Impact of air staging along furnace height on NOx emissions from pulverized coal combustion

Experiments were carried out on an electrically heated multi-path air inlet one-dimensional furnace to assess NO_x emission characteristics of an overall air-staged (also termed air staging along furnace height) combustion of bituminous coal. The impact of main parameters of overall air-staged combustion technology, including burnout air position, air stoichiometric ratio, levels of burnout air (the number of burnout air arranged at different heights of the furnace), and the ratios of the burnout air flow rates and pulverized coal fineness of industrial interest, on NO_x emission were simulated to study in the experimental furnace, as well as the impact of air staging on the carbon content of the fly ash produced. These results suggest that air-staged combustion affects a pronounced reduction in NO_x emissions from the combustion of bituminous coal. The more deeply the air is staged, the further the NO_x emission is reduced. Two-level air staging yields a greater reduction in NO_x emission than single-level air staging. For pulverized coal of differing fineness, the best ratio between the burnout air rates in the two-level staging ranges from 0.6 to 0.3. In middle air-staged degree combustion with f_M = 0.75, pulverized coal fineness, R₉₀ (%), has a greater influence on NO_x emission, whereas R₉₀ has little influence on NO_x emission for deep air-staged degree with f_M = 0.61. Air-staged combustion with proper burnout air position has little effect on the burnout. For overall air-staged combustion, proper burnout air position and air-staged rate should be considered together in order to achieve high combustion efficiency.     

NOx and SOx emissions of a high sulfur self-retention coal during air-staged combustion

NO_x and SO_x emissions of air-staged combustion were investigated in a 1 MW tangentially-fired furnace combusting a high sulfur self-retention coal. Two variables including the air stoichiometric ratio of primary combustion zone and the relative location of over-fire air (OFA) injection ports were studied. These results suggest that NO_x reduction efficiency monotonically increases with increasing the relative location of OFA injection ports, and the lowest NO_x emissions are achieved when the air stoichiometric ratio of primary combustion zone is 0.85. In the meantime, SO_x emissions can be effectively reduced when the air stoichiometric ratio of primary combustion zone is 0.85 or 0.95, and SO_x emissions monotonically decrease with increasing the relative location of OFA injection ports.     

A reduced NOx reaction model for pulverized coal combustion under fuel-rich conditions

A reduced NO_x reaction model was developed for analysis of industrial pulverized coal firing boilers. The model was developed from experiments of laminar premixed combustion under a variety of stoichiometric ratios, burning temperatures, coal ranks (from sub-bituminous coal to anthracite) and particle diameters. Calculations agreed with experimental results for NO_x and nitrogen species (NH₃ and HCN), if the model assumed that the hydrocarbon radicals were formed not only from pyrolysis of volatile matter, but also from char oxidation and gasification. The presence of hydrogen in char at the final burnout stage supported this assumption. NO_x reduction by hydrocarbon radicals was the most important reaction in high temperature (>1500 K), fuel-rich, char combustion regions. NO_x reduction from nitrogen species was sensitive to peak NO_x concentration in volatile combustion regions, but NO_x emission downstream had little influence from the peak NO_x concentration. The heterogeneous reaction between char and NO_x was important for fuel-lean or low-temperature conditions.     

Estimation of NOx emissions from coal-fired utility boilers

CFD-based engineering models can be cost-effective tools in determining technical feasibility of clean coal-fired power generation technologies like NO_x mitigation strategies targeting the reduction of acid rain and smog. Their actual effectiveness depends on their capability to provide realistic engineering estimates without arbitrary adjustment of model parameters. This investigation focused on testing a CFD based NO_x model over a variety of coal type, firing configuration and boiler size ranging from 200 MW_e sub-critical to most modern 1000 MW_e ultra supercritical. In most cases, the NO_x estimates based on input data readily available from power plants were found within the range of measured data (with the worst estimate being 22% higher than the maximum measured NO_x level). The CFD results also indicated some sensitivity of the NO_x estimates to the ratio of volatile nitrogen to char nitrogen and the importance of NO reduction by char. However, this study showed that the locations of fuel-bound nitrogen evolution with respect to the stoichiometric condition within the boiler actually governed the overall NO emissions.     

Optimisation of NOx reduction in advanced coal reburning systems and the effect of coal type

The advanced reburning process for NO_x emission control was studied in a down-fired 20 kW combustor by evaluating the performance of 15 pulverised coals as reburning fuels. The proximate volatile matter contents of the coals selected ranged from around 4 to 40 wt% (as received) with elemental nitrogen contents from around 0.6 to 2.0 wt%. The effects of reburn fuel fraction, reburning zone residence time, ammonia agent injection delay time (relative to the reburn fuel and burnout air injection points) and the nitrogen stoichiometric ratio are reported in detail and the optimum configurations for advanced reburning, established as a function of operating condition and coal type. The experimental results show that advanced reburning can reduce NO_x emissions up to 85%. The maximum benefits of advanced reburning over conventional reburning were observed at the lower reburn fuel fractions (around 10%). The results demonstrate that under advanced reburning conditions equivalent or higher levels of NO_x reduction can be achieved while operating the reburn zone closer to stoichiometric conditions compared with conventional reburning operating at high reburn fuel fractions (20-25%). Thus the practical problems associated with fuel-rich staged operation can be reduced. The effect of coal properties on the advanced reburning performance was also investigated. As with conventional reburning, the fuel nitrogen content of the coal used was found to have little influence on the NO_x reduction efficiency except at the highest reburn fuel fractions. There was, however, a strong correlation between the effectiveness of advanced reburning and the volatile content of the reburning fuels, which not only depended on the reburn fuel fraction, but also the mode (rich or lean) of advanced reburning operation. These parameters are mapped out experimentally to enable the best operating mode to be selected for advanced reburning as a function of the reburning fuel fraction and volatile content.     

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.     

The application of FLOX/COSTAIR technologies to reduce NOx emissions from coal/biomass fired power plant: A technical assessment based on computational simulation

Nitrogen oxides (NO_x) is one of the harmful emissions from power plants. Efforts are made to reduce NO_x emissions by researchers and engineers all the times. NO_x emissions are from three resources during the combustion: prompt NO, fuel NO and thermal NO. The last one - thermal NO, which is described by ‘Zeldovich-mechanism’, is the main source for NO_x emissions. The thermal NO emission mainly results from the high combustion temperature in the combustion process. In order to control the NO formation, the control of peak combustion temperature is the key factor, as well as the oxygen concentration in the combustion areas. Flameless oxidation (FLOX) and continuous staged air combustion (COSTAIR) are two relatively new technologies to control the combustion temperature and the reaction rate and consequently to control the NO_x emissions.In this study both FLOX and COSTAIR technologies are assessed based on a 12 MW_e, coal-fired, circulating fluidised bed combustion (CFBC) power plant by using ECLIPSE simulation software, together with a circulating fluidised bed gasification (CFBG) plus normal burner plant. Two different fuels - coal and biomass (straw) are used for the simulation. The technical results from the study show that the application of FLOX technology to the plant may reduce NO_x emissions by 90% and the application of COSTAIR technology can reduce NO_x emissions by 80-85% from the power plant. The emissions from the straw-fuelled plants are all lower than that of coal-fuelled ones although with less plant efficiencies.     

Numerical investigation on the flow, combustion and NOx emission characteristics in a 500 MWe tangentially fired pulverized-coal boiler

The characteristics of the flow, combustion, temperature and NO_x emissions in a 500 MW_e tangentially fired pulverized-coal boiler are numerically studied using comprehensive models, with emphasis on fuel and thermal NO_x formations. The comparison between the measured values and predicted results shows good agreement, which implies that the adopted combustion and NO_x formation models are suitable for correctly predicting characteristics of the boiler. The relations among the predicted temperature, O₂ and CO₂ mass fractions are discussed based on the calculated distributions. The predicted results clearly show that NO_x formation within the boiler highly depends on the combustion processes as well as the temperature and species concentrations. The results obtained from this study have shown that overfire air (OFA) operation is an efficient way to reduce the NO_x emissions of the pulverized-coal fired boiler. Air staging combustion technology (OFA operation) adopted in this boiler has helped reduce fuel NO_x formation as well as thermal NO_x formation under the present simulated conditions. The decrease in the formation of fuel NO_x is due to the decreased contact of the nitrogen from the fuel with the oxygen within the combustion air, while the decrease in thermal NO_x formation is caused by a decrease in temperature. The detailed results presented in this paper may enhance the understanding of complex flow patterns, combustion processes and NO_x emissions in tangentially fired pulverized-coal boilers, and may also provide a useful basis for NO_x reduction and control.     

Rice husk co-firing with coal in a short-combustion-chamber fluidized-bed combustor (SFBC)

This study encompassed the characteristics and performance of co-firing rice husk, a by-product of rice-milling process, with coal in a short-combustion-chamber fluidized-bed combustor (SFBC). Bed phenomena investigated in a cold-flow model combustor showed that with the different mixes of materials, the anticipated offshoot of combustion, the minimum fluidizing velocity (U_mf) was 0.4-0.8 m/s. In concord with axial temperature profiles, axial gas concentration profiles implied that a recirculating ring was able to circumscribe CO within the short-main chamber. The formation, decomposition, and eventual maturity of NO_x characterized the NO_x evolution, inferred from concentration profiles. The impacts of fluidizing velocity and blending ratio on gas emissions and combustion efficiency (E_c) are described. The fluidizing velocity had consequential effect on gas emissions, except NO_x. Surprisingly, NO_x did not hinge much on increased N-content of the mixtures with coal. As expected, increased SO₂ was relevant to increased coal mass. Increased fluidizing velocity adversely affected E_c while increased coal fraction enhanced E_c, mostly >97%.     

Influence of potassium loading at different reaction temperatures on the NOx reduction process by potassium-containing coal pellets☆

The activity of potassium-containing coal pellets and the corresponding free-metal char for NO_x reduction in an oxygen-rich environment has been investigated by temperature-programmed reactions (TPRs) up to 750 °C, 2 h isothermal reactions in the range of 250-475 °C and lifetime tests, (until the samples were completely consumed), for selected samples and temperatures. An interesting ‘reactivity window’, where NO_x reduction is observed, but carbon conversion is negligible, was found from TPRs experiment for a high potassium content sample, at moderate temperatures. This interval was not observed for the char. The catalytic effect of potassium is more dramatic at high temperatures, therefore, metal loading and reaction temperature are very much interrelated. Lifetime tests provide a very valuable information (average selectivity, profitable use of samples for NO_x reduction, etc.), allowing us to check the whole efficiency of the samples. The progressive addition of potassium to the pellets makes samples more effective in terms of: (i) capacity to reduce higher NO_x amounts; (ii) maximum NO_x conversion values and (iii) higher values of average selectivity. In general, the samples studied, exhibit a maximum temperature, very much dependent on their potassium contents, that must not be exceeded with a view to practical applications.     

Numerical analysis of pulverized coal combustion characteristics using advanced low-NOx burner

A three-dimensional numerical simulation is applied to a pulverized coal combustion field in a test furnace equipped with an advanced low-NO_x burner called CI-α burner, and the detailed combustion characteristics are investigated. In addition, the validities of the existing NO_x formation and reduction models are examined. The results show that a recirculation flow is formed in the high-gas-temperature region near the CI-α burner outlet, and this lengthens the residence time of coal particles in this high-temperature region, promotes the evolution of volatile matter and the progress of char reaction, and produces an extremely low-O₂ region for effective NO reduction. It is also found that, by lessening the effect of NO reduction in Levy et al.'s model and taking the NO formation from char N into account, the accuracy of the NO prediction is improved. The efficiency factor of the conversion of char N to NO affects the total NO concentration downstream after the injection of staged combustion air.     

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.     

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.     

Application of the thermal DeNOx process to diesel engine DeNOx: an experimental and kinetic modelling study

Diesel DeNO_x experiments have been conducted using the selective noncatalytic ‘thermal DeNO_x’ process in a diesel fuelled combustion-driven flow reactor which simulated a single cylinder (966 cm³) and head equipped with a water-cooling jacket and an exhaust pipe. NH₃ was directly injected into the cylinder to reduce NO_x emissions. A wide range of air/fuel ratios (A/F=20-40) was selected for NO_x reduction where an initial NO_x of 530 ppm was usually maintained with a molar ratio (β=NH₃/NO_x) of 1.5.The results indicate that a 34% NO_x reduction can be achieved from the cylinder injection in the temperature range, 1100-1350 K. Most of the NO_x reduction occurs within the cylinder and head section (residence time<40 ms), since temperatures in the exhaust are too low for additional NO_x reduction. Under large gas quenching rates, increasing β values (e.g. 4.0) substantially increase the NO_x reduction up to 60%, which is comparable with those achieved under isothermal conditions. Experimental findings are analysed by chemical kinetics using the Miller and Bowman mechanism including both N/H/O species and CO/hydrocarbon reactions to account for CO/UHC oxidation effects, based on practical nonisothermal conditions. Comparisons of the kinetic calculations with the experimental data are given as regards temperature characteristics, residence time and molar ratio. In addition, the effects of CO/UHC and branching ratio (α=k₁/(k₁+k₂)) for the reaction NH₂+NO=products are discussed in terms of NO reduction features, together with practical implications.     

Numerical investigation of NOx emissions from a tangentially-fired utility boiler under conventional and overfire air operation

A CFD investigation has been carried out about the performance of a 600 MW_e tangentially coal-fired boiler, focusing on the reduction of NO_x attainable by using overfire air. To this purpose, a comprehensive combination of NO_x chemistry models has been used, coupled with the numerical simulation of fluid and particle flow, solid fuel combustion and heat and mass transfer. Predicted values of gas temperature and species concentration have been adopted to validate the model against actual measurements from the full-scale boiler, under conventional and overfire air arrangements. A reasonable agreement has been attained in most cases. Additionally, modelling sensitivity has been evaluated against variations in some fuel-dependent parameters hard to measure or estimate (devolatilisation rates, nitrogen content in volatiles and char, reburning rates). As a result, an analysis tool is available to study the response of this kind of boilers to a variety of coal feedstock and combustion conditions, in a feasible and economic manner.     

Mitigation of NOx emissions from a jatropha biodiesel fuelled DI diesel engine using antioxidant additives

Biodiesel offers cleaner combustion over conventional diesel fuel including reduced particulate matter, carbon monoxide and unburned hydrocarbon emissions. However, several studies point to slight increase in NO_x emissions (about 10%) for biodiesel fuel compared with conventional diesel fuel. Use of antioxidant additives is one of the most cost-effective ways to mitigate the formation of prompt NO_x. In this study, the effect of antioxidant additives on NO_x emissions in a jatropha methyl ester fuelled direct injection diesel engine have been investigated experimentally and compared. A survey of literature regarding the causes of biodiesel NO_x effect and control strategies is presented. The antioxidant additives L-ascorbic acid, α tocopherol acetate, butylated hydroxytoluene, p-phenylenediamine and ethylenediamine were tested on computerised Kirloskar-make 4 stroke water cooled single cylinder diesel engine of 4.4 kW rated power. Results showed that antioxidants considered in the present study are effective in controlling the NO_x emissions of biodiesel fuelled diesel engines. A 0.025%-m concentration of p-phenylenediamine additive was optimal as NO_x levels were substantially reduced in the whole load range in comparison with neat biodiesel. However, hydrocarbon and CO emissions were found to have increased by the addition of antioxidants.     

NOx emission from a 71 MWe pressurized fluidized bed combustor

Flue gas NO_x concentration was measured at the outlet of gas turbine (the inlet of de-NO_x catalyst) of a 71 MWe pressurized fluidized bed combustor. The effect of operating parameters on NO_x emission was approximated by assuming a linear combination of three independent parameters, concentration of O₂ in the flue gas, SO₂ removal efficiency, and dense bed temperature. Sensitivity factors for different type of coals were obtained. The relationship between the sensitivity factors and fuel ratio (fixed carbon/volatile matter ratio) was obtained. NO_x emission was well approximated by the present approximation, even for coals/coal mixtures that were not used to determine the sensitivity factors. NO_x emissions for coal/coal mixtures and combustor operation under fly ash recycle conditions can be predicted approximately from the equation derived from other coals under conditions without fly ash recycle.     

Attempts to reduce NOx exhaust emissions by using reformulated biodiesel

Biodiesel is a renewable, domestically produced fuel that has been shown to reduce particulate, hydrocarbon, and carbon monoxide emissions from diesel engines. Under some conditions, however, biodiesel produced from certain feedstocks has been shown to cause an increase in nitrogen oxides (NO_x). This is of special concern in urban areas that are subject to strict environmental regulations. Although soy-based biodiesel may increase the emission of nitrogen oxides, it is the most easily accessible in North America. We investigated two routes to reformulate soy-based biodiesel in an effort to reduce nitrogen oxide emissions. In one of these, soy-oil methyl esters were modified by conversion of a proportion of the cis bonds in the fatty acid chains of its methyl esters to their trans isomers. In the other approach, polyol derivatives of soybean oil were transesterified to form soy methyl polyol fatty acid esters. The NO_x emissions of these modified biodiesels were then examined, using a Yanmar L100 single cylinder, four stroke, naturally aspirated, air cooled, direct injection diesel engine. Using either isomerized methyl oleate or isomerized soy biodiesel, at 20% blend level in petroleum diesel (‘B20’), nitrogen oxide emissions were elevated by between 1.5 and 3 percentage points relative to the combustion of a B20 blend of commercial biodiesel. Nitrogen oxide emissions were reduced in proportion to blend level during the combustion of polyol biodiesel, with a 20% blend in petrodiesel resulting in a reduction of about 4.5 percentage points relative to the emissions of a comparable blend of commercial soy biodiesel.     

Modelling of NOx adsorption over NOx adsorbers

Modelling of the phenomena involved during the adsorption of NO_x on NO_x trap catalysts was developed. The aim of the model is the prediction of the quantity of stocked barium nitrate as well as the emissions of NO and NO₂, as a function of time and temperature. The mechanism of the process is sounded on the adsorption of gas species (NO, NO₂, O₂) on platinum sites, equilibrium reaction between adsorbed species followed by the formation of Ba(NO₃)₂. This formation of barium nitrate is limited by the thermal decomposition reaction which liberates NO in the gas phase. The kinetic constant of decomposition of barium nitrate was determined by temperature programmed thermogravimetry on pure Ba(NO₃)₂, using the method of Freeman and Carroll. Other kinetic constants bound to the mechanism were estimated by fitting the results of the model to experimental results.The mechanism was validated for various values of the molar fraction of O₂, the molar fraction of NO and various values of the NO/NO₂ ratio in the gas entering the reactor. It was also tested with different catalyst compositions (variation of the platinum and BaO concentrations). The importance of oxygen in the process was clearly demonstrated as well as the promoting role of NO₂.     

Evaluation of the use of coal volatiles as reburning fuel for NOx reduction

In this study, computational fluid dynamic (CFD) and kinetic models were used to investigate the relative performances of coal volatiles and natural gas reburning. This modeling approach considers fluid dynamic and non-isothermal effects, which were not considered in past laboratory flow reactor studies. The commercial CFD code FLUENT 6.1 was used to predict the residence times and temperatures for reburning tests in the down-fired combustor (DFC), a 0.5 MMBTU/h research combustor at The Pennsylvania State University. To predict NO_x concentrations within the combustor, this data was then applied to an advanced reburning kinetic model used in past studies. For equal firing rates and stoichiometric ratios, reburning using methane yielded lower concentrations of NO_x (and, therefore, better NO_x reduction performance) than reburning using coal volatiles. The coal volatiles give increased flame temperature over natural gas, which apparently offsets the increased reburn zone hydrocarbon radical yield of coal volatiles over natural gas.