NOx-limited

NOx limit of 190 ppm v/v (measured at 6 per cent oxygen 15°C and 1 bar without correction for water vapor), (approx. 200mg/m ) ... 

This behavior is consistent with the observations of a number of field studies. For example, Jacob et al. (1995) report that in Shenandoah National Park in Virginia (U.S.) in early September, there was a good correlation between 03 and NOz, with a slope of 18 compared to the range of 8.5-14 observed in other studies. In the latter part of September, the correlation was weaker (r2 = 0.23 vs 0.49 earlier) and the slope was only 7. This weakening of the relationship between 03 and NOz was accompanied by a decrease in concentrations of H202 from an average of 0.86 ppb to 0.13 ppb, as expected for a transition from the NOx-limited to the VOC-limited regime. 

NOx Control. NOx control limitations are described in both Tide 1 and Tide 4 of the CAAA of 1990. Tide 4 requirements affect only coal-fired boilers and take effect at the same time that the boilers are impacted by CAAA S02 requirements. As of 1996, EPA had established Tide 4 NOx limits only for tangentially fired and wall-fired, dry-bottom boilers that would be impacted by Phase I of the CAAA S02 regulations (Tide 4). Limits of 0.22 kg/106 kj (0.5 lb/106 Btu) and 0.19 kg/106 kj (0.45 lb/106 Btu) have been set for wall-fired and tangentially fired units, respectively. The EPA based these levels on what was achievable using low NOx burners. However, plants can employ a number of different front- or back-end emissions controls, including a combination of options, to achieve these levels. EPA plans to announce Tide 4 NO requirements for 300 additional boilers by late 1996 or eady 1997. 

The two large open circles on Figure 1 show the ROG and NOx levels chosen to serve as the base case in the incremental reactivity experiments that are currently underway in this chamber to assess ozone impacts of various different types of VOCs (Carter, 2004c). The 25-30 ppb NOx levels were chosen to be representative of pollution episodes of interest in California, based on input provided by the staff of the California Air Resources Board, which is funding most of the current reactivity studies. The ROG/NOx ratios were chosen to represent two sets of conditions of NOx availability relevant to VOC reactivity. The lower ROG/NOx ratio was chosen to represent the relatively higher NOx conditions of maximum incremental reactivity (MIR) where ozone is most sensitive to VOCs, to approximate the conditions used to derive the widely-used MIR ozone reactivity scale (Carter, 1994). The higher ROG/NOx ratio was chosen to be one-half that yielding maximum ozone levels, and is used to represent conditions where ozone is NOx-limited, but not so NOx-limited that VOC reactivity is irrelevant. These are referred to in the subsequent discussion as the MIR and MOIR/2 base eases, respeetively. 

Dependence of P0, on NO, Abundance in CO Oxidation One of the key aspects of tropospheric chemistry is the dependence of ozone production on the NO, abundance. We have derived relationships for Po, for the CO system in the limits of low and high NO,. Here we examine how Po, depends on the NO, abundance over the complete range of NO, levels. To do this, we will fix the rate of HO, production, Pho,. and vary the NO concentration at a fixed N02/NO ratio. Under conditions of high H02 radical abundance relative to NO, the primary chain-terminating reaction is the HO, + HO, reaction, HO2 + H02. This condition is referred to as NOx-limited. At sufficiently high NO, levels, chain termination results from the HO, + NO, reaction, OH 4- N02. This condition is called NOx-saturated. By varying the NO, concentration, we can explore the point at which the system crosses over from NO,-limited to NO,-saturated conditions. The crossover point occurs at the NO concentration where 3Poj/3[NO] = 0. The actual value of the NO concentration at this crossover point depends on the values of Pho, and the NO2/NO ratio. 

The essential role of NOx in ozone formation is evident in the CO oxidation mechanism (Section 6.4). For example, in the low NOx limit (NOx-limited), the rate of O3 formation increases linearly as [NO] increases and the rate is independent of [CO]. In the high NOx limit (NOx-saturated), the rate of O3 formation increases with [CO], but decreases as [NOx] increases. The explanation for the behavior in the high NOx limit is that, with ample NOx available, as NOx increases, the rate of the OH + N02 termination reaction increases, removing both HOx and NOx from the system, limiting OH - H02 cycling, and thereby decreasing the rate of 03 formation. 

In Fig. 7.8, it took nearly 10 hours for O3 to reach its maximum, but it is generally known that the O3 formation rate is roughly proportional to VOC concentration (Akimoto and Sakamaki 1983). According to Eq. (7.52), the O3 formation rate P 0-i) is proportional to the concentration of HO2 and RO2, and the reaction of OH with RH, reaction (7.20), is the rate-determining step for the formation rate of the peroxy radicals. As deduced by the chamber experiment, OH concentration is nearly constant when the VOC/NOx ratio is higher than a certain value (VOC-excess or NOx-limited), so that the reaction rate of OH and VOC is nearly proportional to the VOC concentration (Akimoto et al. 1980). 

It should be noted that by adding unknown VOCs and OVOCs deduced from the measurements of OH reactivity in the model calculation, the calculated results of ozone formation tends to shift from the VOC-limited to NOx-limited regime. 

If in an ozone nonattainment area, does the toller facility actually emit, or have the potential to emit, VOC or NOx in excess of the threshold limit value ... 

Advances in combustion technology now make it possible to control the levels of NOx production at source, removing the need for wet controls. This of course opened up the market for the gas turbine to operate in areas with limited supplies of suitable quality water, e.g., deserts or marine platforms. 

NB There is no limit for NOx for gas-fired plant. In practice, little or no such plant currently exists. The conditions for measurement of NOx are clearly defined.)... 

Estimates of urban NOx emissions and trends are generally limited to those provided by the developed countries which have the detailed emission Inventories. As In the case of other pollutants, the USA contributes the most on a per-country basis to the global NOx emissions per year. Because of the Inaccuracy of the data base used. It Is difficult to discern trends In these emissions. However, with new control technologies being Implemented for both stationary and mobile sources, downward trends In the developed countries may be more prevalent In the future years. Unfortunately, the opposite trend Is likely to occur In the developing countries. 

A gas turbine with power output of 10.7 MW and an efficiency of 32.5% burns natural gas. In order to reduce the NOx emissions to the environmental limits, 0.6 kg steam is injected into the combustion per kg of fuel. The airflow through the gas turbine is 41.6 kg-s 1. The composition of the natural gas can be assumed to be effectively 100% methane with a molar mass of 16 kg-kmoU1. The kilogram molecular volume can be assumed to occupy 22.4 m3 at standard conditions. 

If EuroV finally leads to a moderate reduction of the NOx emissions limit (s 20%), when regarding to US Tier 2-bin 5 levels3, we can presume that EuroVI will consist of much more stringent NOx limitation4. A NOx after-treatment system in combination with a DPF (which is mandatory since EuroV) in the exhaust line should become ordinary with EuroVI. The PM limit values will be unchanged from EuroV. 

In summary, the NOx mass to convert (difference between engine-out NOx emissions and NOx emissions limit imposed by Diesel Euro standards) governs the requested duration for the rich conditions in the exhaust line (NOx regeneration duration), and thus dictates directly the quantity of methane released in the exhaust line. 

The CH4 emissions can reach significant values and even lead directly to a cleanup unfeasibility in certain cases, since these amounts can be close to the unburnt HC threshold imposed by Diesel Euro standards. However, even if the CH4 values do not reach the unburnt HC limit, these emissions are added in the vehicle exhaust gas and limit the global unburnt HC that the engine can release in lean conditions. Thus, with the NOxTrap, a new constraint is appearing, which will have an impact on the engine-out HC emissions during the lean conditions. As NOx emissions levels increase, CH4 emissions in rich mode increase, and it is then necessary to reduce HC emissions in lean mode to stay within the limits of the DOC conversion ability. This context is specific to... 

The severity of the Diesel European standards for HC, CO, NOx and PM has a first consequence car manufacturers seem to reach the limits of combustion/injection improvement. Some particle and NOx after-treatment systems will become mandatory in EuroV and EuroVI context. Consequently, this implies a cost and generates a complex implementation. 

Another important catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, or the lean-NOx trap . In the lean-NOx trap, the formation of N02 by NO oxidation is followed by the formation of a nitrate when the N02 is adsorbed onto the catalyst surface. Thus, the N02 is stored on the catalyst surface in the nitrate form and subsequently decomposed to N2. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SO, adsorbs more strongly on N02 adsorption sites than N02, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. The presence of S03 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Furthermore, catalytic oxidation of NO to N02 can be operated in a limited temperature range. Oxidation of NO to N02 by a conventional Pt-based catalyst has a maximum at about 250°C and loses its efficiency below about 100°C and above about 400°C. 

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