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Module 1: Introduction to Natural Gas

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Hydrogen Production from Natural Gas

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Hi friends, now we will discuss on the topic hydrogen production from natural gas.In the last classes, we have discussed on the utilization of natural gas and we haveseen that it can be used through the conventional route for electricity production or it canbe used to produce syngas and further liquid fuels and it can also be produced to hydrogen.As you know that the hydrogen is considered as a green fuel and the future fuel also.So in this class, we will discuss on this content the hydrogen as a green fuel, thenhydrogen production routes, then natural gas as a feedstock for hydrogen, and the routesfor hydrogen production from natural gas that are reforming and decomposition of methane.So as you know that the hydrogen is having very high energy density and heating valueis 142 megajoule/kg and we also know that hydrogen can be combusted and there will beno carbon dioxide emission.One important feature of hydrogen is that it can be used in fuel cell and electricitycan be generated.So here, this figure shows us the electricity generation in the hydrogen fuel cell.So when hydrogen is used as a fuel in this fuel cell where in the anode this hydrogenis coming and then in this here it is converted to H+ and e-.Then H+ is transported from anodic to cathodic chamber through this membrane, hydrogen transfermembrane, and it is coming to this cathode where oxygen is supplied.So oxygen reacts with H+ and e-, e- or electron which is generated here in the anode, it istransported through the external circuit and comes here, and then this electron and thenH+ which is transported to this membrane and this oxygen reacts and converts to H2O.As a result what we get, get electricity here.So electricity is the very cleaner mode of energy, that is why hydrogen is consideredas the green energy.So this hydrogen can be produced from different resources and we will briefly discuss that.So this can be produced from reforming of hydrocarbons or biomass, gasification of carbonaceousfeedstock, decomposition of hydrocarbons like say methane, then thermolysis of water, electrolysisof water, photolysis, then dark fermentation and microbial electrolysis.So these are the different methods, which have been reported for the production of hydrogenand we will see the comparison of these processes.So reforming process as you know it may be of different types, just we have discussedin the previous class; it may be auto thermal, it may be steam reforming, it may be dry reformingor tri deforming, anything it may be.We will be comparing those things say steam reforming, partial oxidation, autothermalreforming, plasma reforming, biomass gasification, methane catalytic decomposition, and coalgasification.If we see here, these are the different feedstocks, some are hydrocarbons, biomass, and coal.The efficiency for hydrogen production is also different and maturity level is alsodifferent.Some other examples are say biomass reforming, plasma cracking, liquid phase reforming, thermolysis,electrolysis, and photolysis.So if we compare, we can see that either hydrocarbons or biomass or coal are the carbonaceous materialsthat have been used for the reforming process and efficiency is also different.Most important is that their maturity level if you see some are very few are in commercialscale, all are in maturity, either long term maturity or medium term or under development.So these are the different technologies which are available for hydrogen production.The commercial processes are reforming, then biomass gasification, then coal gasification,and here we are having electrolysis.So basically if we see the methods, then we will be having some important data that isthe steam reforming, partial reforming that captures maximum share for the hydrogen production,followed by gasification of coal and biomass, and then electrolysis.Others are very less, contribution of other processes is very less in commercial scale,they are under development.Now we will discuss on reforming because it will be using hydrocarbons and basically inour case it is natural gas.So natural gas or methane will be converting to hydrogen through the reforming processes.Different reforming processes are dry forming, steam reforming, partial oxidation, autothermalforming and tri reforming.As we have discussed in a previous class and as evident here also, we see each H2/CO ratiois maximum in case of steam deforming.As we are interested to get the hydrogen through this route, so obviously the steam deformingcan be more suitable route for the hydrogen production.So, we will be discussing some more on steam reforming and already we have discussed otherreforming in previous class.So in steam reforming, steam is used for the reforming purpose.So, this is our say hydrocarbon and then this is our steam, it will gives us nCO and H2rich gas and this is ΔH value.So what we are getting here, so if it is C1, then n1, then it is methane, this is representingmethane.Then SMR reactor, it requires steam.What would be the ratio of the steam and methane that is very important, that is it requiresis S/C, steam by methane ratio as 1:1 and if we give more steam and more S/C ratio,we can get more hydrogen.The SMR has efficiency of around say 70-80% and carbon dioxide emission is 7 kg carbondioxide per kg of hydrogen produced.In this case catalyst as we have discussed in the previous class that nickel based catalystare used, so mostly used, but other catalyst are transition metals that is iron, cobalt,nickel, copper and the noble metals these are have been reported and these are the activeingredients for the catalyst and some support is required, so basically gamma alumina, CaOAl2O3,MgO, MgAl2O4, TiO2, ZrO2, SiO2 have been used.Among them, these are very commercially available.So these catalysts also coke deposition is a problem for this catalyst and if sulphurand halogen compounds may also deactivate the catalyst.Some industrial catalyst suppliers we see that is Baden Aniline and Soda Factory BASF,Haldor Topsoe and United Catalysts these are the players for the catalyst supply in commercialscale for SMR.Now, we will see the mechanism.What is the mechanism for the production of hydrogen through this route?So we have hydrocarbon and we have steam, so steam, and we have some catalyst.So it is gas and solid phase reactions.So we will be having steam, so that steam reacts with surface nickel atoms which isavailable in the catalyst and then it forms adsorbed oxygen atoms and gaseous hydrogen,this is the first step it is assumed.So H2O it is in the support of the catalyst sides, the nickel sides, it is giving us H2+O(s)which is adsorbed.Then again methane we have also, so methane reacts with surface nickel atoms and thenit yields CH2 radicals which is adsorbed and adsorbed H atom, so CH4 with catalyst sideit is giving us CH2 in adsorbed form and hydrogen also as adsorbed form.So these adsorbed radicals and H and O will further react.So we will see here, the adsorbed radicals and adsorbed oxygen react to yield adsorbedCHO.So this is the reaction CH2(s)+O(s) it is giving CHO(s)+H(s).Then CHO which is adsorbed that can also be converted to other forms, so as mentionedhere that may be possible that CHO is converted to Co(s)+H(s) adsorbed CO(s) and adsorbedhydrogen and CO is with adsorbed oxygen that can give CO2 adsorbed + sides and then CHOadsorbed can react with oxygen adsorbed and also it can give a CO2 adsorbed and hydrogenadsorbed.So these are the different species which are generated during the process in the catalystand those with the help of the catalyst and these intermediates the CO(s) can give a COand the sides and then CO2 (s) can give a CO2 and sides and H(s) can gives us H2 andsides.That means after desorption of those gas, the sides are made free, so catalyst is nowfree, our feedstock is converted to product and that is fuel rich of hydrogen and carbonmonoxide, also having carbon dioxide.So this is the mechanism of the SMR reactionNow, we will see how the natural gas can be processed through this route.So here we have say natural gas, so that natural gas certainly will have some sulphur and thatsulphur will be the poison to the catalyst, so we have to remove it.So for this removing, we are using hydrogen here, so desulphurization unit, DSU, as afirst step of the process.Then some catalyst are used that is cobalt molybdenum catalyst, Co-Mo catalyst, and thenit is 290-370 degree centigrade and then this is your catalyst where then zinc oxide bedis also used and then alumina guard.So these are the series of oxides are used in which the DSU reaction is taking placeand H2S and halides are separated from the natural gas.The natural gas will be going to now reformer SMR and before going to that, it has gonethrough the pre-reforming state to raise the temperature from 290 to say it is up to 525degrees centigrade and then it will be going to SMR reactor where the temperature is higherthat is 800-900 degrees centigrade.So here after this SMR reaction, so CO and H2 gas will be there.So there will be some shift reactions to produce more hydrogen.So high temperature shift, water gas shift reaction is taking place here with the helpof Fe/Cr2O3 catalyst at 340 degree to 360 degree centigrade, That is the high temperaturewater shift reaction with a S/C ratio of 2.5-3.Then, we will be getting more hydrogen rich gas and that will go for pressure swing adsorptionfor the separation of hydrogen and then some of it is going to low temperature water gasshift reaction and here Fe/Cr2O3 catalyst and 2.5-3 that is S/C ratio.Then it is going for carbon dioxide absorption.Then after SMR, it is going to shift reaction, it may be at a high temperature water gasor it may be at low temperature water gas reaction, then after that it is going forcarbon dioxide separation and then we are getting 97 to 99% purity of hydrogen or herewe also get PSA through the pressure swing adsorption we can get the 99.99% purity ofhydrogen.So then we will see how the PSA works, so pressure swing adsorption.Pressure swing adsorption works to separate the impurities present in the hydrogen.Different types of adsorbents are used for the separation of different types of impuritiesin the gas stream like say silica gel, activated carbon, molecular sieve, and activated aluminahave been reported.These components are suitable to remove different type of impurities like silica gel for ethane,propane, butane, etc; activated carbon for methane, carbon dioxide; molecular sieve formethane, carbon dioxide, nitrogen and activated alumina for water.So these are the different adsorbents, which are used for the separations of other gascomponents.So here, we use the feed gas hydrogen plus impurities at high pressure.Then it is going through different adsorbent beds and we are getting hydrogen rich gashere and this is our tail gas, hydrogen lean gas, so there is less hydrogen in this gasstream.So this PSA works in a cycle.So if we have some adsorption part, so gas adsorption takes place in this case.Then depressurization takes, so high pressure pressurize, gas adsorptions, then we are depressurizingit and then purging it and again re-pressurizing it.So it has some cycle and different operations have certain span in the whole cycle.So, this is the way the hydrogen is produced from the natural gas through steam reforming.Now we will discuss on the methane decomposition.How the methane can be converted to hydrogen through decomposition.So this can be done.The decarburation dry natural gas cracking is a promising alternative and carbon freetechnology and this can be converted to solid carbon plus hydrogen as per these reactions.So ΔH is 75.6 kilojoule/mol.These reactions requires high temperature, you know it is around say 1200 degrees centigradetemperature is required because the carbon and hydrogen bond is just double bond andso break it, so it requires high temperature.In this case, carbon dioxide emission from burning of process fuel that is your methaneis around 0.05 mol CO2 per mol of hydrogen, which is very less with respect to that usedfor SMR that is equal to 0.43 mol carbon dioxide per mol of hydrogen.So these things methane to carbon formations and hydrogen production, it is a thermal process,but it is difficult to perform without the presence of catalyst.So use of catalyst reduces the temperature from 1200 to say 500 degrees centigrade anddifferent types of catalyst can be used.For this, the reactor configurations may also be varied, fixed bed to fluidized bed, peoplehave used fixed-bed reactors and alternating between different conditions or a fluidizedbed or regenerator combination they have used.Out of these 2 reactors, the fluidized bed offers the flexibility and ease of continuousoperation.Now if we think about the methane decomposition, then we can get different methods.One is your thermal cracking, another is your catalytic methane cracking or you can getplasma cracking or you can get molten media cracking.So this thermal cracking, thermal black process and advanced thermolysis.This is catalytic methane, metal catalyzed and carbon catalyzed cracking.You have plasma cracking, thermal cracking and electrochemical plasma.You have molten media cracking, thermal molten media and catalyzed molten media.So these are different types of methods people have used for the production of hydrogen bythe decomposition of methane.Now, we will see the thermal cracking.As you mentioned that it is a thermal process, we have to apply heat and around 1200 degreescentigrade, the methane will be converted to carbon and hydrogen, but as you mentionedthat it is endothermic process and also it is not that efficient, the high temperaturerequirement is there and this temperature can be reduced by application of catalyst,and then it will be termed as catalytic method not a thermal one.Thermal cracking of methane, the first industrial implementation of this thermal cracking ofmethane was initiated by the European Commission as this SOLHYCARB project in 2009 to producehydrogen and carbon black, and in this project, the solar power reactor with capacity of 10kilowatt was tested for a temperature range of say 1467-1997 degrees centigrade and thepressure of 1 bar and the 55% energy efficiency was achieved.So the major problem in continuous operation of thermal cracking of natural gas is theplugging of reactor due to the carbon deposition.So then the catalytic methane decomposition developed, and in this case, the temperaturerequirement can be reduced from 1200 degrees centigrade to 500 degrees centigrade and differenttypes of catalyst have been tested as shown here using nickel based, iron based, carbon-basedcatalyst and then these are some other catalyst and this is your say non-catalytic decomposition.So when we are not using any catalyst, our requirement is 1100-1300 degrees centigrade.Whereas we are using the catalyst, the temperature requirement is decreasing and ultimately weare able to get the 500-700 degrees centigrade by using nickel-based catalyst.So that is the advantage of the use of the catalyst, and when we use these differentcatalysts, the type of carbon which is deposited on the catalyst bed is also different, wesee here.So for this nickel-based catalyst, we can get carbon filaments, for Fe-based catalystwe can get carbon filaments; for carbon-based turbo static carbon and carbon filaments,and here graphite carbon and turbo static carbon we can get.So, different types of carbon can be formed by this way.Obviously, the transition metals have been used for the production of catalyst and nickel-basedcatalyst is having more capacity to get the cracking at lower temperature.Then plasma cracking of methane, so as you know that plasma is an ionized gas and itcan be produced at high temperature and this can be used for the cracking of the methanealso.So based on the method applied to generate plasma and the temperature difference betweenthe electrons and heavy particles, the plasma technology is classified into 2 types, oneis your high temperature thermal and non-thermal, one is thermal by using high temperature andanother is your non-thermal, the electrochemical plasma.So 2 types of plasma we can produce.So out of these 2 if you use a thermal plasma, the temperature will be very high and usesan electrical power of around 1 kilowatt and the reactor attains temperature around 5000-10,000degrees centigrade, so use of this technology is not possible for this case.So, the non-thermal electrochemical plasma can be produced and used for the conversionof this methane to hydrogen, and in this case, the temperature is around 350-750 degreescentigrade.This plasma cracking of the methane has some advantage also.It has high thermal efficiency, it has high purity of hydrogen, simultaneous productionof valuable byproduct carbon, and very low carbon dioxide emissions.So these are the advantage of this, but the consumption of relatively high electric energyin plasma process can make this apparently uneconomical and make their applicabilityrestricted to large-scale application.Then, molten salt media cracking of the methane.So people tried to get the high temperature of a molten salt media to crack the methane,and when the carbon is produced by the cracking of methane, that carbon is also lighter thanthe density of the molten salt, so it can be go up and can be easily separated.So here, we see methane is coming from the bottom.So this is our molten salt, this is where the reaction takes place, gas and liquid phasereaction, then carbon is formed and hydrogen is formed.So hydrogen as a gas it is going out and carbon as a solid it is floated as the density ofcarbon is lesser than the density of the salt media.So carbon will goes up and it will be separated from the top.So this is a mechanism through which carbon is separated from the salt media.This technology offers the opportunity to operate the hydrogen production reactor incontinuous mode and deactivating of catalyst is eliminated there, there is no chance ofthe deactivating of catalyst.The compatibility between hydrogen, carbon, and molten media and the stability of moltenmedia at various temperature ranges will be the major challenge for this process and presentlythis technology is under development.So we have discussed different types of processes which can be used for the production of hydrogenfrom the methane or the natural gas, and once we can get hydrogen that can be consideredas a green fuel, so up to this in this class.Thank you very much for your patience.