Emission characteristics of PAHs, benzene and phenol group hydrocarbons in O2/RFG waste incineration processes

This study applies the oxygen/recycled flue gas (O₂/RFG) combustion technology for waste incineration in a laboratory-scale fluidized bed incinerator to investigate the effects of different RFG percentages and O₂ concentrations on the emission characteristics of organic pollutants (PAHs, phenol and benzene hydrocarbons). Experimental results show that most PAHs with high-ring structures were present in solid-phase and most low-ring PAHs were present in gas-phase. The major compounds of benzene and phenol hydrocarbons were benzene, toluene, trichlorobenzene and 2,4-dinitrophenol, phenol, dichlorophenol, respectively. As the O₂ concentration in feed gas was increased from 21% to 40%, the emissions of solid- and gas-phase PAHs and phenol compounds were decreased but not for benzene compounds. Increasing RFG percentages would decrease the emissions of gas-phase PAHs, benzene and phenol compounds, but increased those of solid-phase pollutants. The best operating conditions of such O₂/RFG combustion system to reduce the emissions of PAHs and phenol compounds were 40% O₂, 35% RFG, and that for benzene compounds was 21% O₂, 75% RFG. Comparing with conventional air combustion system, the best diminution efficiencies of PAHs, benzene and phenol compounds at such O₂/RFG conditions were 59.54%, 70.97% and 52.60%, respectively. With proper feed gas compositions and RFG percentages, the combustion efficiency and destruction efficiency of organic pollutants can be improved by this O₂/RFG combustion technology.     

Combustion characteristics of lignite-fired oxy-fuel flames

This experimental work describes the combustion characteristics of lignite-fired oxy-fuel flames, in terms of temperature distribution, gas composition (O₂, CO₂, CO, total hydrocarbon concentration and NO) and ignition behaviour. The aim is to evaluate the flame structure of three oxy-fuel cases (obtained by changing the flue gas recycle rate) including a comparison with an air-fired reference case. Measurements were performed in Chalmers 100 kW test unit, which facilitates oxy-fuel combustion under flue gas recycling conditions. Temperature, O₂ and CO concentration profiles and images of the flames indicate that earlier ignition and more intense combustion with higher peak temperatures follow from reduction of the recycle rate during oxy-fuel operation. This is mostly due to higher O₂ concentration in the feed gas, reduced cooling from the recycled flue gas, and change in flow patterns between the cases. The air case and the oxy-fuel case with the highest recycle rate were most sensitive to changes in overall stoichiometry. Despite significant differences in local CO concentration between the cases, the stack concentrations of CO are comparable. Hence, limiting CO emissions from oxy-fuel combustion is not more challenging than during air-firing. The NO emission, as shown previously, was significantly reduced by flue gas recycling.     

CO2 capture using oxygen enhanced combustion strategies for natural gas power plants

This paper describes a series of experiments conducted with natural gas in air and in mixtures of oxygen and recycled flue gas, termed O₂/CO₂ recycle combustion. The objective is to enrich the flue gas with CO₂ to facilitate its capture and sequestration. Detailed measurements of gas composition, flame temperature and heat flux profiles were taken inside CANMET's 0.3 MWth down-fired vertical combustor fitted with a proprietary pilot scale burner. Flue gas composition was continuously monitored. The effects of burner operation, including swirling of secondary stream and air staging, on flame characteristics and NO_x emissions were also studied. The results of this work indicate that oxy-gas combustion techniques based on O₂/CO₂ combustion with flue gas recycle offer excellent potential for retrofit to conventional boilers for CO₂ emission abatement. Other benefits of the technology include considerable reduction and even elimination of NO_x emissions, improved plant efficiency due to lower gas volume and better operational flexibility.     

Modeling of oxy-fuel combustion for a western Canadian sub-bituminous coal

The motivation of this research is to develop practical oxy-coal combustion techniques in order to facilitate the conversion of coal-fired utility power plants so as to recover a CO₂ rich flue gas stream for use and/or sequestration. The objective of this study is to ascertain the applicability and accuracy of a modeling tool to assist with future pilot scale oxy-fuel combustion experiments and burner scale-up studies. Two modes of oxy-coal combustion, O₂ enriched air (OEA) and recycled flue gas (RFG), were experimentally tested in a 0.3 MW_th pilot-scale combustor using a western Canadian sub-bituminous coal. The computational fluid dynamic tool was utilized to model the combustion, heat transfer and pollutant formation characteristics of these test cases and to examine the impact due to changes in the combustion medium, burner swirl and burner configuration. The model provided insights for the observed variation in NO_x production among the test cases: the dramatic increase in the OEA mode, the drop at higher burner swirl settings and the surprisingly small reduction in the RFG mode. Overall the model results compared well with measured data in all test cases and established confidence in using the model to explore new design concepts for oxy-coal combustion.     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.     

Experimental investigation of NOx emissions in oxycoal combustion

This work presents the results of an experimental investigation on NO_x emissions from coal combustion in a pilot scale test facility. Three oxidiser atmospheres have been compared, namely air, CO₂/O₂, and O₂ enriched recirculated flue gas. NO_x emissions from two different combustion modes have been studied, swirl flame and flameless combustion. The influence of the burner oxygen ratio and the oxidiser O₂ concentration on NO_x formation and reduction have been analysed. With increasing burner oxygen ratio, an increase of NO_x emissions has been obtained for air and CO₂/O₂ in both, swirl flame and flameless combustion. In case of the swirl flame, flue gas recirculation leads to a reduction of NO_x emissions up to 50%, whereas in case of flameless combustion this reduction is around 40% compared to CO₂/O₂. No significant impact of the oxidiser O₂ concentration in the CO₂/O₂ mixture on NO_x emissions is observed in the range between 18 and 27 vol.% in swirl flames. An analysis of NO_x formation and reduction mechanisms showed, that the observed reduction of NO_x emissions by flue gas recirculation cannot be attributed to the reduction of recirculated NO_x alone, but also to a reduced conversion of fuel-N to NO.     

Combustion characteristics of coal in a mixture of oxygen and recycled flue gas

The combustion of coal in a mixture of pure O₂ and recycled flue gas is one variant of a novel combustion approach called oxy-fuel combustion. With the absence of N₂, this approach leads to a flue gas stream highly enriched in CO₂. For many applications, this flue gas stream can then be compressed and sequestered without further separation. As a result, oxy-fuel combustion is an attractive way to capture CO₂ produced from fossil fuel combustion. When coal is burned in this O₂ and CO₂ rich environment, its combustion characteristics can be very different from conventional air-fired combustion. In CETC-O, a vertical combustor research facility has been used in the past years to investigate the combustion characteristics of several different coals with this variant of oxy-fuel combustion. This included flame stability, emissions of NO_x, SO_x and trace elements, heat transfer, in-furnace flame profiles and flue gas compositions. This paper will report some of the major findings obtained from these research activities.     

The effect of O2 enrichment on NOx formation in biomass co-fired pulverised coal combustion

Oxygen enrichment of the combustion air in pulverised coal combustion for power plant is seen as a possible retrofit measure to improve CO₂ scrubbing and capture. This technique produces a reduced volume of flue gas with higher CO₂ concentration than normal air combustion that will contributes to the enhancement of amine scrubbing plant efficiencies. We report in this article the results of a study at the small pilot scale into the effect of these combustion modifications on the formation of NO_x and associated carbon burnout changes. Experiments were performed using a Russian coal, typical of that used in some UK power stations with shea meal and Pakistani cotton stalk as biomass fuels co-fired at a fraction of 15%th. The down-fired pulverised coal combustor was operated at 20 kWth under air-staged conditions for NO_x control and the secondary and over-fire air flows were both enriched by up to 79% (100% O₂) for a range of splits giving a 35% overall O₂ concentration for full enrichment. When the same enrichment process was applied to biomass/coal combustion different behaviour was observed with respect to NO_x formation. We have shown that oxygen enrichment can achieve benefits of improved carbon burnout with a positive impact on NO_x emissions over and above the primary aim of increasing CO₂ concentration in the flue gas for enhanced capture efficiencies. With all other conditions of overall stoichiometry, OFA levels and O₂ enrichment levels remaining the same, NO_x levels at 22% OFA initially increased over the range of secondary air enrichment, particularly for shea meal/coal co-firing. At 31% OFA the trends were to lower NO_x at high enrichment levels. However, co-firing with shea meal initially showed an increase in NO_x emission at lower levels of enrichment (up to 40% O₂) followed by overall lower NOx emissions at 100% O₂ in the secondary air. The results show that NO_x emissions can either increase or decrease depending on the operating conditions. The differences in behaviour are attributed, not only to the effects of enrichment on the stoichiometry of the near-burner zone, but also on the flame dynamics and intensity of combustion related to the associated reductions in gas velocity and swirl intensity by the transition from air to pure O₂ in the secondary oxidant stream.     

Simultaneous easy CO2 recovery and drastic reduction of SOx and NOx in O2/CO2 coal combustion with heat recirculation

Coal combustion with O₂/CO₂ is promising because of its easy CO₂ recovery, extremely low NO_x emission and high desulfurization efficiency. Based on our own fundamental experimental data combined with a sophisticated data analysis, its characteristics were investigated. It was revealed that the conversion ratio from fuel-N to exhausted NO in O₂/CO₂ pulverized coal combustion was only about one fourth of conventional pulverized coal combustion. To decrease exhausted NO further and realize simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x, a new scheme, i.e. O₂/CO₂ coal combustion with heat recirculation, was proposed. It was clarified that in O₂/CO₂ coal combustion, with about 40% of heat recirculation, the same coal combustion intensity as that of coal combustion in air could be realized even at an O₂ concentration of as low as 15%. Thus exhausted NO could be decreased further into only one seventh of conventional coal combustion. Simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x could be realized with this new scheme.     

Catalytic syngas production from greenhouse gasses: Performance comparison of Ru-Al2O3 and Rh-CeO2 catalysts

In this work, 3% Ru-Al₂O₃ and 2% Rh-CeO₂ catalysts were synthesized and tested for CH₄-CO₂ reforming activity using either CO₂-rich or CO₂-lean model biogas feed. Low carbon deposition was observed on both catalysts, which negligibly influenced catalytic activity. Catalyst deactivation during temperature programmed reaction was observed only with Ru-Al₂O₃, which was caused by metallic cluster sintering. Both catalysts exhibited good stability during the 70 h exposure to undiluted equimolar CH₄/CO₂ gas stream at 750 °C. By varying residence time in the reactor during CH₄-CO₂ reforming, very similar quantities of H₂ were consumed for water formation. Reverse water-gas shift (RWGS) reaction occurred to a very similar extent either with low or high WHSV values over both catalysts, revealing that product gas mixture contained near RWGS equilibrium composition, confirming the dominance of WGS reaction and showing that shortening the contact time would actually decrease the H₂/CO ratio in the syngas produced by CH₄-CO₂ reforming, as long as RWGS is quasi equilibrated. H₂/CO molar ratio in the produced syngas can be increased either by operating at higher temperatures, or by using a feed stream with CH₄/CO₂ ratio well above 1.     

EFFICIENT DESULFURIZATION IN O2/CO2 COMBUSTION: DEPENDENCE ON COMBUSTION CONDITIONS AND SORBENT PROPERTIES

Based on experiments on desulfurization, CaSO₄ decomposition, and a system approach using theoretical analysis, efficient in-furnace desulfurization in O₂/CO₂ combustion was investigated. The influence of combustion conditions and sorbent properties on system desulfurization efficiency was clarified. The global desulfurization efficiency was found to increase with O₂ purity. The global desulfurization efficiency in a dry recycle was higher than that in a wet recycle. The global efficiency of in-furnace desulfurization decreased with initial O₂ concentration. As the temperature increased, the global desulfurization efficiency increased first and then decreased due to the decomposition of CaSO₄. In the temperature range investigated, the global desulfurization efficiency in O₂/CO₂ coal combustion was much higher than that of conventional coal combustion in air. The global desulfurization efficiency decreased with sorbent size. When the particle radius decreased to one quarter, the global desulfurization efficiency doubled, becoming as high as 80%. The global desulfurization efficiency was very different among the three sorbents investigated, whether in O₂/CO₂ combustion or in conventional air combustion. The global desulfurization efficiency increased in the order of Ca(OH)₂, scallop, and limestone in O₂/CO₂ combustion, but in the order of scallop, Ca(OH)₂, and limestone in conventional air combustion. Nevertheless, all three sorbents demonstrated much higher desulfurization efficiency in O₂/CO₂ combustion than in conventional air combustion.     

Flame and radiation characteristics of gas-fired O2/CO2 combustion

This paper presents an experimental study on the flame properties of O₂/CO₂ combustion (oxy-fuel combustion) with focus on the radiation characteristics and the burn-out behaviour. The experiments were carried out in a 100 kWth test unit which facilitates O₂/CO₂ combustion with real flue gas recycle. The tests comprise a reference test in air and two O₂/CO₂ test cases with different recycled feed gas mixture concentrations of O₂ (OF 21 @ 21 vol.% O₂, 79 vol.% CO₂ and OF 27 @ 27 vol.% O₂, 73 vol.% CO₂). In-furnace gas concentration, temperature and total radiation (uni-directional) profiles are presented and discussed. The results show that the fuel burn-out is delayed for the OF 21 case compared to air-fired conditions as a consequence of reduced temperature levels. Instead, the OF 27 case results in more similar combustion behaviour compared to the reference conditions in terms of in-flame temperature and gas concentration levels, but with significantly increased flame radiation intensity. The information obtained from the radiation and temperature profiles show that the flame emissivity for the OF 21 and OF 27 cases both differ from air-fired conditions. The total emissivity and the gas emissivity of the OF 27 and the air-fired environment are discussed by means of an available model. The gas emissivity model shows that the increase in radiation intensity (up to 30%) of the OF 27 flame compared to the air flame can partly, but not solely, be explained by an increased gas emissivity. Hence, the results show that the OF 27 flame yields a higher radiative contribution from in-flame soot compared to the air-fired flame in addition to the known contribution from the elevated CO₂ partial pressure.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Gas absorption of carbon dioxide in a hollow fiber contained liquid membrane absorber

Experiments on the absorption of CO₂ into a hollow fiber contained liquid membrane absorber were performed. The feed gas was a mixture of CO₂ and N₂, absorbent liquid was 2-amino-2-methyl-l-propanol and the hollow fiber was a microporous hydrophobic polytetrafluoroethylene membrane. Outlet concentration of CO₂ from the absorber decreased as absorbent concentration increased, gas flow rate increased and were held constant for speed of agitation, but had a maximum value in the range of inlet concentration of CO₂ from 5 to 40 mole%. The reaction rate constant obtained for CO₂-amine system was 231 I/mol · s at 25 °C using a flat stirred vessel, and the membrane-side-mass-transfer coefficient was 1.217 × 10⁻⁵ mol/cm² · s · atm in CO2/N2-amine system. A diffusion model based on mass transfer with fast-reaction was proposed to predict the performance of the absorber.     

Chemical-looping combustion in a 300 W continuously operating reactor system using a manganese-based oxygen carrier

Chemical-looping combustion (CLC) is a method for the combustion of fuel gas with inherent separation of carbon dioxide. This technique involves the use of two interconnected reactors. A solid oxygen carrier reacts with the oxygen in air in the air reactor and is then transferred to the fuel reactor, where the fuel gas is oxidized to carbon dioxide and water by the oxygen carrier. Fuel gas and air are never mixed and pure CO₂ can easily be obtained from the flue gas exit. The oxygen carrier is recycled between both reactors in a regenerative process. This paper presents the results from a continuously operating laboratory CLC unit, consisting of two interconnected fluidized beds. The feasibility of the use of a manganese-based oxygen carrier supported on magnesium stabilized zirconia was tested in this work. Natural gas or syngas was used as fuel in the fuel reactor. Fuel flow and air flow was varied, the thermal power was between 100 and 300 W, and the air ratio was between 1.1 and 5.0. Tests were performed at four temperatures: 1073, 1123, 1173 and 1223 K. The prototype was successfully operated at all conditions with no signs of agglomeration or deactivation of the oxygen carrier. The same particles were used during 70 h of combustion and the mass loss was 0.038% per hour, although the main quantity was lost in the first hour of operation. In the combustion tests with natural gas, methane was detected in the exit flue gases, while CO and H₂ were maintained at low concentrations. Higher temperature or lower fuel flows increases the combustion efficiency, which ranged from 0.88 to 0.99. On the other hand, the combustion of syngas was complete for all experimental conditions, with no CO or H₂ present in the gas from the fuel reactor.     

The electronic effects of Pr on La1−xPrxNiAl11O19 for CO2 reforming of methane

The CO₂ reforming of CH₄ to synthesis gas by using praseodymium modified hexaaluminate La_1−xPr_xNiAl₁₁O₁₉ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) as catalysts was studied. The modifier Pr improved the reducibility and catalytic activity of Ni ions as active component in the hexaaluminate lattices, especially the conversion of CH₄ and CO₂ reached 89.62% and 92.94%, respectively over La_0.8Pr_0.2NiAl₁₁O₁₉. It was found that the addition of Pr can promote the electronic transformation between the Ni ions and the La ions to maintain Ni at a lower valence, which promotes the activation of CH₄.     

Results from trialling aqueous NH3 based post-combustion capture in a pilot plant at Munmorah power station: Absorption

Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO) and Delta Electricity have developed, commissioned and operated an A$7 million aqueous NH₃ based post-combustion capture (PCC) pilot plant at the Munmorah black coal fired power station in Australia. The results from the pilot plant trials will be used to address the gap in know-how on application of aqueous NH₃ for post-combustion capture of CO₂ and other pollutants in the flue gas and explore the potential of the NH₃ process for application in the Australia power sector. This paper is one of a series of publications to report and discuss the experimental results obtained from the pilot plant trials and primarily focuses on the absorption section.The pilot plant trials have confirmed the technical feasibility of the NH₃ based capture process. CO₂ removal efficiency of more than 85% can be achieved even with low NH₃ content of up to 6 wt%. The NH₃ process is effective for SO₂ but not for NO in the flue gas. More than 95% of SO₂ in the flue gas is removed in the pre-treatment column using NH₃. The mass transfer coefficients for CO₂ in the absorber as functions of CO₂ loading and NH₃ concentration have been obtained based on pilot plant data.     

Effects of operation parameters on NO emission in an oxy-fired CFB combustor

Oxy-fuel Circulating Fluidized Bed (CFB) combustion technology, a very promising technology for CO₂ capture, combines many advantages of oxy-fuel and CFB technologies. Experiments were carried out in a 50 kW_th CFB facility to investigate how operation parameters influence the NO emission in O₂/CO₂ atmospheres. The simulated O₂/CO₂ atmospheres were used without recycling the flue gas. Results show that NO emission in 21% O₂/79% CO₂ atmosphere is lower than that in air atmosphere because of lower temperature and higher char and CO concentrations in the dense bed. Elevating O₂ concentration from 21% to 40% in O₂/CO₂ atmosphere enhances fuel-N conversion to NO. Increasing bed temperature or oxygen/fuel stoichiometric ratio brings higher NO emission in O₂/CO₂ atmosphere, which is consistent with the results in air-fired CFB combustion. As primary stream fraction increases, NO emission increases more rapidly in O₂/CO₂ atmosphere than that in air atmosphere. Stream staging is more efficient for controlling NO emission in oxy-CFB combustion than that in air combustion. Oxygen staging provides an efficient way to reduce NO emission in oxy-CFB combustion without influencing the hydrodynamic characteristic in the riser.     

Ca-based sorbent looping combustion for CO2 capture in pilot-scale dual fluidized beds

To demonstrate process feasibility of in situ CO₂ capture from combustion of fossil fuels using Ca-based sorbent looping technology, a flexible atmospheric dual fluidized bed combustion system has been constructed. Both reactors have an ID of 100 mm and can be operated at up to 1000 °C at atmospheric pressure. This paper presents preliminary results for a variety of operating conditions, including sorbent looping rate, flue gas stream volume, CaO/CO₂ ratio and combustion mode for supplying heat to the sorbent regenerator, including oxy-fuel combustion of biomass and coal with flue gas recirculation to achieve high-concentration CO₂ in the off-gas. It is the authors' belief that this study is the first demonstration of this technology using a pilot-scale dual fluidized bed system, with continuous sorbent looping for in situ CO₂ capture, albeit at atmospheric pressure. A multi-cycle test was conducted and a high CO₂ capture efficiency (> 90%) was achieved for the first several cycles, which decreased to a still acceptable level (> 75%) even after more than 25 cycles. The cyclic sorbent was sampled on-line and showed general agreement with the features observed using a lab-scale thermogravimetric analysis (TGA) apparatus. CO₂ capture efficiency decreased with increasing number of sorbent looping cycles as expected, and sorbent attrition was found to be another significant factor to be limiting sorbent performance.     

UV/H2O2氧化联合Ca(OH)2吸收同时脱硫脱硝

在小型紫外光-鼓泡床反应器中,对UV/H₂O₂氧化联合Ca(OH)₂吸收同时脱除燃煤烟气中NO与SO₂的主要影响因素[H₂O₂浓度、紫外光辐射强度、Ca(OH)₂浓度、NO浓度、溶液温度、烟气流量以及SO₂浓度]进行了考察。采用烟气分析仪和离子色谱仪分别对尾气中的NO₂和液相阴离子作了检测分析。结果显示:在本文所有实验条件下,SO₂均能实现完全脱除。随着H₂O₂浓度、紫外光辐射强度和Ca(OH)₂浓度的增加,NO的脱除效率均呈现先大幅度增加后轻微变化的趋势。NO脱除效率随烟气流量和NO浓度的增加均有大幅度下降。随着溶液温度和SO₂浓度的增加,NO脱除效率仅有微小的下降。离子色谱分析表明,反应产物主要是SO₄^2-和NO₃^-,同时有少量的NO₂^-产生。尾气中未能检测到有害气体NO₂。