Combustion Principles and Control , Student ID: E34007101 Instructor : Professor Hsin ChuDepartment of Environmental Engineering,National Cheng Kung UniversityJanuary, 2014

TITLE:A study on two types of Low NOx Burners: Technology and Application

OUTLINE:

IntroductionNew technology: the creation of an ultra-lean premixing of the fuel with air and furnace flue gas.An application: Advanced CI-a Low NOx Burner in pulverized- coal combustion Conclusion/Summary References

Introduction:(review for exam!)

In order to better understand the NOx reduction technique of Low NOx Burner (LNB), a brief review of the basic theory of NOx formation should first be discussed.

Nitrogen Oxides, Nox

N2Onitrous oxideNOnitric oxideNO2nitrogen dioxide Only NO and NO2 relate to combustion systems

Routes by which NOx is formed Thermal NOx: formed by the oxidation of nitrogen present in the combustion air Fuel NOx.: produced by oxidation of nitrogen bound molecules contained in the fuelPrompt NOx: produced by high speed reactions at the flame front

The need to study oxides of nitrogen (NOx) and its routes of formation is because it has been found to be one the greenhouse gases that greatly contributes to the phenomenon of global warming. It also has been categorized as a contributor to acid rain and photochemical smog.

EC directive 88/609/EC sets the following emission limits for large plant (> 50MW):

FuelNOx Emission (mg/m3)

Solid fuels650High-ranking coal1,300Liquid fuels450Gaseous fuels350

Of all the techniques presently being available, LNBs stand out because of its low operating cost, is compatible with flue gas recirculation and can be used in combination with SNCR or SCR.

Conventional burner vs. LNB?

Conv. Burner incorporates single point injection of the fuel, followed by the rapid mixing of the fuel and air by a swirling air supply.

LNB- Upon combustion, the fuel and air are split into stages giving for a more gradual mixing; firstly under fuel rich conditions which serves to reduce the NOx then there is a supply of additional air (fuel-lean) so that all the HC is combusted; tf is lowered

Due to tightened NOx, regulations in California and newly proposed regulations in Texas, processing facilities are forced in some areas to reduce Nox, emissions below 10 ppm on large heat release units.

A new generation of low NOx burner technology has been developed for eductor-style, radiant-wall burners used in ethylene cracking furnaces.

The key difference, however, between previous burner technology and the new technology is the creation of an ultra-lean premixing of the fuel with air and furnace flue gas.

NEW TECHNOLOGY:

The new low NOx burner passes 100 percent of the combustion air through the eductor system using the motive energy of the primary fuel gas. As the excess air levels in the primary combustion zone are increased (making the mixture more fuel-lean), more air is present to absorb additional heat from the flame, and NOx emissions continue to reduce

Secondly, staged fuel is injected at a location inside the furnace so that it is able to mix with inert combustion products before encountering the remaining oxygen in the ultra-lean primary combustion.

Then, upon combustion, the high level of excess air cools the flame by absorbing heat, resulting in a reduction in NOx emissions.

Reliable air entrainment performance is achieved via semi-empirical modeling (computerized).

Theoretical and experimental evidence has shown that thermal NO, formation is highly dependent on the flame temperature

Two burners firing under identical conditions in the same furnace at a heat release of 0.94 MM BTU/hr.

TREND:

As the secondary fuel split increases, the NOx, levels decrease.

As we increase the fuel split in the secondary, we decrease the primary fuel flow rate, raising the air fuel ratio in the eductor. Recall an increase in the air-fuel ratio reduces the adiabatic flame temperature and, therefore, the NO, formation rate.

Coal-fired thermal power plants mainly involve pulverized coal combustion, for which environmental pollutants, particularly NOx emissions, must be reduced.

Among low-NOx combustion technologies, the staged

combustion method is the most effective for pulverized coal combustion.

Some of the combustion air is separated from the burner is supplied via injection ports mounted on the rear of the furnace, and hence the generation of NOx near the burner is suppressed due to the weakened oxidation atmosphere

Advanced CI-a Low NOx Burner in pulverized- coal combustion

However, unburned carbon tends to increase, which decreases the combustion efficiency and makes it difficult to utilize fly ash as a component of cements or concretes.

It has therefore been desired to develop a new combustion technology, by which NOx can be reduced without the increase of unburned carbon

Hence, an advanced low-NOx burner called CI-a was developed.

CI-a Burner:

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The primary air has a straight motion, and secondary and tertiary air have strong swirling motions.

This recirculation flowlengthens the residence time of pulverized coal particles in the high-temperature field near the burner outlet accelerates the evolution of volatile matter and the progress of char reaction

Therefore, the amount of unburned carbon is effectively reduced, but the NOx concentration increases in this region.

However, NOx is immediately reduced to N2 in the reduction flame existing after the recirculation zone. Furthermore, this NOx reduction effect is promoted by the staged combustion method, in which some of the combustion air separated from the burner is supplied via injection ports mounted on the rear of the furnace.

The test furnace used is that at the Yokosuka Research Laboratory of CRIEPI, in which the CI-a burner with a coal combustion capacity of about 0.1 t/h is installedComputational Domains and Conditions:

The swirl vane angles for secondary and tertiary air are set at 81 and 72 , respectively, which are optimum values for bituminous coal (these values are zero when the swirl force is zero).

The pulverized coal is carried by the primary air.

Conditions:The computational conditions are given to correspond

with our experiment .The coal feed rate is about 97 kg/h). The air ratio is 1.24, and the excess O2 concentration at the furnace outlet is 4.0%.

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Conditions:The staged combustion air ratio is set at 30%. The mass ratio of the pulverized coal (dry base) to the primary air is 1:2.2, and the mass ratio of secondary air to tertiary air is 1:6. The test fuel is Newlands bituminous coal.

The computation is performed using the STAR-CD code.

Fig. 5(b) shows that the recirculation flow can be seen in the region near the burner.

The CI-a burner was designed so that the large recirculation flow forming in the central region lengthens the residence time of pulverized coal particles in the high-temperature field, which accelerates the evolution of volatile matter and the progress of char reaction.

Only the coal

particles containing unburned carbon are plotted.

the small particles with diameters of 5 and 20 vanish due to the evolution of volatile matter and char reaction in this near-burner zone, whereas the larger particles are carried downward while undergoing reaction

This is attributed to the fact that these particles are caught by the recirculation flow, which appears in the central region (arrow B in Fig. 5(b) ) and their residence times in this zone are lengthened.

On the other hand, the regular motions for the larger coal particles are due to the fact that they have large inertias and hence are hardly affected by the recirculation flow.

It can be concluded that the recirculation flow formed by the CI-a burner effectively increases the particle residence time in the region near the burner.

Conclusion:It was verified that recirculation flow is formed in the upstream high-gas-temperature region near the CI-a burner outlet, and this lengthens the residence time of coal particles in the high-temperature region, promotes the evolution of volatile matter and the progress of char reaction, and produces an extremely low-O2 zone for effective NOx reduction.It was determined that this new low NOx burner technology can generate NO x emissions < 10 ppm (at 3% O2) without significant effect on the thermal efficiency of the conventional system.

References:Bussman, W., Poe, R., Hayes, B., McAdams, J., & Karan, J. (2002). Low NOx burner technology for ethylene cracking furnaces. Environmental Progress, 21(1), 1-9. doi: 10.1002/ep.670210107Khanafer, K., & Aithal, S. M. (2011). Fluid-dynamic and NOx computation in swirl burners. International Journal of Heat and Mass Transfer, 54(2324), 5030-5038. doi: http://dx.doi.org/10.1016/j.ijheatmasstransfer.2011.07.017Kurose, R., Makino, H., & Suzuki, A. (2004). Numerical analysis of pulverized coal combustion characteristics using advanced low-NOx burner. Fuel, 83(6), 693-703. doi: http://dx.doi.org/10.1016/j.fuel.2003.07.003Moore, M. J. (1997). Nox emission control in gas turbines for combined cycle gas turbine plant. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 211(1), 43-52. doi: 10.1243/0957650971536980Rrtveit, G. J., Zepter, K., Skreiberg, ., Fossum, M., & Hustad, J. E. (2002). A comparison of low-NOx burners for combustion of methane and hydrogen mixtures. Proceedings of the Combustion Institute, 29(1), 1123-1129. doi: http://dx.doi.org/10.1016/S1540-7489(02)80142-0Zhou, H., Yang, Y., Liu, H., & Hang, Q. (2014). Numerical simulation of the combustion characteristics of a low NOx swirl burner: Influence of the primary air pipe. Fuel, 130(0), 168-176. doi: http://dx.doi.org/10.1016/j.fuel.2014.04.028

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