The phase diagram characteristics of Indonesian coal ash were studied with thermodynamic equilibrium calculations and the Na2O-Al2O3-SiO2 ternary system. Baikuang coal was selected as the blended coal, and the blending ratio was optimized. Gasification experiments were carried out in a horizontal tube furnace system; the microstructure, mineral crystalline phases and chemical compositions of residues were analyzed; and the retention ratios of sodium and potassium were calculated. The particles underwent three stages, namely, (1) internal crushing and quick pore growth, (2) the formation of dense layers on the surface, and (3) the coverage and blockage of pores. These three stages corresponded to the release of the internal gas phases, the formation of silicates and aluminosilicates, and the low-temperature co-melting with other metallic elements (Ca, Mg, Fe), respectively. The main crystalline phases in the residues were converted from SiO2 to NaAlSiO4. During the reaction, the chemical form of Na changed from H2O-soluble to insoluble and HCl-soluble. The gasification temperature had a pronounced effect on the sodium conversion rate and the conversion degree, and the retention ratio of sodium and potassium was significantly reduced as the temperature was increased. After coal blending, the slagging tendency was significantly alleviated, the peak of NaAlSiO4 decreased, and the crystal phase of mullite (3Al2O3·2SiO2) was determined. In addition, the sodium content in the ash was remarkably reduced. Adjusting the coal ash components by coal blending was an effective way to alleviate or even avoid slagging of Indonesian coal, and a mixing ratio of 25% was optimal.

Ash-related problems may induce serious slagging problems in gasification boilers, especially in the presence of alkali and alkaline earth metals at high concentrations. Suitable additives have shown to be promising in suppressing the slagging characteristics of high-alkali coal. In this study, the results of lab-scale experiments for Zhundong coal were demonstrated, especially in Yihua coal (YHc) here, with six common additives, which are kaolin (A), boehmite (B), corundum (C), quartz (D), raw vermiculite (E) and ripe vermiculite (F), at fractions of 3%, 6%, and 9%. The priority of reactions involving AAEM-based species was investigated by characterizing ash samples and thermodynamic equilibrium calculation by Factsage 7.2. The gasification residue of YHc alone (O) was adopted as the reference sample. All six additives could affect the release and transformation of Na, the capture order from high to low was A, F, E, O, B, D, and C, and the slagging risks of different samples were F, E, D, O, A, B, and C in sequence. Given both the ability of additives to capture alkali/alkaline earth metal-based species and the ATFs of the coal ashes, kaolin was proved as a suitable additive for YHc gasification. Furthermore, 6% (mass ratio) was considered the most reasonable addition ratio.

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 (850(O)C950(O)C) 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. (C) 2020 Energy Institute. Published by Elsevier Ltd. All rights reserved.

The release behavior of sulfur during coal gasification was studied in a bench-scale self-heated circulating fluidized bed gasifier. With the increase of the O2/C molar ratio, gasification temperature increases, which promotes sulfur release rate and the formation of H2S. The conversion reaction between H2S and COS is far from equilibrium and the yield of COS is excessive. Under the same molar ratio of O2/C, the increase of coal feeding rate can elevate the gasification temperature, promote the release of sulfur and the transformation of gaseous sulfur to H2S.

Two kinds of high alkali and alkaline earth metal (AAEM) lignite (TTc and ZDc) were used as fuels, and three atmospheres of CO2, CO/CO2, and H2O (g) were used in the horizontal tube furnace experiments to simulate the gasification conditions in different parts of the boilers. The relevant thermodynamic calculations on the atmospheres and ash compositions were conducted in FactSage 7.2. The results show that both two coals have a high risk of slagging. Decreasing the relative content of SiO2 or increasing the relative content of Al2O3 can increase the ash fusion temperatures (AFTs) of TTc; increasing the relative content of CaO can increase the AFTs of ZDc. Besides, it is not recommended to adjust the relative content of Fe2O3. The proportion of the liquid phase of ZDc ash in CO/CO2 atmosphere is much higher than that of CO2 and H2O (g) in a typical circulating fluidized bed (CFB) gasification temperature interval, while the generation of liquid phase of TTc ash is less affected by atmospheres. The bottom of boilers is the most severe slagging zone, and purging with H2O (g) is a good way to alleviate the low-temperature slagging of the return feeder.

In order to provide basic knowledge for the gasification utilization of low-rank Zhundong (ZD) coal in the process of circulating fluidized bed (CFB), this study focuses on gasification reactivity and ash related agglomeration of ZD coal mainly induced by sodium transformation. Typical ZD coal was pyrolyzed in a horizontal tube reactor by a rapid pyrolysis method at 900 degrees C. Isothermal and non-isothermal thermogravimetric analyses were used to study the gasification reactivity of ZD char under CO2 atmosphere. The effect of temperature, ranging from 850 degrees C to 1050 degrees C, on isothermal gasification was investigated, and the obtained gasification residues were analyzed to observe microscope morphology and evaluate agglomeration degree. The ternary system relationship of Na2O-SiO2-Al2O3 at each gasification temperature was calculated with FactSage to compare with the experimental results. The results show that: the gasification reactivity of ZD coal char is greatly affected by the gasification temperature. Higher temperature can increase the average specific gasification rate, thus shorten the reaction time that the carbon conversion reaches equilibrium. The gasification reactivity of coal char is proved to be better at higher temperature. The agglomeration characteristics were also affected by the gasification temperature. There is almost no melting and agglomeration phenomenon for 850 degrees C gasification residue, while a strong tendency to agglomeration is showed in other samples. However, when the mass fraction of local CaO on the surface of large particles is more than 40%, the melting phenomenon can be alleviated. According to the Na2O-SiO2-Al2O3 phase diagram, the positions of ZD coal and ZD char are located near the complete Slag-liq region, indicating that using ZD coal and ZD char at the operation temperature of CFB above 950 degrees C has strong risk of slagging, and a relatively lower gasification temperature, similar to 900 degrees C, is suitable for ZD coal/char gasification. (C) 2020 Energy Institute. Published by Elsevier Ltd. All rights reserved.

This work reports a newly preheated system with pulverized coal oxy-fuel moderate or intense low-oxygen dilution (MILD) combustion for low NOx emissions. During the experiment, high-temperature fuel preheated by a circulating fluidized bed unit would burn out in a down-fired combustor. This preheating process of pulverized coal tended to form a strong reducing atmosphere and promoted the fuel temperature above the ignition temperature, which contributed to NOx reduction and MILD combustion formation. During the oxy-fuel MILD combustion process, the CO2 concentration of flue gas reached approximately 95%, the NO2 and N2O emissions were nearly zero, and the NO emissions were 107.99 mg/MJ. The combustion efficiency was 98.21%, which indicates that the combustion efficiency would not decrease in MILD combustion. Also, NO emissions were further reduced to 37.25 mg/MJ through a rational arrangement of tertiary gas positions. For pulverized coal combustion, the accurate definition of MILD combustion should consider the disappearance of flame front besides the feature of temperature distribution.

This study examined the combustion of pulverized char under an O2/CO2 atmosphere to (1) attempt to achieve a moderate or intense low-oxygen dilution (MILD) combustion process and (2) investigate the effects of gas distribution modes on NO emissions. First, fuel was preheated in a circulating fluidized bed (CFB) and then high-temperature preheated fuel from the CFB was burned in a down-fired combustor (DFC). The study was conducted with two secondary gas nozzle positions, three tertiary gas position arrangements, and four secondary oxygen ratios in the DFC. During the test process, the combustion temperature was uniform in the CFB and the CO2 concentration in the flue gas reached approximately 90%. MILD combustion was achieved when the secondary gas nozzle position was center and the char combustion reaction dispersed into the upper low-oxygen space in the DFC. The obvious flame front disappeared and the temperature profile in the DFC was more uniform which is also one of the important features of MILD combustion. This reduced NO emissions by around half while maintaining high combustion efficiency. NO emissions were further reduced by a particular arrangement of tertiary gas positions, but at the cost of reducing combustion efficiency.