According to the supply trend of primary energy resources, the resources of such fossil fuels as oil, natural gas and coal have still been consumed steadily worldwide. Among those fossil fuels, the coal resource has the highest ratio of reserves to production. On the other hand, the global environmental issues have strongly been focused recently. In these points of view, the coal must be utilized efficiently and ecologically as well as carbon neutral resources like biomass and solid wastes should also be utilized more effectively. The coal, biomass and solid wastes are generally categorized as solid fuel. Even for the biomass, it is necessary to convert it high-efficiently and environment-friendly. However, those solid fuels have not only their own structural difficulties, but also compositional diversity. In order to solve those difficulties, the harmonization between the theory or model and the social needs is necessary, and the interdisciplinary cooperation will also be effective.
Particle combustion of solid carbon in the quiescent atmosphere has been overviewed by use of accomplishment in the aerothermochemical analyses, in which not only the surface C-O2 and C-CO2 reactions but also the gas-phase CO-O2 reaction is taken into account. By virtue of the generalized species-enthalpy coupling functions, close coupling of those reactions has been elucidated. It has been identified that the combustion response in the three limiting situations, such as the Frozen, the Flame-detached and the Flame-attached modes, can be described analytically, by using the generalized coupling functions. Explicit combustion-rate expressions by use of the transfer number in terms of the natural logarithmic term, just like that for droplet combustion, have further been obtained for the combustion response in the limiting situations. In addition, by examining establishment of CO-flame over the carbon particle, it has been confirmed that the combustion rate can fairly be represented by the expression in the Frozen mode when the particle diameter is 100 μm or less. Since this expression is explicit and has fair accuracy, various contributions are anticipated not only qualitatively but also quantitatively. Combustion response in the transient situation has further been examined, from which an existence of the critical size for the particle burn-out has even been derived. A fair degree of agreement in experimental comparisons indicates that the present formulation has captured the essential features of the particle burn-out.
Pyrolysis of cylindrical woody biomass has been investigated both numerically and experimentally with emphasis on intra-particle transport phenomena. Intra-particle structure was visualized with x-ray CT images, and pore along axial direction was seen. To make a calculation, a two-dimensional, unsteady state, single particle model was used, and the convective and radiative heat fluxes were given to the top and side surface of all wood cylinders. Both calculation and experiment showed that tar yield was decreased when the length of the biomass was increased and the diameter was kept constant. The calculation showed that, first, tar was formed in the wood cylinder, and then it moved to outwards during decomposition. When the intra-particle temperature gradient was large, primary tar, which has been formed in the biomass with low temperature, passed through the side surface layer at high temperatures, enough to advance intra-particle tar decomposition before the tar was released. This resulted in the enhancement of intra-particle tar decomposition.
The thermal radiation between fly ash and the solid wall of high temperature or low temperature, was applied to decomposition of dioxins contained in fly ash in an exhaust gas. We used a solid radiation type heating apparatus designed to carry out the temperature control of the solid wall with a tubular flame and an electrically heated wire, the maximum processing gas volume of 350Nm3/h, which has a radiation wall of the cylinder type which consists of 450 mm in inside diameter, and the effective length of 1000 mm. We investigated the decomposition characteristics of the dioxins contained in fly ash using imitation gas and actual exhaust gas of an industrial-wastes incinerator. We found that the surface unevenness becomes small notably by radiation heating of fly ash, and the maximum dioxins decomposition rate of about 66%, in the holding time 0.2s, without change of the homologue distribution of dioxins, was obtained. The value of the chloride concentration after radiation processing of fly ash decreased below in half in before processing.
There are various forms in solid combustion, and sometimes interesting combustion behavior may be seen there. When paper is burned by smoldering combustion on the conditions where the natural convection was restricted such as in a microgravity environment, or inside a narrow space, a fingering pattern of combustion may be formed due to combustion instability. Although it is supposed that this instability arises by almost the same mechanism as the thermal-diffusive instability (the Lewis number effect) and some stability analyses are carried out so far, there have been few researches performed where the mechanism is examined by reproducing a phenomenon by a numerical simulation. In the present research, the objective is to elucidate the mechanism of fingering pattern formation, by building the numerical model of the fingering pattern formation in smoldering combustion, and by establishing the prediction method of a fingering pattern. It is expected that clarifying the details of this phenomenon deepens combustion study understanding, and also gives the useful information for fire safety.
The numerical works about ultra-micro combustors and related fundamental phenomena are introduced. Numerical simulation can be very powerful tool because the physical quantities which can be measured by experiments are often limited. Wall effects become stronger as the size of the combustor decreases; thus, thermal and radical quenching on the wall may have a large influence on the combustion characteristics in an ultra-micro combustor. We have featured wall effects for combustion characteristics. First, the results of gas-phase H2-air or CH4-air reaction near a wall using numerical simulation with or without surface reactions in a narrow parallel-plate channel are shown. Second, the numerical simulation of a combustor with porous medium injector is shown. Finally, the numerical simulation modeled real ultra-micro combustor with new approach for porous medium injector and surface reaction is introduced.
Since energy density of hydrocarbon fuels is tens of times larger than that of lithium-ion batteries, micro-scale combustion attracts much attention to develop mobile high-added-value energy source with prolonged operation time. In micro-scale combustion, however, surface to volume ratio becomes large, so that gas-phase combustion is difficult to sustain due to both wall thermal and chemical effects. In this paper, recent progress on our study of the wall thermal/chemical quenching effects is presented. First, oscillating flames induced by the thermal effect in micro quartz plate channels are introduced. Second, effects of wall material on the chemical quenching behavior in narrow quartz plate channels are reported. Quartz, alumina and platinum are chosen as the wall surface materials. Alumina and platinum thin films ∼100 nm in thickness are deposited on quartz substrates using atomic layer deposition or vacuum arc plasma gun techniques to establish equivalent thermal boundary condition with different wall chemical reactions. Through the OH-PLIF measurement, it is shown that the chemical effect starts to take over the thermal effect at high wall temperature at around 800 °C, resulting in the highest OH* concentration in the vicinity of the alumina surface. By using a radical quenching model, the initial sticking coefficient associated with radical adsorption is estimated to be 0 and 0.01 for the alumina and the quartz surfaces, respectively. Thus, radical quenching does exist on the quartz surface, while the alumina works as an inert surface.
Observations of coaxial oxygen-jet diffusion flames stabilized in a high pressure environment and measurements of flame stabilization limits, flame base positions and stream lines as well as flow turbulence were performed when jet turbulence and co-flow turbulence were enhanced. Comparing the results with those of the flames with no turbulence enhancement, the flame stability characteristics and stabilization mechanism for turbulence enhanced diffusion flames were discussed. For co-flow turbulence enhanced flames, flame stabilization limits, flame base positions and stream lines were not so different from those of the flames with no turbulence enhancement. On the other hand, jet velocity at the flame stabilization limits drastically decreased when jet turbulence was enhanced. From turbulence measurements and combustion experiments, it was proven that turbulence intensity near the burner lip for the jet turbulence enhanced flow significantly increased and the flame base position is located closer to the burner lip in comparison with the flames with no turbulence enhancement and co-flow turbulence enhancement, the latter being due to enhanced mixing between jet and co-flow when the flame is stabilized. From stream line observations, it was found that jet turbulence enhancement has significant effects on the weakening the recirculation zone which is formed near the burner lip and plays an important role for the flame stabilization especially at high pressure. It was also proven that flame base stability for jet turbulence enhanced flames at high pressure is strongly related to the characteristic scale of the recirculation zone formed near the burner lip.
Turbulent burning velocity of spherically propagating premixed turbulent flame keeps increasing during flame propagation although that of steady flame is constant for a given turbulence intensity. In this study, the variation of turbulent burning velocity of spherically propagating turbulent flame during flame propagation was investigated. As the size of flame becomes larger, scales of turbulent eddies effective to turbulent burning velocity may vary. The effective turbulence intensity was adopted in order to evaluate the energy of these eddies only among the entire energy of turbulence. The flame front area is considered to be one of the dominant parameters for turbulent burning velocity. The perimeter of cross-sectional image of turbulent flame which may correlate with the turbulent flame front area was evaluated using effective turbulence intensity. Experiments were carried out for stoichiometric iso-octane/air flames at initial mixture pressures of 0.10, 0.25 and 0.50 MPa. The cross-sectional images of spherically propagating premixed turbulent flame were obtained by laser tomography technique. It was found that the ratio of the perimeter of cross-sectional image of turbulent flame to that of laminar one increased with the increase in effective turbulence intensity. The increase in turbulent burning velocity of spherically propagating premixed turbulent flame during flame propagation may be caused by the increase in the ratio of turbulent flame front area to laminar one during flame propagation.