Research Article pubs.acs.org/journal/ascecg
Wet Air Oxidation of Hydrothermal Carbonization (HTC) Process Liquid M. Toufiq Reza,*,†,‡ Ally Freitas,§ Xiaokun Yang,‡ and Charles J. Coronella‡ †
Department of Mechanical Engineering, 251 Stocker Center, Ohio University, Athens, Ohio 45701, United States Department of Chemical and Materials Engineering, University of Nevada, Reno, 1664 N. Virginia Street, Reno, Nevada 89557, United States § Department of Civil and Environmental Engineering, University of Nevada, Reno, 1664 N. Virginia Street, Reno, Nevada 89557, United States ‡
ABSTRACT: Hydrothermal carbonization (HTC) is a promising thermochemical treatment of wet biomass. The byproduct HTC process liquid often contains a very high total organic carbon (TOC). Various toxic organic compounds (e.g., phenol, furfural, 5-HMF, etc.) can be found in HTC process liquid. As a result, the HTC process liquid often requires further treatment prior to discharge into the environment. In this study, wet air oxidation (WAO) is applied to process liquid produced from HTC of dairy manure and wastewater biosolids. Different oxygen loadings (25 and 50 atm partial pressure) were charged for WAO at 260 °C for 30 min. Furthermore, the oxidation capability of copper oxide (CuO) was evaluated at the same temperature and time. In all cases, the resulting liquid product was clear and has reduced TOC. With an increase in oxygen partial pressure, TOC is reduced up to 60% compared to the control run and 74% compared to the original HTC process liquid. The primary products in the WAO liquid product are short chain organic acids (formic, acetic, succinic, propionic, and glycolic acid), which are not considered as toxic substances. KEYWORDS: Hydrothermal carbonization, Wet air oxidation, HTC process liquid, Total organic carbon, HPLC
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INTRODUCTION Hydrothermal carbonization (HTC) is a promising thermochemical treatment of biomass to solid fuel and value-added chemicals. During HTC, biomass is treated with hot compressed water (temperature range 180−280 °C) slightly above water saturation pressure to keep it in the liquid state in an inert atmosphere.1 Subcritical water at this temperature range behaves like a mild acid, mild base, and a nonpolar solvent simultaneously.2 As a result, the solid feedstock degrades in the form of various dissolved organic compounds as well as crosslinked macromolecular compounds.1 Depending on the process condition, HTC process liquor contains up to 15% of the initial carbon mainly in the form of acetic and formic acid.3,4 However, individual organic toxic compounds like phenol, furfural, 5-HMF, and other phenol and furan derivatives are often observed in concentrations of up to several grams per liter depending on feedstock and process parameters.5−7 The HTC process by itself cannot eliminate all these organic compounds to negligible concentrations as they contribute to the HTC process.8 With respect to biological activity, it is important to note that the temperature and time of HTC both exceed those used in standard lab autoclaves, and thus, the HTC products are considered sterile. Organic chemical compounds detected in manure- and sludge-derived HTC process liquids produced at 220 °C include furfural, 5-HMF, phenol and phenolic derivatives, short © 2016 American Chemical Society
chain carboxylic acids (formic, acetic, propionic, lactic acids, etc.), and proteins and protein-degraded N-containing compounds (e.g., amino acids).9,10 The presence of these compounds contributed to very high TOC and COD contents (up to 80 g L−1) of HTC process liquid. Treating HTC process liquor by common anaerobic degradation (AD) techniques to diminish its environmental toxicity seems adequate since most of the organic compounds present are biologically available; AD is capable to reduce BOD with an efficiency of over 85%.11,12 However, AD is relatively a slower process compared to thermochemical conversion, with a correspondingly larger reactor volume. Moreover, the digestate produced from AD requires further treatment.13 On the contrary, wet air oxidation (WAO) is similar to wet incineration, where excess air/oxygen is fed to the reactor.14,15 At high temperature and pressure, oxygen dissolves in the aqueous system and reacts with organic substances to produce CO2 via organic acid intermediates. As a result, the waste liquid has been treated into clean water. Like incineration, WAO is an exothermic process and with proper engineering, WAO requires no additional heat and can be self-sustaining or even useful for producing heat from wastes.15 A few recent studies Received: February 10, 2016 Revised: March 28, 2016 Published: May 12, 2016 3250
DOI: 10.1021/acssuschemeng.6b00292 ACS Sustainable Chem. Eng. 2016, 4, 3250−3254
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ACS Sustainable Chemistry & Engineering
Figure 1. TOC, TC, IC, and TN of WAO products of (a) sewage sludge and (b) dairy manure at various oxygen partial pressures.
degradation of toxic substances. Also, the effect of CuO after WAO and thermal regeneration was observed as well.
show that WAO can be applied for HTC and a cascaded HTC−WAO has better potential of generating heat and also treat the organic wastes in the process liquid.16,17 However, the maximum WAO temperature was 200 °C in both studies, but WAO kinetics are faster at higher temperature.18 As HTC temperature is typically between 200 and 260 °C, higher WAO temperature might be preferable for process integration. Again, the effect of various oxygen loadings or use of oxidizing agents (e.g., copper oxide) might be useful for the degradation of organic matter as well. CuO was used to degrade phenol previously and around 40%−80% degradation was achieved, but copper leaching and fouling of the catalyst was observed.19 Therefore, the main objective of this study is to study the WAO of HTC process liquids produced from dairy manure and digestate sewage sludge at 220 °C in different oxygen loadings and the use of CuO as oxidizer. A further objective was to analyze the WAO product quantitatively to ensure the
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MATERIALS AND METHODS
HTC Process Liquid. HTC process liquid was produced from dairy manure and wastewater sludge digestate. Fresh cow manure and digested biosolids were collected from the Agricultural Experimental Station at the University of Nevada, Reno, and a local wastewater treatment plant (Truckee Meadows Water Reclamation Facility, Reno, NV), respectively. The feedstocks were not dried, as residual moistures (85% in manure and 87% in sludge) act as liquid medium for HTC. A Parr 2 L batch reactor (4520 series, Moline, IL) was used for HTC. Around 800 g of wet manure or sludge water were put into the reactor. The experimental condition was set at 220 °C with a reaction time of 5 min and an 8 °C min−1 heating rate. The reactor was stirred continuously at 100 rpm throughout HTC. At the end of reaction period, the reactor was quenched in an ice−water bath. HTC process liquid was then vacuum-filtered from slurry using Whatman 3 filter paper for 15 min. The detailed procedure of HTC can be found 3251
DOI: 10.1021/acssuschemeng.6b00292 ACS Sustainable Chem. Eng. 2016, 4, 3250−3254
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ACS Sustainable Chemistry & Engineering elsewhere.9 As HTC process liquid is the point-of-interest for this study, corresponding hydrochars were not further analyzed. HTC process liquid was then diluted five times and refrigerated in an amber glass jar until WAO. The main reason behind the dilution is to reduce the amount of theoretical oxygen or CuO required to convert all TOC to CO2 in laboratory experiments; clearly, no dilution would be expected in a industrial-scale deployment. All the HTC experiments were at least duplicated for consistency. Wet Air Oxidation (WAO). A Parr 160 mL batch reactor (4560 series, Moline, IL) was used for WAO. Pure oxygen at various partial pressures was used as the oxidation agents. For WAO, 20 mL of HTC process liquid (diluted, as described above) and 20 mL of deionized (DI) water were poured into a glass liner, and the liner was placed into the reactor. Either 0, 25, or 50 bar of oxygen were charged into the reactor. The WAO experimental condition was set at 260 °C and 30 min with a 10 °C min−1 heating rate. After completion of the WAO, the reactor was quenched in an ice−water bath. Gaseous products were purged in a fume hood, and the liquid product was refrigerated for chemical characterization. Copper(II) oxide (13%) on alumina was purchased from SigmaAldrich (St. Louis, MO) and used as the oxidation agent in one series of experiments. In this case, the WAO temperature was 260 °C, but the initial pressure was atmospheric air pressure. During WAO, in the presence of CuO, CuO was reduced to Cu as it reacted with dissolved organic compounds. Ideally, reduced Cu can be regenerated (i.e., oxidized) into CuO, which is an exothermic reaction and reused for WAO.17 In this study, a stoichiometric amount of CuO (12−14 g, based on TOC of HTC of process water) was added into the WAO reaction. After the reaction, Cu particles were filtered, washed, and dried. The liquid product was characterized with other WAO samples. The WAO products are denoted as WAO-S/M-p, where S stands for sludge, M for manure, and p denotes oxygen partial pressure. For CuO runs, the samples are denoted as CuO-S and CuO-M for sludge and manure derived oxidations, respectively. Characterization of WAO Products. The samples were analyzed for total organic carbon (TOC), total carbon (TC), total inorganic carbon (IC), and total nitrogen (TN). Prior to analysis, samples were diluted with DI water, and the dilution factor was recorded. TOC and TN analysis was conducted using a Shimadzu TOC analyzer equipped with a TNM-1 option module (TOC-VCPN, Shimadzu Corp., Kyoto, Japan). Calibration curves were run prior to each sequence of sample analysis. Calibration samples of known concentrations were regularly run in each analysis method using standard stock solution to verify the method and calibration. The aqueous products of WAO were analyzed separately for each run by using a Shimadzu high performance liquid chromatography (HPLC). After the reaction, the aqueous phase was filtered through a 0.45 μm syringe filter then diluted 15 times with ultrapure water. HPLC analysis was performed using a Shimadzu HPLC system equipped with a UV−vis detector (Shimadzu SPD 10-AV) and refractive index detector (Shimadzu RID-10A). For analysis of organic acids and reaction intermediates, the samples were separated in an Aminex 87-H Ion exclusion column from Bio-Rad, using 5 mM of H2SO4 as the mobile phase, at a 0.7 mL/min flow, and a column temperature of 55 °C. For quantitative identification and results, the UV−vis detector was utilized at 208 and 290 nm.
time. Furthermore, both hemicellulose and cellulose in the biomass are hydrolyzed above 220 °C, and lignin remains relatively inert throughout the HTC reaction.25 Physical appearances of both HTC process liquids were very dark, and the liquid was pungent. The pH of the manure-derived HTC process liquid was slightly acidic (5.2), whereas the sludge-derived one was slightly basic (8.0). WAO of HTC Process Liquid. The TOCs of manure and sludge-derived HTC process liquids were 5.6 and 6.5 g L−1 (diluted five times from the original HTC process liquid), respectively, which were reduced to 2.2 and 2.8 g L−1 with the control run of 1 atm of air with no additional oxygen (Figure 1). It was previously reported that at higher HTC temperature (∼260 °C) organic compounds, especially furfural substances, degrade into smaller compounds (organic acids, water, CO2, etc.).5 With the addition of pressurized oxygen, TOC is reduced to 1.8 and 1.7 g L−1 for manure and sludge HTC process liquids, respectively. An increase in oxygen loading for WAO leads to a further reduction in TOC. In one previous study, WAO was applied to the whole HTC slurry, including solids, to improve heat efficiency.17 From that study, the COD of paper-derived HTC slurry was reduced as much as 70% when WAO was performed at 200 °C and with 20 bar oxygen loading. In this study, much lower oxygen loading but a higher WAO temperature yielded a 74% reduction of TOC. Note, however, that the concentration of dissolved oxygen is the dominant factor in reaction rates and products. Only if the oxygen is present in excess can it be assumed that the dissolved oxygen concentration is constant throughout the period of reaction. The physical appearance of WAO-M-50 and WAO-S50 are much clearer than the HTC process liquid, although it still contains a fair amount of TOC. From Figure 1, it can also be noticed that WAO has a minimal effect on inorganic carbon as IC remains similar throughout WAO experiments. As a result, total carbon (TC) is larger than TOC but follows the similar trend of TOC. In the case of HTC manure, WAO has a negligible effect on total nitrogen. However, TN is reduced in the case of HTC sludge. Potentially, this is of concern if it indicates production of NOx. Further analysis of gaseous products is indicated. It is shown in Figure 1 that CuO successfully reduced TOC of both liquids. Manure-derived HTC process liquid is slightly more reactive with CuO than sludge-derived HTC process liquid. The basic pH of sludge HTC process liquid may have degraded CuO, as reduction of CuO is possible for neutral to acidic environments at higher temperature.19,26 Furthermore, CuO was found stripped from the Al2O3 support during WAO of the sludge HTC process liquid. This is an unwanted side effect, as regeneration of CuO will be compromised. Therefore, WAO in the presence of CuO may not be feasible for sludgederived HTC process liquid, unless adding some acid buffer solution. WAO Products. The overall WAO mechanism includes two steps. The first one, a physical step, is the mass transfer of oxygen from the gas phase to the liquid phase. When designing a WAO reactor, one usually considers that oxygen diffuses rapidly within the gas phase. The only significant transfer resistance is located at the gas−liquid interface (film model).27 The second step is the chemical reaction between the organic matter and dissolved oxygen. Although the ultimate product of an efficient WAO is water and CO2 just like incineration, various organic acids are produced as intermediate products.28 To quantify the intermediate products from WAO, HPLC was
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RESULTS AND DISCUSSION HTC Process Liquid. During HTC, biomass undergoes a number of complex chemical reactions including hydrolysis, dehydration, decarboxylation, condensation, and isomerization in the liquid state.20,21 Kinetics of each reaction differ depending on HTC temperature, reaction time, feedwater pH, and feedstock itself.5,22−24 Therefore, composition of the HTC process liquid varies significantly with process parameters and feedstock composition. As hydrolysis is the first HTC reaction, the strength of the HTC process liquid (in terms of TOC and COD) is maximized at relatively shorter reaction 3252
DOI: 10.1021/acssuschemeng.6b00292 ACS Sustainable Chem. Eng. 2016, 4, 3250−3254
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Figure 2. Equivalent TOC calculated from HPLC results of WAO products of sewage sludge and dairy manure at various oxygen partial pressures.
organic acids. Sludge-derived WAO products contain more undetected compounds than manure-derived WAO products. Both sludge- and manure-derived WAO products contain significant amounts of formic, acetic, and propionic acids. Among them, acetic acid concentration was increasing with oxygen loading. For both feedstocks, formic acid concentration was the highest at the control runs and reduces significantly with oxygen addition. Unlike the sludge-derived WAO liquid, more lactic acid was present in manure-derived WAO products. Besides these C1−C4 carboxylic acids, lactic, glycolic, succinic, and malic acid were detected in different WAO products at lower concentrations. The presence of different organic acids in the WAO liquid is consistent with previous studies.16,17 WAO in the presence of CuO produces similar compounds as dissolved oxygen. In addition, the presence of furfural, succinic acid, and 2-hydroxy butyric acid were observed. The presence of unconverted furfural indicates that WAO was incomplete. The physical observation of CuO-treated WAO liquid was opaque and colored. As furfural and its derivatives cause color in the liquid,29 the unconverted furfural likely accounts for the opacity of the WAO process liquid.
performed for organic acids, simple sugars, and sugar derivatives. Theoretical TOC was calculated from HPLC results based on stoichiometry of complete combustion, and stacked results are presented in Figure 2. It is obvious from the comparison of Figure 2 and Figure 1 that most of the compounds are detected by HPLC, as up to 70% TOC was detected by calculating TOC from HPLC results except for control runs (0.21 bar O2 partial pressure). As stated in the earlier section, the WAO sample was filtered with 0.45 μ filter paper prior to the HPLC, but TOC was measured for the original HTC process liquid. The filtration may lead to the discrepancy of the TOC and HPLC results. Further, the HTC process produces organic macromolecules, which are difficult to detect and quantify in HPLC. That is why only 35% of the measured TOC of the products from the WAO control run were identified by HPLC. The HTC process liquid contains various products other than organic acids and sugar derivatives listed in Figure 2. In particular, mono- and disaccharides can be significant in HTC process liquids.1,7,9 No detectable sugar was found even at the control runs. Sugars, at 260 °C, may have degraded into smaller compounds like 3253
DOI: 10.1021/acssuschemeng.6b00292 ACS Sustainable Chem. Eng. 2016, 4, 3250−3254
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and Nitrogen Distributions in HTC products. Environ. Prog. Sustainable Energy 2016, n/a. (10) Danso-Boateng, E.; Shama, G.; Wheatley, A. D.; Martin, S. J.; Holdich, R. G. Hydrothermal carbonisation of sewage sludge: Effect of process conditions on product characteristics and methane production. Bioresour. Technol. 2015, 177, 318−327. (11) Wirth, B.; Reza, M. T. Continuous anaerobic degradation of liquid condensate from steam-derived hydrothermal carbonization of sewage sludge. ACS Sustainable Chem. Eng. 2016, 4, 1673. (12) Wirth, B.; Mumme, J. Anaerobic digestion of waste water from hydrothermal carbonization of corn silage. Applied Bioenergy 2013, 1, 1−10. (13) Reza, M. T.; Mumme, J.; Ebert, A. Characterization of Hydrochar Obtained from Hydrothermal Carbonization of Wheat Straw Digestate. Biomass Convers. Biorefin. 2015, 5, 425. (14) Luck, F. A review of industrial catalytic wet air oxidation processes. Catal. Today 1996, 27 (1−2), 195−202. (15) Bhargava, S. K.; Tardio, J.; Prasad, J.; Foger, K.; Akolekar, D. B.; Grocott, S. C. Wet oxidation and catalytic wet oxidation. Ind. Eng. Chem. Res. 2006, 45 (4), 1221−1258. (16) Riedel, G.; Koehler, R.; Poerschmann, J.; Kopinke, F. D.; Weiner, B. Combination of hydrothermal carbonization and wet oxidation of various biomasses. Chem. Eng. J. 2015, 279, 715−724. (17) Baskyr, I.; Weiner, B.; Riedel, G.; Poerschmann, J.; Kopinke, F. D. Wet oxidation of char-water-slurries from hydrothermal carbonization of paper and brewer’s spent grains. Fuel Process. Technol. 2014, 128, 425−431. (18) Lefevre, S.; Boutin, O.; Ferrasse, J. H.; Malleret, L.; Faucherand, R.; Viand, A. Thermodynamic and kinetic study of phenol degradation by a non-catalytic wet air oxidation process. Chemosphere 2011, 84 (9), 1208−1215. (19) Fortuny, A.; Bengoa, C.; Font, J.; Fabregat, A. Bimetallic catalysts for continuous catalytic wet air oxidation of phenol. J. Hazard. Mater. 1999, 64 (2), 181−193. (20) Reza, M. T.; Andert, J.; Wirth, B.; Busch, D.; Pielert, J.; Lynam, J. G.; Mumme, J. Hydrothermal Carbonization of Biomass for Energy and Crop Production. Applied Bioenergy 2014, 1 (1), n/a. (21) Reza, M. T.; Uddin, M. H.; Lynam, J.; Hoekman, S. K.; Coronella, C. Hydrothermal carbonization of loblolly pine: reaction chemistry and water balance. Biomass Convers. Biorefin. 2014, 4 (4), 311−321. (22) Reza, M. T.; Rottler, E.; Herklotz, L.; Wirth, B. Hydrothermal carbonization (HTC) of wheat straw: Influence of feedwater pH prepared by acetic acid and potassium hydroxide. Bioresour. Technol. 2015, 182, 336−344. (23) Wiedner, K.; Naisse, C.; Rumpel, C.; Pozzi, A.; Wieczorek, P.; Glaser, B. Chemical modification of biomass residues during hydrothermal carbonization - What makes the difference, temperature or feedstock? Org. Geochem. 2013, 54, 91−100. (24) Reza, M. T.; Yan, W.; Uddin, M. H.; Lynam, J. G.; Hoekman, S. K.; Coronella, C. J.; Vasquez, V. R. Reaction kinetics of hydrothermal carbonization of loblolly pine. Bioresour. Technol. 2013, 139, 161−169. (25) Reza, M. T.; Becker, W.; Sachsenheimer, K.; Mumme, J. Hydrothermal carbonization (HTC): Near infrared spectroscopy and partial least-squares regression for determination of selective components in HTC solid and liquid products derived from maize silage. Bioresour. Technol. 2014, 161, 91−101. (26) Eftaxias, A.; Font, J.; Fortuny, A.; Fabregat, A.; Stuber, F. Catalytic wet air oxidation of phenol over active carbon catalyst Global kinetic modelling using simulated annealing. Appl. Catal., B 2006, 67 (1−2), 12−23. (27) Debellefontaine, H.; Chakchouk, M.; Foussard, J. N.; Tissot, D.; Striolo, P. Treatment of organic aqueous wastes: Wet air oxidation and wet peroxide oxidation(R). Environ. Pollut. 1996, 92 (2), 155−164. (28) Lau, F. S.; Roberts, M. J.; Rue, D. M.; Punwani, D. V.; Wen, W. W.; Johnson, P. B. Peat Beneficiation by Wet Carbonization. Int. J. Coal Geol. 1987, 8 (1−2), 111−121. (29) Badollet, M. S. Furfural (Furfuraldehyde) and Some of Its Derivatives. Master’s Thesis, Missouri S&T University, 1921.
CONCLUSIONS Wet air oxidation proved to be effective to reduce TOC in HTC-treated cow manure and HTC-treated wastewater sludge process liquids. The resulting clear liquid has TOC degradation up to 60% compared to the control run. An increase in oxygen loading from 25 to 50 bar proved to be more effective on TOC degradation for both HTC process liquids. It is likely that providing additional oxygen will further reduce TOC. WAO products contain primarily short chain organic acids, e.g., formic, acetic, propionic, and lactic acids. Therefore, WAO can be used for treating HTC process liquid. CuO as an oxidant in WAO seems less efficient than 50 bar oxygen loading. The resulting WAO product with the presence of CuO contains unreacted furfural and furfural derivatives. Furthermore, Cu was leached during WAO of sludge-derived HTC process liquid, which questioned the application of CuO as the oxidant for WAO.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +1 (740) 593 1506. Fax: +1 (740) 593 0476. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research is supported by the Western Sun Grant Initiative (Grant 2010-38502-21839) and Nevada NSF EPSCoR seed grant (Grant NSHE-15-88). Mr. Bo Kindred is acknowledged for providing fresh cow manure throughout this study. The authors acknowledge Akbar Saba from Chemical Engineering and Dr. Sage Hiibel from Civil and Environmental Engineering from UNR for their support in experimental and analytical tasks.
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REFERENCES
(1) Funke, A.; Ziegler, F. Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering. Biofuels, Bioprod. Biorefin. 2010, 4 (2), 160−177. (2) Bandura, A. V.; Lvov, S. N. The ionization constant of water over wide ranges of temperature and density. J. Phys. Chem. Ref. Data 2006, 35 (1), 15−30. (3) Uddin, M. H.; Reza, M. T.; Lynam, J. G.; Coronella, C. J. Effects of water recycling in hydrothermal carbonization of loblolly pine. Environ. Prog. Sustainable Energy 2013, 33 (4), 1309−1315. (4) Stemann, J.; Putschew, A.; Ziegler, F. Hydrothermal carbonization: Process water characterization and effects of water recirculation. Bioresour. Technol. 2013, 143, 139−146. (5) Reza, M. T.; Wirth, B.; Luder, U.; Werner, M. Behavior of selected hydrolyzed and dehydrated products during hydrothermal carbonization of biomass. Bioresour. Technol. 2014, 169, 352−361. (6) Becker, R.; Dorgerloh, U.; Helmis, M.; Mumme, J.; Diakite, M.; Nehls, I. Hydrothermally carbonized plant materials: Patterns of volatile organic compounds detected by gas chromatography. Bioresour. Technol. 2013, 130, 621−628. (7) Reza, M. T.; Yang, X.; Coronella, C. J.; Lin, H.; Hathwaik, U.; Shintani, D.; Neupane, B. P.; Miller, G. C. Hydrothermal Carbonization (HTC) and Pelletization of Two Arid Land Plants Bagasse for Energy Densification. ACS Sustainable Chem. Eng. 2016, 4, 1106. (8) Kruse, A.; Funke, A.; Titirici, M. M. Hydrothermal conversion of biomass to fuels and energetic materials. Curr. Opin. Chem. Biol. 2013, 17 (3), 515−521. (9) Reza, M. T.; Freitas, A.; Yang, X.; Hiibel, S.; Lin, H.; Coronella, C. J. Hydrothermal Carbonization (HTC) of Cow Manure: Carbon 3254
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