Equilibrium calculator

The online equilibrium calculator can handle coal as a mixture of C, H2, O2, S and N2 (gasses) and as pure elements in the same way as the MatLab program discussed in 4.2.2 MatLab calculations. There are run calculations on both scenarios.

4.2.4.1 Coal fed as a mixture of elements and molecules

Table 4.18 shows the equilibrium composition when the coal wis fed as a mixture and the hydrogen, oxygen and nitrogen are on molecular form. The carbon and sulfur are fed as pure elements.

Table 4.19. Syngas composition, coal fed as elements and molecules.

Coal Browncoal Browncoal Bituminous Bituminous Anthracite Anthracite Temperature C 1481 1481 1572 1572 1233 1233

Pressure Bar 42 42 42 42 42 42

The results from table 4.18 show a relation between the PRO/II calculations and the calculator. The case of Anthracite and Bituminous give much the same values. The main deviation is that the online calculator gives an amount of CH4 in the products which again decreases the amount of H2 since 1mole of CH4 contains 1C and 4H. In PRO/II these

substances are distributed among CO, CO2 and H2 and therefore it will give a higher fraction of H2 due to the molar balance. CH4 is favored by low temperature and this is a likely

explanation of the differences between the two calculations. In PRO/II the temperature is a function of the fed and will vary during the gasification process, while the calculator does calculation for given temperatures. For the special case of Browncoal the differences can also be a result of the feeding of water. For the Dry feed gasifiers the water is fed separately at 255˚C while the Slurry Fed gets the water part included in the coal feed at lower temperature.

In the calculator there is no feeding procedure that differs if the water is a part of the coal or a separate reactant.

4.2.4.2 Coal fed as elements only

Table 4.20 gives the results from a calculation with pure elements, which means for example that the hydrogen part of the feed is defined as H instead of H2. That gives different molar fractions in the feed compared to the case where the feed is presented as both elements and molecules. An example of a transcript from the online chemical calculator is given in Appendix L.

Table 4.20. Syngas composition, coal fed as elements only.

Coal Browncoal Browncoal Bituminous Bituminous Anthracite Anthracite Temperature C 1481 1481 1572 1572 1233 1233

Pressure bar 42 42 42 42 42 42

The results from table 4.20 are quite similar to the results from table 4.19. The main trend is that the last one produces more CO2 and therefore less CO. Also here the Browncoal values differ more than the other ones. But here they are closer to the values from the PRO/II simulation. Anyway the results all in all give satisfactory results and it indicates that the PRO/II model is useful. Compared with the MatLab program the results are very close. This proves that the MatLab calculator is useful for simplified coal compositions.

4.2.5 Water shift reforming

4.2.5.1 Water gas shift reactor

Three coals are studied. For further simulations and calculations only one coal is used. This is the Bituminous. The syngas from this specific coal is used as feed for the water shift reactor.

There are modeled two stages of the shift reactor. Water steam is added to increase the H2

production. The amount of water and the compositions before, between and after the whole process is presented in table 4.21.

Table 4.21. Molar compositions for different stages. Phase Vapor Vapor Vapor Vapor Vapor Temperature C 250.000 255.000 498.535 294.105 36.670 Pressure bar 41.400 41.400 41.000 40.400 40.200 Flowrate kmol/s 1.097 1.07 2.167 2.167 1.772 Composition

The product gas from the first stage has a temperature of 499˚C and this is cooled to 225˚C before the second stage. This is called the intermediate gas inn the table. The column to the left shows the composition after the water has been removed. In the first stage there is fed 1.07kmol of steam for this amount of syngas. It should be kept in mind that these numbers are valid for a coal feed of 1kmol. When it comes to the total plant the amounts will be adjusted to fit the turbine. The ratio between fed steam and fed CO will be independent of the coal feed and is for this shift reactor given in equation 4.19.

2 1.097 0.014 1.070 1.000

Compared to literature [21] this is a realistic value. The ratio may be as high as 2, however this will give more H2O in the product gas that has to be condensed before the CO2 capture unit anyway. There are also limitations according to the fraction of water in the product gas. It should not exceed 0.2 [10]. In addition is a CO fraction of 0.02 a realistic value. Addition of more steam is also an energy requiring process so this is seen as a satisfactory result.

With these conditions one mole of coal will give 2.167*0.427=1.031kmol of H2. The 2 % of CO has also heating values and can be fed to a turbine. Normally is H2O an excellent diluter, but in this case will the combustion process form a relatively high amount of water so the total amount will be too high for the turbine. The high amount of CO2 makes it relatively easy to remove it. A molar fraction of 0.390 after the water removing and a total pressure of 40.2bar gives a partial pressure of CO2 of 15.68bar. This is advantageous for the removing process.

The N2 is low and anyway a good substance to feed together with the H2 rich fuel. More N2

will be fed after the CO2 capture unit before the combustion chamber in the gas turbine.

4.2.5.2 Integrating the gasification and water shift

The temperature of the syngas can be reduced by adding more water into the gasifier. This again will give more H2O in the syngas and reduce the additional steam demand in the shift reactor. Allocating the steam/water in other ways gives no considerable changes. The composition from table 4.21 is therefore sent further to the CO2 capture unit before it is diluted with nitrogen and fed to the power cycle.

The chosen coal going through the gasifier and two stages of water shift reactors generates heat in all processes. This is because the overall reactions are more exothermic than

endothermic. This can be expressed in the term of cold gas efficiency. The heating value for the coal using equation 3.23 is given in equation 4.20.

0.736 393520 0.228 241820 0.003 296800 345656

coal

LHV kJ

kmol

⎡ ⎤

= i + i + i = ⎢⎣ ⎥⎦ (4.20)

The bituminous has a molar weight of 10.455kg/kmol and the LHV becomes on mass basis 345956/10.455=33090kJ/kg. This is much closed to the tabulated value from table 2.3 in 2.2.1.1 composition which operates with a LHV of 33400kJ/kg for Bituminous. LHV for the CO and H2 mix is then given by equation 4.21.

This gives a CGE of:

290185

0.840 345656

CGE= = (4.22)

This value corresponds to the literature. It is stated that a gasification process requires a CGE of at least 0.78 to make IGCC attractive [6].

The LHV for the H2 rich fuel is loosing some more heating value and for this case it is given by equation 4.23.

[ ]

2 1.967 0.02 282990 0.527 241820 261806

H rich

LHV kJ

kmol

⎡ ⎤

= i i + i = ⎢⎣ ⎥⎦ (4.23)

This indicates that there is a loss in heating values through the process. In an IGCC some of the heat that is released can be utilized in other processes and improve efficiency. This will be discussed more in 4.4.2.2 Adding steam to the steam turbine.

4.3 Acid Gas Removal