Carbon Capture During Power Generation PDF

Carbon Capture during Power Generation Fuel N2 CO2 Separation Power Generation Post Combustion CO2 Air H2O Air/O2/Steam Air Pre-combustion Fuel Gasification Reformer + CO2 Separation Power Generation N2 H2O CO2 H2O Fuel Power Generation CO2 H2O Air O2 Separation N2 Oxy-fuel • T...

Carbon Capture during Power Generation Fuel N2 CO2 Separation Power Generation Post Combustion CO2 Air H2O Air/O2/Steam Air Pre-combustion Fuel Gasification Reformer + CO2 Separation Power Generation N2 H2O CO2 H2O Fuel Power Generation CO2 H2O Air O2 Separation N2 Oxy-fuel • Technologies which produce H2/CO gas mixtures (synthesis gas or ‘syngas’) at high temperatures (>700oC) using an oxygen starved feed gas and a feedstock containing carbon e.g. coal, biomass, waste, natural gas (steam reforming) C(s) + H2O → CO + H2 (steam gasification) and/or CH4 + H2O → CO + 3H2 (steam reforming) CO + H2O  CO2 + H2 (water-gas shift – fast equilibration in the gas phase) with C + O2 → CO2 (combustion) from oxygen lean feed to provide energy for the gasification reactions. • The separation of H2 from the predominantly H2/CO2 mixture after the water-gas shift reaction can be achieved by physical separation processes (liquid phase absorption, membrane separations) • Common to Integrated Gasification Combined Cycle (IGCC) power plants. Water-gas shift Air separation unit Heat recovery steam generator Schematic of a coal-fired IGCC plant with pre-combustion CO2 capture. SEAI (2005) Steps: 1. Natural gas recovery 2. Steam Reforming: CH4+H2O → CO+3H2 CO+H2O →CO2+H2 3. Carbon capture (Separation primarily involves H2 recovery) 4. Transport to long term storage. 5. Enhanced oil recovery. CCS with enhanced oil recovery from the depleted Miller oil field, UK North Sea (BP 2010). Types of solids gasification unit Gas out Solids in Solids flow Gas out Gas in Solids in Solids in Gas flow Gas in Solids flow Gas flow Counter-current moving bed (Low throughput, moderate temperature gas output, high tar levels) Gas flow Gas in Gas out Ash out Solids flow Ash out Ash out Co-current moving bed Fluidised bed (Low throughput, high temperature gas output, low tar levels) High throughput, moderate-high and uniform temperatures Gas out Solids in Solids flow Gas flow Gas in Ash out Advantages: •The primary advantage of fluidised beds lie in the intimate mixing which takes place. Heat transfer is very rapid and control of temperature is easily achieved resulting in temperature profiles which are relatively uniform (no hot-spots). • Higher throughput than for fixed or (slow) moving beds • Suitable for gasification of both low-grade and higher grade solid fuels as well as fuels which form highly corrosive ash. Disadvantages •Conversion efficiency may be low if due care is not paid to the elutriation of fines •Conversion is also influenced by the back-mixing process New designs are being developed (e.g. Entrained Flow Gasifiers, Transport Gasifiers etc) to overcome some of the drawbacks in traditional fluidised bed design (see for example http://www.netl.doe.gov/publications) Fluidisation velocity At a specific upward gas velocity within the bed, the downward gravitational force on the particles is balanced by the upward drag (shear) force exerted by the flowing gas on the particles in the packing – the particles in the packed bed become ‘fluidised’. The minimum gas velocity, umf, at which this occurs is obtained from the Ergun equation (see Coulson and Richardson (1978))  1.75  2  (1 − em f )  G (  S − G ) gd 3p   Rem f + 150  Rem f = 3 3 2 e    e  mf  mf    Energy loss term Shear loss term where emf is the voidage of the packing at the point of fluidisation (typically ~ 0.4), G and S are the gas and solid densities,  is the fluid viscosity and dp is the average particle size in the packing. The Reynolds number Remf = G umf dp/ Behaviour of reacting particles (two models) t=0 Partially reacted particle Ash (a) Time (b) t=0 Flaking ash or gaseous products cause shrinkage Particle disappears Reaction Equilibria: H ro (298 K ) = + 131.3 kJ/mol (endothermic) K = exp(17.644 – (16,811/T)); (1.2 x 10-7 (500K); 280 (1400K)) C + O2 → CO2 H ro (298 K ) = − 393.5 kJ/mol (highly exothermic) Rate Processes (Shrinking Core Model): Time Gas film Shrinking unreacted core Ash layer Surface of particle Moving reaction surface cH 2O cH 2O cCO or cH 2 cCO or cH 2 R rc 0 rc R With mass transfer within the gas film defined by the flux N i (mol/m 2 .s) = k g (ci − ci r =R ) diffusion within the ash layer defined by dci N i (mol/m .s) = D dr 2 and the kinetics at the moving reaction surface given to a reasonable approximation by rH 2O ( mol/m 2 .s) = ks C cH 2O r =rc It may be shown (Froment and Bischoff (1990)) that the time for the reacting core to shrink from r = R to r = rc is R c t= cH 2O  1  rc3  R   rc2   rc3   1  rc   31 − 2  − 21 − 3   + 1 − 3  + 1 −      3k g  R  6D   R   R   ks C  R  The time taken for the core to shrink to zero, rc = 0 (the gasification process is complete), is R C  = cH 2O  1 R 1  + +    3k g 6D k s  C  Three resistances in series Based on parameter estimates from Treybal (1980) and from a specific example of a coal gasification process reported by Wen and Chaung (1979) (with pressure, p, in Pa, temperature, T, in K, and particle radius, R, in metres) we have 1.75 T k g =1.2 x10−4 ( m/s); D / R =  2.5k g ; pR k s  C =1.13 x103 T exp( −21,060 / T ) 1.0E+005 1.0E+004 (sec) In fluidised bed processes the particles are ~ 0.1 – 2 mm with ash porosities ~ 0.5. Results for R = 1 mm and p = pH2O = 101.325x103 Pa with ρC = 2.267 g/cc are shown in the figure. At temperatures of ~ 1400K all resistances are equally limiting and above this temperature, where mass transfer limits the rate process, it is seen that there is little to be gained. Reaction kinetics limiting 1.0E+003 Mass transfer kinetics limiting 1.0E+002 1.0E+001 1000 1200 1400 1600 1800 T(K) Typically, the gasification temperature is maintained in the range 1200-1400K to ensure thermodynamic equilibrium is well to the right for the gasification reaction and that the kinetics are fast. 2000 Gasification Unit Design: Gas out Based on the shrinking core model, the extent of conversion of material within a given particle is Solids in Solids flow Gas flow Gas in Ash out  rc  XC = 1−   R R C t= c H 2O 3  1 R X + 3 1 − (1 − X C ) 2 / 3 − 2 X C  C 6D  3k g 1 1/ 3  ( + 1 − (1 − X C ) ) (I) ks C  (( ) ) In designing a solid phase gasification unit one of the key characteristics to be taken into consideration is the average conversion of the carbon based feedstock. This is largely determined by the residence time distribution (RTD), E(t), of the particles of the feedstock within the unit. The average conversion of particles leaving the gasification process is given by  1 − X C =  (1 − X C ) E (t )dt ( X C  1) (II) 0 Various expressions may be developed for the RTD. For fluidised bed systems, wherein the solids and gas phase may, to a first approximation, be considered to be well-mixed, the RTD is simply (Levenspiel (1980)) exp( t / t ) E (t ) = t (t = particle mean residence time, VFB /Fs ) (III) Given the desired flow of solids, Fs, for a specific power output then the size (VFB) of the fluidised bed may be estimated by trial and error using Eqs. (II) and (III). Results for the integral in (II) are provided by Levenspiel (1972). The CO2 capture itself involves the separation of the CO2 from H2. For the situation in which pure carbon is the component gasified the overall stoichiometry after the water-gas shift reaction (neglecting a small proportion of CO2 from the heat providing combustion step) C(s) + 2H2O → CO2 + 2H2 i.e. a high CO2 composition of 33%. This suggests that a technology which does not involve the reactive absorption approach should be used. Physical absorption or one or other of the techniques to be described in the next lectures should be considered. Current status: The gasification of coal, a long-term primary fossil fuel reserve, is an emerging technology. Currently it is 30% dearer (capital cost) than conventional coal-fired generation and only 4 large scale plants worldwide. DoE/NETL Front End Engineering Design testing of next generation technologies (Allam Cycle) 2019. Advantages of gasification: 1. Syngas can be combusted at higher temperatures than normal/waste solid/liquid fuels, thus higher Carnot efficiencies are applicable TC  =1− TH or employed in fuel cells (Carnot efficiency inapplicable) 2. Syngas may be used to produce low molecular weight synthetic fuels (Fischer-Tropsch). 3. The high temperature combustion process separates out ash elements (including chlorides, metals)  clean gas production. Costs of Carbon Capture The IGCC plant replaces the traditional PCC power plant in generation, producing ‘clean’ energy in the form of Hydrogen. Note that the overall power output from either traditional PCC combustion or gasification is the same since internally within IGCC energy is required to drive the steam gasification reaction. The two results for Moneypoint given in the SEAI (2006) report indicate that the capture costs are essentially the same for absorption capture in the post-combustion capture and precombustion capture involving IGCC. The advantages of IGCC lie in the cleaner technology. As noted above, another very distinct advantage with IGCC is that the Hydrogen could be employed in fuel cells (maximum efficiency 83% compared with ~ 50% for thermal power generation). https://www.volker-quaschning.de/datserv/CO2-spez/index_e.php CO + 3H2 CH4 Reformer (700-1100oC) WGS CO2 + 4H2 CO2,H2 Separation 4H2 CO2 Steam Steam Based on total CO2 emissions of 2,470 kmol/s with fully reversible separation of CO2 and H2 and compression of the pure CO2 product to 100 bar at 298K  p    yH 2 2  Minimum Power = FT G = ln   − ln yCO2 + ln y H 2  RTFCO2 (GW )  yCO2   p1    = (28.2 + 15.3) = 43.5 GW + EU power demand (total) = 2320 GW EU electric power demand ~ 450 GW Carbon capture: Separation primarily involves H2 recovery – liquid phase CO2 absorption or membrane (e.g. Pd) separations. Additional power demand arises in the reformer operation at high temperature and in the water-gas shift process.