Membranes

Membranes are thin selective barriers through which some compounds pass more easily than others. Membranes can be used either to capture CO2 from various gas mixtures or to capture N2 to increase the concentration of CO2 in combustion air or in flue gases to

facilitate the capture of CO2. Membrane capture has several advantages such as modular and compact nature of the equipment, simplicity, lack of regeneration, waste streams and phase changes, as well as low energy consumption. (Wilcox, 2012; IEAGHG 2019a.) Nakao et al. (2019) consider membrane capture to be one of the most promising capture technologies from the point of view of capital and operational costs.

The capture process is driven by a pressure difference over the membrane. The process is more efficient when the pressure difference across the membrane is high, meaning that with mixtures of low CO2-concentration (i.e., low partial pressure of CO2) the capture process can be challenging. In combustion processes concentration of CO2 in the flue gases is relatively low at around 3–15 %, which is problematic for sole membrane capture systems. Post-combustion capture applications might have to utilize feed-side compression, permeate-side vacuum pumping, and larger membrane areas to create efficient capture in low-pressure flue gas conditions, which significantly increase capture costs. (IEAGHG 2019a; Nakao et al. 2019.) To reduce costs in conditions where partial pressure of CO2 is low, high membrane permeability should be preferred over high selectivity (Nakao et al. 2019).

In pre-combustion capture applications, the concentration of CO2 in the syngas mixtures is higher compared to combustion flue gases, making the capture conditions more suitable. In high-pressure conditions there is no need for compressors or vacuum pumps, which expectedly results in significant cost reductions compared to post-combustion capture applications. In these conditions both permeability and selectivity of the membrane should be focused upon. (Nakao et al. 2019.)

Several membrane materials are applicable for carbon capture, such as polymers, microporous organic polymers, fixed site carrier membranes, mixed matrix membranes, carbon molecular sieve membranes and inorganic membranes. Membrane material choice depends on factors like capture conditions and desired operational properties, lifetime, stability, and selectivity of the membrane. For example, polymer-based membranes typically provide good separation performance with low cost but weak stability, which

means poor resistance to impurities and challenging conditions like high pressure and temperature. Inorganic membranes are better at withstanding challenging conditions but have more complex module construction and thus higher cost. (He 2018.)

4.8.1

MTR is one of the leading developers of membrane capture with their commercially available high-permeable and CO2-selective Polaris membrane, suitable for pre- and post-combustion processes. (MTR 2020a.) Material of the Polaris membrane is based on thin-film composite polymer structures (Figueroa et al. 2008). Gen-1 Polaris membranes have reached commercial-scale and have been used, for instance, in natural gas applications and refineries. Gen-2 Polaris membranes are more permeable compared to Gen-1, which reduces capital cost of the system since the required capture area is smaller. Gen-2 membranes have been demonstrated in several pilot-scale projects, for example at National Carbon Capture Center at 1 MWe scale capturing 20 tCO2/d from coal-derived flue gases. Development of Gen-3 membranes with even higher permeability is currently at laboratory-scale. (Merkel 2018.)

Power requirement for a two-stage Gen-2 Polaris membrane post-combustion capture with CO2 selective flue gas recycling is estimated to be as low as 277 kWh/tCO2

(= 1 GJ/tCO2) with a capture rate of 80 % (Baker et al. 2018). Based on data collected from related literature, IEAGHG (2019a) estimate capture costs to be 47 €/tCO2 in coal-firing and 80 €/tCO2 in gas-firing for MTR’s Polaris membranes. However, it is noted that the calculations were done for a capture rate of 90 % in conditions of relatively low CO2-concentration and that the cost benefits of membrane capture emerge in hybrid processes, partial capture or combustion air enriching. (IEAGHG 2019a.) A capture cost of 30 $/tCO2 is considered feasible for scaled-up Gen-2 membranes (Merkel 2018).

A large pilot-scale post-combustion capture project designed to capture 140 tonnes of CO2 per day in coal-firing at Wyoming Integrated Test Center is currently underway. The project aims to successfully operate a partial post-combustion capture process based on

Gen-2 Polaris membranes, while reaching a capture rate of ~70 % and a capture cost of

~40 $/tCO2. (MTR 2020b.) Successful demonstration at Wyoming ITC will advance TRL of Polaris membranes in carbon capture from 6 to 7–8.

4.8.2

MTR and TDA Research have developed a hybrid carbon capture system combining MTR’s Polaris membranes and TDA’s mesoporous carbon sorbent based on physical absorption. The hybrid process is developed to reach a 90 % capture rate in post-combustion applications. Polaris membranes and TDA’s sorbent have been separately evaluated at various pilot-scale projects, whereas the hybrid system has been evaluated at bench-scale (TRL 5) with coal-fired flue gases at Western Research Institute, capturing 20–40 kgCO2 per day. A capture cost of $35.5/tCO2 has been calculated for the process based on these preliminary evaluations. (NETL 2018; IEAGHG 2019a.) Further demonstration will be conducted at 1 MW scale at TCM in Norway, where the process is designed to capture 20+ CO2 tonnes per day from industrial flue gases (NETL 2018), which will advance TRL of the technology to 7.