South African technology for the short term capture of CO2 from flue gases: Synthesis, structure and CO2 capture

properties of zeolites Grace Muriithi, Nicholas M. Musyoka

and Leslie F. Petrik

Cape Town, South Africa

University of the Western Cape

CO2 levels • In 2005, the measured atmospheric CO2 concentration was 379

ppm (IPCC synthesis report, 2007).

• This translates to an increase in global average temperatures of between 1.4 and 5.8 ºC (Watson, 2001).

• CO2 levels required for stabilization, currently estimated at 400-450 ppm

• CO2 levels predicted to increase over the next century from 369 ppm to between 540-970 ppm (International Panel on Climate Control (IPCC)).

• The USA emissions of 6000 million tons per year are projected to increase to 8000 million tons per year by 2030 due to increased consumption of fossil fuels.

• China’s 2006 CO2 emissions surpassed those of the USA by 8 % (Netherlands Environmental Assessment Agency).

Now 500mil ton per annum

Now 228 mil tons

Contribution of the various energy types towards SA’s primary energy consumption for 2009 (Cloete, 2010).

Coal mining areas

Coal burning power stations in RSA

South Africa

• South Africa accounted for 39 % of CO2 emissions from coal combustion in Africa in 2007

• This percentage represents only 1.2 % of the global CO2 emissions

• South Africa is ranked as one of the world's top 15 most energy intensive economies, with a significant contribution to greenhouse emissions at a continental level

Not just CO2 emissions!!

If we cannot even manage to control these particulate emissions……….

How things go wrong

CO2 capture during coal-derived power generation

• Three technological pathways that can be pursued in coal-derived power generation:

– post-combustion capture, e.g scrubbing with monoethanolamine (MEA) solution

– capture during combustion (oxy-fuel combustion); coal is burned with excess oxygen

– pre-combustion capture; coal is pre-gasified with oxygen and steam at high pressure

CO2 Separation Technologies

• Chemical looping combustion; by repeated reduction/oxidation of metal-based oxygen carriers e.g. Fe, Co, Cd and Ni

• Chemical absorption; acid-base neutralization reactions using basic solvents-e.g. mixed amines, aqueous ammonia scrubbing system for CO2 capture

• Physical absorption; cold methanol (Rectisol process), dimethyl ether of polyethylene glycol (Selexol process), propylene carbonate (Fluor process), N-methyl-2-pyrollidone (Purisol process) and sulpholane

• CO2 physical adsorption; pressure swing adsorption (PSA), temperature swing adsorption (TSA), vacuum swing adsorption (VSA) and electrical swing adsorption (ESA)

CO2 Separation Technologies (cont) • Membrane systems; gas phase is separated due to the

selectivity of the membrane to some gases and not others over organic (polymeric), or inorganic (carbon, zeolites, ceramic and metallic) forms

• Cryogenic fractionation; fractional condensation and distillation at low temperatures

• Biological fixation: photosynthetic functions of micro-organisms such as microalgae by the formation of new biomass

• Solid sorbents; range from carbon based, mesoporous silica and microporous materials such as zeolites, membranes (polymeric and ceramic), porous crystals (zeolitic imidazolate frameworks, metal organic frameworks) and anionic clays such as hydrotalcites (HT).

Power plant

Conveyor

Dump site

Fly ash for CO2 capture

Transport

Collection

Ash: a valuable resource?

• Bulk ameliorant – Acid mine drainage (AMD) treatment

– Circumneutral mine water treatment

– CO2 sequestration (mineral carbonation)

• Feedstock for zeolites production – as adsorbents for toxic metals

– as acid catalysts (Friedel Crafts)

– for biodiesel production

– for CO2 storage

Why fly ash?

• “when the properties of waste materials make their use possible in specific high added value applications, these products can successfully compete with products made from primary materials and reduce the environmental costs of waste disposal as well as protecting the environment” (Vilches et al., 2003).

• Journal of Environmental Management 127 (2013) 212-220. • Journal of Environmental Science & Health, Part A.245-259 • Journal of Environmental Management 10/2010; 92(3):655-

64.

Test site: Secunda

Also done at Tutuka and Kragbron

Drilled core samples

CO2 capture in Secunda ash dams

Geophysical mapping of Secunda ash disposal site: Conductivity of the dumping site shows the interaction between the ash leachate and the water table, to a depth of approximately 80 m

EC, Moisture content, and pH

Profiles for electrical conductivity (EC), moisture content (MC) and pH for core S2.

NOT a sustainable salt sink!

Energy Science and Technology, Volume 2, Number 2, November 30, 2011. ISSN 1923-8460 [

Natural CO2 storage capacity of the Secunda ash dam

CO2 sequestration of between 6.5-6.8% using Chittick test (volumetric evolution of CO2 when carbonates react with dilute HCl) as an indicator of the calcite content in the ash along the depth profile in the ash dam for cores S1, S2 and S3.

Journal of Environmental Management 92 (2011) 655-664

Accelerated Ex-situ Mineral Carbonation

Accelerated carbonation experiment conducted under various CO2 pressures and temperatures; varying S/L; particle size. Highest % CO2 content that was observed upon accelerated carbonation was 6.5 wt %;

Sequestration potential of the different size fractions of ash

Correlation of mass % CaCO3 by quantitative XRD and Chittick test

for accelerated carbonation

Fly ash characteristics: XRD Fresh, weathered (core S1) and carbonated

Major crystalline phases; M = mullite; Q = quartz; C = calcite; H = hematite; T = magnetite; A = aragonite; L= lime

CO2 capture by bulk fly ash

• Every ton of CaO can theoretically sequester up to 799 kg of CO2

• Theoretically, assuming all available oxides react to form carbonates, the sequestration capacity of bulk fly ash is 8.2537%

• i.e., 82.54 kg of CO2 can be sequestered in every 1 ton of bulk FA

• 37mil tons FA produced per annum = 3.054 mil ton CO2 per annum

• Mineral carbonation =semipermanent capture

• Natural carbonation in the ash dams could be modelled by accelerated carbonation

• Natural carbonation is significant and can be effective in capturing CO2 in the form of calcite and aragonite

• Issues: ash management; acidification over time; leaching

NaP zeolite/ Gismondine (GIS) using fly ash or solid residues – low temp

ZEOLITE A (LTA) – Hydrothermal CANCRINITE (CAN) by ultrasound - patented

Zeolite synthesis from fly ash

ZEOLITE FAUJASITE (Zeolite Y, or X, or FAU) – high temp.

SODALITE (SOD) – Hydrothermal

Advanced Materials Research. (2012) Vol. 512 – 515, pp. 1757-1762.

Journal of Environmental Science and Health, Part A (2012) 47: 337–350.

Catalysis Today, (2012) volume 190, issue 1, 38 – 46

Research on Chemical Intermediates, (2012) 38: 471-486,

Characteristics of fly ash based Na A

•Fly ash based Na A compared to commercial Na A •Identical XRD •FTIR shows 550 cm-1 peak of D4R rings connected to sodalite cages

Characterization of hierarchical Na X: SEM analysis

Hierarchical FA based zeolite NaX (A and B) and (C) commercial zeolite NaX. (A and B) show that the fly ash based zeolite NaX had a new hierarchical

morphology different from the usual pyramidal octahedral morphology of commercial zeolite NaX (C).

The new hierarchical type zeolite NaX synthesized was characterized by plate-like structures arranged in ball-like clusters with crystal width between 63 nm and 99 nm.

HRTEM analysis

Agglomerated plate-like structures

Individual crystal width of about 100 nm.

Similar crystal widths observed with SEM analysis.

HRTEM image of zeolite X HRTEM-EDS spectra of zeolite X

EDS spectrum confirmed the aluminosilicate nature of FA based zeolite NaX

Na was the most abundant exchangeable cation

Trace presence of Ca, K, S, and Fe from fly ash. The presence of Cu due to the copper sample

grid

HRTEM analysis of micro pores

Micropore channels/cages on average = 0.7 nm; Definitely NOT small pore zeolite (sodalite, NaP or A)

Mineralogy by XRD

XRD comparison of commercial and fly ash based zeolite X

FA based zeolite hierarchical NaX

Compared with commercial X – no mixed phase - replicated

Correspondence of all peak positions

Successful conversion of FA feedstock to high purity zeolite NaX

Patented process

FTIR: Structural configuration of Hierarchical Na X

FTIR comparison of commercial and fly ash based zeolite Na X.

Correspondence of the transmittance bands indicates successful conversion of the fly ash feedstock to zeolite X.

The 561 cm-1 peak is the fingerprint peak for zeolite X associated with D6R ring connection to the cages

Crystallinity and thermal stability

SAED image of zeolite X

The presence of dots and diffuse rings confirmed the high polycrystallinity of FA based hierarchical zeolite NAX

Supports XRD analysis

TGA comparison of commercial and fly ash based zeolite X.

Thermal profiles indicated the successful conversion of the raw FA feedstock to zeolite NaX with <15% moisture loss ~150deg C

The 6 % difference due to higher external and lower micropore area = N2BET.

Fly ash based hierarchical Na X compared with Na A

Surface area analysis by N2-BET

Sorption isotherm of hierarchical zeolite X Incremental mesoporous surface area of hierarchical zeolite X

Isotherm is indicative of a highly microporous zeolite with micropore void filling with N2 occurring to 55 cm3/g.

The zeolite also exhibited mesoporosity as indicated by the presence of a hysteresis loop.

The majority of the mesopores ranged between 3 and 14 nm.

Comparative N2-BET for fly ash based zeolites hierarchical NaX and NaA

Comparison of N2-BET sorption isotherms for zeolites A and X.

The N2 volume adsorbed for hierarchical zeolite X is 50 times more than that of zeolite A = high micropore area.

Both zeolites (synthesized from fly ash) have mesopores as indicated by presence of hysteresis loops (class iv isotherms).

Increase in the height and width of the hysteresis loop in zeolite X is indicative of increasing mesopore size and volume.

CO2 adsorption on hierarchical zeolite Na X by TPD analysis

CO2-TPD profile of hierarchical zeolite Na X

Only one CO2 desorption peak at mild temperatures between 100 to 250 °C.

Thus hierarchical zeolite X is suitable for low temperature-swing adsorption where the zeolite can easily be regenerated and applied for further adsorption cycles.

The peak at approximately 150 °C to 160 °C is representative of desorption of physisorbed CO2.

At around 400 °C, all the adsorbed CO2 is evolved and no further evolution of CO2 was observed at higher temperatures up to 750 °C.

The desorption peak tailing at 250 °C is indicative of the time taken for adsorbed CO2 to be released and diffuse out of the micropores.

CO2 adsorption fly ash based zeolite Na A.

TPD profile of fly ash based zeolite A

Two main desorption peaks centered around 150 °C and 450 °C.

Arbitrary desorbed volume for the main peak (150 °C) was found to be 0.06 au.

This implies a lower adsorption capacity compared to zeolite X (0.25 au).

Both physisorption and chemisorption processes are taking place in the adsorption of CO2 on zeolite NaA.

Comparative CO2 adsorption for hierarchical zeolite Na X vs Na A

Comparative CO2 adsorption of zeolites A and X.

The principal mode of CO2

adsorption in zeolite X is via physisorption.

The relative physisorbed volume of CO2 by zeolite X is much higher than zeolite A.

Zeolite NaX seems suitable for low temperature swing CO2 adsorption applications.

Conclusions

Zeolite X with unique morphology (hierarchical) was synthesized directly from waste coal fly ash.

The CO2 desorption profile of zeolite X gave a single desorption peak in the temperature range of between 50 °C and 150 °C.

Hierarchical Zeolite X had a much higher capacity for low temperature swing adsorption of CO2 compared to zeolite NaA from the same fly ash feedstock.

The hierarchical zeolite Na X was thus suitable for low temperature swing adsorption of CO2. for reversible storage.

Overall conclusions

• Possibilities exist at power generation points for carbon capture and storage IF properly managed

• Low cost FA adsorbents for separation of gases • BUT- longterm acidification is a big problem in

any CO2 storage facility: – carbonates form at particular pH, CO2 and Ca

concentration, – carbonic acid formation drives the pH down; – low pH dissolves carbonates and calcites – Various ways to define risk – Does low risk = evading liability?

THANK YOU!