• Energy Research

A numerical simulation of a pulverized coal jet flame was performed to investigate the physics and flame structure in detail by means of large-eddy simulation with an elementary reaction mechanism....

Abstract The most favorable condition for NOx reduction in the swirl burner of wall-fired type boilers is to increase the residence time of pulverized coal particles in the internal recirculation zone which should be fuel-rich environments. The purpose of this study is to elucidate fundamental mechanisms for NOx reduction in pulverized coal swirling flames by means of the detailed observation of the flow field of the oxygen lean internal recirculation zone. The structure of pulverized coal swirling flames with secondary swirl intensity is studied experimentally by particle image velocimetry and local flame colors based on OH∗, CH∗, and C2∗ chemiluminescence intensities. The results show that the internal recirculation zone is enlarged with increasing the outer swirl intensity because the location of the stagnation point moves toward downstream. Also, the exhaust tube vortex is observed along the centerline in the flames, and started from near the stagnation point. For the combustion characteristics, the maximum temperature reduces as the outer swirl intensity increases, and the flame then moves downstream. The higher outer swirl intensity would be more effective in the low NOx swirl burner systems because of enhancing a dimension of the internal recirculation zone and reducing a local rate of chemical reaction.

The characteristics of NO production mechanisms in counter-flow flames with gasified coal syngas have been computationally studied through detailed chemical reactions and transport properties. The combustion of gasified coal syngas with N2/O2 mixture causes a significant increase in nitric oxide emission due to the higher flame temperature and considerable amount of nitrogen in the oxidant. The nitric oxide emission can be reduced efficiently by using a CO2/O2 mixture instead of an N2/O2 mixture as the oxidant. The NO production mechanism was analyzed in detail through the overall chemical reaction pathways. The phenomenon of NO decomposition was observed for flue gas recirculation when the CO2/O2 mixture was used as the oxidizer and coal was gasified with O2/steam. The adoption of O2/steam gasification in the oxy-firing condition could considerably reduce the thermal NO and fuel NO due to the low nitrogen in the fuel and low flame temperature. However, reverse reactions of fuel NO as in NO→HNO and HNO→NH were observed when combustion flue gases were recirculated. Also, the fuel-N conversion rate of coal syngas combustion was investigated in the oxy-firing condition.

In this study, combustion from the co-firing of coal and wood biomass, and thermal characteristics such as ignition temperature, burn-out temperature, and activation energy were discussed using a thermogravimetric analyzer (TGA). We investigated the effects of biomass blending with two kinds of pulverized coal (bituminous Shenhua, and sub-bituminous Adaro) under air and oxy-fuel conditions. The coal fraction in the blended samples was set to 1, 0.8, and 0.5. The oxygen fraction in the oxidant was set to 0.21, 0.3, 0.5, and 0.8. The ignition temperature was governed by the fuel composition, particularly in the blended biomass which has a much higher content of volatile matter comparing to coal. However, the burnout temperature, which shows a strong relationship with char combustion, depended on the oxidant ingredients rather than on the fuel components. Thermal characteristics such as ignition, burnout temperature, reaction region, and heat flow were very similar between air and a 0.3 oxygen concentration under oxy-fuel conditions with Shenhua coal.

In this study, evaporator optimization, via both experimental and simulation methods was conducted. To evaluate the evaporator performance, under the optimal system, the compressor operating time and the effects of oil on the refrigerator system were studied. If the temperature of the refrigerator chamber reaches the setting value, the compressor stops working and it leads to the temperature of the refrigerator chamber slowly increasing, due to the heat transfer to the ambient. When the refrigerator temperature is out of the setting range, the compressor works again, and the refrigerator repeats this process until the end of its life. These on/off period can be controlled through the compressor piston movement. To determine the optimal compressor operating conditions, experiments of monthly power consumption were conducted under various compressor working times and the lowest power consumption conditions was determined when the compressor worked continuously. Lubricating oil, the refrigerator system, using oil, also influenced the system performance. To evaluate the effect of oil, oil eliminated and oil systems were compared based on cooling capacity and power consumption. The cooling capacity of the oil eliminated system was 2.6% higher and the power consumption was 3.6% lower than that of the oil system. After determining the optimal operating conditions of the refrigerator system, visualization experiments and simulations were conducted to decide the optimal evaporator and the conventional evaporator size can be reduced by approximately 2.9%.

A direct carbon fuel cell (DCFC) system directly converts the chemical energy of solid carbonaceous fuel into electrical energy. The electrochemical reaction of this system has an influence on the properties of solid fuel, such as crystal structure, element composition, and surface properties. In addition, when using raw coals as DCFC fuel, the volatile gases released from coal at a high temperature affect cell performance. The purpose of this study is to investigate the effect of fuel characteristics on the resistance of the inner DCFC system by electrochemical impedance spectroscopy (EIS) and equivalent circuits. Two kinds of solid carbon, graphite and sub-bituminous coal, were prepared to compare electrochemical characteristics by EIS measurement. The equivalent circuits were applied to the constant phase element (CPE) in a Randle circuit to explain the correlation between the fuel characteristic and electrochemical reaction resistance.

A numerical simulation was performed with the two competing model in the devolatilization process for a pulverized coal combustion jet flame by means of LES. The target was a simple jet burner flame in CRIEPI. To solve the LES equations, a CFD code FFR. Comb was employed with the dynamic Smagorinsky SGS turbulent model. A simple global kinetic mechanism was implemented to predict combustion of the gas and solid phase combustion. The interactions between the two phases were calculated by PSI-Cell model while the reaction rate in turbulent flow was established by SSFRRM. The simulation was validated by comparing results to the experimental data in terms of particle dispersion and velocity as well as gaseous velocity. The flame structure was discussed with temperature, mole fraction of major species. In addition, the effect of the devolatilization model was investigated simultaneously by comparing to another simulation that employed the single first order reaction model because the devolatilization was one of the major processes in coal combustion and it had an influence on the flame structure from all reactive regions. The release rate was calculated by two different parameter sets in the Arrhenius rate equation for the two competing model that were corresponding different temperature regions whereas the released rate was determined by only one fixed parameter set in the single first order reaction model. From the simulation, it was revealed that the main reactions took place at the upstream and the first fuel oxidation was stronger at the inner reaction zone comparing to the far side combustion. It was confirmed as well that the two competing model could capture the quick devolatilization faster than the single first order reaction model though the dominant part appeared later than the single first order reaction model.

Abstract Refuse derive fuel and refuse plastic/paper fuel were evaluated in a direct carbon fuel cell (DCFC) system as energy sources, and two different grades of coals were also employed for comparison. The maximum power density of refuse fuels was reached up to 43–62% level comparing to that of coals, despite their significantly low carbon content. Significant properties such as thermal reactivity, nitrogen gas adsorption characteristics, and functional groups on the surface of the fuel were investigated using the TGA (thermogravimetric analysis), BET (Brunauer–Emmett–Teller test), and XPS (X-ray photoelectron spectroscopy) techniques, respectively. The correlation between fuel properties and electrochemical reactions was investigated, and it was found that the total carbon content, surface area, pore volume, and oxygen functional groups on the surface might have an influence on the reactions in the DCFC system. The effect of temperature increase from 973 K to 1023 K was restricted in the RDF (refuse derived fuel) because of its highly activated gasification phenomenon. The stirring effect could improve the performance of the RDF and the RPF only at 1023 K and 923 K, respectively, because of thermal characteristics and certain substances that affected the viscosity of the electrolyte.

In this study, we investigate the effect of surface characteristics on the electrochemical resistance of various grades of coal, i.e., Glencore (bituminous) and Adaro (sub-bituminous) coals, used a...