Homogeneous formation of NO and N2O from the oxidation of HCN and NH3 at 600–1000°C

The oxidation of HCN and NH₃ with CO, CH₄, or H₂ addition has been studied in the temperature range between 600 to 1000°C. In most of the tests 10% oxygen was used. The experiments were carried out under well-defined conditions in a flow tube reactor made of quartz glass. The effects of NO addition and oxygen level have been tested. To study the importance of O/H radicals in the reaction mechanism and to confirm previous studies, iodine was added in some tests. A detailed chemical kinetic model was used to analyze the experimental data. In general, the model and experimental results are in good agreement. The results show that under the conditions tested CO significantly promotes NO and N₂O formation during HCN oxidation. During NH₃ oxidation carbon-containing gaseous species such as CO and CH₄ are important to promote homogeneous NO formation. In the system with CH₄ addition, the conversion of HCN to N₂O is lower compared to the other systems. In the HCN/NO/CO/O₂ system NO reduction starts at 700°C and the maximum reduction of approx. 40% is obtained at 800°C. For the NH₃/NO/CO/O₂ system the reduction starts at 750°C and the maximum reduction is 50% at 800°C. Iodine addition shifts the oxidation of HCN, NO, and N₂O formation as well as NO reduction to higher temperatures. Under the conditions tested, it was found that iodine mainly enhances the recombination of the O-radicals. No effect on NO formation was found in the HCN/CH₄/O₂ system when oxygen was increased from 6% to 10%, but when oxygen was increased from 2% to 6% NO formation decreased. The role of hydrocarbon radicals in the destruction of NO is likely to become important at low oxygen concentrations (2%) and at high temperatures (1000°C).     

Nitrous oxide formation and destruction in lean, premixed combustion

Concentraion profiles of N₂O, NO, and N₂ from atmospheric pressure, flat-flame burner experiments are presented. Axial profiles of species and temperature are described for CH₄-air and H₂---O₂---Ar flames doped with either NH₃, NO, or N₂O.

Species concentrations were determined by microprobe sampling and direct analysis by gas chromatography or chemiluminescence analysis, for flames ranging in temperature from 1300 to 2000 K. Burner surface temperatures were also estimated for these flames, using heat transfer analysis and an optical method. Nitrogen-atom balances were achieved in each of the H₂---O₂---Ar flames to within experimental error and better than 6%.

Axial profiles of N₂O were similar for all flames. At sampling locations nearest to the burner (0.5–1.0 mm), N₂O concentrations were highest. Concentrations decreased monotonically in the downstream direction, with N₂O destruction essentially complete in the postflame region (height greater than 3 mm) for all flames except the one at the lowest temperature (1300 K). With NH₃ as dopant, early-flame and postflame N₂O concentration varied inversely with temperature. The lowest early-flame N₂O concentration was observed with NO as dopant, and the highest was observed with direct N₂O addition. Increased initial concentrations of NH₃ led to higher early-flame N₂O concentrations. With N₂O as dopant, product branching to NO and N₂ in the postflame region is approximately 8% and 92%, respectively. An NO removal process involving NH_i is active in lean, NO-doped flames.     

Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O₂/N₂ and O₂/CO₂ atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O₂/CO₂ (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O₂/N₂ (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O₂/N₂ condition or the O₂/CO₂ condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.     

The devolatilization of coal and a comparison of chars produced in oxidizing and inert atmospheres in fluidized beds

This is a study of the devolatilization of coal in a laboratory-scale bed of silica sand, fluidized with either air or N₂ and electrically heated to 750 or 900°C. Coal particles (diameter 1.4–1.7 or 2.0–2.36 mm) were fed in batches to the surface of the bed and allowed to devolatilize in either an oxidizing atmosphere of air or inert N₂. In the first case, combustion of the volatiles occurred, but there was only thermal decomposition (pyrolysis) in the second situation. The resulting chars were recovered and analyzed for composition and structure, so that comparisons could be made between the effects of devolatilization with combustion and of pyrolysis in an inert atmosphere. It was found that the fractions of C and H in the char were only slightly sensitive to the type of fluidizing gas used. The amount of nitrogen in the recovered char and also the devolatilization time showed no dependence on the type of fluidizing gas, whereas BET areas were slightly larger after combustion in air. It is concluded that these effects are small relative to other errors, inherent in experiments on coal combustion, so that chars prepared in a bed fluidized by hot N₂ are very similar to those formed during coal combustion at nominally the same temperature. Equally, the overall composition of the volatile matter released during combustion in a fluidized bed is the same as in pyrolysis in nitrogen. The effects of other parameters, such as the temperature of the bed, the flow rate of the fluidizing gas and the size of the coal particles are also discussed in detail. It is concluded that most of the volatiles burn in a fluidized bed (at or less than 900°C) far away from the original coal particle. Also, NO_x is in effect a primary product of devolatilization, being produced in appreciable amounts when coal is heated in inert N₂. The ratio of C/N in the volatiles is found to be a constant during the latter stages of devolatilization, but beforehand at lower temperatures, carbon species are preferentially released. Overall, devolatilization of small particles (< 2.4 mm) in a fluidized bed at 900°C is kinetically controlled. The activation energy is small, being 15 ± 6 kJ/mol.     

Comparison of species profiles between O2 and NO2 oxidizers in premixed methane flames

The profiles of the species H, OH, CH, NH, CN, NCO, NO₂, and CH₃O are compared in a series of five premixed stoichiometric 15-torr CH₄/O₂/NO₂/N₂ flames with NO₂ comprising between 0% and 40% of the oxidizer. Relative species concentrations were measured by laser-induced fluorescence (LIF) and these results are compared with calculations using measured temperature profiles. The reaction mechanism of Miller and Bowman incorrectly predicts the standoff from the burner in flames containing more than 20% NO₂; addition of several reactions involving NO₂ and HONO produces excellent agreement with experiment for most species. The reaction CH₃ + NO₂ → CH₃O + NO is found to be particularly important in the reaction mechanism. LIF profiles of CH₃O show this species to be present in far larger quantities in the NO₂ supported flames than in the CH₄/O₂ system. The nitrogen-containing intermediates CN, NCO, and NH are all overpredicted by a factor of two in the 40% NO₂ flame relative to the 10% NO₂ flame. This indicates an inaccuracy in either the reburn reactions or the fuel nitrogen chemistry when large amounts of NO are present. The kinetic modeling shows that in the 40% NO₂ flame, the dominant pathway to N₂ formation is through N₂O, which is produced primarily by the reaction of NCO with NO. Comparison of emission profiles of NO₂* for the various flames indicates that the appearance of an orange-yellow luminous zone at the base of NO₂ supported flames is caused by thermal excitation of NO₂, not by a chemiluminescence mechanism.     

流化床燃烧中N2O的排放控制研究进展

随着温室效应问题的日益严重,人类活动造成过量温室气体的排放受到普遍关注。本文阐述了流化床燃烧中N₂O的来源、生成和分解机理、影响N₂O生成量的各种因素以及减少N₂O排放量的技术措施。对国内外关于N₂O最新研究进展进行了综述,指出了未来研究的方向并为节能减排提供理论依据。     

Decomposition of nitrous oxide at medium temperatures

Flow reactor experiments were done to study the decomposition of N₂O at atmospheric pressure and in a temperature range of 600–1000°C. Dilute mixtures of N₂O with H₂, CH₄, CO with and without oxygen with N₂ as carrier gas were studied. To see directly the relative importance of the thermal decomposition versus the destruction by free radicals (i.e.: H, O, OH) iodine was added to the reactant mixture suppressing the radicals’ concentrations towards their equilibrium concentrations. The experimental results were discussed using a detailed chemistry model. This work shows that there are still some uncertainties regarding the kinetics of the thermal decomposition and the reaction between N₂O and the O radical. Using the recommendations applied in this work for the reaction N₂O + M ↔ N₂ + O + M and for N₂O + O ↔ products, a good agreement with the experimental data can be obtained over a wide range of experimental conditions. The reaction between N₂O and OH is of minor importance under present conditions as stated in latest literature. The results show that N₂O + H ↔ N₂ + OH is the most important reaction in the destruction of N₂O. In the presence of oxygen it competes with H + O₂ + M ↔ HO₂ + M and H + O₂ ↔ O + OH, respectively. The importance of the thermal decomposition (N₂O + M ↔ N₂ + O + M) increases with residence time. Reducing conditions and a long residence time lead to a high potential in N₂O reduction. Especially mixtures of H₂/N₂O and CO/H₂O/N₂O in nitrogen lead to a chain reaction mechanism causing a strong N₂O reduction.     

铁基载氧体的污泥化学链气化过程中氮迁移热力学模拟与实验研究

基于吉布斯自由能最小化原理,采用HSC Chemistry 6.0软件,对污泥化学链气化过程中NO_x前驱物（NH₃和HCN）与Fe₂O₃载氧体的氧化还原行为进行了热力学模拟。基于污泥热解实验中NO_x前驱物的含量,计算载氧体与污泥的摩尔比（OC/SS）对NH₃、HCN以及NH₃和HCN混合气氧化过程的影响。热力学模拟结果表明：Fe₂O₃能显著促进NO_x前驱物的氧化和裂解,主要生成N₂,几乎无NO_x生成;当NH₃、HCN以及混合气（NH₃和HCN）分别作为还原剂时,其最优OC/SS分别为0.02、0.04和0.05;由于HCN还原性强于NH₃,其氧化速率较快。基于Fe₂O₃/Al₂O₃混合物（FeAl）载氧体,实验对比了污泥化学链气化与污泥热解过程中NO_x前驱物的释放特性,发现Fe₂O₃能显著降低烟气中NO_x前驱物的产率,NH₃和HCN产率分别下降32%和62%。实验结果与热力学模拟结果一致。     

Nongray radiation from gas and soot mixtures in planar plates based on statistical narrow-band spectral model

The nongray behavior of combustion products plays an important role in various areas of engineering. Based on the statistical narrow-band (SNB) spectral model with an exponential-tailed inverse intensity distribution and the ray-tracing method, a comprehensive investigation of the influence of soot on nongray radiation from mixtures containing H₂O/N₂+soot, CO₂/N₂+soot, or H₂O/CO₂/N₂+soot was conducted in this paper. In combustion applications, radiation transfer is significantly enhanced by soot due to its spectrally continuous emission. The effect of soot volume fraction up to 1×10^-6 on the source term, the narrow-band radiation intensities along a line-of-sight, and the net wall heat fluxes were investigated for a wide range of temperature. The effect of soot was significant and became increasingly drastic with the increase of soot loading.     

高水分煤在流化床中燃烧时NO_x的排放特性

通过数值计算,研究了高水分煤在流化床中燃烧时ＮＯ的排放特性及其形成的主要影响因素。结果表明煤中的水分,床内的ＣＯ浓度和空－燃比等都对于ＮＯ转换率有较大影响。     

Fluidised bed combustion: Improved system design leading to reduced pollutant emissions

Various fluidised bed technologies are discussed, their advantages and drawbacks being identified. Reductions in both NO_x and SO₂ emissions can be achieved by employing two-stage combustion in the fluidised bed, but these desired attributes have not, as yet, been realised with large, commercially viable fluidised bed combustors.     

An experimental study of propane combustion in a fluidized-bed gasifier

This paper reports an experimental study of the combustion of volatiles from coal, simulated by propane, and its interaction with char gasification reactions in a fluidized-bed coal gasifier (FBG). Experiments were performed under conditions of propane pyrolysis (in a bed fluidized by nitrogen and steam), propane reacting with oxygen and steam (in a bed fluidized by air and steam), char gasification only (in a bed fluidized by air and steam without propane) and in char gasification (in a bed fluidized by air and steam with propane). Axial concentration profiles of various species were obtained at 750, 850 and 950 °C. It was observed that the combustion of propane in an FBG, but without char, behaves as reported in the literature for fluidized-bed combustion (FBC). However, upon introducing char into the bed to simulate the reducing atmosphere in an FBG, oxygen was rapidly consumed within a short distance of the distributor, by significant partial oxidation of both propane and its decomposition products to carbon monoxide. The char was found to aid the pyrolysis of propane, limiting the amount of hydrocarbons surviving into the freeboard. The experimental results reported here are believed to be the first observations on the combustion of volatiles under conditions in an FBG.     

Effect of Fe on NO release during char combustion in air and O2/CO2

The chemistry of char was probed by studying nitrogen release under the reactions with air and oxy-fuel combustion. The experiments were conducted in a drop tube furnace and a fixed bed flow reactor. NO was observed during those experiments. The results show that the particle size of char generated at 1073 K in CO₂ is larger than that in N₂. However, at 1573 K, it is smaller in CO₂ atmosphere due to particle breaking by gasification of char and CO₂. The Fe addition increases the NO conversion ratio, and the effect of Fe rises steeply with the process going until it becomes stable in the end. The results also indicate that the release of NO increases more significantly with the Fe addition in oxy-fuel environment.     

Thermodynamic basis for the choice of working fluids for solar absorption cooling systems

Through the application of the first and second laws of thermodynamics upper and lower limits for the coefficient of performance (COP) of absorption cooling cycles are derived. These upper and lower limits, besides being dependent on the environmental temperatures of components of the cycle, are also dependent on the thermodynamic properties of refrigerants, absorbents, and their mixtures. With the use of these upper and lower limits of COP it is now possible to make a quantitative comparative study of different refrigerant-absorbent combinations. The technique developed is applied for the comparative evaluation of NH₃ + H₂O, NH₃ + NaSCN and H₂O + LiBr combinations which are the favorable candidates used in solar absorption cooling cycles.     

Ignition of CO/H2/N2 versus heated air in counterflow: experimental and modeling results

Nonpremixed ignition in counterflowing CO/H₂ vs. heated air jets is experimentally and computationally investigated. The experiments confirm the numerical modeling observation of the existence of three ignition regimes as a function of the hydrogen concentration. In all three regimes, we first detect experimentally the onset of chemiluminescent glow due to excited CO₂ followed by flame ignition, as the temperature of the air jet is raised gradually. The temperature extent of the glow regime, however, is progressively reduced with increasing hydrogen addition; no glow is detected for H₂ concentrations in excess of 73%. The temperatures for glow onset and flame ignition are represented by the boundary air temperatures for each threshold. The variation of these temperatures with system pressure and flow strain rate is explored, for pressures between 0.16 and 5 atm, and strain rates of 100 to 600 s⁻¹. The pressure variation is found to result in three p-T ignition limits, similar to the ignition limits observed in the H₂/O₂ system. This similarity is also observed on the effects of aerodynamic transport on ignition: within the second limit the ignition temperatures are found to be essentially insensitive to flow strain rate, whereas the other two limits are significantly affected by strain. The transport insensitivity is maintained even in the limit of very low H₂ concentrations, where an analogous H₂/N₂ mixture would fail to ignite. This behavior is explained computationally by the replacement of the shift reaction OH + H₂ → H₂O + H with the reaction CO + OH → CO₂ + H, thereby minimizing the effect of diminishing H₂ concentration. The experimental data are found to agree well with the calculated results, although discrepancies are noted in modeling the onset of chemiluminescence and its response to pressure variations.     

Potential GHG mitigation options for agriculture in China

China's agriculture accounts for about 5–15% of total national emissions of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Land-use changes related to agriculture are not major contributors of greenhouse gas emissions in China.

Mitigation options are available that could result in significant decrease in CH₄ and N₂O emissions from agricultural systems, and are likely to increase crop and animal productivity. Implementation has the potential to decrease CH₄ emissions from rice paddies, ruminants, and animal waste by 4–40%. Improving the efficiency of plant utilization of fertilizer N could decrease N₂O emissions from agriculture by almost 20%. Analyses of several of the proposed options show positive economic as well as environmental benefits.     

Comminution of carbons in fluidized bed combustion

The study surveys fifteen years of research on carbon comminution in bubbling fluidized bed combustors. It is carried out on the premise that comminution can be seen as the result of at least four phenomena occurring in series-parallel with each other and with combustion, namely: the primary fragmentation of coals or other carbonaceous materials, the secondary fragmentation, the fragmentation by uniform percolation and the attrition of chars, cokes and graphite. In combination with combustion, these phenomena control size reduction of feed carbon particles into fines of elutriable size and, as a consequence, combustion efficiency and particulate emissions.

Information on fragmentation and attrition behaviour is conveniently obtained by means of laboratory and pre-pilot scale combustors when carbon is charged batchwise to the bed. Some of the literature data separately reported in the paper for each of the four phenomena taken into consideration can be used, as they are, or after appropriate modification, in the design of larger units. To this end, submodels directed to extrapolate comminution data beyond the ranges of experimental conditions in which they have been determined are thoroughly examined.

The paper also discusses how comminution parameters are embodied into the equation model of bubbling fluidized bed combustors. The starting point is the carbon particle population balance. Depending on whether secondary fragmentation is or is not relevant, the population balance is expressed by means of an integro-differential or an ordinary differential equation. Reference is also made to a simplified model which contains essential comminution effects and adequately describes the performance of combustors charged with coals of different rank.     

A high-efficiency, compound NH3/H2O-H2O/LiBr absorption-refrigeration system

A high-efficiency, compound absorption-refrigeration system is considered, which is composed of two cooperating absorption units using NH₃/H₂O and H₂O/LiBr solutions, respectively. The heat output from the NH₃/H₂O unit is employed to drive the H₂O/LiBr unit. The thermodynamics of the new system are simulated by using a procedure which showed that very high theoretical coefficients of performance may be obtained (up to 230%) compared to the corresponding theoretical values for the usual single absorption units, which do not exceed 100%.     

焦炭流化床燃烧条件下氧化亚氮生成过程的实验研究

在小型流化床实验台上对一种无烟煤焦炭燃烧过程中氧化亚氮的生成途径进行了实验研究,实验结果表明,在焦炭燃烧过程中HCN的氧化反应是焦炭氮向N2O转化的一条途径。同时,NO与焦炭表面的多相反应也是N2O的一条生成途径。HCN的析出是焦炭燃烧过程中进一步脱挥发份的结果,当焦炭脱挥发份过程结束后,N2O来源于NO和焦炭表面的气固多相反应。     

循环流化床中颗粒内循环与循环流化床锅炉的设计

本文从循环床锅炉密相区的热平衡计算出发，探讨了密相区内受热面面积及密相区高度与飞灰循环倍率、密相区燃烧份额的关系，并以４种典型煤种为例，分析了煤种变化对密相区高度的影响。设计计算和运行经验相结合，在密相区热平衡分析中，引入了床内粒子循环的概念，从而对密相区内热平衡和受热面面积的确定有更深入的理解。