Search

This paper reports on the recent experimental and numerical results from a 35 MW front wall-fired large pilot boiler. The boiler features a first-of-its-kind compatible design of oxy-fuel combustion and conventional air combustion (air-combustion). A sub-bituminous coal is used as fuel in the experiment. Wet and dry flue gas recycle oxy-fuel conditions are investigated, while air-combustion is also examined for reference. Measured in-furnace temperature, species concentration, exhaust emission, and transferred heat to membrane-wall and superheater are compared between oxy-fuel and air-combustion. Detailed numerical studies of combustion and heat transfer are performed using in-house developed sub-models for oxy-fuel condition, including a radiative property model and a global reaction mechanism. The predicted results agree well with the experimental data. The experimental and numerical results show that stable oxy-fuel combustion is achieved in the 35 MW large pilot boiler. Importantly, the oxy-fuel combustion and air-combustion can operate well with the especially designed burner system at similar heat loads in the experiment. Moreover, similar average temperatures and heat flux distributions are achieved under wet and dry flue gas recycle oxy-fuel conditions with an initial O 2 concentration of 28% in the oxidant. Transferred heat to membrane-wall and superheater in oxy-fuel combustion is slightly greater than in air-combustion. Furthermore, the flue gas recycle ratio significantly influences the heat transfer in oxy-fuel combustion. Increasing the recycle ratio from 0.71 to 0.73 reduces the transferred heat to membrane-wall by approximately 6%, while that of superheaters increases by approximately 4%. The selection of the optimized recycle ratio is a trade-off between boiler performance and flame stability of the burner system. [ABSTRACT FROM AUTHOR]

• A certain amount CaCO 3 exists under oxy-fuel combustion. • The viscosity of fly ash particles was higher under oxy-fuel combustion. • The particles were stickier and more easily coalesced under oxy-fuel combustion. • The slagging trend under oxy-fuel combustion was greater. Global warming has attracted extensive attention due to increasing CO 2 emissions. Oxy-fuel combustion has become an important topic in the domain of energy saving and emission reduction due to its high efficiency in CO 2 reduction. The slagging characteristics of coal ash under oxy-fuel combustion conditions are the premise for the successful application of this technology. In this study, we relied on a tubular furnace and flat flame-assisted entrained flow reactor to test coal ash under different atmospheres. The slagging trend of Shenhua coal under oxy-fuel combustion, including its melting point, mineral species, particle size, and morphology, was analyzed. A comparative analysis of different ash samples obtained from different experimental platforms was also conducted. Results showed that the oxy-fuel combustion atmosphere exerted a minimal effect on the melting point of Shenhua coal ash. Moreover, a high oxygen concentration corresponded to a relatively high melting point of ash. Ca existed in the form of CaO under air combustion, and a certain amount of undecomposed CaCO 3 appeared under oxy-fuel combustion. Given the same oxygen concentration, the average particle size under oxy-fuel combustion was smaller than that under air combustion. Moreover, particle adhesiveness was stronger and particles more easily gathered under oxy-fuel combustion than under air combustion. Particle aggregation was also more obvious under oxy-fuel combustion than under air combustion. Therefore, the slagging trend under oxy-fuel combustion was obviously higher than that under air combustion. [ABSTRACT FROM AUTHOR]

[Display omitted] • A benchmark model to evaluate the performance of pressurized oxy-coal systems is developed. • The performance and process characteristics of two pressurized oxy-coal systems are compared. • Detailed analysis of the factors affecting the pressurized oxy-coal efficiency is conducted. • The efficiency increases of two pressurized oxy-coal systems are obtained and analyzed. Process simulation and analysis are demonstrated to provide significant benefits for pressurized oxy-fuel combustion systems. This study compares the performance of pressurized oxy-fuel combustion systems and a first-generation oxy-combustion system under the same standard conditions. On this basis, the increase in efficiency of pressurized oxy-fuel combustion systems is evaluated. Process simulation and analysis are carried out for the first-generation oxy-combustion system in a 600 MW supercritical primary intermediate reheat power plant without heat integration (Base Case) and with heat integration (Case A). The validity of the simulation results is then verified. Considering Case A as a benchmark model, the process simulations of a pressurized oxy-coal combustion system and a staged pressurized oxy-combustion system are performed. The simulation results are compared with results in other literature studies. Compared with the benchmark model, the results indicate that the gross generation efficiency of the pressurized oxy-coal combustion system is improved by 1.12 % (LHV), and the net generation efficiency is improved by 0.47 % (LHV). In addition, the gross generation efficiency of the staged pressurized oxy-combustion system is enhanced by 1.78 % (LHV), and the net generation efficiency is improved by 2.4 % (LHV) compared with the benchmark model. [ABSTRACT FROM AUTHOR]

Abstract: The emission of mercury by coal-fired power plants has become a recent concern on the part of the electric utility industry. Knowledge of mercury kinetic mechanisms is imperative for the research of predicting mercury transformation and finding its effective control methods in coal combustion flue gas. Near the end of the flue gas path, mercury exists as a combination of elemental vapor and HgCl2 vapor. HgCl2 is more likely to be removed from the flue gas. Thus, the degree of oxidation is considered to be a critical factor that tends to reduce emission. In the present work, the microcosmic kinetic mechanisms of reactions between mercury and oxidizing species were investigated by ab initio calculations of quantum chemistry. The geometry optimizations of reactants, transition states, intermediates and products were made by the quantum chemistry MP2 method at SDD basis function level. All molecule energies were calculated at QCISD(T)/SDD level and corrected with zero point energy. The activation energies and heat of reactions were calculated. The reaction rate constants were calculated from transition state theory (TST). The performance of the ab initio calculations of quantum chemistry was assessed through comparisons with the literature data. The comparisons showed that the ab initio calculations of quantum chemistry were in agreement with the literature data. The results showed that quantum chemistry was an effective means for investigating kinetic mechanism of mercury interaction with combustion-generated flue gas. [Copyright &y& Elsevier]

Particle radiation characteristics have a strong wavelength-dependence. However, the gray particle assumption is widely used for coal combustion simulations, which cannot reflect the non-gray radiative property of the particles. In this study, based on the measured complex index of refraction from literatures (Gupta and Wall, 1985, Goodwin and Mitchner, 1989, and Lohi et al., 1992), a new non-gray particle radiative property model for fly ash is proposed by combining the features of full-spectrum k -distribution (FSK) model with the weighted sum of gray gases (WSGG) model. Four gray particles with different absorption and scattering efficiencies are used to replace the non-gray particles, for which absorption efficiency, scattering efficiency and weighting factor are directly obtained from the k -distribution, with model parameters obtained based on rational polynomials. Simultaneously, a gray particle model based on the Planck’s law is also obtained for comparison. The new model is systematically validated by comparing the radiative source terms and radiative heat fluxes, with those predicted by the line-by-line (LBL) integration of Mie-data in a one-dimensional plane-parallel slab system. The maximum relative error of radiative source term is 8% for the new non-gray radiative property model, 13% for Planck mean coefficients, 14% for modified Johansson model (Johansson, 2017), and 18% for empirical-constant model in non-isothermal inhomogeneous particle media, respectively. Combining the new non-gray radiative property model with the non-gray formulation of WSGG-SK model (Guo et al., 2015), the prediction accuracy is further validated by LBL solutions in the gas and particle mixture. Moreover, the contribution of gases and particles to radiative heat transfer is discussed at different path lengths, which shows the accuracy of the particle radiative property model determines the prediction accuracy of the radiative heat transfer in large-scale furnace. [ABSTRACT FROM AUTHOR]

Predicting thermal radiation for oxy-coal combustion highlights the importance of the radiation models for the spectral properties of gases and particles. This study numerically investigates radiation behaviours in small and large scale furnaces through refined radiative property models, using the full-spectrum correlated k (FSCK) model and Mie theory based data, compared with the conventional use of the weighted sum of grey gases (WSGG) model and the constant values of the particle radiation properties. Both oxy-coal combustion and air-fired combustion have been investigated numerically and compared with combustion plant experimental data. Reasonable agreements are obtained between the predicted results and the measured data. Employing the refined radiative property models achieves closer predicted heat transfer properties to the measured data from both furnaces. The gas-phase component of the radiation energy source term obtained from the FSCK property model is higher within the flame region than the values obtained by using the conventional methods. The impact of using non-grey radiation behaviour of gases through the FSCK is enhanced in the large scale furnace as the predicted gas radiation source term is approximately 2–3 times that obtained when using the WSGG, while the same term is in much closer agreement between the FSCK and the WSGG for the pilot-scale furnace. The predicted total radiation source term (from both gases and particles) is lower in the flame region after using the refined models, which results in a hotter flame (approximately 50–150 K higher in this study) compared with results obtained from conventional methods. In addition, the predicted surface incident radiation reduces by using the refined radiative property models for both furnaces, in which the difference is relevant with the difference in the predicted radiation properties between the two modelling techniques. Numerical uncertainties resulting from the influences of combustion model, turbulent particle dispersion and turbulence modelling on the radiation behaviours are discussed. [ABSTRACT FROM AUTHOR]

The catalytic hydrogasification of coal char was investigated in a high-pressure fixed-bed tube reactor using CaO, CaCO 3 , Na 2 CO 3 , and K 2 CO 3 as catalysts. Experimental results showed that the char reactivity could be greatly improved with the addition of catalysts. Analysis of char structural evolution was performed to further explore the catalytic mechanism by employing Raman spectroscopy and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX). The results revealed that CaO catalyst inhibited the aromatic ring condensation of the char structure during hydrogasification. The catalytic reaction of CaO only occurred on the char surface. Na 2 CO 3 or K 2 CO 3 was reduced to metallic Na or K and diffused into the char under hydrogen atmosphere, enhancing the cracking of large aromatic ring systems into small ones, which were the active species for hydrogasification. The catalytic reaction of alkali catalyst occurred simultaneously on the surface of and inside the char. Therefore, the Na 2 CO 3 and K 2 CO 3 displayed higher catalytic activities than CaO in hydrogasification. [ABSTRACT FROM AUTHOR]

In pulverized coal combustion, the participating radiative media includes gas, soot, and coal/char particles; a fundamental understanding of their radiation behavior is essential for the prediction of radiation heat transfer and control of radiation in the furnace. In this study, the individual contributions of gas, soot, and coal particles to the total radiation were examined based on the large eddy simulation of the CRIEPI pulverized coal jet flame. The radiation transfer equation was solved using the discrete ordinate method with state-of-the-art non-gray radiative property models of multi-phase media. The full spectrum correlated k -distributions (FSCK) model was used to calculate the radiative property of the gas-soot mixture. A burnout-based particle radiative property model was applied for coal/char particles. The results showed that the predicted spatial distributions of coal particles, tar, and soot were in good agreement with the experimental data. The gray gas radiation model predicted temperatures approximately 50 K lower in this flame, and the peak soot volume fraction was predicted at approximately 22% lower. For the laboratory scale burner under examination, the individual radiative contributions of gas, soot, and coal/char were found to be 35%, 40%, and 25%, respectively, so that it was necessary to consider all components for accurate prediction. Moreover, these contribution ratios varied with the flame position. Because of the high particle concentration, the individual contribution of coal/char particles accounted for up to 65% in the devolatilization region. Soot radiation played an important role in the pulverized coal flame. In the region where the ensemble-averaged soot volume fraction reached the maximum of 2.7 ppm, the individual contribution of soot was 50% of the total radiation, which accounted for 70% of the total particles (soot and char) radiation. [ABSTRACT FROM AUTHOR]

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]

Co-firing residual char and pulverized coal can apply the residual char in pulverized coal combustion equipment, whose difficulties lie in poor ignition performance and low burnout efficiency. For the first time, residual char and bituminous coal fuel blend is experimentally co-fired under moderate or intense low-oxygen dilution (MILD) conditions. MILD co-combustion is achieved with low-temperature preheating air in a pilot-scale furnace. The co-firing experiments include swirl low-NO flame combustion, MILD combustion through symmetrical secondary air jets, and MILD combustion through an asymmetrical secondary air jet. The pilot-scale tests reveal that MILD co-combustion through the asymmetrical single nozzle achieves the lowest peak temperature and the most uniform distributions of temperature, heat flux, and gas compositions in the furnace. The NO emission of MILD co-combustion through the asymmetrical air jet is decreased by 57% compared with swirl low-NO flame co-combustion. The burnout rate of MILD co-combustion through the asymmetrical air jet reaches 94%. Considering more uniform in-furnace distribution, lower NO emission, and higher burnout rate, MILD combustion through the asymmetrical single nozzle shows more obvious advantages than that through symmetrical air nozzles. Overall, the present study further widens the fuel flexibility of MILD combustion even to residual char fuel blend. • Co-firing residual char under MILD combustion is realized for the first time. • MILD co-combustion of residual char can reduce fuel-NO emission by at least 54%. • Fuel flexibility of MILD combustion is even expanded to residual char fuel blend. [ABSTRACT FROM AUTHOR]

A Lattice-Boltzmann model coupled with boundary method is proposed for the hydrodynamic resolved simulation of a single char particle combustion. A pure lattice Boltzmann scheme with multi distribution functions is presented for this low Mach reactive flows. Real varying thermodynamic and transport properties are considered, and the fluid density can bear significant change depending on the varying temperature and species concentrations. The recently proposed boundary-thickening based direct forcing-immersed boundary method is extended to implement the boundary conditions of velocity, temperature, and species concentrations at the char particle surface. Two heterogeneous reactions at particle surface and one homogeneous reaction in fluid are adopted to describe the combustion. The Stefan flow near the particle surface caused by the heterogeneous reactions is also considered. A satisfactory agreement can be found between the present simulation results and the previous experimental and numerical results. Three flame modes of a char particle combustion and the transition mechanisms between them are investigated. Furthermore, the effect of hydrodynamic interaction and the oxygen concentration in O 2 /CO 2 atmosphere on particle combustion behavior are explored in detail. The present work establishes the foundation for the efficient simulations of particle combustion with size change and motion. • IB-LBM is developed for particle combustion with varying thermodynamic and transport properties. • Results are validated by comparing with previous experiments and simulations. • Particle combustion behaviors in O 2 /N 2 and in O 2 /CO 2 atmosphere are compared. [ABSTRACT FROM AUTHOR]

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]

Pressurized oxy-fuel combustion technology is the second generation of oxy-fuel combustion technology and has low energy consumption and low cost. In this research, a visual pressurized flat-flame reaction system was designed. A particle-tracking image pyrometer (PTIP) system based on a high-speed camera and an SLR camera was proposed. Combining the experimental system and data-processing method developed, the ignition and combustion characteristics of a single coal particle between 69 and 133 μm in size were investigated. The results indicated that at atmospheric pressure, the ignition delay time of ShanXi (SX) anthracite coal was longer than that of ShenHua (SH) bituminous coal, while that of PRB sub-bituminous coal was the shortest. As the pressure rose, the ignition delay time of the PRB sub-bituminous coal and SX anthracite coal showed a continuous increasing trend, while the ignition delay time of SH bituminous coal showed a trend of first increasing and then decreasing. Moreover, pressure also affects the pyrolysis process of coal. As the pressure increases, it became more difficult to release the volatiles produced by coal pyrolysis, which reduced the release rate of volatiles during the ignition stage, and prolonged the release time and burning duration time of volatiles. [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 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]

• CFD results of a swirl oxy-coal flame are compared with detailed experiment. • Nonlinear variation particle radiative property is first used in oxy-coal modeling. • Contributions of gas and particle radiation are illustrated in oxy-coal flame. • Constant model ten times over-predicts the particle absorption after burnout. This paper reports a comprehensive computational fluid dynamics simulation of the IFRF's swirling oxy-fuel flame with pulverized coal using wet flue gas recycle. Special attention is given to the influence of particle radiative property model. Based on the burnout rate of pulverized coal, a recent proposed nonlinear conversion-dependent particle radiative property model (Guo et al., 2018) is used in the simulations. The influences of the high CO 2 concentration on the sub-models are also considered in detail. The gas-phase global reaction mechanism, kinetic parameters of char oxidation and gasification, and gas radiative property model are modified for oxy-fuel condition. The results show the prediction results agree well with the measurements, in terms of the temperature and composition of the flue gas. The particle radiation has an important effect on the flame temperature. Even in the small furnace studied here (optical length is approximately 1.6 m), the radiation in the flame zone is still dominated by particle radiation. Linear variation assumption (Yin et al., 2015) under-predicts the particle absorption in combustion zone. Ignoring the effect of burnout rate on particle radiative property will 10 times over-predict the particle absorption after burnout. [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]