Energy Saving and Pollution Reducing Effects of Coal Combustion Catalysts

TSINGHUA SCIENCE AND TECHNOLOGYISSN1007-021405/12pp156-159Volume 6, Number 2, June 2001Energy Saving and Pollution Reducing Effects ofCoal Combustion CatalystsWU Zenghua(武增华), YU Zhiwu(尉志武), ZHU Wentao(朱文涛), ZHOU Rui(周蕊)Department of Chemistry, Tsinghua University, Beijing 100084, ChinaAbstract: Coal catalytic agents (CCS type) have been prepared to improve coal combustion and reduce airpollution. The energy and pollution reductions resulting from the catalysts have been examined with thermalnalysis and chromatography. the cCs agents lower the ignition temperature by 30-80 c and improve thecoal combustion efficiency by 10%-25%. The agents also reduce the release of carbon monoxide, sulfurdioxide, and coal particles to environment. the working mechanisms of the catalysts are discussed in terms oftheir participation in various physico-chemical processes during combustion.Key words: coal; catalytic effect environmental protectionIntroductionare based on chemical principles of coal combustionand should have the potential to lower the ignitionCoal has been the main energy supply throughout temperature, to facilitate heat and matter transfermodern China history and will remain so for at within the material, and to reduce pollutionleast another fifty years. Although efforts have simultaneously. The additives are often referred asbeen made to convert coal to non-solid fuels such coal combustion catalysts. This paper continuesas alcohol-based fuels, coal is still used primarily our previous paper which investigated thefor direct combustion in China and many other effects of individual alkali salts and alkaline earthcountries,but two main problems remain salts on the ignition temperature of coalunsolved The first problem concerns thecombustion efficiency and the generation of a1 Materials and Methodsvariety of gaseous pollutants. At present about The present investigation mostly used Taixi coal85% of the air pollutants in China arise from direct from Ningxia Hui Autonomous Region of Chinacoal combustion. The second problem is the The general efficacy of the additives on the coalextremely high ignition temperature of some coals. combustion was investigated using other raw coalsResearch is focused mainly on new designs of from different areas of China, Datong mixed coalcombustion equipment. However, this has the Shanxi Province ) Jinzhou coal Liaoningdrawbacks of expensive investments and long Province), Yongan coal (Fujian Province ) andconstruction periodsYibin coal (Sichuan Province). The compositionsEfforts have also been made to solve the of the coals, i. e, the carbon content, volatileproblem chemicallyThe objective is to matter, ash level, and moisture, are listed inimprove the combustion efficiency by introducing Table 1. Among them, Taixi coal and Yongan coalnts of chemical additives without have the highest carbon contents and Jinzhou coalupgrading or replacing old furnaces. The additives has the lowest. The Yongan coal is unusual in thatits low volatile content makes it the most difficultReceived:1999-05-12; revised:2000-05-20coal to ignite. The grain size of all the coal samplesLaboratory of Cl Chemical Technology中国煤化工 eferred to CCSCNMH Gture of inorganicalkalieSr, and Ba)andWU Zenghua(武增华)etal: Energy Saving and Pollution Reducing Effects of Coal…157transitional metals (Mn, Fe, Ni, Cr, Zn, and lowered the ignition temperature. The bestCu)lo, Four mixtures were used in this studydditive concentration was 2%(mass fraction),i. e, CCS-1, CCS-2,CCS-3, and CCS-4. Raw at which the ignition temperature was reducedcoal was added to solutions containing the CCs from 565c to 500-510C with cCS-1 ands and sufficient time was allowed for CCS-2homogenization. The solution was dried in air atThe combustion enhancers can also influence120℃the combustion process behavior. The burn-up ofTable 1 Composition of the Chinese coals usedTaixi coal and other coals in the presence andin this study(% absence of CCS-1 is shown in Fig. 2.ThreeCarbon Volatile matter Ash Water features can be seen clearly from the burn-up DTA0112.301.54curves in the presence of CCS-1 when comparedDatong mixed coal 52.47 28. 10 18. 00 1.43with the raw coals, i.32.4134.8931.651.05temperatures, lower end temperatures, and largerYongan copeak areas of the combustion events. As shown inYibin coal61.304.05 18.80 Fig. 2(a), the Taixi coal initially burned very10.0727.101.53slowly without the additive. A peak temperatureA thermobalance(WCT-1, Beijing AnalyticalWith 1.5%CCS-1Instruments, China) was used for the differentialthermal analysis (DTA)study. The tg curveswere used to determine ignition temperaturesWithout CCS-1Identical experimental conditions were used for allthe measurements with a heating rate of 50 C/minand air flow rate of 100 mL/min. The sample sizewas(5.8=+0. 1)mg. The structural morphology of(a) Taixi coalthe ash was examined using a scanning electronwith 1.5%CCS-lmicroscope(Hitachi $-450)ithout CCS-12 Results and Discussion2. 1 Characterization of combustionIgnition temperature is one of the most importantT/℃properties in coal combustion. The data in Fig. 1(b) Datong mixedshows the effect of the four CCS agents on theignition temperature of Taixi coal. The datawith 1.5% CCS-1clearly demonstrate that the addition of CCs agentwithout cCs-1T/℃540CCS-3With 1.5%CCS-1CCS-2Without CCS-1CCS-l%中国煤化工Fig. 1 Effect of the additive CCS on the ignitionCNMHGtemperature of Taixi coalg2 Errect of CCS-1 on the burn-up behavlour158Tsinghua Science and Technology, June 2001, 6(2): 156-159of 720c was recorded, indicating extensive 750C indicates the early completion of theburning of the material. The addition of 1. 5% combustion process at that temperature,inCCS-1 accelarated the burning process at a much comparison with about 850 C without CCS-1earlier stage with the most extensive burning attemperatures around 670 C. The end temperatureAbsence of cCS-was also lowered. The larger peak area of thedta curve in the presence of CCS-1 indicates morecomplete combustion after addition of thePresence of CCs-1enhancerSimilar effects of CCS-1behaviour of coal were also observed using Datong●mixed coal (Fig. 2(b)),Jinzhou coal (Fig. 2(c))1000and Yongan coal (Fig. 2(d)). The ignition and endT/℃temperatures of the four coals in the presence anda)Carbon monoxideabsence of CCS-1, as determined from the TGcurves, are listed in Table 2. These measurementsdemonstrate that the CCS agents improve the coalAbsence of ccs-1burning process by lowering the ignitiontemperature and increasing the heat release.Industrial tests showed that CCS-1 was able toimprove the combustion efficiency by about 10%Presence of CCS.and to reduce coal consumption by 10%-25%. Forthe same amount of coal, the enhancer raised thefurnace temperature by 60-100 c(b)OxygenTable 2 Effect of CCS-1(1.5%)on the ignition and endtemperatures of different raw coalsFig 3 Concentrations of carbon monoxide and oxygenin the exhaustof Taixi coalRaw coal With CCs-1 Raw coal With CCS-1Sulfur-containing gases, such as SO2, are theTaixi coal 560main source of acid rain, CCS agents were found toJinzhou coal 520reduce the release of such gases by bindingchemically with SO2, in addition to their ability toimprove the combustion characteristics. The sulfur2.2 Air pollution reductionreduction ability of CCs agents was studied using amixture of equal mass of Taixi coal and Yibin coalRelease of carbon monoxide, sulfur dioxide, and containing 1.74% sulfur. The data in Table 3micro particles to the environment during coal illustrates the sulfur-removing effect of CCS-1combustion causes air pollution. In addition, the added to the coal mixture. More than half of thegeneration of carbon monoxide is an indicator of sulfur was removed when only 5%CCS-1 wasincomplete combustion. The concentration added to the coal mixtureequivalents of carbon monoxide and oxygen in theexhaust gases with or without CCS-1 in Taixi coalTable 3 Sulfur-removing effect of CCS-1 at 900 Cclearly show that the addition of CCS-1CCS content/%1significantly reduced the generation of carbon Sulfur-removing(%) 28 36 42 48 52monoxide, Fig, 3(a). This is in agreement withthe results in Fig. 2, where the larger peak areasThe addition of CCs agents to raw coal notwith the additives implied the more complete only reduced the level of carbon monoxide in theburning of the coal. The oxygen level in the exhaust gas, but also reduced the release of carbonexhaust gas which can be used to monitor the micro中国煤化工nt, as evidencedburning process of coal in situ n is shown in Fig 3 by thCNMHGexhaust gas An(b). For CCS-1 mixed with Taixi coal(1.5%), examplell ruK. 4 iur Taixi coal. Thethe sharp increase in the oxygen level at about decreased soot actually results from the completeWU Zenghua(武增华)etal: Energy Saving and Pollution Reducing Effects of Coal…combustion of the coallow as200℃a.Sometransitional cations, for example Fet, are electronacceptors due to their empty" d"orbital, while O2and CO are electron donors, which may help ozand CO adsorb on the additive surfaces and thuscatalyze the further oxidation of co to COThe CCS agents are also able to increase theevolution of volatile materials. Zhang et alreported that some salts such as Nacl can catalyzethe splitting of organic compounds during thethermal decomposition of coal and thus canFig. 4 Blackness of coal gases from raw Taixi coalincrease the release of volatile components(A)and Taixi coal mixed with 1% of CCS-1(B)Because the volatile materials have lower ignitiontemperatures and can be more completely burned2. 3 CCS agent mechanismshe catalytic property of the additive salts makesThe morphology of the coal ash was examined tothem combustion enhancers. In addition, theunderstand the working mechanisms of the CCSadditives may also improve the thermalagents used in this study. Electron microscopicconductivity and mechanical properties of coalphotos of the ash from the combustion of taixiwhich will also promote complete combustion ofcoal and its mixture with 1%of CCS-1 are showncoal at lower temperatures. Finally, metal oxidesin Fig. 5. Numerous micropores can be seen clearlyformed from CCS agents during combustion canon the ash sample when CCS-1 was used. This is react with sulfur to form sulfate so as to reduce theelease of sulfur to the airbelieved to be due to the effect of additives on thecoal microenvironment. After thorough mixing Referenceswith the coal, the additive material was spreadover the coal particle surface. Along with the [1] Lu Yansun. Current situation of coal combustion andincreased temperature, active oxygen ( oxygenits improvement policies. Coal Conversion, 1992atoms)will be released from the oxides such as the15(1):1-5.(in Chinesenitrate salt, which will increase the oxygen [2] Wagner R, Muhlen H J. Effect of a catalyst oncombustion of char and anthracite. Fuel, 1989concentration in the microenvironment and, as a68(2):251-253result, will facilitate combustion reaction[3] Reuther JJ, Daley R D, Warchol J J, et al. 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