Calcium species evolution mechanism during coal pyrolysis and char gasification in H2O/CO2

The release mechanism of Ca during coal pyrolysis and char gasification in H₂O, CO₂ and their mixtures was studied. Sequential chemical extraction was used to determine the modes of occurrence of Ca in coal and char. The released Ca from coal pyrolysis and char gasification were captured and analyzed by activated carbon adsorption and X-ray photoelectron spectroscopy (XPS). Model compounds CaS and CaSO₄ were adopted to further reveal the released form of Ca under different atmospheres. The results indicate that Ca in coal is mainly released as CaCl₂ during the pyrolysis process, and the possible migration mechanism of Ca during pyrolysis was proposed. Ca in coal is mainly released in the form of CaCl₂, CaCO₃, and CaSO₄ during the gasification, and Ca is released as CaCl₂ under all conditions. In addition, Ca will be released as CaCO₃ under CO₂ atmosphere, as CaSO₄ under H₂O and H₂O/CO₂ atmospheres at 800 °C and 900 °C, released as CaSO₄ under all conditions at 1000 °C. This is closely related to the formation of CaO₂ intermediates during the gasification process.     

Experimental study on ash deposition of Zhundong coal in oxy-fuel combustion

To facilitate the large-scale utilization of high-alkali and -alkaline earth metals (AAEMs) coals in power generation, the ash deposition behaviors of a typical Zhundong coal in oxy-fuel combustion were experimentally investigated using a drop tube furnace. A wall-temperature-controlled ash deposition probe by which the bulk gas temperature could be measured simultaneously was designed and employed in the experiments. The deposition tendencies, ash morphologies, chemical compositions of deposited ash particles were studied respectively under various oxygen concentrations, bulk gas temperatures, probe surface temperatures and probe exposure times. The experimental results revealed that the oxygen concentration had a significant influence on the deposition behavior during oxy-fuel combustion of high-alkali coal. Compared with air case, more fine ash particles were generated during the combustion of Zhundong coal in 21% O₂/79% CO₂ atmosphere but the deposition tendency was weaker. However, a higher oxygen concentration could aggravate the tendency of ash deposition. The high contents of iron (Fe), calcium (Ca), sulfur (S), and sodium (Na) in Zhundong coal could result in the generations of low-melting point compounds. Calcium in flue gas existed as CaO and was captured prior to SO₃ by the probe surface during the ash deposition process. At the initial 30 min of the ash deposition process, the dark spherical fine ash particles rich in Fe, Na, oxygen (O), and S were largely produced, while in the range of 60–90 min the light spherical fine ash particles with high contents of Ca, barium (Ba), O, and S were generated on the other hand. The deposition mechanisms at different stages were different and the melted CaO (BaO)/CaSO₄ (BaSO₄) would give rise to a fast growth rate of ash deposit.     

Hydrogen and syngas production from catalytic steam gasification of char derived from ion-exchangeable Na- and Ca-loaded coal

Catalytic steam gasification of char derived from low-rank coal possesses substantial potential as a source of hydrogen energy and syngas feedstocks, and its performances are largely associated with the employed catalysts. Therein, ion-exchangeable Na or Ca species are always regarded as excellent in-situ catalysts in low-rank coal. In this paper, gasification of Na-Char, Ca-Char and a Na/Ca-Char mixture with different partial pressures of steam was performed within a temperature range of 700–900 °C using a micro fluidized bed reaction analyzer. The results indicate that Na and Ca species could accelerate the gas release rate during gasification and even significantly increase H₂ production, in sharp contrast to non-catalytic gasification. Variations in the product gases during Na-Char and Ca-Char gasification were completely different, which associated with the different deactivation pathways and catalytic reaction mechanisms of Na and Ca catalysts. With an increasing gasification temperature, the decreasing trend of H₂ production for Na-Char gasification was mainly due to the loss of Na during gasification. Conversely, the enhancement of Ca activity promoted the H₂ production. The H₂/CO ratio of Ca-Char gasification at 700 °C approximately ranged from 1.0 to 2.0 as a function of the partial pressure of steam, which suggested catalytic gasification can be suitable for hydrogen-rich production and subsequent synthesis reactions. In addition, gasification of Na/Ca-Char mixture produced a higher hydrogen content in the product gases than that of Na-Char or Ca-Char gasification alone, particularly for the 30%Na/70%Ca-Char mixture. It implies that the high H₂ production of 70%Ca30%Na-Char mixture was attributed to the cooperative effects of the Na and Ca species on the catalytic activity. This study provides comprehensive information regarding the effects of ion-exchangeable Na, Ca and a Na/Ca mixture on the hydrogen production and syngas composition during steam gasification, which provides new insight into the utilization of low-rank coal.     

Numerical analysis on combustion process and sodium transformation behavior in a 660 MW supercritical face-fired boiler purely burning high sodium content Zhundong coal

CHEMKIN software was used to optimize the reaction mechanism of sodium in flue gas to study the influence of targeted design for purely burning Zhundong (ZD) coal on boiler characteristics. Then, the optimized 32-step elemental reaction was combined with CFD software. An eddy-dissipation concept model considering detailed chemical reactions was used to simulate the transformation behavior of sodium-containing substances. The combustion characteristics of the 660 MW face-fired boiler under various loads were also simulated. The field distribution in the furnace and the migration path of sodium along the track of pulverized coal particles were obtained. The results show that the interference between each burner in the furnace is small at the BMCR load, and the phenomenon of “wind wrapping fire” is distinctly clear. The temperature at furnace outlet is approximately 970.98 °C. At a low load, the combustion in the furnace is stable, and the temperature at the furnace outlet reaches the design value. The sodium present in ZD coal is involved in the reaction after it is released in the form of Na and NaCl. Sodium is present in different forms in the main burner zone, mainly NaCl (67%), NaOH (12%), Na (9%), and Na₂SO₄ (7%). The forms of sodium at the furnace outlet are NaCl (50%), Na₂SO₄ (37%), Na₂Cl₂ (9%) and NaHSO₄ (4%). A small amount of Na₂SO₄ is formed by NaHSO₄ reaction in the main burner zone. It then reacts to form NaSO₄, wherein NaHSO₄ is formed by path 2. Na₂SO₄ is mainly generated in the burnout zone through path 1, and paths 2, 3, and 4 are hardly observed. The findings of this research can provide reference for the design of a purely fired ZD coal boiler and further studies on slagging observed on the heating surface.     

Investigation on ash deposition characteristics during Zhundong coal combustion

The reserves of Zhundong (ZD) coal in China are huge. However, the high content of Na and Ca induces serious slagging and fouling problems. In this study, the ZD coal was burned in a DTF (drop tube furnace), and the ashes collected at different gas temperature with non-cooling probe were analyzed to obtain the ash particle properties and their combination mode. The results showed that Na, Ca and Fe are the main elements leading to slagging when the gas temperature is about 1000 °C during ZD coal combustion, but their mechanisms are quite different. Some sodium silicates and aluminosilicates and calcium sulfate keep molten state in the ashes collected at 1000 °C. These molten ash particles may impact and adhere on the bare tube surface, and then solidified quickly. With the growth of slag thickness, the depositing surface temperature is increased. The molten ash particles might form a layer of molten film, which could capture the other high fusion temperature particles. The Fe₂O₃ sphere were captured by the formed molten slag and then they blended together to form a new molten slag with lower melting temperature.     

Desulfurization behavior of Ca-based absorbents under periodically changing condition between reducing and oxidizing atmosphere

The capture of H₂S or SO₂ with Ca-based absorbents, limestone and dolomite, was investigated under a periodically changing condition between reducing and oxidizing atmospheres, taking account of the coexistence of these atmospheres in a fluidized-bed coal gasifier. The degree of desulfurization in the reducing atmosphere was large compared with that in the oxidizing atmosphere. Apparent difference in desulfurization behavior was scarcely observed between limestone and dolomite. Stabilization of CaS in in situ desulfurization residues derived from both limestone and dolomite to harmless CaSO₄ was also investigated. Absorbents maintaining porous-structure, such as dolomite, will be preferable for desulfurization in the coal gasification/combustion types of hybrid power systems.     

Effect of high temperature and separated combustion on SO2 release characteristics during coal combustion under O2/CO2 atmosphere

This paper studied the effect of high temperature (up to 1873K) and separated combustion mode (volatile combustion and char combustion are separated) on SO₂ release characteristics during pulverized coal combustion under O₂/CO₂ atmosphere. Coal combustion experiments were conducted at different combustion environment temperatures utilizing a high temperature fixed-bed setup. The results show that as temperature rises, the SO₂ release curve is transformed from a single-peak process to a double-peak process. In separated combustion, temperature has little effect on the volatile-SO₂ (SO₂ released during volatile combustion) but brings about a significant effect on char-SO₂ (SO₂ released during char combustion). Char-SO₂ release amount and the ratio of it to fuel-SO₂ release amount (total SO₂ released during coal combustion) increase with temperature rising. The increase of temperature leads to a dramatic decreasing of sulphur mass fixed in the ash and causes SO₂ release amount to rise when temperature is lower than 1573 K. Separated combustion causes a higher SO₂ release amount than coupled combustion (the same as conventional combustion, volatile combustion and char combustion are simultaneous). Thermochemistry equilibrium composition calculation results show that alkali metals and alkaline-earth metals are significant in sulphur retention. CaSO₄ and Na₂SO₄ are the main sulphates at high temperatures.     

Study on utilizing zinc and lead-bearing metallurgical dust as sulfur absorbent during briquette combustion

Zinc and lead-bearing metallurgical dust (ZLMD) exhibit strong desulfuration ability due to their higher content of metal oxides, such as Fe₂O₃, ZnO, PbO and CaO. In present work, their performances as a sulfur absorbent used in the combustion process of briquette, which is made up of coal and ZLMD, are investigated. Experimental results show that a large part of H₂S and a small part of COS, CS₂ are volatilized from briquette and react with ZnO and PbO to form ZnS and PbS at the earlier stage of combustion, and that O₂ oxidizes FeS₂ to form SO₂ at the later stage. The adsorption reaction of sulfur depends on the content of CaO. ZnO, ZnS and CaSO₄ are stable during the combustion process at temperatures lower than 1100 °C. When the weight percent of ZLMD in the briquette is kept at around 2.5%, and sulfur in coal at around 2.1%, the absorption efficiency of sulfur can reach 90%. These results suggest that utilizing ZLMD as a sulfur absorbent in the combustion process of briquette is a cheap and highly efficient method to treat both ZLMD and toxic emission such as H₂S/SO₂ released during briquette combustion processes.     

The evolution characteristic of sulfur-containing gases during coal thermal conversion

In this study, the evolution characteristics of sulfur-containing gases during thermal conversion of two coals under different atmospheres were studied through temperature-program decomposition (TPD) and rapid-heating decomposition (RHD) coupled with online mass spectrum (MS). The releasing profiles of H₂, CH₄ and CO were also measured. Results showed that the effect of atmosphere and heating rate on evolution of sulfur-containing gases was very significant. It was found that Ar atmosphere was more favorable to the formation of sulfur-containing gases than CO₂ atmosphere by using TPD-MS. In CO₂, the formation of H₂S and SO₂ was restrained in 260–650 °C, but was promoted in 880–980 °C; the formation of COS was promoted during the whole process. In Ar, high releasing intensity of H₂ and CH₄ could stabilize sulfur-containing radicals which led to high amount of H₂S and SO₂; while high releasing intensity of CO in CO₂ resulted in high amount of COS. By using RHD-MS, it was found that the steam atmosphere was highly favorable for the transformation of H₂S, SO₂ and COS during the entire reaction period. However, the CO₂ atmosphere was disadvantageous to the transformation of H₂S, SO₂ and COS at the initial stage, but slight favorable for the transformation of H₂S, SO₂ and COS during the later stage. These was resulted from the gasification reaction of steam/CO₂ with coal. The key factor was the releasing amount of H₂ and CO, which promoted the formation and transformation of H₂S, SO₂ and COS.     

Effect of CO2 atmosphere on Ca-containing mineral transformation behaviors during a Ca dominated coal co-firing with a Si/Al dominated coal in oxy-fuel condition

Coal blending is an effective method to reduce the slagging tendency. The mineral transformation within blended coal ashes has an important influence on the slagging behaviors, which have been well studied under conventional air-fuel condition. Oxy-fuel combustion is recognized as one of the most promising CO₂ reduction technologies. However, the mineral transformation within blended coal ashes in oxy-fuel condition has yet to be fully understood. This work is aimed to comprehensively study the influence of CO₂ atmosphere on Ca-containing mineral transformation behaviors during a Ca dominated coal co-firing with a Si/Al dominated coal in O₂/CO₂ condition by TG-DSC, XRD and HSC simulation. These results show that the atmosphere significantly affects on the transformation of Ca-containing mineral and the final existing form of Ca in the blended coal ashes. When the high content Ca coal is blended with the high content Al/Si coal in the O₂/CO₂ atmosphere, the CO₂ promotes the transformation of Ca into CaCO₃ and Ca under low temperature mainly exists in the form of CaCO₃ (little CaSO₄) in the blended coal ashes. CaCO₃ under high temperature is decomposed to produce highly active CaO, and CaO reacts with Al/Si to form the low melting point minerals, which may aggravate the slagging tendency. The present study can provide useful information to reduce the slagging behavior of the blended coals in the O₂/CO₂ atmosphere.     

NOx and H2S formation in the reductive zone of air-staged combustion of pulverized blended coals

Low NO_x combustion of blended coals is widely used in coal-fired boilers in China to control NO_x emission; thus, it is necessary to understand the formation mechanism of NO_x and H₂S during the combustion of blended coals. This paper focused on the investigation of reductive gases in the formation of NO_x and H₂S in the reductive zone of blended coals during combustion. Experiments with Zhundong (ZD) and Commercial (GE) coal and their blends with different mixing ratios were conducted in a drop tube furnace at 1200°C–1400°C with an excessive air ratio of 0.6–1.2. The coal conversion and formation characteristics of CO, H₂S, and NO_x in the fuel-rich zone were carefully studied under different experimental conditions for different blend ratios. Blending ZD into GE was found to increase not only the coal conversion but also the concentrations of CO and H₂S as NO reduction accelerated. Both the CO and H₂S concentrations inblended coal combustion increase with an increase in the combustion temperature and a decrease in the excessive air ratio. Based on accumulated experimental data, one interesting finding was that NO and H₂S from blended coal combustion were almost directly dependent on the CO concentration, and the CO concentration of the blended coal combustion depended on the single char gasification conversion.Thus, CO, NO_x, and H₂S formation characteristics from blended coal combustion can be well predicted by single char gasification kinetics.     

Mineral transformation during combustion of coal blends

Mineral behaviour for two individual coals (I, J) and their two‐component coal blends and 800°C ash blends heating were studied. Ash samples were heated progressively from 800°C to IT (initial deformation temperature) at 100°C intervals under different conditions. Coal samples were heated from room temperature to the corresponding temperature. Mineral transformation at each temperature was determined by X‐ray diffraction and SEM measurements. The results show that Si, Al, Fe and Ca compounds have a great form variation during heating. Their forms at different temperatures depend on the chemical composition of the ash, the blending ratio and the atmosphere. For different coal ashes, the main mineral matters at 800°C were quartz, anhydrite, hematite, calcite and feldspar. As the temperature increased, oxidation, thermal decomposition, transformation and reaction occurred between the components. Comparing a 40% I+60% J ash blend with individual ashes, fayalite was formed at 1100°C for the blend; the reaction product existed in a glassy phase at 1300°C. For a coal blend having the same ash ratio as the ash blend, FeO reacted with amorphous SiO₂ or Al₂O₃ to form fayalite and hercynite at 1000°C. As the temperature increased to 1100°C, fayalite and hercynite increased obviously. At 1200°C, some iron inclusion compounds melted to become glassy phase matter. Compared with the ash blend, iron species undergo a different change during coal blend heating: fayalite and hercynite formed earlier, iron compounds melted to form a glassy phase at lower temperature. This may be caused by early combustion of the more reactive coal (J coal) in the blend inducing local variation in oxygen concentration gradients around the less reactive coal and consequently affecting the reaction atmosphere and Fe mineral behaviour and interaction. That is to say, for coal blends, the mineral transformation was affected by both the mineral species interaction and the combustion behaviour. The calculations were performed to examine the fate of mineral matter under different combustion conditions using a thermodynamic chemical equilibrium calculation program. Calculations from coal blends were comparable with experiments from ash blends, this is because the calculation program only considers the interaction among the mineral species but does not consider the combustion reaction. It indicates that combustion and the relative volatiles also affected the mineral behaviour and slagging during coal blend combustion. Meanwhile, the mineral species evaporations were measured at high temperature: the main evaporated species were Na, K pure species and compounds, Fe, FeO, SiO and SiO₂. The evaporation of Fe has an important effect on initial deposition. Calculations were comparable with the experiments. Copyright © 1999 John Wiley & Sons, Ltd.     

Enhanced hydrogen-rich gas production from steam gasification of coal in a pressurized fluidized bed with CaO as a CO2 sorbent

Steam gasification of a typical Chinese bituminous coal for hydrogen production in a lab-scale pressurized bubbling fluidized bed with CaO as CO₂ sorbent was performed over a pressure range of ambient pressure to 4 bar. The compositions of the product gases were analyzed and correlated to the gasification operating variables that affecting H₂ production, such as pressure (P), mole ratio of steam to carbon ([H₂O]/[C]), mole ratio of CaO to carbon ([CaO]/[C]) and temperature (T). The experimental results indicated that the H₂ concentration was enhanced by raising the temperature, pressure and [H₂O]/[C] under the circumstances we observed. With the presence of CaO sorbent, CO₂ in the production gas was absorbed and converted to solid CaCO₃, thus shifting the steam reforming of hydrocarbons and water gas shift reaction beyond the equilibrium restrictions and enhancing the H₂ concentration. H₂ concentration was up to 78 vol% (dry basis) under a condition of 750 °C, 4 bar, [Ca]/[C] = 1 and [H₂O]/[C] = 2, while CO₂ (2.7 vol%) was almost in-situ captured by the CaO sorbent. This study demonstrated that CaO could be used as a substantially excellent CO₂ sorbent for the pressurized steam gasification of bituminous coal. For the gasification process with the presence of CaO, H₂-rich syngas was yielded at far lower temperatures and pressures in comparison to the commercialized coal gasification technologies. SEM/EDX and gas sorption analyses of solid residues sampled after the gasification showed that the pore structure of the sorbent was recovered after the steam gasification process, which was attributed to the formation of Ca(OH)₂. Additionally, a coal-CaO–H₂O system was simulated with using Aspen Plus software. Calculation results showed that higher temperatures and pressures favor the H₂ production within a certain range.     

Full-scale evaluation of SO2 capture increase for semi-dry FGD technology

This paper reports on a full-scale investigation of a possible increase of the SO₂ capture ratio in current semi-dry flue gas desulphurization (FGD) technology. The FGD unit is used for two 130 t/h steam PC boilers burning a blend of lignite and hard coal with dry ash-free sulphur content at 1.83%. The FGD unit has been designed to reach an SO₂ emission limit of 1350 mg/Nm³. The aim of the experimental work presented in this paper was to investigate the possibility of reaching SO₂ emission targets of 500 and 200 mg/Nm³. The investigation sought to determine the real correlation of SO₂ capture ratio with Ca/S and with the difference of dry-bulb temperature and dew point in the absorber Δt_AD. Generally, the SO₂ capture correlation with Ca/S has flat characteristics at a capture ratio >90%, which is required to reach the 200 mg/Nm³ target. In this case, lowering the Δt_AD in the absorber has only a weak effect. The 500 mg/Nm³ target requires an SO₂ capture ratio of about 80%; in this case lowering the Δt_AD by about 7 °C increases the SO₂ capture by 10% points at the same Ca/S ratio.     

Transformation characteristics of Na and K in high alkali residual carbon during circulating fluidized bed combustion

In this study, the transformation characteristics of sodium (Na) and potassium (K) during combustion of Zhundong coal gasification fly ash in circulating fluidized bed (CFB) reactors were investigated by examining gasification fly ash (TCf) from a 0.1-MW CFB test system. Experimental results indicated that TCf was rich in Na and K, with water-soluble and insoluble Na the main Na forms. Insoluble K was the major K form in TCf, accounting for 70.6% of total K. Reactor bed temperature exerted important effects on Na release during combustion such that, as bed temperature increased, the proportions of Na in bottom and circulating ash decreased while the Na in fly ash increased. Hydrochloric acid-soluble and insoluble Na in ash accounted for a large fraction of total Na. However, insoluble K was the principle K form in ash and bed temperature showed little influence on K release and distribution in ash during combustion. With decreased flue gas temperature, the Na content in deposition ash initially increased, then decreased, and eventually stabilized, while the K content in deposition ash was basically unchanged. Agglomeration of ash particles occurred during combustion, being more apparent at higher gas temperatures, and the agglomerates were rich in Na, K, sulfur (S), chlorine (Cl), and calcium (Ca). Deposition ash Na was mainly contained NaCl and Ca/Na sulfates. The enrichment of these salts as well as of Ca sulfate in ash was the main cause of ash agglomeration and deposition.     

Boiling coal in water: Hydrogen production and power generation system with zero net CO2 emission based on coal and supercritical water gasification

Coal is the single most important fuel for power generation today. Nowadays, most coal is consumed by means of “burning coal in air” and pollutants such as NO_x, SO_x, CO₂, PM2.5 etc. are inevitably formed and mixed with excessive amount of inner gases, so the pollutant emission reduction system is complicated and the cost is high. IGCC is promising because coal is gasified before utilization. However, the coal gasifier mostly operates in gas environments, so special equipments are needed for the purification of the raw gas and CO₂ emission reduction. Coal and supercritical water gasification process is another promising way to convert coal efficiently and cleanly to H₂ and pure CO₂. The gasification process is referred to as “boiling coal in water” and pollutants containing S and N deposit as solid residual and can be discharged from the gasifier. A novel thermodynamics cycle power generation system was proposed by us in State Key Laboratory of Multiphase Flow in Power Engineering (SKLMFPE) of Xi'an jiaotong University (XJTU), which is based on coal and supercritical water gasification and multi-staged steam turbine reheated by hydrogen combustion. It is characterized by its high coal-electricity efficiency, zero net CO₂ emission and no pollutants. A series of experimental devices from quartz tube system to a pilot scale have been established to realize the complete gasification of coal in SKLMFPE. It proved the prospects of coal and supercritical water gasification process and the novel thermodynamics cycle power generation system.     

High quality syngas production from pressurized K2CO3 catalytic coal gasification with in-situ CO2 capture

The enhanced K-catalytic coal gasification by CO₂ sorption reaction (EKcSG) was proposed to produce syngas with high content of H₂ and CH₄ and perform in-situ CO₂ capture. CO₂ is reduced dramatically with the introduction of the CaO into the reactor under typical K-catalytic coal gasification condition (3.5 MPa, 700 °C). The carbonation reaction of CaO can promote the syngas production by improving the equilibrium of the water-gas shift reaction and supplying heat for coal gasification reaction. In the presence of the CaO sorbent (Ca/C = 0.5), the CO₂ concentration in the product gas decreased from 25.61% to 12.80% compared with that without CaO. Correspondingly, the total concentration of H₂ and CH₄ is improved from 65.61% to 82.99% and the carbon conversion reached above 95%. The effect of Ca/C ratio and reaction temperature was investigated during the EKcSG process. It is considered that Ca/C ratio of 0.5 is the best proportion in terms of carbon conversion and CO₂ absorption in our experimental conditions.     

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 (850^OC-950^OC) 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.     

Circulating fluidized bed gasification of low rank coal: Influence of O<Subscript>2</Subscript>/C molar ratio on gasification performance and sulphur transformation

To promote the utilization efficiency of coal resources, and to assist with the control of sulphur during gasification and/or downstream processes, it is essential to gain basic knowledge of sulphur transformation associated with gasification performance. In this research we investigated the influence of O₂/C molar ratio both on gasification performance and sulphur transformation of a low rank coal, and the sulphur transformation mechanism was also discussed. Experiments were performed in a circulating fluidized bed gasifier with O₂/C molar ratio ranging from 0.39 to 0.78 mol/mol. The results showed that increasing the O₂/C molar ratio from 0.39 to 0.78 mol/mol can increase carbon conversion from 57.65% to 91.92%, and increase sulphur release ratio from 29.66% to 63.11%. The increase of O₂/C molar ratio favors the formation of H₂S, and also favors the retained sulphur transforming to more stable forms. Due to the reducing conditions of coal gasification, H₂S is the main form of the released sulphur, which could be formed by decomposition of pyrite and by secondary reactions. Bottom char shows lower sulphur content than fly ash, and mainly exist as sulphates. X-ray photoelectron spectroscopy (XPS) measurements also show that the intensity of pyrite declines and the intensity of sulphates increases for fly ash and bottom char, and the change is more obvious for bottom char. During CFB gasification process, bigger char particles circulate in the system and have longer residence time for further reaction, which favors the release of sulphur species and can enhance the retained sulphur transforming to more stable forms.     

Comparison of metallic oxide,natural ore and synthetic oxygen carrier in chemical looping combustion process

As one of clean coal combustion ways, chemical looping combustion (CLC) showed high CO₂ capture efficiency with lower energy penalty. But these processes were limited by the low reaction rate between oxygen carriers (OCs) with coal char. This study evaluated the performances of Cu-based OCs with coal in in-situ gasification chemical-looping combustion (iG-CLC) and chemical-looping with oxygen uncoupling (CLOU) process. CuO modified by iron ore and chrysotile were employed as OCs which the addition of chrysolite improved the char gasification and iron ore enhanced the stability of CuO at high temperature. Results showed that CuO supported by ores (chrysolite and iron ore) had better H₂ and CO conversion under H₂O atmosphere than CuO and iron ore. Chrysolite decorated CuO can convert almost all H₂ to H₂O at 850 °C. Synthetic OCs showed better stability and high temperature tolerance during 10 redox cycles.