direct capture of Co2 from ambient air using solid sorbents

Qian Yu is a PhD student in the research group Sustainable Process Technology. Her supervisors are D.W.F. Brilman and prof.dr. S.R.A. Kersten from the faculty of Science and Technology.

CO2 emissions related to human activities utilizing fossil fuels are generally believed to cause climate changes. Conventional CO2 capture technologies focus on capturing CO2 at large pointed sources, such as power plants. But distributed sources account for around one-third to one-half of the total emissions, which cannot be captured by conventional CO2 capture technologies. Those emissions can be mitigated by one technology – CO2 capture direct from ambient air (DAC), which attracts increasing attention nowadays.

The use of DAC to combat climate change has been suggested in the 1990s by Klaus Lackner. Since then, a few different technologies have been suggested. For these, the system developed by Keith and coworkers, comprising of CO2 capture with aqueous alkali solvent and regenerated in a calcination process (above 700 °C) is probably most far in its development.

For DAC, a process based on the use of regenerative solid sorbents may be an attractive, energy efficient alternative to the use of aqueous solvents due to a lower specific heat. Among solid sorbents, amine-functionalized sorbents have been identified as promising sorbents for DAC, due to their relatively high CO2 capacities under the air capture conditions and a moderate desorption temperature (100 – 120 °C).

The research on DAC is still in an early stage. The majority of studies reported in the literature on amine-functionalized sorbents focus on improving the equilibrium CO2 adsorption capacity. Others topics such as sorbent tolerance towards water (focusing on mechanism and adsorption rate) and sorbent stability under various conditions need more attention. Also DAC studies focusing on process development issues (such as reactor design, efficient sorbent/gas contacting methods, kinetics, optimal adsorption-desorption time, etc.), where the sorbent is in a non-equilibrium state, are rare. Sorbent testing for DAC processes is mostly done at a small-scale (mg to g of sorbent), while their large-scale (~kg of sorbent) feasibility, together with applications of the captured CO2, remains to be proven. The aforementioned knowledge gaps on DAC are addressed in this thesis.

In this thesis, a novel process is developed and experimentally demonstrated, for CO2 capture from ambient air to produce CO2 enriched air to enhance microalgae cultivation. First, an amine functionalized sorbent is selected, initially based on its water and CO2 equilibrium adsorption capacity. Subsequently, the selected sorbent is characterized on its stability under different conditions for a wide range. After selecting the sorbent, a selection of operating conditions for the adsorption step is made, targeting fast sorbent saturation and a low pressure drop. After establishing proper adsorption conditions as well as a suitable adsorber configuration, sorbent desorption is investigated in view of the production of CO2 enriched air for microalgae cultivation. The established process uses a radial flow reactor for CO2 adsorption and a fluidized bed for desorption using air as the sweep gas, with sorbent circulation between adsorber and desorber. This process is evaluated and, based on the operating costs, found to be an economically competitive way to capture CO2 from ambient air for use in microalgae cultivation.

Chapter 2 describes a newly developed method using a coupled thermogravimetric (TG) – Fourier-transform infrared spectroscopy (FTIR) analysis system. This method is able to evaluate small amount of various types of solid sorbents on their CO2 and H2O equilibrium capacity, in view of their suitability for a DAC process. An amine functionalized sorbent, Lewatit VP OC 1065, is selected for (1) its high CO2 capacity (1.40 mol/kg) and a high selectivity of CO2 over H2O (0.24 mol CO2/ mol H2O) in comparison to other alkali carbonate sorbents and physical sorbents under air capture conditions (PCO2 = 40 Pa, 20 °C, relative humidity (RH) = 58 %) and (2) its commercial availability in large amounts.

The effect of water on CO2 adsorption is further studied using the selected sorbent. The presence of water (due to RH in air) actually increases the equilibrium CO2 capacity, but as water also co-adsorbs on the sorbent, it also increases heavily the desorption energy required due to the large heat of vaporization for water. From the FTIR spectra, no additional absorbance peaks are identified in the sorbent saturated with CO2 under the humid conditions compared with dry air, which indicates that water co-adsorption does not alter the mechanism for CO2 adsorption. The dynamic performance showed that the sorbent is much faster at its equilibrium loading for water than for CO2, both for the adsorption as well as for the desorption step. The adsorption rate of CO2 is only slightly affected by the presence of water. Again, those results tested for the dynamic performance suggest the CO2 adsorption mechanism does not change in the presence of water. The additional CO2 capacity in humid air is presumably due to an increment of accessible (active) amine in the presence of water.

Chapter 3 presents a stability study of the selected sorbent, which is useful for selecting desorption conditions. The sorbent was tested by two different methods. In the first method, the sorbent is subjected to constant desorption conditions. In the second method, the sorbent is subjected to consecutive adsorption-desorption cycles. For the first method, the sorbent was treated at different temperatures in a continuous flow of air, different O2/CO2/N2 mixtures, concentrated CO2 and steam. Subsequently, the remaining CO2 adsorption capacity was tested in standardized adsorption-desorption cycles. For the latter method, the sorbent was treated in the presence of air and pure CO2 in adsorption-desorption cycles, mimicking more realistic operating conditions. To characterize the fresh sorbent and treated sorbent samples, elemental analysis, BET/BJH analysis, Fourier transform infrared spectroscopy, and thermogravimetric analysis were applied.

As a result, it was found that the sorbent does not degrade when subjected to steam at 100 °C. However, significant oxidative degradation occurs at moderate temperatures (above 70 °C). CO2-induced degradation occurs at 120 °C, which can be partially prevented by adding moisture to the concentrated CO2 stream. A finding of practical importance is that sorbent degradation using the cyclic treatment does not differ from the one using the continuous treatment at the same desorption conditions, when evaluated at the same total desorption duration.

Starting from Chapter 4, the research focus moves from a sorbent perspective to a process perspective.  Chapter 4 studies various parameters such as particle size, superficial velocity, and bed length for DAC in a small-scale fixed bed reactor. The optimal conditions are found out to be at an adsorption duration of 0.5-1.5 times the stoichiometric time (minimum time required to load the sorbent fully based on air supply and sorbent conditions), which can be calculated in advance to practical operation. A design strategy using the stoichiometric time as the parameter for design and scaling up is proposed in this chapter. With this, a design for a larger-scale DAC process is made for a radial flow type of adsorber.

Chapter 5 evaluates the performance of the designed, kg-sorbent-scale, radial flow reactor (RFR) for capturing CO2 from ambient air. The design of the RFR was based on experimental results obtained from a small, 1-3 g-scale fixed bed reactor (FB) presented in Chapter 4. It was found that the RFR performs with good similarity compared with the results obtained from the FB, when operated under comparable conditions. The RFR itself is characterized by a low pressure drop, a uniform flow distribution, a very short gas phase contacting time (< 0.1 s), and has the ability to operate both in a fixed-bed mode as well as in a moving-bed mode. These features make the RFR a versatile adsorber for further process development. The sorbent used exhibits faster adsorption rate in comparison to other air-capture sorbents. The RFR possesses a low contacting energy of 0.7 – 1.5 GJ/ton at a relatively short adsorption time. Thus, the combination of the sorbent used and the RFR seems a good candidate for future air capture applications. In the last section of Chapter 5, desorption using air as sweep gas is preliminarily studied at relatively low temperatures (60 – 65 °C). This approach seems a feasible option to produce CO2 enriched air, for application in e.g. microalgae cultivation. The ‘Proof of concept’ is part of the study in Chapter 6.

In Chapter 6, we evaluate the desorption process for producing CO2 enriched air for enhancing microalgae cultivation. This can be seen as an alternative to e.g. utilization of flue gas directly or indirectly via flue gas CO2 capture. The experimental work on the desorption process is performed in a kg-scale of sorbent lab unit. The lab unit comprises, next to the RFR for adsorption, a separate desorption unit and fast sorbent circulation between these units. Countercurrent gas-solid (G-S) contacting is applied during desorption in a (moving) fluidized bed desorber, while intermittent adsorption is carried out simultaneously in the RFR adsorber. With this, a relatively constant CO2 concentration in the product gas out of the desorber (CO2 enriched air) and a uniform temperature distribution inside the desorber are realized. Operation conditions such as gas flow rate and solid residence time (for desorption) were varied and shown to affect desorption performance. The targeted concentration of 1% CO2 is obtained in the product gas, which is applied in microalgae cultivation for demonstration purpose. The product gas is successfully applied in algae cultivation and found to enhance significantly the algae growth rate. In an optimization effort, the desorber was tested in both fixed bed and fluidized bed configurations. A strong effect of sweep gas air flow rate and desorption time on desorption efficiency and energy consumption was found, and guidelines for further optimization are provided.

Chapter 7 provides a brief economic evaluation of the established direct-air-capture (DAC) -system presented in Chapter 6. Based on actual experimental data, the total energy required and the operating cost are estimated and compared with other DAC-systems using amine functionalized sorbents reported in literature. Economic calculations show that the established DAC process, though not yet optimized, is already competitive and worthwhile for further development, optimization, and scaling up. Recommendations for future work are provided.