Entrained Flow Coal Gasifier Research Articles

Numerical simulation of ash deposition behavior in radiant syngas cooler coupled with impact surface temperature

Radiant syngas cooler (RSC) is a crucial heat recovery device for entrained-flow coal gasification technology. However, ash deposition in an RSC can severely affect operating efficiency and safety. An ash deposition model coupled with the impact surface temperature was established based on the ash particle deposition morphology. The effects of different operating conditions on ash deposition characteristics are discussed. The simulated data agreed well with the industrial data. The results indicate that ash particles were primarily deposited in the collision deposition zone. The maximum deposition thickness of radiation screen and cylinder membrane wall is 9.82 mm and 1.88 mm, respectively. The ash deposition hindered heat transfer process and the highest heat flux was observed in the middle part of the RSC. Ash deposition around 5.3 m increases significantly with an increase in the inlet syngas temperature. Increasing the load increased probability of ash particle collision and the area of influence.

Study on gasification reaction and energy conversion characteristics of the entrained flow coal gasification based on chemical kinetics simulation

Coal gasification that converts coal into syngas is a promising technology for efficient and clean utilization of coal. Current simulations of coal gasification are mostly based on equilibrium reactions, which cannot reflect the residence time and carbon conversion rate. In this paper, the reliable chemical kinetics simulation of the coal gasification is carried out considering the solid residence time and the effects of the main parameters on the gasification performance are analyzed. The results show that the higher the O2/coal ratio, the higher the carbon conversion rate until it reaches 100 %. However, an excessively high O2/coal ratio will reduce the cold gas efficiency. Therefore, the ideal cold gas efficiency is further proposed, revealing the energy saving potential of the system by reusing the ungasified coal. Based on this, the energy conversion characteristics of the combined cycle system were analyzed, showing that the effective conversion of coal to synthetic gas is the key to improving efficiency.

Comparative study of chemical speciation and leaching feature of typical heavy metals in coarse and fine slags from the entrained-flow coal gasification

ABSTRACT The dissimilarity in chemical speciation and release properties of heavy metals will result in varying levels of environmental hazards and potential applications of coal gasification coarse and fine slags. The distribution, chemical speciation, leaching features, and potential environmental risks of heavy metals (Cr, Cu, Cd, Zn, Pb, and Mn) in coarse and fine slags from different coal gasification processes were revealed and compared by the sequential chemical extraction method, Horizontal vibration method, and pH-dependent leaching experiments. The results show that Cr (516.78 mg/kg) in coarse slag, Zn (348.50 mg/kg) and Pb (214.78 mg/kg) in fine slag are highly enriched and the content exceeds the screening value, indicating the need for better management during stockpiling and landfilling. The distribution of Cr, Cu, Pb, and Mn in residual state, Zn in Fe-Mn oxide bound state, and Cd in unstable bound state is the highest in both coarse slag and fine slag. The leaching toxicity of heavy metals (HMs) does not exceed the standard, but the leachate is classified as Level V surface water or groundwater quality due to the leaching concentration of Pb exceeding 0.1 mg/L. Furthermore, leaching of Cr, Zn, Cd, and Pb in fine slag is much more dependent on pH compared to coarse slag, and the concentrations of Zn and Pb exceed emission standards at pH < 3.0. The high concentration of heavy metals caught on the surface of fine slag is more harmful to the environment. The properties of HMs in the slags and pH of the application environment must be comprehensively considered before coal gasification slag disposal, especially fine slag.

A new viscosity model for the high-iron coal slag based on the bond distribution

The coal slag viscosity, intrinsically determined by the slag structure, is the key to the smooth slag tapping process of an entrained flow coal gasifier. However, the slag structure parameters obtained by the experiments (Raman, NMR, and XPS) or by the theoretical calculation (Molecular dynamics simulation and First-principles calculation) are very time-consuming. This study employed thermodynamic modeling as a convenient method to determine the short-range-ordered structure of high-iron coal slag. The viscosity prediction performance of the structure parameters obtained by the thermodynamic modeling was compared with that obtained by the experiments (Raman and XPS). Our results demonstrated that the short-range-ordered structure of coal slag was described as the distribution of the seven bonds determined by thermodynamic modeling. A new structure parameter (RNF/NM) based on the bond distribution was proposed to quantify the network depolymerization degree of slag. The activation energy (Eη) for the viscous flow, which was predicted by RNF/NM, was much more accurate than the structure parameters (Qn distribution, Al coordination, and NBO/(BO + NBO)) that were determined by the experiments, even the effect of Fe3+-tetrahedron was considered. Finally, a novel slag viscosity model as a function of RNF/NM was proposed. This study provides new insights into the structure analysis and viscosity modeling of coal slag by thermodynamic modeling. Additionally, this method can be applied to the viscosity prediction of other slag systems, such as metallurgical slag, glass, etc.

Effect of Minor Oxide and Residual Carbon on Coal Ash Crystallization Behavior and Slag Properties during Coal Gasification: Critical Literature Review and Thermodynamic Simulation.

Coal-ash-associated problems seriously limit the industrial application of entrained flow coal gasification. There have been many studies of this issue. However, many problems still need to be fully understood, especially regarding the effect of minor oxides and residual carbon on the crystallization of slag in coal ash systems. Crystals formed in slag affect the slag properties, especially its viscosity. This article reviews the crystallization behavior of coal ash slag and its impact on ash fusion temperature (AFT) and temperature of critical viscosity (Tcv). A thermodynamic simulation using FactSage 7.3 thermochemical software was performed to study the effect of minor oxides (i.e., Na2O, K2O, and TiO2) and residual carbon at a temperature range of 1000-1700 °C on the phase transformation in the oxide system. The literature review reveals that the minor oxides and residual carbon in coal ash affect the AFT and the coal slag's Tcv. The thermodynamics simulation indicated that Na2O, K2O, and TiO2 tend to decrease Tcv, while the residual carbon increase Tcv.

Crystallization-controlled fusion mechanism of the amorphous fly ash from the Shell coal gasifier by particle sieving

The mineral in coal is transformed to the fly ash of different particle sizes during the entrained flow coal gasification, causing the ash deposition problem on the radiant syngas cooler. In this study, the fusion behavior of fly ash collected from a Shell coal gasifier was analyzed by particle sieving. Results indicated that the fusibility of amorphous fly ash was influenced by the anorthite crystallization at high temperature. The fusion process of fly ash particle was divided into the sintering stage, plateau stage, and melting stage, which were caused by the flow of the amorphous fly ash, anorthite crystallization, and anorthite melting, respectively. The high theoretical anorthite content and the low fly ash viscosity improved the anorthite crystallization reaction in the fly ash particles <45 μm, resulting in a wide plateau stage, narrow melting stage, and high ash fusion temperatures (AFTs). Therefore, the fusion behavior of fly ash particles <45 μm followed “melting-dissolution” mechanism. In contrast, the fusion of fly ash particles >45 μm belonged to the “softening-melting” mechanism for the weak anorthite crystallization behavior. The fly ash particles <45 μm may cause the serious ash deposition problems for its low viscosity.

Polycyclic aromatic hydrocarbons (PAHs) and soot formations under different gasifying agents: Detailed chemical kinetic analysis of a two-stage entrained flow coal gasifier

The injection of CO2 with coal into coal gasifier is a promising approach to achieve CO2 recirculation. Predicting the formation and evolution of polycyclic aromatic hydrocarbons (PAHs) and soot during coal gasification is particularly important as their presence poses challenges to the gasifier operation and environment. However, the effect of CO2 injection on their formation must be elucidated in more detail to optimize the gasifier design. This work investigates PAHs and soot formations in gas-phase reactions under different gasifying agents (Air, O2/CO2, O2/H2O) in a two-stage entrained flow coal gasifier using a detailed chemical kinetic model (DCKM). Under the O2/CO2– and O2/H2O-blown conditions, aromatics reforming was progressed whereas the yields of tar and soot were effectively suppressed. The benzene was observed as the most abundant PAHs species, followed by naphthalene and acenaphthalene. Regarding the relative oxygen concentration, the conversion behavior of aromatics was significantly influenced in the O2/CO2-blown condition, while the somewhat effect was found in the O2/H2O-blown condition. Tar yield peaked at 50% relative oxygen concentration in the O2/CO2-blown condition. The present finding may have practical significance in the O2/CO2-blown gasifier to facilitate CO2 recirculation for the oxy-fuel IGCC system.

Separation and characterization of different types of residual carbon in fine slag from entrained flow coal gasification

A large amount of coal gasification fine slag (CGFS) with substantial carbon is produced in the entrained flow coal gasification process. Understanding the type and activity of the residual carbon in the slag is essential for its efficient use. Coal undergoes a series of complex reaction in a gasifier. The residue carbon contains not only incompletely reacted semi-char, but also soot formed by the condensation of volatile fractions. In this study, the ultrasound-assisted anhydrous ethanol method was used to separate the different types of carbon in the fine slag. Light carbon deposit (LCD) and heavy carbon residue (HCR) were obtained from the top and bottom of the centrifuge tube respectively. The results showed a dry basis yield of 47 % for LCD and 30 % for HCR. LCD particle size is concentrated in the range of 0.2 μm to 5 μm, and the specific surface area is 116 m2/g after de-ashing, in the form of flocs. HCR particle size is concentrated in the range of 20 μm to 100 μm, and the specific surface area is 179 m2/g after de-ashing, composed of irregular particles. The TPO results of the residual carbon in the fine slag were fitted by peak splitting, and three main types of residual carbon were found: polymerization soot, amorphous carbon and graphitization carbon, corresponding to maximum CO2 release peaks of 596.3 °C, 626.8 °C and 705.6 °C respectively. LCD is composed of polymerization soot. HCR is composed of amorphous carbon and graphitization carbon. Raman characterization reveals that LCD has more active sites than HCR, thus leading to their higher reactivity. This study is an innovative approach to the separation of residual carbon in fine slag. It helps to understanding the formation mechanism of residue carbon in the gasifier.

Computational Investigation on the Impact of Coal Feed Size in a Tangentially Fired Gasifier

ABSTRACT Using a validated comprehensive model, the effect of coal particle size on two-phase flow inside a tangentially fired, Mitsubishi Heavy Industry entrained-flow coal gasifier is investigated. It is observed that the length of the central recirculation zone (CRZ) formed in the gasifier flow increases with an increase in the mean size of the injected coal particles. The CRZ lengths are 5.6, 4.5 & 2.1 m, respectively, for the cases with particle size distributions (PSDs) having mass mean diameters of 80, 40 & 20 µm. The generic swirl number, , captures this impact of particle size on the CRZ length, whereas the conventional swirl number, , is unable to do so. The value of does not vary significantly over most of the gasifier’s constant diameter zone in each case simulated in the present work, thereby underlining the utility of this swirl number to characterize similarity in swirl dynamics. Further, the analysis of the impact of coal particle size on char conversion yields that the PSD with finer particles leads to higher conversion due to the better combustibility of finer particles.

CFD modeling of ash deposition distribution in an industrial radiant syngas cooler to enhance the operation performance

Radiant syngas cooler (RSC) is a crucial heat recovery equipment for entrained-flow coal gasification. Ash depositions in the industrial RSC endanger its operation efficiency and operation safety. In this work, the critical viscosity model and the ash shedding model are coupled to explore the ash deposition behavior in the industrial RSC for a 2000 t/d entrained-flow coal gasifier. Simulation results are in good agreement with the industrial conditions. The results show that ash depositions in RSC are mainly distributed in the upper part and the bottom cone, and the maximum ash thickness are 9.67 mm and 36.71 mm respectively. Due to ash depositions, the outlet temperature of RSC increases by 19.76%, the total heat transfer rate decreases by 21.88%, and the steam yield decreases by 22.41%. Employing low-ash coal as gasification raw coal is a good suggestion to enhance the operation performance of the industrial RSC. Moreover, when the industrial RSC is fully loaded, an appropriate increase in syngas mass flow rate can also contribute to a better operation performance.

Sophisticated interplay of operating conditions governs flow field transition and optimal conversion inside tangentially fired gasifiers

A computational model is developed to simulate reacting environment inside the pilot-scale, tangentially-fired, Mitsubishi Heavy Industry entrained-flow coal gasifier. Using this comprehensive model, the impact of three operating parameters, namely, swirl number (SN), reactor pressure and mass throughput on the gasifier flow field and coal gasification process is investigated. We demonstrate that for given values of any two of these parameters, it requires a threshold value of the third parameter to form the central recirculation zone (CRZ), the absence of which lowers char conversion. Therefore, a modified SN incorporating the combined effect of conventional SN, reactor pressure and mass throughput on the onset of CRZ formation is proposed for tangentially-fired gasifiers. Furthermore, the impact of variation in reactor pressure and mass throughput on char conversion is shown to be mediated through the corresponding change in particle residence time and pressure effect on kinetics. The results demonstrate that the operating parameters interact in a complex state space that governs the gasifier flow physics and optimal operation.

Multifaceted evaluation of distribution, occurrence, and leaching features of typical heavy metals in different-sized coal gasification fine slag from Ningdong region, China: A case study

The coal gasification fine slag (CGFS) from the entrained-flow coal gasification unit faces the challenge of safe disposal and clean utilization in the Ningdong region, China. This study aims to provide complete and thorough understanding of the distribution features, chemical speciation, environmental impact, and leaching behavior of typical heavy metals (i.e., V, Cr, Mn, Ni, Cu, Zn, Ba, and Pb) in the CGFS with different size fractions. The results show that the distribution of selected heavy metals in the CGFS has evident particle size dependence. Except for Zn, the other heavy metals in different size fractions mainly exist in chemical speciation of residual form with the ratio of 50.11–86.69 wt%. Moreover, it is found that the heavy metals in the different-sized CGFS show different RAC (risk assessment code) environmental risk levels and TCLP (Toxicity Characteristic Leaching Procedure) leaching concentrations. Especially, Zn in SGFS-C and SGFS-D posed a high-risk level to the environment, while the heavy metal elements of Cr, Mn, Ni, Zn, and Ba in other size fractions are classified as a medium environmental risk. In addition, the TCLP test results indicate that the leaching concentration of Cr, Mn, Ni, Zn, Ba, and Pb exceeds the groundwater-related regulatory limit in China. The pH-dependent leaching experiments suggest that Pb shows the amphoteric behavior, while the leaching mode of other heavy metals seems to be the cationic pattern. Furthermore, the leachability of the selected heavy metals in small-size fractions of the CGFS should be given more consideration at both acid and alkaline pH ranges. The leaching kinetic results demonstrate that the most effective mechanism to describe the leaching process of Cr, Ni, Zn, and Pb in different CGFS size fractions is the diffusion-controlled theory, which is supported by the different morphological traits of spherical mineral particles and carbon particles in the CGFS.

Reactions and transformations of mineral matters during entrained flow coal gasification using oxygen-enriched air

Clarification of the reactions and transformations mechanisms of mineral matters during entrained flow gasification using oxygen-enriched air is necessary for the reliability of the gasifier. To explore the impacts of different gasification temperatures (1000 and 1200 °C) and oxygen concentrations (21, 50, 100 vol % O2 in N2) on the mineral behaviors, the gasified chars obtained from a pilot-scale gasifier were characterized by the means of ICP-OES, XRD, PSD, SEM-EDX, and AFT methods. Results indicated that gasification with oxygen-enriched air not only promoted the retention of inorganic elements in the solid phase but also changed the mineral types and contents. The main components of gasified chars were SiO2 and Ca-bearing minerals, and the increase in gasification temperature and oxygen concentration would promote the transformation of CaCO3 and CaSO4 to CaO, CaS, and Ca2Al2SiO7. With increasing oxygen concentration under 1000 °C gasification, the average particle size of gasified chars was getting smaller and the high-melting mineral Al6Si2O13 was likely to form, leading to the increase in the fusion temperature. However, when the gasification temperature rose to 1200 °C, the re-carbonation of CaCO3 was largely promoted with the increase of oxygen concentration due to the more abundant CO2 in the reaction atmosphere. Meanwhile, the morphologies of apparent agglomerate and sintering began to appear in the 1200 °C gasified chars, leading to a significant increase in particle size. It is deduced that the enrichment of Ca and Fe in the 1200 °C gasified chars would accelerate the formation of low-temperature eutectic mixtures, thus reducing the ash fusion temperatures of 1200 °C samples. Overall, the ash fusion temperatures of the gasified chars are closely linked to mineral composition, particle size distribution, and agglomeration behavior during the gasification process.

Study on Chemical Kinetics Modeling and Thermodynamics Characteristics of Entrained Flow Coal Gasification

Study on Chemical Kinetics Modeling and Thermodynamics Characteristics of Entrained Flow Coal Gasification

Polycyclic Aromatic Hydrocarbons (Pahs) and Soot Formations Under Different Gasifying Agents: Detailed Chemical Kinetic Analysis of a Two-Stage Entrained Flow Coal Gasifier

Polycyclic Aromatic Hydrocarbons (Pahs) and Soot Formations Under Different Gasifying Agents: Detailed Chemical Kinetic Analysis of a Two-Stage Entrained Flow Coal Gasifier

Characterization of Mineral Phase Changes from Feed Coal to Gasification Residues in Entrained-Flow Coal Gasification Based on an Individual-Particle-Basis Analysis

Characterization of Mineral Phase Changes from Feed Coal to Gasification Residues in Entrained-Flow Coal Gasification Based on an Individual-Particle-Basis Analysis

Mineralogical examination of the entrained-flow coal gasification residues and the feed coals from northwest China

Minerals are the dominant impurities in coal and their changes in gasifiers could cause many problems such as fouling, slagging, and ash deposition. The mineralogy of the coal gasification residues derived from two commercial entrained-flow coal gasification plants from the Ningdong Energy and Chemical Industry Base were determined. A range of minerals were detected by the analyses of X-ray diffraction and scanning electron microscope with associated energy-dispersive X-ray spectrometer (SEM–EDS). The mineral species formed in General Electric (GE) gasification were more abundant than that in the Gaskombinat Schwarze Pumpe (GSP) gasification. The neoformed minerals such as grossular and hedenbergite detected in the residues indicated that the gasification processes share some similarities with magmatism in geological system. The existence of wustite in the GE coal gasification residues indicated the reductive environment in the GE gasifier. Compared to the GSP feed coals, the higher proportions of calcite, pyrite, and siderite and the higher Na contents in the GE feed coals indicated that the ash problems probably happen in the coal-water slurry gasification. Reducing the proportions of those Na-, Ca-, and Fe-containing mineral from the feed coal is necessary for the improvement of the coal gasification process.

Water distribution and adsorption behaviors of two typical coal gasification fine slags from Ningxia Region

In this study, the water distribution and adsorption behaviors of two typical coal gasification fine slag (GFS) samples with different moisture contents were studied. Both of the samples were collected from entrained-flow coal gasifiers with the same coal as feedstock from Ningxia Region. The GFS sample collected from a GSP gasifier (2.5–4.0 MPa, 1400–1750 °C), with lower moisture content, was named as GFS-L, and the sample collected from a Texaco gasifier (4.0–6.5 MPa, 1300–1400 °C), with higher moisture content, was GFS-H. As the water distribution and adsorption behaviors in GFS samples, largely depending on the pore structure, had not been extensively analyzed, this study was investigated by the means of nitrogen adsorption analysis, mercury intrusion porosimetry (MIP), low field nuclear magnetic resonance (LF NMR) and dynamic water vapor sorption analysis (DVS). Experimental results revealed that the surface area, pore volume and average pore diameter of GFS-L were all lower than those of GFS-H. The fractal dimension analysis results illustrated that the GFS-L sample had smoother surface but more complicated internal pore structure than the GFS-H sample. Accordingly, the moisture content in GFS-L (45.42%) was slightly lower than that in GFS-H (48.78%). Water distributed in macropores decreased significantly during dehydration process, whereas decrease of water in mesopores was not apparent. And negligible water was found in micropores. Moreover, it was found that the water adsorption behaviors in both of the GFS samples could be characterized by the monolayer adsorption and multilayer adsorption over the relative humidity (RH) range of 0–0.7. Beyond the RH range of 0.7, it was found that water adsorption in the GFS-H sample transitioned to the capillary condensation, leading to higher moisture content. This study is envisaged to provide some new insights into developing GFS dehydration technology.

The application of thermal-calculation methods in the design and syngas prediction of entrained-flow coal gasifiers

A novel thermal-calculation method for membrane wall entrained-flow coal gasifier based on the equilibrium model was proposed in this study, and the method takes into account the heat transfer of the gasifier. The equilibrium model was established by the Gibbs free energy minimization method to avoid solving the intermediate chemical reaction process. The concepts of effective radiation heating area of furnace and flame center modification factor were introduced into the thermal-calculation method. This study not only analyzes the influence of oxygen-to-coal and water-to-coal ratios under normal operating conditions, but also investigates the impact of structural characteristics of a membrane wall entrained-flow gasifier on gasification characteristics. The model was effectively used to predict the syngas composition, the syngas temperature at the outlet of the chamber, and the energy distribution of the gasifier. The syngas composition and heat loss of gasifier obtained by thermal-calculation method are in good agreement with the experimental data of industrial-scale plants. The results show that the oxygen-to-coal ratio significantly impacts the syngas composition and heat loss than the water-to-coal ratio. Increase in the effective radiation heating area of furnace results in the decrease in the syngas temperature at the outlet of chamber, thus reducing the heat loss of flue gas and improving the heat absorption of the membrane wall tube. However, simultaneously, it can also reduce the CO concentration in syngas. Reduction in the flame center modification factor is helpful to increase the CO concentration; however, it also causes colossal heat loss of flue gas.

Numerical Simulations of Solidification Characteristics of Molten Slag Droplets in Radiant Syngas Coolers for Entrained-Flow Coal Gasification.

The high-temperature syngas and molten slag droplets discharged from entrained-flow coal gasifiers contain a large amount of heat energy, which can be efficiently recovered by radiant syngas coolers (RSCs). However, it is hard to know the solidification degree of molten slag droplets at the outlet of an RSC during industrial operations. In this work, the industrial-scale RSC and molten slag droplet models are established to predict the solidification degree of slag droplets at the outlet of the RSC. Then, the effects of slag diameter, syngas flow field, slag initial temperature, slag porosity, and slag pore structure are investigated by numerical simulations, and residence time as well as complete solidification time are calculated by coupling of a discrete-phase model and a solidification model. The results indicate that as the slag droplet diameter increases, the residence time of the slag droplet shortens, but the complete solidification time increases. When the slag droplet diameter is greater than or equal to 3.0 mm, the complete solidification time is larger than the residence time, and the slag droplet cannot solidify completely at the outlet of the RSC. The solidification degree in the windward zone is greater than that in the leeward zone. Although the slag initial temperature has little effect on the solidification, a lower slag initial temperature is still conducive to a greater solidification degree. Additionally, the pore structure facilitates solidification, and the promoting effect of penetrated pores is more remarkable than that of closed pores. A larger porosity is also beneficial to accelerate the solidification of molten slag droplets and increase the solidification degree.