REV. CHIM. (Bucureºti) ♦ 59 ♦ Nr. 6 ♦ 2008698

The Nitrogen Monoxide Generation and Diminishingto the Co-combustion Corn-coal

IONEL PΪÃ*Bucharest Polytechnical University, 313, Splaiul Independentei, 060032, Bucharest, România

The study analyses the nitrogen monoxide generation and reducing during the co-combustion of biomass(corn-cob) in a power plant fired with pulverised coal. The physical and mathematical model shows thenitrogen monoxide generation and the reducing of its concentration by the air staging and by the combinationof the primary methods.

Keywords: biomass, combustion installations, co-combustion, pollution

The developing of the sources of renewable energy is atop priority of the European Union. Within this framework,the biomass area will bring a major contribution in solvingthe request of energy for the future due to the biomasspotential and to the available technologies. One of theopportunities of development the biomass is itscombustion together with the coal. In this paper, the authorstudied the co-combustion process and the generation andreducing the concentration of nitrogen monoxide duringthe combustion of the coal-corn mixture (the corn is usedas fuel under the form of cob, being crushed and thenground in the coal mill). In the physical and chemicalprocesses from the furnace, the appearance of the thermaldecomposition and the combustion of the releasedvolatiles lead to a developed mathematical model, in orderto be closer to the analysed real processes. In comparisonwith previous models [1], this paper takes into accountthe fact that not all the nitrogen quantity is transformed inNO. This decrease is studied in different types of equationsin [2]. The exposed model considers the NO concentrationdecreasing caused by the heterogeneous reaction of thenitrogen monoxide on carbon surface. The NO realconcentration is thus obtained.

The purpose was to calculate the nitrogen monoxideconcentration and its main influences, in order to decreasethe nitrogen monoxide concentration. The nitrogenmonoxide results from the nitrogen oxidation existing inthe fuel organic matters and from the oxidation of thenitrogen introduced together with the combustion air. Theinfluence of these characteristics was quantified after themathematical model was obtained and applied. This paperpaid attention to the primary air excess coefficient, to theinitial diameter of the particles of equivalent fuel and tothe gravimetric participation of the corn to the mixture.The internal recirculation of the combustion gases at theburning inlet, the recirculation of the combustion gases atthe furnace end-part, the value of the air excess coefficientand the introduction time of the secondary air have beenconsidered constants.

The method of successive air injections (air staging) atburning inlet (or after an established law) was consideredthe primary reducing method (basic). Other primarymethods were, also extended, in the same time, in orderto obtain, if it is possible, a NO concentration lower thenthe value allowed by environment protection norms. Thus,the secondary methods, which are expensive and requestsupplementary locations, are eliminated.

* email: ipisa@caz.mecen.pub.ro

The mathematical modelThe co-combustion total time was divided into three

distinct stages and the secondary air introduction wasconsidered to be done according to a linear time low. Theco-combustion took place by an infinite number of stages,as it was shown in [1]. This co-combustion is called also“co-combustion with continue access of secondary air”.Thus, the equation structure [3], which forms themathematical model, was changed. The spray corn-coalco-combustion layout, completed by the interferingparameters, is shown in figure 1.

Also, the mathematical model functions have adynamics related to the period of co-combustion. Thisdynamics is presented in figure 2. The co-combustion timewas divided in three stages, Z1, Z2, Z3, resulting, for eachone of them, the characteristic mathematical model fortheir own physical and chemical processes.

The mathematical model for Z1This stage corresponds to the interval 0 < τ < τ1, where

τ1 is the moment when the introduction of the secondaryair starts. It mentions that, τ1 is immediately after τaprv,volatiles ignition time.

The equations for Z1 are:- the released volatiles dynamics:

(1)

- the volatiles combustion dynamics:

(2)

Fig.1. The sprayed coal combustion layout

[kg/(kg.s)];

[kg/(kg.s)];

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- the equivalent fuel temperature dynamics:

Fig.2. The combustion dynamics of thesolid fuel particle (qualitative

presentation)

(3)

where:Qk is calorific power of coke (heating value), kJ/kg;ck - specific coke heat, kJ/(kg.K);

- the powdered equivalent fuel apparent density, kg/m3;

ρk - coke density, kg/m3;

(4)

where :GFL is the thermal radiation coefficient, between the

radiant bodies within the furnace and the burning powderjet;

Vgz1 - specific combustion gases volume of Z1 stage, referring to one kg of dust equivalent fuel;

V0aum - theoretical wet air volume requested for the

combustion process, Nm3/kg;vv - specific volatiles volume, Nm3/kg;

Qv - calorific power of volatiles, kJ/kg; cCO2, cO2

- specific volatiles heat of carbon dioxide, respectively of oxygen kJ/(kg.K).The computing relation for Vgz1 is:

(5)

Tfl - flame temperature,K; ak - coke energy emission coefficient;αg - thermal convective coefficient [4], kW/(m2.K);Vie - initial volatiles volume of the equivalent fuel, kgvolatiles/kgfuel;- the combustion gases temperature dynamics:

where:VO2 is the specific oxygen volume required by the co-

combustion process, as follows: “Cfi” index referring to 1

kilo fix equivalent fuel combustion and “v” index referringto 1 kilo of volatiles;

Vgo

- resulted combustion gases volume, on the samereference base;

Cfie - the initial content of fix equivalent fuel;

λ i ,λ f ,λev ,λp - air excess coefficient at the burning inlet,at the furnace end-part, at the “discharge”(economiserarea), respectively the primary air excess coefficient;

revp, ri, rf – the recalculation degree from the “discharge”(combustion gases for primary inlet), from the burninginlet, respectively from furnace end-part;

ζ - combustion gases rising volume coefficient.- the diameter dynamics of the equivalent fuel particle

is described by the relationship:

(6)[Nm3/kg];

[m/s]

REV. CHIM. (Bucureºti) ♦ 59 ♦ Nr. 6 ♦ 2008700

where MO2k is the quantity of oxygen required by coke co-combustion, kgO2 / kg coke.

The model takes into consideration the nitrogenmonoxide generation from air and equivalent fuel:

- kinetics of the nitrogen volatilisation of the equivalentfuel organic mass:

(7)

where: Nie is the concentration of the atomic volatilised nitrogen

existing in the initial equivalent fuel organic mass, kgnitrogen/kgfuel;

γ - fraction of the initial nitrogen which can be volatilised.

- the dynamics of the molecular nitrogen generationfrom the volatilised nitrogen:

(8)

- the generation velocity of the released nitrogenmonoxide from the nitrogen of the equivalent fuel organicmass, taking into account its heterogeneous reduction onthe coke basis surface – negative term – is expressed by:

(9)

- the nitrogen monoxide generation from the nitrogenof the combustion air, according to the Zeldovici’s low:

[kg/(Nm3.s];

[kg/(Nm3.s];

[kg/(Nm3.s];

(10)

This stage includes the initial moment, τ = 0. At thismoment, the oxygen concentration resulted from primaryair combustion, from combustion gases recirculated at the

furnace end-part and from the combustion gasesrecirculated at the steam generator end-part, is:

(11)

where MO2 is the quantity of oxygen required forcombustion, “Cf

i” index refers to the combustion of 1 kiloof fix equivalent fuel, while “v” index refers to thecombustion of 1 kilo of volatiles.

In this stage, the concentration of the molecularnitrogen resulted from air, is:

(12)

The mathematical model for Z2This stage corresponds to the interval τ1 ≤ τ ≤ τ2 (fig. 2).

After the volatiles were ignited (We > 0) the secondary airand the recirculated combustion gases are introduced.

The equations of this stage are as follows:- equation (1) for the released volatiles dynamics;- equation (2) for the volatiles combustion dynamics;-equation (3) for the equivalent fuel temperature

dynamics;- for the gaseous phase temperature variation:

(13)

where:cam is the specific heat of mixed air and combustion gas

at “discharge”, kJ/(Nm3.K);tam - the secondary mixing temperature at the burning

inlet, °C;cg - the specific combustion gas heat, kJ/(Nm3.K);Vgz2- gaseous phase volume, corresponding to Z2. Vgz2 is

computed by the formula:

(14)

which shows the linear ascending (in time) of secondaryair introduced in jet.

- the diameter variation of the equivalent fuel particle ispresented in relation (6);

- equation of nitrogen volatilisation dynamics is:

(15)

The other equations are identical with the ones of Z1,while for Z2 the difference is given by oxygen concentration,which is calculated by the relationship:

(16)

[kg/(Nm3 .s)];

[Nm3/kg];

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while the concentration of the molecular nitrogen resulted from the air is:

(17)

The Z2 stage end, corresponding to τ2 = τ1+∆τ , isconsidered at the moment when the volatiles were burned.Also, at the moment t2 the secondary air introductionended. The Z2 can have variable length, depending of thefuel type. The time interval ∆t is established at thebeginning of the computation. In this case, the optimalvalue was considered 0.2 s.

The mathematical model for Z3The Z3 stage starts at the moment τ2 and ends at the

moment when the coke basis has burnt. The Z3 stage liesin the interval τ2 ≤ τ t ≤ τa, where, the total burning time ofequivalent fuel particle is τa. Z3 equations, that set thedifference between this stage and the other two, arepresented below:

- gaseous phase temperature variation:

(18)

- nitrogen existing in equivalent fuel volatilisationdynamics:

[kg/(Nm3.s)]. (19)

For this stage the gaseous phase volume is calculatedby: [Nm3/kg]; (20)

- current oxygen concentration:

(21)

while the current nitrogen concentration is:

(22)

The equations contain the following symbols: δ, δ0, -current, respectively initial diameter of the equivalent fuelparticle, µm; τ, τa, τ1, τ2 - current, respectively burning time,at the ends of Z1 and Z2, s; kdv0, kav0, kon, kor, koon - pre-exponential factor of volatiles releasing velocity, volatilescombustion, nitrogen volatilisation, molecular nitrogenrecombined in atomic nitrogen and nitrogen oxidation; Edv,Eav, Evn, Er, Eon - activation energy of these reactions, kJ/kmol; K - global coefficient of equivalent fuel particlecombustion velocity [3], m/s.

Results and discussionsThe system for different variants has been solved with

the aim to make comparisons between them starting from

the equation representing the dependence of the value ofthe NO concentration upon different independent variantsconsidered for different variable parameters:

(23)

where:δ0 is the initial diameter of the equivalent fuel particle;λp – the primary air excess coefficient;gp - the gravimetric participation of the corn to the

mixture.To solve the system the following fuels with the

elementary analysis at the primary stage have beenselected:

Based on the data above and depending on thegravimetric participation of the corn, the analysis of theequivalent fuel is determined. The results are presentedin table 1. It is mentioned that the maximal gravimetricparticipation of the corn, gp, has been considered as 30%,because over this value there are problems related to theobstruction of the grounding mills.

The data in table 1 allow the calculation of the values

needed for solving the equations system (example:Voaum ,

Vog, etc.). The system has been solved, in different variants,

for the following cases: δ0 = 60; 90; 120; 160 mm; λp =0.20; 0.30; 0.40 and, obviously, gp = 0.10; 0.20; 0.30. Theresults are presented in the graphs in figures 3-6 (the NOconcentration is expressed in mg/Nm3 and the diameterof the particle of equivalent fuel in µm).

REV. CHIM. (Bucureºti) ♦ 59 ♦ Nr. 6 ♦ 2008702

Fig..4.The variation of NO vs f (δ0,λp);gp=0.10

Fig..3.The variation vs NO=f (δ0,λp) ; gp=0.00

Fig..5.The variation of NOvs f (δ0,λp) ; gp=0.20 Fig..6.The variation of NOvs f (δ0,λp) ; gp=0.30

ConclusionsThe physical model proposed, namely the linear

introduction of the secondary air divides the combustiontime in three bounded stages, like: the first one from theinitial moment until after the volatile substances ignition,the second one ending at the moment of total introductionof the secondary air and the third one at the completecombustion of the particle of equivalent fuel;

- the combustion of the mixture corn-coal leads toreducing the total combustion time by 10-20% comparedwith the combustion of the coal due to the higher contentof volatile substances of the corn than of the coal;

- the value of the nitrogen monoxide concentration inthe mixture of fuels depends, among others, on the initialdiameter of the particle of equivalent fuel, δ0, on theprimary air excess coefficient, λp, and on the gravimetricparticipation of the corn in the mixture, gp;

- the influence of the corn combustion over theconcentration of NO is a complex and contradictory one,thus: the higher content of nitrogen in the organic massleads to the increase of NOc; the higher calorific powerleads to the increase of the theoretical combustion

temperature, thus favouring the thermal NO; the highcontent of oxygen in the composition of corn leads toreducing the volume of oxygen introduced, a fact thatdiminishes the concentration of NO;

- the value of NO concentration increases with theincrease of the initial diameter of the particle of equivalentfuel up to about 120 µm after which it becomes constant,and after 160 µm a decreasing tendency can be seen;

- also, the value of NO is influenced by the quantity ofair introduced under the form of primar y air (theconcentration of NO diminishes with the increase of λp)and by the gravimetric participation of the corn in themixture (the concentration of NO increases with theincrease of gp). These influences meet the expectationsbecause by the increase of gp the content of volatilesubstances of the equivalent fuel (NO increases) increasesand this increase of Vie entails a increase of λp (NOdiminishes), being known that one of the conditions forobtaining a low NO concentration is the observance of thecondition, λp ≈ Vi e [ 5 ];

- by the combustion of the mixture coal-corn under theabove mentioned circumstances, its have been obtainedvalues of the NO concentration between 350 and 530mg/

Table 1 THE COMPSITION OF THE EQUIVALENT FUELS

REV. CHIM. (Bucureºti) ♦ 59 ♦ Nr. 6 ♦ 2008

Nm3, values close to the limit values of emission. Anyhow,the decision to implement the corn (or any other type ofbiomass) in producing energy implies the analysis of theeconomic factors (the production and the transport costfor the corn, the benefit on the market of green certificates,etc.).

References1. NEAGA, C., PΪÃ, I., The influence of the linear introduction of thesecondary air over the dynamic of formation of thermal NO, Energetics,nr.1, Bucharest, 1999, p. 27

2. PΪÃ, I., Researches on the generation and reducting the nitrogenoxides in the process of combustion of powered coal, DoctorateDissertation, p. 150, Bucharest,19983. PΪÃ, I., The Flue Gas Recirculation to Suppress Nitrogen MonoxideFormation in Combustion Installations, Rev. Chim. (Bucureºti), 57, nr.4, 2006, p.3874. EPPLE, B., Modellbildung und Simulation von Stromungs-Reactions-und NO –Bildungsvorgangen in technischen Feuerungen, Univ.Stuttgart,1993, p.1975. PΪÃ, I., Generation and reducing NOX- during the coal combustionin boilers, PERFECT Publishing House, Buc., 2003, p. 218

Manuscript received: 6.08.2007

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