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Module 1: Gassificazione del carbone

    Study Reminders

    Hi friends, now we will discuss on the topic cleaner route for energy production from coal.In the last class, we have discussed on the conventional route for energy production fromcoal and we have seen that combustion based thermal power plant is the most conventionalroute for energy production from coal.And in this process, coal is burnt in a furnace and then flue gas which is having high temperatureis passed into the boiler for the production of steam, and then the steam is used in turbinefor electricity production.Now, here we will discuss what are the possibilities to make more cleaner energy than the conventionalone, because in the conventional process, we have seen that the emissions are higher,the SOx, NOx and CO2 emission are higher.So, if we want to reduce the emissions, what can be the possible routes, that part we willdiscuss in this class.As you know prevention and control these are two important philosophy for the managementof something or to achieve our target quality.So, in this case, we can try to prevent the emission level or we can take measures tocontrol the concentration of the pollutants present in the flue gas by its cleanup.So, one is our prevention that means, that is related with feedstock that is the coalwhich we are using we may clean it or during the process of combustion when the flue gasis forming at that time, we can take some modification in the process, so, that we canget less emission through this process.Or we can also use some alternate techniques in place of combustion.So, that way we can control the emission level.Now, these are the contents of our discussion, that pre treatment of; the pre treatment ofcoal that means here we are interested to clean the coal before its application andmodification in process, we can use the different types of reactor, improved reactor and thenthe boiler device that can be of supercritical type and flue gas cleanup.And we can use some advanced techniques like oxy-fuel combustion, chemical looping combustion,and then gasification direct liquefaction and underground coal gasification.Now we will see this is the conventional flowsheet, say, here coal is burnt in the furnace andthen flue gas is going and here we are recovering the heat.Then it is going for clean up section and coal is put here.So, this is a conventional process.Now, what we can do, we can clean the coal in higher extent, we can use more efficientfurnace or we can use more efficient heat recovery system or we can use more superiorquality of gas cleanup processes or some advanced techniques.As shown here these lines are related with oxy fuel combustion.Oxy fuel combustion, another method which is the replacement of the combustion process.So, here nitrogen is separated at first from the air and so, only oxygen we can use.So, if we use more oxygen, the flue gas will be having more carbon dioxide it will notbe having nitrogen.So, once it is not having nitrogen, its capture will be very easy.So carbon dioxide capture we can get here and the whole process will be more cleaner.So, these are the scopes, which we have to improve the cleanliness of the process orto reduce the emission level.So, we will discuss all those things.At first we will see how the coal is cleaned As of today in the conventional process alsocoal cleaning is performed to remove the ash and sulphur.So, we have to take more action, if we can reduce the pollutants level like say sulphur,ash content, nitrogen content in the coal.And what are those methods used, we will be discussing now, if you consider the sulphur;then sulphur is present in coal in terms of inorganic and organic form.In the inorganic form basically it is pyrites FeS2 and some others are also there that isZnS, PbS, and FeS2 this is this is your marcasite and another is your pyrite.So, pirate and marcasite, these are 2 minerals are having same formula, but they are differentin their crystal structure.So, these are the source of Sulphur.So, we can remove this sulphur from the sources by different methods, one is physical andchemical and biological.How we can remove in the physical method, we can use the help of jigging we can usethe tabling we can use the dense medium processing, hydrocycloning and air classification.So, in the jigging we take the advantage of the difference in the initial accelerationwhen different particles start to fall from a certain place from its rest position theninitially they possess different velocity.So, that will be utilised to remove the different particles with different densities and sizeetcetera, that is the jigging operation and dense media processing, in this case, we usecertain slurry that is having certain specific gravity, now coal is heterogeneous sampleand it will be having ash, carbon and etc.So number of particles if it has then that will be having different density, say, Rho1.Say, different densities are there for coal, say, Rho1 Rho2 Rho3 Rho4 etc.So, different particles have different density, if we use one slurry solution that is thespecific gravity is equal to say Rho3.So, in that case, the lighter than Rho 3 that will float and heavier than Rho3 will fall.So, this is a way we can get if Rho 4 are higher than this Rho 3 then Rho4 will comeat the bottom and this will be at the top.So, gradually if we take another solution, which is having lesser density, so, more lighterparticles will also come up.So that way we can clean the coal from the impurities like say, ash contents, mineralcontents basically mineral contents which are having higher specific gravity than thecoals particle.So, now the chemical methods, these methods the physical method are suitable to removethe minerals, but it is the sulphur which is available in organic form, those are notvery easy to separate by this method so chemical methods are used in this case.So, some examples are oxy desulfurization process, in this case, oxygen in dissolvedform in some solution aqua solution at elevated temperature and pressures are used.So that the sulphur of the pyrites is oxidised or organic sulphur can also be oxidised tosome extent and then hydro desulfurization where if we supply hydrogen and at high temperaturecontrol, if we have good control on it, so, we can get some conversion of the sulphitesto the sulphur dioxide.So, next is caustic treatment.So, SOx as you know it is acidic in nature.So, if we use some alkaline solution, caustic solution in control amount, so, some sulphurcan also be removed and then biological methods, some microbes are used for the degradationof the sulphur compound, some example are some thermophilic or mesophilic bacteria.Now, we are coming to the changes which you can have during the process, one is Fluidizedbed reactor.If fluidized bed reactor is used then we can get more reaction, possibilities of more reactions,how because if you see here it is a fixed bed and these are the fluidized bed.So, in fixed bed the particles are very close and in a fluidised bed the particles are notvery close it is separated.So, coal combustion is a solid and gas phase reaction.So, when the solid particles will be surrounded by more gas molecules, so, there will be goodchance of the reaction.So, that that is why we get more chance of reactions in case of fluidized bed reactorand in this case the particle sizes are also less.So, once the particle size is small, then one way we can reduce the loss of the finesof the coal particles, fine coal particles and secondary we can get more interactionof the solid particles with the gas molecules and more rate of reaction.Apart from this, this fluidized bed reactor can also give some; gives us some other advantagelike say we can add limestone here with the feedstocks.So, feedstocks and limestone is coming here.So, all our solid particle are fluidized and then in situ SOx capture is possible.In situ SOx capture, so, the limestone will react to this SO2 and it will be settled here.So, there is one additional advantage and if the fine particles are coming here andit is also getting some recycling possibilities.So, more conversions of the carbon we will get here in this fluidized bed reactor, butif we use fluidized bed reactor, then we need smaller particles.So, during crushing we have to use more energy.So, on economic aspect this process may have some less efficiency or it may cost more thanthe conventional one.Now fluidized bed size we have in market that is up to 320 megawatt power plant is possibleby this technology.And now we will come into boiler part, conventionally sub critical boiler is used at 171 atmosphereand 540 degrees centigrade.So, in this case we get 35% net energy efficiency and heat rate is 9,751 Btu per kilowatt hour.If we use supercritical, then operational be 250 atmosphere and 565 degrees centigradeand here 37% efficiency we are getting if we use advanced supercritical then 336 atmosphereand 600 degrees centigrade, we can get 42% efficiency.If we use Ultra-supercritical 393 atmospheres at 760 degrees centigrade, then we can get44% energy efficiency, but you see heat rate is decreasing.So, it is very clear to us, we can improve the efficiency of the process, if we replacethe sub critical boiler to advance or ultra supercritical boiler.In this case, we can improve the emission level like the in case of sub critical CO2926 and a supercritical it is 835 gram per kilowatt hour emission in case of carbon dioxidefor NOx are also here.So it is very clear that for all the cases, we are getting some reductions in supercriticalboiling condition.Now we will come to flue gas clean up, what we can do with the flue gas cleanup.Now, conventional methods also have some cleanup, but we have to apply more advanced techniquefor the more efficient removal of the pollutants from the flue gas.One example is say, duct injection versus FGD, Flue Gas Desulphurization.So, what happens in case of duct injection, we also use some material that is, sorbentis used in duct.Dry sorbent is injected in the duct and then it is humidified by the water downstream ofthe injection and upstream of the preheater from the particulate collection equipment.So, this is the conventional method.But replacing this if we can use flue gas desulphurization where spray takes place,so scrubbers are used in that case, the efficiency improves for the removal of SOx from the fluegas.Regenerative and non regenerative reagents, we can use the reagents which are regenerativein nature, so that it will help the economy of the process wet and dry methods are there.So, wet method, flue gas cleanup and dry method is also developed in recent years.So, if we want to use the wet process, then we have to reduce the temperature of the fluegas and in dry process, it can remove the pollutants at high temperature also, thatis why there will be some variation in the economy also.The improved reagent reactivity, the reagent reactivity can be more if we can select asuitable reagent, the reactivity will be more so less reactant requirement will be there.An improved mixing design to lower calcium is to sulphur ratios.So just fluidized bed reactor if we can change the design say, circulatory fluidized bedreactor, so calcium is to sulphur ratio can be reduced and larger reactor vessels alsohelps to get more removal of pollutants.And design removals steadily improving up to 98%.So, an example is given here and average flue gas desulphurization emission rate is 0.34pounds sulphur dioxide per MMBtu.So, this is one; these are some examples, where we have some possibility to improvethe efficiency of the cleanup system.Now, this is one example of flu gas desulphurization process taken from this reference.So, here we see flue gas is coming.So, we have lime stone slurry, so it is pumped and sprayed here.So we are getting clean flue gas and then this slurry part of it is taken back for thickeningand then from this we can get gypsum.And the supernatant from this thickener it is again recycled back to this to reduce thewater use.Then after SOx removal, the particulates removal is also important.So, particulates are removed, different types of devices are used to remove particulates.And in case of coal based power plant, mercury may exist as particulate form.So, that removal is also very, very important.And mercury can be available in oxidised form, in elemental form and in particulate form.And these different forms of mercury have different solubility, this is water solubleoxidise mercury and elemental non water soluble.So, the techniques to remove this mercury will also be different.So, we will try to develop effective systems for the removal of complete mercury from theflue gas.And some examples are given here, some configurations on ESP and the hot side ESP, cold side ESP,fabric filter and cold side ESP plus spray drying, cold side ESP plus wet flu gas desulphurisation.Fabric filters plus spray drying, fabric filter plus wet flue gas desulphurization and ESPplus WFGD plus SCR.SCR is selective catalytic reduction.So, these methods have been used for different types of samples and these are the removallevel.Now, we will be discussing on the use of advanced techniques in place of conventional combustionmethod.So, one example is oxy fuel combustion.In oxy-fuel combustions as the term says we will be using oxygen in place of air.So nitrogen will be separated from the air first then oxygen will be used.So, nitrogen is separated from the air and oxygen is used.So, this nitrogen may not be completely removed.So it is used for the combustion process and then the flue gas is formed.So, fuel is added here then flue gas forms it is going to condenser for heat recovery.So then the flue gas is again recycled back.So, by this recycling partially, we are able to enrich the carbon dioxide concentrationin this case.So more than 90% carbon dioxide we can achieve here.So it is very easy to sequester.So, that is why this process is very attractive, but research is going on, the commercial plantsare yet to come and then another method is chemical looping combustion.So in this case, some metal are oxidised in one reactor and in other reactor, metal oxidisedreacts with the feedstocks, carbonaceous feedstocks to convert it into CO2 and H2O and metal oxideis reduced to metal again.So, if we can use two reactors in a cycle, then that can meet our requirement for theproduction of energy from the carbonaceous materials in one reactor and other its regeneration,regeneration of the metal to metal oxide.So, this is one reduction and another is oxidations.In this oxidation air is used and metal is converted to metal oxide and in reductionmetal oxide is reduced to metal and fuel is converted to CO2 and H2O.So this is the chemical looping systems, it is very clear to us in this case, when thereaction is taking place, we do not need any air.So, nitrogen is not required and only metal oxides are helping for the conversion.So, in that way that will be more cleaner.So, what happens the metal is oxidised, an example at 700 to 900 degrees centigrade.So, 2MeO say, Me is the metal and then MeO is the metal oxide and then metal oxide reactswith carbon gives CO2 and metal it reacts with hydrogen gives metal and H2O these arethe basic reactions.So, we can represent the phenomena in this figure that this is our fuel.So, this is our metal oxide.So, it will give us H2O + CO2 and this metal is coming here and air so it is it is givingus oxidation and we are getting metal oxide.So, this metal to metal formation, this is exothermic process, overall the process isexothermic and here it is endothermic process.So, these two types of processes are going in a cycle.So, we can get some efficiency of the cycle and that can be presented by this that isthe Qhot, the amount of heat which is released here and Qcold, overall energy used in thisprocess.And Q, this is released heat and this is work done during this.So, this is energy balance and we can get this relationship that Q hot is equal to Qcold plus Qo plus W What is this Qo?Qo is equal to T into del SO.So, there may be some entropy change, so, that is equal to T into del SO is equal toQo.So, this is the chemical looping combustion and different types of metals and metal oxidesare used like copper, iron.When copper is used this reaction takes place in your oxidizer that is, Cu +O2 gives 2CuOand CuO + H2 gives Cu + H2O in the fuel reactor and 2CuO + C gives 2Cu + CO2 in the fuel reactor.Similarly, for Fe2O4 these are the reactions which take place in fuel reactor and thisis in the air reactor.So, this chemical looping system is not yet commercialised in a bigger scale, but demonstrationhas taken place.And total you see 34 pilots of 0.3 to 3 mega watt has done in more than 9000 hour and thisis the technology was demonstrated in 2003 and later with solid fuels in 2006.Initially, it was demonstrated with gas fuels and 2006 it was demonstrated with solid fuels.And from this data on the demonstration plant, it is very clear that this can be used incommercial scale for the conversion of coal.Now, this slide gives us an example of chemical looping applications in thermal power plant.So, this is in Spain, Germany, Taiwan, US and this is also US.These are the capacities and these are the features.So it is clear this one we are having calcium based looping system.Here we are having limestone based chemical looping system.And this ITRI Taiwan is also using limestone sorbents and with spent CaO.And Alstom, U.S they are using CaS CaSO4 system and Ohio state university is using iron oxidebased high pressure syngas chemical looping.So, that is for gasification applications.Now, we will see one problem on this.So, calculate the amount of work done in the burning of carbon in a nickel based chemicallooping combustion, when the change in entropy is 356 joule per K and working temperatureis 300 K.So, this is our cyclic process, it is given we have to calculate the work done.These values we have to calculate.So, now if we have nickel based chemical looping, so, what will be the reactions or what willbe the reactions here or what will be the reactions here that we will consider first.So, here nickel will be oxidise to NiO the del H equal to -396 K Joule kilojoule andin this carbon will be reacted to NiO(s).So, then CO2 + 2Ni(s) So, this reactions is 136 kilo joule.Then how will get Qhot that is equal to Qcold + Qo + W as we have discussed.So, Qhot is equal to del H exothermic that is 396 kilo joule and Qcold del H endothermic136 kilo joule and what is Qo?T into del SO del S value is given, T value is given.So, you can calculate the Qo T del S that is equal to this one so, this joule you aregetting.So, then we have the relationship that is Qhot is equal to Qcold + Qo + W and by rearrangingW is equal to this much.So, now, we are able to find out the work done during this process.Next is gasification, another replacement of combustion is gasification.In combustions we use excess amount of oxygen, in gasification we use controlled amount ofoxygen.So, R that is equal to oxygen by coal ratio, mass by mass ratio, for combustion it is greaterthan 2.5 and for gasification it is .68 to 2.5 and another is pyrolysis, there is nooxygen in that case we can get R equal to 0.68.So, if we use this gasification method, then what we will get, we will be getting CO andH2 but in case of combustion we are getting is H2 CO2 which is also available in gasificationplus oxygen.So, CO H2 which we are getting here, so, CO2 concentration will be less.So, carbon dioxide production in gasification plant will be less.And we are having CO + H2 rich gas that can be used for the production of electricityas well as for the production of different types of chemicals.So, we will be having an opportunity to capture carbon dioxide in situ in between and alsoto make the syngas in more economical way.So, I will discuss this in next classes in more detail.Now, direct liquefaction, another replacement of oxidation can be direct liquefaction.How it is because you see in coal, we have seen in our introductory module that it containsvery complex structure containing number of aromatic rings.So, if we can break this aromatic ring and can make some hydrogen addition, so, we canget number of organic compounds with aromatic rings and that is available in conventionalliquid fuels.So, direct Coal liquefaction can be possible, may be possible if we use higher pressureand hydrogen and required temperature.So, this is done and efforts are going on to get more scale up version of DCL, alreadyone plant has been commercialised in China and I have discussed this in detail.Next is underground coal gasification, another technology possibilities is underground coalgasification, in this case, gasification will take place.But that will not take place in any gasification plant on the surface, but it will be takingplace underground.So, by that way, we can remove the possibilities of emissions etc.So, that is a philosophy of underground coal gasification.So, in this case, air or oxygen and steam can be sent underground, where coal is availableand this coal will be combusted and ash will form and the syngas will form, so that syngaswill goes up.So, it will come out as CO2, CO, H2, CH4, Steam and tar.So, this is the concept and people have studied on it mostly on simulation based studies.And some plants are also available for the underground coal gasification around the world.I will discuss that.But now I will see what are the advantages of these and what are the disadvantage.So advantages over conventional processes are, no reactor to be manufactured, low dustand noise, no ash handling at power stations, no coal stocking and transportations, largercoal resource exploitation, it converts sulphur to H2S and nitrogen to NH3 instead of SO2and NOx.And it has some disadvantages that are, a potential for surface subsidence, possibleaquifer contamination and expensive drilling and well linking technologies.So, on the basis of the data, that people have walked on it, it is evident now, thatcoal seam is more than two metre thick.In that case, this technology is feasible.It also requires seam depth more than 300 metre and seam has more than 100 metre verticalseparation from aquifers.These are the requirement for the UCG and these are some example of the UCG based plantswhere the seam depth is provided, thickness of the seam is provided and quality of thecoal is also provided.So, it is very clear that this method can be used for any type of coal, the qualityof coal does not matter.So, that is the advantage of this technique.But we have some challenges in India that is we lack of understanding of the technologyand unavailability of proven track record and implementation guidelines, then regulatoryframework governing UCG operations in the formulation stage and the lack of understandingof the environmental impact of UCG and concern over the safety.So these are the issues related with underground coal gasification.So this is not a commercialised one.It is it has some issues also.So far, we have discussed the possible options to produce cleaner energy from the coal.Thank you very much for your patience.