Hydrothermal Dewatering Research Articles

Hydrothermal dewatering of low-grade coal: Product evaluation and primary component analysis of ash deposition

The hydrothermal dewatering (HTD) process was carried out using 16 coal samples obtained from several regions in Indonesia. This research aims to identify the dominant parameters that influence ash deposition during coal combustion in a boiler or gasifier due to the HTD process. This research was conducted due to the lack of clear and comprehensive information regarding this issue. Therefore, to understand the effect of HTD on ash deposition, an analysis of the chemical composition of ash and the ash melting temperature (AFT) was carried out. To verify the data obtained from the experimental analysis, statistical methods such as paired sample t-test and primary component analysis were applied to obtain the dominant parameters influencing ash deposition due to HTD. Eight parameters, namely SiO2, Fe2O3, slagging viscosity index (SR), Babcock index (Rs), initial deformation temperature, softening temperature, hemispherical temperature, and fluid temperature under oxidative conditions, have the greatest influence on ash deposition due to the HTD process. Based on the average values of SR and Rs, raw coal and processed coal samples have the same ash deposition trend. On the other hand, based on AFT under oxidation conditions, processed coal has a higher AFT, indicating that the tendency for ash deposition is lower than raw coal. Therefore, the HTD process can be used to improve the quality of low-grade coal. As an implication, low-grade coal, which has not been widely utilized in Indonesia, due to several constraints regarding its characteristics, through the HTD process can be optimized where coal utilization is a bridge towards the use of green energy which is being intensively pursued.

Combustion performance of lignite after efficient upgrading via deep eutectic solvent‐assisted hydrothermal dewatering

AbstractIn order to increase the efficiency of hydrothermal dewatering (HD), a deep eutectic solvent (DES, synthesized by ChCl and ZnCl2) was employed to promote the removal of moisture and oxygen from lignite. The variation of physicochemical structure of upgraded coals greatly affected the combustion performance. The effects of addition amount of DES, upgrading temperature, and the molar ratio by ChCl and ZnCl2 on combustion performance of upgraded coals were analyzed via thermogravimetric analysis. Compared with HD lignite upgraded at 280°C, the combustion curves of DES‐HD lignite were shifted and delayed toward a higher temperature zone as DES additions increased. DES‐HD lignite had lower spontaneous combustion tendency and higher combustion reactivity. When upgrading temperature increased from 250 to 300°C, the combustion curves of lignite upgraded with 3.0‐g DES (ChCl:ZnCl2 = 1:1) first moved to the low‐temperature zone and then shifted to the high‐temperature zone. DES‐HD lignite maintained the advantages of raw coal in terms of combustion characteristics. For lignite upgraded with different molar ratios of ZnCl2 in DES, the difference in combustion behavior was mainly manifested in the burnout stage. The burnout stage was advanced slightly as the molar ratio of ZnCl2 increased.

A quantum chemistry study of the deoxidation of lignite during hydrothermal dewatering treatment

ABSTRACT To investigate the deoxidation mechanism of lignite under hydrothermal dewatering (HTD) condition, the removal mechanism of carboxyl and methoxy groups was calculated at the B3LYP-D3/6-311 G(d,p) level by using density functional theory. The feasibility of deoxidation was analyzed from the perspective of thermodynamics and kinetics, and the influences of hydrogen bond on the deoxidation process were analyzed. The results showed that the activation energy of decarboxylation was lower than that of demethoxide, indicating that the carboxyl group had high reactivity. Through the hydrogen bonding between the carboxyl group and water, the activation energy of indirect decarboxylation (lignite-C6H5-COOH→lignite-C6H6-COO→lignite-C6H6+ CO2) decreased by 10 kJ/mol, whereas the activation energy of direct decarboxylation increased by 8 kJ/mol. The activation energy of demethylation was reduced by 25 kJ/mol, and the change in activation energy of subsequent decarbonylation was similar to that of decarboxylation. The two-step elementary reaction benefitted the deoxidation reaction.

Evaluation of hydrothermal treatment on physicochemical properties and re-adsorption behaviors of lignite

Lignite is a kind of low-rank coal with abundant reserves and low price in China, but the high moisture content and low calorific value greatly restrict the large-scale utilization of it. Hydrothermal dewatering (HTD) is a promising drying method for dewatering and upgrading lignite. Meanwhile, re-adsorption is an important index to evaluate feasibility of a drying project. In this study, the effects of drying properties of an Inner Mongolia lignite under HTD and moisture re-adsorption of the upgrading lignite after HTD were investigated. The results indicate that HTD upgrading treatment has a significant effect on lignite composition, oxygen-containing functional groups, and pore structure. The effective diffusion coefficient of moisture and activation energy processed with equilibrium moisture decrease with the increasing of temperature. The drying process of equilibrium water can be described by thin-layer dynamics model and the Arrhenius formula. There existed fairly good linearity between the oxygen content, pore structure and moisture re-adsorption rate. The re-adsorption rate of the upgraded lignite was affected by oxygen-containing functional groups with a descending order of Ph-OH, COOH, CO and Ar-OH.

Mechanism insights into hydrothermal dewatering of food waste digestate for products valorization

Poor dewaterability is a bottleneck of the disposal of digestate from food waste (DFW). However, the dewatering mechanism remains unclear due to the complex composition of DFW. Understanding the dewatering mechanism, as well as the transformation of organic/inorganic matters is essential for the DFW management and valorization. In this study, the distribution, transformation, and complex interplay of organic and inorganic matters at different Hydrothermal treatment (HTT) temperatures were comprehensively analyzed to explore the hydrothermal dewatering mechanism of DFW. When HTT was conducted in the temperature range of 120–180 °C, the interstitial water was released as surface or free water because of membrane breaking and size reduction of the solid substrate. Releasing divalent cations increased the Zeta potential of the bulk solution. The weaker electrostatic repulsion between suspended particles made them easier to settle as the centrifugation cake. When the temperature of HTT was above 180 °C, polymerization and aromatization reactions took place gradually for organic matters, and the bound water was further removed. The generated humic substances were more hydrophobic than the raw material. In addition, the humic substance could combine with cationic metals, which decreased the zeta potential of the bulk solution but promoted the aggregation of nanoparticles and enhance the dewaterability of DFW.

Study on combustion performance of hydrothermally dewatered lignite by thermal analysis technique

The high water content of lignite has raised serious environmental and efficient issues, which are obstacles to its large scale utilization. In this study, hydrothermal dewatering (HTD) method was used to upgrade a lignite from Inner Mongolia in China at 230–330 °C. The combustion behaviors of raw lignite and HTD samples were investigated by thermogravimetry with differential scanning calorimetry (TG-DSC). By changes of mass and heat flow, the lignite combustion process was described from three stages: water evaporation, coal devolatilization and char combustion. Compared with raw lignite, the combustion reactivity of HTD samples upgraded at 230–300 °C was improved, while that of sample upgraded at 330 °C decreased. The temperatures at maximum combustion rate of 230–300 °C samples (below 397.6 °C) were lower and that of 330 °C samples (448.9 °C) was higher than that of raw lignite (399.8 °C). The released heat at char combustion stage increased for HTD samples. Then the physico-chemical structures of lignite before and after HTD process were analyzed to investigate the effect of HTD on coal combustion characteristics. Results showed that HTD process could affect the heat at water evaporation and initial devolatilization stages during coal combustion through removing oxygen functional groups in lignite. The enhancement in aromaticity of lignite after HTD increased activation energy of intrinsic reaction, whereas the large pore volume and high surface area of 230–300 °C samples decreased diffusion activation energy at char combustion stage. The effect of pore structure predominated on combustion reactivity compared with aromaticity for HTD samples.

Treatment of digestate residues for energy recovery and biochar production: From lab to pilot-scale verification

Hydrothermal pretreatment was used for dewatering food waste digestate residue, and biochar/biogas were generated from the separated solid and liquid phases via pyrolysis and anaerobic digestion, respectively. Increasing hydrothermal pretreatment temperature (110–200 °C) clearly improved dewaterability, whereas enhancing treatment duration (30–90 min) had little impact. The optimal condition of 160 °C/30 min gave the best dewatering performance with relative lower energy consumption and was chosen for pilot-scale verification achieving 61.7 wt% dry weight content after mechanical squeezing. Moreover, the filtrate and filter cake obtained at optimal condition were applied for biogas and biochar production in lab scale. The methane yield of the filtrate was 335 mL/g COD. Pyrolysis temperature of 500 °C gave better biochar performance and was verified in a pilot scale test. Additionally, the heavy metals in digestate were effectively immobilized during hydrothermal dewatering and pyrolysis processes. In the end, mass/energy balance calculated using pilot-scale data presented the combined systems provided a promising strategy for accomplishing energy recovery and resource reuse of digestate residue.

The Effect of Hydrothermal Dewatering Temperature on Hydro-Char Obtained from Oily Scum

Oily scum was treated by hydrothermal dewatering (HTD) under 120-240°C. The changes of surface characteristics and physico-chemical structure of oily scum were investigated and the effect of temperature on the properties of the hydro-char obtained from oily scum was analyzed through different characterization techniques. Results show that the moisture content of HTD treated hydro-char decreases as the temperature increases, which implies that the oily scum can be dewatered and upgraded through HTD method. The morphologies of the HTD pretreated oily scum present less oil content, smaller particle size, and become darker. As the reactor temperature increases from 120 to 240°C during the HTD process, the moisture content of hydro-char obtained decreases from 35.51 to 11.31%, while the liquid content and hydro-char content increase from 58.53 to 79.76% and from 2.50 to 5.29%, respectively, and the released gas content slightly varies in the range of 3.14 - 4.31%. The breaking and gathering effects of the HTD upgrading on oily scum result in a wide particle size distribution of products, which indicates that the overall structure of raw oily scum has been destroyed. With the increase of HTD temperature, the shorten vibration of methylene groups in hydro-char products is weakened.

Changes in Physicochemical Properties and the Release of Inorganic Species during Hydrothermal Dewatering of Lignite

Hydrothermal dewatering (HTD) is a nonevaporative drying method mainly useful for lignite, which in general has high moisture content. The lignite upgraded with HTD is expected as a clean fuel for ...

Evaluation of combustion behaviour for Indonesian low-rank coals treated hydrothermally

Hydrothermal dewatering process has been made to produce dry-processed coals, which are comparable to bituminous coal. Two types of coals, i.e. low rank and high-rank coals. The low-rank coal came from West Papua while the high one was from Central Kalimantan. The behaviour of raw and processed coals were observed using thermogravimetry and differential scanning calorimetry techniques The change in chemical properties that are based on proximate, ultimate, calorific value and Fourier-transform infrared spectroscopy analyses are studied. Those are closely related to some combustion problems. This process was conducted in a laboratory scale using an autoclave with 5,000 ml/batch in capacity at the temperature of 300 and 330°C for one hour. The results indicate that the processed coals generally have a better combustion behaviour than that of the raw coals. The processed coals have a lower reactivity than that of raw ones, due to the higher ignition temperature (Tig), char burnout temperature (Tbo) as the end of combustion and maximum combustion rate (Rmax) of processed coals. The processing temperature of the process was a slight effect on combustion behaviour. The process is very effective to improve the quality of low-rank coal, nonetheless to high-rank coal, which has low moisture content and high calorific value, and the combustion behaviour of processed coals was not significantly changed.

Transformation behaviors of C, H, O, N and S in lignite during hydrothermal dewatering process

Lignite resources are abundant in China, but high moisture contents limit their utilization. With a view to making optimal use of beneficial elements (C, H, O) and effectively control the release of harmful elements (N and S) in lignite during drying, two lignites, one from Inner Mongolia (IM) and one from Yunnan (YN) province, were hydrothermally dewatered (HTD) at 230 °C, 270 °C and 300 °C. The solid, liquid and gaseous products were characterized by proximate and ultimate analyses as well as Fourier transform infrared spectroscopy (FT-IR), chemical oxygen demand (COD) analysis, Raman laser analysis and gas chromatography. Based on these data, the transformation behaviors of C, H, O, N and S contained in the raw lignite as they partition into the HTD products (solid, liquid and gas) were investigated. Results show that the effects of HTD on the elemental transformations depend on the coal properties. HTD is more effective for YN lignite than for IM lignite. IM liquid products have lower COD contents, resulting from less dissolution of IM lignite. CO2 and CO are released during HTD and their amounts increase with temperature for both IM and YN lignites. High temperature leads to a significant loss of carbon in the solid product; this corresponds with increases of the carbon content in the gas and liquid phases. Hydrogen, nitrogen and sulfur in lignite are mainly transferred into the liquid phase. A large proportion of the oxygen-containing functional groups in the lignite decompose, causing its transfer out of the solid and into the gas and liquid phases.

Hydrothermal dewatering of low-rank coals: Influence on the properties and combustion characteristics of the solid products

The presence of increased moisture content, low energy density and maximum spontaneous combustion tendency has raised serious environmental issues, which are obstacles to large scale utilization of coals. Hydrothermal dewatering (HTD) is a promising method for upgrading low rank coals (LRCs). The influences of HTD, executed at different temperatures on solid product yield, removal rate of elements, organics and inorganics, and combustion characteristics in LRCs were studied. HTD treatment of LRCs was carried out in 2 L cylindrical autoclave at 200–300 °C for 1 h. For upgraded coals, the inherent moisture and oxygen content decreased significantly while fixed carbon content and calorific value increased. FTIR results indicate the conversion of LRCs near to high-rank coals (HRCs). The organic matter was removed due to loss of Methylene, Methyl groups and O-containing functional groups, whereas inorganic matter was removed mainly in the forms of Fe and Ca-containing minerals. The upgraded coals had higher ignition and burnout temperatures, while their combustion was shifted and delayed toward higher temperature zone. Activation energy of treated coals increased in ignition stage while decreased in combustion segment. Prominent changes were found in coal structure, composition, rank, and combustion characteristics at HTD temperature of 300 °C.

Effect of hydrothermal dewatering on the pyrolysis characteristics of Chinese low-rank coals

This paper describes the effect of the hydrothermal dewatering (HTD) on the pyrolysis characteristics of low-rank coals (LRCs). The effect of HTD on the characteristics of LRCs was explored based on scanning electronic microscopy and N2 adsorption analyses. The pyrolysis behaviors and gas products were determined using a TG-FTIR instrument. The results show that the crosslink structure and overall gel structure were broken. The pore structure expanded to the micropore region, and the surface area and total pore volume initially increased and then decreased as the treatment temperature increased. The TG-DTG results confirmed that the pyrolysis process moved towards the high-temperature and bituminous coal region. The characteristic parameters of pyrolysis indicated that the thermal stability of the coal structure was improved. According to the TG-FTIR results, the amount of released gas products from stable and saturated components (such as p-xylene, phenol and CH4) increased, whereas the amount of released gas products from unstable and unsaturated components (such as CO2, CO and formic acid) decreased. The unstable structure and components of the LRCs were decomposed and transformed, and a stable structure and phase were created. Moreover, pyrolysis activity declined, and thermal stability improved.

Hydrothermal dewatering of lignite water slurries: Part 2 surface properties and stability

AbstractIn this paper, the mechanism of the stability of lignite lignite water slurry after hydrothermal dewatering/treatment (HTD) was studied by the measurement of the yield stress and the fractal dimension of the slurry. The effects of HTD on the surface properties of low rank coal were characterized by the contact angle, zeta potential, and Fourier transform infrared (FTIR) spectroscopy. After HTD, the hydrophobic attraction between coal particles was found to increase, as shown by the increase of the contact angle, and the electrical double layer repulsion was found to decrease, due to the decrease of zeta potential. These effects lead to an increase on the inter‐particle attractions, which is also proven by the theoretical calculations on the interaction energy using the extended DLVO theory. The stability measurements show that the low rank coal water slurry (CWS) becomes more stable after HTD, which is probably due to the formation of a network structure (soft sediment) by coal aggregating across the whole volume of the slurry to prevent coal particles from settling down. The network structure (soft sediment) in coal water slurry after HTD was verified and quantified by the yield stress and fractal dimension. Our findings indicated that the static stability of coal water slurry may be acquired by the network structure of the aggregated coal particles.

Effects of hydrothermal dewatering of lignite on rheology of coal water slurry

AbstractMaximizing the dry‐based solid loading of coal particles in water is essential in increasing the burning efficiency of coal water slurry, which has been widely used as a liquid fuel. Understanding the rheology of coal water slurry could provide fundamental guidance on designing and optimizing coal water slurry formulation. The rheological studies have shown that coal water slurries made with lignite samples after hydrothermal dewatering (HTD) exhibit a stronger shear thinning behaviour as compared with those made with raw lignite samples. The viscosity of coal water slurry at the shear rate of 100 s−1 decreases with an increasing HTD temperature, which is probably due to the decrease of volume of lignite particles caused by the permanent reduction of both bound and non‐freezable water (inherent moisture) after the HTD process. The reduction of the inherent moisture of lignite samples after HTD treatment was elucidated by differential scanning calorimetry (DSC) under the temperature well below the freezing point. A lignite water slurry with a solid loading of 62 wt% db (dry basis) is obtained after hydrothermal dewatering at 300 °C with the addition of 1.2 wt% of polycarboxylate ether (PCE). Our findings indicated that hydrothermal dewatering of lignite has profound impacts on the inherent moisture of lignite and the rheological properties of coal water slurry.

Changes in the physicochemical characteristics and spontaneous combustion propensity of Ximeng lignite after hydrothermal dewatering

AbstractSpontaneous combustion behaviour and physicochemical characteristics of Ximeng lignite dewatered by hydrothermal dewatering (HTD) were investigated. In addition, effect of upgrading temperature, as well as the mechanism for evolution of spontaneous combustion propensity was discussed. The results showed that after HTD, fixed carbon content of lignite increased, whereas equilibrium moisture content and volatile content decreased; aromatic carbons increased relatively at the expense of oxygen‐containing functional groups and aliphatic hydrocarbons. Pore structures of lignite were developed by different extents after HTD. Spontaneous combustion propensity of upgraded lignite was evaluated using the crossing‐point temperature method. The results indicated that the spontaneous combustion propensity of HTD‐upgraded lignite significantly depended on upgrading temperature. When upgrading temperature was lower than 230 °C, spontaneous combustion propensity was raised due to development in pore structure. However, when upgrading temperature exceeded 230 °C, spontaneous combustion propensity was suppressed because active sites involved in coal oxidation were extensively removed. In conclusion, a higher upgrading temperature is recommended to extend the upgrading effect and to further suppress the spontaneous combustion propensity of lignite.

Distribution of trace elements during coal Gasification：The effect of upgrading method

Abstract A combination of upgrading and gasification process seems quite promising to cleanly utilize low-rank coal (LRC). The distribution of trace elements in this process has rarely been researched. In this study, four upgrading methods for LRC (evaporative drying, pyrolysis upgrading, microwave drying and hydrothermal dewatering (HTD)) were used and then the upgraded coal (UC) was gasified with investigation on the distribution and emissions of 13 trace elements (beryllium, vanadium, chromium, manganese, cobalt, nickel, copper, zinc, arsenic, selenium, cadmium, mercury and lead). The relative enrichment factors (RE) were introduced to quantitatively compare four different upgrading methods. The results show that gasification of UC helps controlling trace element of LRC by reducing its contents in feedstock and enhancing the ash enrichment. Different upgrading methods have their specific effects on different trace elements, respectively. Microwave drying presents to be best for most trace elements enriched to ash, in particular for beryllium, vanadium, chromium, manganese, cobalt, copper. Majority of 13 elements could reduce roughly 50% of the content through microwave drying method, and the reduction of lead reaches 72.5%. It seems the upgrading is quite effective in controlling emissions of trace elements from coal gasification.

Effect of hydrothermal dewatering on the moisture content of brown coal

ABSTRACTBrown coal was dewatered under hydrothermal dewatering (HTD) conditions. The variation in moisture content and distribution of brown coal were studied. The results showed that equilibrium moisture content decreased from 28.2% to 9.6% after hydrothermally treated at 300 ºC. Three types of moisture existed in brown coal and its HTD products according to the coal-water binding energy (Eb): Surface adsorption water (Eb < 2000 kJ/kg), interparticle water (2000 < Eb < 9000 kJ/kg), and interior adsorption water (Eb > 9000 kJ/kg). Drying rates of these types of moisture​ respectively followed logarithmic, linear, and indicial equations.

The effects of hydrothermal dewatering on Indonesia lignite characteristics for fixed-bed gasification

ABSTRACTHydrothermal dewatering of lignite derived from Pendopo, South Sumatera, Indonesia, was evaluated at residence time ranging from 30 to 90 minutes and size ranging from 1 to 7 cm, with a pressure of 30 bar and a temperature of 225°C. The total moisture (TM) content of the raw lignite was approximately 37.56 wt.%, the volatile matter content was 37.24 wt.%, ash content was 1.8 wt.%, fixed carbon content was 23.4 wt.%, and the calorific value (CV) was 3,337 kcal/kg. In addition, the thermal fragmentation of raw lignite was 42%. Lignite was hydrothermally dewatered with the addition of water in the laboratory scale. The solid products yield was between 48.00 and 57.88 wt.%, and the CV of hydrothermally dewatered lignite was between 4,781 and 5,308 kcal/kg. Meanwhile, the thermal fragmentation of HTD lignite was 0.1–1% and TM was 11.53–20.91wt.%. Change in the carbon-functional groups and their properties, as determined by Fourier-transform infrared spectroscopy, shows the significant decrease of hydroxyl group intensity that appeared in the region 3,100–3,600 cm−1, which was attributed to the dehydration phenomena. The HTD lignite has low water content and is resistant to thermal fragmentation, which makes it promising to be used as a fuel in the fixed-bed gasifier.

Relationship between physicochemical properties and dewaterability of hydrothermal sludge derived from different source

Sewage sludge (SS) and deinking sludge (DS) were used to comparatively study the hydrothermal dewatering of sludge with different components. For a better overview, an insight into the relationship between physicochemical properties and dewaterability of hydrothermal sludge was provided. Results found that not all kinds of sludge were suitably conditioned by hydrothermal treatment (HT) in term of the elevation of dewaterability. Higher hydrothermal temperature tended to enhance the dewaterability of SS rather than DS, which was supported by the variation of their physicochemical properties (including water distribution, bonding energy, extracellular polymeric substance (EPS), particles size, acid functional groups and zeta potential in this study). In addition, the changes in surface morphology suggested that the reverse effect of HT on sludge dewaterability was mainly due to their dewatering behavior. For SS, the destruction of EPS structure leaded to the release of bound water, thereby strengthening sludge dewatering. Conversely, “Bridging effect” generated by lignocellulose in DS was beneficial for sludge dewatering; however, the increasing hydrothermal temperature degraded part of lignocellulose and weakened “bridging effect”, finally resulting in worse dewaterability of DS.