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MILD (Moderate or Intense Low-oxygen Dilution) combustion is a novel technology to reduce NO X emission from combustion. With the development of this technology, it is expected to combine this technology with oxy-fuel combustion to jointly control the pollutants from fossil fuel combustion. The present paper investigates the combustion characteristics of coal under oxy-MILD operation with the aid of CFD simulations. A seven-step global reaction mechanism is used for modeling coal combustion involving gaseous volumetric reaction and particle surface reaction. The predicted results using the adopted models for the reference case agree well with the experimental results. It is revealed that, the combustion temperature level increases with the enhancement of initial oxygen concentration because of the reduced injection momentum and promoted heat release. The in-furnace CO formation increases but the final CO emission reduces when enhancing the oxygen concentration under oxy-MILD combustion. Moreover, oxy-MILD combustion shows a greater potential on NO X formation reduction than air-MILD combustion, and this potential is promoted at higher initial oxygen concentration. [ABSTRACT FROM AUTHOR]

Abstract: Chemical looping combustion (CLC) of coal has received great attention for its verified advantage in the inherent separation of CO2 without great cost penalty. It would be meaningful to adopt anthracite as fuel for CLC application in China due to its abundant reserve in China and also a great challenge for its low reactivity. A typical Chinese anthracite as Yang Quan (YQ) coal was selected in this research as a model fuel and its reaction with both synthesized CuO/Al2O3 and Fe2O3/Al2O3 oxygen carriers (OCs) was performed in a thermogravimetric analyzer (TGA) to investigate the reaction peculiarities involved. Fourier transform infrared spectroscopy (FTIR) was used to in-situ detect the emitted gasses from TGA. Field scanning electron microscopy/energy-dispersive X-ray spectrometry (FSEM-EDX) was used to study the morphology and elemental compositions present in the solid residues collected from the reaction of YQ with these two OCs, and the related phase was identified by X-ray diffraction (XRD). In order to further explore the reaction mechanisms involved, a more realistic reaction system with 426 species was designed for thermodynamic simulation. Through all these measures, two reaction stages were observed for the reaction of YQ with Fe2O3/Al2O3 or CuO/Al2O3 at 300–600°C and 600–850°C after dehydration, respectively. The maximum weight loss rate for YQ with CuO/Al2O3 at the second stage was pronounced enough to reach up to 2.8wt.%/min, greatly higher than that of YQ with Fe2O3/Al2O3. The mixture conversion index for YQ with CuO/Al2O3 was far bigger than that of YQ with Fe2O3/Al2O3, which further indicated that CuO/Al2O3 was more suitable to YQ in CLC. At these two stages throughout 400 to 1100°C, CuO and CuAl2O4 contained in CuO/Al2O3 OC were mainly reduced to Cu and Cu2O or CuAlO2 and Al2O3 by transfer of the lattice oxygen [O] involved to YQ in a sequential mode; however, above 800°C, Cu2O and CuAlO2 were also produced through direct decomposition of CuO or CuAl2O4 by emission of gaseous oxygen O2, which was beneficial to the direct combustion of coal. During reaction of YQ with CuO/Al2O3 OC, active CuO was found to tend to react with various sulfur species produced from YQ pyrolysis and formed to Cu2S, but the Al2SiO5 was formed through the interaction of Al2O3 with SiO2 in the YQ, which resulted in the loss of the inert support involved in the CuO/Al2O3 OC and further degraded its reactivity to YQ due to the lower resistance to sintering. [Copyright &y& Elsevier]

In this study, a particle-tracking image pyrometer (PTIP) system, based on a high-resolution high-speed camera and a digital single lens reflex (SLR) camera, was proposed to investigate the ignition characteristics of pulverized coal particles. The goal of the PTIP system was to obtain reliable simultaneous measurements of the velocity, diameter, and temperature of individual burning particles. A modified particle-tracking velocimetry (PTV) algorithm was developed to collect the image sequences of individual particles, and it could weed out defocused particles and other odd particles by low-frequency filtration. Moreover, a template matching method was adopted to distinguish particles undergoing different combustion states (homogeneous volatile combustion or heterogeneous coal/char combustion). The experimental validation was carried out under an O 2 /CO 2 atmosphere in an optically accessible flat flame-supported entrained flow reactor. Particles of Shenhua (SH) bituminous coal, in the size range of 70–133 μm, were combusted in the flue gas with gas temperature of 1400 K and O 2 concentration of 20%. The diameter-temperature patterns of particles along their trajectories were discussed. Several criteria based on statistical analysis were adopted to divide the combustion process and obtain the ignition delay time (IDT) and devolatilization duration time (DDT). • Speed, diameter and temperature of a burning particle are measured simultaneously. • Modified PTV method is used to capture the combustion sequence of single particle. • Different stages of particle combustion are distinguished by particle templates. • Statistical analysis is adopted to obtain characteristic time for combustion. • Combination of micro and macro photography improves the credibility of diagnosis. [ABSTRACT FROM AUTHOR]

In situ gasification chemical looping combustion (i G-CLC) was a promising way to inherently sequestrate CO 2. Due to the nitrogen in coal, the NOx emission was a key issue in i G-CLC. In this study, we aimed to identify the nitrogen conversion and distribution characteristics in two elementary stages of coal conversion (pyrolysis and subsequent char gasification) during the i G-CLC process in a tailor-made two-stage fluidized bed reactor. HCN was the dominant precursor of NO x in i G-CLC, almost all the NO x precursors were rapidly released during the coal pyrolysis stage, and no gaseous nitrogen pollutant was detected within the subsequent CO 2 -char gasification process. The generated HCN could be completely converted by the Fe-based oxygen carrier (OC) to either NO or N 2. The high CO 2 concentration and reducing reaction atmosphere in the fuel reactor were found to exert certain inhibitory effects on the formation of NO. Meanwhile, the operating conditions, that is, the O/fuel ratio and temperature, exhibited insignificant effects on the fate of fuel-N. Finally, the fate of nitrogen in volatiles, char, and tar of coal during i G-CLC and the potential homogeneous nitrogen conversion in the i G-CLC of coal were proposed based on the results obtained from both experiments and thermodynamic simulations. • The main NOx precursor (HCN) was only released in pyrolysis process in iG-CLC. • HCN was converted to NO or N 2 by Fe-based oxygen carrier. • Limited effects of O/fuel ratio and temperature on the NOx emission are observed. [ABSTRACT FROM AUTHOR]

Although fuel-NO formation is significant for the combustion of pulverized coal, biomass, and heavy distillate fuels, its mechanism has not been systematically validated under air and oxy-fuel combustion. This is the first study that quantitatively evaluates detailed fuel-NO mechanisms against large experimental datasets from a jet stirred reactor (JSR) under both air and oxy-fuel combustion. The mechanism with the best overall performance is thoroughly reduced and validated. A skeletal fuel-NO mechanism including only 35 species is obtained using methods of directed relation graph with error propagation (DRGEP), DRGEP-aided sensitivity analysis, and computational singular perturbation. The skeletal mechanism is comprehensively validated and agrees well with the detailed mechanism and experiments predicting the ignition delay, temperatures, and concentrations of important species in JSRs, laminar flame speeds, and flame extinction under both air and oxy-fuel combustion. Moreover, the skeletal mechanism is successfully applied to the finite-rate modeling of moderate or intense low-oxygen dilution (MILD) combustion of pulverized coal. Unlike conventional semi-empirical post-processing modeling of fuel-NO production, the fuel-NO calculation in this study is coupled with coal combustion by using the well-validated skeletal mechanism. A highly precise in-furnace distribution of fuel-NO critical precursors (i.e., NH 3 and HCN) and detailed fuel-NO reaction pathway under the MILD combustion are obtained. • Detailed mechanisms for fuel-NO are evaluated for the first time. • A 35-species skeletal fuel-NO mechanism is obtained. • Skeletal mechanism agrees well with detailed mechanism and experiments. • Mechanism is used in finite-rate modeling of coal combustion for NO formation. • Skeletal mechanism is suitable for fuel-NO under air and oxy-fuel conditions. [ABSTRACT FROM AUTHOR]