INTRODUCTION
Hydrothermal Carbonization of Lignocellulosic Biomass
Crude oil, a crucial energy source derived from ancient solar energy captured by plants, faces challenges due to its finite reserves and environmental impacts, particularly greenhouse gas emissions like CO2 These issues, coupled with fluctuating market prices and limited availability, have prompted a shift towards renewable and alternative energy sources.
Biomass energy production has been a fundamental practice since the dawn of human civilization, primarily utilizing wood The efficient use of organic solid waste has been extensively documented in literature, leading to the exploration of innovative technologies like hydrothermal processes Hydrothermal carbonization (HTC) has gained significant attention globally as a thermochemical method that transforms biomass waste into high-value products, often referred to as wet torrefaction due to its use of wet biomass, distinguishing it from other methods such as pyrolysis and incineration The resulting product, known as hydrochar, represents a combination of water and char derived from the conversion of carbon-rich organic materials into valuable resources.
Hydrothermal carbonization refers to the natural geological processes that lead to the formation of oil and coal (Silakova, 2018) This process mirrors the transformation of plant and animal remains into coal or crude oil, occurring over time in nature.
The laboratory-scale process effectively manages agricultural, industrial, and municipal waste with high moisture content, utilizing water as the thermal reaction medium This innovative approach allows for the processing of biomass without the need for moisture removal, thereby eliminating the drawbacks of costly and energy-intensive pretreatment, the necessity for dried feed, and the requirement for separate systems to handle wet and dry biomass.
This study focuses on the hydrothermal carbonization (HTC) of biomass from three major waste-producing industries: restaurants, paper mills, and sawmills By treating restaurant food waste, paper mill sludge, and sawdust, the research aims to enhance their properties for solid fuel applications The investigation examines how process temperature and reaction time affect the final product's characteristics, providing valuable insights for future research on biomass utilization The findings highlight the potential of HTC to convert low-value biomass into solid fuel, showcasing its advantages and applications The research comprises two main components: a literature review in Chapter 2 that explores the HTC process, its conditions, and the characteristics of input and output materials, and an experimental analysis in Chapters 3 and 4 that 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
LITERATURE REVIEW
Historical overview of HTC process
The HTC process, first described by German chemist Friedrich Bergius in 1913, aimed to simulate the natural formation of coal from organic materials in a laboratory setting Bergius, alongside Carl Bosch, was awarded the 1931 Nobel Prize in Chemistry for their pioneering work in high-pressure chemical conversion methods He detailed the hydrogenation effects on coal and oil formation under elevated pressure and temperature, known as the Bergius process, and also explored the hydrothermal transformation of cellulose into coal-like substances in the same year (Funke et al., 2010).
Following the Nobel Prize, researchers like Burl and Schmidt explored hydrothermal carbonization (HTC) on various biomass types at temperatures ranging from 150 °C to 350 °C (Marchetti, 2012) Schuhmacher later investigated the impact of pH on HTC reactions, revealing differences in the basic components (C, H, and O) (Titrici et al., 2007) Subsequently, research on this thermochemical process experienced a decline in prominence.
In the past decade, hydrothermal carbonization (HTC) has emerged as a promising method for converting biomass into sterile fuel, capturing renewed interest as an alternative energy source (Fink and Andrin, 2011) German scientist Markus Antonietti's research in 2006 detailed a straightforward approach to HTC, enabling the transformation of biomass into coal-like material through increased temperature and pressure (Libra et al., 2011) This innovative process, dubbed the "black revolution," has since attracted significant attention from the scientific community However, despite its growing popularity, research on hydrothermal carbonization remains limited compared to other thermal conversion techniques (Libra et al., 2011).
Biomass And Thermochemical Processes
The energy production from biomass can be divided into biological and thermochemical treatments Biological treatment utilizes microorganisms like bacteria and fungi to oxidize and stabilize organic matter, resulting in valuable energy streams (Basso, 2016) In contrast, thermochemical treatment focuses on the combustion of biomass, employing methods such as gasification, pyrolysis, and co-firing with woody and herbaceous materials This process involves drying lignocellulose-rich biomass to generate electricity or heat through boilers and gasifiers, while the biological method transforms protein and water-rich biomass into compost or energy.
Torrefaction is a thermochemical conversion process that utilizes biomass as the primary input, resulting in the production of biochar as the main product, along with liquid and gas byproducts This process occurs in an oxygen-free environment, leading to the degradation of organic compounds into 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, contrasts with dry torrefaction, which is the conventional method Dry torrefaction operates at temperatures between 200°C and 300°C, utilizing atmospheric pressure and an inert nitrogen gas environment devoid of oxygen This technique is also recognized 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 causes the biomass to decompose into various phases, resulting in the simultaneous production of solids, liquids, and gases The yield and ratio of these products are significantly influenced by the speed of the pyrolysis process.
Conventional pyrolysis primarily focuses on charcoal production through a slow phase to enhance solid output In contrast, fast pyrolysis emphasizes hydrogen production by rapidly exposing biomass to high temperatures for mere seconds, resulting in liquid and gas products This process generates combustible gases that can be condensed into pyrolysis oil (bio-oil) Overall, biomass pyrolysis yields gaseous byproducts such as CO2, CO, H2, and light hydrocarbons, along with liquid products and a solid residue enriched in carbon content.
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 process that converts organic matter into a gaseous product known as syngas through oxidation at high temperatures exceeding 700°C This process primarily produces hydrogen (H2), carbon monoxide (CO), and small quantities of methane (CH4), along with water vapor, carbon dioxide (CO2), nitrogen (N2), and tar Various oxidants, including air, pure oxygen, steam, or a combination of these gases, can be utilized in gasification (Choo et al., 2020).
(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 product gas with a high nitrogen concentration and low heating value, while oxygen and steam-based gasifiers produce gas rich in hydrogen and carbon monoxide, resulting in 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 hot, pressurized water environment, emphasizing the importance of maintaining conditions that allow for the direct treatment of wet substrates while eliminating the need for any drying pretreatment.
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 solid product yields while enhancing the production of gas and liquid These conditions are crucial for achieving the desired liquid output, which primarily consists of liquid hydrocarbons and heavy oils.
Increasing temperatures enhance hydrothermal gasification, primarily resulting in gaseous products These gases, including hydrogen (H2), methane (CH4), and carbon dioxide (CO2), can be combusted after a cleaning process, such as in a gas turbine or Biomass Integrated Gasification systems, and can also be utilized to generate pure hydrogen (Basso, 2016).
Hydrothermal carbonization operates at temperatures ranging from 180°C to 250°C, resulting in minimal gas production (1-5%) and a significant yield of solid char This process involves the chemical decomposition of feedstock, producing a carbon-rich solid material characterized by enhanced chemical stability.
The hydrothermal carbonization process involves submerging biomass in a liquid phase under high temperature and autogenous pressure for several hours Throughout this process, water remains in liquid form due to the elevated pressure and temperature When conditions reach a critical threshold, water transitions into a supercritical state, where it exhibits properties of both a liquid and a gas (Marchetti, 2012).
The ideal temperature for the carbonization process is approximately 200°C, as this helps prevent 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 in determining the outcomes.
The HTC process significantly enhances and preserves the carbon content of original biomass, which is a key advantage, especially as the reduction of material can lead to increased carbon levels (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 refers to the total mass of living organisms, encompassing plants, animals, and microorganisms From a biochemical standpoint, it includes essential organic compounds 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 the hydrothermal carbonization (HTC) process, as it facilitates a series of critical reactions akin to those in pyrolysis A key reaction is hydrolysis, which effectively breaks the ester and ether bonds in cellulose (above 200°C), hemicellulose (above 180°C), and lignin (above 200°C) by incorporating water molecules.
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 further converting into 5-hydroxy-methyl-furfural (5-HMF), which ultimately forms formic or levulinic acid Similarly, cellulose hydrolysis also produces 5-HMF, which is then converted into formic or levulinic acid after breaking down into glucose Additionally, lignin degradation results in the formation of phenolic compounds.
The decarboxylation and dehydration reactions significantly impact the H/C and O/C ratios of hydrochar during the hydrothermal carbonization (HTC) process Dehydration primarily occurs around the hydroxyl (OH) groups, while decarboxylation involves carboxyl and carbonyl groups In the Van Krevelen diagram, dehydration shifts the ratios from the top right to the bottom left, while decarboxylation moves from the bottom right to the top left Consequently, dehydration decreases both H/C and O/C ratios, whereas decarboxylation increases the H/C ratio while reducing the O/C ratio Among these reactions, dehydration plays a crucial role.
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
According to Qadariyah et al (2011), the formation mechanism of aromatic structures is governed by two primary reactions: ionic reactions, which dominate during subcritical and near-critical temperatures, and free-radical reactions, which prevail at supercritical temperatures The formation of these aromatic structures occurs under specific temperature and pressure conditions, and can be experimentally verified using 13C-NMR measurements Additionally, the residence time of the hydrothermal carbonization (HTC) process significantly influences the aromatization process, as the formation of aromatic bonds decreases the carbon content of hydrochar Therefore, it is crucial to investigate the impact of reaction time on these aromatization reactions (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
The pyrolytic reaction occurs in fragments that cannot contact water due to the precipitation of condensed materials This reaction takes place at temperatures exceeding 200°C and leads to the formation of carbonaceous products (Qadariyah et al., 2011).
The HTC process yields a final product that exists in three distinct states: a solid phase consisting of carbon-enriched hydrochar, a liquid phase comprising a mixture of phenolic compounds and furan derivatives, and a gas phase that primarily contains a small amount of carbon dioxide (CO2).
Hydrochar undergoes decarboxylation and dehydration reactions, resulting in a reduced number of carboxyl and hydroxyl groups, as well as lower H/C and O/C ratios compared to the original biomass Despite this, hydrochar contains more functional groups than natural bituminous coals and exhibits greater hydrophobicity than the initial 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 hydrothermal carbonization (HTC) process retains a significant portion of carbon from the raw feed, with hydrochar containing up to 80% by weight of the original carbon (Berge et al., 2011; Li et al., 2011; Kruse et al., 2013) This retention substantially boosts the energy content of hydrochar, enhancing its energy value by approximately 1.01 to 1.41 times on a weight basis and between 6.39 to 9 times on a volume basis compared to the feedstock (Lu et al., 2011) Additionally, the higher heating value of hydrochar is reported to increase by 1.50 to 1.70 times on a weight basis relative to the feedstock (Roman et al., 2012) This significant energy enhancement highlights 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
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 aromatic substances such as furanic and phenolic compounds (Lu et al., 2011) Studies by Heilmann et al (2011) also reveal the presence of non-agglomerated colloidal carbonized materials and a tar fraction rich in 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, as well as lignin-derived phenolic 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 leachate from landfills Additionally, the presence of organic acids contributes to the acidic pH of the liquid phase, 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 decarboxylation reaction is responsible for gas formation, yet the overall yield of this gas phase is limited due to the restricted availability of oxygen 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 can be efficiently transferred and stored by the water in the medium.
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 temperature rises, the viscosity, surface tension, density, and dielectric constant of water decrease Conversely, the diffusion and self-ionization capacity of water increase with higher temperatures (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 considered subcritical, while above it is termed supercritical In the subcritical region, as temperature increases, the 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 transforms as infinite hydrogen bonds are broken, resulting in distinct clusters with chain-like formations.
Near the critical point, the dielectric constant of water decreases, leading to an increase in its viscosity and self-diffusion coefficient At temperatures ranging from 227°C to 327°C, depending on pressure, the ionic products of water reach their maximum values Consequently, under subcritical conditions, an increase in temperature results in higher pressure and enhanced dielectric properties This unique behavior of subcritical water enables it to effectively extract and facilitate the 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 overall 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 culmination of numerous processes, and wasting it means squandering all the resources involved, particularly water Water is essential at every stage of food production, from irrigating crops to hydrating livestock, as well as in the packaging and transportation of food For instance, discarding just one apple results in a staggering waste of approximately 170 trillion liters (or 45 trillion gallons) of water annually (Let’s Talk Science, 2019) As one of humanity's fundamental needs, water is vital for survival, underscoring the importance of reducing food waste to conserve this precious resource, as emphasized by the World Health Organization.
The minimum daily water requirement per person is 15-20 liters Reducing food waste significantly contributes to conserving water resources, which can help ensure that more water is available for those in need globally.
A significant portion of food waste is disposed of in landfills, where it decomposes and generates greenhouse gases like methane and carbon dioxide This waste not only contributes to emissions during decomposition but also during transportation Furthermore, the conversion of forests and natural habitats into agricultural land adversely affects biodiversity Researchers estimate that eliminating food waste could reduce greenhouse gas emissions from the food system by up to 11%.
Figure 3.2 Paper mill sludge, Peninsular Malaysia
The paper mill industry is the fifth largest energy consumer globally, accounting for 4% of total energy production In the United States, it ranks as the sixth-largest industry based on emissions into air, water, and land In 2015, the industry released approximately 79,000 tonnes of pollutants, representing 5% of all industrial emissions in the country Of these emissions, 66% were released into the air, 10% into water bodies, and 24% ended up on land, primarily in landfills.
The Vietnam paper industry, one of the oldest in the country, dates back to 284 B.C and evolved significantly until the 20th century when handmade paper was widely used for writing and folk painting The establishment of the first industrial paper mill in Viet Tri in 1912 marked a turning point, with a production capacity of 4,000 tonnes per year The 1960s saw the emergence of small and mid-sized paper mills, each producing under 20,000 tonnes annually A major milestone occurred 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, solidifying the 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 by 11.8% to 687,570 tonnes, while paper sales rose by 9.8% to 837,855 tonnes Additionally, paper imports reached 327,474 tonnes, up 11.9%, and exports soared by 26.3% to 167,684 tonnes (Duc, 2020) Despite being a significant industry, the pulp production generates waste that contributes to water and land pollution, with various types of waste produced at each production stage In Ho Chi Minh City, 8.2% of total municipal waste comprises paper, which often ends up in landfills without any reuse.
Figure 3.3 Forestry waste Terrace Community Forest, Northwest British Columbia
Timber is a renewable and sustainable resource, with the United Nations Food and Agriculture Organization predicting a 45% increase in global industrial timber consumption by the end of 2020 Furthermore, the World Bank forecasts that global timber consumption will quadruple by 2050, highlighting the growing demand for this valuable resource.
The harvesting and disposal of timber products can lead to significant environmental impacts throughout the supply chain In urban areas, various sources contribute to timber waste, including commercial and industrial activities, construction and demolition processes, as well as pallets and packaging materials that are discarded, ultimately affecting the environment.
Timber waste, when not reused, recycled, or refurbished, ultimately ends up in landfills, contributing to greenhouse gas (GHG) emissions during transportation This waste occupies significant space in landfills, increasing the demand for new disposal sites and leading to the use of timber modified with synthetic materials.
Wood waste disposal in landfills contributes significantly to toxic waste, with Sydney and Melbourne estimated to dispose of approximately 446,000 and 623,000 tonnes annually, respectively (Tucker et al., 2009) Additionally, the burning of timber waste releases harmful smoke containing carbon, CO2, and dioxins (Adhikari et al., 2018).
Efficient utilization of wood waste is crucial for minimizing environmental impact while meeting timber product demands without further ecological degradation Research by Dionco et al (2001) indicates that for every cubic meter of trees harvested, approximately 50% is wasted, including damaged residuals (3.75% from abandoned logs, 10% from stumps, 33.75% from tops and branches, and 2.5% from butt trimmings) Thus, wood waste constitutes a substantial segment of overall waste materials.
Wood waste primarily originates from low-quality logs, bark, off-cuts, sawdust, slabs, and edged trimmings from sawn timber Utilizing these low-quality logs with advanced technology can significantly minimize wood recovery, yet such technology is predominantly available in developed countries In contrast, developing nations like Vietnam face challenges due to outdated equipment, inefficient production methods, and management practices, leading to substantial wood waste To address these issues, innovative approaches to timber waste utilization are essential One effective strategy involves altering energy sources, such as converting sawmill by-products into thermal energy, which can reduce reliance on fossil fuels and promote bioenergy production on-site For instance, sawdust can be transformed into hydrochar, enhancing 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, containing significant amounts of water, food scraps, and paper napkins Additionally, paper mill sludge was sourced from Hanoi Paper and Packaging Production Trading Co., Ltd., comprising clay and paper pulp after dewatering Sawdust was obtained from a sawmill in Ngọc Đại, Đai Mễ, Từ Liêm, Hà Nội In the laboratory, all samples underwent moisture removal following ASTM D3173 standards and were ground using an electrical blender before being sieved through a 5-millimeter opening in preparation for the HTC process.
Figure 3.4 Raw dried restaurant food waste, raw dried paper mill sludge and raw dried saw dust
The uniformity of the samples, crucial for optimizing the HTC process, was attained through careful grinding To prevent moisture exposure, the dried feed was stored in polythene bags within a desiccator The processed milled 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 45 ml These reactors consisted of a Teflon inner tube and a stainless steel outer column with a screw cap The inner tube measures 17 cm in height and 28 mm in diameter, while the outer tube stands at 18 cm tall with a diameter of 30 mm, 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
In the HTC process, 4.0 g of biomass was combined with 36 mL of deionized water in a Teflon tube, maintaining a water-to-biomass ratio of 9:1 The Teflon tube was secured within a stainless steel tube, and the assembly was placed in a furnace where the temperature was gradually increased Experiments were conducted at reaction times of 3, 4, and 6 hours, and at 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 optimal temperature for the HTC process, and selecting reaction temperatures within this range aims to enhance yield while minimizing energy consumption Additionally, it was noted that hydrochar yield and properties stabilize after 6 hours, leading to the conclusion that a maximum reaction time of 6 hours is ideal for the HTC process.
Following 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 and kept in a refrigerator at 4°C 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 conducting carbonization experiments, solid and liquid samples were collected for analysis The hydrochar and untreated biomass samples underwent measurements for ash content, dry solid content, volatile solid content, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and gross calorific value, adhering to standard procedures Liquid samples were analyzed for pH, electrical conductivity (EC), total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD) Each measurement was replicated three times, with the average value reported as the official characteristic of each sample SEM and EDS analyses of solid products were performed at the Nanotechnology Laboratory of Vietnam Japan University, while gross calorific value assessments were conducted 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
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 Measured in units of μS/cm (microsiemens per centimeter) or mS/cm (millisiemens per centimeter), EC values in this study were obtained using a portable multi-parameter digital meter, following 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
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, start by taking a 50 mL sample in an Erlenmeyer flask, then add 1 mL of 11N sulfuric acid and 0.4 g of ammonium persulfate Heat the mixture gently in an autoclave at 121 °C for 30 minutes After cooling, filter the sample and introduce 1 mL of 11N sulfuric acid, 4 mL of ammonium molybdate-antimony potassium tartrate, and 2 mL of ascorbic acid, mixing thoroughly After a 5-minute wait, 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
Allow the samples to cool to room temperature before transferring them to the spectrophotometer cuvettes Measure the absorbance at 600 nm and calculate the 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 Before this process, the samples were ground into small pieces The raw samples did not require any pretreatment, as they contained minimal impurities that necessitated such a process.
Before placing the samples, the silica crucibles underwent cleaning, calcination, and measurement A dry spatula was used to add 1.0 g of each sample into the crucibles, and the combined weight of the crucible and sample was recorded The covers were then secured until the crucibles were ready to be transferred to the oven Once the covers were removed, the crucibles were swiftly placed into a preheated oven.
To effectively remove moisture, samples were heated at temperatures ranging from 104 to 110°C for a duration of 1 to 2 hours After this process, the samples were cooled in a desiccator containing a desiccant and weighed immediately once the silica crucibles reached room temperature The dried samples were then stored in the cooled crucibles for accurate measurement.
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 determined following EPA METHOD 1684, which assesses total, fixed, and volatile solids A 1.0 g sample was placed in each crucible, and the initial weights of the crucible and sample were recorded After covering the crucibles, they were placed in a preheated oven at 550 °C for 2 hours Once cooled, the crucibles were weighed again, and the ash content and volatile matter on a dry basis were calculated 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 determine the moisture content of a sample, the following measurements are essential: m1 represents the mass of the empty crucible with its lid in grams, m2 indicates the mass of the crucible, lid, and sample prior to analysis in grams, and m3 denotes the mass of the crucible, lid, and remaining matter after analysis in grams The moisture content of the sample, expressed as a percentage, is calculated using these values.
(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 the study by Sahito et al (2013), titled "Estimating Calorific Values of Lignocellulosic Biomass from Volatile and Fixed Solids," the authors found a strong positive correlation in their findings, indicated by an R² value of 0.822 The research outlines a mathematical approach to calculating 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 affected the biomass composition and color, highlighting the crucial role these factors play in the HTC 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, highlighting the relationship between volatile solids and fixed carbon in assessing combustion efficiency An increased presence of volatile solids facilitates easier combustion but can lead to instability in flameless conditions, while a higher fixed carbon content helps alleviate this issue Consequently, a lower volatile to fixed carbon ratio is advantageous for producing a more effective 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, as shown in figure 4.2, indicates that maintaining a consistent input feed of 4.0g leads to higher yields at lower temperatures and extended reaction times during hydrothermal carbonization This trend is attributed to the formation of secondary char, which facilitates the conversion of dissolved carbon back into solid form Conversely, increased temperatures and longer reaction times result in greater mass loss due to the decomposition of organic materials, with conversion rates ranging from 42.5% to 52.51%, indicating that approximately half of the feed material transitions into liquid and gas phases While there is only a slight variation in yield among samples, optimal yields are achieved at lower reaction temperatures where degradation is minimal Ultimately, it can be concluded that temperature and residence time significantly impact the mass yield of hydrochar derived from paper mill sludge.
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 %
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, increasing it from 25.48 MJ/kg in untreated biomass to a range of 25.16 – 27.25 MJ/kg under temperatures of 180°C to 220°C and reaction times of 3 to 6 hours The peak gross calorific values were recorded at 27.22 MJ/kg for a sample processed at 180°C for 6 hours and 27.25 MJ/kg at 200°C for 4 hours, marking a 6.95% energy increase compared to the raw sample This improvement positions the hydrochar close to the energy content of bituminous coal, indicating its potential for energy applications alongside coal The hydrothermal process is particularly effective at lower temperatures and extended reaction times, resulting in hydrochar with low ash content, higher volatile solids, and total solids, which contribute to the increased gross calorific value The rise in higher heating value during hydrothermal carbonization is attributed to hydrolysis and carboxylation reactions that enhance carbon content while reducing oxygen levels in the substrate.
GCV RFW HC with time and temperature
The hydrothermal carbonization process operates effectively at lower temperatures, which are influenced by the substrate's components Restaurant food waste primarily contains easily degradable molecules such as proteins, carbohydrates, and hemicellulose At these lower temperatures, the process can convert these materials into hydrochar with minimal mass loss, unlike higher temperature methods As a result, a greater amount of carbon is retained in the solid phase, which is essential for achieving higher gross caloric values.
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, as illustrated in Figure 4.6, indicates that maintaining a consistent input feed of 4.0g results in varying yields during hydrothermal carbonization (HTC) Notably, the mass yield decreases with increased temperatures and extended reaction times, likely due to the decomposition of organic materials transitioning from the solid phase to other phases The conversion rates range from 80.6% to 93.5%, demonstrating that a substantial portion of the feed material remains in the solid phase throughout the HTC process Furthermore, the disparity between the maximum and minimum yields is considerable 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 impact 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 %
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 enhances the gross calorific value of paper mill sludge hydrochar The untreated biomass sample has a calorific value of 18.5 MJ/kg However, when subjected to temperatures between 180°C and 220°C for reaction times of 3 to 6 hours, the gross calorific value of the hydrochar increases to between 21.84 and 22.40 MJ/kg The highest calorific value recorded was 22.40 MJ/kg from a hydrochar sample processed at 220°C for 3 hours, representing a 21.08% increase compared to 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% increase in energy, the highest among the three biomass feeds analyzed This energy elevation approaches that of sub-bituminous coal, which has a gross calorific value of less than 24 MJ/kg (He et al., 2013), indicating that these hydrochar samples can be effectively utilized for energy production alongside coal Additionally, paper mill sludge samples demonstrate optimal results when subjected to higher temperatures and shorter reaction times, yielding hydrochar with low ash content and elevated levels of volatile solids and total solids, ultimately contributing to a higher gross calorific value.
The increase in higher heating value during hydrothermal carbonization is primarily due to hydrolysis and carboxylation reactions, which enhance carbon content while reducing oxygen levels in the substrate Notably, the paper mill sludge hydrochar exhibits a higher gross calorific value at elevated temperatures, where significant mass loss and decomposition occur The substrate's complex molecules, including cellulose, hemicellulose, and lignin, require these higher temperatures for effective degradation, leading to increased carbon conversion and enrichment of the solid phase Consequently, achieving a higher gross calorific value during the hydrothermal carbonization process depends not only on temperature and reaction time but also on the feed composition.
Figure 4.10 Yield of SD hydrochar with increasing time and temperature at 220 o C (n 3, triplicate)
The mass yield of hydrochar derived from raw sawdust, as illustrated in Figure 4.10, indicates that with a consistent input feed of 4.0g, the yield decreases at elevated temperatures and extended reaction times during hydrothermal carbonization (HTC) This decline is attributed to the decomposition of organic material transitioning from the solid phase to other phases The conversion rates observed range from 72.7% to 79.7%, highlighting that a significant portion of the feedstock remains in the solid phase throughout the HTC process Notably, the difference between the maximum and minimum yields is considerable when compared to yields obtained from food waste.
The 22S3 sample demonstrates the highest conversion rates due to minimal degradation observed at lower reaction times and temperatures This indicates 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 %
Figure 4.13 Gross calorific values of SDHC with time and temperature at 220 o C (n = 3, triplicate)
Hydrothermal carbonization results in a slight reduction of the gross calorific value of sawdust hydrochar, with untreated biomass measuring 27.97 MJ/kg Treated hydrochar, subjected to temperatures between 180°C and 220°C for 3 to 6 hours, maintains a calorific value 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 value aligns closely with that of paper mill sludge hydrochar, indicating that both materials exhibit similar energy characteristics The calorific values of sawdust hydrochar remain comparable to those of bituminous coal, ranging from 24 to 35 MJ/kg, showing no significant change due to the hydrothermal process Additionally, both sawdust and paper mill sludge hydrochar samples benefit from higher temperatures and shorter reaction times, resulting in lower ash content and higher levels of volatile and total solids.
The consistent gross calorific value of the sample is primarily due to its feed composition, which contains a significant percentage of lignin This lignin requires temperatures exceeding 220°C for effective degradation, ultimately contributing to a higher gross calorific value.
GCV of SD HC with time and temperature
In summary, achieving a higher gross calorific value in 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), specific samples were selected for analysis based on their ultimate analysis The samples included the initial feed materials and the resulting hydrochar, specifically the restaurant food waste hydrochar (sample 18R6), paper mill sludge hydrochar (sample 22P3), and sawdust (sample 22S3).
The ultimate analysis of 65 samples revealed that hydrothermal carbonization significantly enhances the gross calorific value (GCV) of various feeds, as shown in Table 4.4 Notably, restaurant food waste hydrochar experienced a 23.44% increase in GCV, while sawdust hydrochar saw a 13.3% increase Although both calculated and analyzed GCV values are closely aligned, paper mill sludge hydrochar exhibited the lowest increment in GCV post-carbonization, with substantial discrepancies between calculated and analyzed values This indicates that the equation used for GCV calculation may not be reliable for feeds like paper mill sludge, which contain lower biomass and higher inorganic content However, for samples with higher biomass, the equation provides relatively accurate GCV estimates, making it a useful tool for preliminary screenings.
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 is notably acidic, ranging from 3.30 to 4.14, compared to the initial measurement of 4.26 This acidity fluctuates with temperature and reaction time; lower temperatures and extended reaction times result in decreased pH, while higher temperatures can lead to increased pH due to the formation of organic acids like acetic and glycolic acid However, elevated temperatures can also reduce acidity by degrading intermediate organic acidic products It is essential to neutralize the process water's acidity before environmental release Alternatively, in line with circular economy principles, this process water can be utilized for anaerobic digestion, provided its pH is 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, following carbonization, demonstrates a significant increase from an initial value of 17.26 g/L to a range of 35.53 g/L to 70.14 g/L, reflecting the degradation of biomass during the process This increase in COD is particularly pronounced at lower temperatures, where the transformation of organic material from solid to liquid is intensified Conversely, at higher temperatures, COD values decrease over time due to the formation of secondary char and concurrent absorption processes during hydrochar formation Ultimately, the COD characteristics of this process water resemble those of landfill leachate, indicating the necessity for 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 following carbonization is illustrated in Figure 4.16, with initial samples measuring 6.13 mS/cm, while subsequent hydrothermal samples exhibit increased conductivity values ranging from 6.84 to 9.80 mS/cm These elevated conductivity levels indicate a higher concentration of inorganic compounds in the liquid phase Notably, electrical conductivity rises with both reaction time and temperature, reflecting the degradation of solid-phase materials and the release of organic and inorganic compounds into the liquid Although this study did not analyze the specific inorganic composition, it focused on Chemical Oxygen Demand (COD), Total Nitrogen (TN), and Total Phosphorus (TP) to assess potential biological treatment options for the process liquor However, there is a risk that the process water may contain harmful heavy metals, necessitating a pretreatment process before environmental discharge.
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 content in process water after carbonization shows a significant increase, rising from 48.49 mg/L in the initial sample to between 148.97 and 353.32 mg/L in post-carbonization samples, as illustrated in Figure 4.17 This surge 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, while higher temperatures eventually limit degradation due to the formation of secondary char and the absorption of already formed hydrochar Notably, 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 show a significant increase, with initial samples measuring 9.28 mg/L and post-carbonization samples ranging from 10.27 to 25.51 mg/L This rise in total phosphorus 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 Lower temperatures and extended reaction times enhance the degradation of organophosphorus compounds, whereas higher temperatures limit degradation over time due to the formation of secondary char in the liquid phase, resulting in phosphate becoming insoluble and depositing 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 hydrothermal carbonization (HTC) shows a decrease from an initial value of 6.71, ranging between 4.55 and 6.26, indicating increased acidity This reduction in pH is attributed to higher temperatures and longer reaction times during the HTC process, which promotes the degradation of organic compounds and the formation of organic acids like acetic and glycolic acid (Choo et al., 2020) Consequently, similar to process water from food waste, the acidity of the 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 following carbonization is illustrated in Figure 4.20, with the initial sample exhibiting a conductivity of 0.42 mS/cm In contrast, samples after the hydrothermal process demonstrate significantly higher conductivity values ranging from 0.92 to 1.96 mS/cm These elevated conductivity levels indicate an increased presence of inorganic compounds in the liquid phase, which correlates with extended reaction times and higher temperatures, reflecting the degradation and release of both organic and inorganic substances Although the specific composition of these inorganic compounds was not analyzed in this study, there is a concern that the process water may contain harmful heavy metals, necessitating a pretreatment process before its environmental discharge.
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 COD values of paper mill sludge process water after the carbonization process, as shown in Figure 4.21, initially measured 2.02 g/L, with subsequent values ranging from 4.27 to 12.46 g/L, indicating significant biomass dissolution during carbonization Lower temperatures resulted in reduced COD concentrations due to insufficient transformation of organic material from solid to liquid phases Conversely, higher temperatures and extended reaction times led to increased COD values, suggesting enhanced dissolution of organic compounds In comparison to restaurant food waste, the COD values of paper mill sludge process water are lower, attributed to the complex molecular structure of its organic components, which are more challenging to degrade.
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 levels in process water after carbonization show a significant increase, rising from an initial 0.15 mg/L to a range of 33.47 mg/L to 106.23 mg/L post-process, as illustrated in Figure 4.22 This surge 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, while higher temperatures limit degradation 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 to mitigate the risk of eutrophication (Funke et al., 2010).
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 levels in process water after carbonization are illustrated in Figure 4.23, showing an initial concentration of 0.72 mg/L, while post-carbonization samples ranged 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 exhibited minimal dissolved phosphorus in the liquid phase This phenomenon may be attributed to the formation of insoluble phosphate, which accumulates on the hydrochar surface under such 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) shows a significant decrease, dropping from an initial value of 4.54 to a range of 2.69 to 3.00, indicating increased acidity This reduction in pH is attributed to higher temperatures and longer reaction times, which enhance the degradation of organic compounds and lead to the formation of organic acids such as acetic and glycolic acid (Choo et al., 2020) Consequently, similar to the process water from food waste, the acidity of sawdust hydrochar process water 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 after carbonization, as illustrated in Figure 4.25, indicates significant changes The initial sample exhibited a conductivity of 0.049 mS/cm, while subsequent hydrochar samples showed increased values between 0.22 and 0.45 mS/cm These elevated conductivity levels suggest a higher concentration of inorganic compounds in the liquid phase, which correlates with extended reaction times and temperatures that facilitate the degradation of solid-phase materials Although the specific inorganic composition of the liquid phase was not analyzed in this study, there is a potential risk of heavy metal contamination that could pose environmental hazards Consequently, it is essential to implement further pretreatment processes before discharging 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 COD values of SD process water after carbonization, as illustrated in figure 4.26, show a significant increase from an initial measurement of 2.35 g/L to a range of 7.43–13.79 g/L, indicating substantial biomass dissolution during the process Lower temperatures result in reduced COD concentrations due to insufficient transformation of organic material from solid to liquid Conversely, higher temperatures and extended reaction times enhance the conversion and dissolution of organic compounds, leading to elevated COD levels in the process water Notably, the COD values for SD hydrochar process water are lower than those 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 show a significant increase, with initial samples measuring 0.14 mg/L and post-carbonization samples ranging from 3.44 mg/L to 11.25 mg/L, as illustrated in Figure 4.27 This rise in total nitrogen is attributed to the degradation of nitrogen-containing organic compounds during the hydrothermal carbonization (HTC) process While lower temperatures and extended reaction times enhance the breakdown of these compounds, higher temperatures eventually limit dissolution due to the formation of secondary char and the absorption of already formed 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 (MSW), wet agricultural residues, human waste, sewage sludge, algae, and aquaculture residues, even with moisture contents ranging from 75-90% Often referred to as wet pyrolysis, hydrothermal carbonization offers significant advantages over traditional pyrolysis, which is restricted to dry biomass feedstocks.