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Conversion of water hyacinth to value-added fuel via hydrothermal carbonization
Journal Pre-proof Conversion of water hyacinth to value-added fuel via hydrothermal carbonization Chaoyue Zhang, Xiaoqian Ma, Xinfei Chen, Yunlong Tian, Yi Zhou, Xiaoluan Lu, Tao Huang PII:
S0360-5442(20)30300-5
DOI:
https://doi.org/10.1016/j.energy.2020.117193
Reference:
EGY 117193
To appear in:
Energy
Received Date: 5 August 2019 Revised Date:
10 November 2019
Accepted Date: 17 February 2020
Please cite this article as: Zhang C, Ma X, Chen X, Tian Y, Zhou Y, Lu X, Huang T, Conversion of water hyacinth to value-added fuel via hydrothermal carbonization, Energy (2020), doi: https://doi.org/10.1016/ j.energy.2020.117193. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Conversion of water hyacinth to value-added fuel via hydrothermal carbonization Chaoyue Zhang, Xiaoqian Ma*, Xinfei Chen, Yunlong Tian, Yi Zhou, Xiaoluan Lu Tao Huang Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization, School of Electric Power, South China University of Technology, Guangzhou 510640, China Postal address: School of Electric Power, South China University of Technology, No. 381, Wushan Road, Tianhe District, Guangzhou, 510640, China *Corresponding author Tel.: +86 20 87110232; fax: +86 20 87110613 E-mail address: [email protected]
Abstract: Hydrothermal carbonization (HTC) of water hyacinth (WH) was investigated to elucidate the effects of reaction temperature, residence time and pH (acid and alkali catalysts) on the chemical properties, combustion behavior and emission properties of hydrochar. Results found that high reaction temperature, long residence time and catalysts were beneficial to ameliorate the fuel properties of hydrochar in terms of calorific value and energy densification, albeit the yield and energetic recovery efficiency got deteriorated. The lower H/C, O/C and N/C ratios of hydrochar reflected more severe dehydration, decarboxylation and denitrogenation reactions within HTC process. SEM images represented that HTC could lead to the fragmentized structure of hydrochar. As HTC progressed, the vibration of hydroxyl and carboxyl groups in hydrochar weakened, which was conductive to improving the
hydrophobicity of hydrochar. The combustion characteristics of hydrochar got remarkable upgraded after HTC, whose combustibility index S and combustion stability index Rw were both superior to that of WH. The hydrochar obtained from the addition of alkali catalyst (NaOH) possessed lower emission concentration of SO2 and NOX during combustion, thus demonstrating better emission properties. Overall, HTC was a feasible way to bridge the gap from WH to alternative renewable fuel.
Graphical abstract
Keywords: Hydrothermal carbonization; Water hyacinth; Hydrochar; Acid and alkali
catalysts; Combustion and emission characteristics
1. Introduction
With the acceleration of urbanization and industrialization process in China, a large amount domestic sewage and industrial wastewater are discharged into water bodies without proper purification. The wastewater is rich in nitrogen, phosphorous, potassium and organic pollutants [1], thereby leading to eutrophication of water bodies. Water hyacinth (WH), known as a monocotyledonous freshwater aquatic plant with flourishing roots, can reproduce abundantly in eutrophic water bodies and thus has become one of the primary invasive aquatic plants in China [2, 3]. The massive growth of WH leads to the loss of fishery and tourism, the hindrance to water transport as well as the blockage of irrigation channels. The rotten remains of WH deteriorates water quality, making water unfit for drinking by humans, livestock and wildlife [4]. Moreover, WH areas provide a large number of breeding places for vector (mosquito), causing the outbreak of epidemic diseases [5]. Meanwhile, on account of high moisture and heavy metal in WH, traditional disposal methods, such as incineration, landfill and fertilization, face technological difficulties, resulting in emission of contaminative gases and runoff of heavy metal into aqueous or edaphic systems. As a consequence, how to effectively deal with the remains of WH has become a major issue in urban environmental management. According to relevant report [6], WH is mainly composed of lignin, crystalline cellulose and hemicellulose polymer, indicating that it can be regarded as a potential
biomass resource for energy utilization. UP to now, the effective conversion methods of biowastes mainly include thermochemical and biological conversions. Among two conversion technologies, thermochemical conversion has attracted extensive attention on account of its advantages in terms of energy production, including low cost, short treatment process and convenient product recovery [7]. Pyrolysis and hydrothermal carbonization (HTC) are common methods in thermochemical treatment. Hu et al. [8] mainly researched the conversion effects of catalytic and non-catalytic pyrolysis of WH with the purpose of energy upgradation. It was pointed out that traditional pyrolysis would induce several technological problems such as particle agglomeration and heated surface deposition, further leading to high inefficiencies of pyrolysis [9]. Moreover, the drying pretreatment before pyrolysis would also increase conversion cost. Compared with pyrolysis, HTC has its unique advantages in terms of biomass utilization: (1) HTC has relatively mild reaction temperature (180-350 ºC), autogenous pressure (2-10 MPa) and low carbon dioxide emissions [10, 11]. Hydrolysis of biomass under subcritical condition can effectively solve the problem of agglomeration. (2) HTC is one such technology that develops as an ideal pretreatment method for biomass with high moisture and high organic content, thus saving the pre-drying cost of raw materials. (3) HTC can minimize the environmental hazard caused by biowastes, because of the removal of undeserved nutrients, the stabilization of heavy metal, and the elimination of malignant bacteria during hydrothermal process [12]. (4) Water is used as not only solvent but also catalyst to improve the utilization rate of biomass. During HTC process, the dielectric constant
of water decreases [13], thus improving the solubility of organic compounds. Meanwhile, hydrogen bonds in water decrease, while isothermal compression rate of water increases, which can promote the reactions of ions, polar non-ions and free radicals [14, 15]. In addition, under the subcritical environment, the ionization of water strengthens with the increase of temperature and pressure, leading to high concentration of H+ and OH-, thus forming a catalytic environment to promote the hydrolysis reaction [16, 17]. (5) HTC is able to convert biowastes into carbon-rich micro-nano particles (i.e. hydrochar) by simulating natural coalification process. Hydrochar has high energy content, superior grindability and hydrophobicity [18, 19], which can settle the transportation and storage problems induced by biowastes and thus benefits in the subsequent application aimed at energy production [20, 21]. (6) According to the techno-economic assessment of HTC, because of the huge output and low price of biowastes, treating biowastes via HTC can achieve lower breakeven cost [22, 23], which is conductive to the large-scale promotion of HTC technology. In summary, HTC has tremendous potential to bridge the gap from WH to value-added solid fuel. To our knowledge, Gao et al. [9] have reported the effect of residence time on the surface properties of hydrochar obtained via HTC of WH. As expect, a significant uptrend in higher heating value of hydrochar was observed by prolonging residence time. However, except for residence time, the variations in reaction temperature and catalysts also deserve great consideration, since they may affect the characteristics of solid product in different aspects and degrees. Besides, most of available reports
usually focused on surface properties and combustion properties of hydrochar, while the influence of various reaction conditions on emission properties of hydrochar obtained from HTC of WH was unfortunately lacking. Nowadays, most countries have clearly stipulated the emission standards of air pollutants from coal-fired boilers, and thus the exploration of hydrochar emission properties is necessary for the application of HTC in industrial scale. Consequently, on the basis of verifying the previous experimental results, this paper extends its research and systematically analyzes the effects of reaction temperature, residence time and pH (acid and alkali catalysts) on hydrochar characteristics (including chemical properties, surface properties,
combustion properties
and
emission properties).
The chemical
compositions of hydrochar were investigated through proximate and elemental analysis. The surface morphology, functional group properties and combustion characteristics
of hydrochar were deeply explored
by SEM,
FTIR
and
thermogravimetric analysis. Furthermore, the emission profiles of several typical gases during the combustion of hydrochar were detected through TG-FITR experiments. The results obtained in this paper are beneficial to further explore the feasibility of treating water hyacinth through hydrothermal carbonization and provide referential opinions for designing, optimizing and scaling up the comprehensive utilization of water hyacinth.
2. Materials and methods
2.1. Materials
In this work, the raw material was fresh WH salvaged from the Taihu Lake Basin in Jiangsu Province. Prior to the experiment, the collected WH was cleaned by deionized water, and the non-combustible materials, such as gravel and sand, were screened out. The remaining leaves, trunks and roots were dried, pulverized and sieved, in order to harvest target samples with uniform particle size in the range of 0-178µm, and then stored in a drying oven at 105 ºC for 24 hours.
2.2. Hydrothermal treatment
The HTC experiments of WH were carried out in a 250 ml autoclave reactor (Model SLM250, Beijing Shi Ji Sen Lang Experimental Instrument Co., Ltd., Beijing, China). To analyze the effects of reaction temperature and residence time on HTC, four reaction temperature (180, 210, 240 and 270 ºC) and four residence time (10, 30, 60 and 90 min) were adopted. Besides, in order to investigate the effects of acid and alkali catalysts on HTC, CH3COOH and NaOH were used to adjust the pH of deionized water to 3, 5, 7, 9 and 11. Prior to each experiment, 8 g of oven-dried raw material was blended with 80 ml of deionized water in a reaction vessel and manually stirred for 10 min to achieve a slurry densification of 10% w/w. After that, the reactor was sealed and purged with high purity nitrogen for 3 minutes to create an oxygen-starved atmosphere. Subsequently, the reactor was heated up to the pre-set reaction temperature by an external electric heating device and maintained for the desired residence time, while the heating rate and stirring speed were at 5 ºC/min and 500 rpm, respectively. Once the reaction elapsed, the autoclave reactor was removed
from furnace and quenched to ambient temperature by cooling water. The pressure relief valve was then opened, and the gas product quickly released. The slurry inside reactor was filtered to obtain hydrochar. Subsequently, the hydrochar was dried at 105 ºC for 24 hours and pulverized into a particle diameter of less than 178µm for later analysis. All the experiments were performed three times to ensure repeatability and accuracy. Notably, the hydrochar was denoted as “reaction temperature-residence time-pH”. For example, the hydrochar obtained at 210 ºC for 60min and pH of 7 was labeled as “210-60-7”.
2.3. Analytical approach
The elemental and proximate analysis of WH and hydrochar were investigated by an elemental analyzer (Vario EL cube, Germany) based on ASTM D5373 standard and a muffle furnace (MXX1100-30, shmicrox Co., Ltd., China) based on China GB/T28731-2012 Standard, respectively. The higher heating value (HHV) of solid samples were determined by the 5E-KC5410 rapid calorimeter. Additionally, in order to better assess the HTC process, the yield, energy densification and energetic recovery efficiency of products were calculated by the following formulas [24]: Hydrochar yield = (Mass of hydrochar / Mass of raw material) × 100%
(1)
Energy densification = HHV of hydrochar / HHV of raw material
(2)
Energetic recovery efficiency = Hydrochar yield × Energy densification × 100% (3)
The surface morphology of the samples was characterized by scanning electron microscope (SEM, LEO-1530). The variation of functional groups was analyzed by Fourier transform infrared spectrometer (Nicolet™ iS™ 10 FT–IR spectrometer).
2.4. TG-FTIR method
The combustion and emission characteristics of the samples were characterized by a thermal gravimetric analyzer (TGA, METTLER TOLEDO) in combination with a FT-IR spectrometer (Nicolet™ iS™ 10 FT–IR spectrometer). In each test, the initial weight of the sample was kept at 8±0.5 mg. The air was continuously supplied with a fixed rate of 100 mL/min throughout the entire combustion process. Under air atmosphere, the experimental temperature was heated from 50 ºC to 800 ºC with a rate of 30 ºC/min. As gaseous products, generated from the combustion of the sample, entered the gas chamber through a transmission line, FTIR spectra from 400 cm-1 to 4000 cm-1 were detected for further analysis. The ignition temperature (Ti) and burnout temperature (Tf) of the sample could be determined by methods in Ref. [25]. Besides, the combustibility index S was calculated, for the purpose of better evaluating the combustion performance, whose equation was defined as follow [26]: S = ((dw/dt)max × (dw/dt)mean) / (Ti2 × Tf)
(4)
Where (dw/dt)max and (dw/dt)mean referred to the maximum weight loss rate and average weight loss rate, respectively. The combustion stability index Rw could evaluate the burning stability of the sample, whose equation was showed as follow [27]:
Rw = (dw/dt)max / (Ti × Tf)
(5)
In general, the higher the index S and Rw, the superior the combustion characteristics of the sample [7].
3. Results and discussion
3.1. Hydrochar yield
After HTC, the yield of hydrochar were 47.9%, 48.0%, 33.1% and 28.8% at 180 ºC, 210 ºC, 240 ºC and 270 ºC, respectively. A downtrend in hydrochar yield by elevating reaction temperature might be attributed to the more intense thermal decomposition of hydrochar via hydrolysis, dehydration and decarboxylation under higher temperature [20]. Specifically, the viscosity of water dropped with increased temperature [28, 29], facilitating the contact between solvent and biomass and thus accelerating the degradation of biomass. For different residence time, the hydrochar yield were 53.7%, 51.6%, 48.0% and 42.9% at 10 min, 30 min, 60 min and 90 min, respectively. It could be concluded that the influence of residence time on hydrochar yield was not as significant as that of reaction temperature. This phenomenon might be explained as the slow heating rate leading to excessive time to reach the required reaction temperature. The decomposition reaction of long-chain molecules and the repolymerization reaction of small organic molecules have already reached dynamic equilibrium during the heating process. Thus, after the heating process was completed, the effect of prolonged residence time on hydrochar yield was very slight.
As for different pH, the hydrochar yield were 40.3%, 44.6%, 48.0%, 44.3% and 41.6% at pH of 3, 5 7, 9 and 11, respectively. It could be observed that the hydrochar yield decreased in acidic and alkaline environment. This phenomenon might be attributed to the fact that H+ and OH- were conductive to ameliorating the miscibility between small organic compounds and water solvents, thus contributing to the dissolution of small organic compounds from the biowaste.
3.2. Characteristics of hydrochar
3.2.1. Proximate analysis Results relevant to the proximate analysis were summarized in Table 1. A stable reduction of the volatile matter (VM) in hydrochar was observed with increasing reaction temperature and residence time, dropping from 67.26% and 64.97% to 45.11% and 58.33%, respectively; on the contrary, that of fixed carbon (FC) was accumulated steadily from 19.52% and 19.54% to 35.11% and 25.11%, respectively. This phenomenon reflected that dehydration, decarboxylation as well as carbonization reactions occurred during HTC treatment obviously lead to the decomposition of VM and the enrichment of FC in biomass. Both Zhuang et.al [30] and Lin et.al [31] believed that the released VM had two destinations, part of it was converted into gaseous phase, while the remaining was dissolved into aqueous phase and further generated FC via repolymerization reaction. Likewise, the reduction of VM and enrichment of FC in hydrochar was observed with the addition of acid and alkali catalysts. According to relevant reports[32, 33], acid catalysts (organic acid) could
catalyze the degradation of cellulose, while alkali catalysts could destroy the crystal structure of cellulose and accelerate its breaking and cracking, thus contributing to the more intense devolatilization of the biowaste. Besides, another noteworthy finding was the obvious reduction of ash content in hydrochar compared with WH (19.76%) (except for 270-60-7). Specifically, an increment of ash content in hydrochar was clearly observed as a function of increased reaction temperature, jumping up from 13.22% to 19.78%; on the contrary, the ash content got dropped with the addition of acid and alkali catalysts. The ash was the main factor responsible for fouling, slagging and agglomeration when burning in boilers [7]. Consequently, ash removal was the indispensable step to upgrade the biowaste into alternative renewable fuel. Meanwhile, compared with WH, the HHV of hydrochar jumped up to 17.62 MJ/kg-20.93MJ/kg, which might be ascribed to the fracture of low-energy chemical bands and the formation of high-energy ones during HTC process. Moreover, it could be observed that an obvious upgradation in HHV of hydrochar was achieved with increasing reaction temperature and residence time. This phenomenon could be explained as cellulose and hemicellulose in feedstocks decomposed gradually as HTC progressed, thus leaving products with higher lignin content. Specifically, lignin, cellulose and hemicellulose began to decompose at 265 ºC, 220 ºC and 180 ºC, respectively. Thereby, lignin was superior in thermal stability compared with cellulose and hemicellulose [29, 34, 35]. Moreover, lignin (26 MJ/kg) possessed higher HHV than cellulose (16.5 MJ/kg) and hemicellulose (13.9 MJ/kg) [36]. For these reasons, higher lignin content in hydrochar definitely contributed to the elevation of HHV. The
HHV of hydrochar was also improved in acidic and alkaline environment. It could be speculated that H+ or OH- would strengthen the catalytic effect of water and thus create a highly reactive solvent environment within HTC process. At this time, the characteristics of water were similar to those of polar organic solvents, which could promote the dissolution of small organic molecules from biomass [37]. Therefore, dehydration and decarboxylation of cellulose and hemicellulose were accelerated, leading to the higher HHV. Furthermore, as reaction temperature and residence time elevated, the energy densification of hydrochar steadily climbed from 1.24 and 1.20 to 1.38 and 1.39, respectively. The acid and alkali catalysts also benefited in the upgradation of energy densification. It was visible that the evolution of energy densification coincided exactly with the results of HHV. Besides, the energetic recovery efficiency climbed slightly from 59.29% to 63.30% with the severity of reaction temperature from 180 ºC to 210 ºC, followed by a sharp decline to 39.77% as temperature further increased to 270 ºC. Likewise, as residence time elevated, the energetic recovery efficiency was slightly fluctuated, reaching the minimal value of 59.65% at 90 min. The acid and alkali catalysts could also lead to the inferior energetic recovery efficiency. It could be speculated that with increased HTC severity, the significant reduction in hydrochar yield was responsible for the loss of energetic recovery efficiency. To our knowledge, the over loss of energetic recovery efficiency could be an obstacle for energy upgrading. Therefore, it was particularly important to select suitable reaction conditions.
Table 1. Proximate analysis, HHV, energy densification and energetic recovery efficiency of WH and hydrochar on dry basis. Energy Proximate analysis/wt.% HHV/(MJ/kg) densification VM FCa Ash WH 68.42 11.82 19.76 14.68 180-60-7 67.26 19.52 13.22 18.17 1.24 210-60-7 58.64 24.03 17.33 19.36 1.32 240-60-7 53.23 27.81 18.96 19.72 1.34 270-60-7 45.11 35.11 19.78 20.27 1.38 210-10-7 64.97 19.54 15.49 17.62 1.20 210-30-7 63.66 21.85 14.49 18.81 1.28 210-90-7 58.33 25.11 16.56 20.41 1.39 210-60-3 55.12 28.81 16.07 20.69 1.41 210-60-5 56.28 27.11 16.61 19.52 1.33 210-60-9 56.14 27.54 16.32 20.01 1.36 210-60-11 55.46 29.25 15.29 20.93 1.43 a By difference: Fixed carbon% = 100% − volatile matter% − ash%. Sample
Energetic recovery efficiency (%) -
59.29 63.30 44.46 39.77 64.45 66.12 59.65 56.80 59.30 60.38 59.31
3.2.2. Elemental analysis Elemental compositions of WH and hydrochar were illustrated in Table 2. With the elevation of reaction temperature and residence time, the carbon (C) content in hydrochar got improved from 45.90% and 44.87% to 51.24% and 47.21%, respectively; on the contrary, that of hydrogen (H) and oxygen (O) decreased as HTC progressed. In fact, the accumulation of C content was owing to carbonization, whereas the removal of H and O contents were attributed to continuous strengthening dehydration and decarboxylation during HTC process [28]. The hydrogenation and oxidation components in biomass were decomposed, cracked, and eventually evolved out as gases such as H2, CH4, CO2 and H2O [33]. In addition, an increase in C content of hydrochar was achieved, while the O content decreased with the addition of
catalysts. The reason responsible for this variation was corresponded exactly to that of HHV change after adding catalysts. H+ or OH- would facilitate the dissolution of small organic molecules from biomass to solvent, thus accelerating the carbonization, dehydration and decarboxylation during HTC process. Compared with WH, the removal of nitrogen (N) content in hydrochar was also identified (except for 240-60-7 and 270-60-7). Excessive content of N might lead to higher emissions of contaminative gases such as NO and NO2 when hydrochar was adopted as the solid fuel. Consequently, HTC could be indeed considered as an available mean to generate clean biofuel.
Table 2. Elemental analysis of WH and hydrochar on dry basis. Elemental analysis/wt.% C H Oa N S WH 36.65 5.86 34.20 3.53 0.250 180-60-7 45.90 5.84 32.03 3.02 0.028 210-60-7 47.21 5.50 26.69 3.35 0.021 240-60-7 50.31 5.00 21.66 4.07 0.020 270-60-7 51.24 4.71 20.15 3.97 0.030 210-10-7 44.87 5.55 30.86 3.18 0.032 210-30-7 47.21 5.68 29.72 2.90 0.028 210-90-7 46.57 5.38 28.33 3.09 0.031 210-60-3 49.92 5.65 25.35 3.02 0.023 210-60-5 48.70 5.48 25.82 3.40 0.019 210-60-9 48.53 5.58 26.58 3.00 0.024 210-60-11 49.99 5.53 26.32 2.88 0.027 a By difference: O% = 100% − C% − H% − N% − ash%. Sample
H/C 1.919 1.527 1.399 1.191 1.104 1.484 1.444 1.387 1.357 1.349 1.379 1.327
Atomic ratio O/C 0.700 0.523 0.424 0.323 0.295 0.516 0.472 0.456 0.381 0.398 0.411 0.395
N/C 0.083 0.056 0.061 0.069 0.066 0.061 0.053 0.057 0.052 0.060 0.053 0.049
The variation in H/C and O/C atomic ratios of hydrochar under different reaction conditions were visualized in the Van Krevelen diagram (Fig. 1(a)). Besides, the positions of several kinds of typical coals, cellulose, hemicellulose and lignin were
also marked in Fig. 1(a) for comparison [25]. As could be observed from the diagram, the position of WH was closer to that of cellulose and hemicellulose regions. As reaction temperature and residence time elevated, a steady reduction of H/C and O/C atomic ratios in hydrochar was observed and the position was closer to lignin and lignite regions, in turn, upgrading the HHV and energy densification. A noteworthy finding was that from 210 ºC to 240 ºC, the H/C and O/C atomic ratios had a large mutation from 1.399 and 0.424 to 1.191 and 0.323, respectively. This represented that the hydrolysis of cellulose and hemicellulose in WH was most severe at 240 ºC. In addition, the reduction in H/C and O/C atomic ratios of hydrochar under acidic and alkaline environment also confirmed that H+ and OH- could promote the dehydration and decarboxylation of biowastes. Moreover, the relationship between N/C and O/C atomic ratios under different reaction conditions was illustrated in Fig. 1(b). It could be clearly observed that the evolution of N/C and O/C atomic ratios from WH to hydrochar followed the paths of reduction and denitrogenation. Notably, it was found that the 210-60-3 and 210-60-11 possessed the relatively lower N/C atomic ratio of 0.052 and 0.049 among all the hydrochar, representing that acid and alkali catalysts could obviously accelerate the denitrogenation degree of the samples.
Fig. 1. The H/C, N/C and O/C ratios of WH and hydrochar.
3.2.3. SEM analysis of hydrochar SEM micrographs could intuitively reveal the substantial change in inherent properties during the conversion from WH to hydrochar. As could be observed, HTC could significantly affect the surface morphology and physical structure of solid products. The untreated WH had compact structure, smooth surface and clear texture, whereas hydrochar had loose structure, rough surface and blurred texture. Besides, many small molecular fragments could be observed in hydrochar. As already mentioned above, lignin had relatively high chemical stability and was difficult to degrade until the temperature exceeded 265 ºC. Therefore, it could be inferred that these rough fragments might be the original skeleton of non-discomposed lignin. Specifically, with the elevation of reaction temperature and residence time, small molecular fragments in hydrochar increased, which could be attributed to the enhanced decomposition reaction. Besides, the surface of hydrochar became obviously rougher as HTC progressed, which could be explained by the release of volatile matter. The catalysts strengthened the destruction degree of surface morphology. Therefore, it could be speculated that H+ and OH- could accelerate the decomposition of biomass by promoting the dissolution of small organic molecules in water solvent.
3.2.4. FT-IR analysis of hydrochar
The FT-IR analysis of WH and hydrochar could reflect the evolution of functional groups in samples under different reaction conditions. The absorbance peaks from 3700 cm-1 to 3200 cm-1 represented the stretching vibration of hydroxyl (─OH). The vibration intensity experienced downtrend as reaction temperature increased, indicating strengthened dehydration of feedstock. The absorbance peaks between 2935 cm-1 and 2915 cm-1 were attributed to the antisymmetric stretching vibration of aliphatic methylene (─CH2). The absorbance intensity was significantly enhanced at 270 ºC, which might be attributed to the high temperature promoting the fracture of aliphatic side chains, thereby resulting in the formation of methylene groups. The absorption bands from 1770 cm-1 to 1650 cm-1 were associated to stretching vibration of carbonyl (─C=O). Its intensity was weakened by elevating reaction temperature, indicating enhanced decarboxylation reaction as HTC progressed. Moreover, the decrease of hydroxyl and carboxyl groups in hydrochar could effectively improve its hydrophobicity [38]. The absorption bands from 1380 cm-1 to 1310 cm-1 and from 1214 cm-1 to 1030 cm-1 were mainly ascribed to the vibration of methoxy group (R─O─CH3) and ether group (C─O─C), respectively. Similarly, the vibration intensity was weakened with the severity of temperature, indicating that high temperature might accelerate the deoxygenation reaction of the biowaste during HTC process. The effect of prolonged residence time on the peak intensity of hydrochar had similarity in that of elevated HTC temperature, but to a lesser extent, indicating that temperature had a more prominent impact on HTC.
As for different pH, it could be seen that the absorption bands from 900 cm-1 to 650 cm-1 corresponding to the substituted ─CH in benzene ring were obviously enhanced in acidic and alkaline environment, which might be owing to the fact that H+ and OH- could promote the aromatization process of hydrochar during HTC process.
3.2.5. Combustion behavior and thermal characteristics The WH and hydrochar were combusted in thermogravimetric analyzer within 50-800 ºC under air atmosphere. The TG and DTG curves at different reaction conditions were represented in Fig. 2. It was visible that all hydrochar suffered from two steep weight loss peaks during combustion process. In general, the combustion of hydrochar was a complicated process involving multiple chemical reactions that affected mutually. Thus, for better evaluating the combustion characteristics of hydrochar, the process could be divided into two consecutive stages based on two weight loss peaks. The stage 1 was devolatilization in the range of 260-340 ºC, while the stage 2 was decarbonization within 350-450 ºC, corresponding to oxidation processes of volatile matter and fixed carbon, respectively. Moreover, the detailed information of combustion parameters was summarized in Table 3. After HTC, the ignition temperature (Ti) and burnout temperature (Tf) of hydrochar increased to different degrees compared with WH (except for 270-60-7). It could be deduced that the combustion of hydrochar was transferred to a higher temperature region after HTC treatment. Specifically, the Ti dropped from 285.9 ºC to 277.6 ºC by elevating reaction temperature from 180 ºC to 210 ºC, followed by a promotion from 277.6 ºC to 311.5
ºC with temperature further increasing from 210 ºC to 270 ºC. The explanation below was cited for this phenomenon: It was well established that the ignition temperature of hydrochar was mainly ascertained by porosity and organic constituents (mainly volatile matter) [39]. From perspective by Kambo and Dutta [36], in the range of 180-210 ºC, albeit the VM of hydrochar decreased, the creation of pores dominated, thereby enlarging specific surface area, making oxygen react with organic matters more sufficiently [19]. Consequently, the samples could be ignited more easily. However, after 230 ºC, the pores were deteriorated and blocked. Meanwhile, the non-volatile organic compounds were accumulated and thus hydrochar was more difficult to ignite. Besides, the Tf for hydrochar was elevated in acidic and alkaline environment. Two reasons might be responsible for this behavior: (1) Catalysts could accelerate the formation of ordered and condensed structure in hydrochar via cross-linking as well as repolymerization of biomass during HTC. (2) FT-IR analysis indicated that H+ and OH- could promote the aromatization reaction during HTC process, thus generating more aromatic compounds with benzene ring structure. The above changes enhanced the thermal stability of hydrochar, thereby eliminating active sites and elevating burnout temperature. Furthermore, the first weight loss rate (DTG1) maintained a gradual downtrend in response to the elevation of reaction temperature and residence time or the addition of catalysts, resulting from the more severe hydrolysis of VM in hydrochar. The second weight loss rate (DTG2) demonstrated a downward trend with increased HTC temperature from 180 ºC to 210 ºC. This might be attributed the enhancement of aromatization and repolymerization within HTC
process to form chemical substances with stable benzene ring structure and low reaction activity, thus dropping the combustion intensity in stage 2. Contrarily, the uptrend in DTG2 was observed at over 210 ºC, which could be explained by the excessive decomposition of aromatic compounds at higher HTC temperature. After HTC, the combustibility index S of hydrochar was greatly improved, confirming that HTC was conductive to ameliorating the combustion performance of hydrochar. However, a reduction in the combustibility index S of hydrochar was clearly observed as a function of increased HTC severity. Downtrend in here was closely followed the variation of VM. To our knowledge, VM could improve the flame burning and reaction activity. Thus, as HTC severity increased, the VM decreased sharply and the S was correspondingly declined. Compared with WH, an obvious increase in the combustion stability index Rw of hydrochar was also achieved, representing better flame stability of hydrochar. In addition, the evolution of Rw with increased HTC severity showed great similarity with that of S. After evaluating the S and Rw comprehensively, it could be concluded that the combustion characteristics of hydrochar were obviously superior to that of WH. Besides, it could be observed that the reaction condition of 180-60-7 achieved the highest index S value of 25.81 (10-7×min-2׺C-3) and index Rw value of -28.43 (10-5×min-1׺C-2) and thus possessed the optimal combustion characteristics.
Fig. 2. TG and DTG of WH and hydrochar obtained at: (a) different reaction temperature; (b) different residence time; (c) different pH.
Table 3. The combustion characteristics parameters of WH and hydrochar. S RW Tia Tfb Mfc T1d DTG1e T2d DTG2e T3d DTG3e DTGmeanf -7 -2 -3 -5 (10 ×min ׺C ) (10 ×min-1׺C-2) (ºC) (ºC) (%) (ºC) (%/min) (ºC) (%/min) (ºC) (%/min) (%/min) WH 262.2 663.3 14.91 286.3 -11.88 437.5 -11.04 525.9 -4.26 -2.36 6.14 -6.83 180-60-7 285.9 672.5 11.09 295.0 -54.67 375.2 -24.49 -2.60 25.81 -28.43 210-60-7 277.6 665.1 11.36 288.7 -41.51 360.2 -17.61 -2.61 21.10 -22.48 240-60-7 288.5 665.8 19.37 283.2 -19.58 366.9 -20.11 -2.27 8.02 -10.47 270-60-7 311.5 659.6 26.85 277.7 -19.56 356.4 -24.71 -2.12 6.46 -9.52 210-10-7 284.3 678.8 14.13 296.5 -34.13 371.5 -18.85 -2.50 15.53 -17.69 210-30-7 288.8 688.4 13.43 300.4 -29.71 390.3 -16.80 -2.43 12.57 -14.94 210-90-7 277.0 668.5 16.41 290.4 -25.24 369.5 -15.74 -2.36 11.59 -13.63 210-60-3 282.9 672.2 12.55 296.5 -27.62 381.6 -18.64 -2.49 12.78 -14.52 13.28 293.5 -35.97 380.5 -21.43 -2.48 16.47 -18.93 210-60-5 284.9 667.1 210-60-9 279.9 669.6 15.56 289.1 -31.26 371.6 -19.49 -2.42 14.40 -16.68 290.1 672.6 14.95 303.2 -25.75 383.4 -19.27 -2.42 11.01 -13.20 210-60-11 a Ti, the ignition temperature. b Tf, the burnout temperature. c Mf, the residual mass. d T1, T2, T3, the temperature according to the first loss peak, the second loss peak and the third loss peak. e DTG1, DTG2, DTG3, the weight loss rate according to the first loss peak, the second loss peak and the third loss peak. f DTGmean, the average weight loss rate
Sample
3.2.6 TG-FITR analysis of gaseous products during combustion process Chinese emission standard of air pollutants for boilers (GB 13271-2014) clearly stipulated the maximum allowable emission concentration of sulfur dioxide, nitrogen oxides and other polluting gases in the coal-fired boilers. For this reason, the TG-FIRT analysis of emission gaseous products was necessary during the combustion of WH and hydrochar, not only for the upgradation of solid products, but also for the application of HTC in industrial scale. The emission profiles of several typical gases were visualized in Fig. 3(a-e). According to research by Chen et al. [40], the absorption peaks at 2200-2400cm-1 and 669cm-1 indicated the generation of CO2. The absorption bands at 2060-2240cm-1 suggested the emission of CO. The absorption peaks at 1300-1400 cm-1 were attributed to the release of SO2. The absorption peaks at 2965 cm-1 and 1762 cm-1 represented the existence of NO2 and NO, respectively. Based on Fig. 3(a), it could be concluded that CO2 was the prime component in gaseous products, whose emission concentration occupied the maximum level. The release of CO2 from the combustion of hydrochar mainly occurred at 260-330 ºC and 350-400 ºC, which respectively corresponded to the two combustion stages in thermogravimetric analysis. The emission concentration of CO2 in stage 2 was significantly higher than that in stage 1. This phenomenon could be ascribed to the fact that fixed carbon, consisting of covalent cross-linking and aromatic compounds, mainly combusted in stage 2. These compounds had relatively high carbon content compared with that of volatile matter combusted in stage 1 and thus the emission concentration of CO2 was maintained at higher fraction in stage 2.
As shown in Fig. 3(b), it was visible that the emission of CO from hydrochar was mainly concentrated in the ranges of 280-340 ºC and 350-410 ºC, corresponding to incomplete combustion of VM and FC, respectively. Besides, it could be concluded that the CO emission of hydrochar was lower than that of WH. This might be due to the dehydration and decarboxylation of long-chain macromolecules within HTC stage to form small molecules, such as furan, acid, aldehyde, ketone, alcohol and phenol [7], which could be burnt out easily. Fig. 3(c) illustrated the emission of SO2 from the combustion of WH and hydrochar. It could be observed that the emission concentration of SO2 showed relative low level, mainly occurring in the range of 300-350 ºC. In opinion of Lin et al. [41], the emission of SO2 from 250 ºC to 350 ºC was mainly derived from the combustion of organic sulfur such as aliphatic-S, aromatic-S and sulfoxide-S. Compared with prolonged residence time, the increased HTC temperature could reduce the emission concentration of SO2 more effectively, which might be ascribed to the more severe decomposition of organic sulfur components at higher HTC temperature. Previous study [42] also proved that high reaction temperature was conducive to promoting the conversion of organic sulfur in coke to inorganic salt sulfur in ash. Moreover, another noteworthy finding was that alkali catalyst could greatly reduce the emission concentration of SO2, which might be attributed to the great sulfur retention characteristics of alkali metal salts (sodium salts) during combustion [43].
Fig. 3(d-e) presented the emission curves of NOX (NO and NO2) for WH and hydrochar. It was visible that the emission curves of NOX from hydrochar demonstrated one peak within 280-310 ºC. Thus, it could be inferred that the emission of NOX could be attributed to the oxidative combustion of high nitrogen-containing volatile matter. The NO emission of hydrochar was obviously lower than that of WH. Lin et al. [41] believed that NO could be reduced on the surface of char to generate → CO + N2), thereby resulting in low CO and N2 during combustion (NO + C
emission of NO. Similar to SO2, the alkali catalyst could effectively inhibit the emission of NOX. This might be attributed to the fact that alkali metal salts (sodium salts) could significantly catalyze the reduction of NOX by NH3 (NOX + NH3 → N2 + H2O) during combustion [44]. In summary, CO2 was the primary gaseous product in the combustion process of the samples. The alkali catalyst could effectively reduce the emission concentration of SO2 and NOX, in turn, ameliorating the emission properties of hydrochar and minimizing the environmental hazard.
Fig. 3. Gaseous emissions during combustion of WH and hydrochar. (a) CO2, (b) CO, (c) SO2, (d) NO, (e) NO2.
4. Conclusions
In summary, the increased reaction temperature, prolonged residence time as well as acid and alkali catalysts could promote HTC intensity. In comparison to the downtrend of hydrochar yield (from 53.7% to 28.8%) and energetic recovery efficiency (from 66.12% to 39.77%), the HHV (from 17.62 MJ/kg to 20.93 MJ/kg) and energy densification (from 1.20 to 1.43) of hydrochar followed a reverse trend as HTC progressed. Besides, compared with WH, the fixed carbon and C content in hydrochar were enriched to 19.52%-35.11% and 44.87%-51.24%, respectively, while the volatile matter, H content and O content declined to 45.11%-67.26%, 4.71%-5.84% and 20.15%-32.03%, respectively. Thermogravimetric analysis showed that 180-60-7 achieved the optimal combustion performance, with an index S value of 25.81 (10-7×min-2׺C-3) and an index Rw value of -28.43 (10-5×min-1׺C-2). The alkali catalyst could greatly reduce the emission of typical pollutants (SO2 and NOX) during the combustion of hydrochar, thereby ameliorating the emission properties.
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (51476060); Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization (2013A061401005); Key Laboratory of Efficient and Clean Energy Utilization of Guangdong Higher Education Institutes (KLB10004).
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Highlights
Hydrothermal carbonization of water hyacinth was studied.
The increased reaction temperature, prolonged residence time as well as acid and alkali catalysts could promote HTC intensity.
The HHV and energy densification of hydrochar followed an uptrend as HTC progressed.
The reaction condition of 180-60-7 achieved the optimum combustion performance.
The alkali catalyst could reduce the emission of typical pollutants (SO2 and NOX) during the combustion of hydrochar.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S0360-5442(20)30300-5
DOI:
https://doi.org/10.1016/j.energy.2020.117193
Reference:
EGY 117193
To appear in:
Energy
Received Date: 5 August 2019 Revised Date:
10 November 2019
Accepted Date: 17 February 2020
Please cite this article as: Zhang C, Ma X, Chen X, Tian Y, Zhou Y, Lu X, Huang T, Conversion of water hyacinth to value-added fuel via hydrothermal carbonization, Energy (2020), doi: https://doi.org/10.1016/ j.energy.2020.117193. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Conversion of water hyacinth to value-added fuel via hydrothermal carbonization Chaoyue Zhang, Xiaoqian Ma*, Xinfei Chen, Yunlong Tian, Yi Zhou, Xiaoluan Lu Tao Huang Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization, School of Electric Power, South China University of Technology, Guangzhou 510640, China Postal address: School of Electric Power, South China University of Technology, No. 381, Wushan Road, Tianhe District, Guangzhou, 510640, China *Corresponding author Tel.: +86 20 87110232; fax: +86 20 87110613 E-mail address: [email protected]
Abstract: Hydrothermal carbonization (HTC) of water hyacinth (WH) was investigated to elucidate the effects of reaction temperature, residence time and pH (acid and alkali catalysts) on the chemical properties, combustion behavior and emission properties of hydrochar. Results found that high reaction temperature, long residence time and catalysts were beneficial to ameliorate the fuel properties of hydrochar in terms of calorific value and energy densification, albeit the yield and energetic recovery efficiency got deteriorated. The lower H/C, O/C and N/C ratios of hydrochar reflected more severe dehydration, decarboxylation and denitrogenation reactions within HTC process. SEM images represented that HTC could lead to the fragmentized structure of hydrochar. As HTC progressed, the vibration of hydroxyl and carboxyl groups in hydrochar weakened, which was conductive to improving the
hydrophobicity of hydrochar. The combustion characteristics of hydrochar got remarkable upgraded after HTC, whose combustibility index S and combustion stability index Rw were both superior to that of WH. The hydrochar obtained from the addition of alkali catalyst (NaOH) possessed lower emission concentration of SO2 and NOX during combustion, thus demonstrating better emission properties. Overall, HTC was a feasible way to bridge the gap from WH to alternative renewable fuel.
Graphical abstract
Keywords: Hydrothermal carbonization; Water hyacinth; Hydrochar; Acid and alkali
catalysts; Combustion and emission characteristics
1. Introduction
With the acceleration of urbanization and industrialization process in China, a large amount domestic sewage and industrial wastewater are discharged into water bodies without proper purification. The wastewater is rich in nitrogen, phosphorous, potassium and organic pollutants [1], thereby leading to eutrophication of water bodies. Water hyacinth (WH), known as a monocotyledonous freshwater aquatic plant with flourishing roots, can reproduce abundantly in eutrophic water bodies and thus has become one of the primary invasive aquatic plants in China [2, 3]. The massive growth of WH leads to the loss of fishery and tourism, the hindrance to water transport as well as the blockage of irrigation channels. The rotten remains of WH deteriorates water quality, making water unfit for drinking by humans, livestock and wildlife [4]. Moreover, WH areas provide a large number of breeding places for vector (mosquito), causing the outbreak of epidemic diseases [5]. Meanwhile, on account of high moisture and heavy metal in WH, traditional disposal methods, such as incineration, landfill and fertilization, face technological difficulties, resulting in emission of contaminative gases and runoff of heavy metal into aqueous or edaphic systems. As a consequence, how to effectively deal with the remains of WH has become a major issue in urban environmental management. According to relevant report [6], WH is mainly composed of lignin, crystalline cellulose and hemicellulose polymer, indicating that it can be regarded as a potential
biomass resource for energy utilization. UP to now, the effective conversion methods of biowastes mainly include thermochemical and biological conversions. Among two conversion technologies, thermochemical conversion has attracted extensive attention on account of its advantages in terms of energy production, including low cost, short treatment process and convenient product recovery [7]. Pyrolysis and hydrothermal carbonization (HTC) are common methods in thermochemical treatment. Hu et al. [8] mainly researched the conversion effects of catalytic and non-catalytic pyrolysis of WH with the purpose of energy upgradation. It was pointed out that traditional pyrolysis would induce several technological problems such as particle agglomeration and heated surface deposition, further leading to high inefficiencies of pyrolysis [9]. Moreover, the drying pretreatment before pyrolysis would also increase conversion cost. Compared with pyrolysis, HTC has its unique advantages in terms of biomass utilization: (1) HTC has relatively mild reaction temperature (180-350 ºC), autogenous pressure (2-10 MPa) and low carbon dioxide emissions [10, 11]. Hydrolysis of biomass under subcritical condition can effectively solve the problem of agglomeration. (2) HTC is one such technology that develops as an ideal pretreatment method for biomass with high moisture and high organic content, thus saving the pre-drying cost of raw materials. (3) HTC can minimize the environmental hazard caused by biowastes, because of the removal of undeserved nutrients, the stabilization of heavy metal, and the elimination of malignant bacteria during hydrothermal process [12]. (4) Water is used as not only solvent but also catalyst to improve the utilization rate of biomass. During HTC process, the dielectric constant
of water decreases [13], thus improving the solubility of organic compounds. Meanwhile, hydrogen bonds in water decrease, while isothermal compression rate of water increases, which can promote the reactions of ions, polar non-ions and free radicals [14, 15]. In addition, under the subcritical environment, the ionization of water strengthens with the increase of temperature and pressure, leading to high concentration of H+ and OH-, thus forming a catalytic environment to promote the hydrolysis reaction [16, 17]. (5) HTC is able to convert biowastes into carbon-rich micro-nano particles (i.e. hydrochar) by simulating natural coalification process. Hydrochar has high energy content, superior grindability and hydrophobicity [18, 19], which can settle the transportation and storage problems induced by biowastes and thus benefits in the subsequent application aimed at energy production [20, 21]. (6) According to the techno-economic assessment of HTC, because of the huge output and low price of biowastes, treating biowastes via HTC can achieve lower breakeven cost [22, 23], which is conductive to the large-scale promotion of HTC technology. In summary, HTC has tremendous potential to bridge the gap from WH to value-added solid fuel. To our knowledge, Gao et al. [9] have reported the effect of residence time on the surface properties of hydrochar obtained via HTC of WH. As expect, a significant uptrend in higher heating value of hydrochar was observed by prolonging residence time. However, except for residence time, the variations in reaction temperature and catalysts also deserve great consideration, since they may affect the characteristics of solid product in different aspects and degrees. Besides, most of available reports
usually focused on surface properties and combustion properties of hydrochar, while the influence of various reaction conditions on emission properties of hydrochar obtained from HTC of WH was unfortunately lacking. Nowadays, most countries have clearly stipulated the emission standards of air pollutants from coal-fired boilers, and thus the exploration of hydrochar emission properties is necessary for the application of HTC in industrial scale. Consequently, on the basis of verifying the previous experimental results, this paper extends its research and systematically analyzes the effects of reaction temperature, residence time and pH (acid and alkali catalysts) on hydrochar characteristics (including chemical properties, surface properties,
combustion properties
and
emission properties).
The chemical
compositions of hydrochar were investigated through proximate and elemental analysis. The surface morphology, functional group properties and combustion characteristics
of hydrochar were deeply explored
by SEM,
FTIR
and
thermogravimetric analysis. Furthermore, the emission profiles of several typical gases during the combustion of hydrochar were detected through TG-FITR experiments. The results obtained in this paper are beneficial to further explore the feasibility of treating water hyacinth through hydrothermal carbonization and provide referential opinions for designing, optimizing and scaling up the comprehensive utilization of water hyacinth.
2. Materials and methods
2.1. Materials
In this work, the raw material was fresh WH salvaged from the Taihu Lake Basin in Jiangsu Province. Prior to the experiment, the collected WH was cleaned by deionized water, and the non-combustible materials, such as gravel and sand, were screened out. The remaining leaves, trunks and roots were dried, pulverized and sieved, in order to harvest target samples with uniform particle size in the range of 0-178µm, and then stored in a drying oven at 105 ºC for 24 hours.
2.2. Hydrothermal treatment
The HTC experiments of WH were carried out in a 250 ml autoclave reactor (Model SLM250, Beijing Shi Ji Sen Lang Experimental Instrument Co., Ltd., Beijing, China). To analyze the effects of reaction temperature and residence time on HTC, four reaction temperature (180, 210, 240 and 270 ºC) and four residence time (10, 30, 60 and 90 min) were adopted. Besides, in order to investigate the effects of acid and alkali catalysts on HTC, CH3COOH and NaOH were used to adjust the pH of deionized water to 3, 5, 7, 9 and 11. Prior to each experiment, 8 g of oven-dried raw material was blended with 80 ml of deionized water in a reaction vessel and manually stirred for 10 min to achieve a slurry densification of 10% w/w. After that, the reactor was sealed and purged with high purity nitrogen for 3 minutes to create an oxygen-starved atmosphere. Subsequently, the reactor was heated up to the pre-set reaction temperature by an external electric heating device and maintained for the desired residence time, while the heating rate and stirring speed were at 5 ºC/min and 500 rpm, respectively. Once the reaction elapsed, the autoclave reactor was removed
from furnace and quenched to ambient temperature by cooling water. The pressure relief valve was then opened, and the gas product quickly released. The slurry inside reactor was filtered to obtain hydrochar. Subsequently, the hydrochar was dried at 105 ºC for 24 hours and pulverized into a particle diameter of less than 178µm for later analysis. All the experiments were performed three times to ensure repeatability and accuracy. Notably, the hydrochar was denoted as “reaction temperature-residence time-pH”. For example, the hydrochar obtained at 210 ºC for 60min and pH of 7 was labeled as “210-60-7”.
2.3. Analytical approach
The elemental and proximate analysis of WH and hydrochar were investigated by an elemental analyzer (Vario EL cube, Germany) based on ASTM D5373 standard and a muffle furnace (MXX1100-30, shmicrox Co., Ltd., China) based on China GB/T28731-2012 Standard, respectively. The higher heating value (HHV) of solid samples were determined by the 5E-KC5410 rapid calorimeter. Additionally, in order to better assess the HTC process, the yield, energy densification and energetic recovery efficiency of products were calculated by the following formulas [24]: Hydrochar yield = (Mass of hydrochar / Mass of raw material) × 100%
(1)
Energy densification = HHV of hydrochar / HHV of raw material
(2)
Energetic recovery efficiency = Hydrochar yield × Energy densification × 100% (3)
The surface morphology of the samples was characterized by scanning electron microscope (SEM, LEO-1530). The variation of functional groups was analyzed by Fourier transform infrared spectrometer (Nicolet™ iS™ 10 FT–IR spectrometer).
2.4. TG-FTIR method
The combustion and emission characteristics of the samples were characterized by a thermal gravimetric analyzer (TGA, METTLER TOLEDO) in combination with a FT-IR spectrometer (Nicolet™ iS™ 10 FT–IR spectrometer). In each test, the initial weight of the sample was kept at 8±0.5 mg. The air was continuously supplied with a fixed rate of 100 mL/min throughout the entire combustion process. Under air atmosphere, the experimental temperature was heated from 50 ºC to 800 ºC with a rate of 30 ºC/min. As gaseous products, generated from the combustion of the sample, entered the gas chamber through a transmission line, FTIR spectra from 400 cm-1 to 4000 cm-1 were detected for further analysis. The ignition temperature (Ti) and burnout temperature (Tf) of the sample could be determined by methods in Ref. [25]. Besides, the combustibility index S was calculated, for the purpose of better evaluating the combustion performance, whose equation was defined as follow [26]: S = ((dw/dt)max × (dw/dt)mean) / (Ti2 × Tf)
(4)
Where (dw/dt)max and (dw/dt)mean referred to the maximum weight loss rate and average weight loss rate, respectively. The combustion stability index Rw could evaluate the burning stability of the sample, whose equation was showed as follow [27]:
Rw = (dw/dt)max / (Ti × Tf)
(5)
In general, the higher the index S and Rw, the superior the combustion characteristics of the sample [7].
3. Results and discussion
3.1. Hydrochar yield
After HTC, the yield of hydrochar were 47.9%, 48.0%, 33.1% and 28.8% at 180 ºC, 210 ºC, 240 ºC and 270 ºC, respectively. A downtrend in hydrochar yield by elevating reaction temperature might be attributed to the more intense thermal decomposition of hydrochar via hydrolysis, dehydration and decarboxylation under higher temperature [20]. Specifically, the viscosity of water dropped with increased temperature [28, 29], facilitating the contact between solvent and biomass and thus accelerating the degradation of biomass. For different residence time, the hydrochar yield were 53.7%, 51.6%, 48.0% and 42.9% at 10 min, 30 min, 60 min and 90 min, respectively. It could be concluded that the influence of residence time on hydrochar yield was not as significant as that of reaction temperature. This phenomenon might be explained as the slow heating rate leading to excessive time to reach the required reaction temperature. The decomposition reaction of long-chain molecules and the repolymerization reaction of small organic molecules have already reached dynamic equilibrium during the heating process. Thus, after the heating process was completed, the effect of prolonged residence time on hydrochar yield was very slight.
As for different pH, the hydrochar yield were 40.3%, 44.6%, 48.0%, 44.3% and 41.6% at pH of 3, 5 7, 9 and 11, respectively. It could be observed that the hydrochar yield decreased in acidic and alkaline environment. This phenomenon might be attributed to the fact that H+ and OH- were conductive to ameliorating the miscibility between small organic compounds and water solvents, thus contributing to the dissolution of small organic compounds from the biowaste.
3.2. Characteristics of hydrochar
3.2.1. Proximate analysis Results relevant to the proximate analysis were summarized in Table 1. A stable reduction of the volatile matter (VM) in hydrochar was observed with increasing reaction temperature and residence time, dropping from 67.26% and 64.97% to 45.11% and 58.33%, respectively; on the contrary, that of fixed carbon (FC) was accumulated steadily from 19.52% and 19.54% to 35.11% and 25.11%, respectively. This phenomenon reflected that dehydration, decarboxylation as well as carbonization reactions occurred during HTC treatment obviously lead to the decomposition of VM and the enrichment of FC in biomass. Both Zhuang et.al [30] and Lin et.al [31] believed that the released VM had two destinations, part of it was converted into gaseous phase, while the remaining was dissolved into aqueous phase and further generated FC via repolymerization reaction. Likewise, the reduction of VM and enrichment of FC in hydrochar was observed with the addition of acid and alkali catalysts. According to relevant reports[32, 33], acid catalysts (organic acid) could
catalyze the degradation of cellulose, while alkali catalysts could destroy the crystal structure of cellulose and accelerate its breaking and cracking, thus contributing to the more intense devolatilization of the biowaste. Besides, another noteworthy finding was the obvious reduction of ash content in hydrochar compared with WH (19.76%) (except for 270-60-7). Specifically, an increment of ash content in hydrochar was clearly observed as a function of increased reaction temperature, jumping up from 13.22% to 19.78%; on the contrary, the ash content got dropped with the addition of acid and alkali catalysts. The ash was the main factor responsible for fouling, slagging and agglomeration when burning in boilers [7]. Consequently, ash removal was the indispensable step to upgrade the biowaste into alternative renewable fuel. Meanwhile, compared with WH, the HHV of hydrochar jumped up to 17.62 MJ/kg-20.93MJ/kg, which might be ascribed to the fracture of low-energy chemical bands and the formation of high-energy ones during HTC process. Moreover, it could be observed that an obvious upgradation in HHV of hydrochar was achieved with increasing reaction temperature and residence time. This phenomenon could be explained as cellulose and hemicellulose in feedstocks decomposed gradually as HTC progressed, thus leaving products with higher lignin content. Specifically, lignin, cellulose and hemicellulose began to decompose at 265 ºC, 220 ºC and 180 ºC, respectively. Thereby, lignin was superior in thermal stability compared with cellulose and hemicellulose [29, 34, 35]. Moreover, lignin (26 MJ/kg) possessed higher HHV than cellulose (16.5 MJ/kg) and hemicellulose (13.9 MJ/kg) [36]. For these reasons, higher lignin content in hydrochar definitely contributed to the elevation of HHV. The
HHV of hydrochar was also improved in acidic and alkaline environment. It could be speculated that H+ or OH- would strengthen the catalytic effect of water and thus create a highly reactive solvent environment within HTC process. At this time, the characteristics of water were similar to those of polar organic solvents, which could promote the dissolution of small organic molecules from biomass [37]. Therefore, dehydration and decarboxylation of cellulose and hemicellulose were accelerated, leading to the higher HHV. Furthermore, as reaction temperature and residence time elevated, the energy densification of hydrochar steadily climbed from 1.24 and 1.20 to 1.38 and 1.39, respectively. The acid and alkali catalysts also benefited in the upgradation of energy densification. It was visible that the evolution of energy densification coincided exactly with the results of HHV. Besides, the energetic recovery efficiency climbed slightly from 59.29% to 63.30% with the severity of reaction temperature from 180 ºC to 210 ºC, followed by a sharp decline to 39.77% as temperature further increased to 270 ºC. Likewise, as residence time elevated, the energetic recovery efficiency was slightly fluctuated, reaching the minimal value of 59.65% at 90 min. The acid and alkali catalysts could also lead to the inferior energetic recovery efficiency. It could be speculated that with increased HTC severity, the significant reduction in hydrochar yield was responsible for the loss of energetic recovery efficiency. To our knowledge, the over loss of energetic recovery efficiency could be an obstacle for energy upgrading. Therefore, it was particularly important to select suitable reaction conditions.
Table 1. Proximate analysis, HHV, energy densification and energetic recovery efficiency of WH and hydrochar on dry basis. Energy Proximate analysis/wt.% HHV/(MJ/kg) densification VM FCa Ash WH 68.42 11.82 19.76 14.68 180-60-7 67.26 19.52 13.22 18.17 1.24 210-60-7 58.64 24.03 17.33 19.36 1.32 240-60-7 53.23 27.81 18.96 19.72 1.34 270-60-7 45.11 35.11 19.78 20.27 1.38 210-10-7 64.97 19.54 15.49 17.62 1.20 210-30-7 63.66 21.85 14.49 18.81 1.28 210-90-7 58.33 25.11 16.56 20.41 1.39 210-60-3 55.12 28.81 16.07 20.69 1.41 210-60-5 56.28 27.11 16.61 19.52 1.33 210-60-9 56.14 27.54 16.32 20.01 1.36 210-60-11 55.46 29.25 15.29 20.93 1.43 a By difference: Fixed carbon% = 100% − volatile matter% − ash%. Sample
Energetic recovery efficiency (%) -
59.29 63.30 44.46 39.77 64.45 66.12 59.65 56.80 59.30 60.38 59.31
3.2.2. Elemental analysis Elemental compositions of WH and hydrochar were illustrated in Table 2. With the elevation of reaction temperature and residence time, the carbon (C) content in hydrochar got improved from 45.90% and 44.87% to 51.24% and 47.21%, respectively; on the contrary, that of hydrogen (H) and oxygen (O) decreased as HTC progressed. In fact, the accumulation of C content was owing to carbonization, whereas the removal of H and O contents were attributed to continuous strengthening dehydration and decarboxylation during HTC process [28]. The hydrogenation and oxidation components in biomass were decomposed, cracked, and eventually evolved out as gases such as H2, CH4, CO2 and H2O [33]. In addition, an increase in C content of hydrochar was achieved, while the O content decreased with the addition of
catalysts. The reason responsible for this variation was corresponded exactly to that of HHV change after adding catalysts. H+ or OH- would facilitate the dissolution of small organic molecules from biomass to solvent, thus accelerating the carbonization, dehydration and decarboxylation during HTC process. Compared with WH, the removal of nitrogen (N) content in hydrochar was also identified (except for 240-60-7 and 270-60-7). Excessive content of N might lead to higher emissions of contaminative gases such as NO and NO2 when hydrochar was adopted as the solid fuel. Consequently, HTC could be indeed considered as an available mean to generate clean biofuel.
Table 2. Elemental analysis of WH and hydrochar on dry basis. Elemental analysis/wt.% C H Oa N S WH 36.65 5.86 34.20 3.53 0.250 180-60-7 45.90 5.84 32.03 3.02 0.028 210-60-7 47.21 5.50 26.69 3.35 0.021 240-60-7 50.31 5.00 21.66 4.07 0.020 270-60-7 51.24 4.71 20.15 3.97 0.030 210-10-7 44.87 5.55 30.86 3.18 0.032 210-30-7 47.21 5.68 29.72 2.90 0.028 210-90-7 46.57 5.38 28.33 3.09 0.031 210-60-3 49.92 5.65 25.35 3.02 0.023 210-60-5 48.70 5.48 25.82 3.40 0.019 210-60-9 48.53 5.58 26.58 3.00 0.024 210-60-11 49.99 5.53 26.32 2.88 0.027 a By difference: O% = 100% − C% − H% − N% − ash%. Sample
H/C 1.919 1.527 1.399 1.191 1.104 1.484 1.444 1.387 1.357 1.349 1.379 1.327
Atomic ratio O/C 0.700 0.523 0.424 0.323 0.295 0.516 0.472 0.456 0.381 0.398 0.411 0.395
N/C 0.083 0.056 0.061 0.069 0.066 0.061 0.053 0.057 0.052 0.060 0.053 0.049
The variation in H/C and O/C atomic ratios of hydrochar under different reaction conditions were visualized in the Van Krevelen diagram (Fig. 1(a)). Besides, the positions of several kinds of typical coals, cellulose, hemicellulose and lignin were
also marked in Fig. 1(a) for comparison [25]. As could be observed from the diagram, the position of WH was closer to that of cellulose and hemicellulose regions. As reaction temperature and residence time elevated, a steady reduction of H/C and O/C atomic ratios in hydrochar was observed and the position was closer to lignin and lignite regions, in turn, upgrading the HHV and energy densification. A noteworthy finding was that from 210 ºC to 240 ºC, the H/C and O/C atomic ratios had a large mutation from 1.399 and 0.424 to 1.191 and 0.323, respectively. This represented that the hydrolysis of cellulose and hemicellulose in WH was most severe at 240 ºC. In addition, the reduction in H/C and O/C atomic ratios of hydrochar under acidic and alkaline environment also confirmed that H+ and OH- could promote the dehydration and decarboxylation of biowastes. Moreover, the relationship between N/C and O/C atomic ratios under different reaction conditions was illustrated in Fig. 1(b). It could be clearly observed that the evolution of N/C and O/C atomic ratios from WH to hydrochar followed the paths of reduction and denitrogenation. Notably, it was found that the 210-60-3 and 210-60-11 possessed the relatively lower N/C atomic ratio of 0.052 and 0.049 among all the hydrochar, representing that acid and alkali catalysts could obviously accelerate the denitrogenation degree of the samples.
Fig. 1. The H/C, N/C and O/C ratios of WH and hydrochar.
3.2.3. SEM analysis of hydrochar SEM micrographs could intuitively reveal the substantial change in inherent properties during the conversion from WH to hydrochar. As could be observed, HTC could significantly affect the surface morphology and physical structure of solid products. The untreated WH had compact structure, smooth surface and clear texture, whereas hydrochar had loose structure, rough surface and blurred texture. Besides, many small molecular fragments could be observed in hydrochar. As already mentioned above, lignin had relatively high chemical stability and was difficult to degrade until the temperature exceeded 265 ºC. Therefore, it could be inferred that these rough fragments might be the original skeleton of non-discomposed lignin. Specifically, with the elevation of reaction temperature and residence time, small molecular fragments in hydrochar increased, which could be attributed to the enhanced decomposition reaction. Besides, the surface of hydrochar became obviously rougher as HTC progressed, which could be explained by the release of volatile matter. The catalysts strengthened the destruction degree of surface morphology. Therefore, it could be speculated that H+ and OH- could accelerate the decomposition of biomass by promoting the dissolution of small organic molecules in water solvent.
3.2.4. FT-IR analysis of hydrochar
The FT-IR analysis of WH and hydrochar could reflect the evolution of functional groups in samples under different reaction conditions. The absorbance peaks from 3700 cm-1 to 3200 cm-1 represented the stretching vibration of hydroxyl (─OH). The vibration intensity experienced downtrend as reaction temperature increased, indicating strengthened dehydration of feedstock. The absorbance peaks between 2935 cm-1 and 2915 cm-1 were attributed to the antisymmetric stretching vibration of aliphatic methylene (─CH2). The absorbance intensity was significantly enhanced at 270 ºC, which might be attributed to the high temperature promoting the fracture of aliphatic side chains, thereby resulting in the formation of methylene groups. The absorption bands from 1770 cm-1 to 1650 cm-1 were associated to stretching vibration of carbonyl (─C=O). Its intensity was weakened by elevating reaction temperature, indicating enhanced decarboxylation reaction as HTC progressed. Moreover, the decrease of hydroxyl and carboxyl groups in hydrochar could effectively improve its hydrophobicity [38]. The absorption bands from 1380 cm-1 to 1310 cm-1 and from 1214 cm-1 to 1030 cm-1 were mainly ascribed to the vibration of methoxy group (R─O─CH3) and ether group (C─O─C), respectively. Similarly, the vibration intensity was weakened with the severity of temperature, indicating that high temperature might accelerate the deoxygenation reaction of the biowaste during HTC process. The effect of prolonged residence time on the peak intensity of hydrochar had similarity in that of elevated HTC temperature, but to a lesser extent, indicating that temperature had a more prominent impact on HTC.
As for different pH, it could be seen that the absorption bands from 900 cm-1 to 650 cm-1 corresponding to the substituted ─CH in benzene ring were obviously enhanced in acidic and alkaline environment, which might be owing to the fact that H+ and OH- could promote the aromatization process of hydrochar during HTC process.
3.2.5. Combustion behavior and thermal characteristics The WH and hydrochar were combusted in thermogravimetric analyzer within 50-800 ºC under air atmosphere. The TG and DTG curves at different reaction conditions were represented in Fig. 2. It was visible that all hydrochar suffered from two steep weight loss peaks during combustion process. In general, the combustion of hydrochar was a complicated process involving multiple chemical reactions that affected mutually. Thus, for better evaluating the combustion characteristics of hydrochar, the process could be divided into two consecutive stages based on two weight loss peaks. The stage 1 was devolatilization in the range of 260-340 ºC, while the stage 2 was decarbonization within 350-450 ºC, corresponding to oxidation processes of volatile matter and fixed carbon, respectively. Moreover, the detailed information of combustion parameters was summarized in Table 3. After HTC, the ignition temperature (Ti) and burnout temperature (Tf) of hydrochar increased to different degrees compared with WH (except for 270-60-7). It could be deduced that the combustion of hydrochar was transferred to a higher temperature region after HTC treatment. Specifically, the Ti dropped from 285.9 ºC to 277.6 ºC by elevating reaction temperature from 180 ºC to 210 ºC, followed by a promotion from 277.6 ºC to 311.5
ºC with temperature further increasing from 210 ºC to 270 ºC. The explanation below was cited for this phenomenon: It was well established that the ignition temperature of hydrochar was mainly ascertained by porosity and organic constituents (mainly volatile matter) [39]. From perspective by Kambo and Dutta [36], in the range of 180-210 ºC, albeit the VM of hydrochar decreased, the creation of pores dominated, thereby enlarging specific surface area, making oxygen react with organic matters more sufficiently [19]. Consequently, the samples could be ignited more easily. However, after 230 ºC, the pores were deteriorated and blocked. Meanwhile, the non-volatile organic compounds were accumulated and thus hydrochar was more difficult to ignite. Besides, the Tf for hydrochar was elevated in acidic and alkaline environment. Two reasons might be responsible for this behavior: (1) Catalysts could accelerate the formation of ordered and condensed structure in hydrochar via cross-linking as well as repolymerization of biomass during HTC. (2) FT-IR analysis indicated that H+ and OH- could promote the aromatization reaction during HTC process, thus generating more aromatic compounds with benzene ring structure. The above changes enhanced the thermal stability of hydrochar, thereby eliminating active sites and elevating burnout temperature. Furthermore, the first weight loss rate (DTG1) maintained a gradual downtrend in response to the elevation of reaction temperature and residence time or the addition of catalysts, resulting from the more severe hydrolysis of VM in hydrochar. The second weight loss rate (DTG2) demonstrated a downward trend with increased HTC temperature from 180 ºC to 210 ºC. This might be attributed the enhancement of aromatization and repolymerization within HTC
process to form chemical substances with stable benzene ring structure and low reaction activity, thus dropping the combustion intensity in stage 2. Contrarily, the uptrend in DTG2 was observed at over 210 ºC, which could be explained by the excessive decomposition of aromatic compounds at higher HTC temperature. After HTC, the combustibility index S of hydrochar was greatly improved, confirming that HTC was conductive to ameliorating the combustion performance of hydrochar. However, a reduction in the combustibility index S of hydrochar was clearly observed as a function of increased HTC severity. Downtrend in here was closely followed the variation of VM. To our knowledge, VM could improve the flame burning and reaction activity. Thus, as HTC severity increased, the VM decreased sharply and the S was correspondingly declined. Compared with WH, an obvious increase in the combustion stability index Rw of hydrochar was also achieved, representing better flame stability of hydrochar. In addition, the evolution of Rw with increased HTC severity showed great similarity with that of S. After evaluating the S and Rw comprehensively, it could be concluded that the combustion characteristics of hydrochar were obviously superior to that of WH. Besides, it could be observed that the reaction condition of 180-60-7 achieved the highest index S value of 25.81 (10-7×min-2׺C-3) and index Rw value of -28.43 (10-5×min-1׺C-2) and thus possessed the optimal combustion characteristics.
Fig. 2. TG and DTG of WH and hydrochar obtained at: (a) different reaction temperature; (b) different residence time; (c) different pH.
Table 3. The combustion characteristics parameters of WH and hydrochar. S RW Tia Tfb Mfc T1d DTG1e T2d DTG2e T3d DTG3e DTGmeanf -7 -2 -3 -5 (10 ×min ׺C ) (10 ×min-1׺C-2) (ºC) (ºC) (%) (ºC) (%/min) (ºC) (%/min) (ºC) (%/min) (%/min) WH 262.2 663.3 14.91 286.3 -11.88 437.5 -11.04 525.9 -4.26 -2.36 6.14 -6.83 180-60-7 285.9 672.5 11.09 295.0 -54.67 375.2 -24.49 -2.60 25.81 -28.43 210-60-7 277.6 665.1 11.36 288.7 -41.51 360.2 -17.61 -2.61 21.10 -22.48 240-60-7 288.5 665.8 19.37 283.2 -19.58 366.9 -20.11 -2.27 8.02 -10.47 270-60-7 311.5 659.6 26.85 277.7 -19.56 356.4 -24.71 -2.12 6.46 -9.52 210-10-7 284.3 678.8 14.13 296.5 -34.13 371.5 -18.85 -2.50 15.53 -17.69 210-30-7 288.8 688.4 13.43 300.4 -29.71 390.3 -16.80 -2.43 12.57 -14.94 210-90-7 277.0 668.5 16.41 290.4 -25.24 369.5 -15.74 -2.36 11.59 -13.63 210-60-3 282.9 672.2 12.55 296.5 -27.62 381.6 -18.64 -2.49 12.78 -14.52 13.28 293.5 -35.97 380.5 -21.43 -2.48 16.47 -18.93 210-60-5 284.9 667.1 210-60-9 279.9 669.6 15.56 289.1 -31.26 371.6 -19.49 -2.42 14.40 -16.68 290.1 672.6 14.95 303.2 -25.75 383.4 -19.27 -2.42 11.01 -13.20 210-60-11 a Ti, the ignition temperature. b Tf, the burnout temperature. c Mf, the residual mass. d T1, T2, T3, the temperature according to the first loss peak, the second loss peak and the third loss peak. e DTG1, DTG2, DTG3, the weight loss rate according to the first loss peak, the second loss peak and the third loss peak. f DTGmean, the average weight loss rate
Sample
3.2.6 TG-FITR analysis of gaseous products during combustion process Chinese emission standard of air pollutants for boilers (GB 13271-2014) clearly stipulated the maximum allowable emission concentration of sulfur dioxide, nitrogen oxides and other polluting gases in the coal-fired boilers. For this reason, the TG-FIRT analysis of emission gaseous products was necessary during the combustion of WH and hydrochar, not only for the upgradation of solid products, but also for the application of HTC in industrial scale. The emission profiles of several typical gases were visualized in Fig. 3(a-e). According to research by Chen et al. [40], the absorption peaks at 2200-2400cm-1 and 669cm-1 indicated the generation of CO2. The absorption bands at 2060-2240cm-1 suggested the emission of CO. The absorption peaks at 1300-1400 cm-1 were attributed to the release of SO2. The absorption peaks at 2965 cm-1 and 1762 cm-1 represented the existence of NO2 and NO, respectively. Based on Fig. 3(a), it could be concluded that CO2 was the prime component in gaseous products, whose emission concentration occupied the maximum level. The release of CO2 from the combustion of hydrochar mainly occurred at 260-330 ºC and 350-400 ºC, which respectively corresponded to the two combustion stages in thermogravimetric analysis. The emission concentration of CO2 in stage 2 was significantly higher than that in stage 1. This phenomenon could be ascribed to the fact that fixed carbon, consisting of covalent cross-linking and aromatic compounds, mainly combusted in stage 2. These compounds had relatively high carbon content compared with that of volatile matter combusted in stage 1 and thus the emission concentration of CO2 was maintained at higher fraction in stage 2.
As shown in Fig. 3(b), it was visible that the emission of CO from hydrochar was mainly concentrated in the ranges of 280-340 ºC and 350-410 ºC, corresponding to incomplete combustion of VM and FC, respectively. Besides, it could be concluded that the CO emission of hydrochar was lower than that of WH. This might be due to the dehydration and decarboxylation of long-chain macromolecules within HTC stage to form small molecules, such as furan, acid, aldehyde, ketone, alcohol and phenol [7], which could be burnt out easily. Fig. 3(c) illustrated the emission of SO2 from the combustion of WH and hydrochar. It could be observed that the emission concentration of SO2 showed relative low level, mainly occurring in the range of 300-350 ºC. In opinion of Lin et al. [41], the emission of SO2 from 250 ºC to 350 ºC was mainly derived from the combustion of organic sulfur such as aliphatic-S, aromatic-S and sulfoxide-S. Compared with prolonged residence time, the increased HTC temperature could reduce the emission concentration of SO2 more effectively, which might be ascribed to the more severe decomposition of organic sulfur components at higher HTC temperature. Previous study [42] also proved that high reaction temperature was conducive to promoting the conversion of organic sulfur in coke to inorganic salt sulfur in ash. Moreover, another noteworthy finding was that alkali catalyst could greatly reduce the emission concentration of SO2, which might be attributed to the great sulfur retention characteristics of alkali metal salts (sodium salts) during combustion [43].
Fig. 3(d-e) presented the emission curves of NOX (NO and NO2) for WH and hydrochar. It was visible that the emission curves of NOX from hydrochar demonstrated one peak within 280-310 ºC. Thus, it could be inferred that the emission of NOX could be attributed to the oxidative combustion of high nitrogen-containing volatile matter. The NO emission of hydrochar was obviously lower than that of WH. Lin et al. [41] believed that NO could be reduced on the surface of char to generate → CO + N2), thereby resulting in low CO and N2 during combustion (NO + C
emission of NO. Similar to SO2, the alkali catalyst could effectively inhibit the emission of NOX. This might be attributed to the fact that alkali metal salts (sodium salts) could significantly catalyze the reduction of NOX by NH3 (NOX + NH3 → N2 + H2O) during combustion [44]. In summary, CO2 was the primary gaseous product in the combustion process of the samples. The alkali catalyst could effectively reduce the emission concentration of SO2 and NOX, in turn, ameliorating the emission properties of hydrochar and minimizing the environmental hazard.
Fig. 3. Gaseous emissions during combustion of WH and hydrochar. (a) CO2, (b) CO, (c) SO2, (d) NO, (e) NO2.
4. Conclusions
In summary, the increased reaction temperature, prolonged residence time as well as acid and alkali catalysts could promote HTC intensity. In comparison to the downtrend of hydrochar yield (from 53.7% to 28.8%) and energetic recovery efficiency (from 66.12% to 39.77%), the HHV (from 17.62 MJ/kg to 20.93 MJ/kg) and energy densification (from 1.20 to 1.43) of hydrochar followed a reverse trend as HTC progressed. Besides, compared with WH, the fixed carbon and C content in hydrochar were enriched to 19.52%-35.11% and 44.87%-51.24%, respectively, while the volatile matter, H content and O content declined to 45.11%-67.26%, 4.71%-5.84% and 20.15%-32.03%, respectively. Thermogravimetric analysis showed that 180-60-7 achieved the optimal combustion performance, with an index S value of 25.81 (10-7×min-2׺C-3) and an index Rw value of -28.43 (10-5×min-1׺C-2). The alkali catalyst could greatly reduce the emission of typical pollutants (SO2 and NOX) during the combustion of hydrochar, thereby ameliorating the emission properties.
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (51476060); Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization (2013A061401005); Key Laboratory of Efficient and Clean Energy Utilization of Guangdong Higher Education Institutes (KLB10004).
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Highlights
Hydrothermal carbonization of water hyacinth was studied.
The increased reaction temperature, prolonged residence time as well as acid and alkali catalysts could promote HTC intensity.
The HHV and energy densification of hydrochar followed an uptrend as HTC progressed.
The reaction condition of 180-60-7 achieved the optimum combustion performance.
The alkali catalyst could reduce the emission of typical pollutants (SO2 and NOX) during the combustion of hydrochar.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.