Mass transfer of ammonia escape and CO2 absorption in CO2 capture using ammonia solution in bubbling reactor
Graphical abstract
Introduction
The climate change caused by greenhouse gas became a global environmental issue, in order to deal with the excessive emissions of CO2 from plants, many CO2 capture methods were proposed [1], [2], [3], [4], [5], [6], [7]. Among these methods, the ammonia method was regarded as a feasible and pragmatic direction to precipitate a reduction of the CO2 emissions from power plants, it was applied in CO2 capture, such as ECO2 of Powerspan Ltd, US [8], [9], [10], [11], [12], [13]. Though ammonia method has the advantages of a high removal efficiency and adequate adaptability for operating conditions, this technology has an ammonia escape problem, leading to a less favorable view of the application [4], [14]. Ammonia escape leads to a loss of absorbent barriers, allowing ammonia to escape into the atmosphere (the limit standard of factory boundary in China is 5 mg/m3), which causes secondary environmental pollution. The extra water or energy required to remove the ammonia, such as water washing or condensation, are wasted as well [15], [16], [17], [18], [19]. Therefore it is important to study ammonia escape in CO2 capture using ammonia.
Though there were many studies of CO2 capture using an ammonia solution, majority of them were focused on the CO2 removal efficiency and the capacity [9], [10], [20], [21], [22], [23], [24], [25]. Meanwhile there were a lot of researches on ammonia escape inhibition, but studies on mass transfer of ammonia escape was seldom [26], [27], [28], [29], [30]. The CO2 capture using an ammonia solution in a super-gravity bed with disks was studied by Zhang [31], under the conditions of a 4–20% ammonia solution; the molar fraction of ammonia in the outlet gas was 1.3–3.6%. Aspen Plus™ was applied to simulate a pressing CO2 capture system using an ammonia solution by Corti [12], under the condition of a 2–4% ammonia solution, a CO2 removal efficiency was beyond 80%, and the ammonia molar fraction in the outlet gas was 0.4–1.2%. Moreover, the higher the ammonia concentration in the solution, the higher the leaching effect of the ammonia into the environment. In the desorption experiments by Resnik [32], ammonia escaped significantly in the thermal desorption process; the total ammonia loss obtained was 43.1% with an ammonia concentration of 14%, and the ammonia escape increased significantly when correlated with a rise in temperature. Among relative researches, there were few results about mass transfer.
In order to study the mass transfer of ammonia escape, as related to the CO2 capture, and providing basic data, the volumetric mass transfer coefficient of ammonia escape in a bubbling reactor was studied in this paper. In addition, the mass transfer of CO2 absorption and the interface area in the bubbling reactor were also investigated simultaneously. These studies can provide the basic data as a reference guideline for the process application.
Section snippets
The mechanisms of CO2 absorption and ammonia escape
The total reaction in the absorption is reflected as follows [10]:where the production of NH2COOH is rapid, an irreversible second-order reaction primarily occurred in liquid film. The reaction kinetics of ammonia and CO2 under 273–313 K were investigated by Pinsent et al. [33], which was referenced in this paper.
CO2
Reagents
The ammonia solution (AR) and concentrated sulfuric acid (AR) were purchased from Tianjin Xintong chemical Co., Ltd, China, anhydrous calcium chloride (AR) was purchased from Tianjin Kermel chemical reagent Co., Ltd, China, Soda lime (AR) was purchased from Tianjin FuChen chemical reagent Co., Ltd, CO2 (99.995%), and N2 (99.996%) gas was purchased from North Special Gases Co., Ltd, Baoding, China.
Apparatus and method
Fig. 1 shows a schematic view of the overall experimental apparatus, with all the various parts of
Overall mass transfer coefficient of ammonia escape
According to Fick’s first law, the flux of NH3 is proportional to the NH3 concentration gradient in the diffusion direction opposite to CO2 absorption. The proportionality factor is the diffusion coefficient of NH3 in the medium.
This representation is inspired by the two-film model. The principal characteristic of these models are the two thin films of gas and liquid (Fig. 1) at the side of gas–liquid interface for molecular transfer between water and gas, respectively. All the resistance to
Curves trend and analysis
In these experiments, the molar fractions of NH3 and CO2 in outlet gas were measured by analyzers. The online curves of the ammonia escape, with different ammonia concentrations, are illustrated in Fig. 3 (No CO2 loading, 293 K) and Fig. 4 (15% inlet CO2 molar fraction, 293 K).
Based on Fig. 3, Fig. 4, ammonia escape significantly increased with the enhancement of ammonia concentration; therefore, the concentration of free ammonia was the key factor relating to the ammonia escape. Under a 15%
Conclusions
- (1)
Based on the data in this paper, the molar fraction in the outlet gas of escaped ammonia reached 16%, which was more than the data from previous studies. The ammonia escape was a serious problem in CO2 capture using an ammonia solution.
- (2)
The temperature and inlet CO2 molar fraction were the important factors to , and the ammonia concentration in solution had little impact on , while it was the key factor to the amount of ammonia escape. The volumetric mass transfer coefficients of
Acknowledgements
The authors are highly thankful for the financial support of the National Natural Science Foundation of China (21176064) and the Fundamental Research Funds for the Central Universities of North China Electric Power University (2014ZD39).
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