INTRODUCTION
Hydrothermal Carbonization of Lignocellulosic Biomass
Crude oil, a vital energy source derived from ancient solar energy captured by plants, faces challenges due to its finite reserves and environmental impact, particularly greenhouse gas emissions like CO2 These issues, along with volatile market prices and limited availability, have prompted a shift towards renewable and alternative energy sources.
Biomass energy production has been utilized since the dawn of human civilization, primarily through wood Historically, the efficient use of the organic fraction of solid waste has been explored in various literature reviews Recently, scientists worldwide have been investigating hydrothermal processes, particularly hydrothermal carbonization (HTC), which has gained significant attention over the past decades This thermochemical process transforms biomass waste into high-value products and is often referred to as wet torrefaction, as it employs wet biomass unlike other methods such as pyrolysis and incineration The resulting product, known as hydrochar, combines "hydro" for water and "char," representing the conversion of carbon-containing organic materials into usable forms.
Hydrothermal carbonization refers to the natural geological processes that lead to the formation of oil and coal (Silakova, 2018) This process resembles the way plant and animal remains transform into coal or crude oil, albeit on a larger scale.
The laboratory-scale process effectively manages agricultural, industrial, and municipal waste with high moisture content, utilizing water as a thermal reaction medium This innovative approach allows for the processing of biomass without the need for moisture removal, eliminating the drawbacks associated with costly and energy-intensive pretreatment methods, the requirement for dried feed, and the necessity for separate systems to handle wet and dry biomass.
This study focuses on the hydrothermal carbonization (HTC) of biomass from three industries—restaurant food waste, paper mill sludge, and sawdust—that contribute significantly to solid waste The research aims to enhance the properties of these materials for solid fuel applications, examining how process temperature and reaction time affect the final product's characteristics The findings may inform future research on utilizing biomass for various purposes, highlighting HTC as an effective method for converting low-value biomass into solid fuel The investigation comprises two main components: a literature review in Chapter 2, which explores the HTC process, its conditions, input and output characteristics, and potential applications, and experimental work in Chapters 3 and 4, which identifies the properties of hydrochar and process water.
5 reasoning behind some of the observations and the potentiality of the samples as a solid fuel were also presented h
LITERATURE REVIEW
Historical overview of HTC process
The HTC process, first described by German chemist Friedrich Bergius in 1913, aimed to simulate natural coal formation from organic materials in the lab Alongside Carl Bosch, Bergius was awarded the Nobel Prize in Chemistry in 1931 for their pioneering work in high-pressure chemical conversion methods He elucidated the hydrogenation effects on coal and oil formation under extreme conditions, known as the Bergius process, and also detailed the hydrothermal transformation of cellulose into coal-like substances in the same year.
Following the Nobel Prize, researchers like Burl and Schmidt explored hydrothermal carbonization (HTC) of various biomass types at temperatures ranging from 150 °C to 350 °C (Marchetti, 2012) Schuhmacher later investigated the impact of pH on HTC reactions, identifying variations in the fundamental components (C, H, and O) (Titrici et al., 2007) Subsequently, research on this thermochemical process experienced a decline in focus.
In the past decade, hydrothermal carbonization (HTC) has emerged as a promising alternative energy source, particularly for converting biomass into sterile fuel (Fink and Andrin, 2011) German scientist Markus Antonietti's research in 2006 highlighted a straightforward method for transforming biomass into a coal-like material through increased temperature and pressure (Libra et al., 2011) This process, dubbed the "black revolution," has garnered significant attention within the scientific community However, despite its rising popularity, the body of literature on hydrothermal carbonization remains limited when compared to other thermal conversion techniques (Libra et al., 2011).
Biomass And Thermochemical Processes
The production of energy from biomass involves two main processes: biological treatment and thermochemical treatment Biological treatment utilizes living organisms like bacteria and fungi to oxidize and stabilize organic matter, ultimately generating valuable energy streams (Basso, 2016) In contrast, thermochemical treatment focuses on the combustion of biomass through methods such as gasification, pyrolysis, and co-firing of woody and herbaceous materials This process typically involves drying lignocellulose-rich biomass to produce electricity or heat via boilers and gasifiers, while the biological process transforms protein and water-rich biomass into compost or energy.
Torrefaction is a thermochemical conversion process that utilizes biomass as the primary input, yielding biochar as the main product along with liquid and gaseous byproducts This process involves the degradation of organic compounds in a low-oxygen environment, resulting in the production of biochar.
8 conditions Compared with pyrolysis, torrefaction manages to produce higher caloric value output which can be utilized as high-quality solid fuel (Choo et al.,2020)
Torrefaction can be divided into two categories, namely,
Wet torrefaction, also referred to as the hydrothermal process, differs from dry torrefaction, which is the conventional method for torrefaction In dry torrefaction, temperatures between 200°C and 300°C are employed under atmospheric pressure and in an inert nitrogen gas environment, completely devoid of oxygen This method is also commonly known as mild pyrolysis or low-temperature pyrolysis.
Table 2.1 Characteristics of biomass torrefaction
Parameter Features of torrefaction process
Reaction time Several hours to a days
Biomass feed Any type of dried organic material
Advantages Simple process, less energy intensive
Pyrolysis is a thermochemical process that involves heating biomass to temperatures between 450°C and 850°C in an oxygen-free environment This process results in the biomass decomposing into various phases, yielding a mixture of solids, liquids, and gases The proportions of these products are significantly influenced by the rate of the pyrolysis process.
Conventional pyrolysis primarily focuses on charcoal production through a slow phase that enhances solid yield In contrast, fast pyrolysis emphasizes hydrogen production by rapidly exposing biomass to high temperatures for mere seconds, resulting in liquid and gaseous products This process generates combustible gases that can be condensed into pyrolysis oil (bio-oil) Overall, biomass pyrolysis yields permanent gases such as CO2, CO, H2, and light hydrocarbons, alongside liquid products and a carbon-rich solid residue.
Table 2.2 Characteristics of biomass pyrolysis
Reaction time From seconds to hours (flash, fast and slow)
Biomass feed Small (2mm) dried feed
Advantages Biochar can be used as a soil amendment or energy carrier, bio-oil can be used as fuel in engines and turbines
Disadvantages High cost of investment, require energy supply, process water need to post treat
Fuel produced Bio-oil and biochar
Gasification is a thermal conversion process that transforms organic matter into a gaseous product known as syngas through oxidation This process occurs at high temperatures exceeding 700°C and primarily produces hydrogen (H2), carbon monoxide (CO), and small quantities of methane (CH4), along with water vapor, carbon dioxide (CO2), nitrogen (N2), and tar (Choo et al., 2020) Various oxidants, including air, pure oxygen, steam, or a combination of these gases, can be utilized in the gasification process.
(Marchetti, 2012) According to the composition of the oxidant, there are two types of gasifiers and the final product varies with the dimensions of the gasifier
Air-based gasifiers generate a product gas with a high nitrogen concentration and low heating value, while oxygen and steam-based gasifiers yield a product gas rich in hydrogen and carbon monoxide, offering a higher heating value.
Table 2.3 Characteristics of biomass gasification
Parameter Features of gasification - process
Biomass feed Dried and uniform size of feedstock
Oxidizing agent Air or oxygen
Disadvantages High cost of investment, complex and sensitive process, biomass feed need to reduce the its size
Fuel or energy produced Syngas
Hydrothermal processes occur in a high-pressure, hot water environment, emphasizing the importance of directly treating wet substrates without the need for drying pre-treatments.
The hydrothermal process is divided into three categories related to the condition parameters (Basso, 2016)
Raising the temperature to 400 °C during hydrothermal liquefaction significantly reduces the solid product yield while enhancing the production of gas and liquid outputs This temperature is crucial for optimizing the HTL process, where the primary goal is to generate liquid products The liquid outputs from HTL primarily consist of liquid hydrocarbons and heavy oils.
Increasing temperatures enhance hydrothermal gasification, primarily resulting in gaseous products such as hydrogen (H2), methane (CH4), and carbon dioxide (CO2) These gases can be combusted after a cleaning process, for instance, in a gas turbine or Biomass Integrated Gasification, and are also viable for pure hydrogen production (Basso, 2016).
Hydrothermal carbonization operates at temperatures ranging from 180°C to 250°C, yielding a minimal gas output of 1-5% while producing a significant amount of solid char This process involves the chemical decomposition of feedstock, resulting in a carbon-rich solid material characterized by enhanced chemical stability.
The hydrothermal carbonization process involves submerging biomass in a liquid phase, subjecting it to high temperatures and autogenous pressure for several hours Throughout this process, water remains in liquid form due to elevated pressure and temperature When conditions reach a certain threshold, water transitions into a supercritical state, exhibiting properties that are neither purely liquid nor gas (Marchetti, 2012).
The ideal temperature for the carbonization process is around 200 °C, as it prevents liquefaction and gasification Both temperature and pressure significantly impact this process In hydrothermal carbonization, the oxygen content, indicated by the O/C ratio, and the hydrogen content, represented by the H/C ratio, play crucial roles.
The HTC process offers a significant advantage by improving and preserving the carbon content of original biomass, even as 12% of the material is reduced, resulting in an increased carbon concentration (Basso, 2016).
In summary, the main process conditions for hydrothermal carbonization reactions are noted as (Marchetti, 2012) :
Biomass surrounded by water in all reactions
The liquid phase water reacts with high pressure (at least saturation pressure)
The temperature range is in the range of 180-250 ° C with a pressure of about 20 bar
The HTC reaction process time is about 1-72 hours
Table 2.4 Characteristics of biomass Hydrothermal carbonization
Reaction time 30 minutes to days
Biomass feed Any type of organic material
Advantages Can use wet biomass without pre-drying, simple process, the higher conversion efficiency
Disadvantages High cost of investment, require energy supply, process water needs to post- treatment
2.2.2 Biomass: definition, properties, and comparison
Biomass, as defined in the Encyclopedia of Ecology (2008), refers to the total mass of living organisms, encompassing plants, animals, and microorganisms From a biochemical standpoint, it includes essential components such as cellulose, lignin, sugars, fats, and proteins.
Overview of HTC Process
2.3.1 Reaction mechanism of HTC with biomass
The subcritical temperature is essential for facilitating key reactions in the hydrothermal carbonization (HTC) process, many of which resemble those occurring during pyrolysis A significant reaction is hydrolysis, which involves the breakdown of ester and ether bonds in cellulose (at temperatures above 200°C), hemicellulose (above 180°C), and lignin (above 200°C) through the addition of water molecules (Basso, 2016).
16 suggested by many researchers During hydrolysis, the major components of biomass degrade into various molecules
Figure 2.2 Degradation products and sub products during hydrolysis of lignocellulosic biomass (Qadariyah et al.,2011)
The hydrolysis of hemicellulose yields acetic acid, D-xylose, D-mannose, D-galactose, and D-glucose, with D-mannose, D-galactose, and D-glucose subsequently transforming into 5-hydroxymethylfurfural (5-HMF), which ultimately produces formic or levulinic acid Similarly, cellulose hydrolyzes into glucose, leading to the formation of 5-HMF and its conversion into formic or levulinic acid (Basso, 2016) Additionally, lignin degradation results in the production of phenolic compounds.
The decarboxylation and dehydration reactions significantly influence the H/C and O/C ratios of hydrochar during the hydrothermal carbonization (HTC) process Dehydration primarily occurs around hydroxyl (OH) groups, while decarboxylation involves carboxyl and carbonyl groups In the Van Krevelen diagram, the dehydration line moves from the top right to the bottom left, whereas the decarboxylation line shifts from the bottom right to the top left Consequently, dehydration decreases both H/C and O/C ratios, while decarboxylation increases the H/C ratio and decreases the O/C ratio, with dehydration playing a crucial role in these transformations.
17 reaction counts as the important reaction, since it enhances the formation of highly reactive hydrolysis fragments which later condense and polymerize to form hydrochar
Research by Qadariyah et al (2011) indicates that the mechanism of formation is governed by two key reactions: ionic reactions, which dominate at subcritical and near-critical temperatures, and free-radical reactions, prevalent at supercritical temperatures This temperature variation allows for the formation of aromatic structures, which can be confirmed through 13C-NMR measurements Additionally, the residence time during the hydrothermal carbonization (HTC) process significantly influences the aromatization process, as the formation of aromatic bonds reduces the hydrochar's carbon content Therefore, the impact of reaction time on aromatization reactions warrants further investigation (Basso, 2016).
Other mechanisms occurring during the hydrothermal carbonization can be summarized as:
During the Demethylation reaction, the phenol removes the methyl group from its structure and transforms into the catechol-like structure of the coal
The transformation reactions particularly take place in lignin with a crystalline structure and oligomer fragments (Mok and Antal, 1992) when successive condensation reactions cannot occur during the hydrolysis reaction h
The pyrolytic reaction occurs in fragments that are isolated from water due to the precipitation of condensed materials This reaction takes place at temperatures exceeding 200°C, leading to the production of carbonaceous substances (Qadariyah et al., 2011).
The HTC process results in three distinct states of matter: the solid phase, which consists of carbon-enriched hydrochar; the liquid phase, comprising a mixture of phenolic compounds and furan derivatives; and the gas phase, which contains a small amount of gas, primarily CO2.
Hydrochar exhibits a reduced number of carboxyl and hydroxyl groups, along with lower H/C and O/C ratios, due to decarboxylation and dehydration reactions compared to the initial biomass Despite this, hydrochar contains more functional groups than natural bituminous coals and demonstrates increased hydrophobicity relative to the original biomass According to Berge et al (2011), the carbonization process enhances the structural stability of hydrochar through the formation of fused aromatic rings, making it suitable for use as an amendment.
Research indicates that the hydrochar produced from the hydrothermal carbonization (HTC) process retains a significant amount of carbon from the raw feed, with studies showing retention rates of up to 80% by weight (Berge et al., 2011; Li et al., 2011; Kruse et al., 2013) This high carbon retention contributes to a notable increase in the energy content of hydrochar, with enhancements ranging from 1.01 to 1.41 times by weight and 6.39 to 9 times by volume compared to the original feedstock (Lu et al., 2011) Additionally, the higher heating value of hydrochar is reported to be 1.50 to 1.70 times greater on a weight basis than that of the feedstock (Roman et al., 2012) These findings underscore the potential for energy exploitation through the hydrochar process.
N2 adsorption at 77K, the surface of hydrochar ranges between 25 and 30 m 2 g -1 (Bernardo et al.,2011) and adsorption isotherms correlate with the type 2 adsorption in accordance to h
IUPAC classification Therefore, hydrochar clearly shows the capability of work as an activated carbon adsorbent
The process water derived from hydrochar is a complex mixture of organic compounds, primarily including acetic acid, aldehydes, alkenes, and various aromatics such as furanic and phenolic compounds (Lu et al., 2011) Research by Heilmann et al (2011) indicates that this water also contains non-agglomerated colloidal carbonized materials and a tar fraction with high molecular mass polar compounds GC-MS analyses conducted by Xiao et al (2012) identified sugar-derived compounds like Furfural and 2-ethyl-5-methyl-furan, alongside lignin-derived phenol monomers (Basso, 2016) The chemical oxygen demand (COD), biochemical oxygen demand (BOD), and total organic carbon (TOC) levels in this process water are comparable to those found in landfill leachate Additionally, the presence of organic acids contributes to its acidic pH, with a BOD/COD ratio exceeding 0.3 (Lu et al., 2011).
The gaseous phase in the Hydrothermal Carbonization (HTC) process constitutes 5% of the final products, primarily consisting of gases like CO2, CO, CH4, H2, and hydrocarbons such as ethane, ethene, and propene (Lu et al., 2011) The predominant gas produced is CO2, while the lower yield of the gaseous phase is attributed to the limited oxygen availability within the reactor during HTC, in contrast to processes like direct combustion or pyrolysis.
2.3.2 The role of water in HTC
In the HTC process, water serves as a catalyst rather than a solvent, enhancing both the reaction rate and heat transfer compared to other mediums Due to its stability, the removal of water from biomass during HTC requires energy, which water can efficiently transfer and store.
20 this heat effectively Moreover, it can serve as a distribution medium for homogeneous and heterogeneous catalysts/additives (Funke et al.,2010)
The chemical properties of water change notably with temperature; as the temperature rises, the viscosity, surface tension, density, and dielectric constant of water decrease, while its diffusion and self-ionization capacity increase (Marchetti, 2012).
Water can function as both an acidic and basic catalyst, with its critical point occurring at 374°C and 22.1 MPa (Marchetti, 2012) Below this critical point, water is in a subcritical state, while above it, it enters a supercritical phase As temperature increases in the subcritical region, hydrogen bonds in water weaken, leading to the formation of acidic hydronium ions (H3O+) and basic hydroxide ions (OH−) At the critical point, the structure of water changes significantly as infinite hydrogen bonds are broken, resulting in the formation of distinct clusters with chain-like structures.
Near the critical point, the dielectric constant of water decreases, leading to an increase in its viscosity and self-diffusion coefficient At this critical juncture, ionic products peak at temperatures ranging from 227°C to 327°C, influenced by pressure Consequently, under subcritical conditions, rising temperatures also elevate pressure and enhance dielectric properties This behavior allows subcritical water to effectively extract and facilitate hydrolysis and oxidation of organic compounds, which are essential reactions in the hydrothermal carbonization (HTC) process.
The hydrothermal process is primarily influenced by temperature and reaction time, while other factors like pressure, solid-to-liquid ratio, and pH value also play significant roles Additionally, the type of feedstock used is crucial in determining the effectiveness and results of the HTC process.
MATERIALS AND METHODS
HTC applicable industries
The HTC process can be applied to any industry where there are biomass wastes This research mainly focuses on three industries and there potential applicability of the HTC process
Figure 3.1 Food waste collecting site, Canada
The food we consume is the result of numerous production steps, and wasting it also wastes the resources used in these processes, particularly water Water is essential for growing crops, raising livestock, and packaging and transporting food For instance, discarding an apple contributes to the loss of approximately 170 trillion liters (or 45 trillion gallons) of water annually Given that water is a vital resource for human survival, its conservation is crucial.
The minimum daily water requirement per person is 15-20 liters Reducing food waste significantly contributes to conserving water, which can help provide essential water resources to those in need globally.
Food waste predominantly ends up in landfills, where it decomposes and generates greenhouse gases like methane and carbon dioxide This waste contributes to greenhouse gas emissions not only during decomposition but also throughout the transportation process Furthermore, the conversion of forests and natural areas into agricultural land negatively impacts biodiversity Scientists estimate that halting food waste could prevent up to 11% of the greenhouse gas emissions associated with the food system.
Figure 3.2 Paper mill sludge, Peninsular Malaysia h
The paper mill industry is the fifth largest energy consumer globally, representing 4% of total energy production In the United States, it ranks as the sixth-largest industry based on emissions to air, water, and land In 2015, the industry emitted approximately 79,000 tonnes of pollutants, accounting for 5% of all industrial emissions in the country Of these emissions, 66% were released into the air, 10% into water bodies, and 24% were disposed of in landfills.
The Vietnam paper industry, one of the country's oldest sectors dating back to 284 B.C., evolved significantly over the centuries By the 20th century, handmade paper was widely used for writing and folk painting The establishment of Vietnam's first industrial paper mill in 1912 in Viet Tri marked a turning point, with a production capacity of 4,000 tonnes per year Throughout the 1960s, numerous small and mid-sized paper mills emerged, each producing under 20,000 tonnes annually The industry's growth was further solidified in 1982 with the Bai Bang Paper Factory, which boasted an annual capacity of 53,000 tons of pulp and 55,000 tons of paper, establishing the paper industry as a key sector in Vietnam's economy.
2000 – 2006 the paper industry saw an 11% (Habubank Securities, 2009) increase while contributing 64% of the annual paper demand of Vietnam
According to the Vietnam pulp and paper association (VPPA) in the first two months of
In 2020, Vietnam's paper production increased to 687,570 tonnes, marking an 11.8% rise from 2019, while paper sales reached 837,855 tonnes, up 9.8% Additionally, paper imports rose by 11.9% to 327,474 tonnes, and exports surged by 26.3% to 167,684 tonnes (Duc, 2020) However, the pulp industry, a significant sector in Vietnam, generates waste that contributes to water and land pollution, with various waste types produced at each production stage In Ho Chi Minh City, paper constitutes 8.2% of total municipal waste, predominantly ending up in landfills without any recycling efforts.
Figure 3.3 Forestry waste Terrace Community Forest, Northwest British Columbia
Timber is a renewable and sustainable resource, essential for various industries The United Nations Food and Agriculture Organization projected a 45% increase in global industrial timber consumption by the end of 2020, while the World Bank anticipates that this consumption will quadruple by 2050.
The harvesting and disposal of timber products can lead to significant environmental impacts throughout the supply chain Urban areas contribute to this issue through the generation of commercial and industrial waste, construction and demolition activities, as well as discarded pallets and packaging, resulting in timber waste being improperly disposed of in the environment.
Timber waste often ends up in landfills due to a lack of reuse, recycling, or refurbishment, contributing to greenhouse gas emissions during transportation Once in the landfill, this waste occupies significant space, increasing the demand for new landfill sites, particularly when timber is modified with synthetic materials.
Wood waste significantly contributes to toxic waste, with landfill data indicating that Sydney and Melbourne dispose of approximately 446,000 and 623,000 tonnes annually (Tucker et al., 2009) Additionally, burning timber waste releases harmful smoke containing carbon, CO2, and dioxins (Adhikari et al., 2018).
Efficient utilization of wood waste plays a crucial role in minimizing environmental impact while meeting the demand for timber products without causing further ecological harm Research by Dionco et al (2001) reveals that for every cubic meter of trees harvested, approximately 50% is wasted, which includes damaged residuals, abandoned logs (3.75%), stumps (10%), tops and branches (33.75%), and butt trimmings (2.5%) This highlights that wood waste constitutes a substantial fraction of overall waste materials.
Wood waste primarily originates from low-quality logs with significant defects, as well as bark, off-cuts, sawdust, slabs, and edged trimmings from sawn timber The use of advanced technology to process these low-quality logs can enhance wood recovery; however, such technology is often limited to developed countries In developing nations like Vietnam, the main contributors to wood waste include outdated technology, inefficient production methods, and management practices, alongside administrative challenges To address these issues, innovative approaches for utilizing timber waste are essential One effective strategy involves altering energy sources and consumption patterns, such as converting sawmill by-products into thermal energy, which can reduce reliance on fossil fuels and promote bioenergy production at sawmill sites For instance, sawdust can be transformed into hydrochar, significantly increasing its heating value from 16.94 MJ/kg to 27 MJ/kg (Adhikari et al., 2018).
Materials
In this research, it mainly focuses on three kinds of biomass feeds, All of which consist of more or less cellulose, hemicellulose, and lignin
Food waste samples were collected from a fried rice and noodle restaurant near the MEE laboratory on Nguyen Co Thach street, comprising a significant amount of water, food scraps, and paper napkins Additionally, paper mill sludge was sourced from Hanoi Paper and Packaging Production Trading Co., Ltd., consisting of dewatered clay and paper pulp Sawdust was obtained from a sawmill in Ngọc Đại, Đai Mễ, Từ Liêm, Hà Nội In the laboratory, all samples underwent moisture removal according to ASTM D3173 standards, and prior to the HTC process, they were ground using an electrical blender and sieved through a 5-millimeter opening.
Figure 3.4 Raw dried restaurant food waste, raw dried paper mill sludge and raw dried saw dust
The grinding process ensured the uniformity of the samples, significantly impacting the efficiency of the HTC process To prevent moisture exposure, the dried feed was stored in polythene bags within a desiccator The milled dried samples are illustrated in Figure 3.4.
Description of the HTC reactor
The experiments were conducted using HTC batch reactors in the Environmental Engineering laboratory, each with a total volume of 45ml These reactors featured a Teflon inner tube and a stainless steel outer column and cap, designed for durability and precision The inner tube measures 17cm in height and 28mm in diameter, while the outer tube has a height of 18cm and a diameter of 30mm, as illustrated in figure 3.5.
Figure 3.5 Hydrothermal carbonization reactor with Teflon inner compartment and stainless steel outer cover
The Carbolite Gero laboratory furnace, capable of reaching temperatures up to 1200°C and featuring an integrated timer, was utilized in the experiments to effectively control temperature and reaction time.
Experimental methods and principles
The HTC process involved mixing 4.0 g of biomass with 36 mL of deionized water in a Teflon tube, maintaining a 9:1 water-to-biomass ratio The Teflon tube was secured within a stainless steel tube and placed in a furnace where the temperature was gradually increased Experiments were conducted at reaction times of 3, 4, and 6 hours, and temperatures of 180 °C, 200 °C, 220 °C, and 250 °C The biomass primarily comprised proteins, cellulose, hemicellulose, and lignin, with optimal degradation temperatures identified as 150 °C for proteins, 180 °C for cellulose, 220 °C for hemicellulose, and 250 °C for lignin Research indicates that 200 °C is the optimum temperature for the HTC process, suggesting that maintaining temperatures around these values can enhance yield and reduce energy consumption Additionally, yields and properties of hydrochars stabilized after 6 hours, indicating that extending the reaction time beyond this point had minimal impact on hydrochar yield Thus, a maximum reaction time of 6 hours was selected.
After the hydrochar process, the solid and liquid mixture was separated using Newstar 15D filter paper The resulting liquid fraction, referred to as process water, was stored in 50ml plastic centrifuge tubes at 4°C in the refrigerator Meanwhile, the solid fraction, known as Hydrochar, was dried at 105°C for 24 hours and then stored in a desiccator to avoid moisture exposure The collected solid and liquid samples were systematically named for identification.
After each carbonization experiment, solid and liquid samples were collected for measurement The hydrochar and untreated biomass samples underwent analysis for ash content, dry solid content, volatile solid content, SEM, EDS, and Gross calorific value, adhering to standard procedures Liquid samples were tested for pH, Electrical conductivity (EC), Total Nitrogen (TN), Total phosphorus (TP), and Chemical oxygen demand (COD) Each measurement was triplicated, and the average value was used to represent the characteristics of the samples SEM and EDS analyses of solid products were performed at the Nanotechnology Laboratory of Vietnam Japan University, while Gross calorific value was determined at the Physical Chemistry Laboratory of Vietnam French University Prior to Gross calorific value measurement, samples were ground and sieved through a 60 mesh according to ASTM D3172 standards.
3.4.1 pH pH value is an indicator of the amount of H ion in the medium in the terms of negative logarithm of the hydrogen ion concentration This value helps us to determine whether the medium is acidic or basically relevant to the amount of H + ions In the research, the pH value was measured by a portable parameter digital meter following ASTMD1293-B4 standard method For each temperature-time constraint, average pH values were calculated h
44 and the obtained value presented as the pH value of the sample represents subsequent temperature and time constraints
Electrical conductivity (EC) quantifies a material's ability to conduct electric current, providing insights into the concentration of inorganic substances in process water The measurement units for EC are microsiemens per centimeter (μS/cm) or millisiemens per centimeter (mS/cm) In this study, EC values were obtained using a portable multi-parameter digital meter, adhering to Standard Method 2510.
B For each temperature-time constraint, average EC values were calculated and the obtained value was presented
3.4.3 Total Nitrogen (NCASI Method TNTP-W10900)
Reagents Solution A (NaOH and 3g K2S2O8), Hydrochloric acid solution, Stock Nitrogen solution, Standard nitrogen solution
Solution A: prepared by dissolving 4g of NaOH and 3g K2S2O8 in 100 mL of distilled water Hydrochloric acid solution: prepared by (HCl: H2O = 1: 19): mixing 10 mL HCl into 190 mL of distilled water
Stock nitrogen solution: prepared by dissolving 0.722g of pre-dried (105 o C for one hour) KNO3 in distilled water and diluting it to 1000 mL (1 mL= 0.1mgN = 100 mgN/L)
Standard nitrogen solution (10 mg/L): prepared by diluting from Stock nitrogen solution 10 times Measure the absorbance at 220 nm with a spectrophotometer h
Figure 3.7 Calibration curve for total nitrogen
Take 50 mL of sample into the 125 mL Erlenmeyer flask Add 10 mL solution A Cover the flask and boil gently approximately for 30 -40 minutes at 120 o C Cool and filter the sample Take 30 mL solution into 50 mL volumetric flask and dilute to 50ml mark with distilled water Add 6.5 mL HCl and after 15 minutes, measure the absorbance at 220 nm with a spectrophotometer and determine the total nitrogen concentration using the standard curve The color is stable for at least one hour For concentration in the range less than 3 mgN/L
3.4.4 Total phosphorus (NCASI Method TNTP-W10900)
Reagents preparation Ammonium molybdate-antimony potassium tartrate solution, Ascorbic acid solution, Sulfuric acid, 11 N solution
Ammonium molybdate-antimony potassium tartrate solution: Dissolve 8g of ammonium molybdate and 0.2g antimony potassium tartrate in 800 mL of distilled water and dilute to
Ascorbic acid solution: dissolve 60g of ascorbic acid in 800 mL of distilled water and dilute to 1000 mL Add 2 mL of acetone This solution is stable for two weeks
Sulfuric acid, (11 N): slowly add 310 mL of concentration of H2SO4 appropriately to 600 mL distilled water Cool and dilute to 1000 mL y = 0.2048x + 0.0144 R² = 0.9982
Figure 3.8 Calibration curve for total phosphorus
To analyze phosphorus concentration, begin by taking a 50 mL sample in an Erlenmeyer flask and adding 1 mL of 11N sulfuric acid and 0.4 g of ammonium persulfate Gently heat the mixture in an autoclave at 121°C for 30 minutes, then cool and filter the sample Next, incorporate 1 mL of 11N sulfuric acid, 4 mL of ammonium molybdate-antimony potassium tartrate, and 2 mL of ascorbic acid, mixing thoroughly After allowing the mixture to sit for 5 minutes, measure the absorbance at 710 nm using a spectrophotometer to determine the phosphorus concentration based on a standard curve.
3.4.5 Chemical Oxygen Demand (EPA Method 410.3)
Solution A: Dissolve 10.216g K2Cr2O7 of pre-dried (103 o C for two hours), add 167 mL
H2SO4 and 33.3g HgSO4 Cool and dilute to 1000 mL
Solution B: Dissolve 10.012g Ag2SO4 into 1000 mL H2SO4 Keep the solution 1-2 days for completely soluble
Solution C: Dissolve 0.85g KHP (potassium hydrogen phthalate) into 1000 mL distilled water
Take 2.5 mL of sample, add 1.5 mL of solution A and finally add 3.5 mL of solution B into the COD vial Place the vials in the incubator at 150 o C for 2 hours and allow the digestion process to take place After heating, remove the samples from the incubator and then let h
Allow the samples to cool to room temperature before transferring them to spectrophotometer cuvettes Measure the absorbance at 600 nm and calculate the Chemical Oxygen Demand (COD) value using the calibration curve.
Figure 3.9 Calibration curve for Chemical oxygen demand
Initially, all raw and hydrochar samples underwent moisture removal in accordance with the ASTM D3173 standard Prior to this process, the samples were ground into small pieces Notably, the raw samples did not require any pretreatment, as they contained minimal impurities.
Before sample placement, the silica crucibles underwent cleaning, calcination, and measurement A dry spatula was used to add 1.0 g of the sample to each crucible, followed by calculating the combined weight of the crucible and sample The covers were secured until the crucibles were transferred to the oven Once the covers were removed, the crucibles were promptly placed in a preheated oven.
To effectively remove moisture, samples were heated to a temperature range of 104 to 110°C for 1 to 2 hours Following this heating period, the samples were placed in a desiccator containing a desiccant to cool Once the silica crucibles reached room temperature, the dried samples were weighed promptly.
48 weighed again and mass was recorded The dry solid content and moisture content were determined by equation (2)
3.4.7 Ash content and volatile matter calculation
The ash content of biomass and hydrochar samples was measured following EPA METHOD 1684, which assesses Total, Fixed, and Volatile Solids in various materials A 1.0 g sample was placed in each crucible, and initial weights were recorded After covering the samples, they were heated in a preheated oven at 550°C for 2 hours Once cooled, the crucibles containing ash were weighed again to determine the ash content and volatile matter on a dry basis using specific equations.
Where m1 - is mass of empty crucible [g]; m2 - mass of the crucible with sample before analysis [g]; m3 - mass of the crucible with ash [g]; Med - moisture content of the sample [%]
To calculate the moisture content of a sample, use the following masses: m1 represents the mass of the empty crucible with its lid in grams, m2 is the mass of the crucible with the lid and sample prior to analysis, and m3 is the mass of the crucible with the lid and the sample after analysis The moisture content of the sample, denoted as Mad, is expressed as a percentage.
(2) w – Weight of the sample before drying d – Weight of the sample after drying
The calculation of the gross caloric values was calculated using an equation obtained by
In their 2013 study, Sahito et al demonstrated a strong correlation in estimating the calorific values of lignocellulosic biomass, achieving an R² value of 0.822, indicating high accuracy The research outlines the mathematical calculation of the Gross Calorific Value (GCV) based on the proximate values of volatile solids, fixed solids, and ash content.
GCV (MJ/kg) = 0.21575 (VS) + 0.07492 (FS) - 0.08426 (ash)
Figure 3.10 Thermal analysis procedure for Biomass fuel (Hydrochar) Fixed solid, Ash,
Proximate values were calculated according to the equations (6), (7), (8) and (9),
RESULT AND DISCUSSION
Moisture Removal
The samples were analyzed for approximate analysis prior to the HTC treatment The result for the Restaurant food waste, Paper mill sludge, and Saw dust presented in table 4.1
Table 4.1 Moisture content, Amount of dry solid Volatile, ash and Gross caloric values calculated by equation (5) of biomass feeds
Moisture of the wet feed % 65.1 78.1 29.9
Dry solid amount dried sample % 34.9 21.9 70.1
Volatile solid of dried sample % 61.8 84.7 99.5
Ash of dried of dried sample % 38.2 15.3 0.45
Gross calorific value MJ/kg 18.5 25.5 27.9
The HTC treatment sample exhibited a darker color, signaling structural changes during the process Variations in parameters like temperature and residence time significantly altered the biomass composition and color Thus, the impact of temperature and residence time is crucial in this transformation process.
Table 4.2 Proximate analysis of biomass after the hydrothermal carbonization gross caloric values were calculated by equation (5) n = 3
Gross calorific value MJ/kg 21.8 - 22.4 25.2 - 27.3 27.3 - 27.5
Dried raw restaurant food (left) waste sample and subsequent hydrochar sample, 18RFW6
Dried raw paper mill sludge sample (left) and subsequent hydrochar sample, 22PMS3
Dried raw saw dust sample (left) and subsequent hydrochar sample, 22SD3 (right)
Figure 4.1 Raw biomass feed samples and subsequent hydrochar samples
Table 4.2 displays the proximate analysis results of biomass before and after hydrothermal carbonization The combustion efficiency of the biomass is indicated by its volatile solids and fixed carbon content A higher level of volatile solids facilitates easier combustion, while it may also lead to instability in the flame Conversely, an increased amount of fixed carbon can counterbalance this issue Thus, a lower volatile to fixed carbon ratio is advantageous for producing a more combustible fuel.
Hydrochar
4.2.1 Restaurant Food waste (RFW/R) hydrochar
Figure 4.2 The dependence of yield of RFW hydrochar from dried food waste with temperature and time (n = 3, triplicate)
The mass yield of hydrochar from raw restaurant food waste was analyzed, revealing that maintaining a consistent input feed of 4.0g leads to increased yields at lower temperatures and longer reaction times during hydrothermal carbonization This phenomenon is attributed to the formation of secondary char, which allows dissolved carbon to revert to solid form.
4.2.1.2 Total solid(WB), Ash content and Fixed carbon content(DB) in RFW hydrochar with time (DB: Dry Basis, WB: Wet basis)
The following figures 4.3, 4.4 show the values obtained from the approximate analysis of Restaurant food waste hydrochar
Figure 4.3 Percentages of total solid content and moisture content of RWFHC with time and temperature (n = 3, triplicate)
Figure 4.4 Percentages of Volatile solid content and ash of RWFHC with time and temperature (n = 3, triplicate)
Total solid content and moisture content of RWF HC with time and temperature
Volatile solid content and ash of RWF HC with time and temperature
Moisture content % Fixed Solid content %
Ash content % Volatile Solid content % h
Figure 4.5 Gross calorific values of RFWHC with time and temperature (n = 3, triplicate)
Hydrothermal carbonization significantly enhances the gross calorific value of restaurant food waste hydrochar, with untreated biomass showing a value of 25.48 MJ/kg Under conditions of 180 to 220 °C and reaction times of 3 to 6 hours, the calorific value of hydrochar ranges from 25.16 to 27.25 MJ/kg, peaking at 27.22 MJ/kg for samples processed at 180 °C for 6 hours and 27.25 MJ/kg at 200 °C for 4 hours This represents a 6.95% increase in energy compared to raw samples, making hydrochar a viable energy source alongside bituminous coal The hydrothermal process is particularly effective at lower temperatures and longer reaction times, resulting in hydrochar with low ash content, high volatile solids, and total solids, all contributing to its elevated gross calorific value The increase in higher heating value during hydrothermal carbonization is attributed to hydrolysis and carboxylation reactions, which enhance carbon content while reducing oxygen levels in the substrate.
GCV RFW HC with time and temperature
Hydrothermal carbonization at lower temperatures effectively transforms restaurant food waste, primarily composed of easily degradable molecules such as proteins, carbohydrates, and hemicellulose, into hydrochar with minimal mass loss This process preserves a significant amount of carbon in the solid phase, which is essential for achieving higher gross caloric values, making it an efficient method for waste conversion.
4.2.2 Paper mill sludge (PMS/P) hydrochar
Figure 4.6 Percent conversion of PMS hydrochar with time and temperature (n = 3, triplicate)
The mass yield of hydrochar derived from raw paper mill sludge shows a notable decrease with increasing temperatures and extended reaction times during hydrothermal carbonization (HTC) This trend is attributed to the decomposition of organic material transitioning from the solid phase to other phases under these conditions The conversion rates observed range from 80.6% to 93.5%, indicating that a substantial portion of the feed material remains in the solid phase throughout the HTC process Additionally, the disparity between maximum and minimum yields is significant when compared to hydrochar yields obtained from food waste.
Higher yields of hydrochar from restaurant food waste can be achieved at temperatures that minimize degradation In summary, it can be concluded that variations in temperature and residence time significantly affect the mass yield of hydrochar derived from paper mill sludge.
4.2.2.2 Total solid(WB), Ash content and Fixed carbon content(DB) in PMS hydrochar with time (DB: Dry Basis, WB: Wet basis)
The following figures 4.7, 4.8, show the values obtained from the approximate analysis of Paper mill sludge hydrochar
Figure 4.7 Total solid content and moisture content of PMSHC with time and temperature
Total solid content and moisture content of PMS HC with time and temperature
Moisture content % Fixed Solid content % h
Figure 4.8 Percentages of Volatile solid content and ash of PMSHC with time and temperature (n = 3, triplicate)
Figure 4.9 Gross calorific values of PMSHC with time and temperature (n = 3, triplicate)
Hydrothermal carbonization significantly increases the gross calorific value of paper mill sludge hydrochar The untreated biomass sample has a calorific value of 18.5 MJ/kg, while hydrochar produced at temperatures between 180°C and 220°C and reaction times of 3 to 6 hours shows a calorific range of 21.84 to 22.40 MJ/kg The highest gross calorific value recorded is 22.40 MJ/kg from hydrochar processed at 220°C for 3 hours, representing a 21.08% improvement over the raw sample.
Volatile Solid content and ash content of PMS HC with time
GCV of PMS HC with time and temperature
Ash content % Volatile Solid content %
The hydrothermal carbonization process results in a remarkable 60% energy increase in biomass feeds, the highest among the three studied This energy boost approaches that of sub-bituminous coal, which has a gross calorific value below 24 MJ/kg (He et al., 2013), indicating that hydrochar samples can be effectively utilized for energy alongside coal Additionally, paper mill sludge samples benefit from the hydrothermal process when subjected to higher temperatures and shorter reaction times, resulting in low ash content and elevated levels of volatile solids and total solids, ultimately leading to an increased gross calorific value.
The increase in higher heating value during hydrothermal carbonization is attributed to hydrolysis and carboxylation reactions, which enhance carbon content while reducing oxygen content in the substrate Notably, the gross calorific value of paper mill sludge hydrochar is significantly higher at elevated temperatures, where mass loss and decomposition are pronounced The substrate's complex molecules, including cellulose, hemicellulose, and lignin, require higher temperatures for effective degradation, leading to increased carbon conversions Consequently, this enrichment of carbon in the solid phase results in elevated gross calorific values at higher temperatures Ultimately, achieving a higher gross calorific value in the hydrothermal carbonization process depends not only on temperature and reaction time but also on the composition of the feedstock.
Figure 4.10 Yield of SD hydrochar with increasing time and temperature at 220 o C (n 3, triplicate)
The mass yield of hydrochar from raw sawdust, as illustrated in Figure 4.10, shows that with a constant input feed of 4.0g, the yield decreases at higher temperatures and longer reaction times during hydrothermal carbonization (HTC) This decline can be attributed to the decomposition of organic material transitioning from the solid phase to other phases The conversion rates range from 72.7% to 79.7%, indicating that the majority of the feed material remains in the solid phase throughout the HTC process Notably, the difference between the maximum and minimum yields is significant when compared to yields obtained from food waste.
The 22S3 sample demonstrates the highest conversion rates, indicating minimal degradation at lower reaction times and temperatures This suggests that variations in temperature and residence time significantly impact the mass yield of hydrochar.
4.2.3.2 Total solid(WB), Ash content and Fixed carbon content(DB) in SD hydrochar with time (DB: Dry Basis, WB: Wet basis)
Figure 4.11 Total solid content and moisture content of SDHC with time and temperature at 220 o C (n = 3, triplicate)
Figure 4.12 Percentages of Volatile solid content and ash of SDHC with time and temperature at 220 o C (n = 3, triplicate)
Fixed Solid content and ash content of SD HC with time and temperature
Volatile Solid content and ash content of SD HC with time and temperature
Moisture content % Fixed Solid content %
Ash content % Volatile Solid content % h
Figure 4.13 Gross calorific values of SDHC with time and temperature at 220 o C (n = 3, triplicate)
Hydrothermal carbonization reduces the Gross Calorific Value (GCV) of sawdust hydrochar samples, with untreated biomass at 27.97 MJ/kg Treated hydrochar, subjected to temperatures between 180°C and 220°C for 3 to 6 hours, maintains a GCV of approximately 27.30 to 27.45 MJ/kg, peaking at 27.45 MJ/kg for samples processed at 220°C for 3 hours This GCV aligns with that of paper mill sludge hydrochar and falls within the bituminous coal range of 24 to 35 MJ/kg (He et al., 2013), indicating minimal impact from the hydrothermal process Both sawdust and paper mill sludge hydrochar benefit from higher temperatures and shorter reaction times, resulting in lower ash content and higher levels of volatile and total solids.
The stable gross calorific value of the sample is primarily due to its composition, particularly the high percentage of lignin present in the feed This lignin requires temperatures exceeding 220°C for effective degradation, contributing to an increased gross calorific value.
GCV of SD HC with time and temperature
In summary, achieving a higher gross calorific value during the hydrothermal carbonization process relies not only on factors such as temperature and reaction time but also significantly on the composition of the feed.
Table 4.3 Comparison of gross calorific value from the research with literature review
Table 4.4 GCVs of the feed and chosen hydrochar samples from ultimate analysis
GCV raw feed MJ/kg 17.9368 6.2159 18.6317
GCV hydrochar sample MJ/kg 23.4275 6.6748 21.4902
After calculating the Gross Calorific Values (GCVs) using equation (1), selected samples were analyzed based on their ultimate analysis The analysis included initial feed samples and subsequent hydrochar samples Specifically, the samples examined were the restaurant food waste hydrochar (18R6), paper mill sludge hydrochar (22P3), and sawdust (22S3).
The ultimate analysis of 65 samples revealed that hydrothermal carbonization significantly increases the gross calorific value (GCV) of various feeds Specifically, restaurant food waste hydrochar exhibited a 23.44% increase in GCV, while sawdust hydrochar showed a 13.3% increase, with both values demonstrating no significant difference between calculated and analyzed results Conversely, paper mill sludge hydrochar, despite showing an increase, had the lowest increment and a substantial discrepancy between calculated and analyzed GCV values This indicates that the calculation method (equation 1) may be less reliable for feeds like paper mill sludge, which contain lower biomass and higher inorganic content However, equation 1 remains effective for samples with higher biomass, providing a rough estimate of GCV values during the screening process.
Characteristics of hydrochar process water
4.3.1 Restaurant food waste (RFW/ R) process water
4.3.1.1 pH of RFW process water
Figure 4.14 pH of RFW process water with time and temperature (n = 3, triplicate)
The pH of process water following the hydrothermal carbonization (HTC) process ranges from 3.30 to 4.14, indicating increased acidity compared to the initial measurement of 4.26 During HTC, lower temperatures and longer reaction times lead to a decrease in pH, while higher temperatures can cause an increase due to the formation of organic acids like acetic and glycolic acid However, at elevated temperatures, the degradation of intermediate organic acids can reduce acidity It is essential to neutralize the acidity of the process water before its environmental release Alternatively, in line with circular economy principles, this process water can be utilized in anaerobic digestion, although the pH must be adjusted to suitable levels for microbial growth.
Sample Name pH of Restaurant Food Waste Process water
4.3.1.2 COD of RFW process water
Figure 4.15 COD of RFW process water with temperature and time (n = 3, triplicate)
The COD (Chemical Oxygen Demand) of restaurant food waste process water, post-carbonization, shows significant changes, with initial values at 17.26 g/L and post-process values ranging from 35.53 g/L to 70.14 g/L, reflecting biomass degradation Lower temperatures lead to higher COD values due to increased conversion of organic material from solid to liquid, while higher temperatures result in a decrease in COD over time, attributed to secondary char formation and absorption during hydrochar production Ultimately, the COD characteristics of the process water resemble those of landfill leachate, necessitating pretreatment before environmental discharge.
COD (g/L) of RFW process water
4.3.1.3 Electrical conductivity of RFW process water
Figure 4.16 Electrical conductivity of restaurant food waste process water with temperature and time (n = 3, triplicate)
The electrical conductivity of process water post-carbonization is illustrated in Figure 4.16, revealing an initial conductivity of 6.13 mScm -1, which increases to between 6.84 and 9.80 mScm -1 after hydrothermal treatment This rise in conductivity indicates a higher concentration of inorganic compounds in the liquid phase The study shows that electrical conductivity correlates with increased reaction time and temperature, suggesting solid-phase degradation that releases both organic and inorganic compounds into the liquid While the focus was on analyzing COD, TN, and TP for potential biological treatment of the processing liquor, there is a concern that the process water may also contain harmful heavy metals Thus, a pretreatment process is essential before discharging the water into the environment.
EC (mScm -1 ) RFW process water
4.3.1.4 Total nitrogen of RFW process
Figure 4.17 Total nitrogen of RFW process water with temperature and time (n = 3, triplicate)
The total nitrogen concentration in process water after carbonization significantly increased from an initial 48.49 mg/L to a range of 148.97 – 353.32 mg/L, as shown in Figure 4.17 This rise is attributed to the degradation of nitrogen-containing organic compounds, such as proteins, during the hydrothermal carbonization (HTC) process Lower temperatures and extended reaction times enhance the breakdown of these compounds; however, at higher temperatures, degradation slows due to the formation of secondary char and the absorption of hydrochar The total nitrogen levels in the process water are comparable to those found in landfill leachate, necessitating pretreatment before environmental discharge (Funke et al., 2010).
TN (mg/L) of RFW process water with time
4.3.1.5 Total phosphorus of RFW process water
Figure 4.18 Total phosphorus of RFW process water with temperature and time (n = 3, triplicate)
The total phosphorus levels in process water after carbonization significantly increased, with initial samples showing 9.28 mg/L and post-carbonization samples ranging from 10.27 to 25.51 mg/L This rise is attributed to the breakdown of phosphorus-containing organic compounds, such as phospholipids, nucleic acids, and proteins, alongside the release of inorganic phosphorus during the hydrothermal carbonization (HTC) process While lower temperatures and extended reaction times enhance the degradation of organophosphorus compounds, higher temperatures limit degradation over time due to the formation of secondary char in the liquid phase, resulting in insoluble phosphate that deposits on the hydrochar surface.
TP (mg/L) of RFW process water
4.3.2 Paper mill sludge (PMS/P) process water
4.3.2.1 pH of PMS process water
Figure 4.19 pH of PMS process water with temperature and time (n = 3, triplicate)
The pH of process water after the hydrothermal carbonization (HTC) process, as shown in Figure 4.19, initially measured 6.71 but ranged from 4.55 to 6.26, indicating increased acidity This decrease in pH is attributed to higher temperatures and longer reaction times during HTC, which promotes the degradation of organic compounds and the formation of organic acids such as acetic and glycolic acid (Choo et al., 2020) Therefore, similar to process water from food waste, the acidity of paper mill sludge hydrochar process water must be neutralized before being released into the environment.
Sample Name pH of Paper Mill Sludge process water
4.3.2.2 Electrical conductivity of PMS process water
Figure 4.20 Electrical conductivity of PMS process water with temperature and time (n 3, triplicate)
The electrical conductivity of process water after carbonization is illustrated in Figure 4.20, revealing an initial conductivity of 0.42 mS/cm, which increases to between 0.92 and 1.96 mS/cm following the hydrothermal process This rise in conductivity indicates a higher concentration of inorganic compounds in the liquid phase, which correlates with increased reaction time and temperature, suggesting the release of both organic and inorganic compounds from the solid phase Although the specific inorganic composition of the liquid phase was not analyzed in this study, there is a concern that the process water may contain harmful heavy metals Consequently, it is essential to implement a pretreatment process before discharging the process water into the environment.
EC (mScm -1 ) of Paper Mill Sludge
4.3.2.3 COD of PMS process water
Figure 4.21 COD of PMS process water with temperature and time (n = 3, triplicate)
The Chemical Oxygen Demand (COD) of paper mill sludge process water after carbonization is illustrated in Figure 4.21, showing an initial COD value of 2.02 g/L, which increases significantly to a range of 4.27 – 12.46 g/L during the process This rise indicates a higher dissolution of biomass as temperatures increase, with lower COD values observed at lower temperatures due to insufficient transformation of organic material from solid to liquid Conversely, elevated temperatures and prolonged reaction times enhance the dissolution of organic compounds, leading to higher COD values in process water Notably, the COD values of paper mill sludge (PMS) process water are lower than those of restaurant food waste, attributed to the complex molecular structure of the substrate, which poses challenges for degradation.
COD (g/L) of PMS process water
4.3.2.4 Total nitrogen of PMS process water
Figure 4.22 Total nitrogen of PMS process water with temperature and time (n = 3, triplicate)
The total nitrogen content in process water after carbonization shows a significant increase, with initial levels at 0.15 mg/L rising to between 33.47 mg/L and 106.23 mg/L post-carbonization This surge is attributed to the breakdown of nitrogen-containing organic compounds, such as proteins, during the hydrothermal carbonization (HTC) process Lower temperatures and extended reaction times enhance the degradation of these compounds; however, at higher temperatures, degradation plateaus due to the formation of secondary char and hydrochar absorption The total nitrogen levels in the process water fall within the range of landfill leachate, necessitating pretreatment before environmental release to mitigate eutrophication risks.
TN (mg/L) of PMS process water
4.3.2.5 Total phosphorus of PMS process water
Figure 4.23 Total phosphorus of PMS process water with temperature and time (n = 3, triplicate)
The total phosphorus content in process water after carbonization is illustrated in Figure 4.23, showing an initial concentration of 0.72 mg/L, while post-carbonization samples range from 0.40 to 2.37 mg/L These findings suggest that phosphorus tends to remain in the solid phase rather than dissolving into the liquid medium Notably, samples subjected to higher temperatures and longer reaction times exhibit minimal dissolved phosphorus in the liquid phase, likely due to the formation of insoluble phosphate that deposits on the hydrochar surface under these conditions.
TP (mg/L) of PMS process water
4.3.3 Saw dust (SD/ S) process water
4.3.3.1 pH of SD process water
Figure 4.24 pH of SD process water with time and temperature at 220 o C (n = 3, triplicate)
The pH of process water after hydrothermal carbonization (HTC) is significantly lower than the initial sample, which measured 4.54, with values ranging from 2.69 to 3.00, indicating increased acidity This decrease in pH is attributed to higher temperatures and extended reaction times during the HTC process, leading to the degradation of organic compounds and the formation of organic acids such as acetic and glycolic acid (Choo et al., 2020) Therefore, similar to the process water generated from food waste, the acidity of process water from sawdust hydrochar requires pretreatment before being released into the environment.
Sample Name pH of Saw dust Process water
4.3.3.2 Electrical conductivity of SD process water
Figure 4.25 Electrical conductivity of SD process water with time and temperature at
The electrical conductivity of process water post-carbonization, illustrated in Figure 4.25, shows an initial value of 0.049 mS/cm, with subsequent samples from the hydrochar process exhibiting increased conductivity values between 0.22 and 0.45 mS/cm These elevated conductivity levels indicate a higher concentration of inorganic compounds in the liquid phase Furthermore, the increase in electrical conductivity correlates with longer reaction times and elevated temperatures, suggesting the breakdown of solid-phase materials and the release of both organic and inorganic compounds into the liquid Although the specific inorganic composition of the process water has not been analyzed in this study, there is a potential risk of harmful heavy metals present Consequently, it is essential to implement additional pretreatment processes before discharging the process water into the environment.
EC (mScm -1 ) of Saw dust Process water
4.3.3.3 COD of SD process water
Figure 4.26 COD of SD process water with time and temperature at 220 o C (n = 3, triplicate)
The Chemical Oxygen Demand (COD) of SD process water after carbonization shows a significant increase, with initial values at 2.35 g/L rising to a range of 7.43–13.79 g/L, indicating substantial biomass dissolution Lower temperatures result in reduced COD concentrations due to limited organic material transformation from solid to liquid phases Conversely, higher temperatures and extended reaction times enhance the conversion and dissolution of organic compounds, leading to increased COD values In comparison, the COD of SD hydrochar process water is lower than that of restaurant food waste, attributed to the complex organic molecules in the substrate that are more challenging to degrade.
COD(g/L) of SD process water
4.3.3.4 Total nitrogen of SD process water
Figure 4.27 Total nitrogen of SD process water with time and temperature at 220 o C (n 3, triplicate)
The total nitrogen levels in process water after carbonization, as shown in Figure 4.27, reveal a significant increase from an initial 0.14 mg/L to a range of 3.44 mg/L – 11.25 mg/L This rise is attributed to the degradation of nitrogen-containing organic compounds during the hydrothermal carbonization (HTC) process Lower temperatures and extended reaction times enhance the breakdown of these compounds, while higher temperatures limit dissolution due to the formation of secondary char and the absorption of hydrochar The total nitrogen concentration in the process water is comparable to that of landfill leachate, necessitating pretreatment before environmental discharge to mitigate the risk of eutrophication.
TN(mg/L) of SD process water
Hydrothermal carbonization is a thermochemical process that converts various types of biomass into a coal-like material with increased higher heating value (HHV) and carbon content This innovative method effectively utilizes both dry and wet biomass, including municipal solid waste, wet agricultural residues, human waste, sewage sludge, algae, and aquaculture residues, even with moisture contents ranging from 75% to 90% Often referred to as wet pyrolysis, hydrothermal carbonization offers significant advantages over traditional pyrolysis, which is restricted to dry biomass feedstocks.