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Nox emission modelling from industrial steam boilers
Abstract
Previous researches often emphasize on the sources and effects of air pollutants in the environment and human population. A part of those studies were done in order to explore the spread or distribution pattern of those pollutants, especially regarding the emission from industrial steam boilers. It is very important to evaluate the transfer trend of air pollutants at both local and global scales. In this study, Industrial Source Complex Short Term Version 3 (ISCST3) model has been used to predict the distribution of NOx emitted from industrial steam boilers in the District of Hulu Langat, Selangor. The result of analyses indicates that the emission rates for steam boilers were ranging from 0.0083 kg NOx/hour to as high as 0.2771 kg NOx/hour while the total emission load was 1.9969 kg NOx/hour. The evaluation on dispersion contour shows that the concentration of NOx was higher in 1-hour reading than 24-hour value. The highest concentration of NOx was predicted to be within 104.65 km2 from the sources of NOx (steam boilers).
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Masaya volcano (560 m a.s.l.), Nicaragua, resumed its degassing activity in mid-1993 with the continuous emission of SO2 at rates increasing from 600 metric tons (t) day-1 (7.0 kg s-1) in 1995 to 1800 t day-1 (21.0 kg s-1) in 1999. The low-altitude gas plume is typically blown westward by the prevailing wind across the Masaya caldera and Las Sierras highlands, which are at a higher elevation than the gas vent. In this study, the areal distribution of atmospheric SO2 concentrations was monitored within 44 km of the vent with a network of passive samplers. Measured SO2 air concentrations ranged from <2 to 90 ppbv in 1998 and from <2 to 230 ppbv in 1999. The data suggest that the volcanic emissions influenced air quality across a 1,250-km2 area downwind. Local topography exerts a strong control on plume dispersal, and hilltops are particularly prone to fumigation and thus, to high ambient SO2 levels. In a zone 22 km2 in size located within 15 km of the source, the response of vegetation to sustained exposure to high atmospheric dose of volcanic SO2 and HF resulted in a strong reduction in the number of plant communities. A transition zone of somewhat indefinite boundary surrounds the devastated zones and exhibits vegetation damage in the form of leaf injury. In addition to the environmental impacts of the volcanic emissions, both short- and long-term public health hazards may exist in areas most exposed to the plume. The harmful effects of the volcanic emissions on cultivated vegetation could be diminished by using windbreaks made of gas-tolerant trees and shrubs such as Eugenia jambos, Brosimum utile and Clusia rosea. The current gas crisis at Masaya volcano provides an unique opportunity for investigating the atmospheric, environmental and medical impacts of volcanic gases and aerosols.
Most pulp and paper mills have boilers that burn hogged fuels to generate steam for mill use. The use of these fuels is important for off-setting the costs of purchasing fossil fuels and for disposing of wastes. Emissions from hog fuel boilers that are commonly regulated include carbon monoxide (in the range of 300 ppm to 800 ppm) and nitrogen oxides (ranging from less than 0.1 lb/million Btu to over 0.40 lb/million Btu). Nitrogen oxide (NOx) emissions can be reduced by decreasing excess air but after a certain point this will lead to an increase in carbon monoxide (CO). In an example shown in this paper, NOx was reduced from 0.32 lb/million Btu at 6% O2 to 0.18 lb/million Btu at 3% O2. But, at 3% O2, CO increased to over 2,000 ppm. Many boilers have poorly designed combustion air systems resulting in poor mixing of air and combustibles. As a result, the boilers are fired with too much excess air, particularly undergrate air. This increases the amount of particle carryover as well as NOx emissions. Computational Fluid Dynamics (CFD) models can be used for designing improved combustion systems for hog fuel boilers. Recent work has resulted in enhanced algorithms for prediction of CO and NOx. The algorithms are based on fundamental equations which are then refined using data from operating hog fuel boilers. Modeling CO predictions for approximately 20 hog fuel boilers gave qualitative agreement with measurements, but also point to boiler operating factors such as unsteady or non-uniform fuel and air delivery that cannot be easily replicated in models. Quantitative calculations of NOx emissions are challenging, but algorithms incorporated in CFD models show the mechanism of air staging on NOx formation. Before and after air system upgrade comparisons of emissions from hog fuel boilers are often not meaningful due to differences in operating conditions and changes in boiler configuration. CFD models can also be useful in projecting intermediate or theoretical operating conditions for making more direct comparisons. In one example of an air system upgrade, where data was available, O2 could be decreased to 3% without any increase in CO above 200 ppm. At this level of excess air, NOx was approximately 0.26 lb/million Btu.
The water/oil emulsified fuel consists of base fuel oil and water doped with or without a trace content of surface active agents. In this study, the water/oil emulsified fuel was successfully produced without usage of any surface active agents. An experimental study has been implemented in an industrial steam boiler to investigate the combustion characteristics of this emulsified fuel by analyzing the exhaust gas emissions and the thermal efficiency of the boiler. The experiment demonstrated that the water/oil emulsified fuel is effective for NOx and soot emission suppression as well as thermal efficiency improvement. These effects were also demonstrated for the commercially operated water heating boiler.
Dilution has long been considered a solution to many problems of toxic/flammable material releases. It implies diluting to a concentration that is below physiologically dangerous levels for a toxic substance (generally below TLV), or to a level below LFL for a flammable material release, ensuring that the process adopted for dilution does not itself enhance the risks.In this paper, we discuss the dilution of a gaseous release by deliberate and cautious mixing with air to reduce its concentration to a harmless level. The idea bears its origin to the Bhopal Gas Tragedy where some families saved themselves by turning the ceiling fans on when MIC reached their bedrooms at the dead of very cold night on December 2–3, 1984. The air pushed in by the fans diluted the MIC to below the harm level.Some of the advantages of using air dilution are: no cost of air, no air storage needed, no need to treat the air after use as in case of water curtains; required equipment, its maintenance and staff training in its use are very likely to cost less than in other ways of handling a release.Air dilution may not be feasible in all cases, such as gaseous release within a congested equipment layout, release that forms a liquid pool, etc. The method needs to be evaluated for each case.
Modeling of effects of fuel quality on the emissions of major pollutants (NOx, SO2, CO2 and PM) and eight trace elements (As, Co, Cr, La, Mo, Ni, Sb and U) from a 300-MW boiler unit fired with Thai lignite was the main focus of this study. The NOx and SO2 emission models were validated with the use of experimental data. Emission rates and specific emissions (per MWh) of the major pollutants and trace elements were quantified by including efficiencies of the flue gas desulphurization system and electrostatic precipitators in the computations. As shown in this work, the contributions of 300-MW boiler units fired with Thai lignite to the “greenhouse” and “acid rain” gas emissions in the region are significant. Additionally, substantial amounts of hazardous As, Cr and Ni are emitted from the boiler units into the atmosphere via fly ash particles.
The problem of plume dispersion around urban buildings has been investigated by physical modelling using arrays of building-like obstacles at two scales—1 : 100 in a boundary layer wind tunnel and 1 : 10 at a field site. The particular effect of obstacle width-to-height ratio (W/H) was examined for a fixed obstacle packing density. At short distances from the plume source the concentration profiles in the obstacle arrays were quite variable, but at distances beyond two rows the concentration profiles were well approximated by a Gaussian distribution laterally and a reflected Gaussian vertically. A comparison of the Gaussian plume parameters as a function of nondimensional distance from the source shows the influence of the obstacles. Compared to plumes in the open terrain, the plumes in the arrays were typically 2–4 times as wide. At short to intermediate distances the plume width was closely related to the width of the obstacles. An effective vertical displacement of the plume was also found; it was typically about one building height. This resulted in a net vertical growth of the plume which was typically 2–3 times larger than that in the open terrain at similar distances from the source. In the arrays of obstacles with W/H>1, significant bodily deflection of the plume occurred for nonorthogonal wind directions. Compared to the wind tunnel, there was more scatter in the field results, and some effects of the larger scales of turbulence were observed. However, the field results were qualitatively and in most cases quantitatively the same as those from the wind tunnel.
Urban air quality is an issue of major concern across many cities in India. In particular, high levels of particulate matter (both SPM and RSPM) are responsible for noncompliance to air quality standards. Air quality modeling is an effective tool to simulate the air quality of a region and to predict air quality concentrations under different scenarios. Kanpur city which is a top-ten urban conglomerate in India (based on population) is chosen for the application of the ISCST3 model and simulation of air quality. Sectored emission loads are estimated for transport, industrial, power, and domestic sectors, which provide an estimate of the major contributors to air pollution with specific reference to particulate matter, which is a major pollutant of concern. A detailed scenario analysis is carried out to estimate the changes in emissions that would take place due to various interventions. Dispersion modeling is carried out using the ISCST3 model, to estimate the concentrations of SPM all over the city under different scenarios. Emission inventory and meteorological data served as input to the model, and the air quality is predicted for various seasons and intervention scenarios. The modeled values for the scenario without intervention results in an underestimation of 48%, which is due to unaccountable or unidentified sources, trans-boundary movement of SPM, and model calibration errors. To overcome the error, the model is calibrated with the observed values and results are obtained for other scenarios using the calibration factor. The paper demonstrates only the research direction currently used to simulate air quality in Indian cities. However, further refinement and research is required before it could be used for more accurate predictions.
To meet the coming, more stringent NOx emission standards for small boilers, a self-recirculation type burner, in which a fresh air is injected through eight nozzles, arrayed peripherally around a liquid fuel-spray nozzle, while the hot burned gas is recirculated within the furnace and entrained into the injected air streams has been experimentally investigated. The flowfield has been determined with a PIV system and spatial distributions of NOx, temperature, O2 and CO have been determined with use of chemi-luminescence, thermocouple, magnetic pressure type analysis, and non-dispersive infrared analysis, respectively. 2-D flame chemi-luminescence images have been obtained by an ICCD camera synchronized with the PIV system. Results show that the burned gas is recirculated in the furnace and successively entrained into the air streams. As a result, NOx emission is drastically decreased. In the present prototype burner, the amount of a recirculating gas entrained into the air streams attains 50% the fresh air by volume and the NOx level is decreased to 22 ppm from its original value of 108 ppm without recirculation. Further measurements show that besides the recirculating flow outside the air nozzles, another recirculating flow is induced around the center axis inside the injected air streams. Combustion is stabilized around this inner recirculation zone. With an increase in the air flow rate while keeping the spraying fuel flow rate constant, this inner recirculation zone expands and its centre approaches the fuel nozzle. The evaporation of liquid fuel is promoted and a blue flame zone is established. The NOx level is further reduced to 18 ppm when the air excess ratio is increased to 1.6. Thus, the present, simple device of self-recirculation type burner can successfully reduce NOx emission without great cost and space.
Air-surrounding-fuel is a well-known concept used within tangential and wall-fired boilers. Here, we report for the first time on industrial experiments performed to study the effects of this concept on a 660 MWe full-scale down-fired boiler. Data are reported for the gas temperature distributions along the primary air and coal-mixed flows, furnace temperatures, gas compositions, for example O2, CO and NOx, and gas temperatures in the near-wall region. The influence of concentration control valve (CCV) opening on combustion and NOx emission in the furnace were determined. The results show that the flame stability, temperature distribution, unburnt carbon are influenced by both concentration ratios and fuel-rich flow velocities. As CCV opening increases, NOx emissions decrease from 2594 mg/m3 to 1895 mg/m3. Considering altogether economic benefits and environmental protection issues, 30% is the optimal value for the CCV opening.Research highlights► The application of air-surrounding-fuel concept on down-fired boiler. ► Experiments on a 660 MWe down-fired boiler, of the largest capacity in the world. ► The concentration ratio of the fuel-rich flow should be neither too large nor too small in down-fired boiler. ► A 30% CCV opening appears to be the better option for this boiler retro-fitted with air-surrounding-fuel concept.
Full-scale experiments were performed on a 300Â MWe utility boiler retrofitted with air staging. In order to improve boiler thermal efficiency and to reduce NOx emission, the influencing factors including the overall excessive air ratio, the secondary air distribution pattern, the damper openings of CCOFA and SOFA, and pulverized coal fineness were investigated. Through comprehensive combustion adjustment, NOx emission decreased 182Â ppm (NOx reduction efficiency was 44%), and boiler heat efficiency merely decreased 0.21%. After combustion improvement, high efficiency and low NOx emission was achieved in the utility coal-fired boiler retrofitted with air staging, and the unburned carbon in ash can maintain at a desired level where the utilization of fly-ash as byproducts was not influenced.
The potential air pollution from the fuel combustion in the uncontrolled industrial boilers in Kocaeli, the most industrialized area in Turkey, was evaluated by using the emission factors determined based on the flue gas measurements conducted at over 100 industrial plants in the area. The emissions were modeled for conventional combustion pollutants by a Gaussian dispersion model to determine the possible contribution of these emissions to the background air pollution. Results show that the emission factors calculated for the pollutants including CO, SO2, NO2, PM and formaldehyde are comparable with those given in literature for other countries. On the other hand, the emission factors are laid in a wide range due to the presence of many factors affecting the emission of the pollutants, including the characteristics of fuel, boiler and combustion practice. The Gaussian air dispersion model applied by using the emission factors indicated that the effect of the industrial fuel combustion on the background air pollution was low for the conventional combustion pollutants despite the high number of industrial boilers in the area, suggesting the significant contribution of other sources, i.e. residential heating and mobile sources.
The present work introduces an approach to predict the nitrogen oxides (NOx) emission characteristics of a large capacity pulverized coal fired boiler with artificial neural networks (ANN). The NOx emission and carbon burnout characteristics were investigated through parametric field experiments. The effects of over-fire-air (OFA) flow rates, coal properties, boiler load, air distribution scheme and nozzle tilt were studied. On the basis of the experimental results, an ANN was used to model the NOx emission characteristics and the carbon burnout characteristics. Compared with the other modeling techniques, such as computational fluid dynamics (CFD) approach, the ANN approach is more convenient and direct, and can achieve good prediction effects under various operating conditions. A modified genetic algorithm (GA) using the micro-GA technique was employed to perform a search to determine the optimum solution of the ANN model, determining the optimal setpoints for the current operating conditions, which can suggest operators’ correct actions to decrease NOx emission.
The near-wake entrainment behaviour of a limited range of isolated obstacles, representative of those found in urban areas, was investigated in the field at a nominal scale between 1/10 and 1/20. Tracer gas was released from a continuous point source located upwind of the obstacles. It is shown that concentrations in the lee of different obstacles depend on the proportion of the plume entrained and on the dimensions of the region of recirculating flow. The presence of taller obstacles results in a reduction of ground level concentrations. The effect of longitudinal, lateral and vertical source displacement was also examined for the different obstacles in the field. The field experimental programme was supported by physical modelling in the wind tunnel at a nominal scale of approximately 1/100. Dispersion around a single cube was mainly investigated in the wind tunnel and the results confirm in general the findings of the field trials. However, comparisons between wind tunnel and field results clearly show that the plume is more dispersed in the field, attributable to the effect of additional wind meander occurring in the atmosphere. Accordingly, centreline concentrations are higher in the wind tunnel and the effect of lateral source displacement is more pronounced in the wind tunnel than in the field.
A nonlinear five-dimensional mathematical model is proposed and analyzed to study the removal of a gaseous pollutant and
two different particulate matters by rain from the atmosphere of a city. The atmosphere, during rain, is assumed to consist
of five interacting phases namely, the raindrops phase, the gaseous pollutant phase, its absorbed phase and the phases of two
different particulate matters, one being formed by the gaseous pollutant.We assume that the gaseous pollutant is removed from the
atmosphere by the processes of absorption while the two particulate matters are removed only by the process of impaction with
different removal rates. By analyzing the model, it is shown that under appropriate conditions, these pollutants can be removed
from the atmosphere and their equilibrium levels, remaining in the atmosphere, would depend mainly upon the rates of emission
of pollutants, growth rate of raindrops, the rate of raindrops falling on the ground, etc. It is found that if the rates of conversion of
gaseous pollutant into the particulate matter and rainfall are very large, then the gaseous pollutants would be removed completely
from the atmosphere.
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