Elsevier

Waste Management

Volume 28, Issue 11, November 2008, Pages 2320-2328
Waste Management

Sorbents for CO2 capture from high carbon fly ashes

https://doi.org/10.1016/j.wasman.2007.10.012Get rights and content

Abstract

Fly ashes with high-unburned-carbon content, referred to as fly ash carbons, are an increasing problem for the utility industry, since they cannot be marketed as a cement extender and, therefore, have to be disposed. Previous work has explored the potential development of amine-enriched fly ash carbons for CO2 capture. However, their performance was lower than that of commercially available sorbents, probably because the samples investigated were not activated prior to impregnation and, therefore, had a very low surface area. Accordingly, the work described here focuses on the development of activated fly ash derived sorbents for CO2 capture. The samples were steam activated at 850 °C, resulting in a significant increase of the surface area (1075 m2/g). The activated samples were impregnated with different amine compounds, and the resultant samples were tested for CO2 capture at different temperatures. The CO2 adsorption of the parent and activated samples is typical of a physical adsorption process. The impregnation process results in a decrease of the surface areas, indicating a blocking of the porosity. The highest adsorption capacity at 30 and 70 °C for the amine impregnated activated carbons was probably due to a combination of physical adsorption inherent from the parent sample and chemical adsorption of the loaded amine groups. The CO2 adsorption capacities for the activated amine impregnated samples are higher than those previously published for fly ash carbons without activation (68.6 vs. 45 mg CO2/g sorbent).

Introduction

In 2005 around 70 million tons of fly ash were generated by electric utilities in US, and around 60% was disposed (ACAA, 2005). Similarly, in Western Europe (15 EU countries), the total production of fly ash was around 44 million tons in 2003, with only 47% being used in the construction industry (ECOBA, 2003). Moreover, the concentration of unburned carbon present in fly ash has risen drastically over the last few years, due to the implementation of increasingly stringent Clean Air Act Regulations regarding NOx emissions, which are mainly addressed in coal combustion furnaces by a combination of low-NOx burners and catalytic reduction systems. Although low-NOx burner technologies efficiently decrease the emissions level by lowering the temperature of combustion, they also reduce the combustion efficiency with a corresponding increase in the concentration of unburned carbon in the fly ash (Maroto-Valer et al., 1998). This has restricted the principal use of ash in the cement industry, since the unburned carbon tends to adsorb the air-entrainment agents, which are added to the cement to prevent crack formation and propagation (Hill et al., 1997). Consequently, fly ashes of high-unburned-carbon content, also referred here to as fly ash carbons, derived from coal–fired combustors are an increasing problem for the utility industry, since they cannot be marketed as a cement extender and, therefore, have to be disposed. However, due to the increasingly restricted landfill use, the coal industry needs to find uses for these chars. Following this demand, the authors have previously developed a one-step activation protocol to produce activated carbons from high carbon fly ashes (Zhang et al., 2003, Maroto-Valer et al., 2002). The authors’ previous studies have shown that fly ash carbons only require a one-step activation process, since they have already gone through a devolatilization process while in the combustor (Zhang et al., 2003, Maroto-Valer et al., 2002). This is an important advantage compared to the conventional two-step activation process that includes a devolatilization of the raw materials, followed by an activation step.

CO2 capture technologies aim to isolate the CO2 from the flue gas into a form suitable for transport and subsequent storage. Existing chemical absorption processes using monoethanolamine (MEA) were developed for enhanced oil recovery and will impose an energy penalty of about 30% and an increase of the price of electricity of at least 60% when applied to power plants (Yeh and Pennline, 2001). Adsorption processes for CO2 capture using high surface area solids have recently been proposed (Siriwardane et al., 2001). Adsorption can be classified as physical and chemical adsorption (chemisorption) based on the nature of the bonding between the adsorbate molecule and the solid surface. Chemisorption involves electron transfer, whereas the bonds formed in physical adsorption are held by van der Waals and coulombic (electrostatic) forces. The later are much weaker, generally below 10–15 kcal/mole, and hence the process is easily reversed. (Yang, 1987). Materials like zeolites and activated carbons have high surface areas (>1500 m2/g) and adsorb selectively different gases depending on their surface area, pore size, pore volume and surface chemistry. They operate in pressure-swing-adsorption (PSA) or temperature-swing adsorption (TSA) modes to desorb the adsorbed gases either by reducing the pressure or increasing the temperature, respectively. However, physical adsorption on zeolites systems may not be attractive for gas- and coal-fired power plants because these adsorption processes are energy intensive and expensive, particularly the PSA and TSA processes. Accordingly, new solid-based sorbents are being investigated, where amine groups are bonded to a solid surface, resulting in an easier regeneration step (Siriwardane et al., 2001, Zinnen et al., 1989, Soong et al., 2001). The supports used thus far include commercial molecular sieves and activated carbons. Previous work has explored the potential development of amine-enriched fly ash carbons (Gray et al., 2004, Arenillas et al., 2005). Although the amine treatment process used improved the CO2 capture capacities of the fly ash carbon samples, their performance was lower than that of commercially available carbon based sorbents (7 vs. 88 mg CO2/g sorbent) (Gray et al., 2004). However, none of the previously published work activated the fly ash carbons prior to amine impregnation, and, therefore, they have a very low surface area (<100 m2/g) compared to commercial activated carbon sorbents (>800 m2/g).

Accordingly, the work reported here focuses on the development of activated fly ash derived sorbents for CO2 capture. In this work, two fly ash carbon samples were collected. Previous work has shown that the sorbent properties of the inorganic fraction is very low compared to the carbon present in ash (Serre and Silcox, 2000, Hassett and Eylands, 1999, Maroto-Valer et al., 2005a). Therefore, the samples were subjected to an acid digestion step to further reduce their ash concentrations. The samples were then activated using the protocols previously developed by the authors (Zhang et al., 2003), and the resultant activated samples were amine impregnated. The parent, activated and treated samples were tested for CO2 capture at different temperatures.

Section snippets

Study samples

Two high carbon fly ash samples, named FA1 and FA2, were procured and characterized for this study. FA1 was collected from a pulverized-coal-fired suspension-firing research boiler equipped with a low-NOx burner and burning high volatile bituminous coal. The FA2 sample was procured from a gasifier that uses a subbituminous coal. These samples were selected due to their high loss-on-ignition (LOI) values (Table 1), as this work focuses on the study of high carbon fly ashes as feedstock for

Sample de-ashing and one-step activation

The LOI values of the studied samples are listed in Table 1. The LOI of the parent samples, FA1 and FA2, are 62% and 38%, respectively, which are higher than those reported in previous studies that are typically <15% (Zhang et al., 2003, Hill et al., 1997).

The de-ashing step used in the present paper can successfully concentrate the unburned carbon, where the resultant samples, FA1–DEM and FA2–DEM, have LOI values as high as 97%. The acid digestion step was used to reclaim the carbon from fly

Conclusions

The objective of this paper is the development of activated fly ash derived sorbents for CO2 capture. Two fly ash carbon samples were collected and subjected to acid digestion to further reduce their ash concentrations. The samples were then activated using the protocols previously developed by the authors, and the resultant activated samples were amine impregnated. The parent, activated and treated samples were tested for CO2 capture at different temperatures.

The one-step steam activation

Acknowledgement

This work was supported by the US DOE through the Combustion Byproducts Recycling Consortium (Project number 01-CBRC-E9).

References (30)

  • M. Seggiani et al.

    Investigation on the porosity development by CO2 activation in heavy oil fly ashes

    Fuel

    (2003)
  • M. Seggiani et al.

    Effect of pre-oxidation on the porosity development in a heavy oil fly ash by CO2 activation

    Fuel

    (2005)
  • K.M. Steel et al.

    Re-generation of hydrofluoric acid and selective separation of Si(IV) in a process for producing ultra-clean coal

    Fuel Process. Technol.

    (2004)
  • ACAA 2005, 2005. Coal Combustion Product Production and Survey,...
  • ECOBA, 2003. Coal Combustion Product Production and Survey, http://www.ecoba.com/index.html (accessed...
  • Cited by (171)

    View all citing articles on Scopus
    1

    Present address: Sorbent Technologies Corp., 1664 East Highland Road, Twinsburg, OH 44087, USA.

    View full text