Emissions of nitrous oxide from combustion sources

Nitrous oxide (N₂0) has recently become the subject of intense research and debate, because of its increasing concentrations in the atmosphere and its known ability to deplete the ozone layer and also to contribute to the greenhouse effect. There are both natural and anthropogenic sources for N₂O; however, the man-made sources are increasing at a much higher rate than natural ones. Until very recently it was believed that the combustion of fossil fuels, especially coal, was the major contributing factor to these anthropogenic sources. For example, 30% of all N₂0 released into the atmosphere was once attributed to combustion sources, with 83% of the combustion sources coming from coal combustion. Correction of a recently discovered sampling artifact, whereby SO₂, H₂O and NO in combustion gases react in a sampling vessel to produce N₂O, has revealed that, in fact, less than 5 ppm of N₂0 are found in most product gases from combustion systems. Fluidized bed coal combustors are the exception, though, yielding N₂O levels of ca. 50ppm in their off-gases.

The gas-phase reactions of N₂0 in flames are reviewed first. It is clear that in most cases N₂0 is a very reactive intermediate, which is quickly destroyed before being emitted from a flame. The important homogeneous reactions removing N₂0 are thermal decomposition to N₂ and O₂ and also radical attack in e.g. N₂O + H → N₂ + OH. Nitrous oxide is formed from nitrogen-containing species by NO reacting with a radical derived from either HCN or NH₃; the reactions are NCO + NO → N₂0 + CO and NH + NO → N₂0 + H. The levels of N₂O observed are a balance betwen its rates of formation and destruction. It turns out that HCN is a more efficient precursor than NH₃ at producing N₂0. The removal of N₂O is fastest at high temperatures and in fuel-rich systems, where free hydrogen atoms are present in relatively large amounts.

When coal burns in a fluidized bed, most of the N₂O detected is produced during devolatilization, rather than in the subsequent stage of char combustion. It is clear that HCN and NH₃ are produced from nitrogenous material released during devolatilization; these two compounds give N₂0 when the volatiles burn. The burning of char, on the other hand, involves the chemi-sorption of O₂ on to sites containing carbon or nitrogen atoms, followed by surface reaction, with one of the products being N₂0, in addition to CO, CO₂ and NO. Fluidized coal combustors have temperatures around 900°C, which is low enough for the thermal decomposition of N₂O to be relatively slow. In addition, the presence of the solid phase provides a large area for radical recombination, which in turn reduces the rate of removal of N₂O by free radicals. Parametric studies of fluidized bed combustors have shown that factors such as: temperature, amount of excess air, carbon content and O/N ratio of the coal, all have a significant effect on N₂O emissions. It is important to note that heterogeneous reactions with solids, such as CaO and char, can cause large decreases in the amount of N₂O produced during the combustion of coal in a fluidized bed. In fact, there are several methods available for lowering the yields of N₂O from fluidized bed combustors generally. Areas of uncertainty in the factors affecting N₂O emissions from fluidized bed combustors are identified.     

The pseudo-catalytic promotion of nitric oxide oxidation by ethane at low temperatures

Experimental work carried out in a flow reactor has shown that, from 700 to 750 K, low concentrations of ethane can act as a virtual catalyst in effecting the oxidation of NO to NO₂. That is, while there is strong promotion of the oxidation of NO, there is very little concurrent degradation of the ethane. This contrasts with other experimental and modeling investigations, in which promotion by the hydrocarbon of NO oxidation has accompanied oxidation of a considerable fraction of the hydrocarbon. The rate of this pseudo-catalytic effect is significantly affected by the concentrations of both ethane and oxygen, which in this study ranged from 0 to 70% for [O₂], 0 to 5000 ppm for [C₂H₆], and 0 to 350 ppm for [NO]. The level of reaction achieved under the various conditions was measured in terms of the rates of formation of NO₂ and C₂H₄. Current kinetic mechanisms, though displaying good accuracy in other temperature regimes, fail to predict this pseudo-catalytic behavior of ethane, indicating that several elementary reactions important at the low temperatures are missing or poorly represented in such mechanisms. Mechanistic modifications are discussed and allow the measurements to be simulated more closely than with existing reaction schemes. It has been shown that the relative rates of the competing reactions C₂H₅ + O₂ → C₂H₄ + HO₂ and C₂H₅ + O₂ (+M) → C₂H₅O₂ + (M) are of critical importance in this situation.     

Performance and durability of metal-supported solid oxide electrolysis cells at intermediate temperatures

Metal-supported solid oxide electrolysis cells (MS-SOECs) operating at 600–700 °C are attractive for storage of intermittent renewable electricity from solar and wind energy due to their advantages of easy sealing and fast startup. This paper reports on the fabrication of MS-SOECs consisting of dense scandium stabilized zirconia (SSZ) electrolytes, Ce_0.8Sm_0.2O_2−δ (SDC)/Ni impregnated 430L/SSZ cathodes and SmBa_0.5Sr_0.5Co₂O_5+δ (SBSCO) impregnated SSZ anodes supported on porous 430L alloys. Such cells demonstrated excellent electrolysis performance with current densities at 650 °C as high as 0.73 A⋅cm⁻² at 1.3 V in 50% H₂O-50% H₂ and 0.95 A⋅cm⁻² at 1.5 V in 90% CO₂-10% CO. Electrochemical impedance measurements indicated that the cell performance was largely limited by the ohmic losses for steam electrolysis and by the cathodic reduction reactions for CO₂ electrolysis, especially at reduced temperatures. Pronounced degradation was observed for both steam and CO₂ electrolysis over the preliminary 90-h stability measurements at 600 °C. SEM examination and EDS mapping of measured cells showed significant aggregation and coarsening of impregnated Ni particles, resulting in smaller activities for H₂O and CO₂ reduction reactions. As evidenced by the almost unaltered ohmic resistances over the measurement durations, the 430L stainless steel substrates demonstrate excellent resistances against corrosions from H₂O and CO₂ and thus show great promise for applications in reduced-temperature MS-SOECs.     

Mechanical properties of solid oxide fuel cell glass-ceramic seal at high temperatures

Mechanical properties of solid oxide fuel cell glass-ceramic seal material, G18, are studied at high temperatures. Samples of G18 are aged for either 4 h or 100 h, resulting in samples with different crystallinity. Reduced modulus, hardness, and time-dependent behavior are measured by nanoindentation. The nanoindentation is performed at room temperature, 550, 650, and 750 °C, using loading rates of 5 mN s⁻¹ and 25 mN s⁻¹. Results show a decrease in reduced modulus with increasing temperature, with significant decrease above the glass transition temperature. Hardness generally decreases with increasing temperature, with a slight increase before Tg for the 4 h-aged sample. Dwell tests show that creep increases with increasing temperature, but decrease with further aging.     

Methane internal steam reforming in solid oxide fuel cells at intermediate temperatures

The internal steam reforming of methane (CH₄) on conventional solid oxide fuel cell (SOFC) anode (nickel-yttria stabilized zirconia or Ni-YSZ) offers significant advantages compared to the external reforming process. However, the technology is currently facing some major issues such as coking and oxidation of anode during operation. Here we report a low-temperature sinterable catalyst, Ce_0·77Ni_0·2Mn_0·03O_2-δ (CNMnO), applied on top of Ni-YSZ to perform the steam reforming reaction. A single cell with CNMnO/Ni-YSZ/YSZ/GDC/LSC configuration produces a peak power density of 492 mW cm^?2 in wet hydrogen and 371 mW cm^?2 in wet methane, at 600 °C. The cell also shows exceptional durability against Ni oxidation when tested in wet methane under 0.2 A cm^?2 for 100 h. The improved performance and durability of the catalyst layer has been attributed to the nanosized precipitated Ni and Mn particles distributed on the surface of individual CNMnO particles.     

Effect of oxygen on decomposition of nitrous oxide over various metal oxide catalysts

The inhibitory effect of oxygen on decomposition of nitrous oxide over various metal oxide catalysts was investigated. The activity of nitrous oxide decomposition significantly decreased over CuO, Co₃O₄, NiO, Fe₂O₃, SnO₂, In₂O₃ and Cr₂O₃ by reversible adsorption of oxygen onto the active sites. On the contrary to this, there was no or small change in the activity of TiO₂, Al₂O₃, MgO, La₂O₃ and CaO. A good correlation was observed between the degree of inhibition and the heat of formation of metal oxides. On the basis of kinetic model, the reduction of catalytic activity in the presence of oxygen was rationalized with the strength of oxygen adsorption on the metal oxide surface.     

Study on Nitrous Oxide Emission in Boiler Furnace

INTRODUCTIONThenitrogenoxides(NOx)yieldus[lallyisdeterlninedbyl1itricoxide(NO).Nitrousoxideconcentra-tiollintotalNOxyieldislow.Experimentalmea-sllrelnentsofN2OconcentratioIlsinexllaustgaswithdifferel1tf1lelsshowedthatN2Olevelsinfurnacesusu-allydollotexceed5-l6ppm[l-2].N2Oconcentrationwithexhaustgasintotlleatn1osphereislower.Forex-amPle,thejettinglevelofN2Ofordifferentfuel-firedboilersare:forpulverizedcoal'o.2-o.8ppm,forft1eloilo.1-o.8ppm,fornaturalgas5o.2ppm['-'1.Thisprovesthatthej…     

Performance and efficiency of a DMFC using non-fluorinated composite membranes operating at low/medium temperatures

In order to increase the chemical/thermal stability of the sulfonated poly(ether ether ketone) (sPEEK) polymer for direct methanol fuel cell (DMFC) applications at medium temperatures (up to 130 °C), novel inorganic–organic composite membranes were prepared using sPEEK polymer as organic matrix (sulfonation degree, SD, of 42 and 68%) modified with zirconium phosphate (ZrPh) pretreated with n-propylamine and polybenzimidazole (PBI). The final compositions obtained were: 10.0 wt.% ZrPh and 5.6 wt.% PBI; 20.0 wt.% ZrPh and 11.2 wt.% PBI. These composite membranes were tested in DMFC at several temperatures by evaluating the current–voltage polarization curve, open circuit voltage (OCV) and constant voltage current (CV, 35 mV). The fuel cell ohmic resistance (null phase angle impedance, NPAI) and CO₂ concentration in the cathode outlet were also measured. A method is also proposed to evaluate the fuel cell Faraday and global efficiency considering the CH₃OH, CO₂, H₂O, O₂ and N₂ permeation through the proton exchange membrane (PEM) and parasitic oxidation of the crossover methanol in the cathode. In order to improve the analysis of the composite membrane properties, selected characterization results presented in [V.S. Silva, B. Ruffmann, S. Vetter, A. Mendes, L.M. Madeira, S.P. Nunes, Catal. Today, in press] were also used in the present study. The unmodified sPEEK membrane with SD = 42% (S42) was used as the reference material. In the present study, the composite membrane prepared with sPEEK SD = 68% and inorganic composition of 20.0 wt.% ZrPh and 11.2 wt.% PBI proved to have a good relationship between proton conductivity, aqueous methanol swelling and permeability. DMFC tests results for this membrane showed similar current density output and higher open circuit voltage compared to that of sPEEK with SD = 42%, but with much lower CO₂ concentration in the cathode outlet (thus higher global efficiency) and higher thermal/chemical stability. This membrane was also tested at 130 °C with pure oxygen (cathode inlet) and achieved a maximum power density of 50.1 mW cm⁻² at 250 mA cm⁻².     

Estimation of methane and nitrous oxide emissions from paddy fields in Taiwan

To investigate the greenhouse gases emissions from paddy fields, methane and nitrous oxide emissions were estimated with the local measurement and the IPCC method during 1990–2006 in Taiwan. Annual methane emission ranged from 9001 to 14,980 ton in the first crop season for 135,314–242,298 ha of paddy fields, and it was between 16,412 and 35,208 ton for 101,710–211,968 ha in the second crop season with the local measurement for intermittent irrigation. The value ranged from 31,122 to 55,729 ton of methane emission in the first crop season, and it was between 29,493 and 61,471 ton in the second crop season with the IPCC guideline for continuous flooding. Annual nitrous oxide emission from paddy fields was between 371 and 728 ton in the first crop season, and the value ranged from 163 to 365 ton in the second crop season with the local measurement. Methane emission from paddy fields in Taiwan for intermittent irrigation was only 26.72–28.92%, 55.65–57.32% and 41.19–43.10% with the IPCC guidelines for continuous flooding and mean temperature of transplanting stage in the first crop, the second crop and total paddy fields, respectively. The values were 53.44–57.84%, 111.29–114.55% and 82.38–86.20% with the IPCC guidelines for single aeration and mean temperature of transplanting stage, respectively; and the values were 133.60–144.61%, 282.56–286.62% and 205.96–215.49% with the IPCC guidelines for multiple aeration and mean temperature of transplanting stage, respectively. Intermittent irrigation in paddy fields reduces methane emission significantly; appropriate application of nitrogen fertilizer and irrigation in paddy fields also decreases nitrous oxide emission.     

Steam reforming on Ni-samaria-doped ceria cermet anode for practical size solid oxide fuel cell at intermediate temperatures

Direct internal and external reforming operations on Ni-samaria-doped ceria (SDC) anode with the practical size solid oxide fuel cell (SOFC) at intermediate temperatures from 600 to 750 °C are carried out to reveal the reforming activities and the electrochemical activities, being compared with the hydrogen-fueled power generation. The cell performance with direct internal and external steam reforming of methane and their limiting current densities were almost the same irrespective of the progress of reaction in the methane reformate at 700 and 750 °C. The durability test for 5.5 h at 750 °C with direct internal reforming operation confirmed that the cell performance did not deteriorate. The operation temperature of the cell controlled the reforming activities on the anode, and the large size electrode gave rise to high conversion due to the slow space velocity of the steam reforming. Direct internal steam reforming attained sufficient level of conversion for SOFC power generation with methane at 700 and 750 °C on the large Ni-SDC cermet anode.     

Friction and wear at low temperatures

For investigations of the friction and wear behaviour of materials under cryogenic conditions a special test device (Cryotribometer) has been constructed. The friction tests are performed with samples in a pin-on-disc configuration in a He-gas environment at temperatures between 8 and 77 K. Most of the tests were performed with filled and unfilled polymers like polyterafluorethylene (PTFE), polyamide (PA), polyoxymethylene (POM) and polyimide (PI) against steel. Compared to room temperature most polymers have an improved frictional behaviour in dry sliding at low temperatures, due to their increasing hardness and mechanical strength. Some experiments with hard carbon coatings indicate that the running-in and wear behaviour of these materials is influenced mainly by temperature.     

Spin-coating derived solid oxide fuel cells operated at temperatures of 500 °C and below

Low-temperature solid oxide fuel cells (SOFCs) operated at a temperature of 500 °C and below are developed by modifying the microstructures of single cells consisting of Ni-cermet anodes, doped ceria electrolytes and strontium-doped samaria cobaltite cathodes. The cell microstructure is optimized by varying the starting powder firing temperature, so that the doped ceria electrolytes have a high sinterability, reducing the spin-coating cycles to decrease the electrolyte thickness to approximately 9 μm, adopting a two-step sintering process so that the electrolytes consist of small grains and have a high density; while the anodes are composed of small particles and have high porosity. In particular, the two-step sintering process depresses the co-firing temperature, thus enhancing the electrolyte conductivity and reducing the electrode polarization resistance. Outstanding performance with peak power density of 476, 319, and 189 mW cm⁻² at 500, 450, and 400 °C is achieved with a typical single cell comprising a 9-μm-thick Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte, a Ni-SDC porous anode, and a Sm_0.5Sr_0.5CoO_3−δ-Sm_0.2Ce_0.8O_1.9 (SSC-SDC) composite cathode. A durability test over 110 h maintained a power density of approximately 150 mW cm⁻² at 400 °C, suggesting optimization of the microstructure has promise for enhancing the performance of low-temperature SOFCs.     

A note on chemiluminescence in low-pressure hydrogen and methane–nitrous oxide flames

Absolute OH(A) and CH(A) concentrations were determined in low-pressure H₂–air and CH₄–N₂O flames, respectively, by measuring absolute chemiluminescence yields at 310 and 430 nm. From spatial profiles and intensities in these and other flames, we deduce that two reactions are responsible in each case, and derive rate constants for all.     

Optimisation of an asymmetric manganese oxide/activated carbon capacitor working at 2 V in aqueous medium

The charge storage mechanism of manganese oxide and activated carbon has been studied in aqueous medium in order to optimise an asymmetric (or hybrid) supercapacitor based on these two materials as positive and negative electrode, respectively. Amorphous manganese oxide can be polarised up to potentials of 1.2 V in neutral medium. Under negative polarisation, a pseudocapacitive behaviour of activated carbon has been demonstrated, that is related with reversible hydrogen adsorption in the pores. It allows carbon to be polarised at potential values far from the thermodynamic decomposition of the electrolyte. Balancing the mass of these two materials with pseudocapacitive properties results in a practical cell voltage of 2 V in aqueous medium, with energy densities close to the values obtained with electric double layer capacitors working in organic electrolytes, while avoiding their disadvantages.     

Recent progress and prospective of medium and high temperatures thermal energy storage materials

开发中高温储热材料及其制备方法是储热技术发展的关键之一.本文结合中高温储热材料的分类,特点,应用及存在的问题对中高温储热材料的研究进展进行了综述,主要包括显热储热材料,热化学储热材料以及潜热储热材料.探讨了复合结构储热材料及其制备工艺,进一步介绍了其最新研究进展,并对中高温储热材料的下一步研究进行了展望,提出开发高性能纳微复合结构储热材料是未来研究的重点.     

Testing aqueous caustic electrolyzers at high temperatures

Increasing the electrochemical cell operating temperature allows a general improvement of the operating energetics of hydrogen production via alkaline solution water electrolysis. Considerations of operation of these advanced cells above 100°C leads to a reexamination of all of the basic materials stabilities. One previous controversy has been with respect to the upper temperature value for a stable asbestos electrochemical cell separator. In addition to testing alternative materials for the cell separator, one must examine which of the structural polymers can be used as a cell frame, reexamine the stability of the selected anode electrocatalyst and finally, determine if the plumbing and piping selected will introduce corrosion products which could affect the cathode electrocatalyst. This paper describes the work performed at Teledyne Energy Systems on the studies of alkaline electrolyzers and materials for operation at temperatures up to 150°C. This program, under contract to the Brookhaven National Laboratory, has proceeded through the design and construction of an applied research sized test system. The materials testing and preliminary test results obtained with the system are described.