| Blending of power coals is a reasonable way to utilize the coal in our country. Ithas great economic and social benefits. The relationship of coal quality indexesbetween blended coals and raw coals was systematically studied in the paper. A morecomprehensive and detailed work was also included to indicate the effects of mineralsin coal on coal’s combustion characteristics and kinetics. The research can providecertain theoretical support for accurate quality prediction of blended coals from rawcoals, and practical guidance for coal-fired boilers running safely and stably.Some typical power coals in China were selected and analyzed by modernanalysis techniques as Coal petrography, SEM, FTIR and XRD. The result shows, theminerals in coal can be classified as inherent minerals and external minerals. Theexternal minerals present an individual distribution in the form of monomer or fiineparticles or crum while the inherent ones were tightly wrapped by organics, otherinherent minerals or external minerals. The main minerals in coal are all but quarts,calcite, kaolinite, pyrite, and some trace compounds of natrium and kalium.The paper shows that the Mad, St,ad, and Aadof low sulfur content blended coalsare linear additive, with high linear correlation coefficient of0.930,0.997,0.990, anderror qualification rate of95.0%,85.0%,90.0%, respectively. The main elements incoal of Cad, Had, Nadare also additive for the reason that the blending process ofdifferent coals is only a physical superposition, and their error qualification rates areseparately83.3%,91.7%,91.7%. Other indexes such as Ad, Vad, Vdaf, Qb,ad, Qgr,ad,Qnet,ad, and Aadof high sulfur content blended coals show a nonlinear additiveproperty. This is because when higher sulfur content coals are used for blending, thesulfur-containing minerals and its pyrolysis product as SO3will make sulfur-retentionreactions with alkaline components, CaO, for example, and produce the sulfur-fixingproduct CaSO4left in coal ashes, causing the nonlinear relationship of blended coalsand raw coals. A strong non-additive property was also demonstrated by the volatileof blended coals for the coal type change and affects of heat and mass transfer involatile release process.Great influence of minerals on coals’ calorific value has been displayed in theresearch. Compared with the allowed error of120J/g by national standard for coal’scalorific value, the inherent minerals in coal result an unqualified maximum error of1170J/g, with a rather low error qualification rate of43.0%. It has been indicated that the thermal decomposition reaction of inherent minerals itself and co-melting withother ones during coal combustion are the principle reasons for the nonlinearrelationship of blended coals and raw coals. All the external minerals make thecalorific value of blended coals decrease linearly as their content rising, and with therank: calcite> kaolinite> quarts> pyrite. The thermal transition behaviors of mainminerals in coal are found as: Quarts to Tridymite or Cristobalite, Kaolinite toMullite, Calcite to Calcium oxide, and Pyrite to Hematite.The influence of inherent minerals on raw coals and blended coals was founddifferently in the process of volatile release. In the former stage that guided bytemperature, the inherent minerals presents good thermal conductivity, thus makingthe volatile release point of high-rank raw coals and blended coals decrease. However,this is not suitable for the latter stages that the inherent minerals mainly present asobstacles during the release of volatile, and reduce the maximum release rate, raisethe peak temperature. For low rank coal of lignite with rich pores and lowerdecomposition temperature, the inherent minerals delay its volatile release point from289.4degrees celsius to333.1degrees celsius, showing an inhibition effect due to itsendothermic effect and heat conduction block during the former stage. But in thelatter stages, the inherent minerals mainly display a good heat conductivity, andreduce the peak temperature by9.2degrees celsius. The high-low rank blended coalsshows the same rule as high-rank coals in this point. All the external minerals exert annegative impact on the volatile release of raw coals and blended coals after thethermal influence of themselves are excluded.Combustion characteristics of raw coals and blended coals were also investigatedin detail. Inherent minerals increase the ignition point and burnout temperature ofhigh-rank raw coals and their blended coals, and lower the burning conversion rateand combustion rate, against coal’s combustion and burnout. But lignite displaysconversely because of the good thermal conductivity of inherent minerals. It shouldbe noted that the blended coals of anthracite and lignite shows a unstable and stagedcombustion process, presenting the combustion characteristics of lignite andanthracite in pre-combustion and post-combustion respectively. The externalminerals make a maximum reduce to the ignition point of blended coals by21degrees celsius at addition level of10%. Within addition amount of50%, theignition point of blended coal rise in proportion to the increase of calcite, quarts, andkaolinite. However, neither of them is higher than the original ignition point349 degrees celsius. All the three external minerals reduce blended coals’ burningconversion rate and combustion rate, and represent a delayed burnout temperaturewith worse burnout degree, blocking the blended coals’ combustion.Finally, the result of combustion kinetics reveals that, the combustion reactionorder of coal varies from0to2, not typical1, because of the inherent minerals. Anincreased activity energy range of4.34KJ/mol~48.21KJ/mol was also observed ofraw coals and blended coals. Additionally, the activity energy of blended coalsshows an growing tendency as the addition amount of external minerals increasingwithin50%, with calcite making the biggest increase of16.18KJ/mol. Both theinherent and external minerals increase the combustion activity energy of blendedcoals, and impede its combustion and burnout. |
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