Oxy-fuel Combustion Atmosphere Research Articles

Insight into the Mercury Removal Mechanism of Copper Salt-Modified Biomass Coke in an Oxyfuel Combustion Atmosphere

Insight into the Mercury Removal Mechanism of Copper Salt-Modified Biomass Coke in an Oxyfuel Combustion Atmosphere

Process Simulations of High-Purity and Renewable Clean H2 Production by Sorption Enhanced Steam Reforming of Biogas

Renewable clean H2 has a very promising potential for the decarbonization of energy systems. Sorption enhanced steam reforming (SESR) is a novel process that combines the steam reforming reaction and the simultaneous CO2 removal by a solid sorbent, such as CaO, which significantly enhances hydrogen generation, enabling high-purity H2 production. The CO2 sorption reaction (carbonation) is exothermic, but the sorbent regeneration by calcination is highly endothermic, which requires extra energy. Biogas is one of the available carbon-neutral renewable H2 production sources. It can be especially relevant for the energy integration of the SESR process since, due to the exothermic sorption reaction, the CO2 contained in the biogas provides extra heat to the system, which can help to balance the energy requirements of the process. This work studies different process configurations for the energy integration of the SESR process of biogas for high-purity renewable H2 production: (1) SESR with sorbent regeneration using a portion of the produced H2 (SESR+REG_H2), (2) SESR with sorbent regeneration using biogas (SESR+REG_BG), and (3) SESR with sorbent regeneration using biogas and adding a pressure swing adsorption (PSA) unit for hydrogen purification (SESR+REG_BG+PSA). When using biogas as fuel (Cases 2 and 3), these configurations were studied using air and oxy-fuel combustion atmospheres in the sorbent regeneration step, resulting in five case studies. A thermodynamic approach for process modeling can provide the optimal process operating conditions and configurations that maximize the energy efficiency of the process, which are the basis for subsequent optimization of the process at the practical level needed to scale up this technology. For this purpose, process simulations were performed using a steady-state plant model developed in Aspen Plus, incorporating a complex heat exchanger network (HEN) to optimize heat integration. A comprehensive parametric study assessed the effects of biogas composition, temperature, pressure, and steam to methane (S/CH4) ratio on the process performance represented by the selected key performance indicators, i.e., H2 purity, H2 yield, CH4 conversion, cold gas efficiency (CGE), net efficiency (NE), fuel consumption for the sorbent regeneration step, and CO2 capture efficiency. H2 with a purity of 98.5 vol % and a CGE of 75.7% with zero carbon emissions can be achieved. When adding a PSA unit, nearly 100% H2 purity and CO2 capture efficiency were achieved with a CGE of 77.3%. The use of oxy-fuel combustion during regeneration lowered the net efficiency of the process by 2.3% points (since it requires an air separation unit) but allowed the process to achieve negative carbon emissions.

Catalytic effects for cellulose-based model fuels under low and high heating rate in air and oxy-fuel atmosphere

The detailed catalytic influence of minerals on solid biomass in oxy-fuel combustion is yet to be fully understood. The catalytic influence of metal sulfates on a mineral-free, cellulose-based model biomass was investigated during slow and high heating in air and oxy-fuel combustion. Measurements were performed in a thermogravimetric setup in air with slow heating rates and in a flat-flame burner in oxy-fuel combustion atmosphere with high heating rates. Temperature-programmed experiments identified the catalytic activity scale of Fe > K > Na > Mg ∼ Ca in synthetic air (20% O2/He) for the sulfates. The highly active metals Fe and K were chosen for more detailed investigations in oxy-fuel combustion experiments using an additional loading of Mg as less-volatile mineral tracer. Samples doped with Fe and Mg (FeMg-MH) exhibited lower thermal stability and higher particle combustion temperatures in the flat-flame burner compared with the undoped model fuel, while the combination of K and Mg (KMg-MH) decreased the particle combustion temperature drastically during oxy-fuel combustion. X-ray diffraction patterns acquired between 25 and 800 °C showed that in FeMg-MH the mineral phases FeSO4 and MgSO4 were still separated and independently active, while the addition of MgSO4 to K2SO4 formed the stable mineral phase Langbeinite inhibiting the K mobility. The influence of metal chlorides and nitrates was also investigated by slow heating rate TGA experiments showing an overlapping of metal salts decomposition and carbon devolatilization and oxidation.

Co-combustion of high and low ash lignites with raw and torrefied biomass under air and oxy-fuel combustion atmospheres

ABSTRACT Co-combustion characteristics of high and low ash lignites blended with raw and torrefied pine woodchips were studied by Thermogravimetric Analyzer (TGA) under air and oxy-fuel conditions. The lignites were blended with biomass samples at the mass fraction of 50/50 wt.%. Three heating rates of 10, 20, and 40°C/min were chosen, and the characteristic temperatures, including initial, ignition, and burnout temperatures, were obtained. In order to estimate the activation energies of the co-combustion of the blends, Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, and Friedman kinetic methods were employed. Additionally, to assess the summative behavior of the fuel blends, the relative error as a degree of synergism was calculated based on the difference between theoretical and experimental DTG profiles. It was seen that co-combustion of torrefied biomass with the low ash Orhaneli lignite in air resulted in the average relative error of 21.41%, indicating the maximum synergism for the blend. This value was 9.59% under oxy-fuel combustion atmosphere. Blending torrefied biomass with the high ash Soma lignite resulted in average relative errors of 1.34% and 1.45% under air and oxy-fuel combustion atmospheres showing an insignificant synergetic effect. An improvement in combustion performance was noticed under oxy-fuel combustion conditions. The average activation energy values for the blend of torrefied biomass and Orhaneli lignite were 54.47 and 112.48 kJ/mol under air and oxy-fuel combustion atmospheres, which were lower than that of the parent fuels indicating higher reactivity of the blends. This trend was not seen for Soma lignite. The associated uncertainty values for the FWO method were in the range of 3.57% to 12.45% making it a proper tool for obtaining the kinetic parameters.

Effectiveness of bed additives in abating agglomeration during biomass air/oxy combustion in a fluidised bed combustor

In this study, molochite (calcined kaolin) and dolomite with particle sizes (ca.1 mm) similar to those of the sand bed materials of industrial-scale fluidised beds, rather than powders which were normally used by previous studies, were investigated as the bed additives to counter agglomeration with the combustion of wheat straw and miscanthus pellets in a 20 kWth bubbling fluidised bed combustor under both conventional air combustion and oxy-fuel combustion conditions. The performance of the additives in abating agglomeration was compared to that achieved with another agglomeration countermeasure, lime addition to fuel. Dolomite was found to be superior to molochite in terms of prolonging defluidisation time when firing wheat straw. When firing miscanthus, similar effects of these two bed additives on defluidisation time were observed. The most significant improvement to the combustion performance was achieved by lime addition to fuel for both fuels. Moreover, the oxy-fuel combustion atmosphere showed a great impact on the effectiveness of dolomite but little influence on the effectiveness of molochite. The agglomeration samples were analysed by CAMSIZER, XRD and SEM/EDX and the results were used to derive the possible anti-agglomeration mechanisms of the additives under both air and oxy combustion conditions.

Interference Effect of H2O on Hg0 Removal by a Mercury Sorbent in an Oxyfuel-Combustion Atmosphere

Interference Effect of H₂O on Hg⁰ Removal by a Mercury Sorbent in an Oxyfuel-Combustion Atmosphere

Influence of the simplifications applied to the surrounding gas composition of evaporating droplets in air-blown and oxy-fuel combustion atmospheres

Influence of the simplifications applied to the surrounding gas composition of evaporating droplets in air-blown and oxy-fuel combustion atmospheres

Effects of metal ions on Hg0 re-emission under air and oxy-fuel combustion atmospheres in a WFGD system

The effects of metal ions, including Fe2+, Fe3+, Mn2+, and Cu2+, on Hg0 re-emission were investigated in a simulated wet flue gas desulfurization (WFGD) system under air and oxy-fuel combustion atmospheres. The results show that in the presence of metal ions, Hg0 re-emission is enhanced under a N2 atmosphere but inhibited under N2/9%O2, air, and oxy-fuel combustion atmospheres. In the inhibition cases, Hg2+ reduction and Hg0 re-emission were accelerated in the initial stages of the experiments. Fortunately, Fe2+, Fe3+, Mn2+, and Cu2+ have a positive impact on mercury stabilization in the liquid phase, and the precipitates of Fe2+/Fe3+ can also adsorb mercury in the solid phase, thus offsetting the acceleration of Hg0 re-emission in the initial stage. Therefore, Hg0 re-emission is suppressed by metal ions under N2/9%O2, air, and oxy-fuel combustion atmospheres. With an increase in CO2 concentration, the inhibition of Fe2+, Fe3+, and Cu2+ was intensified, whereas the suppression by Mn2+ was slightly weakened. Under an oxy-fuel combustion atmosphere, increasing O2 concentration would promote the inhibition of Mn2+ on Hg0 re-emission, but hardly affect the inhibitory effect of Cu2+.

Effect of SO2 and SO3 on the distribution of Na/S/Cl and ashes characteristics of Zhundong coal during circulating fluidized bed O2/CO2 combustion

The high concentration of SO2 and SO3 under oxy-fuel combustion atmosphere is conducive to the sulfation of alkali metals. In order to investigate the effect of SO2 and SO3 on the distribution of Na/S/Cl and ashes characteristics of Zhundong coal during oxy-fuel combustion, a series of experiments were carried out in a 50 kW CFB oxy-fuel combustor system. The results showed that both SO2 and SO3 could realize the sulfation reaction of NaCl. It would enhance the distribution of sodium to the bottom ash and the circulating ash, the chlorine to the flue gas, so as to reduce the content of sodium and chlorine in deposit. In addition, the contents of Ca and Mg increased in the deposits and decreased in the fly ashes as the SO2 and SO3 were added, while the contents of Fe were all decreased in the deposits and fly ashes. Compared with SO2, SO3 is more prone to the sulfation reaction. While increasing the addition of SO2 and SO3 could strengthen the sulfation reaction. In addition, the deposit and fly ash generated by Zhundong coal combustion could be divided into two parts: particles less than 10 μm formed by condensation of sodium, chlorine and other components; particles larger than 10 μm formed by crushing of coal ash. Introducing SO2 and SO3 made the surface structure of deposit more porous, but the BET surface area of fly ash declined. There was no obvious difference of particle size distribution of deposit.

A new WSGGM considering CO in oxy-fuel combustion: A theoretical calculation and numerical simulation application

The HITEMP 2010 databases was employed in line by line (LBL) method to calculate an accurate total emissivity benchmark for oxy-fuel combustion atmosphere. This paper focuses on the effect of CO in the oxy-fuel gas mixture on the total emissivity. CO is significant in the primary combustion zone of boiler, generating from incomplete combustion and gasification reactions in oxy-fuel combustion mode. Based on the total emissivity benchmark from LBL, the new correlation coefficients for weighted sum of gray gases model (WSGGM) are fitted for mole fraction of CO from 0% to 45%, molar ratio of H2O to CO2 between 0.01 and 4, temperature range of 400 K–3000 K and path length varying from 0.01 m to 60 m. Radiative heat flux and radiative source term results from solving the radiation transfer equation (RTE) in one-dimensional slab system are used to evaluate the accuracy and effectiveness of new correlation coefficients for WSGGM. By comparing with the results from LBL and models in other literature, new correlation coefficients for WSGGM are validated no matter in non-isothermal and non-homogenous conditions considering CO in gas mixture. At last, the new correlation coefficients are implemented in the numerical simulation of oxy-natural gas furnace to show the effect of gas radiation model on the wall surface radiative heat flux.

Research on the slagging behaviors of Shenhua coal in different combustion atmospheres

Global warming has attracted extensive attention due to increasing CO2 emissions. Oxy-fuel combustion has become an important topic in the domain of energy saving and emission reduction due to its high efficiency in CO2 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 CaCO3 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.

Characteristics of Mercury Re-emission and Migration in a Lab-Scale Wet Flue Gas Desulfurization Scrubber under Simulated Air and Oxy-Fuel Combustion Atmospheres

Based on a lab-scale wet flue gas desulfurization scrubber, mercury re-emission and migration during the desulfurization process were experimentally investigated under simulated air and oxy-fuel co...

Flow pattern and behavior of 40 kWth pulverized torrefied biomass flames under atmospheric and oxy-fuel conditions

The combustion of torrefied biomass under oxy-fuel conditions is a promising technological alternative, which has the potential to transform electric and heat generation systems into more sustainable and carbon-neutral forms. This combustion approach combines two complex processes, oxy-fuel and biomass combustion, which still require significant understanding. For this, an experimental study has been conducted considering a series of 40 kWth aerodynamically stabilized pulverized torrefied biomass and coal flames. Given the increasing interest and advantages of substituting coal by biomass in different thermal industrial processes, a set of four experimental conditions was studied and compared. Two torrefied biomass flames, under conventional and oxy-fuel combustion atmospheres, are compared to two similar coal (lignite) operating conditions. The combustion behavior of the flames is assessed by conducting detailed measurements of flow fields, gas compositions, and radiative heat fluxes. Experiments are performed employing a combustion chamber with a simplified configuration. Combined results from the different measurements show that the overall combustion characteristics of torrefied biomass under the two atmospheres are fairly similar. Also, they do not significantly differ from the combustion characteristics observed for the coal flames. Comparatively, the radiative heat fluxes produced by the torrefied biomass flames were higher than those produced by coal. In addition, oxy-fuel flames produced 15–20% lower radiative heat fluxes than the air counterparts. Gas concentration measurements near the flames showed diverse results attributed to small differences in the turbulent flow fields and fuel composition. The presented analysis of results can also contribute to the retrofitting of conventional pulverized coal firing systems for operation under oxy-fuel conditions with torrefied biomass.

Numerical investigation of the influence of operating parameters on NOx emission characteristics under oxy-coal combustion atmosphere in a tubular combustor

In this study, NOx emission characteristics of oxy-coal combustion atmosphere have been thoroughly investigated employing the developed and validated numerical model under oxy-coal combustion atmosphere. The influence of oxy-fuel combustion atmospheres, feed gas inlet temperatures and pressures on the NOx emission (ppm) characteristics have been studied in detail. Influence of higher concentration steam addition on NOx emission under oxy-coal combustion atmosphere has also been assessed employing the developed oxy-coal combustion model. Furthermore, a quantitative summary of NOx emission at the outlet of the combustion chamber under above-mentioned operating conditions have been provided. The result showed that oxy-coal combustion cases having higher O2 concentration in the oxidizer had produced higher NOx emission due to an enhanced conversion rate of coal_N to NOx. The wet oxy-fuel combustion case (10–50% H2O in the oxidizer) has 2.5%–75% higher NOx concentration (ppm) compared to the ideal dry-recycle oxy-fuel combustion case (0% H2O in the oxidizer). The oxy-steam combustion case has 99% higher NOx concentration (ppm) compared to the ideal dry-recycle oxy-fuel combustion case.

The effect of Kaolinite’s structure on migration and release characteristics of sodium under oxy-fuel combustion condition

Studying the adsorption properties of kaolinite on sodium chloride is of great significance for application of high sodium coal, but high concentration of steam in the oxy-fuel combustion atmosphere increases the uncertainty of this process. Moreover, under the corresponding condition, the research on the adsorption properties of sodium of kaolinite after calcination and intercalation-exfoliation is still scarce. Therefore, this paper carried out a study on sodium adsorption characteristics of kaolinite. This work further evaluated H2O influence on its Na retention ability via integrating experimental and DFT computational studies. As the proportion of additive increases, the sodium content in the ash also increases, but after the ratio exceeds 9%, the increase is not obvious. The kaolinite’s adsorption performance on sodium chloride has been significantly improved after the intercalation-exfoliation. In order to confirm this conclusion, this paper also carried out adsorption experiments using pure sodium chloride, and obtained similar results. Variations in the concentration of steam (0–45%) in the atmosphere have a strong effect on the adsorption of sodium chloride on the surface of kaolinite. Moreover, both acceleration and competition effects of H2O on surface-bound Na species were substantiated by DFT calculations·H2O showed a stronger interaction with the dehydroxylated surface, can promptly replenish the missing adsorption sites on the surface due to high temperature. The experimental results show that the dynamic balance can be achieved at 20%. However, H2O and sodium chloride compete with each other on the complete adsorption surface, which reduces the fixation of sodium.

Combustion Performance and Ash Compositions during Biomass/Semi-Coke Blended Fuel Oxy-Fuel Circulating Fluidized Bed Combustion

Co-combustion biomass with semi-coke under oxy-fuel combustion is a promising way to reduce CO₂ emission and achieve the integrated utilization of biomass and semi-coke. To provide hints on the further industrial utilization, cornstalk was co-combusted with Shenmu semi-coke under oxy-fuel combustion atmospheres in a 50 kW circulating fluidized bed (CFB) test rig, of which the operating parameters can be well controlled. Results showed that semi-coke can achieve stable combustion under an oxy-fuel combustion atmosphere with 30% inlet oxygen concentration. Although inferior to that of cornstalk/coal blended fuel, the combustion performance (ignition and pollutants emission) of cornstalk/semi-coke blended fuel was superior to that of unblended semi-coke. In addition, the increase in the blending ratio of cornstalk or inlet oxygen concentration under the oxy-fuel combustion atmosphere was conducive to reduce the emission concentrations of CO, N₂O, and NO. K tended to release into the gas phase during co-combustion with the semi-coke process, especially under a high blending ratio. High CaO content in semi-coke or coal would result in enrichment of S in bottom ash and circulating ash, which, in turn, inhibited the reaction between S-containing species and KCl, while Cl would accomplish almost total release from fuel, and then it would be enriched in deposits.

High-temperature corrosion of water-wall tubes in oxy-combustion atmosphere

Oxy-combustion is one of the most promising technology for CO2 capture in coal-fired power plants. However, under oxy-combustion conditions, the concentrations of acid gas species are significantly increased due to the introduction of the flue gas recycle, which aggravates the high-temperature corrosion of heat exchanger materials in boilers. In this study, the early-stage high-temperature corrosion (0–16 h) of two representative water-wall tube materials (20G, 12Cr1MoV) is experimentally tested in a lab-scale furnace with the simulated oxy-combustion atmosphere. The effects of material, temperature, CO2, H2O, SO2, H2S and CO atmospheres on high-temperature corrosion behaviors is investigated. The micro-morphologies and compositions of corrosion layers are characterized by scanning electron microscope with energy dispersive X-ray spectra (SEM-EDS) and X-ray diffraction (XRD). Kinetic analysis shows that the high concentration of CO2 accelerates high-temperature corrosion of water wall materials. In the simulated oxy-fuel combustion atmosphere (CO2/O2/SO2), the mass gain rate can be enhanced by 10%–30% compared to the conventional air combustion atmosphere (N2/O2/SO2), and the major composition of oxide scale is magnetite. In a reducing oxy-fuel atmosphere (CO2/CO/SO2/H2S), the major components of oxide scale are magnetite and ferrous sulfide. The high concentration of moisture in the atmosphere accelerated the corrosion rate by 10–30%. For both model alloys, the corrosion kinetics obey the parabolic law. Water-wall tube material 12Cr1MoV appears superiority in corrosion resistance compared with 20G material.

Release and migration characteristics of sodium and potassium in high alkali coal under oxy-fuel fluidized bed combustion condition

Zhundong coal is promising to replace inferior coal in fossil-fueled power plants, considering its high abundance, low contents of sulfur and nitrogen. However, the relative high contents of alkalis, especially sodium and potassium, damage the operational economy and safety badly since the release of alkali metals in flue gas can lead to fly ash-related problems such as slagging, ash accumulation and corrosion. Although, oxy-fuel circulating fluidized bed (CFB) combustion is beneficial to reduce the release of sodium and potassium by lowering the operation temperature, the ash formation characteristics of Zhundong coal under the oxy-fuel CFB is still scarce. Our results showed that the residual amounts of sodium and potassium in the ash samples, both tend to decrease with increasing temperature no matter under air combustion or oxy-fuel combustion. However, the sodium content in the cinder obtained under the oxy-fuel combustion atmosphere was higher than that under the air combustion atmosphere, and the insoluble sodium, mainly existed in the form of aluminosilicates, was the main form. In addition, the increase of steam content in both atmospheres reduced the sodium content in the cinder while the residual amount of potassium increased. Density functional theory calculations verified the transformation behavior between sodium and potassium and the inhibiting effect of steam. This study elucidates the release and migration characteristics of sodium and potassium in Zhundong coal both experimentally and theoretically and will benefit the popularity of Zhundong coal.

TGA and kinetic study of different torrefaction conditions of wood biomass under air and oxy-fuel combustion atmospheres

Combustion and oxy-fuel combustion characteristics of torrefied pine wood chips were investigated by Thermogravimetric Analysis (TGA). Three torrefaction temperatures (250, 300, and 350 °C) and two residence times (15 and 30 min) were considered. Experiments were carried out at three heating rates of 10, 20, and 40 °C/min. The isoconversional kinetic methods of FWO, KAS, and Friedman were employed to estimate the activation energies. The assessment of uncertainty in obtaining the activation energy values was also considered. The obtained results indicated that due to torrefaction, the O/C and H/C atomic ratios decreased, resulting the 300ºC-30 min and 350ºC-15 min torrefied biomass to be completely embedded in lignite region in van-Krevelen's diagram. Oxy-fuel combustion affected the decomposition of cellulose and lignin components of biomass while the impact on the hemicellulose component was negligible. The kinetic analysis revealed that with the evolution of conversion degree, the activation energy values increased during hemicellulose degradation, remained approximately constant during cellulose decomposition and showed a sharp decrease for lignin decomposition. The activation energy trends were comparable in both air and oxy-fuel combustion conditions, however slight changes in activation energy values were noticed. The highest activation energy value was obtained for 250ºC-30 min torrefied biomass at 183.40 kJ/mol and the lowest value was 72.93 kJ/mol for 350ºC-15 min biomass. The uncertainty values related to FWO method were lower than KAS and Friedman methods. The uncertainty values for FWO and KAS methods were at the range of 5–15%.

Fundamental study of heterogeneous KCl sulfation under oxy‐fuel combustion conditions

Under oxy-fuel combustion condition, SO2 in the flue gas would be accumulated by recirculation, which is conducive to the heterogeneous sulfation reaction of alkali metals. In the present study, experiments were conducted in a fixed bed to investigate the effects of operating parameters and mineral additives (SiO2, CaO, and Fe2O3) on the heterogeneous sulfation of potassium chloride under oxy-fuel combustion atmosphere. According to the results here, the heterogeneous sulfation reaction was a kinetically controlled process, with the activation energy of 93.6 kJ/mol. The reaction orders with respect to SO2, O2, and H2O were determined as 1, 0.6 and 0 (H2O involved in the reaction). While the reaction would be promoted obviously in the absence of H2O. The rate law of heterogeneous sulfation of potassium chloride was derived based on the experimental data. Compared with air combustion, the heterogeneous sulfation rate was lower under oxy-fuel combustion. All the mineral additives employed would affect the sulfation reaction. The sulfation reaction can be catalyzed by Fe2O3. While CaO would suppress the reaction by competing for SO2 with KCl. The reaction between CaO and SO2 could also be catalyzed by Fe2O3. Besides, SO2 was more reactive towards CaO than KCl.