スマートプロセス学会誌 Vol. 5(2016) No. 2

  The conserved scalar approach is widely used for a reaction model in the numerical simulation of turbulent diffusion flames. This model is based on the assumption that all the effective diffusion coefficients of scalar quantities, such as chemical species concentrations and heat, are the same. When turbulent diffusion coefficient is much larger than molecular one, this assumption is applicable. However the assumption would cause serious error if molecular diffusion cannot be neglected and differential diffusion takes place among various species. Then, in the present study, experimental examination was carried out on the diffusion rate of chemical species in turbulent diffusion flames, using two kind of coaxial jet flames which are different from each other in turbulence condition. The mixture of CO and H₂, which have much different diffusivities, was used as fuel. The results of the study can be summarized as follows: (1) The assumption that all the effective diffusion coefficients of scalar quantities are the same can be effectively applied to triple jet diffusion flames, where the combustion field is surrounded by strong turbulence. However, this assumption is difficult to apply to flames with weak turbulence, such as coaxial jet diffusion flames. (2) The findings in above should be considered for the modeling of jet diffusion flames used for biomass-gas fuel and syngas.

  Recently, biomass attracts much attention as a renewable energy resource. Pyrolytic gasification and carbonization are major processes for the utilization of biomass. Biomass gasification and carbonization are already in practical use; however, gasification systems are not enough to be economically viable, and carbonization is low energy efficiency process. So the co-production of carbide and fuel is being examined in late years. To develop the co-production system, the knowledge about biomass gasification under the temperature which is suitable for carbonization is needed. A lot of researches about biomass gasification are conducted so far; however most of them explore the phenomena under the temperature more than around 1073 K while the temperature for carbonization is around from 773 K to 973 K. Further, in order to achieve systematic development of that kind of co-production system, the information about pyrolysis behavior of biomass under various atmosphere gas composition and of various specimen size should be investigated. In this study, we tried to investigate the transient gasification behavior of a biomass under the carbonization temperature 873 K and the coexistence atmosphere of H₂O-CO₂. The experimental results show that the transient gasification behavior of a biomass under carbonization temperature has several similar features as that under gasification temperature qualitatively; however, further information about pyrolysis behavior and the scale effect of the material should be investigated.

  To investigate the influence of briquette size on both flaming combustion and char combustion of highly densified biomass briquette, combustion experiments were carried out with the briquette made of cypress sawdust. In this study, three sizes of cylindrical briquette of which the ratio of height to diameter is kept at 2 ((1) 30 mm in diameter and 60 mm in height, (2) 35 mm in diameter and 70 mm in height and (3) 40 mm in diameter and 80 mm in height) were used as fuel samples. The density of the briquette and the moisture content are kept constant at 1300 kg/m³ and 10 wt%, respectively. A single briquette is inserted into the furnace to which preheated air (673K, 1.42 m/s) is supplied and the mass loss rate and each combustion duration were measured. As a result, in the flaming combustion duration, it is observed that the average mass loss rate ((dM/dt)_mean) of briquette per specific surface area (S/V, V: volume) is proportional to the surface area (S) of the briquette. In the briquette size used in the study, it is demonstrated by introducing Fourier number that the flaming combustion duration (t_f) is proportional to the square of an inverse number of the specific surface area (t_f ∝ (V/S)²). The normalized mass loss rate ((dM/M₀)/(dt/t_f)) in the normalized flaming combustion duration for all three briquette size can be expressed in a single curve. It is shown that the char combustion duration (t_c) is also proportional to the square of an inverse number of the specific surface area (t_c∝ (V/S)²). For char combustion, shrinking-core reaction model was applied to the cylindrical briquette used in the study and the oxygen diffusion coefficients through both the gas boundary layer (K_c) and the ash layer (D_c) were determined by the experimental results. The oxygen diffusion coefficient through the ash layer (D_c) is lower than that reported for lower density one and this may be attributed to the lower porosity of the highly densified briquette in this study. It is successfully demonstrated that the unburnt fraction in the normalized char combustion duration can be expressed by the model.

  Gasification is one of the methods for efficiently utilizing solid fuels such as biomass and coal. A fluidized bed gasifier usually operates at low temperature and can handle fuel more easily. For these reasons, construction cost is low and maintenance is easy. However, the high concentration tar is generated together with syngas. The tar discharged to outside the gasifier is processed by methods for instance, high temperature reformers, scrubbers, or adsorption by activated carbon. This brings about increase in running costs. The traditional evaluation index of the tar is concentration in the syngas, for example “g/m³_N”. It's necessary to elucidate the details of tar composition to optimize a gasification system. The purpose of this study is to analyze in detail the tar produced by using a laboratory-scale fluidized bed gasifier. The experimental condition is steam gasification of lignite at 1123K. The conventional methods for measuring the tar are gas chromatograph mass spectrometry (GC/MS) and gas chromatograph flame ionization detector (GC-FID). These methods are suitable for identification of low-molecular compounds in tar. Additionally, the new measuring methods, which are field desorption mass spectrometry (FD-MS) and matrix assisted laser desorption mass spectrometry (MALDI-MS), were adopted in this study. The molecular weight distribution of tar is clarified by these methods. As results, it was found that the tar, which was obtained by the gasification experiment, contains polycyclic aromatic hydrocarbon (PAH) with molecular weight range in 100-600 mainly. Therefore, details of tar in syngas, which is produced by steam gasification of lignite, was clarified by a number of analytical methods such as GC/MS and FD-MS.

  It is well known that the performance and the safety may be hindered because of the clinker formation, when high ash content biomass solid fuel is applied to small scale combustion appliances. This study is aimed for investigation of the clinker formation mechanism of high ash content biomass solid fuel. It gives the suggestion that is very important to development and the improvement of the combustion apparatus to arrange clinker formation mechanism of the biomass solid fuel. In this study, the clinker formation situation by the difference that was in combustion form was classified through the combustion experiment using the commercial combustion apparatus. Furthermore, important knowledge considered to help development and the improvement of the combustion apparatus was provided.

  Lean premixed combustion is one of the most promising techniques to reduce nitrogen oxide (NO_X) emissions. However, lean premixed combustors have a narrow stable combustion range compared with diffusion combustors and have a risk of flashback. Flashback into the fuel nozzle causes serious damage to combustors. So, in this study we aim to clarify the relationship between the flame behavior and the flow fields in the premixed combustors with swirling flow by applying high-speed stereo particle image velocimetry (PIV) measurement. In this measurement, use of olive oil particles as tracer particles permits us to capture not only the flow fields but also the flame behavior. Then, we prepared two experimental conditions on the flow fields which have different swirl intensity; swirl number S = 0.242 and 0.633. These different conditions on swirl intensity can be realized by using a variable swirler whose vane angle is changeable. This variable swirler makes it possible to independently change of swirl intensity. In S = 0.633, it is clarified that both axial velocity and circumferential velocity of premixed gas upstream the flame tip have relationships with flame propagation velocity. In short, the local strengthening of swirl intensity in the vicinity of flame tip influences the flame propagation velocity when swirl intensity is strong. On the other hand, it is found that flame propagation velocity has relation to only axial velocity of premixed gas under S = 0.242. Briefly, the change in eddy structure of swirling flow influences the flame propagation velocity when swirl intensity is weak.

  We found that the laser irradiated surface of copper substrate becomes fine structure, which we named "surface fine crevice structure". Liquid bismuth on “surface fine crevice structure” of copper substrate occurs “unusual wetting”. In the present work, we investigated the wettability of “surface fine crevice structure” of iron substrate by liquid tin, indium and bismuth. “Unusual wetting”appears on laser irradiated surface of iron substrate in case of liquid indium and tin. Whereas, liquid bismuth does not show “unusual wetting” on laser induced iron surface under the present experimental conditions.