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9Journal of Siberian Federal University. Engineering & Technologies 5 (2011 4) 504-521
УДК 621.181.7
Complex Approach to the Evaluation о f th e Reactivity of Solid Organic Fuels
Evgeny A. Boiko* and Sergey X. Pachkovsky
Siberian Fekeryl University, g9 Svobodry, Krasaoaarsk, 660к4Г Ruesia 1
Re^iji-v^d 1.08.2011Oeceived ^n revisO0 Гоат l^.^^.^Oll, ¡accepted 1.00.20П
nt universal experiment-calculation approach for the evaluation of the kinetic characteristics of thermochemical transformation of solid organic fuels basedonthe application of them ethodsof complex lhermcl aoatysis and mathemvtical vimclation has bevn pnoposed. Thv tecfaiqveof the theomegraaimetuic sxauaimhft data processing and mathematical model description bases itself on a unified calculation scheme of material balances and kinetic processes of natural coal burnout. Satisfactoryoualitalive andqsactiehOiveconvergenoeofoxperemcnOal anCoomerioal rrsultehas bets obtained.
Keywreds: CvoO, Kinetic processes of burning-out, Complex thermal analysis, Mathematical simulaeion.
Nomenclature
H20q initial hygroscopic moisture of fuel (kg kg"1)
H2Oh Quantity of evaporuted hygroscopic moistfre in gas colcmo (kg kg-1)
k0 Preiexporgntiae Oactor (s-1 oc m s-1)
EE Activation energy (J moF)
R Umnertal ras constant (C mof1, K1)
T grfrers CemporaSurt (K)
k q ponstant of. ^kt£ar'os;^oifi<c moisture evepofation tute (s1)
t grurera time is)
V0 irntiaa ooftenrration of poirSrle motterx in fuel (H20|f ^^CaeneiiLCiitlbb Oouag
moioture), COeo, HvCH4o, CO0, C8Hho (pitch))(kgkg-1) Vvol = Vbai + Vcom Concentration of volatile matters emitted from the fuel into gas phase: ballast Ve (H2Och (chemically bound moisture), CO2) and combustible Vcom lH2, CH4, CO, C8H18 epitch))respectively(kgkg-1) kVo Constant of emissionrateof volatile matters (s-1)
i =1.. .zt Number of vapor gas components takenintoconsideration
j =1.. .Zj Number of individual stages responsible for the emission of the i-th component
as aresult of decomposition
* Corresponding author E-mail address: EBoiko@sfu-kras.ru
1 © Siberian Federal University.All rights reserved
O2
Pr„
C
Pc P
PP
P
r =1...zr
ktch
Crs 0
C n
khr d
Constant ofburning rate off^o^;a^]il(sma^iters(sS-i Oxygen concentration (kg kg1)
Co^sen^^ntin^ oftheproductrformed nuriugburningc^CvolatiOe neatters (CarCS>t and H20)(kg lkg)
CarSu^ co^cor^tr^tic^^ in farC (soHd phason (kg kg1)
Co^con^^atia^ ofpronucCsofcomplereburning ofco^gbuso(kO2) (kg egl)
Concentratinn ofcon^du^^tea^i;p^(snu^^s; of coke base gasriiaatkan ^TsTI and ^d
(kglsg-1)
Concentratian of f^i^f^i^cte of burmng of combustible producds oi2 ofks bftr gaslnectionSC0lann H^C^) (fa kgD
Concentration of j^i^oofIharmachemtealeotvergran od fun): dryidg of H^, aad unreartdd an She reactionof eolatilr matteotoxidatioa Fbg f
burmng ofnolntilemntters Pv ,bdrning Pc endgos(ficgtlan ofcoke base Pg and after-burning on combustible gasification products PPg (CO2 andH2O)(kg 1kg"1) Number of reaction groups taken into consideration in the description of the gesificotion pianero This number depends on the number ft eomnoncntt enteriIlg rscat(an with cort>en
Constantofrateofha(eranenccnrbuпlmg ofcoge best (m e')
Constanteofconebeshgesificotionrate(m s1)
Constant of burningrateofcombustiblegasificationproducts^"1)
Constant of topochemical reactionrate(s-1)
Initial concentrationofreactingsubstance(kgkg-1)
Current massofreactedsubstanceuptotemperature T (kg kg-1)
Reactionsequence(takenastheunit)
Constant of heterogeneousreactionrate(ms-1)
Diameterof cokeparticles(m)
Greek symbols v <t>* ß
Stoichiometricratio
Factor allowingformolefraction of component x in gas phase Heating rate of coal sample
¡Superscript symbols
h Hygroscopic
ch Chemically bound
¡Subscript symbols
Initial
Initial content of hygroscopic; moisture in fuel Volatile Ballast Combustible
0
HO
vol bal com
Initial content of volatile matters in fuel Concentration of combustiple volatile matters emitted from fuel Carpon Gasificatkrn
Concentration of combustible gasification products Topochemical reaction Reacting substance Heterogeneous reaction.
Introduction
The experience of using solid organic fuels in thermal power stations shows that till now there are no ways of their preparation and burning that would provide maximum efficiency, reliability and ecological safety of boiler unit operation [1,2]. This problem is aggravated even greater when non-rated or inferior quality coals are burnt [3]. The most practical way of solving this problem can be provided by implementation of new technical processes, in particular, development of monitoring, engineering diagnostics and control systems of furnace processes as well as improvement of the methods, practices and devicesof fuelutilization[4,5].
To determine practical recommendations for the selection of optimum technical solutions of power utilization of solid organic fuels basing on the abovementioned principles it is urgent to establish a correct correlation of the initial quality characteristics of coal substance with the reaction characteristics of its thermochemical transformation basing on the kinetic parameters. The empirical relationships now in use in the majority of studies [6-8] are far from taking into account all basic stages, processes and factors of burning and, that is the most essential, are not always connected in an apparent form to the reaction parameters describing these processes which consequently raises doubts about reliabilityof these techniquesand quality ofsuggested engineeringsolutions.
The purpose of the present study is to develop a complex method of evaluating the kinetic characteristics of the basic stages and phases of thermochemical transformation of a wide range of solid organic fuels as applied to the conditions of real furnace processes to provide qualitative and quantitativeconvergence of numerical and experimentaldata.
When studying the complex physicochemical process of solid organic fuel burnout in the furnaces of the steam boilers being a part of a more general gas-dynamic problem associated with description of heat-mass exchange and aerodynamics processes, there is arising a problem to construct an adequate kinetic mechanism and kinetic model describing behavior of chemically reacting system [9, 10]. The solution of this problem requires the following questions to be answered: what stages of the analyzed kinetic mechanism pre-determine kinetics of the system as a whole; what stages are redundant for the discussed kinetic scheme and may it be simplified; which of the kinetic mechanism alternatives is the most probable one [11, 12].
Assuming the reactivity to be adequate to the total time of the burnout of power-plant fuel [13], it can be stated that during pulverized burning this characteristic is determined by a set of parameters of the overlapping processes, in particular: rate of moisture evaporation, rate of emission and burning of volatile matters, rate of nonvolatile (coke) residue burning. At the same time the scheme of coal substance burnout at certain stages also becomes complicated by such processes as oxygen chemisorption,
tch
gasification of non-volatile residue by carbon dioxide (CO2) and water vapors (H2O), transformation of the chemical components of the mineral fraction of fuel etc. [14, 15]. Consideration of these processes in the evaluation of the extent of the thermochemical transformations determining the burnout of pulverized solid fuel is necessitated by their essential mutual influence upon each other as well as significant contribution of the values of the thermal endo- and exoeffects inherent in the behavior of these processes [16]. The maximum correctness of such evaluation can be achieved by implementation of the complex approach based on experimental determination of the reaction characteristics of various stages and phases, which form the process of fuel burning, with subsequent calculated application of these data for computation of the kinetic parameters of coal thermochemical transformation [13].
The principal feature of the technique proposed is the unity of experimental-methodical ways and the analytical apparatus from the point of view of the structural scheme and the basic mathematical models employed in the description of individual processes and stages responsible for the burnout of solid organic fuel. This correlation, on the one hand, lays down strict requirements to the technique of conducting the kinetic experiment and processing its data and, on the other hand, provides, in the end, the appropriate quality of the mathematical simulation results of the processes of solid fuel thermochemical transformation kinetics in actual practice of processing plants. However, to implement such approach in practice it is necessary to overcome a series of procedural difficulties associated with the determination of the running rates of various stages of coal thermochemical transformation in a wide range of operating conditions (operating environment, temperature, heating rate, etc.), connected with overlapping of several processes and impossibility of their separate evaluation as applied to the conditions of real technical methods of solid organic fuel processing [17].
An effective instrument of solving the abovementioned problem is the use of the complex thermal analysis of solid fuel with continuous recording under non-isothermal conditions [18, 19] and the methods of mathematical simulation of kinetic processes [20, 21]. Here, a general structural physicochemical scheme with a set of basic models of individual processes and stages responsible for solid organic fuel burnout should be assumed as a basis of the method of thermogravimetric experiment data processing and mathematical model description.
Experiment
The plant for the complex thermal analysis of solid fuel (Fig. 1) integrates derivatograph (type 0-1500D) of Hungarian company MOM and chromatographic gas analyzer "Soyuz 3101" in the framework of the complete functional scheme that allows to derive the dynamics of gaseous products emission (CO, CO2, H2, CH4, etc.) under non-isothermal conditions alongside with the total characteristics of coal sample heating process (loss of weight, rate of weight loss, change of temperature, thermal effects) [22]. The derivatograph enables to work in quasi-isothermal mode with the heating rate adjustable from 0,2 to 5 mg min-1 in nine stages. The limit load of the scales is 10 g, the measurement accuracy at 20 mg sample is ±1%. The instrument is equipped with a six-channel self-recorder with the measurement range from 50 |V up to 5 mV 250-1 mm per each channel. Parameters of the derivatograph operating mode were selected and adjusted by evaluation calculations upon the results of repeated running of isothermal and quasi-static procedures in order to ensure running of the heat treatment processes of the investigated material in the kinetic area without any diffusion complications. The operating parameters of the derivatograph during fuel thermal decomposition met the following values: inert
Rotameter Quartz cups
Gas pipette stand
Computer
Fig. 1. Schematic diagram of the plant for complex thermal analysis of solid fuel
medium (He); weight of coal sample 500 mg; heating rate 5-20 K min-1; those during combustion of nonvolatile products of thermal decomposition and thermal-oxidative degradation of initial coal were: oxidizing medium (air); weight of coal sample 50 mg; heating rate 5-20 K min-1. For the investigated processes there was also a series of generalized adjustments employed: inert material Al2O3; platinum crucible; sensitivity of recording (^V) TG 500, DTA 1000, DTG 500; gas flow rate 200 cm3 min-1; size of coal particles: polyfraction (with sieve residue R90 = 45-50 %, R20o = 28-32 %, R1000 < 1 %).
The evaluation technique of the kinetics of the pulverized solid fuel burnout processes is based on the processing of the experimental data obtained in three experiments (Fig. 2) by means of the complex thermal analysis [23]: 1) in inert atmosphere (Fig. 2a) with gas analysis of the composition of emitted volatile matters (Fig. 2d) upon which results the characteristics of moisture evaporation process, the total output and composition of volatile matters are determined. The DTG-curve after subtraction of the differential curve of the total output of gas-vapor components (curve 5 in Fig. 2d) is transformed to the pitch formation curve; 2) in oxidizing medium the nonvolatile residue generated in the previous experiment (Fig. 2b) burns out; 3) in oxidizing medium: thermal-oxidative degradation of the initial coal substance characterized by overlapping of the processes of moisture evaporation, emission and burning of volatile matters as well as burning of nonvolatile base (Fig. 2c).
In consideration of synchronism of several processes running at thermal-oxidative degradation, which total rate is registered as DTG-curve, the process of volatile matters (the "synthetic" volatile ones [17]) emission was separated by subtracting the coke base oxidation rate from the DTG-curve of thermal-oxidative degradation process. The rate of the oxidation reaction of nonvolatile residue was determined upon the results of the second experiment and superimposed on the DTG-curve of the thermal-oxidative degradation process with appropriate correction of temperature intervals.
Then, the total curve of gas emission rate obtained in the first experiment and recalculated with respect to the rates of volatile matters emission in inert and oxidizing mediums on the assumption that
20
40
60
100
120
140
20
40
80
100
120
140
Time / min
Time/mm Temperature/°C
Fig. 2. The data of the complex thermal analysis carried out by the example of Irsha-Borodinsky coal (P = 5 K min-1): a) the thermal decomposition; b) burning of coke base; c) thermal-oxidative degradation; d) dynamics of individual gas components emission in inert atmosphere: 1 -C02; 2 -CO; 3 -CH4; 4 -H2; 5 - total
the total amount of gaseous components is constant was superimposed on the period of volatile matters emission derived during thermal-oxidative degradation. By subtracting the respective curve of gas emission from the differential curve of the process of volatile matters emission in oxidizing medium the differential curve of pitch formation in oxidizing medium ("synthetic" pitches) was produced.
For the kinetic evaluation of gasification processes of the coke base the nonvolatile coal residues generated during the thermal analysis in inert atmosphere with the final heating temperature of 1173 K were subjected to the treatment by carbon dioxide and steam under conditions of continuous temperature increasing in the complex thermal analysis plant. The heating rate in the experiments for determination of the reactivity of coke residues relative to CO2 and H2O was 5 K min-1; concentration of CO2 = 100 %, H2O = 100 % (at atmospheric pressure); weight mass 150 mg; the flow rate of CO2 into the furnace of the derivatograph was 220-240 cm3 min-1, the flow rate of H2O = 250-290 cm3 min-1 [24]. The standard platinum plate-shaped crucibles of the derivatograph were used as object holder.
The above-stated technique of producing experimental data upon the results of experiments of the complex thermal analysis of solid fuels (thermal decomposition, burning of non-volatile residue and thermal-oxidative degradation, coke base gasification) enables to proceed to their processing in order to determine kinetic parameters (activation energy E and pre-exponential factor k0, numbers of individual stages of reaction, share of reacted substance, temperature interval of reaction running etc.) of multiphase overlapping processes of fuel thermochemical transformation [23].
The kinetic analysis and evaluation of the complex experimental curves, representing the dynamics of thermochemical transformation of solid organic fuel, were carried out using a computational software package [25] built on the basis of generalized kinetic mathematical model on the assumption of multiphase process additivity that meets the following physicochemical (Fig. 3) and calculating schemes (Fig. 4).
According to the proposed schematization the process of coal burning is divided into several relatively independent overlapping multiphase stages (Fig. 3): drying and preheating of particle before
Interpretation and discussion of research results
Moisture evaporation (drying}
/
\
\ CO, \
/ 1,
/
\
o,
I
C(
Burning of volatile matters
Burnout and gasification of coke base
o.
Time of complete burnout of solid fuel
0
Fig. 3. Physicochemical model of thermal transformation of solid organic fuel
pH ho" Y
I Coal I—>
__ ! k,
I !
I
k,
Fig. 4. The calculated scheme of the kinetic processes of the thermochemical transformation of solid fuel
emission; emission and burning of volatile matters near the particle; burning of nonvolatile (coke) residue consisting of organic and mineral fractions.
The physicochemical model of thermal transformation of solid organic fuel in wide temperature range implies that the process of wet material drying is characterized by moisture evaporation from the volume of coal particle on the condition of movement of the evaporation surface, as a front of phase transition, inside of the particle and results from an increase of the dry layer surface temperature. Since moisture contained in fuel has bonds of different strength with coal substance it is appropriate to consider two independent evaporation fronts of hygroscopic and chemically bound moisture [26, 27], where the front of the chemically bound moisture evaporation remains behind the front of hygroscopic moisture evaporation.
As the temperature of the surface layer increases there is running the process of thermal destruction (pyrolysis) of organic mass accompanied by volatile matter emission. The up-to-date models of pyrolysis regard the organic mass of coal as an ensemble of condensed aromatic, hydroaromatic and heterocyclic structures (clusters) containing various functional groups as substituents [28]. During heating of fuel there are observed ruptures of the bonds, that attached functional groups to ring clusters, with formation of the volatile gas components (mainly CO2, H2, H2O, CO, CH4, HCN, aliphatic hydrocarbons). Together with the emission of light gas components there is a splitting of bridge-type structure of coal with release of large molecular fragments (pitches C8H18 • 2H2O) [29].
The emitted combustible gaseous volatile components reaching sufficient concentration of stoichiometric mixture react in gas phase with oxygen of air to generate final products CO2 and H2O [30].
During emission of -volatile matters and their subsequent burning there is heating of coal particle accompanied by initialization of the process of its nonvolatile (coke) basis burning. The reaction runs on the coal particle surface and results in adsorption of air oxygen from gas space onto carbon surface. Here the atoms of oxygen ente r a chemical reaction forming compound carfon-oxygen ccmplexes CxOy. Tde latter then dissociates and forms products of complete burning (C<n2).
Steams (H2O) and carbon dioxide (CO2 ) generated from the processes of drying, sublimation of the ballast volatile matters, burning of the combustible volatile matters and coke base of fuel as well as geneeated fdom the after-burning of incompleOe products of combustion can react with the solid phase fesulting in formation CO2 and H2. These reactions ran accoed-ng to the chain-radical mechanism wo ith direct participation of hydroxyls as intermed-ate oompounds resulting in appearance of HCOH complex and also subsequent appearance of donor-acceptor-type valence bonds with monatomic carbon. Collision of atoms and molecules with lattice vacancies and presence of valence forces on active places of surface results in formation of intermediate complexes. The latter stimulates electron transfer, thus peoviding chemisorption od gaseous components an tha surface of coke base with their subseque ntoxidalion to CO2 and H2O [31].
The proposed calcularion scheme enadles to buikl a mathematical model that ensures control of material balance accomplishment of separate stages and phases of coal burning, and also the gross -process as a whole. When the mathematical model was build it was assumed that multiphase processes of thermochsmica1 tomsformatkm oc sohd fuel were additive, bhe functional groups during thermal decomposition ot fuel were emitted independently from earh otliew, the ratio of functional groups in pitch weee the same as in tire; initial coal, the volatile matters and nonvolatile coke residue, emitted during pyrieysis of coal dust, ideally mixed w-th oxidizing agent fair) and then entered chemical reaction.
The mathematical model of the kinetic processes of thermochemical transformation of solid organic fuel based on application of the commonly used stoichiometric equations [16, 17, 22, 26] with regard for the assumptiona made in differeacial form looks as follows.
The equdtion of the kinetics of drying describes tne kineties of hygroscopic moisture evaporation
from coal surface and complies the following stoichiometric equation H2Oj —kH2°h > H2O :
dHOh = -k 0 H2Oq , dt H2°0 2 0 '
( E
where kH O* = ko * exP
H2O0 H2O*
RT (t )
The equation of kinetics of volatile matter emission takes into consideration the contents of residual volafile matters in the fuel tesulting from the multiphase mechanism of bond rupture being accompanied by emission of ballast (H2O and CO2 ) and comb ustible (H2, CH4, CO, pitches (C8Hi8 -2H2O) volapile vapor-eas components durine thermal decomposition of organic coal mass [30] on the basis of
z' zj kV , zi zj sSoichiometric scheme ^^V0 ,-^—> ^^Vvol accosdonaVo expression:
i=1 jr=l '"' '=1 .=1 't
dV z' z'
= -YYk v0 dt ttjR ^ 0t
( E
whe re kV = k0
exp
o
RT (t)
Thr kinetic equction of volatik: matter bufnina describes homogeneous process of changing of combustible volatile matters conceneration in the gas pha(e during theie oxidation accompanie d by formation of final products CO2 and H20 according to lhe following stoichiometric ratio » vP Pv . Tlie firsit summand fovers thv agy namics of accumulation of the
'eSon+v o,°2-
emitted combustible votatiPe matters in the; volume near the particle, whereas tVe secon° one covers their burning:
dt
■ -YY k V -k V VVVcom CfOl
" Ocbeiabiej t^0iJ rVoeyVCOa Vbom ¿=1 j=1
where kv =k0 exp
( E \
'J7
RT(0.
The equation of the k:ine;tic;^ of coke )b;ase; buinout anU gaoification describes the consumption oO cart>on during seaction of fuel resulting in formation of CO2 according to stoichiometric iatio
VcC + Vf2°2-
avP Jfjc and recovery of complete combustion peoduchs (CO2, H2O) ^esirlta^ivee in
indermediatecombustible ]prc5dt:ic;t;s nO and H2 odcardina io the schirne 2h (v c + VPP
■Ot VPK.Pr
The first summand renresents ^^^ ^^irjtir^ic^^ oV coke burnout, the secovd one descaibes gasification oP combustion products [24]:
dC_
dt
RcvcCnCht ^cv^'ip
VP,r
r=1
where kc = k0 expl -
RT(t\
• kC,r = V ,eXP
JC,r
RT (t)
Tho kineCic equation ob aftor-buming of the combustible products of gasification considers the process rrf channsng of tho concenftations oJF thw hombustiMe proclunts resulted frcm the gasification of coke base with final products of the thermhchemical tsohsformation oh solid (organic fuel during their homogenpus oxidation, wliich is ascompanietl by foumation of final products CO2
v e Pe . The first
and H2O according "to tire; fo llowing stoichfometricratio vP Pf r-vC2CSi summand covers the dynamics of accumulation o° the emitted combustible volatile products of gaeification, whereas the second f tte covera their burning:
dPg
idt
■=£k
V C C-rP Co Sit hti l-
Vc-eVp--k^df^ pgpg pg g 2
whccc kp = ds cxp
r=1
( E \ . Epg
Rd(t)
The equation of the dynamics of oxygen concentration shift in the gaseous phase describes the consumption rate of the oxidizing agent during homogeneous and heterogeneous combustion of gaseous components and coke base accordingly. The first and the second summands represent the process of O2 concentration lass as a result of burning o0 volatile matte rs end ooke bnse, ohe third one represents the process of homoge neour oxidation of the gasification products formed:
dO2 dt
kycomVo/OrO2O2 + kcVo2CVcOV2°2 + k V02PgPgO2O2
The concentration of the products of thermochemical transformation of coal - dust particle is taken into consideration according to the equation describmg the dynamics of moisture evaporation, emissien of ballast volatile matters and volatile matters unreacted in oxidatkrn, complete burning of volatile maitett and coke base of fuel as wel l as gasificrtion of nonvotatile pro dusts and aftns-fnrning of combustible products of gasification [28]. The first summang of this relationship represents tie dynamics or ptoducr formation as a result of moisture evaporation, Ihe second tne representi that of ballast and unreacted volatile matters emission, the third one represents that of homogeneous burning rf volatile matters, the fourth one represents that od heterogeneous burnout ot coke base, thr fifth one repneients that on after-burningoO cnmbosiiMr prraducns of garification, tires sixth oite describes the concnntration lors of rnd psoducts (CO2 and H2O) during gasification with coke carboss b a se:
PP zJ
ap rjn2oh+YYv v0 + kv vP vOroJ02^ +
J,Ih 2 01 ¿-n j v0i j 0/, j Vcom PVcom com 2 TVcom
1j 11" 2 U / 1 2 1 Vo . . Ui j V com fV
rit 2 210 , i,j com Vc
Ul i=1 j=1
kcv^o^ +kPsvPp/;:pgov2o^Pc
r=1
The aboveotated system of differential kinetic equations was meant for the following initial
ronditions: HjOh vHjO0n!fiO, t = 0«, - = -W Oj = Oj^, HjCO^ =0, V = 0, P = (r . In addition to the kinetic equations this system meets the law of conservation of mnss
H20o -po+n,+02+P = Hn0rninZ + V0miSii + Otaa1 + OjmfttZ> , f^«1 1he r—tio "r^ + ^ +
HC d0j dP j + — + —— —0 = 0 is met. d0 dl dt
To dereaibe the non-isothermal nature rf thr process rf ccal thermochemicol Iransformation unilar condition o:f the comolex thermal anal—sis ohe mathematiaol jatodee: 1 woe supplemented with the
equatioa oO the linrao law erf henting T = Tf+bi, ^tttett^ P =t gW.
Ot
Ties present mathematical model of indrpendent or,"i;^t;^jtiti^n|t teactatns is assumed rs r basis o, hhe ppixt^eis^ini^ algosilhm of tle gross-process oa thesmoohemical tranrformation o- the sofid oagamc cuel thfE^t enobles (deteer-niirL^rion of the whote aggregate oo ohe reaction cbarstreristics of the elementary stages of thir pro ce s s ir 5 ]. The implementation of ohe al^mtimt oo s ir^t—ea.t^iia.^l calculation and elimination of indrndunl stnges is associnted witii deirsmination of icollated stnae section on tinn rtrumulatot1 kinetic
curve and based on the principle of sequential search and evaluation of the kinetic characteristics of dlementary stages on the basis of using the coerelation and multifactor regression analysis [32] of experimendal points in Arrenius coordinates. In these coordinates the linearized kinetic equation of the marked out stage represents the equation of straight line. Sequential analysis of the correlation factor values of experimental points reveals the aggregate of points meeting the preset correlation value and taken as a stage interval.
In this case the rates of tsp chemical reactions (moisture evaporation and thermal decomposition) were calculated according to the following relationship:
W = ktch (C„o - C„)".
The heterogeneous reactions (burnout of cske bass and gasification of coke) were calculated, in consideration of particle size change, according to the expression [23):
W _ khr (Crs0 ~Crs)
where d = d0 JCr:s0—C
Crs0
Furtheh, the sought kineeic parameters are determined by linear regression factors and the relaekrnship og conversion dhgree at the /'-th stage is build on the calculation basis according tn the following ratio:
C i(0 = exp
t
ghepe F s (i) = J" exp
fS )
de -? integro-exponent function.
After subtraction of the calculated values from the initial experimental kinetic curves the process of feaeching and determination of the paramrtere of next stage begins and the dieect kinetic problem subject to identification of thr rype of global stage nb coef thermochemical transformation (dtying, volatile matters entission, burning of coke base etc.) is sohied according ho tlie a°ohe-stated mathematical ^sitios.
On the batie of the technique stated above a cycle of researches on the kinetics of burning-out of Kansk-Achinsk coats, in partisuler tUose of Borrfinsky, Berezovsky and Nazarovsky deposits, was carried out. The proflem of power-producing utilization ot these fuels sn coniideratlon of scale or" their minsng as well fs immense role iff the development of Siberia's paoduotive forces and lhe fuel and power base oO Russia is of higliest priority. The Kansk-Achinsk basin cnaln are humrc, semi-glittering and remi-dull, lens-shapeh rtreaky, bolong to helyloMc and oonsist of amall fragments and fungmonVary siruoture residues with disstinctia,.^ microlamsnation. The main layer-forming cool iypes of heavy layers of the basm are aotrific and frrgmentary telohielits making up 70-95% of the overall cnpncity. Some distfnctive fealutes of the coal ohemicoS fomposition, m paotioular, low hvdrttgen rontent, larrge; amf iint of humir acitiss, relniiveiy Iriirlts humidity and tmisnion of voiatile matters, low and medium ^sIi percentage, point: out the low-reduction nature of the hetsfied subitanco erf rhe cnals. The trchhh;fl arfiysis i^ndS element joo;]i:"<t€in^iig<2 oO Kfaslo-Achinsn coals are sUown ins Table 1.
Table 1. Characteristics ofKansk-Achinsk coals
Coal field Teclmical specificction, % Element contents, %o per working mass
Wr Ar ydaf Qr, (kJ kg-1) Cr Hr Nr Or Sr
Borodinskoye 33.0 9.95 485.2 15318 41.2 2.89 0.57 12 . 1 9.29
Berezovskoye 35.5 4.0 495.5 15084 42.4 3.0 0.40 14. 4 0. 30
Nazarovskoye 38.5 5.53 49.1 13157 38.8 2.86 0.41 13.5 0.40
Using the above-stated mathematical model and values of the kinetic characteristics found from the results of evaluation of the complex thermal analysis data the calculation analysis of the kinetic processes of thermal decomposition, coke base burnout and thermal-oxidative destructio n of Kansk-Achinsk brown coals has been performed. In this article the results of the kinetic problem solution are given in comparison too the data of the complex thermoanalysis by an example of heat treatment of Kansk-Achinsk coals at herting rates of 5, 10 and 20 K min-1.
Type oof the chemical reaetions faken into censidera-ion according to the calculation srhrme presented in Fig. 4 and values of the respective kinetic parameters oy various procdsrns and dtsges of thermochemical transformction of Kansk-Achlnsk cnals tevedled upan the tesults of the data processing of thc complex thermal analysis are shown ins Table 2. The calculation was carried out on PC Peetirm 4 in programming environment yuilder C ++ 6.0. For the numerical solution of the system of differential equations the RungerKutt-Felberg method with automatic siep selection and accuracy oh 10-4 [322] wat used. Tit calculatirn results in yomparison to the data of the complex thermal analysis are given in Fig. 5. This figure shows the curves or thormydtemical transformation degree changing undet conditions of thermal decomposhon ( see = mitidm0r), burning oS coke base (a>bc = mbclm%c) and thermal-oxidative destruction ( atod =mtod/m0td) depending cf process temp eratitre, where mtcl, mbc anf mtf are the currenS fuel mass dusing thetmal decomposition burning of coke residue; and thermal-oxidative destrurtion accordsngly; h0 m0b aind hi0^^ are the iaitial fuel masf before thermal decomposition, Sunning of coke bate fnd thrmrre-oxidative destrucmion aacordingly. Al the same time, the calculation cotrectness was checked by eontrol cf enuring the strict ratio of ihe material bnlance o0 thermochemical transformaticn process of fuel in terms on analytical maes mr - mW - mr - ma - mtl = 0, where ns iis t-Uoie^ mass rf initiai fuel, mW Is tahie mast of hygroscopic moisture, mf a the mars of molatilm matters, mc is the mass oh roke bnse aird nft isthe mass o° ash residue.
As evident from tfe cbtalned results the obrermed kinetics of moisture evaporation conforms to the competition mechanism of adsorption and desorption of wa-er present in the coal in form of general phase and molecular clusters of coal structure pores. The curnilinear profile o0 changing of fuel sample mass in the temperature range of S93-453 K completety corresponds to the model of evaporation ol moisfure bound with material in the hdgroscoaic form tccordmg Co Rebinder classification. The chantc oof cueH mass in this section rorresponds to cvaposaticrt of cr-Mar moisture tnd nrooi ^ttuiire; of moyomolecular and multfmoleyuiar adtorption caused by hydrogen bonds oS wrtet molecules with active centers of furl surface [26]. The maximum cate of moisiure
- 5t6 -
Table 2. Kinetic mechanism and kinetic characteristics of various stages of thermochemical transformation of Kansk-Achinsk coals
Borodinsky coal Beryosovsky coal Nasarovsky coal
Process Reaction No. Type of reaction Temperature range T(K) Activation energy E (kJ mol-1) Probability factor k° (s-1 ), (m s-1) Temperature range T (K) Activation energy E (kJ mol-1) « fe -r •e s » 2 2 s £<S fi Temperature range T (K) Activation energy E (kJ mol-1) 1 « fe -r •e 3 » 2 2 s £ <8 &
Moisture evaporation (drying) 1 293- -453 18.6±6.83 4.01±2.41 293-448 14.4±1.52 4.01±2.12 293-453 12.2±0.98 2.3±0.11
2 H,O;Ji —ÎÎ-hîHjO 433- -553 28.3±1.86 3.5-102±0.31-102 438-563 30.9±1.13 2.9-102±0.6-102 443-548 29.9±1.3 4.2-102±1.8-102
3 C02p->CO; 613- -838 71.7±2.64 1.1-103±0.26-103 553-743 71.7±2.64 1.1403±0.3-103 723-973 79.4±2.64 1.6 403±0.26-103
H'/'o-^H, 813- 1413 92.4±4.73 45.0±3.72 793-1173 92.4±3.55 45.0±2.72 853-1173 126.5±4.11 1.3 402±0.2402
Hf'Cl— 698- -933 142.3±5.33 4.9-106±0.91-106 763-988 142.3±3.9 4.9 406±1.2406 783-1023 94.2±2.84 1.1405±0.3405
Cltf'o >CH4 613- -863 106.9±3.32 1.4-105±0.68-105 663-978 106.9±2.8 1.4405±0.5405 853-1033 199.2±3.67 2.4 4 07±0.4407
Emission of volatile matters (thermal 5 503- -698 92.3±1.19 5.2-105±1.15-105 558-738 92.3±3.11 5.2405±1.3405 703-998 74.9±2.88 40.9±2.14
decomposition) COIJI0 ka > CO 763- -1293 90.2±2.28 87.9±4.7 863-1163 90.2±2.22 87.9±2.53 758-1148 83.1±1.92 89.2±3.22
6 co""0 ka > CO 693- -928 140.9±3.74 4.5-106±1.03-106 763-993 140.9±4.3 4.5406±0.8406 738-998 116.0±2.0 2.5 4 04±0.6404
CO(///)0 *63 > CO 558- -903 71.5±1.61 4.5-102±0.88-102 553-808 71.5±2.92 4.4402±0.3402 483-788 56.1±1.53 1.3 402±0.2402
CO(/I )0 k<* > CO 598- -1483 43.2±2.39 0.3±0.15 453-613 43.2±1.53 0.35±0.16 - - -
7 608- -1003 81.7±2.16 8.4-104±0.88-104 553-1053 75.9±2.42 2.5 402±0.4402 573-913 78.4±2.2 2.8407±0.3407
Burning of volatile matters 8 IV + 290, >!'*•( ). + 22H,0 693- -1173 41.1±1.13 250±6.73 693-853 43.8±1.32 264±5.18 693-873 44.2±1.54 275±7.15
9 c+o,—^co3 653- -1213 113.0±3.45 1.06-104±0.23-104 663-1223 147.0±5.61 8.5408±0.97408 713-1253 132.8±4.2 2.8408±0.4408
Burnout and 10 C + CÛ, > 2CO 805- -1380 214.2±6.13 79-104±2.3-104 1068-1593 210.1±5.03 58-104±1.35-104 1088-1668 221.9±6.63 36-104±1.15-104
gasification of coke
base 11 C + H30—^-»CO + H, 718- -1223 181.4±5.32 1.6-104±0.21-104 703-1253 161.2±4.29 5.8-104±0.81-104 698-1203 179.4±3.98 2.9-104±0.36-104
12 2P +02 *<->CO; +H30 693- -1173 41.1±1.13 250±6.73 693-853 41.1±1.13 250±6.73 693-873 41.1±1.13 250±6.73
0 100 200 300 400 500 600 700
Temperature / 0C
0 100 200 300 40i 5S0 600 700
Temperature / 0C
0 100 200 300 400 500 600 700
Temperature / 0C
0 100 200 300 400 500 600 700
Tempiraturs / °C
0 100 200 300 400 500 600 700
Temperature / 0C
0 100 200 300 400 500 600 700
Temperature / 0C
0 100 200 300 400 500 600 700
Temperature / 0C
100 200 300 400 500 600 700
Temperature / 0C
0 100 200 300 400 500 600 700
Temperature / 0C
Fig. 5. Calculation results of the thermochemical transformation of Kansk-Achinsk conl (1 - p = 5 K min-1; 2 - p = 10 K min-1; 3 - p = 20 K min-1): I -thermal decomposition; II -coke base burnout; III -thermal-oxidative degradation; coal: a -Irsha-Borodinsky, b -Beresovsky, c -Nrsfrovsky; solid line -thee data ofcomplex the^mciil analysis; dotted line -calculation
2
evaporation in the range of heating rate variations ol 5-20 K mini dueing drying of Borodinsky coal is Wmax= 2.34-2.85 mg g-1,K-1; that oof Berezovsky coal is Wmna 2.22-2.751 mg g-s,KS1 andthat of Nazarovsky coal is Wmax= 2.53-2.98 mg g-1,K-1 with general position of temperature maximum
^nrx = 3 43 -393 K.
The temperature corresponding to the maximum rrte oof thermal decomposition reacSion 1 Tmrx) for all three types of Kansk-Achinsk coals has values of 713-733 K. The maximum ratt of volatile matters emission is Wmax= 1.61-1.95 mg g-1,K-1 for Borodinsky coal; Wmax= 1.87-2.21 mg g-1,K-1 for Berezovsky coal and Wmax= 1.33-1.69 mg g-1,K-1 for Nazarovsky coal. The peak of the total gas emission during thermal decomposition occurs at the temperature of 833 K for Borodinsky coal ; 973 K for Berezovsky coal and 793 K for Nazarovsky coal. It is significant to note that the process of volatile matters emission of Borodinsky coal in oxidative atmosphere runs in a lower temperature range (513-633 K) and at a higher rate (Wmax = 5.4-5.6 mg g-1,K-1) than the similar process in inert atmosphere (573-853 K and
Wmax = 1.29-1.36 mg g-1, K-1). The same effect is observed during the volatile matters emission of Berezovsky and Nazarovsky coals.
Burnout of nonvolatile residues of Nazarovsky coal runs in two stages with the temperature maxima of reaction rate at Tmax1= 673-748 K and Tmax2= 763-818 K, at the rate values Wmax1 = 0.44-0.56 mg g"1,K"1 and Wmax 2 = 0.48-0.6 mg g-1, K-1 accordingly. Burnout of nonvolatile residues of Borodinsky and Berezovsky coals runs in one stage at the maximum reaction rate Wmax= 0.52-0.66 mg g-1,K-1 at Tmax = 703-823 K and Wmax = 0.65-0.79 mg g-1,K-1 at Tmax= 673-798 K accordingly.
The investigations performed and the comparison of calculation results of the kinetics of thermochemical transformation of various coals allows to establish that the proposed experiment-calculation method of evaluating the degree of solid fuel burnout provides satisfactory qualitative and quantitative convergence under the linear law of heating which simulates the conditions of the complex thermal analysis. The relative error of the experimental and calculated parameters of the investigated thermochemical reactions subject to 95% confidence interval of Student statistical test t was 3-5 %.
The comparative analysis of the obtained data on the kinetics of various processes of the thermochemical treatment of Kansk-Achinsk coals shows good conformity with the results of similar investigations of individual stages of burning of a wide range of coals presented by other authors [1, 6, 13, 24, 30, 31] with the maximum correlation of reaction parameters being observed in case of coincidence of the technical composition and the analysis of coals [6, 19, 27, 29, 31]. With that the conduction conditions of the thermogravimetric experiment and the method of processing its data allow to assert that the kinetic characteristics of fuel burnout determined with the help of the proposed approach can be used in calculations of high-speed and high-temperature processes peculiar to real power plants subject to proper consideration and superposition of the diffusion factors on the kinetic model [9]. Thus, the proposed method of determination of kinetic parameters of various burning stages of coal based on its complex thermal analysis can be recommended as a universal and scientifically substantiated approach for the evaluation of the mechanism of thermochemical transformation and the reactivity of solid organic fuel.
Summary and Conclusions
1. An experiment-calculation method of evaluation of the kinetic parameters of burning processes in the context of a unified approach for the conditions of a wide range of fuel utilization plants, particularly such processes as: moisture evaporation, volatile matters emission with separate evaluation of pitch and gas emission, burning and gasification of coke base of solid organic fuel, has been developed.
2. The complex application of both experimental and calculating methods of investigating the burning processes of Kansk-Achinsk coals implemented in the discussed technique enables evaluation and control of kinetics of various overlapping processes and stages of thermochemical transformation of solid fuel in a wide range of practices in consideration of changes of reactivity of coal in comparison with its initial quality.
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Комплексный подход к оценке реакционной способности твёрдых органических топлив
Е.А. Бойко, С.В. Пачковский
Сибирский федеральный университет, Россия 660041, Красноярск, пр. Свободный, 79
Предложен экспериментально-расчетный подход для оценки кинетических характеристик термохимического превращения твердых органических топлив, основанный на использовании методов комплексного термического анализа и математического моделирования. В основу методики обработки данных термогравиметрического эксперимента и описания математической модели положена единая расчетная схема материальных балансов и кинетических процессов выгорания натуральных углей. Получена удовлетворительная качественная и количественная сходимость экспериментальных и численных результатов.
Ключевые слова: уголь, кинетические процессы выгорания, комплексный термический анализ, математическое моделирование.
