Module 2 PDF - Lecture Notes on Heat Engines and Thermodynamic Cycles

Department of Chemical Engineering CHE 5313 / 6313: Energy and the Environment Lecture: Heat Engines and Thermodynamic Cycles Mr. Bradley J. McPherson P. Eng. Department of Chem...

Department of Chemical Engineering CHE 5313 / 6313: Energy and the Environment Lecture: Heat Engines and Thermodynamic Cycles Mr. Bradley J. McPherson P. Eng. Department of Chemical Engineering University of New Brunswick Department of Chemical Engineering In order to generate electricity, we’ve seen that several steps must occur to extract the stored energy in our natural resource … We’ll now consider the “heat engine” term … Thermodynamics! Heat, light Fossil Fuel combustion Electrical Generators Energy Mechanical Heat Engine Energy Transportation, Nuclear Fuel Fission; fusion Motors Industry Department of Thermodynamics Chemical Engineering Thermodynamics is the study of energy (heat) and how it is used to produce tangible things such as work. It stems from some physical laws: – Conservation of energy; – Conservation of mass. It is applied to the production of electricity because most of the methods used to create electricity depend on transferring heat from hot reservoir to a cold reservoir. Department of Chemical Engineering Energy Equation: – The energy equation is a statement of the 1st Law of thermodynamics - conservation of energy; – “Energy cannot be created or destroyed, it may only be transformed from one form into another”. z1g + 21 V12 + u1 + p1v1 + win + qin = z2g + 21 V22 + u2 + p2v 2 + wout + qout Department of Chemical Engineering Potential Energy - this is the energy that a fluid has due to its elevation above a specific point. mzg (J) or zg (J/kg) Total Energy Specific Energy Kinetic Energy - this is the energy that a fluid has due to its velocity. ½ m V2 (J) or ½ V2 (J/kg) Total Energy Specific Energy Department of Chemical Engineering Internal Energy - this is the energy that a fluid has due to the movement and vibration of its individual molecules. U (J) or u (J/kg) Total Energy Specific Energy Flow Work - this is the energy required or emitted from the fluid in moving a slug of fluid of volume V against a pressure p. pV (J) or pv (J/kg) Total Energy Specific Energy Department of Chemical Engineering Mechanical Work - this is the actual work applied to or produced in the system. W (J) or w (J/kg) Total Energy Specific Energy Heat - this describes the heat added to or produced by the system. Q (J) or q (J/kg) Total Energy Specific Energy Department of Chemical Engineering Quite often, some of these terms are negligible and can be omitted. We can also define new functions based on terms that are used frequently. for example - the product of internal energy and pv work occurs often, thus we define the specific enthalpy. h  u + pv (J/kg) Department of Chemical Engineering The energy equation, as applied to power production, generally takes the following forms: – In a steam boiler, if elevation and velocity terms are neglected and no work is exchanged the energy equation becomes: h1 + qin = h2 – Thus: qin = h2 - h1 – In an ideal turbine, if elevation and velocity terms are neglected and no heat is exchanged: h1 = h2 + w out w out = h1 − h2 Department of Chemical Engineering We’ve seen that the First Law states that: “Energy can neither be created nor destroyed but only transformed from one form into another”. The Second Law relates the conversion of heat energy to work and its limitations: “No heat engine can generate work without net rejection of heat to a low temperature reservoir” Department of Chemical Engineering The efficiency () of the thermodynamic QREJ process is then related to the useful work Heat Rejected output produced and the total heat input required to produce it. Heat Input (fuel) Work Output work output QIN WOUT = heat input (mechanical work or electricity) Wout = Qin Department of Chemical Engineering The First Law applied to this situation gives: Wout + Qrejected = Qin The Second Law then states that: Qrejected  0 Wout  Qin Department of Carnot Cycle Chemical Engineering The Carnot Cycle is the thermodynamic ideal of conversion of heat energy into work. It is best represented on a plot of temperature (K) against the specific entropy (J/kg K) – known as the T-s Diagram. Entropy is a useful quantity in thermodynamics, although it cannot be measured directly. We can imagine it as defining the quality or grade of energy in a fluid. Department of Carnot Cycle Chemical Engineering The total heat input to this system is the complete area under points 1 & 2. (Qin) The work output is the area enclosed by the thermo- dynamic cycle, points 1, 2, 3 & 4. (Wout) This leaves the heat rejected as the area under the temp- erature of heat rejection, points 3 & 4. (Qrejected) Department of Chemical Engineering The efficiency of the cycle is given by: Wout ( TH − TC ) s TH − TC carnot = = = Qin THs TH We can also define the Carnot cycle in terms of available energy and unavailable energy. – Available Energy: the energy available to produce useful work. – Unavailable Energy: the energy rejected to the environment. Department of Thermodynamic Cycles Chemical Engineering The Carnot cycle is the thermodynamic cycle to which all other cycles are compared. It is the most efficient heat engine that can be devised. The higher the temperature of the hot reservoir and the cooler the temperature of the cold reservoir the higher the cycle efficiency. Typically, the high temperature is limited by materials of construction while the low temperature is limited by ambient conditions. Department of Chemical Engineering Typical thermodynamic cycles include: – Rankine Cycle – steam generation and use; – Brayton Cycle – combustion turbines; – Otto/Diesel Cycles – engines; – Refrigeration Cycles: Vapour-compression; Etc … We’ll look at the first two in detail … Department of The Rankine Cycle Chemical Engineering This is the basic cycle for a steam turbine. – What happens in a power plant? Department of Chemical Engineering B&W 450 MW Radiant Boiler for pulverized coal Department of Chemical Engineering CHE 5313 / 6313: Energy and the Environment Lecture 7: Typical Rankine Cycles for Power Generation Mr. Bradley J. McPherson Department of Chemical Engineering University of New Brunswick Department of Chemical Engineering B&W 450 MW Radiant Boiler for pulverized coal Department of Turbine / Generator Set Chemical Engineering Department of Rankine cycles Chemical Engineering We can plot the cycle on a T-s diagram, as we did for the idealised Carnot cycle. The T-s diagram for water incorporates a bell-shaped curve depicting the saturated conditions for water. Department of Chemical Engineering Department of Chemical Engineering In real systems, the Rankine cycle for water-steam for example, the heat is not all input at a constant temperature, thus the efficiency is somewhat less than in the idealised Carnot cycle. For the Rankine cycle: – Heat is added to the water in preheaters and the boiler to produce saturated steam; – The steam is expanded from high pressure to low pressure in the turbine producing work, and; – Heat is rejected through condensation of the low pressure steam. This is typically done under vacuum conditions to minimize the temperature of heat rejection. Department of Saturated Rankine Cycle Chemical Engineering Work lost from Carnot cycle The efficiency of the cycle is still calculated in the same manner: Boiling Wout = Qin Condensation But we can see that the efficiency will be somewhat less than in the Carnot cycle. Department of Saturated Rankine Cycle Chemical Engineering The saturated Rankine cycle follows progressively from Point 1 through 5: 1 to 2 – low pressure water pumped, isentropically (at constant entropy), to high pressure; 2 to 3 – water is heated to saturation isobarically (at constant pressure); 3 to 4 – water is boiled to saturated steam, isobarically; 4 to 5 – steam is expanded in the turbine (ideally isentropic); 5 to 1 – wet steam is condensed back to saturated water (isobarically). Department of Chemical Engineering For the Rankine cycle, work is produced through steam expansion in the turbine and used in pumping (compressing) the water to high pressure. The efficiency can be calculated from the specific enthalpy values of the water/steam at each of these points: Note that we assume some of the work produced is used in compression … h= mwturbine - mwpump = ( h 4 ) ( - h5 - h2 - h1 ) mq h4 - h2 Department of Chemical Engineering Consider a typical steam boiler operating under the following conditions: – Pboiler = 4 MPa; – Pexhaust = 0.005 MPa (5 kPa absolute pressure). How do we determine the enthalpies for points 1 through 5 in order to determine the efficiency of this process? Department of Chemical Engineering The Steam Tables can be used to directly evaluate the enthalpies at each stage of the process. Steam Tables are a tabulation of the thermodynamic properties of sub-cooled, saturated and superheated water/steam as a function of temperature and pressure. Department of Chemical Engineering This saturated steam table shows the thermodynamic properties of the saturated liquid, the saturated vapour and the difference between the two. For enthalpy, these are designated as: – hf enthalpy of the saturated fluid (kJ/kg); – hg enthalpy of the saturated vapour or gas (kJ/kg); – hfg the latent heat of vapourization (h g-hf) (kJ/kg). These are similarly defined for the other parameters: – Specific Volume -  (m3/kg) – Specific Internal Energy – u (kJ/kg) – Specific Entropy – s (J/kg K) Department of Chemical Engineering Department of Chemical Engineering Thus, from this saturated steam table we can get the enthalpies required for points 3 and 4. – h3 = hf = 1087.31 kJ/kg; – h4 = hg = 2801.40 kJ/kg; – Tsat = 250.4oC. Likewise, we can go to Pexhaust = 0.005 MPa in the saturation pressure table to determine the enthalpy at point 1: – h1= hf = 137.82 kJ/kg; – Tsat = 32.88oC. Department of Saturated Steam Table Chemical Engineering Department of Chemical Engineering For points 2 & 5 we must consider a sub-cooled liquid and a wet steam mixture respectively. This will generally require interpolation in the steam tables. For Point 2: – P = 4 MPa, T ≈ 33oC (tabulated values are at 2.5 MPa and 5 MPa and between 20oC and 40oC); – h2=141.91 kJ/kg. Department of Chemical Engineering Department of Chemical Engineering For Point 5 we go to the saturated steam tables again and make the assumption of isentropic expansion (ideal case – no frictional losses) in the turbine from Point 4. Thus: s4 = s5 = 6.0701 J/mol K We now interpolate to estimate the quality (x) of the steam at Point 5: s x − s f h x − hf s x = sf + x sfg x= = sfg hfg Thus, x = 0.706 and h5 = 1849.90 kJ/kg Department of Saturated Water Table Chemical Engineering Department of Chemical Engineering Putting these all together: h1 = 137.82 kJ/kg; h2 = 141.91 kJ/kg; h3 = 1087.31 kJ/kg; h4 = 2801.40 kJ/kg; h5 = 1849.90 kJ/kg. h= ( h 4 ) ( - h5 - h2 - h1 ) h4 - h2 h= ( 2801.40 -1849.90) - (141.91-137.82) 2801.40 -141.91 h = 0.356 Department of Chemical Engineering Note that the exit steam quality from the saturated Rankine cycle is low (~70%) indicating a large moisture content (~30% moisture – water droplets). This is undesirable because the water droplets will impinge on the turbine blades and cause increased friction (loss in efficiency) and erosion (premature wear). This deficiency of the saturated cycle can be overcome if we superheat the steam before expanding it in the turbine. Department of Superheated Rankine Cycle Chemical Engineering The superheated Rankine cycle follows progressively from Point 1 through 7: 1 to 2 – low pressure water pumped, isentropically, to high pressure; 2 to 3 – water is heated to saturation isobarically; 3 to 4 – water is boiled to saturated steam, isobarically; 4 to 6 – steam is superheated isobarically; 6 to 7 – steam is expanded in the turbine (ideally isentropic); 7 to 1 – wet steam is condensed back to saturated water (isobarically). Department of Chemical Engineering Consider a steam boiler operating under the following conditions: – Pboiler = 4 MPa; – Pexhaust = 0.005 MPa; – Tsuperheat = 450oC. From the steam tables (same procedure as above) we get: – h1 = 137.82 kJ/kg; h2 = 141.91 kJ/kg; h3 = 1087.31 kJ/kg; h4 = 2801.40 kJ/kg; h6 = 3316.2 kJ/kg; h7 = 2079.0 kJ/kg. – x = 0.80. Department of Chemical Engineering The efficiency is calculated the same way: wturbine - wpump h= q h= ( h 6 ) ( - h7 - h2 - h1 ) h6 - h2 h= ( 3316.2 - 2079.0) - (141.91-137.82) 3316.2 -141.91 h = 0.388 Department of Chemical Engineering The superheated Rankine cycle has: – Led to an improved efficiency of the overall cycle (38.8% vs 35.6%); – Improved the conditions at the turbine exhaust (steam quality of 80% vs 70% for the saturated cycle). Further improvements can be made by going to a superheated-reheated cycle (this is the boiler – turbine system shown earlier).  Superheated-Reheated Department of Chemical Engineering Rankine Cycle The superheated-reheated Rankine cycle follows progressively from Point 1 through 10: 1 to 6 – same as the superheated Rankine cycle; 6 to 8 – steam is expanded to an intermediate pressure; 8 to 9 – steam is reheated to the initial temperature at the new, intermediate pressure; 9 to 10 – steam is expanded in the turbine to the original exhaust pressure. 10 to 1 – wet steam is condensed back to saturated water. Department of Chemical Engineering The superheated-reheated Rankine cycle usually operates at higher pressures and higher temperatures than what we have shown for the previous two cycles, but the principles of operation are the same. The efficiency of this cycle is calculated in the same manner, except we must now account for two stages of work production and an additional stage of heating. w HPturbine + w LPturbine − w pump Thus: = qsup erheat + qreheat = (h6 − h8 ) + (h9 − h10 ) − (h2 − h1 ) (h6 − h2 ) + (h9 − h8 ) Department of Chemical Engineering This leads to: – An improved overall efficiency of the process; – A much improved steam quality at the turbine exhaust (this is the main reason for using this cycle – can typically get to 5% moisture or less i.e., > 95% steam quality); – More work output per unit mass of steam. Department of Chemical Engineering CHE 5313 / 6313: Energy and the Environment Lecture 8: Feedwater Heating and Non-ideal Energy Conversion in Turbines and Pumps Mr. Bradley J. McPherson P. Eng. Department of Chemical Engineering University of New Brunswick  Regenerative Saturated Department of Chemical Engineering Rankine Cycle If we could extract steam directly from the turbine, as it was producing work, and use it to preheat the feedwater we would get the diagram show on the right. Since the energy represented by areas GHKF & MBEJ are equal, the Rankine cycle represents the ideal Carnot cycle – HBCL. Department of Chemical Engineering This heat input to the feedwater would require an infinite amount of feedwater heaters, thus this is considered the idealised case of the Rankine cycle with regenerative feedwater heating. Obviously, this isn’t possible but most plants will make use of a certain number of feedwater heaters to help improve the overall efficiency of the system.  Open/Direct Contact Feedwater Department of Chemical Engineering Heaters The fraction of steam required to heat the feedwater from Point 2 to Point 3 is given by m. This can be calculated by a simple energy balance on the heater.  Department of Chemical Engineering The energy balance around this heater just 6 equates the heat given up by the steam to the heat m gained by the feedwater: 3 2 1-m Heat lost by steam = heat gained by feedwater m ( h6 − h3 ) = (1 − m )( h3 − h2 ) Open Heater or Deaerating Heater m= ( h3 − h2 ) ( h6 − h2 ) Department of Chemical Engineering Once m is known, then the efficiency of the cycle is calculated as before, only taking into account the difference in flow rates between different points: w turbine − w pump w1st stage + w 2nd stage − w pump1 − w pump2 = = qin qin = ( h5 − h6 ) + (1 − m )( h6 − h7 ) − (1 − m )(h2 − h1 ) − (h4 − h3 ) (h5 − h4 ) Department of Chemical Engineering Most plants will only have one open/direct contact heater which is used, primarily, for deaeration (oxygen removal) – called the deaerator. Department of Chemical Engineering The most common type of feedwater heater is a closed or surface heater. This is essentially a shell-and-tube heat exchanger/condenser. Its operation is significantly different than the open heater, however the method of analysis is the same. Department of Chemical Engineering Department of Chemical Engineering As with the heat balance on the open heater, the fraction of steam required, m, is calculated by an energy balance around the heater: 6 m ( h6 − h8 ) = (1)( h3 − h2 ) m m= ( h3 − h2 ) 3 2 (h6 − h8 ) Surface Heater 1 8 9 Note: throttling from Point 8 to Point 9 creates no work and is a constant enthalpy process (h8 = h9). Department of Chemical Engineering The efficiency of this cycle is given by: w5®6 + w 6®7 - w1®2 h= qin3®5 = ( h5 − h6 ) + (1 − m )( h6 − h7 ) − ( h2 − h1 ) (h5 − h3 ) In real plants, several feedwater heaters of this type are used to increase the overall efficiency of the process. Analysis can be performed on the cost/benefit of each additional feedwater heater.  Department of Chemical Engineering Parameters for chart: Steam Pressure = 4 MPa Steam Temperature = 500oC Saturation Pressure = 250oC Exhaust Pressure = 0.0042 MPa Exhaust Temperature = 30oC Turbine Internal Efficiency = 0.85 Department of Chemical Engineering Depending on plant size and desired configuration, six to eight feedwater heaters are typically used. One of these heaters is typically a surface heater used for deaeration. Department of Chemical Engineering Department of Chemical Engineering Department of Saturated-reheated cycle Chemical Engineering A nuclear power plant operates on a saturated-reheated cycle since only saturated steam can be produced due to its configuration. Thus, they employ moisture separation and reheating: Note – points 3, 10 & 11 are at the same point in the deaerator n – fraction of flow bled from HP turbine s – fraction of flow separated as moisture r – fraction of flow used to reheat Department of Configurations of Nuclear Reactors Chemical Engineering Boiling Water Reactor (BWR) Department of Chemical Engineering Pressurized Water Reactor (PWR) Department of Chemical Engineering CANDU (CANada Deuterium Uranium) … steam cycle much like a PWR … Department of Chemical Engineering Calculating efficiency on real systems is really just an accounting procedure. – Heat balances on all the feedwater heaters give equations describing h and fractional flow (r, n, s etc.); – Any separations are also accounted for; – In the end, you come up with a system of linear equations to solve. You’ll always have one equation for each unknown. Department of Turbine & Pump Efficiency Chemical Engineering To accurately calculate cycle efficiency, we also must take into account the internal efficiency of the turbines and pumps. Friction on the turbine blades and pump impellers cause irreversible heating resulting in an increase in entropy (remember – steam expansion in the turbine and compression in a pump are only ideally isentropic processes). In the case of the work output and expansion occurring in the steam turbine, it is more convenient to plot the process on an Enthalpy-Entropy Diagram (h-s diagram – commonly known as the Mollier Chart). This is similar to the T-s diagram only the saturation “bell” curve is slightly skewed. Department of h-s diagram (Mollier Chart) Chemical Engineering Department of Chemical Engineering On this chart, throttling (an isenthalpic process, ie. h=0) is simply represented by a horizontal line from the high pressure to lower pressure. The constant entropy process of ideal steam expansion in a turbine is thus represented by a vertical line from the initial pressure to the final pressure. Department of Chemical Engineering The internal efficiency of the turbine is defined as the ratio of actual work produced to the ideal work produced: Dhactual hinternal = Dhideal Thus, if internal is given or known the work output in the overall cycle efficiency can be adjusted accordingly. Department of Chemical Engineering This diagram illustrates different turbine expansion lines for different applications. The internal efficiency of modern turbines is typically greater than 80%. Blue line: supercritical steam cycle Red line: conventional fossil steam cycle Green line: nuclear steam cycle  Example – Non-ideal turbine Department of Chemical Engineering calculations The internal efficiency of the turbine is defined as the ratio of actual work produced to the ideal work produced: Dhactual hinternal = Dhideal Thus, if internal is given or known the work output in the overall cycle efficiency can be adjusted accordingly. Department of Chemical Engineering The previous slide describes the definition of efficiency in a turbine. To apply this equation, follow these steps: – Assume ideal turbine behaviour and calculate Δhideal upon expanding from the high pressure to the low pressure. You can do this using a Mollier chart or directly from steam tables. If you use steam tables, you’ll have to calculate the enthalpy at point 2 using interpolation assuming the entropy is constant from point 1 to point 2. – Given a turbine efficiency, calculate the Δhactual for the expansion. – Once Δhactual is known, evaluate the final enthalpy value and use this in calculating the actual work output. Department of Example Chemical Engineering Point 1: 0.3 MPa superheated steam at 500oC: h1 =3486.0 kJ/kg s1= 8.3215 kJ/kgK Point 2: 0.005 MPa wet steam: h2=? s2=? (assume 8.3215 kJ/kgK) In the saturated liquid steam tables for a pressure of 0.005 MPa: hf = 137.8 sf = 0.4764 hfg = 2423.7 sfg = 7.9187 hg = 2561.5 sg = 8.3951 Department of Chemical Engineering To perform the calculation, under ideal conditions, s2 = 8.3215 kJ/kg K thus: s2 = sf + xsfg 8.3215 = 0.4764 + x7.9187 8.3215 - 0.4764 x= 7.9187 x = 0.991 This gives the exact point of extraction and the steam quality. Department of Chemical Engineering Since we now know the steam quality, we use it to calculate the ideal enthalpy in the same manner: h2 = hf + xhfg h2 = 137.8 + 0.991´ 2423.7 = 2539.7 Giving Δhideal = 3486 - 2539.7 = 946.3 kJ/kg Now, assuming 85% efficiency in the turbine: Dhactual = hinternalDhideal = 0.85 ´(946.3) = 804.4 kJ / kg Department of Chemical Engineering Now that we know the actual enthalpy change, we easily calculate the actual final enthalpy: h2acutal= h1 - Δhactual = 3486 - 804.4 = 2681.6 kJ/kg This is the value you’d use to evaluate the work output in a non-ideal turbine. Department of Chemical Engineering A similar process is used for evaluating non-ideal pumps in compressing the fluid. The efficiency of a pump is defined as the ratio of the work required to pump the fluid under ideal conditions to the actual work needed to pump the fluid. Dhideal hpump = Dhactual Department of Chemical Engineering ChE 5313 / 6313: Energy and the Environment Lecture 9: Brayton Cycles, Combined Cycles and Co-generation Mr. Bradley J. McPherson P. Eng. Department of Chemical Engineering University of New Brunswick Department of Gas Turbines Chemical Engineering Brayton Cycle: this is the thermodynamic cycle used in gas turbines. Another form of the “heat engine” converting stored chemical energy to useful work (mechanical or electrical). Heat, light Fossil Fuel combustion Electrical Generators Energy Mechanical Heat Engine Energy Transportation, Nuclear Fuel Fission; fusion Motors Industry Department of Chemical Engineering In the Brayton cycle, the gas (compressed air), receives heat from the combustion of a fuel (natural gas, jet fuel etc...). Expansion of these hot gases in the turbine section create work, however, some of this work is used in compressing the air to begin with. Not widely used for power production until recently when high-temperature materials for the turbine blades were developed. Department of Chemical Engineering Note: Wnet is commonly used for propulsion, jet aircraft engines or to generate electricity Department of Chemical Engineering Again, the efficiency is still calculated in the same manner: Wout h= Qin But we must account for the work being used to drive the compressor, thus: Wturbine - Wcompressor h= Qin h= ( h 3 ) ( - h4 - h2 - h1 ) = 1- (h - h ) 4 1 (h 3 - h2 ) (h - h ) 3 2 Department of Chemical Engineering To a large extent, the efficiency of a gas-turbine system is dependent on the compression ratio … i.e. the pressure in the combustion chamber over the ambient pressure. p2 p3 Compression ratio: = p1 p4 From thermodynamics, for an ideal gas: where k = specific heat ratio Cp P = rRT R = Cp - Cv k= Cv where T (k -1) k and æ P1 ö 1 =ç ÷ T2 è P2 ø Department of Chemical Engineering Assuming constant Cp & Cv of the gasses over the temperature range (not typically valid but good for rough estimates) the efficiency of a gas turbine may be estimated by: 1 1 h = 1- h = 1- æ p2 ö (1-Cv Cp ) or æ p2 ö ( k -1) k çp ÷ çp ÷ è 1ø è 1ø Department of Chemical Engineering Based upon the previous equation, the efficiency of an ideal air-standard Brayton cycle is easily calculated: Department of Chemical Engineering Real gas turbine systems have considerable efficiency losses, as do steam turbines, due to non-isentropic compression in the compressor and non-isentropic expansion in the turbine section: Compression Losses Expansion Losses Department of Chemical Engineering For the compressor, actual compression efficiency is: (k -1) k æ p2 ö ç ÷ -1 h1 - h2s è p1 ø hcompressor = = h1 - h2 T2 -1 T1 Work required for the compression is (note this is a negative representing work input): æ T ö ( ) Wcompressor = Cp T1 - T2 = CpT1 ç 1- 2 ÷ è T1 ø Combining, actual compressor work is: CpT1 æ æ p ö ( k -1) k ö Wcompressor = ç 1- ç 2 ÷ ÷ hcompressor çè è p1 ø ÷ø Department of Chemical Engineering For the turbine, actual expansion efficiency is: T4 1- h3 - h4 T3 hturbine = = ( k -1) h3 - h4s æ p4 ö k 1- ç ÷ è p3 ø Work output is: æ T ö ( ) Wturbine = h3 - h4 = Cp T3 - T4 = CpT3 ç 1- 4 ÷ è T3 ø Thus, combining the actual turbine work is: æ æ p ö (k -1) k ö Wturbine = hturbineCpT3 ç 1- ç 4 ÷ ÷ çè è p3 ø ÷ø Department of Chemical Engineering By knowing or estimating the compression and expansion efficiencies, the temperatures at point 2 and point 4 are readily calculated: ì éæ p ö (k -1) k ù ü ï 1 ê 2 ú ïý T2 = T1 í1+ - 1 ïî hcompressor êçè p1 ÷ø úï ë ûþ ì é æ p ö (k -1) k ùü ï ú ïý T4 = T3 í1- hturbine ê1- ç 4 ÷ ïî ê è p3 ø úï ë ûþ Department of Chemical Engineering Gas turbine efficiency can be increased by incorporating multiple compression stages with cooling in between … Department of Chemical Engineering Or, by recuperating some of the exhaust heat to reduce fuel consumption … Department of Combined Cycles Chemical Engineering As we’ve seen, steam and gas turbine cycles typically have thermodynamic efficiencies in the range of 30-35%. This costs money as a more efficient use of the heat input would yield more useful work and reduce environmental emmissions. Some efficiency improvement can be realized by utilizing the hot exhaust gasses from a gas turbine Brayton cycle to generate steam in a Rankine cycle in an intermediate Heat Recovery Steam Generator (HRSG). This is called a combined-cycle mode of operation. Department of Chemical Engineering Department of Chemical Engineering Department of Chemical Engineering Department of Chemical Engineering The efficiency of a combined-cycle plant is obviously higher than that for each of the individual cycles since we are using more of the combustion gas heat to generate electricity. To evaluate, consider: hg - gas-cycle efficiency hs - steam-cycle efficiency hcc - combined-cycle efficiency Department of Chemical Engineering The amount of heat exiting the gas turbine for use in the steam cycle is: qs = qin - wg = qin (1- hg ) Thus the work output of the steam cycle is its efficiency multiplied by this available energy: ws = hs qin (1- hg ) Department of Chemical Engineering Using our basic relation for efficiency: wout w g + ws wg ws hcc = = = + qin qin qin qin ( ) hcc = hg + hs 1- hg = hg + hs - hghs The combined cycle efficiency will always be lower than the sum of the efficiencies of the individual cycles, however we are achieving much more efficient use of the input heat. Department of Chemical Engineering Consider the combined-cycle system shown. What is its efficiency? Gas-turbine parameters: – Tamb = 20oC – P2 = 1 MPa – T3 = 1000 K – T2 & T 4 = ? – P4 = 0.1 MPa  compressor = 87%  turbine = 90% Steam turbine parameters: – Tsteam = 450oC – Psteam = 4 MPa – Pdeareator = 0.2 MPa – Pcondenser = 0.005 MPa – Turbine efficiency = 90% Department of Chemical Engineering We first, can easily calculate the ideal gas-turbine efficiency based on the compression ratio (10:1): where Cv is 0.717 & Cp is 1.004 1 hg = 1- (1-0.715) = 0.481 10 ( ) However, using the given compression/expansion efficiencies we find: æ æ p ö (k -1) k ö Wturbine = hturbineCpT3 ç 1- ç ÷ 4 ÷ çè è p3 ø ÷ø Using the average Cp & k of air and the combustion gasses (Cp ~ 1.148 kJ/kgK; k ~ 1.334): Wturbine = 453.0 kJ/kg Department of Chemical Engineering For the compressor work: CpT1 æ æ p ö ( k -1) k ö Wcompressor = ç 1- ç 2 ÷ ÷ hcompressor çè è p1 ø ÷ø Using the Cp & k of air (Cp = 1.004 kJ/kg K; k = 1.4): Wcompressor = -314.7 kJ/kg Next we need the heat input from T 2 – T3: ì éæ p ö (k -1) k ù ü ï 1 ê 2 ú ïý = 606 K T2 = T1 í1+ - 1 ïî hcompressor êçè p1 ÷ø úï ë ûþ Qin = Cp (T3 -T2 ) = 395.1 kJ/kg Department of Chemical Engineering So, the actual gas turbine efficiency is: Wturbine - Wcompressor 453.0 - 314.7 hg = = = 0.35 Qin 395.1 Note the large difference between the ideal (isentropic) gas turbine versus this more realistic number … 35.0% vs 48.1% Department of Chemical Engineering We next, evaluate the steam cycle … wturbine - wpump hs = qs w1st stage + w 2nd stage - wpump1 - wpump2 = ( qin 1- hg ) hs = ( h5 ) ( )( ) ( )( - h6 + 1- m h6 - h7 - 1- m h2 - h1 - h4 - h3 ) ( ) (h 5 - h4 ) Using actual turbine efficiency this gives ηs= 0.357 … Department of Chemical Engineering The overall, combined cycle efficiency is: hcc = hg + hs - hghs ( )( = 0.350 + 0.357 - 0.350 0.357 ) = 0.582 58.2% … that’s quite a bit better use of the fuel input! Department of Chemical Engineering CHE 5313 / 6313: Energy and the Environment Lecture 10: The Heat Input (QIN) Term – Combustion Analysis Mr. Bradley J. McPherson P. Eng. Department of Chemical Engineering University of New Brunswick Department of Chemical Engineering In order to generate electricity, we’ve seen that several steps must occur to extract the stored energy in our natural resource … We’ll now consider the “heat input” term in the overall efficiency of an energy-producing system. Heat, light Fossil Fuel combustion Electrical Generators Energy Mechanical Heat Engine Energy Transportation, Nuclear Fuel Fission; fusion Motors Industry Department of Chemical Engineering The conventional fuels (fossil fuels) comprise about two thirds of all fuels used to produce electricity: – Coal (40.6% of electricity production worldwide); – Oil (4.6% of electricity production worldwide); – Natural gas (22.2% of electricity production worldwide); – Biomass (wood etc. - < 1% for electricity production). Department of Chemical Engineering A fossil fuel is a hydrocarbon, thus consists of carbon and hydrogen, along with other minor (or sometimes not- so-minor) elemental constituents. Coal, for example, is mined from the surface or underground and contains impurities such as ash, sulfur and heavy metals (lead, mercury, bismuth, uranium …); Upon combustion (a highly exothermic chemical reaction with oxygen), most of the constituents of the fuel are oxidised, releasing heat in the process. Department of Chemical Engineering Example: hydrocarbon combustion – Fuel: 80 % C (by weight) 15 % H 5%S The combustion of 100 kg (basis) of this fuel with stoichiometric oxygen (from air) results in: Note: 3.76 = 79/21 the fractions of nitrogen and oxygen in air 80kg C + 15kg H + 5kg S + 338 kg (O2 + 3.76 N2 ) ® 293kg CO2 + 135kg H2O + 10kg SO2 + 1113kg N2 Department of Chemical Engineering Mass before: Mass after: – C = 80 kg – CO2 = 293 kg – H = 15 kg – H2O = 135 kg – S = 5 kg – SO2 = 10 kg – O2 = 338 kg – N2 = 1113 kg – N2 = 1113 kg – Total = 1551 kg – Total = 1551 kg The mass balance is conserved … Department of Chemical Engineering This is easily calculated considering the mole balances on each component: C: 80/12 = 6.667 kmol ® 6.667 kmol CO2 ® 6.667 kmol O2 H: 15/1 = 15 kmol ® 7.5 kmol H2O ® 3.75 kmol O2 S: 5/32 = 0.156 kmol ® 0.156 kmol SO2 ® 0.156 kmol O2 Total: 10.57 kmol O2 Thus, air input for stoichiometric combustion is: O2 : 10.57 kmol ´ 32 kg/kmol = 338.3 kg N2 : 10.57 kmol ´ 79/21 ´ 28 kg/kmol = 1113.1 kg Department of Chemical Engineering The ideal, or stoichiometric, air-to-fuel ratio is then easily calculated: mair 338.3 kg O2 +1113.1 kg N2 = = 14.5 mfuel 100 kg fuel Theoretical air required for various fuels: Fuel Theoretical Air (kg air/ kg fuel) Wood 3.94 Subbituminous coal 6.05 Bituminous coal 9.07 Carbon 11.5 Oil 13.7 Natural Gas 15.7 Hydrogen 34.3 Department of Chemical Engineering In practice, excess air is needed for complete combustion of the fuel. The amount of excess air used in combustion is typically determined by analyzing the combustion gas for its CO, CO2, SO2 and NOx content. A complete mass balance allows determination of the excess air used in the combustion process. Department of Chemical Engineering The combustion mass balance yields the following information: – Air/fuel ratio (mair/mfuel) – Excess O2 (or excess air) – Elemental constituents of the fuel Mass balance on hydrocarbon combustion: ( ) ( ) aC + bH + cS + d O2 + 3.76 N2 ® aCO2 + xH2O + xsO2 + cSO2 + 3.76d N2 Balance from Measured dry gas analysis Department of Chemical Engineering Example: combustion gas analysis of fuel from previous example with unknown excess air: Gas Analysis: CO2 – 11.8% (by volume) (dry basis) O2 – 3.73% SO2 – 0.275% N2 – balance - 84.195% For gases, we know mole fraction is equivalent to volume fraction, so we can easily calculate the water and actual air input … this also gives us the weight fractions of the components in the fuel. Department of Chemical Engineering ( Thus: aC + bH + cS + d O2 +3.76 N2 ) ( ® aCO2 + xH2O + xsO2 + cSO2 + 3.76d N2 ) ( aC + bH + cS + d O2 +3.76 N2 ) ( ® 11.8CO2 + xH2O + 3.73O2 + 0.275SO2 + 84.195 N2 ) We can immediately calculate: d = O2 Total = 84.195/3.76 = 22.39 kmol Completing the mass balance on O 2: O2 Total = 11.8 + x 2+ 3.73 + 0.275 = 22.39 x = 13.17 kmol Department of Chemical Engineering We now know all the products of combustion and can calculate the weight fractions of the fuel: Products: CO2 = 11.8 kmol x 44 kg/kmol = 519.2 kg H2O = 13.17 kmol x 18 kg/kmol = 237.1 kg SO2 = 0.275 kmol x 64 kg/kmol= 17.6 kg Fuel: C = 11.8 kmol x 12 kg/kmol = 141.6 kg H = 2x13.17 = 26.34 kmol x 1 kg/kmol = 26.3 kg S = 0.275 kmol x 32 kg/kmol = 8.8 kg Total fuel mass: 141.6 + 26.3 + 8.8 = 176.7 kg Department of Chemical Engineering Thus the weight fraction of the fuel components is: C – 141.6/176.7 = 0.801 (80.1%) H – 26.3/176.7 = 0.149 (14.9%) S – 8.8/176.7 = 0.050 (5.0%) You’ll note this the original composition of the fuel in the previous example (with minor rounding errors …). Excess air: O2 remaining O2 remaining 3.73 Excess air = = = = 0.20 O2 for stoichiometric combustion O2 in - O2 remaining 22.39 - 3.73 Department of Chemical Engineering And the air-to-fuel ratio is: mair 716.48 kg O2 +2358.4 kg N2 = = 17.4 mfuel 176.7 kg fuel Note the effect of 20% excess air in increasing the air-to- fuel ratio from 14.5 to 17.4. Department of Fuel Analysis Chemical Engineering The ultimate analysis of hydrocarbon fuels is typically determined in this manner: – Combust a known mass of the fuel with a known mass of air or oxygen (in excess); – Perform the mass balance calculation to determine the weight fraction of elements in the fuel. The proximate analysis of a hydrocarbon fuel gives: – Fixed carbon content; – Moisture content; – % volatile matter; – Ash content. Department of Chemical Engineering In addition to the elemental compositions, ultimate and proximate analyses of a fuel will also give the fuel’s higher heating value (HHV – kJ/kg). This is also know as the fuel’s calorific value (CV) and is a measure of the energy released upon complete combustion of the fuel. Department of Chemical Engineering The HHV includes the energy liberated when the water combustion product condenses back to a liquid (latent heat), which does not occur for most combustion situations of interest. The fuel’s lower heating value (LHV) is the energy released, keeping the water in the gas phase – a more practical measure in hydrocarbon combustion. The HHV and LHV are related and one can be calculated from the other by knowing the hydrogen content of the fuel and multiplying by the latent heat of vapourisation of water. Department of Chemical Engineering The CV’s of a few common fossil fuels … Fuel HHV (kJ/kg) Beech wood 20 380 Douglas Fir 21 050 Maple 19 960 Poplar 20 750 Gasoline 46 880 Coal – Pittsburgh Seam 31 820 Charcoal 34 390 Peat 22 000 Methane 55 380 Ethane 56 630 Hydrogen 142 080 Department of Boiler Efficiency Chemical Engineering We’ve seen that the efficiency of the energy conversion process depends upon the efficiency of the steam cycle and the efficiency of the mechanical-to-electrical energy conversion. We can now include the efficiency of the steam generation process, and the boiler efficiency in the overall calculation of total power plant efficiency: hplant = hboiler ´ hRankine cycle ´ hgenerator Department of Chemical Engineering The two primary methods for calculating the efficiency of a boiler system are: Input-Output: Wout Qout hboiler = = Qin Qin Heat Loss: (Steam) Qout = Qin − Qlost Qlost hboiler = 1- Qin Department of Chemical Engineering Input-Output Method: – For a boiler system, the energy out is the energy contained in the steam; – Heat input is the energy released by combustion of the fuel. hboiler = = ( Qout msteam hs - hfw ) Qin mfuel CV The steam and feedwater enthalpies are determined by measurement of the temperatures and pressures at the inlet and outlet of the boiler respectively. These are dependent on the configuration of the Rankine cycle. Department of Chemical Engineering Heat Input (Qin): – Measured by burning a known quantity of fuel and measuring the heat uptake by a water bath. – This would typically be the HHV of the fuel. – Device known as a bomb calorimeter … Department of Chemical Engineering Known mass of fuel ignited in bomb vessel; Energy released warms the pressure vessel and heat is conducted to the surrounding water bath; Temperature of the water bath monitored and the precise energy released may be calculated – this is the fuel’s HHV since combustion water is condensed. Department of Chemical Engineering Heat Loss Method: Qlost Qstack + Qconvection + Qradiation hboiler = 1- = 1- Qin Qin The heat input term is the same as described above … Since convection and radiation heat losses are typically < 1%, they are commonly neglected: Qstack hboiler = 1- Qin Department of Chemical Engineering Heat lost to the stack is dependent on the composition of the stack gases and their total mass flow rate. Remember that: h = Cp T Therefore: ( Qstack = mgasCp Tout - Tin ) And: hboiler ( æ mgas ö Cp Tout - Tin = 1- ç ) ÷ è fuel ø m CV Department of Chemical Engineering Note: – in the previous equation, the air-to-fuel ratio is a necessary parameter and can be determined quite accurately from the combustion equations. – The gas heat capacity is commonly assumed to be the heat capacity of air at the average temperature between the input air and the stack gases, which is readily measured. Department of Example Chemical Engineering It is proposed that the coal given below be burnt in a boiler that supplies steam to a Rankine cycle with a total desired output of 250 MWe. The steam cycle efficiency is expected to be 36% and the electrical generator efficiency is 98%. If the air-to-fuel ratio for this process is 14, and the stack temperature is 230oC calculate the following: a)The net efficiency for this process. b)The mass of fuel required for one day’s operation. c)The yearly mass of CO2 emitted in the process (350 days of operation). d)The yearly mass of SO2 emitted in the process (350 days of operation). Department of Chemical Engineering Knowns: Rankine = 36% generator = 98% net output = 250 MWe Tstack = 230oC air:fuel = 14 First, we calculate the net efficiency of the process: hplant = hboiler ´ hRankine cycle ´ hgenerator To do so, we need to calculate the boiler efficiency … Department of Chemical Engineering hboiler ( æ mgas ö Cp Tout - Tin = 1- ç ) ÷ è fuel ø m CV We estimate Cp of the stack gases as 1.4 kJ/kg K (air) thus: hboiler = 1- 14 ´ ( 1.4 kJ / kgK 230o C - 20o C ) = 0.863 30000 kJ / kg Knowing the boiler efficiency is 86.3%, we next calculate the overall efficiency of the process: hplant = 0.863 ´ 0.36 ´ 0.98 = 0.304 Department of Chemical Engineering Now that we know the net efficiency of the process, the heat input required is easily calculated: Net work out hplant = Qin Net work out 250 MW Qin = = = 822.4 MWthermal hplant 0.304 Noting 1 MW = 106 J/s, we divide this value by the CV of the fuel to get the required fuel input … 822400 kJ / s mfuel = = 27.41kg / s 30000 kJ / kg Department of Chemical Engineering This gives a daily fuel requirement of 2 368 512 kg … or 2,368.5 tonnes of coal per day! CO2 emitted in the process is calculated from the ultimate analysis of the fuel: – per 1 kg fuel (basis) we have 0.75 kg C – Thus: 0.75 kg C ® 0.0625 kmol CO2 ´ 44 kg/kmol ® 2.75 kg CO2 12 kg/kmol Department of Chemical Engineering On a yearly basis, we multiply this by the mass flow rate of the coal fuel: 2.75 kgCO2 CO2 produced = ´ 27.41kg / s = 75.38 kg CO2 / s kg coal On a yearly basis (350 days per year operation) this amounts to 2.279x109 kg CO2 or 2.28 Million tonnes of CO2 emitted. Using the same calculation for SO2 yields 38.1x106 kg SO2 per year or 38100 tonnes SO2 emitted. Department of Chemical Engineering CHE 5313 / 6313: Energy and the Environment Lecture 11: Environmental Remediation – Flue-gas Clean-up Bradley J. McPherson P.Eng Department of Chemical Engineering University of New Brunswick Department of Chemical Engineering All forms of electricity production produce environmental effects that need remediation in one form or another. Conventional fossil-fired stations are of the most concern for their environmental impacts and releases: – Flue gases: CO2, H2O, CO, SO2, NOx ; Flyash (small particulates of ash and unburned fuel carried out of the boiler). – Solid waste: Ash and clinker. Nuclear plants have a need to handle radioactive spent fuel for 100 ’s or 1000’s of years. Renewable energy resources have more-or-less benign electricity production once in place but also have environmental effects through CO2 emissions and other environmental pollutants during production and fabrication … Department of Fossil Plants Chemical Engineering Effective environmental remediation is required for flue gas clean up to comply with Federal and Provincial regulations on air quality. Typical arrangements employed for flue-gas clean up at coal fired stations include: 1. Low NOx burners in the furnace to reduce excessive NOx production and selective catalytic (or non-catalytic) NOx reduction in the hot flue gas; 2. Complete removal of particulate flyash (~99.9% solids removal); 3. Scrubbing for SO 2 removal (can be wet or dry process); 4. CO2 sequestration (currently being tested on an industrial scale, see Boundary Dam project - SaskPower); http://saskpowerccs.com/ccs-projects/boundary-dam-carbon-capture-project/ Also, “clean” coal technologies are being developed to gasify the coal to remove most of the impurities before combustion. Department of Chemical Engineering 2 4 1 3 1. Low NOx burners; 2. Flyash Collection; 3. FGD system; 4. CO 2 capture Department of Chemical Engineering Low NOx burners: – The term NOx refers to several oxides of nitrogen consisting primarily of NO with some NO2 present; – It is formed readily at the high temperatures present in a boiler system and comes from both oxidation of the nitrogen in the combustion air (thermal NOx) and nitrogen contained in the fuel (fuel NOx). – Without proper remediation, concentrations in the part per thousand level are present. – NOx is a major contributor to urban smog; not so major contributor to acid rain (HNO3). Department of Chemical Engineering NOx production is a complicated set of chemical reactions that rely on free radicals (O*, H* etc) present in the flame to sustain the reactions: O + N2 NO + N N + O2 NO + O N + O2 NO2 In a conventional burner, the combustion air is fed entirely with the pulvarised coal creating; – Long burn time; – High burn temperature; Both allow significant time for NOx to form. Department of Low NOx Burners Chemical Engineering The low NOx burner employs staged combustion to keep overall flame temperature down and to create reducing radicals that react the NOx formed back to N2 & O2 Department of Selective NOx Reduction Reactors Chemical Engineering Selective catalytic reduction (SCR): – The NOx-containing flue gas is dosed with ~ 2-5% ammonia vapour (in air) and passed over a vandium- titania catalyst, typically operating around 370oC. – The primary reactions are: 4NO + 4NH3 + O2 ¾catalyst ¾¾¾ ® 4N2 + 6H2O 2NO2 + 4NH3 + O2 ¾catalyst ¾¾¾ ® 3N2 + 6H2O – NOx reduction is typically ~ 90%. – Care must be taken if significant SO2 is present which can be oxidised to SO3 on the catalyst … Department of Chemical Engineering “Selective catalytic reduction” component added to flue gas exit before the primary air heater. Department of Chemical Engineering Selective non-catalytic reduction (SNCR): – The same NOx reduction reactions occur as in the catalyzed process only, without the presence of the catalyst, temperatures in the 900-1100oC range are required. – These temperatures are only present within the convective-pass tube banks. – Conversion efficiencies are quite low compared to the SCR process (~ 70-80% under optimum conditions). Typically operate around 30-40% removal efficiency. Department of Chemical Engineering Ash removal: – Ash is produced in large quantities in a boiler and must be removed from various locations: Bottom ash – falls to the bottom of the boiler and constitutes 10-30% of the overall solids waste from a typical boiler Economizer ash – accumulates on the economizer tube pass and drops into hoppers for periodic removal. Constitutes up to 10% of solids waste. Flyash – consists of relatively small particles and is typically ~ 70-90% of all solids waste. Removal is facilitated through one of two methods: – Electrostatic precipitation; – Fabric filters; Department of Chemical Engineering Electrostatic precipitators (ESP): – ESPs induce a charge on the small flyash particles by subjecting them to a strong electric field (generated by DC voltages ~ 55-75 kV). – Negatively charged wires are hung vertically between grounded plates (positive electrode), the electric field on the wires creates a corona discharge around the wires, ionizing the gas close to the wire and emitting electrons. – The free electrons are readily absorbed by the gasses which become absorbed onto the flyash particles. – The net effect is the flyash particles become negatively charged and are attracted to the collecting plates which are periodically rapped to remove the accumulated dust film. Department of Chemical Engineering Corona Department of Chemical Engineering Department of Chemical Engineering An estimate of the collector area can be made through the Deutsch-Anderson Equation: V æ 1 ö A = ln ç w è 1- E ÷ø Where: A – collection surface area (m2) V – gas flow rate (m3/s) w – migration velocity (m/s) E – collection efficiency The migration velocity is the theoretical average velocity that the particles migrate toward the collector plates. Department of Chemical Engineering Fabric filters (Baghouses): – Baghouses are essentially long, cylindrical supported bags through which the dust-laden air is passed. – They have three typical configurations that are characterised by their cleaning method: Reverse gas – flue gas flow through the inside of the bag which is periodically cleaned by reversing the gas flow direction; Shaker – same configuration as reverse gas but cleaning is facilitated by shaking; Pulse jet – flue gas flows from outside – in, cleaning is facilitated periodically by directing a pulse of high pressure air to the inside of the bag … can be done while still online; Department of Chemical Engineering Department of Chemical Engineering Department of SO2 Removal Chemical Engineering Flue gas de-sulfurisation (FGD) or Scrubbers: – SO2 is produced in the furnace through the combustion of the impure sulfur in the fuel. – SO2 is a toxic gas and is the primary cause of acid rain. – The FGD units are intended to reduce the SO2 concentration in the exit gas to essentially zero and come in two forms: Wet scrubbers; Dry scrubbers. Department of Chemical Engineering Wet scrubbers: – These typically use a slurry of lime or limestone to react with the SO2 in the flue gas. Overall: SO2(g) + CaCO3(s) + 12 H2O ® CaSO3 × 12 H2O(s) + CO2 or SO2(g) + CaCO3(s) + 12 O2 + 2H2O ® CaSO4 × 2H2O(s) + CO2 gypsum – Flue gas enters the side of the spray chamber and is intimately contacted with the lime slurry through a spray column on a perforated plate. – Several spray levels are typically employed. – Final mist eliminators are used to keep the droplet carryover to the stack as low as possible. – The second reaction is promoted by air injection in the slurry at the bottom of the reactor. Department of Chemical Engineering Department of Chemical Engineering Dry scrubbers: – This process injects an atomized mist of dissolved calcium hydroxide directly into the hot flue gas upstream of the baghouse or ESP. – The reaction between the hydroxide and SO2 occurs quickly as the droplets evaporate and the product is solid calcium sulfite. Ca(OH)2 + SO2 ® CaSO3 × 12 H2O + 1 2 H2O – The product is a fine particulate that is collected and disposed of along with the flyash. Department of Chemical Engineering CO2 capture: – Three main processes have been proposed for capturing the vast amounts of CO2 coming from fossil fuelled power plants: Air separation & CO2 recycling; CO2 absorption; Membrane separation; – All processes reduce the overall efficiency of the power production and increase capital, production and maintenance costs considerably. Department of Chemical Engineering Air separation; – Instead of trying to separate a small amount of CO2 (relatively speaking) from a large amount of N2 in the flue gas, the air separation process proposes to remove the N2 prior to combustion. – An Air Separation Unit (ASU) would be employed to produce pure, liquified O2 that would be used for: Coal gasification – produces syngas (CO & H2) Combined cycle turbine system Department of Chemical Engineering This plant has a thermal efficiency of ~ 37% The levelized generation cost is expected to be at least 50% higher than a conventional coal plant. Department of Chemical Engineering Solvent absorption: – CO2 is soluble in several solvents – particularly ethanolamines … Department of Chemical Engineering Drawbacks include: – Need for a flue gas devoid of SO2 and NOx as these gases poison the absorption process. – Traditionally only suitable for CO2 capture from natural gas combustion or from gassified coal (syngas combustion) – Thermal efficiency ~ 30%. – LGC >> 50% over conventional coal