Reducing Energy Cost of NOx Production in Air Plasmas

ArticleinThe Chemical Engineering Journal · January 2019with 261 Reads
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Abstract
Current worldwide fixed nitrogen utilization efficiency (NUE) is low, leading to significant environmental problems. An important aspect of this problem involves NH3 released from bacterially degraded, N-containing organic waste such as animal manure. Acidifying organic waste with nitric acid traps NH3 as NH4NO3, thus increasing the nitrogen content of the resulting organic fertilizer and reducing the environmental effects of NH3 emission. Air plasmas have been proposed for this purpose through production of NOx from air which can be readily converted to nitric acid. However, a major challenge remains in reducing the energy cost of NOx production in air plasmas to make the technology economically competitive. Different types of discharges such as arcs, sparks, dielectric barrier discharges have been investigated for NOx production but they are challenging to compare due to differences in their structure, geometry and excitation modes etc. Different discharges also utilize potentially different chemical reaction pathways to produce NOx. As of yet, there are no general evaluation criteria to determine the qualities of an appropriate discharge configuration for energy-efficient NOx production. In this study, we report on four types of discharges; dielectric barrier, glow, spark and extending arc discharge to examine NOx production efficiency under different conditions. Based on our results and previously published results from the literature, we propose a dimensionless parameter to guide the design of appropriate plasma sources for reduced energy cost NOx production.

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    Full-text available
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    Full-text available
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    The knowledge of the lightning-induced nitrogen oxides (LNOx) source is important for understanding and predicting the nitrogen oxides and ozone distributions in the troposphere and their trends, the oxidising capacity of the atmosphere, and the lifetime of trace gases destroyed by reactions with OH. This knowledge is further required for the assessment of other important NOx sources, in particular from aviation emissions, the stratosphere, and from surface sources, and for understanding the possible feedback between climate changes and lightning. This paper reviews more than 3 decades of research. The review includes laboratory studies as well as surface, airborne and satellite-based observations of lightning and of NOx and related species in the atmosphere. Relevant data available from measurements in regions with strong LNOx influence are identified, including recent observations at midlatitudes and over tropical continents where most lightning occurs. Various methods to model LNOx at cloud scales or globally are described. Previous estimates are re-evaluated using the global annual mean flash frequency of 445 s-1 reported from OTD satellite data. From the review, mainly of airborne measurements near thunderstorms and cloud-resolving models, we conclude that a “typical” thunderstorm flash produces 15 (2-40)1025 NO molecules per flash, equivalent to 250 mol NOx or 3.5 kg of N mass per flash with uncertainty factor from 0.13 to 2.7. Mainly as a result of global model studies for various LNOx parameterisations tested with related observations, the best estimate of the annual global LNOx nitrogen mass source and its uncertainty range is (53) Tg a-1 in this study. In spite of a smaller global flash rate, the best estimate is essentially the same as in some earlier reviews, implying larger flash-specific NOx emissions. The paper estimates the LNOx accuracy required for various applications and lays out strategies for improving estimates in the future. An accuracy of about 1 Tg a-1 or 20%, as necessary in particular for understanding tropical tropospheric chemistry, is still a challenging goal.
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    An assessment of the global distribution of nitrogen oxides is required for an adequate description of tropospheric chemistry and in the evaluation of the global impact of increasing anthropogenic emissions of NOx. In the mathematical models utilized for this purpose, one needs to specify as inputs the natural as well as man-made sources of nitrogen oxides in the atmosphere. Lightning is one of the main natural sources of nitrogen oxides in the atmosphere, and it may be the dominant source of nitrogen oxides in the troposphere in equatorial and tropical South Pacific regions. Thus, an accurate quantification of nitrogen oxide production by thunderstorms is necessary for further development of the chemical models of the troposphere and in the evaluation of the effects of the man-made nitrogen emissions in the terrestrial atmosphere.
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    Atmospheric pressure air plasmas are often thought to be in local thermodynamic equilibrium owing to fast interspecies collisional exchange at high pressure. This assumption cannot be relied upon, particularly with respect to optical diagnostics. Velocity gradients in flowing plasmas and/or elevated electron temperatures created by electrical discharges can result in large departures from chemical and thermal equilibrium. This paper reviews diagnostic techniques based on optical emission spectroscopy and cavity ring-down spectroscopy that we have found useful for making temperature and concentration measurements in atmospheric pressure plasmas under conditions ranging from thermal and chemical equilibrium to thermochemical nonequilibrium.
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    Lightning is thought to represent an important source of tropospheric reactive nitrogen species NOx (NO + NO2), but estimates of global production of NOx by lightning vary considerably. We evaluate the production of NOx by lightning using a global chemical/transport model, satellite lightning observations, and airborne NOx measurements. Various model calculations are conducted to assess the global NOx production rate of lightning by comparing the model calculations with airborne measurements. The results show that the simulated NOx in the tropical middle and upper troposphere are very sensitive to the amount and altitude of the lightning NOx used in the model. A global lightning NOx production of 7 Tg N yr(-1) uniformly distributed in convective clouds or 3.5 Tg N yr(-1) distributed in the upper cloud regions produces good agreement between calculated and measured NOx concentrations in the tropics.
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    Nitric monoxide (NO) is increasingly being used in medical applications. Currently, a gas cylinder of N<sub>2</sub> mixed with a high concentration of NO is used. This arrangement is potentially risky due to the possibility of accidental leak of NO from the cylinder. The presence of NO in air leads to the formation of nitric dioxide (NO<sub>2</sub>), which is toxic to the lungs. Therefore, an on-site generation of NO would be very desirable for patients with acute respiratory distress syndrome and other related illnesses. We have recently reported on the production of NO using a pulsed arc discharge. In the present work, the discharge reactor was made simpler and smaller. NO was generated using a pulsed arc discharge in dry air and in mixtures of oxygen and nitrogen. The composition of the gas mixture after treatment with an arc discharge followed by exposure to heated molybdenum was 540 ppm of NO, 48 ppm of NO<sub>2</sub>, and the balance dry air at 0.1 MPa and 300 ± 3 K. No ozone was detected at the outlet of the system by UV absorption. The density of the brass particles emitted from the electrodes, which had diameters over 0.3 μm, was less than 1.39 μg/L. A filter could readily capture and thus remove the brass particles.