Industrial wastes derived solid adsorbents for CO2 capture: A mini review

chemical engineering research and design 9 0 ( 2 0 1 2 ) 1632–1641

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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd

Industrial wastes derived solid adsorbents for CO2 capture: A mini review Aveen Kaithwas a , Murari Prasad b,∗ , Ankita Kulshreshtha b , Sanjay Verma a a b

Department of Chemical Engineering, Ujjain Engineering College, Ujjain, M.P., India Environmental Chemistry Division, CSIR – Advanced Materials & Processes Research Institute, Hoshangabad Road, Bhopal, India

a b s t r a c t Coal combustion in thermal power plants throughout the world produces large amounts of fly ash. Disposal of fly ash is a serious threat to the environment and hence is a worldwide concern for conversion of these wastes into useful products. Synthesis of mesoporous silica materials from coal fly ash has already been proposed as an option which can be utilized as an adsorbent. Adsorption is considered to be one of the more promising technologies for capturing CO2 from flue gases. This paper reviews the recent development of solid adsorbents from industrial waste materials with special reference to fly ash for post-combustion capture of CO2 . © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Greenhouse gas; Fly ash; Adsorption; CO2 capture

1.

Introduction

Human activities result in the generation of greenhouse gases (GHGs) into the atmosphere. Greenhouse gases composed of mainly carbon dioxide (CO2 ), methane (CH4 ), chlorofluorocarbons (CFCs), and nitrous oxide (N2 O) that are contributing to the global warming phenomena considerably. The global warming caused by the increased levels of these gases is one of the most serious environmental threats to the human race at present. Out of these GHGs, the contribution of CO2 is maximum. CO2 emitted into the atmosphere is assumed to cause the greatest adverse impact on the observed green house effect accounting for approximately 55% of the observed global warming. The flue gas emitted from the thermal power plants (TPPs) is growing because 30% of the total global fossil fuel is being used for power generation that emits considerable amount of CO2 (Bandyopadhyay, 2010). The emissions of anthropogenic CO2 have increased the CO2 concentration on the atmosphere with over 30% compared to pre-industrial levels (Keeling and Whorf, 1998). Furthermore, it is estimated that future global CO2 emissions will increase from ∼7.4 GtC (billion tons of atmospheric carbon)/year in 1997 up to ∼26 GtC/year in 2100 (Mercedes et al., 2004). Carbon dioxide sequestration provides a mid-term solution to mitigate environment impacts and allows human

∗

continue to use fossil energy until renewable energy technology mature. Carbon capture and sequestration (CCS) is one of the most suitable techniques for long term technology policies (Riahi et al., 2004). Since the CO2 separation is the first and most energy intensive step of CCS, many researches have targeted at improving the current technologies or developing new approaches of CO2 separation and capture (Yang et al., 2008a). Carbon dioxide capture is widely studied with a view to its application in energy generation systems as a means of reducing greenhouse gas emissions; solid sorbents, capable of being regenerated, could provide an effective means for carrying out various operation (Felice et al., 2011; Harrison, 2008; Florin and Harris, 2008). CO2 capturing from flue-gas streams is an essential parameter for the carbon management for sequestrating of CO2 from our environment. Current technologies being considered for CO2 sequestration include: disposal of CO2 in deep oceans; depleted oil and gas fields; deep saline formations (aquifers); and recovery of enhanced oil, gas, and coal-bed methane. However, the current cost for the utilization of these types of technologies has proven to be too expensive. Consequently, reducing the cost for the capture of CO2 will be a critical step in the overall carbon management program. The physical and chemical adsorption of CO2 can be achieved by using solvents chemical (gas–liquid) absorption,

Corresponding author. Tel.: +91 755 2459343; fax: +91 755 2457042. E-mail address: [email protected] (M. Prasad). Received 28 September 2011; Received in revised form 18 January 2012; Accepted 20 February 2012 0263-8762/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2012.02.011

chemical engineering research and design 9 0 ( 2 0 1 2 ) 1632–1641

physical adsorption, cryogenic separation, membrane separation, biological fixation, and oxyfuel combustion with CO2 recycling (Wolsky et al., 1994; Kimura et al., 1995; Nishikawa et al., 1995; Bandyopadhyay, 2010). Technical developments of CO2 capture and storage using adsorption processes is of much interest due to the low energy requirement and low capital cost. All over the world a coal based thermal power plants are releasing bulk quantity of solid waste as a by-product during the production of electricity by burning of pulverized coal in the boiler at about 1500 ◦ C. During this process most of the carbon is burned and the inorganic non combustible materials release different solid fractions called ash (Bandyopadhyay, 2010). Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal. Fly ash is generally captured by electrostatic precipitators or other particle filtration equipments before the flue gases reach the chimneys of coal-fired power plants, and together with bottom ash removed from the bottom of the furnace is in this case jointly known as coal ash. Depending upon the source and makeup of the coal being burned, the components of fly ash vary considerably, but all fly ash includes substantial amounts of silicon dioxide (SiO2 ) and calcium oxide (CaO). Large quantities of coal fly ash are produced in electric power plants throughout the world every year. The amount of coal fly ash formed is approximately 500 million tons per year and is predicted to increase. Efficient disposal of coal fly ash has been a worldwide issue because of the massive amount of ash produced and harmful effects on environment (Hui and Chao, 2006). CO2 capture technologies aim to isolate the CO2 from the flue gas in to a form suitable for transport and subsequent storage. To date, all commercial CO2 capture plants use processes based on chemical absorption with an aqueous alkanolamine solvent, (i.e. Econamine FGSM, KerrMcGee/ABB Lumus Crest MEA) (Chapel et al., 2001; Leci, 1996; Mignard et al., 2003); monoethanolamine (MEA) is the most popular solvent (Herzog, 2003). However, they have a number of shortcomings for treating flue gases, which manifest themselves in the form of high capital and running costs (the typical energy penalty incurred by the operation of the MEA capture process is an estimated 15–37% of the net power output of a plant) (Herzog and Drake, 1993). As well as raising the operating costs, this parasitic load generates more CO2 emissions, hence decreasing the overall benefit of sequestration (Herzog, 2003). The focus of the present work is on the development and the use of adsorbents from fly ash for their effective use as CO2 sorbents. This paper reviews the current state-of-theart and potential future developments in the application of solid adsorbents from waste materials with special reference to fly ash for post-combustion capture of CO2 .

2.

Literature review

Wastes may be generated during the extraction of raw materials, the processing of raw materials into intermediate and final products, the consumption of final products, and other human activities. Industrial solid wastes are produced several million tons around the world per day. Improper disposal of these solid wastes is a serious threat to the environment and hence it is a worldwide concern (Ferraiolo et al., 1990;

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Chandrasekar et al., 2008). The conversion of these wastes into useful products has been actively pursued to achieve economic gains and protection of the environment (Rio et al., 2005; Criado et al., 2007; Kuceba and Nowak, 2005). Such useful products/materials which have been developed are; Mesoporous MCM-68 Solid sorbent materials (Devadas et al., 2010), Zeolite from waste fly ash (Lee and Jo, 2010), Mesoporous silica SBA-15 (Chandrasekar et al., 2008), Zeolite 4A (Hui and Chao, 2006) and MCM-41 aluminosilicate (Chang et al., 1999). In addition, waste materials have been utilized as such for different applications; blast furnace (BF) slag, dust and sludge as adsorbents for the removal of dyes (Jain et al., 2003), RM I, RM II, RM III (red mud three different size fractions) for sequestration of CO2 (Yadav et al., 2010), red mud as adsorbent for the removal of toxic pollutants from water and wastewater (Bhatnagar et al., 2011).

2.1.

Industrial solid waste

The major generators of industrial solid wastes include; thermal power plants producing coal ash, the integrated iron and steel mills producing blast furnace slag and steel melting slag, non-ferrous industries like aluminium, zinc and copper producing red mud and tailings, sugar industries generating press mud, pulp and paper industries producing lime and fertilizer, allied industries producing gypsum, etc. are shown in Table 1, and their possible route of disposal are shown in Table 2. All these wastes are used for various beneficial purposes. Out of these waste, Fly ash finds a useful application in CO2 capture technology. In the following sections, characteristics, chemical analysis and bulk utilization of fly ash shall be discussed before its application in carbon capture technology is discussed.

2.1.1.

Fly ash

Fly ash is one of the residues generated in the combustion of coal, it is generally captured from the chimneys of coal-fired power plants, and is one of two types of ash that jointly are known as coal ash; the other, bottom ash, is removed from the bottom of coal furnaces. The large quantity of coal burned in thermal power plants generates huge quantity of fly ash. Fly ashes with high-unburned-carbon content, referred to as fly ash carbons, are an increasing problem for the utility industry. It is, therefore, necessary that fly ash should be utilized wherever possible to minimize environmental degradation. The research and development work carried out for utilization of fly ash for making building materials has proved that fly ash can be successfully utilized for production of bricks, cement and other building materials. Fly ash particles are very fine, light weight (density 1.97–2.89 g/cc) and spherical (specific surface area 4000–10,000 cm2 /g; diameter, 1–150 ␮m), refractory and have pozzolanic ability. Fly ash grey to blackish grey and is dependent on coal type and combustion process. Fly ash has dielectric property (dielectric constant, 104) and can be used in electronic application.

2.1.1.1. Chemical composition of fly ash. Fly ashes are generally highly heterogeneous, consisting of a mixture of glassy particles with various identifiable crystalline phases such as quartz and mullite. Fly ash particles are generally spherical in shape and range in size from 0.5 ␮m to 100 ␮m. They consist mostly of silicon dioxide (SiO2 ), present in two forms: amorphous, which is rounded and smooth and crystalline, which

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Table 1 – Generation of some major industrial waste. S. no

Name

Source/origin

1

Steel and blast furnace

2 3 4 5 6

Brine mud Copper slag Fly ash Kiln dust Lime sludge

Conversion of pig iron to steel and manufacture of Iron Caustic soda industry By product from smelting of copper Coal based thermal power plants Cement plants Sugar, paper, fertilizer tanneries, soda ash, calcium carbide industries Mica mining areas Phosphoric acid plant, ammonium phosphate Mining and extraction of alumina from bauxite Coal mines Iron ore Lime stone quarry

7 8 9 10 11 12

Mica scraper waste Phosphogypsum Red mud/bauxite Coal washery dust Iron tailing Lime stone wastes

is sharp and pointed, aluminium oxide (Al2 O3 ) and iron oxide (Fe2 O3 ). Chemical composition of fly ash generated by three different types of coal has been shown in Table 3. Toxic constituents depend upon the specific coal bed makeup, but may include one or more of the following elements or substances in quantities from trace amounts to several percent: arsenic, beryllium, boron, chromium vi, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium, along with dioxins (Managing Coal Combustion Residues in Mines, 2006). Fly ash material solidifies while suspended in the exhaust gases and is collected by electrostatic precipitators or filter bags. Since the particles solidify while suspended in the exhaust gases.

2.1.1.2. Utilization of fly ash. The extensive research carried out in recent years for finding further applications of fly ash have led to the development of processes for making, hollow/masonry/concrete blocks, glass ceramics, ceramic wares, silicon carbide, silicon nitride, sialon, cordierite, mullite and separation of cenospheres for use as extenders for plastic compounds, synthetic foams with better mechanical properties, automobile industries, paints, coatings, fire and heat protection devices, etc. Bulk utilization of fly ash has been in practice for Manufacturing of Bricks, Embankment and Fills, Road Pavement, Portland Pozzolana Cement, Cement Concrete and Mortar, Back filling of Open Cast Mine, Stowing of Under Ground Mines, Agriculture, Manufacture of Alum, Paint, Ceramic, Asbestos Cement Products, Plastic industries. The recent use of fly ash is the development of sorbents (Arenillas et al., 2005; Gray et al., 2004; Mercedes et al., 2008) and mesoporous silica (Ahmaruzzaman, 2010) for CO2 capture.

3.

CO2 capture technologies

The rise in the global surface temperature is widely attributed to an increase in atmospheric levels of CO2 , for which the burning of fossil fuels is commonly held to be responsible (Arenillas et al., 2005). Carbon capture and storage (CCS) is widely seen as a critical technology for limiting atmospheric emissions of carbon dioxide (CO2 ) the principal “greenhouse gas” linked to global climate change from power plants and other large industrial sources. CO2 has been captured from a portion of the flue gases produced at power plants burning coal or natural gas. Since most anthropogenic CO2 is a by-product of the combustion of fossil fuels, CO2 capture technologies

References urbanindia.nic.in/publicinfo/swm/chap6.pdf urbanindia.nic.in/publicinfo/swm/chap6.pdf http://urbanindia.nic.in/publicinfo/swm/chap6.pdf urbanindia.nic.in/publicinfo/swm/chap6.pdf urbanindia.nic.in/publicinfo/swm/chap6.pdf urbanindia.nic.in/publicinfo/swm/chap6.pdf urbanindia.nic.in/publicinfo/swm/chap6.pdf urbanindia.nic.in/publicinfo/swm/chap6.pdf urbanindia.nic.in/publicinfo/swm/chap6.pdf urbanindia.nic.in/publicinfo/swm/chap6.pdf urbanindia.nic.in/publicinfo/swm/chap6.pdf urbanindia.nic.in/publicinfo/swm/chap6.pdf

are commonly classified as either pre-combustion or postcombustion systems, depending on whether carbon (in the form of CO2 ) is removed before or after a fuel is burned. For the post-combustion technology, the exhaust gas contains CO2 with low concentration (4–14%, v/v) represents an important limitation for CO2 capture. A third approach, called oxyfuel or oxy-combustion, does not require a CO2 capture device. This concept is still under development and is not yet commercial.

3.1.

Post-combustion processes

As the name implies, these systems capture CO2 from the flue gases produced after fossil fuels or other carbonaceous materials (such as biomass) are burned. Combustion-based power plant provides most of the world’s electricity today. In a modern coal-fired power plant, pulverized coal (PC) is mixed with air and burned in a furnace or boiler. The heat released by combustion generates steam, which drives a turbine-generator. The hot combustion gases exiting the boiler consist mainly of nitrogen (from air) plus smaller concentrations of water vapour and CO2 formed from the hydrogen and carbon in the fuel (Folger, 2010; Rao and Rubin, 2002). There are several methods for CO2 capture from gas streams such as the use of: chemical and physical solvents, adsorption on solids, membrane and cryogenic/condensation system. Different plant configurations will require different CO2 strategies as shown in Fig. 1. These all techniques are used in experiment but adsorption is considered to be promising techniques for capture of CO2 from flue gas (Drage et al., 2009).

3.1.1.

Adsorption

Adsorption is a process in which molecules contained in liquid or gaseous mixtures adhere on the surface of solid adsorbent. The properties of the adsorbed particles (molecular size, molecular weight and polarity) and the adsorbent surface (polarity, pore size and spacing) determine the adsorption quality. As the adsorption is an exothermic process, the regeneration of the adsorbents through desorption can be performed by rising the temperature. Solid sorbents have the potential for significant energy savings over liquid solvents, in part because they avoid the need for the large quantities of water that must be repeatedly heated and cooled to regenerate the solvent solution (Sigueroa, 2008). However, adsorption presents lower energy requirements and avoids the shortcomings when compared to absorption (Drage et al., 2009). In post-combustion process, adsorption is recognized to be an attractive process for CO2 capture from

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Table 2 – Waste that have been utilized and their usual disposal routes. Waste Slag

Description

Usual disposal and utilization routes Aggregate manufacturing

References Bertos et al. (2004)

BFS

Secondary products from metal refining Granulated blast furnace slag

SS

Steel production

Galligu

By-product of the manufacture of sodium carbonate Ash from combustion of municipal solid wastes. There are two kinds, bottom and flyash Ash from paper recycling process Air pollution control waste

Dumped to pits and covered with ash Disposal of in landfills. Incorporation into materials for construction applications

Bertos et al. (2004)

Dumped and landfill

Bertos et al. (2004)

Landfilled

Bertos et al. (2004)

Waste dust coming from furnace for metal casting Residue deposited on a permeable medium when slurry is forced against the under pressure Powder of burnt coal in thermal power plants The insoluble residue deposited on the air pollution control devices of a cupola furnace By-product produced during the combustion of dewatered sewage sludge in an incinerator Waste product from wastewater treatment Partially calcined mineral mixture Combined material collected either in electrostatic precipitators or fabric filter devices Residue coming from coal burning power plants Largest solid waste stream produced by steel mills Dust coming from the breaking of the sand treated in the reclaim units for its reuse in casting processes Dust coming from the lining of castings in a puddling furnace By product from the cleaning of finished casting Bag house dusts are collected from emissions from the furnace or sand reclamation plant By product of the foundry casting process of metals Filter sludge from wet cleaning plants from foundry, iron and steel industries By product of the molten metal injection processes Filter dust from sand regeneration and fettling shop plants Waste product of the electrolytic process in the smelting of aluminium

Landfilled

Bertos et al. (2004)

Landfilled in general. Certain treatments depend on the nature of the cake

Bertos et al. (2004)

Additive in the building industry

Bertos et al. (2004)

Landfilled

Bertos et al. (2004)

Landfilled, concrete production, mineral filler and soil conditioner

Bertos et al. (2004)

Stabilization with cement

Bertos et al. (2004)

Landfilled, agricultural applications Landfilled and used for cement production

Bertos et al. (2004)

Cement products and landfilled

Bertos et al. (2004)

Recycled and landfilled

Bertos et al. (2004)

Used as fillers, for concrete manufacturing for asphalt manufacturing

Bertos et al. (2004)

Recycled in plant and landfilled

Bertos et al. (2004)

Used as roadbase and the rest landfilled Landfilled. Used as raw material floor cement and concrete manufacturing

Bertos et al. (2004) Bertos et al. (2004)

Partly reused and partly landfilled

Bertos et al. (2004)

Part recycled, and the rest sent to landfill

Bertos et al. (2004)

Mostly recycled

Bertos et al. (2004)

Landfilled and recycled

Bertos et al. (2004)

Vitrified and landfilled

Bertos et al. (2004)

MSWI ash

Deinking ash Cyclone dust Cupola furnace dust Filter cake

Pulverized fly ash Cupola arrester filter cake

Sewage sludge ash

Sewage sludge Cement kiln dust Air pollution control residues

Coal fly ash Arc furnace dust Thermal reclaim dust

Fettling shop extraction dust Shot blast dust Bag house dust

Foundry sands Blast furnace flue dust

Melting dust Mill scale

Silica pot liner

Cement production and concrete admixture Amour stone and soil conditioning

Bertos et al. (2004)

Bertos et al. (2004)

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– Table 2 (Continued) Waste Coke breeze Red mud

Fly ash

CaO and MgO

Description Filter dust from hall dust extraction plants The extraction of alumina from bauxite ore results in a highly alkaline bauxite residue slurry known as red mud

Fly ash is one of the residues generated in the combustion of coal

Ca(OH)2 and CaO from steel slag or concrete waste

flue gases, due to its lower energy requirements (Aaron and Tsouris, 2005). Numerous CO2 adsorbents like zeolites (Cavenati et al., 2005; Konduru et al., 2007; Brandani and Ruthven, 2004; Bonenfant et al., 2008) and carbons (Himeno et al., 2005) are used commercially for the removal of CO2 (Belmabkhout et al., 2011). In addition, considerable research effort has been made in recent years to develop novel CO2 adsorbents like basic oxides (Feng et al., 2007; Wang et al.,

Usual disposal and utilization routes Mostly recycled and the rest landfilled or land applied Red mud can be effectively utilized for sequestration of CO2 , tantalite and can crinite are mineral phases responsible for carbonation of red mud and carbonation capacity was evaluated to be 5.3 g of CO2 /100 g of RM II (i) Low cost carbon materials derived from fly ash as effective CO2 sorbents (ii) The capture of CO2 from gas streams has been achieved by the utilization of amine-enriched fly ash carbon sorbent system. The initial fly ash carbon sorbents were generated by the chemical treatment of carbon-enriched fly ash concentrates with a 3chloropropylaminehydrochloride(COACH) solution at 25 ◦ C (iii) Alcohol amines have been used to impart an amine functionality to activated carbons and, therefore, to enhance their CO2 adsorption capacity. A primary (monoethanolamine, MEA), a secondary (diethanolamine, DEA), and a tertiary (methyldiethanolamine, MDEA) were used to modify the activated fly ash carbon (iv) Converting fly ash into mesoporous molecular sieves. Due to their uniform molecular pore sizes and large surface areas, the mesoporous materials can be very useful for a wide range of applications such as molecular sieves, adsorbents, and catalysts CO2 in ambient air to capture and store carbon safely and permanently in the form of stable carbonate minerals (CaCO3 )

References Bertos et al. (2004) Yadav et al. (2010)

Belmabkhout et al. (2011)

Gray et al. (2004)

Mercedes et al. (2004)

Chang et al. (1999)

Stolaroff et al. (2005)

2008; Lee et al., 2008; Symonds et al., 2009), metal-organic frameworks (MOFs) (Millward and Yaghi, 2005; Llewellyn et al., 2006, 2008; Yang et al., 2008b; Arstad et al., 2008), organosilicas and surface-modified silicas (Zelenak et al., 2008; Harlick and Sayari, 2006, 2007; Hicks et al., 2008; Chaffee et al., 2007; Gray et al., 2005; Yue et al., 2008; Belmabkhout and Sayari, 2009; Serna-Guerrero et al., 2008; Chang et al., 2009).

Table 3 – Chemical composition of fly ash. Component SiO2 (%) Al2 O3 (%) Fe2 O3 (%) CaO (%) LOI (%)

Bituminous 20–60 5–35 10–40 1–12 0–15

Sub bituminous 40–60 20–30 4–10 5–30 0–3

Lignite 15–45 20–25 4–15 15–40 0–5

chemical engineering research and design 9 0 ( 2 0 1 2 ) 1632–1641

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Fig. 1 – Technologies for CO2 capture (Rao and Rubin, 2002). In general, the aim of solid sorbent research is to reduce the cost of CO2 capture by designing durable sorbents with efficient materials handling schemes, increased CO2 carrying capacity (DOE, 1999). The CO2 carrying capacity is a key sorbent parameter that depends on the total microscopic surface area of the material. Researchers are thus attempting to identify and design sorbents with very high surface area, pore size and pore volume such as mesoporous silica for CO2 capture. The capture mechanism can be either a chemical or physical surface interaction as shown in Fig. 2.

4.

Mesoporous silica for CO2 capturing

Mesoporous silica is a form of silica and a recent development in nanotechnology. The most common types of mesoporous nanoparticles are MCM-41 (Mobil Composition of Matter No. 41) and SBA-15 mesoporous silica. MCM-41 is formed by surfactant treatment procedure in which fly ash is treated with surfactant Cetrimonium bromide (C-TAB, C16 H33 (CH3 )3 NBr) with NaOH (Chang et al., 1999). In other way MCM-41 mesophase with surface area 1138 m2 /g synthesized by using Cab-O-Sil M5 fumed silica as the silica source,

Fig. 2 – CO2 capture mechanism (Gray et al., 2008).

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Table 4 – Surface properties of adsorbents. Sample

Surface area (SBET) m2 /g

MCM-41 PE-MCM-41 EDA SBA-15 Activated fly ash carbon (AC) (untreated) Activated fly ash carbon (AC) with MDEA Activated fly ash carbon (AC) with DEA Activated fly ash carbon (AC) with MEA Activated fly ash carbon (AC) with AMP

1138 917 700 818 204 265 241 245

Pore volume, ml/g Vmi

Vme

– – – 0.400 0.110 0.126 0.143 0.118

– – – 0.265 0.092 0.162 0.254 0.084

Total pore volume ml/g

1.03 2.03 – 0.665 0.203 0.288 0.397 0.201

References

Franchi et al. (2005) Franchi et al. (2005) Zheng et al. (2004) Mercedes et al. (2004) Mercedes et al. (2004) Mercedes et al. (2004) Mercedes et al. (2004) Mercedes et al. (2004)

Fig. 3 – Proposed reactions for preparation of the amine-enriched fly ash sorbent (Gray et al., 2004). cetyltrimethylammonium bromide (CTAB) as the surfactant template, and a 25% solution of tetramethylammonium hydroxide in water (TMAOH) (Sayari and Yang, 2000). Mesoporous material is used for CO2 adsorption by the amine (MDEA, MEA, DEA, AMP) treatment for increasing their adsorption capacity. Recently PE-MCM-41 with surface area 917 m2 /g is synthesized by with N,N-dimethyldecylamine (DMDA) (Franchi et al., 2005) and EDA SBA-15 mesoporous silica with surface area 700 m2 /g is synthesized by N-[3(trimethoxysilyl)propyl]ethylenediamine (EDA) (Zheng et al., 2004). These mesoporous materials show significantly increase in the CO2 adsorption capacities (Xu et al., 2002) such as; 12.6 mg CO2 /g by PE-MCM-41 and 20 mg CO2 /g by EDA SBA-15.

5. Fly ash derived materials for CO2 capturing Present study focuses on the use of activated carbons derived from fly ash for their realization as effective CO2 sorbents through impregnating them with chemicals that have a high affinity for CO2 . To date, all commercial CO2 capture plants use processes based on chemical absorption with an aqueous alkanolamine solvent (i.e. Econamine FGSM, Kerr-McGee/ABB Lumus Crest MEA) (Chapel et al., 2001; Leci, 1996; Mignard et al., 2003) monoethanolamine (MEA) is the most popular solvent (Herzog, 2003). However, they have a number of shortcomings for treating flue gases, which manifest themselves in the form of high capital and running costs (the typical energy penalty incurred by the operation of the MEA capture process is an estimated 15–37% of the net power output of a plant) (Herzog and Drake, 1993). As well as raising the operating costs, this parasitic load generates more CO2 emissions, hence

decreasing the overall benefit of sequestration (Herzog, 2003). In total, 75–80% of the cost of capturing and sequestering 90% of the CO2 from a power plant is attributable to the capture and compression stage (David, 2000). The development of a low-cost means of capturing CO2 is therefore crucial to the realization of the goal of sequestering CO2 emissions on an industrial scale. Adsorption is considered to be one of the more promising technologies for capturing CO2 from flue gases. The success of such an approach is however dependent on the development of an adsorbent that, at high temperatures, has both a high CO2 selectivity and adsorption capacity. With their low cost, high amenability to functionalization, large surface area and easily modified pore structure, activated carbons have the potential to act as effective CO2 sorbents. However, their use for this purpose is hampered by their high sensitivity to temperature-at the temperatures commonly associated with power plant flue gases (50–120 ◦ C), their CO2 adsorption capacity drops dramatically. Consequently, before they can be utilized to capture CO2 , they must first be modified in order to enhance the adsorbate–adsorbent interactions. Activated carbons from fly ash are modified in order to introduce chemical adsorption sites towards CO2 capture. Alcohol amines such as; monoethanolamine (MEA), diethanolamine (DEA), 2-amino-2-methyl-1-propanol (AMP) and methyldiethanolamine (MDEA) are among the most commonly used solvents for CO2 adsorption processes (Smith, 1999; Yeh and Pennline, 2001). Alcohol amines have previously been used to impart an amine functionality to activated carbons, and therefore, to enhance their CO2 capacity (Zinnen et al., 1989). The activated fly ash carbons (AC) can be impregnated by immersing them in an amine solution of the desired amine compound. A desired amount of activated carbon samples are saturated by the solution of chemicals (MDEA,

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Table 5 – Comparison of CO2 adsorption capacity of adsorbents. Adsorbent

Chemical treatment

MCM-41 PE-MCM-41

12.6 23.6

Franchi et al. (2005) Franchi et al. (2005)

EDA-SBA-15

Untreated N,Ndimethyldecylamine (DMDA) Ethylenediamine (EDA)

20.0

Zheng et al. (2004)

AC-30 AC-70 AC-100 AC-120

Untreated Untreated Untreated Untreated

41.8 18.5 – 7.7

Mercedes et al. (2004)

AC-MDEA-30 AC-MDEA-70 AC-MDEA-100 AC-MDEA-120

MDEA MDEA MDEA MDEA

17.1 30.4 40.6 16.1

Mercedes et al. (2004)

AC-DEA-30 AC-DEA-70 AC-DEA-100 AC-DEA-120

DEA DEA DEA DEA

21.1 37.1 16.3 4.2

Mercedes et al. (2004)

AC-MEA-30 AC-MEA-70 AC-MEA-100 AC-MEA-120

MEA MEA MEA MEA

68.6 49.8 25.3 5.5

Mercedes et al. (2004)

AC-AMP-30 AC-AMP-70 AC-AMP-100 AC-AMP-120

AMP AMP AMP AMP

22.3 14.0 5.20 2.80

Mercedes et al. (2004)

95A carbon 95B carbon 95C carbon 95C carbon (regenerated) Polymer-modified mesoporous materials (MCM-41)

3CPAHCL and KOH KOH only 3CPAHCL only 3CPAHCL only Polyethylenimine (PEI)

MEA, DEA, AMP). The activated fly ash carbons before and after chemical loading were characterized and comparison of surface properties of adsorbents are shown in Table 4.The activated carbons from fly ash have been modified with loading of chemicals in order to introduce chemical adsorption sites towards CO2 capture. In other way fly ash carbon concentrate samples 95A, 95B, and 95C are treated with 3chloropropylamine-hydrochloride (3-CPAHCL) salt solution with and without potassium hydroxide for 1 h at 25 ◦ C. The treated amine-enriched fly ash carbon different concentrate is filtered and dried in an oven for 1 h at 105 ◦ C. After that amine treated fly ash carbon concentrates samples (95A, 95B, and 95C) are examined as CO2 capture sorbents. The adsorption of CO2 increased with the amine chemical treatment. The amine-enriched fly ash concentrations are then tested as CO2 capture sorbents in the DRIFTS/TPD reactor systems as shown in Fig. 3 (Gray et al., 2004). The best sample is 95C (surface areas of 27 m2 /g) which has the CO2 capture capacity of 174.5 mg CO2 /g and can be regenerated for an additional test (140.6 mg CO2 /g). In comparison to commercially available sorbents with surface areas of 1000–1700 m2 /g and CO2 capture capacities of 1800–2000 mg CO2 /g, the 95C was only able to achieve 9% of the commercial adsorbent CO2 capture capacity. The comparative study of surface properties of adsorbents is shown in Table 5 and CO2 adsorption of mesoporous material and fly ash derived adsorbent before and after the amine (DMDA, EDA, MEA, DEA, AMP, MDEA and 3-CPAHCL) treatment are shown in Table 5.

6.

CO2 adsorption mgCO2 /g sorbent

81.1 117.9 174.6 140.6 111.7

References

Gray et al. (2004) Gray et al. (2004) Gray et al. (2004) Gray et al. (2004) Kuceba and Nowak (2010)

Conclusion and recommendation

The removal of anthropogenic CO2 from the post combustion flue gas at large point sources has been spotlighted in recent years as a potential way to reduce greenhouse gas emissions. Among a range of separation technologies including membrane separation, absorption, cryogenic distillation, and others, adsorption with solids appears to be one of the most promising CO2 capture strategies. Of the adsorption processes studied over the past decades, adsorptive separation of CO2 by PSA techniques has been most extensively studied. However, a temperature swing process is often envisioned for post-combustion capture of flue gas from existing coalfired power plants, owing to the low pressure of most flue gas streams. This work does not seek to suggest that adsorption is the ideal way to achieve efficient CCS. Rather, if adsorption is to be used for large-scale implementation of CCS, the physical and chemical properties of the fly ash derived adsorbent must be well understood. To this end, this review categorizes the available fly ash derived adsorbent materials that have been reported to date. There have been many recent advancements in the development of suitable and cost-effective process/materials (adsorbents) for the post combustion capture of CO2 . Fly ash derived adsorbents prepared by different chemical procedures such as: surfactant treatment process for mesoporous materials, amine treatment process, and impregnation of activated fly ash by alcohol amines appear to have good promise for the

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capture of CO2 . Amine (MDEA, DEA, MEA and MDEA + MEA) treatment and impregnation can improve significantly the CO2 adsorption of the samples. The highest CO2 adsorption capacity achieved was 174.6 mg CO2 /g adsorbent, for activated carbons (95C) among all activated carbons (95A, 95B and 95C) derived from fly ash. Polymer-modified mesoporous materials (MCM-41) from fly ashes have yielded a good result for CO2 adsorption (capacity of 111.7 mg CO2 /g adsorbent). So both amine treated activated carbons and polymer modified mesoporous materials may be possible candidates for post combustion CO2 adsorbents.

Acknowledgement The authors are grateful to the Director A.M.P.R.I. Bhopal for encouragement of the present research work and kind permission to publish this paper.

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