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Dynamic modeling and simulation of carbon dioxide capture from flue gas pdf
Post: #1

Dynamic modeling and simulation of carbon dioxide capture from flue gas


.pdf  Dynamic modeling and simulation .pdf (Size: 1.05 MB / Downloads: 30)

Introduction

In the light of increasing fears about climate change, greenhouse gas mitigation
technologies have assumed growing importance. In the United States, energy related
CO2 emissions accounted for 98% of the total emissions in 2007 with electricity
generation accounting for 40% of the total1. Carbon capture and sequestration (CCS)
is one of the options that can enable the utilization of fossil fuels with lower CO2
emissions. Of the different technologies for CO2 capture, capture of CO2 by chemical
absorption is the technology that is closest to commercialization. While a number of
different solvents for use in chemical absorption of CO2 have been proposed, a
systematic comparison of performance of different solvents has not been performed
and claims on the performance of different solvents vary widely. This thesis focuses
on developing a consistent framework for an objective comparison of the
performance of different solvents.

Carbon dioxide, and climate change

The greenhouse effect stimulating a change in climate with potentially devastating
effects for the planet’s inhabitants. The IPCC (Intergovernmental Panel on Climate
Change) has identified six anthropogenic gases with climate change potential: CO2,
CH4, N2O, SF6, CFC’S (chlorofluorocarbons), and HFC’S (hydro fluorocarbons).
CO2 has the lowest Global Warming Potential

Chemical absorption

Chemical absorption systems at present are the preferred option for post-combustion
capture of CO2. Chemical absorption systems have been in use since the 1930s for
the capture of CO2from ammonia plants for use in food applications and hence, are
a commercially realized technology, though not at the scale required for power
plants. CO2 is separated from the flue gas by passing the flue gas through a
continuous scrubbing system. The system consists of an absorber and a desorber.
Absorption processes utilize the reversible chemical reaction of CO2 with an
aqueous alkaline solvent. In the desorber, the absorbed CO2 is stripped from the
solution and a pure stream of CO2 is sent for compression while the regenerated
solvent is sent back to the absorber. The process of chemical absorption with
different solvents is discussed in detail in the later chapters of this thesis. Heat is
required in the reboiler to heat up the solvent to the required temperature; to provide
the heat of desorption and to produce steam in order to establish the required driving
force for CO2 stripping from the solvent. This leads to the main energy penalty on
the power plant. In addition, energy is required to compress the CO2 to the conditions
needed for storage and to operate the pumps and blowers in the process.

Current status of CO2 capture technology

While there are a number of possible routes for carbon dioxide capture from power
plants, a number of them are still in the developmental stage. All these technologies
need to be evaluated when choosing the best one to incorporate in a power plant to
be built in the future. However, the only immediately realizable capture technology
for flue gases from power plants appears to be chemical absorption.
It is expected that in the near future, if oxyfuel combustion is employed, it will only
be in a power boiler with an integrated steam cycle. A conceptual process flowsheet
for a pulverized coal steam boiler operating on a supercritical steam cycle with CO2
capture has been developed. It has been found that the overall thermal efficiency on
a LHV basis is reduced from 44.2% to 35.4%. For oxyfuel combustion to be
incorporated, some modifications to the burner design are required. In addition, lines
for recirculation of the CO2need to be provided. One of the other challenges is the
lower purity of CO2 produced in oxyfuel combustion. For the production of ultra-
pure CO2 (matching that produced in an amine absorption process)

ABSORPTION ASSOCIATED WITH CHEMICAL
REACTION


The process of absorption of the gas in the liquid has been entirely a physical one.
There are, however, a number of cases in which the gas, on absorption, reacts
chemically with a component of the liquid phase. The topic of mass In the
absorption of carbon dioxide by caustic soda, the carbon dioxide reacts directly
with the caustic soda and the process of mass transfer is thus made much more
complicated. Again, when carbon dioxide is absorbed in an ethanolamine solution,
there is direct chemical reaction between the amine and the gas. In such processes
the conditions in the gas phase are similar to those already discussed, though in the
liquid phase there is a liquid film followed by a reaction zone. The process of
diffusion and chemical reaction may still be represented by an extension of the film
theory by a method due to HATTA. In the case considered, the chemical reaction
is irreversible and of the type in which a solute gas A is absorbed from a mixture
by a substance B in the liquid phase, which combines with A according to the
equation A + B → AB. As the gas approaches the liquid interface, it dissolves and
reacts at once with B. The new product AB, thus formed, diffuses towards the main
body of the liquid. The concentration of B at the interface falls; this results in
diffusion of B from the bulk of the liquid phase to the interface. Since the chemical
reaction is rapid, B is removed very quickly, so that it is necessary for the gas A to
diffuse through part of the liquid film before meeting B.
Post: #2
Thermal energy needs to operate on a daily basis with frequent and rapid load changes to balance large variations in intermittent energy sources such as wind and solar energy. In order for the integration of carbon sequestration into power plants to be economically and technically feasible, the carbon sequestration process must be able to follow these rapid and large load changes without diminishing the overall performance of the carbon capture plant. Therefore, dynamic models for simulation, optimization and design of the control system are essential.

In this paper we compare the transient behavior of the model with the dynamic pilot data for the absorption and desorption of CO2 for changes in the flow rate of the combustion gases. In addition, we investigated the dynamic behavior of a full scale post-combustion capture plant using monoethanolamine (MEA) and piperazine (PZ). This analysis demonstrates the good agreement between the developed model (dCAPCO2) and the pilot measurements under both transient and stationary conditions. It describes how the time needed to reach a new steady state varies with respect to the type of amine and concentration. The simulation study reveals that it is essential to control the flow of lean solvent to avoid sudden changes in the rate of CO2 removal and to avoid the increase in the demand for heat from solvent regeneration. In addition, it shows how storage tanks (system retention liquids) can be designed to accommodate significant changes in the direction of the power plant. This flexibility is especially necessary for operation in the future mixed green energy market.
 

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