Behaviors Of Minerals In Coal During The Processes Of Thermal Transformation

As a component of coal, minerals and other inorganic matters have an important impact on coal gasification and combustion. Although some minerals have catalytic effect on the processes of coal gasification, the problems regarding process safety, stability as well as pollution, which result from the deposition, slagging, fouling, and erosion caused by minerals and other inorganic matters, deserve more attention. With the development of the modern large-scale coal gasification technology, the influence of minerals on the process of gasification will be even more significant, especially at the late stages of the process in which high temperature and high pressure are typically employed for coal gasification. Coal is a heterogeneous mixture. The diversity of mineral components in various coal particles result in the unpredictability of ash chemistry at high temperature due to different transformation of minerals, causing slagging and fouling and giving rise to the problems in terms of process stability, safety and environmental pollution.Limited by the increasing concern about environment and the suitability of gasification technology, the utilization of coals with high ash content, high ash fusion temperature (HAHT) or with high alkali content has become a bottleneck. There are even more difficulties for the HAHT coal gasification using existing technology. In the processes of combustion and gasification, the slagging and fouling caused by alkali elements from the high-alkali coal (HA) are common problems. Therefore, seeking solutions for the utilization of these coals is currently the urgent task for industrial application.There are extreme cases for the contents of alkali metal and alkaline earth metal in HAHT coal and HA coal. In this article, the occurrence, distribution and thermal evolution characteristics of minerals in different coal particles from the representative types of Jincheng anthracite coal and high alkali coal (Xinjiang Zhundong sub-bituminous coal) were analyzed after float-sink experiments. The evolution mechanism of minerals from the coal particles with different density were revealed, which provide the basic theoretical support for industrial utilization. The change of sinters structures of Jincheng anthracite ash and high alkali coal (Indonesia coal) ash at high temperature were investigated and the effects of this change on its heat transfer efficiency were analized. The mechanism of wetting and corrosion of molten ash on the surface of refractory material were revealed, which lays a theoretical foundation for the industrial operation of gasifier. The main results obtained are as follows:1) The investigation of the composition of coal samples with different densities shows that the main portion of Jincheng anthracite corresponds to the coal sample with a density of 1.3-1.5 g/cm3 which accounts for 74.9 wt.% of the raw coal, while the D4 fraction(> 1.7 g/cm3) contains the main portion of raw coal ash(60.86 wt.%). The mineral contents (or particles) and the size of mineral particles increase with increasing density, elevating the possibility of the contact between particles.. Kaolinite exists in every coal sample and the heaviest coal sample has the highest content of quartz.2) Carbon residue has an important influence on sintering mechanism of ash particles from Jincheng anthracite. If the ash particle containing a large number of carbon residue, an appropriate amount of quartz and high concentration of ash accelerates sintering processes and the segregation of minerals of coal particles with different composition has important effect on the sintering and fusion of coal ash. The sintering process of coal ash is mainly dependent on the thermal evolution behavior of anorthite when the ash particle do not contain carbon residue, and the iron-enriched particles generated on the surface of the ash sinters could participate in and promote the sintering reaction with further increasing temperature, leading to the higher content of Si and Al in the iron-enriched particles and sintering worsening.3) The study on the mixed coals consisting of Jincheng anthracite and blending coals or fluxing agent shows that the content of coal plays an essential role in the AFT change of blended coals. Due to the heterogeneity of minerals in coal and the reactions of minerals at high temperature, the quantitative correlation between the decrease in AFT of mixed coal and the adding amount of blending coal remains uncertain. In general, the lower the ash contained in raw coal is, the easier the AFT of blended coal changes. The addition amounts of blending coal needed to considerably decrease the AFT are 20% and 30%(wt%) for coal C(low ash) and coal G(high ash), respectively. The fluxing effect of K for coal C is stronger than that for coal G. There are noticeable characteristic peaks of feldspar minerals at the deformation temperature. The decreasing contents of mullite as well as appreciably increasing amount of feldspar are the main reasons for the decline of AFT. The thermodynamic analyses indicate that the production of liquid matter is related to the rapidly decreasing contents of anorthite and quartz together with slowly decreasing content of mullite at high temperature. The melt of anorthite contributes to the generation of liquid matter. The fluxing effect of anorthite is a function of the contents of matter which react with anorthite to produce melt with lower fusion point.4) In order to figure out the existence state and thermal transformation properties of AAEM, the coal samples of S and W were separated into different density fractions by float-sink experiments with organic heavy liquid. The BSE-EDX and XRD were employed to analyze mineral matter of coal samples. The soluble AAEM in coal with different density were examined by fractional extraction with deionized water, ammonium acetate solution and hydrochloric acid solution. The mineral and chemical compositions of coal ash samples were determined by XRD and XRF. The results show that the coal sample with a density of 1.4～1.5 g.cm-3 is the major portion of S and W. The matteuccite and kalicinite were found in the coal samples with lower density. The content of soluble AAEM increases with increasing density of S coal. In addition, the main soluble AAEM species in S coal encompass water soluble Na, water and ammonium acetate soluble K, and ammonium acetate and hydrochloric acid soluble substance of Ca and Mg. When it comes to W coal, ammonium acetate soluble substance is the primary constituent of soluble AAEM for K, Ca and Mg, while Na exists mainly in the form of water and ammonium acetate soluble substance. The contents of soluble Na and K are higher in low density fraction than that in high density fraction, which is contrary to the case of soluble Ca and Mg. There was abundant Na contained in low density coal fraction at lower temperature. The existence state of soluble sodium in different density fractions is crucial for its volatility at low temperature.5) Three typical coal ashes (i.e., X, J and A) with various compositions were employed to clarify the conductive properties of coal ash depositing on the heat transfer or inner wall of reactor. The dependence of the thermal conductivity of sintered coal ash on the chemical composition, micro-structure, and morphology was systematically investigated using XRF, XRD, SEM-EDS and thermal conductivity analyzer, etc. Results show that the thermal conductivity and bulk density are positively correlated with temperature, and the increasing trend varies according to the chemical composition. The thermal conductivities and bulk densities exhibit a linear correlation for the same sintered ash. At high temperature, the existence of refractory minerals, such as mullite and quartz, give rise to relatively small changes in physical and chemical structure for acid ashes (X/J), which causes a slow increase in sintering rate and thermal conductivity. On the other hand, the formation of fluxing minerals, such as clinopyroxene and anorthite, brings about a large variation on bulk density for basic ash (A), which leads to an increased thermal conductivity.6) The interfacial properties between three types of coal slags (A, J1 and J2 with various base/acid ratio) and two refractory materials (mullite and corundum) were explored.The results indicate that the wertability of slag A on both two refractory materials is good. In contrast, the slags J1/J2 would experience a gradual process from partial wetting to complete wetting. The contact angle decreases with increasing interface width of slag and refractory materials (S-R). The contact angle of Jl on mullite decreases with increasing inclination angle from the deformation temperature to 1520℃, while significantly increasing when the inclination angle reaches20°. The SEM-EDS analysis shows that the quantity of slag A permeating into the refractory material is higher than that of J1 slag because of interfacial reaction. The penetration and acid-base reactions are the key factors for the refractory corrosion.