Hisao Makino

Large eddy simulation of a solid-fuel jet flame

Three-dimensional large eddy simulation (LES) is applied to a turbulent jet flame of a solid fuel, and the interactions among the dispersion, evaporation and combustion of the solid-fuel particles are investigated. The SGS turbulence and combustion models used are based upon the Smagorinsky model and the conserved scalar approach. The results show that the evaporation of the solid-fuel particles and the heat release produced by the reaction between the fuel vapor and oxidizer tend to increase the mean centerline gas velocity and the jet width and to decrease the rms velocity fluctuations from the values for a particle-laden jet without evaporation. In spite of the different particle-laden and heat release conditions, the radial profiles of the axial gas velocity, product mass fraction and gas temperature normalized by their centerline values and jet widths approach a Gaussian profile as the streamwise distance increases. The peak of the particle number density of the laden particles tends to shift outside as the streamwise distance increases, and the trend is observed further upstream for the higher heat release rate case. The evaporation of the fuel particles for the lower heat release rate takes place in the intermittent regions around the centerline, whereas the rate of evaporation for the higher heat release rate is proportional to the streamwise distance. These behaviors could be explained by considering the mass transfer from the particles to the fluid, the particle inertial force and the variation of the kinematic viscosity of fluid.

Effects of moisture in coal on pulverized coal combustion characteristics

Effects of moisture in coal on pulverized coal combustion characteristics are investigated by means of a three-dimensional numerical simulation. The results show that as the moisture increases, flame temperature and NOx mole fraction decrease, while O2 mole fraction increases in the region near the burner, and the peaks of the flame temperature and NOx mole fraction shift downstream. Also, the increase in the moisture increases the unburned carbon fraction and decreases the NOx conversion at the outlet of the furnace. These trends agree well with the previous experimental results. It is found that the contribution of thermal NOx to the total NOx drastically decreases with an increasing moisture content, whereas the emission of fuel NOx increases.

Emission characteristics of NOx and unburned carbon in fly ash during combustion of blends of bituminous/sub-bituminous coals☆

Because of large recoverable reserves, sub-bituminous coal is anticipated to become a substitute for the bituminous coal used presently as fuel in Japan. Most sub-bituminous coal contains more than 20% moisture. In the case of the utilization of sub-bituminous coal in Japan, it is considered that, in the near future, sub-bituminous coal will be applied to conventional boilers designed for bituminous coal combustion. We have already clarified that moisture in coal causes low combustion efficiency and a high conversion ratio of fuel nitrogen to NOx compared with a coal having the same fuel ratio with little moisture, in pulverized coal combustion. In particular, oxygen consumption during combustion of sub-bituminous coal with high moisture is more delayed than that during combustion of bituminous coal. Therefore, sub-bituminous coal is utilized in blended combustion with bituminous coal. In this paper, the emission characteristics of NOx and unburned carbon in fly ash in blended combustion of sub-bituminous coal with bituminous coal is investigated using a pulverized coal combustion test furnace with a single burner (coal combustion rate: 0.1 t/h). NOx concentration in blended combustion approaches the NOx concentration in sub-bituminous coal combustion with the increase of the amounts of sub-bituminous coal. As the remaining moisture content in sub-bituminous coal increases, NOx concentration becomes higher in blended combustion. On the other hand, the unburned carbon concentration in fly ash in blended combustion is higher than that in non-blended combustion of each coal. Furthermore, unburned carbon concentration in fly ash becomes much higher as the moisture in sub-bituminous coal increases. This tendency is caused by the influence of the moisture in the sub-bituminous coal on the decrease of the combustion efficiency of bituminous coal, since moisture in sub-bituminous coal delays oxygen consumption at the burner exit and the shape of the combustion flame is diffused. When the swirl vane angle of the burner is controlled in order to suppress diffusion of the combustion flame, oxygen consumption progresses and emissions of both NOx and unburned carbon in fly ash are reduced.

Effects of outflow from the surface of a sphere on drag, shear lift, and scalar diffusion

A three-dimensional numerical simulation of an isothermal flow past a solid sphere with outflow in a linear shear flow is performed to investigate the effects of the outflow on drag and shear lift. In addition, the effects of the outflow and the fluid shear on diffusion and reaction of reactant from the surface of the sphere are also discussed. The results show that the outflow reduces the drag, and, in the linear shear flow, acts to push the sphere to the lower fluid velocity side and promote the negative lift for the high particle Reynolds numbers. The diffusion and reaction of the reactant from the surface of the sphere are strongly affected by the outflow and the fluid shear because these factors cause the deformation of vortices appearing behind the sphere.

Measurement of adhesive force of coal flyash particles at high temperatures and different gas compositions

Abstract To evaluate the adhesive property of coal flyash particles, a high temperature shear tester capable of measuring up to 950°C has been developed. Shear test can be done at different gas conditions such as air, N2, CO2 and N2–CO2 mixture at different ratio and all manipulations can also be done from the outside the furnace. Shear stress of flyash particles from pulverized coal combustion but different lime addition was measured from ambient to 950°C. As a result, adhesive force and internal friction angle are found to change slightly with temperature and gas conditions up to 700°C. However, a noticeable increase in adhesive force was observed at temperature higher than 800°C, especially higher than 850°C. Furthermore, an interesting phenomenon was observed when limestone was tested under CO2 containing condition, i.e., adhesive force of lime contains flyash increases at most about three-halves depends on CO2 content of gas.

Extraction of PCBs and water from river sediment using liquefied dimethyl ether as an extractant.

We investigated whether polychlorinated biphenyls (PCBs) and water could be simultaneously removed from river sediment by solvent extraction using liquefied dimethyl ether (DME) as the extractant. DME exists in a gaseous state at normal temperature and pressure and can dissolve organic substances and some amount of water; therefore, liquefied DME under moderate pressure (0.6-0.8 MPa) at room temperature can be effectively used to extract PCBs and water from contaminated sediment, and it can be recovered from the extract and reused easily. First, we evaluated the PCB and water extraction characteristics of DME from contaminated sediment. We found that 99% of PCBs and 97% of water were simultaneously extracted from the sediment using liquefied DME at an extraction time of 4320 s and a liquefied DME/sediment ratio of 60 mL g(-1). The extraction rate of PCBs and water was expressed in terms of a pseudo-first-order reaction rate. Second, we estimated the amount of DME that was recovered after extraction. We found that 91-92% of DME could be recovered. In other words, approximately 5-10% of DME was lost during extraction and recovery. It is necessary to optimize this process in order to recover DME efficiently. The extraction efficiency of the recovered DME is similar to that of the pure DME. From the results, we conclude that solvent extraction using liquefied DME is suitable for extracting PCBs and water from contaminated sediment.

Observation of detailed structure of turbulent pulverized-coal flame by optical measurement - (Part 2, instantaneous two-dimensional measurement of combustion reaction zone and pulverized-coal particles)

The purpose of this study is to elucidate of the primary air combustion zone in pulverized-coal combustion by means of advanced laser-based diagnostics with high temporal and spatial resolutions. An open-type burner is fabricated to apply various optical measurement techniques. In this paper, simultaneous measurement of OH-planar laser-induced fluorescence (PLIF) and Mie scattering images of pulverized-coal particles is performed, and the spatial relationship between the combustion reaction zone and the pulverized-coal particle zone is examined. It is found that, in the upstream region, combustion reaction occurs only in the periphery of the clusters of pulverized-coal particles where the high-temperature burnt gas of a methane pilot flame is entrained and oxygen supply is sufficient, and that, in the downstream region, however, combustion reaction can be seen also within the clusters of pulverized-coal particles. This is because, in the downstream region, the devolatilization process of the coal particles proceeds with the temperature rise of the particles, and the mixing process between the volatile matters and ambient air is prompted. From these results, it can be said that the present diagnostic techniques are effective for evaluating the pulverized-coal flames.

Observation of dust release behavior from ceramic filter elements

The dust-releasing behavior from a ceramic candle filter, which is a key technology of the hot gas cleaning system in advanced coal power generation processes such as pressurized fluidized bed combustion and integrated gasification combined cycle, is discussed based on the observation of the dust-releasing process using a high-speed video camera and the measurement of time change of pressure inside the filter. Time changes of dust-released area and geometrical characteristics and motion of released dust cake fragments were investigated by analyzing photo images from the video camera, compared with the time change and distribution of pressure inside the filter. In order to discuss the influence of porosity of the dust layer, a consolidated dust layer was prepared by clean air filtration at an elevated filtration velocity after the dust filtration. The dust-releasing process was found to be a multi-mode process, i.e. release occurred in a short time after the injection of cleaning air, followed by one or two intermittent releases afterwards. The amount of dust released by the first strike increased with tank pressure. The dust-releasing behavior was sensitive to the cake porosity: the mean size and perimeter of cake fragments increased with porosity and tank pressure for the consolidated dust layer. The initial radial velocity of cake fragment released just after the first strike increased with tank pressure and was almost independent of the filter location.

Characteristics of particulate matter generated in pressurized coal combustion for high-efficiency power generation system

Characteristics of particulate matter in pressurized coal combustion for high-efficiency power generation system are investigated by using the percolation model and the results are compared with experiments performed using drop tube furnace facilities (DTF). The results show that conversion for Plateau coal (high volatile matter and low ash contents) rises earlier than that for Newlands coal (low volatile matter and high ash content). For both coals, volatile consumption rate increases with increasing the gaseous temperature, whereas the char combustion rate increases with increasing both the gaseous temperature and the pressure. At the char-combustion-dominant stage after devolatilization, the characteristics of the particulate matter such as specific surface and porosity area are not affected by the gaseous temperature, but they are affected by the pressure and the coal properties. These results are of general agreement with the experiment by DTF and hence the present percolation model is valid for the prediction of the characteristics of the particulate matter in coal combustion.

Numerical analysis of a flow passing through a ceramic candle filter on pulse jet cleaning

Ceramic filters are often utilized to collect dust particles under the very high temperature and high pressure conditions in flue gases for gas turbines or fuel cells in combined cycle power generation systems, and for their long-term use the dust cakes attached on the filter surface should be removed at adequate intervals. At present, the pulse jet-type dust cleaning method is considered to be the most useful. In this study, a numerical simulation is applied to a flow passing through a ceramic candle filter on the pulse jet cleaning, and the validity of it and the detailed flow behaviors are investigated by comparison with experiment. The results show that the present computation is valid in predicting the flow characteristics in the ceramic candle filters for arbitrary pulse jet and dust cake packing density conditions. Also, the dust-removal behaviors observed in the experiment can be well explained by using the time variation of pressure (static pressure) between the filter and dust cake.