Oxy-coal Combustion Research Articles

Adsorption and Desorption Behaviors of Nitrous Oxide on Various Rank Coals: Implications for Oxy-coal Combustion Flue Gas Sequestration in Deep Coal Seams

Injecting oxy-coal combustion flue gas into deep coal seams is viable to simultaneously reduce the main anthropogenic greenhouse gas (GHG) CO2 and gaseous contaminants SO2 and NOx. This paper inves...

Optimizing complexity in the kinetic modelling of integrated flue gas purification for pressurized oxy-combustion

A 5-step mechanism and a corresponding kinetic model are presented for integrated flue gas purification, where SOx and NOx are removed during pressurized oxy-coal combustion. Three coupled subsystems (‘blocks”) of gas–liquid transformations are considered: 1) oxidation of NO to NO2 and dissolution of NO2 in water; 2) dissolution of SO2 in the water; and 3) interactions between S- and N- containing substances in the liquid phase. The model has been formulated in terms of only species that can be experimentally measured (model of optimal complexity). Subsystem I is limiting regarding NO removal. Subsystem II is very fast in comparison with the other subsystems and can be treated separately. Analytical expressions for all independent subsystems have been found and compared with computational results for the whole system. The temporal domain of validity of all these subsystems is presented. Finally, the validity of the NO2 quasi-steady-state assumption is verified, and it is reported that variables such as NO, NO2 and HNO3 can be considered decoupled from the whole system since they appear in only Subsystem I, while HSO3− and HNO2 are coupled. The presented kinetic model was verified experimentally by determining the 1:1 ratio of HNO2 and HNO3, in gas–liquid NO-O2 interaction. Additionally, new experimental facts on kinetic orders and stoichiometry between HNO2 and HSO3− have been used to optimize the model. It is concluded that all NOx- SOx interactions can be essentially grasped by a sequence of only 4 irreversible reactions considering, in addition, the 5th reaction, i.e. the fast ‘SO2-water’ equilibrium.

Reactive force field molecular dynamics (ReaxFF MD) simulation of coal oxy-fuel combustion

Understanding the reaction mechanism of oxy-coal combustion is fundamentally important to the CO2 capture and storage in coal-fired industrial processes. In this paper, the microscopic reactive behaviors of brown coal combustion in O2/CO2 atmosphere were simulated by combining the molecular dynamics (MD) simulations and the reactive force field (ReaxFF). With thoroughly tracing the motion trajectories of atoms, as well as the formation, transition and breaking of bonds between atoms, the generation pathways of CO2 and the effects of temperature (1600–2000 K) and oxygen concentration (21%~30%) on the coal oxy-fuel combustion process were studied. Results showed that the main process of CO2 generation consists of the decomposition, oxygenation, and dehydrogenation. The atmosphere with a high concentration of CO2 can accelerate the reaction rate of coal combustion and reduce the consumption of O2. The higher temperature will promote the production of major intermediates with more fragments being released earlier, and finally increase the generation of CO2. The increase of O2 concentration obviously hastens the decomposition of coal molecule and the generation of H2O, but shows very weak influence on the distributions of other major products and intermediates.

Formation characteristics of ultra fine particles during oxy-coal combustion with flue gas recirculation

A laboratory-scale oxy-fuel combustion experimental apparatus was built up to simulate the real oxy-fuel combustion. Under conditions with three kinds of recycles (no-RFG, de-ash and ash), combustion experiments of DT bituminous coal (rich in Si and Al) and SH bituminous coal (rich in Ca) were conducted to investigate the influence of recycled ash on formation of ultrafine particle matters. The results show that, compared with the no-RFG condition, under condition with de-ash flue gas recycle, the yields of ultrafine particle matters of the DT and SH coal were both basically invariant, while that under condition with ash recycle decreased by about 12.03% (DT coal) and 7.73% (SH coal), respectively. Under condition with ash recycle, the silicon aluminate, which was rich in the ash of DT coal, would absorb the Na and K vapor released during the combustion, and the rich CaO in the SH coal may also absorb the S element during the combustion to restrain the S from migrating to the ultrafine particles, both of the reactions decreased the formation of the ultrafine particles.

Experimental study and demonstration of pilot-scale oxy-coal combustion at elevated temperatures and pressures

High pressure oxy-fuel firing is a promising next-generation combustion technology for electricity generation, allowing net efficiency improvement compared to traditional oxy-fuel firing systems. Oxy-coal combustion has been extensively investigated at ambient pressure, but limited experimental data is available regarding this combustion method at elevated pressures at pilot or full scale. The study presented here investigates the operational performance of a high pressure, high temperature pilot-scale oxy-fuel combustor. Pulverized Utah bituminous coal in form of coal-water slurry was combusted at pressures up to 14 bar, temperatures up to 1600 °C and firing rates up to 300 kW. Eight different runs were selected for presentation and the measurements include flue gas composition (CO, CO2, CH4, SO2, O2), carbon and sulfur content of fly ash, combustor temperature profile and flame radiation profile. The flame radiation measurements were carried out with a novel narrow beam radiometer which measures local radiant heat flux at a specific location in the combustor. The radiation ranged from 108 to 279 kW/m2 depending mainly on combustor temperature. Additionally, the temperature significantly impacted the carbon content of fly ash: 2.66% was measured near 1600 °C, while the concentration increased to 36.7% when the temperature was decreased to 1300 °C. The successful experimental demonstrations showed that increasing pressure is a viable combustion method and a step towards enhancing the efficiency in oxy-fuel firing based power plants.

Radiative energy flux characteristics and model analysis for one-dimensional fixed-bed oxy-coal combustion

This paper describes the radiative energy flux characteristics of fixed-bed oxy-coal combustion for the purpose of guiding the quality-splitting conversion of combustion energy. An experiment was performed in a tube furnace at a temperature range of 800–1200 °C in O2/N2 and O2/CO2 atmospheres, and the radiative intensity was measured. It was found that an increase in oxygen concentration and temperature could increase the radiative intensity more than 1.5 to 2 fold during combustion, and the radiative energy flux was higher for semi-coke than coal by about 16%–27%. The radiative energy results could be described by a semi-empirical model and an artificial neural network (ANN) model. The results showed that the errors of the ANN were less than 0.01%, and demonstrated the superiority of the ANN. This study provides guidance for subsequent research on quality-splitting conversion of combustion energy.

Ignition and combustion characteristics of coal particles under high-temperature and low-oxygen environments mimicking MILD oxy-coal combustion conditions

The experiments of coal particles combustion were conducted to mimic moderate and intense low-oxygen dilution (MILD) oxy-coal combustion on a diffusion-flamelet-based Hencken burner under different O2/CO2 and O2/N2 environments with the coflow temperatures of 1400–1800 K and oxygen concentrations of 5%–20%. The images of coal particles combustion flame were captured by a digital camera to determine the ignition standoff distances of coal particles. The radiation spectrums of burning coal particles at different coflow temperatures and oxygen concentrations were measured by a fiber optic spectrometer and the temperatures of burning coal particles were deduced with the two-color method. It is found that either increasing the coflow temperature or the oxygen concentration leads to brighter flame luminosity, shorter ignition standoff distance and higher burning particles temperature. The ignition standoff distance of coal particles is increased by 0.07–0.49 cm when carbon dioxide is used as diluent gas instead of nitrogen. The temperatures of the burning coal particles under O2/CO2 environments are lower by 51–185 K than those under O2/N2 environments at the same conditions due to the combined physicochemical properties of CO2 with higher heat capacity and the endothermic reaction of char gasification. Moreover, a method based on the coal particles temperature uniformity coefficient is proposed to quantitatively identify the combustion modes of MILD coal combustion and MILD oxy-coal combustion. All coal combustion flames are visible under both O2/CO2 and O2/N2 environments, and the MILD coal combustion flames under high-temperature and low-oxygen concentration conditions are weaker and more uniform with dark red color.

System Analysis on Supercritical CO2 Power Cycle with Circulating Fluidized Bed Oxy-Coal Combustion

Supercritical carbon dioxide (S-CO2) Brayton power cycle is a competitive technology to achieve high efficiency in a variety of applications. However, in coal power applications, the CO2 generated from coal combustion still discharges into the atmosphere causing a series of environment problems. In this work, an 300 MWe S-CO2 power cycle with circulating fluidized bed (CFB) oxy-coal combustion was established including air separation unit (ASU), CFB boiler, recuperator system and carbon dioxide compression and purification unit (CPU). Based on the material and energy conservation, the cycle efficiency of S-CO2 (620°C, 25 MPa) Brayton power cycle with CFB oxy-coal combustion is evaluated compared to the oxy-coal combustion steam Rankine cycle and S-CO2 Brayton power cycle with the 31.65 kg/s coal supply. After that, the influence of several factors, e.g., exhaust flue gas temperature, split ratio in recuperator system and the oxygen supply on the cycle efficiency was investigated and analyzed. The results show that the net efficiency of S-CO2 power cycle with CFB oxy-coal combustion (32.67%) is much higher than the steam Rankine cycle utilizing CFB with 17.5 Mpa, 540°C steam (27.3%), and 25 Mpa, 620°C steam (30.15%) under the same exhaust flue gas temperature. In addition, lower exhaust flue gas temperature and higher split ratio are preferred to achieve higher cycle efficiency. Lower oxygen supply can reduce the energy consumption of ASU and CPU, further increasing the system net efficiency. However, the energy losses of ASU and CPU are still very large in oxy-coal combustion and need to be improved in further work.

Investigation of particle radiation and its effect on NO prediction in a pilot-scale facility for both air and oxy-coal combustion

Radiation heat transfer plays an important role in pulverised coal combustion, influencing the overall combustion efficiency, pollutant formation and flame ignition and propagation. In this paper, the radiation properties of the particles as well as gas property models on the overall influence of the prediction of the formation of NOx pollutants in a pulverised coal combustion have been investigated. The non-grey weighted sum of grey gases (WSGG) model has been employed to calculate the radiation of the gas phase coupled with the radiation interaction from the particulate phase. The Mie theory, as well as constant or linear models, have been employed to describe the particle radiative properties. The prediction results, calculated from the data from a 250 kW pilot scale combustion test facility (CTF), are compared against experimental measurements under air-fired condition and a range of oxyfuel conditions. The results show that the choice of radiation solution can have a considerable impact on the radiative heat transfer results, in which the Mie theory shows a significant improvement in the incident wall heat flux compared to the constant or linear models. Also, the more accurate solution employed for radiation of gases and particles considerably improves the NOx prediction in the flame region.

Oxy-Coal Combustion Fly Ashes: Leachability of Inorganic Trace Contaminants

The leachability and speciation of inorganic trace contaminants of oxy-coal combustion fly ash from a 20 MWth oxy-pulverized coal combustion (oxy-PCC) demonstration plant have been evaluated. Colle...

Numerical simulations towards optimizing the burner design for efficient staged pressurized oxy-coal combustion.

Staged pressurized oxy-coal combustion (SPOC) – a technology developed at Washington University at St. Louis (WUSTL) – is nowadays a promising tool for low-cost/emissions, high-efficiency power generation. The present computational simulations, performed at West Virginia University (WVU) by means of the ANSYS Fluent package, compliment the experimental research for better understanding of the impacts of turbulence, fluid flow, flame/particle dynamics, and heat transfer on oxy-coal combustion. Here, CO2 is injected alongside the coal, for its carriage, while a small amount of CH4 is also injected alongside the coal to maintain a steady flame. Out of 100 kW in a lab-scale reactor, 10 kW originates from CH4-air burning and 90 kW comes from coal combustion. Steady and unsteady RANS simulations are performed resulting in an asymmetric flame shape. Three causes of the flame asymmetry are hypothesized: (i) coal injection, (ii) onset of the shear-layer instability due to various stream velocities and densities in a shear layer, where mixing occurs, and (iii) presence of the vortex shedding due to the flow past a bluff body, which is used to stabilize the flame. As a result, an impact of the presence of coal on the flame symmetry is demonstrated, and a benchmark at which further increase of coal would result in flame asymmetry is found. The present simulations also shows occurrence of the vortex shedding when the flow passes over a disk in the reactor used to stabilize the flame, constituting a potential reason for the periodic flame oscillations experienced in the result.This work is performed in cooperation with the Washington University at St. Louis (WUSTL). It is sponsored by the US Department of Energy (DoE) through the US-China Clean Energy Research Center – Advanced Coal Technology Consortium (CERC-ACTC).

Reynolds-Averaged Navier-Stokes (RANS) simulation of a staged pressurized oxy-coal combustion (SPOC)

Staged pressurized oxy-coal combustion (SPOC), which is a promising technology being developed at the Washington University at St. Louis (WUSTL) aiming to be used for low-cost, low-emission, high-efficiency power generation, is investigated numerically using the ANSYS Fluent commercial package. The modelling supports the WUSTL experimental endeavors in a 100 kW lab-scale reactor, with 90% of energy coming from the coal and 10% originating from methane-air combustion. To be specific about the burner design, carbon dioxide is injected alongside the coal, for its carriage. In addition, a small amount of methane is also injected alongside the coal to maintain a steady flame. Steady and unsteady Reynolds-Averaged Navier-Stokes (RANS) simulations are performed resulting in an asymmetric flame shape. We propose several potential causes of such a flame asymmetry including peculiarities of the coal injection and the onset of the shear-layer instability occurring because the densities and velocities of the mixing streams. As a result, an influence of the presence of coal on the flame symmetry is demonstrated. In addition, a benchmark at which the further increase of coal would result in flame asymmetry has been found.This work is performed in cooperation with the Washington University at St. Louis (WUSTL). It is sponsored by the US Department of Energy (DoE) through the US-China Clean Energy Research Center – Advanced Coal Technology Consortium (CERC-ACTC).

Displacement behaviors of adsorbed coalbed methane on coals by injection of SO2/CO2 binary mixture

Direct injection of oxy-coal combustion flue gas into coal reservoirs is viable to mitigate CO2, SO2 and NOx emissions, with the simultaneous benefit to improve coalbed methane (CH4) recovery. To study the feasibility of this technology, the displacement process of adsorbed CH4 on coals by SO2/CO2 binary mixture injection, termed as SO2/CO2-ECBM, was investigated in laboratory. The results show that the adsorption amount of CO2 on coals at displacement equilibrium state for SO2/CO2 injection slightly decreases compared with that for pure CO2 injection. The total adsorption amount of SO2/CO2/CH4 shows approximate linear correlation with coals' micropore parameters. However, a downward parabolic relation is found between the adsorption amount of CO2 and micropore parameters. In comparison with CO2 injection, the molar composition of CH4 in the bulk phase rises while that in the adsorbed phase drops at displacement equilibrium state for SO2/CO2 injection. All these changes in molar compositions of CH4 in the two phases lead to more CH4 molecules desorption from coal matrix, and demonstrate that implementing SO2/CO2-ECBM can achieve superior CH4 recovery. XPS characterizations confirm that the injected SO2 contained in SO2/CO2 occupying the oxygen-containing functional groups, i.e. C–O and CO on coals, is the main interaction mechanism between SO2 and coals. This chemisorption effect may account for the enhanced CH4 recovery via SO2/CO2 injection. To summarize, injection of SO2/CO2 rather than CO2 into coal seams has potential to generate higher CH4 recovery and mitigate SO2 and CO2 emissions into the atmosphere.

Energy efficiency analysis of pressurized oxy-coal combustion system utilizing circulating fluidized bed

Oxy-fuel combustion with circulating fluidized bed (CFB) is one of the most important technology for carbon dioxide capture and sequestration. However, its development and application are seriously hindered by the low net efficiency mainly resulting from the use of air separation unit (ASU) and carbon dioxide compression and purification unit (CPU). In this work, a pressurized oxy-coal CFB combustion cycle was established mainly including ASU, CFB combustion unit, heat exchangers, power island and CPU. Based on the material and energy conservation, the detailed energy penalties of ASU, CPU, and recirculating compressor were investigated under the varying combustion pressures from 0.105 MPa to 3 MPa. Results revealed that with the increase of combustion pressure, the energy consumption of ASU increases while that of CPU decreases. Compared with PC boiler, the influence of the recirculating compression work on CFB system is negligible in lower pressures. Overall, Higher combustion pressure is favorable to the oxy-coal CFB combustion. By minimizing the penalties and recover more latent heat, the net efficiency rises up from 27.2% to 30.5% in this work while the optimum pressure being about 1.1 MPa. Meanwhile the mole fraction of carbon dioxide can reach 92% which is satisfied with the demands.

Experimental Investigation of NO Reburning During Oxy-Coal Burner Staging

This study presents an investigation into the impact of varied burner staging environments on an oxy-fuel flame and the rate of the NO formation and destruction processes. The experimental data were extracted from the use of a 250 kWth down-fired combustion test facility with a scaled-down model of an industrial low-NOx burner. Two oxy-coal combustion regimes were investigated by varying a fixed flow of oxidant between the secondary and tertiary registers, so as to impact the stoichiometry in the fuel-rich region and flame structure, and using various NO recycling regimes, to test the impact of these different burner configurations on NO reburning. The data were collected by monitoring key emissions in the flue gas and in the flame, as well as temperatures throughout the furnace and the unburned carbon content of the ash. A detailed investigation encompassing the impact of secondary oxidant proportion for different oxidants on NO emissions, together with the quantification of recycled NO destruction, is discussed. This investigation finds that 85–95% of the recycled NO is destroyed at a range of burner configurations using OF 27 and OF 30 at 170 kWth. In addition to this, NO formation and carbon burnout are found to be significantly affected with changing burner configurations. Further to this, OF 30 flames appear to be more sensitive to burner configuration than OF 27 flames with regards to both NO formation and destruction, possibly due to the decreased density of the OF 30 oxidant. Radial profiles of two burner configurations at OF 27 and OF 30, as well as an axial profile of two burner configurations at OF 30, were analyzed. The profiles appear to show that burner staging aids in controlling the products of NO reburning, hence maximizing the destruction of recycled NO.

Observation of simultaneously low CO, NOx and SO2 emission during oxy-coal combustion in a pressurized fluidized bed

Pressurized oxy-fuel combustion is a promising technology for CO2 capture with respect to its high combustion efficiency and the simultaneous reduction of gaseous pollutants. A 10 kWth bubbling fluidized bed reactor with continuous coal-feeding was designed, and effects of pressure, temperature and fuel types on pollutant emission were investigated in detail. Generally, the relatively low carbon content in the ash and CO concentration in the flue gas demonstrated that the combustion efficiency was improved by high pressure. The concentration of NO, N2O and SO2 showed decreasing trends with the increase of pressure. Moreover, the effect of pressure on the emission of NO and SO2 in the lower pressure (≤0.3 MPa) was more pronounced than that in the higher pressure. The concentrations of NO and SO2 correlated positively with temperature, while for N2O, it had a negative correlation. Compared with air combustion, NO and SO2 emission dropped sharply in 21%O2/79%CO2 atmosphere. However, N2O concentration during oxy-combustion was slightly higher than that in air combustion in the range of experimental pressure.

Investigation of gas and particle radiation modelling in wet oxy-coal combustion atmospheres

In this study, the impact of various treatments of gas and particle radiative properties on modelling radiative heat transfer in a 2D-squared enclosure with conditions representative to the post-burner region of an oxy-coal furnace with wet flue-gas recirculation is systematically investigated. The benchmark solutions for the radiative source terms and the net wall heat fluxes are computed with a narrow-band correlated-k (NBCK) model to account for the non-gray gas radiation in conjuction with the Mie-theory to compute the particle radiative properties. Comparisons with this benchmark solution show that the use of gray Henyey-Greenstein functions obtained with the Planck-mean values for the asymmetry factors of the coal/char and ash particles is sufficient to model accurately the scattering phase functions. In addition, the use of Planck-mean values for all the particle radiative properties provides predictions within 20% of the reference solutions. On the other hand, model results show that the application of a constant refractive index for ash particles induce large discrepancies, whereas such approximation can be made for coal particles without loss of accuracy. Finally, a significant amount of computational cost can be saved with no significant loss of accuracy by considering a wide-band correlated-k model instead of the NBCK model. The full-spectrum correlated-k (FSCK) scheme proposed by Modest and Riazzi (2005) based on gray scattering coefficients and phase functions improves further the computational saving with higher mean relative errors of about 5% for the radiative source terms and net wall heat fluxes. Thus, it is concluded that the FSCK model is appropriate to model the radiative heat transfer including gas and particle radiation for engineering applications.

Experimental Investigation of Oxy-coal Combustion in a 15 kWth Pressurized Fluidized Bed Combustor

Pressurized oxy-coal combustion is considered as a promising carbon capture technology as a result of its potential of high efficiency and low cost in CO2 capture. However, experimental investigations of oxy-coal combustion under pressurized conditions are far less common than those under atmospheric pressure conditions, and hence, further research is needed to elucidate the effects of pressure on oxy-coal combustion in terms of combustion performance, emissions, etc. In this study, a series of oxy-coal combustion experiments were carried out under the pressures from 0.1 to 0.4 MPa on a 15 kWth fluidized bed combustion system. The effects of the combustion pressure and oxygen concentration in the oxidant on the temperature profile, unburnt carbon, combustion efficiency, fly ash composition, and NOx emission were investigated. The experimental results have shown that the CO2 concentration in the oxy-combustion flue gas under different combustion pressures have all exceeded 90%, which is beneficial to the c...

Combustion characteristics of char from pyrolysis of Zhundong sub-bituminous coal under O2/steam atmosphere: Effects of mineral matter

In the oxy-coal combustion steam system (OCCSS), the demineralized coal is consumed by reacting with O2 and steam. The stability and efficiency of demineralized coal combustion at high steam concentrations could be a critical issue to the development of mature OCCSS technology. In this study, Zhundong coal and its acid-washed sample were used for conducting experiments. Pyrolysis chars were firstly prepared from the raw coal and the demineralized coal in a high-temperature drop tube furnace at 1200 °C. The effects of the reaction atmosphere (O2/steam and O2/N2) and temperature (1100–1400 °C) on the combustion characteristics of the two char samples were then investigated. The results show that compared to the char from pyrolysis of raw coal, the char from demineralized coal contained abundant macropores with low specific surface area. For the pyrolysis char from demineralized coal at 1200 °C, the steam-char gasification reaction rate was significantly lower than that of O2-char reaction. The presence of high concentration of steam could alter the heat/mass transfer on the char surface, thus indirectly reducing the rate of O2-char reactions. The combustion of char without minerals in an O2/steam atmosphere accelerated significantly with increasing temperature, while the increase in combustion rate with increasing reaction temperature for char containing minerals was very limited. The steam-char gasification reaction of demineralized char at 1300 °C has promoted the carbon conversion of char. The demineralized char combustion reaction required a relatively high oxygen concentration (i.e. above 40%) to show high burning rate.

Pressure and particle property impacts on radiation in oxy-coal combustion

A numerical study evaluating the significance of scattering phase function, particle size, particle distribution and pressure in an oxy-coal combustor was conducted. Particle absorption and scattering coefficients were calculated from particle properties and Mie theory. Mie theory was also used to identify appropriate scattering phase functions as a function of particle size. A strongly forward scattering function represented by a Henyey-Greenstein function was found to match scattering behavior for particle diameters greater than approximately 10 μm. A comparison of highly forward scattering based on two mono-sized particle distributions (15 µm and 119 µm) and a polydisperse size distribution (27.9 µm SMD) showed small particles produced peak incident wall heat fluxes 30–40% greater than large particles due to the increased number density with the smaller particles. The polydisperse distribution produced flux profiles similar but slightly higher than the 15 µm mono-sized particles. Although strongly forward scattering was found to best represent particle scattering for this system, data did suggest that phase function/scattering coefficient influence becomes more important with increasingly small particle size. Two particle distribution profiles (narrow and wide) were used to represent different coal flame profiles. A wide particle distribution across the combustor diameter produced higher incident wall heat fluxes than a narrow distribution limited to the center region of the combustor. Increased particle size decreased the effect of distribution. Incident wall heat flux was found to increase with increasing combustor pressure up to approximately 5 atm and 10 atm, respectively, for wide and narrow particle distributions. This was primarily due to increased particle absorption and scattering coefficients. Additional pressure increases beyond this up to 20 atm had minimal impact on wall fluxes due to saturated values of the particle absorption coefficient.