(57a) Analysis of Post-Combustion Inertial CO2 Extraction System

Analysis of Post-Combustion Inertial CO₂
Extraction System

Adam
H. Berger¹  Yuqi Wang^1*  Abhoyjit S. Bhown¹

Robert
Kielb,² Anthony Castrogiovanni²  Vladimir Balepin³

¹Electric Power Research Institute
(EPRI), Palo Alto, CA, USA

²ACENT Laboratories, Bohemia, NY, USA

³Orbital ATK, Ronkonkoma, NY, USA

Abstract

One method for
removing CO₂ from flue gas that has been proposed for
post-combustion CO₂ capture is cryogenic separation. To cool the
flue gas to the necessary temperature to remove CO₂ as a solid, Orbital
ATK and ACENT Labs have proposed the Inertial CO₂ Extraction System
(ICES) as shown in Figure 1. ICES uses the cooling effect from accelerating
flue gas to supersonic speeds in order to promote the formation of solid CO₂
particles. The solids can then agglomerate to form larger particles that grow
sufficiently to be inertially separated from the remaining gas flow by an
appropriately-curved gas flow duct. At the duct curve, CO₂ solids
can be removed using a simple knife edge separator, sent to a cyclone, and
pressurized in dense-phase, minimizing the energy of compression. The remaining
CO₂-free gas stream that is still at high speed in the duct but at low
temperature and pressure can recover its pressure in a diffusor for emission to
the atmosphere. This work provides a thermodynamic analysis of this process and
the effects of condensing material in supersonic, two phase flow in the duct.

When gas is
isentropically accelerated to supersonic speeds through a nozzle, the
temperature and pressure of the gas are significantly lowered to a point where CO₂
forms a solid as described by isentropic supersonic flow equations. However,
these simple relations are complicated by the introduction of multi-phase flow,
heat of sublimation released during phase change, friction losses, and other
non-idealities. In addition, to ensure that the solid particles that are formed
can grow to the appropriate mass to inertially migrate towards the
solid-capture duct, nucleation sites can be introduced in the form of recycled
solid CO₂ particles. In order to analyze the performance of the ICES
capture system for post-combustion CO₂ capture, the accelerating
flow through a converging-diverging nozzle, solid CO₂ capture, and
subsequent pressure recovery in a decelerating diffusor were computationally
simulated at a range of initial conditions and with different levels of solid CO₂
recycle.

Figure 1. 
Inertial CO₂ Extraction System.

To simulate the
ICES process, an equation of state for CO₂ valid in the solid-gas
region was implemented. This was combined with real-gas property data to
calculate the phase behaviour and thermodynamic state of the system at the
temperatures and pressures required for solid CO₂ formation. The
equation of state with phase equilibrium calculations was used to calculate the
thermodynamic state of the process at each velocity during acceleration and
deceleration. The solids separation and gas deceleration in the diffusor were
subsequently calculated as well using similar techniques. The simulation
results were verified through forwards and backwards comparison to isentropic
flow, single-phase supersonic flow, and Rayleigh flow. This integrated
simulation provides an understanding of the ideal flow case and allows analysis
of the ICES process.

This paper will
present the results of these calculations, including the effect of inlet
temperature, inlet pressure, and the presence of a recycled medium to increase
nucleation. Several lessons learned from the simulations and parametric tests
of the process will be described. Thermodynamic limits to supersonic cryogenic CO₂
capture processes will also be discussed, for post combustion CO₂ capture
from coal-derived flue gas as well as for other gas streams of interest.

*Present
Address:  Department of Mechanical Engineering, University of California, Los
Angeles