Toshiyuki Suda

A study of combustion behavior of pulverized coal in high-temperature air

High-temperature air combustion is a promising technology to increase the usage of combustion energy and to improve combustion efficiency. This technology has been mainly developed for gaseous fuels, and recently application of this technology to solid fuels like pulverized coal has also become of interest. For the development of high-temperature air combustion technology for pulverized coal, it is important to experimentally investigate the combustion behavior of pulverized coal in high-temperature air. In this study, high-temperature air is applied to a pulverized coal burner to investigate the effect of air temperature on ignition, coal burnout, and NOx emission. Pulverized coal is introduced into a cylindrical furnace of 1 m diameter and 3 m height using a water-cooled stainless nozzle of 15 mm diameter. Combustion air is preheated using a heat exchanger with a gas burner and electrical furnace. The temperatures of the combustion air are set to 623 or 1073 K in order to compare the effect of air temperature. It is observed that ignition delay decreases as the air temperature increases, which is due to the more rapid devolatilization caused by higher particle heating rates. It is possible to form a stable flame even for low-volatile coals like anthracite. The difference in measured peak flame temperatures between 623 and 1073 K air is about 100 K, which is smaller than expected. Coal burnout is improved in the 1073 K air condition, which seems to be due to the increase of porosity of the particle. NOx concentration decreases for higher temperature due to enhancement of the reduction zone by rapid devolatilization of coal, as the volatile and fuel nitrogen release is enhanced in high-temperature air.

Effects of particle sizes on transport phenomena in single char combustion

A one-dimensional char combustion model including pore structure effects is used to numerically investigate single char particle combustion for several different types of char samples. Previously, it is expected that small char particles have less combustion time. However, the present work shows that this is true only if the combustion time is defined as that needed for a char particle diameter diminished below a certain value. If the combustion time is defined as time needed for the carbon conversion ratio higher than a certain value, there are optimal particle sizes in a limited combustion period. Just reducing the char particle sizes may not get high carbon conversion ratios. It has also been found that, in general, the larger particles have higher temperatures at the exterior surfaces.

Numerical Investigations of CO/CO2 Ratio in Char Combustion

The CO/CO2 ratio of reactions during char combustion was investigated numerically. The simulations used the pore model with fractal properties, a gas diffusion model suitable for fractal pores, and a carbon-oxygen reaction model to describe the char oxidation. The CO/CO2 ratio in the primary reactions was derived assuming Gaussian distributions of the activation energies and Maxwell distribution of the oxygen molecular velocity. The numerical simulations of the char combustion for various particle sizes, pore structures, and temperatures revealed that the effects of the secondary reactions and the pore structure on the CO/CO2 ratio are the main reasons why different researchers obtain different experimental results for of the CO/CO2 ratio.

Numerical Study of the Relationships Between Pore Structures and Reaction Parameters for Coal Char Particles

Fractal pore models, gas molecular movement models, and reaction models are combined to simulate char oxidation in char pores. The numerical results show the relationship between the pore structure and the apparent char reaction parameters. A modulus number φ is introduced to characterize the diffusion properties in fractal porous media with a relationship given for the apparent reaction parameter in terms of φ. The results show that the char combustion process is strongly impacted by the pore structure, and that the fractal dimension is an important parameter that should be considered in char combustion. Comparison with experimental data shows that this model is more accurate than the classical intrinsic combustion model.

Combustion Rate for Char with Fractal Pore Characteristics

A char combustion model was derived by assuming constant intrinsic activation energy for the different chars. The char combustion rates are affected by various diffusion processes within the different char pores. The fractal pore diffusion effects were characterized by three pore structure parameters (porosity, specific surface area, and fractal dimension) and the mean particle diameter. Three-dimensional numerical results for various char particles burned in drop tube furnaces obtained using the model compare well with experimental results to verify the model. The comparisons show that the model has good accuracy for a wide range of char, from lignite to anthracite.

AGGLOMERATION BEHAVIOR IN A BUBBLING FLUIDIZED BED AT HIGH TEMPERATURE

Minimum fluidization velocity and agglomeration behavior were investigated at high temperature in an 80 × 30 mm two-dimensional quartz fluidized bed and in an 82 mm i.d. circular fluidized bed. Bed materials tested were two sizes of glass beads as well as three sizes of fluidized bed combustor (FBC) ash. The minimum fluidization velocity decreased with increasing bed temperature, whereas the minimum sintering fluidization velocity increased with the bed temperature. The sintering of glass beads belongs to visco plastic sintering, the first type. FBC ash agglomerate has higher amounts of SiO2, Al2O3, Na2O, K2O, and SiO2 than in the original ash, indicating that low melting eutectics were formed and that the liquid phase in a silicate system was formed. The agglomeration of FBC ash belongs to the second type, an excessive quantity of liquid being formed by melting or chemical reaction.

Phase-plane invariant analysis of pressure fluctuations in fluidized beds

Partial agglomeration is a major problem in fluidized beds. A chaotic analytical method based on the phase-plane invariant of the pressure fluctuations in the fluidized beds has been used to warn of agglomeration at an early stage. Cold tests (no combustion) and hot tests (combustion) in fluidized beds show that the phase-plane invariant of the pressure fluctuations can distinguish the dynamic behavior of fluidized beds with different flow rates in cold tests. With combustion, when the flow rate was kept constant, agglomeration was detected very early by looking at the phase-plane invariant. The phase-plane invariant can be used to distinguish changes in fluidized beds due to changes in the flow rate, agglomeration, or various other factors. Therefore, this reliable agglomeration early warning system can be used for better control of circulating fluidized beds.

Biomass Gasification in Dual Fluidized Bed Gasifier

The dual fluidized bed gasification technology is prospective because it produces high caloric product gas free of N2 dilution even when air is used to generate the required endothermic heat via in situ combustion. This study is devoted to providing the fundamentals for development of a bubbling fluidized bed (BFB) gasifier for biomass fuels coupled to a pneumatic transported riser (PTR) char combustor. Gasification test of 1.0 g dried coffee grounds containing 10.5 wt.％ water using steam-blown fluidized bed clarified first the characteristics of fuel pyrolysis and steam gasification at 1073 K. With this clarification as the known input, a process simulation using the software package ASPEN then revealed that the dual bed gasification plant is able to sustain its independent heat and mass balances to allow cold gas efficiencies higher than 75％. The experiment using 5.0 kg/h dual fluidized bed pilot gasification facility was held according to the operation condition obtained from the simulation. Stable operation can be achieved and syngas with higher heating value can be produced. It was revealed that the necessary reaction time for the fuel can be lower than 160 s. Also it was found that impregnation of Ca catalysis will further increases the carbon conversion of the fuel compared with the gasification by simple mixing of fuel and catalysis.

Energy Flow of Advanced IGCC With CO2 Capture Option

Conventional IGCC (integrated gasification combined cycle) employs a cascaded energy flow with a high efficiency, yet it is difficult to achieve over 50% HHV (higher heating value). The current study proposes an alternative model of exergy recuperated Advanced IGCC (A-IGCC) to achieve higher plant efficiency by applying an autothermal reaction in the gasifier. This requires an additional heat supply from the gas turbine exhaust and the steam extracted from the steam turbine. System and performance analyses were studied on base IGCC and A-IGCC cases incorporating the heat (exergy) recuperation concept with an air-blown twin circulating fluidized bed gasifier for the gasification of sub-bituminous coal, both with and without the post combustion carbon dioxide (CO2 ) capture option. A-IGCC could deliver sufficient energy in the gasifier to the gas turbine without losing heat as resulted in IGCC. Chemical absorption methods using monoethanolamine (MEA) and methyldiethanolamine (MDEA) were selected as a CO2 absorbent. A-IGCC demonstrated a significantly higher system efficiency (51%) than IGCC (43%) without CO2 separation, provided the gas purification was at high temperature. The thermal efficiency penalty by CO2 capture was −8% using MDEA (56% absorption) and −11% using MEA (90% absorption).Copyright

Fundamental Study of the Pulverized Coal Char Combustion in Oxyfuel Mode with Drop Tube Furnace

The combustion characteristics of coal char particles in either O2/N2 or O2/CO2 conditions were experimentally investigated. Especially, the char burnout, the char particle temperature and the shrinkage of the char particles were discussed. A Drop Tube Furnace (DTF: whose wall temperature was set at 873, 923 and 973 K) was used as the experimental apparatus. The experimental results revealed that, in equivalent oxygen concentration, the char burnout and the char particle temperature were higher in O2/N2 conditions than those in O2/CO2 conditions. The shrinkage of the char particle did not show the large difference in either O2/N2 or O2/CO2 conditions. Up to 15% of char burnout, the char particle diameters were reduced gradually. Up to 80% of char burnout, the char particle diameters were not changed. This is supposed that the chemical reaction is mainly occurred not on the external surface but on the internal surface of the char particle. Over 80% of char burnout, sudden shrinkage could be seen. Finally, an empirical equation for the prediction of the char particle shrinkage was introduced. Further investigation is required in high operating temperature, where CO2 gasification may have a large influence on the char burnout.