Microscopic insight into catalytic HCN removal over the CuO surface in chemical looping combustion

Hydrogen cyanide (HCN) is well-accepted as a main nitrogen-containing precursor from fuel nitrogen to nitrogen oxides. When using coal as fuel with a CuO-based oxygen carrier in chemical looping combustion (CLC), complex heterogeneous reactions exist among the system of HCN, O₂, NO, H₂O, and CuO particles. This work performs density functional theory (DFT) calculations to systematically probe the microscopic HCN heterogeneous reactions over the CuO particle surface. The results indicate that HCN is chemisorbed on the CuO surface, and the third dissociation step within the consecutive three-step HCN dissociations (HCN*→CN*→NCO*→N*) is the rate-determining step. Namely, the CN*/NCO* radicals can be deemed as an indicator of the performance of HCN removal due to their quite higher dissociation energies. With the existence of O₂, H₂O, and NO, the reaction mechanism of HCN conversion becomes extremely complex. Both DFT calculations and kinetic analyses determine that O₂, NO, and H₂O all significantly accelerate the consumption of CN*/NCO* radicals to produce various N-containing species (NOx or NH₃) to different extents. Finally, a skeletal reaction network in a system of O₂/NO/H₂O/HCN is concluded, which clearly elucidates that CuO exhibits excellent catalytic activity toward HCN removal.     

Nitrogen release during reaction of coal char with O2, CO2, and H2O

The chemistry of char-N release and conversion to nitrogen-containing products has been probed by studying its release and reactions with O₂, CO₂, and H₂O. The experiments were performed in a fixed bed flow reactor at pressures of up to 1.0 MPa. The results show that the major nitrogen-containing products observed depend on the reactant gas; with O₂, NO, and N₂ being the major species observed. Char-N reaction with CO₂ produces N₂ with very high selectivity over a broad range of pressures and CO₂ concentrations, and reaction with H₂O gives rise to HCN, NH₃, and N₂. Observed distributions of nitrogen-containing products are little affected by pressure when O₂ and CO₂ are the reactant gases, but increasing pressures in the reaction with H₂O results in the formation of increasing proportions of NH₃. Formation of NH₃ is also promoted by increasing concentrations of H₂O in the feed gas. The results suggest that NO and HCN are primary products when O₂ and H₂O, respectively, are used as the reactant gases, and that the other observed products arise from interactions of these primary products with the char surface.     

Development of a wide range-operable,rich-lean low-NOx combustor for NH3 fuel gas-turbine power generation

Low-NOx NH₃-air combustion power generation technology was developed by using a 50-kWe class micro gas-turbine system at the National Institute of Advanced Industrial Science and Technology (AIST), Japan, for the first time. Based on the global demand for carbon-free power generation as well as recent advances involving gas-turbine technologies, such as heat-regenerative cycles, rapid fuel mixing using strong swirling flows, and two-stage combustion with equivalence ratio control, we developed a low-NOx NH₃-air non-premixed combustor for the gas-turbine system. Considering a previously performed numerical analysis, which proved that the NO reduction level depends on the equivalence ratio of the primary combustion zone in a NH₃-air swirl burner, an experimental study using a combustor test rig was carried out. Results showed that eliminating air flow through primary dilution holes moves the point of the lowest NO emissions to the lesser fuel flow rate. Based on findings derived by using a test rig, a rich-lean low NOx combustor was newly manufactured for actual gas-turbine operations. As a result, the NH₃ single fueled low-NOx combustion gas-turbine power generation using the rich-lean combustion concept succeeded over a wide range of power and rotational speeds, i.e., below 10–40 kWe and 75,000–80,000?rpm, respectively. The NO emissions were reduced to 337?ppm (16% O₂), which was about one-third of that of the base system. Simultaneously, unburnt NH₃ was reduced significantly, especially at the low electrical power output, which was indicative of the wider operating range with high combustion efficiency. In addition, N₂O emissions, which have a large Global Warming Potential (GWP) of 298, were reduced significantly, thus demonstrating the potential of NH₃ gas-turbine power generation with low environmental impacts.     

Influence of wall heat loss on the emission characteristics of premixed ammonia-air swirling flames interacting with the combustor wall

The influence of wall heat loss on the emission characteristics of ammonia-air swirling flames has been investigated employing Planar Laser-Induced Fluorescence imaging of OH radicals and Fourier Transform Infrared spectrometry of the exhaust gases in combustors with insulated and uninsulated walls over a range of equivalence ratios, ?, and pressures up to 0.5 MPa. Strong influence of wall heat loss on the flames led to quenching of the flame front near the combustor wall at 0.1 MPa, resulting in large unburned NH₃ emissions, and inhibited the stabilization of flames in the outer recirculating zone (ORZ). A decrease in heat loss effects with an increase in pressure promoted extension of the fuel-rich stabilization limit owing to increased recirculation of H₂ from NH₃ decomposition in the ORZ. The influence of wall heat loss resulted in emission trends that contradict already reported trends in literature. NO emissions were found to be substantially low while unburned NH₃ and N₂O emissions were high at fuel-lean conditions during single-stage combustion, with values such as 55 ppmv of NO, 580 ppmv of N₂O and 4457 ppmv of NH₃ at ? = 0.8. In addition, the response of the flame to wall heat loss as pressure increased was more important than the effects of pressure on fuel-NO emission, thereby leading to an increase in NO emission with pressure. It was found that a reduction in wall heat loss or a sufficiently long fluid residence time in the primary combustion zone is necessary for efficient control of NH₃ and N₂O emissions in two-stage rich-lean ammonia combustors, the latter being more effective for N₂O in addition to NO control. This study demonstrates that the influence of wall heat loss should not be ignored in emissions measurements in NH₃-air combustion, and also advances the understanding of previous studies on ammonia micro gas turbines.     

Effects of Ca and Na acetates on nitrogen transformation during sewage sludge pyrolysis

The study reports the effects of Ca and Na acetates on the transformation of nitrogen species during sewage sludge pyrolysis. Sludge samples, with or without acetates, were pyrolysed in a fix-bed reactor at 150–550 °C, and the nitrogen species in the pyrolysis products (char, tar, and gas) were characterised and quantified. Ca and Na acetates distinctly affect nitrogen transformation during sludge pyrolysis, which is ascribable to their different catalytic activities for the decomposition of nitrogen species in sludge. The addition of Ca acetate is found to increase nitrogen retention in char and reduce the formation of nitrogen species in tar, which is mainly due to the suppressed decomposition of protein-N as well as the promoted formation of stable nitrogen species in char. On the other hand, the addition of Na acetate enhances the decomposition of nitrogen species in sludge, such as protein- and inorganic-N. The levels of both Ca and Na acetates are significantly reduced in the nitrogen-containing gas emissions because acetone is produced when acetates are heated, and acetone readily reacts with NH₃ to produce binary clusters or amines. Our results show that acetate addition is an important strategy for the reduction of NH₃ emission during sludge pyrolysis.     

Towards the development of an efficient low-NOx ammonia combustor for a micro gas turbine

Recent studies have demonstrated stable generation of power from pure ammonia combustion in a micro gas turbine (MGT) with a high combustion efficiency, thus overcoming some of the challenges that discouraged such applications of ammonia in the past. However, achievement of low NOx emission from ammonia combustors remains an important challenge. In this study, combustion techniques and combustor design for efficient combustion and low NOx emission from an ammonia MGT swirl combustor are proposed. The effects of fuel injection angle, combustor inlet temperature, equivalence ratio, and ambient pressure on flame stabilization and emissions were investigated in a laboratory high pressure combustion chamber. An FTIR gas analyser was employed in analysing the exhaust gases. Numerical modeling using OpenFOAM was done to better understand the dependence of NO emissions on the equivalence ratio. The result show that inclined fuel injection as opposed to vertical injection along the combustor central axis resulted to improved flame stability, and lower NH₃ and NOx emissions. Numerical and experimental results showed that a control of the equivalence ratio upstream of the combustor is critical for low NOx emission in a rich-lean ammonia combustor. NO emission had a minimum value at an upstream equivalence ratio of 1.10 in the experiments. Furthermore, NO emission was found to decrease with ambient pressure, especially for premixed combustion. For the rich-lean combustion strategy employed in this study, lower NOx emission was recorded in premixed combustion than in non-premixed combustion indicating the importance of mixture uniformity for low NOx emission from ammonia combustion. A prototype liner developed to enhance the control and uniformity of the equivalence ratio upstream of the combustor further improved ammonia combustion. With the proposed liner design, NOx emission of 42?ppmv and ammonia combustion efficiency of 99.5% were achieved at 0.3?MPa for fuel input power of 31.44?kW.     

Characterizing methane and nitric oxide interaction in oxygen-free outwardly propagating spherical flame

Co-firing methane (CH₄) and ammonia (NH₃) has attracted growing concerns as a feasible greenhouse gas reduction strategy in gas turbine-based power generation, which raises the need to better understand the interaction of methane and nitric oxide (NO) under flame conditions. In this work, laminar flame propagation of CH₄/NO mixtures at initial pressure (P_u) of 1 atm, initial temperature (T_u) of 298 K and equivalence ratios of 0.4–1.8 was experimentally investigated using a constant-volume combustion vessel. Laminar burning velocities (LBVs) and Markstein lengths were experimentally determined. A kinetic model of CH₄/NO combustion was developed with rate constants of several important reactions updated, presenting reasonable predictions on the measured LBVs of CH₄/NO mixtures. The modeling analyses reveal that the reduction of NO can proceed through two mechanisms, i.e. the hydrocarbon NO reduction mechanism and non-hydrocarbon NO reduction mechanism. Among the two mechanisms, the non-hydrocarbon NO reduction mechanism which includes reactions NO+H = N+OH, NO+O = N + O₂ and NO+N = N₂+O has a higher contribution to NO reduction at the equivalence ratio of 0.6, while the hydrocarbon NO reduction mechanism with hydrocyanic acid (HCN) as the key intermediate plays a more important role at the equivalence ratio of 1.8. NO+H = N+OH and CH₃+NOHCN+H₂O are found to be the two most sensitive reactions to promote the flame propagation, while the LBVs measured in this work are demonstrated to provide strong constraint for these reactions. Furthermore, previous CH₄/O₂/NO oxidation data measured in flow reactor and rapid compression machine were also simulated, which provides extended validation of the present model over wider conditions.     

NOx and N2O precursors (NH3 and HCN) from biomass pyrolysis: Co-pyrolysis of amino acids and cellulose, hemicellulose and lignin

Nitrogen in biomass is mainly in forms of proteins (amino acids). Glycine, glutamic acid, aspartic acid, leucine, phenylalanine and proline are the major amino acids in agricultural straw. The six amino acids were pyrolyzed individually at 800 °C in a tubular reactor in an argon atmosphere. Each amino acid sample was then pyrolyzed individually with cellulose, hemicellulose or lignin with 1:1 mixing ratio by weight under the same condition. The emissions of HCN and NH₃ were detected with a Fourier transform infrared (FTIR) spectrometer. The extent of interaction between the amino acids with cellulose, hemicellulose or lignin was determined by comparing the yields of HCN and NH₃ from co-pyrolysis with those from single amino acid pyrolysis under the same condition. The results indicate that the structure of the amino acid has a significant effect on the nitrogen transformation during pyrolysis. The mixtures undergo solid-state decomposition reactions during co-pyrolysis. The extent of interaction between the amino acids with cellulose, hemicellulose or lignin depends on the amino acid types and the components in biomass. Although single proline and leucine form no char, they give a significant amount of nitrogen-containing char when co-pyrolyzed with cellulose, hemicellulose and lignin. HCN and NH₃ yields and nitrogen conversion pathway from amino acid pyrolysis are influenced by cellulose, hemicellulose and lignin.     

Effect of alkali metals on nitrogen oxide emission: Role of Na and its occurrence in coal

High-alkali coal contains relatively high contents of alkali metals, which can be usually released in the form of gaseous chlorides and hydroxides during combustion. The effect of alkali metals on NO formation is analyzed in an electrical heated drop-tube furnace at 800–1200 °C during coal combustion. Based on experiments and simulations, the mechanisms underlying the effects of Na salts on NO emission are clarified in CO/NH₃/O₂/H₂O/Na additive (NaCl, Na₂SO₄, and NaAc) systems. The results indicate that the yield of NO initially increases and then decreases as the furnace temperature increased. As the temperature increased from 800 to 1000 °C, NO precursors (HCN and NH₃) undergo accelerated oxidation to form NO. When the furnace temperature is greater than 1000 °C, due to the rapid precipitation of volatiles, a local reducing atmosphere is present around the pulverized coal particles, which inhibits NO formation. NaCl and NaAc addition significantly inhibit NO formation. However, the inhibitory effect is weakened at higher temperatures (>1000 °C). The Na₂SO₄ additive exerts little effect on NO generation during combustion because of its stable chemical properties. The same conclusion is also obtained from gaseous experiments showing that NaCl and NaAc significantly inhibit NH₃ oxidation to form NO. Based on the results of calculations, NaCl and NaAc addition inhibits NO formation by promoting the recombination of H, O and OH and reducing the concentrations of radicals. According to the analysis of chemical reactions, the effect of NaCl and NaAc on NO formation is mainly determined by the competitive relationships among multiple reactions.     

Combustion of ethylamine,dimethylamine and diethylamine: Theoretical and kinetic modeling study

Aliphatic amines are an important class of nitrogen-containing compounds present in renewable fuels such as bio-oils. Conversion of this fuel-bound nitrogen can lead to the formation of HCN and NH₃, as well as NO_x emissions. In this work, the combustion chemistry of small aliphatic amines is investigated via a combination of quantum chemical calculations, chemical kinetic modeling and experimental validation. The influence of the degree of substitution on the nitrogen atom and the alkyl chain length on the reactivity and product distribution is studied via three model compounds, i.e. ethylamine (EA, CH₃CH₂NH₂), dimethylamine (DMA, (CH₃)₂NH) and diethylamine (DEA, (CH₃CH₂)₂NH). A detailed kinetic model containing 258 species and 2274 reactions is developed to describe their combustion over a wide range of conditions. The proposed model captures the trends in ignition delay times and species concentrations over a temperature range from 500 to 2000 K and pressures from 4 to 170 kPa. The ignition delay data are predicted with an average deviation of 10%. The difference between experimental and simulated species concentrations from laminar premixed flames is for the major species on average 50%, while the agreement for the JSR is even better, with an average deviation of 10%. The dominant decomposition pathway under all the studied conditions is a set of hydrogen abstractions from the C_α and N positions followed by β-scission of the fuel radicals. Among the unimolecular decomposition pathways, the homolytic CC and CN scissions and four-centered elimination play a minor role. HCN is the main intermediate and at temperatures above 1100 K the amines are completely converted to N₂ and NO.     

Effects of residence time on the NOx emissions of premixed ammonia-methane-air swirling flames at elevated pressure

In this study, a bespoke single-stage swirl burner was used to experimentally investigate the effects of residence time on emissions from premixed ammonia-methane-air flames. The residence time was altered in two ways: by modifying the combustion chamber's length or by modifying the swirl number. Exhaust emissions of O₂, CO₂, CO, NO, NO₂, and N₂O were measured at an absolute pressure of 2 bar for equivalence ratios between 0.50 and 0.95 and ammonia fractions in the fuel blend between 0 and 100%. Spatial distributions of NO and OH radicals were also imaged using PLIF inside the combustion chamber at different heights above the nozzle. Data shows that increasing residence time can further advance chemical reactions, as evidenced by a reduction in O₂ concentration in the exhaust. Increasing the swirl number reduces emissions of NO, NO₂, and N₂O more efficiently than tripling the chamber's length. However, a decrease in the combustion efficiency may be responsible for a fraction of this NOx reduction when the swirl number is increased for some equivalence ratios. NO emissions are not modified when the chamber's length is increased, which is consistent with the fact that the NO-LIF signal does not decay when the distance from the nozzle increases. Therefore, NO formation is somehow restricted to within the main reaction zone of the swirling flame, that is, the zone whose height does not exceed 60 mm for this burner. Conversely, tripling the chamber's length reduces the concentrations of NO₂ and N₂O. This reduction is not reflected in a measurable increase in NO concentration because NO is present in much larger quantities than NO₂ and N₂O in flames examined here. Consistent with the fact that OH promotes NO formation via fuel-NOx pathways, a positive correlation is found between NO- and OH-LIF intensities.     

Realization of oxyfuel combustion for near zero emission power generation

Oxyfuel combustion is one of the promising carbon capture and storage (CCS) technologies for coal-fired boilers. In oxyfuel combustion, combustion gas is oxygen and recirculating flue gas (FGR) and main component of combustion gas is O₂, CO₂ and H₂O rather than O₂, N₂ in air combustion. Fundamental researches showed that flame temperature and flame propagation velocity of pulverized cloud in oxyfuel combustion are lower than that in air with the same O₂ concentration due to higher heat capacity of CO₂. IHI pilot combustion test showed that stable burner combustion was obtained over 30% O₂ in secondary combustion gas and the same furnace heat transfer as that of air firing at 27% O₂ in overall combustion gas. Compared to emissions in air combustion, NOx emission per unit combustion energy decreased to 1/3 due to reducing NOx in the FGR, and SO_x emission was 30% lower. However SO_x concentration in the furnace for the oxyfuel mode was three to four times greater than for the air mode due to lower flow rate of exhaust gas. The higher SO₃ concentration results that the sulphuric acid dew point increases 15–20 °C compared to the air combustion. These results confirmed the oxyfuel pulverized coal combustion is reliable and promising technology for coal firing power plant for CCS.In 2008, based on R&D and a feasibility study of commercial plants, the Callide Oxyfuel Project was started in order to demonstrate entire oxyfuel CCS power plant system for the first time in the world. The general scope and progress of the project are introduced here. Finally, challenges for present and next generation oxyfuel combustion power plant technologies are addressed.     

Experimental study on co-firing characteristics of ammonia with pulverized coal in a staged combustion drop tube furnace

Utilizing ammonia as a co-firing fuel to replace amounts of fossil fuel seems a feasible solution to reduce carbon emissions in existing pulverized coal-fired power plants. However, there are some problems needed to be considered when treating ammonia as a fuel, such as low flame stability, low combustion efficiency, and high NO_x emission. In this study, the co-firing characteristics of ammonia with pulverized coal are studied in a drop tube furnace with staged combustion strategy. Results showed that staged combustion would play a key role in reducing NO_x emissions by reducing the production of char-NO_x and fuel(NH₃)-NO_x simultaneously. Furthermore, the effects of different ammonia co-firing methods on the flue gas properties and unburned carbon contents were compared to achieve both efficient combustion and low NO_x emission. It was found that when ammonia was injected into 300 mm downstream under the condition of 20% co-firing, lower NO_x emission and unburnt carbon content than those of pure coal combustion can be achieved. This is probably caused by a combined effect of a high local equivalence ratio of NH₃/air and the prominent denitration effect of NH₃ in the vicinity of the NH₃ downstream injection location. In addition, NO_x emissions can be kept at approximately the same level as coal combustion when the co-firing ratio is below 30%. And the influence of reaction temperature on NO_x emissions is closely associated with the denitration efficiency of the NH₃. Almost no ammonia slip has been detected for any injection methods and co-firing ratio in the studied conditions. Thus, it can be confirmed that ammonia can be used as an alternative fuel to realize CO₂ reduction without extensive retrofitting works. And the NO_x emission can be reduced by producing a locally NH₃ flame zone with a high equivalence ratio as well as ensuring adequate residence time.     

Atomic and molecular adsorption on Pd(111)

The adsorption properties of a variety of atomic species (H, O, N, S, and C), molecular species (N₂, HCN, CO, NO, and NH₃) and molecular fragments (CN, NH₂, NH, CH₃, CH₂, CH, HNO, NOH, and OH) are calculated on the (111) facet of palladium using periodic self-consistent density functional theory (DFT–GGA) calculations at ¼ ML coverage. For each species, we determine the optimal binding geometry and corresponding binding energy. The vibrational frequencies of these adsorbed species are calculated and are found to be in good agreement with experimental values that have been reported in literature. From the binding energies, we calculate potential energy surfaces for the decomposition of NO, CO, N₂, NH₃, and CH₄ on Pd(111), showing that only the decomposition of NO is thermochemically preferred to its molecular desorption.     

Interactions of hydrogen and nitric oxide in outwardly propagating spherical flame: Insight into non-hydrocarbon NOX reduction mechanism

Co-firing ammonia (NH₃) and hydrogen (H₂) or H₂-rich fuel and partially cracking NH₃ are promising non-carbon combustion techniques for gas turbines and marine engines, raising a growing need to understand the interactions of H₂ and nitric oxide (NO) as well as the non-hydrocarbon nitrogen oxides (NO_X) reduction mechanism under flame conditions. In this work, the outwardly propagating spherical flame method was used to investigate the laminar flame propagation of H₂/NO and H₂/NO/nitrogen (N₂) mixtures at initial pressure (P_u) of 2 atm, initial temperature (T_u) of 298 K and equivalence ratios of 0.2-1.4. The laminar burning velocities (LBVs) of H₂/NO mixtures are generally 5-10 times lower than those of H₂/air mixtures, while the dilution of N₂ can dramatically inhibit the laminar flame propagation. A kinetic model of H₂/NO combustion was constructed and validated against the new data in this work and other types of experimental data in literature. The modeling analyses reveal that NO+H=N+OH becomes the most important chain-branching reaction in H₂/NO reaction system, while the LBV data of H₂/NO mixtures in this work can provide highly sensitive validation targets for the kinetics in H₂ and NO interactions. Furthermore, the NO reduction to N₂ mainly proceeds through NO+N=N₂+O under various H₂/NO ratios, and NO+O=N+O₂ is found to have a significant contribution to NO reduction under NO-rich conditions.     

Treating NOx emission of hydrogen fueled combustion engines by NOx storage and reduction catalysts: A transient kinetic study including PLIF measurements

NOx storage and reduction (NSR) catalysts are a well-known and broadly used technology to reduce NOx emissions from combustion engines, which may also be applied for hydrogen fueled engines in the future. In this study, Pt- and Pd-based NSR-catalysts were investigated in the absence and presence of water to understand how NO oxidation as well as the storage and reduction phases are influenced by the gaseous environment with H₂ as a reductant. A planar channel configuration was chosen for conducting planar laser-induced fluorescence experiments during the storage phase in addition to steady-state oxidation measurements and transient lean/rich cycles in a packed bed reactor. The presence of steam significantly decreases the NO oxidation activity of both noble metal catalysts. The Pt/BaO/Al₂O₃ catalyst is more active during transient lean/rich cycles, however, it suffers an activity loss during repeated cycles, whereas the activity of the Pd/BaO/Al₂O₃ sample is slightly more stable in the wet gas feed over time. All experiments showed a strong correlation between the NO₂ formation over the catalyst and its storage capability. The influence of water in the exhaust gas on the NSR-catalysts shows a strong temperature dependency on storage and reduction of NO for both catalysts containing Pt and Pd. The storage behavior is also strongly influenced by both the experimental configurations chosen revealing the significance of the interaction of intrinsic catalytic kinetics and mass transfer in the surrounding flow field.     

Characterizing ammonia and nitric oxide interaction with outwardly propagating spherical flame method

With the growing attention on ammonia (NH₃) combustion, understanding NH₃ and nitric oxide (NO) interaction at temperatures higher than DeNOx temperature region or even flame temperature becomes a new research need. In this work, the outwardly propagation spherical flame method was used to investigate the laminar flame propagation of NH₃/NO/N₂ mixtures and constrain the uncertainties of the specific kinetics. The present experiments were conducted at initial pressure of 1 atm, temperature of 298 K and equivalence ratios from 1.1 to 1.9. A kinetic model of NH₃/NO combustion was updated from our previous work. Compared with several previous models, the present model can reasonably reproduce the laminar burning velocity data measured in this work and speciation data in literature. Based on model analyses, the interaction of NH₃ and NO was thoroughly investigated. As both the oxidizer and a carrier of nitrogen element, NO frequently reacts with different decomposition products of NH₃ including NH₂, NH and NNH, and converts nitrogen element to the final product N₂. It is found that the laminar burning velocity experiment of NH₃/NO/N₂ mixtures using the outwardly propagating spherical flame method can provide highly sensitive validation targets for the kinetics in NH₃ and NO interaction.     

Understanding the effect of CaO on HCN conversion and NOx formation during the circulating fluidized combustion process using DFT calculations

Hydrogen cyanide (HCN) is an important intermediate during the conversion of fuel nitrogen to NO_x. The mechanism of HCN oxidation to NO, N₂, and N₂O on the CaO (100) surface model was investigated using density functional theory calculations to elucidate the effect of in-furnace SO_x removal on HCN oxidation in circulating fluidized bed boilers. HCN adsorption on the CaO (100) surface releases as high as 1.396 eV and the HC bond is strongly activated. The CaO (100) surface could catalyze the oxidation of CN radical to NCO with the energy barrier decreasing from 1.560 eV for the homogeneous case to 0.766 eV on the CaO (100) surface. The succeeding oxidation of NCO by O₂ forming NO is catalyzed by the CaO (100) surface with the energy barrier decreasing from 0.349 eV (homogeneous process) to 0.026 eV on the CaO (100) surface, while the reaction between NCO and NO forming either NO or N₂ is prohibited in comparison with corresponding homogeneous routes. The rate constants of these reactions under fluidized bed combustion temperature range are provided, and the calculation results lead to the conclusion that CaO (100) surface catalyzes the HCN conversion and improves the NO selectivity during HCN oxidation in the HCN/O₂/NO atmosphere, which could well explain previous experimental observations. Kinetic parameters of HCN oxidation on the CaO (100) surface are provided in the Arrhenius form for future kinetic model development.     

程序升温气化过程中氮的释放规律研究

采用 U 型管反应系统研究了不同煤种在程序升温条件下在四种气氛中反应时燃料氮的释放规律.研究发现热解时主要的含氮产物为 HCN,如果煤中含有较多的氧,即使在惰性气氛下也会释放出 NO.没有检测到 NH3 的生成,证实NH3 来源于 HCN 在焦表面的二次反应.N2O 仅在有氧气氛中气化时生成,说明氧气是 N2O 生成的必要条件.气化时的总固定氮 TFN 高于热解时,且气化剂中 O2 含量越高则总固定氮 TFN 越高.     

An experimental and numerical investigation of the catalytic-rich/gaseous-lean combustion of H2/CO/air mixtures at 8 bar

The catalytic-rich/gaseous-lean (R/L) combustion concept was investigated experimentally and numerically for syngas fuels with H₂:CO volumetric ratios 1:0, 4:1 and 1:2, catalytic-rich stoichiometries φ_rich = 2–10 (including operation without air), pressure of 8 bar and air preheat of 673 K. Experiments were performed in a subscale R/L burner with optical access to both catalytic-rich and gaseous-lean stages. OH-PLIF monitored the turbulent combustion in the gaseous-lean stage, OH*-chemiluminescence assessed the propensity for homogeneous ignition in the catalytic-rich stage, and exhaust gas analysis provided the NOx and CO emissions. Two-dimensional simulations were carried out for both stages, while a 1-D opposed-jet code modeled the NOx emissions. The exothermicity of the heterogeneous reactions promoted homogeneous ignition and flame anchoring in the upstream parts of the catalytic-rich stage and allowed for complete consumption of the deficient O₂ reactant, a process that could not be achieved by the catalytic pathway alone due to transport limitations. Homogeneous combustion in the catalytic-rich stage was beneficial for attaining the highest possible fuel pre-conversion. The catalyst not only initiated gaseous combustion but also mitigated potential NOx emissions from the catalytic-rich stage at the highest pre-conversions (lowest φ_rich) and highest CO-content mixtures. Two-sided diffusion flames were established in the gaseous-lean stage due to the recirculation of O₂-rich combustion products, which was advantageous for the burner compactness. It was shown that cardinal to the R/L concept was the fact that a decreasing φ_rich led to an increased heat transfer from the catalytic-rich stage to the bypass air, which reduced the enthalpy in the fuel stream of the gaseous-lean stage and thus lowered the peak flame temperatures (by 400 K for H₂:CO = 1:0). The reduction in flame temperatures with decreasing φ_rich led to a six-fold drop in NOx emissions, while CO emissions were less than 5 ppmv.