Oxy-fuel Combustion Conditions Research Articles

Ash characteristics during oxy-fuel fluidized bed combustion of a Victorian brown coal

This paper investigates, for the first time, the characteristics of ashes resulting from oxy-fuel fluidized bed combustion of a Victorian brown coal. The experimental results showed that the bed ash composition was independent of the reactor temperature and feed gas concentration in the combustion environment. No bed agglomeration was observed under oxy-fuel combustion conditions, even with the addition of steam, at 800–900°C temperatures during the period of the tests. It is expected that agglomeration is unlikely to be a major problem during oxy-fuel fluidized bed combustion of this Victorian brown coal.

Effect of dilution of fuel in CO2 on the conversion of NH3 to NO x during oxy-fuel combustion

The indirect chemical effects of fuel dilution by CO2 on NO formation were investigated numerically in this paper. CH4 doped with NH3 was used as fuel, while CO2 and O2 were mixed as oxidant. The dilution effect of CO2 was then investigated through adding extra CO2 to the reaction system. An isothermal plug flow reactor was used. An unbranched chain reaction mechanism is proposed to illustrate the chemical effects of CO2 on the H/O/OH radical pool and NO x . Due to the reaction between CO2 and H, extra NO will be formed in fuel-rich conditions, while NO will be inhibited in fuel-lean conditions and high CO2 dilution conditions. The reaction affected the radical pools of OH, H, and O of the branched chain reaction, and then the formation and reduction of NO. The pool of H had the greatest effect on NO reduction. The results suggest that the indirect chemical effects on NO formation differ between diluted fuel oxy-fuel combustion conditions and normal oxy-fuel conditions.

산소부화용 공업로의 운전조건이 열효율에 미치는 영향

After a reheating furnace installation, the modification of the size and the heat capacity is very difficult. Therefore, the development of design package tool is required for the computation on the correct specifications before the design and the installation. Prior to development of the design tool, a module that calculates the amount of heat loss of each part according to the specifications for determining the thermal efficiency of a continuous heating furnace was developed and applied to the oxy-fuel industrial furnace. Through this, the effects of fuel type, oxygen fraction and recirculation on the efficiency of the furnace of which the output is 110Ton/hour were analyzed. In oxy-fuel combustion condition, the efficiency was 15% higher than air combustion conditions. With the using COG(Coke Oven Gas) instead of LNG, the efficiency was slightly increased. In the air combustion condition, the efficiency was increased about 33% with the preheated air. But, in oxy-fuel condition, the amount of exhaust gas was reduced, so the efficiency was increased about 7%.

Morphological analysis of sulfated Ca-based sorbents under conditions corresponding to oxy-fuel fluidized bed combustion

The use of Ca-based sorbents in circulating fluidized beds (CFB) allows the in-situ desulfurization in oxy-fuel combustion processes. The sulfation process involves important changes in the sorbent morphology, which could vary depending on the operating conditions and be different to those observed in conventional air combustion. This work analyzes the morphological variations observed during limestone and dolomite sulfation at typical oxy-fuel combustion conditions (high CO2 concentration, higher temperatures than in air combustion) in CFB combustors (long reaction times). Sulfated samples prepared in a thermogravimetric analyzer were analyzed by Scanning Electron Microscope (SEM). The space limitations due to the higher molar volume of CaSO4 compared to CaO in the external surface of the particles make that the CaSO4 product layer trend to grow outwards to form a honeycomb-shaped structure. This structure appeared for limestone at both calcining and non-calcining conditions. A strong effect of the CaSO4 sintering phenomenon was observed at temperatures above 950°C. Moreover, the honeycomb structure was never observed working with dolomite in spite of the high sulfation conversions reached with this sorbent.

Interactions of HCN with NO in a CO2 Atmosphere Representative of Oxy-fuel Combustion Conditions

The knowledge of NOx concentration in the exhaust gas of an oxy-fuel combustion system is very important for an appropriate design of the subsequent carbon capture process. NOx final emissions in oxy-fuel combustion depend on the reburn reactions of the recycled NO when it returns to the boiler. HCN is the main intermediate during this process, and thus, it may interact with NO significantly. The present work aims to evaluate the gas-phase interactions between HCN and NO in a CO2 atmosphere, characteristic of oxy-fuel conditions. A flow reactor experimental and kinetic modeling study of the oxidation of HCN in the presence of NO has been performed in a CO2 atmosphere and in the 973–1423 K temperature range. The influence of the stoichiometry and NO inlet concentration is analyzed. The results indicate that the HCN–NO interaction results in the diminution of both HCN and NO in a CO2 atmosphere. The conversion of NO is higher as temperature, oxygen concentration, and HCN/NO inlet ratio increase. The maximum...

CO2 capture performance of CaO–based pellets in a 0.1 MWth pilot-scale calcium looping system

Post-combustion CO2 capture performance of CaO-based pellets tested in a pilot-scale calcium looping system is investigated. The pellets were prepared from two natural limestones with 10wt.% cement as a binder. The pilot-scale system setup consists of a circulating fluidized bed calciner operating at 900°C and a bubbling bed carbonator operating at 650°C. The flue gas was supplied to the carbonator from a natural gas-fired combustor, while the oxy-fuel combustion condition was established in the calciner using wood pellets as the fuel source. The test campaign results showed that CO2 capture efficiencies in the range of 80–90% can be achieved for each sorbent. The carbonation efficiency with limestone was steady throughout the ~5h steady-state operation period, whereas efficiencies obtained with pellets exhibited a gradual decline over the duration of the test. An average carbonation conversion over the steady-state duration indicated the superiority of limestone with a conversion of 7.8%, compared to that of 6.0–6.8% achieved by pellets. Neither the limestone type nor the make-up mode showed any significant effect on the performance of the pellets. The inferior performance of pellets compared to that of limestone was attributed to the rapid and sever sintering of the pellets at the early stages of the test. It was concluded that, under the test conditions tested here, the use of CaO-based pellets in a pilot-scale calcium looping system does not seem to be a promising approach due to the significantly higher cost of production with comparable CO2 capture performance to that of limestone. Nonetheless, the test campaign provided important insights into the behavior of synthetic/modified sorbents under pilot-scale calcium looping conditions, which has not been reported elsewhere.

Effects of total pressure on non-grey gas radiation transfer in oxy-fuel combustion using the LBL, SNB, SNBCK, WSGG, and FSCK methods

The effects of total pressure on gas radiation heat transfer are investigated in 1D parallel plate geometry containing isothermal and homogeneous media and an inhomogeneous and non-isothermal CO2–H2O mixture under conditions relevant to oxy-fuel combustion using the line-by-line (LBL), statistical narrow-band (SNB), statistical narrow-band correlated-k (SNBCK), weighted-sum-of-grey-gases (WSGG), and full-spectrum correlated-k (FSCK) models. The LBL calculations were conducted using the HITEMP2010 and CDSD-1000 databases and the LBL results serve as the benchmark solution to evaluate the accuracy of the other models. Calculations of the SNB, SNBCK, and FSCK were conducted using both the 1997 EM2C SNB parameters and their recently updated 2012 parameters to investigate how the SNB model parameters affect the results under oxy-fuel combustion conditions at high pressures. The WSGG model considered is the recently developed one by Bordbar et al. [19] for oxy-fuel combustion based on LBL calculations using HITEMP2010. The total pressure considered ranges from 1 up to 30atm. The total pressure significantly affects gas radiation transfer primarily through the increase in molecule number density and only slightly through spectral line broadening. Using the 1997 EM2C SNB model parameters the accuracy of SNB and SNBCK is very good and remains essentially independent of the total pressure. When using the 2012 EM2C SNB model parameters the SNB and SNBCK results are less accurate and their error increases with increasing the total pressure. The WSGG model has the lowest accuracy and the best computational efficiency among the models investigated. The errors of both WSGG and FSCK using the 2012 EM2C SNB model parameters increase when the total pressure is increased from 1 to 10atm, but remain nearly independent of the total pressure beyond 10atm. When using the 1997 EM2C SNB model parameters the accuracy of FSCK only slightly decreases with increasing the total pressure.

Mercury Adsorption and Oxidation over Cobalt Oxide Loaded Magnetospheres Catalyst from Fly Ash in Oxyfuel Combustion Flue Gas.

Cobalt oxide loaded magnetospheres catalyst from fly ash (Co-MF catalyst) showed good mercury removal capacity and recyclability under air combustion flue gas in our previous study. In this work, the Hg(0) removal behaviors as well as the involved reactions mechanism were investigated in oxyfuel combustion conditions. Further, the recyclability of Co-MF catalyst in oxyfuel combustion atmosphere was also evaluated. The results showed that the Hg(0) removal efficiency in oxyfuel combustion conditions was relative high compared to that in air combustion conditions. The presence of enriched CO2 (70%) in oxyfuel combustion atmosphere assisted the mercury oxidation due to the oxidation of function group of C-O formed from CO2. Under both atmospheres, the mercury removal efficiency decreased with the addition of SO2, NO, and H2O. However, the enriched CO2 in oxyfuel combustion atmosphere could somewhat weaken the inhibition of SO2, NO, and H2O. The multiple capture-regeneration cycles demonstrated that the Co-MF catalyst also present good regeneration performance in oxyfuel combustion atmosphere.

Experimental and numerical study of oxygen separation and oxy-combustion characteristics inside a button-cell LNO-ITM reactor

A combined experimental and numerical study is performed on a button-cell LNO-ITM reactor. A semi-empirical model for oxygen permeation is considered, ABn model, and the values of the empirical constants are calculated based on the fitting of the available experimental data in the literature. A validation study for the present model is performed using the present experimental data. A detailed numerical study is presented on an LNO-ITM button-cell reactor under oxy-fuel combustion conditions. CH4 is used as the working fuel forming a mixture with CO2 at the permeate side inlet. The model results showed reasonable agreements under different operating conditions. The effect of reactivity in the permeate side of the membrane on oxygen permeation flux is considered. It is found that the oxygen permeation flux is increased by about 50% for the case of reacting flow as compared to the case of non-reacting flow. Distinct behavior of oxygen permeation flux values through the present button-cell ITM reactor is encountered while varying the operating conditions as compared to other reactors in the literature. This may be attributed to the complicated design of the flow path close to the membrane surface which maximizes the effects of flow momentum on the oxygen flux.

Dynamic Behaviors in Methane MILD and Oxy-Fuel Combustion. Chemical Effect of CO2

The oxidation process of CH4/O2 mixtures diluted in CO2 under moderate or intense low-oxygen dilution (MILD) and oxy-fuel combustion conditions was numerically investigated in a perfectly stirred flow reactor at atmospheric pressure. The analysis aimed to investigate the kinetics involved in fuel oxidation in systems highly diluted and strongly preheated. Particular attention was focused on the effects of CO2 on oxidation routes because it can significantly alter the kinetic pathways participating directly in key reactions or indirectly in termolecular reactions as a third body species. Furthermore, adiabatic flame temperatures are lower with respect to air conditions because of the higher thermal heat capacity of CO2 in comparison to that of N2, thus modifying the kinetics promoted by temperature in combustion processes. The analyses were realized as a function of main system parameters, systematically changing inlet temperatures and mixture compositions. Results showed the establishment of complex dynam...

Thermo-oxidation characteristics of oil shale and oil shale char under oxy-fuel combustion conditions

The current research involves characteristics of pyrolysis and thermo-oxidation of Estonian oil shale (OS) and its char. Non-isothermal and isothermal thermal analysis methods were used to characterise and compare the specifics of these processes under air and oxy-combustion conditions using Ar, CO2, O2 and their mixtures. The study of reaction kinetics including pyrolysis and oxidation characteristics is crucial to understand the complex mechanism of OS oxy-fuel combustion. The experimental results showed that the pyrolysis behaviour is highly similar in Ar and CO2 until the end of decomposition of organic part. There is no visible char carbon and CO2 reaction during the char oxidation stage under oxy-fuel conditions. The release of CO related to the char carbon and CO2 reaction in 100 % CO2 atmosphere increases notably at temperatures above 650–700 °C. The role of inorganic minerals in the oxidation and gasification of char was described as well as the effect of oxy-fuel conditions on gaseous emissions. It was found that the rate constants for char oxidation in CO2/O2 are approximately 1.2–1.3 times higher as compared to Ar/O2 atmosphere and that char oxidation under oxy-fuel conditions takes place with lower activation energies as compared to Ar/O2.

MSW oxy-enriched incineration technology applied in China: Combustion temperature, flue gas loss and economic considerations

To investigate the application prospect of MSW oxy-enriched incineration technology in China, the technical and economical analyses of a municipal solid waste (MSW) grate furnace with oxy-fuel incineration technology in comparison to co-incineration with coal are performed. The rated capacity of the grate furnace is 350tonnes MSW per day. When raw MSW is burned, the amount of pure oxygen injected should be about 14.5wt.% under 25% O2 oxy-fuel combustion conditions with the mode of oxygen supply determined by the actual situation. According to the isothermal combustion temperature (Ta), the combustion effect of 25% O2 oxy-enriched incineration (α=1.43) is identical with that of MSW co-incineration with 20% mass ratio of coal (α=1.91). However, the former is better than the latter in terms of plant cost, flue gas loss, and environmental impact. Despite the lower costs of MSW co-incineration with mass ratio of 5% and 10% coal (α=1.91), 25% O2 oxy-enriched incineration (α=1.43) is far more advantageous in combustion and pollutant control. Conventional combustion flue gas loss (q2) for co-incineration with 0% coal, 20% coal, 10% coal, 5% coal are around 17%, 13%, 14% and 15%, respectively, while that under the condition of 25% O2 oxy-enriched combustion is approximately 12% (α=1.43). Clearly, q2 of oxy-enriched incineration is less than other methods under the same combustion conditions. High moisture content presents challenges for MSW incineration, therefore it is necessary to dry MSW prior to incineration, and making oxy-enriched incineration technology achieves higher combustion temperature and lower flue gas loss. In conclusion, based on technical and economical analysis, MSW oxy-enriched incineration retains obvious advantages and demonstrates great future prospects for MSW incineration in China.

Experimental investigation on ash deposition of a bituminous coal during oxy-fuel combustion in a bench-scale fluidized bed

A bituminous coal was burned in a bench-scale fluidized bed to investigate the associated fly ash depositions under both oxy-fuel and air combustion conditions. The results showed that the highest deposition propensity appeared under 21% O2/79% CO2 atmosphere, and only a small difference was observed between 30% O2/70% CO2 and air atmosphere. The chemical compositions and micro-morphologies of ash deposits and fly ash were analyzed by inductively coupled plasma-atomic emission spectrometry and by scanning electron microscopy, while particulate matter with an aerodynamic diameter less than 10μm (PM10) was measured by electrical low pressure impactor. The results showed that there were no obvious differences in the chemical compositions of fly ash and ash deposits between combustion conditions, except for the slight variations in K, Na and S contents. The concentration of PM10 decreased in the order of 30% O2/70% CO2>21% O2/79% CO2>air.

Nitrogen oxides, sulfur trioxide, and mercury emissions during oxy-fuel fluidized bed combustion of Victorian brown coal.

This study investigates, for the first time, the NOx, N2O, SO3, and Hg emissions from combustion of a Victorian brown coal in a 10 kWth fluidized bed unit under oxy-fuel combustion conditions. Compared to air combustion, lower NOx emissions and higher N2O formation were observed in the oxy-fuel atmosphere. These NOx reduction and N2O formations were further enhanced with steam in the combustion environment. The NOx concentration level in the flue gas was within the permissible limit in coal-fired power plants in Victoria. Therefore, an additional NOx removal system will not be required using this coal. In contrast, both SO3 and gaseous mercury concentrations were considerably higher under oxy-fuel combustion compared to that in the air combustion. Around 83% of total gaseous mercury released was Hg(0), with the rest emitted as Hg(2+). Therefore, to control harmful Hg(0), a mercury removal system may need to be considered to avoid corrosion in the boiler and CO2 separation units during the oxy-fuel fluidized-bed combustion using this coal.

Effect of Al Enrichment by Pack Cementation of FeCr Coatings Deposited by HVOF

A great contribution to CO2 emissions comes from coal fired power generation. Combination of carbon capture sequestering technologies with sustainable biomass conversion constitutes a decisive boost in limiting rise in global temperature. Co-firing alternative materials with pulverized coal and using oxy-fuel combustion conditions (oxy-fuel co-combustion) is a very attractive process for power industry. Materials with both high mechanical properties and high environmental resistance are required by such advanced combustion systems. One approach to improve high-temperature oxidation/corrosion resistance is to apply protective coatings. In the present work, low and high Cr content Fe-based alloys have been deposited in order to investigate the influence of Cr content on coating protective performance in oxy-fuel co-combustion conditions. Grade 91 steel has been assumed as reference substrate. Effect of Al enrichment on coating environmental resistance has also been analyzed. Activities have been performed within the framework of Macplus Project (Integrated Project co-founded by the European Commission under the 7th Framework Program in the Energy area).

Detailed analysis of the CO oxidation chemistry around a coal char particle under conventional and oxy-fuel combustion conditions

The purpose of this article is to analyze in detail the homogeneous chemistry involving the CO oxidation in the gas around a burning char particle. Namely, the model presented in a previous work (Gonzalo-Tirado et al., 2014) [1] has been applied to the case of a 120μm and a 600μm subbituminous char particle in a 24% O2, 1673K atmosphere under both conventional and oxy-fuel combustion conditions. The CO+OH↔CO2+H reaction is shown to be the prevailing reaction in the conversion of the CO in the boundary layer; the high CO2 concentrations typical of oxy-combustion affect the equilibrium in this reaction and reduce its overall rate, which explains the lower ‘intensity’ of the flame in those conditions. As for the release/absorption of heat in the gas, the reactions in which the OH radicals participate as reactants or products are predominant; the OH chemistry is somehow more intense in N2 and higher flame temperatures and OH concentrations are thus attained in conventional combustion conditions. Relatively low moisture concentrations in the bulk gas are sufficient to activate this boundary layer chemistry; with [H2O] larger than ∼3% no substantial changes are observed in the CO conversion. The combustion history of the particles has been also studied. A logical sequence oxidation–gasification is observed; whereas the CO-to-CO2 oxidation occurs first contiguous to the particle, the onset of char-CO2 gasification results in a detachment of the flame from the surface and a decrease in the oxidation rate, especially for large particles.

Accuracy of the single-film model in the prediction of coal char conversion rates under oxy-fuel and conventional combustion conditions

The accuracy of the single-film model in the calculation of char burning rates and particle temperatures in conventional and oxy-fuel combustion conditions has been investigated taking as a reference the results obtained with a model with detailed gas-phase kinetics. Both models have been applied to subbituminous coal and anthracite char particles in different sizes (60–1000μm), in N2 and CO2-rich atmospheres with a low and a high oxygen concentration. For 120μm char particles, the models’ predictions have been also compared over an ample range of ambient gas temperatures (1373–1823K) and oxygen concentrations (4–50%). High particle diameters, bulk gas temperatures and oxygen concentrations result in CO-flames closer to the particle surface; however, the specific effects of the flame strongly depend on the coal rank and combustion conditions. In general, the single-film model accuracy is quite high in terms of burning rate predictions, except for fine anthracite particles burning at moderate bulk gas temperatures and high O2 concentrations; greater deviations have been found regarding particle temperatures, even in cases where the corresponding combustion rates are very similar. The oxy-combustion of the reactive coal char is found to be faster than its conventional combustion, except for the finest particles; although qualitatively similar, the difference in rates is overestimated if the single-film model is considered.

Modeling CO2 Chemical Effects on CO Formation in Oxy-Fuel Diffusion Flames Using Detailed, Quasi-Global, and Global Reaction Mechanisms

Interest in oxy-fuel combustion as one of the leading carbon capture technologies has grown significantly in the past two decades. Experimental studies have shown higher CO concentration in oxy-fuel diffusion flames than in traditional air-fuel flames of both gaseous and solid fuels. The higher CO concentration changes the flame profiles, and it may have impacts on pollutants formation. This article presents a numerical study regarding the chemical effects of CO2 on CO formation in the flame region, and their modeling approaches in CFD simulations. Equilibrium calculation confirms higher CO concentration associated with fuel-rich stoichiometry in CO2 diluted combustion environment. One-dimensional counter-flow diffusion flame simulation using detailed reaction mechanisms reveals that the reaction enhances CO formation in the presence of high CO2 concentration, leading to a significantly higher CO concentration under oxy-fuel combustion conditions. High CO2 concentration also impacts the reaction via OH radical and results in lower H2 and higher H2O concentrations in the flame profile. Computational fluid dynamics (CFD) simulations of a swirling diffusion flame under air-fired and oxy-fuel conditions were conducted using the eddy dissipation model and the eddy dissipation concept model with quasi-global and global kinetic mechanisms. Results show that reasonable CO predictions can only be obtained using finite-rate approach with appropriate mechanisms considering the CO2 chemical effects. The Westbrook–Dryer two-step mechanism consistently underestimates the CO concentrations. In contrast, the Westbrook–Dryer multiple-step mechanism captures the chemical effects of CO2, and improves the predictions.

A comprehensive evaluation of the influence of air combustion and oxy-fuel combustion flue gas constituents on Hg0 re-emission in WFGD systems

This paper evaluates the influence of the main constituents of flue gases from coal combustion (CO2, O2, N2 and water vapor), in air and oxy-fuel combustion conditions on the re-emission of Hg0 in wet scrubbers. It was observed that the concentration of water vapor does not affect the re-emission of mercury, whereas O2 and CO2 have a notable influence. High concentrations of O2 in the flue gas prevent the re-emission of Hg0 due to the reaction of oxygen with the metals present in low oxidation states. High concentrations of CO2, which cause a decrease in the pH and the redox potential of gypsum slurries, reduce the amount of Hg0 that is re-emitted. As a consequence, the high content of CO2 in oxy-fuel combustion may decrease the re-emission of Hg0 due to the solubility of CO2 in the suspension and the decrease in the pH. It was also found that O2 affects the stabilization of Hg2+ species in gypsum slurries. The results of this study confirm that the amount of metals present in limestone as well as the redox potential and pH of the slurries in wet desulphurization plants need to be strictly controlled to reduce Hg0 re-emissions from power plants operating under oxy-fuel combustion conditions.

Mercury emission characteristics from coal combustion by supplying oxygen and carbon dioxide with limestone injection

Coal combustion experiments at a lab-scale furnace with a capacity of 10,000kcal/h were conducted in order to investigate the speciation changes and mass distribution of mercury with limestone injection in conditions of air and oxy-fuel combustion. Mass distribution and concentration of mercury in flue gas at oxy-fuel combustion were higher than those at air combustion. With limestone injection, the reduction of mercury from flue gas by adsorption into limestone could be clearly observed at both air and oxy-fuel combustion conditions. In the speciation of mercury, the particulate mercury in flue-gas was dominant with around 50% and the portion of elemental mercury was higher than that of oxidized mercury as shown at other coal combustion cases. At oxy-fuel combustion, most of particulate mercury seemed to be bounded to limestone more easily in comparison to air combustion, so that the elemental mercury in flue gas resulted to be more dominant at oxy-fuel combustion.