 O þ32 NH3 þ 50 SO2

ð1Þ

Table 3 Concentrations of inorganic elements in the input BSG-substrate and HTC-biocoals (concentration data given in lg g1 DM).

BSG Biocoal at 200 °C Recovery in biocoal (%) Biocoal at 240 °C Recovery in biocoal (%) a b

K

Ca

Na

Mg

Si

Al

Fe

P

S

Mn

Cu

Zn

R (Xi⁄Ci)a

650 180 14 165 12

5750 9565 85 11020 92

Characterization of biocoals and dissolved organic matter phases obtained upon hydrothermal carbonization of brewer's spent grain.

The wet biomass brewer's spent grain was subjected to hydrothermal carbonization to produce biocoal. Mass balance considerations indicate for about tw...
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