CO2 Capture in Solid Form by an Alcohol-Water Solution of Ammonia

1st Post Combustion Capture Conference

Giacomo A Pellegrini Susinia*, Robert Strubeb, Daniele Fiaschib, John E Oakeya

aCentre for Energy and Resource Technology, Cranfield University, Cranfield, MK43 0AL, UK bDepartment of Energy Engineering, University of Florence, 50134 Florence, Italy

Keywords: Ammonia-Ethanol-Water Solution; CO2 Capture ; Ammonia Salts; Carbamate; Aspen Plus

1. Introduction The still rising demand for energy and the increase in CO2 emissions typically involved has become one of the most important environmental topics. In order to reduce the environmental impact of power generation from fossil fuels, CO2 can be captured from the flue gases exiting the energy conversion process. The most mature technology of CO2 absorption in gas purification processes uses amines such as Monoethanolamine (MEA) as absorbent (Kohl, 1985). A novel method of reducing CO2 emissions from power plants is given by the use of an alcohol-water solution with ammonia (EAA), which makes it possible to capture CO2 as valuable solid product (Lee, 2003 and Pellegrini et al, 2004). The authors focused their research on the capture of ammonia salts, as this allows for a much more rational use of the CO2 than liquefaction and storage. The energy penalty for postcombustion CO2 capture is currently in the range of 6-10% of the energy efficiency, mainly due to the necessary input of thermal energy for the regeneration of the solvent (IEA, 2004 and Abu-Zahra et al, 2007). Energy requirements are greatly reduced in the absorption process with aqueous ammonia solutions, since no heat for solvent regeneration and no compression of the separated CO2 are necessary. However, a great amount of water is required for washing the exhaust gas stream in order to reduce emission of ammonia into the atmosphere. 2. EAA CO2 Capture Model A postcombustion CO2 capture system using a EAA solution to produce stable ammonia salts was investigated. Aqueous ammonia or amines solutions are not able to produce stable ammonia salts due to their high solubility, while EAA solution produce stable salts reducing their solubility, using the great affinity to react between CO2 and ammonia, and the great miscibility between the three liquids. Ethanol was chosen due to its similar characteristics to water and its good availability. All simulations were carried out with Aspen Plus®, which has become a widely used standard tool in chemical process. To simulate the CO2 capture process, outputs from 500 MWe coal-fired power plant were used and the process consists in an absorption tower, a crystallizer and a filter to promote and separate the salts, and a mixer to reintegrate the solvent losses (Table 1). The removal process is based on chemical reactions of the systems CO2 – H2O – NH3, using the “shuttle mechanism” proposed by Astarita (1981). In this way, equilibrium reactions lead to the formation of unstable salts, such as ammonium carbamate that starts to decompose at ambient temperature. To avoid this problematic and for a more accurate simulation of the process, kinetic reactions were added.

* Corresponding author. Tel.: +44(0)1234754124; fax: +44(0)1234751671. E-mail address: [email protected]. 2

The gas was cooled before entering the absorption tower, where CO2 into the gas reacts with the solvent (i.e. CO2 removal efficiency is 89.4%) and creating the basis for the salts formation, but they still cannot be separated from the liquid. To separate these salts, a crystallizer and a filter were used (i.e. Ammonia salts production are 51.6 kg/s). Subsequently, ammonia was reintegrated into a mixer to bring it back to initial conditions. Concentrations and fluxes used in the simulation are shown in Table 1. GASIN SOLVENT FLUEGAS Temperature [°C] 20 Temperature [°C] 1.7 PUREFLUE WASH-H2O PURVAPOR

Mass Flow [kg/s] 121.2 Mass Flow [kg/s] 1358.5 STACK DEAMMON NH3-WASH HX-GAS SEDIMENT [%wt.] [%wt.] PUREFWAT SDFILTER

SWEETGAS NH3WATER N 68.54 NH 2.44 NH3 MXMAKEUP 2 3 SOLVENT SOLIDS2

GASIN SOLVMIX H2O H2O 2.08 H2O 2.52 SOLVPUMP

CO2 19.64 CO2 0.23 ABSORBER REFLUX O2 3.48 ETHANOL 94.81 LIMITER LIQOUT

LIQUI D CO 6.10 SLFILTER EXCESS

SOLIDS1

SO2 0.16 Table 1 Concentrations and fluxes used in the simulation, Aspen Plus flowsheet 3. Results and Discussions Analyzing the results of the simulations, CO2 capture efficiency is function of loading (i.e. ratio between initial CO2 and initial NH3) and inlet solvent temperature. In fact, when loading increase, the removal efficiency increase, viceversa the inlet solvent temperature produces the opposite effect. Also, the salts formations is influenced by the inlet solvent temperature and water concentration, because by increasing these parameters increase the salts solubility and this reduce the salts formation and the ability to capture CO2. The results of the simulations are subsequently compared with the results of a series of experiments carried out previously (Pellegrini et al, 2010).

CO2 losses vs temperature (water-alcohol solution 2% ammonia concentration)

] 50 % [

s 40 e

s 30 s

o T = -5°C l

2 20

O T = 0°C

C 10 T = 20°C 0 0 50 100 150 200 250 300 Time [min]

Table 2 Experimental apparatus and experimental results As found in the simulations, experiments revealed the great influence of the inlet solvent temperature on CO2 removal efficiency and salts formation (Table 2). Also ammonia concentration has a great impact on these two parameters. In fact, an increase of the ammonia concentration produces and increase of CO2 removal efficiency and salts formation, but also the losses of the system increase. The experiments were carried out using a semi-continuous flow reactor (Table 2) containing 250 – 300 ml of ethanol-water solution and a substitute flue gas (10% v/v CO2) was fed continuously to the bottom of the absorber. The CO2 capture performance of the system was easily determined by weight measurements of all system components before the tests and after certain time intervals. Species in solution have been analysed with 13C-NMR apparatus (Mani et al, 2006) and a gas chromatograph was used to measure the losses of CO2 from the system. In a final comparison between experiments and simulations, for a ethanol-water solution of ammonia of ca.2% wt. and at an absorption temperature of ca. 0 °C, the Aspen Plus model demonstrated higher salts production (1.56, compared to 1.23 kgsalts/kgNH3IN of the experiments) and lower CO2 captured rate (0.76 compared to 1.09 kgCO2REM/kgNH3IN of the experiments). This could be due to the amount of liquid used in the simulations that is excessive and the model is not fully optimized. However, simulations and experiments demonstrated that is possible to apply this capture system to the existing power generation process and use in more rational way to captured CO2.