Scholarly article on topic 'Doped phenol-formaldehyde resins as precursors for precombustion CO2 capture adsorbents'

Doped phenol-formaldehyde resins as precursors for precombustion CO2 capture adsorbents Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — C.F. Martín, S. García, J.J. Pis, F. Rubiera, C. Pevida

Abstract The use of solid sorbents for precombustion CO2 capture, which implies the removal of CO2 from the shifted-syngas prior to electricity generation, has acquired increasing interest in recent years. As an alternative method for precombustion CO2 capture, adsorption can be considered a promising technology, offering potential energy savings compared to absorbent systems. Solid sorbents are currently used in pressure swing adsorption (PSA) systems for the purification of hydrogen in petrochemical industries. In the present work, phenol formaldehyde resin-based adsorbents have been prepared to be applied in precombustión CO2 capture applications. The produced carbons showed superior performance when compared to commercial CO2 adsorbents and a great selectivity towards CO2.

Academic research paper on topic "Doped phenol-formaldehyde resins as precursors for precombustion CO2 capture adsorbents"

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Procedía

Energy Procedía 4 (2011) 1222-1227 :

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GHGT-10

Doped phenol-formaldehyde resins as precursors for precombustion CO2

capture adsorbents

C.F. Martín, S. García, J.J. Pis, F. Rubiera, C. Pevida1

Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain

Abstract

The use of solid sorbents for precombustion CO2 capture, which implies the removal of CO2 from the shifted-syngas prior to electricity generation, has acquired increasing interest in recent years. As an alternative method for precombustion CO2 capture, adsorption can be considered a promising technology, offering potential energy savings compared to absorbent systems. Solid sorbents are currently used in pressure swing adsorption (PSA) systems for the purification of hydrogen in petrochemical industries. In the present work, phenol formaldehyde resin-based adsorbents have been prepared to be applied in precombustion CO2 capture applications. The produced carbons showed superior performance when compared to commercial CO2 adsorbents and a great selectivity towards CO2.

© 2011 Puiblished by Elsevier Ltd.

Keywords: CO2 adsorption, phenolic resins; precombustion capture

1. Introduction

Commercial methods to capture CO2 have long been used in the chemical and petrochemical industries. These methods, applied to the removal of H2S and CO2 from gaseous streams, are mostly based on chemical absorption with aqueous solutions of alkanolamines (MPA, DPA, MDPA), in the case of combustion flue gases, or physical absorption with solvents (Rectisol®, Selexol®), in the case of high CO2 partial pressures. However, these processes present several drawbacks such as high energy consumption, degradation, short life and high solvent cost [1]. Alternative methods have been proposed for CO2 capture but, currently, advanced research is focused on membranes and adsorption with solid sorbents [2,3]. Adsorption is considered a promising technology for CO2 capture applications since adsorbents present high adsorption capacity, great selectivity, good mechanical properties and they remain stable over repeated adsorption-de sorption cycles [4,5]. Activated carbons are suitable candidates for CO2 capture,

1 Corresponding author. Tel.: +34 985 11 90 90; fax: +34 985 29 76 62. E-mail address: cpevida@incar.csic.es

ELSEVIER

doi:10.1016/j.egypro.2011.01.177

their adsorption performance being dependent on the pore structure and the properties of the surface chemistry [6]. Although capture capacities of activated carbons are, in general, lower than those of zeolites and molecular sieves under low pressure and ambient conditions, they present a series of advantages as CO2 adsorbents such as larger capacities at higher pressures, ease of regeneration, potential low-cost and less sensitiveness to moisture. Phenolic-resin-based activated carbons offer further advantages in that they can be produced in a wide variety of physical forms (including granular or extruded, fibres, monolithic structures), allow close control of porosity, present a very low level of impurities and good physical strength [7,8].

In this work, phenol formaldehyde resins were synthesised with and without the addition of organic additives. These resins were carbonised and activated with CO2 to produce phenolic-resin-based activated carbons. Samples were chemically and texturally characterised and their CO2 capture performance evaluated under pure CO2 flow (adsorption isotherms) and CO2/H2 mixtures (breakthrough curves).

2. Experimental

2.1. Synthesis of materials

In this work, phenol formaldehyde resin (PF) samples have been prepared following different synthesis pathways. During the synthesis, phenol and formaldehyde were mixed and heated in the presence of a catalyst. In some cases, an organic additive, ethylene glycol (EG) or polyethylene glycol (PEG), was also added. The following samples were produced: samples NA and CLA were synthesised without additive using NH4OH and HCl, respectively, as catalyst; samples E1A and PE10 were prepared adding EG (1 wt.%) and PEG (10 wt.%), respectively, during the acid-catalysed synthesis route. Afterwards, the resins were cured in a rotary evaporator by heating under vacuum [9-12].

2.2. Carbonisation and activation with CO2

The cured resins were then carbonised and activated with CO2 to develop microporosity in the samples. Carbonisation was performed in a horizontal furnace up to 800°C under a stream of 50 mL min-1 of N2. Activation with CO2 was conducted in a vertical furnace, in a flow rate of 10 mL min-1 of CO2 at 900°C. Activated carbons with various degrees of burn-off were prepared.

2.3. Characterisation

The obtained carbons were characterised in terms of texture and chemical composition. For the textural characterisation, the micropore volume, W0 [13], the average micropore width, L0 [14], and the BET surface area, SBET (m2g-1), were determined from the N2 and CO2 adsorption isotherms at 77 and 273 K, respectively.

Chemical characterisation involved proximate and ultimate analyses and evaluation of the point of zero charge (pHpzc).

2.4. CO2 capture capacity

The capture performance of the adsorbents was determined under pure CO2 and CO2/H2 mixtures, to evaluate the equilibrium capacities and the maximum working capacities of the samples, respectively. CO2 capture tests at 25°C and atmospheric pressure were conducted in a thermogravimetric analyser by recording the mass uptake of the samples when exposed to pure CO2 atmosphere. High pressure CO2 and H2 adsorption isotherms were evaluated in a magnetic suspension balance at 25°C and pressures up to 30 (CO2) and 40 bar (H2). The maximum CO2 uptakes of the samples at high pressures and their selectivity towards CO2 were assessed.

The CO2 capture performance of the adsorbents in a binary mixture of CO2 and H2 was also evaluated by obtaining the breakthrough curves of the samples in a purpose-built bench-scale system. The maximum working capacities were evaluated after seven consecutive adsorption-desorption cycles that involved an adsorption step at 15 bar and room temperature, followed by complete regeneration of the adsorbent at atmospheric pressure and 80 °C.

3. Results and discussion

3.1. Chemical analysis

Ultimate analysis and pHPZC of the prepared carbons are included in Table 1. The main element in the samples composition is carbon (~97 wt.%). The obtained carbons show basic character (pH > 7), due to the devolatilisation of oxygenated compounds during carbonisation. Basicity also increased after activation with CO2, because part of the oxygen that was incorporated to the surface of the char during the activation step is in the form of basic oxygen groups (i.e., pyrone ) [12,15].

Table 1 Chemical characterisation of the prepared adsorbents

Sample

Ultimate Analysis (wt.%, daf)

NA 97.3 0.3 0.9 1.5 8.8

CLA 97.4 0.1 - 2.5 9.4

E1A 97.2 0.1 - 2.7 8.8

PE10 95.7 0.1 - 4.2 7.5

O*: calculated by difference

3.2. Textural characterisation

Table 2 summarises the textural parameters calculated from the N2 and CO2 adsorption isotherms at 77 and 273 K, respectively, and the helium density of the PF carbons.

Helium density in PF samples is around 2 g cm-3. Activation with CO2 resulted in a significant textural development. The prepared carbons are mainly microporous. BET surface areas (SBET) and total micropore volumes (Wo N2) are similar for the activated samples, but lower for the carbonised sample PE10. Analysing the narrow microporosity in the PF carbons, here assessed by the CO2 adsorption isotherms, differences between the activated samples can be observed. E1A shows the highest narrow micropore volume (W0 CO2) (0.33 cm3 g-1), most probably as a consequence of the addition of EG during the synthesis. Regarding the average narrow micropore widths (L0 CO2), the activated samples present average values around 0.7 nm while sample PE10 exhibits the smallest average narrow micropore width.

Table 2 Textural parameters calculated from the N2 and CO2 adsorption isotherms of PF carbons

Sample dHe (g cm-3) N2 adsorption at 77 K CO2 adsorption at 273 K

SBET (m2 g1) W0 N2 (cm3 g1) W0 CO2 (cm3 g1) L0 co2 (nm)

NA 2.08 1211 0.45 0.26 0.70

CLA 2.11 1381 0.51 0.29 0.74

E1A 1.96 1369 0.51 0.33 0.73

PE10 1.92 472 0.25 0.23 0.58

Since the molecular size of CO2 is 0.209 nm, only pores less than 1.0 nm are effective towards CO2 capture at atmospheric pressure. Thus, the textural characterisation suggests that the produced carbons present suitable porous structure to be applied as CO2 adsorbents.

3.3. CO2 capture capacity

Figure 1 shows the CO2 uptake for each sample at 298 K and atmospheric (1 bar) and high pressure (30 bar). CO2 capture capacities are expressed in mmol of CO2 per gram of adsorbent (mmol g-1).These values correspond to the equilibrium CO2 uptakes under the tested conditions. Figure 1 shows the

correlation between the CO2 uptakes and the total micropore (Wo N2) and narrow micropore (W0 CO2) volumes.

The maximum CO2 uptake at atmospheric pressure was achieved by sample E1A, 2.46 mmol g-1, closely followed by samples CLA and NA that reached 2.16 and 2.07 mmolg-1, respectively (see Figure 1). These values are comparable or slightly superior to those assessed on commercial activated carbons [4,16-18].

C02 uptake (mmol g"1) 0 1 bar, 298 K

12- □ 30 bar, 298 K

10- & / ~

8- -I-! :::: s -

6- -V. ::: y-

2 0- i ::::

NA CLA E1A PE10

Figure 1. CO2 uptakes (mmolg- ) at 298 K, and 1 and 30 bar, respectively.

At 30 bar, the maximum CO2 uptake corresponded to E1A that reached 10.30 mmolg-1, followed by CLA, 9.89 mmolg-1. High-pressure CO2 capture capacities are greater than those of commercial CO2 adsorbent materials like zeolite 13X (7.4 mmol g-1 at 32 bar and 298 K [19]) or BPL carbon (8.4 mmol g-1 at 55 bar and 298 K [20]).

CO2 uptake

.£3 4

R2 = 0,68

12 10 8 6 4 2 0

— —

R2 = 0,91

4 K3 .a

R2 = 0,98 2

♦ 30 bar □ 1 bar

Figure 2. Correlation between the CO2 uptakes at 1 and 30 bar, and W0 N2 (a) and W0 CO2 (b).

R = 0,93

W0N2 (cm g" )

W0C02 (cm3g)

On the other hand, the carbonised sample, PE10, presented the lowest CO2 uptakes: 1.86 and 8.18 mmolg-1 at 1 and 30 bar, respectively.

Relating the results of these tests to the textural characteristics of the adsorbents (see Figure 2) it can be observed that there is a good correlation between CO2 uptake and narrow micropore volume (W0 CO2). In the particular case of CO2 adsorption at atmospheric pressure, the higher W0 co2, the greater the CO2 uptake (Figure 2b). At high pressure there is also a good correlation between the CO2 uptake and the total micropore volume (W0 N2) that, on the other hand, does not exist at atmospheric pressure (see R2 in Figure 2a). At atmospheric pressure the presence of narrow microporosity plays a key role on the CO2 adsorption performance. However, at higher pressures, CO2 is also adsorbed on the super-microporosity range (pore sizes between 0.7 and 2 nm). In addition, these correlations show that it is possible to assess the maximum CO2 adsorption capacity of an adsorbent from its textural characterisation [17].

It is worth to draw attention on sample PE10 that shows a CO2 uptake at atmospheric pressure comparable to the activated resins. This may be due to the organic additive (PEG) that enhances narrow microporosity development.

The CO2 capture performance of sample E1A in a ternary mixture containing 20-40 vol.% of CO2 and 5070 vol% H2 (N2 balance), was also evaluated. The values of CO2 adsorption capacities shown in Table 3, were obtained from the average capacity of seven consecutive breakthrough curves.

Table 3 CO2 capture capacities of E1A at ambient temperature and 15 bar in CO2/H2/N2 mixtures

Cases P (bar) T (K) CO2 (vol%) H2 (vol%) CO2 capture capacity (mmol CO2 g-1)

1 2 15 15 298 298 20 40 70 50 4.57 6.14

CO2 adsorption capacity is reduced when the partial pressure of CO2 is diminished, as expected for conventional adsorption processes. According to the high pressure CO2 adsorption isotherms, the maximum CO2 uptake of E1A was 4.77 mmol g-1 at 3 bar and 6.45 mmol g-1 at 6 bar. Cases 1 and 2 correspond to CO2 partial pressures in the feed stream of 3 and 6 bar, respectively. Thus, the CO2 capture capacities assessed during these experiments are close to those of the equilibrium, showing the great selectivity of the carbon towards CO2.

4. Conclusions

Addition of organic additives, ethylene or polyethylene glycol, during the synthesis of phenolformaldehyde resins enhanced the textural characteristics of the prepared activated carbons. The CO2 capture performance of the phenol-formaldehyde resin-based carbons showed promising results at atmospheric and high pressures, when compared to commercial CO2 adsorbents. In addition, it has been demonstrated that CO2 adsorption relies on both the width and volume of micropores. The sample produced with addition of ethylene glycol, E1A, reached the greatest CO2 uptakes (2.46 mmol g-1 at atmospheric pressure and 10.30 mmol g-1 at 30 bar). Great selectivity of the carbons to separate CO2 from CO2/H2 mixtures was observed.

From the results presented here it can be concluded that phenol-formaldehyde resin carbons show promising characteristics to be applied in precombustion CO2 capture processes. Further research is currently being undertaken in our research group to tailor and improve the characteristic of these resin adsorbents, without increasing the cost of the production process.

5. Acknowledgements

This work was carried out with financial support from the Spanish MICINN (Project ENE2008-05087).

C.F.M. acknowledges support from the CSIC JAE-Predoc Program.

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