High-calcium Coal Research Articles

Experimental study on reactivity and inorganic component transformation of activated fuels in a fluidized bed

The gasification and combustion of activated fuels produced from fluidized bed are beneficial for achieving clean and efficient coal utilization. In this study, a high-calcium coal was used for the activation process, carried out in a high-temperature vertical fluidized bed. The carbon and ash characteristics of activated fuels were studied. The reactivity of activated fuels was characterized using Raman test, and scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS). Inorganic components were characterized using X-ray diffraction (XRD), and X-ray fluorescence spectrometry (XRF). With the increase of temperature and equivalence ratio (ER), the graphitization degree of activated fuels decreases, and a higher proportion of active sites leads to, a better activation effect. The activation effect is optimized at the equivalence ratio of 0.45. As the temperature rises, the calcium-containing minerals in the raw coal are gradually transformed into anorthite (CaAl2SiO7), and the anhydrite (CaSO4) reacted with the reducing gas (CO) to produce oldhamite (CaS); Fe2O3 as a fluxing agent, is prone to melting with silica-aluminates at high temperature. As the particle size of activated fuel increased, the relative enrichment index (REI) of heavy metals decreases.

Effect of High Calcium and High Iron Coal on Ash Fusion Characteristics of Petroleum Coke during Cogasification.

Entrained flow gasification provides a more efficient utilization method for high-sulfur petroleum coke. The operation temperature of the entrained flow gasifier must be above the ash fusion temperature (AFT) of petroleum coke due to the liquid slag discharge. In this work, petroleum coke was blended with high-calcium coal and high-iron coal, respectively, under a reducing atmosphere, and the variations in AFTs were recorded by an ash fusion temperature analyzer. The influence of mineral transformations on the ash fusion characteristics of blended ash was analyzed by X-ray diffraction and FactSage. The results showed that both high calcium coal and high iron coal could efficiently reduce the AFTs of petroleum coke. When the ratio of high calcium coal and high iron coal reached 60 wt %, the corresponding flow temperature (FT) of mixed ash decreased to 1225 and 1312 °C, respectively. With the content of high calcium coal increasing, coulsonite (FeV2O4), vanadium trioxide (V2O3) and nickel (Ni) with high-melting points tended to decrease, causing the decrease of AFT for mixed ash. As high iron coal was added, Ni and V2O3 continuously kept decreasing. In particular, the percentage of FeV2O4 first increased and thereafter decreased with high iron coal above 40 wt %.

Ash fusion behavior modification mechanisms of high-calcium coal by coal blending and its ash viscosity predication

In this paper, the ash fusion temperature (AFT) modification of high calcium Huolinhe lignite (HLH) with the addition of high aluminum Datong coal (DTC) and their corresponding mechanisms were investigated by an AFT tester, X-ray diffractometer, Raman spectrometer, and FactSage software. The AFTs of HLH mixtures increased gradually with the increasing DTC mass ratio due to their corresponding decrease in the total basic oxide content of CaO, MgO, Fe2O3, Na2O, and K2O. The formations and content increases of high melting-point minerals (anorthite, cordierite, and mullite), the gradual decrease in the ratios of no bridging-oxide bond/bridging-oxide bond (NBO/BO), and an increase in the R (R = (Q3 + Q2)/(Q1 + Q0)) value resulted in the increases in the AFT and ash viscosity. Considering the variations in the characteristics of AFT and ash viscosity, the 40.0–50.0 % DTC mass ratio might be suitable for HLH entrained-flow bed (EFB) gasification. The combination of AFT measurement and ash viscosity predication using FactSage provided a simple method to determine whether ash characteristics of coal were suitable for EFB gasification.

Impact of inherent calcium in coal on the structure and performance of activated carbon in flue gas activation: The enhanced mechanism of calcite on the methylene blue adsorption

To investigate the correlation and mechanism of inherent calcium species in coal on the preparation and performance of activated carbon (AC) when using flue gas activation. In this study, XCaCO3/AC (X means calcite contents in coal, 0–8 wt.%) was successfully produced by mixing different contents of calcite and demineralized Taixi coal and then using flue gas activation. The textual evolution and methylene blue (MB) removal of AC were conducted by a series of characterizations and tests, and revealed its mechanism. Coal with a high content of calcium could result in a low yield of AC. Calcite could transform into calcium oxide. During AC preparation by flue gas activation, calcium oxide also could react with CO2 and transform into calcium carbonate. Calcite in coal reduced the degree of AC's graphitization. The calcite in coal promotes the increment of CO, COO, and π-π* of AC linearly as the calcite content increases. Due to the excessive catalytic effect of calcite, high-content calcium could result in inferior porosity through promoting the collapse of pore topology. Calcite in coal strengthens the methylene blue (MB) removal capacity of AC. The 5CaCO3/AC possesses the utmost MB adsorption capacity, up to 118.8 mg/g, which is higher by 122.9% than AC of 53.3 mg/g. The various kinetics models were also verified and data was well matched to Pseudo second-order kinetics (R2=0.9165 for AC and R2=9729 for 3CaCO3/AC). Calcite in coal increases the content of CO and π–π* of AC, which strengthens chemical adsorption by functional groups complexation and π–π* stacking. Meanwhile, the existence of calcium species, OH− and CO32− is beneficial for ion exchange and complexation, respectively. Thus, the removal ability of MB by AC is enhanced. The results of this investigation could cast novel light on the high-performance AC design by utilization of high-calcium coal, and further multiple pollutants treatment.

Controlling the ash adhesion strength of zhundong high-calcium coal by additives at high temperature

Herein, ash adhesion strength has been investigated via a new self-designed tester for solving the severe ash deposition problems of Zhundong high-calcium coal. The results indicate that the ash adhesion strength of Zhundong high-calcium coal appears at 800 °C and increases with the temperature rising. The ash adhesion strength increases rapidly at about 1100 °C, and reaches 184.933 kPa at 1200 °C. Above 800 °C, the melting of sulfate results in the ash adhesion strength of Zhundong high-calcium coal. As the temperature increases, feldspar minerals, such as gehlenite and akermanite, form a low-temperature eutectic, causing greater ash adhesion strength. The ash adhesion strength is positively correlated with ash shrinkage. Kaolin and vermiculite can effectively inhibit the formation and melting of sulfate and decrease the ash adhesion strength of Zhundong high-calcium coal. At 1200 °C, kaolin and struvite can increase the content of minerals with high melting temperature and decrease the ash adhesion strength. Kaolin additive is more suitable for the Zhundong high-calcium coal combustion below 1200 °C and can effectively reduce the ash adhesion strength by 55.9% at 1200 °C. Vermiculite additive is well suited for circulating fluidized bed boilers and can almost eliminate the ash adhesion strength below 1000 °C. Struvite additive only shows good ash adhesion strength inhibition above 1000 °C.

Impact of gas impurities on the Hg0 oxidation on high iron and calcium coal ash for chemical looping combustion.

Coal-based mercury pollution from power plants has received increasing attention. In a previous study, high iron and calcium coal ash (HICCA) was found as a promising oxygen carrier (OC) for chemical looping combustion (CLC). The purpose of this study was to investigate the catalytic effect of HICCA on Hg0 removal as well as the impacts of several gas impurities, such as HCl, SO2, and NO. Experiments on Hg0 removal efficiencies for different atmospheres were performed in the fixed-bed reactor at 850°C. Based upon the characterization of BET, SEM, XRD, XPS, and EDS of reaction products, the reaction mechanisms of different gases with the HICCA samples were established. The mechanisms were further explained using the thermodynamic equilibrium calculations. The experimental results showed that the Hg0 removal efficiency using HICCA was 11.60%, while the corresponding value in the presence of 50ppm HCl was 90.46%. Hg0 removal by HICCA involving HCl is mainly attributed to homogeneous reaction between Hg0 and HCl as well as the formation of reactive species (Cl, Cl2, Cl2O, O, S, and SCl2) through the reactions of HCl with Fe2O3 and CaSO4 in HICCA. The formation of C-Cl bond is not the main pathway for the promotional effect of HCl on Hg0 removal. SO2 played a negative role in Hg0 removal by HICCA. The inhibition of SO2 may be attributed to its effect on the reduction of Fe2O3 and its bonding with C-O, COOH, and C(O)-O-C. NO enhanced Hg0 removal by HICCA primarily through the homogeneous reactions of Hg0 with N2O and O. In addition, NO also interacted with HICCA and promoted the heterogeneous oxidation of Hg0 by producing more C-O, C=O, and COOH/C(O)-O-C on HICCA surface. This study proved the effectiveness of HICCA on Hg0 removal in iG-CLC and revealed the mechanisms of the interaction between HCl/SO2/NO and MxOy/CaSO4 as well as carbon-oxygen groups.

Indispensable role of inherent calcite in coal on activated carbon (AC)’s preparation and applications

Calcite (CaCO3) is the main calcium-containing mineral in coal, activated carbons (ACs) were obtained through blending Alxa coal with different content of CaCO3 (2, 4, 6, 8%), molding, carbonization at 600 °C and activation at 900 °C under water–vapor atmosphere. As-derived ACs were investigated physiochemical structure and performance by XRD, SEM, BET, XPS, SO2 and rhodamine-B (RhB) removal test. The results show that: (1) CaCO3 promotes the development of AC’s physiochemical structure and surface chemistry. CaCO3 is beneficial to pore evolution through etching carbon surface, which causing the evolution of micropore/mesopore to mesopore/macropore and/or pores collapse. The total specific surface area and micropore’s specific surface area has decreased from 895 m2/g and 674 m2/g to 671 m2/g and 564 m2/g with CaCO3 addition up to 8%. The total volumes and micropore’s volumes (<2 nm) also decreased from 0.40 cm3/g and 0.33 cm3/g to 0.31 cm3/g and 0.24 cm3/g, respectively. The O/C and π-π* are increased linearly with the addition CaCO3; (2) CaCO3’s addition promotes sulfur capacity through enhancement of pore structure, oxygen-containing functional groups and π-π*. Moreover, the CaO in ACs participates in the desulfurization process and finally forms CaSO4, which further promotes desulfurization, and the mechanism was proposed; (3) CaCO3’s addition slightly decrease the RhB adsorption capacity from 96.7 mg/g to 87.9 mg/g due to competition of OH− with COO−. The results of this work indicate that AC prepared from high-calcium coal is more suitable for desulfurization other than RhB removal.

Investigating the Effect of Flux on Ash Fusibility of High-Calcium Coal.

The primary aim of this study is to understand the effect of metal oxide flux on the fusibility of high-calcium coal ash. Based on the decomposition rate, the evolution of mineral matters in high-calcium coal has been investigated. The ash fusion temperatures of samples are measured by adding different flux Al2O3, Na2O, K2O, MgO, and TiO2. The results show that Na2O is the most effective in lowering ash fusion temperatures and its flow temperature could be 110 °C lower than that of the original ash. FactSage is used to calculate the proportion of solid phase and the mineral compositions as a function of the ash compositions and temperature. With the increase of Na2O, mineral matters with a low melting point form in the mixture. Furthermore, the decomposition rate of mineral matters increases in the first stage. The phase diagrams and relative mineral variation illustrate that the mineral and the decomposition rate variations are the main reasons for the change of ash fusion temperatures.

Investigation into the flow behavior of high-temperature ash and low-temperature ash of high calcium coal

The flow behavior of high-temperature ash (HTA) and low-temperature ash (LTA) of high calcium coal in the heating process was investigated systematically. By means of the heating stage microscope, the behavioral changes of samples were studied visually. The composition and mineral matters transformations of HTA and LTA samples were analyzed by X-ray fluorescence and X-ray diffraction. The results show that the original composition of HTA sample is different from that of LTA sample. In addition, the HTA and LTA samples experience the shrinkage, fusion and spreading processes in succession. However, the volume change of HTA sample is different from that of LTA sample. The volume of LTA sample shows a slow change at the temperature lower than 800 °C, while the volume of HTA sample is unchanged. In the temperature range of 800°C–1100 °C, the remarkable shrinkage of HTA and LTA samples are demonstrated. The formation of srebrodolskite and gehlenite attributes to this volume change. Moreover, the sharp shrinkage of HTA and LTA samples is indicated at 1100°C–1300 °C. This is caused by the formation of eutectics. Because of the diverse content and species of mineral matters in LTA sample, the volume change of LTA sample is more remarkable than that of HTA sample. The maximal shrinkages of LTA and HTA sample are 57% and 53%, respectively.

Regulation of ash fusion behaviors for high aluminum coal by high calcium coal addition

ABSTRACTTo meet the requirement of liquid slag tapping entrained‐flow gasification, two kinds of high ash fusion temperatures (AFTs) coals with significant difference in ash chemical composition (high aluminum coal: Al2O3 &gt; 38%, Sunjiahao coal, and Shajuzi coal; high calcium coal: CaO &gt; 35%, Hebi coal, and Xinjiang coal) were selected. The ash fusion behaviors of blending coal were investigated. The results showed that blending coal could significantly reduce the AFTs of high‐AFT coal. The formation of high melting point minerals led to the increase of AFTs (high aluminum coal: mullite, corundum, and cristobalite; high calcium coal: lime and periclase). Xinjiang coal reduced AFTs more obviously than that of Hebi coal because of the differences in the total amount of alkali oxides, Fe2O3 and MgO. The effect of CaO on AFTs was determined by the amount of CaO. CaO, Fe2O3, and MgO promoted the formation of low‐melting eutectic [anorthite, gehlenite, spinel, pyrope, and ringwoodite (Fe‐rich)] and made AFTs low when the mass ratios of high calcium coal were low. On the contrary, the high content of CaO increased AFTs. FactSage7.0 software is a very effective way to explore the effect of mineral transformation on AFTs.

Calcium-Bearing Minerals Transformation during Underground Coal Gasification

Calcium-bearing minerals are one of the main typical minerals in coal and coal ash. In the process of coal thermal conversion, calcium-bearing minerals undergo different morphological transformation in which the reaction temperature, pressure, and atmosphere are important factors affecting their transformation. The reaction process of underground coal gasification (UCG) could be clearly divided into pyrolysis, reduction, and oxidation and the typical calcium-bearing minerals are expected to indicate the actual reaction conditions of UCG. A high-calcium coal, Zhundong coal, was used in this research. The products of UCG were prepared and the minerals were identified by X-ray diffraction (XRD) and a scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic calculation was used to assist in understanding the transformation behaviors of calcium-bearing minerals. The experimental results show that the calcium-bearing mineral is gradually converted from gypsum (CaSO4·2H2O) in the raw coal into anhydrite (CaSO4) during the pyrolysis process. In the reduction stage, anhydrite reacts with the reducing gas (CO) to produce oldhamite (CaS), and the oldhamite is stably present in the reduction ash. During the oxidation process, oldhamite is first transformed into CaSO4, and then CaSO4 is converted into CaO. Finally, CaO reacts with Al2O3 and SiO2 to produce gehlenite (Ca2Al2SiO7) at 1100 °C. As the oxidation temperature rises to 1400 °C, gehlenite is transformed into the thermodynamically stable anorthite (CaAl2Si2O8). With the further progress of the reaction, anorthite will co-melt with iron-bearing minerals above 1500 °C. The ternary phase diagram of SiO2–Al2O3–CaO system proves that anorthite and gehlenite are the typical high-temperature calcium-bearing minerals when the mole fraction of SiO2 is higher than 0.6. Moreover, the gehlenite is converted to anorthite with the temperature rise, which is consistent with experimental results. This study provides a scientific basis for understanding the UCG reaction conditions.

High Iron and Calcium Coal Ash As the Oxygen Carrier for Chemical Looping Combustion

The potential of Zhundong’s coal ash as a novel oxygen carrier (OC) in chemical-looping combustion (CLC) was evaluated in a thermogravimetric analysis (TGA) under both the CO/Ar and air atmospheres. A single reduction–oxidation cycle was carried out in the TGA reactor to determine the optimum reaction temperature, which was 1123 K. The cyclic stability and agglomeration tendency of ash also was evaluated by consecutive redox cycles. On the basis of the kinetic analysis, the apparent activation energy of ash was about 26–29 kJ/mol and the reduction process of ash was divided into three stages. The reaction control steps for these stages were gas diffusion, diffusion and kinetics, and kinetics, respectively. Besides, the reactions in the first and second stages corresponded to the reduction of Fe2O3 to Fe and CaSO4 to CaS. In the third stage, the reaction occurred between the CaS and CaSO4.The products of the equilibrium reaction between CO and ash were obtained using Factsage (software), while the transiti...

A Novel Method for CO2 Sequestration via Indirect Carbonation of Coal Fly Ash

Coal fly ash is a potential candidate for CO2 mineral sequestration. If calcium is extracted selectively from coal fly ash prior to carbonation (namely indirect carbonation), a high-purity and marketable precipitated calcium carbonate (PCC) can be obtained. In the extraction process, recyclable ammonium salt (i.e., NH4Cl/NH4NO3/CH3COONH4) solution was used as a calcium extraction agent in this study. The influence of time, temperature, agent concentration, and solid-to-liquid ratio on calcium extraction efficiency was explored. NH4Cl/NH4NO3/CH3COONH4 are confirmed to be effective calcium extraction agents for the high-calcium coal fly ash investigated, and about 35–40% of the calcium is extracted into the solution within an hour. The calcium extraction performance is best for CH4COONH4, followed by NH4NO3 and NH4Cl. Increasing temperature from 25 to 90 °C and agent concentration from 0.5 to 3 mol/L only subtly increases calcium extraction efficiency for NH4Cl and NH4NO3, while the positive effect of incre...

CO&lt;sub&gt;2&lt;/sub&gt; Sequestration by Indirect Carbonation of High-Calcium Coal Fly Ash

The use of coal fly ash for CO2sequestration has been proposed as a promising option for its utilization. However, research is quite necessary for advancing this technology. Indirect carbonation of high-calcium coal fly ash for CO2sequestration was investigated in this study. In these processes, calcium was first extracted from a high-calcium coal fly ash sample with NH4Cl solution. The obtained leachate was subsequently carbonated by bubbling CO2. It was shown that NH4Cl could extract about 35% of the total calcium into the solution under the conditions investigated. The dissolution of calcium is nearly accomplished in half an hour. Further increasing the extraction solution temperature (10-90°C) and the concentration of NH4Cl (0.5-3mol/L) only has subtle positive effects on the calcium extraction efficiency. The carbonation efficiency of the extraction leachate, namely the percentage of the calcium in the solution transformed into calcium carbonates is about 47%. The sharp drop in pH after bubbling CO2due to weak acid buffering capacity of the solution hinders the further precipitation of Ca2+. Calcium carbonate samples with a purity of up to 97% are obtained, meeting the purity requirements for industrial use.

Evaluation of the Effect of Municipal Sewage Sludge Compost and Furnace Waste on the Quality of Amur Silver Grass Miscanthus Sachariflorus Biomass

Abstract A single-factor field experiment was carried out at the Cultivar Evaluation Station in Szczecin-Dabie in 2008-2010. The soil on which this experiment was set up is formed from light loamy sand (lls). In respect of granulometric composition, it is classified to the category of light soils, of soil quality class IV b and good rye complex. In the experiment, compost produced with municipal sewage sludge by the GWDA method was used. This compost contained clearly more nitrogen and phosphorus in relation to potassium. The content of heavy metals (Cd, Cu, Mn, Ni, Pb and Zn) in composts did not exceed standards of the Regulation of the Minister of Agriculture and Rural Development (Official Journal of Laws No. 165, item 765 of 2008) referring to organic fertilisers. In the study design, the following fertilisation treatments were applied: I - carbonate lime (CaCO3) at a dose of 1.5 Mg CaO · ha-1, II - high-calcium brown coal ash at a dose of 1.5 Mg CaO · ha-1, III - municipal sewage sludge compost at a dose of 250 kg N·ha-1, IV - municipal sewage sludge compost at a dose of 250 kg N · ha-1 + high-calcium brown coal ash at a dose of 1.5 Mg CaO · ha-1 (1st year of study), V - high-calcium brown coal ash at a dose of 1.5 Mg CaO · ha-1 (1st year of study), and 0.75 Mg CaO · ha-1 in following study years each, VI - municipal sewage sludge compost at a dose of 250 kg N · ha-1 + high-calcium brown coal ash at a dose of 1.5 Mg CaO · ha-1 (1st year of study), and 0.75 Mg CaO · ha-1 in following study years each. In addition, mineral fertilisation was applied annually in the form of multi-component fertiliser Polifoska 20, complex fertiliser Polimag S and ammonium nitrate. A test plant was perennial grass - Amur silver grass (Miscanthus sachariflorus). The obtained results show that Amur silver grass biomass contained on average the most nitrogen, ie 6.87 g·kg-1 d.m., in 2008, while the most phosphorus (0.39 g P·kg-1 d.m.), potassium (7.82 g K·kg-1 d.m.), magnesium (0.98 g Mg·kg-1 d.m.) and sulphur (1.19 g S·kg-1 d.m.) in 2010, whereas the most calcium ie 4.13 g Ca kg-1 d.m., in 2009. Significantly more nitrogen, calcium and sulphur was contained by Amur silver grass biomass from the objects where municipal sewage sludge compost had been applied without and with addition of high-calcium brown coal ash when compared to calcium carbonate or high-calcium brown coal ash being applied at a dose of 1.5 Mg CaO · ha-1. Differences in average phosphorus, potassium and magnesium contents in test plant biomass from particular fertilisation objects were not significant. The biomass of Amur silver grass contained significantly more cadmium, nickel, lead and zinc as affected by organic fertilisation without and with addition of high-calcium brown coal ash when compared with the objects where solely calcium carbonate or high-calcium brown coal ash had been introduced into soil. Differences in the average content of cadmium, nickel and zinc in test plant biomass from the objects fertilised with municipal sewage sludge compost without and with addition of high-calcium brown coal ash were not significant. The uptake of heavy metals by Amur silver grass biomass, ie its mean value of three harvest during three years of its cultivation, can be arranged in the following descending order of values: Zn &gt; Mn &gt; Pb &gt; Cu &gt; Ni &gt; Cd. The degree of cadmium, copper, manganese, nickel, lead and zinc bioaccumulation in test plant biomass differed, depending on the fertilisation applied. The average degree of cadmium, nickel, lead and zinc accumulation after three study years was intense for all fertilisation objects, whereas average for copper and manganese.

Increased Leaching of As, Se, Mo, and V from High Calcium Coal Ash Containing Trona Reaction Products

The trona (trisodium hydrogendicarbonate dihydrate) injection method is being tested to control SOx emission from coal-fired power plants. A pair of moderately high calcium coal fly ashes, with and without trona injection, were used to determine the impact of trona injection on the leaching characteristics of anionic elements, including As, Se, Mo, and V, under different pH conditions. The results showed that trona ash (trona reaction products captured with fly ash) leached significantly more As, Se, Mo, and V than the control fly ash did, especially under the natural pH condition. In addition, trona ash had a significantly greater soluble fraction than the control ash. Further investigation indicated that greater amounts of the studied anionic elements in the trona ash were associated with the soluble, trona reaction products. Moreover, the insoluble fraction of the trona ash lost its capability to adsorb these elements under the natural pH condition. The competition from other major anions in the trona ...

Microstructure and Mechanical Performance of Belite Cements from High Calcium Coal Fly Ash

The microstructure and mechanical performance of two low energy fly ash belite cements (FABC‐1‐W and FABC‐1‐N) obtained from high calcium‐coal fly ash as raw material is studied in this work. The FABC were obtained after a previous hydrothermal treatment of the fly ash in demineralized water and NaOH 1M solution. The hydration of both FABC‐1‐W and FABC‐1‐N during 180 days was followed by thermal analysis, X‐ray diffraction, FT infrared spectroscopy, scanning electron microscopy and Brunauer‐Emmett and Teller‐N2 surface area measurements. The mechanical properties were studied under different curing conditions: normal (21°C and &lt;95% RH) and accelerated (40°C and &lt;95% RH). Porosity and pore‐size distribution were also studied. The results showed a better mechanical performance for FABC‐1‐N when the curing was normal and a better mechanical performance for FABC‐1‐W when the curing was accelerated.

Iron sulfide deposition during coal gasification

Previous analysis of ash generated during coal gasification using a high calcium coal (SUFCo) suggested that iron sulfide was the dominant sulfide species in the system. During a recent analysis of ash material removed from a gasification pilot unit using Pittsburgh No. 8, a high iron coal, sub-μm to 5 μm euhedral iron sulfide crystals were identified on the outer surface of the ash. X-ray diffraction suggests both pyrrhotite and troilite were present in the ash. No other sulfides were identified. Within the iron sulfide crystals were siliceous fly-ash particles. Possible modes of formation which include vapor deposition, accumulation and annealing of fume particles, or recrystallization of iron sulfide droplets were investigated.