Oxy-fuel Circulating Fluidized Bed Combustion Research Articles

Theoretical Study on CO Participated N2O Reduction

ABSTRACT N2O is the main gaseous pollutant in circulating fluidized bed (CFB) combustion. Density functional theory (DFT) of quantum chemistry was used to theoretically study the role of CO interaction with N2O on the char surface in this paper. The rate parameters were calculated based on the transition state theory (TST) for kinetic analysis. The armchair configurations with six aromatic ring clusters were selected as the carbonaceous surfaces. This paper identifies a novel redox reaction pathway for N2O pre-adsorption on the char surface. By calculating the difference in activation energy between N2O-CO homogeneous and char catalyzed N2O-CO heterogeneous reduction, it is concluded that the char surface provides an active site for N2O reduction by CO and significantly reduces the activation energy for N2O abatement. In oxy-fuel CFB combustion, localized surface C-CO2 gasification of the char causes high concentrations of CO to enrich and defend the char surface, creating a Stefan flow that prevents O2 gas from approaching the char surface, which makes CO pre-adsorption possible. The reaction mechanism of the heterogeneous reduction on the char surface is discussed more comprehensively in terms of both N2O pre-adsorption and CO pre-adsorption conditions, which also enriches the mechanism of N2O abatement during CFB combustion.

Effect of particle size on in-situ desulfurization for oxy-fuel CFBC

Oxy-fuel circulating fluidized bed combustion (Oxy-CFBC) has been widely recognized as an economically promising way to capture carbon dioxide by injecting pure O2 and recycling CO2 from the flue gas because Oxy-CFBC has the advantages of fuel flexibility, in-situ desulfurization and NOx emission reduction. In this study, domestic limestone samples with a particle size range of 0–1 mm (0–75 μm, 75–106 μm, 106–212 μm, 212–300 μm and 500–1000 μm) were reacted in a thermogravimetric analyzer (TGA) under 60% CO2/40% O2 and 80% CO2/20% O2 at 1073 K and 1123 K for a direct desulfurization reaction. The surface and cross-section of limestone and CaSO4 samples with different conversion ratios were analyzed using a field emission scanning electron microscope (FE-SEM) and an energy dispersive spectrometry (EDS). FE-SEM images and EDS analysis of limestone and CaSO4 showed that direct desulfurization reaction starts from surface and progresses to center of particles, and the layer of CaSO4 expands into the whole particle. Since the experimental conditions of all particles were the same, the difference in CaSO4 conversion ratio by particle size confirmed through the TGA results depends only on the specific surface area. Also, the CaSO4 layer was formed at a uniform thickness regardless of the particle size.

Ash and Flue Gas from Oil Shale Oxy-Fuel Circulating Fluidized Bed Combustion

Carbon dioxide emissions are considered a major environmental threat. To enable power production from carbon-containing fuels, carbon capture is required. Oxy-fuel combustion technology facilitates carbon capture by increasing the carbon dioxide concentration in flue gas. This study reports the results of calcium rich oil shale combustion in a 60 kWth circulating fluidized bed (CFB) combustor. The focus was on the composition of the formed flue gas and ash during air and oxy-fuel combustion. The fuel was typical Estonian oil shale characterized by high volatile and ash contents. No additional bed material was used in the CFB; the formed ash was enough for the purpose. Two modes of oxy-fuel combustion were investigated and compared with combustion in air. When N2 in the oxidizer was replaced with CO2, the CFB temperatures decreased by up to 100 °C. When oil shale was fired in the CFB with increased O2 content in CO2, the temperatures in the furnace were similar to combustion in air. In air mode, the emissions of SO2 and NOx were low (<14 and 141 mg/Nm3 @ 6% O2, respectively). Pollutant concentrations in the flue gas during oxy-fuel operations remained low (for OXY30 SO2 < 14 and NOx 130 mg/Nm3 @ 6% O2 and for OXY21 SO2 23 and NOx 156 mg/Nm3 @ 6% O2). Analyses of the collected ash samples showed a decreased extent of carbonate minerals decomposition during both oxy-fuel experiments. This results in decreased carbon dioxide emissions. The outcomes show that oxy-fuel CFB combustion of the oil shale ensures sulfur binding and decreases CO2 production.

Three-dimensional CFD simulation of oxy-fuel combustion in a circulating fluidized bed with warm flue gas recycle

Among various CO2 capture methods, the oxy-fuel combustion technology exhibits great potential for CO2 capture in an economical way. Based on a pilot-scale circulating fluidized bed (CFB) combustor, computational fluid dynamics (CFD) is applied to simulate the oxy-fuel CFB combustion with warm flue gas recycle (WFGR). In this work, the WFGR in the oxy-fuel CFB combustion is realized by the Eulerian-Eulerian CFD simulation. Detailed profile of the hydrodynamics, temperature, gas characteristics and pollutant emission is analyzed with good agreement between the simulated result and the experimental data achieved. Special attention is paid to the comparison of gas distribution under the combustion conditions with and without WFGR, illustrating the unique advantages of the oxy-fuel CFB combustion with WFGR in the CO2 capture and pollutant reduction. Besides, influence of the oxy atmosphere with different O2 concentrations (from 21% to 40%) on the overall performance is conducted, successfully predicting the oxy-fuel combustion with WFGR under a high O2 concentration and enriching the investigation method of the CFB combustion study.

A Novel Method of Pure Oxy-fuel Circulating Fluidized Bed Combustion with Zero Recirculation Flue Gas – Experimental Validation

Applying oxy-fuel combustion requires more advanced combustion control methods to avoid inadmissible high flam temperature. In fluidized beds and pulverized unites, enhanced heat transfer and recirculation flue gas are used. On other hand, higher oxygen concentration has pluses viz. better heat transfer, higher efficiency, compact setup and lower installation and operating costs. In pulverized power unites, pure oxy-fuel combustion is used with 100% O2 in the oxidant. In contrast, the highest experimental O2 % in oxy-fuel circulating fluidized bed (CFB) combustor is 70%. To the best of authors’ knowledge, there is no single CFB power plant operating under pure oxygen condition. In this work, we are aiming to use pure oxygen for oxy-CFB combustion, with new temperature controlling method for CFBs depending on combustion staging by fuel staging rather than using RFG. Fuel staging allows controlling combustion and varying SR. At the first stage, the used oxidant is 100% O2, and fuel is fed to achieve over SR (λ>1), where the excess oxidant absorbs heat and does not take a part in the reaction. The products of the first stage are reach of O2 and subsequently it is used as an oxidant for the second stage. For validation, a series of experiments are conducted using mini-CFB, and an oxidant of 100% O2 concentration is used with three SR ratios λ=1.25, 2.0, and 3.0. The resulted average temperatures along the riser for biomass are 1031°C, 950°C, and 798°C; and for coal 1129 °C, 1051 °C, and 961 °C respectively. The controlling of AFT with pure oxy-fuel combustion eliminates the recycled flue gas (RFG) in oxy-fuel CFB combustion and flue gas recirculation section; this simplifies the power plants’ design, fabrication and its installing-operating costs. Familiarising this concept can accelerate adapting oxy-fuel combustion in CFB power plant for Carbon Capturing and Sequestration (CCS). This contribution can commence and commercialise the third generation of oxy-fuel CFB combustion with zero recycled flue gas. Finally, the concept of controlling AFT by SR (λ) is validated experimentally.

Experimental study on N2O emission in O2/CO2 combustion with high oxygen concentration in circulating fluidized bed

To characterize the N2O formation and fuel nitrogen conversion in an oxy-fuel circulating fluidized bed (CFB) combustor with high oxygen concentration, tests were carried out by analysing the axial concentrations of N2O in a 50 kWth CFB combustor. The conversion ratios from fuel nitrogen to gaseous N-containing pollutants were calculated. The initial N2O concentrations in the bottom of the combustor were similar between oxy-fuel firing and air-firing. The axial N2O formation was more in oxy-fuel combustion than in air-firing, improving the N2O emission during 50% O2/50% CO2 combustion. The atmospheric variation significantly affected the conversion ratio from fuel nitrogen to N2O. In addition, the conversion from fuel nitrogen to N2O was much higher than that to NO. As a result, the N2O emission during oxy-fuel CFB combustion cannot be ignored. Gas staging little influenced the N2O emission. With the increasing ratio of secondary gas, the initial N2O formation in the dense zone increased, while the axial N2O formation along the combustor declined. By analysing the conversion ratios of fuel nitrogen, it was also found that gas staging obviously affected the conversion ratio from fuel nitrogen to NO by enhancing the NO to N2 conversion. However, gas staging did not impact the conversion ratio from fuel nitrogen to N2O.

Carbon dioxide emission factors for oxy-fuel CFBC and aqueous carbonation of the Ca-rich oil shale ash

Oil shale (OS) is a low-calorific-value fossil fuel. Today, Estonia’s OS usage is the largest in the world. Approximately 76 % of the electricity is produced from Ca-rich OS. Yearly, approximately 12 million tons of OS is used for power generation utilizing pulverized combustion (PC) and circulating fluidized bed combustion (CFBC) technologies that produce nearly 6 million tons of Ca-rich ash.Estonian kukersite OS consists about one third of carbonate minerals, mainly calcite. Therefore, in addition to the combustion of organic carbon, the carbonaceous minerals decompose and release additional CO2. The extent of decomposition of carbonaceous minerals depends on combustion technology. Using oxy-fuel CFBC technology alters ash properties, including decomposition of carbonate minerals. By means of aqueous carbonation of Ca-rich ash, CO2 can be stored safely and leakage-free for very long time.In order to understand the changing CO2 sequestration potential, oil shale ashes were produced at a 60 kWth CFBC facility in oxy-fuel mode. The ash was treated with water and CO2 in order to mimic the ash treatment technology currently in use in the industry. The ash bound 81 kgCO2/tash it means that 6 % of the CO2 emitted would be bound at the ash fields. Due to decreased decomposition of carbonates, when using oxy-fuel CFBC, the CO2 specific emission of combustion would decrease by 5 % compared to regular CFBC and 19 % compared to PF. Decreased CO2 production would result in reduced CO2 transportation and further utilization or storage cost.

Influence of operating parameters on N2O emission in O2/CO2 combustion with high oxygen concentration in circulating fluidized bed

Oxy-fuel circulating fluidized bed (CFB) combustion combining the advantages of oxy-fuel combustion and CFB technology is one of the most promising technologies for capturing CO2. However, the emission of N2O in oxy-fuel CFB combustion can be high enough to raise some related environmental issues. This study investigated N2O emission in O2/CO2 CFB combustion with a high oxygen concentration based on the experimental results using a the 0.1MWth oxy-fuel CFB. This study focused on the operating parameters. It was found that increasing the combustion temperature and overall oxygen concentration can lead to lower N2O emission, while increasing the excess oxygen ratio can lead to higher N2O emission levels. Both gas staging and oxygen staging have slight adverse influences on controlling N2O owing to the higher oxygen concentration in the secondary gas. Finally, the excess oxygen ratio in the primary zone was determined in order to assess the actual effects of gas and oxygen staging on both N2O and NO emissions.

The experimental study on nitrogen oxides and SO2 emission for oxy-fuel circulation fluidized bed combustion with high oxygen concentration

Oxy-fuel circulating fluidized bed combustion (CFBC) has an advantage of being operated with high oxygen concentration, making it a better choice for CO2 capture. In order to study the emission of nitrogen oxides and SO2 under O2/RFG (recycle flue gas) firing mode with an overall O2 concentration of 50%, oxy-fuel combustion experiments with Datong coal were carried out in a 0.1MWth oxy-fuel CFBC apparatus. The effects of different operating conditions on gaseous pollution emission, including gas staging combustion, oxygen staging combustion, and excess oxygen ratio were investigated. The experimental results show that a lower secondary flow ratio improves the sulfur capture efficiency, although the fuel N conversion increases slightly. Oxygen staging combustion, which means decreasing the oxygen concentration in the primary flow and increasing it in the secondary flow, is a new and safe method for oxy-fuel combustion with high oxygen concentration. A reasonably low excess oxygen ratio for the oxy-fuel CFBC operation is beneficial to simultaneously enable a decrease in the fuel N conversion and an improvement in the sulfur capture efficiency.

Combustion Characteristics of Coal and Coke Under High Oxygen Concentration Oxy-Fuel CFBC Conditions

Oxy-fuel circulating fluidized bed combustion (CFBC) is one of the emerging technologies for reducing green house gas emissions from fossil fuel power generation. To improve the overall efficiency of an oxy-fuel CFBC plant and reduce the cost of CO2 capture, high oxygen concentration combustion is desired, as it means a lower flue gas recycle ratio and a smaller combustion chamber. To support the study of high oxygen concentration oxy-fuel research, combustion characteristics of different fuels were determined in a thermogravimetric analyzer (TGA). The fuels tested include lignite, bituminous coal, anthracite, and petroleum coke. Results showed that under the conditions tested, oxygen concentrations had only weak effect on the ignition of all the fuels. However, higher oxygen levels, as expected, reduced the burnout time. It was noticed that at typical CFBC combustion temperatures (˜900 °C), direct reaction between CO2 and carbon is negligible, even though the CO2 concentrations in the gas mixtures were high.

Progress of Combustion in an Oxy-fuel Circulating Fluidized-Bed Furnace: Measurements and Modeling in a 4 MWth Boiler

Oxy-fuel combustion, which is a promising technology for the abatement of carbon dioxide emissions, can be applied in circulating fluidized-bed (CFB) power plants. In this study, the effects of operational conditions on the progress of oxy-fuel CFB combustion were investigated by means of a mathematical model for CFB oxy-fuel combustion together with experimental data from a 4 MWth oxy-fuel CFB, currently representing the largest oxy-fuel CFB combustion experiments in the literature. Modeled in-furnace profiles for carbon monoxide (CO) and oxygen (O2) were compared to the corresponding measurements, yielding a general good agreement for both air- and oxy-fuel-fired conditions. The developed model was also used to investigate the effects of varying the inlet O2 concentration over a wider range than that applied in the experiments. The experimental results show that, for an equivalent inlet O2 concentration, the peak CO concentration is higher under oxy-fuel-fired conditions than under air-fired conditions. The model result shows that a higher inlet O2 concentration generates combustion of greater intensity up through the furnace with a lower level of CO at the furnace exit.

O2/CO2 coal combustion characteristics in a 50 kWth circulating fluidized bed

Combustion experiments of two Chinese coals were conducted in a 50kWth circulating fluidized bed (CFB) combustor in air and O2/CO2 atmospheres, and the effects of atmosphere and O2 staging on combustion characteristics including temperature distribution profiles, combustion efficiency, gaseous pollutant emission and desulfurization efficiency were investigated. Results show that O2 enrichment in the atmosphere elevates the temperature level in the riser and improves the combustion efficiency. Without limestone addition, O2 enrichment in the atmosphere augments SO2 and NO emission and reduces CO emission. The desulfurization efficiency under higher O2 concentration is higher with the same Ca/S molar ratio in O2/CO2 atmosphere. O2 staging provides more operation flexibility for oxy-fuel CFB combustion. As O2 concentration in the primary gas increases, combustion efficiency and desulfurization efficiency are improved. O2 staging can reduce the NO emission to a large extent in O2/CO2 atmosphere.

Simultaneous Carbonation and Sulfation of CaO in Oxy‐Fuel CFB Combustion

AbstractFor anthracites and petroleum cokes, the typical combustion temperature in a circulating fluidized bed (CFB) is > 900 °C. At CO2 concentrations of 80–85 % (typical of oxy‐fuel CFBC conditions), limestone still calcines. When the ash which includes unreacted CaO cools to the calcination temperature, carbonation of fly ash deposited on cool surfaces may occur. At the same time, indirect and direct sulfation of limestone also will occur, possibly leading to more deposition. In this study, CaO was carbonated and sulfated simultaneously in a thermogravimetric analyzer (TGA) under conditions expected in an oxy‐fuel CFBC. It was found that temperature, and concentrations of CO2, SO2, and especially H2O are important factors in determining the carbonation/sulfation reactions of CaO.

Carbonation of fly ash in oxy-fuel CFB combustion

Oxy-fuel combustion of fossil fuel is one of the most promising methods to produce a stream of concentrated CO 2 ready for sequestration. Oxy-fuel FBC (fluidized bed combustion) can use limestone as a sorbent for in situ capture of sulphur dioxide. Limestone will not calcine to CaO under typical oxy-fuel circulating FBC (CFBC) operating temperatures because of the high CO 2 partial pressures. However, for some fuels, such as anthracites and petroleum cokes, the typical combustion temperature is above 900 °C. At CO 2 concentrations of 80–85% (typical of oxy-fuel CFBC conditions with flue gas recycle) limestone still calcines, but when the ash cools to the calcination temperature, carbonation of fly ash deposited on cool surfaces may occur. This phenomenon has the potential to cause fouling of the heat transfer surfaces in the back end of the boiler, and to create serious operational difficulties. In this study, fly ash generated in a utility CFBC boiler was carbonated in a thermogravimetric analyzer (TGA) under conditions expected in an oxy-fuel CFBC. The temperature range investigated was from 250 to 800 °C with CO 2 concentration set at 80% and H 2O concentrations at 0%, 8% and 15%, and the rate and the extent of the carbonation reaction were determined. Both temperature and H 2O concentrations played important roles in determining the reaction rate and extent of carbonation. The results also showed that, in different temperature ranges, the carbonation of fly ash displayed different characteristics: in the range 400 °C < T ⩽ 800 °C, the higher the temperature the higher the CaO-to-carbonate conversion ratio. The presence of H 2O in the gas phase always resulted in higher CaO conversion ratio than that obtainable without H 2O. For T ⩽ 400 °C, no fly ash carbonation occurred without the presence of H 2O in the gas phase. However, on water vapour addition, carbonation was observed, even at 250 °C. For T ⩽ 300 °C, small amounts of Ca(OH) 2 were found in the final product alongside CaCO 3. Here, the carbonation mechanism is discussed and the apparent activation energy for the overall reaction determined.

99/00593 Experience with combustion tuning and fuel system modifications to inexpensively reduce NO x emissions from eleven coal-fired tangential boilers

Oxy-fuel combustion of fossil fuel is one of the most promising methods to produce a stream of concentrated CO2 ready for sequestration. Oxy-fuel FBC (fluidized bed combustion) can use limestone as a sorbent for in situ capture of sulphur dioxide. Limestone will not calcine to CaO under typical oxy-fuel circulating FBC (CFBC) operating temperatures because of the high CO2 partial pressures. However, for some fuels, such as anthracites and petroleum cokes, the typical combustion temperature is above 900 °C. At CO2 concentrations of 80–85% (typical of oxy-fuel CFBC conditions with flue gas recycle) limestone still calcines, but when the ash cools to the calcination temperature, carbonation of fly ash deposited on cool surfaces may occur. This phenomenon has the potential to cause fouling of the heat transfer surfaces in the back end of the boiler, and to create serious operational difficulties. In this study, fly ash generated in a utility CFBC boiler was carbonated in a thermogravimetric analyzer (TGA) under conditions expected in an oxy-fuel CFBC. The temperature range investigated was from 250 to 800 °C with CO2 concentration set at 80% and H2O concentrations at 0%, 8% and 15%, and the rate and the extent of the carbonation reaction were determined. Both temperature and H2O concentrations played important roles in determining the reaction rate and extent of carbonation. The results also showed that, in different temperature ranges, the carbonation of fly ash displayed different characteristics: in the range 400 °C < T ⩽ 800 °C, the higher the temperature the higher the CaO-to-carbonate conversion ratio. The presence of H2O in the gas phase always resulted in higher CaO conversion ratio than that obtainable without H2O. For T ⩽ 400 °C, no fly ash carbonation occurred without the presence of H2O in the gas phase. However, on water vapour addition, carbonation was observed, even at 250 °C. For T ⩽ 300 °C, small amounts of Ca(OH)2 were found in the final product alongside CaCO3. Here, the carbonation mechanism is discussed and the apparent activation energy for the overall reaction determined.

99/00585 Burning low volatile coals in utility boilers with high efficiency and clean environment

Oxy-fuel combustion of fossil fuel is one of the most promising methods to produce a stream of concentrated CO2 ready for sequestration. Oxy-fuel FBC (fluidized bed combustion) can use limestone as a sorbent for in situ capture of sulphur dioxide. Limestone will not calcine to CaO under typical oxy-fuel circulating FBC (CFBC) operating temperatures because of the high CO2 partial pressures. However, for some fuels, such as anthracites and petroleum cokes, the typical combustion temperature is above 900 °C. At CO2 concentrations of 80–85% (typical of oxy-fuel CFBC conditions with flue gas recycle) limestone still calcines, but when the ash cools to the calcination temperature, carbonation of fly ash deposited on cool surfaces may occur. This phenomenon has the potential to cause fouling of the heat transfer surfaces in the back end of the boiler, and to create serious operational difficulties. In this study, fly ash generated in a utility CFBC boiler was carbonated in a thermogravimetric analyzer (TGA) under conditions expected in an oxy-fuel CFBC. The temperature range investigated was from 250 to 800 °C with CO2 concentration set at 80% and H2O concentrations at 0%, 8% and 15%, and the rate and the extent of the carbonation reaction were determined. Both temperature and H2O concentrations played important roles in determining the reaction rate and extent of carbonation. The results also showed that, in different temperature ranges, the carbonation of fly ash displayed different characteristics: in the range 400 °C < T ⩽ 800 °C, the higher the temperature the higher the CaO-to-carbonate conversion ratio. The presence of H2O in the gas phase always resulted in higher CaO conversion ratio than that obtainable without H2O. For T ⩽ 400 °C, no fly ash carbonation occurred without the presence of H2O in the gas phase. However, on water vapour addition, carbonation was observed, even at 250 °C. For T ⩽ 300 °C, small amounts of Ca(OH)2 were found in the final product alongside CaCO3. Here, the carbonation mechanism is discussed and the apparent activation energy for the overall reaction determined.