1 Aug 2018 - Myint, M. T., Tchinda, D.,Henkanatte-Gedera, S.M.,, Karbakhshravari, M., Mallick, K., Nirmalakhandan, N. (2017) Pilot Scale. Demonstration of ...
An ASABE Meeting Presentation DOI: https://doi.org/10.13031/aim.201801158 Paper Number: 1801158
Hydrothermal Liquefaction of Algae Grown on Brackish Dairy Wastewater Meshack Audu1, Maung Thein Myint2, Feng Cheng1, Kwonit Mallick1, Umakanta Jena1, * Nagamany Nirmalakhandan,2 Catherine E. Brewer1 1
New Mexico State University, Chemical & Materials Engineering, PO Box 30001 MSC 3805, Las Cruces, NM 88003; 2New Mexico State University, Civil Engineering, PO Box 30001 MSC 3CE, Las Cruces, NM 88003 .
Written for presentation at the 2018 ASABE Annual International Meeting Sponsored by ASABE Detroit, Michigan July 29-August 1, 2018 ABSTRACT. Hydrothermal Liquefaction (HTL) is a thermochemical process that uses subcritical water (270-350 °C and 818 MPa both as a solvent and a reaction medium to convert organic biomass constituents into energy-rich bio-crude oil. HTL is most suitable for conversion of wet feedstocks including algae. In this study, we compare the oil yields and energy recoveries from the HTL of a microalgal polyculture used to remove excess nutrients from brackish dairy manure effluent. The biomass was cultured from a sample taken from the U.S. Bureau of Reclamation’s Brackish Groundwater National Desalination Research Facility (BGNDRF), Alamogordo, NM in a photo bioreactor system. Batch HTL experiments were carried out in a 1.8 L autoclave reactor at reaction temperatures of 310 and 350°C for 30 and 60 minutes. Product yields and the higher heating values (HHV) of the bio-crude oils were measured for different HTL conditions. Prior to HTL, the microalgae biomass was diluted with deionized water to lower the electrical conductivity of the reaction medium to approximately 1000 μS/cm and centrifuged; this pretreatment was used to lower the ash content of the feedstock and to decrease corrosion of the reactor surfaces. The light bio-crude oil (LBO) yield ranged from 9-24 wt. %, heavy bio-crude oil (HBO) yield and char yield were ranged from 3-7 wt. % and 10-26 wt. % respectively. The feedstock was still relatively high in ash and presented low-moderate potential for energy recovery as liquid fuels. Future work will focus on the evaluation of the combined process of brackish dairy effluent wastewater treatment and renewable energy production. Keywords. algae, brackish, biofuels, dairy wastewater treatment, energy recovery, hydrothermal liquefaction.
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Introduction Algae in Dairy Wastewater Treatment Controlling the input of nitrogen and phosphorus from dairy operations into the water creates challenges for the environmental community (Adey, Luckett and Jensen 1993, Kaiser 2001, Van Horn et al. 1994). When manure wastewater is used for land applications, large amounts of nitrogen (N) are lost to the atmosphere by volatilization of ammonia (Pizarro et al. 2006). Recent water quality declines due to eutrophication are partially caused by animal waste, such as manure (Horton 2013). Ecologically-sound manure use on farms is important to reduce losses of valuable plant nutrients; a challenge for ecological engineering is finding technologies to economically transform manure into useful products (Pizarro et al. 2006). One manure treatment option is to grow algae using the manure effluent as a nutrient source. Most of the efforts in wastewater treatment have focused on using suspended microalgae for biological uptake of N and P (Benemann and Oswald 1994, Green, Lundquist and Oswald 1995). Previous laboratory work has demonstrated the use of Algal Turf Scrubber (ATS) system to remove N and P from dairy manure effluents, as well as the use of the cultivated algal biomass as an organic filter (Kebede-Westhead et al. 2004, Mulbry et al. 2005). Pizarro et al (2006) proposed a system for nutrient flow in an ATS system: collection of manure by mechanical scrapping or flushing with water followed by separation of solids from the manure slurry, subsequently subjecting the solids to composting and the effluent to anaerobic digestion. The anaerobic digestion effluent is then drained into the ATS raceway for algal biomass growth. Algal biomass can then be converted into fuels to offset the energy needed for treating the wastewater. Algae in Brackish Wastewater Desalination Current systems of wastewater desalination involve the use of membrane based technology. These systems are not cost effective for reclaiming irrigation- quality water from brine/brackish wastewater, especially for brine/brackish wastewater effluents from agricultural farmlands. An alternative method for brackish dairy wastewater treatment using algal biomass production has been suggested (Myint et al ., 2017). Algae require dissolved carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, sodium, magnesium, calcium, and chloride to grow (Raven, 1980, Mandalam and Palsson, 1998, Concas et al., 2012). The algae typically carry a negative surface charge of -7.5 to - 40 mV (Liu et al., 2013), and thus tend to bind with cations. Phosphorus (as phosphate) chemically reacts with cations (mostly, calcium and magnesium) as cation phosphate species at high pH, and precipitate from the aqueous phase (Cerozi and Fitzsimmons, 2016, Zhou et al., 2006). This reaction results in less free phosphates and cations ions in solution and, hence, lower dissolved solids. Divalent cations (Ca2+ and Mg2+), as well as metal ions, bind to the negatively charged groups from extracellular polymeric substances, together forming bonds onto which cells and particles can adhere (Bruus et al., 1992, Modin et al., 2016). Microalgae species like Scenedesmus and Chlorella have been reported to be effective for brine/brackish wastewater desalination. These species have high inorganic uptake and high tolerance for the brine/brackish nature of the wastewater. Analysis of the water contaminant removal efficiencies when Scenedesmus microalgae was used for brine wastewater treatment showed that, with a retention time of 14 days, the total dissolved solids (TDS), phosphates, and NaCl content decreased by 97 %, while sulfate and nitrate concentration decreased by 93% (El Nadi et al., 2011). Use of algae can be a cost-effective solution for brackish wastewater desalination and provide additional opportunity for biofuel production. Hydrothermal Liquefaction (HTL) Pathways used to convert algal biomass into liquid fuels include carbohydrate fermentation (Ueno et al. 1998; Agarwal 2007), fast pyrolysis and hydrothermal liquefaction (Miao and Wu 2004, Miao et al. 2004, Shuping et al. 2010), and transesterification of lipids (Xu et al. 2006). After several comparison studies, hydrothermal liquefaction (HTL) of algae appears to be the most promising technology. Dote et al. (1994) were the first team to report hydrothermal liquefaction of algae. In the following years, several Japanese researchers found that some properties of algae-derived bio-crude oil are similar to that of the petroleum crude oil (Minowa et al. 1995, Zhou et al. 2010, Inoue, Sawayama and Ogi 1996, Sawayama et al. 1999). Since 2009, algal HTL has drawn interest of the University of Michigan, New Mexico State University, the University of Illinois, and the University of Aalborg (Guo et al. 2015). Studies on HTL have focused mainly on the effects of reaction time, reaction temperature, reactant selection, and solids loading. HTL of algae is operated at high pressure and moderate temperatures (> 250°C) in liquid water to avoid water vaporization. As a wet process, the energy efficiency of algae HTL is higher than a process requiring drying of the algae (Guo et al. 2015). At HTL conditions, the macromolecules in the biomass break into smaller hydrophilic and hydrophobic ASABE 2018 Annual International Meeting
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molecules, which polymerize into larger compounds that form bio-crude oil. At high temperature, the strength of the hydrogen bonds in water is much lower than that at the room temperature, and the dielectric content is lower (Guo et al. 2015), which enables more dissolution of low-polarity organic components. At the same time, the lower dielectric constant leads to a higher ionic product constant, which is better for the acid or base-catalyzed hydrolysis reaction of organic molecules (Faeth et al. 2013). Most studies have found that the bio-crude oil yield in HTL of algae or other biomass surpasses the lipids content in the raw material (Minowa et al. 1995). Although the lipid fraction is the main part that contributes to bio-crude oil production, HTL technology can convert other components such as carbohydrates and protein into bio-crude oil, with a higher energy density in comparison to the initial materials (Duan and Savage 2010, Biller and Ross 2011, Li et al. 2014, Cheng et al. 2014). Study Objectives The goals of this study were to compare the product yields and characteristics at different HTL reaction conditions for conversion of an algal polyculture grown on brackish water with dairy wastewater effluent as a nutrient source, and characterize the HTL products characteristics in terms of the energy content, energy recovery, and elemental distribution.
Materials & Methods Algae Culitivation and Harvest The microalgae polyculture was grown at New Mexico State University (Las Cruces, NM) using a photobioreactor of 19 cm internal diameter and 151 cm height. About 37 L of growth medium occupied the bioreactor. Mixing in the reactor was achieved by sparging air bubbles at a flow rate of 1.5 standard L/min. The initial mesotrophic microalgae polyculture was isolated from desalination concentrate in an evaporation pond at the Brackish Groundwater National Desalination Research Facility (BGNDRF) in Alamogordo, NM; the same brackish groundwater was used for the experiment. Nutrients were obtained from a dairy farm in Mesquite, NM. About 300 g of manure was mixed in about 700 mL of the brackish ground water and let to sit for two days for extraction of the nutrients into the aqueous phase. The supernatant from the mixture was added to the medium on a daily basis: approximately 440 mL was in the morning and 260 mL in the evening. As per Myint et al. (2017), the biomass growth rate was measured using standard methods; biomass was harvested and wastewater treatment characterization analyses were conducted daily. Total dissolve solids (TDS) of the samples were measured using electrical conductivity. Results of the cultivation experiments showed that the TDS, ammonia-nitrogen, phosphate, and chemical oxygen demand (COD) were reduced by approximately 30%, 90%, 80%, and 90%, respectively. Hydrothermal Liquefaction of Algal Biomass The hydrothermal liquefaction experimental runs were carried out in a high pressure 1.8 L stainless steel batch reactor (Model 4572, Parr Instrument Co, Moline, IL). The reactions were run at 10% solids loading, varying temperatures (310°C and 350°C), and retention times (30 min. and 60 min.). Before each experimental run, the biomass was thawed and diluted with de-ionized water for a 500 ml working sample volume. The reactor was initially purged with nitrogen gas to remove residual air and pressurized to about 200 psi to prevent water evaporation during heating. The reactor was then heated to the desired reaction temperature, held at the reaction temperature for the specified retention time, and then cooled to room temperature. After cooling, the reactor was depressurized by venting the head space gases. Hexane (200 mL) was added to the HTL products and stirred for 5 min. to extract the light bio-crude oil (LBO). An additional two rinses of 100 ml hexane were used to clean the reactor and the rinseates added to the HTL product mixture. The HTL product mixture was vacuum filtered through a Whatman® No. 4 filter paper (pore size=20-25 µm) using a Buchner funnel to separate the char. The filtrates (organic and aqueous phases) were separated using a separatory funnel and the hexane evaporated from the LBO using a rotary evaporator unit at 40°C-50°C. The solid residues were dried in a fume hood, then extracted with acetone at 56 °C using a Soxhlet extraction unit to recover heavy bio-crude oil (HBO). Acetone was evaporated from the HBO using the rotary evaporator unit at 40°C-50°C. The HBO-free solid residues (chars) were dried overnight (12-15 hours) in a fume hood. All samples were stored in glass vials prior to characterization. Feedstock and HTL Products Characterization Moisture content of the algae was determined by drying the samples in a drying oven for 24 hours at 105-110 °C. Energy content as higher heating value (HHV) of the algal feedstock, LBO, HBO, and chars were measured using a model 6725 semi-micro bomb calorimeter (Parr Instrument Co.); samples were analyzed in duplicate. Elemental composition (CHNS) of the algal feedstock was measured using a Series II 2400 elemental analyzer (Perkin Elmer, Waltham, MA); samples were analyzed in triplicate. Electrical conductivity and pH of the aqueous phase were measured using an Orion Star A111 pH benchtop meter (ThermoFisher Scientific, US). Total organic carbon of the aqueous phase was measured ASABE 2018 Annual International Meeting
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using a Model TOC-VCPH analyzer (Shimadzu Corp.,Kyoto,Japan).
Results & Discussion Feedstock Characteristics Table 1 summarizes the composition of the algal feedstock. The ash content was high and the lipid content low, which is typical for algae grown on wastewater. Elemental analysis shows that the oxygen content of the feedstock is high (46.2 wt. %), which accounts for a low to moderate HHV (24.1 MJ/kg) of the algal biomass. Table 1. Proximate and elemental composition of the filamentous algae polyculture grown on dairy wastewater effluent Proximate Analysis Ash (dry wt.%)
20.5
Salinity (mg/L, as received)
1500-2000
Moisture (wet wt.%)
90.0
HHV (MJ/kg, dry basis)
24.1 Elemental Analysis (dry wt. %)
C
40.2
H
5.7
N
5.4
S
2.4
O (by difference)
46.2
HTL Yields Table 2 summarizes of the HTL product yields at different reaction conditions. LBO yields ranged from 9-24 wt.%, which is low compared to high-lipid algal species like Nanocholoropsis salina,where bio-crude oil yields can reach up to 56.5% (Li et al, 2014). The HBO and char yields ranged from 3-7 wt. % and 10-26 wt. %, respectively. There were some losses of HBO during the extraction process, so yields are slightly underestimated. The reaction conditions with the highest LBO yield were 350°C and 60 min.. Table 2. Yields of HTL products at different reaction conditions for solids loading of 10 wt.%; all yields reported on a dry feedstock weight basis. LBO, light bio-crude oil; HBO, heavy bio-crude oil. Temperature Retention Time LBO HBO Char (°C) (min.) (wt.%) (wt.%) (wt.%) 310 60 15.14 7.93 25.81 310
60
9.93
3.21
10.86
350
60
24.22
5.60
26.20
350
60
19.82
3.46
10.98
350
30
22.69
4.40
11.59
350
30
19.30
4.30
11.45
Bio-Crude Oil and Char Characteristics Table 3 shows the energy content (HHV) and the energy recovery (ratio of the energy content of each product compared to the energy content of the biomass) of the HTL products at different reaction conditions. The HHV of LBO, HBO, and char ranged from 39-44 MJ/kg, 35-39 MJ/kg, and 23-26 MJ/kg, respectively. The HHV for the LBO is similar to that of petroleum crude oil (42-44 MJ/kg) (U.S. Energy Information Administration, 2011). HHV values of the HBO and char are typical for wastewater algae. The energy recovery ration for LBO, HBO, and char ranged from 18-43 %, 6-8 %, and 10-25 %), respectively. Aqueous Phase Characteristics Characteristics of HTL aqueous phase samples are summarized in Table 4. The electrical conductivity was relatively high in the range of 46-76 mS/cm—values comparable to brackish water (London, 2018). The high electrical conductivity ASABE 2018 Annual International Meeting
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can be attributed to the relatively high ash content in the biomass (Table 1). The pH was almost neutral or slightly basic, which indicates that the salts in solution are close to neutral. The total carbon content in the aqueous phase samples ranged from 4.7-7.0 g/L. The total organic carbon ranged from 4.8-6.0 g/L, which represents 81-85% the total carbon in the aqueous phase. This indicates the presence of hydrophilic oily compounds rather than inorganic carbons. Table.3 Higher heating values (HHV) and energy recovery (ratio of energy content of HTL product to the energy content of the biomass) of HTL products at different operating conditions at a 10% solids loading. LBO: light bio-crude oil; HBO: heavy bio-crude oil. LBO Retention HBO Energy Temperature LBO HHV Energy HBO HHV Char HHV Char Energy Time Recovery (°C) (MJ/kg) Recovery (MJ/kg) (MJ/kg) Recovery (%) (min.) (%) (%) 310
60
41.7± 1.1
26.3
35.5 ± 0.5
11.7
24.2±3.3
25.9
310
60
44.0± 0.3
18.1
35.2 ± 0.1
4.7
23.3±5.6
10.5
350
60
43.5± 0.6
43.8
35.5 ± 5.4
8.3
26.4±1.2
28.8
350
60
39.4± 2.2
32.4
30.5±10.3
4.4
24.8±1.6
11.3
350
30
41.3± 1.0
39.0
35.7 ± 4.7
6.5
23.8±0.5
11.5
350
30
43.6± 0.9
35.0
39.3
12.0
24.2±5.7
11.5
Table 4. Characteristics of the aqueous phase products of HTL at different operating conditions. Total carbon (TC), total organic carbon (TOC), electrical conductivity (EC). Temperature Reaction Time EC pH TC TOC (°C) (min) (mS/cm) (g/L) (g/L) 64.8 7.8 7.0 6.0 350 30 76.2 7.7 4.7 4.0 350 30 350
60
46.8
7.3
6.1
5.0
350
60
71.2
7.5
5.7
4.8
310
60
69.5
7.7
6.7
5.6
310
60
72.2
7.9
6.6
5.4
Conclusions Algae growth in desalination ponds is a cost-efficient method of managing nutrients from dairy manure effluent. HTL is an efficient method for converting algae biomass into biofuels as drying of the feedstock is not required and wet biomass can be transformed in a short time into bio-crude oil that can be further upgraded into a fuel. Among the conditions tested in this study, the highest light bio-crude oil yield of 24.2% was obtained at 350°C and 60 min. Further research is required for investigating the overall energy balance for combined wastewater treatment and energy recovery using algal HTL. . Acknowledgements This research is supported by NM EPSCoR, “Energize New Mexico,” funded by the National Science Foundation (NSF) #1301346. The authors wish to thank their Brewer, Jena, Khandan, Holguin and Van Voohries group colleagues for their assistance with biomass handling, reactor operation, and sample characterization, especially Jacob Usrey, Scott Woolf, Kailey Garland, Barry Dungan, Stephanie Willette, Feng Cheng, Zheng Cui, Kwonit Mallick, Mark Chidester, and Nicholas Soliz, Cesar Martinez.
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