2.3. Marco conceptual
2.3.2. Factores de producción
For both Shell and Texaco gasifiers, the cool syngas from quench module will subsequently enter the WGS (water gas shift reactor) for the following reasons:
1) WGS is important for the preparation of CO2 sequestration because the CO2 concentration rises, which is ideal for PSA adsorption capture which is necessary for the PSA carbon capture process;
2) CO2 can be used as diluent in gas turbine and reduce the NOx emission; 3) Conversion of gaseous impurities such as COS, HCN;
4) The shift reaction is exothermic, the heat release by shift reaction can be used to heat HP and LP steam which will be used in HRSG to produce mechanical power. This
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can improve the efficiency of IGCC power plant.
The water gas shift reaction is shown below:
2 2 2
COH OH CO (5.37)
In industry, shift reactor adopts variable catalyst for the different working conditons. The most widely used catalysts for high temperature WGS are Fe2O3, Cr2O3 and MgO (Newsome, 1980). For low temperature WGS, CuO, ZnO and Al2O3 are reported as the typical choice (Smith et al.). Shift reactor can be located either prior to AGR (Acid gas removal) or after AGR since the catalyst poisoning issue caused by sulphide has been solved (M.Karmarkar, 2005). In this study, the shift reactor is located prior to AGR.
The primary aim to build shift reactor model in this study is to prepare high H2 and CO2 content shifted syngas stream for the combined cycle power plant, then investigate the performance of IGCC power plant. The detailed chemical reaction process with catalysts is out of the scope of this work, thus a reaction rate-controlled reactor block developed in Thermolib toolbox is adopted to simulate the WGS reaction process. The block can predict the syngas contents and temperature changes. Two shift reactors blocks are connected in series in this model and their reaction rates are defined respectively as 0.9 and 0.8 (Wang et al., 2015), respectively.
Heat exchanger modules are used for raising HP and LP steam by recovering the heat from the both stages of shift reactor model. The syngas temperature from the first stage reactor is normally over 683K (M.Karmarkar, 2005); this syngas stream is used to generate HP steam. Its temperature drops to around 523K and then it will enter the second stage reactor. The heat release from the second reactor is much less than the first stage due to a much lower CO
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concentration in the current syngas. The syngas temperature is around 550K, and then it will be cooled to 303K for the following sulphur removal process. The Schematics of WGS with heat recovery configurations are shown below in Figure 5.9 and Figure 5.10, respectively.
Figure 5.9 Schematics for WGS with heat recovery block for Shell gasifier
Figure 5.10 Schematics for WGS with heat recovery block for Texaco gasifier
The shift reactor model is developed based on mass balance and energy balance. The mass balance equation for gas contents i is shown below:
, , i i in in i out out i dM x M x M R dt
(5.38)123
where Mi(kg) denotes the mass of content i,xi in, and xi out, denote the mole concentration of content i in the input and output streams respectively. Minand Moutdenote the mass flow rate (kg/s) of input and output streams respectively while Ri (kg/s) denotes the net mass
production rate of content i by chemical reactions. There is no mass accumulation considered in this reactor model. The energy balance equation derived based on first law (Eutech, 2013) of the reactor is shown below:
, , i in j out k m i j k dU H H Q P dt
(5.39)where U denotes the internal energy (kJ) within the reactor, Hi in, (kW) denotes the enthalpy
flow rate of content i in the input stream, Hj out, (kW) denotes the enthalpy flow rate of content j in the output stream, Qk (kW) denotes the heat flow rate caused by heat transfer and
m
P (kW) denotes the mechanical power, which equals to zero for the reactor.
In terms of dynamic simulation, the results of shift reactor model in Shell-based GEM plant is presented. It is assumed that the mole flow rate of syngas stream generated by Shell gasifier is assumed to reach the full load value in 100s (Wang et al., 2015), the dynamic change of first and second stage reactor temperature and the contents concentrations are shown in the Figure 5.11:
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Figure 5.11 1st stage WGS shifted syngas temperature dynamic change
As shown in Figure 5.11. the syngas temperature rises to 761 K when it passes through the 1st stage WGS reactor, which is caused by the heat released from the exothermic shift reaction. The steam used for the first stage WGS is the exhaust steam from the steam turbine (Kreutz et al., 2010), which is preheated and compressed before it is mixed with the syngas to meet the reaction demand. The mixing process is simulated with a mixer block in Thermolib with a pressure loss factor of 1e-5 considered.
The CO2 and H2 concentration dynamic changes of the first stage WGS are shown in Figure 5.12 and Figure 5.13:
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Figure 5.13 1st stage WGS shifted syngas H2 concentration dynamic change
As shown in Figure 5.12 and Figure 5.13, the CO2 and H2 concentration in the wet syngas rises, which are caused by water gas shift reaction. The reaction rate of the first stage is set to be 0.9, hence it will convert most of CO to CO2 and generate extra H2. The high hydrogen and CO2 concentration is favoured by the pressure swing adsorption process since CO2 has a high partial pressure in the shifted syngas.
Figure 5.14 2nd stage WGS shifted syngas temperature dynamic change
As shown in Figure 5.14, the temperature of outlet syngas of the second stage low temperature WGS has experienced a drop first and then rises to the final temperature around 550K. The first temperature drop is caused by heat transfter from syngas to HP steam in the heat exchanger. As the syngas input gradually rises in the following time until it reaches maximum value after 100 seconds, the syngas temperature will rise to around 550K.
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Figure 5.15 2nd stage WGS shifted syngas CO2 concentration dynamic change
Fig. 5. 16 2nd stage WGS shifted syngas H
2 concentration dynamic change
The concentration of CO2 and H2 rises in the LT WGS as well, the final H2 mole concentration is 46.4%, and CO2 mole concentration is 31.7%. In terms of mole concentration in the dry stream, the ratios for these two contents are 57.1% and 39.1% respectively.
For the Texaco gasifier, there is no additional steam added for WGS, since there is enough saturated steam is carried by the syngas from quench chamber. Hence the changes of syngas contents concentration in WGS are transient. The detailed syngas parameters from HL and LT WGS outlets of Shell and Texaco gasifier are listed in Table 5.5:
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Table 5.5 Parameters of two stages WGS outlet syngas parameters of Shell and Texaco gasifier
Parameter HT WGS Shell LT WGS Shell HT WGS Texaco LT WGS Texaco Gas temperature (K) 770.685 549.753 698.698 556.379 Pressure (bar) 26.943 26.943 59.8 59.8
Mole flow rate ndot (mol/s) 3911 3911 8619 8619
Heat capacity rate Cpdot (W/K) 147334 140072 321479 310702
H2S mole concentration (%) 0.067 0.067 0.263 0.263
COS mole concentration (%) 0.008 0.008 0.030 0.030
N2 mole concentration (%) 1.782 1.782 0.507 0.507
CO mole concentration (%) 3.154 0.631 4.008 1.463
H2 mole concentration (%) 43.874 46.397 29.843 32.388
CO2 mole concentration (%) 29.204 31.727 21.423 23.968
H2O mole concentration 21.344 18.821 43.509 40.964
Internal research report (M.Karmarkar, 2005) provides the data of shifted syngas for Texaco- based GEM plant. The comparison of Texaco gasifier simulation and reference data are shown below:
Table 5.6 Parameters of two stages WGS outlet syngas parameters of Texaco gasifier
Parameter Shifted syngas R Shifted syngas S
Gas temperature (K) - 556.379
Pressure (bar) 59.78 59.8
H2S mole concentration (dry %) 0.51 0.445
H2S mole flow rate (kmol/h) 94.06 81.60
COS mole concentration (dry %) 0.00 0.05
COS mole flow rate (kmol/h) 0.2 9.07
N2 mole concentration (dry %) 0.77 0.507
N2 mole flow rate (kmol/h) 143.14 157.44
CO mole concentration (dry %) 2.47 2.48
CO mole flow rate (kmol/h) 456.36 453.90
H2 mole concentration (dry %) 55.5 54.7
H2 mole flow rate (kmol/h) 10259.69 10121.2
CO2 mole concentration (dry %) 39.87 40.60
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As shown in Table 5.6, the simulation results show high accuracy for the major parameters of shifted syngas stream including the gas flow rate and syngas contents concentration such as CO, CO2 and H2, etc. Relative large error exist for the COS and H2S simulation results. This is caused by the 9:1 assignment of sulphur assumption in syngas prediction model, which is introduced in Chapter 4. Since the total mole concentration of COS and H2S in the syngas stream is less than 0.5% in dry stream and 0.25% in wet steam, the error in sulphides is ignored and the simulation results will be used in the following COS and H2S removal process.