Oxy-steam Combustion Research Articles

Towards oxy-steam combustion: The effect of increasing the steam concentration on coal reactivity

Oxy-steam combustion and devolatilization performance of three different coals (anthracite, blend of bituminous coals and low-rank coal) was studied in a thermobalance with a water vapour furnace under variable steam concentrations (0, 20, 40 and 70 vol% H2O) in N2 and CO2. Two oxygen concentrations were used during the combustion tests (20 and 30 vol%). Devolatilization behaviour was similar under N2 and CO2 atmosphere and increasing steam concentration did not affect significantly the observed reactivity and the reaction temperature range. Replacing CO2 with 20 vol% H2O during combustion tests produced a decrease in ignition and burnout temperatures. When H2O content increased to 40 and 70 vol%, this effect was only found with high-rank anthracite. Maximum mass loss rates for the low-rank coal with high volatile content were up to four times higher than with the other two coals. During direct oxidation experiments, DTG curves of anthracite and the coal blend showed a double peak, corresponding to devolatilization and oxidation reactions. This allowed determining independent kinetic parameters (Ea and A) for both stages. An only DTG peak was detected with the low-rank coal since devolatilization and oxidation reactions took place simultaneously owing to its high reactivity. Devolatilization and oxidation kinetics followed a first-order reaction using the Coats-Redfern integral method. Significant differences were not observed between the activation energies of the three coals when comparing conventional (N2), oxy-fuel (CO2) and oxy-steam conditions (70 vol% H2O in CO2), although Ea values were higher for the devolatilization stage than for the oxidation process.

Experimental investigation of ignition and combustion characteristics of single coal and biomass particles in O2/N2 and O2/H2O

Oxy-steam combustion is a potential new-generation option for CO2 capture and storage. The ignition and combustion characteristics of single coal and biomass particles were investigated in a flow tube reactor in O2/N2 and O2/H2O at various oxygen concentrations. The ignition and combustion processes were recorded using a CCD camera, and the two-color pyrometry was used to estimate the volatile flame temperature and char combustion temperature. In O2/N2 and O2/H2O, coal ignites heterogeneously at <O2> = 21–50%. In O2/N2, biomass ignites homogeneously at <O2> = 21–30%, while it ignites heterogeneously at <O2> = 40–50%. In O2/H2O, biomass ignites homogeneously at <O2> = 21–50%. With increasing oxygen concentration, the ignition delay time, volatile burnout time and char burnout time are decreased, and the volatile flame temperature and char combustion temperature are increased. At a certain oxygen concentration in both atmospheres, the ignition delay time, volatile burnout time and char burnout time of biomass are shorter than those of coal. Moreover, biomass has a higher volatile flame temperature but a lower char combustion temperature than coal. The ignition delay time, volatile burnout time and char burnout time in O2/H2O are lower than those in O2/N2 for coal and biomass. The presence of H2O can improve the combustion rates of coal and biomass. The volatile flame shows a lower temperature in O2/H2O than in O2/N2 at <O2> = 21–50%. The char combustion shows a lower temperature in O2/H2O than in O2/N2 at <O2> = 21–30%, while this behavior is switched at <O2> = 40–50%. The results contribute to the understanding of the ignition and combustion characteristics of coal and biomass in oxy-steam combustion.

Conversions of fuel-N, volatile-N, and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O at low temperature

Abstract Oxy-steam combustion is a promising next-generation combustion technology. Conversions of fuel-N, volatile-N, and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O were studied in a tube reactor at low temperature. In O2/N2, NO reaches the maximum value in the devolatilization stage and N2O reaches the maximum value in the char combustion stage. In O2/H2O, both NO and N2O reach the maximum values in the char combustion stage. The total conversion ratios of fuel-N to NO and N2O in O2/N2 are obviously higher than those in O2/H2O, due to the reduction of H2O on NO and N2O. Temperature changes the trade-off between NO and N2O. In O2/N2 and O2/H2O, the conversion ratios of fuel-N, volatile-N, and char-N to NO increase with increasing temperature, and those to N2O show the opposite trends. The conversion ratios of fuel-N, volatile-N, and char-N to NO reach the maximum values at = 30 vol% in O2/N2. In O2/H2O, the conversion ratios of fuel-N and char-N to NO reach the maximum values at = 30 vol%, and the conversion ratio of volatile-N to NO shows a slightly increasing trend with increasing oxygen concentration. The conversion ratios of fuel-N, volatile-N, and char-N to N2O decrease with increasing oxygen concentration in both atmospheres. A higher coal rank has higher conversion ratios of fuel-N to NO and N2O. Anthracite coal exhibits the highest conversion ratios of fuel-N, volatile-N, and char-N to NO and N2O in both atmospheres. This work is to develop efficient ways to understand and control NO and N2O emissions for a clean and sustainable atmosphere.

Comparison of the Reburning Chemistry in O2/N2, O2/CO2, and O2/H2O Atmospheres

The reburning chemistry in oxy-fuel and oxy-steam combustion of methane was investigated both experimentally and numerically. Comparison experiments in O2/N2, O2/CO2, and O2/H2O atmospheres were performed in a flow reactor at atmospheric pressure with equivalence ratio ranging from fuel-rich to fuel-lean and temperature from 973 to 1773 K. Experimental results showed that compared with N2 and CO2 atmospheres NO reduction observed in H2O atmosphere is the lowest under fuel-rich and stoichiometric conditions, while it is the highest under fuel-lean conditions. The NO reduction intensity in CO2 atmosphere lies between N2 and H2O atmosphere under fuel-rich and fuel-lean conditions; however, it is the highest under stoichiometric conditions. A chemical kinetic mechanism, which was hierarchically structured and updated in our previous work, captured the main characteristics and quantity of CO and NO formation satisfactorily even under fuel-lean conditions. According to the analysis from a chemical kinetic point...

Effects of steam and CO2 on the characteristics of chars during devolatilization in oxy-steam combustion process

Oxy-steam combustion is considered as one of promising technologies for the next generation oxy-fuel combustion, in which coal mainly burns in O2/H2O atmosphere. This work aims to investigate the devolatilization characteristics of coal in oxy-steam combustion and clarify the potential effects of steam and CO2 on the char characteristics. Chars of a Chinese lignite were prepared in a fixed-bed reactor at 1050°C under various steam/CO2/N2 atmospheres and characterized by a FT-IR/Raman spectrometer and thermogravimetric analyzer. The results indicate the char yield during devolatilization in oxy-steam combustion was lower than that in N2 atmosphere, and the steam gasification reaction played the key role. The high concentration of steam would significantly accelerate the condensation of aromatic rings in the char, and more condensed char would form during devolatilization in oxy-steam combustion compared to that in N2 atmosphere. CO2 in high concentration of steam had little effects on the char yields but it could participate in the cross-linking reactions on the char surface and partly reduce the condensation of the chars during devolatilization. Steam and CO2 gasification reactions can not only speed up the consumption of the original O-containing functional groups in the coal but also bring some additional O-containing functional groups in the char. The reactivity of the char formed during devolatilization in oxy-steam combustion was lower than that in N2. A good linear relationship between the Raman band area ratio I(Gr+Vl+Vr)/ID and char reactivity confirmed the decrease of the char reactivity was mainly attributed to the condensation of the char.

Experimental and Numerical Study of the Effect of High Steam Concentration on the Oxidation of Methane and Ammonia during Oxy-Steam Combustion

The effect of high H2O concentration during oxy-steam combustion on the oxidation of methane and ammonia was investigated both experimentally and numerically. Comparison experiments between O2/N2 and O2/H2O atmosphere were performed in a flow reactor at atmospheric pressure covering fuel-rich to fuel-lean equivalence ratios and temperatures from 973 to 1773 K. Experimental results showed that the presence of high H2O concentration dramatically suppressed CO formation at temperatures above 1300 K. High H2O concentrations inhibited NO formation under stoichiometric and fuel-lean conditions but enhanced NO formation under fuel-rich conditions. The chemical kinetic mechanism, which was hierarchically structured and updated, satisfactorily reproduced the main characteristics of CO and NO formation. High H2O concentrations significantly alter the structure of radical pool and subsequently the formation of CO and NO. Ultralow CO concentrations above 1300 K are attributed to the enhancement of CO + OH ⇄ CO2 + H b...

Limestone Decomposition in an O2/CO2/Steam Atmosphere Integrated with Coal Combustion

Ca looping is one of the most promising technologies for CO2 capture. Calcined limestone can be used in the Ca-looping system. It is a low-cost approach to decompose limestone integrated with coal combustion in an O2/CO2/steam atmosphere. In this approach, coal combustion heat is continuously supplied for limestone decomposition. We aim to obtain high-purity CO2 exhaust gas and high-reactivity CaO sorbent. High-purity CO2 exhaust gas is then further compressed or used. High-reactivity sorbent is then used to capture CO2 from the coal power plant. However, little research was found to investigate limestone decomposition behavior under oxy-steam combustion conditions. In this study, limestone and coal particles were mixed together and decomposed/combusted in a continuously operating fluidized bed reactor in an O2/CO2/steam atmosphere. The results show that increasing O2 and steam supply reduces the emission of CO, H2, and SO2, which enhances the purity of CO2 exhaust gas. Limestone decomposition conversion ...

A study of coal chars combustion in O2/H2O mixtures by thermogravimetric analysis

Oxy-steam combustion is a new oxy-fuel combustion technology which involves fuels burn in pure oxygen, and the high temperature is moderated using either water or steam. In this study, FG and XS char samples were prepared in a horizontal tube furnace at 1073 K under argon atmosphere. The combustion characteristics and kinetic parameters of FG and XS char in O2/H2O atmosphere were studied using non-isothermal thermogravimetric analysis. The results indicated that replacing N2 by H2O caused the improved in the combustion reactivity and performance of FG and XS char with the identical oxygen concentration. The ignition temperature, peak temperature and burnout temperature in O2/H2O atmosphere were lower than those in O2/N2 atmosphere with the identical oxygen concentration. The activation energy values of FG and XS determined by three mode-free methods decreased with the increasing conversion level, and the activation energy of FG char was less than that of XS char at the same conversion. The kinetic mechanism function calculated result based on the combination of the Popescu method and the Coats–Redfern integral method showed the combustion of FG char in O2/H2O atmosphere followed the first-order chemical reaction kinetic.

The characteristics and mechanism of the NO formation during oxy-steam combustion

Abstract Oxy-fuel combustion has been deemed a promising technology to reduce CO2 emissions from coal-fired power stations, and oxy-steam combustion technology is regarded to be the next-generation oxy-fuel combustion technology. The characteristics and mechanism of NO formation were numerically studied in the counter-flow laminar diffusion flame using methane as fuel. The comparisons of the N conversion rate in the oxy-steam combustion with that in air combustion under a similar temperature profile show that the emission of NO is enhanced due to high concentration of H2O during oxy-steam combustion compared with air combustion. The chaperone effect of H2O and a high mole fraction of H2O are important reasons for the enhancement of NO formation during oxy-steam combustion because H + NO + M ⇔ HNO + M and HNO + OH ⇔ NO + H2O are enhanced dramatically. The key elementary reaction for the enhancement of NO production during oxy-steam combustion is NH + H2O ⇔ HNO + H2 because it produces a large amount of HNO due to a high mole fraction of H2O. The nitrogen conversion rate through the pathway NH2 → NH → HNO → NO is dramatically enhanced and predominant during oxy-steam combustion. The nitrogen conversion rate increases with the increase in the mole fraction of H2O in O2, and equivalence ratio has a minimal impact on the NO formation during oxy-steam combustion.

Comprehensive investigation of process characteristics for oxy-steam combustion power plants

Oxy-steam combustion, as an alternative option of oxy-fuel combustion technology, is considered as a promising CO2 capture technology for restraining CO2 emissions from power plants. To attain its comprehensive process characteristics, process simulation, thermodynamic assessment, and sensitivity analysis for oxy-steam combustion pulverized-coal-fired power plants are investigated whilst its corresponding CO2/O2 recycled combustion (oxy-CO2 combustion) power plant is served as the base case for comparison. Techno-economic evaluation and integration with solar parabolic trough collectors are also discussed to justify its economic feasibility and improve its thermodynamic performance further, respectively. It is found that oxy-steam combustion exhibits better performance than oxy-CO2 combustion on both thermodynamic and economic aspects, in which the cost of electricity decreases about 6.62% whilst the net efficiency and exergy efficiency increase about 0.90 and 1.01 percentage points, respectively. The increment of oxygen concentration in oxidant (20–45mol.%) and decrease of excess oxygen coefficient (1.01–1.09) in a certain range are favorable for improving oxy-steam combustion system performance. Moreover, its thermodynamic performance can be improved when considering solar parabolic trough collectors for heating recycled water, even though its cost of electricity increases about 2$/(MWh).

Numerical and experimental studies on the ignition of pulverized coal in O2/H2O atmospheres

Oxy-fuel combustion is regarded as a promising technology for CO2 abatement in coal-fired power generation. Oxy-steam combustion technology is regarded as the nextgeneration of oxy-fuel combustion technology because it has numerous advantages over O2/CO2 recycled combustion. The ignition of coal particles was investigated using numerical and experimental analysis in O2/N2 and O2/H2O atmospheres with different oxygen concentrations (21%, 30%, 35%, 40% and 50%). The experiments were performed in a drop tube furnace (DTF) using a high-speed camera. The experimental results show that the pulverized coal ignites homogeneously in both the O2/H2O and O2/N2 atmospheres, and the ignition of pulverized coal occurs sooner in the O2/H2O atmospheres than in the O2/N2 atmosphere with an identical O2 mole fraction. The model proposed in the paper was validated by comparison of the predicted locations of the coal ignition with the experimental results. The effects of the physicochemical properties of H2O on the ignition of coal were analyzed in detail by numerical simulation. The results of numerical simulation showed that the steam shift reaction is the primary reason for the advanced ignition of coal in the O2/H2O atmospheres compared with the corresponding O2/N2 atmospheres, and the steam gasification reaction is a minor reason for the phenomena because it produces CO and H2, which are beneficial in the ignition.

Ignition behaviors of pulverized coal particles in O2/N2 and O2/H2O mixtures in a drop tube furnace using flame monitoring techniques

Oxy-steam combustion technology is regarded to be the next generation of oxy-fuel combustion technology. Because the physical and chemical properties of steam are very different from that of CO2 or N2, especially its reactivity, the ignition characteristics of coal in oxy-steam combustion are expected to be different from those in conventional combustion or O2/CO2 recycled combustion. This paper presents experimental investigations of the ignition behaviors of coal in O2/N2 and O2/H2O atmospheres using different oxygen fractions (21%, 30%, 35%, 40% and 50%). Ignition experiments were performed in a drop tube furnace. The ignition behaviors were recorded using a high-speed camera and a two-color pyrometer. According to the images of the ignition, the pulverized coals ignite homogeneously in all cases. The ignition of pulverized coal in the O2/H2O atmospheres occurs sooner than in the O2/N2 atmospheres at identical oxygen fractions. A high concentration of steam may enhance the steam gasification reaction (C+H2O→CO+H2) and the steam shift reaction (CO+H2O→CO2+H2) in O2/H2O atmospheres. These two reactions can form a large amount of H2 and CO in O2/H2O atmospheres in the gaseous mixture around the coal particle. This result is beneficial to reducing the ignition delay times of volatiles mixtures in O2/H2O atmospheres.

Simulation and comparative exergy analyses of oxy-steam combustion and O2/CO2 recycled combustion pulverized-coal-fired power plants

Oxy-steam combustion is proposed as an alternative for the next generation oxy-fuel combustion technology. In order to justify the economic feasibility of this novel oxy-fuel combustion, a conventional 600MWe pulverized-coal-fired power plant in China is selected to be retrofitted to oxy-steam combustion and O2/CO2 recycled combustion separately, whilst the O2/CO2 recycled combustion system serves as the base case for comparisons. The modeling and simulation of these two systems are carried out with Aspen Plus version 11.1. Simulation results indicate that on the same boundary conditions, the mass flow of the main steam and reheated steam in the oxy-steam combustion system must be reduced by 0.84%, which makes the net energy efficiency of the oxy-steam combustion system decrease by 0.15%. The comparative exergy analyses reveal that the net exergy efficiency of the oxy-steam combustion system is 0.153% lower than that of the O2/CO2 recycled combustion system. The marked increase of the exergy destruction happening in the oxy-steam combustion furnace is the fundamental cause of the decrease of the exergy efficiency.

The chemical mechanism of steam’s effect on the temperature in methane oxy-steam combustion

A numerical study with a detailed chemistry mechanism has been conducted to investigate the chemical effects of steam on the temperature in methane oxy-steam combustion; steam is used to moderate the high flame temperature produced by combustion of the fuel using oxygen. The temperature profiles of the oxy-steam combustion are consistent with those of traditional combustion when the mole fraction of steam in the oxidant is 72.5%. The comparisons between the maximum temperatures when steam is added and the corresponding artificial material X addition reveal that when the mole fraction of H2O (or X) is between 0.60 and 0.66, the overall chemical effects of the steam peak. High steam concentration in the atmosphere enhances the three-body reaction, raises the concentration of OH, and lowers the concentration of H during oxy-steam combustion, in contrast to traditional combustion. Consequently, when the steam’s mole fraction is between 0.5 and 0.7, the three-body reaction (H+O2+H2O⇔HO2+H2O) (R35) is the key elementary reaction that determines the combustion temperature during oxy-steam combustion; this reaction also provides sufficient HO2 to R287, giving R287 the highest heat release rate. When the steam mole fraction exceeds 0.75, the reduced oxygen mole fraction enhances the rate of CO production. Concurrently, R38 is the key elementary reaction for determining the combustion temperature because it supplies a large amount of radical OH species. Therefore, R99 dominates R287 and has the highest heat release rate. When the steam mole fraction is 0.725, R35 and R38 play a very important role affecting the combustion temperature; R99 makes the largest contribution to temperature, while R287 and R43 also make significant contributions to the temperature.

A study of combustion characteristics of pulverized coal in O2/H2O atmosphere

Oxy-steam combustion technology in which the mixtures of oxygen and steam are used as oxidizer instead of air is a new generation oxy-fuel combustion technology. The combustion characteristics of SH and PDS pulverized coal in O2/H2O mixtures are studied using non-isothermal thermogravimetric analysis technology in this paper. The effect of combustion atmosphere, oxygen concentration, heating rate and particle size on the combustion characteristics of SH and PDS pulverized coal in O2/H2O mixtures are analyzed based on the combustion profiles and kinetic analysis. The results show that replacing the inert N2 gas in the oxidizer with steam delayed the burning process of SH and PDS pulverized coal. With the increasing of the oxygen concentration, the ignition Ti and burnout temperature Th decrease and the comprehensive combustibility index S increases both in O2/H2O and O2/N2 atmosphere. In O2/H2O atmosphere, the combustion of SH and PDS coal occur in a higher temperature region as the heating rate increases, but the increase in heating rate can enhance the coal combustion property. As the particle size decreases, the maximum mass loss rate increases and the ignition and burnout temperature decrease. The results of kinetic analysis indicate that the combustion reactions of SH and PDS pulverized coal in O2/H2O mixtures follow the first-order kinetics through linear fitting of experimental data using Coats–Redfern integral method, and compensation effect exists between apparent activation energy and pre-exponential factor in different oxygen concentrations during coal combustion in O2/H2O mixtures.

Remarks on the different modes of applying power for locomotive purposes

Oxy-steam combustion is a potential new-generation option for CO2 capture and storage. The ignition and combustion characteristics of single coal and biomass particles were investigated in a flow tube reactor in O2/N2 and O2/H2O at various oxygen concentrations. The ignition and combustion processes were recorded using a CCD camera, and the two-color pyrometry was used to estimate the volatile flame temperature and char combustion temperature. In O2/N2 and O2/H2O, coal ignites heterogeneously at <O2> = 21–50%. In O2/N2, biomass ignites homogeneously at <O2> = 21–30%, while it ignites heterogeneously at <O2> = 40–50%. In O2/H2O, biomass ignites homogeneously at <O2> = 21–50%. With increasing oxygen concentration, the ignition delay time, volatile burnout time and char burnout time are decreased, and the volatile flame temperature and char combustion temperature are increased. At a certain oxygen concentration in both atmospheres, the ignition delay time, volatile burnout time and char burnout time of biomass are shorter than those of coal. Moreover, biomass has a higher volatile flame temperature but a lower char combustion temperature than coal. The ignition delay time, volatile burnout time and char burnout time in O2/H2O are lower than those in O2/N2 for coal and biomass. The presence of H2O can improve the combustion rates of coal and biomass. The volatile flame shows a lower temperature in O2/H2O than in O2/N2 at <O2> = 21–50%. The char combustion shows a lower temperature in O2/H2O than in O2/N2 at <O2> = 21–30%, while this behavior is switched at <O2> = 40–50%. The results contribute to the understanding of the ignition and combustion characteristics of coal and biomass in oxy-steam combustion.