Research Article pubs.acs.org/journal/ascecg
Hydrothermal Carbonization of Digestate in the Presence of Zeolite: Process Efficiency and Composite Properties Jan Mumme,*,† Maria-Magdalena Titirici,‡ Anke Pfeiffer,§ Ulf Lüder,§ M. Toufiq Reza,∥ and Ondřej Mašek† †
UK Biochar Centre, School of GeoSciences, University of Edinburgh, Crew Building, King’s Buildings, Edinburgh EH9 3JN, U.K. School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, U.K. § Leibniz Institute for Agricultural Engineering Potsdam-Bornim, Max-Eyth-Allee 100, 14469 Potsdam, Germany ∥ Department of Chemical and Materials Engineering, University of Nevada, Reno, 1664 North Virginia Street, Reno, Nevada 89557, United States ‡
S Supporting Information *
ABSTRACT: A systematic experimental study on hydrothermal carbonization (HTC) of digestate was conducted to evaluate the catalytic impact of natural zeolite and the properties of the produced hydrochar−zeolite composites (HZCs). An agricultural digestate and, as reference, microcrystalline cellulose (MCC) were treated at HTC temperatures of 190, 230, and 270 °C. HZCs were analyzed for their elemental composition, NMR structural properties, thermogravimetric behavior, N2 adsorption porosity, and scanning electron microscopy morphology. The results indicate distinct catalytic effects of zeolite on carbonization. For digestate, catalytic effects of zeolite increased the degree of carbonization equally to a 9−29 K higher HTC temperature. Zeolite increased the energy and carbon recovery in solid products for digestate, whereas MCC showed a lower recovery. Interestingly, zeolite preserved the cellulose fraction of digestate. This was attributed to physical and chemical shielding by formation of a visible zeolite layer on organic particles. Compared to pure hydrochar, the HZCs showed less aromatic and thermally stable carbon but higher surface area and pore volume. Potential areas of applications for the HZCs range from energetic use (e.g., gasification) to soil amendment and additive in bioprocesses (e.g., growing media). KEYWORDS: Biochar, Biogas, Carbon−mineral composites, Catalysis, NMR, Proximate analysis, Porosity
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waste biomass like sewage sludge or municipal solid waste.18 For municipal solid waste, commercial application of HTC has already been demonstrated.19 However, when compared to other waste treatments like anaerobic digestion, composting, or incineration, commercial applications of HTC are still developing. Another interesting feedstock for thermochemical conversion is digestate. Its traditional use as soil amender and organic fertilizer could become more and more restricted because of environmental and economic reasons.20,21 Furthermore, the feasibility of hydrothermal conversion of digestate into hydrochar has been repeatedly demonstrated.16,22,23 A known drawback of HTC are losses of carbon and energy by the production of soluble and gaseous byproducts usually ranging between 20 and 50%.10−12,16,23,24 In addition, several volatile organic compounds (VOCs) contained in HTC liquids
INTRODUCTION Hydrothermal carbonization (HTC) is not a new field of research; it has its origin in the first decades of the twentieth century when it was used to learn the mechanisms of natural coalification.1 Later, hydrothermal degradation of organic matter and synthesis of basic chemicals as well as liquid and gaseous fuels gained scientific interest.2−4 Research activities that focused on the solid product, termed hydrochar, began in recent years. It was found that the process can yield nano- and microsize carbon particles with distinct properties such as high energetic value, high chemical and thermal stability, and moderately high surface area and adsorption capacity.5−7 Consequently, hydrochars are discussed as solid biofuels8−11 and for various industrial, environmental, and agricultural applications.12−15 Furthermore, integration of HTC in bioconversions (e.g., for biogas or bioethanol production) is assumed to considerably improve the overall energy efficiency by making use of its organic wastes.16,17 With respect to the economic feasibility of HTC, the most promising area of application is the treatment of high-moisture © XXXX American Chemical Society
Received: August 26, 2015 Revised: October 6, 2015
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DOI: 10.1021/acssuschemeng.5b00943 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering are toxic,25,26 which increases the cost of wastewater treatment and, in the case of a material use, of the post-treatment of hydrochar products. Furthermore, large-scale application is also hindered by immature HTC technology, which is still mostly operating in batch-mode, and high costs associated with hightemperature, high-pressure processing of heterogenic wastes. To tackle these obstacles, a range of catalysts (both homogeneous and heterogeneous) has been investigated including various organic acids and noble metals.27−29 A wellknown natural catalyst is the mineral zeolite, which derives its functionality from a high specific surface area and its strong acid sites resulting from substitution of Si by Al in its framework.30 Today, technically produced zeolites are among the most important catalysts in chemical conversion (e.g., for catalytic cracking, biomass conversion into chemicals, etc.). It was shown that all of these different catalysts are capable of catalyzing certain reactions involving HTC, e.g., use of zeolite as sacrificial template to produce porous carbons.31 But this faces limits with respect to energy and cost efficiency. In light of the current state of the art, it is of high interest to investigate to what extent natural zeolite can be used for catalysis of HTC and production of hydrochar−zeolite composites (HZC) with distinct properties for environmental and other applications. Consequently, the overall aim of the present study was to assess the effect of natural zeolite as an alternative catalyst on the HTC process and solid product. Individual objectives were (i) to characterize and quantify potential catalytic effects of zeolite on HTC of digestate and microcrystalline cellulose (MCC); (ii) to describe the impact of zeolite on the yields and recovery rates of hydrochar, carbon, minerals, and energy; (iii) to describe the material properties of HZCs concerning elemental composition, chemical structure, morphology, and porosity; and (iv) to classify the thermal stability of carbons within the HZCs and pure hydrochars.
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from porosity measurements, the zeolite has a Brunauer−Emmett− Teller (BET) surface area of 38.2 m2 g−1. Design of Experiment and Hydrothermal Carbonization. For the HTC experiments, a full factorial experimental design was used comprising two feedstocks (digestate and MCC), addition of zeolite in a dosage of 1 g zeolite per gram of feedstock and no zeolite addition (control), and three HTC temperatures (190, 230, and 270 °C). Further details including the dry matter (DM)-based zeolite-tofeedstock ratios are shown in Table 1. The HTC experiments were carried out in a Parr 1-L 4520 Series stirred reactor (Moline, IL, US) equipped with an external resistance heater. For the zeolite-containing experiments, a mixture of 50 g of feedstock, 50 g of zeolite, and 600 g of distilled water was poured into the reactor. The feedstock-only controls were carried out with 80 g of feedstock and 620 g of distilled water. A heating rate of 2 K min−1 was applied to reach set temperatures of 190 °C after 80 min, 230 °C after 100 min, and 270 °C after 120 min (starting temperature, 30 °C). The set temperature was maintained for 120 min, after that the heater was turned off and the reactor was left for natural cooling overnight. During the whole experiment, the stirrer was operated at 90 rpm. The next day, the slurry was vacuum filtered (max. 100 mbar) with a filter paper (particle retention 5−8 μm). Afterward, the solids were dried for 15 h at 105 °C and stored under dry conditions for subsequent analyses. Analytical Methods and Calculation Basis. Contents of DM and organic dry matter (ODM) were determined gravimetrically by drying the samples at 105 °C for 24 h and ashing at 550 °C for 4 h, respectively. The elemental composition (C, H, N, and S) was determined using a Vario EL III elemental analyzer (Elementar Analysensysteme GmbH, Germany). The DM-based content of O was calculated by subtracting the measured elements and ash from 100%. For correction purposes, the zeolite’s CW is of particular interest. Based on TGA (see Figure S1), losses of CW during drying are about 5% of initial mass (dry, without external bound water), and another 6% are lost during ashing. As described earlier, the total CW content of zeolite was assumed to be 11% of initial dry mass. This includes the elements H and O, which stoichiometrically represent 1.22% and 9.78% of the dry but CW-containing mass of the zeolite. The content of H was confirmed by a measured value of 1.27% (see Table 1). All corrections were carried out with the stoichiometric values. The equations used to calculate and correct the HZC’s contents of DM, ODM, ash, and C, H, N, and S elements are provided in the Supporting Information. 13 C CP/MAS solid-state NMR was performed with a Bruker Avance 300 MHz spectrometer, operated at a magic-angle spinning (MAS) rate of 14 kHz. For rotation, 4 mm zirconia sample holders were used. TGA was conducted with a Mettler-Toledo TGA/DSC1 instrument, equipped with an autosampler. The 7−24 mg samples of each ground and dried material were weighed into 70 μL alumina crucibles. The program comprised the following consecutive steps: (1) equilibration for 2 min at 25 °C, (2) water evaporation with temperature increased to 110 °C (heating rate 25 K min−1) and held for 10 min, (3) pyrolysis with temperature increased to 900 °C (heating rate 25 K min−1) and held for 10 min, and (4) oxidation (ashing) for 30 min at 900 °C. Steps 1−3 were conducted under N2 atmosphere (flushed at 50 mL min−1), whereas step 4 was aerobic (flushed with air at 50 mL min−1). All analyses were conducted in triplicate. The measurement data was pre-evaluated and converted to ASCII format by means of Mettler-Toledo STARe evaluation software v13. Final data assessment was performed by MS Excel 2013. This included proximate analysis with normalization of data and attribution of mass losses to the individual phases. Fixed carbon content was calculated on a mass basis by subtracting moisture, volatile, and ash content from the original mass of the sample. Porosity and surface analyses were conducted for HZCs as well as for raw digestate and raw zeolite by using a QuadraSorb SIMP analyzer (Quantachrome Instruments, US). The specific surface area was measured based on N2 adsorption and multipoint BET method according to ISO Standard 9277. Barrett−Joyner−Halenda (BJH),
EXPERIMENTAL SECTION
Origin and Properties of Feedstocks. In this study, natural digestate and, as reference, MCC were used as feedstock for HTC. The digestate was obtained in dried state from an on-farm biogas plant (Hof Karp, Rastow, Germany), which uses cow manure and maize at a mass ratio of 4:3 as feedstock. A more detailed description of the digestate origin can be found elsewhere.32 Before being processed, the digestate was again dried for 24 h at 105 °C and then ground to a particle size below 1 mm with cutting mill SM 100 from Retsch (Haan, Germany). Industrial grade MCC (Avicel PH-101 Fluka) was obtained from Sigma-Aldrich, Switzerland. Avicel PH 101 is a microcrystalline, powdery material with an average particle size of 50 μm and a bulk density of 0.28 g cm−3. The chemical properties are shown in Table 1. Zeolite. A natural hydrated zeolite with particle size 0−100 μm mainly composed of clinoptilolite (90−92%), cristobalite (5−7%), feldspar (2−4%), mica (1−2%), and quartz (traces) was obtained from Zeolithversand LTD (Germany). Based on the empirical chemical formula Ca4K8Na8Mg4(Si40Al8)O96·24H2O provided by Zeolithversand LTD, the zeolite’s crystal water (CW) amounts to a mass portion of 10.6% of the zeolite. This was confirmed by thermogravimetric analysis (TGA), which showed a mass loss of 11.0 ± 0.2% at 900 °C (see Figure S1 in the Supporting Information) and is in good agreement with data from another report.33 In consequence, it was assumed that the whole mass loss of 11.0% represents CW and that the eventual presence of external unbound water can be neglected. Further material information provided by the supplier is as follows: geometric bulk density, 1.60−1.80 kg L−1; specific weight, 2.20−2.44 kg L−1; porosity, 24−32%; effective pore size average, 0.4 nm; compression strength, 33 MPa; thermal stability,
