Hiroshi Moritomi

A comprehensive interpretation of solid layer inversion in liquid fluidised beds

Abstract Solid mixing and segregation in liquid fluidised beds containing binary mixtures of spherical particles of different density and size has been studied for a range of liquid velocities, bulk bed compositions and particle properties. It was shown that a bed of denser particles expands with liquid velocity independently of the presence of the lighter particles. When the bulk volume fraction of the lighter particles is high and the liquid velocity is relatively low, the bed forms two layers, i.e. the upper layer consisting almost entirely of the lighter and the lower mixed layer consisting of both components in which the volume of the lighter increases with liquid velocity. A completely mixed bed is obtained at a certain velocity and then a further increase of the velocity causes “layer inversion”. The liquid velocity at which complete mixing occurs depends on the bulk bed composition, and at that velocity the volume fraction of the lighter in the lower mixed layer is constant regardless of the bulk bed composition. It is shown that layer inversion occurs for a given particle mixture when the liquid velocity passes through a value at which the volume fraction of the lighter in the lower layer becomes equal to the bulk bed composition; or for a given velocity, when the bulk bed composition becomes equal to the fraction of the lighter component which exists in the lower layer. The dependency of the fraction on the liquid velocity and the particle properties is examined to some extent.

Prediction of complete mixing of liquid-fluidized binary solid particles

Abstract In a liquid-fluidized bed consisting of density- and size-different binary solid particles with a given bulk composition, complete mixing may be observed at a liquid velocity less than the terminal velocities of the constituent particles. In such cases a change either in liquid velocity or in bulk composition results in stratification of the bed. A theoretical model to predict relationships between the bulk composition and the critical velocity for complete mixing is developed on the basis of momentum equations for each particle component. The voidage function involved in the equations is independently given from a unit cell model with assumptions of creeping flow around the particle within the fictitious spherical cell and of a potential flow outside the cell. Although the voidage function estimates monocomponent bed expansions to an accuracy comparable to that of the empirical Richardson-Zaki equation, it is not accurate enough when used for predicting the volume fractions of binary particles at complete mixing. The prediction is highly sensitive to monocomponent bed-expansion characteristics, and closer agreements between the predicted and observed results are obtained when experimental expansion data are used in the prediction. Nonetheless, the present model gives correct predictions for totally segregating particle systems. Thus, a segregation map is then illustrated which predicts the mixing state of any given binary particle system and is well comparable with the observed results not only from the present work but also from other existing investigations.

Influence of ash composition on heavy metal emissions in ash melting process

Two kinds of fly ash, discharged in the combustion of either refused derived fuel (RDF) or car shredder dust (SD), were examined for the emission of heavy metals in melting process under oxidizing and reducing conditions. The residual fractions of heavy metal in slag were experimentally estimated. As a result, it was confirmed that several volatile heavy metals were readily emitted during melting process. The type of atmosphere provided for the melting process was found to affect the emission of some volatile metals in RDF ash, but not in SD ash. The emission of volatile heavy metals in RDF ash under oxidizing conditions was lower than under any other conditions in this study. The emission behavior of iron and heavy metals in RDF ash under reducing conditions was similar to that in SD ash. These facts indicated that phosphorous in RDF ash had the property of fixing the volatile metals in the slag only under oxidizing conditions. Then the mixture of SD ash with phosphorous oxide powder was also tested in a melting process, and the result was consistent with the above inference of the effect of phosphorous.

A Mechanism for Mercury Oxidation in Coal-Derived Exhausts

Abstract This paper evaluates an elementary reaction mechanism for Hg0 oxidation in coal-derived exhausts consisting of a previously formulated homogeneous mechanism with 102 steps and a new three-step heterogeneous mechanism for un-burned carbon (UBC) particles. Model predictions were evaluated with the extents of Hg oxidation monitored in the exhausts from a pilot-scale coal flame fired with five different coals. Exhaust conditions in the tests were very similar to those in full-scale systems. The predictions were quantitatively consistent with the reported coal-quality impacts over the full range of residence times. The role of Cl atoms in the homogeneous mechanism is hereby supplanted with carbon sites that have been chlorinated by HCl. The large storage capacity of carbon for Cl provided a source of Cl for Hg oxidation over a broad temperature range, so initiation was not problematic. Super-equilibrium levels of Cl atoms were not required, so Hg was predicted to oxidize in systems with realistic quench rates. Whereas many fundamental aspects of the heterogeneous chemistry remain uncertain, the information needed to characterize Hg oxidation in coal-derived exhausts is now evident: complete gas compositions (CO, hydrocarbons, H2O, O2, NOx, SOx), UBC properties (size, total surface area), and the ash partitioning throughout the exhaust system are required.

Correlation of energy efficiency of NO removal by intermittent DBD radical injection method

Ammonia radicals are produced by a dielectric barrier discharge (DBD) in a chamber separate from the chamber that NO gas flows, and are injected in the NO gas flowing field to decompose NO gas. The power source for generating the DBD is a one-cycle sinusoidal (OCS) waveform so as to easily control the power consumed in the DBD plasma. The fundamental frequency of the OCS power source is 150 kHz. The correlation of the DeNOx characteristics was discussed, where the residence time of the ammonia gas in the radical injector and the power density consumed in the DBD plasma were considered. Their product was called residence energy density (RED). It was confirmed that the parametric data of the DeNOx energy efficiency were clearly correlated by the RED. Currently, the energy efficiency of 250 g/kWh was attained at a NO gas temperature of 600/spl deg/C. In order to obtain a high-energy efficiency in this system, the suppression of the energy consumed in the DBD plasma is effective, and instead, the ammonia flow rate decreases, compensating the accepted energy of the ammonia particles by the residence in the radical injector.

Coal gasification with a subcritical steam in the presence of a CO2 sorbent: products and conversion under transient heating

Pulverized Taiheiyo coal (Japanese subbituminous coal) was gasified with steam in the presence of CO2 sorbent (Ca(OH)2) under relatively high pressure (approximately 20 MPa; subcritical condition) using a tubing-bomb microreactor (TB reactor). The transient characteristics in the conversion of coal and the formation of gaseous products during the gasification were investigated. During initial heating period, 40–50 wt.% of the coal initially loaded was rapidly converted to gas or tar. In the presence of Ca(OH)2, the yield of gaseous products was apparently increased owing to the catalytic effects of Ca(OH)2 on coal gasification. No CO2 was observed in the produced gas at any soaking time in the gasification with Ca(OH)2, suggesting that CO2 sorption by Ca(OH)2 took place effectively under high-pressure conditions. A rigid agglomeration of char and CO2 sorbent was observed at relatively high temperature, which is attributed to the formation of melts of the CO2 sorbent.

Wake solids holdup characteristics behind a single bubble in a three-dimensional liquid-solid fluidized

Abstract A light transmittance technique using a dual optical fiber probe was used to measure the local solids holdup in a three-dimensional gas-liquid-solid fluidized bed. The bubble can be clearly identified in the same signal, thus permitting the simultaneous determination of the local solids holdup profile in the wake of a single bubble and the rise velocity and chord length of the bubble. The solid holdup behavior was studied in the wake of single bubbles rising in a liquid-solid fluidized bed for different liquid velocities, particle sizes and bubble sizes. Over the range investigated in this study, the wake solids holdup was found to decrease with decreasing mean solids holdup in the bed, to depend only slightly on bubble Reynolds number and to decrease with increasing particle size.

Optimum conditions for NO reduction using intermittent dielectric barrier discharge at atmospheric pressure

NO in N2 gas was removed by injecting ammonia radicals, which were externally generated by flowing NH3 gas diluted with Ar gas through a dielectric barrier discharge (DBD) with a one-cycle sinusoidal-wave power source. The discharge was intermittently formed between coaxial cylindrical electrodes at an applied peak-to-peak voltage of 2–15 kV. The generated radicals were introduced in a reaction chamber and reacted with NO. In order to find optimum parameters for NO reduction and energy efficiency, the reaction temperature in the mixing zone, the voltage applied to the gap of the electrodes for DBD generation and its repetition rate, the NO gas concentration, and the ammonia concentration and flow rate were varied. A maximum energy efficiency of 140 g/kWh at a NO reduction of over 99% is obtained at a voltage slightly higher than the discharge firing voltage and a repetition rate of 5 kHz, which corresponds to a duty cycle of 5%. Thus it is found that the use of the intermittent power source is an advantage for obtaining a high energy efficiency of NO reduction.

Mercury Transformation Behavior On A Bench-scale Coal Combustion Furnace

The mercury release behavior in bituminous coals, and the partitioning rate of mercury in solids and gaseous in flue gases have measured to develop technologies for evaluating the partitioning of mercury in coal combustion process and develop in-situ adsorption and removal technologies using three kinds of experiment equipments - a thermo-balance, a drop-tube furnace (DTF); a benchscale pulverized coal combustion furnace. The results showed that about 20 to 60% of the mercury in coal was released between 573 K and 673 K, which was the range of temperature in which the release of the volatile matter of coal began. And more than 90% of the mercury was released at 773 K, the temperature at which the release of the volatile matter was completed. The rate of mercury partitioned into bottom ash in a bench-scale pulverized coal combustion furnace was the smallest irrespective of the type of coal. The rate of mercury partitioned into cyclone ash was also low for all types of coal with values generally below 10%. The rest of the mercury was partitioned into mercury in gaseous fonn, but the rate partitioned into dust, oxidized mercury and elemental mercury varied slightly depending on the flue gas temperature and the type of coal.

Reduction of N2O emissions from circulating fluidized bed combustors by injection of fuel gases and changing of coal feed point

Abstract This paper describes the reduction techniques of N 2 O emitted from circulating fluidized bed combustion (CFBC) of coal. Two methods, injecting the fuel gases into the riser and changing the position where coal was supplied, were tried to decrease N 2 O emission. A small lab-scale CFBC whose whole parts were made of quartz was used. In the first method, methane, propane and hydrogen were used as a fuel gas. By injecting these gases into the riser, N 2 O emission was decreased remarkably. Reduction rate was almost in proportional to the volumetric flow rate of injected gas. At this time, NO emission did not increase. Required flow rate of injected gas to achieve the same N 2 O reduction was different among the above gases. N 2 O emission was able to be decreased remarkably by changing the position where coal was supplied. When coal was fed to the top of the downcommer, devolatilization took place and volatile matters were burnt in the cyclone. As the results, the temperature in the cyclone became high enough to reduce N 2 O.