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Module 1: Produzione di energia da carbone

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Conventional Route

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Hi friends now we will discuss on the topic conventional route for energy production fromcoal.In the introductory module we have discussed that coal can be used through different routeslike say combustion, gasification or direct liquification for the production of energy.Out of these routes the most conventional one is combustion based thermal power plant.So, in this module we will be discussing what is the flow sheet and what is the mechanismfor the production of electricity in a thermal power plant?What is its efficiency?What are the thermodynamic cycles and what types of reactors are used?And the contents of these are combustion based power generation flow sheet then thermodynamiccycles for power generation, then overall plant efficiency, factors influencing powerefficiency, performance of coal based power plant and conventional reactor process conditionsand efficiency.Now we will see the flowsheet, the conventional flowsheet.Here we have combustion chamber from which the flue gas goes into heat exchanger or boilerwhere combustion takes place.So, heat is released, high temperature flue gas goes to heat exchanger then from heatexchanger water comes and it forms steam.So, steam goes to turbine, steam turbine coupled with generator so power to Grid that is oneway we get the electricity.And this flue gas which is going into the boiler after exchanging the heat it will goto the environment as exhaust gas.And the steam which is coming out from turbine that will be of low pressure, that steam iscondensed and condensed water is used for the production of steam again or some partof it can be sent to the process in a cooling tower and heat recovery can be possible.So, this is conventional process.And on energy perspective if we think, then pulverized coal we have thermal processingthen we get flue gas and solid residue then flue gas heat transfer in boiler and steamand electricity production.Now we can use gas turbine also or we can use the heat of the flue gas for the productionof heat only by heat exchanger for heat application or we can produce steam only or we can produceelectricity only or we can do the combination of these things.So, we may have some options; coal combustion then boiler, the flue gas is going to boiler,it can give a steam and steam to electricity that is one route.Another is steam to heat exchanger then hot air for space heating.There is also one option, this is our combination of both, heat and steam then electricity.So, steam is coming to steam turbine, we are generating electricity and supplying it togrid and this steam is also used in the process for heating purpose.So, this is called co-generation when we are using both electricity as well as heat ina plant.Now this steam turbine can also be replaced by gas turbine, air after coal combustion,it can go to gas turbine and gas turbine to exhaust of the gas turbine, it can go to heatexchanger and then it can increase the temperature of air which can be used as a preheated airalso and then it goes to vent.And when the turbine runs then it is connected with generator and produces electricity.So, number of possibilities are there; we can use for the production of electricityor other type of energy for its application.On the basis of these different possibilities we have different types of cycling, that isthermodynamic cycles, that is topping cycle, bottoming type cycle and combined cycle.What is the topping cycle?We have fuel, air then we are using it in furnace and flue gas is produced at high temperaturethen it is used in turbine for electric power generation then the exhaust from the turbineis further added with some supplementary fuel and that is combusted and temperature of thisstream is also increased and then it is used in heat exchanger for steam generation.So, in this case at first we are getting mechanical energy then heat recovery so that is why itis called topping cycle.But for bottoming cycle we are producing heat here then that is first heat recovery takingplace in the first case whether you are using water to produce steam and then the turbinethen electricity so this is called bottoming cycle.Now in combined cycle it is a hybrid cycle and contains both topping and bottoming cycles.Now how the cogeneration helps?cogeneration means we are producing electricity as well as we are producing steam for differenttypes of applications.So, in that case we can reduce the loss of energy and we can get more efficiency of theprocess.cogeneration indicates combined heat and power generation and tri generation concept is alsodeveloped that is combined cooling heat and power, heat application cooling applicationsand electricity generations so by that way the people have seen that the efficiency canbe improved.Now this efficiency is equal to in this case electric power output plus heat output pluscooling out divided by total heat input so, that way we can calculate the efficiency.Now this table gives us some example.That how we can get overall efficiency, higher overall efficiency by using suitable combinationof different types of energy utilisation.If we use it for heat only application then recovery is 80% and overall efficiency 80%steam only there is also 80% power only 35%.So, power is the most usable form of energy so that is our major concern but here ourefficiency is less 35%.But if we combine these power applications with steam application or steam + power +cooling application then we will see that this 35% can be increased to 35 to 75% orin some cases it is 85%.So, that is why the cogeneration and tri generation concepts are being implemented.Now on the basis of this discussion it is clear to us that we can use steam turbineor we can use gas turbine or we can use both turbines for the production of electricityfrom the coal.So, for the steam turbine is governed by the thermodynamic cycles that is called Rankinecycle.So, here we will see the Rankine cycles, how the entropy and temperature changes as perthe Rankine cycle.And here it has the 4 steps the steam turbine has four steps, the step 1 adiabatic compression,what happens in this case? the pump is used to compress the water in the boiler.And then step 2 isobaric expansions, that the pressure is constant and then we producethe steam here.And then adiabatic expansion, the steam is going to the turbine and expansion is takingplace, the adiabatic process.And then isobaric compression, after that that is condensed, condensed air is sent tocondenser and this transform vapour to liquid in a constant pressure heat transfer process.So, these are the four processes of steam turbine and Rankine cycle is shown here.So, what we see from this? we see that an ideal condition can be achieved when no fluidfriction and heat loss is considered so this is the case dotted line.But this is the ideal one but this does not happen in reality because of loss due to thefriction.So, friction, fluid frictions in the boiler and pipe lines etcetera reduces the energyand we need to provide additional energy to maintain it.Now as per this Rankine cycle efficiency, thermodynamic efficiency is equal to ratioof net power output to heat input and Carnot cycle efficiency is 1 minus cold temperatureby hot temperature TC is equal 1 minus cold temperature by hot temperature.Now if we increase the temperature hot temperature then the efficiency is more that we have discussedin the introductory module also that supercritical conditions if we can use then we can enhancethe efficiency.But how much? that is guided here if we use steam turbine NT temperature is typicallyaround 565 degree centigrade and steam condenser temperatures are around 30 degree centigradethen Carnot efficiency is 63%.But actually overall thermal efficiency up to 42% is available it is possible with moderncoal-fired power station.In the previous slide I have shown you it is 35% subcritical conditions but the supercriticalconditions which may be increased.So, thermal Carnot cycle efficiency is 63% that is a maximum but so far there is no turbinewith the steam turbine with this much of efficiency this is up to 42% as you put it here.Now we will see the Brayton cycle who did used to explain the gas turbine.So, it also has 4 steps in this case what happens now air is used and compressed sothen it is sent to the furnace for the heat exchanger to heat it up and then basicallythe combustion chamber so it is the isobaric process so it expands.So, expansions and then we are going for step 3 that is isentropic process, the heated andpressurized air then gives up its energy in the turbine blade so that is the case expandingto the turbine.And then isobaric process, heat rejection in the atmosphere so these are the steps ofthe gas turbine cycles and efficiency is equal to 1 – T 1 by T 2 just like Carnot cycleefficiency or T 1 by T 2 can be replaced by P 1 by P 2 the power 1 - gamma by gamma alreadywe have discussed we studied this in thermodynamics subjects and here gamma is the heat capacityratio.So, what we can guess from this relationship? if we can increase the temperature then wecan get more efficiency.but how much?We are using the air in a combustion chamber and then in the combustion process the materialwhich you are using that that will be having some resistance power so maximum is a 1500to 1800 degree centigrade can be used or we can convert the pressure ratio change thepressure ratio P1 by P2.By pressure ratio change we can change the efficiency of it and it has been reportedthat the pressure ratio of gas turbine ranges from 11 to 16.So, pressure ratio as well as operating condition temperature both influence the performanceof the gas turbine.But in actual case the Brayton isentropic process is not applicable we get adiabaticprocess, adiabatic that is compression then isobaric process heat addition and adiabaticprocess expansion in turbine and then isobaric process heat rejection.So, these are the different steps of the thermodynamic cycles of steam and gas turbines.Now we will see now what is a performance of a coal based power plant how can we defineit and how this will be influenced by other factors obviously we are using coal.So, if we have high heating value then the performance will be higher and the thermalconversion units which we are using, the combustor in some cases, the combustion or the furnaceand boiler are combined in some cases, 2 are different units.So, there are variations in the technology and the heat recovery is also different.So, that way the efficiency of the thermal processing unit will also influence the overallefficiency of the process.And then thermal efficiency of the boiler, the thing as I already discussed, and thenefficiency of the turbine and generator, in the boiler we are getting steam and then thatsteam is used in turbine and generator.So, all those units’ turbines and generators have their own limitations and efficiencyso that will also be considered into the overall efficiency of this, it will influence.Then energy requirements of the air pollution control systems as I have shown after theheat recovery from the flue gas it is allowed to go into the atmosphere the exhaust gas.So this exhaust gas temperature at which temperature we are releasing it and what type of fluidswe are using for the heat exchanger purpose all those things will be influencing the performanceof the power plant as just we have discussed, that if we use the cogeneration or tri generationthen I have sensible increase.So, all those factors influence the performance of the thermal power plant, coal based thermalpower plant.And overall efficiency can be expressed as that is efficiency of boiler efficiency ofturbine efficiency of generator and emissions of works; works efficiency.Now it is found that full heating value and what is the size of the plant that will influencethe economy or the performance of the process.Apart from this, notice the pressure, the steam is produced that will also influencejust we have discussed, the pressure ratio.So, pressure it is used that will also influence the performance.The type of condensing device air or water so heat transfer coefficient is differentso more the transfer coefficient more efficient to the heat transfer or recovery, so thatalso influence the performance of the process in the heat source to preheat combustion airthat will also if we are using recovering some amount of energy or not.Actually 60 to 80 degree centigrade, this temperature stream in the industries are verypoorly utilized.So, if we can have some arrangement to utilize this heat then also increase the efficiency.And steam or energy recovered from flue gases, the number of boilers and turbines, that wehave discussed whether it is a bottoming cycle, topping cycle or combined cycle.The combined cycle will give us more efficiency.Now we will discuss the performance of coal based power plant.The performance of a power plant can be expressed by heat rate as well as thermal efficiency.So, what is heat rate?heat rate is nothing but it is the ratio of the energy input to electrical energy output.So, HR a heat rate is heat energy input supplied by the fuel to the power plant for a periodso that we can calculate in kilocalorie divided by energy output from the power plant in thatperiod that is kilowatt hour.So, heat rate is this one the heat energy input supplied by the coal, that how muchfuel you have used during this time and what is the heating value of it.So, MC how much fuel we have used and CV calorific value of it, that is the heat used and thenhow much we are getting that energy generated into time.How much time the plant is running and during this time how much energy is produced, thatis kilowatt hour.Now what is the thermal efficiency?thermal efficiency is the inverse of this, your heat recovery, that is 100 by HR intokilo calorie per kilowatt hour.So, thermal efficiency is defined as the energy generated into time by amount of fuel usedinto the calorific value of the fuel.Now how much energy we're getting per unit amount of heating value of the fuel, thatis an energy efficiency.So, thermal efficiency is equal to 100 divided by, we are getting MC into CB divided by energygenerated into time.So, here we see MC divided by energy generated by time is nothing but the HR so we can get100 divided by HR, so this is kilo calorie per kilowatt hour.But this if we want to get in percentage then these two units have to be same.So, the unit conversion is indeed , we know that one kilowatt hour is equal to 859.846kilo calorie so we can put this value here and this is a relationship for the determinationof thermal efficiency in percentage.So we will put the value HR in kilocalorie per kilowatt hour and multiply and we putthis in numerator and denominator heat rate in kilo calorie per kilowatt hour then wewill get the thermal efficiency in percentage.So, I will show you some example, if the average heat rate of a power plant in 2018 is 2611kilo calorie per kilowatt hour then calculate the average efficiency of the plant.So, how we can do it? thermal efficiency we know 100 divided by MC into CV divided byenergy generated into time.So, 100 divided by heat energy inputs applied by fuel to the power plant for a period byenergy generated for the period.So, what is in our case?HR is given 2611 kilocalorie per kilowatt hour we will be using this formula 859.5856divided by HR so 100 into 859.8456 divided by 2611 as given in the statement, it is comingto 33%.Now we will see whatever reactors are used for the production of electricity in a thermalpower plant using coal.The conventional reactor type is fixed bed type and in some cases we also get fluidizedbed.so, fixed bed and fluidized bed , these are mostly used reactor type in thermal powerplant.And in this case what is the condition of the boiler operation? it is mostly in subcritical conditions and some advanced plants are using supercritical boiler and also gettingmore efficiency than the subcritical boiler.And direct use of coal without washing so the coal is having high ash and sulphur content.If so we need to remove the ash by washing, after washing we will be using it for thermalpower plant.And then in this case the efficiency is around 43%.So, up to this in this class thank you very much for your patience.