Process for removing carbon dioxide from combustion exhaust gas

A process for removing carbon dioxide (CO.sub.2) from a combustion exhaust gas of a boiler (1) which generates steam for driving high (3), intermediate (7), and low (8) pressure turbines. The process comprises the steps of removing CO.sub.2 in the combustion exhaust gas by absorption with a CO.sub.2 -absorbing liquid (19), liquefying the removed CO.sub.2 (28) by compression (42) and cooling (48), storing (52) the CO.sub.2, and regenerating the CO.sub.2 -absorbing liquid by a CO.sub.2 -absorbing liquid regeneration column (24) equipped with a reboiler (30). In the process, a part (40) of steam discharged from the high pressure turbine (3) is used to drive turbines (41, 43) for compressors (42, 44) that compress the CO.sub.2, and a refrigerant for cooling the CO.sub.2, and steam (45) discharged from the compressor turbines is supplied as a heating source to the reboiler (30) for the regeneration of the CO.sub.2 -absorbing liquid. A decrease in the overall power plant efficiency due to the removal of carbon dioxide from the exhaust gas can be reduced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for efficiently removing and recovering 
carbon dioxide (CO.sub.2) from combustion exhaust gases leaving the 
boilers of thermal power plants. 
2. Description of the Related Art 
In recent years the greenhouse effect of CO.sub.2 has arrested attention as 
a factor contributing to the global warming. Counteracting this effect is 
urgently needed throughout the world so as to protect the global 
environment. The source of CO.sub.2 is omnipresent in every area of human 
activities that involve combustion of fossil fuels, and the tendency is 
toward stricter emission control than before. In view of these, factors 
many studies are under way on the recovery of CO.sub.2 from combustion 
exhaust gases, especially from those emitted by power-generating 
installations such as steam power plants that burn huge volumes of fossil 
fuels, and on the storage of the recovered CO.sub.2 without releasing it 
to the atmosphere. 
The present applicant previously proposed a process for the removal and 
recovery of CO.sub.2 from combustion exhaust gases with less energy 
consumption, as illustrated in FIG. 3 (Japanese Patent Provisional 
Publication (Kokai) No. 3-193116). 
In FIG. 3, CO.sub.2 -containing combustion exhaust gas from a boiler 1 is 
boosted to a high pressure by a boiler combustion gas fan 14 and delivered 
to a combustion gas cooler 15, where it is cooled with cooling water 16 
and transferred to a CO.sub.2 -absorption column 18, while the spent 
cooling water 17 is discharged out of the system. 
Inside the CO.sub.2 -absorption column 18, the combustion exhaust gas comes 
in countercurrent contact with regenerated CO.sub.2 -absorbing liquid 19 
containing an alkanolamine, and through a chemical reaction the CO.sub.2 
in the gas is absorbed by the liquid. The gas 21 freed of CO.sub.2 is 
discharged from the system. The absorbing liquid 20 that has absorbed 
CO.sub.2 is sent, after pressure boost by a rich solvent pump 22, to a 
rich/lean solvent heat exchanger 23, where it is heated by the regenerated 
absorbing liquid and then supplied to a CO.sub.2 -absorbing liquid 
regeneration column 24. 
At a lower portion of the regeneration column 24, the CO.sub.2 -absorbing 
liquid is heated in a reboiler 30 by low pressure steam (at an absolute 
pressure of 4 kg/cm.sup.2 G) 13 extracted from a low pressure turbine 8. 
CO.sub.2 gas entraining steam is conducted from the top of the CO.sub.2 
-absorbing liquid regeneration column 24 to an overhead condenser 25. A 
condensate of low pressure steam, condensed by the reboiler 30, is boosted 
by a reboiler condensing pump 32, mixed with preheated boiler feed water 
to raise the temperature of the feed water, and the mixture is fed to the 
boiler 1. 
The CO.sub.2 discharged, entraining by steam, from the CO.sub.2 -absorbing 
liquid regeneration column 24 preheats in the overhead condenser 25 the 
boiler feed water whose pressure has been boosted by the boiler feed water 
pump 12. The steam-entraining CO.sub.2 is then cooled by an overhead 
cooler 26 and separated from water by a separator 27, and CO.sub.2 alone 
is led through line 28 to another process step for recovery. The water 
separated by the separator 27 is pumped back to the CO.sub.2 -absorbing 
liquid regeneration column 24 by a condensing water circulating pump 29. 
The regenerated CO.sub.2 -absorbing liquid is boosted to a high pressure by 
a lean solvent pump 31, cooled in the rich/lean solvent heat exchanger 23 
with the absorbing liquid that has absorbed CO.sub.2, cooled further by a 
lean solvent cooler 33, and then supplied to the CO.sub.2 -absorption 
column 18. 
In the meantime steam 2 at a high pressure and high temperature that has 
been generated and heated by the boiler 1 is caused to drive a high 
pressure steam turbine 3, heated by a reheater 5 in the boiler 1 as an 
emission 4 from the turbine, and delivered as reheated intermediate 
pressure steam 6 to the low pressure turbine 8. 
Part of the steam is extracted at line 13 from the low pressure section of 
the low pressure turbine 8 and supplied to the reboiler 30. The rest of 
the steam 9 exhausted from the low pressure turbine is condensed by a 
condenser 10, and the condensate 11 is led to the overhead condenser 25 by 
the boiler feed water pump 12. 
Examples of alkanolamines that absorb CO.sub.2 include monoethanolamine, 
diethanolamine, triethanolamine, methyldiethanolamine, diisopropanolamine, 
and diglycolamine. An aqueous solution of such a single alkanolamine or of 
two or more such alkanolamines is used. Usually, an aqueous 
monoethanolamine solution is preferred. 
The above-described process reduces the power generation efficiency of a 
power plant compared with a plant that does not adopt the process for 
CO.sub.2 removal, but the degree of efficiency drop can be kept low. For 
example, when 90% of CO.sub.2 in the combustion exhaust gas from the 
boiler of a natural gas-fired power plant is to be removed, if the supply 
of heat for heating the reboiler 30 is obtained by combustion of fuel, the 
required fuel would amount to 18.9% of the heat of combustion in the 
boiler of the power plant. Consequently, the power generation efficiency 
for the same quantity of heat of combustion would decrease by 6.3%, from 
36.4% for non-CO.sub.2 removal operation to 30.1% with CO.sub.2 removal. 
According to the process proposed as above, however, steam at a pressure 
of 4 kg/cm.sup.2 G is extracted from the low pressure steam turbine 8 to 
heat the reboiler 30, and the condensate of the steam can heat boiler feed 
water. Moreover, the heat exchange in the overhead condenser 25 between 
the steam-entraining CO.sub.2 from the CO.sub.2 -absorbing liquid 
regeneration column and the boiler feed water renders it possible to 
decrease the quantity of steam extraction otherwise required to heat the 
boiler feed water. Thus, while the axial power of the low pressure steam 
turbine decreased to some extent, a drop in the power generation 
efficiency for the same quantity of heat of combustion could be limited to 
4.5%, attaining a 1.8% improvement in the power generation efficiency over 
the conventional process. Also, when a combined cycle gas turbine is 
adopted, an improvement of 3.4% was shown to be achieved. 
Although the above-described proposed process can limit the deterioration 
of the power generation efficiency owing to the removal and recovery of 
CO.sub.2 to some extent, there is strong demand for more improvements 
which would lessen the penalty of efficiency drop further. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to further reduce a decrease in 
the overall power generation efficiency due to the recovery of carbon 
dioxide from the combustion exhaust gas. 
In view of the problem associated with the power generation with concurrent 
removal and recovery of CO.sub.2 by absorption from the combustion exhaust 
gas leaving the boiler, intensive research has been made. As a result, it 
has now been found that the problem can be solved through an improvement 
in the method of securing a steam source for heating the reboiler. It is 
upon this basis that the present invention has just been perfected. 
The present invention provides a process for removing carbon dioxide 
(CO.sub.2) from a combustion exhaust gas of a boiler which generates steam 
for driving high, intermediate, and low pressure turbines, comprising the 
steps of removing CO.sub.2 in the combustion exhaust gas by absorption 
with a CO.sub.2 -absorbing liquid, liquefying the removed CO.sub.2 by 
compression and cooling, storing the CO.sub.2, and regenerating the 
CO.sub.2 -absorbing liquid by a CO.sub.2 -absorbing liquid regeneration 
column equipped with a reboiler, wherein a part of steam discharged from 
the high pressure turbine being used to drive turbines for compressors 
that compress and cool the CO.sub.2, and steam discharged from the 
compressor turbines being supplied as a heating source to the reboiler for 
the regeneration of the CO.sub.2 -absorbing liquid. 
It is a preferred modification in this invention that, where necessary, 
steam extracted from the low pressure turbine is added to the steam 
discharged from the turbines of the compressors for CO.sub.2 compression 
and cooling. 
The invention will now be described in detail with reference to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1 and 2 are two parts of a block flow diagram illustrating the 
process of the invention for CO.sub.2 removal. In these figures only major 
components are shown, and ancillary devices are omitted. As necessary, 
tanks, valves, pumps, heat exchangers, etc. are to be provided. Low, 
intermediate, and high pressure tanks, which are usually installed in 
pairs, are indicated here by single numerical values, with their 
associated generators omitted. Numerals like those used in FIG. 3 
designate like parts in these figures. 
The CO.sub.2 separated from the absorbing liquid in a separator 27 in FIG. 
1 is conducted through line 28 to a compressor 42 in FIG. 2, where it is 
compressed, and cooled by a cooler 46. It is then dehydrated in a 
dehydrating step 47. The dehydrated CO.sub.2 is further cooled by a cooler 
48 to a liquefied state and stored in a liquefied CO.sub.2 tank 52. A 
steam turbine 43 shown in FIG. 2 is provided to drive a compressor 44 for 
compressing a refrigerant. The compressed refrigerant is cooled by a heat 
exchanger 49 and supplied to a drum 50. The refrigerant is flush cooled by 
a flush valve 51 and supplied to the cooler 48. Thus, in order to recover 
and store in a liquid state the CO.sub.2 that has been removed by 
absorption in the process steps of FIG. 1, it is necessary to provide the 
compressors 42, 44 for CO.sub.2 compression and cooling as shown in FIG. 
2. 
Under the invention, part of the steam 4 returning from the high pressure 
turbine 3 to the reheater 5 of the boiler 1 is utilized as a source of 
steam for the steam turbines that drive these compressors. For this 
purpose provided is a branch line 40 through which the steam is led to 
drive the turbines 41 and 43 in FIG. 2. The proportion of the steam 4 to 
the total steam supply is so set as to optimize the overall thermal 
efficiency of the process and usually ranges between 15 and 20%. 
Another feature of the invention is the use of the steam leaving the 
compressor-driving turbines 41, 43 as a source of heat for the reboiler 
30. 
As an alternative to the heating steam source for the reboiler 30 according 
to the invention, it might appear possible to supply the steam exhausted 
or extracted from the low pressure or intermediate pressure turbine 7 to 
the CO.sub.2 -compressing or cooling turbine and then utilize the steam 
exhausted or extracted from such a turbine. However, a temperature of 
about 150.degree. C. is high enough as a steam source for heating the 
reboiler 30, and the steam thus exhausted or extracted has an 
unnecessarily high temperature (about 260.degree. C.). Too high a heating 
source temperature would raise the surface temperature of heating tubes 
inside the reboiler 30 and give unfavorable results such as decomposition 
of monoethanolamine. If such hot steam were used, it would rather be 
necessary to cool it, e.g., by mixing it with the condensate of a 
condenser installed downstream of the low pressure turbine, a practice not 
advisable in terms of the energy balance of the whole system. According to 
the process of the invention, the steam emission (about 270.degree. C.) 
from the high pressure turbine is used, without reheating, in the other 
two (low and intermediate pressure) turbines, and the steam discharged 
(usually wet vapor) from the latter is utilized in the reboiler 30. 
Consequently, the steam has a pressure of between 3 and 4 ata and a 
relatively low temperature of about 140.degree. C. It requires no cooling 
with the condensate from the condenser and permits effective utilization 
of energy. 
Since the invention uses the steam discharged from the high pressure 
turbine as the source of heating steam for the reboiler 30, the steam 
pressure is relatively high and accordingly reduces the required amount of 
steam. This may, in an extreme case, cause a shortage of steam supply for 
the heating of the reboiler 30. In that case, as indicated in FIG. 1, it 
is desirable to use the steam 53 extracted from the low pressure turbine 
as the steam for heating the reboiler 30 in addition to the steam 
discharged by the CO.sub.2 -compressing or cooling compressor. 
Experimental Example 
The conventional process illustrated in FIG. 3 and the process of the 
invention (FIGS. 1 and 2) were applied, separately, to an LNG-fired steam 
power plant having a generating capacity of 600,000 kW. The generated 
output decreased as shown in Table 1. As is clear from the table, the 
process of the invention makes possible an improvement in generated output 
by about 2%. 
TABLE 1 
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Itemized decreases in the power 
Process of 
Process of 
output as a result of CO.sub.2 recovery 
FIG. 3, the invention, 
and liquefaction kW kW 
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Output decrease due to steam 
41,300 
extraction from low pressure turbine 
Auxiliary power requirement for 
8,581 8,581 
CO.sub.2 removal 
Power requirement for CO.sub.2 
22,300 
compressor 
Power requirement for CO.sub.2 
13,400 
refrigeration compressor 
Power requirement for liquid CO.sub.2 
50 50 
pump 
Boiler reheater load reduction 
-35,000 
Combined output decrease of inter- 
100,000 
mediate and low pressure turbines 
Total 85,631 73,631 
Decrease from 600,000 kW, % 
14.27 12.27 
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As described above, the present invention renders it possible to lessen the 
decrease of power generation efficiency of a steam power plant due to the 
removal and recovery of CO.sub.2 from the combustion exhaust gas.