Oxy-steam Combustion Research Articles

NO Emission Characteristics of Pulverized Coal Combustion in O2/N2 and O2/H2O Atmospheres in a Drop-Tube Furnace.

Oxy-steam combustion is a new oxy-fuel combustion technology. This paper focuses on the NO emission characteristics during the combustion of SF (Shen Fu) coal in O2/N2 and O2/H2O mixtures. Experiments were performed in a drop-tube furnace. Combustion tests were carried out in O2/N2 and O2/H2O atmospheres for various O2 concentrations (21%, 30%, 40%, and 60%) at different temperatures (1173 K, 1273 K, and 1373 K). In addition, combustion experiments at different excess oxygen ratios (λ) were conducted in O2/N2 and O2/H2O atmospheres. The influences of the atmosphere, oxygen concentration, temperature, and excess oxygen ratio on NO emissions were analyzed. The results show that the NO concentrations of SF coal combustion in the 21% O2/79% H2O atmosphere were much lower than those in the 21% O2/79% N2 atmosphere at the three temperatures considered. This was because a large amount of NO was decomposed during the SF coal combustion in the O2/H2O atmospheres. The reasons for the decomposition of NO include the selective non-catalytic reaction (SNCR) mechanism and char's important role as a catalyst for the destruction of NO, either directly or by reacting with CO or H2. In oxy-steam combustion, the NO concentrations significantly increased with the increase in the oxygen concentration from 21 vol.% to 60 vol.% and the temperature from 1173 K to 1373 K. The excess oxygen ratio (λ) slightly impacted the NO emissions in the O2/H2O atmosphere.

Integration of an Autothermal Outer Electrified Reformer Technology for Methanol Production from Biogas: Enhanced Syngas Quality Production and CO2 Capture and Utilization Assessment

Biogas has emerged as a valid feedstock for biomethanol production from steam reforming. This study investigates an alternative layout based on an auto-thermal electrified reforming assuming a 1 MW equivalent anaerobic digestion plant as a source for methanol synthesis. The process considers an oxy-steam combustion of biogas and direct carbon sequestration with the presence of a reverse water–gas shift reactor to convert CO2 and H2 produced by a solid oxide electrolyzer cell to syngas. Thermal auto-sufficiency is ensured for the reverse water–gas shift reaction through the biogas oxy-combustion, and steam production is met with the integration of heat network recovery, with an overall process total electrical demand. This work compares the proposed process of electrification with standard biogas reforming and data available from the literature. To compare the results, some key performance indicators have been introduced, showing a carbon impact of only 0.04 kgCO2/kgMeOH for the electrified process compared to 1.38 kgCO2/kgMeOH in the case of biogas reforming technology. The auto-thermal electrified design allows for the recovery of 66.32% of the carbon available in the biogas, while a similar electrified process for syngas production reported in literature reaches only 15.34%. The overall energy impact of the simulated scenarios shows 94% of the total energy demand for the auto-thermal scenario associated with the electrolyzer. Finally, the introduction of the new layout is taken into consideration based on the country’s carbon intensity, proving carbon neutrality for values lower than 75 gCO2/kWh and demonstrating the role of renewable energies in the industrial application of the process.

Insights into optimal gas-ash-energy nexus: Oxy-steam combustion of spent pot lining

Altering the atmosphere type may boost the physicochemical drivers of the combustion, thus contributing to the circularity and sustainable management of hazardous wastes. This study aimed to compare the combustions of spent pot lining (SPL) in the oxy-steam and air atmospheres. The SPL combustion with 21% (Oxy21), 30% (Oxy30), and 40% (Oxy40) O2 in the oxy-steam atmosphere improved the performance indices. Weighted mean activation energy (Em) fell from 148.36 kJ/mol in the air atmosphere to 118.04 kJ/mol in Oxy21, with the lowest Em value occurring in Oxy30 (referred to as energy-saving pathway). With the temperature rise, the pattern of fluorine distributions in the air atmosphere (the oxy-steam atmosphere) exhibited 65.60 → 92.27% (65.87 → 9.08%) of fluorine in bottom ash, 7.33 → 31.84% (22.31 → 72.83%) in off-gas, and 0.4 → 2.56% (11.82 → 22.22%) in fly ash. The increased O2 concentration slightly affected the combustion characteristic indices and fluorine distributions. Steam appeared to enhance the pore expansion. The advance decomposition of Na3AlF6, the new emergence of NaAlO2, and the disappearance of CaF2 characterized the oxy-steam combustion of SPL. The gas-ash-energy nexus was jointly optimized via artificial neural network. The oxy-steam combustion destroyed the stability of both soluble and insoluble fluorine, thus greatly releasing fluorine-bearing gas.

Oxy-combustion characteristics of torrefied biomass and blends under O2/N2, O2/CO2 and O2/CO2/H2O atmospheres

The combined use of bio-fuels along with CO2 capture techniques is the basis for the so-called negative emissions energy systems. In this paper, oxy-fuel combustion of two torrefied biomasses is experimentally investigated in a lab-scale entrained flow reactor. The torrefied biomasses are fired alone, and co-fired with coal (50%). Two oxygen concentrations (21% and 35%) and four steam concentrations are tested: 0% (dry recycle oxy-combustion), 10% (wet recycle oxy-combustion), 25% and 40% (towards the concept of oxy-steam combustion). The tests are designed to get the same mean residence time for all the fuels and conditions. Burnout degrees are significantly increased (up to 9 and 16 percentage points) when the share of torrefied biomass is raised, with a slightly better behavior of the torrefied pine in comparison to the torrefied agro-biomass. C-fuel conversion to CO2 follows a similar trend to the observed for the burnout degrees. NO formation rates are reduced when oxy-firing torrefied biomass alone in comparison to the blends, with maximum diminutions of 16.9% (torrefied pine) and 8.5% (torrefied agro-biomass). As regards the effect of steam, the best results are found for the 25% H2O atmospheres in most of the cases, yielding maximum conversions along with minimum NO levels.

Effect of CO2/H2O addition on laminar diffusion flame structure and soot formation of oxygen-enriched ethylene

Oxygen-enriched combustion technology has an important application prospect, in which Oxy-steam combustion is a new generation of potential carbon capture technology. In this paper, the effects of O2/CO2 and O2/H2O atmosphere on soot formation in laminar diffusion flame were studied. The flame image was taken by a color (CCD) camera in the visible band, the directional emission power in R and G bands was calibrated, and the two-dimensional distribution of temperature and soot volume fraction in the flame was reconstructed. The experimental results are compared with the numerical results, and the numerical results well predict the temperature of oxygen-enriched flame in O2/N2/CO2 atmosphere, but overestimate the soot volume fraction. The laminar diffusion flame of ethylene in two kinds of oxygen-rich atmosphere was numerically studied by using a step-by-step decoupling method to separate the density, transport, thermal and chemical effects of the diluent by introducing four virtual substances. The calculation results show that combustion in O2/H2O and O2/CO2 atmosphere reduces the flame peak temperature, but the effect on the flame centerline temperature distribution is different. The addition of H2O enhanced the concentration of OH through the reverse reaction rate of OH + H2=H + H2O, while the addition of CO2 decreased the concentration of OH.O2/CO2 combustion leads to the increase of flame height and radius, while in O2/H2O atmosphere, the flame height is shortened. Both CO2 and H2O have a good inhibitory effect on soot formation. CO2 in the oxidant affects soot nucleation and surface growth rate mainly by reducing flame temperature and H mole fraction, but not by promoting soot oxidation process. Water vapor also inhibits soot formation by increasing OH concentration through chemical effect.

Oxy-steam combustion of torrefied biomass under high water vapour concentrations

Oxy-steam combustion of torrefied biomass under high water vapour concentrations

The role of H2O in NO formation and reduction during oxy-steam combustion of bituminous coal char

To improve the CO2 capture efficiency and reduce energy penalties in conventional oxy-fuel combustion systems, one possibility is to eliminate the use of recycled flue gas, by using steam and O2. This is known as oxy-steam combustion. However, the role of H2O in NO formation and reduction under oxy-steam combustion is not yet clear. Here, NO emissions during oxy-steam and air combustion of coal char were compared using both experiments and theoretical calculations, with an emphasis of the role of H2O and its resulting radicals in NO formation and reduction reactions. High concentration of H2O promotes both OH radicals and hydroxyl groups formation during oxy-steam combustion, which in turn effects NO formation and reduction reactions and eventually NO emissions. Specifically, NO formation under air combustion is largely determined by HCN conversion to NCO and then NO, whereas under oxy-steam combustion, high concentrations of OH radicals cause HCN to form NH2 and eventually NO as the dominant route. However, the conversion of HCN to NH2 and then NO is faster, and hence these OH radicals accelerate HCN oxidation to NO under oxy-steam combustion. Given that OH radicals and hydroxyl groups have little influence on NO reduction to N2, there is more NO production in oxy-steam combustion that air combustion.

The role of H2O in structural nitrogen migration during coal devolatilization under oxy-steam combustion conditions

Oxy-steam combustion is a promising technology for reducing CO2 emissions. During coal devolatilization under oxy-steam condition, the high steam concentration has a considerable influence on the migration of structural nitrogen. In this paper, the transformation of nitrogenous functionalities and the release of nitrogen-containing precursors in N2 and N2/H2O atmospheres at 1173–1373 K were measured experimentally. Using density functional theory calculation, the reaction networks of nitrogen migration under N2 and N2/H2O conditions were established, with the corresponding thermodynamics data. On this basis, a detailed kinetics analysis was conducted. Results showed that compared to a N2 atmosphere, the char prepared in a N2/H2O atmosphere exhibits a lower content of pyridinic N and higher contents of quaternary and pyrrolic N. This is because high OH radical concentrations inhibit the transformation of N-Q to N-6. Moreover, the adsorption of OH on pyridinic N leads to the formation of 2-pyridone, thereby making the conversion of N-6 to N-5 feasible. Furthermore, the decomposition of both N-6 and N-5 leads to the formation of HCN, while N-5 is the major source of NH3. In this process, the presence of hydroxyl groups inhibit the release of HCN and NH3, leading to lower emissions under N2/H2O than N2 conditions.

Experimental and numerical modelling of solid and hollow biomass pellets high-temperature rapid oxy-steam combustion: The effect of integrated CO2/H2O concentration

Oxy-steam combustion behaviour of rice straw (RS) pellets was studied at high temperatures experimentally in a new plasma combustion system and numerically using an upgraded one-dimensional unsteady-state single particle combustion model with a new homogeneous devolatilization volatile combustion and heterogeneous char oxidation sub-models under different O2/H2O/CO2/N2 gas compositions. Experimental results showed that the temperature gradient profiles for both the surface and center decreased significantly when the steam was added to the gas mixture at 20 and 40%. At 30% H2O, the heat consumption of the gasification reaction decreased which resulted in a higher particle temperature and a higher char oxidation reaction rate. Combustion characteristics for heating, ignition, devolatization, and combustion stages were obtained and confirmed through the time of recorded pictures by high-speed camera. The obtained experimental results were validated through fitting the measured values of surface and center temperature-time profiles, ignition time, final pellet diameter, and time for the maximum amount of released volatiles, under different H2O and CO2 concentrations, with those predicted from the model solution. Also, the mass of the released volatiles and pellet diameter history were predicted from the model solution. Results of the model solution proved the reality and credibility of the measured and calculated experimental findings.

Wet oxy-combustion of blends of coal and biomass

Oxy-fuel combustion of blends of coal and biomass has been experimentally investigated, largely replacing CO2 with H2O as temperature moderator in the firing atmosphere. The novel concept of the so-called oxy-steam combustion, i.e. combustion in O2/H2O atmospheres, merges the advantages of reducing auxiliary consumptions and lowering air in-leakages. Two blends of coal and biomass have been tested, in 80:20 mass ratio. Forestry wood residues from pine and raw residues from vineyard pruning have been selected. The experiments have been carried out in lab-scale electrically-heated entrained flow reactor for a set of O2/CO2 and O2/CO2/H2O atmospheres, with steam contents up to 40 % and oxygen contents up to 35%. When 20% of coal is substituted with biomass, fuel conversion rates increase and NO formation rates decrease, with a different extent depending on the biomass type. The higher volatile and lower nitrogen contents of the biomass determine that observation. As regards the effect of CO2 replacement by H2O, it can be observed that particular steam concentrations lead to the maximum fuel conversion and the minimum NO formation rates. This behavior is related to the effects of steam when replacing carbon dioxide: increase of flame temperature, oxidant diffusivity and char gasification, as well as active participation in NOx reduction mechanisms in the gas-phase.

Mechanism of kaolinite’s influence on sodium release characteristics of high-sodium coal under oxy-steam combustion conditions

Oxy-steam combustion technology with high steam concentration is one of Oxy-fuel combustion. In terms of the distinct physicochemical properties of steam, the high steam concentration substantially affected the combustion by changing the heat capacity, radiation and gasification. This work aims to numerically investigate characteristics of Na under Oxy-steam atmosphere, mainly related to temperature, steam concentration, particle size and modification methods of kaolinite. Results indicated that the high temperature promoted the release of Na: compared with 750 °C, the Na content in ash at 950 °C has dropped by more than 27%. With the increase of particle size, the Na content increased obviously: when the particles increased from 0.25 ~ 0.27 mm to 1.7 ~ 4.75 mm, Na content increased by about 14%. Furthermore, steam enhanced pore structure of the particles, promoted the release of Na inside the coal. Compared with the steam concentration of 45%, the Na content at 70% was reduced by about 10%. The sequence of adsorption for Na was: KAc-kao > C-kao > R-kao > H-kao. KAc-kao has significantly increased the W-soluble and Insolubel content, about 27% and 65% respectively. Monte Carlo simulation results indicated that temperature showed significant influence on NaCl adsorption by KAc-Kao: temperature had obvious influence on adsorption. Concentration of steam exhibited negligible to NaCl adsorption. When the distance between layers was less than 9.8 Å, NaCl couldn’t enter the kaolinite’s layer. In the range of 11.5 Å to 13.3 Å, NaCl could enter and form adsorption. In synergy with experimental results discussed hereinbefore, some interesting points of NaCl behaviors in Kaolinite were obtained.

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

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

Oxy-steam combustion: The effect of coal rank and steam concentration on combustion characteristics

This paper addresses the experimental investigation of coal combustion characteristics (ignition, burnout and NO formation) under oxy-steam combustion conditions. Two coals are selected in order to compare the effect of the rank: bituminous and sub-bituminous ones. The experiments have been conducted in an electrically-heated entrained flow reactor for a set of O2/N2, O2/CO2 and O2/H2O/CO2 atmospheres, with O2 concentrations up to 35% and H2O concentrations up to 40%. Regarding ignition, 10% H2O reduces ignition temperature (max. 16–19 K) but the trend is reversed when supplying additional steam to 25% and 40%. This behaviour is similar for both coals, with slight larger variations in the case of the low rank coal. Burnout degree of the sub-bituminous coal is barely affected by the steam concentration since all observed conversions are very high. Larger increments (up to 6.1 percentage points) are obtained for the bituminous coal, with a maximum burnout degree for the 25/35% H2O/O2 atmosphere. A very different effect of steam on NO formation is found depending on the coal rank. Significant reduction rates are observed for the bituminous coal in comparison to the dry O2/CO2 atmospheres, with a maximum diminution of 24% when 40% H2O replaces CO2. On the contrary, the higher volatile content in the sub-bituminous coal leads to NO increments up to 9%. For all the combustion characteristics studied, the increase of O2 concentrations attenuates the effects caused by the steam addition.

Numerical Investigation of the Effect of CO2/H2O Composition in Oxidizer on Flow Field and Combustion Behavior of Oxy-pulverized Coal Combustion

ABSTRACT Oxy-coal combustion is the most promising technology for the reduction of greenhouse gases from pulverized coal-fired power plants. Oxy-coal combustion employs recirculated flue gas mainly consisting of CO2 and water vapor as a diluent. Thus, the pulverized coal particles are surrounded and burned under steam rich atmosphere. The addition of H2O would have a significant impact on the combustion characteristics of oxy-coal combustion. The chemical and physical properties of steam are different than the CO2, replacement of CO2 with steam will alter heat capacity, gasification, and radiation properties considerably. The present article numerically compares ideal dry recycle oxy-coal combustion (0% H2O) with wet recycle oxy-coal (10-50% H2O) and oxy-steam combustion (H2O replaces whole CO2 from oxidant) in terms of flow field, temperature distribution, oxidizer distribution, radiative heat transfer, char consumption, and species concentration. Higher flame temperatures under enriched steam oxy-coal combustion cases were found due to lower volume heat capacity of H2O than CO2. Steam enrichment also enhanced char gasification reaction, which has affected temperature distribution and incident radiation profile inside the combustion chamber. Peak temperature obtained under oxy-steam case is around 10% higher than ideal dry recycle case (0% H2O) and 2-5% higher than the wet oxy-coal combustion cases having 50-10% H2O in the oxidizer.

Effect of CO2 and H2O on Char Properties. Part 2: In Situ and Ex Situ Char in Oxy-Steam Combustion

The physical and chemical properties of H2O (oxy-steam combustion) and N2 (air combustion) are different, which has influence on the char properties in oxy-steam combustion. Measuring in situ char (without any cooling) kinetic parameters is very important, especially at oxy-steam atmosphere. Both in situ and ex situ char properties were investigated in this paper. Ex situ char was obtained at different cooling rates. Char properties were measured by X-ray diffraction, H/C ratio, Raman spectrum, and specific surface area. The effect of steam on in situ and ex situ char combustion kinetics were determined. We found that the combustion reaction rates of different chars are in order of in situ > rapid cooling (103–104 K/s) > medium cooling (10–100 K/s) > slow cooling char (0.1 K/s). The rapid cooling char structure is disordered and has a high active surface area, which results in the high reactivity. In situ char has a shorter burn-out time, a faster combustion rate, and a higher reaction constant. For example, at 903 K, it appears the reaction rate of 0.048 s–1 for ex situ char (Vientiane coal char) in oxy-steam atmosphere (30% O2 + 15% H2O), while that of in situ char is 0.07 s–1. The reactivity of in situ char is higher than that of ex situ char. The activation energy of in situ char (e.g., 20.9 kJ/mol) is much lower than that of ex situ char (e.g., 82.24 kJ/mol) in oxy-steam atmosphere (e.g., 30% O2 + 10% H2O). The reaction reactivity of both ex situ and in situ char increase in oxy-steam atmosphere when compared with those from O2/CO2 combustion.

Microwave gasification and oxy-steam combustion for using the biomass char

To use the biomass char as energy, we carried out a study on biomass carbon receptor-applied microwave reforming to secure energy conversion technology using carbon dioxide, a major greenhouse gas. In the case of the microwave reforming, it was shown that the carbon dioxide can be converted into gas fuel, which is carbon monoxide, and fixed carbon fuel through CO2 gasification reforming on the carbon receptor, and the conversion rate of carbon dioxide was 65% at a reforming temperature of 900 °C. We applied the oxy-steam combustion technology to use the biomass char as a carbonized fuel and performed numerical calculations on the changes in oxygen supply and steam supply to identify these combustion characteristics. As the oxygen supply increased, the combustion temperature remained constant after initially showing the maximum temperature rapidly. Residual char, carbon monoxide and carbon monoxide were also relatively increased in good combustibility and decreased after initial generation. When the steam supply was increased, the temperature increase rate and the maximum temperature were relatively low. Residual char and carbon monoxide were reduced by combustion after remaining until the latter half by combustion.

Experimental study of a single char particle combustion characteristics in a fluidized bed under O2/H2O condition

Oxy-steam combustion is a potential new route for oxy-fuel combustion with carbon capture from coal-fired power plants. In the present work, the combustion behavior of single char particles were investigated in a transparent fluidized bed combustor under different operating conditions (i.e., gas atmosphere, oxygen concentration, coal rank, location, fluidization number, particle size, and bed temperature). Both pre-calibrated two-color pryrometry and a flexible thermocouple were used to measure the char particle temperature in the combustion tests. Results indicated that the pore structure of the char generated in H2O atmosphere was better than that generated in CO2 and N2 atmospheres. As expected, with increase of oxygen concentration, the burnout time (tb) decreased, and the particle temperature (Tp) increased. The sequence of burnout times for different rank coal char particles was: anthracite > bituminous coal > lignite. Interestingly, comparing O2/CO2 and O2/N2 combustion, a shorter tb and a lower Tp of char could be achieved simultaneously in O2/H2O combustion, regardless of location and oxygen concentration. Furthermore, the increase of fluidization number strengthened the mass and heat transfer between the char and the environment, thereby reducing the tb and Tp of char. With increasing of particle size, the Tp slightly decreased, the tb increased markedly, and the gasification reactions became more and more significant. As the bed temperature increased, the gasification rate increased exponentially, and the mass transfer coefficient increased gradually.

Investigating the effect of integrated CO2 and H2O on the reactivity and kinetics of biomass pellets oxy-steam combustion using new double parallel volumetric model (DVM)

Thermal characteristics and kinetic parameters of rice straw (RS) and sawdust (SD) raw materials were investigated under an oxidizing atmosphere using a non-isothermal thermogravimetric analysis (TGA). The effect of H2O and CO2 on the oxy-steam combustion was studied by performing the TGA experiments using a mixture of (O2/CO2/H2O/N2) at different compositions. Kinetic parameters were determined using a volumetric model (VM), random pore model (RPM), double parallel random pore model (DRPM), mixed volumetric random pore model (MVRPM) and a new proposed double parallel volumetric model (DVM). The good fit between the experimental and modeled data was obtained from the parallel and mixed models results when the fractional conversion was used in these models. While the models’ solutions didn’t fit with the experimental results when they were solved based on the conversion rate. The new proposed DVM model gave the best fit with experimental conversion (α). The ignition delayed at high concentration of H2O due to the highest specific heat of H2O that lowered the temperature of the fuel particles. The activation energy and the frequency factor of RS were lower than that of SD. The char conversion rate of SD exceeded that of RS.

New approach to materials behaviour studies in high-speed flue gas from oxy-steam combustion

Some new proposals are being considered aiming at improving the efficiency and operability of oxy-combustion power plants, such as the so-called oxy-steam combustion. This process consists in replacing the carbon dioxide by steam in the firing atmosphere. One of the main uncertainties for the feasibility of the oxy-steam technology is the materials resistance to steam corrosion at high temperature, mainly due to chromium vaporization. The novel approach to the issue presented in this work is the generation of realistic flue gas velocity (20 m·s−1) in flames produced with a thermal spray gun using H2 as fuel. Gas composition was selected to match expected final H2O content after combustion: 40 and 60% H2O in CO2 with excess of O2. A set of metallic steels and alloys (SS304, SS310, I800HT, Kanthal and IN617) was preoxidised in air to generate external oxide scales. After preoxidation, the samples were treated under steam flames at 700 °C, placed in a position either parallel or perpendicular to the flame axis. Surface oxides composition was compared to that obtained in a quasi-static furnace, where metallic samples were exposed to steam atmosphere at 700 °C for 20 h (50% steam, 45% CO2 and 5% O2). The characterisation of the surfaces was performed using low-angle XRD and SEM-EDX for a precise measuring of the variation of oxide amount and chromium content. The analyses showed that mixed Cr-Mn and Cr-Fe external oxides formed during the preoxidation stage suffered the depletion of the surface chromium when they were exposed parallel to the steam flame axis, as a simulation of the lateral flue gas pass in the heat-exchanger tubing. Cr depletion was also observed in the windside of tubes (perpendicular to flame axis) but in minor extent. The reduction in the Cr amount of the samples when tested in the vapour furnace was negligible compared to the Cr loss found after treatment under high velocity flames. The presence of external (Mn,Cr)3O4 in preoxidised SS310 and SS304 did not provide an effective protection against Cr volatilization in the steam flame treatment, whereas surface alumina in Kanthal seemed to prevent Cr volatilization.

Effect of steam concentration on demineralized coal char surface behaviors and structural characteristics during the oxy-steam combustion process

Abstract Oxy-steam combustion, fuel combusts in O2/H2O atmosphere, is treated to be a novel and promising technology for the next generation oxy-fuel combustion. The purpose of this study is to identify the effects of steam concentration on NO emission characteristics, functional group distribution and char NO reducibility during oxy-steam combustion process. A typical bituminous coal (SH) was employed for demineralization, devolatilization, combustion in O2/H2O environment, and investigated by Temperature Programmed Desorption/Reduction (TPD/TPR) methods and Raman spectrometer. The combustion results illustrated that char N/NO conversion during the oxy-steam combustion tests initially decreased with an increasing steam concentration in low range (1.2–8.5 vol.%), while increased apparently in high steam range (8.5–20 vol.%). Decomposition of H2O molecules promoted the formation of C(O) and small aromatic rings on particle surface at the low steam range; the high concentration of steam could promote the condensation of aromatic ring structures in char, leading to significant generation of large aromatic ring structures with a lower reactivity and the reduction of surface active sites (Cf). The TPD results illustrated that the chars oxidized at moderate steam concentration (8.5 vol.%), and the chars with moderate burnout degrees (such as S3, S6 and S11) had the largest amount of C(O). Due to the decomposition of the C(O) into new Cf and massive CO molecules, a rapid acceleration of NO reduction rate occurred in the temperature range of 1000–1400 K during the TPR process of each sample obtained from identical reaction conditions. Thus, variations in char surface behavior and microstructure might be the primary reasons affecting char-N emission.