Leached Coal Research Articles

Effects of leaching and additives on the formation of deposits on the heating surface during high-Na/Ca Zhundong coal combustion

Zhundong coal, high-quality coal with large reserves in China, faces great challenges on its large-scale utilization in utility boilers due to the ash-related problems caused by high alkali and alkali earth metals (AAEMs). In this research, to simulate ash deposition behavior on heating surfaces in high-temperature flue gas zone (panel superheater) and that in medium-temperature flue gas zone (superheater or reheater), deposition experiments were carried out during Zhundong coal combustion in a drop tube furnace (DTF) at the deposition temperature of 1200 °C and 900 °C. Meanwhile, water leaching, Na2CO3 additive and kaolin additive were employed to elucidate the formation mechanism of ash deposition and alleviate deposition and slagging. Results showed that leaching reduced the contents of troublesome elements such as AAEMs and S and increased the relative contents of Si and Al, thus alleviating ash deposition. Compared with raw coal, the deposition rate of leached coal decreased from 5.82 mg h−1 cm−2 to 0.88 mg h−1 cm−2 at the deposition temperature of 900 °C. The primary reason for the decreased deposition rate might be that the dominant minerals in leached coal ash were refractory SiO2, CaO and Ca2SiO4. The addition of Na2CO3 would aggravate the slagging and fouling on the deposition surface significantly. The gaseous Na2O decomposed from Na2CO3 reacted with SO2 and SiO2 in the flue gas to form low-melting Na2SO4 and Na2Si3O7, which were easy to cause severe slagging on the deposition surface. The inhibition effect of kaolin addition on the formation of deposition and slagging was similar to that of leaching. After the addition of kaolin, the dominant compounds of fly ash altering to high-melting minerals such as CaSiO3, CaSO4, Ca2MgSi2O7 and NaCaAlSi2O7, resulting in a lower deposition rate than raw coal. Besides, when deposition temperature increased from 900 °C to 1200 °C the slagging degree of deposition surfaces was aggravated, while the deposition rate of four fuels followed different change trends. Specifically, the deposition rate of raw coal and leached coal decreased, whereas that of the case with Na additive and kaolin additive presented an increasing trend. At last, the role of water leaching and additives on the formation of ash deposition during Zhundong coal combustion was clarified.

Desulfurization of Lakhra coal by combined leaching and catalytic oxidation techniques

ABSTRACT Desulfurization of the Lakrah coal was investigated by combined leaching and catalytic oxidation. For leaching various concentrations of different acidic and base solutions including H2SO4, HCl, HNO3, KOH, NaOH, and Na2CO3 were used. Among acids, 0.4 N HNO3 at room temperature and 30 min stirring time, provided about 19.3% of sulfur removal, whereas the base solution of 0.4 N KOH was most effective, decreasing the total sulfur by 29%. The leached coal was subjected to oxidative desulfurization using H2O2 as oxidant. The optimum conditions were found to be 70°C temperature, 60 min reaction time and using toluene as a solvent, which led to decrease in the total sulfur upto 50%. The oxidative desulfurization was further investigated in the presence of Fe loaded on bentonite catalyst. The oxidative desulfurization of the Lakhra coal in the presence of 3% Fe loaded on bentonite clay was enhanced to about 83%.

Effects of leaching and additives on the ash fusion characteristics of high-Na/Ca Zhundong coal

Considering the intensive application of Zhundong coal and its serious slagging during combustion in pulverized coal fired boilers, the effects of water leaching and different dosages of additives (Na2CO3, CaO, SiO2, and kaolin) into the leached coal on ash fusion characteristics were studied using sintering tester, X-ray Fluorescence, and X Ray Powder Diffraction. After leaching, the contents of Na2O, K2O, CaO, MgO, SO3, and Cl decreased, whereas the contents of SiO2 and Fe2O3 increased. As a result, the slagging and fouling index of Basic/Acid ratio decreased from 1.6 in the raw coal ash to 1.1 in the leached coal ash, and alkali-induced slagging was reduced. However, the decreased ash fusion temperatures (AFTs) of the leached coal worsen the melt-induced slagging. Detailed analysis on the effects of ash compositions on the AFTs by additions of Na2CO3, CaO, SiO2, and kaolin additives into the leached coal showed that Na2CO3 and SiO2 improved the formation of low-melting Na2SO4 and Na2SiO3, and thus decreased the AFTs; However, overdosed Na2CO3 (for instance 9 wt%) improved the formation of relative high-melting Na4Al2Si2O9 and thus increased the AFTs. Overdosed SiO2 as a high-melting contaminant also increased the AFTs. Similar with the function of overdosed SiO2, CaO in the ash of leached coal was excess, and the AFTs increased with increased CaO dosage. Kaolin, which decomposed as high-melting mullite and cristobalite, caused an elevation of the AFTs. Therefore, water leaching decreased the AFTs of Zhundong coal and consequently worsen melt-induced slagging, lime and kaolin additives increased the AFTs effectively, but Na2CO3 and SiO2 additives showed ‘V’ shaped functions on the AFTs. These general results are useful for the pretreatment of Zhundong coal during combustion and alleviating slagging.

Na&Ca removal from Zhundong coal by a novel CO2-water leaching method and the ashing behavior of the leached coal

Zhundong coalfield in China is the largest intact coalfield in the world. However, serious fouling and slagging issues occurred when the Zhundong coal combustion, because of its high Na and Ca contents. In this study, a novel CO2-water leaching method was proposed to remove Na and Ca in Zhundong coal. The removal efficiencies of Na and Ca and the ash characteristics of the leached coal were investigated in detail. The results show that the CO2-water leaching can be easily achieved at room temperature under atmospheric pressure. The removal efficiencies of Na and Ca by the CO2-water leaching were significantly higher than those of water leaching. It is attributed to the effective removal of the organic Na and Ca in the coal. In addition, the CaCO3 in the coal was also effectively removed by CO2-water leaching. The volatility of Na from CO2-water leached coal is only 25% of that of raw coal during the combustion at 900°C. The alkali-acid ratio (B/A) of the CO2-water leached coal ash was significantly lower than those of the raw coal and the water leached coal ashes. The fusion temperature of the CO2-water leached coal ash significantly increased, compared with those of the raw coal and the water leached coal ashes. The deformation temperature of the CO2-water leached coal ash was as high as 1202°C, much higher than those of the raw coal and water leached coal ashes, which were 1098°C and 1142°C, respectively. Besides, the agglomeration and sintering phenomena of coal ash was also inhibited significantly by the CO2-water leaching. Therefore, the CO2-water leaching method was found to be an effective and promising method for AAEM removal from Zhundong coal, and can markedly mitigate the fouling and slagging during Zhundong coal combustion.

Two-stage coal flotation to remove pyritic sulfur, arsenic, selenium and mercury

This study focuses on partial leaching of coal pyrite with sodium sulfite in order to increase its degree of liberation from the coal matrix, which might help to increase the removal of coal pyrite, together with trace elements of arsenic (As), selenium (Se) and mercury (Hg) by froth flotation. The U.S. Bureau of Mines’ two-stage reverse flotation process was applied to further reject coal pyrite in the second stage, where pyrite was floated while coal was depressed. The residts show that the leached coal flotation with sodium sulfite produced lower pyritic sulfur content in the clean coal by approximately 25% compared to the flotation of unleached coal. The synergism mechanism between the leaching and the two-stage flotation is delineated. The rejection of trace elements (As, Hg and Se) after the two-stage flotation process with unleached coal followed approximately that of pyritic sidfur (64.6%). However, after the two-stage flotation with leached coal, the rejection ofSe was as high as that of pyritic sulfur (77.9%), while the rejections of As and Hg were 20% lower than that of pyritic sulfur, or followed approximately the same rejections obtained through the two-stage flotation with unleached coal.

Removal of Ash from Indian Assam Coking Coal Using Sodium Hydroxide and Acid Solutions

Mineral matter (ash) removal from Assam coking coal by leaching with different concentrations of sodium hydroxide and acid (HCl, H2SO4, HNO3, and HF) solutions has been investigated at a temperature of 75oC. The parameters tested were concentration of NaOH, type of acid, concentration of acids, and number of acid leaching steps. Total ash removed increased with increase of NaOH and acid concentrations up to the range studied. For the same experimental conditions, treatment of caustic leached coal in HCl acid resulted in better demineralization than in H2SO4 or HNO3 acid. In the NaOH-HNO3 leaching method, a higher concentration (> 20%) of HNO3 acid had an adverse effect on the de-ashing of coal. The NaOH-HF leaching process has been found to be the most effective method of coal de-ashing. The two acid treatment steps (HCl-H2SO4 / HCl-HNO3) after caustic leaching are the next most effective methods of coal de-ashing. The removal of mineral matter (including S) from coal is expected to decrease the graphite reactivity and thus the atmospheric pollution (due to the generation of smaller quantities of CO and SO2 gases).

Demineralization Study of Indian Assam Coking Coal by Sodium Hydroxide Leaching

The purpose of this study was to examine the ash removal efficiency of sodium hydroxide leaching from Assam coking coal so that it could be used for the manufacture of graphite electrodes. The parameters tested were sodium hydroxide concentration, panicle size of coal, NaOH solution / coal ratio, leaching temperature and time, and heat treatment of coal. Increase in NaOH concentration up to 500 gL −1 was found to increase the extent of ash removal. The ash content of the leached coal decreased with decrease in particle size, but very finely ground (− 72 mesh) particles reduced the ash removal rate. Increase in leaching temperature (50-120°C) and time (30-180 min) considerably reduced the ash content of the leached coal. Heat-treated (at 1300°C) coal (i.e., coke) has much lower ash removal efficiency than raw coal.

Influence of coal source and treatment upon indigenous microbial communities

The leaching of six Eastern coals was investigated using experimental coal columns subjected to simulated leaching events. Measurements of CO2 assimilation and specific enrichment cultures indicated that the microbial communities of all leachates were dominated by iron- and sulfur-oxidizing chemoautotrophic bacteria. Comparison of CO2 assimilation rates in leachates and core samples of leached coal indicated that most chemoautotrophs remained within coal columns during leaching. Mean numbers of chemoautotrophic bacteria in leachate samples were correlated with concentrations of dissolved iron and sulfate. Leachates from unwashed, run-of-mine coals contained more chemoautotrophs and more iron and sulfate than did leachates from washed, final product coals. After several leachings, the ratio of sulfur oxidizers to iron oxidizers tended to increase. These data suggest that the chemoautotrophic community of final product coals may be pyritelimited. Aerobic heterotrophs constituted a minor component of the microbial community in leachates from the six coals and their abundance and metabolic activity were apparently not influenced by the beneficiation history of the coal. Changes in rates of acetate metabolism may have been related to microbial succession within the heterotrophic community of coal columns. In all leachates, rates of tritiated methylthymidine assimilation were correlated with rates of acetate incorporation but not with CO2 assimilation, even though autotrophs dominated the microflora. Thus, thymidine assimilation rates appear to reflect activities or growth of mainly heterotrophic microorganisms in leachate.

Coal demineralization using sodium hydroxide and acid solutions

A coal product which exceeds the purity requirements for ash, iron and silicon in Hall Cell anode carbon was prepared by a unique leaching sequence involving caustic treatment, followed by treatments with sulphuric and nitric acids. The two acid leaching steps for the three-step process can also be combined with no adverse effect on the coal quality for anode purposes. In the process, the majority of the pyrite reacts with NaOH, forming Fe 2O 3 and Na 2S. Na 2S is dissolved in the leaching liquor and Fe 2O 3 is trapped in the coal matrix. The incorporation of sodium with organic sulphur increases the ash content and decreases the heating value of caustic leached coal on a dry basis. The silicon and aluminium contents are lowered by reactions with NaOH. While most of the reaction products are soluble sodium silicates and aluminates, the remaining silica and alumina form noselite. Dilute sulphuric acid dissolves nearly all of the sodium salts. The remaining iron compounds are dissolved by nitric acid.

A Kinetic study of leaching of coal pyrite with nitric acid

The leaching of coal pyrite with nitric acid has been investigated. The temperature ranged from 313 to 363 K, and the concentration of nitric acid was varied from 0.154 to 1.54 mol/l. A coal sample of 50 grams was leached in a reactor containing 500 ml of solution in an open system. It was observed that the leaching reaction could remove 47 pct of the pyrite sulfur in seven minutes and 88 pct in 30 minutes at 343 K with 1.54 mol/l of nitric acid. The reaction order with respect to hydrogen and nitrate ion activity was found to be first order. The activation energy for the initial stage of the reaction was determined to be 14.7 K cal/mol (61.5 kJ/mol). A mathematical model was developed on the basis of mixed kinetics (reaction zone model) to explain the leaching rates. Good agreement between experimental rate data and predicted rate curves by the developed model was obtained. Ultimate analysis was used to determine the extent of nitration of the leached coal. This nitration was found to be insensitive to the reaction temperature and acidity of the solution.