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Module 1: Produzione di energia da carbone

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Characterization of Coal

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Hi, friends now we will discuss on the topic characterization of coal. In introductorymodule we have discussed about the different properties of coal and we have seen that someof the properties are very important to fix its price. And as we are interested to usecoal as an energy source so heating value is very, very important. And this heatingvalue is dependent on carbon, hydrogen, oxygen, present in it and it also depends upon theash content, moisture content etc.So, in this class we will see what are the different methods that can be used to measurethe properties of coal like say what is the moisture content what is the fixed carbon?What is the volatile matter content? And what is the carbon hydrogen nitrogen sulphur oxygenpresent in coal? And ultimately we will see how the heating value is determined.So, the content of this class is the approximate analysis, ultimate analysis and heating valuedetermination. Now we will see what is proximate analysis? This is a standard method whichgives us the procedure through which we can analyze the moisture content of coal, ashcontent of coal, volatile matter content and fixed carbon.So, these 4 properties moisture, volatile matter, ash and fixed carbon are determinedby proximate analysis. So, how it is possible? we have to take small amount of sample andthen we will heat it under certain condition that is prescribed in the method, that is105 degree centigrade for 24 hour, then the mass loss is equivalent to moisture content.And after the loss of moisture the dry material if we put it in a crucible and cover it withthe lead and then put it inside a furnace at high temperature for small period of timethen the volatile present in the coal sample goes off and the difference between the initialmass and remaining mass will give us the measure of volatile matter present in the sample.In this case the condition is given as 950 degree centigrade for 7 minutes in absenceof oxygen that is why we have given the lead on it.Then we will take this material and heat in presence of oxygen so that the carbon, hydrogenpresent in it will be converted to CO2 and H2O so that carbon and hydrogen basicallyis from the volatile matters. During this process in presence of oxygen if we heat fora longer period at moderately lower temperature maybe that it is given here 750 degree centigradeand half an hour duration so that all this carbon and hydrogen will be converted to CO2and H2O and the remaining part which we will get that is nothing but ash content.So, after that we can determine the fixed carbon by the difference that is 100% minusthe percentage of ash minus percentage of volatile matter and minus percentage of moisture.Now we will see the volatiles fixed carbon as a moisture content of some materials likepeat, lignite sub-bituminous, bituminous and anthracite. So, from this table we see thevolatile matters is decreasing gradually with the increase of rank.Fixed carbon is increasing gradually but for moisture and ash there is no such specifictrend because as you know that volatiles and fixed carbon these are the inherent properties,these are present in the plant biomass and it is converted to coal through coalificationprocess. So, with the stage of maturity these two are changing, volatiles is reducing andthe fixed carbon is increasing as discussed in the introductory module.But moisture and ash these two are not the inherent property every time. So, it alsodepends upon the exterior external factors like say how we are handling the coal, howwe are storing it so all those things and where from we are getting it so all thosefactors, that is why there is no specific trained on it. Now we will see how to determinethe elemental composition of coal that is called ultimate analysis, sometimes it isalso called CHNS analysis, C for carbon H is for hydrogen N for nitrogen S for sulphur.So what is the mechanism for this analysis? what do you do in this case? the full sampleis combusted at higher temperature, the specific temperature is given here that is 990 degreecentigrade temperature, when we heat it the carbon is converted to CO2, H is convertedto H2O, N is converted to Nox, S is converted to Sox. So, now we are getting a mixture ofgas components.So, if we put the material in small container and then we apply oxygen and heat it here,then the gas will be available in this container. So, now we need one carrier gas so this carriergas will carry the gas and it will mix well the components and then we will use the methodologyfor the separation and identification of these gas molecules which is possible by using agas chromatography.But in this case we see we have carbon dioxide H2O, N2 and SO2 so if I want to separate weneed the variation in the retention period of the different gas molecules in the GC columnthat means the interaction of these gas molecules with the stationary phase of GC column shouldbe different that is why NOx is reduced to nitrogen prior to its entry into the GC column.So, one silica tube packed with copper granules is used in the path of this gas.So, carrier gas along with the components passes through this and NOx is converted tonitrogen by reduction then the mixed gas is going through this separation stage in theGC and detection stage. So, you see here, sample carrier gas, combustion, then reductionfor NOx reductions and then mixing, then separation and detection. So, these are the differentsteps and mechanism for the analysis of carbon, hydrogen, nitrogen, oxygen and sulphur.Now you see this figure is giving us different peak for nitrogen, CO2 H2O and SO2 becauseof the variation of the interaction of these gas molecules with the stationary phase ofthe GC column. So, retention period is different as nitrogen is inert so its retention timeis less then carbon dioxide, then H2O, then SO2. So, these peaks area gives us the relativeconcentration of these different gas components.So, we calculate how much carbon, hydrogen, nitrogen, sulphur is present in the originalsample. Now we will see some example of the ultimate analysis of different types of materialincluding peat, lignite, sub-bituminous, bituminous and anthracite. Here also the hydrogen contentis decreasing gradually, oxygen content is decreasing gradually, and carbon content isincreasing gradually. But for nitrogen, sulphur there is very less changes, 0.5 to 1.5 sothese small changes may be because of the limit of analysis or the instrument errorlimit.Here we see ash content also does not have any specific trend because it also dependsupon the external factors and the handling of the material. Now we are coming to heatingvalue so what is the heating value? that is most desirable parameter of the sample, weneed higher heating value so that we can get more heat. So, if we take a coal sample andthen combust it in presence of sufficient amount of oxygen then it will release Heat,C+O2, CO2.And if hydrogen is available H + O2 is H2O so these are the basic reactions which generatesheat and that heat we are interested to get in useable form. So, heating value is obviouslythe amount of heat energy released per unit amount of sample due to the combustion. Thenhere we can get two types of heating value as we have discussed in our introductory modulealso, one is your higher heating value or gross heating value and another is your lowerheating value or net heating.So, what these two are? after combustion we are getting flue gas and during the machineheat is released so flue gas is having high temperature it may be more than 600, 500 likethis degree centigrade. that high temperature of flue gas is used to produce steam fromthe water and then the steam will be available in different temperature as per the need ofthe application in downstream. So, one possibility is that the steam is condensed, now the fluegas which is produced to the combustion that is cooled down to atmospheric temperature.So the heat recovered from this flue gas will be the maximum that is called high heatingvalue and low heating value is the difference of the heat required to vaporize the wateravailable in the flue gas.So, low heating high heating value and lower heating value the difference is;We have to subtract the energy required to vaporize the water, now how this heating valuecan be expressed what is the unit of it? Unit may kilo Joule per kg, kilo Joule per moleor kilocalorie per kg, BTU per pound and there are some conversion factors kilocalorie perkg 2 mega Joule per kg if we want to convert this relationship we can use. So, people havetried to find out the relationship between high heating value and lower heating valueand it is found that HHV high heating value is equal to low heating value plus H v intonumber of H2O out by number of fuel.This number of means number of moles of water produced during the combustion and numberof moles of fuels so this divided by this n H2O per mole of fuel that will be the moleof water produced and if we multiply it with the heat of vaporization of water. So, thisamount of heat if we add with the lower heating value then that will give us higher heatingvalue. Now let us see how can you determine it.For the determination of higher heating value we use bomb calorimeter and bomb calorimeterhas been shown, in this figure you see this is the calorimeter and inside this calorimeterwe have water bucket and then in the water bucket we put bomb inside the bomb we putthe material in this crucible and then fix it and then with the electric devices we ignitethis one, the coal sample, and it combust inside it in presence of excess amount ofoxygen.Then it releases heat during the combustion and that heat is taken up by the water putin this water bucket as well as this bomb of the calorimeter. Now what is the principle?the amount of heat released due to the combustion of the material inside the bomb is taken upby the bomb material as well as the water kept inside the water bucket. So, this isthe principle for the determination of heating value in the bomb calorimeter.Now what type of heating value this is? high heating value or low heating value? obviouslyit is high heating value, why? because inside this bomb we are putting the material andwe are putting oxygen inside it in sufficient amount at higher pressure. So, we are assumingthat complete combustion is taking place and when the combustion will take place the localheat will be very high. But that heat is being released through this material of the bombwall the heat will also come to the water inside the water bucket.And there is some starring arrangement so there is some change in the water temperature.Now if the sample is very small so this variation in temperature may not be very high, it willvary within small range. So, what is happening during combustion? moisture is formed andagain it is cooled down to atmospheric temperature. So, that is why we are able to get completeheat released by the material that is why it is high heating value. So, from bomb calorimeterwe can determine high heating value.Now I will explain the steps. what we will do at first? we will take 1 to 1.5 gram ofmaterial in the crucible and then we will be using some thread or wire to ignite itand after ignition we will be getting the temperature change. How the temperature ischanging and what is the equilibrium temperature what is the difference between equilibriumtemperature and initial temperature. Then we will do the energy balance. So, how dowe do the energy balance?Here we have heat released by this equal to say we have taken M amount of material. soM into HHV heating value, that amount of heat is released and that is taken up by the calorimeter,that is equal to mass of calorimeter say M bomb mass, MB into CP or bomb MCP del T, thisis the heat taken up by the bomb and the heat taken up by the water that is W into CP winto del T. so this is the total heat taken up by the system.And this is the total heat released by the material. So, if we ignore any other typeof corrections then this is the basic formula. Now MbCp and this from this relationship wecan get Mb Cpb + w into C pw into del T this whole term is replaced by w that is waterequivalent of the calorimeter and this is the property of this material whatever materialit is taking and in short the properties of the calorimeter. So, we are getting w intodel T this is the heat gained by the calorimeter bombas well as the water body.So this is the basic principle and on basis of which we determine the HHV value excludingthe correction factors. But here when you are using the coal and we are using some wirefor the ignition, then that will also release some amount of heat so that correction isrequired. And if the coal contains some sulphur and nitrogen then that SO2 will be formedand then a SO2 will be converted to H2 SO4 or NO2 can be converted to HNO3.So these acid converts and they will also contribute some amount of heat in this processso that has to be excluded to determine the HHV of this sample. Now I will show you avideo which is self explanatory. (Video Start: 20:54)Which explain us how the bomb calorimeter is used to determine the heating value ofcoal or any other solid sample. So, now you see the crucible part is getting ready, thewire connection is being made. Now the sample is pulled into the crucible and the sampleis put inside the bomb and it is present with the lead. Then we will put oxygen inside itthe bomb is connected with the oxygen supply line from the oxygen cylinder.Now oxygen will go, it is started to go and set the bomb and pressure will increase, pressureis increased. So, this high pressure is basically needed to ensure that all the samples willbe combusted. Now the bomb will be put inside a bucket and water will be filled in the bucket.This video is made for demonstration purposes only, thewater has to fill so that the bomb will be completely immersed in the water.And then electricity connection is provided and the calorimeter is covered. The temperaturemeasurement device is inserted and studying arrangement is made. Now we have to provideelectricity inside it and this is the device which provides electricity and also measuresthe differential temperature. The temperature difference now I have to put auto zero sodifference has to be made zero first, now you see 0, gradually it will reach the equilibriumpoint.The temperature difference will be fixed, you see here 0.683 degree centigrade, thisis the equilibrium temperature difference that t1 - t2 the initial and final temperaturedifference we are getting point 683 it is now constant it will not change. So, thatway we can get the Delta t value. Now we are taking the recording, recorded, we are recordingthe value. (Video Ends: 28:17)So, this is the video for demonstration for the measurement of heating value using bombcalorimeter. So, from this video it is clear to asses, after the process we will be havingsome parameters or some values.Like say temperature at the temperature at time of firing and final maximum temperature.See in our case we do not have two different but we have del so you are already gettingthe difference in the temperature during the process. And then C 1 we get milliliter ofstandard alkali solution used in acid titration. So, what will be happening after the reaction?after the completion of this will release the oxygen from the bomb and will take thesample out, there will be no sample theoretically.But in some cases some soots may be available, in that case this run is discarded, not considered.Then it is washed, the wall is washed with water and that is titrated with alkali solutionto get the acid deposited on the wall of it and how much acid is formed. That way we gethow much how many milliliters of alkali solution is required that we can measure, and thenwe can get percentage of sulphur in the sample that we should have some idea about the elementalanalysis of the sample.And then what is the length of the wire which was fused during this process that we haveto determine initially we take 10 centimeter then after this process how much is left thatwe measure, the balance is used we assume. So that this value we get and mass of samplehow much sample you have taken that also it is known to us. Now, on the basis of thisinformation we can proceed further to determine the heating value of it.And as I discussed that without incorporation of any error the energy balance formula wehave discussed but now there will be some error.So, heat of formation of nitric acid heat of formation of sulphuric acid and heat ofcombustion of fuses so these three correction factors we have to consider and this H 2 SO4 this is basically we measure from the presence of sulphur in the sample and HNO 3 heat howmuch can be released that is measured on the basis of acid titration value the titrationvalue how much milliliter NaOH KOH or Na2 CO3 is used. heat of combustion of fuse wirealso is determined. And the supplier of this manufacturer of this instrument they providethe E3 value or corresponding how to calculate E3 value.So how can we calculate E 1 E 2 and E 3 ? E 1 it is nothing but dependent on the C 1 value,what is C 1? that is the milliliter of alkali solution used to get that titrate value ofthe wall wash of the bomb and then E 2 we can calculate by this formula 13.7 into C2 into M so C 2 is the percentage concentration of sulphur in the sample and M is the amountof sample I have taken in gram. And E 3 is measured through different equations for differenttypes of fuse wire used. So, nickel chromium if you use then this formula 2.3 into C 3.If we use iron 34 B and ash gauge iron then 2.7 into C 3 or if we use 34 gauge platinumwire then is equal to 0. So, these are provided by the manufacturer of the bomb calorimeter.Then how we will get the gross heating value? gross heating value into mass of the materialthat will be the heat released and that is equal to del T into W that is equal to waterequivalent into the temperature difference minus the amount of energy is released dueto the formation of nitric acid that is E 1. Due to the formation of sulphuric acidE 2 and due to the heat released by the combustion of the fused wire. So, this is the formulathrough which we can calculate the gross heating value. Then this w value that is the waterequivalent value of the bomb calorimeter that is the property of a calorimeter so we needto determine it. So, the same process is followed but in this case we use a known sample theheating value of the sample is known to us.So one such example is benzoic acid, so benzoic acid does not have any sulphur so G 2 componentwill be 0 so E 2 will also be 0 so in that case we will be getting W into T that is equalto H into M plus E 1 + E 3, E 2 is 0 see if we use this formula so now H is known heatingvalue of the benzoic acid is known, how much benzoic acid you have taken that is known,E 1 we can calculate from the titer value.And E3 we can calculate on the basis of the length of wire fuse wire used during the combustionin centimeter unit and we can calculate the W value. Then this way we can calculate grossheating value or high heating value then how can we calculate the net heating value?. So,some formula is given H in negative value is equal to 1.8 into H g that is high heatingvalue and gross heating value minus 91.23 into H where H is the percentage of hydrogen.So, this formula is in this unit BTU per pound but if we want to get it in kilocalorie perkg we have to convert it and this relationship we can use for the conversion of for the determinationof net calorific value. Then people try it to find out some relationship of these twohigh heating value and low heating value with the amount of hydrogen, oxygen and moisturepresent in it. So, this is one empirical relationship which is provided.So, what we have come to know on the basis of this discussion that to measure the heatingvalue we need to perform it limit. So, it requires time and investment of manpower andmoney also. So, for general purpose for the comparison of different types of coals wemay use some empirical relationships which have been reported by different researchers.And provided in this table different researchers they have provided different empirical relationshipfor different types of coal and some are based on approximate analysis some are based onultimate analysis basis. So, these will not be used to determine the actual calorificvalue of whole sample and to fix its price but it will be useful they will be usefulto screen coal samples when it is needed.If we have number of coal samples we can screen it that can be a suitable one by these applyingthis formula.Then I will give you some example so a coal sample with 1% sulphur is combusted in a bombcalorimeter. The temperature of the bucket water increases from 25 to 28 degree centigrade.The water equivalent of the calorimeter is 2402 calorie per degree centigrade 1 gramsample is used for the test and power 45 CT no wire is used for ignition. Out of the 10centimeter wire 2.6 centimeters are unused. To titrate the calorimeter washing 24.2 mlof 0.0709N sodium carbonate is required this is equivalent to N by 10 NOH.So, calculate the gross heating value of the waste consider the thermometers are workingperfectly. So, here there is no correction for thermocouple or thermometer is required.So, if we want to calculate the gross heating value obviously we will use the formula justwe have discussed. So this is the formula H g equal to T into W – E 1 – E 2 – E3 by M. Now we will see how can you get the value of E 1 E 2 E 3 in this case.And what is the M, T and W value. So, here we have W value is given, M value is giventhat is 1 gram and temperature difference T a and T f it is also given so differencetemperature we are getting now T is 3 degree centigrade, W is equal to 2402 calorie perdegree centigrade, E 1 how can you calculate? the value of E 1, E 1 is dependent of C 1,so C 1 is given how much it is given 24.2 ml.So, this will be E 1 will be 24.2 calorie then what is E 2? E 2 dependent sulphur contentso we have 1% sulphur so we will use the formula E 2 13.7 into 1 into how much sample you havetaken 1 gram so 13.7 calorie is our E 2 value and E 3 value is dependent on the wire whichwe have used that 7.4 centimeter power 45 C 10 wire so E 3 will be 2.3 into 7.4 equalto 17.0 calorie so we have to subtract this with the TW.S,o Hg is T into 2402 – 24.2 - 13.7 into – 17, so we are getting 7150 calorie pergram. So, this way we can measure the heating value. So, in this module we have discussedhow to determine the approximate and ultimate analysis. How to determine the concentrationof moisture, volatile matter, fixed carbon and ash and also how to measure the heatingvalue of the coal sample, so thank you very much for your patience.