Experimental trends of NO in circulating fluidized bed combustion

Experimental trends for the dependence of CO, NO and N₂O emissions on bed temperature and oxygen concentration in circulating fluidized bed combustion (CFB) are presented. The main focus is on the nitrogen emission formation in the lower furnace area. A test campaign including seven tests with a laboratory scale CFB test rig were carried out to produce appropriate data of the phenomena. These experiments show that NO emissions above the dense bed decrease with decreasing temperature or oxygen concentration. Instead, N₂O emissions increase when the bed temperature is decreased and decrease when the oxygen concentration is decreased. These trends can partly be explained by heterogeneous reactions between NO and char, since decrease in temperature or oxygen concentration increases the bed char inventory. However, oxygen and temperature also affect directly on NO emissions. Correlations for the CO, NO, N₂O, NH₃ and HCN concentrations at the exit of dense bed were developed. This type of correlations can, among other things, be applied as boundary conditions to the more sophisticated CFD models that are usually applied to modelling of diluted part of the furnace. CFD modelling of the dense bed area is complicated and accuracy is not sufficient, thus simplified experimental correlations can aid in the development of furnace design towards better emission performance.     

Modelling of NO and N2O emissions from biomass-fired circulating fluidized bed combustors

A model for NO and N₂O emissions from biomass-fired circulating fluidized bed (CFB) combustors has been developed and evaluated in this study. All the model parameters were chosen for a typical woody biomass-pinewood chips. Both drying and devolatilization of biomass particles were modelled with limited rates, which were selected from the literature based on woody biomass fuels. The partition of fuel-nitrogen between volatiles and char was also specifically chosen for pinewood based on available experimental data from the literature. Volatile nitrogen was assumed to consist of NH₃, HCN and N₂ with the distribution between three species as input parameters to the model. Twenty-five homogenous and heterogeneous global chemical reactions were included in the model, of which 20 reactions represents the global fuel-nitrogen reactions. Both gaseous and solid phase were assumed to be in plug flow. The model has been applied to the modelling of a 12 MW_th CFB boiler. The predicted N₂O emissions were always less than 5 ppmv for pinewood combustion, which was consistent with the experimental results. The predicted NO emissions increased with the total excess air of the riser and the fuel-N content while the predicted percentage conversion of fuel-N to NO decreased with increasing fuel-N content. The NO emissions were also predicted to decrease with increasing primary zone stoichiometry. These predictions agree with the experimental results. The predicted NO emissions decreased slightly with increasing bed temperature, whereas experiments showed that NO emissions slightly increased with bed temperature for birch chips combustion and did not change with bed temperature for fir chips combustion. Sensitivity analyses reveal that the reaction between NO and char is the key reaction to determine the NO emissions.     

NOx formation in natural gas combustion—a new simplified reaction scheme for CFD calculations

Computational fluid dynamics is a widely used tool in optimizing natural gas burners, for instance, for emission issues. Especially, a further reduction of NO_x emissions is of interest. However, due to computational efforts calculating three-dimensional turbulent flames, there is the necessity for simplified models in order to simulate the combustion reactions and the NO_x formation, respectively. Hitherto, models describing thermal NO and prompt NO formation, respectively, were applied in a post-processing step. Beforehand, the flow field including combustion has been determined in the three-dimensional geometry. However, in the former work, it was shown that prompt NO formation is of minor significance. For temperatures higher than 1600 °C, thermal NO formation is dominating. At lower temperatures, the N₂O/NO and NNH route have significant contribution.Though, the widely applied prompt NO model captures the observed trends acceptable, it lacks of physical bases. Besides low temperature NO formation is more related to N₂O/NO and NNH route, it assumes the prompt NO formation to be proportional to the fuel concentration. The detailed reaction mechanism show NO formation more related to fuel oxidation rate, i.e. radical concentration.Thus, in this work, a new simplified model combining thermal NO formation, N₂O/NO, and NNH route is proposed. It applies steady-state approximation for the intermediate species, i.e. N, N₂O, NNH, and NH. In this way, their concentrations can be obtained by four algebraic equations and rate of NO formation can be calculated without any model parameter, solely based on reaction kinetics. Moreover, the concentrations of O₂, N₂, H₂, and H₂O as well as the radicals O, H, OH, and HO₂ have to be known from combustion calculations.The model was evaluated against the predictions of a detailed reaction mechanism, showing good agreement in a wide range of conditions. Neglecting prompt NO formation affects predicted NO emissions only under very fuel rich conditions. Under these circumstances, total NO formation is low, anyway. Thus, the performance of the presented model is not influenced by the lack of prompt NO formation.     

Computational study of the combustion process and NO formation in a small-scale wood pellet furnace

This paper presents a computational study of the combustion process of wood pellets in a small-scale grate fired furnace. The objectives were to obtain detailed information on the combustion characteristics and NO formation in the furnace, and to examine the effect of secondary air on the combustion process. The simulation results were compared with experimental data in terms of flame temperature and distributions of species concentrations, including CO and NO. It was shown that the combustion process is strongly controlled by the inflow turbulence from the secondary and tertiary air jets. The combustion process is not sensitive to the bed combustion process in the present test case. The high speed air flow from the secondary and tertiary air inlets ‘destroys’ the history of the effluent volatile gases from the fuel bed. Different paths for the NO emission were investigated, including the thermal NO, the fuel-NO and NO from the N₂O intermediate mechanisms. The fuel-NO path is responsible for the rapid NO increase and the high NO peak near the fuel bed. Fuel-NO is rather low far downstream owing to the rather low nitrogen content in the fuel (less than 0.1% on mass basis), and the de-NO_x reactions with NH₃. NO is likely formed from the N₂O intermediate mechanism far downstream.     

N2O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions

The number of published N₂O and NO emissions measurements is increasing steadily, providing additional information about driving factors of these emissions and allowing an improvement of statistical N-emission models. We summarized information from 1008 N₂O and 189 NO emission measurements for agricultural fields, and 207 N₂O and 210 NO measurements for soils under natural vegetation. The factors that significantly influence agricultural N₂O emissions were N application rate, crop type, fertilizer type, soil organic C content, soil pH and texture, and those for NO emissions include N application rate, soil N content and climate. Compared to an earlier analysis the 20% increase in the number of N₂O measurements for agriculture did not yield more insight or reduced uncertainty, because the representation of environmental and management conditions in agro-ecosystems did not improve, while for NO emissions the additional measurements in agricultural systems did yield a considerable improvement. N₂O emissions from soils under natural vegetation are significantly influenced by vegetation type, soil organic C content, soil pH, bulk density and drainage, while vegetation type and soil C content are major factors for NO emissions. Statistical models of these factors were used to calculate global annual emissions from fertilized cropland (3.3 Tg N₂O-N and 1.4 Tg NO-N) and grassland (0.8 Tg N₂O-N and 0.4 Tg NO-N). Global emissions were not calculated for soils under natural vegetation due to lack of data for many vegetation types.     

Evaluation of the performance of a commercial circulating fluidized bed boiler by using IEA-CFBC model: Effect of primary to secondary air ratio

The performance of a commercial circulating fluidized bed boiler in the Yeosu thermal power plant, which has been operating since October 2011 by KOSEP, has been evaluated by using the IEA-CFBC model. To validate the calculation procedure of the model, the calculated results were compared with the operation values such as the temperatures, pressures, emissions of SO₂ and NO, particles size distribution and unburned carbon fraction of the CFB boiler at a certain actual condition. The calculated results were comparable to measured values from the CFB boiler, so these could conform to acceptable formats with a good accuracy. The effect of the primary to secondary air ratio on the performance of the CFB boiler was also determined. As the primary air ratio increased, the solid fraction and temperature in the furnace freeboard increased. As a result, the solid circulation rate and the heat absorption in the furnace increased with increasing the PA ratio. In the case of the amount of heat absorption, the wall tube of the furnace absorbed much more generation heat in the furnace than the wing wall tube. The SO₂ emission decreased due to increase of the limestone hold up in the furnace, and the combustion efficiency somewhat increased with increasing the PA ratio. Therefore, from these results, we could expect to control the boiler performance such as the furnace temperature, steam temperatures of superheater or reheater, gaseous emissions and combustion efficiency through the changing the PA ratio of the CFB boiler.     

Effect of co-combustion of chicken litter and coal on emissions in a laboratory-scale fluidized bed combustor

Co-combustion of chicken litter (CL) with coal was performed in a laboratory-scale fluidized bed combustor to investigate the effect of CL combustion on pollutant emissions. The emissions of major gaseous pollutants including CO, SO₂, H₂S and NO and temperature distribution along the combustor were measured during the tests. Effects of CL fraction and secondary air on combustion characteristics were studied. The experimental results show that CL introduction increases CO emissions and reduces the levels of SO₂. The ratio of H₂S/SO₂ increases with increasing fraction of CL. NO emissions either increase or decrease depending on the percentage of CL in the mixed fuels. The temperature in the freeboard region increases with increasing the fraction of CL while the reverse is true for the bed temperature.     

Measurements of N2O and NO emissions during tomato cultivation using a flow-through chamber system in a glasshouse

Nitrous oxide (N₂O) and nitric oxide (NO) fluxes resulting from long-term tomato cultivation in a glasshouse were continuously determined using the flow-through chamber method over the course of three cultivation periods. Gas concentrations were measured using an nondispersive infrared (gas filter correlation/infra-red) analyzer and a chemiluminescence-based analyzer, respectively. Following a basal application of fertilizer, daily N₂O and NO emission rates increased, with peaks lasting from 40 to 140 days. Short-term fluctuations in daily N₂O and NO emissions were affected by differences in nitrogen application, soil water, and soil temperature. Diurnal changes in N₂O and NO fluxes during the period of peak emissions depended primarily on soil temperature. Following the application of a top dressing (N as urea or calcium nitrate) in the irrigation water, the N₂O and NO fluxes increased immediately, with a very short period of peak emissions (1–5 h) after urea application. The duration of the peak period in daily accumulated N₂O and NO emissions following application of the top dressing ranged from 3 to 10 days.     

Detailed measurements in a pulverized-coal-fired large-scale laboratory furnace with air staging

The aim of the present work was to evaluate the performance of a pulverized-coal-fired large-scale laboratory furnace with air staging. New data are reported for gas phase species concentration, temperature and particle burnout for two primary zone stoichiometric ratios, 1.15 (unstaged flame) and 0.95 (staged flame), other operating conditions being fixed. The results revealed that the reduction in primary zone stoichiometric ratio caused a decrease in NO_x emissions from 421 to 180 mg/N m³@6%O₂, an increase in CO emissions from 51 to 168 mg/N m³@6%O₂ and a reduction in particle burnout from 81.8% to 76.5%. It was concluded that the reduction of the O₂ availability in the primary zone inhibits the NO formation, mainly via the fuel mechanism, but it affects negatively both the CO and the char oxidation processes because, under staging conditions, both processes tend to occur in the vicinity of the over fire air injection region, where the temperatures are relatively low.     

Measurement and modeling of nitrous and nitric oxide emissions from a tea field in subtropical central China

Tea fields represent an important source of nitrous oxide (N₂O) and nitric oxide (NO) emissions due to high nitrogen (N) fertilizer applications and very low soil pH. To investigate the temporal characteristics of N₂O and NO emissions, daily emissions were measured over 2½ years period using static closed-chamber/gas chromatograph and chemiluminescent measurement system in a tea field of subtropical central China. Our results revealed that N₂O and NO fluxes showed similar temporal trends, which were generally driven by temporal variations in soil temperature and soil moisture content and were also affected by fertilization events. The measured average annual N₂O and NO emissions were 10.9 and 3.3 kg N ha^?1 year^?1, respectively, highlighting the high N₂O and NO emissions from tea fields. To improve our understanding of N-cycling processes in tea ecosystems, we developed a new nitrogenous gas emission module for the water and nitrogen management model (WNMM, V2) that simulated daily N₂O and NO fluxes, in which the NO was simulated as being emitted from both nitrification and nitrite chemical decomposition. The results demonstrated that the WNMM captured the general temporal dynamics of N₂O (NSE = 0.40; R² = 0.52, RMSE = 0.03 kg N ha^?1 day^?1, P < 0.001) and NO (NSE = 0.41; R² = 0.44, RMSE = 0.01 kg N ha^?1 day^?1, P < 0.001) emissions. According to the simulation, denitrification was identified as the dominant process contributing 76.5% of the total N₂O emissions, while nitrification and nitrite chemical decomposition accounted for 52.3 and 47.7% of the total NO emissions, respectively.     

The NO and N2O formation mechanism under circulating fluidized bed combustor conditions: From the single particle to the pilot-scale

The NO and N₂O formation mechanism is studied starting from a single fuel particle burning under well-defined conditions up to a pilot-scale circulating fluidized bed combustor (CFBC). The fuel, petroleum coke, was the same in all tests and care has been taken to obtain chemical similarity between the different units: a formation rate unit, a laboratory-scale and a pilot-scale CFBC. A detailed single particle NO/N₂O formation model has been developed and incorporated in a CFBC NO/N₂O emission model. To thoroughly test the modeled NO/N₂O mechanism, the iodine addition method has been used in all units.     

A kinetic study of NOx reduction over Pt/SiO2 model catalysts with hydrogen as the reducing agent

Flow reactor experiments and kinetic modeling have been performed in order to study the mechanism and kinetics of NO_x reduction over Pt/SiO₂ catalysts with hydrogen as the reducing agent. The experimental results from NO oxidation and reduction cycles showed that N₂O and NH₃ are formed when NO_x is reduced with H₂. The NH₃ formation depends on the H₂ concentration and the selectivity to NH₃ and N₂O is temperature dependent. A previous model has been used to simulate NO oxidation and a mechanism for NO_x reduction is proposed, which describes the formation/consumption of N₂, H₂O, NO, NO₂, N₂O, NH₃, O₂ and H₂. A good agreement was found between the performed experiments and the model.     

Effect of chemical fertilizer form on N2O, NO and NO2 fluxes from Andisol field

N₂O, NO and NO₂ fluxes from an Andosol soil in Japan after fertilization were measured 6 times per day for 10 months from June 1997 to April 1998 with a fully automated flux monitoring system in lysimeters. Three nitrogen chemical fertilizers were applied to the soil–calcium nitrate (NI), controlled-release urea (CU), and controlled-release calcium nitrate (CN), and also no nitrogen fertilizer (NN). The total amount of nitrogen applied was 15 g N m^–2 in the first and the second cultivation period of Chinese vegetable. In the first measuremnt period of 89 days, the total N₂O emissions from NI, CN, CU, and NN were 18.4, 16.3, 48.7, and 9.60 mgN m^–2, respectively. The total NO emissions from NI, CN, CU, and NN were 48.4, 33.7, 149, and 13.7 mgN m^–2, respectively. In the second measurement period of 53 days, the total N₂O emissions from NI, CN, and CU were 9.66, 7.23, and 20.6 mgN m^–2, respectively. The total NO emissions from NI, CN, and CU were 24.7, 2.60 and 34.2 mgN m^–2, respectively. The total N₂O emission from CU was significantly higher than CN. In the third cultivation period, all plots were applied with 10 g N m^–2 of ammonium phosphate (AP) and winter barley was cultivated. In the third measurement period of 155 days, the total N₂O and NO emissions were 9.02 mgN m^–2 and 10.2 mgN m^–2, respectively. N₂O and NO peaks were observed just after the fertilization for 30 days and 15 days, respectively. N₂O, NO and NO₂ fluxes for the year were estimated to be 38.6 81.5, 48.2 181, and –24.8 to –39.3 mgN m^–2, respectively. NO₂ was absorbed in all the plots, and a negative correlation was found between NO₂ flux and the NO₂ concentration just after the chamber closed. NO was absorbed in the winter period, and a negative correlation was found between NO flux and the NO concentration just after the chamber closed. A diurnal pattern was observed in N₂O and NO fluxes in the summer, similar to air and soil temperature. We could find a negative relationship between flux ratio of NO-N to N₂O-N and water-filled pore space (WFPS), and a positive relationship between NO-N and N₂O-N fluxes and temperature. Q₁₀ values were 3.1 for N₂O and 8.7 for NO between 530 °C.     

Combustion gas and NO emission characteristics of hazardous waste mixture particles in a fixed bed

Experiments with fixed-bed incinerators were carried out to model the combustion characteristics and gas emission characteristics of hazardous waste mixture particles in a grate furnace. The results indicate that combustion can be divided into three stages: ignition, main combustion and combustion completion stage. According to the various concentrations of O₂, CO₂ and CO, the main combustion stage can be subdivided into pyrolysis gas combustion and char combustion. Primary air rate, moisture and particle size have significant effects on concentrations of combustion gases and NO. Bed height has no effect on CO₂ concentrations but does have an effect on other combustion gases and NO emissions.     

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.     

Modeling Trace Gas Emissions from Agricultural Ecosystems

A computer simulation model was developed for predicting trace gas emissions from agricultural ecosystems. The denitrification-decomposition (DNDC) model consists of two components. The first component, consisting of the soil climate, crop growth, and decomposition submodels, predicts soil temperature, moisture, pH, Eh, and substrate concentration profiles based on ecological drivers (e.g., climate, soil, vegetation, and anthropogenic activity). The second component, consisting of the nitrification, denitrification, and fermentation submodels, predicts NH₃, NO, N₂O, and CH₄ fluxes based on the soil environmental variables. Classical laws of physics, chemistry, or biology or empirical equations generated from laboratory observations were used in the model to parameterize each specific reaction. The entire model links trace gas emissions to basic ecological drivers. Through validation against data sets of NO, N₂O, CH₄, and NH₃ emissions measured at four agricultural sites, the model showed its ability to capture patterns and magnitudes of trace gas emissions.     

N2O and NO emissions from different N sources and under a range of soil water contents

Emissions of nitrous oxide (N₂O) and nitric oxide (NO) have been identified as one of the most important sources of atmospheric pollution from grasslands. Soils are major sources for the production of N₂O and NO, which are by-products or intermediate products of microbial nitrification and denitrification processes. Some studies have tried to evaluate the importance of denitrification or nitrification in the formation of N₂O or NO but there are few that have considered emissions of both gases as affected by a wide range of different factors. In this study, the importance of a number of factors (soil moisture, fertiliser type and temperature) was determined for N₂O and NO emissions. Nitrous oxide and NO evolution in time and the possibility of using the ratio NO:N₂O as an indicator for the processes involved were also explored. Dinitrogen (N₂) and ammonia (NH₃) emissions were estimated and a mass balance for N fluxes was performed. Nitrous oxide and NO were produced by nitrification and denitrification in soils fertilised with and by denitrification in soils fertilised with . Water content in the soil was the most important factor affecting N₂O and NO emissions. Our N₂O and NO data were fitted to quadratic (r=0.8) and negative exponential (r=0.7) equations, respectively. A long lag phase was observed for the N₂O emitted from soils fertilised with (denitrification), which was not observed for the soils fertilised with (nitrification) and was possibly due to a greater inhibiting effect of low temperatures on microbial activity controlling denitrification rather than on nitrification. The use of the NO:N₂O ratio as a possible indicator of denitrification or nitrification in the formation of N₂O and NO was discounted for soils fertilised with . The N mass balance indicated that about 50 kg N ha⁻¹ was immobilised by microorganisms and/or taken up by plant roots, and that most of the losses ocurred in wet soils (WFPS >60%) as N₂ and NH₃ losses (>55%).     

Reduction of nitric oxide at a flow-through mercury plated nickel electrode

The electrochemical reduction of nitric oxide at a flow-through mercury-plated nickel gauze electrode in sulphuric acid was investigated. The current efficiencies of hydroxylamine, nitrous oxide and of hydrogen formation were determined. The main experimental results are: 1. The ratio between the NH₂OH and N₂O formation depends on the cd and on the flowrate of the electrolyte through the electrode, but does not depend on the H₂SO₄ concentration in the investigated range from 0.25 to 2.0 M and likewise not on the temperature. 2. The rate of the reduction of nitric oxide to NH₂OH and N₂O increases with increasing cd up to a maximum value, thereafter this rate decreases with increasing cd. 3. The ratio between the current efficiency of the NH₂OH formation and the current efficiency of the N₂O formation increases slowly with increasing cathodic potential.It seems that at low cd (much lower than the cd where the rate of the reduction of NO reaches its maximum) the reduction of NO is affected by both the electrochemical parameters and by the transport of NO to the electrode surface. However, at high current densities the reduction is dominated by mass-transport of NO only. NOH is an intermediate for both the NH₂OH and the N₂O formation.     

Heavy fuel oil combustion in a cylindrical laboratory furnace: measurements and modeling

The finite-volume based commercial CFD-code Fluent was used to simulate the reacting flow in a heavy fuel oil fired laboratory furnace. Both the standard k−ε turbulence model and the Reynolds stress model (RSM) were tested. The combustion model was based on the conserved scalar (mixture fraction) and prescribed probability density function approach. The heavy fuel oil droplet trajectories were predicted by solving the momentum equations for the droplets using the Lagrangian treatment. The soot distribution in the furnace was calculated by solving a transport equation for the soot mass fraction. Simple expressions for the soot formation and oxidation rates were employed. The radiation heat transfer equation was solved using the finite volume method. The formation of thermal NO from molecular nitrogen was modeled according to the extended Zeldovich mechanism. Fuel-based NO was modeled assuming that all the nitrogen in the fuel is released as hydrogen cyanide (HCN), which then further reacts forming nitric oxide NO or molecular nitrogen N₂, depending on the local combustion conditions. The formation of prompt NO was also included in the calculations. The CFD-code was validated against experimental data for a combustor fired by an industry-type swirl burner for which the initial conditions of the spray have been characterized. It was found that the standard k−ε model does not satisfactorily predict the highly swirling flow field in the furnace. The RSM was able to improve the prediction of the flow field. The predicted gas species concentrations were found to be in a reasonable agreement with the measurements, except near the burner and in the vicinity of the furnace axis where discrepancies were found.     

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.