(254ab) Synthesis of Immobilized Amine Sorbent Pellets from Poly (chloroprene) and Fly Ash Binders for Post-Combustion CO2 Capture

Synthesis of Immobilized Amine
Sorbent Pellets from Poly (chloroprene) and Fly Ash Binders for Post-Combustion
CO₂ Capture

Walter Christopher Wilfong¹,
McMahan L. Gray², Brian W. Kail³, Thiago F. De Aquino⁴,
Bret H. Howard², Sabrina T. Estevam⁴

¹National
Energy Technology Laboratory (NETL), USA, Pittsburgh, PA; Oak Ridge National
Laboratory, Oak Ridge, TN

²National
Energy Technology Laboratory (NETL), USA, Pittsburgh, PA

³National
Energy Technology Laboratory (NETL), USA, Pittsburgh, PA; AECOM, Pittsburgh, PA

⁴Beneficent
Association of the Santa Catarina Coal Association (SATC), Brazil, Criciúma, Santa
Catarina.

            Post-combustion CO₂
emissions primarily from coal-fired power plants constitute 31% of total CO₂
greenhouse gas emissions, where CO₂ represents 82.7% of all emitted
greenhouse gases.[1] With the predicted use of coal as an energy
source to continue for decades to come, it is paramount to remove these CO₂
emissions in parallel with the current development of cleaner energy sources. Basic
Immobilized amine sorbent (BIAS) processes are a promising technology to solve
this CO₂ problem. However, problems such as both the large pressure
drops across tall sorbent beds and the failure of mechanical valves and
conveyors due to particle agglomeration require pelletization of sorbents into
larger millimeter-sized materials for practical application. Pelletization of
these immobilized amine sorbents has been achieved by different methods [2, 3], including
combining the sorbents with an inorganic strength additive, such as fly ash,
and bonding the dry mixture with different polymer binders, namely poly (vinyl
chloride) (PVC).[4] High crush strength of the FA/PVC/BIAS pellets
in the study resulted from chemical interactions of a PVC polymer network with
the BIAS and FA particles, where the smaller FA particles interlocked the
larger BIAS particles.

            Although PVC with
-Cl functional groups produced the strongest pellet of all polymers tested,
these pellets degraded during semi-long (intermediate) term testing from
reactions between PVC and BIAS. Therefore, an alternative chlorinated polymer
is needed to produce strong and stable CO₂ capture pellets. In this
work immobilized amine sorbents were pelletized with a fly ash additive and
poly (chloroprene) (PC, chlorinated rubber) polymer binder, which produced
pellets with high attrition resistance and stable CO₂ capture
capacity during adsorption-desorption cycling under humid conditions.

            Pelletization of a
dry mixture of FA/[ground 50 wt% ethylenimine E100 (Huntsman)/silica]-20/80
with a poly (chloroprene) polymer binder (PC) (Sigma-Aldrich, Mooney
viscosity=40) was accomplished in four steps. Step 1 was preparing multiple batches
of polymer binder solutions containing 10-13 wt% PC dissolved in 1,4-dioxane at
105 ^oC for 20-90 min. In step 2, 1.1 g of each PC solution was mixed
with 1.0 g of the FA/sorbent dry mixture to form a putty-like paste, which was
extruded into ropes in step 3. In step 4, the ropes were dried at room
temperature for 1 hr then dried at 105^oC for 1 hr. The final 1.5 to
2.5 mm diameter pellets contained 17-18 wt% FA, 10-13 wt% PC, and the balance of
BIAS. CO₂ adsorption of the pre-treated pellets (105^oC, N₂,
10 min) was performed in a thermogravimetric analyzer (TGA) with a 60 mL/min
flow of 14 vol% CO₂/N₂ at 55^oC. Figure
1
shows that slightly increasing the PC content of the pellet from 10.9 to 12.6
wt% diminished the relative CO₂ uptake kinetics (normalized TGA
weight profiles) and overall CO₂ capture capacities from 3.20
(particle sorbent) to 1.46 mmol CO₂/g (12.6 wt% PC), respectively.
This reduction is attributed to pore blockage by PC. Here, PC likely interacted
with the hydroxyl and amine groups of both the FA and BIAS inside the pore on
the external particle surface, which imparted CO₂ diffusion
limitations within the pellet and blocked previously accessible amine sites of
the initial particle sorbent.[4] The procedure yielding the optimum combination
of strength, relatively minimal structural flaws, and good CO₂
adsorption behavior (1.56 mmol CO₂/g) for the 12.2 wt% PC pellet was
further improved upon to give a strong pellet, FA/E100-S_(20/80)_12.2PC, with a
higher CO₂ capture of 1.76 mmol CO₂/g capture capacity.

Figure 1: Normalized TGA weight profiles during CO₂
adsorption over PC/FA/BIAS pellets containing 17-18 wt% FA and different
amounts of PC. The numbers in parenthesis indicate the total CO₂
capture capacity of the sorbents.

            Figure
2
(a) shows the scanning electron microscope (SEM) cross-section of the optimized
FA/E100-S_(20/80)_12.2 wt% PC pellet along with energy-dispersive X-ray
spectroscopy (EDS) elemental maps. These images and maps reveal that the
attrition resistance/strength of the pellet is attributed to the interactions
among a well-dispersed poly (chloroprene) network (Cl), and the FA (Ca, Fe, Al)
and sorbent (Si) particles. The flexibility of this robust PC (chlorinated
rubber)/FA/sorbent network, in contrast to the rigidity of the commercial
pellets, could allow our pellets to absorb and dissipate the impact force
experienced during attrition testing.

Figure 2: (a) SEM image of the FA/E100-S_(20/80)_12.2PC
pellet cross section along with EDS elemental mapping of key
sorbent, FA, and PC components.

            Attrition
resistance of FA/E100-S (20/80)_12.2PC pellets was ascertained from 24 hour
attrition testing of 2.0 g of the pellets in an in-house constructed ball mill,
utilizing a 39 rpm rotating jar (D=7 cm, Vol=230 mL) affixed with a 0.5” Al
baffle and filled with 46, 3/16” steel ball bearings. A <0.5 wt% attrition
for our pellets was significantly smaller than attrition of two commercial
silica pellets (14.5-80.0%, 1 hour) and one commercial brand zeolite 13x pellet
(36.6%, 24 hour). These results suggest that flexibility of our pellets,
imparted by the PC network, facilitated their high attrition resistance. Both
the good CO₂ capture stability of our pellets under practical
conditions in the fixed-bed system and the high pellet attrition resistance
make PC/FA/BIAS materials a promising candidate for larger-scale CO₂
capture processes.

Figure 3: Fixed-bed reactor system loaded with the FA/E100-S_(20/80)_12.2PC
pellets, along with a typical dry CO₂ adsorption-desorption cycle.

            To assess the
pellet performance under practical humid CO₂ adsorption-desorption
cycling conditions, the pellets were placed into a fixed bed reactor system
illustrated in Figure 3 and were first pre-treated at 105^oC in
flowing N₂ then cooled. In a typical cycle, CO₂ was
adsorbed by the pellets at 55^oC from a 10% CO₂/He flow for
20 min, where the adsorbed species were subsequently desorbed by flowing He at
55^oC for 15 min (pressure swing desorption) and then by heating at
105^oC in He for 20 min (temperature swing desorption). Two dry
cycles (D1, D2) were followed by two wet cycles (~5 vol% H₂O), and
finally two dry cycles (D3, D4); total testing time was 13 hrs (6 hrs humid).
Results of the CO₂ capture tests, shown in Table 1,
reveal the stability of the pellets during CO₂ adsorption-desorption
cycling in the presence of steam.

Table 1: Results of the fixed-bed (FB) reactor system tests on FA/E100-S
(20/80)_12.2PC pellet. PCR values were calculated from the CO₂ MS
profiles (1 g system) and weight change (TGA) methods.

Excellent agreement between
the fresh CO₂ capture of the pellets as determined from the CO₂
MS profiles (fixed-bed, 1.74 mmol CO₂/g) and from the TGA (1.78 mmol
CO₂/g) methods highlight the reliability of calculating CO₂
capture capacity from the MS gas profiles. The high CO₂ capture
capacity of the pellets after testing (~1.67 mmol CO₂/g) along with
no change in the gray color of the pellet confirms the stability of the PC/FA/BIAS
pellets under practical adsorption-desorption conditions. The low 8 percentage
point difference in these FA/PC/BIAS pellets PCR values calculated by the CO₂
MS profiles (96%) and TGA (88%) further highlights reasonably good agreement
between the two CO₂ capture capacity analysis methods. Overall the
pellets exhibited excellent stability after 13 hrs of adsorption-desorption
cycling, where exposure of the pellets to a humid environment was about 6 hrs.

References

1.         US.
Environmental Protection Agency, U.S. Greenhouse Gas Inventory Report:
1990-2014. 2014.

2.         Knowles,
G.P., Z. Liang, and A.L. Chaffee, Shaped polyethyleneimine sorbents for CO₂
capture. Microporous and Mesoporous Materials, 2016.

3.         Rezaei,
F., et al., Shaping amine-based solid CO₂ adsorbents: Effects of
pelletization pressure on the physical and chemical properties. Microporous
and Mesoporous Materials, 2015. 204(0): p. 34-42.

4.         Wilfong, W.C., et al., Pelletization of
Immobilized Amine Carbon Dioxide Sorbents with Fly Ash and Poly(vinyl
chloride). Energy Technology, 2016, DOI: 10.1002/ente.201500419