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Hydrothermal carbonization of agricultural residues
Bioresource Technology 142 (2013) 138–146
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Hydrothermal carbonization of agricultural residues Ivo Oliveira ⇑, Dennis Blöhse, Hans-Günter Ramke University of Applied Sciences – Hochschule Ostwestfalen-Lippe, Campus Höxter, Faculty of Environmental Engineering and Applied Informatics, Professorship of Waste Management and Landfill Technology, An der Wilhelmshöhe 44, 37671 Höxter, Germany
h i g h l i g h t s HTC converts organic residues into a solid fuel comparable to brown coal. Most of the tested agricultural residues are suitable for the HTC process. Blending of different agricultural residues ensures a successful carbonization. Dewatering and drying properties of most HTC-Biochars are very good. HTC-process water can be used in biogas plants increasing process energy efficiency.
a r t i c l e
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Article history: Received 12 January 2013 Received in revised form 29 April 2013 Accepted 30 April 2013 Available online 10 May 2013 Keywords: Hydrothermal carbonization Agricultural residues HTC-Biochar Process water Biogas potential
a b s t r a c t The work presented in this article addresses the application of hydrothermal carbonization (HTC) to produce a solid fuel named HTC-Biochar, whose characteristics are comparable to brown coal. Several batch HTC experiments were performed using agricultural residues (AR) as substrates, commonly treated in farm-based biogas plants in Germany. Different AR were used in different combinations with other biomass residues. The biogas potential from the resulting process water was also determined. The combination of different AR lead to the production of different qualities of HTC-Biochars as well as different mass and energy yields. Using more lignocellulosic residues lead to higher mass and energy yields for the HTC-Biochar produced. Whilst residues rich in carbohydrates of lower molecular weight such as corn silage and dough residues lead to the production of a HTC-Biochar of better quality and more similar to brown coal. Process water achieved a maximum of 16.3 L CH4/kg FM (fresh matter). Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Over the years, fossil fuels have been the cheapest source for the petroleum and petrochemical industries, which led to the development of millions of petroleum based products. However, fuel prices demand have reached record heights in recent years triggered by factors like the depletion of easily accessible deposits and also increasing demand by emerging economies. Biomass was once the global energy source before the arrival and the unbridled expansion of fossil fuels during the Industrial Revolution in the 1800–1900s. Germany’s decision to shut down all the nuclear reactors by 2022 increases the country’s demand for other sorts of energy like: coal fired plants, natural gas imports and renewable energy production (Spiegel, 2011; Euractiv, 2011). Therefore, the adoption
⇑ Corresponding author. Present address: University of Southwales, SERC Laboratories, Upper Glyntaff CF37 4BD, United Kingdom. Tel.: +44 (0) 1443482163. E-mail addresses: [email protected], [email protected] (I. Oliveira), [email protected] (D. Blöhse), [email protected] (H.-G. Ramke). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.04.125
of decentralized systems like many small interconnected units harvesting wind, solar, hydroelectric and biomass power is expected in the near future. Hydrothermal Carbonization (HTC) was discovered by Bergius in 1913, and has been re-discovered and further developed under the direction of Professor Antonietti, director of the Department of Colloid Chemistry at the Max-Planck Institute of Colloids and Interfaces in Golm/Potsdam (MPI). HTC is now being mentioned as a promising technology to convert biomass into multiple bioproducts: a solid fuel compared to brown coal (Parshetti et al., 2013; Funke and Ziegler, 2010; Kleinert et al., 2009; Ramke et al., 2009; Titirici et al., 2007); liquid fuel or bio-oil (Akhtar and Amin, 2011; Heilmann et al., 2010, 2011a,b; Hoekman et al., 2011; Titirici et al., 2007; Xiao et al., 2012) as a soil amendment to increase soil fertility and crop yields (Du et al., 2012; Kleinert et al., 2009; Libra et al., 2011; Rillig et al., 2010); carbon material that could be either activated to work as an adsorbent for water purification systems or CO2 sorption (Libra et al., 2011) and as a low cost adsorbent or permeable reactive barrier for Uranium(VI), Copper and cadmiumcontaminated waters (Kumar et al., 2011; Regmi et al., 2012); nanostructure carbon material (Cui et al., 2006; Inagaki et al.,
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2010; Libra et al., 2011); carbon catalyst, which could be used in the production of fine chemicals (Xiao et al., 2012); and lastly a carbon material that could increase a fuel cells efficiency (Libra et al., 2011). The objective of this research work was to give more data about the use of HTC in the production of a solid fuel similar to brown coal (HTC-Biochar) using different agricultural residues. Agricultural residues1 (AR) are a type of biomass that could present considerable heterogeneity depending on the country and region. We considered AR as energy crops (corn and grass silage), livestock manures from poultry, cattle, and piggery farms, bedding material used in barn stalls and digestate (or Biogas Slurry) from biogas plants all present in the region of Höxter, Germany. In Germany this type of AR are being treated through farmbased biogas plants which produce a renewable fuel (biogas) used to generate heat and power, and a digestate (solid and liquid) used as fertilizer. However, several problems have been reported related to the use of energy crops. Due to its lignocellulosic structure these substrates are not fully hydrolyzed. Hydrolysis of lignocellulosic material has been referred to for years as the rate limiting step in anaerobic digestion of lignocellulosic biomass (Alvarez et al., 2000; Chynoweth and Isaacson, 1987; Hendriks and Zeeman, 2010; Lindorfer et al., 2007; Prochnow et al., 2009; Ward et al., 2008). As a result, the material tends to float upon the fluid surface in the digester, leading to increased stirring expenses. For instance wrapping of longer grass particles around moving devices can cause failures in operating the biogas plant (Prochnow et al., 2009). Therefore, long retention times (30–80 days) are utilised increasing the operating volumes and digestate storage costs (Lindorfer et al., 2007; Prochnow et al., 2009). However, these technical problems arising from energy crops utilization for anaerobic digestion have not been the subject of systematic scientific investigation thus far. HTC is an exothermic process that lowers both the oxygen and hydrogen content of the feed (described by the molecular O/C and H/C ratio) by 5 main reaction mechanisms which include hydrolysis, dehydration, decarboxylation, polymerization and aromatization (Funke and Ziegler, 2010; Hoekman et al., 2011). This is achieved by applying temperatures of 180–220 °C to a suspension of biomass and water at saturated pressure for a couple hours. At the end of the process the solid phase, denominated as ‘‘HTC-Biochar’’, can be easily separated from the water. Approximately 75– 80% of the carbon input is found in the solid phase (HTC-Biochar); about 15–20% is dissolved in the liquid phase (process water); and the remaining 5% are converted to gas (mainly carbon dioxide). The liquid phase is highly loaded with organic components, which for example could be easily degradable through anaerobic digestion (Ramke et al., 2009). Carbonization of Biomass has a number of advantages when compared with common biological treatment. It generally takes only hours, instead of the days or months required for biological processes, permitting more compact reactor design. When compared to fermentation and anaerobic digestion, HTC is referred to as the most exothermic and efficient process for carbon fixation (Titirici et al., 2007). Therefore, HTC is now seen as a promising technology also for CO2 sequestration. In addition, some feedstocks are toxic and cannot be converted biochemically. The high process temperatures can destroy pathogens and potential organic contaminants, such as pharmaceutically active compounds that could be present for example in AR (Libra et al., 2011). It is therefore important to study the quality2 of the ‘‘HTC-Biochar’’ produced
1 The term AR was adopted to combine different streams of Biomass in the Agricultural sector. 2 Quality of HTC-Biochar is used in the manuscript to avoid describing elemental composition (C/H/N/O) or molar ratios as well Higher Heating Value. The quality of HTC-Biochar is then compared to Lausitz brown coal.
Biogas Plant Digestate Agricultural Residues
Separated Digestate
HTC reactor
Process Water
Liquid Digestate
Fertilizer
HTC-Biochar
Fig. 1. Presumed combination of anaerobic digestion and hydro-thermal carbonization of agricultural residues.
through carbonization of AR, envisioning its application as a solid fuel. Recently, a first study has been reported using digestate produced from maize silage previously treated at 55 °C in a two-stage solid-state reactor system (Mumme et al., 2011), however with different experimental conditions from the work presented here. The present work was performed considering a hypothetical situation (Fig. 1) where an HTC reactor could be used to improve the efficiency of an existing farm-based biogas plant. In this case, AR and separated digestate are used as input material for the HTC reactor to produce a high quality solid fuel (HTC-Biochar) compared to brown coal, while process water (liquid phase) is treated by anaerobic digestion. This preliminary work has the objective to present the first results about the HTC of AR and the quality of the HTC-Biochar produced (Trial I). Additionally, carbonization tests with different combinations of AR and some industrial organic wastes and silvicultural residues were also performed (Trial II). The results of biogas and biomethane potential determination of the liquid phase are also given.
2. Methods 2.1. Substrates Several biomass substrates were used in this work: Agricultural Residues, Industrial Organic Waste (IOW) and Silvicultural Residues (SR). The AR used in this work were collected from a farmbased biogas plant located in the town of Marienmünster in Höxter district. The AR used were: corn silage (CS), poultry manure (PM), separated (solid) digestate (SD), bedding material (BM; a heterogeneous mixture mainly composed by hay, straw and manure), and also dry straw (DS). The IOW used was collected from different local small industries. Those substrates were: cabbage (CR) and dough (DR). The SR were collected from the main biomass supply platform in the region of Höxter located in the city of Borlinghausen. The following SR were chosen: forest (FWC) and landscape (LWC) wood chips with low market value, and forest wood chips with high market value (FWCH). Every substrate was representatively sampled and transported in closed barrels of 200 L. Table 1 shows the characterization of each substrate used. AR and IOW were only stored for a short period of time before being used in the HTC experiment due to their susceptibility to degradation. To avoid any losses of material (organic and water) the IOW were only stored for 3 days while AR such as CS and SD were stored for a period no longer than two weeks.
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Table 1 Characterization and Identification of the substrates used in Trials I and II. Substrates
Aba
(a) Characterization of the substrates used Corn silage CS Poultry manure PM Bedding material BM Separated digestate SD Dry straw DS Cabbage residues CR Dough residues DR Forest wood chips FWC Landscape wood chips LWC Forest wood chips (high quality) FWCH IDc
DM (%)
oDM (%)
C (%)
H (%)
N (%)
O (%)
HHVb (J/g DM)
26.9 53.2 26.0 26.9 90.8 10.8 51.2 56.9 38.8 71.4
96.2 84.4 78.2 88.7 95.3 90.0 97.1 90.1 95.2 98.6
46.9 46.8 40.7 45.1 46.8 45.8 44.1 46.2 49.6 49.3
6.2 6.3 5.1 5.5 6.0 5.4 6.5 5.6 5.8 6.2
1.2 1.3 1.9 1.4 0.4 3.7 2.1 0.7 0.5 0.4
41.9 30.1 30.5 36.7 42 35.2 44.4 37.6 39.4 42.6
19.600 18.800 16.900 18.400 18.700 18.000 17.800 18.400 19.400 19.300
Amount of each substrate in the autoclave (% of relative DM)
(b) Description of the HTC experiments combining two different substrates for Trial II Substrate 1 Substrate 2 X1 20 80 X2 50 50 Y1 20 80 Y2 50 50 Z1 20 80 Z2 50 50 K1 20 80 K2 50 50 W1 20 80 W2 50 50 A 50 50 B 50 50 C 50 50 D 50 50 E 70 30 F 62 38 a b c
Substrate combinations
Substrate 1 CS
Substrate 2 SD
CS
PM
CS
BM
CS
FWCH
CR
SD
SD FWC SD SD SD SD
BM BM FWC LWC DS DR
Abbreviations for each substrate. Higher heating value. Identification of each substrate combination.
2.2. Hydrothermal carbonization experiment procedure The hydrothermal carbonization experiments were carried out in a custom-built 25 L autoclave located in the Laboratory of Waste Management and Landfill Technology (Ramke et al., 2009). The autoclave is equipped with external and internal thermometers, digital and analog manometer, cooling system and a computer supported programmable logic controller (PLC). For every HTC batch experiment an operation volume of 20 L, a dry matter content of 15%, and the same HTC operational procedure were adopted. The HTC operation procedure was divided into three phases: heating phase, reaction phase and cooling phase. The experiment started with the heating phase where the autoclave was heated at maximum power until the suspension reached a temperature of 220 °C. After that only 10% of heat power is provided to allow enough time and temperature (4 h > 180 °C) for the reaction phase. After the substrate had been kept at 180 °C (reaction phase) for 4 h the heaters were turned off and the cooling phase was started. The process parameters: temperature, pressure and energy consumption were continuously recorded. The experiment ended once ambient temperatures (<25 °C) were reached inside the autoclave. The gas produced during the experiment was collected in a 250 L bag and the autoclave was opened. The resulting wet product was transferred and stored in closed plastic buckets until separation. The HTC experiments were performed with all the substrates mentioned on Table 1 (Trial I). Different substrates combinations were also performed as shown in Table 1b (Trial II). 2.3. Biogas potential test 2.3.1. Method and test apparatus Specific biogas yield was determined using the biogas potential test specified in a German Norm for fermentation of organic
materials (VDI 4630, 2006). Test apparatus used is specified by a DIN norm (DIN 38414, 1985). Two batch tests were performed and in each test a total number of 29 vessels were used. Two working volumes were used for every batch test: 1000 ml and 3000 ml. The studied substrate samples and reference (microcrystalline cellulose) were conducted in duplicate and for the inocula a triplicate was used. Tests were undertaken at mesophilic temperatures (38 °C ± 2 °C) controlled inside a climatic room. The volume of biogas produced, temperature, and atmospheric pressure were recorded daily. Mixing was carried out manually every day by shaking each vessel for a couple of seconds. The test ended when the daily biogas rate was equivalent to only 1% of the total biogas volume produced until that time. The biogas quality of each vessel was assessed after 15 days of retention time. 2.3.2. Substrate and inocula The Inocula was collected and transported in a closed barrel of 200 L from a waste water treatment plant in Beverungen, Germany. Inocula was stored using the procedure described in (DIN 38414-8). The substrates used in the biogas potential test were obtained after separation of the whole wet HTC product. The process water was stored in plastic bottles of 1000 ml. The process water was characterized in terms of dry matter content, TOC, pH and conductivity. 2.4. Analytical methods All the HTC product phases (HTC-Biochar, process water, and gas) were analyzed. HTC-Biochar and process water were obtained after separation and stored in separated plastic sealed buckets. All the substrates used in the HTC and HTC-Biochar were characterized in terms of dry matter content (DM), organic dry matter
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I. Oliveira et al. / Bioresource Technology 142 (2013) 138–146 Table 2 Distribution of the carbon fraction in each HTC product phase and the carbon yield in HTC-Biochar for Trial I and Trial II. Trial
Substrate
C – solid phase (% DM)
C – liquid phase (% DM)
C – gas phase (% DM)
HTC-biochar yielda (% DM)
Trial I
CS PM BM SD DS CR DR FWC LWC FWCH X1 X2 Y1 Y2 Z1 Z2 K1 K2 W1 W2 A B C D E F
70.1 60.9 51.7 74.0 72.3 51.9 74.3 79.1 79.5 80.9 72.6 70.2 59.7 58.5 63.8 68.5 78.4 75.2 59.9 56.6 81.4 75.1 79.7 82.5 76.6 63.6
24.5 32.4 34.1 17.4 20.4 38.8 20.8 16.6 16.1 15.7 24.7 22.1 34.0 32.9 26.4 24.2 17.1 19.7 29.8 34.9 13.3 18.2 15.8 12.7 17.4 26.0
5.6 6.7 10.1 8.3 6.7 8.1 4.6 3.9 3.1 3.3 9.8 7.4 6.5 8.6 8.8 7.0 5.0 5.8 10.7 8.3 5.9 7.3 4.5 5.0 7.7 10.5
52.5 52.5 30.9 58.5 49.5 34.1 43.9 60.5 46.0 64.4 54.0 50.8 52.7 44.0 42.6 48.8 62.2 65.0 48.4 43.0 77.3 65.1 69.7 72.3 81.7 45.6
Trial II
a
HTC-Biochar yield is calculated after dewatering step.
content (oDM), higher heating value (HHV) and coke content using DIN norms (DIN EN 12880; DIN EN 12879; DIN 51900-3). For Higher Heating Value determination a IKA-Calorimeter C4000 Adiabatic was used. Elementary analysis was determined using Vario Macro Elementar Analyzer. All the analyses were done in duplicate with exception of elementar analysis which was done in triplicate. Dewatering and drying properties of the HTC-Biochar and substrates were accessed using a method developed by the Waste Management Laboratory in the University of Applied Sciences of Ostwestfalen-Lippe. Special test equipment has been developed for this purpose (Ramke et al., 2009) by using a press, normally used for testing building material, and a cylinder with a standard volume capacity inside where the sample is placed. The samples are put under constant pressure of 15 bar for 30 min and the discharged water is passed over a filter plate inside the cylinder and caught in electronic scales. The scale is connected to a notebook computer and the weight of the discharged water is continuously recorded. In the drying test the pressed samples obtained from dewatering are placed in an enclosed cell connected to a drying machine which provides a continuously air flow with a temperature of 80 °C. The test lasts 120 min and every 30 min the weight of the sample is recorded. The dewatering and drying tests were done in duplicate. Process water was analyzed in terms of dry matter content, TOC, pH and conductivity. Gas samples were analyzed in terms of CH4, CO2 and CO content using a GC TCD Argilent 6850 with a fused silica capillary column carboxen-1010. Temperatures of oven, inlet and detector were respectively 50 °C, 200 °C, and 230 °C. Hydrogen was used as a carrier gas using a column flow of 3 ml/min.
3. Results and discussion 3.1. Mass and carbon balances The total mass balances between in- and output material for all the experiments gave a minimum value of 94% and a maximum of
98% recovery. The resulting losses can be plausibly explained as wetting, evaporation and due to spill- or droplet losses when emptying the autoclave and during separation of the output material. In analogy to this, the recovery-value of organic carbon is close to 100%. However, its distribution in the solid (HTC-Biochar), liquid (process water) and gas phases is dependent on the type of substrate used as shown in the Table 2. It is possible to observe a tendency where the majority of the carbon is present in the solid phase (52–80%) and the remaining organic carbon is transferred to the liquid phase (16–39%) and gas phase (3–10%). The substrates BM and CR present very low carbon in the solid phase and the remaining substrates reached higher carbon percentage in the solid phase in particular SR. The HTC-Biochar yield had a minimum of 31% and maximum of 64% (% DM) for BM and FWCH respectively. The HTC-Biochar yield is calculated on a dry matter basis considering a dewatering step after HTC. This consideration results in lower yields however it is necessary to take into account this step for the application of the HTC-Biochar as a solid fuel. To study the influence of adding other substrate different combinations were performed as shown in Table 1b. The results of the carbon distribution from those experiments and HTCBiochar yield are presented in Table 2. In regard to the carbon content and the yield of the HTC-Biochars produced it is evident that the combination of SD with BM (A), SD with DS (E) and the combinations of SD and BM with SR (B, C and D) resulted in a HTC-Biochar yield of more than 65% on a dry matter basis. However, using SD, BM, DS and SR (FWC and LWC) separately resulted in HTC-Biochar yields lower than 60% (Table 2). When combining CS with SD (X1 and X2) and PM (Y1 and Y2) it seems that with increasing amounts of CS, X1 to X2 and Y1 to Y2, the HTC-Biochar yield decreases. But the opposite happens when combining CS with BM (Z1 and Z2) and FWCH (K1 and K2). Further experiments however are necessary to scrutinize the reason for the increase and decrease in the yield of HTC-Biochar when combining different types of AR. Analysis of cellulose, hemicellulose, lignin, sugars and also organic nitrogen of the Inputs should be considered in future works.
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Fig. 2. Comparison of HTC-Biochars fuel characteristics of Trial I against brown coal. (a) Carbonization diagram for Trial I. (b) Carbon content and higher heating value correlation diagram for Trial I.
3.2. Extent of carbonization and fuel characteristics 3.2.1. TRIAL I The hydrothermal carbonization process or the coalification process that takes place can be described (Fig. 2) by the van Krevelen diagram or Coalification diagram (Ramke et al., 2009). During the course of carbonization as a result of the intensification of dehydration and decarboxylation reactions there is a reduction of H and O content in the input substrate described by the molar ratios of H/C and O/C. In the carbonization diagram, those ratios move from upper right to lower left as the carbonization advance. The process goes from cellulose and wood, via the interim steps peat and lignite to the various forms of brown coal and finally to coal and anthracite (Behrendt, 2009). In addition, the intensity of
the process is shown by the length of the vector, starting at the input analysis and ending at the output analysis. It is evident by looking to the vector that all the substrates reacted intensively. Under the same HTC process conditions all the HTC-Biochars from different substrates with exception of PM nearly reached the region3 of brown coal as it is the case of HTC-Biochar from CS, CR and DR. This last one even fell in the region of lignite coal which is a type of brown coal with higher quality. In contrary, HTC-Biochar from PM and BM reached the region of bitumen-rich brown coal between brown coal and bitumen-rich brown coal respectively. Finally, it is possible to see that the HTC-Biochar from SR, SD and DS fall in the region of lignite brown coal. Analyses
3
The term region is adopted to avoid specifying molar ratios of H/C and O/C
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I. Oliveira et al. / Bioresource Technology 142 (2013) 138–146 Table 3 Fuel characteristics of the HTC-Biochars produced in Trial I and Trial II after dewatering.
Energy yielda (%)
Trial
HTC-Biochar
DM (%)
C (%)
H (%)
N (%)
O (%)
HHV (J/g DM)
Trial I
CS PM BM SD DS CR DR FWC LWC FWCH
48.7 61.4 51.1 41.5 49.0 28.0 51.7 46.0 39.9 41.0
64.4 54.4 48.0 55.7 62.7 61.1 70.1 55.2 59.0 61.3
5.4 6.2 4.3 5.1 6.0 5.2 5.0 5.2 5.3 5.4
1.6 4.4 2.5 3.2 0.9 4.9 3.1 0.8 0.6 0.3
26.9 18.4 12.8 21.3 23.7 23.2 21.2 27.3 28.7 31.8
26.600 24.300 20.900 23.500 26.300 25.300 28.300 21.500 24.300 24.600
72.7 67.6 36.6 76.1 69.5 48.0 69.8 70.6 57.7 72.7
Trial II
X1 X2 Y1 Y2 Z1 Z2 K1 K2 W1 W2 A B C D E F
53.0 50.2 52.3 63.7 47.5 44.2 30.3 36.4 54.0 44.2 41.5 39.7 46.5 46.5 47.2 51.9
57.8 61.8 55.3 62.1 60.4 61.3 62.9 64.5 57.5 57.1 52.9 57.0 53.8 55.8 54.2 62.2
5.2 5.3 6.2 6.2 5.2 5.4 5.5 5.5 5.3 5.2 5.3 5.1 5.4 5.2 5.6 4.7
3.5 2.6 4.5 4.8 2.5 2.3 0.7 1.1 4.1 4.2 2.5 1.7 1.8 1.9 2.4 3.8
20.4 22.4 21.0 18.0 17.8 21.4 29.4 27.0 17.7 22.8 25.1 22.4 27.6 27.6 27.1 19.1
25.100 26.000 24.300 27.500 25.100 26.300 25.500 26.300 24.700 24.100 22.200 23.300 22.200 22.300 22.800 27.100
73.2 69.3 67.8 64.1 57.3 67.3 82.8 89.6 67.1 57.9 92.9 82.5 84.0 85.0 102.7 69.5
LAUBAG-Briquette
NA
60.2
5.1
0.9
28.9
22.400
–
Lausitz brown coal used as a reference. a Energy yield = HHVHTC-biochar HTC-Biochar yield/HHVinput
substrate.
of the element contents and higher heating values of HTC-Biochars allow a better comparison of its fuel characteristics against brown coal. In Table 3 and Fig. 3 the significant correlation between the carbon content and the higher heating values of HTC-Biochar is evident. In addition, it can be seen with regard to C-content and calorific value that most of the HTC-Biochars, with exception of the ones produced with PM, BM, SD and FWC can be classified as being similar to brown coal. However, considering mainly the calorific value, all of the tests have led to HTC-Biochars with calorific values in the range of brown coal. 3.2.2. TRIAL II During the experiments performed in TRIAL II different substrate combinations from TRIAL I were performed with the hypothesis that higher HTC-Biochar yields and better quality, similar to brown coal, could be achieved. The carbonization diagrams for all the experiments are presented in the Fig. 3. The yield and quality of the HTC-Biochar can change drastically depending on the input substrates. Fig. 3a shows the influence of combining CS with PM and BM. It’s evident looking to the diagram that with increasing amounts of CS from 20% to 50% of DM content in the input substrate the quality of HTC-Biochar from PM and BM is improved. A similar effect is apparent when combining BM with FWC (B). Although, the combination of SD and BM (A) was not enough to reach the region of lignite brown coal falling in the region of lignin, the correspondent mass yield (77%) and energy yield (93%) for the HTC-Biochar derived from combination A is quite high. Fig. 3b exhibits the influence of combining different substrates with SD. In this case it is possible to observe the same improvement effect as when combining substrates with CS. The HTC-Biochar quality from those combinations (X1, X2) fall in the region of brown coal. When using SD alone in the HTC process the same quality is not achieved. On the contrary, when combining CR with SD (W1 and W2) a decay effect is observed. Combination W1 produced a HTC-Biochar with a lower quality than using SD or
CR separately. A strong decay effect is clear when combining DS with SD (E) where the quality of the HTC-Biochar produced did not even reach the region of lignite brown coal being only similar to lignin. However, as happened with combination A (SD with BM) the mass yield and energy yields for the HTC-Biochar derived from combination E is very high and respectively 82% and 103%. These observations could reflect the influence of combining these types of lignocellulosic rich substrates like straw and silvicultural residues (SR), which seem to increase HTC-Biochar mass yields (Reza et al., 2012). The combination between SD and DR (F) gave the best result in terms of HTC-Biochar quality. In this case, the HTC-Biochar quality fell in the region of lignite coal. The mass and energy yields achieved are however lower than the ones achieved with other combinations respectively 45% and 69%. Looking to Fig. 3c once again, the effect of adding CS is apparent. With increasing amounts of CS in the input substrate the quality of the HTC-Biochar produced is very similar to brown coal. In contrary, mixing SR with SD gives a low quality of the HTC-Biochar or almost nothing happens. A possible explanation for the supposed positive effect of CS and especially of DR could be due to their content in easily hydrolyzed carbohydrates or carbohydrates of low molecular weight in contrast with SR, SD and DS where their content in lignin is much higher (Heilmann et al., 2010). In this way, the carbonization of wet or liquid organic wastes such as poultry manure can actually take place through HTC by the addition of carbohydrates rich substrates such as corn silage or dough residues. Nevertheless, more studies are necessary where the contents of lignin, hemicellulose, cellulose and different sugars composition are analyzed. 3.3. Dewatering and drying properties of HTC-biochar The possible advantages of HTC for energetic use of AR or any type of biomass with high moisture content can only be assessed
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Fig. 3. Carbonization diagrams for Trial II. (a) Carbonization diagram of PM and BM combinations performed with corn silage, solid separated digestate and forest wood chips [Y1 = 20% CS + 80% PM; Y2 = 50% CS + 50% PM; Z1 = 20% CS + 80% BM; Z2 = 50% CS + 50% BM; A = 50% SD + 50% BM; B = 50% FWC + 50% BM]. (b) Carbonization diagram of SD combinations performed with corn silage, cabbage residues, dry straw and dough residues [X1 = 20% CS + 80% SD; X2 = 50% CS + 50% SD; W1 = 20% CR + 80% SD; W2 = 50% CR + 50% SD; F = 70% SD + 30% DS; E = 70% SD + 30% DR]. (c) Carbonization diagram of SR combinations performed with corn silage and solid separated digestate [K1 = 20% CS + 80% FWCH; K2 = 50% CS + 50% FWCH; C = 50% SD + 50% FWC; D = 50% SD + 50% LWC].
I. Oliveira et al. / Bioresource Technology 142 (2013) 138–146
if the whole process chain including conditioning and utilization is considered. It is therefore necessary to consider the dewatering and drying properties of the HTC-Biochar produced. In this work test equipment which the method was previously developed in the Waste Management Laboratory of University of Applied Sciences Ostwestfalen-Lippe (Ramke et al., 2009). The dewatering tests gives absolute data which can be compared to real dewatering plants (i.e. chamber filter press) while the drying test gives a relative measure of dry matter content after 120 min of drying under defined conditions. The average resulting curves of the dewatering and drying tests for different samples can be drawn. It is known that the dewatering properties of substrates with high moisture content are improved after carbonization (Acharjee et al., 2011; Libra et al., 2011; Ramke et al., 2009), and it is evident when comparing the resulting curves of the dewatering and drying tests from the input substrates and resulting HTC-Biochar produced. The average relative increase of DM content of the HTC-Biochar produced in all the experiments after dewatering and drying tests are between 60% and 80% (% DM). However, depending on the input substrate the increase in DM of the HTCBiochar after the dewatering and drying step could be completely different as it is the case of the HTC-Biochars produced from CR and DR where a relative increase of 47% and 98% of DM was achieved respectively. Another interesting result is the improvement of the dewatering and drying properties reached when combining SD with CS or DR. The achieved relative increase in dry matter of the HTC-Biochar after dewatering and drying is nearly 80% for those combinations, whilst the input material or the HTC-Biochar of SD only reached a dry matter of 50% and 64% respectively. It is therefore interesting to note that the combination of substrates in HTC improves not only the fuel characteristics in comparison with brown coal but also the dewatering and drying properties of the HTC-Biochar to be used as solid fuel for industrial purposes. 3.4. Biogas potential of the process water Analysis showed that the majority of the experiments resulted in a process water predominantly acidic (pH 3–5) with a high TOC (13–26 g/l). The resulting process water could either be used to heat up the reactor or introduced in the process for reaction optimization (Libra et al., 2011). However, sooner or later it will be necessary to find ways to dispose off the process water. With the idea of combining two technologies, HTC and anaerobic digestion, a set of anaerobic batch tests were performed to determine the resulting biogas and methane potential of the HTC process water. The resulting cumulative biogas and CH4 yield had a minimum of 9.6 L biogas/kg FM and 6 L of CH4/kg FM and a maximum of 21.1 L of biogas/kg FM and 16.3 L of CH4/kg FM for C and X2 respectively. It is possible to refer a tendency where the cumulative methane yield is higher when TOC is higher and vice versa. Further studies should assess the use of process water when it is reused for HTC process optimization. In this case, higher cumulative CH4 yields could be achieved due to higher TOC values of the process water. It would also be interesting to study the co-digestion of the process water with different biomass and organic residues. Notwithstanding this, the results show that anaerobic digestion is a suitable method to treat the process water. 4. Conclusions HTC of agricultural residues produced a solid fuel (HTC-Biochar) comparable to brown coal, which could be used to complement farm-based biogas plants. Combinations of different agricultural residues improved carbonization. Most of the HTC-Biochars
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produced presented good dewatering and drying properties. The process water could be used in biogas plants, reducing its environmental impact, which could increase the energy efficiency of the whole process. More studies are necessary to optimize HTC-Biochar yields and quality, and the feasibility of a continuous process. Different uses for the HTC-Biochar produced should be also studied for example as soil amendment or biogas scrubbing material.
References Acharjee, T.C., Coronella, C.J., Vasquez, V.R., 2011. Effect of thermal pretreatment on equilibrium moisture content of lignocellulosic biomass. Bioresour. Technol. 102, 4849–4854. Akhtar, J., Amin, N.A.S., 2011. A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass. Renew. Sustain. Energy Rev. 15, 1615–1624. Alvarez, J., Mace, S., Llabres, P., 2000. Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresour. Technol. 74, 3– 16. Behrendt F., 2009. Direktverflüssigung von Biomasse – Reaktionsmechanismen und Produktverteilungen, Insitut für Energietechnik, Technische Universität Berlin Studie im Auftrag der Bundesanstalt für Landwirtschaft und Ernährung. Projektnummer 114-50-10-0337/05-B. Chynoweth, D.P., Issacson, R., 1987. Anaerobic digestion of biomass. Elsevier Applied Science, London, ISBN 1-85166-069-0, p. 279. Cui, X., Antonietti, M., Yo, S.-H., 2006. Structural effects of iron oxide nanoparticles and iron ions on the hydrothermal carbonization of starch and rice carbohydrates. Communication in Wiley-VCH Cerlag GmbH & Co. KGaA 2006, vol. 6, pp. 756–759. DIN 38414, part 8: German standard methods for the examination of water, waste water and sludge; sludge and sediments (group S): determination of the amenability to anaerobic digestion; 1985. DIN 51900-3. ICS 75.160.10; 75.160.20. Testing of solid and liquid fuels: determination of gross calorific value by the bomb calorimeter and calculation of net calorific value. Part 3: method using adiabatic jacket. January 2005. DIN EN 12879. ICS 13.030.20. Bestimmung des Glüverlustes der Trockenmasse. February 2001. DIN EN 12880. ICS 13.030.20. Bestimmung des Trockenrückstandes und des Wassergehalts. February 2001. Du, Z., Hu, B., Shi, A., Ma, X., Cheng, Y., Chen, P., Liu, Y., Lin, X., Ruan, R., 2012. Cultivation of a microalga Chlorella vulgaris using recycled aqueous phase nutrients from hydrothermal carbonization process. Bioresour. Technol. 126, 354–357. Euractiv, 2011:. Funke, A., Ziegler, F., 2010. Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering. Biofuels, Bioprod. Biorefin. 4, 4160–4177. Heilmann, S.M., Jader, L.R., Harned, L.A., Sadowsky, M.J., Schendel, F.J., Lefebvre, P.A., et al., 2011a. Hydrothermal carbonization of microalgae II: fatty acid, char and algal nutrient products. Appl. Energy. http://dx.doi.org/10.1016/ j.apenergy.2010.12.041. Heilmann, S.M., Davis, H.T., Jader, L.R., Lefebvre, P.A., Sadowsky, M.J., Schendel, F.J., Keitz, M.G.V., Valentas, K.J., 2011b. Hydrothermal carbonization of distiller’s grains. Biomass Bioenergy 35, 2526–2533. Heilmann, S.M., Davis, H.T., Jader, L.R., Lefebvre, P.A., Sadowsky, M.J., Schendel, F.J., Keitz, M.G.V., Valentas, K.J., 2010. Hydrothermal carbonization of microalgae. Biomass Bioenergy 34, 875–882. Hoekman, S.K., Broch, A., Robbins, C., 2011. Hydrothermal carbonization (HTC) of lignocellulosic biomass. Energy Fuels 25, 1802–1810. http://dx.doi.org/10.1021/ ef101745n. Hendriks, A., Zeeman, G., 2010. Review: pretreatments to enhance the digestibility of lignocellulosic biomass. Bioresour. Technol. 100, 10–18. Inagaki, M., Park, K.C., Endo, M., 2010. Carbonization under pressure. New Carbon Mater. 25 (6), 409–420. Kleinert, M., Wittmann, T. Carbonisation of biomass using a hydrothermal approach: state-of-the-art and recent developments. 17th European Biomass Conference, Hamburg, 2009, June 29–July 3. Kumar, S., Loganathan, V., Gupta, R., Barnett, M., 2011. An assessment of U (VI) removal from groundwater using biochar produced from hydrothermal carbonization. J. Environ. Manage. 92, 2504–2512. Libra, J.A., Ro, K.S., Kammann, C., Funke, A., Berge, N.D., Neubauer, Y., et al., 2011. Hydrothermal carbonization of biomass residuals: a comparative review of the chemistry, processes and applications of wet and dry pyrolysis. Biofuels 2 (1), 71–106. Lindorfer, H., López, C.P., Resch, C., Braun, R., Kirchmayr, R., 2007. The impact of increasing energy crop addition on process performance and residual methane potential in anaerobic digestion. Water Sci. Technol. 56 (10), 55–63. Mumme, J., Eckervogt, L., Pielert, J., Diakite, M., Rupp, F., Kern, J., 2011. Hydrothermal carbonization of anaerobically digested maize silage. Bioresour. Technol. 102, 9255–9260.
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I. Oliveira et al. / Bioresource Technology 142 (2013) 138–146
Parshetti, G.K., Hoekman, S.K., Balasubramanian, R., 2013. Chemical, structural and combustion characteristics of carbonaceous products obtained by hydrothermal carbonization of palm empty fruit bunches. Bioresour. Technol. 135, 683–689. Prochnow, A., Heiermann, M., Plochl, M., Linke, B., Idler, C., Amon, T., et al., 2009. Bioenergy from permanent grassland – a review: 1. Biogas Bioresour. Technol. 100, 4931–4944. Ramke, H.-G., Blöhse, D., Lehmann, H.-J., Fettig, J. 2009, Hydrothermal carbonization of organic waste, in : Cossu, R., Diaz, L.F., Stegmann, R. (Eds.). Sardinia 2009: Twelfth International Waste Management and Landfill Symposium Proceedings, CISA Publisher (ISBN 978-88-6265-007-6). Regmi, P., Moscoso, J.L., Kumr, S., Cao, X., Mao, J., Schafran, G., 2012. Removal of copper and cadmium from aqueous solution using switchgrass biochar produced via hydrothermal carbonization process. J. Environ. Manage. 109, 61–69. Reza, M.T., Lynam, J.G., Vasquez, V.R., Coronella, C.J., 2012. Pelletization of biochar from hydrothermally carbonized wood. Environ. Prog. Sustain. Energy 31, 225– 234. http://dx.doi.org/10.1002/ep.11615.
Rillig, M.C., Wagner, M., Salem, M., Antunes, P.M., George, C., Ramke, H.-G., et al., 2010. Material derived from hydrothermal carbonization: effect on plant growth and Arbuscular mycorrhiza. Agric. Ecosyst. Environ. Appl. Soil Ecol. 45, 238–242. Spiegel International online, 2011.. Titirici, M.-M., Thomas, A., Antonietti, M., 2007. Back in the black: hydrothermal carbonization of plant material as an efficient chemical process to treat the CO2 problem? New J. Chem. 31, 787–789. VDI 4630. Stand 2007–03. ICS 13.030.30; 27.190, 2006. Fermentation of organic materials, characterisation of the substrate, sampling, collection of material data, fermentation tests Beuth-Verlag, Berlin. Ward, A., Hobbs, P., Holliman, Hobbs., Holliman, P., Jones, D., 2008. Optimization of the anaerobic digestion of agricultural resources. Bioresour. Technol. 99, 7928– 7940. Xiao, L.-P., Shi, Z.-J., Xy, F., Sun, R.-C., 2012. Hydrothermal carbonization of lignocellusic biomass. Bioresour. Technol. 118, 619–623.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Hydrothermal carbonization of agricultural residues Ivo Oliveira ⇑, Dennis Blöhse, Hans-Günter Ramke University of Applied Sciences – Hochschule Ostwestfalen-Lippe, Campus Höxter, Faculty of Environmental Engineering and Applied Informatics, Professorship of Waste Management and Landfill Technology, An der Wilhelmshöhe 44, 37671 Höxter, Germany
h i g h l i g h t s HTC converts organic residues into a solid fuel comparable to brown coal. Most of the tested agricultural residues are suitable for the HTC process. Blending of different agricultural residues ensures a successful carbonization. Dewatering and drying properties of most HTC-Biochars are very good. HTC-process water can be used in biogas plants increasing process energy efficiency.
a r t i c l e
i n f o
Article history: Received 12 January 2013 Received in revised form 29 April 2013 Accepted 30 April 2013 Available online 10 May 2013 Keywords: Hydrothermal carbonization Agricultural residues HTC-Biochar Process water Biogas potential
a b s t r a c t The work presented in this article addresses the application of hydrothermal carbonization (HTC) to produce a solid fuel named HTC-Biochar, whose characteristics are comparable to brown coal. Several batch HTC experiments were performed using agricultural residues (AR) as substrates, commonly treated in farm-based biogas plants in Germany. Different AR were used in different combinations with other biomass residues. The biogas potential from the resulting process water was also determined. The combination of different AR lead to the production of different qualities of HTC-Biochars as well as different mass and energy yields. Using more lignocellulosic residues lead to higher mass and energy yields for the HTC-Biochar produced. Whilst residues rich in carbohydrates of lower molecular weight such as corn silage and dough residues lead to the production of a HTC-Biochar of better quality and more similar to brown coal. Process water achieved a maximum of 16.3 L CH4/kg FM (fresh matter). Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Over the years, fossil fuels have been the cheapest source for the petroleum and petrochemical industries, which led to the development of millions of petroleum based products. However, fuel prices demand have reached record heights in recent years triggered by factors like the depletion of easily accessible deposits and also increasing demand by emerging economies. Biomass was once the global energy source before the arrival and the unbridled expansion of fossil fuels during the Industrial Revolution in the 1800–1900s. Germany’s decision to shut down all the nuclear reactors by 2022 increases the country’s demand for other sorts of energy like: coal fired plants, natural gas imports and renewable energy production (Spiegel, 2011; Euractiv, 2011). Therefore, the adoption
⇑ Corresponding author. Present address: University of Southwales, SERC Laboratories, Upper Glyntaff CF37 4BD, United Kingdom. Tel.: +44 (0) 1443482163. E-mail addresses: [email protected], [email protected] (I. Oliveira), [email protected] (D. Blöhse), [email protected] (H.-G. Ramke). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.04.125
of decentralized systems like many small interconnected units harvesting wind, solar, hydroelectric and biomass power is expected in the near future. Hydrothermal Carbonization (HTC) was discovered by Bergius in 1913, and has been re-discovered and further developed under the direction of Professor Antonietti, director of the Department of Colloid Chemistry at the Max-Planck Institute of Colloids and Interfaces in Golm/Potsdam (MPI). HTC is now being mentioned as a promising technology to convert biomass into multiple bioproducts: a solid fuel compared to brown coal (Parshetti et al., 2013; Funke and Ziegler, 2010; Kleinert et al., 2009; Ramke et al., 2009; Titirici et al., 2007); liquid fuel or bio-oil (Akhtar and Amin, 2011; Heilmann et al., 2010, 2011a,b; Hoekman et al., 2011; Titirici et al., 2007; Xiao et al., 2012) as a soil amendment to increase soil fertility and crop yields (Du et al., 2012; Kleinert et al., 2009; Libra et al., 2011; Rillig et al., 2010); carbon material that could be either activated to work as an adsorbent for water purification systems or CO2 sorption (Libra et al., 2011) and as a low cost adsorbent or permeable reactive barrier for Uranium(VI), Copper and cadmiumcontaminated waters (Kumar et al., 2011; Regmi et al., 2012); nanostructure carbon material (Cui et al., 2006; Inagaki et al.,
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2010; Libra et al., 2011); carbon catalyst, which could be used in the production of fine chemicals (Xiao et al., 2012); and lastly a carbon material that could increase a fuel cells efficiency (Libra et al., 2011). The objective of this research work was to give more data about the use of HTC in the production of a solid fuel similar to brown coal (HTC-Biochar) using different agricultural residues. Agricultural residues1 (AR) are a type of biomass that could present considerable heterogeneity depending on the country and region. We considered AR as energy crops (corn and grass silage), livestock manures from poultry, cattle, and piggery farms, bedding material used in barn stalls and digestate (or Biogas Slurry) from biogas plants all present in the region of Höxter, Germany. In Germany this type of AR are being treated through farmbased biogas plants which produce a renewable fuel (biogas) used to generate heat and power, and a digestate (solid and liquid) used as fertilizer. However, several problems have been reported related to the use of energy crops. Due to its lignocellulosic structure these substrates are not fully hydrolyzed. Hydrolysis of lignocellulosic material has been referred to for years as the rate limiting step in anaerobic digestion of lignocellulosic biomass (Alvarez et al., 2000; Chynoweth and Isaacson, 1987; Hendriks and Zeeman, 2010; Lindorfer et al., 2007; Prochnow et al., 2009; Ward et al., 2008). As a result, the material tends to float upon the fluid surface in the digester, leading to increased stirring expenses. For instance wrapping of longer grass particles around moving devices can cause failures in operating the biogas plant (Prochnow et al., 2009). Therefore, long retention times (30–80 days) are utilised increasing the operating volumes and digestate storage costs (Lindorfer et al., 2007; Prochnow et al., 2009). However, these technical problems arising from energy crops utilization for anaerobic digestion have not been the subject of systematic scientific investigation thus far. HTC is an exothermic process that lowers both the oxygen and hydrogen content of the feed (described by the molecular O/C and H/C ratio) by 5 main reaction mechanisms which include hydrolysis, dehydration, decarboxylation, polymerization and aromatization (Funke and Ziegler, 2010; Hoekman et al., 2011). This is achieved by applying temperatures of 180–220 °C to a suspension of biomass and water at saturated pressure for a couple hours. At the end of the process the solid phase, denominated as ‘‘HTC-Biochar’’, can be easily separated from the water. Approximately 75– 80% of the carbon input is found in the solid phase (HTC-Biochar); about 15–20% is dissolved in the liquid phase (process water); and the remaining 5% are converted to gas (mainly carbon dioxide). The liquid phase is highly loaded with organic components, which for example could be easily degradable through anaerobic digestion (Ramke et al., 2009). Carbonization of Biomass has a number of advantages when compared with common biological treatment. It generally takes only hours, instead of the days or months required for biological processes, permitting more compact reactor design. When compared to fermentation and anaerobic digestion, HTC is referred to as the most exothermic and efficient process for carbon fixation (Titirici et al., 2007). Therefore, HTC is now seen as a promising technology also for CO2 sequestration. In addition, some feedstocks are toxic and cannot be converted biochemically. The high process temperatures can destroy pathogens and potential organic contaminants, such as pharmaceutically active compounds that could be present for example in AR (Libra et al., 2011). It is therefore important to study the quality2 of the ‘‘HTC-Biochar’’ produced
1 The term AR was adopted to combine different streams of Biomass in the Agricultural sector. 2 Quality of HTC-Biochar is used in the manuscript to avoid describing elemental composition (C/H/N/O) or molar ratios as well Higher Heating Value. The quality of HTC-Biochar is then compared to Lausitz brown coal.
Biogas Plant Digestate Agricultural Residues
Separated Digestate
HTC reactor
Process Water
Liquid Digestate
Fertilizer
HTC-Biochar
Fig. 1. Presumed combination of anaerobic digestion and hydro-thermal carbonization of agricultural residues.
through carbonization of AR, envisioning its application as a solid fuel. Recently, a first study has been reported using digestate produced from maize silage previously treated at 55 °C in a two-stage solid-state reactor system (Mumme et al., 2011), however with different experimental conditions from the work presented here. The present work was performed considering a hypothetical situation (Fig. 1) where an HTC reactor could be used to improve the efficiency of an existing farm-based biogas plant. In this case, AR and separated digestate are used as input material for the HTC reactor to produce a high quality solid fuel (HTC-Biochar) compared to brown coal, while process water (liquid phase) is treated by anaerobic digestion. This preliminary work has the objective to present the first results about the HTC of AR and the quality of the HTC-Biochar produced (Trial I). Additionally, carbonization tests with different combinations of AR and some industrial organic wastes and silvicultural residues were also performed (Trial II). The results of biogas and biomethane potential determination of the liquid phase are also given.
2. Methods 2.1. Substrates Several biomass substrates were used in this work: Agricultural Residues, Industrial Organic Waste (IOW) and Silvicultural Residues (SR). The AR used in this work were collected from a farmbased biogas plant located in the town of Marienmünster in Höxter district. The AR used were: corn silage (CS), poultry manure (PM), separated (solid) digestate (SD), bedding material (BM; a heterogeneous mixture mainly composed by hay, straw and manure), and also dry straw (DS). The IOW used was collected from different local small industries. Those substrates were: cabbage (CR) and dough (DR). The SR were collected from the main biomass supply platform in the region of Höxter located in the city of Borlinghausen. The following SR were chosen: forest (FWC) and landscape (LWC) wood chips with low market value, and forest wood chips with high market value (FWCH). Every substrate was representatively sampled and transported in closed barrels of 200 L. Table 1 shows the characterization of each substrate used. AR and IOW were only stored for a short period of time before being used in the HTC experiment due to their susceptibility to degradation. To avoid any losses of material (organic and water) the IOW were only stored for 3 days while AR such as CS and SD were stored for a period no longer than two weeks.
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Table 1 Characterization and Identification of the substrates used in Trials I and II. Substrates
Aba
(a) Characterization of the substrates used Corn silage CS Poultry manure PM Bedding material BM Separated digestate SD Dry straw DS Cabbage residues CR Dough residues DR Forest wood chips FWC Landscape wood chips LWC Forest wood chips (high quality) FWCH IDc
DM (%)
oDM (%)
C (%)
H (%)
N (%)
O (%)
HHVb (J/g DM)
26.9 53.2 26.0 26.9 90.8 10.8 51.2 56.9 38.8 71.4
96.2 84.4 78.2 88.7 95.3 90.0 97.1 90.1 95.2 98.6
46.9 46.8 40.7 45.1 46.8 45.8 44.1 46.2 49.6 49.3
6.2 6.3 5.1 5.5 6.0 5.4 6.5 5.6 5.8 6.2
1.2 1.3 1.9 1.4 0.4 3.7 2.1 0.7 0.5 0.4
41.9 30.1 30.5 36.7 42 35.2 44.4 37.6 39.4 42.6
19.600 18.800 16.900 18.400 18.700 18.000 17.800 18.400 19.400 19.300
Amount of each substrate in the autoclave (% of relative DM)
(b) Description of the HTC experiments combining two different substrates for Trial II Substrate 1 Substrate 2 X1 20 80 X2 50 50 Y1 20 80 Y2 50 50 Z1 20 80 Z2 50 50 K1 20 80 K2 50 50 W1 20 80 W2 50 50 A 50 50 B 50 50 C 50 50 D 50 50 E 70 30 F 62 38 a b c
Substrate combinations
Substrate 1 CS
Substrate 2 SD
CS
PM
CS
BM
CS
FWCH
CR
SD
SD FWC SD SD SD SD
BM BM FWC LWC DS DR
Abbreviations for each substrate. Higher heating value. Identification of each substrate combination.
2.2. Hydrothermal carbonization experiment procedure The hydrothermal carbonization experiments were carried out in a custom-built 25 L autoclave located in the Laboratory of Waste Management and Landfill Technology (Ramke et al., 2009). The autoclave is equipped with external and internal thermometers, digital and analog manometer, cooling system and a computer supported programmable logic controller (PLC). For every HTC batch experiment an operation volume of 20 L, a dry matter content of 15%, and the same HTC operational procedure were adopted. The HTC operation procedure was divided into three phases: heating phase, reaction phase and cooling phase. The experiment started with the heating phase where the autoclave was heated at maximum power until the suspension reached a temperature of 220 °C. After that only 10% of heat power is provided to allow enough time and temperature (4 h > 180 °C) for the reaction phase. After the substrate had been kept at 180 °C (reaction phase) for 4 h the heaters were turned off and the cooling phase was started. The process parameters: temperature, pressure and energy consumption were continuously recorded. The experiment ended once ambient temperatures (<25 °C) were reached inside the autoclave. The gas produced during the experiment was collected in a 250 L bag and the autoclave was opened. The resulting wet product was transferred and stored in closed plastic buckets until separation. The HTC experiments were performed with all the substrates mentioned on Table 1 (Trial I). Different substrates combinations were also performed as shown in Table 1b (Trial II). 2.3. Biogas potential test 2.3.1. Method and test apparatus Specific biogas yield was determined using the biogas potential test specified in a German Norm for fermentation of organic
materials (VDI 4630, 2006). Test apparatus used is specified by a DIN norm (DIN 38414, 1985). Two batch tests were performed and in each test a total number of 29 vessels were used. Two working volumes were used for every batch test: 1000 ml and 3000 ml. The studied substrate samples and reference (microcrystalline cellulose) were conducted in duplicate and for the inocula a triplicate was used. Tests were undertaken at mesophilic temperatures (38 °C ± 2 °C) controlled inside a climatic room. The volume of biogas produced, temperature, and atmospheric pressure were recorded daily. Mixing was carried out manually every day by shaking each vessel for a couple of seconds. The test ended when the daily biogas rate was equivalent to only 1% of the total biogas volume produced until that time. The biogas quality of each vessel was assessed after 15 days of retention time. 2.3.2. Substrate and inocula The Inocula was collected and transported in a closed barrel of 200 L from a waste water treatment plant in Beverungen, Germany. Inocula was stored using the procedure described in (DIN 38414-8). The substrates used in the biogas potential test were obtained after separation of the whole wet HTC product. The process water was stored in plastic bottles of 1000 ml. The process water was characterized in terms of dry matter content, TOC, pH and conductivity. 2.4. Analytical methods All the HTC product phases (HTC-Biochar, process water, and gas) were analyzed. HTC-Biochar and process water were obtained after separation and stored in separated plastic sealed buckets. All the substrates used in the HTC and HTC-Biochar were characterized in terms of dry matter content (DM), organic dry matter
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I. Oliveira et al. / Bioresource Technology 142 (2013) 138–146 Table 2 Distribution of the carbon fraction in each HTC product phase and the carbon yield in HTC-Biochar for Trial I and Trial II. Trial
Substrate
C – solid phase (% DM)
C – liquid phase (% DM)
C – gas phase (% DM)
HTC-biochar yielda (% DM)
Trial I
CS PM BM SD DS CR DR FWC LWC FWCH X1 X2 Y1 Y2 Z1 Z2 K1 K2 W1 W2 A B C D E F
70.1 60.9 51.7 74.0 72.3 51.9 74.3 79.1 79.5 80.9 72.6 70.2 59.7 58.5 63.8 68.5 78.4 75.2 59.9 56.6 81.4 75.1 79.7 82.5 76.6 63.6
24.5 32.4 34.1 17.4 20.4 38.8 20.8 16.6 16.1 15.7 24.7 22.1 34.0 32.9 26.4 24.2 17.1 19.7 29.8 34.9 13.3 18.2 15.8 12.7 17.4 26.0
5.6 6.7 10.1 8.3 6.7 8.1 4.6 3.9 3.1 3.3 9.8 7.4 6.5 8.6 8.8 7.0 5.0 5.8 10.7 8.3 5.9 7.3 4.5 5.0 7.7 10.5
52.5 52.5 30.9 58.5 49.5 34.1 43.9 60.5 46.0 64.4 54.0 50.8 52.7 44.0 42.6 48.8 62.2 65.0 48.4 43.0 77.3 65.1 69.7 72.3 81.7 45.6
Trial II
a
HTC-Biochar yield is calculated after dewatering step.
content (oDM), higher heating value (HHV) and coke content using DIN norms (DIN EN 12880; DIN EN 12879; DIN 51900-3). For Higher Heating Value determination a IKA-Calorimeter C4000 Adiabatic was used. Elementary analysis was determined using Vario Macro Elementar Analyzer. All the analyses were done in duplicate with exception of elementar analysis which was done in triplicate. Dewatering and drying properties of the HTC-Biochar and substrates were accessed using a method developed by the Waste Management Laboratory in the University of Applied Sciences of Ostwestfalen-Lippe. Special test equipment has been developed for this purpose (Ramke et al., 2009) by using a press, normally used for testing building material, and a cylinder with a standard volume capacity inside where the sample is placed. The samples are put under constant pressure of 15 bar for 30 min and the discharged water is passed over a filter plate inside the cylinder and caught in electronic scales. The scale is connected to a notebook computer and the weight of the discharged water is continuously recorded. In the drying test the pressed samples obtained from dewatering are placed in an enclosed cell connected to a drying machine which provides a continuously air flow with a temperature of 80 °C. The test lasts 120 min and every 30 min the weight of the sample is recorded. The dewatering and drying tests were done in duplicate. Process water was analyzed in terms of dry matter content, TOC, pH and conductivity. Gas samples were analyzed in terms of CH4, CO2 and CO content using a GC TCD Argilent 6850 with a fused silica capillary column carboxen-1010. Temperatures of oven, inlet and detector were respectively 50 °C, 200 °C, and 230 °C. Hydrogen was used as a carrier gas using a column flow of 3 ml/min.
3. Results and discussion 3.1. Mass and carbon balances The total mass balances between in- and output material for all the experiments gave a minimum value of 94% and a maximum of
98% recovery. The resulting losses can be plausibly explained as wetting, evaporation and due to spill- or droplet losses when emptying the autoclave and during separation of the output material. In analogy to this, the recovery-value of organic carbon is close to 100%. However, its distribution in the solid (HTC-Biochar), liquid (process water) and gas phases is dependent on the type of substrate used as shown in the Table 2. It is possible to observe a tendency where the majority of the carbon is present in the solid phase (52–80%) and the remaining organic carbon is transferred to the liquid phase (16–39%) and gas phase (3–10%). The substrates BM and CR present very low carbon in the solid phase and the remaining substrates reached higher carbon percentage in the solid phase in particular SR. The HTC-Biochar yield had a minimum of 31% and maximum of 64% (% DM) for BM and FWCH respectively. The HTC-Biochar yield is calculated on a dry matter basis considering a dewatering step after HTC. This consideration results in lower yields however it is necessary to take into account this step for the application of the HTC-Biochar as a solid fuel. To study the influence of adding other substrate different combinations were performed as shown in Table 1b. The results of the carbon distribution from those experiments and HTCBiochar yield are presented in Table 2. In regard to the carbon content and the yield of the HTC-Biochars produced it is evident that the combination of SD with BM (A), SD with DS (E) and the combinations of SD and BM with SR (B, C and D) resulted in a HTC-Biochar yield of more than 65% on a dry matter basis. However, using SD, BM, DS and SR (FWC and LWC) separately resulted in HTC-Biochar yields lower than 60% (Table 2). When combining CS with SD (X1 and X2) and PM (Y1 and Y2) it seems that with increasing amounts of CS, X1 to X2 and Y1 to Y2, the HTC-Biochar yield decreases. But the opposite happens when combining CS with BM (Z1 and Z2) and FWCH (K1 and K2). Further experiments however are necessary to scrutinize the reason for the increase and decrease in the yield of HTC-Biochar when combining different types of AR. Analysis of cellulose, hemicellulose, lignin, sugars and also organic nitrogen of the Inputs should be considered in future works.
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Fig. 2. Comparison of HTC-Biochars fuel characteristics of Trial I against brown coal. (a) Carbonization diagram for Trial I. (b) Carbon content and higher heating value correlation diagram for Trial I.
3.2. Extent of carbonization and fuel characteristics 3.2.1. TRIAL I The hydrothermal carbonization process or the coalification process that takes place can be described (Fig. 2) by the van Krevelen diagram or Coalification diagram (Ramke et al., 2009). During the course of carbonization as a result of the intensification of dehydration and decarboxylation reactions there is a reduction of H and O content in the input substrate described by the molar ratios of H/C and O/C. In the carbonization diagram, those ratios move from upper right to lower left as the carbonization advance. The process goes from cellulose and wood, via the interim steps peat and lignite to the various forms of brown coal and finally to coal and anthracite (Behrendt, 2009). In addition, the intensity of
the process is shown by the length of the vector, starting at the input analysis and ending at the output analysis. It is evident by looking to the vector that all the substrates reacted intensively. Under the same HTC process conditions all the HTC-Biochars from different substrates with exception of PM nearly reached the region3 of brown coal as it is the case of HTC-Biochar from CS, CR and DR. This last one even fell in the region of lignite coal which is a type of brown coal with higher quality. In contrary, HTC-Biochar from PM and BM reached the region of bitumen-rich brown coal between brown coal and bitumen-rich brown coal respectively. Finally, it is possible to see that the HTC-Biochar from SR, SD and DS fall in the region of lignite brown coal. Analyses
3
The term region is adopted to avoid specifying molar ratios of H/C and O/C
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I. Oliveira et al. / Bioresource Technology 142 (2013) 138–146 Table 3 Fuel characteristics of the HTC-Biochars produced in Trial I and Trial II after dewatering.
Energy yielda (%)
Trial
HTC-Biochar
DM (%)
C (%)
H (%)
N (%)
O (%)
HHV (J/g DM)
Trial I
CS PM BM SD DS CR DR FWC LWC FWCH
48.7 61.4 51.1 41.5 49.0 28.0 51.7 46.0 39.9 41.0
64.4 54.4 48.0 55.7 62.7 61.1 70.1 55.2 59.0 61.3
5.4 6.2 4.3 5.1 6.0 5.2 5.0 5.2 5.3 5.4
1.6 4.4 2.5 3.2 0.9 4.9 3.1 0.8 0.6 0.3
26.9 18.4 12.8 21.3 23.7 23.2 21.2 27.3 28.7 31.8
26.600 24.300 20.900 23.500 26.300 25.300 28.300 21.500 24.300 24.600
72.7 67.6 36.6 76.1 69.5 48.0 69.8 70.6 57.7 72.7
Trial II
X1 X2 Y1 Y2 Z1 Z2 K1 K2 W1 W2 A B C D E F
53.0 50.2 52.3 63.7 47.5 44.2 30.3 36.4 54.0 44.2 41.5 39.7 46.5 46.5 47.2 51.9
57.8 61.8 55.3 62.1 60.4 61.3 62.9 64.5 57.5 57.1 52.9 57.0 53.8 55.8 54.2 62.2
5.2 5.3 6.2 6.2 5.2 5.4 5.5 5.5 5.3 5.2 5.3 5.1 5.4 5.2 5.6 4.7
3.5 2.6 4.5 4.8 2.5 2.3 0.7 1.1 4.1 4.2 2.5 1.7 1.8 1.9 2.4 3.8
20.4 22.4 21.0 18.0 17.8 21.4 29.4 27.0 17.7 22.8 25.1 22.4 27.6 27.6 27.1 19.1
25.100 26.000 24.300 27.500 25.100 26.300 25.500 26.300 24.700 24.100 22.200 23.300 22.200 22.300 22.800 27.100
73.2 69.3 67.8 64.1 57.3 67.3 82.8 89.6 67.1 57.9 92.9 82.5 84.0 85.0 102.7 69.5
LAUBAG-Briquette
NA
60.2
5.1
0.9
28.9
22.400
–
Lausitz brown coal used as a reference. a Energy yield = HHVHTC-biochar HTC-Biochar yield/HHVinput
substrate.
of the element contents and higher heating values of HTC-Biochars allow a better comparison of its fuel characteristics against brown coal. In Table 3 and Fig. 3 the significant correlation between the carbon content and the higher heating values of HTC-Biochar is evident. In addition, it can be seen with regard to C-content and calorific value that most of the HTC-Biochars, with exception of the ones produced with PM, BM, SD and FWC can be classified as being similar to brown coal. However, considering mainly the calorific value, all of the tests have led to HTC-Biochars with calorific values in the range of brown coal. 3.2.2. TRIAL II During the experiments performed in TRIAL II different substrate combinations from TRIAL I were performed with the hypothesis that higher HTC-Biochar yields and better quality, similar to brown coal, could be achieved. The carbonization diagrams for all the experiments are presented in the Fig. 3. The yield and quality of the HTC-Biochar can change drastically depending on the input substrates. Fig. 3a shows the influence of combining CS with PM and BM. It’s evident looking to the diagram that with increasing amounts of CS from 20% to 50% of DM content in the input substrate the quality of HTC-Biochar from PM and BM is improved. A similar effect is apparent when combining BM with FWC (B). Although, the combination of SD and BM (A) was not enough to reach the region of lignite brown coal falling in the region of lignin, the correspondent mass yield (77%) and energy yield (93%) for the HTC-Biochar derived from combination A is quite high. Fig. 3b exhibits the influence of combining different substrates with SD. In this case it is possible to observe the same improvement effect as when combining substrates with CS. The HTC-Biochar quality from those combinations (X1, X2) fall in the region of brown coal. When using SD alone in the HTC process the same quality is not achieved. On the contrary, when combining CR with SD (W1 and W2) a decay effect is observed. Combination W1 produced a HTC-Biochar with a lower quality than using SD or
CR separately. A strong decay effect is clear when combining DS with SD (E) where the quality of the HTC-Biochar produced did not even reach the region of lignite brown coal being only similar to lignin. However, as happened with combination A (SD with BM) the mass yield and energy yields for the HTC-Biochar derived from combination E is very high and respectively 82% and 103%. These observations could reflect the influence of combining these types of lignocellulosic rich substrates like straw and silvicultural residues (SR), which seem to increase HTC-Biochar mass yields (Reza et al., 2012). The combination between SD and DR (F) gave the best result in terms of HTC-Biochar quality. In this case, the HTC-Biochar quality fell in the region of lignite coal. The mass and energy yields achieved are however lower than the ones achieved with other combinations respectively 45% and 69%. Looking to Fig. 3c once again, the effect of adding CS is apparent. With increasing amounts of CS in the input substrate the quality of the HTC-Biochar produced is very similar to brown coal. In contrary, mixing SR with SD gives a low quality of the HTC-Biochar or almost nothing happens. A possible explanation for the supposed positive effect of CS and especially of DR could be due to their content in easily hydrolyzed carbohydrates or carbohydrates of low molecular weight in contrast with SR, SD and DS where their content in lignin is much higher (Heilmann et al., 2010). In this way, the carbonization of wet or liquid organic wastes such as poultry manure can actually take place through HTC by the addition of carbohydrates rich substrates such as corn silage or dough residues. Nevertheless, more studies are necessary where the contents of lignin, hemicellulose, cellulose and different sugars composition are analyzed. 3.3. Dewatering and drying properties of HTC-biochar The possible advantages of HTC for energetic use of AR or any type of biomass with high moisture content can only be assessed
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Fig. 3. Carbonization diagrams for Trial II. (a) Carbonization diagram of PM and BM combinations performed with corn silage, solid separated digestate and forest wood chips [Y1 = 20% CS + 80% PM; Y2 = 50% CS + 50% PM; Z1 = 20% CS + 80% BM; Z2 = 50% CS + 50% BM; A = 50% SD + 50% BM; B = 50% FWC + 50% BM]. (b) Carbonization diagram of SD combinations performed with corn silage, cabbage residues, dry straw and dough residues [X1 = 20% CS + 80% SD; X2 = 50% CS + 50% SD; W1 = 20% CR + 80% SD; W2 = 50% CR + 50% SD; F = 70% SD + 30% DS; E = 70% SD + 30% DR]. (c) Carbonization diagram of SR combinations performed with corn silage and solid separated digestate [K1 = 20% CS + 80% FWCH; K2 = 50% CS + 50% FWCH; C = 50% SD + 50% FWC; D = 50% SD + 50% LWC].
I. Oliveira et al. / Bioresource Technology 142 (2013) 138–146
if the whole process chain including conditioning and utilization is considered. It is therefore necessary to consider the dewatering and drying properties of the HTC-Biochar produced. In this work test equipment which the method was previously developed in the Waste Management Laboratory of University of Applied Sciences Ostwestfalen-Lippe (Ramke et al., 2009). The dewatering tests gives absolute data which can be compared to real dewatering plants (i.e. chamber filter press) while the drying test gives a relative measure of dry matter content after 120 min of drying under defined conditions. The average resulting curves of the dewatering and drying tests for different samples can be drawn. It is known that the dewatering properties of substrates with high moisture content are improved after carbonization (Acharjee et al., 2011; Libra et al., 2011; Ramke et al., 2009), and it is evident when comparing the resulting curves of the dewatering and drying tests from the input substrates and resulting HTC-Biochar produced. The average relative increase of DM content of the HTC-Biochar produced in all the experiments after dewatering and drying tests are between 60% and 80% (% DM). However, depending on the input substrate the increase in DM of the HTCBiochar after the dewatering and drying step could be completely different as it is the case of the HTC-Biochars produced from CR and DR where a relative increase of 47% and 98% of DM was achieved respectively. Another interesting result is the improvement of the dewatering and drying properties reached when combining SD with CS or DR. The achieved relative increase in dry matter of the HTC-Biochar after dewatering and drying is nearly 80% for those combinations, whilst the input material or the HTC-Biochar of SD only reached a dry matter of 50% and 64% respectively. It is therefore interesting to note that the combination of substrates in HTC improves not only the fuel characteristics in comparison with brown coal but also the dewatering and drying properties of the HTC-Biochar to be used as solid fuel for industrial purposes. 3.4. Biogas potential of the process water Analysis showed that the majority of the experiments resulted in a process water predominantly acidic (pH 3–5) with a high TOC (13–26 g/l). The resulting process water could either be used to heat up the reactor or introduced in the process for reaction optimization (Libra et al., 2011). However, sooner or later it will be necessary to find ways to dispose off the process water. With the idea of combining two technologies, HTC and anaerobic digestion, a set of anaerobic batch tests were performed to determine the resulting biogas and methane potential of the HTC process water. The resulting cumulative biogas and CH4 yield had a minimum of 9.6 L biogas/kg FM and 6 L of CH4/kg FM and a maximum of 21.1 L of biogas/kg FM and 16.3 L of CH4/kg FM for C and X2 respectively. It is possible to refer a tendency where the cumulative methane yield is higher when TOC is higher and vice versa. Further studies should assess the use of process water when it is reused for HTC process optimization. In this case, higher cumulative CH4 yields could be achieved due to higher TOC values of the process water. It would also be interesting to study the co-digestion of the process water with different biomass and organic residues. Notwithstanding this, the results show that anaerobic digestion is a suitable method to treat the process water. 4. Conclusions HTC of agricultural residues produced a solid fuel (HTC-Biochar) comparable to brown coal, which could be used to complement farm-based biogas plants. Combinations of different agricultural residues improved carbonization. Most of the HTC-Biochars
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produced presented good dewatering and drying properties. The process water could be used in biogas plants, reducing its environmental impact, which could increase the energy efficiency of the whole process. More studies are necessary to optimize HTC-Biochar yields and quality, and the feasibility of a continuous process. Different uses for the HTC-Biochar produced should be also studied for example as soil amendment or biogas scrubbing material.
References Acharjee, T.C., Coronella, C.J., Vasquez, V.R., 2011. Effect of thermal pretreatment on equilibrium moisture content of lignocellulosic biomass. Bioresour. Technol. 102, 4849–4854. Akhtar, J., Amin, N.A.S., 2011. A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass. Renew. Sustain. Energy Rev. 15, 1615–1624. Alvarez, J., Mace, S., Llabres, P., 2000. Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresour. Technol. 74, 3– 16. Behrendt F., 2009. Direktverflüssigung von Biomasse – Reaktionsmechanismen und Produktverteilungen, Insitut für Energietechnik, Technische Universität Berlin Studie im Auftrag der Bundesanstalt für Landwirtschaft und Ernährung. Projektnummer 114-50-10-0337/05-B. Chynoweth, D.P., Issacson, R., 1987. Anaerobic digestion of biomass. Elsevier Applied Science, London, ISBN 1-85166-069-0, p. 279. Cui, X., Antonietti, M., Yo, S.-H., 2006. Structural effects of iron oxide nanoparticles and iron ions on the hydrothermal carbonization of starch and rice carbohydrates. Communication in Wiley-VCH Cerlag GmbH & Co. KGaA 2006, vol. 6, pp. 756–759. DIN 38414, part 8: German standard methods for the examination of water, waste water and sludge; sludge and sediments (group S): determination of the amenability to anaerobic digestion; 1985. DIN 51900-3. ICS 75.160.10; 75.160.20. Testing of solid and liquid fuels: determination of gross calorific value by the bomb calorimeter and calculation of net calorific value. Part 3: method using adiabatic jacket. January 2005. DIN EN 12879. ICS 13.030.20. Bestimmung des Glüverlustes der Trockenmasse. February 2001. DIN EN 12880. ICS 13.030.20. Bestimmung des Trockenrückstandes und des Wassergehalts. February 2001. Du, Z., Hu, B., Shi, A., Ma, X., Cheng, Y., Chen, P., Liu, Y., Lin, X., Ruan, R., 2012. Cultivation of a microalga Chlorella vulgaris using recycled aqueous phase nutrients from hydrothermal carbonization process. Bioresour. Technol. 126, 354–357. Euractiv, 2011:
146
I. Oliveira et al. / Bioresource Technology 142 (2013) 138–146
Parshetti, G.K., Hoekman, S.K., Balasubramanian, R., 2013. Chemical, structural and combustion characteristics of carbonaceous products obtained by hydrothermal carbonization of palm empty fruit bunches. Bioresour. Technol. 135, 683–689. Prochnow, A., Heiermann, M., Plochl, M., Linke, B., Idler, C., Amon, T., et al., 2009. Bioenergy from permanent grassland – a review: 1. Biogas Bioresour. Technol. 100, 4931–4944. Ramke, H.-G., Blöhse, D., Lehmann, H.-J., Fettig, J. 2009, Hydrothermal carbonization of organic waste, in : Cossu, R., Diaz, L.F., Stegmann, R. (Eds.). Sardinia 2009: Twelfth International Waste Management and Landfill Symposium Proceedings, CISA Publisher (ISBN 978-88-6265-007-6). Regmi, P., Moscoso, J.L., Kumr, S., Cao, X., Mao, J., Schafran, G., 2012. Removal of copper and cadmium from aqueous solution using switchgrass biochar produced via hydrothermal carbonization process. J. Environ. Manage. 109, 61–69. Reza, M.T., Lynam, J.G., Vasquez, V.R., Coronella, C.J., 2012. Pelletization of biochar from hydrothermally carbonized wood. Environ. Prog. Sustain. Energy 31, 225– 234. http://dx.doi.org/10.1002/ep.11615.
Rillig, M.C., Wagner, M., Salem, M., Antunes, P.M., George, C., Ramke, H.-G., et al., 2010. Material derived from hydrothermal carbonization: effect on plant growth and Arbuscular mycorrhiza. Agric. Ecosyst. Environ. Appl. Soil Ecol. 45, 238–242. Spiegel International online, 2011.