**4. Strategies to control climate change**

Global climate change, also called global warming or the greenhouse effect, may be the most significant problem ever faced by humankind. Global climate change is caused by adding certain gases including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and many chlorofluorocarbons (CFCs). The added gases absorbed infrared radiation leading to excess thermal energy within the earth's biosphere. The largest driver of global warming is carbon dioxide (CO2) emissions from fossil fuel combustion, cement production, and land use changes such as deforestation. Therefore, the strategies to control CO2 emission are also the main strategies to control global climate change.

#### **4.1. Upstream control**

energy of the TiO2, an electron could be excited from the valence band to the conduction band of TiO2, leaving holes behind on the TiO2 surface. The leaved holes could react with surround‐

oxidation reaction to destroy many organic contaminants (CxHyOz) completely [26]. The

\*

*OH C H O xCO yH O*

2 2

... *x yz*

+ ®++

The three common reactor types designed to use a photocatalyst for air purification purposes are the honeycomb monolith, fluidized-bed, and annular reactors [27]. A honeycomb monolith reactor contains a certain number of channels, each of which typically has an internal dimen‐ sion of the order of 1 mm. The cross-sectional shapes of the channels are square or circular. The photocatalyst is coated onto the walls of channels in a very thin wash coat. Fluidized-bed reactors are designed to treat a high gas feed rates directly passing through the catalyst bed. Based on reactor design, the solid photocatalyst could directly contact with the UV irradiation as well as gaseous reactants. The fluidized-bed reactors generally consisted of two concentric cylinders, which form an annular region with a certain gap. The photocatalyst is deposited onto the interior wall of the outer cylinder. The light source is usually located at the center. The thickness of the deposited photocatalyst film is sufficiently thin ensuring that all of the

The applications of the photocatalyst for photocatalytic oxidation processes to reduce air pollutants have been considered as alternatives to conventional air pollution control technol‐ ogies. However, they have yet to overcome the problems of low energy efficiency and poor cost competitiveness. Therefore, numerous methods for modifying photocatalysts have been developed and investigated to accelerate the photo-conversion, enable the absorption of visible light, or alter the reaction mechanism to control the products and intermediates [29]. In this regard, metals or nonmetals were used as doping agents to implant or coprecipitate on the surface or in the lattice of TiO2. Electron donors or hole scavengers have been added to such photocatalytic systems. In addition, another semiconductor was integrated with TiO2 to establish a suitable two-semiconductor system [29]. The modifications not only change the mechanism and kinetics of the photocatalytic processes under UV irradiation but also enhance the photocatalytic activities of the photocatalyst, thereby enabling the photocatalytic oxidation

−


OH), while the excited electrons could react with O2

). These oxy radical species can participate in the

ing H2O to produce hydroxyl radicals (\*

242 Current Air Quality Issues

to produce superoxide radical anions (*o*<sup>2</sup>

photocatalytic process mainly follows the following reactions:

2

2 2 \*

photocatalyst could be illuminated by UV irradiation [28].

processes to proceed even under visible light [30].

2 2 2 2 2 2 \*

*H O OH*

*eO O*

+ ®

*TiO h e h h H O H OH*

+®+ + ®+

n

+ + - - -- +

2 2

++ ® ®

*O e H HO*

Upstream strategies to control CO2 emission include energy conservation, alternative and renewable fuels, and oxy-fuel combustion.

#### *4.1.1. Energy conservation and efficiency use*

Energy conservation refers to reducing energy consumption through using less of an energy service. The strategies concerned to energy conservation include energy taxes, building design, transportation, and consumer products.

**•** Energy taxes

Some countries employ energy or carbon taxes to motivate energy users to reduce their consumption.

**•** Building design

Energy conservation in building could be improved by using of an energy audit, which is an inspection and analysis of energy use and flows in the building, process, or system to reduce the amount of energy input into the system without negatively affecting the output(s). In addition, a passive solar building design in which windows, walls, and floors are made to collect, store, and distribute solar energy in the form of light or heat in the winter and reject solar heat in the summer. The design, leading to decreased use of mechanical and electrical devices, is also a solution for energy conservation.

**•** Transportation

The zoning reform and designs for walking and bicycling could allow greater urban density leading to reduce energy consumption concerning to transportation. The application of telecommuting is also a sufficient opportunity to conserve energy. For example, with people who work in service jobs, they could work at home instead of commuting to work each day.

**•** Consumer products

Because the consumers usually lack the information concerning to saving by energy-efficient products, we must inform the consumer in understanding the problems. For example, many consumers choose cheap incandescent bulbs, failing to take into account their higher energy costs and lower lifespan. However, as compared to modern compact fluorescent and LED bulbs, these products have a higher upfront cost, with their long lifespan and low energy.

### *4.1.2. Alternative and renewable fuels*

Use of alternative energy sources could prevent CO2 emission from fossil fuel. The alternative energy sources include wind, solar, hydropower, biomass, and geothermal energy.

**•** Wind energy

Airflows can be used to run wind turbines to produce electric energy. Globally, the long-term technical potential of wind energy is believed to be five times total current global energy production, or 40 times current electricity demand.

**•** Solar energy

Solar energy, radiant light and heat from the sun, is harnessed using a range of ever-evolving technologies such as solar heating, photo-voltaic, concentrated solar power, solar architecture, and artificial photosynthesis.

**•** Hydropower

Hydropower is the power derived from the energy of falling water and running water, which may be harnessed for useful purposes. Hydropower could be also captured from ocean surface waves and tidal power.

**•** Biomass

Biomass is the biological material derived from living or recently living organisms. Biomass could be used as energy source by either used directly via combustion to produce heat or indirectly after converting it to various forms of biofuels.

**•** Geothermal energy

Geothermal energy could be generated from thermal energy, which is stored in the earth. Because of the difference between temperature of the core of the planet and its surface, it drives a continuous conduction of thermal energy in the form of heat from the core to the surface.

#### *4.1.3. Oxy-fuel combustion*

When burning coal or other fossil fuel using ambient air, the air contains a huge amount of nitrogen as well as the oxygen needed for combustion (4:1). Raising the temperature of the nitrogen to the combustion temperature requires a great deal of heat. Therefore, reducing nitrogen content in the air input could be a good strategy to reduce fuel consumption, leading to reduced CO2 emission. In the strategy, oxy-fuel combustion, a process of burning a fuel using pure oxygen instead of air as the primary oxidant, is applied. There are several researches being done in firing fossil-fueled power plants with an oxygen-enriched gas mixture instead of air. Almost all of the nitrogen is removed from input air, yielding a stream that is approxi‐ mately 95% oxygen.

#### **4.2. Downstream control**

consumers choose cheap incandescent bulbs, failing to take into account their higher energy costs and lower lifespan. However, as compared to modern compact fluorescent and LED bulbs, these products have a higher upfront cost, with their long lifespan and low energy.

Use of alternative energy sources could prevent CO2 emission from fossil fuel. The alternative

Airflows can be used to run wind turbines to produce electric energy. Globally, the long-term technical potential of wind energy is believed to be five times total current global energy

Solar energy, radiant light and heat from the sun, is harnessed using a range of ever-evolving technologies such as solar heating, photo-voltaic, concentrated solar power, solar architecture,

Hydropower is the power derived from the energy of falling water and running water, which may be harnessed for useful purposes. Hydropower could be also captured from ocean surface

Biomass is the biological material derived from living or recently living organisms. Biomass could be used as energy source by either used directly via combustion to produce heat or

Geothermal energy could be generated from thermal energy, which is stored in the earth. Because of the difference between temperature of the core of the planet and its surface, it drives a continuous conduction of thermal energy in the form of heat from the core to the surface.

When burning coal or other fossil fuel using ambient air, the air contains a huge amount of nitrogen as well as the oxygen needed for combustion (4:1). Raising the temperature of the nitrogen to the combustion temperature requires a great deal of heat. Therefore, reducing nitrogen content in the air input could be a good strategy to reduce fuel consumption, leading to reduced CO2 emission. In the strategy, oxy-fuel combustion, a process of burning a fuel using pure oxygen instead of air as the primary oxidant, is applied. There are several researches being done in firing fossil-fueled power plants with an oxygen-enriched gas mixture instead of air. Almost all of the nitrogen is removed from input air, yielding a stream that is approxi‐

energy sources include wind, solar, hydropower, biomass, and geothermal energy.

*4.1.2. Alternative and renewable fuels*

production, or 40 times current electricity demand.

indirectly after converting it to various forms of biofuels.

**•** Wind energy

244 Current Air Quality Issues

**•** Solar energy

**•** Hydropower

**•** Biomass

and artificial photosynthesis.

waves and tidal power.

**•** Geothermal energy

*4.1.3. Oxy-fuel combustion*

mately 95% oxygen.

Technologies to downstream control CO2 or remove CO2 from atmospheric include biological capture, wet scrubbing, CO2 absorption, and mineral carbonation.

#### *4.2.1. Biological capture*

Biological capture of CO2 is a process in which photosynthetic organisms are used to absorb the CO2 gas from air and convert it into a solid carbonaceous compound. The strategies to conduct for biological capture of CO2 include:

**•** Reforestation

Reforestation means that tree could be replanted on marginal crop and pasturelands leading to incorporate atmospheric carbon (CO2) into biomass. For a successful process, the incorpo‐ rated carbon could not return to the atmosphere from burning or rotting when the trees die. Finally, the trees grow in perpetuity or the wood from them must itself be sequestered in the forms of biochar or bio-energy with carbon storage or landfill.

**•** Agriculture

Agricultural activity to capture CO2 is also called as "capture" energy of the sun. Under solar light, the artificial plants could capture and convert the CO2 in atmosphere into biomass, which can be storage or used as food or also used as raw material to make biofuels to replace the use of fossil fuel. Land-based plants such as corn and soybeans can be grown as energy crops, in particular to make biodiesel. Because of the limitations of land-based plants, there has been much interest over the years in developing systems that utilize microalgae for engineered biological CO2 capture systems. Microalgae can fix CO2 up to ten times faster than trees, and utilize sunlight much more efficiently than the land-based energy crops.

#### *4.2.2. Wet scrubbing of CO2*

A carbon dioxide scrubber, which uses various amines such as monoethanolamine as absorb‐ ents, could absorb CO2 to capture them from the atmosphere. The design and principle of wet scrubber have been presented in Section 3.2.2. Amines could be used as absorbents to absorb CO2 based on following reactions:

> 2 22 32 3 32 3 2 2 3 3 2 2 3 2 ( ) ( ) 2 2 *RNH CO H O RNH CO RNH CO CO H O RNH HCO RNH O RNHCOONH R* ++® ++® + «

#### *4.2.3. Mineral carbonation*

Many chemical processes, known as carbon sequestration by mineral carbonation or mineral sequestration, could capture and store CO2 from the atmosphere in stable carbonate mineral forms. In the process, CO2 could react with abundantly available metal oxides such as MgO or CaO to form stable carbonates. These reactions are mostly exothermic and occur naturally.

$$\begin{aligned} \text{CaO} + \text{CO}\_2 &\rightarrow \text{CaCO}\_3\\ \text{MgO} + \text{CO}\_2 &\rightarrow \text{MgCO}\_3 \end{aligned}$$

CO2 could also react with calcium and magnesium silicates including forsterite and serpen‐ tinite by the following the reactions:

$$\begin{aligned} Mg\_2SiO\_4 + 2CO\_2 &\rightarrow 2MgCO\_2 + SiO\_2\\ Mg\_3Si\_2O\_5(OH)\_4 + 3CO\_2 &\rightarrow 3MgCO\_3 + 2SiO\_2 + 2H\_2O \end{aligned}$$

These reactions are slightly more favorable at low temperatures. This process occurs naturally over geologic time frames and is responsible for much of the earth's surface limestone.
