The present invention is in the technical field of process engineering. More particularly, the invention concerns capturing acid gases such as from a flue gas stream. There are well-known environmental or regulatory reasons for industrial operations to reduce their emissions of acid gases such as CO2 or H2S.
A method for removing acid gas has been known since at least 1926, though this apparently original process did not regenerate the sorbent. A method for removing acid that incorporates sorbent regeneration has also been known since at least 1930 and is shown in U.S. Pat. No. 1,783,901.
While functional, early methods for acid gas capture suffered from excessive energy consumption. Much subsequent literature and many efforts have focused on reducing this energy consumption. The first approach to reducing energy consumption would be heat integration through the addition of heat exchangers. The central cross heat exchanger, which exchanges heat between the unloaded and loaded sorbent streams is a common staple present in most processes.
The general process of heat integration can be extended through the incorporation of heat pumps. An absorption-driven aqueous lithium bromide heat pump was integrated with an acid gas capture system in 1982. Other efforts have considered different types of heat pumps and methods for implementing them.
Process simulation software such as AspenTech's Aspen Plus V8.5 (Aspen Technology, Inc., Bedford, Mass.) can be used to design and assess the merits of various designs. Process simulators typically rely on iterative numerical methods. Simulations may require excessively long time spans, diverge, or otherwise fail to converge, making the process of finding a solution nontrivial. It is often necessary to build simulations up from simpler cases to avoid divergence or an undesired solution to systems with multiple solutions.
The primary goal of a capture system can be quantified by the capture rate,
  R  =                    CO                  2          ⁢                                                    ⁢      captured                      CO        2            ⁢                          ⁢      in      ⁢                          ⁢      flue      ⁢                          ⁢      gas      
Target capture rates for the present invention range from 80% to 95%, and preferably range from 75%-99%. Energy optimization is often quantified in terms of one or more of the following: regeneration energies; energy penalty; and equivalent work. Regeneration energy can be described in terms of thermal and electrical consumption per tonne of CO2 captured,
            E      t      regen        ≡                                        ∑                                          ∀                consumers                                  utility                  ⁢                                                                          ⁢                  heat                                            ⁢              i                                                                      ⁢                                          ⁢                      D            i                                    F                      captured            ⁢                                                  ⁢                          CO              2                                          ⁢                          ⁢      and      ⁢                          ⁢              E        e        regen              ≡                            ∑                                    ∀              consumers              electricity                        ⁢            i                                                          ⁢                                  ⁢                  B          i                            F                  captured          ⁢                                          ⁢                      CO            2                                ⁢        
where Etregen is the thermal regeneration energy; Eeregen is the electrical regeneration energy; Di is the heat duty of unit i; Bi is the electrical consumption of unit i; and Fcapture CO2 is the material flow rate of CO2 in the flue gas.
Generally, the primary contribution to Etregen comes from the regeneration tower's one or more reboilers while the primary contribution to Eeregen comes from the one or more compression units which may be compression trains.
There are two commonly used definitions for energy penalty. First is the production-loss energy penalty EPPROD-loss, defined as the portion of a power plant's energy production lost when a CCS system is installed. The second is the size-up energy penalty EPsize-up, defined as the factor by which a power plant must be sized up to still produce the same amount of electricity after a CCS unit is installed.
Energy penalties can be calculated as
                                          EP                          prod              ⁢                              -                            ⁢              loss                                ⁢                    ≡                      1            -                                          η                                  with                  ⁢                                                                          ⁢                  CCS                                plant                                            η                                                      w                    /                    o                                    ⁢                                                                          ⁢                  CCS                                plant                                                                                            ⁢                      =                          R              ⁢                                                          ⁢                                                ɛ                  e                  CCS                                ⁡                                  (                                                            E                      e                      regen                                        +                                                                  ∑                                                                              ∀                                                          steam                              ⁢                                                                                                                                                                ⁢                                                                                                                                                              ⁢                              sinks                              ⁢                                                                                                                                                                ⁢                                                                                                                                                              ⁢                              i                                                                                ⁢                                                                                                                                            ⁢                                                                                                                                                                                                                                                              ⁢                                                                                          ⁢                                                                        η                          i                          steam                                                ⁢                                                  E                                                      t                            ,                            i                                                    regen                                                                                                      )                                                                          and                    EP                  size          ⁢                      -                    ⁢          up                    ≡                                    η                                          w                /                o                            ⁢                                                          ⁢              CCS                        plant                                η                          with              ⁢                                                          ⁢              CCS                        plant                          -        1              =                  EP                  prod          ⁢                      -                    ⁢          loss                            1        -                  EP                      prod            ⁢                          -                        ⁢            loss                              
where ηiplant (i=reference plant with or without CCS) is the ratio of net electrical energy output of plant i to the thermal energy it obtains by burning fuel; ηisteam the ratio of electrical energy obtainable to the thermal energy used for steam i; and εeCO2 is the thermal emission rate: the mass flow rate of CO2 emitted per unit of thermal energy produced by burning fuel in the base case plant.
The plant-specific factors ηref and εeCO2 can vary widely between different power plants.