Article pubs.acs.org/IECR
Catalytic Supercritical Water Gasification: Continuous Methanization of Chlorella vulgaris Gael̈ Peng,† Frédéric Vogel,†,‡ Dominik Refardt,§ and Christian Ludwig*,†,∥ †
Energy and Environment Research Department, Paul Scherrer Institut (PSI), 5232 Villigen PSI, Switzerland University of Applied Sciences Northwestern Switzerland (FHNW), 5210 Windisch, Switzerland § Zurich University of Applied Sciences (ZHAW), 8820 Wädenswil, Switzerland ∥ ENAC-IIE, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland ‡
ABSTRACT: Continuous catalytic supercritical water gasification (CSCWG; 400 °C, 28 MPa) of microalgal biomass (Chlorella vulgaris) was carried out at the microalgae production site of ZHAW in Wädenswil (Switzerland) nonstop over a period of 100 h. Microalgae slurries (3−15 wt %) were successfully gasified to a methane-rich gas for 55 h. The low total organic carbon of the reactor effluent ( 22.1 MPa) offer several advantages: (i) a low salt solubility allowing the separation of the minerals prior to gasification;5,6 (ii) tar and coke formation is reduced because of the high solubility of organic substances in SCW.7,8 Many research papers have Received: Revised: Accepted: Published: 6256
January 4, 2017 April 27, 2017 May 1, 2017 May 1, 2017 DOI: 10.1021/acs.iecr.7b00042 Ind. Eng. Chem. Res. 2017, 56, 6256−6265
Article
Industrial & Engineering Chemistry Research
Figure 1. Simplified flow scheme of the continuous-flow system KONTI-C.
nickel and of the spent catalyst showed the low effectiveness of Raney nickel for removing sulfur since a sulfur concentration gradient (increasing from the reactor inlet) along the catalyst bed was detected. More recently, Bagnoud-Velásquez et al.20 in our group studied continuous CSCWG (420 °C, 32 MPa) of Phaeod. tricornutum over a 2% Ru/C catalyst at a weight hourly space velocity of 0.34 galgae gcat−1 h−1. Unlike Elliott’s setup, no sulfur removal unit was used for protecting the catalyst. Although a salt separator located upstream of the catalytic reactor was used for removing the salts, a fast deactivation of the catalyst caused by sulfur poisoning, coking, and salt deposits was observed. The carbon gasification efficiency was only 31% and the gas composition was far from the calculated thermodynamic chemical equilibrium. In this work, significant progress regarding the development of CSCWG for methane production from microalgae is presented. A new design of the salt separator was implemented in order to improve removal of the inorganic sulfur species. A detailed description of the performance of the salt separator can be found elsewhere.21 Since reduced sulfur compounds (e.g., H2S) cannot be mineralized in the salt separator and thus cannot be removed, a sulfur removal unit using a ZnO adsorbent between the salt separator and the catalytic reactor was added. In the frame of the SunCHem project,15 a gasification campaign of 100 h was carried out at the microalgae production site of the Zurich University of Applied Sciences (ZHAW) in Wädenswil (Switzerland). The main objective of the campaign was to assess the technical feasibility of continuous CSCWG over longer times on stream.
reported continuous SCWG of several microalgae species at high temperatures (600−700 °C).9−13 Elsayed et al. 9 performed SCWG (600−650 °C, 28 MPa) of Scenedesmus obliquus (2.5−5% dry matter, DM) over a continuous period up to 50 h. The carbon gasification efficiency was above 90% and the gas consisted of H2 (46 vol %), CH4 (19 vol %), CO2 (29 vol %), and C2H6 (4.5 vol %). Patzelt et al.10 have gasified (650 °C, 28 MPa) Acutodesmus obliquus (2.5−5% DM) over a period of 50 h with a flow rate of 4.8 g min−1 and observed a gasification efficiency up to 98%. Caputo et al.13 showed that the addition of K2CO3 and Na2CO3 was beneficial for both the gasification and hydrogen production when gasifying (663 °C, 24 MPa) Nannochloropsis gaditana (3−5% DM) with a flow rate of 2.5 mL min−1 using a down flow reactor. Unlike SCWG, catalytic supercritical water gasification (CSCWG) is typically performed at moderate temperatures (374−500 °C) with metal-supported catalysts for enhancing the gasification rate. Under these milder conditions, instead of hydrogen, the target gas product is methane. Using metalsupported catalysts makes the process development much more complex since the minerals contained in biomass, especially sulfur, are responsible for the fast deactivation of the catalyst. Hence, the implementation of efficient sulfur removal techniques by developing sulfur-resistant catalysts, efficient regeneration methods of the deactivated catalyst, and sulfur removal methods (e.g., precipitation of inorganic sulfur species via a salt separator, sulfur adsorbent material) is essential in order to achieve high periods on stream. While many studies have investigated CSCWG of microalgae over metal-supported catalysts in unstirred batch reactors,14−18 only a few research papers reported on their work using a continuous-flow reactor. Elliott et al.19 were the first to report continuous hydrothermal gasification (350 °C, 20 MPa) of different microalgae species (e.g. Spirulina, Nannochloropsis salina) over a 7.8% Ru/C catalyst at a liquid hourly space velocity of 1.4−1.9 L Lcat−1 h−1. They reported a relatively good short-term stability (3.5−11.2 h) of the catalyst in which a high level of carbon conversion to gas was obtained. Although a mineral separation unit and a sulfur scrubber bed (Raney nickel) were used for preventing damages to the catalyst, some signs of deactivation were observed. In fact, ex situ characterization of the spent Raney
2. METHODS AND MATERIALS 2.1. Experimental Setup. Continuous CSCWG was carried out with a catalytic reactor system (KONTI-C) having a feeding capacity of 1−2 kg h−1. A simplified flow scheme is depicted in Figure 1. The plant consists of six main sections: feeding section, salt separator, salt removal section, reactor, pressure control, and release, phase separator. The feeding section consists of a tank for the deionized (DI) water and the feed, and a piston pump (slurry feeder). The slurry feeder has two cylinders (SS316LN) with a working volume of 2.6 L each. The pistons in the cylinders are moved 6257
DOI: 10.1021/acs.iecr.7b00042 Ind. Eng. Chem. Res. 2017, 56, 6256−6265
Article
Industrial & Engineering Chemistry Research
conductivity detector. Gas samples were withdrawn automatically every 5 min and passed through two parallel columns (A, B). CH4, CO, and H2 were separated by column A, whereas CO2 and higher hydrocarbons (C2H6, C3H8) were separated by column B. The TC, TOC, and TIC in the liquid phase were measured by the same TOC analyzer as the one used for the brine effluent. Finally, the liquid phase was collected in a tank which was placed on an analytical balance (KERN & SOHN GmbH, IFS 60K0.5DL) for monitoring the mass flow rate. In addition to the above-mentioned equipment, the pressure was measured at several locations. All temperatures, pressures, balance signals, electrical conductivities of the effluents, carbon content in the brine and reactor effluents, gas composition, and gas flow rate were monitored online. The laboratory plant was operated by remote control using a Labview-based control program. Chlorella vulgaris strain CASSIE/CCAP 211-52 was grown in an open thin-layer photobioreactor,24 situated in a green house on the Grüental campus of the Zürich University of Applied Sciences in Wädenswil, Switzerland. In short, the reactor consists of an inclined (1.7%) culture surface made of glass sheets in a steel frame (length, 2 × 9 m; width, 1 m) on which the algal suspension is circulated (layer thickness, 6 mm; speed, 0.5 m s−1). At the lower end of the surface, the suspension is collected in a tank before being pumped up again. Approximately 200 L of algal suspension are circulated in the reactor and a mineral fertilizer is added regularly such that nutrients never become limiting.25 CO2 is sparged into the culture and kept at a minimal partial pressure of 10 mbar during the daytime. When required, density of the algal suspension was increased by means of a conical plate centrifuge (GEA Westfalia) after harvesting. Dry matter content was determined with a moisture analyzer (Mettler Toledo). 2.2. Characterization Methods. The elemental analysis of the feedstock was performed at ETH Zürich (Microelemental Analysis Laboratory). The following analyzers were used: carbon, nitrogen, hydrogen (LECO CHN-900), oxygen (LECO RO-478), sulfur (LECO CHNS-932), phosphorus (photometer), and chloride (ion chromatography). The carbon, nitrogen, and sulfur (CNS) elemental analyses of the liquid effluents were performed with an elemental analyzer (Vario EL cube, Elementar). Elemental screening and quantitative elemental analysis of aqueous samples were performed by an ICP-OES device (SPECTRO Ciros Vision SOP). The ammonium concentration of the aqueous samples was measured with a Nanocolor 300D photometer (MachereyNagel) in combination with Nanocolor Ammonium 50 tube test kits. Elemental analysis of the solid residue were carried out with wavelength dispersive X-ray fluorescence (WDXRF) spectroscopy (S4 Explorer, Bruker AXS). Prior to the analysis, the samples were dried in a vacuum oven at 60 °C and 0.02 MPa during 2 h. Then, the samples were mixed with boric acid in a mortar and pressed to obtain a solid pellet. The ash content of the feedstock and solid residue was determined by TPO. The sample was heated to 900 °C under flowing O2/Ar (10:90, 10 mL min−1). The pH measurements were performed by colorfixed indicator sticks (Macherey-Nagel). Gas chromatography− mass spectroscopy (GC−MS; Agilent 5975C) of the solid residue (after its extraction with toluene) was carried out by the following procedure: the solid residue (10 g) was added to a toluene solution (50 mL) and heated under reflux at 120 °C for 12 h. After filtration, the toluene phase of the filtrate (3 mL) was diluted with toluene (6 mL) and analyzed by GC−MS.
by a hydraulic system, pushing out the biomass slurry at the desired flow rate. It was designed and constructed in-house; a detailed description can be found elsewhere.22 After being pressurized, the feed passes through a filter (pore size 25 μm) and then enters the salt separator (SITEC, stainless steel grade 1.4980; inner length, 300 mm; inner diameter, 40 mm) via a standpipe (stainless steel grade 1.4404) which extends 100 mm into the salt separator. The standpipe has an inner diameter of 3 mm and an orifice of 1 mm at the top. The salt separator is heated electrically by two heating cartridges. The temperature is measured at the inner wall at different heights by thermocouples and along the axis by a temperature lance (length, 200 mm; outer diameter, 3 mm), which is introduced from the top of the salt separator. The salt separator has two outlets, one for delivering the liquefied feed to the reactor (located at the top) and another for extracting the concentrated brine (located at the bottom). After being cooled to 20−30 °C (Huber, Unichiller MPC006), the brine effluent passes through a particle trap, a filter (pore size 25 μm), and a mass flow controller (Bronkhorst, Liquiflow), which are located before a pressure relief valve. The mass flow controller was calibrated for a flow between 0 and 3.2 g min−1, corresponding to a set point range of 0−100%. After the pressure relief, the brine effluent enters a gas/liquid phase separator. The total carbon (TC), the total organic carbon (TOC), and the total inorganic carbon (TIC) in the liquid were measured online by a TOC analyzer (GE Analytical Instruments, Sievers InnovOx Online TOC Analyzer). The liquid phase was harvested in a tank which was placed on an analytical balance (KERN & SOHN GmbH, IFS 30K0.2DL) for monitoring the mass flow rate. The liquefied feed leaving at the top of the salt separator was transferred via a heated tube (stainless steel grade 1.4404; approximately length, 1600 mm; inner diameter, 2.4 mm) to the catalytic reactor (SITEC, stainless steel grade 1.4435; inner length, 1515 mm; inner diameter, 36 mm). The reactor was heated electrically by heating clamps. The temperature was measured along the axis by a temperature lance (length, 1450 mm; outer diameter, 3 mm), which was introduced from the top of the reactor. The feed entered the reactor from the bottom. The lower part of the reactor was filled with 713 g (on a wet basis, 5.2 wt % H2O) of a commercial ZnO adsorbent (Johnson Matthey Catalysts, KATALCO 32−5) containing 60−100 wt % of ZnO, and the upper part was filled with 493 g (on a dry basis) of a commercial 5% Ru/C catalyst (BASF). Data of the fresh catalyst and the fresh zinc oxide adsorbent can be found elsewhere.23 After being cooled to 20−30 °C the reactor effluent passes through a particle trap and a filter (pore size 25 μm). A second filter was installed in parallel for switching between them by a three-way valve in case of plugging. The reactor effluent is depressurized to atmospheric pressure by a manually adjustable spring-loaded relief valve (SITEC). A control valve (Kämmer), combined with a pressure controller (Flowserve) located upstream of the relief valve, regulated the pressure. The depressurized fluid enters a gas/liquid phase separator (borosilicate glass, 2000 mL) which hangs on a digital spring balance (HiTec Zang GmbH, GraviDos) used for recording the mass flow rate. The gas leaving the phase separator at the top is cooled to 8 °C (JCT Analysentechnik GmbH, JCP-S) and the gas flow rate is recorded by a wet test meter (Wohlgroth). The gas composition is measured online by a microGC (INFICON, 3000 Micro GC) with Ar as the carrier gas using a thermal 6258
DOI: 10.1021/acs.iecr.7b00042 Ind. Eng. Chem. Res. 2017, 56, 6256−6265
Article
Industrial & Engineering Chemistry Research Table 1. Elemental Composition of Chlorella vulgaris Calculated on a Dry Matter Basis wt % C
H
N
O
S
P
Cl
ash content
50.0 ± 0.3
7.1 ± 0.1
5.8 ± 0.1
33.8 ± 0.2
0.53 ± 0.04
0.65 ± 0.01
0.08 ± 0.03
3.4 ± 0.3
Table 2. List of the Process Parameters feed DM
time on stream
wt %
h
2.8b 3.6b 6.5c 14.8c 6.0c 3.0c 3.0c 6.0c
0−13a 18−19a 25−50 50−56a 59−83 83−84a 89−95 95−99
WHSVd
feed rate −1
kg h
gOrg gCat
1.4 N.A. 1.5 1.5 1.0 N.A. N.A 1.4
−1
Tsalt sep. −1
h
0.08 N.A. 0.20 0.45 0.12 N.A. N.A 0.17
bottom
Tsalt sep. top
Treactor bottom
Treactor top
P
°C
°C
°C
°C
MPa
274 267 299 287 264 325 306 318
414 418 412 396 368 414 414 440
407 404 420 415 394 411 407 413
390 385 379 372 363 385 388 385
28−29 28−29 28−29 28−29 25−26 25−28 27−29 26−30
a
Started to rinse the plant with water. bFeed coming directly from the algae reactor. cFeed coming from the freezer. dWHSV = weight hourly space velocity defined in units of hourly rate of grams of organic processed over the grams of catalyst. N.A. = not available.
2.3. Calculation of Carbon Gasification Efficiency. The carbon gasification efficiency (GEC) establishes a relation between the total amount of carbon in the gas phase and the total amount of carbon entering the reactor, it is defined as GEC (%) =
ṁ C,gas ṁ C,feed − ṁ C,brine
observed in that period may be explained by salt precipitation at the reactor entrance. 3.3. Gas Analysis. In Figure 2, the gas composition recorded online is depicted. During the first 10 h of the
100 (1)
2.4. Calculation of the Carbon and Sulfur Recoveries. The carbon and sulfur recoveries were estimated for the overall campaign (0−100 h) as follows: recoveryC (%) =
recoveryS (%) =
mC,brine + mC,reactor + mC,gas mC,feed mS,brine + mS,reactor mS,feed
100 (2)
100 (3)
3. RESULTS AND DISCUSSION 3.1. Feedstock Composition. In Table 1, the chemical composition of Chlorella vulgaris is listed. Note that the composition of the feedstock does not solely represent the chemical composition of the microalgae but also the one of the cultivation medium (ratio of main nutrients: 100N:10.5P:5.2S). 3.2. Process Parameters. The different process parameters for the gasification campaign are listed in Table 2. The first day (0−19 h), microalgae were directly taken from the photobioreactor without any further treatment. Then, until the end of the campaign (25−99 h), the feed slurry was prepared from microalgae that had been harvested from the same reactor, dewatered, and stored at 4 °C. Depending on the desired feed concentration, microalgae were diluted with pure water. To improve the stability of the microalgae suspension, a small amount (
