NOx Emissions
 
Utility Boiler Computer Modeling Experience in the USA for Practical Furnace Air port and Low NOx Burner Field Design
B Breen ESA Pittsburgh PA USA; J Urinch ESA Pittsburgh PA USA; and B Krippene BCK  of North America Inc, USA
Practical field experience with low NOx combustion systems and equipment on existing coal-fired utility boilers has presented significant engineering challenges to the designer. Many of these boiler units have required extremely imaginative or "high tech" engineered solutions in order to accomplish the necessary field design modifications for successful NOx emissions control at minimum impact and cost to the operator. Many of these challenging units have been investigated for optimum low NOx combustion system design through the use of computer modeling.
 
One example of these engineering challenges is how to minimize the emissions of both NOx and carbon monoxide while maintaining design or lower excess combustion air levels leaving the boiler unit and simultaneously minimizing the unburned carbon content or carbon loss in the ash. With some coals, this is not a problem because the coals are fairly porous, very reactive and will burn out in almost any furnace. With other coals the low NOx burner and furnace combustion air injection port settings, as optimized to achieve acceptable carbon monoxide emissions and carbon levels in the boiler ash, often compromise the boiler unit’s true potential for NOx emissions reduction.
 
These "hard-to-burn" or un-reactive coals, therefore, need to utilize all of the available furnace volume in a very effective manner. Computer simulations lead to a better understanding of practical combustion air addition and/or air injection profiles which help to maintain good combustion through efficient burnout while minimizing NOx formation.
A second example for engineering challenge is where very low NOx emissions leaving the boiler unit are required to be achieved by way of deep air-staging of the boiler-furnace burner zone area. This results in the potential for heavy flame impingement and reducing conditions on the lower furnace pressure part enclosures with a corresponding increase in furnace slagging and corrosion of the pressure parts. This is especially a problem on the higher pressure and temperature – more thermally efficient boiler units which, because of their higher use or capacity factors, are also required to be in-service more of the time. This requirement for higher boiler unit availabilities and reliabilities
creates an incentive for both the boiler operator and the system designer to investigate design changes in the low NOx burner and air port configurations which will minimize reducing environment type flame impingement on the furnace walls. Math modeling investigations not only of operating equipment settings or adjustments but also of moving or re-sizing burner and air port configurations often leads to the most practical low NOx combustion system applications.
 
Other practical investigations include the computer modeling of Fuel Lean Gas Reburn for NOx reduction in the upper furnace as well as the effect of primary air on burner settings and adjustments for minimum NOx when coal heating value or moisture content changes. This paper presents several examples of where effective furnace and low NOx burner modeling has produced substantial advantages to the low NOx combustion system designer. Using practical boiler furnace air injection port and low NOx burner math modeling as an integral part of the design process has often made the difference between a successful low NOx combustion system field conversion project and an unsuccessful one.

Formation of HCN and NH3 during the Pyrolysis and Gasification of Coal and Biomass
Z Xie, W Zhao, J  Feng and C Li, Monash University, Melbourne; K Pratt, Swinburne University of Technology, Melbourne; and K Xie Taiyuan University of Technology, China
A small set of rank-ordered Chinese (the Northern Hemisphere) and Australian (the Southern Hemisphere) coal samples were pyrolysed in Ar and gasified in CO2. A pair of Chinese and Australian coals of identical carbon content but very different petrographic composition behaved very differently in terms of the formation of HCN and NH3 during pyrolysis. The use of CO2 instead of Ar was seen to drastically affect the formation of NOx precursors at 800°C.  The exact effects of CO2 depend on the rank of the original coal substrate as well as the freshness of the char while it is held at high temperature. Reaction mechanisms leading to the formation of HCN and NH3 during pyrolysis and gasification are discussed.

Influence of Volatile Matter on NOx Emissions Behavior of Pulverized Coals and Blends
S Pisupati  Pennsylvania State University, USA
Coal properties are known to affect NOx emissions, ash behavior in the furnace and unburnt carbon loss.  Volatile matter content is the most important coal property that affect NOx emissions.  The objective of this study was to determine the influence of volatile matter content of coals and coal blends on NOx emissions.  NOx emissions are a strong function of temperature in the flame zone and the operation conditions (ie air staging, burner configurations etc).  The study used six coals with volatile matter content ranging from 19 to 36%, and two coal blends simulating the volatile matter content of tow medium volatile coals.  This study was conducted in 1,000 lb steam/h water-tube research boiler located at the Energy Institute of The Pennsylvania State University.  The study employed two burner configurations (a high swirl and a low swirl).  Most of the combustion air was admitted in three stages (zones) near the burner
 
The results showed that for all the coals and blends tested, air staging by increasing the percentage of air admitted down stream from the coal nozzle, the NOx emissions decreased.  Even at both extreme conditions (i.e. without any air staging and with maximum possible air staging) in the study, a guner4al trend of lower NOx emissions with increasing volatile matter content was observed.  The NOx emissions ranged from 0.79 to 1.15 lb/MMBtu with no air staging, and . 0.54 to 0.74 lb/MMBtu with maximum air staging for the low swirl burner configuration.  The NOx emissions ranged from 1.1 to 1.25 lb/MMBtu with no air staging and 0.6 to0 0.84 lb/MMBtu with maximum air swirl tested for the high swirl burner configuration

Measurements of Nitrogen Species, Evolutions and Reaction in Coal-fired Utility Boiler Flames
A Williams, J Pohl, B Stanmore, D Szczepanski, University of Queensland Brisbane; S Visona Sigma Process Solutions, Brisbane
CFD modelling may become a valuable tool for prediction of nitric oxide emissions in power stations around the world. Verification of these codes is an important step to determining their usefulness, accuracy and applicability to designing and optimising industrial boilers. Currently there is a lack of data in the mainstream literature of gas species concentrations, and char properties measured in an industrial sized, coal fired utility boiler. This paper aims to present measurements suitable for CFD code verification and to examine the assumptions made about the evolution and reaction of nitrogen species within an opposed-fired, pulverized coal-fired boiler. The assumptions discussed include whether
· nitrogen evolution from char is proportional to char burnout, and
· the importance of NH3, HNCO and HCN as NO precursors.
A water-cooled probe was used to collect gas and char samples within the flame region of a coal-fired boiler. Gas species concentrations were determined on site by FTIR spectroscopy and conventional measurements. Char samples were collected and analysed off site. The data collected suggested that the nitrogen evolution is proportional to coal burnout although an accurate estimate of the proportionality constant could not be determined. An estimate of this value found was 1.2. The role of HNCO and NH3 in the formation of NO was found to be small at the power stations tested, however only bituminous coals were tested.

Utilization of Advanced Low NOx Burner for Pulverized Coal
H Makino, H Tsuji and M Kimoto, Yokosuka Research Laboratory Japan
T Hoshino and T Kig, Harima Heavy Industries Co., Ltd Japan
Y Otake  The University of Queensland Brisbane
The advanced low NOx burner for pulverized coal combustion, which can reduce both NOx concentration and unburned carbon in fly ash, was applied to an industrial boiler. Highly-intensified internal recirculation zone, which accelerates the early stage of thermal decomposition of coal and thus reduces carbon content in char, was formed in the flame generated by this advanced burner. The burner has been developed with the following procedure. The influence of the burner configuration on formation of the internal recirculation zone was investigated by cold flow tests. Combustion tests were, then, conducted with two sizes of test furnace (coal feed rate; 0.1 and 1.5 t/h) in order to evaluate the influence of burner operating conditions on the emission of NOx and unburned carbon in fly ash. The scale-up effect was also investigated in these combustion tests. For the purpose of a practical application, sixteen advanced low NOx burners were installed to an industrial boiler and an Australian coal was fired. Fuel ratio of the coal was 1.5 and ash content was approximately 10%. NOx concentration and unburned carbon in fly ash were measured under various boiler operating conditions. NOx concentration with the burner decreased to about half compared that with a conventional low NOx burner when unburned carbon in fly ash with the advanced burner was the same as that with the conventional burner.

Gas-Diesel (Dual-fuel) Modeling in Diesel Engine Environment
A Bounif, A Aris and C Mansour  University des Sciences et de la Technologie Algerie
The aim of this paper is to investigate the emission and performance characteristics of a commercial diesel engine (Deutz FL8 413F) being operated on natural gas with pilot diesel ignition. A computer program has been developed to model the experimental data using a chemical kinetic reaction mechanism of the Gas-Diesel (Dual-fuel) combustion. A detailed chemical kinetic reaction mechanism of natural gas oxidation and NOx reduction was used to predict the main combustion characteristics temperature, pressure and species concentrations) under the conditions of this study. The following sections include a description of the experimental facilities, discussion of numerical simulation and engine test results. The performance in terms of accuracy of the networks is assessed by comparison with the experiments. A reasonably good prediction of performance and emission was obtained by computation covering the whole range of the engine operating conditions. It can be summarized that the results of this study are satisfactory.

NOx Predictions
P Bennett, Energy Tactics, Brisbane
Coal properties, such as volatile and nitrogen content, influence NOx emissions but there is no simple relationship between these coal properties and NOx emissions that fits all operational power plant. To help in the understanding of the interactions that occur during NOx formation several empirical models (fitting an arbitrary equation to data), simple reaction engineering models (combinations of CSTR and PFR react ors) using simplified chemical kinetics and complex computational fluid dynamics (CFD) models for the prediction of NOx emissions have been developed by organisat ions aro und the world. All methods have their limitat ions and advantages.
In a recent ACARP project 1 several models were formulated, these were:
• Reaction Engineering Model - used to model NOx formation in a pilot scale furnace,
• Empirical Model (coal properties) - used to show the influence of coal properties on NOx formation in a pilot scale furnace, and
• Empirical Model (power plant) - used to show the influence of power plant operation on NOx formation.
 
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