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Static model of coal pyrolysis in a circulating fluidised-bed reactor
Applied Energy 71 (2002) 455–465 www.elsevier.com/locate/apenergy
Static model of coal pyrolysis in a circulating fluidised-bed reactor Marek Sciazko* Institute for Chemical Processing of Coal, 41-803 Zabrze, 1 Zamkowa Str., Poland
Abstract Four different coals were investigated: two sub-bituminous, one bituminous and lignite, which were processed in the temperature range 750–950 C. The heat for pyrolysis was generated by partial gasification of the char produced. Air was used as the gasifying medium with amounts of 0.6–1.5 m3/kg of coal, depending on the required gasification-temperature. Two sequential phenomena were taken into account: char gasification and coal devolatilisation in respect of temperature. The experimental data on carbon dioxide and monoxide concentrations in a LCV gas produced were used for the correlation of Boudouard’s equilibrium and the data on carbon burn-off and final volatile matter content in char were used for the solidproducts yield. The equations for the quasi-equilibrium state were developed and calculated values were compared with the measurements. The model takes into account the equations developed and the total energy-balance assuming the heat losses of the experimental system. The investigated coal throughput amounted to 200–300 kg/h depending on the coal properties. Process characteristics were discussed, namely: the effect of air/coal ratio on the pyrolysis temperature; char and gas yield, volatile matter and ash content in a char; as well as the gas calorific value. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Pyrolysis; Gasification; Fluidisation; Modelling
1. Introduction Considerable research on coal pyrolysis and gasification has been conducted over the years, but the results are widely dispersed because of the complex chemistry of coal. Time related coal-pyrolysis modelling assumes basically first-order kinetic equations, or less sensitive for heating rate, the distributed activation energy model or energy activation dependent on the heating rate [1,2]. The last two more * Tel.: +48-32-271-5151; fax: +48-32-271-0809. E-mail address: offi[email protected] (M. Sciazko). 0306-2619/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0306-2619(02)00200-3
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Nomenclature Ak Gg Gk Gw Ke ms Qg Qk Qh Vk Vp Vw T Wd Wdc V
ash content in a char (kg/kg) gas flow rate (kg/h) char flow rate (kg/h) coal flow rate (kg/h) experimental equilibrium constant (0.1 MPa1) mass of carbon reacted in gasification (kg/kg of coal) gas chemical enthalpy (kJ/kg of coal) char chemical enthalpy (kJ/kg of coal) heat evolved in a process (kJ/kg of coal) volatile matter content in a char (kg/kg) air flow rate (m3/h) volatile matter content in a coal (kg/kg) temperature (K) gas’s calorific value (kJ/m3) coal’s calorific value (kJ/kg) volatile-matter loss due to coal pyrolysis (kg/kg of coal)
advanced models need three and four constants respectively, which basically depend on the coal properties but also cover to some extent, the effect of heat-and-mass transfer phenomena. That is the reason for the different values of the activation energy and pre-exponential factor cited in the literature and the lack of generally valid data. The same situation exists in the case of coal-char gasification. The reaction rate of char is influenced mainly by chemical and physical factors, which include coal type, catalytic effect of the ash and the specific surface area of char, which changes during the reaction course with the development of internal pores, and finally, their destruction [3,4]. In the case of the scaling-up procedure, the uncertainty of a complex model of the reacting system may be very high and it is reasonable under some conditions to use a methodology based on quasi-equilibrium conditions, which can be reflected at a larger scale. This approach assumes basically that the process itself, with all specific features, is a decisive factor for the path of the reactions of coal decomposition. Therefore a static model of char gasification and coal pyrolysis was developed: it is based on the assumption that the final process temperature is a decisive factor for the required volatile-matter content in the char being in a quasi-equilibrium state with respect to the gas temperature. To achieve this, it is necessary to create conditions of internal circulation of the transported coal and char in a riser, where the average concentration of solids amounts to 0.05–0.15 m3/m3, i.e. the conditions for residence time are long enough for the thermal decomposition of coal and intensive mixing so enhancing mass and heat transfers. On the other hand, the gasification of circulated char is mainly chemically controlled due to the relatively short residence time of the gas. Additionally, the characteristic feature for a given circulating fluid-bed reactor is a very narrow range of operational
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residence time, both for the gas and coal, which allows us to use the quasi-equilibrium approach. The model developed reflects the kinetic processes (transport phenomena of coal species, reaction rate) by constants of the developed equations, which depend mainly on the coal properties.
2. Coal pyrolysis in a circulating fluidised-bed A circulating fluidised-bed reactor was used as a coal pyrolysis reactor. The process was tested at a scale of 200–300 kg/h: collecting operational and design data to build an industrial installation [5]. A technological diagram of the coal-pyrolysis process development unit (PDU) is presented in Fig. 1. The reactor comprises basically two parts, which are distinguished geometrically. Air and circulating char are introduced to the bottom section of a large diameter reactor and coal is fed to the base of the riser—the upper section, which is connected with the bottom one. Due to oxygen availability, the char undergoes gasification reactions so generating heat for the coal pyrolysis. The region of the reactor’s operation depends primarily on the velocity of the gas phase, and so for a specific reactor’s design, on the air flow rate and on the concentration of the solid phase. With regard to changes of the gas-phase flow rate, the reactor’s operation regimes are confined to the region between the pneumatic transport and the turbulent or bubble fluidised-bed including internal solid circulation in the riser. Such aerodynamic conditions of the reactor’s operation make it possible to achieve high values of heat-and-mass transfer coefficients. This leads to a better utilisation of the reaction volume, a reduction of the overall reactor’s dimensions and its easy control. 2.1. Process demonstration unit This process is directed primarily towards the production of gas and char. The installation (see Fig. 1) consists of a pyrolysis reactor with circulating fluidised-bed
Fig. 1. Coal pyrolysis PDU system.
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(2), coal hopper (1), a system of char separation from the hot process-gases (3,4,5), heat-recovery system, as well as the means for tar and HCl removal from the gas. A two-stage system of solid products separation from the process gas has been used in the installation, resulting in obtaining a coarse char (particle size exceeding 0.5 mm) and a cyclone dust with particle size below 0.5 mm. The cleaned gas is burnt in a 0.5 MWth burner, in which the combustion dynamics were studied. The tests were conducted in the temperature range 750–950 C using coals of average diameter 0.160–0.560 mm, which resulted in a gas residence-time of 0.7–0.9 s and for coal 90–120 s. Char was sampled from the char bin and from the secondary cyclone. Char gasification data were collected and the concentrations of [CO] and [CO2] measured at the outlet of the reactor cooling gas immediately by means of a two-stage ice-cooler [6]. Four different coals were used, namely bituminous ‘‘Rozbark’’, two subbituminous coals ‘‘Wieczorek’’ and ‘‘Janina’’, and lignite ‘‘Borsod’’. The demonstration installation is a part of a development project to design a pilot plant for smokeless-fuel production. The technology consists in the briquetting of a mixture of char obtained from a steam coal in a pyrolysis reactor with a preheated coking coal at approximately 430 C. 2.2. Experimental results In the investigations, it was found that the ratio of air-flow rate Vp to the coal flow rate Gw is the leading process parameter that correlates all the changes of gas and char properties (namely the content of volatile matter in char Vdaf, char output Gk/ Gw, gas calorific value Wd and the ratio of [CO]/[CO2] concentration in the gas) as well as determines the yield of process products (Fig. 2). The direct influence of this parameter on the products’ properties means that, in the studied flow range and for the residence time of gas and coal connected with this range, the process temperature is the leading parameter. 2.3. Static model of coal pyrolysis in a reactor with circulating char Taking into account the operation principle of a coal-gasification reactor with a circulating fluidised-bed, consisting of: partial gasification of circulating char (devolatilised coal), coal pyrolysis, where the heat produced by the reaction of the char’s partial gasification is consumed, and considering the temperature-profile uniformity in the system resulting from the very intensive axial mixing, the logistic diagram of the combined gasification and pyrolysis process presented in Fig. 3 may be considered. In the gasification system, one deals mainly with char and in principle two consequent reactions occur there, the first of which is a combustion reaction and the second is the CO2 reduction—Boudouard’s reaction:
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a1C þ a1O2 ¼ a1CO2 þ H1 b1C þ b1CO2 ¼ 2b1CO þ H2 Assuming that: =a1+b1 stands for the total amount of C kmol, which reacts, the following equations can be obtained: a1C þ a1O2 ¼ a1CO2 and ð a1 ÞC þ ð a1 ÞCO2 ¼ 2ð a1 ÞCO It is assumed that the combustion reaction proceeds until the depletion of oxygen (supplied with air ensues). On the other hand, the Boudouard reaction
Fig. 2. Effects of air–coal ratio on: (a) yield of char, (b) volatile matter content, (c) gas calorific value and (d) CO/CO2 ratio (‘‘Wieczorek’’ coal).
Fig. 3. Schematic diagram of the char-gasification and coal-pyrolysis process.
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[7] proceeds, where the empirically-determined equilibrium constant is described by equation: Ke ¼
1 y2CO y2 CO P yCO2 yCO2
ð1Þ
The empirical equilibrium constant may be correlated by the relationship (2) [8] expressing the quasi-equilibrium conditions. lnKe ¼
A þB T
ð2Þ
The experimental data necessary to determine the coefficients of the above equation have been derived by performing gasification tests on a char from a given coal obtained by its previous devolatilisation at 550 C in an oxygen-free environment in a bubbling fluid-bed reactor. The temperature used is that for the low-temperature pyrolysis of coal, at which primary volatile matter is released. Measurements of [CO] and [CO2] concentrations in the obtained gas allow one to determine appropriate coefficients [Eq. (2)] that for various coals are presented in Table 1. The correlation error is in the range of 11.5%. The most important effect of coal pyrolysis is the volatile-matter evolution. Assuming that we are interested in the Vk =Vw ratio expressed by the absolute loss of coal’s volatile matter (on a as-received basis) V and the mass ms, of carbon reacted in the gasification, one can obtain: #¼
Vk Vw V ¼ Vw ð1 V ms WÞVw
ð3Þ
which can be correlated as a function ln# ¼
C þ DlnT þ E T
ð4Þ
The calculated coefficients for coals of various compositions are shown in Table 2.
Table 1 Empirical coefficients of Eq. (2) Coefficient
A B
Char tested ‘‘Borsod’’
‘‘Janina’’
‘‘Rozbark’’
‘‘Wieczorek’’
0.848 4350
0.799 3826
0.161 2430
2.75 6559
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The correlation error does not exceed the value of 8.2%. To assess the gas’s calorific value, it has then been assumed that its value is determined by the calorific value of coal, less the: enthalpy of the char, enthalpy of the burned and gasified carbon, enthalpy of the hydrogen that reacted with the coals and oxygen. In the last case, it has been assumed that 50% of the oxygen content in the coal reacts with hydrogen producing water. However, this does not affect the heat of the pyrolysis reaction, which is approximately equal to zero [9]. Practically all the Table 2 Coefficients of Eq. (4) Coal tested
Coal composition Carbon Hydrogen Oxygen Sulphur Nitrogen Ash Water Volatile matter Coefficients C D E
‘‘Borsod’’ lignite
0.395 0.027 0.105 0.018 0.012 0.188 0.255 0.291 6160 11.2 82.3
‘‘Janina’’ subbituminous
0.585 0.030 0.110 0.012 0.018 0.095 0.150 0.310 10 510 16.1 121.3
‘‘Rozbark’’ bituminous
0.792 0.017 0.025 0.005 0.012 0.125 0.024 0.134 10 410 17.0 127.1
‘‘Wieczorek’’ subbituminous
0.700 0.030 0.105 0.0085 0.010 0.087 0.060 0.320 47 930 51.1 400.2
Fig. 4. Effects of temperature on air demand, yield of gas, and properties of char according to the model calculations (for ‘‘Wieczorek’’ coal).
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remaining oxygen shifts to the gas in the form of carbon monoxide and dioxide, while the sulphur and nitrogen split proportionally to the char yield between the gaseous and solid phases [10]. However, they have no major influence from the point of view of the calorific value of the generated gas. Taking into account the presented static model of the process of partial gasification and pyrolysis of coal, the behaviours of various coals in the discussed process have been analysed. Differences in the Boudouard reaction kinetics and the process of pyrolysis are allowed for by empirical correlations and thermodynamic properties
Fig. 5. Comparison of experimental data and calculated results of char yield at various temperatures (‘‘for Janina’’ coal).
Fig. 6. Char volatile matter content—comparison of experimental data and calculated results (‘‘for Janina’’ coal).
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of the coal, and the products of its decomposition have been defined according to the procedure suggested by the Institute of Gas Technology (USA) [11]. Fig. 4 presents typical relationships for the gasification of ‘‘Wieczorek’’ coal, which indicates that, at higher temperatures, the ash content in a char increases and the volatile matter decreases. The increase of temperature by 100 K demands ca. 0.4 m3 of additional air. Comparisons of the experimental data with computations are shown in Figs. 5–8 for the gasification of ‘‘Janina’’ coal in a circulating fluidised-bed reactor. Fig. 9 presents, for this coal, the structure of the chemical enthalpy of fuel breakdown; it may be seen that approximately 50% of the chemical energy goes to the solid product, while
Fig 7. Ash content in char—comparison of experimental data and calculated result (‘‘for Janina’’ coal).
Fig. 8. Air demand—comparison of experimental data and calculated results (‘‘for Janina’’coal).
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approximately 35% goes to the gas. The remaining 15% covers the heat of reaction and thermal losses including water evaporation. The heat losses to the environment in the pilot plant (300 kg/h) amounted to 10 kW, on average, i.e. approximately 5% of the heat released in the gasification reaction. The total demand for heat to perform the process in the temperature range 750–950 C requires 2500–4500 kJ/kg of the coal. From the point of view of gas energy utilisation, its calorific value is significant; this quantity in the experimental conditions is calculated on the basis of the gas composition in the ambient state. So neither the contained water vapour nor the condensing hydrocarbons are taken into account; their amount, as results from the performed studies may equal to 5–20 g tar/m3 and 5–10 g m3 of benzene, toluene, xylene (BTX) in process gas. These components increase the gas’s calorific-value by
Fig. 9. Distribution of coal’s chemical enthalpy in pyrolysis products (‘‘for Janina’’ coal).
Fig. 10. Gas calorific value—comparison of experimental data and calculated results (‘‘for Janina’’ coal).
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320–1200 kJ and, in total, allow one to obtain a gas of calorific value of up to approximately 6000 kJ/m3. It was noticed that for the specific Vp/Gw ratio, the maximum gas calorific value is achieved (Fig. 10), which was confirmed experimentally. From Fig. 10, it can be seen that, in the considered temperature range, the calorific value of the gas varies in the range 4500–5000 kJ/kg and achieves the maximum at 830 C. Performed simulations and experimental data show that basically the moisture and ash contents in coal are decisive parameters, which affect the char and gas yield and composition at the determined process-temperature. On the other hand, the temperature of pyrolysis should be controlled by an air/coal ratio adjustment due to the requirement that the char should be suitable for smokeless fuel production.
3. Conclusion A static model of coal pyrolysis in a circulating fluid-bed reactor was developed and validated against experimental data collected at PDU scale for different coals. The main concept for model development was based on the assumption of quasiequilibrium conditions within the investigated range of process parameters. Two basic phenomena were considered, viz. circulating char gasification and coal devolatilisation. Empirical correlations for Boudoard’s reaction and for volatile matter evolution were developed, which were used for the process mass and energy balancing. The model allows one to calculate the volatile matter and ash content in a char, as well as the yield of char and process gas with respect to final pyrolysis temperature. The calculations show that the gas’s calorific value at some process conditions achieves a maximum: this was confirmed experimentally. References [1] Donskoi E, McElwain DLS. Approximate modelling of coal pyrolysis. Fuel 1999;78:825–35. [2] Wiktorsson LP, Wanzl W. Kinetic parameters for coal pyrolysis at low and high heating rates—a comparison of data from different laboratory equipment. Fuel 2000;79:701–16. [3] Wei-Biao Fu, Quing-Hua Wang. A general relationship between the kinetic parameters for the gasification of coal chars with CO2 and coal type. Fuel Processing Technology 2001;72:63–77. [4] Liu G, Benyon P, Benfell KE, Bryant GW, Tate AG, Boyd RK, et al. The porous structure of bituminous coal chars and its influence on combustion and gasification under chemically-controlled conditions. Fuel 2002;79:617–26. [5] Sciazko M, Zielin˜ski H. Circulating fluid—bed reactor for coal pyrolysis. Chem Eng Technol 1995; 18:343–8. [6] Sciazko M, Kubica K. The effect of dolomite addition on sulphur, chlorine and hydrocarbons distribution in a fluid—bed mild gasification of coal. Fuel Processing Technology 2002;77–78:95–102. [7] Ocheˆduszko S. Termodynamika stosowana [applied thermodynamics]. Warszawa: WNT; 1967. [8] Meunier J. Vergasung fester Brennstoffe und oxidative Umwandlung von Kohlenwasserstoffen. Weinheim: Verlag Chemie GmbH; 1962. [9] Johnson JL. Fundamentals of coal gasification. In: Elliott MA, editor. Chemistry of coal utilisation. New York: John Wiley & Sons; 1981. [10] Middleton SP, Patrick JW, Walker A. The release of coal nitrogen pyrolysis and partial gasification in a fluidised bed. Fuel 1997;76(13):1195–200. [11] Coal conversion systems. Technical data book. Chicago (USA): Institute of Gas Technology; 1988.
Static model of coal pyrolysis in a circulating fluidised-bed reactor Marek Sciazko* Institute for Chemical Processing of Coal, 41-803 Zabrze, 1 Zamkowa Str., Poland
Abstract Four different coals were investigated: two sub-bituminous, one bituminous and lignite, which were processed in the temperature range 750–950 C. The heat for pyrolysis was generated by partial gasification of the char produced. Air was used as the gasifying medium with amounts of 0.6–1.5 m3/kg of coal, depending on the required gasification-temperature. Two sequential phenomena were taken into account: char gasification and coal devolatilisation in respect of temperature. The experimental data on carbon dioxide and monoxide concentrations in a LCV gas produced were used for the correlation of Boudouard’s equilibrium and the data on carbon burn-off and final volatile matter content in char were used for the solidproducts yield. The equations for the quasi-equilibrium state were developed and calculated values were compared with the measurements. The model takes into account the equations developed and the total energy-balance assuming the heat losses of the experimental system. The investigated coal throughput amounted to 200–300 kg/h depending on the coal properties. Process characteristics were discussed, namely: the effect of air/coal ratio on the pyrolysis temperature; char and gas yield, volatile matter and ash content in a char; as well as the gas calorific value. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Pyrolysis; Gasification; Fluidisation; Modelling
1. Introduction Considerable research on coal pyrolysis and gasification has been conducted over the years, but the results are widely dispersed because of the complex chemistry of coal. Time related coal-pyrolysis modelling assumes basically first-order kinetic equations, or less sensitive for heating rate, the distributed activation energy model or energy activation dependent on the heating rate [1,2]. The last two more * Tel.: +48-32-271-5151; fax: +48-32-271-0809. E-mail address: offi[email protected] (M. Sciazko). 0306-2619/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0306-2619(02)00200-3
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Nomenclature Ak Gg Gk Gw Ke ms Qg Qk Qh Vk Vp Vw T Wd Wdc V
ash content in a char (kg/kg) gas flow rate (kg/h) char flow rate (kg/h) coal flow rate (kg/h) experimental equilibrium constant (0.1 MPa1) mass of carbon reacted in gasification (kg/kg of coal) gas chemical enthalpy (kJ/kg of coal) char chemical enthalpy (kJ/kg of coal) heat evolved in a process (kJ/kg of coal) volatile matter content in a char (kg/kg) air flow rate (m3/h) volatile matter content in a coal (kg/kg) temperature (K) gas’s calorific value (kJ/m3) coal’s calorific value (kJ/kg) volatile-matter loss due to coal pyrolysis (kg/kg of coal)
advanced models need three and four constants respectively, which basically depend on the coal properties but also cover to some extent, the effect of heat-and-mass transfer phenomena. That is the reason for the different values of the activation energy and pre-exponential factor cited in the literature and the lack of generally valid data. The same situation exists in the case of coal-char gasification. The reaction rate of char is influenced mainly by chemical and physical factors, which include coal type, catalytic effect of the ash and the specific surface area of char, which changes during the reaction course with the development of internal pores, and finally, their destruction [3,4]. In the case of the scaling-up procedure, the uncertainty of a complex model of the reacting system may be very high and it is reasonable under some conditions to use a methodology based on quasi-equilibrium conditions, which can be reflected at a larger scale. This approach assumes basically that the process itself, with all specific features, is a decisive factor for the path of the reactions of coal decomposition. Therefore a static model of char gasification and coal pyrolysis was developed: it is based on the assumption that the final process temperature is a decisive factor for the required volatile-matter content in the char being in a quasi-equilibrium state with respect to the gas temperature. To achieve this, it is necessary to create conditions of internal circulation of the transported coal and char in a riser, where the average concentration of solids amounts to 0.05–0.15 m3/m3, i.e. the conditions for residence time are long enough for the thermal decomposition of coal and intensive mixing so enhancing mass and heat transfers. On the other hand, the gasification of circulated char is mainly chemically controlled due to the relatively short residence time of the gas. Additionally, the characteristic feature for a given circulating fluid-bed reactor is a very narrow range of operational
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residence time, both for the gas and coal, which allows us to use the quasi-equilibrium approach. The model developed reflects the kinetic processes (transport phenomena of coal species, reaction rate) by constants of the developed equations, which depend mainly on the coal properties.
2. Coal pyrolysis in a circulating fluidised-bed A circulating fluidised-bed reactor was used as a coal pyrolysis reactor. The process was tested at a scale of 200–300 kg/h: collecting operational and design data to build an industrial installation [5]. A technological diagram of the coal-pyrolysis process development unit (PDU) is presented in Fig. 1. The reactor comprises basically two parts, which are distinguished geometrically. Air and circulating char are introduced to the bottom section of a large diameter reactor and coal is fed to the base of the riser—the upper section, which is connected with the bottom one. Due to oxygen availability, the char undergoes gasification reactions so generating heat for the coal pyrolysis. The region of the reactor’s operation depends primarily on the velocity of the gas phase, and so for a specific reactor’s design, on the air flow rate and on the concentration of the solid phase. With regard to changes of the gas-phase flow rate, the reactor’s operation regimes are confined to the region between the pneumatic transport and the turbulent or bubble fluidised-bed including internal solid circulation in the riser. Such aerodynamic conditions of the reactor’s operation make it possible to achieve high values of heat-and-mass transfer coefficients. This leads to a better utilisation of the reaction volume, a reduction of the overall reactor’s dimensions and its easy control. 2.1. Process demonstration unit This process is directed primarily towards the production of gas and char. The installation (see Fig. 1) consists of a pyrolysis reactor with circulating fluidised-bed
Fig. 1. Coal pyrolysis PDU system.
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(2), coal hopper (1), a system of char separation from the hot process-gases (3,4,5), heat-recovery system, as well as the means for tar and HCl removal from the gas. A two-stage system of solid products separation from the process gas has been used in the installation, resulting in obtaining a coarse char (particle size exceeding 0.5 mm) and a cyclone dust with particle size below 0.5 mm. The cleaned gas is burnt in a 0.5 MWth burner, in which the combustion dynamics were studied. The tests were conducted in the temperature range 750–950 C using coals of average diameter 0.160–0.560 mm, which resulted in a gas residence-time of 0.7–0.9 s and for coal 90–120 s. Char was sampled from the char bin and from the secondary cyclone. Char gasification data were collected and the concentrations of [CO] and [CO2] measured at the outlet of the reactor cooling gas immediately by means of a two-stage ice-cooler [6]. Four different coals were used, namely bituminous ‘‘Rozbark’’, two subbituminous coals ‘‘Wieczorek’’ and ‘‘Janina’’, and lignite ‘‘Borsod’’. The demonstration installation is a part of a development project to design a pilot plant for smokeless-fuel production. The technology consists in the briquetting of a mixture of char obtained from a steam coal in a pyrolysis reactor with a preheated coking coal at approximately 430 C. 2.2. Experimental results In the investigations, it was found that the ratio of air-flow rate Vp to the coal flow rate Gw is the leading process parameter that correlates all the changes of gas and char properties (namely the content of volatile matter in char Vdaf, char output Gk/ Gw, gas calorific value Wd and the ratio of [CO]/[CO2] concentration in the gas) as well as determines the yield of process products (Fig. 2). The direct influence of this parameter on the products’ properties means that, in the studied flow range and for the residence time of gas and coal connected with this range, the process temperature is the leading parameter. 2.3. Static model of coal pyrolysis in a reactor with circulating char Taking into account the operation principle of a coal-gasification reactor with a circulating fluidised-bed, consisting of: partial gasification of circulating char (devolatilised coal), coal pyrolysis, where the heat produced by the reaction of the char’s partial gasification is consumed, and considering the temperature-profile uniformity in the system resulting from the very intensive axial mixing, the logistic diagram of the combined gasification and pyrolysis process presented in Fig. 3 may be considered. In the gasification system, one deals mainly with char and in principle two consequent reactions occur there, the first of which is a combustion reaction and the second is the CO2 reduction—Boudouard’s reaction:
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a1C þ a1O2 ¼ a1CO2 þ H1 b1C þ b1CO2 ¼ 2b1CO þ H2 Assuming that: =a1+b1 stands for the total amount of C kmol, which reacts, the following equations can be obtained: a1C þ a1O2 ¼ a1CO2 and ð a1 ÞC þ ð a1 ÞCO2 ¼ 2ð a1 ÞCO It is assumed that the combustion reaction proceeds until the depletion of oxygen (supplied with air ensues). On the other hand, the Boudouard reaction
Fig. 2. Effects of air–coal ratio on: (a) yield of char, (b) volatile matter content, (c) gas calorific value and (d) CO/CO2 ratio (‘‘Wieczorek’’ coal).
Fig. 3. Schematic diagram of the char-gasification and coal-pyrolysis process.
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[7] proceeds, where the empirically-determined equilibrium constant is described by equation: Ke ¼
1 y2CO y2 CO P yCO2 yCO2
ð1Þ
The empirical equilibrium constant may be correlated by the relationship (2) [8] expressing the quasi-equilibrium conditions. lnKe ¼
A þB T
ð2Þ
The experimental data necessary to determine the coefficients of the above equation have been derived by performing gasification tests on a char from a given coal obtained by its previous devolatilisation at 550 C in an oxygen-free environment in a bubbling fluid-bed reactor. The temperature used is that for the low-temperature pyrolysis of coal, at which primary volatile matter is released. Measurements of [CO] and [CO2] concentrations in the obtained gas allow one to determine appropriate coefficients [Eq. (2)] that for various coals are presented in Table 1. The correlation error is in the range of 11.5%. The most important effect of coal pyrolysis is the volatile-matter evolution. Assuming that we are interested in the Vk =Vw ratio expressed by the absolute loss of coal’s volatile matter (on a as-received basis) V and the mass ms, of carbon reacted in the gasification, one can obtain: #¼
Vk Vw V ¼ Vw ð1 V ms WÞVw
ð3Þ
which can be correlated as a function ln# ¼
C þ DlnT þ E T
ð4Þ
The calculated coefficients for coals of various compositions are shown in Table 2.
Table 1 Empirical coefficients of Eq. (2) Coefficient
A B
Char tested ‘‘Borsod’’
‘‘Janina’’
‘‘Rozbark’’
‘‘Wieczorek’’
0.848 4350
0.799 3826
0.161 2430
2.75 6559
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The correlation error does not exceed the value of 8.2%. To assess the gas’s calorific value, it has then been assumed that its value is determined by the calorific value of coal, less the: enthalpy of the char, enthalpy of the burned and gasified carbon, enthalpy of the hydrogen that reacted with the coals and oxygen. In the last case, it has been assumed that 50% of the oxygen content in the coal reacts with hydrogen producing water. However, this does not affect the heat of the pyrolysis reaction, which is approximately equal to zero [9]. Practically all the Table 2 Coefficients of Eq. (4) Coal tested
Coal composition Carbon Hydrogen Oxygen Sulphur Nitrogen Ash Water Volatile matter Coefficients C D E
‘‘Borsod’’ lignite
0.395 0.027 0.105 0.018 0.012 0.188 0.255 0.291 6160 11.2 82.3
‘‘Janina’’ subbituminous
0.585 0.030 0.110 0.012 0.018 0.095 0.150 0.310 10 510 16.1 121.3
‘‘Rozbark’’ bituminous
0.792 0.017 0.025 0.005 0.012 0.125 0.024 0.134 10 410 17.0 127.1
‘‘Wieczorek’’ subbituminous
0.700 0.030 0.105 0.0085 0.010 0.087 0.060 0.320 47 930 51.1 400.2
Fig. 4. Effects of temperature on air demand, yield of gas, and properties of char according to the model calculations (for ‘‘Wieczorek’’ coal).
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remaining oxygen shifts to the gas in the form of carbon monoxide and dioxide, while the sulphur and nitrogen split proportionally to the char yield between the gaseous and solid phases [10]. However, they have no major influence from the point of view of the calorific value of the generated gas. Taking into account the presented static model of the process of partial gasification and pyrolysis of coal, the behaviours of various coals in the discussed process have been analysed. Differences in the Boudouard reaction kinetics and the process of pyrolysis are allowed for by empirical correlations and thermodynamic properties
Fig. 5. Comparison of experimental data and calculated results of char yield at various temperatures (‘‘for Janina’’ coal).
Fig. 6. Char volatile matter content—comparison of experimental data and calculated results (‘‘for Janina’’ coal).
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of the coal, and the products of its decomposition have been defined according to the procedure suggested by the Institute of Gas Technology (USA) [11]. Fig. 4 presents typical relationships for the gasification of ‘‘Wieczorek’’ coal, which indicates that, at higher temperatures, the ash content in a char increases and the volatile matter decreases. The increase of temperature by 100 K demands ca. 0.4 m3 of additional air. Comparisons of the experimental data with computations are shown in Figs. 5–8 for the gasification of ‘‘Janina’’ coal in a circulating fluidised-bed reactor. Fig. 9 presents, for this coal, the structure of the chemical enthalpy of fuel breakdown; it may be seen that approximately 50% of the chemical energy goes to the solid product, while
Fig 7. Ash content in char—comparison of experimental data and calculated result (‘‘for Janina’’ coal).
Fig. 8. Air demand—comparison of experimental data and calculated results (‘‘for Janina’’coal).
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approximately 35% goes to the gas. The remaining 15% covers the heat of reaction and thermal losses including water evaporation. The heat losses to the environment in the pilot plant (300 kg/h) amounted to 10 kW, on average, i.e. approximately 5% of the heat released in the gasification reaction. The total demand for heat to perform the process in the temperature range 750–950 C requires 2500–4500 kJ/kg of the coal. From the point of view of gas energy utilisation, its calorific value is significant; this quantity in the experimental conditions is calculated on the basis of the gas composition in the ambient state. So neither the contained water vapour nor the condensing hydrocarbons are taken into account; their amount, as results from the performed studies may equal to 5–20 g tar/m3 and 5–10 g m3 of benzene, toluene, xylene (BTX) in process gas. These components increase the gas’s calorific-value by
Fig. 9. Distribution of coal’s chemical enthalpy in pyrolysis products (‘‘for Janina’’ coal).
Fig. 10. Gas calorific value—comparison of experimental data and calculated results (‘‘for Janina’’ coal).
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320–1200 kJ and, in total, allow one to obtain a gas of calorific value of up to approximately 6000 kJ/m3. It was noticed that for the specific Vp/Gw ratio, the maximum gas calorific value is achieved (Fig. 10), which was confirmed experimentally. From Fig. 10, it can be seen that, in the considered temperature range, the calorific value of the gas varies in the range 4500–5000 kJ/kg and achieves the maximum at 830 C. Performed simulations and experimental data show that basically the moisture and ash contents in coal are decisive parameters, which affect the char and gas yield and composition at the determined process-temperature. On the other hand, the temperature of pyrolysis should be controlled by an air/coal ratio adjustment due to the requirement that the char should be suitable for smokeless fuel production.
3. Conclusion A static model of coal pyrolysis in a circulating fluid-bed reactor was developed and validated against experimental data collected at PDU scale for different coals. The main concept for model development was based on the assumption of quasiequilibrium conditions within the investigated range of process parameters. Two basic phenomena were considered, viz. circulating char gasification and coal devolatilisation. Empirical correlations for Boudoard’s reaction and for volatile matter evolution were developed, which were used for the process mass and energy balancing. The model allows one to calculate the volatile matter and ash content in a char, as well as the yield of char and process gas with respect to final pyrolysis temperature. The calculations show that the gas’s calorific value at some process conditions achieves a maximum: this was confirmed experimentally. References [1] Donskoi E, McElwain DLS. Approximate modelling of coal pyrolysis. Fuel 1999;78:825–35. [2] Wiktorsson LP, Wanzl W. Kinetic parameters for coal pyrolysis at low and high heating rates—a comparison of data from different laboratory equipment. Fuel 2000;79:701–16. [3] Wei-Biao Fu, Quing-Hua Wang. A general relationship between the kinetic parameters for the gasification of coal chars with CO2 and coal type. Fuel Processing Technology 2001;72:63–77. [4] Liu G, Benyon P, Benfell KE, Bryant GW, Tate AG, Boyd RK, et al. The porous structure of bituminous coal chars and its influence on combustion and gasification under chemically-controlled conditions. Fuel 2002;79:617–26. [5] Sciazko M, Zielin˜ski H. Circulating fluid—bed reactor for coal pyrolysis. Chem Eng Technol 1995; 18:343–8. [6] Sciazko M, Kubica K. The effect of dolomite addition on sulphur, chlorine and hydrocarbons distribution in a fluid—bed mild gasification of coal. Fuel Processing Technology 2002;77–78:95–102. [7] Ocheˆduszko S. Termodynamika stosowana [applied thermodynamics]. Warszawa: WNT; 1967. [8] Meunier J. Vergasung fester Brennstoffe und oxidative Umwandlung von Kohlenwasserstoffen. Weinheim: Verlag Chemie GmbH; 1962. [9] Johnson JL. Fundamentals of coal gasification. In: Elliott MA, editor. Chemistry of coal utilisation. New York: John Wiley & Sons; 1981. [10] Middleton SP, Patrick JW, Walker A. The release of coal nitrogen pyrolysis and partial gasification in a fluidised bed. Fuel 1997;76(13):1195–200. [11] Coal conversion systems. Technical data book. Chicago (USA): Institute of Gas Technology; 1988.