Article pubs.acs.org/EF
Investigation on the Regulation Mechanism of Ash Fusion Characteristics in Coal Blending Fenghai Li,*,†,‡,§ Hongli Fan,‡ and Yitian Fang§ †
Department of Chemistry and Chemical Engineering, Heze University, Heze, Shandong 274015, People’s Republic of China College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan 454003, People’s Republic of China § State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, People’s Republic of China ‡
ABSTRACT: Blending coal is a promising way to adjust ash fusion characteristics for coal with a high ash fusion temperature (AFT). Ash fusion characteristics of coal samples and their variation behaviors through coal blending were discussed. The regulation mechanism was investigated by an X-ray powder diffractometer with the assistance of normalized reference intensity ratio software and a scanning electron microscopy analyzer equipped with energy-dispersive X-ray spectrometry. The results show that the variations of ash fusion behaviors were diversified in blending coal type and the mass ratio. It was found that the optimized blending mass ratio was in the range of 30−50% for three blending coals to meet Liangbaoshi (LBS) coal gasification in an entrained-flow bed gasifier (EFB). A very high melting point (MP) mullite formation at a high temperature makes the LBS AFT higher. With an increasing blending coal mass ratio, the formations of low-MP hercynite, anorthite, and gehlenite, their low melting eutectics, and these contents increase, resulting in the AFT decrease of mixed ashes. The content of an abundant amorphous matter increases with a low-AFT coal mass ratio increase, also leading to the AFT decrease.
1. INTRODUCTION The power generation through the combustion of fossil fuels has brought serious environmental problems (e.g., serious global warming, climate change, and their influence on the ecosystem1).2 Although, recently, power generation from renewable and clean sources (e.g., solar, wind, and biomass) has been developed quickly, the technologies extracting clean energy from fossil fuels are still important.3 Nowadays, gasification technology is gaining more attention worldwide in the field of power generation and chemical synthesis,4−8 because of its better controlled heating, higher efficiency in removing pollutant or corrosion, and higher power production efficiency than combustion.9,10 Coal gasification provides a wide range of applications, such as hydrogen, methanol, dimethyl ether (DME), liquid fuels, synthetic natural gas (SNG), integrated gasification combined cycle (IGCC) plants, combined heat and power (CHP), etc.11,12 Coal can be considered as a livelong energy resource because it is abundant and widespread throughout the world.13 IGCC power plants are an efficient method to produce power with the characteristics of coal clean utilization and low energy consumption, which are becoming a major power generation in abundant coal resource countries.14,15 Among three gasification technologies [fixed bed, fluidized bed, and entrained-flow bed (EFB)], an EFB is the most widely used in IGCC power plants because of its wide flexibility in feedstock and high carbon conversion rate.16,17 During the process of coal EFB gasification, organic components convert into syngas and minerals transform into molten slag, flow along the wall of the gasifier, and discharge out from the slag outlet.18,19 Thus, the viscosities and flow behaviors of ash/slag under gasification conditions have become key factors for successful EFB operation.20−23 These characteristics of ash/slag © 2016 American Chemical Society
have close relations to the ash fusion temperature (AFT), because the AFT partially indicates the sintering, fusion, and flow characteristics of ash/slag, which strongly influence the ash agglomeration and slag formation.24−26 It is also fundamental to understand the slag formation mechanism during gasification and to propose its prevention method.27 Thus, although some shortcomings exist, the AFT is still an accepted way to estimate coal slagging propensity during its conversion.28 Consequently, the AFT is confirmed to be an important factor for the design and operation of EFB. As a result of the thermal characteristics of refractory materials of the gasifier (EFB equipped with refractory materials in its inside) or the viscosity−temperature properties of ash/slag (mainly for EFB with a water membrane wall), an ash flow temperature (FT) of 1400 °C), which accounts for more than 57% in Chinese coal resources.30 To discharge slag from the gasifier continuously during high-AFT coal EFB gasification, different coals and additives (flux and refractory) must be added to improve the fusion/flow characteristics and keep it within design specification.31 Additives are widely used in industrial practice, but this method consumes more oxygen and more energy.16 However, blending coal not only improves ash viscosity− temperature characteristics but also is more economic and efficient for raw material complementary and product structure optimization.32 Thus, blending coal has become an important developing direction of coal gasification in EFB. To blend coal gasification in EFB, 20% ash content or so, FT of 1500 1237 1207 1153
>1500 1256 1230 1189
atmosphere of 50% carbon dioxide and 50% hydrogen. The volatile matter contents for all coals are more than 20% and belong to typical bitumite. The ash content of LBS is relatively low (∼15%), while those of ZL, YM, and XY are comparatively high (>20%). LBS belongs to high-AFT coal, while the AFTs of ZL, YM, and XY are low. 2.2. Preparation of Ash Samples. The mixed ash samples were prepared as follows: ZL, YM, and XY coal samples were put into highAFT LBS at a certain mass ratio (0, 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50%) and mixed until reaching uniformity. The mixed coals were transformed into a muffle furnace and ashed according to the Chinese procedure (GB/T1574-2001). The ash samples were made in the AFT analyzer to keep the same condition as that of the AFT measurement.37 First, the ashes (1.00 g) were put into a ceramic crucible, and the crucible was inserted into the AFT analyzer. Then, the mixed gases of 50% hydrogen and 50% carbon oxide were introduced into the AFT analyzer to replace air and keep a reducing atmosphere.38 When the per-setting temperature was reached, the ash samples in the muffle furnace were transformed into ice water quickly to prevent phase variation and crystal segregation. These quenched samples were first set in a vacuum drying oven at 105 °C for 24 h and stored in a cabinet dryer before measurement. 2.3. Measurements and Analytical Method. Sample AFTs were tested on the ALHR-2 AFT analyzer under a reducing atmosphere (1:1 H2/CO2, volume ratio) according to the Chinese standard (GB/T 219-2008). Ash compositions were analyzed on an X-ray fluorescence (XRF) spectrometer (XRF-1800, Shimadzu, Japan), with a relative standard deviation (RSD) of 0.6%. Mineral compositions of samples were performed on a RIGAKU D/max-rB X-ray powder diffractometer (XRD) with Cu Kα radiation (40 kV, 100 mA, and Kα1 = 0.154 08 nm), and the scanning speed was 2θ 5°/min with the step size of 2θ 0.01° in the range of 2θ between 10° and 70°. The mineral contents in ashes were conducted by semi-quantitative normalized reference intensity ratio (RIR). This ratio is the pure phase intensity divided by that of a standard material (Al2O3). The amorphous matter content and composition were achieved by subtracting each oxide aggregate percentage for each crystalline phase from the bulk chemical compositions of respective ash samples. Its precision is estimated as ±10% for strongly diffracting phases and ±25% for weakly diffracting phases.27,39 The composition of amorphous matter was calculated by subtracting the contents of crystalline phases from bulk ash composition.40 The surface morphologies and elemental compositions were investigated on a JEOL JSM-7001F scanning electron microscopy (SEM) analyzer equipped with a Bruker Quantax 200 energy-dispersive X-ray spectrometry (EDX) unit. The fine ash samples were placed on conducting glue carefully and coated with gold vapor to observe the sample microstructure clearly. Although the data value has 0.01 wt % precision as a result of EDX unit retestability, the elemental concentration is only semi-quantitative. 2.4. Thermodynamic Equilibrium Calculations. The multiphase equilibria module in FactSage 7.0 with databases of FACTPS and FToxid was used to calculate the amount of liquid phase of hightemperature ashes under a reducing atmosphere (1:1 H2/CO2, volume ratio). The calculation was conducted from 1100 to 1600 °C, with the interval of 50 °C. The mixed ash compositions, including Al2O3, SiO2, Fe2O3, CaO, MgO, and K2O, were input for the thermodynamic equilibrium calculations.
Table 1. Proximate and Ultimate Analyses of Coal Samples ZL
ST
AFT, ash fusion temperature; DT, deformation temperature; ST, softening temperature; HT, hemisphere temperature; and FT, flow temperature.
2.1. Characteristics of Coal Samples. Four air-dried coal samples were provided by the Institute of Coal Chemistry (ICC), Chinese Academy of Sciences (CAS). They were listed as follows: Liangbaoshi bitumite and Zhaolou bitumite (Shandong, eastern China), Yima bituminte (Henan, central China), and Xiangyang bitumite (Hubei, central China). The coal samples were crushed to CaO > MgO > Fe2O3 > Na2O.42 Therefore, it is reasonable to classify SO3 as a basic composition. High ionic potential acid compositions (e.g., the ionic potentials of Si4+ and Al3+ are 9.5, and 5.9, respectively) are inclined to form the polymers and make the AFT increase, while low ionic potential basic compositions (e.g., the ionic potentials of Mg2+, Ca2+, Na+, and K+ are 3.0, 2.0, 1.1, and 0.75, respectively) serve to terminate the polymer formation and decrease the AFT. Thus, the B values (the total amounts of SO3, CaO, MgO, Fe2O3, Na2O, and K2O) of coal samples in Table 3 might be used to predicate the AFT. The B value decreases in the order LBS < LZ < YM < XY. Moreover, from LBS, LZ, YM, to XY, the increases of the CaO content and SiO2/Al2O3 ratio and the decrease of Al2O3 can obviously be seen. These might explain the AFT difference of the four coal samples. 3.2. AFT Variation of the Mixture by Coal Blending. Figure 1 shows the AFT variation of mixed LBS with an increasing mass ratio of different coals, respectively. As seen that the effects of the three low-AFT coals on the AFT of mixed LBS are almost the same, the only difference in the AFT of mixed LBS decreases obviously for YM (0−10%) compared to those for SM and YM. For three blending coals, with their mass ratio increase, their AFTs decrease obviously (10−30%) and then decrease slowly (30−50%). The decreasing range of their mixture AFT is in the order of XY > YM > ZL, which results from the B value differences in three coals. For ZL and YM, when the mass ratio reaches 30%, the FTs of their mixture decrease below 1380 °C, while it is only 25% for XY as a result of its high contents of calcium (CaO, 29.62%) and iron (Fe2O3, 18.95%). The ash contents of LBS with three coals at different mass ratios are shown in Table 4. Moreover, to form the molten slag protection layer on the wall of the EFB gasifier, about 20% ash content is generally required. Thus, it can be concluded preliminary from Figure 1 and Table 4 that the appropriate coal blending mass ratio of more than 30% is necessary for three coals. 3.3. Mineral Transformation Behaviors of Mixed Ashes. 3.3.1. Mineral Transformation in LBS with an Increasing Temperature. The XRD patterns of LBS ash samples at different temperatures (900, 1000, 1100, 1200, and 1300 °C) are presented in Figure 2. At 900 °C, the mineral matter is mostly in the form of quartz (SiO2), hematite (Fe2O3), and anhydrate (CaSO4). At 1000 °C, the formation of magnetite (Fe3O4) results from the reduction of hematite, and mullite (Al2O3·SiO2) is generated as a result of two reactions.27,43
The reaction of magnetite and calcium oxide (decomposition of anhydrate) leads to the formation of calcium iron oxide at 1100 °C, and hercynite (FeO·Al2O3) results from the interaction of wustite (FeO) and mullite.28 Anorthite emerges at 1200 °C because of the following reaction: mullite (3Al 2O3 ·2SiO2 ) + calcium oxide (CaO) → anorthite (CaO·Al 2O3 ·2SiO2 )
At 1300 °C, hercynite and anorthite mostly transform into amorphous matter, indicating by the distorted baselines at 2θ 15−30° in XRD patterns. 3.3.2. Mineral Composition Comparison for Mixed Ash with Different Coal Blending. During the ash fusion process, the minerals interact and fuse into a liquid, resulting in the variation of the mineral components and their contents.44 Thus, the components and amounts of crystal in ashes at a certain temperature can be used to predicate the fusibility characteristics. To explain the AFT variation difference for mixed LBS ashes with different blending coals at the same mass ratio, the mineral composition characteristics of mixed LBS ashes at 1200 °C are investigated by XRD. When the mass ratio reaches 30% for three coals, the FT of mixed LBS ashes all decrease to
