Fluidized Bed Coal Gasification Research Articles

Preliminary petrographic assessment of the suitability of Bobov Dol coals, SW Bulgaria for fluidized-bed coal gasification

Preliminary petrographic assessment of the suitability of Bobov Dol coals, SW Bulgaria for fluidized-bed coal gasification

Characteristics of the Blockage from Air Nozzle Guide Duct in Circulating the Fluidized-Bed Coal Gasifier and Its Formation Mechanism.

Blockage is often generated in the air nozzle guide duct in a circulating fluidized-bed coal gasifier (CFBG), especially with Zhundong sub-bituminous coal (ZSBC) as the raw material. A typical example is found in one CFBG sample from Xinjiang Yihua Chemical Industry Co, Ltd. The serious blockage can be observed obviously. As so far, it is not clear for the characteristics and generation mechanism of the blockage. For analysis, the blockage can be classified into two parts, wall-layer blockage (WLB) and center-layer blockage (CLB). To inhibit its formation, it is of significance to analyze the composition, surface morphology, and formation mechanism of the two blockages. In our experiments, WLB and CLB were tested by XRF, XRD, FTIR, SEM-EDS, and SEM-mapping methods. Results showed that WLB presents high content of Fe, Cr, and Ni, and Fe mainly existed in the form of metal oxides. CLB is dominated by Si (43.04%), derived from silica and alkali and alkaline-earth metals silicates, and the migration of Fe, Cr, and Ni elements from the duct material was observed. Compared with WLB, from FTIR analysis, CLB contains more inorganic minerals, and the absorption peak of inorganic minerals is mainly attributed to asymmetric Si-O-Si. Many fine particles are attached to the surface of the WLB, while the surface of the CLB is smooth, and there is noticeable raised texture, which is presumed to be the result of particle melting and agglomerating as the bottom ash enters the duct in the gasification process. For the formation of the blockage, this paper speculates that it is mainly due to the difference in flow resistance near the air nozzle outlet, resulting in the formation of a flow dead zone at the bottom of the gasifier, which leads to large amounts of ash overcoming the outlet resistance and leaking into the air nozzle, and next, the ash corrodes in the tube, resulting in wall deposition and ultimately blocking the air guide duct. Two methods can be tried to avoid or inhibit the formation of blockage in the duct, including optimizing air nozzle with more wear-resistant and heat-resistant materials and adjusting the distance between air nozzles to avoid mutual interference from ash particles.

A rational and feasible approach to the co-management of condensates from biomass torrefaction and carbon-rich fly ash from fluidized-bed coal gasification

This study explored a promising approach to the co-management of torrefaction condensates (TCs) from biomass and carbon-rich fly ash (FA) derived from an industrial fluidized-bed coal gasifier. TC was a low-quality liquid product consisting mostly of water and a small amount of organic acids and other oxygenated organics. FA was the ultra-fine particulates with high carbon content (40–70%) and some extreme characteristics such as ultra-low volatiles, low reactivity, and high ignition temperature. The blending of FA and TC induced most of the organic components in TC to be chemically or physically adsorbed by FA, accounting for 3–13 wt% of the resultant mixtures of FA and TC (FATCs) and 11–33 wt% of the TCs. The acidic components in TC dissolved locally aggregated Ca minerals in FA, resulting in more evenly dispersed Ca on the surface of FA as an in-situ catalyst. As a result, FATCs exhibited a many-fold improvement in CO2 gasification reactivity compared to FA. And the syngas evolution rate of FATC gasification in steam-oxygen atmosphere was also remarkably elevated. In addition to the promoting effect on gasification, the combustion performance of FATCs was also greatly improved. Specifically, the ignition and burnout temperatures of FATCs were 46.5–68.5 °C and 12.4–31.7 °C lower than those of raw FA, respectively. The lower activation energy also demonstrated the higher reactivity of FATC gasification and combustion.

Structure and reactivity of residual carbon from circulating fluidized bed coal gasification fine ash

Due to the high carbon and ash content of coal gasification fine ash (CGFA) produced by circulating fluidised bed gasifiers, it is mainly dissipated in landfills, resulting in soil and water pollution and energy wastage. The presence of carbon limits the application of CGFA in building materials and ceramics. Large-scale dissipation of CGFA can be achieved if the carbon in CGFA has active oxidation properties and potential as a material for the preparation of carbon. Therefore, exploring the form of carbon present in CGFA is crucial for the resourceful and harmless use of CGFA. Demineralisation of different types of CGFA from a circulating fluidised bed gasifier with a gas production capacity of 40,000 Nm3/h was carried out to obtain demineralised carbon (DC) and to investigate the structure and reactivity of the DC. The d002 and Lc values of the DC increased, the cross-linkage decreased, the main microcrystalline structural parameters of the aromatic carbon network layer with swelling effect increased in the vertical direction, the aromatic carbon and aliphatic hydrocarbon functional groups increased and the oxygen-containing functional groups all tended to decrease to different degrees. Based on the changes of CGFA chemical structure before and after acid treatment, a CGFA chemical structure model was constructed. Through TG experiments and Raman analysis of the samples, the activation energy value of the DC carbon was less than 70 kJ/mol, and the relationship between the carbon structure and combustion reactivity of the Raman characterisation was constructed. The DC had a specific surface area above 350 m2/g and was prepared as activated carbon after KOH and high temperature activation. The activated carbon had a specific surface area of 1636.2211 m2/g, a pore volume of 1.6796 cm3/g and a density of 1 g/cm3. The above indicates that CGFA has the potential to be used as a fuel for combustion, secondary gasification and DC can be used as an adsorbent or a pressed precursors for activated carbon.

Optimization study on operation conditions of an industrial-scale circulating fluidized bed coal gasification

This research was carried out on a new industrial circulating fluidized bed (CFB) gasifier with output power of 111 MW. In order to realize stable operation and improve economic benefits, a series of experimental studies were carried out during the hot commissioning of the gasifier. The effects of O2/coal ratio, oxygen concentration and secondary gasification agent on gasification indices were investigated. The boundary conditions for the safe and stable operation of the gasifier were obtained, which provides a strong support for the commercial operation of the gasifier. The results showed that, the gasifier can operate stably between 940 and 970 ℃. As the O2/coal ratio increases from 0.31 to 0.36 Nm3/kg, the dry gas yield increases from 2.5 to 2.8 Nm3/kg, carbon conversion rate increases from 78.1 to 82.1% and cold gas efficiency increases from 68.9 to 71.5%, while the effective gas composition (CO, H2 and CH4) decreases from 51.9 to 49.3% and heat value of gas decreases from 6.6 to 6.3 MJ/Nm3. With the increase of oxygen concentration, the effective gas composition and heat value of gas increase. When the oxygen concentration of gasification agent reaches 25%, the temperature is about 960 ℃ and the heat value of gas can reach 6.2 MJ/Nm3. The introduction of secondary gasification agent (O2 + H2O) makes the riser temperature more uniform, but it will reduce the effective gas content and gas heat value to a certain extent.

Structure and Reactivity of Residual Carbon from Circulating Fluidized Bed Coal Gasification Fine Ash

Structure and Reactivity of Residual Carbon from Circulating Fluidized Bed Coal Gasification Fine Ash

Effects of flotation and acid treatment on unburned carbon recovery from atmospheric circulating fluidized bed coal gasification fine ash and application evaluation of residual carbon.

The high carbon content (20-60%) in coal gasification fine ash (CGFA) makes CGFA unable to be directly used in the building materials and ceramic industries, and can only be dissipated in landfills, which brings serious environmental safety problems. This paper uses physical (flotation method) and chemical (multi-stage pickling method) methods to treat CGFA, analyze the separated carbon, and evaluate its application. The carbon content of the filter cake ash (FCA) residual carbon recovered by the flotation method is slightly increased, and the carbon ash separation effect for the water-containing CGFA is poor. The carbon content of dry ash (DA) recovered carbon in acid treatment increased from 16.33% to 89.97%. The specific surface area of the acid-washed recovered carbon is 6-34 times that of the original sample, and the specific surface area of dry ash-HCl/HF/HCl (DA-CFC) is as high as 425.31m2/g, and its pore structure network is more developed than before deashing. After pickling treatment, the microcrystalline structure changes, the carbon skeleton becomes looser, the degree of crosslinking decreases, and the reaction activity increases. The relative content of CC in the acid-washed recovered carbon increased by 6.4-46.3%, and the relative content of functional group CH bonds increased by 48.5-89.5%. Compared with the activation energy of the original sample, the activation energy E of the sample after flotation and acid treatment is reduced, and the reaction activity is enhanced. Flotation and pickling methods can improve the grade of fuel, and multi-stage pickling methods to obtain high specific surface area carbon can be used as a precursor for the preparation of activated carbon. Carbon-ash separation of DA by flotation method can be applied to fuel combustion, fine chemical industry, and road filling. However, for FCA, it is necessary to use a pickling scheme for carbon-ash separation to realize resource utilization and harmlessness.

Study on pore and chemical structure characteristics of atmospheric circulating fluidized bed coal gasification fly ash

The pore and chemical structure characteristics of the raw coal and the coal gasification fly ash (CGFA) in an atmospheric pressure circulating fluidized bed gasifier with a gas production capacity of 20000 and 40000 Nm3 h−1 were studied. The results indicated the presence of developed pore structure and high carbon content of the CGFA. The comprehensive fractal dimension method was proposed to analyze the pore structure of the pore section. The comprehensive fractal dimension was found to be > 2.6. The surface of CGFA is rough and porous. When compared to raw coal, the relative content of the CGFA carbon functional group is lower than 20%, and that of aliphatic carbon gets reduced. The interlamellar spacing (0.3489 nm, 0.3501 nm) of the A-filter cake ash (FCAA), A-dry ash (DAA), B-filter cake ash (FCAB), and B-dry ash (DAB) carbon is higher than that of the standard graphite (0.3354 nm), lean coal (0.3468 nm), and anthracite (0.344 nm). Moreover, the carbon structure is loose. The comprehensive results of physical and chemical structure revealed that CGFA showed a good chemical reaction activity and also the value of secondary combustion and gasification as fuel. This study exhibits significance and importance in the field of waste valorization towards a circular economy.

Effects of the ratio of O2/C and H2O/C on sulfur release behaviors during fluidized bed gasification

Gaseous sulfur released during fluidized bed coal gasification will bring lots of negative effects, control the release of gaseous sulfur or the conversion to inorganic sulfur at source is necessary. In this study, sulfur release behaviors of a middle-sulfur coal called Guizhou coal were quantitatively determined through controlled experiments in a lab-scale fluidized bed during oxygen rich-steam gasification. The effects of the molar ratio of O2/C and the molar ratio of H2O/C on the sulfur release behaviors and the desulfurization performance of limestone were investigated. H2S is the dominate gaseous sulfur. With the molar ratio of O2/C increases, the yield of four kinds of gaseous sulfur increases, the trend of H2S/COS with O2/C is not the same as that of H2/CO, and the yield of H2 is the main influence factor of the variation of H2S/COS. With the molar ratio of H2O/C increases, the yield and concentration of four kinds of gaseous sulfur increases, and the molar ratio of H2S/COS and H2/CO increase. The addition of limestone does not change the trend of gaseous sulfur with O2/C or H2O/C. The reduction rate of gaseous sulfur decreases with the increase of O2/C, but with the increase of H2O/C, the reduction rate of gaseous sulfur increases first and then decreases. Increasing the ratios of O2/C and H2O/C simultaneously is not conducive for the conversion to H2S and the sulfuration of limestone.

Physicochemical properties of bottom ash obtained from an industrial CFB gasifier

To investigate the reasons of high carbon content in bottom ash (BA) and guide the suitable utilization of residual carbon, the physicochemical properties of BA which was obtained from an industrial circulating fluidized bed coal gasifier were primarily studied via Raman spectrometer, N2 adsorption test and thermogravimetric analysis and compared with raw coal (RC) and fly ash (FA). BA has the lowest combustion reactivity than RC and FA, which is attribute to high degree of carbon deactivation, limited specific surface area, low catalysis elements contents and high ash content. In this study, the carbon content of BA first increases and then decreases with increasing BA particle size and the peak value appears at about 2 mm. The degree of carbon deactivation increases and specific surface area decreases with increasing BA particle size. Furthermore, it was found that some BA particles were covered with compact shell, which probably hindered the adequate conversion of inside carbon. The formation of shell was presumably owing to the re-attachment of volatile matter to the char matrix and the migration of coal ash, based on the results of Ramen spectra and SEM.

Kinetics, thermodynamics and gas evolution of atmospheric circulating fluidized bed coal gasification fly ash combustion in air atmosphere

The activation energy of samples was calculated using the model-free method and the model method (typically 46 mechanism functions). The model method is suitable for water scrubbing ash (WSA), and the model-free method is suitable for raw coal (RC), dry ash (DA) and WSA. In the model-free method, the Kissinger method not only guarantees the accuracy of the reduction activation energy E value, but also takes into account the calculation amount of the replacement. The Kissinger method is the better method for the replacement activation energy E value. The average estimated value of DA’s E of Kissinger method is 107.89 kJ/mol, which is lower than 143.30 kJ/mol of Yangquan anthracite. The average estimated value of WSA’s E is 82.10 kJ/mol, which is slightly higher than that of lean coal (77.54 kJ/mol). It shows that the combustion performance of DA is better than that of Yangquan anthracite, and the combustion performance of WSA is equivalent to that of lean coal. Comprehensive combustion characteristics and gas emission results show that the CGFA has combustibility and utilization value.

Effects of temperature and limestone on sulfur release behaviors during fluidized bed gasification

Gaseous sulfur is released during fluidized bed coal gasification, and control the yield of gaseous sulfur or the conversion between gaseous organic sulfur and inorganic sulfur at source is necessary, because it can economically satisfy the requirements of industrial production and protect the environments. In this study, sulfur release behaviors of a middle-sulfur coal called Guizhou coal were quantitatively determined through controlled experiments in a lab-scale fluidized bed during oxygen rich-steam gasification. The measured gaseous sulfur species were H2S, SO2, COS and CS2. The effects of temperature (850OC-950OC) and limestone (Ca/S = 2) on the sulfur release behaviors were investigated. Among the above four gaseous sulfur, the yield of H2S is the highest, followed by COS, while only less than 1.5% of sulfur in coal is released as SO2 and CS2. With the increase in temperature, the yield of H2S increases while that of SO2 decreases, and the change of COS yield and CS2 yield is not obvious. The molar ratio of H2S/COS increases with increasing temperature, which is qualitatively matched by thermodynamic analysis. The addition of limestone reduces the released sulfur but not change the distribution of gaseous sulfur forms. Meanwhile, the molar ratio of H2S/COS increases after adding limestone, while the trend with temperature of H2S/COS does not change. The removal rate of H2S is between 23% and 28%, which increases with temperature. The distributions of sulfur in bottom char and fly ash are similar. The main sulfur species in the bottom char is organic sulfur, and thiophene dominates the organic sulfur. The increase of temperature and the addition of limestone will both promote the increase of inorganic sulfur content, and the decrease of organic sulfur content.

Effect of process parameters on circulating fluidized bed coal gasification using 3D full‐loop CFD simulation

AbstractThis article presents a 3D full‐loop computational fluid dynamics (CFD) simulation of a circulating fluidized bed gasifier (CFBG). The simulation results are validated against the experimental data and found to be in good agreement. Thereupon, the effect of the process parameters, ie, temperature, pressure, air/coal (A/C) ratio, and steam/coal (S/C) ratio, on the performance of the gasifier is analyzed. The effect of temperature on the hydrodynamics was found to be small. The CO and H2 increase, whereas the CO2 and H2O decrease with an increase in temperature. While the effect of pressure on the outlet species mole fraction is negligible, the gas and solid axial velocity decrease with an increase in pressure. With an increasing A/C ratio or decreasing S/C ratio, the combustion products (CO2 and H2O) increase, whereas the gasification products (CO and H2) decrease due to the increase in the O2 concentration. In addition, temperature increases with an increase in the A/C ratio or a decrease in the S/C ratio. The feed velocity increases with an increasing A/C or S/C ratio, and, accordingly, the pressure increases and bed height decreases. The CH4 decreases in all of the cases as it is being consumed in gasification as well as combustion reactions.

Radiotracer investigation in a pilot-scale fluidized bed coal gasifier (FBCG)

Radiotracer investigations were carried out for measurement and analysis of residence time distribution (RTD) of coal particles in a pilot-scale gasifier fitted with a flat air/steam distributor. Measurements were made at different operating conditions using gold-198 (198Au) labeled coal particles as a radiotracer. The measured RTDs were treated and mean residence times (MRTs) were determined. Furthermore, the treated RTDs were simulated using a suitably conceptualized mathematical model and detailed information about hydrodynamics of coal particles within the gasifier was obtained. Results of model simulation indicated that the gasifier behaved as an ideal mixer of fine coal particles exiting from the top of the gasifier. A small fraction of the coarser particles was found to be bypassing at ambient temperature.

Low-rank coal gasification using a bubbling fluidized bed reactor at low operating temperature

Coal gasification is one of coal utilizations that produces less CO2 emission than coal combustion. Coal gasification technology that has been used in Indonesia is generally a fixed bed gasification. Fixed bed is designed for high-rank coal and the majority of Indonesian coal is of a low-rank. Low ash and high moisture content of the Indonesian coal in a fixed bed can affect mechanical and thermal fragmentation, pressure drop, gas and particle flow distribution. The operation of gasifier may cause unstable condition. Another gasification technology is bubbling fluidized bed, which is operated above 1,200°C, so ash can melt. High operating temperature causes agglomeration and makes unstable gasification process. Therefore, in this study, low-rank coal is gasified in bubbling fluidized bed reactor at low operating temperature. The purpose of this study is to determine the optimal conditions of bubbling fluidized bed gasification. The research was conducted in bubbling fluidized bed coal gasification Process Development Unit (PDU) at Coal Utilization Technology Centre of R&D Centre of tekMIRA, Palimanan. Coal was fed continuously as many as 20 kg/hour into a gasifier then was gasified to produce gas using air as a gasifying agent and silica sand as a bed material at 850- 950°C. The produced gas from the gasification was analyzed using the Orsat Analyzer. A simulation using a ChemCAD 7.1 CC steady state was applied to validate the experiment result. From the analyzed result of yield gas composition, the produced CO and CO2 were about 10-15 wt%. Gas compositions that are close to criteria of producer gas, no agglomeration, and stable process condition during study indicate that bubbling fluidized bed gasification at low operating temperature is suitable to be applied as gasification technology for Indonesian low-rank coal.

Influences of operating parameters on the fluidized bed coal gasification process: A coarse-grained CFD-DEM study

In the present study, a comprehensive coarse-grained computational fluid dynamics and discrete element method (CFD-DEM) is developed for modeling the fluidized bed gasifier. Based on the model validations against experimental measurements, a series of simulations are conducted to investigate the effects of different operating parameters on the coal gasification in a bubbling fluidized bed. This study offers new insights into the effects of gas-solid mixing on the gasification reaction. The results show the non-uniform spatial distributions of the gasification rate in both the horizontal and vertical directions, which reflects the preferential distribution of fuel particles. It is found that the gas back-mixing plays an important role in controlling of the pyrolysis gas combustion, which is undesired for producing gasification products. The results show that the sand particle size has the most remarkable effect on the gas back-mixing. Though the uniformity of the fuel particle distribution is not a key factor influencing the average product yields, the horizontal fuel particle mixing is found to be the main cause of the fluctuations of the gasification rate. Fundamentally, it is demonstrated that the fluctuations of the horizontal fuel mixing are mainly dominated by the bubble behaviors.

Effect of bed materials on slagging and fouling during Zhundong coal gasification

The suitable bed material is important to inhibit ash-related problems including agglomeration and slagging in furnace and fouling on tail heated surfaces during coal gasification in fluidized bed....

Study of adsorption characteristics of Au(III) onto coal particles and their application as radiotracer in a coal gasifier

This paper describes a systematic study carried out to investigate adsorption characteristics of 198Au onto the surface of coal particles using a radiotracer technique. The main objective of the study was to optimize labeling conditions and utilize the labeled coal particles as a radiotracer for tracing coal particles in fluidized bed coal gasifiers. The effect of various experimental factors such as pH, amount of adsorbent, initial gold concentration, temperature and contact time between adsorbate and adsorbent were studied to obtain optimum conditions for maximum adsorption of 198Au on coal particles from an aqueous solution. Analysis of the data showed that the Langmuir model was found most suitable to describe the adsorption phenomena. The thermodynamic analysis showed that the adsorption process is feasible, spontaneous and endothermic in nature. The results of the kinetics suggested that the adsorption presumably follows second order kinetics and chemisorption in nature. Based on the optimized conditions, the amount of coal particles and activity to be used for tracing the coal phase in pilot-scale gasifiers was estimated. Accordingly, the radiotracer was prepared and successfully used for measurement of residence time distribution (RTD) in a pilot-scale coal gasifier.

The effect of reactor height on coal gasification

A comprehensive 2-D numerical model has been developed to simulate the coal gasification and investigate the effect of reactor height on the coal gasification in fluidized bed. Gas-solid flow, homogeneous and heterogeneous chemical reactions were considered. An Eulerian model for fluid phase and discrete particle method (Lagrangian) for particle phase were used in this study. The reaction rates of homogeneous and heterogeneous reactions were determined by Arrhenius-eddy dissipation reaction rate and Arrhenius-diffusion rate, respectively. Simulations were performed in a fluidized bed coal gasifier with twelve different reactor heights and with a diameter of 0.22 m. The calculated values were compared with the experimental values for the reactor height of 2 m available in open literature. It shows that the predicted exit gas mole fractions are in a good agreement with the experimental data.

Clean Hydrogen Energy and Electric Power Production with CO2 Capturing by Using Coal Gasification

Clean hydrogen is the major energy carrier for power production. The conversion of CO to CO2 and zero emission during hydrogen energy production causes high capital cost. It is a matter of prestige to optimize the process in order to make zero emission and cost effective production of clean hydrogen energy and electric power. In this era, coal gasification is the most promising technology for the clean hydrogen energy and electric power production with simultaneously capturing of CO2. The experimental set up used in this scheme consists of Fluidized Bed Coal Gasifier (FBCG), syngas treatment unit, electricity generation unit, CO2 capturing unit and clean hydrogen adsorption unit. This paper shows the analysis of low sulphur Makarwal (Pakistan) coal. The Oxygen to Steam (O/S) ratio is optimized in order to produce syngas efficiently in the FBCG. The desulphurization unit reduces the H2S contents below than 1ppm. In this experimental plant, the feed rate 37.5 tons/hr of coal is used and clean hydrogen is produced at the rate of 1.30-1.40 tons/h.