Article pubs.acs.org/IECR
Modeling the 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: A rate-based model is developed to elucidate the chemistry behind the simultaneous absorption of NOx and SOx under pressurized conditions (pressures up to 30 bar) that are applicable to the flue gases obtained from CO2 capture systems. The studied flue gas conditions are relevant to oxy-fuel and chemical-looping combustion systems. The kinetics of the reactions implemented in the model is based on a thorough review of the literature. The chemistry of nitrogen, sulfur, and N−S interactions are evaluated in detail, and the most important reaction pathways are discussed. The effects of pH, pressure, and fluegas composition on the liquid-phase chemistry are also examined and discussed. Simulations that use existing kinetic data reveal that the pH level has a strong influence on the reaction pathway that is followed and the types of products that are formed in the liquid phase. In addition, the pressure level and the presence of NOx significantly affect the removal of SO2 from the flue gas.
1. INTRODUCTION Carbon capture and storage (CCS) is a set of technologies developed to mitigate CO2 emissions from large point sources, such as power plants and other industrial processes. CCS technologies enable the continued use of fossil fuels while complying with the emission-reduction requirements established for greenhouse gases. CCS technologies involve removal of N2 to produce a CO2-rich stream. To ensure cost-effective transportation and storage, the highly concentrated CO2 stream generated from the capture process is purified, cooled, and compressed to produce a supercritical CO2 stream. The compression is performed in multiple stages (normally up to four stages) with intercooling and condensate removal between stages.1 For capture technologies, such as oxy-fuel and chemical looping combustion systems, the CO2-rich stream contains water vapor and is contaminated with nitrogen oxides (NOx) and sulfur oxides (SOx). The control of NOx and SOx is necessary, not only to meet emission limitations but also to avoid corrosion in the flue gas pipelines and the CO2 transport system. SO2 and NO, which are the thermodynamically favored species, are the predominant species of SOx and NOx in the flue gases generated from the combustion process. The high pressure and low temperature of the CO2-conditioning train significantly affects the chemistry of the nitrogen- and sulfurcontaining species. In this respect, the most important phenomenon is the substantial increase in the rate of oxidation of NO to NO2 in the high-pressure zones of the CO2conditioning train. This is attributed to increases in the partial pressures of the reactants. The solubility of NO2 in water is higher than that of NO (by about 1 order of magnitude) and NO2 is easily dissolved in water, which is either introduced to the process or formed during the condensation, compression, and cooling of the flue gas. NO2 is highly reactive in aqueous solutions, reacting with water to form nitrous acid (HNO2) and nitric acid (HNO3). The gas-phase chemistry of nitrogen oxides, as well as the absorption mechanisms of these gases in water, has been widely studied.2−5 The presence of SOx in the © 2015 American Chemical Society
system significantly increases the number of reactions that can occur due to interactions between nitrogen and sulfur species in the liquid phase. These interactions can eventually lead to the oxidation of S(IV) to S(VI).6 The formation of sulfuric acid and nitric acid in the high-pressure zones of the flue gas train in oxyfuel systems has been observed in a number of experimental and modeling investigations.7−12 Furthermore, it has been shown that the increased concentration of NOx enhances the removal of SO2 from the gas phase.11 Several concepts for removal of SOx and NOx during the flue gas compression process have also been proposed.8,13 These concepts, if commercialized, would represent high potentials for cost reductions, since they might make conventional flue gas cleaning processes unnecessary. However, the reaction mechanisms that govern the removal of SO2 and formation of sulfuric acid in high-pressure systems are not fully understood. The gas-phase oxidation of SO2 by NO2, which results in the formation of SO3 and its subsequent absorption into liquid water, is mainly referred to in the literature as a route for the formation of sulfuric acid.11,14 However, it has previously been shown by the present authors that the extent of gas-phase oxidation of SO2 by NO2 is not significant under conditions relevant for the flue gas trains in oxy-fuel combustion systems.15 This is supported by extrapolation of the high-temperature rate data for the gas-phase oxidation of SO2 by NO2 reported by Armitage and Cullis.16 Thus, the above-mentioned pathway cannot contribute significantly to sulfuric acid formation. This suggests that the liquid-phase interactions enhance S(IV) oxidation, causing the formation of sulfuric acid. This is in accordance with the mechanism proposed by Immer et al.17 for pressurized flue gas-cleaning systems. The N−S interactions in the liquid phase have Received: Revised: Accepted: Published: 1216
October 13, 2014 January 14, 2015 January 15, 2015 January 15, 2015 DOI: 10.1021/ie504038s Ind. Eng. Chem. Res. 2015, 54, 1216−1227
Article
Industrial & Engineering Chemistry Research previously been studied under atmospheric pressure in relation to atmospheric chemistry,18−20 in relation to the effect of NOx on sulfur scrubber chemistry,21,22 and with respect to the design of processes for the simultaneous removal of NOx and SO2.6,23 The outcomes of these studies indicate that a number of intermediate products are likely to be formed and that the process is complicated by the interactions that occur between the absorbed SO2 and NO2. In summary, experimental studies performed to date on pressurized flue gas systems have led to the conclusion that there is considerable formation of acids. The gas-phase oxidation of NO to NO2 is crucial to the formation of acids, although the enhanced absorption of SO2 and formation of sulfuric acid are not due to gas-phase reactions. While the liquid-phase interactions between nitrogen and sulfur are known to be complex, careful analyses of the liquid-phase chemistry under relevant conditions are currently lacking. The present work examines the reactions of importance for such systems, and evaluates the liquid-phase chemistry, an understanding which is crucial to the design of removal processes and which forms the basis for predicting the formation of acids during compression. To achieve this goal, a reaction model is constructed to identify the important pathways (based on the kinetics reported in the literature) for the simultaneous absorption of NOx and SOx and for the different products that are formed depending on the operating conditions. The remainder of this paper is organized as follows: Section 2 reviews current knowledge of the gas- and liquid-phase chemistry relevant to pressurized flue gas systems. The main focus is on the interactions between the nitrogen and sulfur species in the liquid phase, although the gas phase and the absorption chemistry of NOx and SO2 are also considered. Section 3 describes the method used, including descriptions of the mixing and mass transfer characteristics of the model and the reaction mechanism. Section 4 presents the results of the reaction system analysis. The effects of the operating conditions (i.e., pressure and initial N−S ratio) on the chemistry, as well as the sensitivity of the results regarding the uncertainties in the kinetics, are examined. Finally, the conclusions are presented in section 5.
NO(g) + NO2 (g) ↔ N2O3(g)
If water vapor is present in the flue gas, the formation of gaseous nitric acid and nitrous acid and the decomposition of nitrous acid may proceed as shown by reactions Rg4 and Rg5:25 (Rg4)
2HNO2 (g) ↔ NO(g) + NO2 (g) + H 2O(g)
(Rg5)
NO2 (g) + SO2 (g) → NO(g) + SO3(g)
(Rg6)
SO2 (g) + 1/2O2 (g) → SO3(g)
(Rg7)
2.2. Liquid-Phase Chemistry. Once the NOx and SOx (almost exclusively SO2) components in the flue gas are absorbed in the liquid phase, numerous reactions may occur. The absorption of NO2 proceeds according to reactions Rl1 and Rl2. Whether NO2 or N2O4 is the main diffusing and reacting species depends on the concentration of NOx in the flue gas. N 2 O 4 is the main reacting species at high concentrations.27 The following reactions are possible for the NOx components:3,4,28 2NO2 (aq) + H 2O(l) → HNO2 (aq) + HNO3(aq)
(Rl1)
N2O4 (aq) + H 2O(l) → HNO2 (aq) + HNO3(aq)
(Rl2)
The produced nitrous acid may decompose to form NO and NO2 (Rl3). This reaction is believed to proceed via reactions Rl4 and Rl5.29 2HNO2 (aq) ↔ NO(aq) + NO2 (aq) + H 2O(l)
(Rl3)
NO2 (aq) + NO(aq) → N2O3(aq)
(Rl4)
N2O3(aq) + H 2O(l) ↔ 2HNO2 (aq)
(Rl5)
The nitrous acid and nitric acid instantaneously dissociate according to reactions Rl6 and Rl7. In contrast to nitric acid, nitrous acid is a relatively weak acid with a pKa of 3.3 at 25 °C.28 HNO2 (aq) ↔ H+ + NO2−
(Rl6)
HNO3(aq) ↔ H+ + NO3−
(Rl7)
The instantaneously reversible reaction between dissolved SO2 and water results in the formation of HSO3− (Rl8),30−32 which can reversibly dissociate according to reaction Rl9.33 SO2 (aq) + H 2O(l) ↔ H+ + HSO3−
(Rl8)
HSO3− ↔ H+ + SO32 −
(Rl9)
HSO3−
SO32−
In addition, and may be oxidized by dissolved O2 in the liquid phase, as shown by the following overall reactions:34,35
(Rg1)
Reaction Rg1 has been studied over a wide range of temperatures and partial pressures, and there is general agreement between the different kinetics reported for this reaction.5 N2O4 and N2O3 can be in equilibrium with NO2 and NO according to reactions Rg2 and Rg3: 2NO2 (g) ↔ N2O4 (g)
N2O4 (g) + H 2O(g) → HNO2 (g) + HNO3(g)
The oxidation of SO2 by NO2 (Rg6) and that by O2 (Rg7) require high temperatures to be of significance.16 The oxidation of SO2 by O2 is significant at temperatures greater than 900 °C.26
2. CHEMISTRY The discussion of the chemistry of a pressurized flue gas that contains NO, NO2, SO2, O2, H2O, and N2 is divided into the gas-phase chemistry, liquid-phase chemistry, and liquid-phase N−S reactions. 2.1. Gas-Phase Chemistry. Substantial gas-phase oxidation of NO to NO2 under pressurized conditions is the most important difference between this pressurized system and atmospheric systems. The gas-phase oxidation of NO to NO2 (Rg1) is reversible and is favored by high pressures. The rate of oxidation increases with decreasing temperature for temperatures less than 330 °C.24 2NO(g) + O2 (g) ↔ 2NO2 (g)
(Rg3)
2SO32 − + O2 (aq) → 2SO4 2 −
(Rl10)
2HSO3− + O2 (aq) → 2SO4 2 − + 2H+
(Rl11)
These reactions have been studied for various pH ranges (see, for example, refs 36 and 37), but they are not yet fully
(Rg2) 1217
DOI: 10.1021/ie504038s Ind. Eng. Chem. Res. 2015, 54, 1216−1227
Article
Industrial & Engineering Chemistry Research understood. Some authors37,38 stress that these reactions only occur in the presence of catalysts (ions of transition metals, such as iron, manganese, and copper) or sources of free radicals and, thus, the presence of these catalysts in experimental setups, even in small amounts, has given rise to the discrepancies noted between the reported kinetics.39,40 On the other hand, Connick et al.35 have presented some evidence that a noncatalyzed reaction can occur, which is relevant to the chemistry studied in the present work. 2.3. Liquid-Phase N−S Reactions. The chemistry of the interactions between dissolved nitrogen oxides and sulfur dioxide is complex. A number of reactions can take place between various species, resulting in the eventual oxidation of S(IV). In addition, there are a number of possible pathways with several species involved. The possible interactions between the nitrogen and sulfur species in the liquid phase fall into two categories: (1) interactions between HNO2 (and NO2−) and hydrogen sulfite (HSO3−); (2) interactions between dissolved NO2 and S(IV) (HSO3− and SO32−). The former interactions have been reported as being important under acidic and mildly acidic conditions (pH 5 and found to increase with pH.42 Other oxidized nitrogen species, i.e., N2O, NO, and nitric acid (NO3−/HNO3), have been reported to have no significant reactions with dissolved S(IV) under low pH conditions (pH
