La nuova app di Alison è ora disponibile su iOS e Android! Scarica ora

Module 1: Gassificazione del carbone

    Study Reminders

    Hi friends now we will discuss on the topic gasification of coal.This is a second part of this topic in the first part we have discussed about the gasificationprocess, its mechanism, then different types of reactors applicable for coal gasificationand their comparison.Now we will see how the syngas produced after gasification can be made suitable for differenttypes of downstream application.And the content of this class is syngas conditioning and cleanup and then syngas utilization andIGCC and poly generation options.As we have seen in the previous class that when syngas is produced in the gasifier itstemperature is very high and it contains number of impurities like say particulates, acidgases like SOx, carbon dioxide, H2S etcetera.So, these need to be removed and another fact is that for different type of downstream applicationswe need different type of syngas quality.So, as shown here in this table if we see for power application for hydro processingand for chemical applications of the syngas we need different types of quality here sulphur,CO2 and CO concentration is provided for 3 different applications so you see here sulphurfor power application requirement is 10 to 15 ppm weight basis whereas for hydro processingit should be less than 1 ppm and for chemical less than 0.01 to 1 ppm.So, the degree of removal of sulphur compounds from the syngas is required in different extentfor carbon monoxide carbon monoxide we see for power application there is no such requirementwhere for hydro processing it is, it should be less than 50 ppm on weight basis.And for chemical applications the carbon monoxide and hydrogen ratio should be maintained asper requirement.Different applications have different requirement of hydrogen is to CO ratio.But if we see the CO and H2 composition in the raw syngas then CO is almost 2 times morethan that of hydrogen but in downstream applications in some cases even we need the reverse ratiothat is H2 is to CO as 2, so some conditioning or conversion of CO to H2 is required andthat is why now in this class we will discuss how we can achieve this target.How the high temperatures syngas can be used and cooled to lower temperature heat recoverycan be done.And then the acid gases can be removed to make it suitable for different downstreamapplications to remove the pollutants which create problems during the processing andto maintain the H2 CO ratio.So, we should we here reduce syngas temperature during cooling we need to reduce the syngastemperature and we need to extract the valuable energy from it.And chemicals are removed which can foul, corrode or erode system components or whichcan poison or deactivate chemical processing agents or are environmentally unacceptable.Now we will see how the syngas can be cooled there are basically two mode of operationone is quench mode operation and another is boiler mode.So, on is quench mode and another is boiler mode, in both the mode the reactor part issimilar but the difference is the cooling process.So, in the quench mode the syngas which is produced here is first pass through the quenchring and it is cooled by shock.The temperature of the syngas is reduced to the temperature of saturated steam then itgoes from the side stream.And the slag or ash materials comes down.This way we can reduce the high temperatures syngas to a lower value.And in this case we see 1.47 to 2.94 mega Pascal steam can be produced.Now in case of boiler mode the syngas is produced here it is going from the side and ash iscoming bottom at the bottom.So, there is no direct quenching but it is sent to a boiler for heat recovery and exchangeof heat as a result the syngas temperature reduces.So, this process gives us some advantage we can get high temperature steam here with respectto the quench mode and high pressure steam here with respect to the quench mode.We see there are 9.81 to 13.73 mega Pascal steam can be generated and as you know higherthe pressure in the steam and higher the temperature there is a possibility to have more efficiencyin the turbine.So, that is why this mode of operation that is boiler mode of operation is preferred inIGCC application.Now we will see the how the particulates can be removed from the syngas.As you know the some of the reactor configurations uses very fine particles of coal and thosefine particles can be carried over if it is not completely burned or in some applicationsalso tar particles comes.So, those particles we need to remove, so for the removal of those particles there aresome configuration that is here char materials along with ash can be removed from syngasin two-stage water wash consisting of a quench pipe carbon separator and a packed tower.Just we are spraying water and removing the particulates in a packed tower.In quench pipe around 95% of the carbon is removed by direct water spray.So, these are two basic operations through which the particulates are removed.So, you see in this case the temperature of the syngas should be down made down priorto its application.Another way of efforts are also there to use different types of oxides at high temperatureto remove the gases or we can use ceramic cross-flow candle filters to remove the particulatesat higher temperature.So, high temperature particulate separation this technology is being developed and extensiveresearch is also going on around the globe for this application.Now by spraying water we can remove the particulates but there are a number of types of spray systems.So, this is here spray scrubber so in this very scrubber so dry gas inlet at the bottomand water injection at the top and then there will be interaction between the droplets aswell as the particulates.So, the mechanism is the inertial impaction and interception on the droplets this is themechanism of the separations of the particulates from the syngas in presence of droplets ofthe water.So, it will settle and in this case no packed bed is used only spray scrubber and its efficiencysay 94% for 5 micrometer particles.And greater than 99% for 25 micrometer particles and the efficiency of this system will dependon droplet size and flow velocity of the gas and liquid is to gas ratio.These are the factors which influence the efficiency of the system.Then packed bed and plate column another type of spray system also used in this packed bedone packing material is used like say raschig rings, saddles, coke or broken stone, so theseare normally used for the capturing of the particulates in this.It is a dry gas will come here and again from the clean gas will goes up and spray systemwill be there but we have one packed bed.So, this is one type of design.Another type of design dry gas or dirty gas from the bottom from the top clean gas andliquid from the top and there are a number of plates.So, plates overflow liquid will come here again plate overflow liquid will come hereplate overflow liquid will come so that way the interactions of gas and liquid takes placeliquid droplets and particulates separates.So, this system also help to remove some gas molecules from the syngas.Now we will see how the acid gas components can be removed acid gas components like saySOx, carbon dioxide, H2S etc present in the syngas that can be removed through specifictreatment steps.So, this flow sheet shows us the raw syngas which is having hydrogen, carbon monoxide,carbon dioxide, H2O, H2S, COS and trace contaminants so the first the COS reacts with H2O to formH2S and CO2 so that is hydrolysis of COS then H2S and CO2 acid gas components we need toremove.So, how we can remove it, carbon dioxide removal if you can selectively separate the carbondioxide then that can be enriched and it can be sequestered.So, carbon dioxide enrichment and then compression and carbon dioxide sequestration so one waywe can get.On the other way we have H2S so we can remove the H2S enrichment specifically then we usethem selectively and then after selective separation of H2S we can use this gas forsulphur recovery and we can get sulphur and once we can get sulphur we can produce sulphuricacid.So, after removal of sulphur or H2S, now we can go for further treatment of the tail gas.So, this is the flow sheet for acid gas removal but now we will see how this can be done.So, the H2S and CO2 removal has been made by absorption and adsorption based technology.So, here this figure shows absorption based technology from the bottom we are sendingdirty syngas and from the top we are using clean solvent.Solvent will be selective to capture say carbon dioxide or H2S or may be able to capture both.So, then the clean syngas will go up with CO and H2 there will be no H2S there willbe no CO2 or very less amount of CO2 and H2S.So, different types of solvent has been used this solvent which capture the acid gaseseither by physical or chemical absorption.So MDEA we are having a chemical absorption what is MDEA one solvent that is methyldiethanolamine,Selexol another solvent that is primarily dimethyl ethers of polyethylene glycol thereis also physical absorption and Rectisol that is nothing but the refrigerated methanol sothat is also physical absorption.Now in the recent years some adsorption based technologies are also developed that is warmsyngas clean up, that RTI and Eastman company they have developed this technology.They use some metal oxides so which captures H2S.Now we will see the comparison among these processes so if we see the MDEA process itis a chemical absorption methods so it removes H2S 98 to 99% whereas CO2 is 30% so more selectivetowards H2S removal and process parameters 30 to 35 degree centigrade and pressure is2.94 mega Pascal or less.These are the technology suppliers who have developed the technologies and here we seethe cost is low it is not high this is not costly method.Then Selexol another method H2S 99% and CO2 can be variable and temperature is -7 to -4degree centigrade whereas pressure is 6.87 mega Pascal.These are technology licensers and here higher costs than MDEA but overall system cost includingsulphur recovery process and tail gas treating could be more cost-effective.Another is your Rectisol process H2S is 99.5 to 99.9% and CO2 is 98.5% so temperature - 35to -60 degree centigrade and pressure 8.04 mega Pascal.But this is developed by Linde AG and it is the most costly method but high selectivityto H2S over CO2 and ability to remove COS.So, these are the characteristics of these different methods and out of these methodsRectisol has been used widely.Now we are coming to warm gas desulphurization as you discussed that some metal oxides areused.So, say ZnO and H2S reacts and ZnS formed at 315 to 530 degree centigrade and this isyour adsorber and then the adsorbent has to be regenerated so regenerator is also therewhen ZnS reacts with oxygen and again form ZnO at higher temperature 590 to 680 degreecentigrade.So, that way it can remove the H2S from the syngas and sulphur recovery options is alsothere.Sulphur can be recovered from the syngas through closed process or direct sulphur recoveryprocess, there are two processes and then direct sulphur recovery process SO2 + H2 orCO that reacts 1 by n Sn + H2O or CO2 at 425 to 680 degree centigrade temperature.And sulphuric acid can also be produced from the sulphur.So, this is the claus process so if we have H2S + 3 by 2 O2 then it is giving us SO2 +H2O this is the del H value of this reaction then SO2 reacts with H2S again and then gives3 by 8 S8 + 2H2O and this reactions is also possible.So, these are the reactions of the claus process through which we can get elemental sulphur.Now we will see the different methods for the removal of acid gas components from thesyngas so physical absorption, chemical absorption, adsorption and molecular sieves.Molecular sieves are also used for the shape selective or size selective separations ofthe other molecules from the syngas.So, if sulphur in product gas is high and partial pressure of H2S or COS and CO2 isalso high then we should go for physical absorption.But if sulphur concentration is high but this pressure is low then you can go for chemicalabsorption.But for adsorption process certainly the sulphur compound concentration must be less, so lowsulphur and then low or high-pressure we can go for adsorption.And for molecular sieves high sulphur and low partial pressure of CO2, COS and H2S.Now after the removal of the acid gas now we will see how the CO is to H2 ratio is maintainedor how the H2 concentration in syngas can be increased.So, that is done by shift reaction water gas shift reaction the reaction is CO + H2O itgives H2 + CO2 and this is the exothermic reaction.So, this reaction is performed in presence of catalyst.And this shift reaction can take place before the removal of acid gas from the syngas orafter the removal of acid gas from the syngas.So, if this water gas shift reaction takes place before the removal of acid gas removalthen it is called sour shift water shift reactions.And if it is placed after the acid gas removal then it is called sweet water gas shift reaction.So, we have two types of reactions sweet shift reactions and sour shift reactions.These two reactions uses different catalyst, uses different temperature range.So, we can have two types of shift reaction also on the basis of temperature one is hightemperature shift reactions another is low temperature shift reactions.So high temperature means it is 320 to 450 degree centigrade low means 200 to 250 degreecentigrade.And you see the catalysts are different Fe2O3 Cr2O3 is used for high temperature shift reactionswhereas copper zinc oxide alumina oxide is used for low shift reaction.Now instead of using separate catalyst in low temperature and high temperature shiftreactions catalysts like cobalt molybdenum alumina oxide can also be used over a widerange of temperatures.So these are the development of the catalyst sites for shift reaction.And these are the comparison of sweet shift and sour shift if we see the typical flowsheet then it contains sweet shift 2 HTS and one LTS and sour shift reaction contains 2to 3 conversion stages which heat exchangers and sometimes with steam addition as required.And conversion if we use CO conversion how the H2 can be; how much H2 can be produced?Within two HTS steps the sweet gas shift can reduce CO concentration from 44.6% to 2.1%.Whereas sour shift can produce 44.6 to 1.8% within two steps.Same both the process we can reduce the concentration of CO below 1% and hydrogen concentrationcan be increased.Now you see the catalyst of sweet shift reactions are more expensive than the sour shift reactionsand reactor size is also smaller in case of sweet shift reactions then sour shift reaction.And this table shows the typical composition of syngas before and after shift gas reactions.If we see hydrogen, carbon monoxide and CO2 this concentration changes significantly CO24.36 to 35.50% then CO 49.98 to 1.12% and here hydrogen 43.01 to 61.53%.Now how we will use the syngas once the syngas is cooled then its acid gas are removed thenconditioning is done then we can use it the syngas can be used in different applications.Like it can be used for hydrogen production that we have seen if we use rigorous shiftwater shift reactors then obviously we will get more hydrogen and all carbon monoxidecan be converted to hydrogen through these reactions.Or we can use it for electricity or steam productions.The syngas can further be combusted and heat can be generated and can be used in electricitygeneration.It can be converted to SNG, it can be converted to FT fuels that is naphtha, waxes, diesel,gas fuels etc.Or it can be converted to methanol, so once methanol is derived as a chemical basic chemicalsthen from methanol we can get a number of chemicals.So, as shown here so number of chemicals we can get from the methanol or this syngas canbe utilized in different way.Now IGCC and poly generation options, this option is becoming very relevant and applicablein recent years.As you see the gasification process is costlier than the conventional combustion based process.So, we have to utilize the syngas in such a way that the economy is improved.So, the high cost of the process can be compensated by the high value of the products, in productswhich is produced from the syngas that is why poly generation electricity, chemicals,hydrogen etcetera all productions are considered and integrated and that is called poly generationoptions.So you see here we use coal or any carbonaceous material then the syngas is produced thenparticulate removal gas clean up then shift reaction conditioning.Then we are getting desired concentration of H2 and CO in the syngas then we can useit for synthesis gas conversion, we can use it for FT synthesis etcetera and then carbondioxide sequestration's we will also get and will be able to produce the electricity inthe gas turbine and again in the steam turbine so electricity we can get.So, that way we can be able to improve the economy of the process.Now we will see how different parts of the gasification system or gasification processcontributes on the overall economy.So, feedstock is around 12% feedstock handling system around 12% share of the constructioncost and gasifier air separation unit and syngas cooling around 30% and then gas cleanupand piping it is around 7% and combined cycle power block that is turbines etc so it willbe giving 33% and then remaining components and control systems it is around 18%.So, these are the contribution of different parts of the gasification units on the overallconstruction cost.So, thank you very much for your patience.