These measures are only a recommendation, and states may develop their own RACT requirements on a case-by-case basis, considering the economic and technical circumstances of individual sources. No Federal laws or regulations preclude states from requiring more stringent controls than those recommended as RACT. Some states may need additional control in order to meet the national ambient air quality standards NAAQS for ozone in some areas.
The key to ensure that VOCs are mitigated is to select compounds that do not contain solvents or high levels of VOCs in them to begin with. In order to activate oxygen for the VOC oxidation by noble metal catalysts supported on conventional substrates SiO 2 , Al 2 O 3 and TiO 2 , the electronic structure or chemical state of the metal must be changed by modifying the particle size, morphology or structure.
For noble metal catalysts supported on transition-metal oxides, the activation of oxygen can be reached by growing the adsorbed oxygen quantity and enhancing the lattice oxygen mobility by doping the substrates with other transition metals [ 42 ].
Among noble-based nano-catalysts, Pt-based nanomaterials had attracted attention as catalysts for VOCs benzene, toluene, xylene, formaldehyde and methane , being able to be easily dispersed on porous substrates. Porous SBA silica with high specific surface was used for Pt loading by impregnation or deposition-reduction, the resulted catalysts being applied for the benzene oxidation [ 51 ].
The VOCs catalytic oxidation is highly dependent on the size of Pt particle. It was observed that Pt Some 1 wt. Pd noble metal has relatively lower activity comparing with Pt in catalytic oxidation of most pollutants, but it is used as an active ingredient in various catalysts due to its high catalytic efficiency in the oxidation of toluene [ 53 ] or halocarbons [ 54 ].
Au was initially considered to be a poor catalyst due to its chemical inertness molecules such as oxygen or hydrogen. However, Au nanoparticles possessing unpredictable and unique catalytic features have been intensively studied for VOC mitigation [ 47 ]. Ag also attracts great interest in the field of VOCs mitigation, especially for the catalytic oxidation of formaldehyde, even though its activity is lower than Pt, Pd or Au.
To improve indoor air quality, catalytic nanomaterials can be used for VOCs reduction from coatings, paints, air filters, building materials, etc. Due to their characteristics such as non-toxic, high chemical stability, low cost, metal oxides such as TiO 2 and ZnO have been widely investigated and dominate the field of VOCs removal application in buildings.
Other nanocatalysts have shown high performance and have been theoretically and lab studied, being considered potential candidates for applications in future environmentally friendly and healthy buildings. Their development will contribute to the additional knowledge and potential in the application of catalytic nanomaterials for VOCs reducing or total removal, so that buildings will be healthier and will contribute to a better indoor and outdoor environment.
Photocatalysis based on TiO 2 or TiO 2 -supported metal catalysts is also intensively studied as a green environmental remediation technique for VOCs removal. The photocatalyst are mainly influenced by the structure and morphology [ 55 ] or by the treatments applied on TiO2 particles [ 56 ].
The influence of synthesis parameters variation on the size and morphology of the TiO 2 nanoparticles was studied; the trichloroethylene degradation over TiO 2 catalyst reached a maximum value at particle size of 7 nm [ 55 ]. The photocatalytic efficiency of TiO 2 nanotubes and TiO 2 nanoparticles on degradation of gaseous toluene and acetaldehyde was also investigated [ 57 ].
The photocatalytic activity of TiO 2 nanotubes was slightly reduced after the five cycles in toluene degradation, compared with TiO 2 nanoparticles which rapid deactivated as after the cycles were repeated.
The explanation resides in the highly ordered open channel structure, TiO 2 nanotubes being able to rapidly provide O 2 molecules to the active sites, preventing the accumulation of carbonaceous residues on the nanotubes surface [ 57 ].
It can be concluded that the structural features of the nanotubes prevent the catalyst deactivation during the photocatalytic removal of the aromatic compounds. The photocatalytic performance of nanotubes containing TiO 2 TNT was compared with the one of TiO 2 nanoparticles TNP film during the repeated cycles of photocatalytic degradation of gaseous toluene and acetaldehyde, resulting that the nanotubes were more efficient for the selected VOCs removal [ 57 ].
Novel nanostructured gas filtering systems based on TiO 2 thin films were developed using atomic layer deposition ALD for VOCs removal, showing higher efficiency for the adsorption of toluene [ 58 ]. Freestanding doubly open-ended TiO 2 nanotubes DNT film were also synthesized, showing an improved performance and durability for the photocatalytic degradation of acetaldehyde and toluene in gaseous phase than classical TiO 2 nanotubes [ 59 ].
As an alternative to TiO 2 , zinc oxide is a fast and efficient chemical decontamination nanomaterial used for VOCs mitigation [ 60 ].
Three synthesis methods for preparing ZnAl 2 O 4 solvothermal, citrate precursor and hydrothermal methods were compared for the photocatalytic degradation of toluene in gaseous phase [ 61 ]. The photocatalytic oxidation of gaseous pollutant over UV-illuminated ZnAl 2 O 4 proved to be a promising technique for air cleaning.
Furthermore, a novel composite catalyst was developed for long-lifetime removal of formaldehyde, by loading manganese oxide MnO x catalysts on a polyacrylonitrile-based activated carbon nanofiber PAN-ACNF support [ 63 ].
Other oxides were also studied, for example iridium oxide particles supported on SiO 2 and they were used for the total oxidation of VOCs [ 64 ]. The obtained results showed that the catalytic activity increased when the size of iridium particles decreased. Graphene and graphene oxide GO have attracted much interest as a proficient matrix for gaseous pollutants adsorption, due to their characteristics, including obtaining, high surface area, pores structure and size, high chemical stability and thermal stability [ 66 ].
The activities of three graphene-based co-catalysts graphene oxide, reduced graphene oxide, and few-layer graphene were tested on gas-phase photocatalytic oxidation of methanol, the reduced graphene oxide having the highest performance with the best rate of conversion [ 68 ]. Additionally, the use of 5. The graphene oxide and reduced graphene oxide had also been studied for the toluene removal [ 70 ]. It was observed that the specific surface area of the nanomaterials also had the same trend.
GP had the maximum selectivity towards toluene, its higher graphitic character being responsible for the increased adsorption to specific surface area ratio. However, rGOMWKOH presented the maximum adsorption capacity for toluene, probably due to its higher specific surface area, having similar adsorption efficiency with active carbons in terms of toluene removal [ 71 ].
Ethanol an alcoholic VOC removal by adsorption on graphene materials was investigated. These nanomaterials were applied as composites with MOFs metal organic frameworks [ 69 ], the high specific surface, porous structure and availability of oxygen functionality resulting in Graphene nanomaterials had also been used for the carbonyl VOCs removal.
The second material was characterized by high concentration of amine groups on the surface comparing with the first one, resulting in its high interaction with the formaldehyde Figure 2.
Amino graphene nanodots decorated functionalized graphene sponge—interaction with formaldehydes [ 73 ]. Other studies reported the use of amino-functionalized graphene aerogel as a composite with carbon nanotubes in order to remove gaseous formaldehyde [ 74 ].
The adsorption of formaldehyde was both chemical and physical. The chemical adsorption process was due to the van der Waals forces between the amino groups of graphene and the carbonyl group from formaldehyde.
The potential mechanism of carbon nanotubes-enhanced graphene aerogel for formaldehyde removal [ 74 ]. Graphene nanomaterials were also tested for the removal of chlorinated VOCs, such as methylene chloride or carbon tetrachloride. Summarizing, the use of graphene nanomaterials highlighted great potential for various VOCs mitigation.
However, these nanomaterials can be underperforming in practice, where the concentration of the pollutants is not as high as in laboratory experiments. To avoid such milestone, it is important to assess the nanomaterials efficiency by reducing the sources of bias. The detection and analysis of VOCs from environment is a major provocation due to the issues raised from their sampling and actual analysis.
These VOCs can be theoretically removed, among other classical techniques, by adsorption, catalysis or photocatalysis. The regeneration and reusability of any adsorbent or catalyst are crucial parameters in evaluating the operational costs and viability for industrial uses.
Catalytic and photocatalytic nanomaterials are gaining interest for VOCs mitigation. Among them, up to now, TiO 2 is the most common, efficient and economical due to the low cost, high chemical stability, and low toxicity, being applied in the catalytic VOCs removal from buildings.
As the photocatalytic efficiency of the metal oxides can be improved by combining with hybrid adsorbents, more catalysts nanomaterials are to be developed. In order to improve the removal of the indoor VOCs, more investigations should be achieved to grow the efficiency in particular conditions such as visible light.
One of the greatest issues is the deactivation of catalysts under real process operation. In order to surpass this milestone, a mandatory step in catalysts preparation must be their aging under harsh pre-treatment, in order to insure their durability and stability.
The understanding of the catalytic mechanisms, for example, the interface boundary sites and the synergetic effect, may be helpful for the development of highly efficacy and stable catalysts with interesting design and tailored functionalities. Up to now, the mechanisms for the catalytic oxidation of VOCs with small molecules have been intensively studied and most of the intermediates were identified. However, the mechanisms for catalytic oxidation of VOCs with large molecules are still under investigation due to the complicated reaction pathways.
The elucidation of the reaction mechanisms will also stimulate the developing of new and improved characterization techniques, especially for in situ analysis.
Furthermore, up to this moment, only few studies were performed for the catalytic oxidation of mixed VOCs from actual industrial processes and from indoor environments, the performance and mechanism being quite different from those performed in laboratory conditions using single VOC. Therefore, more academic and industrial research must be performed in order to be able to extensively apply nanomaterials for VOCs mitigation.
Screening the population for health risks must be periodically achieved, strengthening the awareness about the safe practices for waste disposal and indoor air pollution being able to minimize the risk for unintentional or intentional VOCs contamination.
All authors have read and agreed to the published version of the manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be considered as a potential conflict of interest.
National Center for Biotechnology Information , U. Published online Dec Paul B. Tchounwou, Academic Editor. Author information Article notes Copyright and License information Disclaimer. Received Nov 9; Accepted Dec Abstract Volatile organic compounds VOCs comprise various organic chemicals which are released as gases from different liquids or solids. Keywords: environment, nanomaterial, pollution, VOC.
Introduction Volatile organic compounds VOCs are organic chemical compounds found in various products that easily vaporise and reach in the environment under normal conditions. Open in a separate window. Figure 1. The main sources of VOCs are considered to be the following [ 26 , 27 , 28 ]: i Exploitation and use of fossil fuels, e. Table 1 Classification of VOCs pollutants. VOCs Impact VOCs have a variety of direct and indirect impacts on people and the environment and the main problems refer to: harmful effects on people health and on environment through toxicity; carcinogenicity and other adverse effects; the damage to materials; the tropospheric photochemical oxidant formation; stratospheric ozone depletion; global climate change; odour released.
Table 2 Noble metal-based nano-catalysts for VOCs oxidation. Full access to 1m statistics Incl. Single Account. View for free. Show source. Show detailed source information? Register for free Already a member? Log in. More information. Supplementary notes. Other statistics on the topic. Emissions U. Emissions Global CO2 emissions by select country Emissions Cumulative CO2 emissions from fossil fuel combustion worldwide , by country. Emissions Greenhouse gas emissions of major U.
Profit from additional features with an Employee Account. Please create an employee account to be able to mark statistics as favorites. Then you can access your favorite statistics via the star in the header. A bonfire is formed inside the vessel, and the heat transfer from the fire to the brick wall the brick wall occurs by convection. The challenge with this technique is that there is no precise control occurs by convection.
The challenge with this technique is that there is no precise control on the flame on the flame characteristics, and also the gas composition within the KOBM. The steelmaking ladles on characteristics, and also the gas composition within the KOBM. The steelmaking ladles on the other the other hand are preheated slowly, which lasts up to 40 h. During the first five h, the ladle is purged hand are preheated slowly, which lasts up to 40 h.
During the first five h, the ladle is purged with hot with hot air only. Afterwards, a natural gas burner is used as an energy source to preheat the refractory air only. It was assumed that the vessel is preheated using a natural gas burner from was considered. It was assumed that the vessel is preheated using a natural gas burner from the top the top mouth of the KOBM vessel over a duration of 24 h. The two scenarios are schematically shown mouth of the KOBM vessel over a duration of 24 h.
The two scenarios are schematically shown in in Figure 1. The first procedure reflects current field practices with the second procedure designed to Figure 1. The first procedure reflects current field practices with the second procedure designed to assess the effects of a slower preheating rate on minimizing thermal shock as well as the capability of a assess the effects of a slower preheating rate on minimizing thermal shock as well as the capability top-burner to combust released VOCs and sulfur compounds prior to emission from the furnace.
Figure 1. Showing two preheating scenarios: a conventional injection of fuels through the bottom Figure 1. Showing two preheating scenarios: a conventional injection of fuels through the bottom tuyeres , b proposed preheating approach injection of fuels through a top burner.
The most popular preheating technique for ladle preheating is the use of gas burners, whereThe enthalpy from the preheating most popular combustingtechnique gasses is transformed to heat. Inis the for ladle preheating the steel use industry, natural where of gas burners, gas is often replaced by product gases such that blast furnace gas and coke oven gas [11].
In the preheating enthalpy from the combusting gasses is transformed to heat. In the steel industry, natural gas is often station, replaced radiation heatgases by product loss is prevented such by furnace that blast carefullygas using and ladle coke lids oven [12].
As conventional gas [11]. As heating efficiency ladle such as air preheating, and oxygen enrichment resulting in an increase in combustion temperature preheating has very low efficiency, several efforts have been made to improve its heating efficiency and decrease in gas flow rate [12—14].
In premixed burners, fuel and oxidizer are completely mixed such as air preheating, and oxygen enrichment resulting in an increase in combustion temperature before combustion takes place. As a result, it produces shorter and more intense flames. However, and decrease in gas flow rate [12—14]. In premixed burners, fuel and oxidizer are completely mixed in industrial heating plants, a different type of burner, i.
However, in nonpremixed combustion takes place, as gas and combustion air are not mixed until they leave the industrial heating plants, a different type of burner, i. The fluid mixing process is controlled by designing the nozzle in such a way that it nonpremixed combustion takes place, as gas and combustion air are not mixed until they leave the burner ports. The fluid mixing process is controlled by designing the nozzle in such a way that it significantly decelerates its combustion reactions.
Diffusion burners typically have longer flames with a lower maximum temperature and with uniform heat distribution [11,15,16]. Other types of burners are the staged burners. Secondary and sometimes tertiary injectors are used to supply a portion of the fuel or the oxidizer into the flame and downstream to the root of the flame.
This type of burner is often used to produce longer flames, more control of heat transfer, and control of NOX formation. As a result, it produces a lower peak flame temperature with a uniformly distributed heat flux [16]. In major industrial burners, air acts as an oxidizer and pure oxygen is used to produce a high temperature and cause melting to take place [17,18].
Flameless combustion, i. The use of oxyfuel instead of air-fuel makes the combustion of low calorific fuels viable which is attractive and comparable with air-fuel combustion [21,22]. Other appropriate technology is a combination of air and oxygen, referred to as oxygen-enriched air combustion [21,23].
As a result, flameless combustion takes place in the form of a glowing ceramic foam that can be used as a radiating surface for a homogeneous heat source. In the present work, heat transfer and field VOC emission rates from a steel plant during preheating of a KOBM furnace were combined with existing thermodynamic models into a finite difference computational model in order to obtain a more complete understanding on VOC emission rates at different stages of the preheating cycle.
The heat transfer model describes the temporal variation of temperature as a function of heating flux with the thermodynamic model predicting the mass release of different VOCs as a function of temperature. Combining the two models in series, the overall model predicts the emission rates of different VOCs during the preheating process. The calculation was considered in a ton KOBM vessel with a brick lining thickness of 0.
The total concentrations of organic compounds in the bricks, contained primarily in the carbon-based binders along with the overall brick composition considered, are shown in Table 1. Table 1. Component Wt. Metals , 10, 4 of 13 2. Modeling Approach In the present study, two furnace preheating procedures were considered. In the second procedure considered, it was assumed that In the present study, two furnace preheating procedures were considered. The first procedure it was reflects assumed that the vessel is preheated using a natural gas burner from the top mouth of the KOBM current field practices with the second procedure designed to assess the effects of a slower preheating vessel rate on over a duration minimizing thermalof 24 h.
The shock as two wellscenarios are schematically as the capability shown in Figure of a top-burner 1. The first to combust released procedure reflects current field practices with the second procedure designed to assess the effects of VOCs and sulfur compounds prior to emission from the furnace. In either heating rate, brick preheating temperature, procedure, volatile and composition.
Table 1 shows the boiling and sulfur as an approximate indication of their respective volatilities. Heat Transfer Modeling 2. Heat Transfer Modeling The heat transfer model was used to obtain the temperature distribution through the refractory The heat transfer model was used to obtain the temperature distribution through the refractory wall and steel shell of the KOBM vessel.
The major heat transfer processes involved in the preheating wall and steel shell of the KOBM vessel. The major heat transfer processes involved in the preheating operation are shown in Figure 2, with radiative heat transfer occurring from the flame, and convective operation are shown in Figure 2, with radiative heat transfer occurring from the flame, and convective heat transfer occurring on both sides of the KOBM vessel walls.
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