Lignite Water Slurry Research Articles

The resource utilization of coal gasification wastewater by co-slurry with lignite: Slurryability, dispersion/aggregation behavior, and co-slurrying mechanisms

Coal gasification wastewater (CGW) as a substitute for clean water to prepare coal water slurry, which serves as gasification/combustion feedstock, can simultaneously achieve wastewater management and resource utilization. The purpose of this work was to investigate the effects of internal components (i.e., oils, phenols, nitrogen heterocyclic compounds, and inorganic cations) in CGW on the slurryability and dispersion/aggregation behavior of lignite and their co-slurrying mechanisms through experimental and the extended DLVO (eDLVO) theoretical calculations. Results showed that compared to lignite water slurry (LWS), the maximum solids concentration (ωmax) of coal gasification wastewater slurry (CWWS) increased by 0.80%, with a lower pseudo-plasticity. The organic compounds adsorbed competitively on the hydrophobic regions of lignite improved the slurrying capability of CWWS by decreasing the wettability, Zeta potential, roughness and pore structure of the particles. By contrast, the static stability of CWWS decreased, and its kinetic stability index (TSI) increased from 0.23 to 0.33. From the backscattering spectra, dodecane caused the creaming process of the slurry, while the clarification process was attributed to phenol and nitrogen heterocyclic compounds. Inorganic cations, especially NH4+ and Ca2+, significantly reduced the hydrophilic and negative charges on the particle surface, which were the main unfavorable factors leading to particle aggregation and sedimentation. The eDLVO theoretical calculation results exhibited that inter-particle interaction in CWWS was determined by the hydrophobic energy (Eh) and electrostatic energy (Ee) and their dropped from 1.79 × 10−16 J and 5.16 × 10−17 J to −4.67 × 10−16 J and 2.59 × 10−17 J respectively.

Energy utilization of direct coal liquefaction residue via co-slurry with lignite: Slurryability, combustion characteristics, and their typical pollutant emissions

Direct coal liquefaction residue (DCLR) as an auxiliary feed material with lignite to prepare coal water slurry (CWS), which is then used as combustion fuels, enables recycle resources of DCLR and diminishes its impact on the surrounding environment. This paper aims to study the slurryability, combustion characteristics, and typical pollutant emissions of the lignite water slurry (LWS), DCLR water slurry (DCLRWS), and lignite-DCLR water slurry (L-DWS). Results showed that the DCLR and lignite were complementary in physicochemical properties for preparing the L-DWSs. The maximum solid concentration (ωmax) of the DCLR was 73.19 wt% with poor static stability, whereas that of the LWS was 41.70 wt% with high pseudoplasticity. By contrast, the ωmax of the L-DWS-5 prepared from the lignite and DCLR at mass ratio of 5:5 was 53.91 wt% with lower pseudoplasticity and static stability. Meanwhile, combustion experiments confirmed that the introduced DCLR was effective in improving the ignition, burnout, and comprehensive combustion performance of the L-DWS. The Qnet, p of the slurrying samples increased from 6.70 MJ/Kg for the LWS to 14.41 MJ/Kg for the L-DWS-5. And the addition of the DCLR exhibited slight influences on the emissions of NOx and SO2, but significantly increased HCN and other organic compounds such as CnHm, CO, and NH3 emissions. Furthermore, except for Pb and Zn, more than 85% Cr, Cu, and Ni were intercepted in solid residues.

Improve the slurry ability of lignite through co-slurrying and microwave co-pyrolysis with direct coal liquefaction residue

This study is devoted to demonstrating experimentally the advantages of direct coal liquefaction residue (DCLR) on improving the slurry ability of lignite through co-slurry and microwave co-pyrolysis. The results show that: as the lignite/DCLR mass ratio decreased from 10:0–5:5 the maximum solid concentration (MSC) of the mixed slurry increased from 42.14 wt% to 54.25 wt%, simultaneously, the pseudoplasticity and static stability gradually decreased. This is because the organic matter and abundant minerals in DCLR remarkably changed the slurry ability of mixed slurry. And during microwave pyrolysis with the assist of 12 wt% DCLR the slow drying stage of lignite pyrolysis was shortened, and the constant temperature stage was extended, thus, the coal quality was improved, the oxygen-containing functional groups was removed significantly and the BET specific surface area increased. These changes caused the MSC of upgraded lignite water slurry (ULWS) increased to 64.54 wt%; but the “shear-thinning” characteristic became inconspicuous, and the static stability became slightly worse. In general, DCLR could effectively improve the slurry ability of lignite through co-slurry with lignite at a mass ratio of 5:5 and co-pyrolysis with lignite at a microwave power of 1000 W.

Physicochemical properties and slurry ability changes of lignite after microwave upgrade with the assist of lignite semi-coke

Microwave pyrolysis is a promising technique for the clean and effective utilization of low-rank coal, which has aroused widespread concern during the past decades. This paper applied microwave energy with the assist of 10 wt% lignite semi-coke (LSC) to upgrade lignite and prepared the solid residue into lignite water slurry (LWS). The physicochemical property and slurry ability changes of upgraded lignite were analyzed. The micro-mechanisms on interactions between dispersant molecules and upgraded lignite particles were investigated through Molecular Dynamics (MD) simulations. It turns out that LSC could effectively prompt the rapid heating of lignite in the microwave field. And after microwave upgrade the coal quality of lignite was improved, the oxygen-containing functional groups was removed remarkably. The adsorption conformation of dispersant on the lignite-water interface exhibited a double adsorption, while that on upgraded lignite-water interface gradually showed single-layer and multi-point adsorptions. Besides, the physicochemical property changes varied most when lignite was upgraded under 800 W, which leads to the maximum solid concentration of LWS improved most, from 51.84 wt% to 61.45 wt%. It further increased to 62.43 wt% as the LWS was directly prepared from the solid residues after microwave irradiation. Morever, the pseudoplasticity of LWS became weaker, but the static stability became better. Therefore, LSC, a self-product of lignite, exerted an enormous function on improving the slurry ability of lignite.

An approach for upgrading lignite to improve slurryability: Blending with direct coal liquefaction residue under microwave-assisted pyrolysis

As the composition of direct coal liquefaction residue (DCLR) is complex and difficult to handle, more importantly its dielectric properties are excellent, it is used as consumable wave-absorbents to enhance the microwave pyrolysis of lignite for slurryability improvement. The effects of the different additions of DCLR on the evolution of pyrolysis products were studied by using gas chromatography, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and low-temperature N2 adsorption analysis. The results indicated that microwave-assisted pyrolysis with DCLR was a promising method for improving slurryability of lignite and an effective method for utilizing DCLR. The introduced DCLR accelerated the heating rate of the process of microwave upgrading lignite, facilitating the decomposition of active oxygen-containing functional groups and aliphatic hydrocarbons and then their transformation into stable ether groups and aromatic structures. In the meantime, it was converted into gaseous products, mainly composed of H2, CO, and CH4, and solid products with high quality. Additionally, the cyclization and aromatization of organic structures were improved, as well as the order degree of aromatic systems, especially with 12 wt% of DCLR added. Furthermore, the existing and newly formed structures of micropores and mesopores in the upgraded lignite (UL) were remarkably reduced with the increase of DCLR contents. The re-absorption capacity was dramatically reduced and the slurryability of UL was improved as a result of the changes in chemical properties and pore structures. The maximum solid concentration of lignite water slurry (LWS) increased from 41.73 wt% to 65.42 wt% with lower pseudo-plasticity and static stability.

Effect of lignite semi-coke on lignite microwave upgrade and its slurryability

ABSTRACT To facilitate the microwave upgrade of lignite and increase the solid concentration of lignite water slurry (LWS), lignite semi-coke was used as a microwave absorber. Some properties of lignite before and after microwave upgrade were studied by infrared spectroscopy (FTIR) and low-temperature nitrogen adsorption. Besides, the adsorption characteristics and slurryability of upgraded lignite were investigated, respectively. It turned out that adding lignite semi-coke could effectively accelerate the process of lignite microwave upgrade; and as the irradiation power increased, the moisture content, carboxyl, and hydroxyl functional groups were removed obviously, the pore structures changed, and the number of mesopores and micropores increased from macropores transition. These changes improved the adsorption characteristics of upgraded lignite. Therefore, the above changes enhanced the slurryability of upgraded lignite: the maximum solid concentration (with an apparent viscosity at 1000 mPa.s) of LWSs increased from 49.49wt% (prepared from raw coal) to 63.90wt% (prepared from upgraded lignite at 1100 W), and the pseudoplasticity of LWSs gradually weakened.

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.

Further exploring on aqueous chemistry of micron-sized lignite particles in lignite–water slurry: Effects of humics adsorption

The aqueous chemistry of micron-sized lignite particles has essential influences on the dispersion stability of water-lignite slurry. In this work, the adsorption behaviors of dissolved humics on lignite surface in lignite–water slurry were studied through batch adsorption experiments. The influences of adsorbed humics on the surface properties, particle size distribution and dispersion stability of lignite particles in water-lignite slurry were examined by advanced analysis techniques, EDLVO theory and settlement tests. The adsorption results indicate that the adsorption capacity of humics on lignite increases from 4.11 mg·g−1 to 8.25 mg·g−1 with the humics concentration increasing from 100 mg·L−1 to 200 mg·L−1. The adsorption process can be well described by the pseudo-second order kinetic model. The adsorbed humics significantly improves the hydrophilicity and electronegativity of lignite surface and reduces the apparent particle size of lignite. The EDLVO calculation and settlement tests reveal that the total interaction energy of the humics-adsorbed lignite particles at 18 nm of separation distance is enhanced from 0.85 × 10−17 J to 1.45 × 10−17 J with the initial humics concentration increasing from 100 mg·L−1 to 200 mg·L−1. The dispersion stability of micron-sized lignite particles is improved with humics adsorption on lignite surface.

Petroleum coke facilitate the upgrade of lignite under microwave irradiation for slurryability improvement

To remarkably reduce moisture and volatile content of lignite for the preparation of lignite water slurry (LWS) with high solid concentration, different additions of petroleum coke (PC) were used to increase the heating rate of microwave irradiation. After that the physical and chemical property changes of upgraded lignite were investigated through X-ray photoelectron spectra (XPS) and low temperature N2 adsorption method. The results show that: the coal rank increased, the oxygen-containing functional groups decreased remarkably, the specific surface area and pore volume increased, the average pore diameter decreased, and the macropores gradually transformed to mesopores and micropores. Furthermore, these changes of upgraded lignite reached their maximum when 10 wt% PC was added to facilitate microwave pyrolysis. The maximum solid concentration increased from 51.60 wt% to 66.87 wt% and further increased to 67.51 wt% when the 10 wt% PC was mixed with upgraded lignite to prepare LWS. The LWSs prepared from upgraded lignite showed a weaker shear-thinning characteristic and lower static stability.

Application of Turbiscan LAB to study the influence of lignite on the static stability of PCLWS

To study the influence of different petcoke/lignite (P/L) mass ratios on the static stability of petcoke/lignite water slurry (PCLWS), Turbiscan Lab was employed to quantitatively and objectively characterize the static stability of mixed slurry. Besides, the effect of microwave irradiating the lignite for different times on the static stability of petcoke/upgraded lignite water slurry (PCULWS) was explored at a P/L mass ratio of 6:4. And the chemical property changes of upgraded lignite were measured through proximate and ultimate analysis and X-ray photoelectron spectra to elucidate the influence of lignite particles on the static stability of mixed slurry. With the decrease in P/L mass ratio the pseudo-plasticity and static stability of PCLWS gradually enhanced. Furthermore, the solid concentration of PCLWS with good static stability reached about 60wt% at a P/L mass ratio of 6:4. Small decreases in volatile matter, moisture and oxygen functional group content and small increases in fixed carbon of the lignite were associated with an increase in the static stability of PCULWS, but further changes in these properties following prolonged microwave irradiation were associated with a decrease in static stability, perhaps as a result of the destruction of the three-dimensional network of lignite particles. Therefore, the abundant oxygen-containing functional groups and side chains of lignite particles probably plays an indispensable role in supporting the composite particles formed by lignite, petcoke, moisture and dispersant molecules.

Primary reactions of lignite-water slurry gasification under the supercritical pressure in the electric field

Lignite-water suspension supercritical gasification was carried out in an electric field with an intensity of 0.5V/cm in the presence of additives under the pressure of 24MPa in two temperature ranges: 330–360 and 390–450°C. From 50 to 78% of hydrogen and oxygen of gas products were of water origin. The application of the electric field switched carbon oxidation over to hydrocracking in the range of the subcritical temperature. At the subcritical temperature, the electric field had moderated the lignite hydrolysis and intensified its hydrocracking.The gasification of lignite-water suspension under the supercritical pressure in the electric field had at least five types of interactions: electrolysis of water, thermal decomposition of coal, carbon oxidation by oxygen of water as well as hydrolysis and hydrocracking of coal. Electrolysis and hydrocracking were dominant for the process at the subcritical temperature on water. Hydrolysis and hydrocracking of CC bonds of coal were the main at the supercritical parameters of water.

Improvement on slurry ability of lignite under microwave irradiation

Under 900W, the effects of microwave irradiation time on the solid concentration, rheology and static stability of lignite water slurry (LWS) were investigated. The results showed that with the gradual increase of microwave irradiation time the hydrated film on lignite became thinner and it contributed to the maximum solid concentration of lignite increased from 51.63% to 55.30% (irradiated for 12min). At the same time, the oxygen/carbon and hydrogen/carbon atomic ratios decreased, the moisture content and oxygen-containing functional groups gradually removed, and the lignite surface became smooth. The changes of these interfacial properties improved the pseudo-plasticity and reduced the yield stress of LWS. Besides, the changes in lignite surface caused larger dispersant adsorption on lignite surface and therefore enhanced the static stability of LWS.

Primary reactions of lignite-water slurry gasification under the supercritical conditions

The experimental research of lignite-water suspension gasification under the supercritical conditions of water was carried out. The conversion of lignite was studied under the pressure 24MPa in two temperature range: 290–360 and 390–500°C without and with additives. Calculation of the activation energies of the reactions of gases formations had shown that NiO-MoO3-Al2O3 had acted as a catalyst, sodium and calcium hydroxides had behaved as reactants. The general scheme of the mechanism on formation of the gas products containing H2, CH4 and CO2 was examined. It was shown that at the subcritical temperature the gas products were formed mainly from hydrogen of coal and oxygen of mineralized water. Carbon of lignite behaves like an acid, and water is as a base at the subcritical temperature; they switch their roles at the supercritical temperature. 80% of hydrogen and oxygen of gas products were of water origin at the supercritical temperature. C, O and H conversions at initial rates of reactions with gases distribution have been synchronously analyzed for primary reactions clearing. It had found that the gasification of lignite-water suspension under the supercritical pressure had at least four types of interactions: thermal decomposition of coal, carbon oxidation by oxygen of water as well as hydrolysis and hydrocracking of coal. Oxidation of carbon was dominant for the process at the subcritical temperature on water. Hydrolysis of CC bonds of coal was the main at the supercritical parameters of water.

Exploring on aqueous chemistry of micron-sized lignite particles in lignite–water slurry: Effects of pH on humics dissolution

The aqueous chemistry of lignite can change the properties of coal–water slurry. The aqueous chemistry of lignite–water slurry including dissolution kinetics of humics and properties of the residual lignite was studied using optical microscope imaging, SEM, FTIR, zeta potential and contact angle analysis. Also, the influence of humics dissolution on the interaction of micron-sized lignite particles was discussed by EDLVO theory. The dissolution kinetics demonstrates that humics concentration increases with increasing the slurry pH. The outer diffusion through the product layer is the rate controlling step according to shrinking unreacted core model. The value of D50 defined as the apparent particle size relevant to 50% of cumulative distribution is decreased when the lignite slurry pH is increased. The FTIR, zeta potential and contact angle results show that humics dissolution increases the surface hydrophilicity of the residual lignite. The total energy ET of the residual lignite particles at 10nm of separation distance is enhanced from −0.70×10−17 to 0.34×10−17J with the humics concentration is increased from 2.28 to 5.33mg/L. The EDLVO calculation indicates that the dispersion stability of micron-sized lignite particles is improved with humics dissolution.

Improving slurryability, rheology, and stability of slurry fuel from blending petroleum coke with lignite

Petroleum coke and lignite are two important fossil fuels that have not been widely used in China. Petroleum coke–lignite slurry (PCLS), a mixture of petroleum coke, lignite, water, and additives, efficiently utilizes the two materials. In this study, we investigate the effects of the proportion (α) of petroleum coke on slurryability, rheological behavior, stability, and increasing temperature characteristics of PCLSs. The results show that the fixed-viscosity solid concentration (ω0) increases with increasing α. The ω0 of lignite-water slurry (LWS, α = 0) is 46.7 %, compared to 71.3 % for the petroleum coke–water slurry (PCWS, α = 100 %), while that of PCLS is in between the two values. The rheological behavior of PCLS perfectly fits the power-law model. The PCWS acts as a dilatant fluid. As α decreases, the slurry behaves first as an approximate Newtonian fluid, and then turns into a pseudo-plastic fluid that exhibits shear-thinning behavior. With increasing α, the rigid sedimentation and water separation ratio (WSR) increase, indicating a decrease in the stability of PCLS. When α is 60–70 %, the result is a high-quality slurry fuel for industrial applications, which has high slurryability (ω0 = 57–60 %), good stability (WSR < 2 %), and superior pseudo-plastic behavior (n ≈ 0.9).

Variation with time of cell voltage for coal slurry electrolysis in sulfuric acid

To explain the time dependence of cell voltage in CWS (coal water slurry) electrolysis, the effects of three coal types along with graphite, CWS concentration and catalysts on cell voltage are examined. Results show that the overall cell voltage for CWS electrolysis is lowered as coal rank decreases. The four overall types carbonaceous material show successive reductions in Uθ + η for CWS electrolysis which are greater than those of iR, where Uθ is the theoretical reversible potential for water electrolysis, and η is the sum of the anodic and cathodic over-potentials. The cell voltage for DLWS(demineralized lignite water slurry ) electrolysis is lowered by addition of Fe2+ or Fe3+ ions, the former being more effective. The cell voltage for DBWS (demineralized bituminous water slurry) electrolysis decreases with the addition of transition metal ions in the order Fe2+<Ni2+<Co2+. Additionally, it is found that the DLWS electrolysis cell voltage is lowered with increasing DLWS concentration. Results suggest that cell voltage reduction may be attributed to the decrease in Uθ + η for CWS electrolysis.

Roles of inherent mineral matters for lignite water slurry electrolysis in H2SO4 system

To understand roles of inherent mineral matters in lignite water slurry (LWS) electrolysis, lignite and demineralized lignite electrolyses were carried out in H2SO4 system. The results showed that cell voltage for LWS electrolysis was lower than that for demineralized lignite water slurry (DLWS) under constant current condition. Some inherent mineral matters changed into the corresponding metal ions which entered into electrolyte, and thus improved the electrolysis oxidation reactions for coal organic structure. Meanwhile, the relative amount of O-containing functional groups in demineralized lignite increased with electrolysis time, improving its pyrolysis reactivity. However, the pyrolysis reactivity of raw lignite decreased due to the removal of the inherent mineral matters from electrolysis.

Preparation and Rheological Characterization of Lignite−Water Slurries†

This paper reports the preparation of lignite−water slurries (LWS) and critical characteristics affecting their rheology. The slurries were prepared using wet grinding in a ball mill, having particle size distributions up to 300 μm and concentrations up to 50 wt % in solids. Surfactants and polyelectrolytes were used as additives in the slurryfication process to achieve acceptable rheological behavior and stability. Rheological measurements were performed using parallel disks geometry, and the typical shear rates were on the order from 10−2 to 103 s−1. The effect of the particle size distribution was investigated, and the apparent viscosity of the slurries, for the same solids volume fraction, was found to be lower for broad multimodal distributions. In addition, oscillatory rheological measurements were carried out to investigate the microstructure of the slurries, and it was observed that, for dense LWS (ϕ > 0.40), the viscoelastic behavior was controlled by the elastic contribution than the viscous one. When the experimental results of the lignite slurries were compared to rheological models of viscosity, satisfactory agreement was found, although relatively low values of ϕmax were observed, attributed to the physicochemical properties and the shape of the lignite particles. The results of the present study point out that, with an adequate protocol of measurement and, in turn, reproducible data, the apparent viscosity curves of LWS can be effectively described, qualitatively as well as quantitatively, by a well-established model.

Rheological aspects of dense lignite–water suspensions; structure development on consecutive flow loops

Aspects of dense lignite–water slurries (LWS) rheology were investigated using controlled stress and controlled strain rheometers with parallel disks and Couette geometries. During the preparation of the slurries, the achieved solids volume fractions were up to 0.425 and the particle size distributions were polydispersed with sizes up to 300 μm. In the ascending parts of consecutive flow loops, a slope transition of the flow curve was observed and studied in relation to the solids volume fraction. The obtained results with the different geometries and rheometers were qualitatively the same. By following the model proposed by Cheng (Rheol Acta 42:372–382, 2003) for thixotropic fluids, and taking into account the yield stress appearance, a suitable correlation for LWS is proposed, which is consistent with the experimental flow curves.