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The Fate of Nitrogen during Hydrothermal Carbonization Andrea Kruse, Florian Koch, Katharina Stelzl, Dominik Wüst, and Meret Zeller Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01312 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on September 5, 2016
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The Fate of Nitrogen during Hydrothermal Carbonization Andrea Kruse*,1,2, Florian Koch1, Katharina Stelzl2, Dominik Wüst1, Meret Zeller1 1. Chair of Conversion Technology and LCA of Renewable Resources Institute of Agricultural Engineering University of Hohenheim Garbenstraße 9 70599 Stuttgart Germany 2. Institute for Catalysis Research and Technology Karlsruhe Institute of Technology Hermann-von-Helmholtzplatz 1 76344 Eggenstein-Leopoldshafen Germany
In Memoriam Michael J. Antal Abstract Carrot green, the algae Chlorella pyrenoidosa, and straw, as representatives for different types of biomass are converted by hydrothermal conversion. The amount of nitrogen remaining in the hydrochar as well as in aqueous phase is determined and the amount of nitrate, nitrite, and ammonia in the process water are analyzed. The nitrogen content of hydrochar has an significant impact on the properties of hydrochar, therefore a control of the nitrogen content would be useful to design hydrochar for different application. In regard of the fate of nitrogen, the different biomass feedstocks show significant differences, due to the different chemical nature of nitrogen compounds in the feedstock. A complete removal of nitrogen from the hydrochar could not be achieved. In contrast, wood incorporates nitrogen when impregnated with the amino acid cysteine during hydrothermal carbonization. Introduction Hydrothermal carbonization (HTC) is a process to convert biomass in aqueous medium at elevated pressures and temperatures to a carbon rich material with a heating value like that of lignite1, often called hydrochar. Typical reaction conditions are in the range of 180-220 °C and reaction times between 30 minutes and several hours. Usually the pressure is kept above the vapor pressure of water at the reaction time to avoid evaporation, because in hydrothermal carbonization the reaction has to occur in liquid water, which is important in view of the chemical reactions occurring. Here the hydrolysis of carbohydrates in combination with the formation of double bonds are important characteristic reactions in superheated, liquid water2,3. The reaction pathways occurring are very complex 4; A simplified illustration for hydrochar formation is shown in Fig. 15. The polar bonds connecting the glucose molecules of carbohydrates easily hydrolyze into sugar molecules (path A, Fig. 1), which are solved in the liquid reaction medium. Consecutively, water elimination occurs and double bonds are formed. This process is supported by the presence of acids 6. An essential intermediate formed this way is hydroxymethylfurfural, an important platform chemical itself. Then, ACS Paragon Plus Environment
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polymerization of these intermediates formed by that way to so called hydrochar occurs. The exact mechanism is not clear: May be an emulsion is formed which polymerize or nuclei growing by polymerization 4. At the end, a product is formed which seems to be mainly a product of the aldol condensation of furfurals7. In many cases, the particles observed by the formation of carbohydrates derived intermediates show a spherical shape8. Anyway, biomass not only consists of carbohydrates. Several studies give evidence that lignin contained in biomass decreases the reaction rate and leads to different structures of hydrochar 9-11. Conversion of lignin to hydrochar proceeds mainly by pathway B in Fig. 1., where char particles are formed by solid-solid reactions usually maintaining the original structural elements of the biomass.
Fig. 1: Simplified reaction scheme of hydrochar formation 5 During this process of hydrolysis also nitrogen containing compounds may be hydrolyzed and/or released to the aqueous environment. Now the question occurs, what happens to nitrogen consecutively? Is it found in the liquid, solid or the gaseous phase? The fate of nitrogen is of special importance in view to the desired application of the hydrochar: -
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Fuel: In the case the hydrochar is burned, the nitrogen content should be small to avoid the formation of NOx 12. In this case, the nitrogen containing compounds should be transferred into the aqueous solution and – if possible – used as or to produce fertilizer e.g. to grow algae13,14. Soil improvement. Here higher nitrogen content might be an advantage, because hydrochar is degraded in soil and the nitrogen component might be used as fertilizer by plants. Anyway, the behavior of chars in soil is rather complex 15-17. Adsorbent: An increased nitrogen content changes the polarity of the materials and increase the ability to adsorb ions18.
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Supercapacitors and other electrical applications: Here nitrogen in hydrochar is necessary to achieve the desired properties 19,20.
Therefore it is necessary to understand, what happens to the nitrogen containing compounds during HTC and how the process can be influenced in order to produce materials containing a certain nitrogen content and species. It has to be pointed out here that the nitrogen in the biomass can be bounded in organic compounds like proteins or as salt. In addition the oxidation state may be varying (ammonium, nitrate, and nitrite as example). Likely this influences the release of nitrogen. Many studies characterize the hydrochar produced under different conditions from a variety of biofeedstocks and report also the nitrogen content in the hydrochar, examples are 12,15,21-24. But only a few studies give more detailed information on nitrogen issue. Reza et al. reported the content of nitrogen in the hydrochar and as ammonium in the aqueous phase for swine manure24. Ekbo et al. 25 studied the release of nitrogen by hydrothermal processes in a wide temperature range. As feedstock the microalgae Chlorella vulgaris, biogas digestate, swine and chicken manure were investigated in the temperature range from 170° to 500 °C, which means from conditions for carbonization via liquefaction to gasification. In the case of Chlorella the total nitrogen contained in the aqueous phase decreases while in the case of digestate it increases with temperatures. The manures remain in a similar concentration range. At HTC conditions roughly 35 % (digestate) to 75 % (Chlorella) of the nitrogen is found in the water phase25. Nitrogen was identified to be mainly organic nitrogen and ammonium, and the concentration was found to depend on pH value 26. In this study the concentration of nitrogen compounds in process water and the residual nitrogen content in the hydrochar is investigated for carrot green, a Chlorella algae and wheat straw. These feedstock materials stand for three different types of biomass, available for hydrothermal carbonization: lignin-free agricultural and food residues, algae and lignocelluloses. After studying, how much nitrogen is released into the aqueous phase during HTC, the questions occurs, what happens to the nitrogen containing compounds in the water and whether nitrogen is incorporated during HTC. For this experiment wood is impregnated with an amino acid, and the hydrochar formed is analyzed. Experimental set-up The experiments were conducted in small batch autoclaves with an internal volume of 10 mL. More details about the autoclaves is given in literature5. Eight of these autoclaves are used of which one is equipped with temperature and pressure measurement. The reactors are made of stainless steel and have a metal-metal sealing. They are heated-up in a GC furnace or a muffle furnace to temperatures between 180 – 250 °C. This is the temperature range of hydrothermal carbonization; at lower temperatures the reaction rate is very low and above this temperature, hydrothermal liquefaction occurs. The reaction time was set between 2 and 12 hours. Every data point is an average of two experiments. In the case the deviation was above 5 % between the two data sets, the experiment was repeated. The autoclaves are filled with dried biomass (10 wt.% dry mass, composition see Table 1) and water. As lignocellulose with moderate nitrogen content, straw is investigated. In addition experiments were conducted with beech wood. Here the content of ash and nitrogen is very low. In these
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experiments cysteine was used to impregnate the wood and the results are compared with not treated material. The reaction time was set to 17 hours with this biomass because of the relatively low reactivity of wood. The dry mass content was also 10 wt.% and 1 wt.% of cysteine was added. For comparison in one experiment hydroxymethylfurfural was used as feedstock with the same concentration. Here four hours reaction time at 180 °C was applied. The pressure in the experiments was adjusted by the amount of water added and was always set above the vapor pressure of water at the given temperature 27. After reaction the micro autoclaves were opened in a gas-tide containment and purged with nitrogen. The amount of gases was measured by water displacement. In additions samples were taken to determine the gas composition. The gas composition is measured by GC (for details see 28). Table 1: Feedstock composition of biomass with low lignin content Biomass
Humility
Ash
Lipids
[ wt. %]
[wt. %]
[wt. %]
a)
a)
Nitrogen (N) [g/kg]
Phosphor (P) [g/kg] a)
Potassium (K) [g/kg] a)
Chlorella pyrenoidosa Carrot Green
3.55
9.0
4.7
99
2.0
1.4
5.40
15.0
n.d
27
1.82
45.6
Wheat Straw
8
4.0
n.d.
15
10
3
„n.d.”:not detectable because of low content a) http://www.best-of-bio.de/f/pdf/Chlorella.pdf b) http://www.best-of-bio.de/f/pdf/Spirulina.pdf
The suspension of hydrochar and water was removed from the autoclaves and filtered (and not washed). The yield of solid product was determined after drying at 105 °C over night. Both solid and aqueous product phase are analyzed. The water content of feedstock and solid product is measured following DIN 51718 and the ash content according to DIN 51719. To determine the composition of the ash a temperature of only 539 °C was used to avoid evaporation of ash constituents. The composition of the organic material with regard to nitrogen, carbon, sulfur and hydrogen content was measured by elemental analysis (Vario EL III Elementar Analysensysteme GmbH, Hanau, D). The composition of the ash and of metal ions and phosphate in the aqueous solution was measured by ICP-OES (720/725-ES emission spectrometer from Agilent Technology). In the aqueous solution the total carbon (TC), total inorganic carbon (TIC), and total nitrogen (TN) were measured by a Dimatec 2000 instrument. Total organic carbon (TOC) was calculated by a differential method. To measure nitrite (LCK 341), nitrate (LCK 339) and ammonium (LCK 304) colorimetric methods by Hach-Lange are used. The concentrations measured by colorimetric and by ICP-OS are checked for deviation and have been proven to be below 10 % relative to measured amount. Organic nitrogen was calculated as difference of total nitrogen and the sum of nitrogen in nitrite, nitrate and ammonium species.
Results
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First the results of the hydrothermal carbonization with nitrogen containing biomass are presented. This paper focuses on the fate of nitrogen during HTC, other results like the solid yield and the carbon content in the solids are shown in the supplement. Due to reproducibly tests, the experimental error can be estimated to less than 10 % relative to the data given. Fig. 2 shows the concentration of nitrate and ammonium, given as nitrogen content in the ions, in the process water after HTC of carrot green. With increasing reaction time the content of nitrate decreases and the content of ammonium slightly increase. As function of temperature, the nitrate content decreases. The ammonium content increases significantly, if the temperature is increase from 180 °C to 220 °C. Fig. 3 shows the nitrite content as function of reaction time for three temperatures. At 180 °C, the nitrite content increases with reaction time, at 220°C it decreases. At 250 °C, the nitrite content is much lower and decreases with reaction time. In the case of carrot green the total amount of nitrogen and the sum of nitrogen in nitrate, nitrite and ammonium is nearly identical. Therefore the amount of organic nitrogen in the aqueous phase is very low.
5000
-
NO3 -N [mg/L]
4000
6000
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0
NH4+ [mg/L]
180 °C: Ammonium Nitrat 220 °C: Ammonium Nitrat 250 °C Ammonium Nitrat
6000
0 0
2
4
6
8
10
12
Time [h]
Fig. 2: Nitrogen content in ammonium- and nitrate in the process water, given as concentration, after hydrothermal carbonization of carrot green at three different temperatures.
1,5
Nitrite-N [mg/L]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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190 °C 220 °C 250 °C
1,0
1,0
0,5
0,5
0,0
0,0 0
2
4
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Time [h]
Fig. 3: Nitrogen content in nitrite in the process water, given as concentration, after hydrothermal carbonization of carrot green at three different temperatures.
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Chlorella N in solid / wt. %
9 8
180 °C 7
250 °C
220 °C Carrot Green 220 °C
3
250 °C 180 °C -1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Time [h]
Fig. 4: Nitrogen content in the hydrochar for Chlorella and carrot green as function of reaction time and at three different temperatures. Carrot Green 180 °C 220 °C 250 °C Chlorella 180 °C 220 °C 250 °C
0.60
0.55
N in hydrochar [g]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
0.50
0.05
0.00 0
2
4
6
8
10
12
Time [h]
Fig.5: Amount of nitrogen in the solid for Chlorella and carrot green as function of reaction time and at three different temperatures. Fig. 4 shows the nitrogen content in the hydrochar. For carrot green, the content slightly decreases with reaction time at 180°C and increases at the other two temperatures. The nitrogen content in the hydrochar from Chlorella behaves different. At any temperature measured, the nitrogen content decreases with reaction time. This illustration of the results is misleading, because of the mass reduction during HTC. Therefore in Fig. 5 the absolute amount of nitrogen in the solid is shown. Here in the beginning auf the reaction, the nitrogen content is drastically reduced, followed by a slight decrease with time. The reduction of the nitrogen content for Chlorella is much higher than for carrot green. The temperature dependence is much lower than can be assumed from Fig. 4, because of the corresponding mass reduction during HTC. Fig. 6 shows the nitrogen concentration of ammonium and nitrate after HTC of Chlorella. In contrast to carrot green the ammonium concentration is higher than the nitrate concentration. The nitrate concentration increases at the beginning of the reaction, but then shows no clear dependence on reaction time and temperature. The ammonium concentration in the aqueous product increases with time and temperature.
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180 °C: Ammonium Nitrat 220 °C: Ammonium Nitrat 250 °C Ammonium Nitrat
9000 8000
9000 8000 7000
6000
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5000
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1000
-
NO3 [mg/L]
7000
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0 0
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Time [h]
Nitrit, N [mgl/l]
Fig. 6: Nitrogen content in ammonium- and nitrate in the process water, given as concentration, after hydrothermal carbonization of Chlorella algae at three different temperatures.
3.0
3.0
2.5
2.5
2.0
2.0
1.5
1.5 190 °C 220 °C 250 °C
1.0
1.0
0.5
0.5
0.0
0.0 0
2
4
6
8
10
12
Time [h]
Fig. 7: Nitrogen content in nitrite in the process water, given as concentration, after hydrothermal carbonization of Chlorella algae at three different temperatures.
org. N [mg/L]
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NH4+ - N [mg/L]
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15000
15000
10000
10000
190 °C 220 °C 250 °C
5000
5000
0
0
0
2
4
6
8
10
12
Time [h]
Fig. 8: Nitrogen content in organic compounds in the process water, after hydrothermal carbonization of chlorella algae at three different temperatures. Fig. 7 shows the nitrogen content of nitrite for the HTC of Chlorella. Here nitrite is released at HTC at short reaction time and the content then decreases with reaction time. There is no significant ACS Paragon Plus Environment
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temperature dependence. In the case of Chlorella (Fig. 8), the amount of organic nitrogen is much higher than for carrot green. Here the organic nitrogen content strongly increases at the beginning of the reaction and stays nearly constant, with no obvious temperature dependence (Fig. 8). For straw the concentrations of the nitrogen containing compounds are much lower. Like for carrot green, the nitrate concentration (Fig. 9) is much higher here than that of ammonium (Fig. 10). 500
400
190 °C NO3- [mg/L]
300
220 °C 200
100
250 °C
0 0
2
4
6
8
10
12
Time [h]
Fig. 9: Nitrogen content in nitrate as function of reaction time for HTC of wheat straw at three temperatures. Fig. 9 shows the concentration of nitrogen in nitrate as function of reaction time for three temperatures. During HTC nitrate is released and the concentration decreases with time and temperature.
2.5 2.0
180 °C Nitrit, N Ammonium 220 °C Nitrit, N Ammonium 250 °C Nitrit, N Ammonium
3.0 2.5 2.0
1.5
1.5
1.0
1.0
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0.5
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Ammonium, N [mg/L]
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Nitrit, N [mgl/l]
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12
Time [h]
Fig. 10: Concentration of ammonium-N and nitrite-N after HTC of straw at three temperatures. Up to now it was studied which amount of nitrogen is released from the feedstock during HTC. Now the goal is to investigate if nitrogen is incorporated intro hydrochar during HTC. For this purpose beech wood is impregnated with cysteine. Fig. 11 shows SEM pictures of the wood before and after impregnation. The small cysteine crystals can be seen on the surface of the wood particles.
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Fig. 11: Beech wood particles before (left) and after (right) impregnation with cysteine. The analysis of the hydrochar composition (Fig. 12) shows, that nitrogen and sulfur from the cysteine is found in the hydrochar. Surprisingly in the experiments with cysteine “raspberry-like” structures occur, not found without cysteine addition (Fig. 13). To verify this, experiments with hydroxymethylfurfural are conducted with cysteine addition. Here also “raspberries” are observed (Fig. 14).
Fig. 12: Composition of hydrochar from beech wood after HTC with 17 h reaction time. The experiments with impregnation with cysteine are marked with a cross.
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Fig. 13: SEM picture of hydrochar from beech wood impregnated with cysteine (180°C, 17h).
Fig. 14: SEM pictures of hydrochar from hydroxmethylfurfural without (left) and with (right) addition of cysteine (4h, 180 °C). Discussion Considering the chemical processes shown in Fig. 1, it is clear that the nitrogen compounds are released in the beginning of the HTC reaction. First the biomass is hydrolyzed and this way, also nitrogen compounds are solved. This is visible by the time dependence of all nitrogen species but is likely most obvious in Fig. 5. When particles are formed, no drastic change of the nitrogen content occurs. A small increase at long reaction time might be a consequence of salt precipitation e.g. of nitrates. The nitrogen content measured here in hydrochar are in the range of prior studies 12,15,2123,29 . Also, a decrease of the nitrogen content with rising temperature is observed 21,30 as shown in Fig. 5. This might be a consequence of an increased hydrolysis rate at higher temperatures. A very important aspect is the chemical nature of nitrogen compounds in the biomass. In the case of Chlorella, the protein content is relatively high. By hydrolysis of proteins amino acids are formed and solved in water measured as organic nitrogen (Fig. 8). In addition the amino acids are further hydrolyzed to ammonia, leading to the rather high concentration of ammonium (Fig. 6). The increase with temperature is a consequence of the increased rate of hydrolysis (see e.g. 31-33). Also manure
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has a high content of organic nitrogen and amino acids. Therefore prior studies with manure show similar high concentrations of organic nitrogen and ammonium as found for Chlorella algae 26. Carrot green and straw have low protein content. Here the content of nitrate is much higher than that of ammonium (Fig. 2 and Fig. 10). The decrease of nitrate and at higher temperature also of nitrite with rising temperature and reaction time is a consequence of the precipitation of salts. This effect would be too small to be measured as N-content increase of the hydrochar. In all cases the nitrogen is not completely removed from the solid product. Likely nitrogen containing compounds are formed and incorporated into the hydrochar, perhaps via a type of Maillard reaction 34,35 . This needs further investigations. By this it could be explained why also nitrogen compounds on the surface of the wood leads to a nitrogen content of the hydrochar formed (Fig. 12). Such effects are applied to produce nitrogen containing chars, e.g. an incorporation of nitrogen is also observed for the HTC of tannin with ammonia in the water. Here no “raspberry” structures are observed.36 Also the HTC with glucose or other compounds with the addition N-containing compounds N-doped hydrochar is produced37,38. Hydrothermal carbonization of cellulose in the presence of the globular protein ovalbumin leads to the formation of nitrogen-doped carbon aerogels 18. In the case of conversion of carboxymethylcellulose with urea, the particle size is increased, perhaps because of cross-linking by urea38. In the case of chitosan carbonization, the nitrogen content of the hydrochar is higher, especially at higher temperature than in the original feedstock. Here spheres of different size are formed 39. The explanation of the structural change in the presence of amino acids might be that the amino acids are nuclei for the formation of hydrochar particles or that these compounds disturb coagulation by a change of surface properties. Here are more information about hydrochar formation are necessary. Conclusion During hydrothermal carbonization the partition of nitrogen between aqueous and solid phase as well as the nature of the solved nitrogen species depend on the type of biomass. Proteins and amino acids are hydrolyzed to a certain extent, leading to solved organic compounds and ammonium salts. Partly nitrogen is incorporated into the hydrochar. Inorganic nitrogen compounds are set free during HTC and nitrate/nitrite might precipitate depending on the available counter ion. For other biomass, nitrate is of major importance. A complete removal of nitrogen from the hydrochar was not possible; in contrast biomass with very low nitrogen content incorporates nitrogen compounds. Acknowledgment Financial support by the European Commission is gratefully acknowledged; The BioBoost project was funded by the 7th Framework Programme of the European Union under grant agreement 282873 (www.bioboost.eu)
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