Air Combustion Conditions Research Articles

Coal Char Kinetics of Oxidation and Gasification Reactions

Abstract Experimental investigations and numerical simulations have been conducted in this study to derive and test the values of kinetic parameters describing oxidation and gasification reactions between char carbon and O2 and CO2 occurring at standard air and oxy-fuel combustion conditions. Experiments were carried out in an electrically heated drop-tube at heating rates comparable to fullscale pulverized fuel combustion chambers. Values of the kinetic parameters, obtained by minimization of the difference between the experimental and modeled values of char burnout, have been derived and CFD simulations reproducing the experimental conditions of the drop tube furnace confirmed proper agreement between numerical and experimental char burnout.

Study on the limestone sulfation behavior under oxy-fuel circulating fluidized bed combustion condition

In order to investigate the behavior of limestone sulfation under oxy-fuel circulating fluidized bed (CFB) combustion condition, experiments were conducted in a 50 kW oxy-fuel CFB system under the O2/CO2 and air combustion conditions. A small cage, containing limestone particles, was dipped into the dense zone of the CFB combustor during the experiments. The calcination of limestone, pore structure of the product layer, and calcium conversion were studied. It was found that the increasing of temperature would promote the calcination of limestone and the high concentration of CO2 would inhibit calcination of limestone. The formation process of the product layer was completely different between the direct and indirect sulfation, while it was almost the same during the indirect sulfation under the oxy-fuel and air combustion. However, both the temperature and gas compositions played important roles in determining the pore structures of the product layer during the limestone indirect sulfation process. Under the O2/CO2 combustion condition, the calcium conversion of indirect sulfation was always higher than that of direct sulfation. The highest final calcium conversion after 60 min was found at 900 °C under the O2/CO2 combustion condition.

Development of 3D transient wall filming mechanism during combustion by coupling Eulerian-Lagrangian approach and particle-wall interaction model

In the present study, the developments of 3D wall filming mechanism for coal combustion in a small scale furnace under air and oxy-fuel combustion conditions are presented. The principle objective of this study is to develop the film flow behavior and particle deposition characteristics on the furnace refractory wall using a commercial CFD code coupled with some user-defined sub-routines. Eulerian-Lagrangian approach of the gas-particle flow is coupled with the particle-wall interaction mechanisms including particle capturing, entrainment and wall burning sub-models. A case study has been presented to validate the model in a small scale furnace by comparing the available temperature, species data for coal-water slurry combustion and reasonable agreement has been observed. Visualization of the transient film formation and flow characteristics under air and oxy-firing cases are presented. The film thickness deposited on the refractory wall was found to be reasonably in good range with the available data. The deposited film thicknesses for both the conditions are found in the range of 0–1.0mm. The average molten film velocity was 0.0001m/s due to higher viscosity and greater surface tension properties. Slightly higher amount of films are formed in oxy-firing condition due to slower char oxidation rate.

K–Cl–S chemistry in air and oxy-combustion atmospheres

This paper studies the consequences on high temperature corrosion when biomass is co-fired with coal in oxy-fuel combustion, a concept that would enable negative CO2 emissions by combining renewable fuels with CO2 capture. The particular focus of the work is the sulphation of alkali chlorides which is studied by means of experiments and modelling. The melting point of alkali sulphates is higher than for alkali chlorides and a high degree of alkali sulphation is hence a measure to reduce issues related to high-temperature corrosion. In the experiments a propane flame was doped with SO2 and KCl (in solution) in air and oxy-fuel atmospheres. Two alternative measurement principles were applied to quantify potassium sulphation and the paper thereby contributes with a novel experimental approach as well as unique experimental data for turbulent diffusion air and oxy-fuel flames. The degree of sulphation of the injected KCl increases substantially in oxy compared to air combustion conditions, a fact which favours the use of biomass in oxy-combustion. This is mainly due to the increased concentration of SO2 due to flue gas recycling in oxy-combustion, but also chemical effects introduced by the CO2 as well as a small effect resulting from an increased residence time in the combustor.

The Influence of Slight Protuberances in a Micro-Tube Reactor on Methane/Moist Air Catalytic Combustion

The combustion characteristics of methane/moist air in micro-tube reactors with different numbers and shapes of inner wall protuberances are investigated in this paper. The micro-reactor with one rectangular protuberance (six different sizes) was studied firstly, and it is shown that reactions near the protuberance are mainly controlled by diffusion, which has little effect on the outlet temperature and methane conversion rate. The formation of cavities and recirculation zones in the vicinity of protuberances leads to a significant increase of the Arrhenius reaction rate of CH4 and gas velocity. Next, among the six different simulated conditions (0–5 rectangular protuberances), the micro-tube reactor with five rectangular protuberances shows the highest methane conversion rate. Finally, the effect of protuberance shape on methane/moist air catalytic combustion is confirmed, and it is found that the protuberance shape has a greater influence on methane conversion rate than the number of protuberances. The methane conversion rate in the micro-tube decreases progressively in the following order: five triangular slight protuberances > five rectangular protuberances > five trapezoidal protuberances > smooth tube. In all tests of methane/moist air combustion conditions, the micro-tube with five triangular protuberances has the peak efficiency and is therefore recommended for high efficiency reactors.

Combustion characteristics of waste sludge at air and oxy-fuel combustion conditions in a circulating fluidized bed reactor

Oxy-fuel combustion and circulating fluidized bed (CFB) technologies were applied to waste sewage sludge using a cold-bed and a 30kWth CFB pilot test bed. In the cold-bed tests, the minimum fluidization velocity (umf) and the superficial velocity were determined as 0.120m/s and 2.5m/s, respectively. In pilot tests, the combustion characteristics of waste sewage sludge in oxy-fuel condition were very distinctive compared with that in air condition in terms of operation parameters such as the distribution of pressure drop and temperature, flue-gas temperature, and the composition of ash and flue-gas. Based on cold-bed and pilot bed tests, the oxygen injection rate was optimized in the range from 21% to 25% in oxy-fuel condition for waste sludge combustion to apply oxy-fuel combustion and CFB technologies with considering technologies for stable and economic operation.

산소부화용 공업로의 운전조건이 열효율에 미치는 영향

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

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

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

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

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

Effects of wet torrefaction on reactivity and kinetics of wood under air combustion conditions

This work continues our assessment of wet torrefaction for energy applications, looking at effects of the process parameters (temperature, holding time and pressure) on the reactivity and intrinsic kinetics of wood under air combustion conditions. Woody materials, Norway spruce and birch, were wet torrefied in various conditions (temperature: 175, 200, 225°C; holding time: 10, 30, 60min; and pressure: 15.54, 70, 160bar). The reactivity of the treated and untreated woods was thermogravimetrically examined under a synthetic air environment (21% O2 and 79% N2 in volume). A four-pseudo-component model with different reaction orders was adopted for kinetic modelling and extracting the kinetic parameters. The results showed that when increasing either torrefaction temperature or holding time, the torrefied woods behaved more char-like than the raw fuels. However, pressure did not show significant effect on the reactivity. Relatively longer char combustion stages and higher conversion rates (up to 0.5×10−3s−1) were observed for the woods after torrefaction. The activation energy was decreased for hemicellulose and char, but increased for cellulose after torrefaction, whereas the trend for lignin is not clear. In addition, the hemicellulose mass fraction decreased after torrefaction (from 0.15 to 0.05 for spruce and from 0.23 to 0.06 for birch). The amount of char in the torrefied woods increased gradually with increasing torrefaction temperature or holding time (from 0.24 to 0.40 for spruce, and from 0.18 to 0.34 for birch).

The role of steam in silica vaporization and ultrafine particulate matter formation during wet oxy-coal combustion

Oxy-coal combustion produces a combustion environment that contains large amounts of CO2, H2O compared to conventional air combustion. Silica (SiO2) is a major component of the mineral impurities in coal and the vaporization of silicon containing species controls the initial phase of formation of ultrafine particulate matter (ultrafine PM) via vaporization–condensation mechanism. To investigate the vaporization behavior of silica in wet recycle oxy-coal combustion conditions, synthetic chars with SiO2 inclusions were burned at 1873K in the O2/CO2/H2O environments with steam of up to 10vol.%. Experiments were carried out with a high temperature drop tube furnace (HDTF) system and ultrafine PM was collected with a low pressure impactor (LPI). Meanwhile, modeling work was carried out for further investigation of the effect of H2O on the vaporization of silica. The improved model was developed based on a model built in conventional air combustion conditions and verified by the experiment. The results indicate that, under the wet recycle oxy-coal combustion conditions, H2O in combustion atmosphere significantly enhanced the vaporization of SiO2 and considerably increased the yield of the ultrafine PM. The variation of char combustion characteristics such as burning temperature and gas properties surrounding the mineral inclusions caused by H2O seemed to be the primary cause. Especially, effect of H2 inside the char particle cannot be ignored.

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

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

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

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

Mercury Emissions and Removal by Ash in Coal-Fired Oxy-fuel Combustion

This paper presents results of experiments performed at a 20 kW once-through combustion rig of the Institute of Combustion and Power Plant Technology (IFK) of the University of Stuttgart. A methodology to investigate oxy-fuel process configurations was used in which impurities were injected to the oxidant gas of the once-through reactor to simulate different extents of oxy-fuel recycle gas treatment. Three Australian coals, which had previously been tested in the Aioi furnace of IHI in Japan, were used in the experiments. A comprehensive set of total (Hgtot), elemental (Hg0), and oxidized (Hg2+) mercury concentrations was measured for various air and oxy-fuel combustion conditions. These data enable an evaluation of process parameters that influence the Hg emissions of an oxy-fuel combustion process. A theoretical mass balance between Hg fed to the process (fuel and Hg0 injection) and Hg measured before the filter matched well, indicating that no mercury was captured by fly ash at high temperatures. The capture of Hg0 and oxidized Hg2+ by ash in a baghouse filter has been determined for all experiments. Measured Hg concentrations show an increase when switching from air to oxy-fuel operation for all investigated coals and oxy-fuel settings, even when no additional Hg0 is injected to the oxidant gas. Moreover, the Hg2+/Hgtot ratios in the flue gas are higher during oxy-fuel combustion. The Hg capture by ash in the baghouse filter has been found to reduce the Hg emissions considerably. Reduction rates in a range between 18 and 51% for air and between 11 and 29% for oxy-fuel combustion were observed.

Fly Ash Deposition during Oxy-fuel Combustion in a Bench-Scale Fluidized-Bed Combustor

Ash deposition on heat-exchanger surfaces in boiler systems can cause numerous problems, including slagging, fouling, and corrosion. These deleterious processes can be compounded if the boiler combustion process is changed from air to oxy-fuel. In this paper, fly ash deposition characteristics under both air and oxy-fuel combustion conditions were investigated using a bench-scale fluidized-bed combustor (FBC) based on measurements of ash deposition rates via a temperature-controlled probe. Three different combustion atmospheres were studied, and results demonstrated that, under similar combustion temperature profiles and equivalent fluidization velocities, the deposition rate increased when transitioning from combustion atmospheres consisting of 21% O2/79% CO2 to air to 30% O2/70% CO2. To determine the primary factors associated with the observed variations in deposition rates, the chemical compositions and micromorphologies of ash and fly ash deposits were analyzed by inductively coupled plasma–atomic emission spectrometry (ICP–AES) and scanning electron microscopy (SEM). Particulate matter with aerodynamic diameters less than 10 μm (PM10) was measured by an electrical low-pressure impactor (ELPI), and the particle size distributions (PSDs) and carbon contents of the collected filter ash were also ascertained. The results indicate that the higher deposition propensity associated with a 30% O2/70% CO2 atmosphere can be largely attributed to a wider PSD rather than any changes in the chemical compositions of the fly ash or deposited ash, in which there are no obvious differences between air and oxy-fuel combustion. In addition, the slightly higher concentration of fine particles produced under this atmosphere also promotes the deposition of fly ash.

TG–FTIR analysis of oxidation kinetics of some solid fuels under oxy-fuel conditions

A possible technology that can contribute reduction of carbon dioxide emission is oxy-fuel combustion of fossil fuels enabling to increase CO2 concentration in the exhaust gas by carrying out the combustion process with oxygen and replacing air nitrogen with recycling combustion products to obtain a capture-ready CO2 stream. The laboratory studies and pilot-scale experiments discussed during the last years have indicated that oxy-fuel combustion is a favorable option in retrofitting conventional coal firing. Estonian oil shale (OS) with its specific properties has never been studied as a fuel in oxy-fuel combustion, so, the aim of the present research was to compare thermo-oxidation of OS and some coal samples under air and oxy-fuel combustion conditions by means of thermal analysis methods. Experiments were carried out in Ar/O2 and CO2/O2 atmospheres with two oil shale and two coal samples under dynamic heating conditions. FTIR analysis was applied to characterize evolved gases and emission dynamics. Kinetic parameters of oxidation were calculated using a model-free kinetic analysis approach based on differential iso-conversional methods. Comparison of the oxidation characteristics of the samples was given in both atmospheres and it was shown that the oxidation process proceeds under oxy-fuel conditions by all studied fuels with lower activation energies, however, it can last longer as the same temperatures are compared.

Sulphur Oxide Emissions from Dust-fired Oxy-fuel Combustion of Coal

This article summarizes scientific knowledge and practical experiences on the release and capture of SO2 and SO3 with a special focus on the implications of oxy-fuel combustion conditions on sulphur emission behaviour. Results, obtained at the experimental, 500 kWth atmospheric, pulverized fuel combustion rig (KSVA) of the IFK regarding sulphur oxide (SO2/SO3) emissions and capture behaviour are presented. The experimental plant was operated with pre-dried sulphur rich lignite in air and oxy-fuel combustion mode. The following issues are highlighted in particular:•General and transient behaviour of SO2 emissions under air and oxy-fuel combustion conditions SO3•separation behavior of the ESP under oxy-fuel conditions•Differences of ESP ash qualities from air and oxy-fuel operation

Numerical investigation of the application of high temperature air combustion in an industrial furnace

Numerical simulation of turbulent combustion in the radiation section of an industrial steam cracking furnace is carried out in classical premixed and newly developed high temperature air combustion (HiTAC) regimes. The eddy dissipation concept coupled with the detailed KEE-58 mechanism is employed to resolve turbulence–chemistry interaction and combustion chemistry. Three simulations with highly preheated atmospheric air (19%wt oxygen) and diluted fuel are performed to study the attractive features of the so-called HiTAC regime. It is shown that when the inlet air temperature increases to 1100 K, fuel dilution can provide favorable characteristics of the high temperature air combustion. In this temperature, combustion continues even under extremely fuel-lean conditions such as 1.5%wt of methane in the reactants. Calculation of the Damköhler number determined that a finite-rate reaction zone is volumetrically extended throughout the furnace. Computation of NO x emission from different chemical routes revealed that 80% reduction of NO x formation in fuel-diluted high temperature air combustion regimes could be achieved compared to the normal firing condition. Moreover, the results implied that the emission of NO x in normal firing and high temperature air combustion conditions was mainly attributed to thermal/prompt and prompt/NNH mechanisms, respectively.

Optimum temperature for sulphur retention in fluidised beds working under oxy-fuel combustion conditions

Oxy-fuel combustion is one of the leading options for power generation with CO2 capture. The process consists of burning the fuel with a mixture of nearly pure oxygen and a CO2-rich recycled flue gas, resulting in a product flue gas from the boiler containing mainly CO2 and H2O. Among the possible boiler types, fluidised bed combustors are very appropriate for the oxy-fuel process because they allow the in situ desulphurisation by feeding Ca-based sorbents into the combustor.In this work, the effect of the temperature of the combustor on the retention of the SO2 generated in the combustion of two coals with very different sulphur content (a lignite and an anthracite) has been studied. The experimental facility used was a bubbling fluidised bed (BFB) combustor of ∼3kWth. Tests were conducted under oxy-fuel combustion mode and also under enriched-air combustion mode for comparison reasons. A Spanish limestone “Granicarb” was used as Ca-based sorbent for sulphur retention. The temperatures tested were between 800 and 970°C using Ca/S molar ratios between 0 and 3.It was found that in BFB combustors operating under oxy-fuel combustion conditions the optimum temperature to achieve the highest sulphur retention was 900–925°C, whereas operating with enriched air the optimum combustion temperature was 850–870°C. Working at the optimum temperature, the SO2 retentions were lower in oxy-fuel combustion than in enriched air combustion conditions. It was also observed that working with lignite there was 10–15% of sulphur retention by coal ashes, however, working with anthracite the sulphur retention by coal ashes was negligible. This finding was independent of the combustion mode used, oxy-fuel or enriched air.

Numerical investigation of NO emissions from an entrained flow reactor under oxy-coal conditions

The NO emissions of two anthracitic and three high volatile bituminous coals were experimentally and numerically studied under both air and oxy-fuel combustion conditions in an entrained flow reactor (EFR). Thermal decomposition experiments under N 2 and CO 2 atmospheres were carried out to determine the distribution of fuel-bound nitrogen between the volatile and char and the results compared with those obtained by means of the network pyrolysis model, FG-DVC (Functional Group-Depolymerisation Vaporisation Cross-linking). This code was also used as a pre-processing stage to predict the evolution of HCN and NH 3 during devolatilisation of the coals. A Computational Fluid Dynamic (CFD) model was used to predict NO emissions under different O 2/CO 2 (21–35% O 2) conditions in the EFR. Three different models were used. The first assumed that all of the fuel-bound nitrogen had been converted to HCN. The second assumed that all of the volatile nitrogen would evolve as HCN, and the char–N formed NO by an amount determined by a conversion factor. The third approach was similar to the second but it included NH 3 as a precursor of NO as well. The NO emissions predicted with the third approach were in good agreement with the experimental results. A decrease in NO emissions was observed when N 2 was replaced by CO 2 for the same oxygen concentration for both the experimental and computed results. Higher NO emissions under O 2/CO 2 conditions were observed when the oxygen concentration was 30 or 35%.