• Energy Research

Abstract The evolution processes of the structure and morphology of coal slime spherical particles in the process of combustion in an air atmosphere are examined in this work. A high-definition camera was used to capture changes in the macrostructure of the particles in the process of combustion, and a large amount of educt at the stage of homogeneous combustion was studied. The main component of the educt was graphite, as determined by X-ray diffraction(XRD),which demonstrated the process whereby the organic carbon molecule structures are transformed from a complicated and disordered condensed aromatic nucleus macromolecular structure to ordered graphite molecules with simple structures inside coal slime particles. Using a conducting N2 adsorption test and analysis, this study found that there were a number of mesopores and a handful of micropores inside the coal slime particles. The pore structures experienced a process that varied from complicated at the initial moment to simple, to complicated and finally to simple. Scanning electron microscope (SEM) photos proved the change process of the above pore structures and found the phenomenon whereby pyrolytic products filled the pores at the stage of homogeneous combustion.

Coal slime is a byproduct of the coal preparation process. Research on coal slime combustion is of primary importance for the utilization of coal slime and energy savings. To explore applications in circulating fluidized bed boilers, the combustion characteristics of single coal slime particles in air at different furnace temperatures were studied. The temperature changes in the center of the coal slime particles, and combustion images during the combustion process were obtained. The rapid pyrolysis of the coal slime resulted in the production of small molecules of volatile gas, tar, and other pyrolysis products. Next, the pyrolysis caused an unusual change in the surface structure of the particles. The temperature changes in the different types of coal slime varied; however, those in the same coal slimes with different particle sizes at the same furnace temperature were similar. With an increasing particle size, the diffusion of internal oxygen slowed, which resulted in slower combustion processes and in...

Abstract The O2/CO/CO2 atmosphere and temperature dramatically affect NH3 reducing NO. Thermal DeNOx focuses on the effect of O2, while the temperature is limited below 1400 K. This paper investigated the effect of O2 on the NH3/NO reaction in a wider temperature range (1073–1773 K), especially emphasizing high temperatures above 1400 K, in four specific flow reactors. Additionally, three chemical kinetic models were used in simulations for verifying their applicability. It was proved that efficient NO reduction may be achieved in the temperature rising zone for the premixed reactants. Experimental results of gas-preheated reactors indicate that efficient NO reduction can be achieved beyond 1373 K in the absence of O2. While, in the presence of O2, the NH3/O2 reaction becomes more competitive with temperature increase in general. The SNCR temperature window was observed and PG2018 model made a pretty successful prediction of it. Besides, as temperature increases, the mixing mode begins to influence the final reaction results. In a special mixing mode, the NH3/NO reaction plays the dominant role above 1423 K, leading to the second temperature window. Moreover, near 1773 K, the outlet NO concentration tends to be equal to the initial value in almost all cases in gas-preheated reactors. Additionally, in NH3 oxidation experiments, little NO forms at high temperatures above 1673 K.

Abstract In order to further investigate the second rise phenomenon of NO in pulverized coal boilers, the simulated air-staged combustion atmosphere was used in a high temperature fixed bed reactor to study the heterogeneous reduction mechanism of N2O and NO formation. The results show that proper conditions can converse N2O to a large amount of NO. The reduction ability of char to NO produced by N2O in different atmospheres is not much different, but it will be greatly reduced at high temperature, indicating that the ability of char to prevent the second rise of NO is limited. In the oxidizing atmosphere, three reaction paths of N2O heterogeneous reduction were verified through the reduction of N2O by char under different oxygen concentrations. Low O2 concentration at low temperature improves the char’s reducing capacity, while at high temperature it enhances the oxidation capacity of O2, promoting the oxidation consumption of char. The coexistence of iodine and char will reduce the effect of iodine on inhibiting the formation of NO, as well as char’s reducing capacity. The iodine can promote the decomposition of N2O and inhibit the adsorption of N and O on the char surface.

Abstract This paper presents some experimental results on the integral reburning process including all reburning stages in a down-fired furnace. A kind of high volatile bituminous coal (DT coal) and a kind of low volatile anthracite (SX coal) were focused. Natural gas as reburning fuel was also chosen when DT coal used as the main fuel. Reburning fuel fractions and residence times of reburning zone were considered. The deep and middle degree air-staged combustion cases were also conducted. Results show that the NOx reduction efficiency at the cases with natural gas as reburning fuel is significantly higher than that at the cases with two kinds of coal as reburning fuel. At the residence time of 0.82 s and 1.0 s, the conversion ratio of fuel nitrogen to NOx is reduced by 30–70%. At the coal reburning cases, the reduction of NOx and the oxidation of volatile and char to form NOx occur simultaneously in the reburning zone. The NOx concentration values at the reburning stage of DT coal rise faster and reach higher than that of SX coal. The NOx emission values at air-staged combustion cases are lower than that at all reburning cases. Air staging is more likely to inhibit or dissolve the conversion of fuel nitrogen to NOx during the combustion process. Further analysis shows that the release rate of fuel nitrogen from DT coal is significantly higher than that from SX coal, which leads to a faster and higher increase of NO formation at the latter stage of reburning zone.

Abstract NH3 is an important N-containing intermediate in solid fuel combustion, an efficient NO reducing agent and a carbon-free fuel. The transformation of NH3 is fairly complex in furnace and dominates the final NOx emission level. In this work, the effect of the CO/O2/CO2 system on NH3 transformation was researched in a self-designed flow reactor in a wide temperature range, 1073-1773 K. A small amount of NO net formation was detected in both the CO/NH3 and CO2/NH3 systems, while the mechanisms are different. The reaction between CO and NH3 is complex at low and intermediate temperatures, while the effect of CO2 is limited in the elevated temperature range. Significant amounts of NO forms in the O2/NH3 system, but the net formation of NO depends on the competition between NH3 oxidation by O2 and NO reduction by NH3. With CO oxidation included, the transformation of NH3 is dramatically enhanced. However, due to the rapid chemical reaction in the presence of O2 or CO oxidation, the mixing process in the front end of the reaction tube strongly influence the reaction results, leading to a fairly low conversion ratio of NH3 to NO at elevated temperatures.

Abstract A dynamic model on high-temperature corrosion with fouling-type ash deposition was established to predict the thickness distribution of corrosion layer and the outer/inner layer of ash deposit. It provides an observation on variation of heat transfer characteristics of the alloy test probe over time. This model involves the combustion characteristics of high-chlorine coal, the process of formation, migration, and deposition of fly ash particles, and the corrosion of alloy test probe. In this study, the dynamic model has been integrated into the numerical simulation of high-chlorine coal combustion in a 50 kW long-period corrosion test furnace. The prediction results show that the amount of fly ash deposition on the windward surface is the most while the least amount appears on the side surface. There is a great correlation between the corrosion layer and the inner layer of ash deposit. Because the inner layer of ash deposit and the alloy substrate are spatially adjacent, the high temperature corrosion process is greatly affected by the growth and property of fly ash deposit layer. The simulation results show that the heat flux of probe surface beneath ash deposit is 2.5 kW/m2 which is 50% of the clean surface (5 kW/m2). The changing pattern of thermal resistance versus time at different positions of the alloy surface is consistent with the variation of thickness of the corrosion layer and ash deposit layer. The windward face has the highest thermal resistance and the least heat flux due to the highest thickness of the corrosion layer and ash deposit layer. With the increase of probe wall temperature, the ratio of thermal resistance of corrosion layer to total heat transfer resistance increases, and the ratio at the side face is larger than that at other locations. And the proportion of the corrosion layer in thermal resistance for alloy TP347H reaches 8% at the probe wall temperature of 973 K.