Conventional Hydrothermal Carbonization of Shrimp Waste - Energy

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Conventional Hydrothermal Carbonization of Shrimp Waste Shrikalaa Kannan, Yvan Gariepy, and Vijaya Raghavan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03997 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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Conventional Hydrothermal Carbonization of Shrimp Waste Shrikalaa Kannana*, Yvan Gariepya and G.S. Vijaya Raghavana a

-Department of Bioresource Engineering,

Macdonald campus, McGill University, 21111 Lakeshore road, Sainte-Anne-De-Bellevue, Quebec, Canada H9X 3V9. *All correspondence should be addressed to Shrikalaa Kannan Department of Bioresource Engineering, Macdonald campus, McGill University, Room MS-1 101, Macdonald Stewart Building, 21111 Lakeshore road, Sainte-Anne-De-Bellevue, Quebec, Canada H9X 3V9. E-mail: [email protected] Tel: 1-514-582-8939

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ACS Paragon Plus Environment

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Abstract Shrimp is a seafood that is among the most consumed across the world. Shrimp processing operations generates enormous quantities of waste. Current strategies of shrimp waste management, and the current utilization strategies suffer from several disadvantages especially from being not environmentally friendly. This warrants for alternate methods to completely utilize shrimp waste. Previously, we have shown that microwave hydrothermal carbonization (MHTC) can be used to treat shrimp waste to produce hydrochar. Here, in this study, conventional hydrothermal carbonization (CHTC) using a customized autoclave reactor was performed to treat shrimp waste. Upon using response surface design, it was found that at a holding temperature of 186 °C and a time of 120 min, a maximal hydrochar yield of ~29% was achieved. Further, characterization of elemental, proximate, energy, and surface properties of CHTC shrimp waste hydrochar were found to be comparable to that of the MHTC hydrochar from shrimp waste. Further, the hydrochar properties were comparable to that produced from other wastes such as low grade lignocellulosic waste, and mixed wastes. This study further confirms that non-lignocellulosic wastes such as shrimp waste could be used as a biomass to produce hydrochar by HTC irrespective of the heating medium used. Keywords: Biomass, Hydrochar, Material Science, Shrimp Waste, and Energy Value.

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Introduction The world’s population is well over 7 billion and is expected to reach 9 billion by the year 2050 1

. The impact our growing population has on the environment is not only the rapid consumption

of resources such as fossil fuels, water, food and land but also the production of huge quantities of waste which include garbage, sewage, industrial and municipal waste 2. The waste biomass has tremendous potential to be converted into valuable products that are of commercial value. Biomass derived energy is gaining prominence as an alternative to fossil fuels in order to address the growing environmental problems 3. Direct combustion of waste biomass is not economical as it has a high moisture and ash content. This can also contribute to particulate pollution by generating copious amounts of smoke and add to GHG emissions 3. Therefore pre-treatment of biomass to convert it into a product such as bio-coal or bio-oil will help to overcome the above problems and improve the energy density of the biomass. Hydrothermal carbonization is a biomass conversion process where the raw material is treated at a temperature of about 150-250 °C at elevated pressures in the presence of water

4-6

.

The resulting product is known as hydrochar or bio-coal. This process does not require an energy intensive drying step prior to the carbonization process unlike in pyrolysis 7. It is also reported to exhibit high conversion efficiency and low operation temperatures compared to other thermal methods 8. Lignocellulosic materials owing to their high concentration of carbohydrates has been considered suitable for bioenergy applications. Therefore, HTC has been so far widely used to treat lignocellulosic biomass

9-18

. Wood materials have also been carbonized by submerging

them in water and performing HTC

9-18

. Recently, HTC has also gained importance as an

efficient waste management solution in treating municipal 19-22 and sewage waste 23-25. Municipal

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and sewage wastes could be regarded as a complex waste stream as they contain household materials, which could be a mixture of both lignocellulosic, food, animal and other wastes 19-25. Very little was known about the suitability of the HTC to treat non-lignocellulosic wastes. These wastes are very complex as they contain a mixture of proteins, fats and lesser amounts of carbohydrates compared to plant derived waste. They contain neither lignin nor cellulose in contrast to most waste biomasses. For instance, using proximate analysis it was found that each 100 g of commercial shrimp waste contained ~81-82 g of moisture, ~16-17 g of protein, ~0.8 g fat and ~1.2 g of carbohydrates26. For these reasons, non-lignocellulosic wastes are often ignored with respect to energy-related utilization processes such as the production of solid biofuel. Non-lignocellulosic animal wastes such as seafood waste are rich in moisture. Therefore, non-lignocellulosic wastes are well suited for HTC treatment as there is no requirement for a pre-drying step 7. Despite their poor carbohydrate profile, recently, we have shown that HTC could be effectively used to treat fish

27

and shrimp waste

28

to produce

hydrochar of quality equivalent to that produced from municipal, sewage, and low-grade lignocellulosic wastes. It has been reported by the FAO that about 6 to 8 million tonnes of crab, shrimp and lobster waste are generated every year globally

29, 30

. Shrimp is a high value

aquaculture product that is usually exported in frozen form without the exoskeleton. Thus, during processing and packaging operations, the head and the shell are discarded as waste. It has been reported that during processing 45-48% by weight of the shrimp is wasted depending on the species31. Utilization of seafood waste such as shrimp waste by HTC could prove to an effective way to create an additional biomass for energy-related purposes. Current strategies of shrimp waste utilization involve the production of chitosan32,

33

,

bioactive peptides, extraction of antioxidants and other bioactive compounds34-36. Recently, there

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has been a growing interest in the field of shrimp waste utilization where new technologies to handle shrimp waste are fast emerging37,

38

. However, most of these technologies target one

particular compound of interest, and thus still leave behind effluents in the form of waste sludge or blood/sticky water that further needs treatment39. Interestingly, a recent study had proposed the concept of shell waste biorefinery that recommends fractionation of the major components of the shrimp waste, and extract the maximum possible resources from them40, 41. Similarly, HTC process attempts to use both the solid as well as the liquid parts of the waste thereby minimizing the effluents released into the environment. HTC process is so versatile in that it can handle shrimp waste of any quality including poor quality waste that are deemed unsuitable for bioactive extraction. Therefore, HTC could serve as an excellent stand-alone or additional/supplemental technology to the current strategies to enable holistic utilization of shrimp waste. Utilization of seafood waste such as shrimp waste by HTC could yield multifaceted benefits not only for the seafood industry but also for energy industry. First, the waste management of seafood waste by HTC will become more attractive as it might yield additional source of revenue for the seafood processing industry. Next, the HTC of seafood waste generates minimal or no additional waste unlike other seafood waste utilization technologies such as fish meal or bioactive extraction. Therefore, HTC of seafood waste could minimize the environmental impact that otherwise would have resulted from improper handling. Finally, this study shows that non-lignocellulosic materials could serve as a good source of raw material for to be processed by HTC. This study thus could be extended to other waste biomass that are nonlignocellulosic such as meat waste. Therefore, this study would thus expand the availability of

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waste biomass to produce hydrochar, a carbon rich material that has been shown to possess good energy value28. In our previous study, we used microwaves as the heating medium for HTC

28

. It is of

importance to study the effect of conventional HTC (CHTC) on the yield and quality of the char such as its calorific value, ash content, microstructure and elemental composition. This will help us compare and contrast the effects of different heating methods on the final quality and yield of the hydrochar. Therefore, in this study, we have employed a conventional method of HTC by using custom build autoclave HTC reactor. We have employed a response surface design guided optimization of the process parameters of CHTC in order to determine the optimal conditions to maximize hydrochar yield. Further, we have characterized the energy, chemical and material properties of the hydrochar. Materials and methods Hydrothermal carbonization Shrimp waste was carbonized in a customized cylindrical autoclave reactor of 150 mL capacity, made of stainless steel 316. Controlled heating was carried out by a heating mantle with a rated heating power of 350 W equipped with a thermocouple and a temperature controller. A tubular internal heater made of INCOLOY with a maximum heating power of 300 W (Omega, Canada) was used to ensure efficient and more uniform heating. Further, thermal insulation with glass wool was employed to minimize the loss of heat to the surrounding environment (Fig. 1). Following CHTC under varying process conditions, the end product was then subjected to vacuum filtration to separate the solid fraction (i.e., wet hydrochar) from the liquid, biocrude

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liquor. The wet hydrochar was then oven-dried at 105 °C for 24 h to produce dry hydrochar. The yield of the hydrochar was calculated on a dry basis.

ℎ % =

! " # $

× 100 …………. (1)

HTC optimization protocol Based on the results from a previous study, where shrimp waste was carbonized using MHTC, it is known that the holding temperature, and time of the HTC reaction significantly affects the yield of the hydrochar

28

. Biomass-water ratio did not have any effect on the yield of the

hydrochar during MHTC 28. This effect was further confirmed in a preliminary CHTC screening study that was conducted at 180 °C, and 120 min by varying the biomass-water ratio (0.5, 1, and 1.5) (Supplementary Fig. S1). Therefore, holding time and temperature of CHTC was further optimized using response surface design. As previously described, with two factors, namely the holding temperature and time, the central composite design (CCD) design resulted in 4 factorial points, 4 axial points, and 4 center runs

28

. Due to technical limitations, a face-centered non-

rotatable design was implemented as the CHTC system has a very narrow operating temperature range. For holding temperature, the minimal and maximal limit (-1,1) was set to 150 and 210 ° C. For the holding time, the maximal and minimal limit (-1,1) was set to 60 and 120 min. A 4center run was adapted to improve the reliability of this model. CHTC was performed in a random fashion to account for any random hidden effects that may be present. Significant effects were analyzed using ANOVA (analysis of variance) through DoE suite in JMP. The main effects and a further model building were implemented by using F-test (Fisher test) and probability

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values (α= 0.05). This design resulted in total 12 runs per experiment. The second order linear regression model as below could be used to model the output variable i.e. hydrochar yield (%). = () + (+,+ + (- ,- + (+- ,+ ,- + (++ ,+- + (-- ,-- + . ……………. (2) The experimental results from CHTC trials were fitted to the above equation using SAS statistical software, JMP licensed to McGill University.

Sample preparation for HTC Shrimp waste consisted of waste (heads, tails, and shells) from a different types of shrimps including pink shrimp, tiger shrimp and brown shrimp. The wastes were obtained fresh from the local market, stored, and processed as previously described

27, 28, 42

enzymatically pre-treated before CHTC as previously described

. Shrimp waste was

27, 28, 42

. In short, 30 g of

homogenized shrimp waste was mixed with an enzyme cocktail (20%, w/w of each enzyme in the ratio of 1:1:1) of Viscozyme (catalog no.: V2010), lipase (catalog no.: L0777), and protease (catalog no.: P4860); and the incubation was carried out in a laboratory incubator/shaker (Incushaker mini, Benchmark Scientific, USA) at ~40 °C with rotation at 120 rpm for a period of 6 h. After enzyme pre-treatment, the digested shrimp waste was subjected to CHTC. Chemical and energy properties of hydrochar Hydrochar produced from different process conditions were analysed for their chemical, material, energy, and surface properties. The elemental composition i.e. C, H, N and S were determined using an elemental analyzer (Fisons Instruments CHNS-O EA 1108). As previously described43-45, proximate analysis were performed on the raw shrimp waste, and hydrochars

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derived at varying CHTC operating conditions to determine moisture content, ash content, and volatile solids as per ASTM International Standard protocols. First, to detect the moisture content, 1.0 g of hydrochar was placed in a hot air oven at 105 °C until a constant weight was achieved. Weights were noted initially after 4 h and after every hour after the initial 4 hour period (ASTM 871-82, 2006)43-45. Second, to measure the ash content, 1.0 g of the test sample was taken in an open crucible and placed in a muffle furnace that was at 600 ± 10 °C for 4 h. The weight was noted after the crucible was cooled (ASTM-D 3174-04, 2009)43-45. Finally, to measure the volatile matter, 1.0 g of test sample was placed in a muffle furnace that was at 950 ± 10 °C for 7 min (ASTM-D 3175-07)43-45. The total fixed carbon was determined by the difference from 100, considering the percentile amounts of moisture, volatile matter, and ash content. Energy value was determined using a Parr adiabatic bomb calorimeter (Parr Oxygen Bomb Model 1341EB, Calorimeter Thermometer Model 6772, Parr Instrument Company, Moline, IL, USA) to calculate higher heating value (HHV) as previously described 46, 47. Scanning electron microscope (SEM) The raw shrimp waste samples and recovered hydrochar at various operating conditions were analyzed for their surface morphology and microstructure by a Hitachi TM-3000 (Tokyo, Japan) scanning electron microscope for comparative microstructural analysis. Varying resolutions from 50X to 1500X was used to analyze the morphological structure. Results and discussion Optimization of process parameters of CHTC This study used two independent variables (holding temperature and holding time) to optimize CHTC using response surface methodology (face centered-CCD) with the goal of maximizing

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the hydrochar yield. Due to technical limitations with the equipment, a face-centered CCD (Fig. 2.a) was adapted in this study, and the use of more sophisticated rotatable design could not be implemented. The figure (Fig. 2.b) shows the response surface plot, which makes it evident that the hydrochar yield initially increases and then decreases beyond a certain point upon increasing the CHTC process parameters, i.e. holding temperature and time. Firstly, by individual assessment, the process parameters revealed that both the holding temperature (F(1,11) = 7.82; P = 0.03) and holding time (F(1,11) = 18.95; P = 0.004) significantly affected the hydrochar yield. Secondly, it was noteworthy that there was a lack of interaction effect of holding temperature and time on the hydrochar yield (F(1,11) = 1.27; P = 0.30). Thirdly, the curvature term (temperature X temperature effect) was found to exhibit a significant effect on the yield (F(1,11) = 53.71; P = 0.0003), which is evident from the curved response surface of the hydrochar yield (Fig. 2.b). Such a curved surface is formed due to the fact that the hydrochar yield at the center point (180 °C and 90 min) falls on a different surface plane (indicating a higher yield) than the surface plane that fits the four corner points. The response surface model significantly fitted (P = 0.0016) the experimental yield as is depicted by a high value of R2 (0.94). The percentage error between the actual and predicted hydrochar yield values ranged between 0.3 and 7.7% with mean percentage error of 2.7%, which was not significantly different (P = 0.50, matched-pairs t-test). The quadratic model fit of the hydrochar yield is depicted by the equation below, ℎ % = 1.9257 4 + 0.05469 7 + 0.0007 47 − 0.0054 4 - − 0.00002 7 - − 151.5701……………. (3)

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On observing the surface plot as shown in Fig. 2.b, for the evaluated process parameter space, the maximal yield of hydrochar from shrimp waste (~ 29%) was achieved by conducting CHTC at holding temperature of 186 °C and holding time of 120 min. On comparing this result to a previous study that employed MHTC of shrimp waste in a similar parameter testing space, it was found that MHTC resulted in a higher maximal yield of hydrochar (~42%) at a similar optimal holding temperature of 184 °C and holding time of 112 min. This is strikingly ~44% more than the hydrochar yield that was obtained using CHTC at a similar optimal holding temperature and time suggesting that MHTC produced more solid hydrochar than CHTC at similar process conditions. Further, it was found that CHTC was associated with higher come-up times than MHTC and hence longer experimental times 28. For instance, the come-up time of the CHTC to reach 150 °C, 180 °C, and 210 °C were found to be about ~three to four times of that of the MHTC come-up times (150 °C: CHTC = 31.33 ± 2.01 min, MHTC = 8.33 ± 0.89 min, P = 0.009, student t-test; 180 °C: CHTC = 42.33 ± 2.73 min, MHTC = 12.33 ± 0.58 min, P = 0.01, student t-test; 210 °C: CHTC = 63.33 ± 2.80 min, MHTC = 15.33 ± 0.89 min, P < 0.0007, student t-test) 28. These results strongly suggest that MHTC results in shorter process time, and in higher yield than CHTC for shrimp waste at similar optimal conditions. This is in contrast to other studies which have reported similar yields upon CHTC and MHTC of other tested feedstocks. For instance, MHTC and CHTC of lignocellulosic biomass resulted in a comparable yield of the hydrochar produced from both these processes

14

. Similarly, another study which

subjected human biowaste and to MHTC and CHTC reported that the yield of hydrochar from both processes were comparable 48. Having said that, both these above mentioned studies found that the CHTC exhibited higher come-up times as compared to MHTC, similar to what is observed in this study 14. It is well understood that the feedstock used during HTC determines the

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yield of hydrochar. For instance, a study which evaluated the effect of feedstock categories, i.e. carbohydrates, proteins, and fatty oils, found that depending on the feedstock used, the yield of hydrochar will vary greatly

49

. Taken together, these results strongly suggest two important

notions, 1. the feedstock used and its composition greatly affects the yield resulting from HTC and the type of heating medium used may have either significant or insignificant effect on the yield depending on the feedstock used; and 2. the type of heating medium used will greatly determine the duration of the process, and hence the process efficiency and economy. The yield of the hydrochar produced from HTC of other lignocellulosic wastes also suggested that the yield is dependent on the type of raw material used. For instance, HTC resulted in an yield of ~21% from corn stover, ~29% from corn leaves, ~24% from wheat straw, and ~55% from coffee cake50. Further, upon using model compounds such as glucose, starch and sucrose, it was found that depending on the concentration of the model compounds and HTC process conditions the yield could vary widely between 1.5 to 43%51. Collectively, this suggests that HTC of shrimp waste results in hydrochar yield that is largely comparable to one that is achieved from other lignocellulosic and model raw materials. Characterization of elemental composition The elemental composition of the hydrochar obtained from shrimp waste at various operating conditions of CHTC and that of raw shrimp waste (before enzyme pre-treatment) is shown in Fig. 3.a and Supplementary data Table S1. Upon increasing the holding temperature and the holding time of CHTC from 150 °C and 60 min to 210 °C and 120 min, the amount of atomic carbon in the hydrochar obtained increased from ~41% to ~50%. The maximal atomic carbon in the hydrochar (~50%) is obtained at CHTC operating parameters of 180 °C and 120 min, which

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is the near optimal condition to produce maximal hydrochar yield. The trend of the atomic oxygen was opposite to that of the atomic carbon. With an increase in holding temperature and time from 150 °C and 60 min to 210 °C and 120 min, atomic oxygen in hydrochar varied from 47% to 37%. Atomic hydrogen and nitrogen in hydrochar remained minimally changed at varying operating conditions of CHTC. Sulphur composition in the hydrochar is negligible at different operating conditions of the CHTC. These results indicate that hydrochar is formed by the enrichment of atomic carbon and by the removal of atomic oxygen simultaneously, possibly by decarboxylation, demethylation, and dehydration reactions as suggested by the Van Krevelen diagram (Fig. 3.b.), during CHTC in a holding time and temperature dependent manner. In our previous study28, we found that MHTC of shrimp waste resulted in a similar trend of atomic elements, i.e. increase or decrease of atomic elements when the process parameters varied as seen in the hydrochar produced by the CHTC of shrimp waste. At near optimal conditions (Supplementary Table S4), CHTC hydrochar had mildly higher atomic carbon and lower atomic oxygen (C = ~50%; O = 37%) compared to MHTC (C = ~46%; O = 43%). With respect to the other atomic elements, i.e., hydrogen, nitrogen, and sulphur, it was found that there was no significant difference (Supplementary Table S4) between the hydrochar produced from MHTC and CHTC28. These results suggest that with respect to the elemental composition of the hydrochar, there are no major and striking differences (percentage difference between CHTC and MHTC > 20%) in the elemental composition of the hydrochar produced from CHTC and MHTC. Similar trends in the increase or decrease of the atomic elements were observed in a variety of feedstocks including several lignocellulosic raw material 15, 52-55 and mixed waste such as sewage sludge

24

, food waste

56-58

, and MHTC of fish waste

27

. The absolute atomic carbon content of

the hydrochar produced from glucose ranged from 61-64% depending on the HTC process

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conditions51, which is much higher than the shrimp waste hydrochar as expected. Further, similar increase or decrease in atomic carbon and oxygen trends have also been observed in hydrochar derived from model compounds such as glucose, starch, and sucrose51. We then computed H/C and O/C atomic ratios to better understand the reaction pathways during hydrochar production, and plotted Van Krevelen diagram (Fig. 3.b. and Supplementary data Table S1). At holding temperature of 150°C, upon increasing the holding time, there was an increase in the demethylation reaction, represented by a shift to the right in the O/C ratio. Such demethylation reactions are known to occur in other feedstocks 59. It is also possible that at these temperatures other uncharacterized reactions may also occur in the raw material. Upon increasing the holding temperature to 180°C and 210°C, there was an increase in the decarboxylation reactions at all holding times tested as depicted by the lower O/C values for the hydrochar compared to that of the raw shrimp waste. Irrespective of the holding temperature or time, the lower H/C atomic ratio compared to raw shrimp waste, depicts the occurrence of dehydration reaction during CHTC at all holding temperatures and times, albeit to different extent at different operating conditions. At lower holding temperature of 150°C, the dehydration reactions are less evident than at 180°C and 210°C. Similar decarboxylation, demethylation, and dehydration reactions characterize the formation of hydrochar during MHTC of shrimp waste as previously reported

28

. Similarly, decarboxylation or dehydration reactions occur either

exclusively or in combination during MHTC of fish waste 27, as well as in several lignocellulosic materials and mixed waste streams 14, 24, 52-54. Characterization of proximate composition

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The results for the proximate analysis of the hydrochar produced at different operating conditions of CHTC hydrochar and raw shrimp waste are summarized in Fig. 4.a and 4.b. The amount of fixed carbon in the hydrochar varies very little if any with an increase in holding temperature. For instance, when the temperature increases from 150 °C to 210 °C (at holding time of 60 min), the fixed carbon content in the hydrochar does not vary much (fixed carbon: 150 °C = 35.99 ± 1.42%, 210 °C = 33.02 ± 1.25%). Similarly, at a particular holding temperature, increasing the holding time from 60 to 120 min does not produce any clear trend in the fixed carbon content (Fig. 4.a and Supplementary data Table S2). For instance, at the holding temperature of 180 °C, the fixed carbon varies very little from 39.36 ± 1.52% to 35.89 ± 1.43% as the holding time increases from 60 min to 120 min respectively. Similarly, volatile matter increases very little as the holding temperature and time increases during CHTC. The volatile matter increased from 34.71 ± 1.41% to 38.57 ± 1.69% as the holding temperature and time increases from 150 °C, 60 min to 210 °C, 120 min respectively. The ash content of the hydrochar decreases with increasing holding temperature and holding time up to 180 °C, and then decreased upon further increase in the holding temperature to 210 °C. This trend of ash content suggests that the inorganics present in the shrimp waste is being leached more at 180 °C than at other holding temperature in a holding time dependent manner ratio

23

60

. We then computed the fuel

(fuel ratio = fixed carbon / volatile matter) to understand the fuel characteristics of the

hydrochar produced. As the holding temperature and time increases from 150 °C, 60 min to 210 °C, 120 min, the fuel ratio (Fig. 4.c) varies from 1.04 to 0.99, indicating that the fuel characteristics is indifferent to changes in the holding temperature and time. The optimal process conditions to produce maximal yield in CHTC, which is holding temperature of 180 °C and

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holding time of 120 min, will contain fixed carbon of ~36%, volatile matter of ~39%, ash content of 21%, and moisture content of 4%. During MHTC of shrimp waste, increasing the holding temperature and time resulted in clear trends, i.e. an increase in the fixed carbon, decrease in volatile matter and ash content 27. At similar process conditions (180 °C and 120 min), MHTC resulted in ~11% of total fixed carbon, 65% of volatile matter, 22% of ash content, and ~1.4% of moisture content. On comparing this to CHTC hydrochar, it is clearly evident that CHTC hydrochar contains higher fixed carbon (3.2 times more) and moisture content (2.9 times more) and but lower volatile matter (1.7 times less), and similar ash content (Supplementary Table S4) 28. These results suggest that the medium of heat transfer may affect the proximal composition of the hydrochar owing to differential distribution of components in the solid hydrochar and liquid bio-crude liquor. With respect to the feedstock used, similar trends of fixed carbon, ash content, and volatile matter have been observed in MHTC of fish waste 27, several lignocellulosic materials and mixed waste streams 14, 24, 52-54

. More importantly, it should be noted that as discussed in the next section, there is no

difference in the energy value (determined by bomb calorimetry) of hydrochar produced at optimal conditions of CHTC and MHTC. Characterization of energy values Bomb calorimetry experiments were conducted on hydrochar produced from shrimp waste in order to assess the potential of CHTC hydrochar as a solid fuel, and the results are summarized in the Fig. 5.a. It is evident that the calorific value increases from 19.20 ± 0.05 MJ/kg to 24.02 ± 0.12 MJ/kg as the CHTC holding temperature increases from 150 °C, 60 min to 210 °C, 120 min. Highest calorific value (24.02 ± 0.12 MJ/kg) was obtained from hydrochar produced at holding

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Energy & Fuels

temperature of 210 °C and time of 120 min. The calorific value of the shrimp waste that is comparable to the hydrochar obtained from lignocellulosic biomass

61

, food waste

58

, sewage

sludge 62 and human biowaste 62. Interestingly, in another study, MHTC of pure glucose yielded a calorific value ranging from 16.90 MJ/kg in 15 min to 21.30 MJ/kg in 45 min at a temperature of 200 °C13, which is also comparable to the energy value of shrimp waste hydrochar. In another study, the energy content of the hydrochar produced from coconut fiber and eucalyptus leaves were found to be as high as ~31 kJ/mol, and ~30 kJ/mol depending on the process conditions63. The calorific value as obtained from the bomb calorimetry indicates that hydrochar from CHTC shrimp waste resembles that of lignite 23. The Energy Enrichment Factor was then computed (EEF = calorific value of hydrochar/calorific value of raw shrimp waste), which is a parameter that measures the energy densification in hydrochar. EEF value of >1 represents improved energy densification. EEF value of hydrochar produced at all tested holding temperatures (150°C - 210°C) and holding time (60-120 min) were above 1 except the hydrochar produced at holding temperature of 150°C and holding time of 60 min (EEF = 0.96), indicating a good energy densification of hydrochar produced by CHTC at most operating conditions. Subsequently, we computed energy yield (Energy yield = Hydrochar yield X EEF) for hydrochar made at different operating conditions and found that energy yield increased from 18.04% to 28.69% as the holding temperature and holding time increases from 150 °C, 60 min to 210 °C, 120 min. This increase in energy yield is attributed to the increasing hydrochar yield with increasing holding temperature and time. A maximal energy yield of ~33% was achieved at a holding temperature of 180 °C and a holding time of 120 min, which is also the near optimal conditions for CHTC. Therefore, at near optimal conditions, the CHTC hydrochar has energy value of 23.05±0.13 MJ/kg, EEF of 1.15, and

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energy yield of 32.95±0.89%. Comparing this to the MHTC of shrimp waste, the hydrochar has energy value of 23.22±0.02 MJ/kg, EEF of 1.23, and energy yield of 35.36±1.14%

28

. Taken

together, these results suggest that the hydrochar produced by MHTC has similar energy properties, if anything slightly better, than hydrochar produced by CHTC of shrimp waste. Bomb calorimetry is an expensive process and hence researchers have developed several models that are based on the elemental analysis of the hydrochar. These studies have evaluated the energy value of hydrochar produced from several other feedstocks including lignocellulosic 64

, agro-industrial (grape marc)

65

, and sewage sludge

66

, and found that the energy value

correlated strongly with atomic carbon content. We wondered if the carbon content in hydrochar produced from shrimp waste CHTC correlates with the energy value. Fig. 5.b indicates that the total atomic carbon content in the hydrochar loosely but significantly correlates with calorific value as computed from bomb calorimetry. The R2 value of the best-fit line is 0.46, which is significantly correlated to the observed calorific value (P = 0.0019). Similarly, in our previous study, we found that the carbon content in the hydrochar produced by MHTC of shrimp waste also correlated with the atomic carbon content 28. The model equation of the fit line is as below, ?

9 : ; < , 9> = 0.2690 9BC C7 C7 + 9.53……….. (4) @A A unified model developed by Channiwala and Parikh 2002, have been widely employed in the use of the assessment of energy properties of material produced from different types of feedstock based on the elemental and proximate analysis of such materials. Some of the wastes used in this unified model include lignocellulosic waste such as agricultural waste, woody biomass, and also mixed waste such as sewage sludge, animal waste and municipal waste 67. The

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Energy & Fuels

model equation is as below (where C- atomic carbon; H – atomic hydrogen, S- sulphur; O – oxygen; N – nitrogen; and A – ash content), > = 0.34919 + 1.1783 + 0.1005F − 0.1034G − 0.0151H − 0.0211I

(MJ/kg)

……………....(5) The above equation was used to compute the HHV values based on the elemental and the proximate analysis of the hydrochar produced from shrimp waste. It was noted that the HHV calculated from above equation and the actual calorific value calculated from bomb calorimetry matched well (R2 = 0.68; P < 0.0001; Supplementary data Fig. S2.a and b). Despite a good fit, the model overestimates in predicting the HHV at all operating conditions, as depicted by high values of percentage error that ranges from 0.05-29%. The authors recommend the readers to exercise caution when using the model developed by Channiwala and Parikh 2002

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to predict

the HHV values of hydrochar produced from shrimp waste irrespective of the heating medium used, i.e. either conventional or microwave 28. I7

Conventional Hydrothermal Carbonization of Shrimp Waste - Energy

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