Hydrothermal carbonization of soybean milk residue (okara) nutrient extraction and hydrochar fuel properties

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

CAO THI THUY GIANG

HYDROTHERMAL CARBONIZATION
OF SOYBEAN MILK RESIDUE (OKARA):
NUTRIENT EXTRACTION AND
HYDROCHAR FUEL PROPERTIES

MASTER'S THESIS


VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

CAO THI THUY GIANG

HYDROTHERMAL CARBONIZATION
OF SOYBEAN MILK RESIDUE (OKARA):
NUTRIENT EXTRACTION AND
HYDROCHAR FUEL PROPERTIES

MAJOR: ENVIRONMENTAL ENGINEERING
CODE: 8520320.01

RESEARCH SUPERVISORS:
Dr. NGUYEN THI AN HANG
Dr. NGUYEN VIET HOAI

Hanoi, 2021


ACKNOWLEDGMENTS
After a period of conducting research, I have also completed the content of the
thesis “Hydrothermal carbonization of soybean milk residue (okara): nutrient extraction
and hydrochar fuel properties". The thesis was completed not only by the author's own
efforts but also with the helps and active supports of many individuals and groups.
First of all, I would like to express my sincere and deep thanks to my Principal
Supervisor Dr. Nguyen Thi An Hang, who directly guided my thesis. She gave me a lot
of time and energy as well as many valuable ideas, corrected my thesis with specific
comments. Additionally, she always cared, encouraged and reminded me in a timely
manner, thereby I could complete the thesis on schedule. A special thank also goes to
my Co-Supervisor Dr. Nguyen Viet Hoai, for his valuable comments, concern,
encouragment, and dedicated guidance for analysis of some hydrochar.
The second, I am also very grateful to Ms. Nguyen Thi Xuyen, the assistant to
Dr. Nguyen Thi An Hang’s project, for supporting my experiment set-up and
environmental parameters analysis.
In addition, I would like to thank the professors, officers and staffs of the Master’s
Program in Environmental Engineering, Vietnam Japan University, Vietnam National
University, Hanoi, who have wholeheartedly taught and helped me in my years in
graduate school.
Last but not least, I would also like to express my sincere thanks to my family,
friends, and fellow masters of the Master’s Program in Environmental Engineering –
Batch 4 for always encouraging, caring and helping me during my study and process
thesis execution.
I acknowledge VJU’s JICA research fund (2021-2023 Principle investigator Dr.
Nguyen Thi An Hang) for financially supporting my thesis research.
Ha Noi, June 2021- Cao Thi Thuy Giang

TABLE OF CONTENTS
LIST OF TABLES .......................................................................................................... i
LIST OF FIGURES .......................................................................................................ii
LIST OF ABBREVIATIONS ..................................................................................... iii
INTRODUCTION ......................................................................................................... 1
CHAPTER 1. LITERATURE REVIEW .................................................................... 5
1.1. Hydrothermal Carbonization (HTC) ..................................................................... 5
1.1.1. Fundamentals and advantages of HTC .......................................................... 5
1.1.2. Process parameters of HTC ............................................................................ 6
1.1.3. Application of HTC ......................................................................................... 7
1.2. Nutrient extraction and recovery from agro-byproducts using HTC .................. 11
1.2.1. Factors influencing nutrient extraction by HTC .......................................... 11
1.2.2. Potential of axit humics (HA) and nutrient recovery from HTC water process.. 13
1.3. Fuel properties of agro-byproducts derived hydrochars ..................................... 15
1.3.1 Evaluation of fuel properties of hydrochars .................................................. 15
1.3.2. Factors influencing the fuel properties of hydrochars ................................. 16
1.3.3. Comparison between agro-byproducts derived hydrochars and other
conventional fuels ................................................................................................... 17
1.4. Soybean milk residue (okara) as an agricultural by-product .............................. 19
CHAPTER 2. MATERIALS AND METHODS ....................................................... 21
2.1. Materials .............................................................................................................. 21
2.1.1. Soybean milk residue (okara) ....................................................................... 21
2.2. Methods ............................................................................................................... 22
2.2.1. HTC of okara ................................................................................................ 23
2.2.2. Axit humics and nutrient recovery from HTC process water ....................... 26
2.2.3. Fuel properties .............................................................................................. 27
2.3. Analytical methods and equipment ..................................................................... 27
2.4. Statistical data analysis ....................................................................................... 28

CHAPTER 3. RESULTS AND DISCUSSION ......................................................... 29
3.1. Extraction of nutrients from okara using HTC ................................................... 29


3.1.1. Extraction of total phosphorus (TP) ............................................................. 29
3.1.2. Extraction of phosphate (PO43-) ................................................................... 33
3.1.3. Extraction of nitrogen as amonium (NH4+) .................................................. 37
3.2. Nutrient and acid humic (HA) recovered from HTC process water ................... 40
3.2.1. Potential of nutrients and humic substances in HTC process water ............ 40
3.2.2. Recovery of axit humics and nutrients from HTC process water ................. 42
3.2.3. Recovery of nutrients from HTC process water ........................................... 45
3.3. Fuel properties of okara derived hydrochar ........................................................ 46
CHAPTER 4: CONCLUSION AND RECOMMENDATION ................................ 50
4.1. Conclusion ........................................................................................................... 50
4.2. Recommendation ................................................................................................. 50
REFERENCES ............................................................................................................ 51
APPENDICES .............................................................................................................. 59


LIST OF TABLES
Table 1.1. Evaluations of the state of different approaches for applications of HTC
products. ........................................................................................................................ 10
Table 1.2. Table of HHV values of hydrochar from different materials ...................... 17
Table 1.3. Table of ash content values of hydrochar from different materials ............. 18
Table 1.4. Table of Fixed C values of hydrochar from different materials .................. 18
Table 2.1. Methods for examination of parameters ...................................................... 27
Table 3.1. Values of some indicators of precipitation when changing the
concentration of Fe ..................................................................................................... 43
Table 3.2. Values of precipitation at the optimal P extraction conditions.................... 44
Table 3.3. Values of precipitation at the optimal N extraction conditions ................... 44

Table 3.4. The value of ultimate analysis of hydrochar ............................................... 47
Table 3.5. The value of proximate analysis of hydrochar ............................................ 48
Table 3.6. Comparison table of HHV values of hydrochar from different materials ... 49

i


LIST OF FIGURES
Figure 1.1. Okara as a by-product of the tofu and soymilk production processes ....... 20
Figure 1.2. Image of fresh soybean by-product (okara) ............................................... 20
Figure 2.1. Tofu business household ............................................................................ 21
Figure 2.2. Soybean milk residue (okara) raw ............................................................. 21
Figure 2.3. Soybean milk residue (okara) after preparation process ............................ 22
Figure 2.4. Experimental diagram ................................................................................ 23
Figure 2.5. Vacuum filtration apparatus ....................................................................... 24
Figure 2.6. Hydrochar after drying until unchanged weight ........................................ 25
Figure 2.7. Images of apparatus used in this study....................................................... 28
Figure 3.1. Effect of the solvent categories .................................................................. 29
Figure 3.2. Effect of the solvent concentration ............................................................ 30
Figure 3.3. Effect of the HTC temperature ................................................................... 31
Figure 3.4. Effect of the HTC time ............................................................................... 32
Figure 3.5. Effect of the solvent categories .................................................................. 33
Figure 3.6. Effect of the solvent concentration ............................................................ 34
Figure 3.7. Effect of the HTC temperature ................................................................... 35
Figure 3.8. Effect of HTC time..................................................................................... 36
Figure 3.9. Effect of the solvent categories .................................................................. 37
Figure 3.10. Effect of the solvent concentration .......................................................... 38
Figure 3.11. Effect of the HTC temperature ................................................................. 39
Figure 3.12. Effect of the HTC time ............................................................................. 40
Figure 3.13. Concentrations of PO43- and NH4+ in the HTC process water................. 41

Figure 3.14. Precipitation from recovery humic acid and phosphorus process ........... 44
Figure 3.15. The concentration of N in the solution in the condensate when change the
condition of experiments ............................................................................................... 45
Figure 3.16. The images of ultimate analysis of hydrochar ......................................... 47

ii


LIST OF ABBREVIATIONS
BET:

Brunauer–Emmett–Teller

COD:

Chemical oxygen demand

FC:

Fixed carbon

HA:

Humic acid

HHV:

Higher heating value

HTC:

Hydrothermal carbonization

N:

Nitrogen

P:

Phosphorous

TCVN: Vietnam standard
TN:

Total nitrogen

TP:

Total phosphorous

VM:

Volatile matter

iii


INTRODUCTION
In the food processing, besides the process of creating quality products, the
recovery of waste by-products is very necessary. Besides being meaningful in terms of

environmental protection, the treatment of waste by-products will bring economic
benefits to businesses. One of the by-products of the food industry is soybean milk
residues (okara). Okara is the main by-product obtained after processing soybeans into
soy milk.
Our country has favorable natural conditions and farmers' agricultural experience,
so the planted area and soybean yield are quite high over the years. According to the
Food and Agriculture Organization of the United Nations (FAO), by 2009, soybean area
increased to 98.8 million hectares, production reached 222.3 million tons, yield 22.49
quintals/ha, most concentrated in America (76.0%), followed by Asia (20.6%). In
Vietnam, the General Statistics Office of Vietnam and the Ministry of Agriculture and
Rural Development reported that in 2014, the soybean growing area reached 120
thousand hectares with the total output was 176.4 thousand tons. Along with the growing
area and production of soybeans, the amount of soybean residue obtained in soy milk
production is also expected to increase. Currently, a large amount of soybeans is used
to produce soy milk. In our country, there are big soy milk brands such as Vinasoy,
Vinamilk, Tribeco. With a capacity of 120 million liters per year at Vinasoy factory
(Quang Ngai) and 90 million liters per year at Vinasoy factory (Bac Ninh), experts say:
Vinasoy is leading the production capacity of dairy products. soy bean. The total amount
of soybean residue (okara) discharged annually in two factories of Vinasoy alone can
reach more than 20 thousand tons/year.
Okara contains a large amount of nutrients (phosphorus), protein, glucide, fat and
fiber. However, okara does not last long at room temperature (less than 2 days) and even
under refrigeration due to easy decomposition. This can cause odors and pollute the
environment. Therefore, handling okara is very necessary. On the one hand, this
treatment will help reduce solid waste into the environment. On the other hand, it
contributes to the creation of useful materials (biochar, soil improvement nutrients, etc).
1


In addition, the treatment of soybean milk residues also helps bring economic benefits

to enterprises.
Hydrothermal carbonization (HTC) is a developing but promising innovation for
the treatment of waste biomass as well as agricultural residues with high moisture
(Burguete et al., 2016; He et al., 2015; Yao et al., 2016). The HTC is efficient in nitrogen
(N) recovery through change of organic-N into ammonium-N (He et al., 2015; Huang
et al., 2016), at the same time it make possible the conversion of organic-P into
inorganic-P, it is efficient in P recovery when HTC condition is acidic (Dai et al., 2015).
Okara is a by-product from soybean, which is a nutritious agricultural product.
Therefore, the potential to recover nutrients from okara is very large. In order for the
nutrient recovery process to achieve maximum efficiency, it is necessary to study to find
the optimal extraction conditions. To achieve that, it is necessary to investigate the
factors affecting the extraction process by testing the nutrient recovery method. In
addition, besides liquid phase, HTC also generates the solid product known as hydrochar,
so its energy potential needs to be assessed.

2


Research objectives and scope
This study has three main objectives as follows:
(1) Extract nutrients from okara using HTC
 Optimization of HTC process by studying effects of the addition of acids,
solvent concentration, HTC temperature, HTC time
(2) Evaluate the potential of resource recovery (HA and nutrients) from HTC
process solution
 Investigation of the potential of recovery of acid humics, PO43- and NH4+ from
HTC process water: Recovery of HA and P as solid, N as solution (NH4)2SO4
(3) Investigate the effect of extraction conditions on fuel properties of char
 Evaluation the effects of extraction conditions fuel properties of HTC char by
measuring HHV values, ash content, volatile matter.

Research significance
Vietnam is an agricultural country, where a huge amount of agricultural byproducts is generated annually. In which, to mention bean residues, agricultural byproducts obtained from the production of tofu and soy milk. Thus, recycling of agro
waste as a source of nutrients has dual environmental benefits, such as (i) reducing solid
waste into the environment, (ii) producing useful materials (soil amendments, fertilizers,
environmental materials, etc.) and create additional economic value for the business.

3


Thesis’s outline
This thesis contains of 4 chapters. The main content of each chapter is presented
as follows:
Introduction provides the research background, identifies research objectives,
research scope, main tasks, and research significance.
Chapter 1: Literature review provides information about Hydrothermal
Carbonization (HTC), its advantages and application on many fields. factors which
effect on the nutrient and humic acid extraction. The focus is placed on the nutrient and
humic acid extraction from HTC water process and fuels properties of hydrochar.
Chapter 2: Research materials and methodology describes the materials,
equipment, and methods used in this study. Experiment setting-up is described in detail.
The analytical methods as well as instruments are also introduced.
Chapter 3: Result and discussion provides results on the optimal condition of
total phosphorus, ortho phosphate and ammonium nitrogen extraction from HTC water
process. Results of humic acid and nutrients recovery. Furthermore, in this chapter,
results about fuel properties of hydrochar is also discussed.
Chapter 4: Conclusion and recommendation summaries the main findings,
limitations of this research and further research directions.

4

CHAPTER 1. LITERATURE REVIEW
1.1. Hydrothermal Carbonization (HTC)
1.1.1. Fundamentals and advantages of HTC
Hydrothermal carbonization (HTC) is a thermochemical process for the
pretreatment of high moisture content biomass with hot compressed water and making
it applicable for diversified purposes. It takes 5 minutes to 12 hours in a closed reactor
at temperatures ranging from 180 to 260 °C under pressure (2–6 MPa) condition. HTC
has been used to solve practical problems and produce desirable carbonaceous products
using a wide variety of derived waste from sewage sludge, algae to municipal solid
waste, in addition to lignocellulose biomass as a renewable feedstock. Until now,
hydrothermal processing of macro-algae research has primarily centered on
hydrothermal liquefaction to produce oils and supercritical water gasification to produce
syngas (Torres et al., 2020).
Hydrothermal carbonization produces a coal-like substance called hydrochar as
well as aqueous (nutrient-rich) and gas phases (primarily CO2) as byproducts (Fiori et
al., 2014). This solid product has a higher energy density and can be used to generate
energy or disposed of as fertilizer for soil nourishment. Meanwhile the HTC water
process from some material such as agro- waste can be used to recovery nutrients
because of high nutrients content in agro-waste. The process conditions and feedstock
used determine the distribution of these items. Decarboxylation, dehydration, and
polymerization are the key mechanisms involved in this process (Funke and Ziegler,
2010). Wet biomass contains a lot of water, which makes it an ideal solvent and reaction
medium. Since its ionic product is maximized at temperatures between 200 °C and
280 °C, water can act as both a base and an acid. Furthermore, water's dielectric constant
decreases at these temperatures, making it behave more like a nonpolar solvent (Yu et
al., 2007).
Comparing with other biochar preparation method hydrothermal carbonization
method gives some advantages such as: (1) hydrothermal carbonization employs
relatively lower temperatures (150- 350

o

C), (2) the gases generated during

hydrothermal carbonization process could dissolve in water and it makes no further
pollution to the air, and (3) water in a reactor could dilute the acidic property of bio-oil
5


generated during hydrochar production process and thus declines the deterioration to the
equipment (Kang et al., 2012; Lehmann et al., 2006). The benefit of hydrothermal
carbonization is that biomass can be converted to carbonaceous solids without the use
of an energy-intensive drying process. Hydrochar has a higher energy-to-weight ratio
than the starting material. Besides, toxic organic molecules and residual micropollutants
are also degraded during the HTC process (Wang et al., 2006).
1.1.2. Process parameters of HTC
The HTC process occurs in the subcritical zone. The characteristics of water
change drastically under subcritical conditions, as are well understood.
During HTC process, temperature, pressure, time, pH, and substrate
concentration are all important factors in the hydrothermal carbonization process, while
the kinds of biomass used also has an impact on HTC products. As mentioned earlier,
various feedstocks have been chosen for HTC in recent studies, including cellulose (Gao
et al., 2012; Lu and Berge, 2014), glucose (Lu and Berge, 2014), agricultural residue
(Abel et al., 2013), animal manures (Heilmann et al., 2014), food waste (Erdogan et al.,
2015; Parshetti et al., 2014), aquaculture and algal residues (Du et al., 2012). This proves
that the HTC process is not limited to conventional lignocellulosic biomass; feedstocks
may be more nuanced, and these renewable feedstocks contain large amounts of organic
matter that need proper treatment to avoid environmental contamination.
Temperature is a critical factor in the HTC process because it determines the

water properties that trigger ionic reactions in the subcritical zone. In the supercritical
water field, the reaction mechanism changes from ionic to free-radical reactions above
the critical stage. Temperature has a major impact on product distribution in general.
Although high temperatures reduce solid residue, hydrochar energy densification has
been observed in numerous studies, confirming hydrochar as a viable solid fuel source.
Since a long residence time increases reaction severity, it is an important factor
in hydrochar formation. As compared to temperature, residence time had a similar but
smaller impact on solid and liquid product recovery; solid hydrochar content is highest
when residence time is short and decreases as residence time increases. Meanwhile,
when the residence time is longer, the liquid product is higher.
6


Acid in HTC medium could facilitate hydrolysis, which could affect the HTC
process and the properties of the resulting hydrochar. As a result, acid addition in HTC
medium (in addition to HTC time and temperature) may be a novel technique for
controlling hydrochar properties. Regarding to the influence of pH on nutrient extraction
into HTC water process, Ekpo et al. (2016) had shown that acidic conditions favor
higher phosphorus extraction, particularly when a mineral acid (H2SO4) is added, and
offer the possibility of nutrient recovery. Following hydrothermal treatment, a large
amount of organic nitrogen is removed into the process waters. Furthermore, changes in
pH during the hydrothermal phase can have a significant impact on the characteristics
of hydrochar. In general, the HTC process is considered autocatalytic for the production
of organic acids from biomass, such as formic, acetic, lactic, citric, resulting in a pH
decrease.
Besides the temperature, residence time and pH of extraction solution, variations
in the concentration of substrates during the hydrothermal process can have a marked
influence on the content of nutrient from HTC water process and also characteristics of
hydrochar.
1.1.3. Application of HTC

HTC products (both solid and liquid) have several value-added applications in
the industry and environment.
a) Environmental material fabrication
The use of hydrochar in industry for dye, antibiotics, and heavy metal removal.
HTC of biomass improves soil fertility while also assisting in carbon sequestration. The
adsorption of prescription drugs such as ibuprofen, paracetamol, clofbric acid, cafeine,
and iopamidol is aided by hydrochar, which is activated by K2CO3 and KOH. Hydrochar
that has been reused may be altered in several ways.
Furthermore, hydrochar has a much higher adsorption potential than biochar,
despite its low surface area and porosity due to its surface rich in oxygen-containing
functional groups. Hydrochar has recently been identified as having promising
applications for efficient dye and phenolic compound adsorption from industrial.

7


Experiments for heavy metals removal were also conducted starting from agroindustrial wastes as reported by (Chen et al., 2015; Han et al., 2017; Sun et al., 2015).
(Han et al., 2017) conducted the removal capacity of Antimony (III) and Cadmium (II)
on hydrochar realized from animal manure. The total adsorption potentials for Antimony
were 2.24–3.98 mg/g for hydrochar, while higher capacities were reported for the case
of Cadmium, achieving a removal of 19.80- 27.18 mg/g.
Besides, about removing Congo red and 2-naptol was studied by Li et al. (2018,
2016) in two following works, using bamboo sawdust as material. The results proposed
that, among all, the best adsorption performances were achieved with H2SO4 or H3PO4
as acid medium and were equal to 90.51 and 72.93 mg/g, respectively, for Congo red
and 2-naphtol (Li et al. 2018, 2016).
b) Solid fuel production
Hydrothermal carbonization has been shown to be a useful method for pretreating waste biomass and producing highly carbonaceous materials that can be used in
energy applications (Babinszki et al., 2020; Sharma et al., 2020; Wilk et al., 2020; Zhang
et al., 2020). In the last five years, research projects have focused on the HTC products'

potential energetic use as a carbon-rich material for direct combustion (Gunarathne et
al., 2014; Peng et al., 2016). HTC products can have both applications for direct
combustion; supercapacitors and batteries.
Since hydrochar has stronger physicochemical properties than coal and fuels, it
could be used to replace them in the near future. Hydrochar is made from a variety of
biomass sources. In comparison to the raw original biomass, hydrochar has shown to
have improved energetic properties. In reality, the higher heating value and lower
volatile content ensure better combustion and utilization of the biomass's calorific
properties. Furthermore, the lower ash content, which is due to the removal of part of
the inorganics in the liquid process, reduces fouling and slagging, which can contribute
to boiler inefficiency and increased maintenance (He et al., 2014; Mäkelä et al., 2015;
Reza et al., 2014). Several studies looked into the possibility of using hydrochar
generated from HTC sludge for direct combustion in a boiler.

8


The use of HTC pre-treatment has recently received further attention as a result
of research into the potential use of biomass-derived material as a source for electrode
fabrication. Agricultural wastes, plant wastes, and high lignocellulosic materials have
all been stated to be suitable for energy storage (Guo et al., 2020; Sinan and Unur, 2017).
In recent years, the application of supercapacitors has gotten a lot of attention. Guo et
al.(2020) tested soybean root and found that it has a high electrode capacitance and
cycling stability.
A great potential for solid fuel conversion was proposed in Chen et al. (2018) and
Yang et al. (2016) for the some materials. Yang et al. (2016) showed the efficacy of a
220 oC, 90 min HTC treatment on six different materials. these are olive pomace, walnut
shell, hazelnut shell, apricot seed, tea stalk and wood sawdust. Each of the biomasses
studied, presented better combustion properties with respect to the initial feedstock, in
terms of heating value, carbon content. The best results were proposed to be related to

the olive pomace with an increase in the HHV and ash content were 25.6 MJ/kg and
5.5%, respectively, while hazelnut husks showed the poorest properties with a HHV of
20.6 MJ/kg and an ash percentage of approximately 12%.
c) Nutrient extraction and recovery
Phosphorus is a nonrenewable resource with a limited supply that is an essential
structural element of all living organisms cell membranes and a significant agricultural
fertilizer. As a result, a technology that can both extract and recover this finite resource
is needed. Agriculture residues generally contain high levels of organic matter and vital
plant nutrients especially phosphorus and nitrogen. For instance, Nitrogen present in a
typical soybean and corn, Phosphorus in a concentrated functional form is critical for
agriculture since it is a key component of many industrial fertilizers, which are
increasingly being made from non-renewable phosphate rocks.
Hydrothermal carbonation of waste biomass, especially sewage sludge
containing up to 4% by weight on a dry basis, has shown that up to 95 percent of the
total phosphorous content can be recovered (Becker et al., 2019; Bhatt et al., 2018;
Marin-Batista et al., 2020). It has been shown that during HTC, the phosphorous portion
segregates into the solid phase, and that acidic leaching of the recovered hydrochar
removes the P, which then passes to the aqueous phase. P can then be precipitated by
9


adding CaO (Marin-Batista et al., 2020) or sodium hydroxide solution (Becker et al.,
2019) to the aqueous solution to alkalize it. Prior to alkalization, magnesium chloride
and ammonium were added to the acidic phosphorous solution, resulting in the
precipitation of struvite, which can be used as a solid fertilizer.
In generally, a summary table (table 1.1) of technical prospects for these
processes is provided below:
Table 1.1. Evaluations of the state of different approaches for applications of HTC
products.
Approaches

Energy

Advantages

Challenges

production

(hydrochar)
 Combustion

o Direct and easy
application
o Low investment
o No chemicals required
o No concern of
organics pollutants

 Gasification

o Low requirement of
feedstock
o Energy gas produced
o High conversion rate

 Feedstock selective
 Potential ash
slagging and fouling
 Secondary
contamination

 Nutrient loss
 High investment
 High requirement
 Concentrate
pollutant in residue
 Lack of economic
analysis

 Agricultural
application

o Direct and easy
application
o Alternative cheap
adsorbent
o Excellent performance
after activation

 Environmental and
human health risks
 Potential side effects
on crops
 Lack of long-term
field trails

10


 Farmer’s perception
and concerns

 Adsorption

o Possible direct
application
o Alternative cheap
adsorbent
o Excellent performance

 Potential toxicity
 Limited results
 Low efficiency in
real application
 Lack of validation

after activation
o High stability and
reusability
o No additional reactor
requirement
 Nutrient

recovery o Simple recovery rate

(HTC water process)

 Additional

o High recovery rate

chemicals and

o Emerging technique

equipment required

o Green and sustainable
o Dual benefits: nutrient
recycling and

 Lack of
comprehensive
economic analysis

environmental
protection

1.2. Nutrient extraction and recovery from agro-byproducts using HTC
1.2.1. Factors influencing nutrient extraction by HTC
a) Effect of categories of extraction solutions
The benefits of using acidic additives in enhancing phosphate extraction are
evident. Any plausible explanation for the observed phosphorus activity must take into
account two aspects of the phenomenon. To begin, phosphorus must be stripped from
the feedstock and added to the reaction mixture. Second, after extraction, phosphorus
speciation (i.e., complexation and mineral forms) in the reaction mixture must be taken
into account. Although the exact mechanism of phosphate release remains unknown,
high concentrations of H+ cations in the HTC reaction setting seem to have catalyzed
the

reactions

involved

(dehydration,

deoxygenation,

and

decarboxylation).
11


Hydrothermal processing of biomass feedstock is thought to convert all phosphorus
species to orthophosphates, according to numerous reports (Dai et al., 2015; Zhu et al.,
2011)
In the research of using some different pH levels, these were both acidic and basic
pH levels in hydrothermal treatment of swine manure, (Ekpo et al., 2016a) had also
concluded that the nitrogen extraction was not significantly influenced by pH.
b) Effect of concentrations of extraction solutions
The concentration of additives used in the HTC process has played an important
role in solubilizing total phosphorus, phosphate in the produced liquid phase. The
amount of phosphate rised dramatically when the concentration of acid increased. As
discussed previously, acid addition had no significant effect on nitrogen solubilization.
According to Qaramaleki et al. (2020), when researched about factors affecting on
extraction nutrients, the results showed that the concentration of added substances
utilized within the HTC process has played an imperative role in solubilizing
phosphorus in HTC water process. The sum of phosphate expanded significantly from
339 to 1018 mg/L when the concentration of solvents increased from 0.1 to 0.3 M. Be
that as it may, advance increment within the added substance concentration from 0.3 to
0.5 M showed insignificant change on the amount of phosphate in the liquid phase.

c) Effect of HTC temperature
The effect of temperature on nutrient solubilization is investigated using the same
approach as that used to investigate the effect of time. The phosphate extraction is
significantly reduced when the HTC reaction temperature is raised. The maximum
volume of phosphate (PO43-) (847 mg/L) was recovered in the liquid phase at 170 °C,
according to Qaramaleki et al. (2020). The extraction in H2SO4 was again the highest
and represents 80% of TP however it is beginning to decrease compared to treatment at
170 oC.
The impact of reaction temperature on organic and oxidized nitrogen recovery is
also studied. However, the findings show a very complicated pattern in oxidized
nitrogen, with a highest concentration at 200 °C. Contrary to the trend of phosphorous
extraction, when the temperature increases, the amount of nitrogen in HTC water
12


process rises. The similar results have been published for the hydrothermal treatment of
sewage sludge at temperatures ranging from 150 to 300 degrees Celsius (Zhuang et al.,
2017).
d) Effect of HTC time
(Qaramaleki et al., 2020) depicts the effect of reaction time on phosphorus and
nitrogen recovery. The concentration of phosphate (PO43-) in HTC process water as a
function of reaction time is shown in Qaramaleki et al.(2020). The amount of phosphate
in the liquid phase appears to decrease when the HTC time increases. As a result, it
appears that the impact of reaction time on phosphorus solubilization is of minor
significance, at least for the time period examined in this study. The findings point to a
relatively fast solubilization of phosphorus in the aqueous phase, which is consistent
with other research on manure phosphorus reclamation for soil amendment applications
(Szögi et al., 2015).
In the HTC process, the effect of time on the extraction of organic and inorganic
nitrogen is expressed in a study conducted by Qaramaleki et al.(2020). While

phosphorus extraction has been shown to be relatively fast, total nitrogen solubilization
has been slow.
1.2.2. Potential of axit humics (HA) and nutrient recovery from HTC water
process
a) Types of nutrients to be recovery
A large amount of organic substances, including proteins, polysaccharides,
humic substances, and other materials are in organic waste. Among these organic
substances, proteins, polysaccharides, and lipids are easily utilized by microorganisms,
whereas humic substances, mainly humic acid (HAs), fulvic acids (FAs), and humin,
are not readily degradable during biologic treatment.
Because humic substances are compounds which physical and chemical methods
have to be applied for their removal or decomposition. Advanced oxidation processes
can convert humic substances to biodegradable micromolecular organic substances, or
even inorganic matter such as carbon dioxide, nitrogen oxide, and water (Kurniawan
and Lo, 2009; Wang et al., 2006). On the other hand, humic substances, especially HAs
and FAs, can improve soil quality and stimulate plant growth (Ayuso et al., 1997).
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Therefore, as an alternative to conventional treatments, HAs and FAs could be recovered
from the wastewater (Kliaugaite et al., 2013), which would also improve the
biotreatability of the wastewater. This recovery brings two benefits to the environment:
on the one hand, it will help reduce COD in HTC water processes when released into
the environment, on the other hand, it creates a soil amendment, providing carbon for
the soil.
Phosphorus is known as an important structural element of cell membrane for all
living organisms and an important agricultural fertilizer which is considered a
nonrenewable resource with finite supply (Ekpo et al., 2016a). Reported values for the
P and N content of organic waste show a quite high concentration. In recent years,
phosphorus recovery has piqued people's attention. This is due to a variety of factors.

To begin with, rock phosphate reserves are expected to last no longer than 150 years
(Jia, 2014; Loganathan et al., 2014; Tyagi and Lo, 2013). The rising cost of fertilizer
production is caused by the lack of high-quality phosphorus ores, which has a negative
impact on the global economy (Nieminen, 2010). As a result, there is a need to look for
other phosphorus sources.
b) Humic acids and Nutrient recovery methods
There are some kinds of methods for nutrient recovery. Typical phosphorus
recovery products include calcium phosphate and magnesium ammonium phosphate
(MAP). Phosphate industries can recycle calcium phosphate, while agriculture can use
MAP as a slow-release fertilizer (Tyagi and Lo, 2013). Because of its higher solubility
and ability to retain both nitrogen (NH4+) and phosphate (PO43-), MAP appears to be
favored over other calcium phosphate compounds. Fischer et al. presented a concept for
sustainable decentralized phosphate recycling by microbial fuel cell enables phosphate
recovery from digested sewage sludge as struvite. The process yielded up to 82% or 600
mg/L. The mobilized phosphate was precipitated through the addition of Mg2+ and NH4+
as struvite, NH4MgPO4.6H2O. However, the methods recovery nutrient and humic acid
by using calcium phosphate or magnesium ammonium phosphate enable the recovery
nitrogen (NH4+) and phosphate (PO43-) but do not allow the recovery of humic acids.
Another methods for recovering humic acids and nutrients that they can be
removed from water by reaction with Ca2+, Al3+, and Fe3+ (Kloster et al., 2013; Sun et
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al., 2011). However, aluminum salts are not suitable for fertilizer manufacturing, and
therefore ferric chloride was tested in this study. Although Fe-coagulant species have
been investigated in the field of water supply, their effect on wastewater with a complex
composition and containing high concentrations of humic substances is not clear.
Moreover, the process of recovering humic substances as fertilizer has not been verified
practically. This method can help recover both acid humics and phosphorus from water.
Humic acids and phosphorus will be precipitated in solid phase after recover process.

According to Yang and Li (2016), at the optimal condition for recovering humic acids
and nutrient from dewatering effluent of thermally treated sludge by using FeCl3, the
percentage of precipitated humic substances rised up to 70%. Besides, it was noticed
that the content of phosphorus in the dewatering effluent is high, 160 mg/L, which is
also an important source of inorganic fertilizer. During the process of recovering humic
acid substances and nutrient, the soluble phosphorus in the wastewater, mainly
orthophosphate, was also precipitated. At the optimal condition, the precipitation rate of
phosphorus was almost 100% (Yang and Li, 2016).
Thus, the feasibility of the process was considered in this study. Based on the
characteristics of the HTC water process, the effects and mechanism of humic recovery
were analyzed under different pH and ferric dose conditions. The material recovered
under optimized conditions was examined to determine its compliance with the
standards of organic fertilizer, and its performance as a fertilizer was also investigated
using planting experiments.
1.3. Fuel properties of agro-byproducts derived hydrochars
1.3.1 Evaluation of fuel properties of hydrochars
In recent years, solid char from HTC is emerging as a potential contributor to the
energy sector beacause it has favorable chemical characteristics, high HHV, and
abundant availability in terms of resources (Kambo and Dutta, 2015). In this study, fuel
characteristics such as higher heating value (HHV), proximate and ultimate analyses of
hydrochar from HTC process are discussed.
No matter how biomass is used, direct combustion or co-firing with other fuels
(e.g. coals), the heating value, also known as calorific value or heat of combustion,
defines the energy content of a biomass fuel and is one of the most important
15


characteristic parameters for design calculations and numerical simulations of thermal
systems.
The proximate analysis encompasses the quantitative assessment of fixed carbon,

volatile materials, ash, and moisture content in biomass or fuel (Khan et al., 2009). The
goal of proximate analysis is to discover the ratio of combustible components (fixed
carbon and volatile matter) to non-combustible elements (ash and moisture content),
which permits estimation of the energy content of biomass and fuels. Higher content of
ash and moisture is undesirable for fuels.
Ultimate analysis, on the other hand, refers to the elemental composition in terms
of hydrogen, carbon, nitrogen, oxygen, and sulfur. The goal of final analysis is to
determine the composition and quantity of gas emitted during combustion, as well as
the amount of oxygen required to burn biomass and fuels. The carbon and hydrogen
contents will contribute the HHV of fuels. Nizamuddin et al.(2016) showed that, the
lower hydrogen and carbon contents were, the lower HHV of fuels were. The HHV
values reduced around 9.3 MJ/kg when the carbon and hydrogen declined 23.8 and
1,2 %, respectively.
1.3.2. Factors influencing the fuel properties of hydrochars
The properties and percentage distribution of the final products strongly depend
upon the process conditions (Yan et al., 2010). Although both reaction time and
temperature have been observed to influence the physicochemical characteristics of
products, the reaction temperature remains the governing process parameter. Hydrochar
is the desired product in the HTC process with the mass yield of about 40–70% (Yan et
al., 2010).

It has been reported that with the equivalent mass yield, the energy

densification ratio (ratio of the HHV of biochar to the HHV of raw biomass) obtained
via HTC is significantly higher than obtained via torrefaction (Liu and Balasubramanian,
2014; Yan et al., 2010). The addition of salts and acids has been recommended by few
researchers to improve the physicochemical properties of hydrochar, and to reduce the
reaction pressure and temperature (Lynam et al., 2011).
The carbon substance of hydrochar from peels of Carya cathayensis sarg was
concentrated from 51.5 to 84.8% with expanding of hydrothermal carbonization

16


temperature, at the side of critical diminishing of oxygen substance. In any case, the
hydrogen and nitrogen substance were about the same. Besides, the ash substance was
expanded when the hydrothermal carbonization temperature rised (Yang et al., 2015).
The carbon substance was also increased when peels of Carya cathayensis sarg
were treated at longer times at 280 and 360 oC, separately. The oxygen substance was
indeed decreased. The low oxygen substance was due to the decomposition of
hemicellulose and cellulose which contained high oxygen substance at that treatment
temperature (Yang et al., 2015).
1.3.3. Comparison between agro-byproducts derived hydrochars and other
conventional fuels
The HHV value of hydrochar from agricultural by-products is usually smaller
than the HHV value of conventional energy fuels. Although the HHV of materials from
agro-waste is lower, agrowaste is a renewable source and fossil fuels are non-renewable,
which will be depleted. Therefore, finding another type of fuel to replace fossil fuel is
essential.
Table 1. 2. Table of HHV values of hydrochar from different materials
Number

Type of hydrochar

HHV (MJ/kg)

Author, Year

1

Corn stover

16.2

Fueres, 2010

2

Eucalyptus sawdust

16.69

(Sevilla et al., 2011)

3

Barely straw

17.34

(Sevilla et al., 2011)

4

Coconut fiber

18.4

(Liu et al., 2013)

5

Maize sillage

22.3

(Mumme et al., 2011)

6

Fuel oil

42.9

Wei Yang, 2014

7

Diesel oil

45.7

Wei Yang, 2014

From table 1.1 we can see the difference between the HHV value of hydrochar
from agricultural by-products and the HHV of conventional energies. The HHV values
of hydrochar from agricultural by-products ranged from 16 MJ/kg to 22 MJ/kg.

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