Investigation of Heterogeneous Combustion

8/11/2019 Investigation of Heterogeneous Combustion

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Combustion, Explosion, and Shock Waves, Vol. 38, No. 4, pp. 401408, 2002

Investigation of Heterogeneous Combustion

and Gasification of Carbon and Solid Fuel (Review)

UDC 662.61; 662.87E. S. Golovina1

Translated from Fizika Goreniya i Vzryva, Vol. 38, No. 4, pp. 2534, JulyAugust, 2002.Original article submitted May 15, 2001.

A review of literature data is presented on experimental and theoretical investigationsof carbon and solid-fuel gasification in various gas media at high temperatures (upto 1373 K). Methods of experimental and theoretical investigations are described.A theoretical formulation of the problem of nonstationary gasification of carbon ispresented. The value of the real kinetic constant of carbon gasification by carbondioxide is given.

Key words: carbon, carbon dioxide, active areas, nonstationary gasification, kineticconstant.

Some new approaches to studying the mechanismand laws of heterogeneous reactions of combustion andgasification of carbon and solid fuel have been devel-oped.

Within the framework of the diffusion-kinetic the-ory of heterogeneous combustion and gasification of car-bon and fuel, the laws of the process were deduced fromthe behavior of the gas phase only [14], whereas the be-havior of the solid phase, in addition to the gas phase,is taken into account in the current studies. For thispurpose, the notion of active surface areas (ASA) or a

more extended notion of reactive-active surface areas(RSA) is introduced, which describes the active sur-face entering the reaction. The reactive-active surfacearea is determined by the concentration of active carbonatoms on which a carbonoxygen complex is formed; de-composition of this complex yields a gaseous product.Boundaries of the basis plane and defects of the struc-ture inside this plane are referred to active areas (activesurface).

The reactive surface is determined by methodsof chemisorption and desorption, more exactly, themethod of instantaneous nonstationary kinetics and themethod of temperature-programmed desorption.

The consumption of the substance per unit timenormalized to the reactive-active surface or, otherwise,to the number of reactive-active atoms, determines thetrue reaction rate.

The notion of an active surface was introduced byLeine et al. [5] in the 1960s, but this notion has been in

1Krzhizhanovskii Energy Institute, Moscow GSP-1, 117927.

active use only since early 1980s. The papers [57] areof the greatest interest from this new point of view.

1. The experiments of [5] were performed withgasified soot by means of oxidizer chemisorption atlow temperatures and pressures. The specimens werepreliminary prepared to determine further the overallactive surface area (ASA) and the total surface area(TSA). The specimens made in the form of a layer ofgasified soot interacted with oxygen under kinetic con-ditions at a temperature of 625C to different degrees ofconversion (from 0 to 35%). As a result, specimens with

different numbers of active surface areas were obtained.These specimens were heated in vacuum to 950C dur-ing three hours to remove the oxide formed.

The total surface of the specimens obtained wasdetermined by the BrunauerEmmetTeller (BET)method. Chemisorption of a CO molecule was per-formed later on this surface. The overall active surfacearea of such specimens was determined from the amountof oxides on the surface, which were formed during 24 hunder chemisorption at a temperature of 300C and apressure of 500 m Hg. At this temperature, gaseousoxides are not formed, and only surface complexes areformed. The resultant complex was converted back to

the form of CO and CO2 at T = 950

C and recalcu-lated per atom of carbon. It was admitted that thecomplex exists on one carbon atom located in the plane(100). This means that the carbon atom occupies anarea of 8.3A. The gas analysis was performed using amass-spectrometer.

0010-5082/02/3804-0401 $27.00 c 2002 Plenum Publishing Corporation 401


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402 Golovina

TABLE 1

Degree of entrainmentK

i, 106 sec1 m2 (BET) Ki, 10

3 sec1 m2 (unoccupied ASA)

KO2

KCO2

KCO

KC

KO2 KCO2 KCO KC

0 5.2 2.4 2.7 5.1 9.8 4.8 5.4 10.2

3.3 11.0 3.2 9.0 12.2 5.1 1.4 4.3 5.7

6.4 16.3 3.7 20.1 23.8 4.5 1.1 5.7 6.8

8.5 21.3 4.5 26.5 31.0 5.0 1.0 6.0 7.0

14.4 27.4 6.0 33.1 39.1 4.5 1.0 5.9 6.9

20.8 34.4 7.1 38.0 45.1 4.9 0.9 5.4 6.3

25.8 39.8 9.2 48.8 58.0 4.8 0.9 5.9 6.8

34.9 40.7 11.0 59.1 70.1 4.9 1.0 5.8 6.8

Then, experiments were performed to determinethe reactivity of these specimens at T= 575, 625, and675C and a partial pressure of oxygen equal to 39 mHg. The experiments were short-time, and the entrain-

ment rate was

0.1%. Therefore, the total surface areameasured by the BET technique and the overall activesurface area increased insignificantly.

After determining the reactivity of the specimen atthe temperatures mentioned above, the specimen washeated in vacuum up to 950C to desorb stable oxygencomplexes. Based on the material balance of desorbedproducts, the number of oxygen complexes was found.

Based on these data and results obtained for thetotal reactive surface, it was possible to determine thenumber of free places on the surface and, thus, the reac-tive surface area RSA and the unoccupied surface, i.e.,RSA = TSA (total surface) ASA (occupied surface).

The results obtained show that the initial total sur-face area (TSA) measured by the BET technique was76 m2/g, whereas the active surface area (ASA) of theinitial specimen was only 0.29% of TSA. After the re-action with T = 625C and 35% degree of conversion,TSA was 128 m2/g, and ASA was 4 m2/g. This meansthat ASA increases by a factor of 18, whereas TSA isonly doubled.

Based on these data, the constant KO2 of the rateof consumption of O2 and formation of CO and CO2was calculated:

using the equation

dPO2

dt =K

(PO2)TSA (1)for the case of ASA determined by the BET technique;

using the equation

dP

dt =K(P)ASA(1 ) (2)

if the reactive-active surface RSA was chosen. Here isthe degree of surface covering, i.e., ASA(1 ) = RSA.

The first-order reaction was chosen. The gaseousproducts (CO and CO2) are formed from an unstableoxide; the rate of their formation is decelerated by astable oxygen complex on the active surface of carbon.

The rate constants for reactions of graphitized sootand O2 forT= 625C and different degrees of conver-

sion, which are normalized to TSA determined by theBET technique and to the unoccupied surface are listedin Table 1.

The rate constants obtained using Eq. (1) increasemonotonically with increasing degree of conversion ofgraphite. Vice versa, the rate constants determinedby Eq. (2) are unchanged for degrees of conversion of3.334.9%.

2. The paper of Lizzo et al. [6] became famous be-cause of the RSA investigation technique and the resultsobtained. The authors posed the problem of determin-

ing the reaction rate with a varied degree of conversion.The main expression for the kinetics of gasificationof char carbon in O2, CO2, or H2O were taken in theform

rA =saRsp

MC=KCAS, (3)

wheresis the density of carbon, a is the stoichiometriccoefficient,Rsp [h

1] is the specific rate of gasificationmeasured, MC is the molecular mass of carbon, andCASis the concentration of the gaseous reagent on thereactive surface.

The specific rate of gasification was found by theformula

Rsp= 1

1XC

dXCdt

[h1],

whereXC is the degree of carbon conversion:

XC =(mCO mC)

mCO

(mC is the dry mass without ash). Then, we have


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Heterogeneous Combustion and Gasification of Carbon and Solid Fuel 403

dXCdt

=Rsp(1XC) = KCAS

MCsa

(1XC)

=KCAS(1XC).

If the process is controlled by desorption (zerothorder of the reaction) or the partial pressure of the gasis constant (excess of the oxidizer), the concentrationof the gaseous reagent on the reactive surface is deter-

mined by the chemisorption isothermCAS=KCES,

whereCES is the concentration of active carbon on thereactive surface [mole of reactive carbon/total numberof carbon moles]. Then, we have

dXCdt

=K CES(1XC)

or

dXCdt

1

1XC=KCES=Rsp.

Hence, the constant K = Rsp/CES [h1] should be a

function of temperature only and should not depend onthe type of carbon, temperature processing (pyrolysis),and degree of conversion. Only in this case can the timenecessary for total or partial gasification be obtained byintegrating the expression, i.e.,

t= 1

K

1

0

dXCCES(1XC)

.

For integration, one has to know the relation betweenthe reactive surface of carbon determined via CES andthe degree of its conversion: CES =RSA [m

2/g or(mg C)/(g C)].

Thus, to determine K, one has to know CES RSA.

For this purpose, we posed the problem of direct ex-perimental determination of the reactive-active surfaceRSA. Two methods were recommended: the method ofinstantaneous nonstationary kinetics and the method oftemperature-programmed desorption.

Let us discuss the first technique. A fixed layer withan initial mass of 2070 mg and a particle size smallerthan 75 m was gasified. A quartz flow reactor 20 mmin diameter was used. The test gas was carbon diox-ide. Char of bituminous coal, Saransk coal, and lignitewas chosen for the experiments. Prior to the experi-

ment, the layer was heated in chemically pure nitrogento 1173 K, and then it was cooled to the temperature ofthe experiment. After that, gasification in CO2 at nor-mal pressure to different degrees of conversion occurred.The products were analyzed using the Beckmann tech-nique. The specific rate of gasification was determined.As a result of the experiments, specimens with different

Fig. 1.Desorption of CO after completion of gasificationof CO2+ C.

degrees of conversion were obtained. Short-time exper-iments on determining RSA were performed with thesespecimens as follows. After insignificant gasification inCO2(the degree of entrainment was 0.1%), the flow ofCO2 was replaced by a flow of chemically pure nitrogen(200 cm3/min) at the reaction temperature. The in-stantaneous continuous decrease in the CO content wasanalyzed by the Beckmann technique.

A typical course of the reaction for the char of bi-tuminous and lignite coals is shown in Fig. 1. The areaunder the curve determines the active surface of the re-acting specimen for a given degree of conversion.

The second method (temperature-programmeddesorption) was also used in experiments with a fixedlayer of char of the same coals in a CO2flow atT= 575,625, and 650C. The products were analyzed on a mass-

spectrometer.After the reaction to a desired degree of conversion,the specimen of mass of 200400 mg was cooled downto 393 K in the reacting gas, and then the specimenwas frozen (it is better to freeze the specimen inan inert rather than in the test gas). The remainingCO2 was removed from the system by high-purity ar-gon. Then, the specimen was heated in an argon flowwith a constant heating rate of 5 K/min up to 1273 Kand retained under these conditions for two hours forcomplete desorption of CO and CO2. These spectrayielded the total surface and the total C O complexformed during gasification.

Then, the stable intermediate oxide was deter-mined. The following mechanism of the reaction waschosen:

Cf+ CO2 C(O) + CO,C(O) CO,C(O) CO + Cf

(Cf is the carbon contained in the fuel). It was ad-


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404 Golovina

TABLE 2Total, Stable, and Reactive C O Complexes Formed on Char Partly Gasified in CO2

XCTotal C O complex, Stable C O complex, RSA, Fraction of RSA in the total

(mg C)/(g C) (mg C)/(g C) (mg C)/(g C) C O complex,%

Bituminous coal (1093 K)

0.10 12.7 7.3 5.4 42.5

0.35 23.9 16.7 7.2 30.1

0.80 72.4 63.1 9.3 12.8

Saransk char (1133 K)

0.12 2.4 0.4 2.0 83.8

0.24 2.7 0.5 2.2 81.5

0.47 4.3 0.7 3.6 83.7

0.84 6.7 1.3 5.4 80.6

Lignite (953 K)

0.14 34.7 23.1 11.6 34.0

0.48 45.2 23.3 21.9 48.2

0.87 80.9 44.1 36.8 45.5

mitted that the stable complex is in equilibrium withthe intermediate one. After gasification in CO2 (degreeof entrainment of 0.1%) at a specified temperature anddegree of conversion, an inert gas flow was injected todesorb the stable intermediate oxide C O formed dur-ing gasification.

The data obtained for the total and stable com-plexes allowed us to find the reactive-active complex:

RSA = (total C O) (stable C O).

The change in RSA versus the degree of conversionwas determined in the experiments. The total, stable,and reactive-active complexes for char partly gasified bycarbon dioxide are compared in Table 2.

3. Much attention is paid to determining activereaction sites and true kinetic constants in the Ger-man school (Karlsruhe University [7] and others). Theirprincipal viewpoint is that the concept of active sites forleading heterogeneous reactions has been reliably con-firmed, and the true kinetic characteristics and rates ofheterogeneous combustion and gasification can be de-termined from these positions only.

In their experimental studies, Fritzland and Hut-

tinger [7] followed the mechanismC() + CO2

k1

k1

C(O) + CO,

C(O) k2

C() + CO.

The test results show that the heterogeneous reac-tions C +CO2 can be studied only by investigating the

behavior of surface oxides, which can potentially enterthe reaction on ASA; the true reaction rate is indepen-dent of the partial pressure of CO2 and degree of con-version. The authors confirm that the true reaction ratedoes not depend on the type of carbon in the absenceof catalytically active dopes either.

4. The reference data on heterogeneous gasifica-tion show that the large scatter of kinetic characteris-tics of Russian coals, which reaches several orders ofmagnitude, is a consequence of their determination by

normalization of the mass of the reacted fuel or carbonper unit time of gasification to the initial mass or ini-tial surface rather than to the reactive-active surface.An appropriate classification of these data is an impor-tant applied and scientific problem. Determination oftrue kinetic characteristics requires, first of all, determi-nation of the true reactive surface for a given reaction(C+CO2+ O2 and C+H2). These data for the reac-tion C+CO2 can be found in [8]. We recall that thefollowing relation was obtained for the constant of gasi-fication:

Kg= 1012 exp

91,000

RT

[sec1].

For certain fuels, this general information is extremelynecessary for the norms of calculating boiler units. Con-struction of the nonstationary diffuse-kinetic theory ofcombustion is also impossible without these data.

Khitrin [9] proposed a formulation of a nonstation-ary diffuse-kinetic problem. Nevertheless, taking intoaccount the behavior of the solid phase in the reaction


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Heterogeneous Combustion and Gasification of Carbon and Solid Fuel 405

reduced to allowance for density and porosity of reactingcoal, which, as is shown above, does not allow one to de-termine the true kinetic characteristics. Therefore, theproblem posed in [9] requires new consideration, whichimplies that the system of equations that describe theprocess should take into account the true reactive sur-face and the true fraction of the material participatingin the reaction in various regimes; the primary need is

the determination of the functional relationship betweenthe number of active sites and the degree of conversionof the solid phase, i.e., the dependence of the number ofactive sites on the degree of conversion: CV f(XC).This, in turn, allows one to find the relationship betweenthe rate of the process and the degree of conversion:

Kg = 1

1XC

dXCdt

=KCV.

Such a formulation of the nonstationary problem isgiven below (see the scientific report of Energy InstituteNo. 4, 1997 written by E. S. Golovina).

5. Gasification of carbon (fuel char) with allowance

for the conversion of the solid phase is a nonstationarydiffuse-kinetic problem.

A spherical coal particle of radius R reacts with aCO2 flow. The kinetic regime and molecular transfer ofCO2 are implied. For the first-order reaction in the gasand solid phases, the reaction rate is

K =K CVC mole

cm3 sec

g

g

,

where K is the reaction rate constant [sec1], CV isthe number of active sites of carbon [gram of reactive-active carbon/gram of the total carbon in the specimen],andCis the concentration of the gas phase [mole/cm3],

or the concentration of the gas phase on the reactivesurface [mole of the gas/mole of carbon]. For the zerothand pseudozeroth orders (excess of the oxidizer and highpressures), we have

K =KCV

g/(g sec)

.

According to the oxygen-exchange mechanism ofthe reaction, we have

C + CO2 C(O) + CO, (a)

C(O) CO, (b)

C(O) C(O), (c)

where the intermediate oxide C(O) may be stable (a) or

may form a gaseous product (b). The rate of the processis determined by the amount of the gaseous productformed (CO).

Diffusion in the free space to a sphere of radiusr= R under nonstationary conditions in the absence ofsecondary reactions in the gas phase, which is observedin our case, is described by the equation

C

t =D12

2Cr2

+2

r

C

r

,

where

D12=2.63 103

T3(M1+M2)/2M1M2

p21212T

12

is the coefficient of binary diffusion, p is the total pres-sure [atm],T12= kT/12(kis the Boltzmann constant),

M1and M2are the molecular masses of the components(in our case, CO2 and CO), 12 and 12/k are the pa-rameters of potential energy of molecules 1 and 2, and12 is the interaction potential.

If the micropores of a porous body are in operation,the diffusion coefficient should also reflect the Knudsenflow. Then the diffusion coefficient is determined by theexpression

Deff=

1

1/DKn+ 1/D12,

whereDKn= (2/3)rp

8RT/M (rp is the pore radiusandMis the molecular mass of a component), is theporosity, andis the sinuosity coefficient. For the inter-

nal volume of a particle, diffusion of CO2 is describedby the relation

Cit

=

r(D1,2i (XC))

Cir

+2

rD1,2i (XC)

CirKiCVCi,

whereCi is the concentration of CO2 in the solid phase[mole/cm3], CVis the number of active sites of carbon[g/g], Ki is the reaction rate constant [sec

1], Di =D12

2, andD12is the diffusion in the free space; accord-ing to the data of [10], Di = 2.2 10

3

T /T0

cm2/sec

for = 0.2.For first-order reactions in each component, the

change in the degree of conversion XC (flow rate of the

solid phase) is found by the formula

XCt

=KiCVCi(1XC)M

[sec1],

where M is the molecular mass [g/mole] and is thedensity [g/cm3] [M/is measured in (g/mole)(cm3 g)],andCiM/is the dimensionless concentration. For thezeroth order in the gas component, we have

XCt

=KiCV(1XC) [sec1].

The specific rate of gasification is

1

1XC

dXCdt =

dmCdt

1

mC =Kg,

where

Kg = 1

mC

dmCdt

, XC=(mC)0 mC

(mC)0.

We consider the conditions for the fluxes:

(a) D12C

r =KCV RCR Di(XC)

Cir

forr = R;


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406 Golovina

(b) since the reaction temperature is low and the con-centration of CO is small, we can omit the Stefan fluxand admit the following relation at the boundary:

D12

Cr

R

+D12(XC)Cir

= RdR

dt;

(c)Ci = CiR for r = R, and the value ofCV = f(XC)is determined experimentally.

The problem is solved on a computer.6. Let us consider the latest results that attract

the researchers attention.These studies are most completely described in pro-

ceedings of international symposia and workshops. Wecannot avoid mentioning the 27th International Sympo-sium on Combustion (University of Colorado, August27, 1998) [11] and the XIIth Symposium on Combus-tion and Explosion (Chernogolovka, September 1115,2000) [12].

The main direction of papers presented at theXIIth Symposium in Chernogolovka deals with com-bustion (heterogeneous and homogeneous). It should

be noted, however, that the Combustion session con-sidered mainly condensed systems: solid-phase interac-tion, explosives, and powders. In the present review, weconsider only the traditional technological combustion.This topic was scarcely represented at the Symposium.First of all, this is the paper of Babii et al. [13] onmacrokinetic constants of the processes of decomposi-tion of NO, CO, and H2O in a reducing dusty mediumin the course of combustion and gasification of polydis-perse dust. Babii et al. [13] studied the formation ofNO from nitrogen contained in the fuel, determined themacrokinetic constant of NO decomposition on coal

KNO= 24.3 1036344T

m

3

kg sec

,

and showed that the laws of decomposition are inde-pendent of the type of coal. The process of reduction ofCO2 and H2O on coal was also considered, but the rateconstants for these processes were not given, though thelaws of gasification for these reactions were presented fortemperatures of 13002100 K.

For the purpose of obtaining gas synthesis, Borisovet al. [14] considered partial oxidation of hydrocarbongases, methane, and a mixture of gases close to hydro-carbon raw materials (CH4, C2H2, and C3H8). Theprocess allows one to obtain gas synthesis, a significant

amount of heat is released thereby, and the authors re-fer it to combustion processes. The authors performeda thermodynamic and kinetic modeling of such a mix-ture using the detailed mechanism of high-temperaturehomogeneous reactions, which was proposed in [14].The calculations were performed for an air mixturewith pressures of 525 atm, temperatures of 300900 K,

and air-excess coefficients of 0.30.56. The calculationsshowed that organization of the process in a real cham-ber with an initial temperature of 900 K requires theair-excess coefficient to be greater than 0.45. In thiscase, the time necessary for the combustion productsto reach the equilibrium composition is 102 sec, whichallows one to organize the process in a real combustionchamber.

At the 27th International Symposium on Combus-tion, much attention was paid to studying the processesof fighting against the oxide NO. Thus, Matsunagaet al. [15] performed an experimental and numericalstudy of the influence of hydrocarbons on NO NO2conversion in a flow reactor; the effects of time andtemperature were also examined. Five hydrocarbonswere used methane, ethane, ethylene, propane, andpropene: NO (20 ppm)airhydrocarbon (50 ppm), themeasurement time was 0.161.46 sec, and the tempera-ture was 6001100 K. It was found experimentally thatethylene and propane effectively oxidize NO to NO2;methane is less effective; propane yields the best resultsat lowest temperatures. It was shown by calculationsthat oxidation of NO to NO2 occurs in the reactionNO+HO2 = NO2+OH. Hydrocarbons that form theOH radicals and the O atom in the reaction are themost effective. Propane yields the greatest conversionNO NO2 for the lowest temperatures (this is 600 K inthat paper).

Carlos and Cugel [16] demonstrated the effect ofthe particle size and pressure on conversion of nitro-gen contained in the fuel to NO in the boundary layerof an individual particle during liberation of volatiles.The temperature field inside the particle was nonuni-

form. The influence of the particle size, fuel type, gastemperature, and concentration of O2 was studied. Itwas shown that there is a tendency of NO formationto decrease with increasing pressure for particles 80 min diameter burning in a gas that contains 10% O2 (bymass) at T= 1350 K. The values of pressure were notgiven.

Chambrien et al. [17] considered the reactionC+NO in the presence of O2. The reaction of NOwith pure carbon and amorphous C13 was studied forT= 850C in the presence of O2. A preliminary studyshowed that some amount of nitrogen from NO remainson the surface of carbon in the form of the C(N) complex

and plays an important role in the reaction C + NO. Thepresence of O2 significantly increases the reaction rateof NO reduction, and accumulation of the C(N) com-plex on the surface significantly decreases with increas-ing concentration of O2. The products of the reactionbetween the complex C(N) and O2 at T = 850

C areN2 at the early stage and NO at the late stage. Oxygen


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408 Golovina

Marinov et al. [22] performed an experimen-tal study of the influence of hydrocarbons, reactiontemperature, and time on conversion NO NO2 in aquartz flow reactor at atmospheric pressure. Kineticcalculations were also conducted. Five hydrocarbonswere studied: methane, ethylene, ethane, propane, andpropene. The quantities measured in the experimentswere the concentrations of components in the flow re-

actor for the mixture NO (20 ppm)airhydrocarbon(50 ppm); the duration of an experiment varied from0.16 to 1.46 sec, and the temperatures were 6001100 K.The time evolution of NO, NO2, hydrocarbons, and in-termediate hydrocarbons was evaluated in calculations.The chemical mechanism included 649 reversible reac-tions for 126 components. It was found experimen-tally that ethylene and propane effectively oxidize NOto NO2, and methane is less effective. According tocalculations, all five hydrocarbons oxidize NO to NO2via the reaction NO + HO2 NO2+OH, and addition ofoxidation RO2HORO2has a negative effect on the pro-cess. Hydrocarbons that produce highly reactive radi-cals (i.e., OH and O atom) are most effective. They fa-vor hydrocarbon oxidation and lead to additional forma-tion of NO2. On the other hand, if a hydrocarbon pro-duces radicals, such as methyl, that suppress oxidationby oxygen, then these radicals decrease the content ofNO. Experimental results show that the efficiency ofhydrocarbons changes noticeably with temperature andonly in the region of low temperatures; propane is moreeffective in the process of conversion NO NO2. Thiscapability is mainly due to hydroperoxipropyl and thereaction of O2.

Certainly, the data in the present review do not ex-

haust all issues of heterogeneous combustion and gasi-fication. The most important problem is determiningthe true kinetic constants for chars of Russian coals andcomposing a complete table of these data.

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Sources of unburdened carbon in the lay ash producedfrom low-NOx pulverized coal combustion,ibid., p. 36.

21. Guo Feng, W. C. Hecker, et al., Kinetic of NO reaction

by char: Effects of coal Ra, ibid., p. 78.22. N. M. Marinov, W. J. Pitz, et al., Chemical kinetic

modelling of a methane opposed plow diffusion flame

and comparison to experiment, ibid., p. 41.

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Investigation of Heterogeneous Combustion