Application of Nanomaterials in CО2 Sequestration

INTRODUCTION

The most commonly produced greenhouse gas is carbon dioxide. Atmospheric carbon dioxide is captured through the mechanism called the carbon sequestration. So, the carbon capture and sequestration is the process of reducing atmospheric carbon dioxide, thus getting control over the global climate change. It also enables the low carbon economy and maintains the global carbon budget.

Greenhouse gas emission sources are different. Even the sources vary country-wise and are directly proportional to economic development and population demand. As has been described in the report of U.S. Inventory of Greenhouse Gas Emissions and Sinks, 40% or more CO, emissions in the United States come from the electric power generation sector alone. Source-specific carbon-capture and sequestration mechanisms are being invented to reduce the carbon dioxide potential. Fossil fuel-based power plants using those industry-specific technologies can reduce their carbon dioxide emission from 80% to 90% efficiently, which is comparable to planting more than 62 million trees and to wait for their 10 years’ carbon capture.

The carbon sequestration practices got new outlook considering nanomaterials’ uses in the carbon sequestration process. Scientific methods have been developed using nanotechnology. More insights are needed in this process. This chapter highlights the recent trend of researches in carbon sequestration with the application of nanomaterials and nanocomposites and their pros and cons in comparison with other available technologies.

Terrestrial Carbon Storage and Sequestration

Terrestrial (or biologic) sequestrations imply the atmospheric CO, sequestration and store the same as carbon in the plant parts and into the soil biome. During photosynthesis, plants use carbon dioxide and produce oxygen (0₂), which dissipates it into the atmosphere. So, plants actually hold on to and use the carbon to sustain and grow. After the death of any plant, part of the carbon from the plant gets to be preserved (stored) in the soil environment. Terrestrial sequestration is one of the automatic management practices to maximize the carbon storage in the soil and plant materials. Reforestation, rangeland management, and wetland management are prominent examples of terrestrial carbon sequestration practices (MIT 2007). Terrestrial carbon dioxide sequestration is technically low-cost and easy process compared with resource-intensive and complex technology-based geologic sequestration activities, which in turn also improve the habitat and water quality, and overall, it preserves the forests. Terrestrial sequestration alone offsets one-third of the global anthropogenic carbon dioxide emissions. Soil can store the carbon up to a certain level, and over the next 50-100 years, the soil will be saturated with no more space for excess carbon (Paustian and Cole 1998).

Mechanisms for Terrestrial Storage

Various water and land management practices increase the terrestrial carbon storage; the current processes are conservation tillage, soil erosion and distribution management practices, maintaining the buffer strips all along waterway, consideration of land resources in conservation methods, wetland restoration and management, limiting the summer crops, using winter cover crops and perennial grasses, and increase in afforestation (de Silva et al. 2005).

Geologic Sequestration of Carbon Dioxide

It is a one-step process to sequester the atmospheric carbon dioxide. In this process, carbon dioxide is injected deep into the underground to hold it back permanently. Due to the population growth and rapid industrialization, there is an ever-increasing trend of carbon dioxide production in the environment causing greenhouse condition for the mother Earth. Although working as the carbon dioxide sinks, plants seem not enough to hold all those produced. So scientists are continuously trying new methods to accelerate the carbon sequestration mechanisms. Here lies the importance of the geological carbon dioxide sequestration. In many cases, injection of carbon dioxide into a geological formation increases the hydrocarbon recovery, providing economic values that can offset the carbon dioxide sequestration costs. Geological carbon sequestration involves the separation and capture of carbon dioxide at the point of emissions followed by storage in deep underground geologic formations. The method may be of two types, which are discussed below.

Physical Process of Geological Carbon Sequestration

This process involves carbon dioxide trapping within a cavity of any underground rock. The cavities may be either man-made large cavities (e.g. caverns and mines) or the natural pore space within rock formations (e.g. structural traps in depleted oil and gas reservoirs, aquifers). Precise conditions are needed within the deep subsurface geological environment to store the injected carbon dioxide effectively. The selected space should be with some shield so that it cannot migrate out of the storage to cause some environmental hazards. If the reservoirs are porous and permeable, there must be a confining unit overlying as the shield beneath which carbon dioxide should be stored in its supercritical state. Characteristically, there are two types of reservoirs to support the geological carbon dioxide sequestration:

a. Sandstone or other reservoirs contain salt water,

b. Injection into any hydrocarbon-bearing strata such as oil reservoir, gas reservoir, and coal seam. In these cases, actually replacement of the space by carbon dioxide happens when those economically important materials are taken out of the soil core. The C0₂ has a higher affinity towards coal than does the methane that is usually found in coal beds.

Chemical Process of Geological Carbon Sequestration

Chemical mechanisms of trapping the atmospheric carbon dioxide involve transforming the gas or binding it chemically to another substance in the ground. The chemical binding may be done as follows:

a. Dissolving the carbon dioxide directly into underground water or reservoir oil,

b. Decomposing the carbon dioxide into its ionic parts,

c. Locking the carbon dioxide into a steady mineral precipitate,

d. Trapping by adsorption.

CО2 SEQUESTRATION: NONNANOMATERIAL-BASED PROCESSES

Conventional Adsorbents

Adsorbents such as activated charcoal, natural zeolite, and alkali-based materials have frequently been used for C0₂ capture from air. NaX zeolite (Li et al. 2013), zeolite 5A (Saha et al. 2010), and zeolite 13X have been tested in the laboratory for C0₂ sequestration, and the aforementioned studies indicated that the zeolite-based materials have a very high capacity for C0₂ capture. Although zeolite-based materials show high sorption capacity, they need high temperature for regeneration.

Another emerging conventional adsorbent is activated charcoal. As it is economically feasible and thermally stable, its efficacy in C0₂ sequestration has been investigated. It was noticed that activated charcoal has very high C0₂-capturing capacity at increased pressure (Cheng-Hsiu, Chih-Hung, and Tan 2012; Himeno, Komatsu, and Fujita 2005). Also, its regeneration is easy. However, the use of activated charcoal has drawbacks such as low sorption at low pressure, low selectivity, and low efficacy, which are attributable to the presence of NO, and SO_A.

Alkali-based materials are being used for CO, sequestration for a very long time. Alkali-based К,СО/ПО, (Lee et al. 2006) and Li,ZrO, (Ochoa-Fernandez et al. 2005) have been used for CO, sequestration and found to be workable. These types of materials are unique as they can be easily regenerated for more than 20 regeneration-reuse cycles. Also, the volume change due to recycling is found to be very low. However, these types of sorbents have also some drawbacks such as low sorption efficacy and slow sorption kinetics compared with the other conventional materials.

Ionic Liquids

Ionic liquids are very emerging groups of adsorbents that are being used for CO, sequestration. Ionic liquids are composed of organic or inorganic anions and bulky asymmetric cations. These types of materials are thermally stable, have very minimum vapour pressure, and have tunable physicochemical characteristics with very high CO, solubility. Thus, they have been used for CO, sequestration (Park et al. 2015). As these materials are easy to modify using different functional groups for enhancing their CO, sorption capacity, many researchers modified the ionic liquids with amine (Galan Sanchez, Meindersma, and de Haan 2007) or hydroxyl (Lee et al. 2012) groups and found that the CO,-capturing capacity of the modified ionic liquids is enhanced compared with that for the unmodified ionic liquids. As CO, gas is highly soluble in ionic liquid, the ionic liquid is an ideal choice for CO, capture from the natural environment (Ramdin et al. 2015).

Chemicals used for CO, sequestration along with their CO,-capturing capacities are given in Table 8.1 (Sharma et al. 2019).

Modified Porous Support

Porous support is another emerging CO, adsorbent. Recently, Xia et al. (2011) developed zeolite template N-doped carbons and found that these porous supports

TABLE 8.1

Chemical Agents Used for C0₂ Sequestration (Sharma et al. 2019)

TABLE 8.2

Porous Carbon-Based Materials (Sharma et al. 2019)

have very high C0₂-capturing capacity than carbonaceous or other porous materials. Various researchers (Przepiorski, Skrodzewicz, and Morawski 2004; Kim et al. 2008; Son, Choi, and Ahn 2008) also reported similar observations. The CO_r capturing capacity of different porous materials is given in Table 8.2 (Sharma et al. 2019).

CО2 SEQUESTRATION: NANOMATERIAL-BASED PROCESSES

The nanomaterial is defined as a natural, manufactured, or incidental material that has one nanoscale dimension - the dimension range is in between 1 and 100 nm. Nanodimension of any materials can improve their catalytic, electrical, mechanical, magnetic, thermal, and/or imaging properties, which are highly desirable in the military, medical, commercial, and environmental sectors. Till date, many nano- materials are synthesized and can be categorized as carbonaceous (carbon based) or non-carbonaceous. Often non-carbonaceous nanomaterials mixed with other nanomaterials or macroscopic carbonaceous or non-carbonaceous materials are used for the synthesis of nanocomposites. In these subsections, all of these materials will be briefly described.