Oxy-fuel Combustion Flue Gas Research Articles

Enhanced activity and SO2/H2O resistance of Co‐doping CeMnO3 perovskite for low-temperature mercury removal from oxy-fuel combustion flue gas

Enhanced activity and SO2/H2O resistance of Co‐doping CeMnO3 perovskite for low-temperature mercury removal from oxy-fuel combustion flue gas

Improved mercury removal performance and leaching stability of Fe/Br co-doped biochars synthesized through one-step pyrolysis

Mercury removal from oxyfuel combustion flue gas remains challenging owning to the shortage of cost-effective, sulfur- and water-resistant sorbents without secondary pollution. In this study, Fe/Br co-doped biochars synthesized via one-step pyrolysis of iron-loaded biomass and brominated flame retarded plastic were adopted for Hg0 immobilization from oxyfuel combustion flue gas. After iron doping, the Br content of Fe/Br co-doped biochars was sharply improved. Both highly developed pore structure and abundant active components were acquired because of the catalytic effect of iron. The Fe/Br co-doped biochars presented superior Hg0 removal performance than Br-doped biochars across a broad temperature interval from 25 °C to 240 °C. The optimized pyrolysis temperature and wood /Fe salt mass ratio for the synthesis of Fe/Br co-doped biochars were respectively 500 °C and 8:1. The Fe/Br co-doped biochars exhibited excellent resistance to H2O and SO2. The Hg0 removal mechanism was revealed, where Fe3+, CBr and chemisorbed oxygen served as active components for Hg0 chemisorption. The Hg/Br leaching behavior of Fe/Br co-doped biochars was further compared with Br-doped biochars and traditional brominated activated carbon. Iron doping significantly enhanced the stability of Hg/Br in Fe/Br co-doped biochars because partial inorganic bromine was converted into organic bromine during co-pyrolysis.

Heat Transfer and Thermal Efficiency in Oxy-Fuel Retrofit of 0.5 MW Fire Tube Gas Boiler

Industrial boilers cause significant energy wastage that could be mitigated with oxy-fuel combustion versus traditional air combustion. Despite several feasibility studies on oxy-fuel burners, they are widely avoided in industry due to major infrastructural challenges. This study measured the performance and heat transfer characteristics of each component in a 0.5 MW fire tube gas boiler after retrofitting it with an oxy-fuel burner. Comparisons were drawn across three combustion modes—air combustion, oxy-fuel combustion, and oxy-fuel flue gas recirculation (FGR). The Dittus–Boelter equation was employed to predict heat transfer in the fire tube for all combustion modes at full load (100%). Heat transfer in the latent heat section of the economizer was measured and compared with predictions using the Zukauskas equation. With this retrofit, oxy-fuel combustion improved the thermal efficiency by about 3–4%. In oxy-fuel combustion, the flow rate of exhaust gas decreased. When integrated into an existing fire tube boiler, the fire tube’s heat transfer contribution diminished greatly, suggesting the economic viability of a redesigned, reduced fire tube section. Additionally, a new design could address the notable increase in gas radiation from the fire tube in oxy-fuel and FGR, as well as aid in the efficient recovery of condensation heat from exhaust gases.

In situ self-activation strategy for the synthesis of double-layered S/Cl co-doped sorbents for elemental mercury removal from oxy-fuel combustion flue gas

Mercury capture from oxy-fuel combustion flue gas remains an enormous challenge for carbon capture and storage because of the lack of cost-effective, sulfur-resistant and water-resistant sorbent. In this work, double-layered S/Cl co-doped sorbents (CDSs) were synthesized through an in situ self-activation method i.e. co-pyrolysis of waste tire and PVC plastic, which were adopted for elemental mercury (Hg0) immobilization from oxy-fuel combustion flue gas. A synergistic effect between waste tire and PVC plastic was observed during co-pyrolysis. For this reason, chlorine was successfully doped into the CDSs in the form of covalent chlorine and ionic chlorine. The CDSs showed high Hg0 removal efficiency (>80 %) over a wide temperature range (20–140 °C) and excellent sulfur and water resistance. Both the initial Hg0 adsorption rate and adsorption capacity of CDSs were far higher compared with raw tire char. The Hg0 removal mechanisms were further proposed, where Hg0 preferentially reacted with covalent chlorine instead of active sulfur species. Density functional theory calculations revealed that the Hg0 adsorption over CDSs was chemisorption with an adsorption energy of −249.85 kJ/mol. The synergistic effect between sulfur and chlorine was beneficial for the formation of HgCl2 whereas it had negligible effect on the formation of HgCl.

Modelling and Design of a Novel Integrated Heat Exchange Reactor for Oxy-Fuel Combustion Flue Gas Deoxygenation

The concentration of residual O2 in oxy-fuel combustion flue gas needs to be reduced before CO2 transportation, utilization, or storage. An original application of the printed circuit heat exchanger (PCHE) for catalytic combustion with natural gas (catalytic deoxygenation) is described for reducing the residual O2 concentration. The PCHE design features multiple adiabatic packed beds with interstage cooling and fuel injection, allowing precise control over the reaction extent and temperature within each reaction stage through the manipulation of fuel and utility flow rates. This work describes the design of a PCHE for methane–oxygen catalytic combustion where the catalyst loading is minimized while reducing the O2 concentration from 3 vol% to 100 ppmv, considering a maximum adiabatic temperature rise of 50 °C per stage. Each PCHE design differs by the number of reaction stages and its individual bed lengths. As part of the design process, a one-dimensional transient reduced-order reactor model (1D ROM) was developed and compared to temperature and species concentration axial profiles from 3D CFD simulations. The final design consists of five reaction stages and four heat exchanger sections, providing a PCHE length of 1.09 m at a processing rate of 12.3 kg/s flue gas per m3 PCHE.

Investigation of Mercury Removal Behavior by Modified Biomass Coke in Oxy-fuel Combustion Flue Gas

Investigation of Mercury Removal Behavior by Modified Biomass Coke in Oxy-fuel Combustion Flue Gas

CO2 Adsorption Performance of Activated Coke Prepared from Biomass and Coal

CO2 adsorption is one of the promising CCS technologies, and activated coke is a solid adsorbent with excellent adsorption properties. In this study, activated coke was prepared by using bituminous coal and coconut shells activated with KOH or CaCl2 in a physically activated atmosphere and modified with ammonia. The effect of the active agent impregnation ratio on the physicochemical properties of activated coke was investigated by N2 adsorption isotherms, scanning electron microscopy (SEM) and Fourier transform infrared spectrometry (FTIR). The CO2 adsorption performance of activated coke was tested, and the effect of nitrogen-containing functional groups on CO2 adsorption was investigated by experiments and simulations. The results showed that the specific surface area of activated coke reached 629.81 m2/g at a KOH impregnation ratio of 0.5 and 610.66 m2/g at a CaCl2 impregnation ratio of 1. The maximum CO2 adsorption capacity of activated coke reached 71.70 mg/g and 90.99 mg/g for conventional power plant flue gas and oxy–fuel combustion flue gas, respectively. After ammonia modification, the CO2 adsorption capacity of activated coke was further increased. Simulations showed that pyrrole and pyrrole functional groups changed the polarity of graphene and established weak interactions with CO2.

Experimental study of char gasification characteristics with high temperature flue gas

To explore the feasibility of converting oxy-fuel combustion flue gas into valuable syngas through char gasification process, the reactivity of three types of char (bituminous coal char, biochar, and waste char) were experimentally studied under atmospheric pressure in a fixed-bed reactor. The effects of flue gas flow rate and temperature, CO2/H2O/O2 ratio, char types on the char gasification characteristics were mainly investigated. It was found that when the flue gas temperature is 1200 °C or even higher, no matter which char is used, >90% CO2 conversion rate can be reached in line with the Boudouard equilibrium. The reactivity of biochar-H2O is relatively higher than that of waste char-H2O. Besides, 5% oxygen ratio in CO2/H2O mixture is preferred to promote the gasification reaction to some degree. It is important to balance the fixed carbon conversion rate and CO/H2 yield in syngas to decide the optimum residence time of char. Overall, it is a novel way of CO2 reduction and waste resources utilization.

Measurement and modeling of nitrogen oxides absorption in a pressurized reactor relevant to CO2 compression and purification process

The removal of nitrogen oxides under a high pressure has been widely accepted as an effective method for the treatment of oxy-fuel combustion flue gas. The high pressure promotes the oxidation of NO to NO2, subsequently removed by water to form nitric acid. However, both the measurement and modeling of nitrogen oxides absorption shows uncertainties. To quantify the nitrogen mass balance, gas-phase NO/NO2 and HNO2 were measured online, whereas liquid-phase HNO2 and HNO3 were analyzed offline. Nitrogen mass balance was achieved with a relative error within ±8%. To improve modeling accuracy, a novel multiparameter optimization method was proposed. Mass transfer coefficients of NO2/NO, rate constant of NO oxidation, surface area, and volume of liquid were optimized. The absorption of NO2 into a liquid was dominated by gas-phase mass transfer with the mass transfer coefficient decreasing from 1.77 × 10―4 mol m−2 s-1 Kpa-1 to 1.26 × 10―4 mol m−2 s-1 Kpa-1 with the increase of reactor pressure from 5 bar to 20 bar. The pressure dependent rate constant for NO oxidation agreed with gas-phase NO oxidation experimental measurement. It first decreased with the reactor pressure from 5 bar to 15 bar followed by an increase from 15 bar to 20 bar. The surface area of liquid decreased while the liquid volume increased with the rising reactor pressure indicating that liquid droplets were formed by gas bubbling and attached onto the inner surface of the reactor. This study resolved the uncertainties in measurement and modeling of the nitrogen oxides absorption process.

Simultaneous Absorption of NOx and SO2 into Water and Acids under High Pressures

Scrubbing of SO2 and NOx from oxy-fuel combustion flue gas under a high pressure has been well recognized as an effective technique for flue gas purification. Experiments were performed in a bubbli...

Measurement and modeling of nitrogen oxides absorption in a pressurized reactor relevant to CO2 compression and purification process

The removal of nitrogen oxides under a high pressure has been widely accepted as an effective method for the treatment of oxy-fuel combustion flue gas. The high pressure promotes the oxidation of NO to NO2, subsequently removed by water to form nitric acid. However, both the measurement and modeling of nitrogen oxides absorption shows uncertainties. To quantify the nitrogen mass balance, gas-phase NO/NO2 and HNO2 were measured online, whereas liquid-phase HNO2 and HNO3 were analyzed offline. Nitrogen mass balance was achieved with a relative error within ±8%. To improve modeling accuracy, a novel multiparameter optimization method was proposed. Mass transfer coefficients of NO2/NO, rate constant of NO oxidation, surface area, and volume of liquid were optimized. The absorption of NO2 into a liquid was dominated by gas-phase mass transfer with the mass transfer coefficient decreasing from 1.77 × 10−4 mol m−2 s−1 Kpa−1 to 1.26 × 10−4 mol m−2 s−1 Kpa−1 with the increase of reactor pressure from 5 bar to 20 bar. The pressure dependent rate constant for NO oxidation agreed with gas-phase NO oxidation experimental measurement. It first decreased with the reactor pressure from 5 bar to 15 bar followed by an increase from 15 bar to 20 bar. The surface area of liquid decreased while the liquid volume increased with the rising reactor pressure indicating that liquid droplets were formed by gas bubbling and attached onto the inner surface of the reactor. This study resolved the uncertainties in measurement and modeling of the nitrogen oxides absorption process.

Removal and recovery of SO2 and NO in oxy-fuel combustion flue gas by calcium-based slurry

This study investigates the use of calcium-based slurry for simultaneous removal NO and SO2 from oxy-fuel combustion flue gas, and recovery of the sulfur and nitrogen species in resulting solutions. The experiments were performed in a bubbling reactor in a transient mode under the pressure of 20 bar. The various influencing factors including the CaO amount, carrier gas (N2/CO2), and absorption time on the simultaneous NO and SO2 removal process, and the solution products were studied comprehensively. The results show that the NO2 removal efficiency can be improved by the presence of CO2, and the gas phase HNO2 produces in this process. The addition of CaO has positive effects not only on the NO2 removal efficiency but also on the formation of stable HNO3. With the presence of CO2, CaCO3 is formed in a solution initially. With the decrease of pH, CaCO3 is gradually converted to CaSO4, and in particular CaCO3 can be fully avoided through decreasing the pH of an absorption solution to 1.14. At the same time, the formation of unstable S(IV) and NO2- can be prevented when the solution pH is lower than 1.37. The nitrogen and sulfur compounds in the absorption solution (at pH 1.14) were further separated by the addition of different amounts of CaO. In particular, 95% of SO42- finally can be recovered in the form of CaSO4.2H2O with nitrogen in solution existing as NO3- by controlling the Ca/S ratio at 4.70. The effectiveness of calcium-based slurry on the removal and recovery of SO2 and NO is confirmed.

Understanding the thermochemical behavior of La0.6Sr0.4Co0.2Fe0.8O3 and Ce0.9Gd0.1O_Co oxygen transport membranes under real oxy-combustion process conditions

Oxygen transport membranes (OTM) are a promising alternative to conventional systems of air separation based on cryogenic distillation for oxy-fuel combustion power plants. In this work, a systematic study of the thermochemical stability of La0.6Sr0.4Co0.2Fe0.8O3 (perovskite-type) and cobalt doped Ce0.9Gd0.1O (fluorite-type) is proposed. The experiments were developed in a laboratory scale facility, which is able to mimic realistic oxy-fuel combustion flue gas containing SOx, NOx, H2O and CO2. In order to understand the thermochemical behavior of this type of materials, a full characterization analysis of the tested samples using a wide portfolio of analytical techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), infrared spectroscopy (ATR-FTIR), Raman spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and Brunauer−Emmett−Teller analysis (BET) has been carefully discussed.Our data revealed the superior stability of the CGO samples in comparison with the LSCF at all the test conditions studied in this work. The formation of crystalline and amorphous sulphates and carbonates are evident for the LSCF while for the CGO samples do not react with SOX and barely form carbonates. The presence of silicon species – typically ignored in academic works – has been detected, pointing its relevance for real applications.

The impact of variable carbonation and decarbonation conditions on the CO2 sorption capacity of CaO-based sorbents

The study focuses on the utilisation of natural limestones for repeatable high-temperature chemisorption of carbon dioxide from flue gas. Two methods are compared to improve the problem of the capacity decrease with repeated uses: the steam regeneration of limestone and the conversion of the same limestone to a CaO-based sorbent via an organic intermediate. Moreover, the study extends the knowledge of the sorption behaviour of the limestones by assessing the impact of the composition of the calcination and carbonation atmosphere measured independently using a self-designed laboratory apparatus with a fluidised-bed reactor. After ten cycles, performed without any regeneration procedure, the capacity achieved by the natural limestones decreased from ca 290 to 370 to only 80 and 130 mg g−1. The application of steam regeneration showed good results in capacity stabilisation. From the fifth to the tenth cycles, the limestones had an average capacity of 220 mg g−1. Steam regeneration was not, however, the only method to improve the capacity effectively. The CaO sample prepared by the chemical modification of the natural limestone was resistant to the drop in capacity and achieved the value of 210 mg g−1 after ten cycles. Since the pilot-scale units used worldwide utilise oxy-fuel combustion flue gas as the decarbonation medium, the influence of pure CO2 on sorption capacity has been investigated as well. This atmosphere supports a decrease in the capacity of limestones, which drops to 30–50 mg g−1 after ten cycles. The CaO sample exhibited higher resistance and maintained the capacity of ca 110 mg g−1. In addition, the influence of the residual concentrations of sulphur dioxide present in the simulated flue gas has been tested. It has been confirmed that SO2 seriously deteriorates sorption properties; therefore, its presence in the gas feed must be avoided.

Mercury adsorption and oxidation over magnetic biochar in oxyfuel combustion atmosphere: Impact of enriched CO2 and H2O

The abatement of mercury from oxyfuel combustion flue gas should be paid special attention due to the metal embrittlement and aluminum corrosion for CO2 compression device. The effects of enriched CO2 and H2O in oxyfuel combustion atmosphere on mercury removal over biochar (BC) and magnetic biochar (MBC) was studied. Hg0 was firstly captured by physical adsorption and then oxidized to various mercury compounds. A large amount of organic groups, especially CO group, were formed with the assistance of enriched CO2. As a result, more Hg0 was oxidized over BC and more organic Hg (Hg-OM) was formed on BC and MBC. However, the content of chemisorbed oxygen on MBC was reduced under oxyfuel combustion atmosphere, hence reducing the formation of HgO on MBC compared with air combustion atmosphere. The enriched H2O significantly inhibited the removal of mercury over MBC. Chlorine was demonstrated as the most significant active components for mercury removal, since HgCl2 was the dominant species on MBC. This work clarified the effects of enriched CO2 and H2O on Hg0 adsorption over BC and MBC, which is essential for further improving the adsorption activity.

Gas-phase oxidation of NO at high pressure relevant to sour gas compression purification process for oxy-fuel combustion flue gas

Abstract The removal of NO from oxy-fuel combustion is typically incorporated in sour gas compression purification process. This process involves the oxidation of NO to NO2 at a high pressure of 1–3 MPa, followed by absorption of NO2 by water. In this pressure range, the NO conversion rates calculated using the existing kinetic constants are often higher than those obtained experimentally. This study aimed to achieve the regression of kinetic parameters of NO oxidation based on the existing experimental results and theoretical models. Based on three existing NO oxidation mechanisms, first, the expressions for NO conversion against residence time were derived. By minimizing the mean-square errors of NO conversion ratio, the optimum kinetic rate constants were obtained. Without considering the reverse reaction for NO oxidation, similar mean-square errors for NO conversion ratio were calculated. Considering the reverse reaction for NO oxidation based on the termolecular reaction mechanism, the minimum mean-square error for NO conversion ratio was obtained. Thus, the optimum NO oxidation rate in the pressure range 0.1–3 MPa can be expressed as follows: − d NO / d t = d NO 2 / d t = 0.0026 NO 2 O 2 − 0.0034 NO 2 2 Detailed elementary reactions for N2/NO/NO2/O2 system were established to simulate the NO oxidation rate. A sensitivity analysis showed that the critical elementary reaction is 2NO + O2 ⇌ 2NO2. However, the simulated NO conversions at a high pressure of 10–30 bar are still higher than the experimental values and similar to those obtained from the models without considering the reverse reaction for NO oxidation.

A comparative study on the design of direct contact condenser for air and oxy-fuel combustion flue gas based on Callide Oxy-fuel Project

Direct contact condenser is widely used in oxy-fuel combustion capture systems. Unusually high content of water vapor in the flue gas requires rigorous sizing procedures for the condenser design. Non-linear differential equations for humidity, gas and liquid temperatures were set up to understand the evaporation/condensation process in the condenser. A Quasi-Newton method was adopted to simultaneously solve discrete equations to avoid difficulty in convergence.This model was firstly verified with reported experiments in a packed bed condenser. The significant impacts of L/G ratio on condenser height, packing volume, condenser diameter are identified. The optimum L/G range is obtained by the wet bulb temperature and minimal decrease on packing volume, and this results in the L/G range of 2.5–5.2 and 4.3–6.7 for air and oxy-fuel combustion respectively. The condenser diameter and packing volume corresponding to the optimum L/G range for air-fuel combustion are approximately twice and four times of these for oxy-fuel combustion. While the packing height for air-fuel combustion is slightly lower than that for oxy-fuel combustion.By economic analysis, normalized total capital and annual costs for air-fuel combustion are approximately four times and twice of these for oxy-fuel combustion. The decrease of L/G ratio reduces the normalized total capital and annual costs for both air and oxy-fuel combustion and more significant for air-fuel combustion. Therefore, the L/G ratio is preferably obtained by the wet bulb temperature. This paper sheds light on the rigorous design method and the optimization of design parameters for direct contact condenser.

Dynamic simulation in the process of pressurized denitration based on oxy-fuel combustion

Oxy-fuel combustion is considered as one of the most promising technologies for capturing CO2 from coal-fired power plants. It will greatly reduce the cost of gas purification if we remove NOx in the process of compression, which is the characteristic of oxy-combustion. In this paper, simulation of denitration process of oxy-fuel combustion flue gas was realized by the Aspen Plus software, systematically analyzed the effect of temperature, pressure, initial concentration of O2 and NO in the denitration process. Results show that the increasing of pressure, initial concentration of O2, initial concentration of NO and the decrease of temperature are all beneficial to the denitration process.

Simultaneous removal of NO and Hg0 from oxy-fuel combustion flue gas over CeO2-modified low-V2O5-based catalysts

CeO2-doped low-V2O5-based catalysts (V0.5WCe/Ti) were investigated and showed outstanding stability for the simultaneous removal of NO (92.5%) and Hg0 (75%) from oxy-fuel combustion flue gas in 50h compared to a normal V1.0W/Ti catalyst. The good NH3-SCR performance of the V0.5WCe/Ti catalyst was due to the enhancement in acidity that arose from the introduced vanadium oxide, the better redox capability and greater amount of surface adsorbed oxygen species (O2−, O− and O2−) by Ce-modification, which generated a large amount of NO2, conducting the “fast SCR reaction”. Hg0 oxidation was strongly affected by surface adsorbed oxygen and CO functional groups in the absence of HCl in CO2-enriched flow conditions, while the reaction was mostly between Hg0(ad)/HgO(ad) and gaseous Cl⁎ or Cl2 in the presence of both HCl and O2. In addition, a high concentration of CO2 inhibited NO conversion, because of the rate of diffusion, but promoted Hg0 removal. Furthermore, while a high concentration of HCl prohibited NO conversion due to the formation of VCl and CeCl on the catalyst surface, Hg0 adsorption and catalytic oxidation were inhibited when SO2 and H2O coexisted in the flue gas.

Effects of Acidic Gases on Mercury Adsorption by Activated Carbon in Simulated Oxy-Fuel Combustion Flue Gas

Effects of acidic gases (CO2, SO2, NO and HCl) in coal-fired flue gas on mercury removal by a raw activated carbon (AC) under oxy-fuel atmosphere were studied on a laboratory-scale fixed-bed reactor. The temperature-programmed desorption (TPD) method was used to determine mercury forms in the AC. Some characterization methods (BET, FTIR, XRF and EDS) are adopted to characterize the physical and chemical properties of the AC. Results show that NO and HCl can strongly promote mercury removal, and the mercury species formed on AC are Hg2(NO3)2, HgO and HgCl2 respectively, which all desorb at around 300℃. SO2 is not beneficial for mercury removal because it will change into SO3 to occupy the active sites competing with Hg. In addition, Hg2+ adsorbed on AC would be reduced to Hg0 in the presence of SO2. Two different kinds of HgS (black and red) are generated after SO2 introduced, they desorb at 240℃ and 340℃, respectively. For oxy-fuel combustion atmosphere, high concentration of CO2 almost has no effect on m...