Reduced Mechanism for Nitrogen and Sulfur ... - ACS Publications

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Reduced Mechanism for Nitrogen and Sulfur Chemistry in Pressurized Flue Gas Systems Sima Ajdari,* Fredrik Normann, Klas Andersson, and Filip Johnsson Department of Energy and Environment, Chalmers University of Technology, SE-412 96 Göteborg, Sweden S Supporting Information *

ABSTRACT: The gas- and liquid-phase chemistry of nitrogen and sulfur species under pressurized conditions is of high importance to the design and performance of the pressurized flue gas systems in carbon capture and storage (CCS) schemes. Yet, the available description of this chemistry is complex and difficult to apply in design studies for removal of NOx and SOx during the compression. This work proposes a reduced mechanism for engineering calculations of pressurized flue gas systems, a mechanism that is able to describe the relevant gasand liquid-phase chemistry as well as the S/N-product distribution. The reduced mechanism is derived by identifying the rate-limiting reactions using sensitivity analysis. The performance of the mechanism subsets are compared with results of a detailed mechanism. The identified rate-limiting reactions for the formation of key products form the basis for two different types of reduced mechanisms. The sets include one general reduced mechanism (valid for all pH conditions) and sets of pH-specific mechanisms. The general reduced mechanism and the pH-specific mechanisms perform satisfactorily compared to the detailed mechanism under different pH conditions. The results show that depending on the purpose of the modeling, whether it is to predict the pollutant removal (where sulfurous acid and nitrogen acids are mainly important) or capture the liquid composition, for which the N−S chemistry products are also important, different levels of simplification can be made. The number of reactions is reduced from 34 reactions (39 species) in the detailed mechanism to 12 reactions (20 species) in the general reduced mechanism and 7−8 (14−17 species) in the pH-specific mechanisms.

1. INTRODUCTION CO2 capture and storage (CCS) is an important climate change mitigation technology that allows the continued use of fossil fuels in the power and industrial sector1 while meeting strict CO2 emission reductions. Oxy-fuel and chemical looping combustion systems are a group of CCS technologies that result in a flue gas that mainly consists of CO2 and H2O. The flue gas is compressed and liquefied before the CO2 is transported to storage. The flue gas contains contaminants including nitrogen oxides (NOx) and sulfur oxides (SOx). These contaminants need to be controlled due to environmental concerns, due to transport and storage requirements, and also due to their potential adverse impact on the flue gas cleaning and the CO2 processing unit. The control of NOx and SOx is, thus, an important economic and technical concern.2−5 The presence of these acid gases at elevated pressures will pose corrosion risks in the pipes and equipment due to formation of sulfuric and nitric acids as observed in pilot tests.6,7 The initializing reaction is the oxidation of NO (which is the main NOx species in the flue gas) to NO2 by oxygen, where the oxidation rate is significantly increased by an increase in pressure. NO2 has a relatively high solubility and will be absorbed in any liquid present in the flue gas train. The sulfuric acid formation is a result of the interaction between nitrogen and sulfur chemistry in the presence of water.8 The chemistry in the © XXXX American Chemical Society

liquid phase is complex, and there are a great number of possible reactions that may occur.9 Two types of interactions are possible: interactions between NO2 and S(IV) and interactions between HNO2 and HSO3−. The latter has been studied experimentally, and two reaction paths have been identified. As HNO2 and HSO3− react, nitrososulfonic acid (ONSO3−, referred to as NSS hereinafter) forms,10,11 which either can react with HSO3− to form hydroxylamine disulfonic acid (HADS) and other N−S complexes (pathway I) or hydrolyze in the presence of H+ to form N2O and other products (pathway II). For detailed discussion on the chemistry see Ajdari et al.9 The competition between pathways I and II depends on the pH:12 pathway II dominates at low pH conditions (pH ≤1)13 whereas pathway I dominates at pH ≥4.14 For pH between 1 and 4 both pathways are significant. The interactions between NO2 and S(IV) are expected to be important at pH ≥5. Figure 1 shows the expected pH, resulting from the absorption of acid gases, of a liquid phase that is in contact with a pressurized flue gas stream based on the modeling in our previous work.9 The Received: January 26, 2016 Revised: April 20, 2016 Accepted: April 25, 2016

A

DOI: 10.1021/acs.iecr.5b04670 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Calculated pH of the liquid phase in contact with a pressurized flue gas stream and the relation to liquid-to-gas ratio (L/G), pressure, and flue gas composition. The gas−liquid contact time is 2 min. The calculations are based on our previous work.9

pH of the solution leaving a pressurized flue gas system depends on the liquid-to-gas ratio (L/G), pressure, and flue gas composition. Typical L/G ratios in acid gas absorbers as well as in the flue gas condensate are indicated by vertical dashed lines in Figure 1. The acidity of the liquid formed in such systems will thus be considerable. The acidity and the expected SOx and NOx absorption are highly dependent on the concentration of NOx, SOx, and water vapor as well as the pressure. It should be noted that it is of course possible to control pH in process units with additives. The formation of nitric and sulfuric acids as a result of absorption and interaction of NOx and SOx in the liquid phase could potentially cause severe corrosion problems in the compression section. However, with an appropriate process design, the formation of acids may also be used as an efficient flue gas cleaning technology for simultaneous removal of NOx and SOx. The potential possibility of simultaneous removal of NOx and SOx has led to development of new process concepts in connection to pressurized flue gas systems proposed and evaluated in refs 15−17. So far, evaluations of these removal processes and the description of the N−S interactions in pressurized flue gases have been based on the assumption that the gas phase oxidation of NO by O2 is the rate-limiting step in the mechanism. The N−S interaction has commonly been described as a fast (equilibrium) reaction of NO2 and SO2 in the gas phase as shown below:6,17−19 NO2 (g) + SO2 (g) ⇆ NO(g) + SO3(g)

conditions9 showed that pH (and L/G ratio, see Figure 1), residence time, and NOx/SO2 ratio are of importance with respect to the liquid phase chemistry, which in turn affects the removal of NOx and SOx from the flue gas and the formation of liquid phase chemistry products. Therefore, the mechanism applied in process design studies should be able to capture the pH, gas composition, moisture content, and residence time dependence of the liquid phase interactions, which is not possible if only considering the gas phase reactions. Thus, it is of importance to include the liquid phase chemistry to design and optimize flue gas cooling and compression as well as pressurized NOx and SOx absorbers. Yet, a description of the liquid phase chemistry has to be simplified in order to be practically useful for implementation in process evaluation studies using common process simulation software. The aim of the present work is to present a reduced mechanism of the liquid phase chemistry that considers pH, NOx, and SOx concentration and is efficient in process simulations with a detailed description of mass transfer between gas and liquid phases. The mechanism should be able to predict the NOx and SOx removal and adequately describe the liquid phase products for practical applications. The mechanism reduction is based on identification of the most important reaction paths and the rate-limiting steps of a detailed description of the chemistry published in our previous paper.9

(R1)

2. METHOD Mechanism reduction was based on identification of the dominating reaction paths and the rate-limiting steps of a detailed mechanism published in our previous paper.9 The ratelimiting and necessary reactions for the formation of the key products were the basis for the pH-specific and the general reduced mechanisms. The mechanism reduction was performed in steps. Initially, the insignificant reactions and species revealed by a sensitivity analysis were omitted from the detailed mechanism to form the general reduced mechanism that is valid for the entire pH range (pH 1−5). Next, necessary and ratelimiting reactions identified for each pH condition were used to

which is followed by the absorption of SO3 in the liquid phase and formation of H2SO4. This approach is contrary to recent experimental findings8,20 and available kinetic information for reaction R1,21 which show that the gas phase reaction between SO2 and NO2 is not expected to be significant under conditions relevant for pressurized flue gas systems. This emphasizes the importance of the evaluation of the complex liquid phase chemistry as it will play an important role in the design and evaluation of the process concepts. We previously9 presented a mechanism which was constructed and validated based on the available experimental data in the literature (see refs 10, 13, and 14). The evaluation of the chemistry under pressurized flue gas B


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differencing approximations by a ±5% one-at-a-time perturbation in rate constants of each reaction:

form the pH-specific reduced mechanisms. The reactions were combined to form global reactions when possible. In a final step further simplifications were tested and discussed for different pH conditions. In addition, a gas phase controlled mechanism is also evaluated. This mechanism is based on the common assumption of the gas phase NO oxidation by O2 to be the rate-limiting reaction. The prediction of the concentrations of the key liquid phase chemistry products by the reduced mechanisms are qualitatively compared with the detailed mechanism. The dominating reaction paths and the rate-limiting steps were identified based on their impact on the designated key species (H2SO4, H2SO3, HNO3, HNO2, HADS (hydroxylamine disulfonic acid), and N2O) under different sets of operating conditions. The rate-limiting step is characterized by the fact that increasing its rate coefficient significantly increases the production rate of the key products. The developed reaction mechanisms should apply to a set of conditions ranging from absorption in condensate formed during compression to scrubber systems with pH control; cf. Figure 1. The base case conditions and the range of the operating conditions in the sensitivity analysis are listed in Table 1. The flue gas composition

Sij =

pressure SO2 inlet total NOx inlet O2 inlet H2O(g) inlet temperature L/G residence time pH

evaluated range

15 bar 1000 ppm (v) 400 ppm (v) (10% NO2, 90% NO) 3% 2000 ppm 25 °C 5 L/m3 0−10 min 1−5

5−30 bar

Ci(kj + Δkj) − Ci(kj − Δkj)

Ci(kj)

2Δkj

(1)

where Ci is the calculated concentration of key species i. The key species are based on the identified important products in our previous work.9 As the liquid phase chemistry is highly dependent on pH, the sensitivity analysis was performed at different constant pH levels. The results of the sensitivity analyses were compared with results of the principal component analysis method.25 Both methods identified the same reactions as rate-limiting with respect to the key species. Only the results of the sensitivity analysis are presented here. The sensitivity discussion is focused on the reactions for which the normalized sensitivity coefficients are greater than 10% of the highest calculated normalized coefficients. 2.2. Model. The reactor system was described by a two-phase model in which the gas and liquid phases were modeled as two separate, perfectly mixed batch reactors with constant volume. The change in partial pressure (pi) of each species i in the gas phase was described as

Table 1. Base Case Conditions and the Range of the Operating Conditions in the Sensitivity Analysis base case

kj

dpi dt

= RT[∑ (vi , jR j ,g) − R dissolution, i]

(2)

where (vi,j) is the stoichiometric coefficient, Rj,g is the rate of gasphase reaction j, and Rdissolution,i is the net number of moles of species i that are absorbed into the liquid phase per unit volume (moles per liter per second). The mass transfer was considered not to be rate-limited (i.e., fast), and equilibrium was assumed at the interface, according to Henry’s law. Thus, the following expression was applied to model the net rate of dissolution of the gas species into the liquid phase (per unit volume):

0−10 min 1−5

⎛ C⎞ R dissolution, i = kdiss⎜Pi − i ⎟ Hi ⎠ ⎝

is representative of pressurized flue gas systems as seen in refs 6 and 15. As SO3 is easily absorbed in the flue gas condenser/ cooler prior to compression, no SO3 is assumed to be present in the flue gas. The sensitivity analysis method is described in detail in section 2.1. The mechanism is implemented in a simple gas− liquid reactor model for the sensitivity analysis. The mixing and mass transfer characteristics of the reactor model as well as the complete list of reactions included in the detailed mechanism are provided in section 2.2. 2.1. Sensitivity Analysis. The sensitivity analysis was performed by making small perturbations to the rate coefficient for all the reactions in the detailed mechanism and evaluating the effect on the predicted concentration of key products. The effect on the production rate is quantified by the local rate sensitivity coefficients described below.22 A high sensitivity coefficient of a specific product i to a specific reaction j relative to other reactions indicates the reaction j is rate-limiting with respect to the production of product i. The sensitivity coefficients Sij were identified by local sensitivity analysis where the effect of a small change to the rate coefficient (kj) for reaction j on the production rate of product i was studied. A positive Sij indicates that the predicted concentration of product i increases given an increase in the rate coefficient for reaction j. A negative Sij implies a decrease in the predicted concentration of product i as a result of increase in the rate coefficient of reaction j. The sensitivity analysis was based on the brute force method.23,24 The bruteforce normalized sensitivities were calculated using central

(3)

where Ci is the molar concentration of species i in the liquid phase (moles per liter), and Hi is the Henry’s law constant for species i (moles per liter per bar). The gas-side mass transfer coefficient kdiss was assigned high values. This procedure disregards mass transfer limitations on the reactions. Mass transfer characteristics will be specific to the design of the separation equipment. The change in concentration of the species in the liquid phase is calculated as dCi = dt

⎛ Vg ⎞ ⎟ ⎝ Vl ⎠

∑ (vi ,jR j ,l) + R dissolution,i⎜

(4)

where vi,j is the stoichiometric coefficient of species i in the liquid phase reaction j and Rj,l is the rate of liquid-phase reaction j. The term (Vg/Vl) refers to the ratio of the gas-phase volume to the liquid-phase volume. The model for the reaction system analysis represents an upper limit for the liquid-phase reactions in the sense that no mass-transport limitation for the liquid phase is considered in the simulations. Table 2 presents the complete list of reactions as included in the detailed mechanism.

3. RESULTS This section presents the results from the sensitivity analysis and the evaluation of the reduced mechanisms. The focus is on the prediction of the key liquid phase chemistry products: nitrogen C


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3.1. Sensitivity Analysis. The main results from the sensitivity analysis are discussed in the text accompanied by the figures given below. The complete set of figures is available as Supporting Information. It should be noted that the results shown for H2SO3 represent HSO3− and SO32− (and SO2(aq)) since these components are at equilibrium at all times. The same applies for HNO2 (HNO2 and NO2−), HNO3 (HNO3 and NO3−), and H2SO4 (HSO4− and SO42−). It should be noted that instantaneous and equilibrium reactions in addition to reactions with sensitivity coefficients 1 order of magnitude or more below the largest sensitivity coefficient are not shown in the figures. Figure 2 shows the normalized sensitivity coefficients for the key products at pH 1. As can be seen in Figure 2a−c at pH 1 and base case conditions, the liquid phase reaction between HNO2 and HSO3− (Rl14) controls the sulfur chemistry and formation of N2O. For the formation of nitrogen acids (Figure 2d,e), the gas phase oxidation of NO (Rg1), hydrolysis of NO2 (Rl1) and decomposition of HNO2 (Rl3) are also important, especially for residence times less than 120 s. As the residence time is increased, the values of the normalized sensitivity coefficients for the NOxonly chemistry converge and the slower reaction between HNO2 and HSO3− (Rl14) becomes increasingly important. Therefore, with respect to HNO2 for residence times longer than 120 s, reaction Rg1 is the rate-limiting gas phase reaction and reaction Rl14 is the rate-limiting liquid phase reaction. The concentration of HNO3 is most sensitive to reactions Rg1, Rl3, and Rl1 for the residence times less than 120 s. The negative sensitivity coefficient of reaction Rl3 corresponding to HNO3 at pH 1 (Figure 2b) is due to the fact that the backward reaction (Rl3b) is active at residence times less than 60 s until enough HNO2 builds up and the decomposition reaction becomes important (Rl3f). This will cause a competition between the NO2 hydrolysis and the formation of HNO2 via reaction Rl3b. As the residence time is increased, reaction Rl14 becomes important and reaction Rl3 is no longer a rate-limiting reaction corresponding to HNO3. Although the sensitivity coefficients for reaction Rg1 decrease with increased residence time, the sensitivity coefficients for this reaction are still comparable to those of the liquid phase reactions. At lower pressures (for illustrations which compare 5 and 30 bar conditions, see the Supporting Information) the nitrogen acid chemistry becomes more sensitive to the gas phase NO oxidation (Rg1) while reaction Rl14 becomes increasingly important at higher pressures as the rate of reaction Rg1 is enhanced. For the sulfur acids and N2O formation (Figure 2a,c), reaction Rl14 is the most important for all pressures. In general, pressure does not significantly change the relative importance of reactions for the production rate of sulfur acids. Figure 3 shows the normalized sensitivity coefficients for sulfur acids and N2O at pH 2. The figures for the rest of the key products (HNO2, HNO3, and HADS) are shown in the Supporting Information. The formation of sulfur acids at pH 2 (Figure 3a,b) is similar to the conditions at pH 1 with the one exception that the two pathways now are competing and the formation of the sulfuric acid as well as N2O (and HADS, see Supporting Information) (Figure 3c) are rather sensitive to the first steps in each pathway (reactions Rl15 and Rl16). The rates of these reactions are much higher than the rate of reaction Rl14 and proportional based on the findings of Oblath et al.;10 i.e. it is foremost the relative rates of these reactions that are important for the products of the competing pathways. The same sensitivity coefficients are obtained if both of these reactions are made 10 times faster in the modelimplying that the relative rate plays an important role in the calculation of the N−S chemistry products.

Table 2. Complete List of Reactions in the Detailed Mechanisma reactionb,c

ref

Gas Phase Reactions

2NO + O2 → 2NO2

(Rg1f)

26

2NO2 → 2NO + O2

(Rg1b)

27

2NO2 ⇄ N2O4

(Rg2)

NO + NO2 ↔ N2O3

28

(Rg3)

28

N2O4 + H 2O → HNO2 + HNO3

(Rg4)

NO + NO2 + H 2O ↔ 2HNO2 NO2 + SO2 → NO + SO3

29

(Rg5)

30

(Rg6)

21

Liquid Phase Reactions 2NO2 + H 2O → HNO2 + HNO3 (Rl1)

31

N2O4 + H 2O → HNO2 + HNO3

28

(Rl2)

2HNO2 → NO + NO2 + H 2O

(Rl3f)

32

NO + NO2 + H 2O → 2HNO2

(Rl3b)

32

2SO32 − + O2 → 2SO4 2 − 2NO2 +

HSO3−

(Rl10)

+ H 2O → SO4

2−

33 +

+ 3H + 2NO2

−

2NO2 + SO32 − + H 2O → SO4 2 − + 2H+ + 2NO2−

HNO2 + NSS +

HSO3−

HSO3−

→ NSS + H 2O

→ HADS

(Rl13)

(Rl14)

10

(Rl17)

HADS + H 2O → HAMS + HSO4−

35

(Rl18)

HAMS + H 2O → NH 2OH + HSO4

−

HADS + HSO3− → ATS + H 2O

(Rl19)

HAMS + HSO3− → NH 2SO3− + HSO4−

36 37

ADS + H 2O → NH 2SO3− + HSO4−

(Rl22)

−

+

+ HNO2 → N2 + HSO4 + H + H 2O 2−

37

(Rl21)

HAMS + HSO3− → ADS

HAMS + NO2 → N2O + SO4

36

(Rl20)

HADS + HSO3− → ADS + HSO4−

−

34

10

(Rl16)

HNO + HNO → N2O + H 2O

34

10

(Rl15)

NSS + H 2O → HNO + HSO4−

NH 2SO3−

(Rl12)

(Rl24)

12

(Rl23)

12 38

Equilibrium Reactions

HNO2 ⇄ H+ + NO2−

(Rl6)

32

HNO3 ⇄ H+ + NO3−

(Rl7)

31

+

SO2 + H 2O ⇄ H +

HSO3−

+

2−

+

2−

⇄ H + SO3

−

HSO3−

HSO4 ⇄ H + SO4 +

ADS ⇄ H + N(SO3)2

2NO2 ⇄ N2O4 a

3−

(Rl25)

39

(Rl26)

40

(Rl27)

41

(Rl28)

(Rl29)

42 28

b

The mechanism is based on ref 9. Reactions Rl12 and Rl13 are only active for pH ≥5. cAbbreviations used in reactions Rl14−Rl16, Rl18−Rl22, Rl24, and Rl28 are NSS: ONSO3−; HADS: HON(SO3)22−; HAMS: HONHSO3−; ATS: N(SO3)33−; ADS: NH(SO3)22−.

acids (HNO2 and HNO3), sulfurous acid (H2SO3), and the N−S chemistry products (N2O, H2SO4, and HADS). Two sets of compiled reduced mechanisms in addition to a gas phase controlled mechanism are compared to the detailed mechanism in section 3.2. Further simplifications to the reduced mechanisms are also discussed. D


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Figure 2. Effect of residence time on the normalized sensitivity coefficients corresponding to the most important products for base case conditions (according to Table 1) at pH 1: (a) H2SO3, (b) H2SO4, (c) N2O, (d) HNO2, and (e) HNO3. Dashed lines are gas phase reactions and solid lines are liquid phase reactions. Note that the reactions with sensitivity coefficients that are more than 1 order of magnitude smaller than the largest calculated sensitivity coefficient are not shown.

Figure 3. Effect of residence time on the normalized sensitivity coefficients corresponding to the most important products for base case conditions (according to Table 1) at pH 2: (a) H2SO3, (b) H2SO4, and (c) N2O. Dashed lines are gas phase reactions and solid lines are liquid phase reactions. Note that the reactions with sensitivity coefficients that are more than 1 order of magnitude smaller than the largest calculated sensitivity coefficient are not shown.

This is in line with the findings of Oblath et al.,10 who measured the relative rates of these steps. They reported that reaction Rl14 is the rate-limiting step with respect to HNO2 and HSO3− (since they measured these species in the experiments). The formation of nitrogen acids at pH 2 (Figure 3b) is also rather similar to the formation at pH 1. However, reaction Rl3 is reversed under these condition (see detailed discussion in our previous work9). Figure 4 shows the normalized sensitivity coefficients for H2SO3, HADS, and HNO2 at pH 4. The figures for HNO3 are shown in the Supporting Information. For pH 4 and base case conditions (Figure 4a,b), H2SO3 and HADS concentrations are mainly controlled by reaction Rl14 and as the residence time increases reaction Rg1 becomes increasingly important. The concentration of HNO2 (Figure 4c) is mainly controlled by reaction Rl14 except at residence times less than 60 s when

reaction Rg1 limits the formation of NO2. At higher pressures (see the Supporting Information) reaction Rg1 has a negative sensitivity coefficient with respect to HNO2, as the increased formation of NO2 results in high net consumption of HNO2 in the liquid (through reaction Rl3). It should be pointed out that the high sensitivity coefficients for HNO2 formation after 300 s are mainly caused by its low concentration. Figure 5 shows the normalized sensitivity coefficients for the key products at pH 5. If pH is increased above 4, Rl14 becomes the rate-limiting reaction for H2SO3 (Figure 5a) and HNO2 (Figure 5c) except for residence times up to around 120 s. For HNO2, the same conclusions can be made at other operating pressures (for illustrations which compare 5 and 30 bar conditions, see Supporting information). For H2SO3, as the pressure is increased the residence time at which reaction Rl14 E


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Figure 4. Effect of residence time on the normalized sensitivity coefficients corresponding to the most important products for base case conditions (according to Table 1) at pH 4: (a) H2SO3, (b) HADS, and (c) HNO2. Dashed lines are gas phase reactions and solid lines are liquid phase reactions. Note that the reactions with sensitivity coefficients that are more than 1 order of magnitude smaller than the largest calculated sensitivity coefficient are not shown.

Figure 5. Effect of residence time on the normalized sensitivity coefficients corresponding the most important products for base case conditions (according to Table 1) at pH 5: (a) H2SO3, (b) H2SO4, (c) HNO2, and (d) HADS. Dashed lines are gas phase reactions and solid lines are liquid phase reactions. Note that the reactions with sensitivity coefficients that are more than 1 order of magnitude smaller than the largest calculated sensitivity coefficient are not shown.

conditions are presented in Figure 6 as an example. Comparisons of the “reduced pH 1−5” and the detailed mechanism for other pH conditions are included in the Supporting Information. In general, the predictions made by “reduced pH 1−5” agree with the complete mechanism. The only phenomenon not covered by the “reduced pH 1−5” is the hydrolysis of HADS. As can be seen in Figure 6, formation of HAMS at pH 2 is not predicted by the reduced mechanism, but the HADS + HAMS formation in the complete mechanism is the same as the predicted HADS by the “reduced pH 1−5” mechanism, and the results can be considered satisfactory for the application of interest. 3.2.2. pH-Specific Mechanisms. At pH 1, the sulfur acid chemistry is controlled by the formation of NSS (Rl14) in the liquid phase. Pathway II dominates the N−S chemistry and the formation of NSS (Rl14) is the rate-limiting step in the liquid phase. The faster reactions Rl16 and Rl17 may, thus, be combined with reaction Rl14 by removing the highly reactive intermediates (NSS and HNO), resulting in the reaction

becomes the rate-limiting reaction decreases. Reaction Rg1 is the rate-limiting reaction for H2SO4 (Figure 5b) at low pressures, but Rl14 starts to become limiting as the pressure and residence time are increased. Reaction Rl14 is the rate-limiting reaction for HADS under base case conditions (Figure 5d) and all other operating pressures. The rates of the reactions between NO2 and S(IV) are relatively fast and not expected to be limiting. 3.2. Mechanism Reduction. Scheme 1 shows the reactions identified as necessary and rate-limiting by the sensitivity analysis (see section 3.1) for the complete set of pH conditions. These reactions are included in the mechanism called “reduced pH 1− 5”. This mechanism is further reduced for specific pH conditions to mechanisms named “reduced pH 1” to “reduced pH 5”. Further simplifications as well as a gas-phase controlled mechanism (named “mech 1”) are also discussed. Table 3 presents the reduced mechanisms. The mechanism referred to as the “detailed mechanism” includes the 34 reactions and 39 species listed in Table 2. 3.2.1. pH-Dependent Mechanism. The prediction of the key product profiles by the “reduced pH 1−5” mechanism is compared with the detailed mechanism. The results for pH 2 F


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Industrial & Engineering Chemistry Research Scheme 1. Reactions Identified as Important by the Sensitivity Analysis for the Complete Set of pH Conditionsa

a

These reactions are included in the “reduced pH 1−5” mechanism.

Table 3. List of Reactions in the Reduced Mechanismsa kinetic reactions

equilibrium reactions

mechanism

Rg1

Rl1

Rl3

Rl12

Rl14

Rl15

Rl16

Rl17

reduced pH 1−5 reduced pH 1 reduced pH 2 reduced pH 4 reduced pH 5

× × × × ×

× × × × ×

× × × ×

×

× (×) (×) (×) (×)

×

× (×) (×)

× (×) (×)

×

e

(×) (×) (×)

Rl30b × ×

Rl31c

× × ×

Rl6

Rl7d

Rl25

Rl27

no. of reactions

no. of species

×

(×) (×) (×) (×) (×)

× × × × ×

× × ×

12 7 8 7 7

20 16 17 14 14

× ×

×

× indicates that a certain reaction is included in the mechanism. (×) indicates that the reaction is included in the mechanism but is lumped with other reactions. bReaction Rl30 is a result of lumping reactions Rl14, R116, and R117. cReaction R131 is a result of lumping reactions R114 and R115. d Reaction R117 is combined with reaction R11 since HNO3 is in the dissociated form at these conditions. eOnly active for pH ≥5. a

Figure 6. Comparison of “reduced pH 1−5” mechanism and the detailed mechanism for base case conditions at pH 2.

reactions. The dissociations of HNO2 and HSO3− are discarded since these weak acids will occur in nondissociated form for these pH conditions. This requires adjusting the rate equation for reaction Rl14 so that it is expressed in terms of concentration of HNO2 instead of NO2− and H+. This is done by using the dissociation constant of HNO2. The complete list of reactions in the reduced mechanism “reduced pH 1” can be found in Table 3.

HNO2 + HSO3− → HSO4 − + 0.5N2O + 0.5H 2O (Rl30)

with a rate equal to the rate of reaction Rl14. The remaining N−S reactions are neglected. The rate-limiting step for the nitrogen acid chemistry could vary depending on the pressure and residence time. The hydrolysis reaction (Rl1) should be included in the model as its product (HNO2) takes part in the subsequent G


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Figure 7. Comparison of reduced mechanisms and the detailed mechanism for important liquid products at pH 1 and (a) base case conditions, (b) P = 5 bar, and (c) P = 30 bar. Solid lines correspond to the detailed mechanism, short dashed lines to the “reduced pH 1”, and dotted lines correspond to the “reduced pH 1” excluding reaction Rl3.

The comparison between the “reduced pH 1” mechanism and the detailed mechanism is shown in Figure 7. In order to investigate whether further reduction of the mechanism is possible, the effect of excluding the net decomposition of HNO2 (Rl3) from the mechanism is evaluated. This is done because according to Figure 2 the sensitivities of the important products to the net reaction Rl3 is low relative to the other reactions and

the importance of this reaction at high pressure conditions is not as significant as the hydrolysis of NO2,9 which is necessary for the production of HNO2 for the subsequent reaction with S(IV). As can be seen in Figure 7a, if reaction Rl3 is included in the “reduced pH 1” mechanism the results agree with the detailed mechanism for the base case conditions, whereas omitting reaction Rl3 affects the HNO2 concentration significantly (∼50% H


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Figure 8. Comparison of reduced mechanisms and the detailed mechanism for important liquid products at pH 2 and base case conditions for (a) N−S chemistry products and (b) nitrogen acids and HSO3−. Note that S(VI) represents HSO4− + SO42−.

Figure 9. Comparison of reduced mechanisms and the detailed mechanism for important liquid products at pH 4 and base case conditions for (a) nitrogen acids and (b) S(IV) and HADS.

HNO2 + 2HSO3− → HADS + H 2O

relative increase in its concentration at t = 600 s) and the concentration of sulfuric acid is affected to some extent (∼10% relative increase in its concentration at t = 600 s). The higher peak concentration of HNO2 in the “reduced pH 1” mechanism including reaction Rl3 is due to the initial activity of the backward reaction (Rl3b). For longer residence times the decomposition reaction (Rl3 f) becomes dominating; hence, the higher consumption rate of HNO2 in the case where reaction Rl3 is included. From Figure 7 it can be concluded that the “reduced pH 1” is in good agreement with the detailed mechanism and that the net decomposition and formation of HNO2 according to reaction Rl3 is required to describe the formation of the important products in the liquid phase. At pH 2 (and other pH values ≤4), the competing pathways should be included in the mechanism and as discussed in section 3.1 the relative rate of reaction Rl15 to Rl16 is important for the model. But reaction Rl14 is the rate-limiting step, so reactions Rl30 and Rl31 with the following rate constants are used in the model.

rR l30 =

(Rl31)

rR l14 1.7

(5)

rR l31 = rR l14

(6)

Figure 8 compares the results from the detailed and “reduced pH 2” mechanisms. The effect of assuming equal rates for reactions Rl30 and Rl31 is also evaluated. The latter is done to evaluate the effect of assuming reaction Rl14 as the rate-limiting step for all the products. Relative to the detailed mechanism, the “reduced pH 2” mechanism (Figure 8a) underestimates the concentration of N2O and S(VI) while the concentration of HADS is overestimated. This implies that the division between the pathways is not described correctly by the “reduced pH 2” mechanism. This is expected as the results from the sensitivity analysis for these products (Figure 3b,c) shows high sensitivity to the rates of reactions Rl15 and Rl16. The predicted concentrations of I


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Figure 10. Comparison of predicted (a) SO2 absorption and (b) S conversion into stable liquid products by “mech 1”, “mech 1 V2”, pH-specific reduced mechanisms, and detailed mechanism at base case conditions according to Table 1. The results by reduced mechanisms and the detailed mechanism are shown by solid lines and dotted, symbolled lines, respectively.

nitrogen acids and HSO3− by the “reduced pH 2” mechanism results in relative errors of up to around 25, 18, and 13% for HNO2, NO3−, and HSO3−, respectively, for the base case conditions, depending on the residence time. (Note that the high maximum relative error for HNO2 is caused by its low concentration at high residence times.) The relative error of the “reduced pH 2” mechanism compared to the detailed mechanism is reduced with an increase in pressure. Assuming equal rates for reactions Rl30 and Rl31 results in equal amounts of products of pathways I and II, which is not in agreement with previous experimental observations.10 Assumption of equal rates does not affect the calculated concentrations of nitrogen acids and HSO3− (Figure 8b) as much as the N−S products. This is expected as the sensitivity of these acids is not as high as the N−S products toward these reactions (Figure 3a,b). The results show that depending on the aim of the simulation, whether it is to predict the pollutant removal (where sulfurous acid and nitrogen acids are mainly important) or capture the complete liquid profile (where the N−S chemistry products are also important), different levels of simplification can be done to the detailed mechanism at this pH condition. The “reduced pH 4” mechanism applies the overall reaction Rl31 with a rate equal to that of reaction Rl14. The same approach has previously been used by Petrissans et al.14 The effect of including reaction Rl3 is studied. The predictions of the mechanisms at pH 4 are compared in Figure 9. As seen in Figure 9, the “reduced pH 4” mechanism satisfactorily predicts the concentration of important products in the liquid phase. Removing reaction Rl3 from the mechanism significantly increases the error for all products. The trends in predictions are the same for all pressure levels. The difference between the pH 5 and pH 4 mechanisms is the addition of the overall reaction Rl12. The prediction of the “reduced pH 5” is also satisfactory (for results refer to the Supporting Information). 3.2.3. Gas Phase Controlled Mechanism. Figure 10 compares the SOx removal and S-product conversion of the gas phase controlled mechanism proposed by Iloeje et al.17 (“mech 1”) to the reduced mechanisms presented here. “Mech 1” is based on the common assumption that the N−S interactions are fast compared to the gas phase oxidation of NO (reaction

Rg1) and the SO2 oxidation takes place in the gas phase and is not rate-limiting according to NO2 + SO2 ↔ NO + SO3

(Rx)

SO3 + H 2O ↔ H 2SO4

(Ry)

In general, this mechanism does not comply with previous experimental20,43,44 and modeling observations9 with respect to the N−S interactions in the gas phase. We shortly discuss it here as it is commonly referred to as the general mechanism for the production of sulfuric acid in the liquid phase for pressurized flue gas systems. The prediction of SOx absorption and conversion of the SOx absorbed to stable products in the liquid phase by “mech 1” is compared with the predictions of pH-specific mechanisms in Figure 10. The SOx absorption and conversion to stable products are defined as % SOx absorbed =

mol SO2 (g)in − mol SO2 (g)out × 100 mol SO2 (g)in (7)

S conversion into stable products =

mol H 2SO4 + 2(mol HADS) × 100 mol SO2 (g)in − mol SO2 (g)out

(8)

A second version of “mech 1” is also included in the analysis where it is assumed that SO2 does not dissolve into the liquid phase (called “mech 1 V2”).This is done to evaluate the effect of the NO2 formation in the gas phase on the SO2 removal enhancement which is not captured by “mech 1”. As shown in Figure 10, “mech 1” predicts complete absorption of SOx into the liquid phase but only around 40% of the absorbed SOx is converted to H2SO4 via hydrolysis of SO3. “Mech 1 V2”, on the other hand, predicts up to around 30% of SOx (only SO3) being absorbed into the liquid phase out of which all is converted to H2SO4 after around 3 min. Although the predicted SOx uptake from the flue gas and the conversion to the stable products varies for different pH conditions as predicted by the pH-specific mechanisms, the predicted values by “mech 1” and “mech V2” are independent of the pH. In general, compared to the pH-specific J



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mechanisms, the two versions of “mech 1” oversimplify the chemistry and fail to capture the effect of pH on the SO2 absorption from the gas phase and the different products formed in the liquid phase.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04670. Sensitivity analysis results under different pH and pressure conditions; comparison of the predictions by the reduced mechanisms and the detailed mechanism under different pH conditions (PDF)

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REFERENCES

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4. CONCLUSIONS The present work proposes reduced mechanisms for engineering and process modeling of the gas and liquid phase S/N chemistry applicable to pressurized flue gas cleaning systems. A sensitivity analysis has been performed which shows that the oxidation of NO by O2 in the gas phase is the rate-limiting gas-phase reaction while different liquid-phase reactions are of importance depending mainly on the pH and the liquid-phase product of interest. One general reduced mechanism (12 reactions, 20 species) has been constructed. The general reduced mechanism performs well in line with a previously published9 detailed mechanism (34 reactions, 39 species) which is used for reference in this work. The general reduced mechanism is further simplified by compiling pH-specific reduced mechanisms (7−8 reactions, 14−17 species). In general, the results show that depending on the aim of the simulation, whether it is to predict the pollutant removal (where sulfurous acid and nitrogen acids are mainly important) or to capture the complete liquid product composition (where the N−S chemistry products are also important), different levels of simplification can be made to the chemistry. Simplification of the mechanisms by assuming the N− S interactions to be faster than the NO oxidation in the gas phase fails to capture the effect of the pH and N−S interactions in the liquid phase on the SO2 removal from the gas phase and to capture the liquid phase profile. Thus, the pH-specific mechanisms identified in this work are capable of describing the liquid-phase chemistry satisfactorily while being more efficient for use in process design studies where mass transfer need to be considered. The pH-specific mechanisms have fewer species that are not included in the commonly used physical properties databases (e.g., NSS, HADS, HAMS). This facilitates easier implementation of these mechanisms in engineering process modeling tools. It should be also noted that the identified rate-limiting reactions should constitute the focus of future experimental work.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +46-31-772-1000. Fax: +46-31-772-3592. E-mail: sima. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NORDICCS Centre funded by the Nordic Top-level Research Initiative (NORDICCS Project No. 11029) cofunded by Statoil, Gassco, Norcem, Reykjavik Energy, CO2 Technology Centre Mongstad, and Vattenfall. K


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