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Chemistry of Coal Gasification

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Hi friends now we will discuss on the topic gasification of coal. We will cover this topicin two classes and in the first class we will discuss on and the contents of the first classare definition and basic chemistry of gasification, gasification reaction schemes and steps thensyngas production and efficiency and factors influencing gasification, advantages of gasification,typical process flowsheet and utilization schemes for gasification products, gasifiertypes, advanced gasification and some coal based gasifiers.Now we will see what is gasification, as we have discussed in the introductory modulein the previous class also, the gasification is the process which uses controlled amountof oxygen and converts carbonaceous materials into carbon monoxide and hydrogen rich gasthat is called syngas. See in this process sometimes steam is used, for coal steam isused but for biomass steam requirement may be very less. Because coal is having morecarbon so steam is required for this process with the coal.And the syngas can be used for various applications including the liquid fuel production throughthe Fischer-Tropsch synthesis and if air is used in this process in place of oxygen thenthe syngas contains more amount of nitrogen and that is called a producer gas.As we have already discussed in the previous class that for oxygen and coal ratio for gasificationis within .68 to 2.5 and in this case these are the products but we increase the R thenwhat will happen, we will be getting more CO2 and we reduce the R-value will we getmore CO and H2.So, this is the basics of the gasification process now we will see the chemistry, thereactions what happens in whatever reactions takes place in case of gasification process,so we have here feedstocks coal we have oxygen and we have steam. Now these schemes showus the elemental reactions the coal is having carbon it is having hydrogen it is oxygen,sulphur, nitrogen etcetera but we are considering the major components of the coal that is carbonwe are considering in this case.So, the first reaction carbon plus half O2 controlled amount of oxygen so CO where weare terming it as gasification with oxygen so heating value is -110.5 kilo Joule permole. Combustion with oxygen can take place C + O2 then CO2 so this is again exothermicreaction. So, these two reactions are exothermic reactions which provides us heat which arerequired for other reaction say gasification with carbon dioxide C+ CO2 so that is 2CO,it is endothermic reaction and C + H2O that will also give a CO + H2 again endothermicreaction.And gasification with hydrogen that is exothermic reactions but this heat is very less. So,C + 2H 2 CH4 and at the same time under the same condition some water gas shift reactiontakes place CO + H2O reacts to form H2 + CO2 and some methanation also take place thatis CO + 3H2 gives CH4 + H2O, these two reactions are also exothermic in nature. So, overallif we see it is having both exothermic and endothermic reactions.So, initially we need to provide oxygen the reactions that energy will be released thatwill be used for these reactions.Now what are the composition of the syngas and producer gas, here it is provided hydrogenCO CO2 CH4 these are the ranges for syngas hydrogen is 20 to 30% and producer gas 13to 19% CO for syngas 40 to 60% and producer gas 18 to 22% CO2 5 to 15% for syngas and9 to 12% for producer gas and CH4 0 to 5% for syngas and 1 to 5% for producer gas. Heavierhydrocarbon may be present in this and nitrogen in case of syngas 0.4 0.5 to 4% and here 45to 55% water vapour 8 to 12% for syngas and 4% producer gas.Now the heating value which is most important in our case. So, for syngas it is 9.3 to 14.9mega Joule per meter cube where it is 4.5 to 6 mega Joule per meter cube in case ofproducer gas. Now we will see the mechanism of the gasification, How the gasificationtakes place? What is the mechanism? If we apply heat and controlled amount of oxygenthen what type of changes are going to take place?See if we have feedstock then initially you are heating so then will be char and volatilesformation, volatiles will be going off. So, that volatiles will be further gasified, charwill be further gasified and ultimately it will give us syngas. So, how these gasificationtaking place, the representation of this process is your CnHm + 1/2 nO2 that is 1/2 mH2 + nCOand char gasification, charge gasification means it will be having carbon hydrogen oxygenwe are ignoring other impurities in this case.So, then it will give this is an empirical formula this is a formula so which is explainingthat this will be hydrogen production and this will be the CO production from this char.Now you see what is happening will be trying to understand more inside the mechanism onthe mechanism okay so this is the gasifier, we are putting feed here and we are puttingoxygen or air and steam here. So, gradually feed is getting down and gas goes up. So,here is a grate, this grate is there, so solids are falling and it is getting in contact withhot gases and different types of processes are going on changes are going on and ultimatelyash is formed and ash is stored on this grate or something.It is passed through the opening of this grate and here ash collection takes place. And whatare the different types of changes on the on the coal, when it is getting entry hereit is coming in contact with gradually increasing temperature why because oxygen is providedhere so maximum oxygen is available in this zone. As you go up oxygen concentration reduces.So, once oxygen concentration reduces then there will be less combustion the temperaturewill also fall.So, temperature is highest in this case oxidation zone high temperatureand then the temperature is gradually reducing. So, oxidation after this say it is stoppedthen reduction will take place if oxygen is not available then heat is available endothermicreaction will go on and then reduction will take place. And here pyrolysis will take placehigh temperature is there oxygen is not there so pyrolysis take place and here we will behaving drying, only the moisture will goes up volatiles will goes up and then this volatilewill be gasified.And char will be gasified at this temperature and here char gasification, volatile gasificationwill take place so this is a different types of phenomena which is going on inside thegasifier that is drying, devolatilization and pyrolysis and then reductions, then oxidationand ash cooling. So, unlike incineration in case of gasification what is happening thevolatiles are not completely converted combusted because the volatiles are produced here sayso it is going off it is getting less chance to be converted more.So, for fixed bed type of reactor fixed bed type of gasifier the flue gas is sorry thesyngas which is coming out that will not be of that pure it will be having some volatiles.So, tars will be available with this. In 2006 Valero they have presented this phenomenonnicely considering the presence of ash as well as sulphur nitrogen etcetera they havegiven the molecular presentation of the coal that is CHhOoNnSsZ + aO2 and this is convertedto CO2 CO H2O H2S N2 and Z, Z is our ash. So, this is the reactions they have proposedand they have shown the mass balance and how to calculate a; how to calculate h o n s Zetcetera.So, this is one reaction that is your reactions with oxygen then reactions with carbon dioxidesimilar presentation they have provided and then reactions with steam, steam gasification,these are the proposed reactions and then reactions with hydrogen so these are the proposedreactions. So, if we use this scheme of reactions we can predict the compositions completelywhat is the H2S compositions what is nitrogen compositions what is CO what is CO2 etcetera.And important gas phase reactions which take place here that is your H2S + CO2 COS + H2Ois formed. Now we will discuss about the syngas production and efficiency.What is the syngas production? What will be the fuel gas productions or syngas production?that can be calculated on the basis of nitrogen balance and the expression is fuel gas productionnormal meter cube per kg is equal to air flow rate in normal meter cube per second into.79 divided by 1 - CO + CO2 + H2 + CH4 + C2H2 by 100 into feeding rate. Now we are assumingthat the syngas is containing these gases only it is not having any sulphur or any nitrogenin that case this formula we are using.So what is this 1 - this by this that equal to 100 that is equal to 100 - CO - CO2 - H2- CH4 - C2 H2 divided by 100 that means this is nothing but the nitrogen available in thisas we are providing controlled amount of oxygen. So, we are assuming that all oxygen is consumedso remaining will be nitrogen. So, 100 minus this is equal to nitrogen so this is the percentageof nitrogen. So, percentage of nitrogen into we are getting feeding rate and here fluegas production is given.So that will be the nitrogen available in the fuel gas and this is the nitrogen availablein the feedstock. So, nitrogen balance is there we are assuming that 79% is nitrogen.So, that way we are getting this relationship. Now we can determine the yield of hydrogenand carbon monoxide as hydrogen yield is equal to hydrogen atoms in the syngas divided byhydrogen atoms injected in the feed and CO yield that is carbon atoms in the formed COby carbon atoms injected.Now we will see the efficiency, efficiency is related with heat so obviously LHV of coldgas that is kilojoule per normal meter cube into fuel gas production that is normal metercube per kg / LHV of the coal treated + allothermal power by coal flow rates that means for unitamount of coal how much energy associated with this process is required. For examplesay for size reductions for grinding is required so that energy is needed so that is also consideredhere.So, allothermal energy divided by coal flow rate so this is the expressions of the efficiencyof the syngas production process. The conversion efficiency of thermal power plant is between30% and 40% for a single cycle steam power plant as you have discussed in our previousclass. And this can be increased up to 60% for a combined cycle gas turbine power plantalso we have seen that this is the thermodynamic limit thermodynamically that is 63% efficiencycan be increased.Now we will see the factors which influence the gasification process, the type of feedinjection. What type of feed we are injecting? So, coal may be at the top feed or maybe sidefeed. So, if we change the feeding coal feed positions then efficiency will be change becauseit will be getting different time to be in contact with the hot gas. And the gasificationagent air or oxygen may be used as you have discussed that if we use air more nitrogenwill be available in the syngas resulting producer gas and their energy value will alsobe less.The type of heating, what type of we are heating? it can be done either by partial combustionof the coal in the gasifier directly or from an external source also indirect heated alsopossible. So, both these cases efficiency will change. And the temperature range atwhat temperature we are using in a gasification reactor that will also influence the performanceand the pressure range under which the gasifiers are operated so these are different parameterswhich influence the performance of the gasification process.So different types of gasifiers have been tested and developed and used for the coalgasification and different efficiency have been achieved also.Now what are the advantage of this process? It reduces carbon dioxide emission and completecombustion is not taking place so carbon dioxide emission is reduced with respect to combustionmethod. Compact equipment requirements with a relatively small footprint, the equipmentsare very compact and footprint requirement is less. And then accurate combustion control,as we have already discussed it that we have controlled amount of oxygen we are providingand high thermal efficiency, we will get more efficiency in this method than the combustionprocess.Now we will see that how we will use this, the flowsheet and utilization schemes of gasificationproducts. So, we have coal here then we are sending it to gasifier with steam and oxygenthen we will get syngas. So, syngas has to be cleaned up first, the first stage of cleaningso we are removing the particulates here and then second stage of cleaning we are removingthe sulphur here then it is coming as a pure syngas.And syngas there will be some conditioning and then utilization so that is you can gofor processing that is that is liquid fuels and chemical synthesis. We can separate thehydrogen from the syngas and hydrogen vehicle fuel it can be used, hydrogen can be usedin fuel cells for electric power or syngas can be sent to gas turbine so it will be electricalpower production and then gas turbane exhaust can further be heated by using some additionalfuel and then it can be sent to waste heat boiler.And steam can be produced and can be electricity generated from the steam turbine. And thiscondensed from the boiler the steam can be passed here for the gasification and thatthe bottom what will get that ash that will be slag or metal recovery. So, these are thedifferent processes or schemes which can be used for the utilization of products of thegasification process. So, apart from electricity generation we can have number of opportunitiesto utilize the products which is generated through the gasification.So, we will be having more opportunity to make the process more economic and polygenerationoption is utilized and recommended for application. Now we will see major types of gasifiers forcoal gasification.So, on the basis of the feed inlet gas collection and the operating conditions different typesof gasifiers are reported out of which moving bed, fluidized bed and entrained bed are themajor 3 important gasifiers which have been used widely for the gasification of coal andother carbonaceous feedstocks. In the moving bed coal is feed at the top and the particlesize is higher say 5 to 50 millimetre particle size.So, it falls from the top and from the bottom oxygen, air and steam is sent so reactiontakes place as I have discussed in the previous slide also. So, gradually it falls and ashis getting out from the bottom. So, if we see the temperature profile across the heightof the gasifier then this will be the nature. So, if we increase the height from the bottomso gradually temperature will increase and will attain maximum then again it is decreased.Because oxidation is taking place at the middle of this so that is having the maximum temperatureit is around 1800 degree centigrade temperature it can give. But if we see the fluidized bedcoal is put here so here steam, oxygen or air then there will be some fluidization ofthe particles will be taking smaller particles here and then that will be in fluidized formand due to this reason what will be the temperature profile across its height?This is, see it will gradually increase remain constant most of the part and then it is decreasedafter this part it will be constant and it is decreased this part will temperature willdrastically decrease. And another is entrained flow reactor here we send coal and then steam,oxygen or air very finer particles are used. The particle size is much smaller than thisfluidized bed and both all these coal, steam and or air or air or oxygen flows in the samedirection so it gets less residence time and as collected here and gas goes from this atthe bottom.Due to this feature that if we see the temperature distribution along the height of it, it isalmost constant and maximum on this. But not maximum here we will we will get maximum temperaturebecause oxidation is single in certain zone. But here are uniformly distributed the phenomena.So, these are the temperature profile across the height of the gasifier for different typesof gasifiers.And this we have already come to know that here the less time is less residence timeand smallest particle size with respect to this three gasifiers. But this fluidized bedgasifiers have some advantage that it can in situ capture of sulphur.So, that is why transport reactor gasifier has been developed by KBR so in this casewe have the intermediate properties both the fluidised bed and entrained bed propertiesare available here how? we are putting here fuel then absorbent we are putting some adsorbentalso that is to capture the sulphur and then oxygen or air and steam all are here in cominginto this reactor. Then it is going up this riser section very small time it is gettingfor contact.Then it is coming to this so it is falling again and the gas is coming here particulateseparation it is coming back. So, that way here we are getting the benefits of fluidizedbed we are getting the benefits of entrained bed. So, both features are available in transportreactor so in this case we do not need to use very finer particles than that of entrainedbed. Now we will see the comparison of this type of gasifiers.So, fixed bed, fluidized bed you see these are the technology licensers BGL, Lurgy, dryash that is the fixed bed and HTW, IDGCC and KRW and Mitsui Babcock. So, these are thetechnologies licensers and typical process conditions for these are the combustion temperatureis 1300 centigrade and slurry feed if it is and if it is a dry feed then 1500 to 1800degree centigrade. But gas outlet temperature is less 400 to 500 degree centigrade and pressureis this much and particle size already discussed.Cold gas efficiency is high, for fluidized bed the combustion temperature is more butlesser than this and gas outlet temperature is more than this one. And pressure requirementalso this one and residence time is less, particle size is less, coal gas efficiencyis medium. If we go for entrained bed gasifier then 1500 degree centigrade reaction temperature,gas outlet temperature is also higher than other two types of gasifires.Then pressure requirement this and retention time is also lesser and these are the particlesize less than 200 mesh very fine particles are required. Now we see the transport reactorit is in between so temperature is 900 to 1050 degree centigrade gas outlet temperature590 to 980 and residence time 1 to 10 second just like your entrained bed and size is lessthan 50 microns. So, we do not need to get 200-270 microns and it is also it is having50 micron size.Now we will see the advanced process in gasifications, the plasma gasification is one of the advancementin the gasification process, in this case we use high voltage across the two electrodes.So, in that case the electrons are emitted then electron accelerates free electrons electrodesaccelerates free electron and free electrons helps to oxidize or ionize the ions adjacentions and molecules and then number of electrons are produced which forms a avalanche thatis called streamer.So that is called the plasma in this case torch power levels from 100 kilowatt to 200megawatt and produce energy densities up to 100 mega watt per meter cube. So, in thesecases different free radicals are formed which converts the different types of componentspresent in the coal to its oxidized form. So, C is converted to CO2, H is convertedto H2O, N is converted to NO2 S is SO2 like this but typically it has been shown thatCO and H2 gas remains in higher concentration.These are some coal based gasifiers around the world so China, Germany, Italy, Netherlandand South Africa, Spain and USA in these countries different gasifiers are the plants are there;where the gasification process is in practice and different types of products they are producing.There somewhere we are getting say FT fluids and then electricity somewhere ammonia, somewheretown gas and electricity, somewhere ammonia taking some syngas, methanol okay. And thenwe are getting somewhere FT fluids and methanol and chemicals and SNG and then electricity.So, some diesel and electricity is also produced by some plants.And different types of cleaning options are also used rectisol, sulfinol, MDEA etcetera.So, I will discuss this in next class okay so thank you very much for your patience.