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Article

Physical Activation of Waste-Derived Materials for Biogas Cleaning

by
Davide Papurello
1,*,
Massimo Santarelli
1 and
Sonia Fiorilli
2
1
Department of Energy (DENERG), Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Turin, Italy
2
Department of Applied Science and Technology (DISAT), Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Turin, Italy
*
Author to whom correspondence should be addressed.
Energies 2018, 11(9), 2338; https://doi.org/10.3390/en11092338
Submission received: 25 May 2018 / Revised: 20 August 2018 / Accepted: 30 August 2018 / Published: 5 September 2018
(This article belongs to the Section F: Electrical Engineering)
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Versions Notes

Abstract

:
Biogas produced from biomass is carbon neutral. In fact, the carbon feedstock of biomass is converted into gas phase. Biogas use in high efficient energy systems, such as Solid Oxide Fuel Cells is a viable choice. One of the most important drawbacks for such systems is related to the interaction between trace compounds and anode section. Gas cleaning through physical removal mechanisms is the simplest and cheapest method adopted in the literature. Coupled with this solution, the recovery of waste materials is an efficient application of the circular economy approach. In this work, a physical activation process was investigated experimentally for waste-derived materials at a temperature of 700 °C. The removal of H2S was considered as the most abundant trace compound. Activated biochar showed an adsorption capacity comparable to commercial sorbents, while the performance of ashes are still too poor. An important parameter to be considered is the biogas humidity content that enters in competition with trace compounds that must be removed.

1. Introduction

Scientists believed that the Earth’s climate change could be related to the high concentration of CO2 and other greenhouse gases produced by human activities. How biogas can reduce or mitigate this problem is a scientific challenge. Biogas produced from biomass is considered to be carbon neutral, since the carbon feedstock of biomass is converted into gas phase. In other words, atmospheric carbon (CO2) is captured in a relatively short time. Biogas is a secondary energy carrier and can be produced from many organic substrates, such as from agricultural sources, sewage sludge and animal waste [1,2,3,4,5,6]. In addition to the chemical process, a thermal process (gasification or pyrolysis) can exploit organic material for biogas production purposes. The European Biomass Organisation for 2020 estimated a production of biogas around 39.5 Mtoe, which corresponds to 10% of EU natural gas consumption [7]. Biogas can be adopted as a fuel for several energy generators, such as fuel cells, as well as external and internal combustion engines [8,9]. Alternatively, biogas can be upgraded with highly engineered systems into biomethane to be injected into the gas grid [10,11]. Among energy generators, high temperature solid oxide fuel cells have higher electrical efficiency values, high fuel feeding flexibility, low noise and pollutant emissions [8,12,13,14]. The most important drawbacks for such generators is the low tolerability to trace compounds [3,15,16,17,18,19,20,21,22]. Depending on the nature of the trace compound and its concentration in the gaseous matrix, a gas cleaning section has to be specifically designed. The most frequently detected trace compounds from Organic Fraction of Municipal Solid Waste (OFMSW) biogas are sulphurs, carbonyl compounds, carboxylic acids and terpenes [2,4]. Rasi et al. (2011) studied the trace compounds that affect biogas energy users [6]. They mainly detected halogens, aromatic and sulphur compounds in landfill gas. In addition, they detected sulphur compounds, aromatic and siloxane in biogas from the sewage sludge digestion and from waste water treatment plants [6]. Physical adsorption removal using commercially available activated carbons has been investigated in several studies [23,24,25,26]. Its possible use will be determined by economic evaluation based on factors such as its performance and its production cost [27]. The high removal performance of these materials is related to their physical and chemical characteristics. Their specific surface area, the micropore and mesopore volume, as well as the impregnating agents adopted for their chemical activations are also important [27,28,29]. Generally, alkali metals are adopted to improve removal performance, such as potassium, but also alkaline earth metals, such as calcium and magnesium. Barelli et al. (2017) showed how the choice of the impregnating agent is important in the removal of sulphur compounds. In fact, when the carbon is activated with KI and KOH, the role of water on the adsorption capacity has a positive effect [30]. In Papurello et al. (2016) this effect was not highlighted due to the absence of such agents and the role of water remained negative [31]. Another parameter to be considered in literature is the removal of contemporary pollutants. In literature there are several papers that study the interaction of trace compounds with the sorbent materials, such as: Aromatic, terpenes and carboxyls with sulphur compounds [25,26,32,33,34,35]. Waste materials recovery is an efficient application of the circular economy approach. This approach reduces waste and pollutants. Thermo-chemical processes are applied to biomass, as reported in literature using pyrolysis or gasification systems [36]. Pyrolysis process produces char and oil, with a high potential for recovering energy [37,38]. Another waste material suitable for the gas cleaning of biogas is wood ash [39]. Wood ash is an attractive solution due to the low cost of disposal [39]. Ducom et al. (2013) showed how fly ash from biomass was able to remove some trace compounds from the biogas [40]. Fly ash from wood waste and char have a lower removal capacity for the desulphurisation of biogas compared to commercially activated carbons [39,41,42,43,44]. The waste materials adopted for the purpose of cleaning biogas require more research, especially on the improvement of their physical characteristics, such as microporous volume and specific surface area. In this work, a physical activation process is investigated for SOFC gas requirements. The removal of H2S, considered to be the most abundant sulphur compound from a simulated biogas mixture, was investigated.

2. Experimental Methods

2.1. Description of the Experimental Setup

The pyrolysis of wood waste produces a biochar that Gruppo RM Impianti srl (Arezzo, Italy) exploit internally. The reactor adopted is 200 kWe with operating temperature of around 450 °C (construction details reserved). Ashes from wood-chips from local forest were obtained from a boiler (3.3 MW, Viessman, Allendorf, Germany). The activation of biochar and wood ash was accomplished using an electric furnace of 500 We (C.I.T.T., Milano, Italy) regulated with a PID controller (Horst, GmbH Germany). Technical instructions are reported as follows: heat-up ramp fixed at 7 °C/min until 700 °C, with a CO2 flow rate of 100 mL/min, see Figure 1a. The physical activation step was carried out for a period of 60 min under isothermal conditions. The operating temperature for the reactor was fixed using a furnace regulated by a K-type thermocouple placed in the centered bed. Two additional thermocouples (K-type, Tersid, Italy) were placed at the outlet and inlet of the bed. Experiments were accomplished with a Gas Hourly Space Velocity (GHSV) of 66 h−1.
Figure 1b epicts the sorbent material testing set-up. A simulated biogas mixture was accomplished (CH4/CO2 = 1.5) using gas cylinders (Siad spa, Praxair with mass and liquid flow controllers (MFC, LFC) (Bronkhorst GmbH, Ruurlo, The Netherlands). A controlled evaporator mixer (CEM) was adopted to add water to the heated lines with a relative humidity of 30% (RH). A gas mixture prepared with CO2 and H2S (190 ppm(v)) was used for the removal of trace compounds. The H2S concentration was fixed at 75.9 ppm(v) and obtained from the dilution with pure CO2, in order to simulate an average biogas mixture deriving from the anaerobic digestion of organic waste. This concentration is representative of a real biogas, as reported in literature [5,44,45]. Biochar and ashes were ground and sieved before and after the activation process, to obtain a grain size around 400 μm. These sorbent materials were placed in Teflon tubes with an internal diameter of 4 mm and an L/D ratio around 10. The removal test was accomplished three times, both for the activated and for the not activated sorbent materials.

2.2. Methodology

In order to maintain the correct aspect ratio (~4) between the pellet and reactor diameter, the sorbents were ground and sieved [46]. This procedure was followed before the activation proces. The testing sample was selected using metallic sieves in the range 5.4–400 μm. In supplementary material, the pore distribution of activated carbons and waste-derived materials are reported. To remove residual gases inside the pores the samples were pre-treated using N2 flow (200 NmL/min) for 30 min at 120 °C. A gas flow of 200 NmL/min was used for the removal performance testing. Dynamic tests for the sulphur removal were accomplished for the selected materials at 25 °C. Below the adsorption capacity equation is reported, calculated in terms of mg/g for H2S removal:
  C a d s   = Q t o t · M W · [ c i n · t 1 − ( t 1 − t 0 ) · 0.5 [ V m · m · 10 3
where:
  • t0, is the cleaning service time;
  • t1, is the last detection interval, corresponding to initial concentration for H2S of 1%, 10% and 100%;
  • Qtot = total gas flow rate (NL/h);
  • MW = molecular weight of the trace compound removed (g/mol);
  • Cin = inlet trace compound concentration (ppm(v));
  • Vm = molar volume (22.414 NL/mol);
  • m = mass of sorbent (g).
Considering the work of Barelli et al. (2016), Equation (1) was adjusted. In fact, the difference between the rectangle (t1 (h) × Cin (ppm(v))) and the triangle areas (0.5(t1 − t0) (h) × 1 (ppm(v)) approximates the area enclosed by the breakthrough curve and the saturation line (Cout = Cin) [47]. An electrochemical gas sensor (ES, Meccos eTr—Leopold Siegrist GmbH, Lorsch, Germany) was adopted to measure the H2S concentration. This sensor is able to perform a continuous reading in the range of 0–200 ppm(v). The accuracy of the sensor reading is 0.5%, while a value of 1% is fixed for the electrochemical sensor. Flow meter controllers have an accuracy value of 0.1%, related to the full-scale value.

3. Results and Discussion

Results related to the adsorption capacity were reported below, starting from the characterization to the sorbent selection considering also the waste-derived materials.

3.1. Sorbent Characterisation—EDS/SEM

The sorbent materials adopted were characterised in terms of their surface area, composition, micropore volume and morphology structure. The elemental composition measurements were performed by scanning electron microscopy (SEM) (FEI Inspect, Philips 525 M) coupled with EDS analysis (SW9100 EDAX). The results are reported in Table 1, nitrogen and hydrogen fractions are not reported due to the limits of the instrument.
The most abundant compound for biochar samples is carbon followed by oxygen, potassium, calcium, and magnesium. The activation process does not substantially modify the elemental composition. A decrease in potassium content with a corresponding increase in calcium content was recorded. The most abundant compound for the ash samples is oxygen, followed by silicon, carbon, potassium, calcium, magnesium and aluminium. The activation process does not substantially change the elemental composition.
Tepper and Richardson et al. (2002) demonstrated how the concentrations of K and Ca are important for the sulphur compound removal [48,49]. Transition metals are also important for salt formation with trace compounds.
The adsorption isotherms were determined using a Quantachrome Autosorb 1 (Boynton Beach, FL, USA) using for N2 at 77 K. The specific surface areas were calculated by Langmuir equation in the relative pressure range of 0.04–0.1. The micropore volumes were determined by means of t-plot method in the relative pressure range of 0.15–0.3. For the carbon-based materials, the pore size was evaluated through the DFT method (Density Functional Theory), using the NLDFT equilibrium model for slit/cylindrical pores, see Table 2.
The isotherm graph recorded for the biochar, according to IUPAC classification, is of type I. Such isotherm is typical of microporous materials. In Figure 2 the sharp increase in adsorbed volume at low relative pressures is depicted. This is due to the capillary condensation of the adsorbate inside the micropore. The presence of a hysteresis loop suggests the presence of slit porosities, due to the particles aggregation.
In the following Figure 3, an analysis to SEM images is necessary for the sorbent materials adopted, on the left side before activation and on the right side after the activation process.
The waste-derived biochar and ash were physically activated at a fixed temperature and CO2 flow rate. The structure before this physical process was denser and the homogeneity of the structure cannot be guaranteed by the un-optimized production phase. At the end of the activation process, especially for the biochar sample, the increase of porosity and consequently the specific surface area value was evident. This value after the activation process was around 600 m2/g, while the micro pore volume ranged from 0.04 cm3/g to 0.26 cm3/g; and the total volume ranged from 0.06 cm3/g to 0.37 cm3/g. The ash sample was activated using the same process. The specific surface increased to 6 m2/g, while the micro pore and total pore volume also increased according to a similar ratio recorded for the biochar.

3.2. Adsorption Capacity

The recovery of waste materials is an interesting application of the circular economy approach. In fact, literature studies consider a wide range of wastes, such as from agricultural sector for the production of activated carbons [50]. In this work two waste-derived materials have been selected, such as char from the pyrolysis of wood waste, and ashes from the combustion of wood chips in a boiler. Virgin chars, as reported also in this article have lower adsorption capacity for the removal of trace compounds, especially sulphur compounds [37]. The activation methods are divided into chemical and physical origin. Due to technical simplicity and economical aspects, physical activation was adopted as the activation method for the sorbent materials removal of sulphur compounds [51,52,53]. The pore size distribution and pore fraction of the activated biochar vary according to the quantity and type of biomass, operating temperature and activating gas [54]. According to Nabais et al. (2007), the CO2 activation leads to samples with higher BET surface areas and pore volumes compared with samples produced by steam activation and with similar burn-off values [55]. Zhang et al. (2004) studied the duration of CO2 activation and temperature on the characteristics of the char produced from the wood fast pyrolysis [56]. Chars type selected showed an increasing of specific surface area and micropore volume with increasing activation temperature. The increasing of pore size and surface area was verified up to 900 °C, above this value the carbon characteristics decrease [57].
Table 3 shows the adsorption capacity for commercial carbons and waste-derived materials for the removal of H2S. Commercial carbons have an adsorption capacity of around 2 mg/g while commercial metal oxides, such as ZnO has an average removal capacity of around 1 mg/g at t1 = 1% of initial concentration. The two commercial carbons selected have a similar adsorption capacity also at the saturation condition, t1 = 100%. Carbox compared to RST3 shows a steeper profile for the removal of H2S. Waste derived materials not physically activated show an adsorption capacity lower than commercial materials. Here, the surface area and microporous volume weight strongly on the removal performance, as reported [58]. In fact, as reported in the following Figure 4, the adsorption capacity is directly correlated to the specific surface area and microporus volume. Higher micropore volume along with smaller pores results in enhanced capacity of H2S removal. As reported by Adib et al. (2000), the mesoporous volume seems less important for the adsorption capacity performance [59].
In Figure 4, the coefficient of determination is higher than 0.95 for surface area and adsorption capacity, while this coefficient is higher than 0.98 for microporous volume and adsorption capacity. ZnO sample, as reported in the characterization section is defined by an adsorption isotherm related to chemical mechanisms, demonstrated by not observing the linear correlation reported in Figure 4. This removal mechanism is stronger than physisorption, both for removal and regeneration of material. To explain these poor performances, Aravind et al. (2012) demonstrated how the optimal metal oxides performances are shown at a higher temperature [60].
The physical activation with CO2 and higher temperature allows us to improve the average adsorption capacity from 0.18 mg/g to 1.38 mg/g for biochar, and from 0.05 mg/g to 0.38 mg/g for ashes.
As reported by Nabais et al. (2007), physical activation with CO2 promotes pore size and surface area increasing, with better performance than steam activation [55]. Activated biochar shows an adsorption capacity comparable to commercial sorbents, as reported also from [37,61]. Increasing the saturation zone of the filter, from 1% to 10% up to 100%, the adsorption capacity increases up to the saturation of pores, see Table 3. The latter value is a limit of the filter bed filled by sorbent material. An average value of 10 mg/g was registered for activated biochar. This value is not comparable with those obtained with activated ash, while appears similar with those obtained with activated carbons [39]. Metal oxides are able to promote the dissociation of H2S and the HS− reaction to form sulphides. These reaction are reported in literature studies, as studied from Shang et al. (2016) [37]. This was due to the chemical mechanisms developed between trace compounds to be removed and metals [62].
Table 4 depicts the adsorption capacity of H2S in a wet biogas using commercial activated carbons, waste-derived materials and activated waste-derived materials.
Biogas water vapor (30% RH) enters into competition with hydrogen sulphide that has to be removed. This competition decreases the adsorption capacity [31]. In fact, as reported elsewhere [31], when the RH value is above 50%, the adsorption capacity tended to be zero. At t1 = 1% of the initial H2S concentration, the average adsorption capacity value registered for Carbox decrease from 1.9 mg/g up to 1.39 mg/g. A stronger decrease is obtained using ZnO. In fact, the adsorption capacity in the same conditions decreases of 55%.
Considering waste-derived materials, the effect of water content on the removal performance has the same effect registered for commercial sorbents [63]. Because this material is not activated chemically with alkali metals, water content competes with sulphur for their removal [30]. The adsorption capacities decrease more in the case of not possessing physically activated material. In fact, pores and surface area being lower suffer more from the competition of water content. Biochar shows a decrease of 80%, while with the physical activation, this decrease stands at around 30%.

4. Conclusions

Hydrogen sulphide removal performance of waste-derived material, such as biochar show lower adsorption capacity compared to optimized commercial sorbents. These results, although preliminary are important for the future optimization of waste-derived materials. In fact, such types of materials can be reinserted in the gas production cycle as fertilizer or structuring material for the anaerobic digestion process.
Physical activation with CO2 and temperature promotes for activated biochar an adsorption capacity that increases with values comparable to commercial sorbents, while ashes performance are still too poor.
Humid biogas should be dried, because water enters into competition with hydrogen sulphide that has to be removed. A more pronounced decrease of adsorption capacities is recorded for not physically activated material. This is due to the higher competition between water content and trace compounds to be removed with low surface area and microporous volume.

Supplementary Materials

The following are available online at https://www.mdpi.com/1996-1073/11/9/2338/s1.

Author Contributions

S.F. for the chemical analysis and D.P. and M.S. for the research activity and for the supervision work.

Acknowledgments

This research is part of the BWS project (Biowaste for SOFCs) carried out with Fondazione Edmund Mach and SOLIDpower SpA. The project is funded by the contribution of Fondazione Caritro (TN). This research is also part of the DEMOSOFC project (European project FCH2 JU—grant number 671470) and BIOGAS4ENERGY (National project carried out with FinPiemonte).

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 11 02338 g001 550
Figure 1. Experimental set-up: (a) activation test rig, (b) sorbent material testing for VOCs removal.
Figure 1. Experimental set-up: (a) activation test rig, (b) sorbent material testing for VOCs removal.
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Figure 2. Isotherm graph recorded for biochar characterization, according to IUPAC classification.
Figure 2. Isotherm graph recorded for biochar characterization, according to IUPAC classification.
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Figure 3. Scanning electron microscopy images for the materials tested – before and after the physical activation process.
Figure 3. Scanning electron microscopy images for the materials tested – before and after the physical activation process.
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Figure 4. Adsorption capacity in relation with surface area and microporous volume: (a) SSA vs. Cads.; (b) Vmicro vs. Cads.; (c) Vmeso vs. Cads.
Figure 4. Adsorption capacity in relation with surface area and microporous volume: (a) SSA vs. Cads.; (b) Vmicro vs. Cads.; (c) Vmeso vs. Cads.
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Table
Table 1. Composition in terms of weight fraction for the materials tested.
Table 1. Composition in terms of weight fraction for the materials tested.
BiocharActivated BiocharAshActivated AshCommercial Carbon (RST3)Commercial Carbon (Carbox)ZnO
C77.581.814.2614.891.180.9
O13.9816.6549.2949.45.39.0124.79
Si 23.7322.2 1.40.55
Al 0.600.59 1.980.96
K0.930.339.9910.750.790.78
Ca0.901.631.31.40.751.650.75
Mg0.240.190.620.640.311.19
Cl 0.210.22 0.33
P 0.81
S 0.590.11
Mn 0.41
Fe 1.43
Zn 1.16 72.95
Table
Table 2. Surface and volume characteristics for the materials tested.
Table 2. Surface and volume characteristics for the materials tested.
BiocharActivated BiocharAshActivated AshCommercial Carbon (RST3)Commercial Carbon (Carbox)ZnO
Specific surface area(m2/g)755930.961117123735.8
Microporous volume(cm3/g)0.040.260.010.070.30.328
Total pore volume(cm3/g)0.060.370.0150.10.4470.4070.196
Table
Table 3. Adsorption capacity for waste-derived materials and commercial carbons for H2S removal.
Table 3. Adsorption capacity for waste-derived materials and commercial carbons for H2S removal.
SorbentLoad (g)Cads. (mg/g)-1%Std. DevCads. (mg/g)-10%Std. DevCads. (mg/g)-100%Std. Dev
Commercial sorbents
Airdep CarbOx0.531.881.881.940.036.756.746.770.0112.4512.5412.650.08
Norit RST30.31.952.041.980.042.762.812.830.0312.1012.1112.170.03
Zinc oxides0.330.920.981.020.0424.7424.7524.810.0347.0747.1747.210.06
Waste derived materials
Biochar0.250.120.190.230.050.270.310.400.054.764.854.920.07
Biochar act0.161.301.381.450.061.952.012.110.079.859.8910.010.07
Ash0.950.020.060.080.020.160.100.170.030.230.250.270.02
Ash act1.030.290.370.480.080.650.660.690.021.771.791.750.02
Table
Table 4. Adsorption capacity for waste-derived materials and commercial carbons for H2S + H2O removal.
Table 4. Adsorption capacity for waste-derived materials and commercial carbons for H2S + H2O removal.
SorbentLoad (g)Cads. (mg/g)-1% + H2O (30%)Std. DevCads. (mg/g)-10% + H2O (30%)Std. DevCads. (mg/g)-100% + H2O (30%)Std. Dev
Commercial sorbents
Airdep CarbOx0.171.301.371.490.085.195.275.390.088.658.748.780.05
Zinc oxides0.300.340.430.540.082.452.512.620.0729.4429.4829.550.05
Waste derived materials
Biochar0.310.010.020.080.030.080.120.130.022.652.692.800.06
Biochar act0.280.900.931.040.061.261.291.350.045.935.986.020.04
Ash10.010.020.040.010.140.150.180.020.260.170.200.04
Ash act1.040.170.190.250.030.420.470.500.031.101.171.190.04

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Papurello, D.; Santarelli, M.; Fiorilli, S. Physical Activation of Waste-Derived Materials for Biogas Cleaning. Energies 2018, 11, 2338. https://doi.org/10.3390/en11092338

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Papurello D, Santarelli M, Fiorilli S. Physical Activation of Waste-Derived Materials for Biogas Cleaning. Energies. 2018; 11(9):2338. https://doi.org/10.3390/en11092338

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Papurello, Davide, Massimo Santarelli, and Sonia Fiorilli. 2018. "Physical Activation of Waste-Derived Materials for Biogas Cleaning" Energies 11, no. 9: 2338. https://doi.org/10.3390/en11092338

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