Alkali Coal Research Articles

Study on the behavior of AAEMs removal and the evolution of microscopic characteristics in high alkali coal under different acids

ABSTRACT In this paper, the removal behavior of alkali and alkaline earth metals (AAEMs) and the evolution of microscopic characteristics in high alkali coal under the leaching of inorganic and organic acids are studied. The results show inorganic acids have better effect on the removal of AAEMs because of their stronger ability to convert carboxylate and phenol salts into covalent carboxylic acids and phenols. Citric acid has a slightly weaker effect than inorganic acids, and oxalic acid has the least effect. The improvement of slagging characteristics is consistent with the removal ability. Sulfuric acid, hydrochloric acid and citric acid are more effective in enhancing the calorific value of coal, with an enhancement of about 0.60 MJ/kg. After leaching, the pores and cracks in the coal are developed, and the mesopores have a tendency to shift to a lower pore size range. This trend is most pronounced in citric acid leached coal and hydrochloric acid leached coal. Sulfuric acid seriously damage the surface morphology, which results in a tendency of large pore transfer in S-form coal.

Numerical simulation of solidification of molten slag impacting inclined wall in boiler based on smoothed particle hydrodynamics method

In order to study the process of high temperature liquid slag impinging on the wall, the slagging phase change is produced by combustion of high alkali coal in a boiler chamber. In this paper, based on the smoothed particle hydrodynamics (SPH) method, the flow spreading and solidification model of slag particles hitting the inclined wall are established, and the dynamic process of flow spreading and the phase change of slag particles hitting the wall are analyzed by simulating the deposition process of the phase change of slag particles hitting the wall. The effects of different inclination angles of the wall on its deformation and solidification heat transfer are further discussed. It is shown that the change of inclination angle during the impact of single slag on the wall has a greater influence on the spreading flow process. During the impact of single/double slag on the wall with different inclination angles, the time taken by the double slag to reach the final spreading length and complete phase transition is nearly five times longer than that of the single slag. The direction of slag impact also has an effect on the spreading and phase transition. This SPH method provides a novel numerical simulation idea to study the kinetic behavior of molten slag hitting the wall and the problems related to phase change deposition in boilers.

Numerical simulation of solidification of boiler molten ash impinging on vertical wall based on Smoothed Particle Hydrodynamics method

In order to study the whole process of the high temperature liquid slag fly ash produced after the combustion of high alkali coal as power coal in the boiler furnace, which moves in the furnace and impinges on the water wall, the heat exchange occurs and then the cooling phase transformation deposits and slags, and the possible factors of slagging particles are studied by changing the environmental variables. In this paper, based on Smoothed Particle Hydrodynamics (SPH), the dynamic behavior of boiler molten ash impacting vertical wall is studied, and the flow spreading and solidification model of molten slag impacting vertical wall is established. Thus, the process of phase change deposition of slag particles against wall is simulated. By observing the phase distribution of slag and the change in spreading process, the complete process of the slag particles hitting the wall, spreading, shrinking and solidifying is analyzed. Based on the cohesive and repulsive surface force method, a temperature-dependent surface tension model is established to simulate the effect of surface tension on flow characteristics when molten ash impinges on low-temperature vertical walls. The influences of slag impact velocity, initial temperature and water wall temperature on deformation and solidification heat transfer are discussed. The results show that the initial velocity and initial temperature of slag particles affect the phase change deposition velocity and the final spreading factor of slag, while the temperature of the water wall has little effect on the deposition deformation and solidification process of slag.

Infrared heating and synergistic effects during fast co-pyrolysis of corn stover and high alkali coal

Infrared heating method aids in the investigation of the primary volatiles-volatile interactions during co-pyrolysis by minimizing the secondary reactions. In this study, characteristics and synergistic effects during the rapid co-pyrolysis of high alkali coal (HAC) and corn stover (CS) using infrared heating were systematically investigated. The co-pyrolysis liquid yield initially increased and then decreased with increasing the pyrolysis temperature. The synergistic effects clearly promoted the tar yield and quality. The co-pyrolysis tar yields increased with rising temperature, from 23.29 wt% at 500 °C to 28.87 wt% at 700 °C, before decreasing to 23.57 wt% at 800 °C. At 700 °C, the largest deviation was 63%. Moreover, the co-pyrolysis interaction had a maximal negative deviation of − 35% for CO2 mass at 700 °C, indicating that the co-pyrolysis of CS and HAC reduced carbon emissions. The tetracyclic and tricyclic aromatic hydrocarbons in PAHs were substantially reduced due to CS hydrogen radicals, and the quantities of the binary aromatic rings decreased by 50% with the increase in temperature. The presence of AAEMs in coal affected the bond breaking and reformation of molecular bonds within the tar components. Raman analysis and BET analyses suggested that AAEMs in HAC enhanced the reactivity of char. The interaction between the volatile of CS and HAC increased the amount of ammonia carbon and the number of active sites.

Experimental study on the formation characteristics of PM0.4 from preheating combustion of sewage sludge

Municipal sewage sludge (SS) is gaining increasing attention as a carbon–neutral fuel. Preheating combustion has been applied in coal combustion to reduce both PM0.4 and NOx. It is a promising technology for SS disposal while PM formation from preheating combustion of SS is still unknown. This study investigated the formation characteristics of PM0.4 from preheating combustion of SS. The effects of preheating temperature (Tp) and combustion temperature (Tc) on PM0.4 were analyzed. The experimental results showed that simultaneous reduction of PM0.4 and NOx from preheating combustion of SS can be achieved with Tp ≤ 1300 °C and Tc ≤ 1200 °C. The reduction of PM0.4 in preheating combustion compared to conventional combustion is mainly attributed to reduction in vaporization of Ca. Tc has more significant effect on PM0.4 while Tp can affect NOx more effectively. Because SS has a low content of fixed carbon, vaporization of refractory elements (Ca, Mg, Fe, Si and Al) through reduction reaction of refractory oxides mainly occurs under reductive atmosphere in preheating furnace. Increasing Tc will significantly increase vaporization of alkali metals (Na and K), S and Cl, while has no significant effect on vaporization of refractory elements. These are different with the results from high-alkali coal in our previous research (Reference [28]). Fe-precipitation agent used in waste water treatment results in uniform distribution of Fe in SS particles, which enhanced interaction between Fe and Si-Al containing minerals. This decreases reaction between Si-Al containing minerals and Na containing compounds. Therefore, even SS contains less Na and more Si-Al than high alkali coal, Na is easier to transfer to PM0.4 in preheating combustion of SS than high alkali coal.

Removal of AAEMs from high alkali coal under supercritical CO2 fluid-citric acid extraction system

ABSTRACT In this paper, a method for dealkalization and upgrading of high alkali coal by supercritical CO2 fluid-citric acid extraction system is proposed. The leaching mechanism was explored by microstructural changes of elements and functional groups in coal samples, and single-factor optimization experiment was conducted. The result shows that supercritical CO2 fluid-citric acid extraction can effectively remove alkali and alkaline earth metals. The extraction mechanism is the reaction of H+ with inorganic minerals and the ion exchange of H+ with the form of phenolic hydroxyl and carboxyl groups. Under the optimum conditions, the removal rates of Ca and Mg are 44.95% and 65.75% respectively. The ash content of coal is 2.42%, and the content of Na2O (calculated by ash) is 1.13%, which meet the standards of power coal in China. Supercritical CO2 fluid-citric acid extraction is an effective and promising method for the cleaning of high alkali coal.

The effect of Na/K on the NO adsorption behavior and heterogeneous reduction of internal nitrogen-containing char: A DFT study

Sodium and potassium in high alkali coal have an effect on the heterogeneous reduction of NO with char. The microscopic mechanisms of the effect of Na and K on the adsorption behavior and heterogeneous reduction roads of NO on the surface of internal nitrogen-containing char were investigated at the GGA-PBE level using DFT. The adsorption energy of NO on the surface of nitrogen-containing char doped with Na and K was calculated using zigzag, armchair and saturated structures, and the reaction path of Z-N-2 with NO was also calculated. The results showed that the adsorption effects of the three types of nitrogen-containing char on NO ranked from strong to weak were: Z-N > A-N > G-N. The addition of Na and K both promoted the adsorption of NO with char structure. The highest energy barrier for the reaction of the Z-N-2 structure with NO was 232.80 kJ/mol, which decreased to 180.52 kJ/mol when Na doped and 176.04 kJ/mol when K doped. The exotherm of the reaction increases from 378.59 to 1245.26 kJ/mol, it appears that the addition of sodium and potassium facilitates the reaction. The study will provide a theoretical pavement for the continued study of the heterogeneous reduction process, which is of great significance for the clean use of high alkali coals.

Theoretical Study of NO Adsorption by Hydroxyl-Containing Char with the Participation of Na/K.

The study of the effects of Na and K on the heterogeneous adsorption of hydroxyl-containing char with NO is important for the clean utilization of high alkali coal. In this paper, the effects of Na/K atoms on the adsorption of NO on the char surface were investigated at the GGA-PBE level by choosing zigzag type, armchair type, and saturated hydroxyl-containing char structures based on DFT. It was found that the adsorption stability of NO on structures with active sites was greater for sites close to the hydroxyl group than that for sites far from the hydroxyl group. The stability of char doped by Na/K is related to the char structure and the position of functional groups. The most stable Na/K doped structures are Z-OH-2 (Eads= -350.50 kJ/mol) and A-OH-1-2 (Eads= -339.17 kJ/mol), respectively. The participation of Na/K can increase the adsorption energy of the three structures with NO, and especially the adsorption energy of saturated char with NO is increased by as much as 5 times. The reason for that is the promotion of the hybridization of the C and NO p orbitals. The comprehensive analysis of electrostatic potential, charge transfer, and front orbitals indicates that the effects of decorated sodium and potassium atoms on the char surface are very similar. This study lays a theoretical foundation for the study of the heterogeneous reduction process.

Co-combustion of high alkali coal with municipal sludge: Thermal behaviour, kinetic analysis, and micro characteristic

Blending sludge rich in protein and aliphatic hydrocarbons into the high alkali coal (HAC) has been demonstrated to reduce the ash melting temperature of the HAC/sludge mixture, thereby increasing the effectiveness and efficiency of liquid slagging. However, whether the incorporation of sludge can affect the combustion stability of the original coal-fired boiler is still debatable. As the combustion stability of the fuel can directly affect the operational safety of the boiler, it is of great practical value for exploring the effect of sludge incorporation on the combustion performance of HAC. In this work, the thermal behaviour and microscopic properties of individual HAC, municipal sludge (MS) and HAC/MS mixtures were tested using a Thermogravimetric analyser (TGA) and a Fourier transform infrared (FTIR) spectrometer, respectively. The exothermic, thermodynamic and functional group evolution patterns during the combustion of these samples were also evaluated. Ignition temperatures (Ti) of the HAC/MS mixtures were relatively lower than that of individual HAC, and decreased with the increase in sludge mass ratio (SMR). The synergistic effect of the co-combustion of HAC and MS resulted in a slightly higher total heat release during the combustion of MS10HAC90 (i.e., the mass percentage of MS and HAC is 1:9) than HAC alone, however, the total heat release of the blend decreased progressively with increasing SMR. The experimental values of the average Eα for all four mixtures were lower than the theoretical values, indicating that the addition of MS lowered the reaction energy barriers of the mixtures. Consumption rates of the principal groups in samples during the oxidation and combustion all tended to increase progressively with increasing SMR. There are three major synergistic effects existing during co-combustion of HAC and MS: (1) the reaction of free radicals with benzene molecules; (2) the interaction of free radicals; and (3) the catalytic effect of alkali and alkaline earth metals. These findings can provide theoretical guidance for the determination of key parameters (mixing ratio) for the blending of HAC and MS, and can fill the research gap in terms of microscopic reactivity and synergistic effects during the co-combustion of HAC and MS.

Mechanism on the effect of sodium on the heterogeneous reduction reaction of NO by Char(N)

Sodium in high alkali coal promotes the heterogeneous reduction of NO in nitrogen-containing char. In order to deeply understand the mechanism of the effect of sodium on the reaction, two edge models, armchair and zigzag were employed to investigate the reaction paths before and after sodium doping, based on density function theory. It was found that after the addition of sodium, the reaction energy barriers were lowered and the exotherm of the reaction was increased, thereby the reaction was promoted. Four reaction pathways were obtained, the order of reaction difficulty is as follows: armchair@Na > armchair > zigzag@Na > zigzag. Thermodynamic analysis showed that four reactions were unidirectional reactions proceeding spontaneously. Kinetic analysis showed that the involvement of sodium the reaction rate constant was increased. Therefore, an in-depth study of the mechanism of the effect of sodium on the heterogeneous reduction of NO with nitrogen-containing char is important for promoting the clean utilization of coal and developing new carbon-based materials for detecting and removal of NO.

Combustion and slagging characteristics of hydrochar derived from the co-hydrothermal carbonization of PVC and alkali coal

This work investigated on the effect of co-hydrothermal carbonization (co-HTC) of polyvinyl chloride (PVC) and alkali coal on combustion and slagging characteristics of the generated fuels (hydrochar). Fuel properties were assessed in terms of ultimate and proximate analysis, functional groups evolution, and ash melting characteristics. The loss of C–O functional groups in coal would be accompanied by the liberation of some elements such as Na and Ca, which were often associated with such groups. The hydrochars presented higher ignition temperature and average combustion rates compared with raw coal. The hydrochar generated from co-HTC at 250 °C showed the best combustion performance due to the enhancement of air contacting and transformation, which was attributed to its porous structure, smaller uniform particle, and relative higher surface area. The activation energy of hydrochars was increased at the ignition stage while was decreased at the combustion stage according to the kinetic analysis. The fusion temperature was increased resulting from the removal of alkali and alkaline earth metals (AAEMs) during the co-HTC process. From the perspective of waste-to-energy, the co-HTC of high-alkali coal and PVC seems to be a feasible solution to improve combustion and slagging characteristics.

Value-added products from pyrolysis of hydrochar derived from polyvinyl chloride and alkali coal

This work targets to produce multi value-added products by pyrolysis of hydrochars obtained from co-hydrothermal carbonization (co-HTC) of PVC and alkali coal. The hydrochars pyrolysis was carried out with a downdraft fixed bed reactor. The results indicate that HTC promoted the damage of oxygen functional groups, and the fixed carbon content and calorific value were considerably improved after co-HTC. The entire pyrolysis was delayed for coal hydrochar due to the increase of stable compositions in coal structure and improvement of coal rank. Besides, the PVC addition weakened this delay because of its instability, which could be verified by the activation energy. The H2 and CH4 yields increased in syngas, while the CO2 yield was concurrently reduced because of the removal of oxygen by decarboxylation. The obtained H2/CO ratio of syngas from hydrochar pyrolysis ranged from 1.34 to 4.33 depending on the pyrolysis parameters. The tar yield from hydrochar pyrolysis was lower than that of raw coal, which was ascribed to the lower degree of crosslinking in hydrochar facilitating the release of tar. The aromatic degree and rich pore structures of char indicated its great potential for contaminants adsorption which was further confirmed by the malachite green adsorption experiments. These findings could provide comprehensive knowledge on producing multi value-added products (hydrogen-rich syngas, poly-aromatic tar and adsorbent-char) from hydrochar pyrolysis, which would promote the economical utilization of high alkali coal and chlorinated wastes.

Simulation of Combustion and Alkali Metal Distribution in Zhundong High Alkali Coal Boiler Based on Computer Control System

Restricted by Xinjiang’s special geographical location, economic conditions, transportation and other factors, Zhundong coal can’t be sent out on a large scale, which seriously hinders the development of Zhundong coal base. Meanwhile, due to the coal-forming history and Xinjiang’s special natural geographical environment, the alkali metal content in Zhundong coal is generally over 2%, which is much higher than that of power coal in other parts of China. In this paper, based on computer control system, the combustion and alkali metal distribution in Zhundong high alkali coal boiler are simulated, and the morphological distribution characteristics and migration laws of alkali metals such as Na and K in pulverized coal combustion process of high alkali coal, low alkali coal and their two coal samples are deeply studied. Combustion characteristics and heat flow distribution, the simulation results show that the flue gas temperature at the furnace outlet is 895.07°C, and the flue gas temperature near the wall is low, which is helpful to alleviate the slagging and contamination in the furnace.

Hydrogen-rich gas production from hydrochar derived from hydrothermal carbonization of PVC and alkali coal

This work targets to produce hydrogen-rich syngas by steam gasification of hydrochars obtained from co-hydrothermal carbonization (co-HTC) of polyvinyl chloride (PVC) and alkali coal. Hydrochar properties were assessed in terms of ultimate analysis, proximate analysis and the content of typical elements. The syngas, tar and char properties were investigated to explore the steam gasification performace of hydrochars. The co-HTC facilitated the destruction of the hydrophilic groups based on the O/C atomic ratio. The co-HTC increased the yield of H2 and CH4, while the CO2 yield was concurrently reduced. A relative high H2/CO ratio of around 1.65–3.96 was achieved when gasified hydrochar under the various temperature. This temperature was supposed to be an acceptable operational window for yielding H2-rich syngas. The catalytic effect of alkali and alkaline earth metals (AAEMs) on H2 production was weakened due to the AEEMs removal during the co-HTC, while the H2 production could be remedied by the PVC addition. The evolution of poly-aromatic hydrocarbons (PAHs) convinced the event of primary or secondary tars transformed into the tertiary tar. The rich pore structure of char indicated that char have good potential in adsorption, and the adsorption experiment of malachite green confirmed this point of view.These findings will offer comprehensive data on the coal upgrading and hydrochar gasification.

Moisture re-adsorption characteristics of hydrochar generated from the Co-hydrothermal carbonization of PVC and alkali coal

Our previous work reported to simultaneously remove the alkali and alkaline earth metals, and chlorine by co-hydrothermal carbonization (co-HTC) of high-alkali coal and polyvinyl chloride (PVC) (Fuel Process Technol, 199(2020)106277). This work is to investigate on the moisture re-adsorption characteristics of those hydrochar obtained at various co-HTC operating conditions. The results show that the water re-adsorption capacity of hydrochar was decreased with the circumstance temperature rising from 20 °C to 35 °C, while it was increased with the air humidity increasing from 50% to 80%. During the co-HTC, the addition of PVC could effectively inhibit the moisture re-adsorption behavior of hydrochar resulting from the adhere of the hydrophobic PVC to the surface of hydrochar as evidenced by SEM. The circumstance temperature had a significant influence on the effective diffusivity D over the humidity. The equilibrium moisture content (EMC) and effective diffusivity (D) could be linearly correlated with surface area and most probable pore size, respectively. The EMC of hydrochar was reduced about 0.6 mg/g as the BET surface area decreased 1 m2. The reduction of oxygen-containing groups could reduce the adsorption equilibrium time and adsorption heat. Besides, the higher circumstance humidity would cause hydrochar to release higher adsorption heat. As the humidity was increased from 70% to 80%, the heat released by 1 g hydrochar was increased about 13.82–19.63 kJ/mol. Due to the lower EMC and adsorption heat, 300 °C was supposed to be a suitable temperature for the co-HTC to obtain hydrophobic hydrochar.

Effect of ash on dielectric properties and micro-structure of high alkali coal at different temperature pyrolysis

The effect of ash on dielectric properties and micro-structure of high alkali coal at different temperature pyrolysis was studied, so as to provide theoretical basis for coal deep processing by microwave. An acid-washed method was adopted to remove ash in Zhundong coal for preparing coal chars at 700 °C–1300 °C. X-ray diffraction analysis was used to characterize the microcrystal structure. The thermal stability was characterized by thermogravimetric analyzer, and the dielectric properties were measured by a vector network analyzer. The results showed that when the pyrolysis temperature was below 1100 °C, the presence of ash hindered the development of carbon structure in raw coal char. The main reason is that the alkali metal oxides (K2O and Na2O) in the ash promoted the solution loss reaction during pyrolysis. The structure of the original carbon layer was damaged, thereby the graphitization degree, thermal stability and dielectric properties of raw coal char were weaker than the ash-free coal char. When the pyrolysis temperature reached 1300 °C, the minerals were completely melted. The reaction of phase transition of SiO2 in ash played a catalytic role on raw coal char structure, resulting in tighter arrangement of adjacent carbon layers. The raw coal char showed stronger dielectric properties and thermal stability.

Release and migration characteristics of sodium and potassium in high alkali coal under oxy-fuel fluidized bed combustion condition

Zhundong coal is promising to replace inferior coal in fossil-fueled power plants, considering its high abundance, low contents of sulfur and nitrogen. However, the relative high contents of alkalis, especially sodium and potassium, damage the operational economy and safety badly since the release of alkali metals in flue gas can lead to fly ash-related problems such as slagging, ash accumulation and corrosion. Although, oxy-fuel circulating fluidized bed (CFB) combustion is beneficial to reduce the release of sodium and potassium by lowering the operation temperature, the ash formation characteristics of Zhundong coal under the oxy-fuel CFB is still scarce. Our results showed that the residual amounts of sodium and potassium in the ash samples, both tend to decrease with increasing temperature no matter under air combustion or oxy-fuel combustion. However, the sodium content in the cinder obtained under the oxy-fuel combustion atmosphere was higher than that under the air combustion atmosphere, and the insoluble sodium, mainly existed in the form of aluminosilicates, was the main form. In addition, the increase of steam content in both atmospheres reduced the sodium content in the cinder while the residual amount of potassium increased. Density functional theory calculations verified the transformation behavior between sodium and potassium and the inhibiting effect of steam. This study elucidates the release and migration characteristics of sodium and potassium in Zhundong coal both experimentally and theoretically and will benefit the popularity of Zhundong coal.

Investigation on Sodium Fate for High Alkali Coal during Circulating Fluidized Bed Combustion

For better understanding the release and transformation mechanisms of sodium compounds during high alkali coal combustion in circulating fluidized bed (CFB) boilers, a sodium migration model of coa...

Investigation of characteristics and formation mechanisms of deposits on different positions in full-scale boiler burning high alkali coal

Zhundong coalfield are huge in China; however, high alkali contents induce severe slagging and fouling problems in boiler. To obtain the characteristics and formation of deposits in boiler burning high alkali coal, the deposits at different positions were sampled in a 350MW boiler burning Zhundong coal, and analyzed by X-ray Fluorescence. The results indicated that the slagging was deteriorating from bottom to top at water wall. The slag with obvious layer structure only presented at platen superheater. The deposits in horizontal flue were mainly formed from the solidified particles attachment. To further understand the formation mechanism of layer structure in slag at platen superheater, the slag was separated according to the morphology and each layer was analyzed by X-ray Diffraction. The results suggested that through the condensed CaSO4 was the main compounds for the strong thermophoresis in layer 1, the little fused particles with high sodium content played the key bonding role. For layer 2, the fused particles contain high sodium content adhered to deposit surface and formed melt surface, which capture the other particles. The Fe species captured by melt surface lower the melting temperature of the formed deposit, and then aggravated the deposit fusibility in layer 3.

Experiment and Mechanism Study on the Effect of Kaolin on Melting Characteristics of Zhundong Coal Ash

Methods of experimental research and quantum chemical calculation were combined to study the effect of kaolin on ash melting characteristics of Zhundong high alkali coal and the mineral evolution law during ash melting processes. Furthermore, the behavior mechanism of kaolin capturing alkali metal was also studied from the perspective of mineral microstructure features. The results show that the melting temperature of Zhundong high alkali coal ash have rapid rise first and then gradually decreased with the amounts of kaolin increasing. When the adding proportion of kaolin is greater than 10%, the improving efficiency on the ash melting temperature become weakened. Once kaolin was added in Zhundong high alkali coal, anorthite, anhydrite, albite, and muscovite would be generated among the temperature range from 1000 to 1200 °C, Na+ or K+ that easy to volatile and form eutectics have been captured by the Si–Al system, some mullite generated among the temperature range from 1200 to 1300 °C. Investigating the ...