Oxy-fuel Conditions Research Articles

Effect of CO2/H2O on the Incipient Ultrafine Particulate Matter Formation in Oxy-fuel Combustion of High-Sodium Lignite

In this work, the formation of incipient ultrafine particulate matter (PM) from high-sodium lignite combustion, under both dry and wet oxy-fuel conditions, is intensively investigated using an optically accessible flat-flame burner. The ambient temperature was set at 1500 K, and measurements were performed along the zones with the coal residence time of 0–40 ms. We simultaneously introduced thermophoresis sampling, dilution sampling, and phase-selective laser-induced breakdown spectroscopy (PS-LIBS) to characterize the morphologies, components, and particle size distributions (PSDs) of ultrafine PM as well as the behaviors of Na. The results indicate that incipient particles formed in CO2–O2 ambience are more abundant in Na and Si. The substitution of CO2 may accelerate the mass loss rate of coal in the early devolatilization stage, whereas the Na-based incipient particles form around a time of ∼10 ms that is similar to conventional N2–O2 ambience. In combination with the computational fluid dynamics (CFD...

Assess of biomass co-firing under oxy-fuel conditions on Hg speciation and ash deposit formation

In the last years oxy-fuel combustion has been seen as an attractive method for oxygenated fuels, such as biomass, for controlling simultaneously the excess of heat arising from coal combustion and flue gas emissions (CO2, NOx, SO2…). However there is a lack of information with respect to the fate of other toxic pollutants found in trace concentrations, such as mercury, in oxy-fuel processes operating with biomass fuels. In addition, the modified conditions and flue gas composition leads to more corrosive environments which needs of previous evaluation to minimize possibly damaging processes in the boilers, furthermore in case of employing biomass of high alkaline compounds contents.The present work evaluates the effect of oxy-combustion processes of sub-bituminous coal and coal/ biomass (thistle) blends on mercury partitioning and speciation, under both air and different oxy-fuel combustion atmospheres, by means of a bubbling fluidized bed combustor at laboratory scale. HSC Chemistry 6.1 was supplemented for the interpretation of the experimental results and for the analysis of ash deposits formation.Hg speciation was seen to be more affected by oxy-fuel combustion during the co-firing tests than in coal combustion, with an increment in Hg oxidation at low temperatures (T < 350 °C) and mainly for oxygen concentrations over 30% v/v. This effect is highly related to variations produced in chlorine speciation in the oxy-fuel atmospheres, in which ash deposition and corrosion modifications were seen to be also involved, with a reduction of the chlorinated species against an increased in the sulphated compounds concentration.

Long-term pilot testing of the carbonate looping process in 1 MWth scale

The carbonate looping (CaL) process is an efficient post-combustion carbon capture technology to reduce the amount of CO2 released by fossil fuelled power and industrial plants. This work presents the results gained in long-term pilot tests performed in a 1MWth CaL pilot plant at Technische Universität Darmstadt. The experimental data were obtained from continuous operation under realistic CaL process conditions, i.e. coal originated flue gas from a coal fired furnace to be decarbonized in the carbonator and oxy-fuel conditions in the calciner. Steady-state conditions, achieved while parameters were not changed during periods up to 60h, delivered reliable experimental data to scale up the CaL process to industrial size. The feasibility of the CaL process in semi-industrial size was proven by steady-state CO2 absorption for more than 1200h with absorption rates in the carbonator higher than 90% and overall capture rates higher than 95% under a wide range of operating parameters, e.g. type of fuel, flue gas composition or reactor temperatures. Exemplary operation to achieve steady-state conditions is presented to show the evolution of the carbonator absorption efficiency and solid properties, such as conversion from CaO to CaCO3 by carbonation and to CaSO4 by sulphation. The carbonator performance was assessed by reasonably closing the carbon balances and applying the active space time as a key parameter to interpret the trends observed.

Mercury emissions from coal combustion in fluidized beds under oxy-fuel and air conditions: Influence of coal characteristics and O2 concentration

In this work, the fate of mercury in a bubbling fluidized bed combustor working under oxy-combustion conditions has been studied and compared with air combustion. The influence of burning three different rank coals, with sulphur content ranging from 0.65% to 5.17%, on Hg partitioning has been studied. The presence of limestone as sorbent for SO2 capture as well as the concentration of O2 at the entrance of the combustor were also evaluated.Coal rank does not have a direct influence on coal partitioning. However, sulphur content of the coal is an important parameter to describe not only the high percentage of particle-bound mercury, up to 87%, in presence of limestone but also the prevalence of Hg(0) as the main species in the gas phase when burning the lignite. The O2/CO2 ratio has little effect on Hg partitioning in experiments carried out in presence of limestone at 925°C (the optimum temperature for sorbent sulphating under oxy-combustion conditions) and little differences are found with air combustion at 850°C (the optimum temperature for sorbent sulphating under air combustion conditions).Percentage of particle-bound mercury shows a maximum at 925°C independently of the coal studied, which it is related with the maximum sulphur retention for each coal at this temperature.

The effect of HCl and steam on cyclic CO2 capture performance in calcium looping systems

Calcium looping is CO2 capture technology that is considered to be technically feasible at an industrial scale using a variety of fuels such as natural gas, coals, biomass, refuse derived fuels, and biofuels. Unfortunately, many of these fuels contain significant quantities of chlorine which principally converts to gaseous HCl during combustion or gasification. To date, very few studies have examined the effect of HCl on sorbent CO2 capture performance using calcium-based sorbents under realistic carbonation and calcination conditions. In this work, experiments were conducted using thermogravimetric analysis and fixed bed reactor testing to determine the effect of HCl addition during carbonation and calcination over repeated cycles using a Canadian limestone. The presence of HCl was found to increase sorbent reactivity towards CO2 capture when steam was injected during calcination. The resulting decomposition of CaCl2 to CaO during calcination caused changes in the particle morphology, which in turn decreased the CO2 diffusional resistance during carbonation. Fixed bed test results provided confirmation of full sorbent dechlorination under typical oxy-fuel calcination conditions. It was shown that both particle surface area and pore volume were higher during tests where HCl was present during carbonation and that greater than 99% HCl capture could be achieved without adversely affecting sorbent CO2 capture performance when steam was present during both carbonation and calcination.

Key parameters for the radiative emittance of ashes of solid fuels

The knowledge of the emittance of solid fuel ashes is important for the radiative energy balance in boilers and, hence, crucial for their design. This paper summarizes the most important effects on emittance based on own experiments and makes references to literature. Experimental results will be presented on the spectral emittance of typical minerals (SiO2, CaCO3, MgCO3, SrCO3, CaSO4, MgSO4, Fe2O3) contained in solid fuels ashes, extended by exemplary measurements on natural ashes (coal). The normal emittance is measured in a temperature range between 500 and 1000 °C in the wavelength range from 1.6 to 12 µm in a radiation test rig. The influence of physical surface structure and chemical-mineralogical composition on emittance is discussed. The results show that sizes of ash particles influences emittance. Emittance is increasing with particle size. Surface sintering as well as Fe in the ash also increases emittance. Surface fusion can either increase or decrease emittance based on ash composition. Sulfates and carbonates, typical for ashes under oxyfuel conditions, show characteristic spectral emittance bands. These bands vanish when the sulfates and carbonates being converted to the corresponding oxides at elevated temperatures. These characteristic bands can also be detected in natural ashes which consist of a variety of mineral components.

Oxy-fuel burner investigations for CO2 capture in cement plants

Oxy-fuel conditions may have a considerable influence on the cement production process. This paper presents the results of a first validation work aiming to model the combustion behavior under rich CO2 conditions. The experiments took place in a 500 kWth pulverized fuel combustion rig. A swirl burner is employed to stabilize an air and an oxy-fuel case (29% oxygen in combustion gases). Detailed measurements of temperature and concentrations (O2, CO2, CO) along the furnace length are used to validate the simulation work. A commercial CFD software (Ansys Fluent) is employed to simulate both air and oxy-fuel cases. The results from the simulations indicate a good agreement with the measured values. However, some discrepancies are observed in near burner region that may be also be related to the measurement method employed.

New weighted-sum-of-gray-gases model for typical pressurized oxy-fuel conditions

Pressurized oxy-fuel combustion technology has received considerable attention due to its ability to improve the overall system efficiency and to control CO2 emissions. The characteristics of radiation heat transfer are significant for pressurized oxy-fuel gas mixture and different from those under atmospheric conditions. Therefore, to calculate the radiation characteristics of pressurized oxy-fuel gas mixture quickly and accurately, new weighted-sum-of-gray-gases (WSGG) model for pressurized oxy-fuel conditions was first presented in this paper, which was applied in 3 typical high pressure conditions: 5, 10, and 15 bar. The new WSGG model correlations were suitable for pressurized conditions with a molar ratio range of 0.125-2, temperature range of 400-2500 K, and path length range of 0.1-20 m. Calculations for a variety of typical pressurized oxy-fuel combustion cases showed that the new WSGG model can accurately predict the radiation characteristics and heat transfer characteristics of the gas mixtures compared with the SNB model benchmark. In addition, the application of the previous atmospheric WSGG models yielded non-ideal results under pressurized conditions. Consequently, the new model can provide efficient and accurate radiation heat transfer results for pressurized oxy-fuel conditions and can be used to design pressurized oxy-fuel combustion furnaces or boilers.

SO3 formation and the effect of fly ash in a bubbling fluidised bed under oxy-fuel combustion conditions

This study investigates SO3 formation under air-fired and oxy-fuel combustion environments in a bench-scale bubbling fluidised bed. Flue gas compositions typical of air-fired and oxy-fuel environments were simulated with synthetic gas mixtures. Parameters such as bed temperature and SO2 and H2O concentrations were varied to determine their effect on the formation of SO3. Tests were also performed using three fly ashes produced from three different coals and combustion systems. Oxy-fuel combustion led to increased SO3 concentration as compared to typical air-fired environments. SO3 concentrations increased with combustion temperature and O2 concentrations. SO3 formation also depends on the chemical composition of the fly ash.

Identifying spasticity during gait by surface electromyography

This work presents experimental results of swirling pulverized coal flames from a 60kWth combustion chamber based on spectrometric (UV–VIS) measurements comparing conventional air-fuel and oxy-fuel conditions. Radiation of the flames investigated was collected by two different optical systems: first, by a non-intrusive narrow-angle optical system providing integrated line-of-sight radiation data, and second, by an intrusive cold-background optical probe that collects radiation from integrated line-of-sight radiation within a reduced measurement volume. The collected spectra were processed and fitted to the Planck black body radiation function in order to estimate solid particle temperatures and relative radiant fluxes. Atomic and molecular emission lines observed are also identified and compared between the flames studied. The data obtained provide valuable insight to the different flames investigated and helpful explanations of the significant observed differences between the flames. As expected, oxidant composition was found to have a major impact on flame radiation and measured temperatures. Experimental validation data from this type of configuration and flame conditions are scarce, thus, the data may serve as validation data for numerical models.

Comparative ignition tests of coal under oxy-fuel conditions in a standardized laboratory test rig

Ignitability is important to characterize pulverized coal combustion, as it is directly related to flame stability. The current work describes a practical test rig for rapid laboratory analysis of pulverized coal cloud ignition properties. The system has been designed for conventional coal combustion conditions using air as the oxidant. In the current work, the measurement principle of the device is described and its adaption to and applicability for oxy-fuel combustion tests is demonstrated. Four coals with different rank were measured in air and in oxy-fuel atmospheres containing 20–35vol% O2 in CO2. The major influencing factors for the investigated samples were found to be the coal rank and the gas-phase oxygen concentration, while a minor influence of particle size was observed.

Numerical investigation of oxy-fuel combustion in 700 °C-ultra-supercritical boiler

The combination of the advanced ultra-supercritical (AUSC) and oxy-fuel combustion technologies is considered the most feasible and promising choice for carbon capture and storage. The high efficiency of AUSC boilers compensates for the cost incurred in controlling CO2 emissions. In this study, a new improved model was proposed for gas-radiative properties as a polynomial function of the continuous molar ratio of H2O to CO2 and temperature, which was compiled as user-defined functions in C language. The modified radiative model was employed in the computational fluid dynamics analysis to simulate a tangentially fired 700°C-USC boiler with a capacity of 300MW under the oxy-fuel condition. The results show that the improved model has a good accuracy for gas-radiative properties compared to other classical models. In the oxy-fuel cases, the heat transfer obtained in the simulation is approximately 4–11% lower than that in the thermodynamic calculation because of the char gasification. The radiative capability of the flue gases in the oxy-fuel case is much greater than that in the air–fuel case with the increase in the molar fraction of oxygen. The heat transfer via the water wall is approximately 4–8% higher than that via air, when the molar fraction of oxygen increases from 26% to 29%. The increase in the heat absorption helps in better arrangement of the heat-exchanger surfaces in the furnace of the AUSC boiler.

Combustion of Lignite Coal in a Bubbling Fluidized Bed Combustor under Oxyfuel Conditions

Oxyfuel combustion is one of the leading technologies considered for capturing CO2. The field of oxyfuel combustion in a bubbling fluidized bed is recently addressed only marginally, but it is more suitable for lower capacity loads. The paper presents some results from bubbling fluidized bed oxyfuel combustion of lignite coal from 25kW experimental device and 250kW pilot BFB boiler. The methodology of the results presentation is stated and confrontation of the experimental results with the mathematical model is presented. The experiments were focused on identification of the oxyfuel regime operation and determination of the different CO2/O2 ratio in fluidized medium. The CO2 flue gas concentration was for the majority of experiments kept in the range of 84 to 91% and the oxygen concentration in flue gas from 4 to 8% in case of smaller laboratory combustor. Pilot BFB combustor suffers more with air leakage but the results correspond to the reality of the real operation of these facilities in an industrial scale.

Experimental and Numerical Investigations on Heat Transfer Characteristics of a 35MW Oxy-fuel Combustion Boiler

Oxy-fuel combustion has different combustion and heat transfer characteristics from those of air-combustion, due to the high concentration of CO2 and H2O in the flue gas. This study investigated the heat transfer characteristics in a 35 MW oxy-combustion large pilot plant. The experimental and numerical investigations are carried out with a sub-bituminous coal. The gas temperature, exhaust emission, and heat transfer are measured in conventional air-combustion and oxy-fuel conditions with wet and dry flue gas recycle. Detailed numerical studies of combustion and heat transfer are performed with a modified radiative property model and global reaction mechanism. The results are in good agreement with the experimental data. The effects of char and ash on radiative heat transfer are investigated. The experimental and numerical results show that although the peak temperature decreases, the heat transfer in membrane-wall and superheater in oxy-fuel combustion is slightly greater than that in air-combustion. The absorption coefficient ratios of particle to gas are approximately 2.5 and 2.4 in air-combustion and oxy-fuel combustion. The particle radiation dominates the radiative heat transfer in the pulverized coal combustion, which has a significant influence on the temperature level and distribution.

Study of the Post-combustion CO2 Capture Applied to Conventional and Partial Oxy-fuel Cement Plants

The present work was focused on the application in the cement industry of the post-combustion CO2 capture technology using amine-based chemical absorption. Besides conventional conditions, investigations were performed on a concept called “partial oxy-fuel combustion” applied to cement kilns leading to more CO2-concentrated flue gas (yCO2 up to 60%) and allowing to reduce the solvent regeneration energy consumption. The purpose of the study was to highlight and evaluate this reduction. Firstly, the performances of several solvents were evaluated thanks to screening tests at lab (results with a cables-bundle contactor) and micro-pilot (absorption-regeneration unit) scales considering different CO2 contents (yCO2,in = 20-60 vol.%). Simple and blended solvents were tested in such conditions, such as primary and secondary alkanolamines (benchmark Monoethanolamine (MEA), Diethanolamine (DEA) and cyclical di-amine (Piperazine (PZ)). It was shown that the use of activated solutions (such as DEA 30 wt.% + PZ 5 wt.%) presented particularly high absorption performances both in conventional and high CO2 contents conditions. Secondly, Aspen HysysTM simulations were carried out considering cement plant flue gas. For MEA 30 wt.% an increase of yCO2 from 20% to 44% leads to a 26% decrease of the regeneration energy (from 3.36 to 2.48 GJ/tCO2), which is clearly encouraging. As perspectives, new absorption-regeneration tests at micro-pilot scale and simulations will be performed with other solvents in order to have a more global evaluation of the interest of partial oxy-fuel conditions for the cement industry.

Detailed Analysis of the Velocity Fields from 60 kW Swirl-Stabilized Coal Flames in CO2/O2- and N2/O2-Atmospheres by Means of Laser Doppler Velocimetry and Particle Image Velocimetry

ABSTRACTThis work presents experimental results from flow field measurements of swirl stabilized pulverized-coal flames in oxy-fuel conditions and in conventional atmospheres to characterize the influence of the mixture upon flame aerodynamics. Experiments were carried out on flames inside a cylindrical combustion chamber equipped with a pulverized-coal swirl burner. Flow fields for different flame configurations were measured by means of laser Doppler velocimetry (LDV) and are complemented by particle image velocimetry (PIV). The experimental setups and results are presented, compared, and discussed. LDV measurements provide radial profiles for mean and RMS axial and tangential velocity components at different cross sections inside the combustion chamber. PIV measurements provide planar two-dimensional flow field information about mean values for the axial and the radial velocity components of the flow, as well as qualitative information about solid particle distribution inside the combustion chamber. The performance of LDV and PIV as optical measurement techniques is evaluated for the present case. Coal particles with a wide size distribution are used as flow tracers in the experiments. The influences of particle size upon obtained results, the measured flow fields, and qualitative particle distributions are discussed in detail.

Bioslurry for Stationary Applications: Particulate Matter Emission during Combustion under Air and Oxyfuel Conditions

As a new type of fuel, bioslurry is prepared from mixing bio-oil and biochar produced from biomass fast pyrolysis and is suitable for stationary combustion applications. This study reports the emission of particulate matter (PM) from bioslurry combustion in a lab-scaled drop-tube furnace at 1400 °C under various atmospheres, including air, 20% O2/80% CO2, and 30% O2/70% CO2. Similar to biomass combustion, the particle size distribution (PSD) of PM with an aerodynamic diameter of less than 10 μm (i.e., PM10) during bioslurry combustion also follows a bimodal distribution. Increasing the char loading level in bioslurry leads to an increase in the emission of PM10, especially PM1–10, as a result of the high contents of inorganic species in biochar. Emissions of PM1 and PM1–10 during bioslurry combustion under 20% O2/80% CO2 conditions are lower than those under air conditions, apparently attributed to the lower char combustion temperature. However, under 30% O2/70% CO2 conditions, emission of PM10 during bio...

Assessment of several gas radiation models for radiative heat transfer calculations in a three-dimensional oxy-fuel furnace under coal-fired conditions

Oxy-fuel coal combustion is a promising Carbon Capture and Storage technology that allows retro-fitting of existing power plants. Due to high concentrations of CO2 and H2O and the strong and irregular spectral variation of their absorption coefficients, prediction of radiative heat transfer in oxy-fuel combustion systems requires a non-grey gas radiation model. Several models, including narrow-band correlated-k (NBCK), wide-band correlated-k (WBCK), full-spectrum correlated-k (FSCK), and weighted-sum-of-grey-gases (WSGG) models, were used to predict radiative heat transfer by CO2 and H2O in a three-dimensional real-size virtual coal-fired furnace under air- and oxy-fuel (dry and wet flue-gas recycle) conditions. The best agreement with the benchmark solution calculated by the NBCK model with the updated EM2C parameters is achieved by the FSCK model. The WBCK model is less accurate. The WSGG parameter sets of Kangwanpongpan et al. (2012) and Bordbar et al. (2014) are most suited for oxy-fuel combustion among the six sets considered. The FSCK model offers the best performance in terms of both accuracy and computational efficiency among the considered gas radiation models and is recommended for CFD simulations of real-sized oxy-coal systems.

NO and N2O Emissions during Devolatilization and Char Combustion of a Single Biomass Particle under Oxy-fuel Conditions at Fluidized Bed Temperature

Oxy-fuel fluidized bed combustion is a novel clean-biomass utilization technology. The NO and N2O emissions during oxy-fuel combustion of a single biomass particle at fluidized bed temperature were studied in a flow tube reactor. The method of isothermal thermogravimetric analysis was used to distinguish the devolatilization and char combustion stages of biomass combustion. This work is aimed to study the effects of temperature, CO2 concentration, atmosphere and O2 concentration, H2O vapor addition, and biomass type on the NO and N2O emissions during oxy-fuel combustion of a single biomass particle at fluidized bed temperature. In oxy-fuel combustion, NO is rapidly formed during the devolatilization stage, while N2O is mainly formed during the char combustion stage. In 30% O2/70% CO2 at T = 800 °C, the conversions of fuel-N to NO and N2O are 11.96% and 18.98%, respectively. The conversion of fuel-N to NO reaches the maximum value at T = 800 °C during the devolatilization stage, while it increases with inc...

Effects of oxidant distribution mode and burner configuration on oxy-fuel combustion characteristics in a 600 MWe utility boiler

The application of oxy-fuel combustion in utility boilers has drawn increasing attention in recent years. The previous numerical studies on oxy-fuel utility boilers mainly focused on effects of O2 concentration and flue gas recycle mode. In the present study, the effects of oxidant distribution mode and burner configuration on characteristics of oxy-fuel combustion were numerically investigated. The simulation results showed that a similar aerodynamic field and a higher char burnout compared with those of air-combustion were obtained by closing the separated over fire air (SOFA) nozzles under oxy-fuel condition. After the oxidant staging was employed in oxy-fuel combustion, the NOx emission was decreased to approximately half of that in air, but the momentum of second air (SA) suffered a remarkable reduction. The undesired reduction of second air momentum could result in some serious problems. A burner configuration and a new oxidant distribution mode were proposed to solve the problem of insufficient momentum of SA after the oxidant staging was adopted. The incomplete combustion heat loss could decrease to the same level as that of air combustion after the proposed burner configuration was employed. The present investigation is beneficial for offering guidance in both retrofitting and operation of oxy-fuel utility boiler.