(68d) Economic CO2 Capture with Solid Sorbents

Abstract:

The adsorption and desorption of CO₂ on amine-grafted sorbent has been studied by in situ infrared spectroscopy coupled with mass spectrometry.  Infrared spectroscopy demonstrated CO₂ adsorption on amine grafted sorbent as bidentate carbonate, bidentate and monodentate at 50 ˚C and released at 135 ˚C.  Mass spectroscopy revealed an adsorption capacity of 824µmol CO₂ / g-sorbent.  The solid sorbent developed in this paper is intended as a downstream component in a stationary power plant which uses low pressure 135 ˚C steam to regenerate.

Introduction:

           The available approaches for CO₂ capture and separation include absorption of CO₂ in aqueous amines, membrane separation, and adsorption on solid sorbents.  The current aqueous amine and membrane technologies are cost-effective for separation of CO₂ from natural gas in liquefaction process and ammonia synthesis process due to the high value of end products.  These current technologies when applied for CO₂ capture from coal-fired power plants, increases the cost of electricity by more than 70%.  The cost of CO₂ sequestration can be reduced if an effective CO₂ capture sorbent is developed which has (i) high CO₂ adsorption capacity (> 1000 mmole/g), (ii) long term regeneration capacity in power plant flue gas environment, and (iii) low energy requirement for regeneration compared to large amount of energy required for aqueous amine process.

Our previous study has shown that the amine-grafted SBA-15 has one of the highest CO₂ adsorption capacities compared to the solid sorbents reported in literature.  The key issues that need to be addressed for development of an effective solid amine sorbent for CO₂ capture include:

• effect of the nature of amine and adsorbed CO₂ species on the CO₂ adsorption capacity of the sorbent
• interaction of the support and amine
• thermal stability and long term CO₂ adsorption/regeneration capacity.

           The objectives of this paper are (i) to investigate the nature of adsorbed CO₂ species on the amine-grafted sorbent and a commercial polymer amine based sorbent and to determine the thermal stability of the sorbent.  The CO₂ adsorption/desorption is studied by transient technique and temperature-programmed desorption with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupled with mass spectrometry (MS).  The thermal stability of the sorbents is studied by temperature-programmed degradation with DRIFTS and MS.

Exponential methods:

Preparation of amine grafted sorbent:  Silica, alumina, beta zeolite and mordentile zeolite was prepared by heating to 400˚C for 8 hours to remove adsorbed water.  The cleaned supports where submersed in 25 mL of tetraethylenepentamine at 150 ˚C and stirred for 24 hours in an inert environment.  Vacuum filtration and 2-3 ethanol washings were required to remove the excise amine.  The powder was the grinded and passed through a 100 mesh sieve.

Adsorption:  The experimental setup shown in Figure 1 consists of a gas manifold which includes a 4-port valve, and mass flow controllers, a DRIFTS reactor filled with 40-60 mg of sorbent, and a mass spectrometer for effluent analysis.  The adsorption of CO₂ was carried out by switching the inlet flow from an inert gas stream (Ar) to adsorbing gas stream (10% CO₂ in Ar) using a 4 port valve.  The 4-port valve allows a smooth switch from one flow stream to another.  The inert gas stream was bubbled through a H₂O saturator at room temperature corresponding to 4% D₂O in the gas streams during adsorption and desorption.  Upon saturation of the sorbent with adsorbing gas, the inlet stream was switched back to the inert gas stream. 

Temperature-programmed desorption:  The sorbent containing adsorbed CO₂ was regenerated via temperature-programmed desorption (TPD) by heating the DRIFTS from 50 ˚C to 135 °C in Ar stream at a rate of 20 °C/min.  The CO₂ concentration profile during TPD was monitored by the MS and the amount of CO₂ was quantified by calibrating the CO₂ (m/e=44) response on the MS.  The calibration factor was obtained by injecting 1 mL of CO₂ gas in flowing Ar stream using the 6-port valve and calculating the area corresponding to the amount of CO₂ injected.  The adsorption and TPD were cycled multiple times to check for degradation.

Results and discussion:

           Fig. 2 shows the infrared (IR) absorption spectra of the beta zeolite before and during TPD. The spectra in Fig. 2 were obtained by subtracting the spectra from saturated sorbent at 30 °C from the obtained spectra during the TPD. Prior to the TPD, sample A exhibited prominent bands for the amine  N-H bond at 2806 and 2936 cm^-1, amine C-H bond stretching and bending at 3190 and 3368 cm^-1, bidentate cabonate at 1564 and 1422 cm^-1, monodentate bicarbonate at 1476 cm^-1, and monodentate carbonate at 1409 and 1309 cm^-1.  During the TPD bands 1564, 1476, 1422, 1409, and 1309 cm^-1 associated with the carbonates decreased while bands 2806, 2936, 3190, and 3368 correlated with the amine increased.  As the carbon dioxide dissociated from the amine during the TPD, the carbonate species concentration decreased, resulting in an increase in the amine previously associated with the carbonate.

Conclusions:

                      The present study provides an insight on the mechanism of amine grafted support with in situ IR spectroscopy.  CO₂ adsorbs on the primary and secondary amine groups forming carbonate and bicarbonate acid species.  The major species formed by the adsorption of CO₂ on the surface are monodentate bicarbonate, bidentate bicarbonate, and bidentate carbonate.  The amount of CO₂ desorbed from the carbonate and bicarbonate between 30–135 °C is 824 µmol CO₂/g-sorbent.  Water during adsorption has significant roll in the capture of CO₂, and no degradation effect during repeated cycling.