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Highway Tunnel CO and CO2 Filtration
Traffic tunnel air quality improvements were made recently by one of the largest gas treatment plants in the world for removal of nitrogen oxides (NOx) from vehicle waste combustion gas inside a major inner-city tunnel. The effort was completed inside the M30 Tunnel de Sur, in Madrid, Spain. The project was designed, manufactured, and installed by industry leaders GH Anderl Consulting and Construction, Clean Tunnel Air International, based in Norway, www.cta.no, and Sweden's Camfil Farr, www.camfilfarr.com.
Newest Quality Innovation
To improve tunnel air quality further, we offer filter banks that control concentrations of carbon monoxide (CO) and carbon dioxide (CO2). These emissions by vehicles due to incomplete combustion of diesel or gasoline are not being addressed in world tunnels today. Air pollution by carbon species becomes concentrated in the confined space of road tunnels, escalating pollution levels during rush hour traffic. The high concentrations of CO and CO2 and associated health risks of these carbon species may be avoided as we found by our research and development at Industrial Environmental Carbon (IEC), of Huntington, Vermont, USA.
A large portion of energy entering a modern engine is wasted. Roughly 38% goes out the tailpipe. Chemical waste including carbon monoxide (CO), nitrogen (N2), oxygen (O2), hydrocarbons (HC), hydrogen (H2), nitrates (NOx), sulfates (SOx), carbon soot (C), water vapor with CO2 and CO2 neat are all waste products.
Among vaporous species many hundreds are produced. The difficulty of realistic numerical calculations involves the amount of reactions over a range of time-scales, 10E-9 to 1.0 s (See: Curran, H.J. et al., Combustion and Flame, 129, pp 253-280, 2002; and Ren, Z., Pope, S.B., Geometry of Reaction Trajectories and Attracting Manifolds in Composition Spaces, Combustion Theory and Modeling, 10, 361-388, 2006).
Buffer molecules HCO3 (-) are common as bicarbonate ions. Furthermore, high temperature gives rise to orbital bonding and dissociated transfer resulting in one configuration to the possibility of another.
Exhaust products include carbonic acid (H2CO3). Involved too, are common radicals of hydrocarbons with their various derivatives containing oxygen and nitrogen atoms.
The reference, Evolving criteria: Tunnel Ventilation System Technology Preview and Best Practice, cites tunnel pollution particulate matter (PM) down to about 2.5 micron limitation including CO and NOx.
Very little is mentioned in current literature about CO specifications, and though relevant, no specifications or regulations have been located for CO2 contaminant removal for road tunnels. Therefore, our patent, issued March 29, 2011, appears to be the first and only real effort available.
Further improvements regarding traffic emissions in tunnel environments involve pollutant concentrations of carbon monoxide (CO) and carbon dioxide (CO2). These emissions by vehicles due to incomplete combustion of diesel or gasoline are not being addressed in world tunnels today.
The missing technology is now patented and commercially available for the first time.
RE: MURRAY, et al. - New US Appln. No. 13/073,175 CAPTURED CO2 FROM ATMOSPHERIC... - Our Ref.: P09629US02/DEJ
Additionally, emission removals for carbon monoxide (CO) and associated combustion greenhouse gases, carbon dioxide (CO2), while developed by our company and patent protected, according to Annexure 8 (above), other considerations are in development phases with safety issues being cited. These can be found in Tunnel Ventilation System Technology Review and Best Practice, Section 3.3: Emerging Technologies.
Catalyst Systems: Observation and Chemical Analysis
By comparison, IEC clean airstream methods are solid after trials scaled for exhaust applications. The CO and CO2 control experiments for traffic tunnel ventilation were developed in hostile environments involving extreme heat, velocity, high carbon content (CO2 at 14% continuous volume flow – CO at 7000 ppm continuous volume flow).
The catalysts (hydroxides of metal salts) absorb nearly 50% of the carbon species in the total volume flow at low velocity. This value drops to 30% capture efficiency at higher velocity.
Tunnel filters are made to trap CO and CO2 from tailpipe exhaust. We do this by offering adsorbents. The element potassium (K)(+) is used in combination with the hydroxide radical OH (-) or element Ca (+) with OH may substitute.
The materials, commercially available at about one USD/pound, readily absorb carbon species in the confines of the waste stream – if – placement of the filter, size and shape of filter are carefully considered.
Materials and Methods
Leftover waste from combustion exhaust including
H2O, CO, CO2 degrades air quality. The
water molecules contribute to the relative humidity (RH) in tunnel enclosures. This is an
advantage. The IEC design depends on a portion of RH to capture quantities of carbon oxides.
Before tunnel air leaves the filter banks, carbon species and water molecules react with core
matrix material.
We use stainless steel (SS) support and a mix of potassium hydroxide (KOH) flakes. The KOH charge is enclosed within the SS support insert.
Vehicle exhaust CO, CO2 and H2O in the tunnel airstream meets the KOH charge in a flow-by reaction. Immediately the dominate molecule formed is a water soluble one.
The molecule is potassium bicarbonate (KHCO3) ion. These water soluble ions with additional exposure to KOH + CO2 + RH become locked into solid potassium carbonate (K2CO3).
Measurements: Carbon Capture Efficiency
Appropriate instruments measure before and after, trace greenhouse gas emissions CO, CO2, CH4 and H2O and record efficiency data which provides a history of operation as well as an indication when filter maintenance is required. Carbon capture efficiency (catalyst-dependent) is a function of the amount of CO2 collected in the final form CaCO3 or K2CO3.
When saturated, the filter core is removed and replaced by a recycled or new cells of fresh catalyst material.
Dry Absorption Method
The tunnel air is received first by a stainless steel (SS) mesh that covers filter containers facing upstream air. After the SS mesh the airflow meets the KOH absorbent media where immediately reactions evolve KHCO3 first, then K2CO3. Significant carbon, both oxidized and reduced are trapped in this manner to become locked into solid carbonate. The remainder of the tunnel air continues through a downstream SS mesh before exiting containers.
Each filter is unit of uniform cells housed inside a container. In this case, the container is sized 600 mm X 600 mm X 300 mm accordingly. The containers are stacked in the ventilation space provided. The stack is secured to the station deck as recommended.
The design of the container is to enclose a portion of 16 cylinder cells with each cell providing a high surface area exposed to the untreated airstream.
The filter stack is accessible for filter change-out maintenance by IEC trained personnel.
The tunnel air is measured by performance instruments before filtration units and after to ensure operation integrity.
The costs for the tunnel-customized instrument design, construction, test and installation are optional.
Wet Filter Air Treatment
Alkaline water treatment aerosols will absorb the gases into the carbonate precursor KHCO3 resulting in quantitatively collected CO2 measurements by dissolved oxygen method. The solution circuit’s resulting carbonate - evidence of capture efficiency – is data which agree with laser NDIR measurements.
Both non-dispersive infrared, and carbonate mass values support the findings - efficiency is about 30% of the CO2 continuous flow in the waste column at high velocity where the total volume flow contains a measured 14% CO2.
Carbon Monoxide
Carbon monoxide (CO) is continuously emitted by car exhaust at about 7000 parts per million (ppm) by volume. The nature of CO is to produce gases. So, the molecule changes rapidly when conditions are favorable which in our case they are. CO has a triple bond and is only partially oxidized. It may be the strongest bond in nature; however, it will readily oxidize to CO2. Carbon monoxide forms when there are not enough oxygen atoms (oxygen are usually in pairs) to form carbon dioxide.
Further, CO is a slightly polarized. There are no unpaired bonds, but carbon and oxygen together have a total of 10 valence electrons with 6 shared electrons in 3 bonding orbital's; since 4 of the shared electrons come from oxygen and only 2 from carbon, so a small negative charge on carbon and a small positive charge on oxygen. In fact, CO exists in equilibrium with C doublebond O and C+O-.
The arrangement works well with many other species in the immediate area (H2O, O3, O2, K+, CA+, OH-); with CO readily forming preferred covalent bonding with more electronegative or electropositive species present. These offer persuasion. For example water. Water being slightly negative on one end and slightly positive on the other, we use to our advantage when filtering CO and CO2.
Therefore, sufficient CO has opportunity to change favorably under conditions we provide; that is relative humidity (H2O) and the presence calcium+ or potassium+ each with a single valence electron, in addition hydroxide (OH-).
CO + 2O2 --> CO2 + O3
Carbon Dioxide
Ozone (O3) is a strong oxidizer and will cleave double-bonds. O3 having a short life-span, at least in the Troposphere, is not an issue with road tunnel air CO, CO2 filtration.
Bonds broken by force (internal combustion) re-bond; the job of a CO, CO2 filter is to persuade left-over species in the waste stream of a traffic tunnel is to re-bond again by offering a nonkinetic alternative – carefully chosen hydroxides of Group I and II metals placed in fluid pathways provide persuasion toward other orbital gains and losses.
One method is to introduce K+ and OH- to evolve KHCO3 first, then K2CO3 with additional CO2 + KOH + H2O; this goes to K2CO3 as an alternative to some of the thermo chemical species concentrated in tunnel air. The reaction:
K2CO3 + 1.5H2O + CO2 
2KHCO3 + 0.5H2O + heat
Ebune, G., suggests the empirical relation of moisture plays a significant role in the chemical adsorption process (See: Ebune, Guilbert Ebune, Carbon Dioxide Capture from Power Plant Flue Gas using Regenerable Activated Carbon Powder Impregnated with Potassium Carbonate, ohiolink.edu/etdc 2008). Moisture in flue gases as well as vehicle exhaust may be as high as 17%. The value will effect yield of K2CO3. Ebune dried samples saturated by CO2 after 6 hours and gave an absorption capacity range:
mol CO2 2.5 to 3.5 / mol K2CO3
Most vehicles today run with an Air/Fuel combustion ratio that produces large amounts of CO2 and water vapor. This is considered good by industry standards. The air/fuel ratio is "ideal" (Toyota Motor Corporation Sales, USA), at 14.7/1. This is the A/F ratio “where the CO2 production is highest” and where “CO2 is an excellent indicator of efficient combustion.”
Thermal Analysis
Winter traffic conditions in tunnels are not expected to stop carbon capture.
Henry’s Law states: as pressure above water increases, the concentration of the gas increases. For example, gasses become more soluble; and the solubility of CO and CO2 in water increases as the temperature drops.
Specific quantitative analysis of rate of reaction of CO2 sequestration in metropolitan tunnels in cooler temperature environments must be continuously studied, and evaluated under more extreme temperature conditions. There is not enough evidence, however, to suggest that significant sequestration of CO2 would be affected by winter weather.
KOH is a stable at decreasing temperatures. As temperature levels decrease, the successive stable forms of KOH are the mono-, sesqui-, di-, and tetra-hydrates which remain stable at intervals down to extremely low temperatures of -66 C (See: Lang, A., et al, System KOHK2CO3- H2O At Low Temperatures: I. Phase Equilibria1, NRC Research Press, 2011. http://www.nrcresearchpress.com/doi/pdf/10.1139/v58-153).
In winter, in the Okhotsk Sea, atmospheric CO2 may be actively absorbed; because cooling lowers the fCO2 of surface water and air-sea interface turbulence of wind and waves would enhance gas exchange. This has been investigated by Otsuki, Akihisa, Watanabe, Shuichi, and Tsunogai, Absorption of Atmospheric CO2 and its Transport to the Intermediate Layer in the Okhotsk Sea, Journal of Oceanography, Vol. 59, pp. 709 to 717, 2003) and found to be true: efficient and intense gas exchange due to the cold and stormy winter conditions prevail in the western Okhotsk Sea and the northeastern Japan Sea, similar to conditions in the Arctic or Antarctic “polar seas” at mid-latitudes. So, it follows, CO2 is actively absorbed by a decrease in temperature, and cooling surface water enhances gas exchange at the air-sea interface. High temperature of these waters will decrease production of water density and increase surface water temperature fCO2.
Tunnel Design, Installation, and Test
A detail of the design for tunnel CO and CO2 filtration is shown. The drawing, CO2 filter placement, and the number of filters in the air flow are reveled.
The filter arrangement features cylinders (or cells) of 16 per container. The actual number of containers and how high they are stacked may be revised and is contingent upon space available.
The necessary number of cylinder cells and their dimensions are known, namely 16 cells per 600 mm X 600 mm X 300 mm segmented container; an arrangement whereby CO2 filter cells fit inside container segments.
IEC capabilities in this regard include: contractual design, build, ship, install, final test, and maintenance.
The result is a configuration of IEC carbon oxide filters similar in size to conventional, mechanical filter/pre-filter systems located in the upstream portion of each station.
For final test or commissioning, precision measurements and data retention are required. Responsible, continuous mode sampling of tunnel air conditions is expected for quality control.
Additional design / build / installation of optional, NDIR units may be fitted for tunnel operation. Analyses of upstream gases in comparison with out-going gases are recorded for historical and engineering purposes. Sensors and actuators are available for off data monitoring.
Example formula:
C8H18 + 2O2 ---> energy + CO2 + 2H2O [add KOH] CO2 + 2H2O ---> KHCO3 [add KOH] KHCO3 + H2O ---> K2CO3 carbonate
Tunnel Air Contamination Control
Vented exhaust treatment systems patterned to these specifications are sufficiently sized and positioned to establish, meet and exceed standards for combustion carbon gas removal. The contamination and molecular removal system are safe and ready for road tunnel air with concentrated vehicle or train exhaust to clean air and make a noticeable, quantifiable and positive difference in air quality.
Natural Storage for CO2
Carbonate is a natural process for carbon sequestration. An abundance of safely stored, natural CO2 are found in carbonate formations of potassium (K+) or calcium (Ca+) world-wide. These are chalk deposits from the Cretaceous Period. When CaCO3 is compressed limestone is formed. When limestone is compressed metamorphic marble is formed.
The intent for IEC carbon capture in traffic tunnels is to provide safe storage: one additional segment to the already established array of continuous carbon sinks nature has perfected over millions of years.
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