High Temperature Air Combustion: Sustainable

International Review of Mechanical Engineering (I.RE.M.E.), Vol. 6, N. 5 ISSN 1970 - 8734 July 2012

High Temperature Air Combustion: Sustainable Technology to Low NOx Formation Seyed Ehsan Hosseini, Mazlan Abdul Wahid, Abuelnuor Abdeen Ali Abuelnuor

Abstract – In recent decade, more stringent laws have been ordained to cope with environmental issues and global warming. Industrial sectors have been urged to substitute new combustion methods to decline their emissions, but the cost of pollutant reduction in traditional combustion is efficiency abatement. In the other word, emission and fuel consumption cannot be declined simultaneously by conventional combustion. High temperature air combustion (Hitac) is an innovative substitution for conventional combustion which has been developed to increase combustion efficiency and to decline pollutant formation contemporaneously. Recently, some valuable experimental and numerical analysis have been done to study the variety aspects of Hitac and to study the reasons of the compatibility of high efficiency and low NOx production in Hitac area. The outstanding characteristic of Hitac is its sustainability under low oxygen concentration when the combustion air is preheated more than the fuel auto-ignition temperature. Therefore, it can be observed that thermal NOx is suppressed due to lack of oxygen concentration. This paper is concerned with NOx formation reduction in Hitac systems via physical and chemical analysis. Chemical kinetic, heat transfer concepts, simulation studies and experimental investigations have been employed to analyze NOx formation mitigation in Hitac method. Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved.

Keywords: Hitac, NOx Formation, Preheated Air, Dilution, Flame

Respiratory irritation, asthma attacks, aggravation of heart diseases and lung damage are the main consequences of ground level ozone. . Indeed, acid rain formation, visibility reduction due to increase smog and particulate matter are other disadvantages of raising NOx production [3]. Furthermore, N2O is capable to convert to NOx in suitable conditions and it has been reported that the effect of N2O on global warming is 310 times more than dioxide carbon; consequently NOx has great potential to increase global warming indirectly [4]. By invoking to aforementioned disadvantages of NOx, it can be claimed that NOx is a kind of hazardous gases formed in combustion and it can endanger animal, vegetable and humankind life. Therefore, comprehensive investigation should be done to reduce NOx formation in combustion methods. Approximately, 79% by volume of air is nitrogen, thus in combustion systems which air is applied as an oxidizer, the presence of nitrogen in combustion process is unavoidable. However, under the specific circumstances in the furnace NOx is formed due to oxygen and nitrogen reaction [5]. Thermal NOx, N2O intermediate NO formation mechanism, prompt NOx, and fuelbound nitrogen are named as the main mechanisms for NOx formation in different combustion methods [6].

Nomenclature Ai, Bi, Ci K1,K2,K3 K-1,K-2,K-3 Kv Me Mf Ma

Reaction constant Reaction constant for forward reactions Reverse rate constants Recircultion ratio Flow rate of exhaust gases Flow rate of the fuel Flow rate of oxidizer

I.

Introduction

Generally, NOx which is an abbreviation for the combination of NO2 and NO is usually constituted in presence of nitrogen and oxygen within a locally high temperature conditions. Atmosphere can be jeopardized by increasing NOx formation in industrial sectors. Particularly, acid rain, ozone depletion and smog are mentioned as the main consequences of more NOx production [1]. Ground level ozone or so-call bad ozone is usually formed in presence of NOx according to following reactions [2]: NO2 O +O2

NO+O O3

H=57kJ, G= -51.3 kJ

(1)

H=142 kJ, G = 163.2 kJ

(2)

Manuscript received and revised June 2012, accepted July 2012

Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved

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Seyed Ehsan Hosseini, Mazlan Abdul Wahid, Abuelnuor Abdeen Ali Abuelnuor

II.

II.3.

NOx Formation Mechanisms II.1.

Malte and Pratt [13] introduced N2O intermediate NO formation mechanism which is occurred in lean fuel, moderate temperatures and low pressure combustion conditions. In these circumstances N2O is converted to NO via following formulas [14]:

Thermal NOx

Oxygen and nitrogen in extremely high temperature can react inside the combustion furnace according to following reactions which are called Zeldivich formulation [7]: O + N2

NO + N

(3)

N + O2

NO + O

(4)

N + OH

NO + H

(5)

N2O Intermediate NO Formation

N2 + O + M

N2O + M

(8)

N2O + O

N2 + O2

(9)

N2O + O

NO + NO

(10)

Following processes show the presence of H2O impurities conspicuously affect the N2O decomposition [15]:

where K1, K2, K3 are forward rate constants and K-1, K-2, K-3 are the reverse rate constants according to Table I [2]. Thermal NOx formation is accelerated exponentially according to formula (6) at temperatures more than 1500oC [9], [10]: (6)

H2O + O

OH + OH

(11)

N2O + OH

N2 + H2O

(12)

In Eq.(8), M is general third body. The N2O which was constituted in Eq.(8) decomposes by Eq.(9),(11). Eq.(13) shows the chemical kinetics law for the rate of NOx formation via N2O intermediate mechanism:

The rate of NO formation is achieved by formula (7) when the nitrogen radical (N) assumed in steady state conditions:

(13) The reaction rate constants are calculated by Eq.(14): (7) (14) If where T is temperature(kelvin) and the reaction constant , , where taken in Table I. Form formulas (6) and (7) it can be seen when the rate of reaction decreases, NOx formation declines because combustion takes place in limited time in the furnace. Moreover, NOx constitution mitigates in low temperatures.

The concentration of radicals like O, OH and H affect the concentration rates and NO constitution [1]. The N2O intermediate NO formation is accelerates in low oxygen concentration conditions [16].

TABLE I THERMAL NOX REACTION RATE CONSTANTS K1

[m3/gmol s]

K-1

[m3/gmol s]

K2

[m3/gmol s]

K-2

II.4.

[m3/gmol s]

K3

[m3/gmol s]

II.2.

Fuel Bond NOx Formation Mechanism

Fuel-bound NOx formation mechanism is occurred when the molecular structures of the fuel are constituted of nitrogen species [17]. In the combustion of these fuels the nitrogen atoms are decomposed to intermediate products form which can react with NOx [18].

[m3/gmol s]

K-3

then Eq (14) can be written as:

Prompt NOx

Prompt NOx formation mechanism was introduced by Fenimore in 1971. Prompt or Fenimore NOx formation occurs in fuel rich conditions (equivalence ratio greater than 1.2) [11]. The rate of prompt NOx constitution augments near equivalence ratio of 1.4 [12].

III. Hitac Hitac technology, emerged from 1990, and it has been successfully applied, specially, in metallurgy and steel industries of some developed countries. Flameless


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Oxidation (FLOX) in Germany, also known as High Temperature Air Combustion (Hitac) in Japan, Moderate and Intensive Low oxygen Dilution (MILD) combustion in Italy [19], or Colorless Distributed Combustion (CDC) [21], Low NOx Emission Injection in the US is a new combustion system which accomplishes low NOx emissions and high efficiency among several techniques. Postponed mixing of fuel and air and flue gas utilization in the flame zone are the fundamentals of Hitac [22]. The application of high temperature air combustion has been investigated experimentally [23][28] and numerically [29]-[35]. Fundamentally, Hitac is identified by aspects of turbulence and chemistry strongly [36]. The characteristics of flameless oxidation for various gas type fuels like methane or ethane [37], also for mixtures of gaseous hydrocarbons and hydrogen [38], and biogas [39], [40] has been analyzed. In addition, this method has been applied successfully for liquid fuels [41]-[43] and solid fuels [44], [45].

IV.

Fig. 1. Methane Hitac formation conditions

It can be seen that the high temperature reactants and low temperature products are the most important items in Hitac formation.

V.

In order to maintain the temperature of exhaust gases, the regenerative or recuperative heat exchangers are used in Hitac systems and the energy of flue gases is absorbed by this equipment to increase the temperature of the combustion air [50]. Higher efficiency is achieved by applying this secondary air [53]. According to experimental results NOx constitution decreases from 1500 ppm in conventional combustion to 100 ppm in Hitac by applying recuperative heat exchangers [54]. In Hitac systems the fuel nozzle surrounds by regenerators which are made by honeycomb [55]. These honeycombs recover around 72% exhaust gases energy [56].

Flameless Formation

In Hitac the chemical reaction is occurred between the fuel and highly diluted air inside the combustion furnace in temperature above the self-ignition of the fuel [46], [47]. In these circumstances the structure of the flame is altered and become pale [48], [49]. The energy of exhaust gas which is wasted from chamber stack is applied in Hitac as exhaust gas recirculation (EGR) system. Therefore, the combustion efficiency increases drastically in Hitac systems [40]. The performance of Hitac systems is evaluated by recircultion ratio (Kv) in Eq (15): Kv =Me/ (Ma +Mf)

Hitac Heat Exchangers

VI.

The Role of Diluted Air in Hitac

The color, size, luminosity, visibility and lift of distance of the flame are changed by the amount of oxygen in combustion air. Luminosity and visibility of the flame decrease when the oxygen concentration declines in the oxidizer. Lift-off flame and a large ignition delay occurred in conventional flame due to diluted oxidizer with nitrogen [57], [63]. An ultra-lean mixture is defined as a fuel/air lean premixed gas mixture with the fuel concentration near to the lower flammability limit [65]. Fuel ultra-lean mixture combustion cannot take place in the low concentration of oxygen; therefore air preheating method is the best option to accelerate the reactions [20]. NOx formation is significantly increases when air concentration increases due to flame temperature enhancement. Fig. 2 shows the linearity relation between NOx formation and oxygen availability [18]. The rate of NO production diagram in CFD simulation is more similar to reality when N2O-intermediate mechanism is taken into account. Experimental investigations confirm that Hitac systems are reachable with diluted oxidizer. Low oxygen concentration is the main factor to achieve the flameless combustion regime.

(15)

In this formula Me is the flow rate of exhaust gases before reaction, Mf is the flow rate of the fuel and Ma is the flow rate of oxidizer [50]. In order to have flameless mode it is necessary to heat up the chamber over the selfignition of the fuel. Therefore, the system should be run by traditional combustion at the first step. In order to transient from conventional flame to flameless mode the preheated and diluted oxidizer should be charged to the chamber by very high velocity. In these conditions the visible and audible flame becomes disappear and the reaction region spreads in whole of the combustion chamber [51]. As a result the uniform temperature is observed inside the chamber and hot spots are eliminated and thermal NOx is suppressed [7]. The recirculation ratio should be more than 2.5 and the chamber temperature more than 1100 in Hitac conditions [52]. Moreover, the rate of concentration of oxygen decreases by EGR application [27]. The suitable conditions for Hitac constitution has been shown in Fig. 1.



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To obtainn diluted oxidizer in Hitac systems, fluee gas recirculation by high mom mentum air innjection is on ne of the best optioons [50]. Alsoo, highly prehheated air which is constituted a large amountt of inert gasees such as N2, H2O and CO2 hass been appliedd to obtain Hitac H condition ns as shown in Figg. 3.

VIII.

The Effects E of Heat Transfe fer on NOx Formation in Hitacc

The T role of heat transferr in differentt combustionn furn naces have beeen investigateed by Khalil [65] by CFD D metthod. In flameeless mode, heeat transfer viia radiation iss don ne by suspend ded particles inside i the furrnace and thee cham mber walls which w were heated h during conventionall com mbustion by lu uminous flam me. Figure 5 illustrates i thee heatt transfer mechanisms in a Hiitac furnacee scheematically [58 8].

F 2. Excess air effects on NO formation Fig.

Fig. 5. Heaat transfer mechaanisms in Hitac fu urnace

Nabil N Rafidi et e al [59] stipulated that thhe amounts off partticles in Hitaac furnace are a more thaan traditionall com mbustion. Theerefore, the rate of heat transfer viaa nism from pparticles incrreases. As a radiiation mechan resu ult, hot spots are a omitted an nd the uniform m temperaturee can be observed d inside the furnace due to dispersedd reacction zone. In n these circu umstances the pale flamee temperaturee redu uces and therrmal NOx is surpassed s beccause thermall NOx is constituted in very high h temperature.. Also oxygenn conccentration pro omotes thermaal NOx. Indeeed, the role off resid dent time in thermal NOx formation is unavoidable.. Uniform temperaature inside Hitac H furnace is one of thee pow wer points off this combusstion method. Moreover,, dilu uted high temp perature air is injected insside the Hitacc cham mber in high velocity. v Therefore, T it iss concluded th hat thermal NO N x formationn is suppressed. s T The role of particles insside Hitac iss conspicuously no otable to prov vide moderatee temperaturee enviironment. Mo ore than 90% oof NOx production in Hitacc is reelated to N2O-intermediate mechanism [60]-[62]. It iss noteeworthy that the t reactions (14), ( (15), (16 6) involve thee oxy ygen radical, th hus this mechhanism is favorable in Hitacc with h low oxygeen concentrattion atmosphhere. Figure.66 show ws the summ mery of Hitaac specificatioons and NOx form mation mechanisms and confirms that the N2O-intermediate mecchanism is do ominance in NO N formationn in Hitac. It is i noteworth hy that in conventionall mbustion preh heated air in ncreases the efficiency off com com mbustion but NO N x formation n increases siimultaneouslyy due to increase th hermal NOx fo ormation.

Fig. 3. Schem matic of air preheeating and dilutio on system of MIL LD combusttion technology

Experimenntal investiigations connfirm that air combustion dilution d with H2O shows more m compatib bility with low NOx constitutioon and likew wise dilution with CO2 is more in agreementt with NOx foormation reducction rather than N2 diluter, because the speciific heat of H2O is more than N2 and CO2. Figure 4 depicts the NOx formation trrend in Hitacc system in different dilu ution conditions [558].

Fig. 4. The effe fects of dilution in n various conditio ons on NO x formaation

Copyright © 20 012 Praise Worthyy Prize S.r.l. - Alll rights reserved

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[4]

However, in Hitac high temperature reactant not only increases the efficiency of combustion but also reduces NOx formation dramatically [58].

[5]

[6]

[7]

[8]

[9]

[10] Fig. 6. Hitac specifications and NOx formation mechanisms [11]

VIII. Conclusion Thermal NOx which is named as the most probable mechanism in combustion phenomena is suppressed in Hitac due to low resident time, low oxygen concentration and moderate temperature inside the chamber. In traditional combustion the efficiency of furnace increases significantly by using preheated air in combustion process. However, NOx formation decreases drastically. Hitac has many features that are superior to traditional combustion. Not only fuel consumption but also the nitrogen oxide (NOx ) formation is kept at very low level due to EGR application in Hitac. These exhausted products are led into the fresh reactants inside the chamber; therefore high peak temperature is eliminated. As a result, NOx formation via thermal NOx mechanism is omitted, and other inconspicuous NOx formation methods in conventional combustion are remained. N2O-intermediate NOx formation mechanism is dominance method for NOx formation in Hitac due to its moderate temperature and lean fuel condition. The dual role of small particles inside the Hitac chamber is important. In one hand, heat transfer via radiation is occurred due to presence of these particles; therefore the temperature of pale flame declines dramatically and thermal NOx suppressed. On the other aspect, the presence of these radicals in mediate temperature and lean fuel condition lead the system to N2O-intermediate NO formation mechanism. The fuel consumption and NOx formation in Hitac systems reduce around 30% and 70% respectively compared to conventional mechanisms.

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Authors’ information Seyed Ehsan Hosseini was born in Esfahan IRAN at 23 August 1978. He recieved his Bachelor of Mechanical Engineering, heat transfer and fluid mechanics from Semnan University in Iran at 2001and Master of Mechanical Engineering from Universiti Teknologi Malaysia at 2012. He has around ten years work experiences in steel industries in IRAN. His research interests are conventional and MILD combustion, alternative fuel, renewable and sustainable energies and environmental issues. Mr. Hosseini is one of the members of High Speed Reacting Flow Laboratory (HiREF) in Universiti Teknologi Malaysia. Mazlan Abdul Wahid (Corresponding Author) was born at first of October 1966 in Malaysia. He received his PhD in 2003 from State University of New York at Buffalo and Master Degree in 1994 from University of Leeds, UK. He has co-authored two books and over 80 papers. He currently served as the Head of High Speed Reacting Flow Laboratory at Faculty of Mechanical Engineering, UTM and spearheading fundamental and engineering applications research of high speed flows in the presence of different physical phenomena such as combustion and heat transfer. He is also involved in the research related to turbulence combustion modeling, pulse combustion and supersonic reacting shock wave phenomena. Dr. Mazlan Abdul Wahid was the Chairman of the 10th Asian International Conference on Fluid Machinery, Kuala Lumpur, Malaysia 2009 and 4th International Meeting on Advances of Thermofluids, Melaka, Malaysia 2011. Abuelnuor Abdeen Ali Abuelnuor was born in Argo Sudan at 03 November 1970. He received his High Diploma in Mechanical Engineering University of the Nile Valley Atbara Sudan 1994, Bachelor in mechanical engineering Sudan University of Science and Technology, 1999, M.Sc. in Laser application in mechanical engineering, Sudan University of Science and Technology (SUST) 2007 and Currently he is working toward the Ph.D. degree at the Faculty of Mechanical Engineering (FKM), in Universiti Teknologi Malaysia (UTM) under Dr Mazlan Abdul Wahid supervision. He has around fourteen years work experiences in teaching in Sudan University of Science and Technology. His research interests are conventional and flameless combustion, Heat and mass transfer and Laser application in mechanical engineering.Mr. Abuelnuor is one of the members of High Speed Reacting Flow Laboratory (HiREF) in Universiti Teknologi Malaysia.



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International Review of Mechanical Engineering (I.RE.M.E.), Vol. 6, N. 5 ISSN 1970 - 8734 July 2012

Resolution of an Inverse Problem in Thermal Diffusion Process for the Identification of a Heat Flux Abdelkarim Maamar, Touhami Y., Bounegta B., Khadir M.

Abstract – In this work we propose a temporal determination of a heat flow, within a diffusive 2d linear medium with resolution of an opposite problem. Here 2 different methods of resolution are suggested and a comparison between the two is made. Regarding the first method of inversion that is based on a convolution integral for that it is necessary first of all to solve the direct problem which requires the construction of the step responses and the temperature measurements using the finite differences method. In the reversal phase, the linear system is considered under the form of single-input (the flow to be identified), multi-output (temperature measurements). The 2nd method is the regularizing method of the combined gradient, where the direct problem with all the assumed known parameters has an analytical solution to be used as observation to estimate the unknown parameters with resolution of an inverse problem. Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved.

Keywords: Linear Thermal Diffusion Process, Opposite Problem, Heat Flux, Convolution Integral, Combined Gradient, Finished Difference

Nomenclature a cp eT eSp h H(t) M(t) P u(t), y(t) dn 1 FLi J u, p n T R

I.

Thermal diffusivity, m2s-1 Specific heat, J kg-1K-1 Average standard diversion of the temperature,°C Average standard difference of heat flow, W m-3 Convection coefficient, W m-2K-1 Impulse responses Indexical Responses Heat source, W m-3 Vectors Descente direction The heat flow, W Functional gradient

In several industrial applications, a direct measurement of a surface heat flow or a temperature (inner wall of a combustion chamber of an engine or a piping of a steam generator, interface between plate and disc of brakes, external surface of a spacecraft penetrating the atmosphere, etc...) is delicate and sometimes impossible to realize. The determination of these surface sizes starting from transitional measures of temperatures carried out inside and with the faces of the solid constitutes an inverse problem of heat conduction. In spite of their practical interest, the methods able to solve inverse problems of heat transfer can be numerous in literature. Most techniques which made it possible to solve the inverse problems successfully is based on procedures for minimizing a standard deviation criterion [1][2][3].

Sensitivity function No future time

Greek Letters

n 1

II.

Thermal conductivity m-1K-1 Density, kg m-3 Laplacian Operator Depth descent

index/Exhibitors

Introduction

The System Studies and its Modelling

The geometry studied is L-shaped plate, shown in Fig. 1 it presents the results related to a building material of the concrete type. On all the plate sides, the boundary conditions of the flow field are imposed. One gives the law of variation of flow on the time interval [ 0, 6000s ], the evolutions of these three flows are represented on the figures (Fig. 2, Fig. 3 and Fig. 4). Here:

i, j, k, n, m

Manuscript received and revised June 2012, accepted July 2012


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