Effect of oxy-fuel combustion on flame characteristics of low calorific value coal gases in a small burner and combustor

Oxy-fuel combustion of fossil fuel is one of the most promising methods for producing electricity, together with a stream of concentrated CO2 ready for sequestration. Oxy-fuel fluidized-bed combustion (FBC) can also use limestone as a sorbent for in situ capture of sulfur dioxide. However, although a limited number of studies have been performed on sulfation of limestone under oxy-fuel combustion conditions, there are still a number of important but unanswered questions. Here, the effect of water vapor on the sulfation of limestone was studied, because it has not been examined in detail in previous sulfation studies and past studies on direct sulfation of limestone in FBC either did not explore the influence of H2O or did so under unrealistic conditions for oxy-fuel FBC. The purpose of this study is to identify the effect of water vapor on direct sulfation of limestone under simulated oxy-fuel circulating FBC (CFBC) conditions. Direct sulfation of three limestones was conducted in a thermogravimetric analyzer (TGA) apparatus at 800 and 850 °C. The limestone particle sizes used were 75−125, 125−150, 150−250, and 250−425 μm, and tests were carried out in a synthetic flue gas atmosphere, consisting of 80% CO2, 15%, 10% or 0% H2O, 4% O2, 5000 ppm SO2, and balance N2. Water always improved limestone sulfation, especially at 850 °C. In addition, for some limestones, such as Kelly Rock (Nova Scotia, Canada), when the reaction gas contained no H2O, the calcium conversion ratio was higher at 800 °C than at 850 °C. However, when the reaction gas contained 10% H2O, the conversion ratio and the sulfation reaction rate were always higher at 850 °C than at 800 °C. Because coal-fired boiler flue gases always contain water vapor, the role played by H2O in the limestone sulfation reaction should always be considered in future studies.

Coal contributes to almost forty percent of global power generation. As conventional coal-fired power generation technologies result in large CO2 emission, the pursuit for new technologies focuses on either reducing CO2 emission or that allows easier capture of the emitted CO2 from coal-fired power plants. Oxy-fuel fluidized bed (Oxy-FB) combustion is one such technology due to its ability to produce concentrated CO2 stream in the flue gas. This concentrated CO2 allows easier capture for subsequent transportation and storage. Other important benefits of this technology are the potential for using any type of fuel, and the ability to control SO2 and NOX emissions. Despite its perceived advantages over conventional technologies, very little is known about the applicability of Oxy-FB for brown coal. Brown coal accounts for 91% of Victoria’s current electricity needs. Since Victoria has an estimated reserve of over 500 years of brown coal at the current consumption rate, successful application of Oxy-FB can potentially result in environment friendly power generation in Victoria. This first-ever study investigates the Oxy-FB combustion using Victorian brown coal in a combined experimental and modelling approach. The research involves designing and commissioning of a 10 kWth fluidized bed rig, carrying out experiments in laboratory scale and bench scale equipment, and performing thermodynamic and process modelling. Laboratory scale experiments using single char particle were conducted to investigate the combustion characteristics of individual and large char particle under Oxy-FB conditions. Particle temperature was observed to be higher compared to bed temperature. Up to 48°C difference was noticed between the char particle temperature and the bed temperature using 15% (v/v) steam in oxy-fuel combustion atmosphere. The temperature of the char particle during Oxy-FB combustion has practical implication for agglomeration. The bench scale experiments were carried out to evaluate combustion efficiency, agglomeration characteristics, sulphation characteristics, carbonation characteristics, NOX (NO, NO2 and N2O) emission, SOX (SO2 and SO3) emission, and trace elements (Hg, Se, As and Cr) emissions during Oxy-FB combustion of Victorian brown coal. A high level of CO2 concentration (90-94% in dry flue gas), over 99% combustion efficiency and no bed agglomeration under oxy-fuel combustion conditions including those with the addition of steam at temperatures between 800°C and 900°C. Moreover, the measured NOX and SOX concentration levels in the flue gas are within the permissible limits for coal-fired power plants in Victoria. This implies that additional NOX and SOX removal systems may not be required with Oxy-FB combustion of Victorian brown coal. The gaseous mercury concentrations, however, are considerably higher under oxy-fuel combustion compared to air combustion suggesting that mercury removal system may be required to avoid corrosion in the CO2 separation units if CO2 capture and transportation is intended. These conventional pollutants and trace elements emission characteristics are of great importance for the design of the gas cleaning systems for CO2 capture and storage (CCS) purposes. Furthermore, these results also provide information for selecting the optimum operating condition. Thermodynamic equilibrium modelling was carried out to predict the compounds formed during the combustion of Victorian brown coal under different Oxy-FB combustion conditions. It was predicted that the amount of toxic gaseous Cr6+ species was greater for oxy-fuel combustion than for air combustion. The distribution of toxic Se4+ species, however, remained almost the same in both combustion conditions within the typical temperature range for Oxy-FB combustion (800 - 950°C). A process model on Oxy-FB combustion using Aspen Plus was also developed to predict combustion performance of any coal during Oxy-FB. It was observed that the concentrations of CO and SO2 were higher in the lower dense region of the bed. These levels, however, dropped significantly with the introduction of secondary oxygen. The simulation results were consistent with the experimental data. Overall, this thesis has identified several important issues, for the first time, on Oxy-FB combustion using brown coal. The information generated is useful for academics, industry and policy makers. Future research on Oxy-FB combustion can use the findings of this study while developing Oxy-FB combustion for brown coals.

Oxy-fuel coal combustion—A review of the current state-of-the-art

Oxy-fuel fluidized bed combustion using Victorian brown coal: An experimental investigation

In this study, algorithms with two objective functions are defined considering the TS500 (2000) (Reinforced concrete structures design and construction rules) and TBDY (2018) (Turkey Building Earthquake Regulation) standards for rectangular beam design. These objective functions were determined as CO2 emission and cost. Optimizations were performed in MATLAB program using the Hybrid Algorithm of Teaching-Learning Based Optimization and Jaya Algorithm. In the case of using two objective functions, cases were created by multiplying the coefficient values found in the objective function according to the formula with the cost and CO2 emission values at different rates in order to prevent CO2 emission which is one of the biggest problems for the world. In the objective function, each rate used for CO2 and cost is implemented in a manner that increases or diminishes the impact of these values. In this way, comparisons were made between the cross-section dimensions to be formed according to not only impact rates but also the reinforcement area to be used, the CO2 emission and cost values that will arise as a result of these. Impact rates are related to cost and CO2 rate in the objective function, and the total rate is chosen as 1. Impact rates for cost are chosen as 0.1, 0.3 along with 0.5, and comparisons between the results are checked. In addition, recyclable and non-recyclable steel with different properties were used in separate analyses and the values were compared. Since the CO2 rate released by the non-recyclable steel is very high compared to the recyclable steel, the results show that the CO2 emission value is higher and this causes the objective function value to increase.

A distributed parameter model was developed for an evaporation system in a 35 MW natural circulation pulverized-coal oxy-fuel combustion boiler, which was based on a computational fluid dynamic simulation and in situ operation monitoring. A mathematical model was used to consider the uneven distribution of working fluid properties and the heat load in a furnace to predict the heat flux of a water wall and the wall surface temperature corresponding to various working conditions. The results showed that the average heat flux near the burner area in the air-firing condition, the oxy-fuel combustion with dry flue gas recycling (FGR) condition, and the oxy-fuel combustion with wet flue-gas recycle condition were 168.18, 154.65, and 170.68 kW/m2 at a load of 80%. The temperature and the heat flux distributions in the air-firing and the oxy-fuel combustion with wet FGR were similar, but both were higher than those in the oxygen-enriched combustion conditions with the dry FGR under the same load. This study demonstrated that the average metal surface temperature in the front wall during the oxy-fuel combustion condition was 3.23 °C lower than that under the air-firing condition. The heat release rate from the furnace and the vaporization system should be coordinated at a low and middle load level. The superheating surfaces should be adjusted to match the rising temperature of the flue gas while shifting the operation from air to oxy-fuel combustion, where the distributed parameter analytical approach could then be applied to reveal the tendencies for these various combustion conditions. The research provided a type of guidance for the design and operation of the oxy-fuel combustion boiler.

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

In this study, the effect of additives on particulate matter (PM) and flue gas emissions during the co-combustion of poultry waste and pine woodchips in air and oxy-fuel combustion conditions was examined. The appropriate additive for the fuel mixture to reduce PM emissions has been selected by a fast screening method based on thermogravimetric analysis (TGA) in oxygen environment. Among the additives CaHPO4, MgCO3, MnCO3, MgPO4, kaolin, CaO, and Zn, the most suitable ones were determined as Zn and MgCO3. The thermal degradation performance of biofuels containing 2 wt% additives was examined by TGA method under pyrolysis, combustion, and oxy-fuel combustion conditions. Ashes obtained from the additivated biofuel mixture were examined by XRF, XRD, and SEM-EDS analyses, and the effect of additives on the ash structure was investigated. Flue gas emissions of co-firing biofuel were analyzed by TGA-FTIR. It has been observed that the highest emission is CO2, and the oxy-fuel combustion process and addivation have shown reducing effect on CO2 emissions. Under oxy-fuel combustion conditions, higher CO and lower NOx, SO2, and HCl emissions occurred at a lower rate compared to the air environment, and additives also showed a reducing effect. The composition and crystal structure of Zn-additivated biofuel ash support its reducing effect in PM emissions. It was concluded that Zn is a more suitable additive in terms of PM and flue gas emissions than MgCO3.

The carbon stock, biomass, and CO2 emissions in woody species play crucial roles in understanding and managing ecosystems. Understanding these aspects is crucial for sustainable forest management, conservation, and mitigating the impact of woody species on global carbon dynamics and climate change. This study examined the nexus between carbon stock, biomass, and CO2 emission of woody plant composition in disturbed and undisturbed areas in Southwestern Nigeria. The study involved the random establishment of plots in the disturbed and undisturbed areas and, in each plot, the woody plants were enumerated and identified to the species level. The results showed that total biomass (102.645 Mg ha-1), total carbon stock (51.323 Mg C ha-1), and total CO2 emission (188.354 Mg C ha-1) values of tree species in undisturbed plots were higher than the values of total biomass (70.768 Mg ha-1), total carbon stock (35.384 Mg C ha-1), and total CO2 emission (129.859 Mg C ha-1) recorded in disturbed plots. The results also revealed that total biomass (0.123 Mg ha-1), total carbon stock (0.061 Mg C ha-1), and total CO2 emission (0.225 Mg C ha-1 ) values of shrub species recorded in disturbed plots were higher than values of total biomass (0.067 Mg ha-1), total carbon stock (0.034 Mg C ha-1) and total CO2 content (0.124 Mg C ha-1) recorded in undisturbed plots, respectively. The findings showed that in undisturbed and disturbed plots of shrubs, biomass, carbon and CO2 emissions have a strong positive correlation of 1.000**. While biomass, carbon, and CO2 emission have a very strong positive correlation (0.999**) in undisturbed plots of trees, the biomass, carbon, and CO2 emission have moderate to strong positive correlations (0.458** to 0.974**) in disturbed plots of the tree. The study concluded that while biomass, carbon stock, and CO2 emission values of tree species were higher in undisturbed plots than in disturbed plots, the biomass, carbon stock, and CO2 emission values of shrub species were lower in undisturbed plots than in disturbed plots. It also concluded that the main purpose of establishing reserve forests is not totally achieved as human activities occurring in reserve forests still contribute to the increment of climate change.

Sulfur emission from a Victorian brown coal was quantitatively determined through controlled experiments in a continuously fed drop-tube furnace under three different atmospheres: pyrolysis, oxy-fuel combustion, and carbon dioxide gasification conditions. The species measured were H(2)S, SO(2), COS, CS(2), and more importantly SO(3). The temperature (873-1273 K) and gas environment effects on the sulfur species emission were investigated. The effect of residence time on the emission of those species was also assessed under oxy-fuel condition. The emission of the sulfur species depended on the reaction environment. H(2)S, SO(2), and CS(2) are the major species during pyrolysis, oxy-fuel, and gasification. Up to 10% of coal sulfur was found to be converted to SO(3) under oxy-fuel combustion, whereas SO(3) was undetectable during pyrolysis and gasification. The trend of the experimental results was qualitatively matched by thermodynamic predictions. The residence time had little effect on the release of those species. The release of sulfur oxides, in particular both SO(2) and SO(3), is considerably high during oxy-fuel combustion even though the sulfur content in Morwell coal is only 0.80%. Therefore, for Morwell coal utilization during oxy-fuel combustion, additional sulfur removal, or polishing systems will be required in order to avoid corrosion in the boiler and in the CO(2) separation units of the CO(2) capture systems.

Comparison of the flame stabilities during oxy-methane and air-methane combustion in a two-layer porous burner

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.

Supercritical oxy-fuel combustion, which allows for the high efficiency of power generation with near-zero CO2 emissions, is considered a promising method to reduce the carbon footprint in the power energy sector. One of the problems in the widespread use of oxy-fuel combustion is a lack of comparative studies on the existing oxy-fuel combustion kinetic mechanisms depending on mixture composition, which complicates the choice of a kinetic mechanism for modeling oxy-fuel combustion. In this paper, a comparative verification of the kinetic mechanisms of GRI-Mech 3.0, UoS sCO2 2.0, OXY-NG, and Skeletal was performed using published experimental data on the ignition delay time of methane under conditions of oxy-fuel combustion. A comparative numerical study of the kinetic mechanisms in the wide range of pressures, CO2 mass fractions in oxidizer (γ), and excess oxidizer ratios (α) by the ignition delay time is also carried out. It was found that the limits of applicability of all of the mechanisms studied are absent when modeling the ignition delay time, the most accurate mechanism to model the IDT of methane in oxy-fuel conditions being UoS sCO2 2.0, while the other three mechanisms are overall much inferior to it in terms of accuracy. However, Skeletal and GRI-Mech 3.0 mechanisms can be used to model the IDT during the oxy-fuel combustion of methane under both atmospheric and supercritical conditions, although only in a narrow range of γ.

Basic boiler cumulative distributions (fly ash, circulating material, bottom ash) — oxy-fuel and classical combustion conditions

Oxy-fuel combustion is one of the promising carbon capture technologies considered to be suitable for future commercial applications with stationary combustion plants. Although more and more biomass and waste are now being burned in stationary combustion plants, research on oxy-fuel combustion of biomass has received much less attention in comparison to oxy-fuel combustion of coal. In this work, a series of tests was carried out in a 20 kWth fluidized bed combustor under oxy-fuel conditions firing two non-woody fuels (miscanthus and straw pellets) and one woody fuel (domestic wood pellet). The effects of the combustion atmosphere (air and oxy-fuel) and oxygen concentration in the oxidant of the oxy-fuel combustion on gas emissions and temperature profiles were systematically studied with the overall excess oxygen coefficient in the combustor being maintained roughly constant throughout the tests. The experimental results showed that replacing the air with an oxy-fuel oxidant of 21 vol% O2 and 79 vol% CO2 resulted in a significant decrease in combustion temperature and ultimately led to the extinction of the biomass flame due to the larger specific heat of CO2 compared to N2. To keep a similar temperature profile to that achieved under the air combustion conditions, the oxygen concentration in the oxidant of O2/CO2 mixture had to be increased to 30 vol%. A drastic decrease in CO emissions was observed for all three biomass fuels (up to 80% reduction when firing straw) under oxy-fuel combustion conditions providing that the oxygen concentration in the oxidant of O2/CO2 mixture was above 25 vol%. NOx emissions were found to decrease with the oxygen concentration in the oxy-fuel oxidant, due to i) the increase of bed temperature, which implies more volatile-N released and converted in the dense bed zone and ii) the less dilution of the gases inside the dense bed zone, which leads to a higher CO concentration in this region enhancing the reduction of NOx. Similar NOx emissions to those obtained with air combustion were found when the oxygen concentration in the oxy-fuel oxidant was kept at 30 vol%. Further analysis of the experimental results showed that the gas emissions when firing the non-woody fuels were controlled mainly by the freeboard temperature instead of the dense bed region temperature due to the characteristically high volatile matter content and fines of this kind of biomass fuels.