Climate alteration is straight associated with increased atmospheric concentration of gases, and particularly of CO2 produced chiefly by fossil fuel burning. Therefore, accomplishing important decrease of CO2 emanations is pressing. Many different schemes need to be applied for this intent including the increased usage of renewable energy beginnings and atomic power, the betterment of energy transition efficiency and the usage of C gaining control and storage systems ( CCS ) .
In planetary graduated table, energy production is extremely dependent of fossil fuels. Furthermore, the continuously increasing energy demand in combination with abundant and loosely distributed geographically coal, are factors that prohibit the decrease of conventional energy production ways. Therefore, CCS is the lone option to accomplish important CO2 emanations decreases.
The C gaining control engineerings can be separated into three chief methods: pre-combustion gaining control, post-combustion gaining control and oxy-fuel burning. In this assignment, pre-combustion gaining control procedure will be examined.
By definition, pre-combustion C gaining control refers to the remotion of C from fuel before its burning. Therefore, the burning generates zero CO2 emanations. This engineering can be applied in power workss which use hydrocarbon fuels in order to bring forth electricity. These fuels are coal, natural gas, oil but can besides be used in power workss which use non-hydrocarbon fuels such as biomass or waste. However, pre-combustion C gaining control engineering is more efficient and most developed for usage in Integrated Gasification Combined Cycle ( IGCC ) power workss utilizing coal fuel and in Natural Gas Combined Cycle power workss.
The chief procedure can be divided into three stairss. Initially, the hydrocarbon fuel which is typically natural gas or coal ( oil or biomass ) is reformed or gasified severally into synthesis gas, known as syngas, incorporating H and CO. Then, the syngas is reacted with steam in a Water-Gas Shift reactor ( WGS ) and the C oxide is converted to carbon dioxide and produces more H2O. The resulted syngas after the transition of the C oxide is consisted of C dioxide and H2O and is known as shifted syngas. During the 3rd measure, CO2 is separated from H2 and the rich H gas is produced which can be combusted clearly in combined rhythm power workss. Separated CO2 is so compressed, stored and transported.
Between the three basic stairss, described above, there is a figure of extra units for the optimal operation of the system. These units are gas cleaners and heat money changers. Depending on the fuel used, the place and the sort of these units may differ. Figure 1 depicts the basic units of a combined rhythm power works irrespective of the fuel used as input. The water-gas displacement reactor and the CO2 gaining control unit are the extra units used in an IGCC or a NGCC power works for C gaining control.
Figure 1: CC power works with pre-combustion gaining control.
Figure 2 depicts an IGCC power works with pre-combustion C gaining control. From this figure the extra units can be observed. These units are the Air separation Unit ( ASU ) , the atom and the S remover. All these units will be described exhaustively in the following subdivisions of this assignment.
Pre-combustion Capture Process_Vattenfall.jpg
Figure 2: IGCC power works with pre-combustion gaining control.
The chief difference between the IGCC power works and the NGCC power works is that the IGCC uses a gasifier for the production of the syngas and the NGCC uses a gas reformist.
5.1 Natural gas constituents
In combined rhythm power workss which use natural gas as fuel, a gas reformist is used for the syngas production. At this point it is of import to be mentioned that natural gas is consisted chiefly of methane in a proportion about 94 % . Table 1 cites all the constituents that natural gas is consisted of and their proportion.
Table 1: Natural gas constituents
5.2 Natural gas reformation
Three different commercially available engineerings for natural gas reforming have been developed. Steam reforming ( SR ) , Non-catalytic partial oxidization reforming ( POX ) and Autothermal reforming. For the hereafter, reforming in a membrane reactor could be an option.
5.2.1 Steam Reforming
In steam reforming, methane reacts with H2O ( steam ) ( 700 & A ; deg ; C-1000 & A ; deg ; C ) under 3-25 saloon force per unit area in a presence of a nickel-based accelerator to bring forth H and C monoxide. It is an endothermal reaction which frequently using a nickel-based accelerator. In order to avoid the taint of the accelerator, sulfur compounds from the natural gas demand to be removed before the reformation. The basic steam reforming chemical reactions are:
CH4 + H2O ( +heat ) > CO + 3H2 -?=-206kJ/mol
C3H8+ 3 H2O ( +heat ) > 3CO + 7H2
C2H5OH + H2O ( +heat ) > 2CO + 4H2
The chief drawback of this reformist is that demand a secondary steam supply.
5.2.2 Non catalytic Power Oxidation ( POX )
Partial oxidization procedure relies on an exothermal reaction between natural gas and a limited sum of O. The merchandises of the reaction are chiefly carbon monoxide and H. The O can be supplied as pure O either signifier air or via a conveyance membrane. The basic chemical reactions are:
CH4 + ?O2 > CO + 2H2 ( +heat )
C3H8 + ?O2 > 3CO + 4H2 ( +heat )
C2H5OH + ?O2 > 2CO + 3H2 ( +heat )
Temperature of the reaction is about 1250-1400EsC. This engineering is much faster than steam reforming but increases the entire cost of the works significantly. However, it is a really simple system with small care required.
5.2.3 Autothermal Reforming ( ATR )
The reactor in the autothermal reactor is divided into two different zones, a burning and a thermic and catalytic zone. Oxygen reacts with methane and signifiers syngas. It is an exothermal reaction and the operating temperature is in about 850-110 EsC.
CH4 + O2 – CO + 2H2O ( g ) -=519KJ/mol
These reformer are more widespread and common used than the other two because of its flexible operation, its compact design, its low investing cost and its broad operating conditions. Furthermore, the fact that there is no demand of secondary fuel supply makes the reformer even more efficient than the others.
5.3 Water Gas displacement reaction in NGCC power workss with pre-combustion milliliter.
The Water gas displacement reactor is one of the most of import units of the pre-combustion C gaining control procedure and will be examined exhaustively in subdivision 5555. However it is really of import to be mentioned at this point that in NGCC power workss with pre-combustion C gaining control, the H2O gas displacement reactor take topographic point into the reformist.
7 Water Gas Shift reactor
The H2O gas displacement reactor ( WGS ) is one of the most of import parts of the CO2 gaining control system in IGCC and NGCC power workss. It converts about all the CO in Syngas, into CO2 and H with an exothermal chemical reaction. The input of the WGS reactor is syngas and steam ( H2O ) .
CO + H20 – CO2 + H2 ?H= -41.1kj/mol
In general, the H2O gas displacement reaction occurs in an adiabatic system with the presence of a accelerator which accelerates the reaction rate. Several types of accelerators are commercially available but three of them are the most of import. These three accelerators can be divided into two classs: the clean displacement accelerators and the rancid displacement accelerators.
Clean displacement accelerators:
High temperature displacement accelerators
Active constituent: Fe3O4 with Cr2O3 as stabilizer
Operating temperature: 350 – 500 & A ; deg ; C
Sulphur content syngas lower than 100 ppm.
Low-temperature displacement accelerators
Active constituent: Cu supported by ZnO and Al2O3
Operating temperature: 185 – 275 & A ; deg ; C
Sulphur content syngas lower than 0.1 ppm.
Sour displacement accelerators
Active constituent: Sulphided Co and Mo
Operating temperature: 250 – 500 & A ; deg ; C
Sulphur content syngas lower than 300ppm.
In NGCC power workss with pre-combustion gaining control, clean displacement accelerators are used and the chief WGS operation takes topographic point into the reformist. IGCC power workss with pre-combustion gaining control usage either clean or rancid displacement accelerators but rancid accelerators are more widespread. In these workss, the WGS reactor is an independent unit.
During shift reaction, the temperature rises and restricts the farther procedure. Therefore, the transition in a individual reactor is limited. This job can be solved by utilizing a two phase H2O displacement reaction. This reaction can be achieved with the combination of a high temperature and a low temperature displacement reactor operated in sequence. Consequently, in order to acquire a higher grade of transition of the CO to CO2, two reactors are favourable compared to a one-reactor apparatus. Between these two reactors, an inter-bed chilling is used in order to keep the temperature low in the 2nd reactor. The majority of C monoxide is converted to CO2 by the high-temperature accelerators and the procedure is fulfilled by the low temperature accelerators. Figure 2 depicts a High and Low temperature displacement reactor every bit good as the different temperatures for the different phases.
Figure 2: High ( left ) and Low ( right ) temperature displacement reactors.
Figure ten: WGS composed by two reactors.
The WGS unit influences the entire efficiency of the power works. For standard design it is observed an efficiency loss about 3-4 % . The loss of efficiency caused by the WGS ‘ steam demands necessary for sufficiently high CO transition ratios.
Sulfur Removal Unit
Sulfur remotion is a critical portion in the pre-combustion C gaining control processes and it is used in both IGCC and NGCC power workss. Its chief operation is to take S from the input gas.
Application in IGCC power workss with pre-combustion milliliter.
Syngas produced by the gasification contains assorted drosss. Among these drosss, Ss have a large proportion. During gasification, S is chiefly converted to hydrogen sulphide ( H2S ) and to carbonyl sulphide ( COS ) in a proportion about 3-10 % .
In IGCC power workss with pre-combustion C gaining control, in order to clean the gas from sulfur constituents, a Sulfur Removal unit is used which utilizes acid gas recovery processes ( AGR ) . These procedures can be either physical or chemical based and retrieve S either as sulphuric acid or as elemental S. However, AGR procedures are efficient merely when S is in H2S signifier, COS is highly hard to be removed in this manner. Therefore the sulfur remotion unit must be equipped with a COS hydrolysis unit ( before the AGR ) which catalytically converts COS to H2S. This units converts over 99 % of the of the COS to H2S with the chemical reaction presented below:
COS + H2O – H2S + CO2
With the combination of these two units i.e. COS hydrolysis and AGR process the Sulfur remotion unit achieve about 99 % of sulfur recovery.
In unit 7 it underlined that in IGCC power workss with pre-combustion milliliter the WGS reactor can be composed either of clean or rancid displacement accelerator. In instance of clean displacement accelerators, sulfur constituents must be removed from the syngas before the WGS reactor in order to avoid accelerator taint. Therefore, in this instance the S remotion demand to be placed before the WGS reactor as Figure 66 shows.
Figure 66: WGS reactor with clean displacement accelerators
In instance of rancid displacement accelerators, S is required in the provender gas. Therefore the sulfur remotion unit is placed after the WGS reactor. It is really of import to advert here, that in this instance the WGS reactor except from change overing CO to CO2, besides hydrolyses COS to H2S. Consequently, there is no demand of a separate COS hydrolysis system.
Figure 67: WGS reactor with rancid displacement accelerators.
In every instance, a gas ice chest is necessary to be used before the Sulfur remotion unit.
Application in NGCC power workss with pre-combustion milliliter.
In this instance, both the accelerators of the reformist and of the WGS reactor are clean displacement. Therefore, sulfur remotion is necessary to be placed before the gas reformist. Hence, in NGCC power workss, the input gas of the sulfur remotion unit is natural gas alternatively of syngas ( or shifted syngas ) which was in IGCC power workss.
The first measure of the remotion procedure is the transition of sulfur compounds, such as COS and mercaptans, in the natural gas to gaseous H2S. This can be achieved by utilizing a catalytic hydrogenation. After the transition, the S is removed by fluxing of the natural gas through a bed of Zn oxide granules to organize solid Zn sulphide harmonizing to reaction
H2S + ZnO ( s ) > H2O ( g ) + ZnS ( s )
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