The Misguided Focus on Low Heat of Absorption Solvents

Transcription

1 Jochen Oexmann Alfons Kather The Misguided Focus on Low Heat of Absorption Solvents Session: Carbon Capture Technologies (I) Institute of Energy Systems Prof. Dr.-Ing. A. Kather 4 th International Conference on Clean Coal Technologies Dresden, 19 May 2009

2 CO 2 Capture by Wet Chemical Absorption Heat duty Cooling duty Power duty flue gas from FGD to atmosphere make-up water washing section overhead condenser intercooled compression to CO 2 - storage solvent cooler to make-up water system absorber rich-lean HX desorber blower filter reboiler steam/condensate from/to power plant flue gas cooler to water conditioning or FGD solvent pump (CO 2 -rich) solvent pump (CO 2 -lean) disposal reclaimer Introduction 2

3 Components of Reboiler Heat Duty p q vap,h2o tot p H2O +p CO2 T feed q reb = q abs,co2 + q vap,h2o + q sens T reb Introduction 3

4 The Misguided Focus Power Plant Steam demandd Heat of absorption If the heat of absorption of solvent B is lower than of solvent A, less heat must be provided to regenerate solvent B than to regenerate solvent A. Introduction 4

5 Motivation Solvent screening studies often focus on a low heat of absorption This focus blends out two critical facts The overall regeneration (reboiler) heat duty does not only consist of the heat of absorption! The individual components of the overall regeneration heat duty strongly depend on the chosen process parameters and additionally on each other! Lack of transparent and compact discussion for a wide dissemination that is comprehensible and accessible to a broad audience Introduction 5

6 Components of Reboiler Heat Duty Overall reboiler heat duty can be approximated as a sum of three terms p tot p H2O +p CO2 q vap,h2o T feed q reb = q abs,co2 + q vap,h2o The three terms are NOT independent d of each other! + q sens T reb Simplified analysis 6

7 Simplified Analysis Consider two solvents A & B: 1. Heat of absorption. 2. Sensible heat Assuming the same CO 2 capacity, heat capacity and temperature t difference for both solvents 3. Heat of evaporation chosen, so that Simplified analysis 7

8 Heat of Evaporation Gibbs-Helmholtz Clausius-Clapeyron Water vapour pressure and CO 2 equilibrium partial pressure ratio The heat of evaporation is a function of the heat of absorption as well as of the desorber pressure and temperature! Simplified analysis 8

9 High and Low Heat of Absorption Solvent Solvent A B Δh abs,co2 /Δh vap,h2o q abs,co % 12.5 % A q sens 15.0 % 15.0 % q vap,h2o 35.0 % 72.5 % (Δh abs,co2 < Δh vap,h2o ) B (Δh abs,co2 > Δh vap,h2o ) ( abs,co2 vap,h2o increasing temperature Simplified analysis 9

10 Process Simulations Rigorous thermodynamic model Simulation tool: ASPEN Plus Model: University of Texas (Hilliard, 2008) Fluid phase: Electrolyte Nonrandom Two Liquid (enrtl) Vapour phase: Redlich-Kwong-Soave EOS Flash calculations and overall process analysis Process simulations 10

11 Overall Process Δh abs,co2 25 kj / mol CO 2 Δh abs,co2 85 kj / mol CO 2 increasing i temperature Process simulations of overall process show similar results as predicted by the preceding simplified analysis Process simulations 11

12 H 2 O and CO 2 Partial Pressure Ratio High heat of absorption solvent (7 m MEA) profits from temperature swing, low heat of absorption solvent (3.2 m K 2 CO 3 ) profits from vacuum desorption at lower temperature Process simulations 12

13 Summary Focus on low heat of absorption alone is not reasonable without consideration of process in which the solvent is used High heat of absorption solvents can profit from temperature difference between absorber and desorber. Low heat of absorption solvents can profit from regeneration at low temperature (vacuum desorption). Large impact of capacity of solvent, i. e. sensible heat requirement In the overall process the effect of desorber pressure on required heat and steam quality as well as on the auxiliary power demand of the CO 2 compressor must be considered. Conclusions 13

14 The Misguided Focus Power Plant Steam demandd Heat of absorption Conclusions 14

15 The Complete Picture CO 2 Compressor Power Plant negative coupling positive coupling (with respect to power plant energy penalty) Desorber pressure Reboiler temperature quality Steam demand quantity Low Δh Δh abs,co2 High Δh abs,co2 Heat of absorption overall regeneration heat duty Heat of evaporation Sensible heat Conclusions 15

16 Thank you for you attention! Acknowledgements University of Texas (Gary Rochelle, Marcus Hilliard) Federal Ministry of Economics and Technology (COORETEC initiative) iti 16