Article pubs.acs.org/EF
Ash Fusion Characteristics of a High Aluminum Coal and Its Modification Fenghai Li*,†,‡,§ and Yitian Fang§ †
Department of Chemistry and Chemical Engineering, Heze University, Heze, 274000, People’s Republic of China School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, People’s Republic of China § State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, People’s Republic of China ‡
ABSTRACT: High aluminum coal (HAC, Al2O3 in its ash content ranges 38−51%) is abundant and plays an important role in energy and chemical product in China. The ash fusion characteristics of HAC and its modification behaviors by coal blending or flux were investigated by ash fusion temperature (AFT) analyzer, X-ray fluorescence spectrometry, X-ray diffractometry combined with normalized reference intensity ratio method, and scanning electron microscopy. The AFT variation is manly dependent on mineral composition and its transformation. A large amount of high melting point (MP) mullite formation results in HAC (Sunjiahao coal, SJH) AFT high. Compare with the coal blending of Shenmu (SM) and Xiangyuan (XY), CaO addition provides a better way to adjust ash fusion characteristics of SJH. For coal blending, that calcium and iron element in low AFT coal react with high MP mineral and then evolve into eutectic and amorphous matter makes the AFT of SJH mixtures decrease. The formations of low MP anorthite, and gehlenite, and their eutectic resulting from the reaction of CaO and mullite lead to a decrease in the AFT.
1. INTRODUCTION With the increase of environmental pressure (e.g., air pollution, ozone depletion, acid rain, global warming, and their effects on the eco-system1−3), gasification technologies, with superior environmental impact4,5 have been developed quickly in many countries.6,7Among three gasifiers (fixed-bed,8 fluidized-bed,9 and entrained-flow bed), entrained-flow bed gasifier has been considered as a promising technology with the advantages of flexibility in feedstock and product, high thermal efficiency and carbon dioxide capture, and a high quality syngas with low tar content due to its operation on higher temperature with smaller coal particles.1,10,11At a high temperature, the difference of organic matter gasification reactivity is small, the behavior of ash fusion and flow becomes an important factor that affecting the stable operation of gasification.12 Continuous slag tapping is the key to successful operation on entrained-flow bed gasifier,2 e.g., the viscosity of 2.5−25 Pa·s is required at the temperatures from 1300 to 1500 °C for Shell gasifier. The ash fusion temperature (AFT) of coal partially reflects ash sintering and agglomeration, which strongly influences ash deposition and slag formation in the gasification.13−15Although some shortcomings exists, the AFT test is still a acceptable way to predict coal slagging tendency during its conversion (e.g., gasification, combustion).3 Generally speaking, when coal are used in the industrial entrained-flow gasifier, it is required that its FT < 1380 °C, its ash content ranges from 18 to 22%, and its ash viscosity 1400 °C) accounts for 57% of Chinese retained coal reserves.20Two methods are generally used to decrease coal AFT: flux addition and coal blending.21,22Coal blending provides a superior feedstock for slagging gasifier because of there is no increase in ash content and oxygen consumption, which is necessary for flux addition.23,24 Among Chinese high AFT coal reserves, high aluminum coal (HAC, Al2O3 in its ash content ranges from 38 to 51%) is abundant, nearly reaches one hundred billion tons, and mostly is distributed in Northern China, especially in Xinjiang Autonomous Region, Inner Mongolia, and Shanxi Province. HAC will play an important role in future energy and chemicals supply in China. To utilize HAC cleanly and high efficiently, catalytic gasification reactivity of HAC char was investigated in the Institute of Coal Chemistry (ICC), Chinese Academy of Sciences (CAS).25 Lots of aluminum minerals (e.g., kaolinite and boehmite) were found in the HAC,26 which resulted in the total content of Al2O3 and SiO2 in its ashes >85%. And coal AFT generally increases with the increase of its alumina content.27 Thus, HAC usually has a high AFT (its FT even reaches 1600 °C25), which limits its application in entrainedflow bed gasifier obviously. However, published papers have been done to modify the AFT of HAC are still lacking. The objectives of the work were to investigate the ash fusion characteristics of HAC and to explore its AFT modification behaviors by coal blending or flux addition. It is expected that the work might provide some Received: February 3, 2016 Revised: March 13, 2016 Published: March 29, 2016 2925
DOI: 10.1021/acs.energyfuels.6b00285 Energy Fuels 2016, 30, 2925−2931
Article
Energy & Fuels
their AFTs tests.30 The ash samples (1.0 g) were put into AFA and heated to the preset temperature. Then they were taken out and crushed to a particle size 1500 >1500 >1500
1115 1140 1170 1237
1100 1110 1121 1147
a
DT deformation temperature; ST softening temperature; HT hemispherical temperature; FT flow temperature.
high (DT, ST, HT, and FT are all higher than 1500 °C, respectively). While the AFTs of SM and YM are relatively low (ST, 1150 °C or so), belonging to typical low AFT coal, and slag might be formed during their conversion (e.g., combustion,36 gasification in fixed-bed or fluidized-bed4), which might lead to the shutdown of system. Thus, SM and YM were selected as coal blending to decrease SJH AFT. The ash compositions of three samples analyzed by XRF-1800 are presented in Table 3. SJH is a typical HAC for Al2O3 content is very high (43.28%). While for SM and XY, the total contents of CaO and Fe2O3 in their ashes are >40% (SM 44.57%; XY 48.56%), both are typical coals of high iron and calcium. Among ash composition, the acid compositions with high ionic potential are prone to form polymer and make AFT increase, while basic compositions serve to terminate the formation of
Figure 1. Modified schematic diagram of ash fusion point determination meter: (1) gas valve, (2) mass flow meter, (3) hydrogen gas cylinder, (4) carbon dioxide cylinder, (5) computer, (6) mixed chamber, (7) round bearing, (8) corundum tube, (9) carborunbum tube, (10) aluminum boat, (11) ash cone, (12) thermoelectric couple. 2926
DOI: 10.1021/acs.energyfuels.6b00285 Energy Fuels 2016, 30, 2925−2931
Article
Energy & Fuels Table 3. Ash Compositions and B Values for Three Coal Samples composition (wt %) constituent
SJH
SM
XY
SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O TiO2 P2O5 A/B valuea
41.33 43.28 6.75 4.38 1.09 1.24 0.23 0.66 1.01 0.22 6.53
34.06 14.32 16.95 27.62 1.88 0.97 3.08 0.22 0.62 0.28 0.99
31.09 13.29 20.36 28.20 2.92 0.95 2.14 0.15 0.74 0.16 0.83
a A/B value = (SiO2 + Al2O3 + TiO2)/(Fe2O3 + CaO + MgO + Na2O + K2O).
polymer and lower the AFT.37,38 A good correlation was found that the AFT of coal increases with its ash A/B ((SiO2 + Al2O3 + TiO2)/(Fe2O3 + CaO + MgO + Na2O + K2O)) ratio. The A/ B values for SJH, SM, and XY are also shown in Table 3, which decrease in the order: SJH (6.53) > SM (0.99) > XY (0.83). This might explain the AFT differences of three coals. 3.2. AFT Variation of Mixed Ashes by Coal Blending. Figure 2 shows the AFT variations of SJH mixture with the increase inmass ratios of SM and XY. For SM and XY, the AFT change trends of SJH mixture were similar, it decreased slightly with the mass ratio increased (0−30%) and dropped obviously with adding coal increase from 40 to 60%. However, even the blending coal mass ratio reached 60%, the FTs of mixed ashes did not decrease to meet the requirement of entrained-flow bed gasifier (
