Hirotatsu Watanabe

Impact of Equivalence Ratio on the Macrostructure of Premixed Swirling CH4/Air and CH4/O2/CO2 Flames

Premixed CH4/O2/CO2 flames (oxy-flames) and CH4/air flames (air-flames) were experimentally studied in a swirl-stabilized combustor. For comparing oxy and air flames, the same equivalence ratio and adiabatic flame temperature were used. CO2 dilution was adjusted to attain the same adiabatic temperature for the oxy-flame and the corresponding air-flame while keeping the equivalence ratio and Reynolds number (=20,000) the same. For high equivalence ratios, we observed flames stabilized along the inner and outer shear layers of the swirling flow and sudden expansion, respectively, in both flames. However, one notable difference between the two flames appears as the equivalence ratio reaches 0.60. At this point, the outer shear layer flame disappears in the air-flame while it persists in the oxy-flame, despite the lower burning velocity of the oxy-flame. Prior PIV measurements (Ref. 9) showed that the strains along the outer shear layer are higher than along the inner shear layer. Therefore, the extinction strain rates in both flames were calculated using a counter-flow premixed twin flame configuration. Calculations at the equivalence ratio of 0.60 show that the extinction strain rate is higher in the oxy than in the air flame, which help explain why it persists on the outer shear layer with higher strain rate. It is likely that extinction strain rates contribute to the oxy-flame stabilization when air flame extinguish in the outer shear layer. However, the trend reverses at higher equivalence ratio, and the cross point of the extinction strain rate appears at equivalence ratio of 0.64.Copyright

Experimental and Numerical Investigation of Biomass Pyrolysis Process Focusing on Intra-Particle Heat Transfer

The purpose of this research was to investigate biomass pyrolysis process focusing on intra-particle heat transfer. Thermal decomposition characteristics of wood cylinder with a diameter of 8mm were studied experimentally and numerically. In an experiment, a thermobalance reactor was used to investigate weight loss of wood cylinder during the pyrolysis. Three K-type thermocouples with a diameter of 0.5 mm were placed in the sample to measure the intra-particle temperature. Wood cylinders were heated by infrared furnace under inert gas at 1 Ks−1 and 30 Ks−1 . In a calculation, unsteady two-dimensional heat and mass transfer equations were discretized by using Finite Volume Method with first order implicit scheme. The reaction kinetics of biomass pyrolysis were modeled by using a multi-step kinetic scheme. To investigate the effect of intra-particle heat transfer, calculations with considering temperature gradient and with uniform temperature were carried out. As a result, the calculation results with considering intra-particle heat transfer were in good agreement with experimental ones, while calculation results without considering temperature gradient were quite different from experimental ones at high heating rate. Intra-particle heat transfer mechanism at low heating rate was quite different from that at high heating rate. Both numerical and experimental results showed that there was a distinct peak of intra-particle temperature due to strong exothermic reaction at low heating rate. Meanwhile, endothermic reactions were dominant at high heating rate, and there was no temperature peak. Moreover, an increase in the slope of temperature history observed at high heating rate. It was difficult to explain the slope increase by only weak exothermic reactions. This was because that heat capacity was decreased significantly during pyrolysis. When the heating rate was high, the yield of volatile matter whose heat capacity was quite less than that of char or wood was increased. It was shown that volatile and char formation characteristics were strongly related with intra-particle heat transfer characteristics.© 2010 ASME

A Proposal of Waste Heat Recovery System Through Methanol Steam Reforming Integrated With Absorption Heat Pump

Through the methanol steam reforming (MSR), the energy of low temperature waste heat (100–150°C) can be recovered into that of hydrogen. However, actual MSR requires over 200°C to enable the high conversion of methanol into hydrogen. In this research, two types of combined absorption heat pump (AHP) and MSR systems were proposed: one-pass system and steam recycling system. The role of the AHP is to enhance the temperature level of the waste steam up to 230°C, which is used for the MSR. To evaluate the performances of these systems, “energy enhancement factor” was defined. As a result, the recovered energy of the waste heat was almost three times as much as the required work for the AHP when the reaction temperature and waste heat temperature and S/C ratio were 210°C and 150°C and 4.0, respectively. It was also indicated that the steam recycling was more effective at the higher reaction temperature and lower waste heat temperature and higher S/C ratio.Copyright

Effects of Heterogeneous Reaction and Slow/Rapid Volatilization Process on N2O Formation During the Combustion of Pulverized Biomass

The purpose of this study is to clarify the fundamental and general features of N2 O formation during the combustion of pulverized biomass under low temperature. First, the effect of various important factors, i.e., volatilization process (i.e., either slow or rapid dispersion), oxygen concentration, and solid-gas reaction on N2 O formation were investigated by theoretical analysis. The analysis of the effect of the slow/rapid volatilization process on the formation of nitrous oxide showed that the conversion ratio of biomass-N to N2 O increases with the decrease in the dispersion of volatile matter per unit time; it means that biomass-N is effectively converted to N2 O during slow volatilization. The analysis of the effect of initial oxygen concentration on the formation of nitrous oxide showed that at low temperature (T = 1100K), the level of N2 O emission increases, while that of NO emission decreases, with the decrease in initial oxygen concentration. In other words, there is a trade-off relationship between the formation of NO and that of N2 O. With respect to the effect of solid-gas reaction, the gasification reactions between CO2 , O2 , and C(s) occur simultaneously on the surface of biomass particles during combustion. Further, the N2 O emission level increases with the increase in N-content of the biomass, while the NO emission level remains constant during low-temperature combustion.Copyright

NO Reduction Mechanism Peculiar to O2/CO2 Coal Combustion Characterized by High CO2 Concentration

The characteristics of NOx reduction in O2 /CO2 combustion were investigated experimentally and numerically. Experiments showed that the exhaust CO concentration in O2 /CO2 combustion was almost twice as much as that in air combustion. This was because high CO2 concentration enhanced the reaction (CO2 + H → CO + OH). A mass of OH radicals led to the oxidation of NH3 and HCN. The sum of the nitrogen-species in O2 /CO2 combustion was less than that in air combustion. It meant that NO formation suppression effect was high in O2 /CO2 combustion. N2 conversion ratio became an effective guide to show NO formation suppression effect quantitatively. Calculations showed that N2 conversion ratio was high in O2 /CO2 combustion and the unique NO reduction scheme caused by high CO2 concentration. This scheme proceeded under high CO and OH concentration. It was shown that O2 /CO2 combustion was suited for reducing nitrogen-oxide due to the unique NO reduction scheme.Copyright

The Characteristics of Recycled-NO Reduction in O2/CO2 Coal Combustion

O2 /CO2 coal combustion is a promising technology for easy CO2 recovery and low NOx emission. O2 /CO2 coal combustion system has very high CO2 concentration, and so O2 /CO2 ratio is very important parameter in this system. In this study, the effect of CO2 concentration on recycled NO reduction was investigated experimentally and numerically. It was revealed that the recycled NO reduction ratio was increased with decreasing CO2 concentration. This was because high CO2 concentration enhanced the reaction to produce OH radical (CO2 + H → CO + OH). Both calculations and experiments showed OH radical increased with an increase in CO2 concentration. However, increasing OH radical enhanced the consumption of H2 which is needed for the reduction of secondary NO. It was shown that high CO2 concentration inhibits recycled-NO reduction from the new point of CO2 reactivity.Copyright

The Effect of the Number of Computational Grids on Calculation Results of Co-axial Jet Flows by Large Eddy Simulation Using Dynamic SGS Model

A properly designed co-axial jet would efficiently mix the air and the fuel, and provides the clean combustion. It is important to carry out LES by using the appropriate number of computational grids in order to apply the LES to many combustion systems. In this study, LES using the Smagorinsky model and the dynamic SGS model for co-axial jet flows is carried out, and the effect of the number of computational grids on calculation results is investigated.