THERMAL DECOMPOSITION AND OXIDATION OF COAL PROCESSING WASTE

Transcription

1 THERMAL SCIENCE: Year 2018, Vol. 22, No. 2, pp THERMAL DECOMPOSITION AND OXIDATION OF COAL PROCESSING WASTE by Ksena Yu. VERSHININA a, Sergey Yu. LYRSCHIKOV a,b, and Pavel A. STRIZHAK a * a Natonal Research Tomsk Polytechnc Unversty, Tomsk, Russa b Federal Research Center of Coal and Coal Chemstry of SB RAS, Kemerovo, Russa Orgnal scentfc paper Introducton To expand the database of knetc parameters used for modelng the gnton of coals and ther processng waste, promsng coal-water slurry and coal-water slurry contanng petrochemcals, studes have been performed on an expermental set-up usng thermal gravmetrc analyss. The research nto coals of varous ranks (flame, gas, cokng, low-cakng, and non-bakng) and ther processng waste (flter cakes) has yelded the decomposton parameters of the organc matter of coal and the formaton of volatle substances as well as the oxdaton parameters of the coke resdue of all the coals and flter cakes under study. The studes cover the temperature range of the burnng processes: K (for low-, medum-, and hgh-temperature burnng modes). We have ascertaned the dependence of knetc parameters (pre-exponental factor and actvaton energy) descrbng the thermal decomposton of the organc matter of coal on the rank of coals and flter cakes. The fndngs show that the knetc parameters descrbng the thermal decomposton of the organc matter of coal and ts processng waste are practcally the same. The thermoknetc parameters of coke resdue oxdaton are close for all the coals under study but they dffer sgnfcantly for coke resdue of flter cakes. The values of thermoknetc parameters obtaned n the research are necessary to devse adequate physcal and mathematcal models and perform numercal studes (for mathematcal modelng) of fuel slurry combuston processes n the combuston chambers of power plants. Key words: coal water slurry contanng petrochemcals, coal processng waste, gnton, thermal decomposton, oxdaton, thermoknetc constants Greenhouse effect, global warmng, global dmmng, and ther ramfcatons (rsng sea and ocean levels, land devastaton, human mgraton, dseases, excess mortalty, etc.) have been known to the world snce Almost 200 years have passed. Has humanty found a way to fght ths? Have we solved the key problems or s the global threat stll present? Unfortunately, we are stll n search of answers to these questons. Moreover, the consumpton of electrc and thermal energy has almost trpled over the last 40 years. At the same tme, CO 2, NO x, and SO 2 emssons have only gone down 13-15% per ton of fuel equvalent. The current statstcs look alarmng: on average 4-5 tons of per capta CO 2 emssons. No mprovements n the car ndustry, however, wll solve global envronmental problems. Almost 80% of CO 2, NO x, and SO 2 * Correspondng author, e-mal: pavelspa@tpu.ru

2 1100 THERMAL SCIENCE: Year 2018, Vol. 22, No. 2, pp emssons fall on power-generatng facltes lqud, sold or gaseous fuel combuston plants. The problem of GHG emssons has to be tackled n heat power engneerng [1-3]. The analyss shows that over 40% of heat and power plants as well as other power-generatng facltes around the world work on sold fuels (coals of dfferent qualty) [4, 5] wthout any chance of converson to alternatve fuels for a number of objectve reasons. At the same tme, sold fuels are the most hazardous n terms of consequences for the envronment [6, 7]. Over the last years, the technologes of coal-water slurres (CWS) have been qute actvely developng throughout the world [8-11]. Usng these fuels can reduce the emssons of carbon and sulfur oxdes vs. coal dust and nvolve low-rank coals (even fre-hazardous brown coals) as raw materals. Alongsde CWS, coal water slurry contanng petrochemcals (CWSP) s also perceved as a promsng slurry fuel [12-14]. The producton process of CWSP suspensons also nvolves a lqud fuel component. The latter makes t possble to sgnfcantly expand the range of raw materals for large and small power ndustres as well as transport ndustry (desel fuel can be replaced by CWS) and other sectors. The followng materals can be CWSP components [15-17]: low-qualty coal dust, ol and coal processng waste, ol sludge, heavy coal-tar products, resns, waste ndustral ols, and varous combustble lquds. The most mportant task here s to establsh the opportunty to use flter cakes n CWSP, the volumes of such low-qualty coal processng resdues are estmated n mllons of tons n the world [16-19]. The development of a research-backed technology of CWSP producton wll sgnfcantly expand the scope of raw materals used n heat and power ndustry. As a consequence, there wll be an actual alternatve to hgh qualty energy fuels based on coal, ol and gas. One of the deterrents to usng CWS and CWSP throughout the world s qute complcated sets of nterdependent physcal and chemcal processes nvolved n the gnton and combuston of CWS. We need relevant fundamental understandng of these processes and an extensve nformaton base of relable expermental data. Over the last decades, such data have been obtaned n the feld of preparaton and combuston of CWS [8-10] and CWSP [12-14]. These experments [8-14], however, were devoted to specfc types of coal (matchng the coal deposts of a lmted number of world regons). These data are dffcult to use for a broad-scale development of CWS and CWSP technologes, snce the propertes of components may vary drastcally and, consequently, the gnton and combuston characterstcs wll be sgnfcantly dfferent as well. Therefore, a follow-up on studes [8-14] wll be the development of mathematcal models smulatng the gnton and combuston of CWSP droplets. Ths stage, however, depends qute consderably on the source data (there are extremely many emprcal constants). A number of approaches have been used for modelng the processes nvolved n the gnton of CWS and CWPS droplets over the last years: usng one s own programmng codes [20-22] or mathematcal modelng packages [23-25]. However, a comparson of the modelng results wth the known expermental data often reveals sgnfcant devatons between theoretcal and expermental gnton characterstcs. These devatons usually stem from the lack of relable nformaton on the knetcs of thermal decomposton and oxdaton of coal fuel components. Ths stfles the actve usage of mathematcal modelng software (n partcular, ANSYS Fluent, etc.) to determne the optmal condtons of CWSP combuston n power engneerng. The am of ths work s to obtan an expermental base of thermoknetc constants by thermal gravmetrc analyss for modelng the gnton processes of CWSP based on coal and ts processng waste. It s reasonable to study the thermal decomposton of coals and flter cakes as well as the oxdaton of coke resdue on an expermental set-up.

3 THERMAL SCIENCE: Year 2018, Vol. 22, No. 2, pp Expermental set-up and procedures We studed coals, tab. 1, from Kuznetsk Basn (Kemerovo regon, Russa) and ther processng waste (flter cakes): F flame coal (processng plant of the Lstvyazhnaya mne), G gas (processng plant of the Krov mne), C cokng (Severnaya processng plant), L low-cakng (Cherngovskaya-Koksovaya processng plant), N non-bakng (Kaltanskaya-Energetcheskaya processng plant). Table 1. Results of proxmate and element analyss of coals and flter cakes Coal rank Proxmate analyss Ultmate analyss [% daf] A d, [%] V daf, [%] Q d s,v, [MJ/kg] C H (O+N+S) Flame coal Gas coal Cokng coal Low-cakng coal Non-bakng coal Flter cake of flame coal Flter cake of gas coal Flter cake of cokng coal Flter cake of low-cakng coal Flter cake of non-bakng coal We performed our measurements on coal analyss samples (ar-dry, partcle sze µm) as well as flter cakes dred at a temperature of 380 K, ground and seved (partcle sze µm). These flter cakes were produced durng coal flotaton, whch nvolves washng coal wth a surfactant soluton and further screenng nto separate fractons. Water used for coal washng s fed to specal contaners for clarfcaton, where coal partcles settle. The slurry s pumped out and sent to belt press flters for pressng and flterng through a specal membrane. The resultng wet resdue of coal partcles s known as a flter cake. Proxmate analyss of flter cakes and ntal coals measured the humdty of an ar-dry analyss sample, W a, and dry ash level, A d, and recalculated the VOC concentraton n the fuel mnus humdty and ash level, V daf, usng the correspondng methods [25-27]. Gross heat value of the analyss sample of coals and flter cakes wth a constant volume, Q d s,v, was measured by an IKA C 2000 calormeter accordng to the method descrbed n [28]. The ultmate analyss of flter cakes and analyss sample, by the method from [29], was carred out usng a varo MICRO cube analyzer. We burned the sample n oxygen at a temperature of 1373 K. The resultng gases were separated chromatographcally and detected by means of a katharometer. The software pre-nstalled on the detector measured the mass fractons of C, H, N, and S (oxygen was calculated) and recalculated the VOC concentraton n the fuel mnus humdty and ash level. Thermoknetc parameters (pre-exponental factor and actvaton energy) of the decomposton of the organc matter and oxdaton of the coke resdue were determned n several stages: Stage 1. Inert-atmosphere thermal gravmetrc analyss of a coal sample or flter cake. Stage 2. Oxdzng-atmosphere thermal gravmetrc analyss of coke resdue. Stage 3. Calculaton of thermoknetc parameters from the thermogravmetrc data.

4 1102 THERMAL SCIENCE: Year 2018, Vol. 22, No. 2, pp Thermal analyss of coals was performed on an STA 449F3 synchronous thermoanalyzer (NETZSCH Company) under the followng condtons: a 25-mg sample was placed n a corundum crucble and heated up to 1273 K n argon at a gas-flow rate of 100 ml/mn and n argon-ar mxture (10 and 50 ml/mn, respectvely). Thermogravmetrc analyss (TGA) was carred out for the flame coal n the nert atmosphere at the three dfferent rates of heatng, namely 10, 20, and 30 K/mn. The thermoknetc decomposton parameters of the organc matter of coal have the smlar values for all the studed rates of heatng,. e. there are almost no dfferences between values for the three heatng rates. Thus, we neglect these dfferences between values durng the mathematcal smulaton of CWSP combuston. For ths reason, thermoknetc parameters of a decomposton of the organc matter of coal and a coke resdue oxdaton for all the samples of coals and flter cakes were determned at a heatng rate that equals to 10 K/mn. Knetc decomposton parameters (actvaton energy and pre-exponental factor) were calculated, smlar to the methodology used n [30]. We assumed that the ntal product A decomposes to form ntermedate product B and volatle P 1. Product B then reacts to form the ntermedate product C and lberates the volatle product P 2, etc.: A B+ P C+ P (1) k1 k2 k The lberaton rate of volatle substances at each stage can usually be descrbed by a frst-order chemcal equaton wth the subsequent calculaton of rate constants of the correspondng stages. The process of chemcal destructon wll overall correspond to the scheme m [mg] Fgure 1. Typcal emsson curve of volatle substances durng thermal destructon of a sample τ [mn] of frst-order consecutve reactons. The rate of the process s derved from the overall accumulaton of volatle substances,. e., ther mass s measured over a certan perod, wth two destructon stages, fg. 1. The mass of the volatle substances lberated s equal to the loss of mass of the sold sample. As can be seen from fg. 1, the knetc curve of volatle substance lberaton conssts of two-stages, each represented by an S-shaped lne. They have ths shape, because the reacton rate frst goes up because the rate constant ncreases accordng to the Arrhenus equaton and then the destructon rate goes down due to the reagent decomposton. The mass of volatle substances lberated at each stage s dfferent, and the number of moles partcpatng n the reacton s unknown. Therefore, we can consder by conventon that at each stage, the same number of reagent moles decompose, whch s equal to the mole rate of the ntal substance n A,0. The maxmum mole rate of volatles that can be lberated at each stage wll also equal n A,0 : n = n = n (2) A,0 P1max P2 max The rate of volatle emssons at each decomposton stage wll be determned by the mole number of the ntal or ntermedate sold substance. Then the system of equatons descrbng the two-stage thermal decomposton of the sold phase may look lke ths:

5 THERMAL SCIENCE: Year 2018, Vol. 22, No. 2, pp dn d kn Volatle emsson rates: Materal balance relaton: Then dn kn knb τ =, dnc kn 2 B dτ = (3) A 1 A τ =, B 1 A 2 d dn P1 kn 1 A τ =, P2 kn 2 B dτ = (4) d dn n = n + n + n or n = n + n, n = n (5) A,0 A B C A,0 A P1 C P2 n = n n n or n = n n (6) B A,0 A C B P1 P2 and the system of equatons wll take the followng form: dn P1 k1 ( na,0 np) 1 τ =, P2 k2 ( np n ) 1 P2 τ = (7) d The current mass of volatle substances, m, emtted at ths stage s proportonal to the current number of moles, n P, (where s the number of the stage of the knetc volatle emsson curve): whence t follows that: dn d m mp n P = = (8) m m n max P max A,0 n n A,0 P = m (9) m max If we use ths expresson n the system of equatons descrbng the volatle emsson rates, we obtan: na,0 d m1 m1max na,0 dm 1 m = 1 k1 na,0 m1 = km 1 1max 1 = k1 ( m1max m1 ) dτ m1max dτ m1max na,0 d m2 m2max na,0 na,0 dm 2 m1 m = 2 k2 m1 m2 = km 2 2max (11) dτ m1max m2max dτ m1max m2max In the case of coals and flter cakes, qute a long tme passes between the decomposton stages. Therefore, by the begnnng of the second-stage, the emsson of volatle substances as per the frst equaton has stopped and m 1 = m 1max. The resultng system of equatons then smplfes to: (10) dm1 k 1( m 1max m 1) dτ = (12)

6 1104 THERMAL SCIENCE: Year 2018, Vol. 22, No. 2, pp dm d Here, eq. (10) remans unchanged, whereas the eq. (11) s derved: 2 k2 ( m2max m2 ) τ = (13) dm 2 m1max m 2 m 2 = km 2 2max = k2 m2max m2max dτ m1max m2max m2max The total loss of mass of the ntal substance and the rate of volatle emsson: m= m, n partcular, for two-stages of destructon: (14) dm m = (15) dτ dτ dm dm1 dm2 m = m 1 + m 2, = + (16) dτ dτ dτ The process under study runs under non-sothermal condtons. Therefore, the rate constants k 1 and k 2 change accordng to the Arrhenus equaton n the course of the experment. As a rule, the process s carred out so that the temperature of the sample lnearly depends on the reacton tme wthout varyng wth the volume of the sample: Т = Т 0 +βτ (17) where Т 0 [K] s the ntal temperature and β [Kmn 1 ] the rate of temperature ncrease. Then the Arrhenus dependence of the constants at the th stage wll take the followng form: E k = k0exp R( T0 + βτ) and the rate equaton of the th stage of the frst order reacton: where dm b r = = exp a ( mmax m) dτ T0 + βτ a = ln k 0 and b E R (18) (19) = (20) The rates of changes n the sample mass are derved from the dagram determned expermentally. The parameters a = lnk 0 and b = E /R are selected by lnearzng the rate equaton of the correspondng stage: r b ln = a m m T + βτ max 0 The last expresson s an equaton of a straght lne n axes: where a s the ntercept and b s the slope. r 1 ln m m T max (21)

7 THERMAL SCIENCE: Year 2018, Vol. 22, No. 2, pp Results and dscusson Table 2 presents the thermoknetc parameters of the flame coal. The ranges of varaton of the nvestgated parameters are shown at dfferent heatng rates and temperature ranges. In general, we can note a good correlaton of the obtaned data for dfferent heatng rates of the sample. Table 2. Thermoknetc parameters of a thermal decomposton of the flame coal n the nert atmosphere Temperature range, [K] Heatng rate, [Kmn 1 ] E, [kjmol 1 ] k, [s 1 ] E, [kjmol 1 ] k, [s 1 ] Fgure 2 presents typcal thermal gravmetrc curves n an nert atmosphere for three ranks of the coals under study. There are two loss peaks on the curves of the mass loss rate: the man low-temperature loss peak resultng from the decomposton of the organc matter of coal and formaton of volatle substances and the hgh-temperature loss peak most lkely stemmng from the decomposton of mneral carbonates n coal [31]. As we have establshed, the hgher the coal rank (from flame to non-bakng one), the hgher the peak decomposton temperature of the organc matter. Sample mass [%] Rate of mass loss [%mn 1 ] (a) Temperature [K] Temperature [K] Fgure 2. Thermogravmetrc curves of mass loss (a) and rate of mass loss (b) of the coal samples under study; 1 rank G, 2 rank L, 3 rank N The thermogravmetrc data are lnearzed n axes: max (b) dm d 1000 ln τ m m T The equaton of the straght lne defnes the actvaton energy and pre-exponental factor for the frst and the second decomposton stages, fg. 3. The thermoknetc decomposton parameters of the organc matter of coal and the formaton of volatle substances for all the coals and flter cakes under study are presented n tab. 3.

8 1106 THERMAL SCIENCE: Year 2018, Vol. 22, No. 2, pp ln [r/(mmax m)] R R / T Fgure 3. Lnearzaton of the temperature dependence of the L-rank coal thermal destructon rate As reflected by tab. 3, wth a growng coal rank comes a lnear ncrease n the actvaton energy at the frst stage and an exponental ncrease n the pre-exponental factor, fg. 4. The parameters of the second decomposton stage are ndependent of coal rank. Apparently, the thermoknetc parameters of the second decomposton stage depend on the chemcal composton of the mneral matter. In addton, the thermoknetc decomposton parameters of coals and ther processng waste (flter cakes) are vrtually equal. Table 3. Thermoknetc parameters of thermal decomposton (n an nert atmosphere) Sample Temperature Pre-exponental Actvaton range, [K] factor k, [s 1 ] energy E, [kjmole 1 ] Flter cake of flame coal Flame coal Flter cake of gas coal Gas coal Flter cake of cokng coal Cokng coal Flter cake of low-cakng coal Low-cakng coal Flter cake of non-bakng coal Non-bakng coal In low-rank coals, a macromolecule of coal has the largest pendant groups. An ncreased content of oxygen n pendant groups results n the thermal nstablty of the organc matter. Such bulky sde groups qute easly break away from the nucleus of a large molecule durng coal pyrolyss. Therefore, the thermoknetc parameters of low-rank coals have the lowest values. The hgher the coal rank, the shorter the sde groups. The content of oxygen also drops n functonal groups, and the polyaromatc nucleus of a macromolecule expands. These changes rase the thermal stablty of the organc matter of coal, whch, n turn, ncreases the values of thermoknetc parameters.

9 THERMAL SCIENCE: Year 2018, Vol. 22, No. 2, pp The thermoknetc decomposton parameters of the organc matter of coal and the formaton of volatle substances are close to those of coal decomposton from other deposts of the world, e. g., for Australan coals [32] of smlar ranks, the actvaton energy also ranges from 50 to 90 kj/mole (and smlarly depends on the rank). Turksh and Pakstan coals [33] as well as coals from deposts n other countres, e. g., [31], have comparable values. It s, therefore, qute lkely that same-rank coals (flame, gas, cokng, low-cakng, non-bakng) from other deposts wll have the values of the sad thermoknetc parameters smlar to those establshed n tab. 3. The thermoknetc parameters of more oxdzed coals are lkely to be somewhat lower than those of less oxdzed coals of the correspondng ranks. The thermogravmetrc data of the coke resdue n an oxdzng envronment were processed n a smlar way. Oxdaton was supposed to run n an excess of oxygen as a pseudo-frst-order reacton. The thermoknetc parameters of the heterogeneous coke resdue combuston for all the coals and flter cakes under study are presented n tab. 4. In the case of coke resdue oxdaton, the thermoknetc parameters are close for all the coals under study, whereas these parameters dffer sgnfcantly for coke resdue of flter cakes, tab. 4. Ths must be because the mneral part of the coke resdue affects the oxdaton process. For coals wth moderate ash level, the share of the mneral part n the coke resdue s neglgble and does not seem to have any consderable mpact on the oxdaton process. When t comes to flter cakes, though, whch have hgh ash level n the frst place, the share of mneral part n the coke resdue seems sgnfcant as does ts mpact on the coke resdue oxdaton process. In ths research, we obtaned the thermoknetc parameters of thermal Actvaton energy [kjmol 1 ] Carbon content [%] Fgure 4. Dependence of actvaton energy and pre-exponental factor on carbon content Table 4.Thermoknetc parameters of coke resdue oxdaton Pre-exponental Sample factor k, [s 1 ] Flter cake of flame coal Actvaton energy E, [kjmole 1 ] Flame coal Flter cake of gas coal Gas coal Flter cake of cokng coal Cokng coal Flter cake of low-cakng coal Low-cakng coal Flter cake of non-bakng coal Non-bakng coal decomposton and oxdaton of coke resdue for the coals of varous ranks from Kuznetsk Basn, Russa. The data obtaned allowed us to determne how thermoknetc parameters of the thermal decomposton of the organc matter depend on the coal rank, whch makes t possble to forecast the values of thermoknetc parameters of coals from other deposts of ths basn. We also establshed that the thermoknetc parameters of the thermal decomposton of coal processng waste agree wth the sad values of ntal coals, whch n turn enables us to forecast the values of thermoknetc parameters. If the coke resdue of coal processng waste s oxdzed, t Pre-exponental factor [s 1 ]

10 1108 THERMAL SCIENCE: Year 2018, Vol. 22, No. 2, pp does not seem possble to forecast the values of thermoknetc parameters at ths research stage. Thermoknetc parameters of the ntal coal coke resdue are close for all the coals rrespectve of ther rank. The expermental data obtaned deepen the nsght nto the knetcs of the processes nvolved n the thermal decomposton and oxdaton of sold fuels. In partcular, the database of relable knetc constants s expanded, whch s based on a set of papers publshed over the last 20 years, e. g. we can underlne works [34-38]. The development of laboratory-based measurement equpment wll help us take major steps to create a unfed database of thermophyscal and thermoknetc characterstcs of wdely used sold and composte fuels [39-41]. Conclusons y We obtaned values of thermoknetc parameters (pre-exponental factor and actvaton energy) descrbng the decomposton of the organc matter and oxdaton of the coke resdue of the coals and ther processng waste. Ths nformaton has been systematzed nto an expermental database to cater for the needs of the modern computatonal scence n relable data on the knetcs of the processes nvolved n fuel gnton. The database contanng thermoknetc constants for thermal decomposton and oxdaton of coal processng waste wll smooth the way for usng mathematcal modelng to explore the man deterrents to ther broad-scale applcaton n power engneerng as the prmary or secondary component of promsng coal-water slurry and coal-water slurry contanng petrochemcals. y We showed the maxmum dfferences of values of thermoknetc parameters (pre-exponental factor and actvaton energy) descrbng the decomposton of the organc matter and oxdaton of the coke resdue of the coals and ther processng waste. Ths result s mportant for predctng dfferences of the characterstcs of gnton and combuston of CWS and CWSP, prepared from varous coal components all over the world. Usng the present work, t s possble to predct the dfferences n the thermoknetc characterstcs for slurres, typcal for Chna, Inda, Japan, the USA, Russa and other countres. y Expanson of the created database wth knetc parameters of the thermal decomposton and oxdaton of the components of CWS and CWSP by nvolvng all promsng components (n terms of envronmental, economc and energy crtera), n partcular, coals of dfferent grades, coal waste, used combustble lquds, refnery waste, wood waste, plant wastes, etc. s an mportant drecton of development of ths work. Acknowledgment Ths work was supported by the Russan Scence Foundaton (project ). Nomenclature A ntal substance A d ash level of dry sample, [%] B, C ntermedate sold product C daf fracton of carbon n the sample converted to a dry ash-free state, [%] E actvaton energy, [kj mol 1 ] E actvaton energy of stage, [kj mol 1 ] H daf fracton of hydrogen n the sample converted to a dry ash-free state, [%] k pre-exponental factor, [s 1 ] k 0 pre-exponental factor of stage, [s 1 ] k reacton rate constant of stage, [s 1 ] m total loss of mass (percentage), [%] m current mass (percentage) of volatle substances released at ths stage, [%] m max total mass (percentage) of volatle substances released at ths stage, [%] N daf fracton of ntrogen n the sample converted to a dry ash-free state, [%] n A,0 number of moles of ntal substance n P current number of moles O daf fracton of oxygen n the sample converted to a dry ash-free state, [%] volatle substance P

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