573656
research-article2015
WMR0010.1177/0734242X15573656Waste Management & ResearchJanajreh and Raza
Original Article
Numerical simulation of waste tyres gasification
Waste Management & Research 1–9 © The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X15573656 wmr.sagepub.com
Isam Janajreh and Syed Shabbar Raza
Abstract Gasification is a thermochemical pathway used to convert carbonaceous feedstock into syngas (CO and H2) in a deprived oxygen environment. The process can accommodate conventional feedstock such as coal, discarded waste including plastics, rubber, and mixed waste owing to the high reactor temperature (1000 °C–1600 °C). Pyrolysis is another conversion pathway, yet it is more selective to the feedstock owing to the low process temperature (350 °C–550 °C). Discarded tyres can be subjected to pyrolysis, however, the yield involves the formation of intermediate radicals additional to unconverted char. Gasification, however, owing to the higher temperature and shorter residence time, is more opted to follow quasi-equilibrium and being predictive. In this work, tyre crumbs are subjected to two levels of gasification modelling, i.e. equilibrium zero dimension and reactive multi-dimensional flow. The objective is to investigate the effect of the amount of oxidising agent on the conversion of tyre granules and syngas composition in a small 20 kW cylindrical gasifier. Initially the chemical compositions of several tyre samples are measured following the ASTM procedures for proximate and ultimate analysis as well as the heating value. The measured data are used to carry out equilibrium-based and reactive flow gasification. The result shows that both models are reasonably predictive averaging 50% gasification efficiency, the devolatilisation is less sensitive than the char conversion to the equivalence ratio as devolatilisation is always complete. In view of the high attained efficiency, it is suggested that the investigated tyre gasification system is economically viable. Keywords Gasification, biomass, syngas, cold efficiency, reactive flow
Introduction Human development sounds positive, unfortunately it brings environmental and ecological stress and strain on energy–water– food resources, adding to the augmentation and stockpile of waste that continue to raise CO2, CH4, and other greenhouse (GH) gas emissions. Recycling, reusing, and reducing is an early intellectual practice that lessens the societal trans-developments side-effect, while concurrently being more considerate to the finiteness of earth resources. For example, the making of the wheel has carried civilisation forward, but less emphasis was put into its disposal until recently. Used tyres, besides being unsightly and taking up valuable land space, their illegal dumps and stockpiles raise many environmental concerns. It is estimated that the annual worldwide generation of discarded tyres is over 1.2 billion, with only a fraction being re-treaded, granulated, or subjected to energy recovery, while the majority being openly incinerated, dumped, or stockpiled. In the US, it is estimated that one tyre is generated per capita annually (i.e. over 300Mill.) and the current stockpiles is estimated to be more than 1.5 billion. In Japan around 100Mill. is generated annually and in Australia about 18Mill., with over 1 year of stockpile for both countries. In Europe, in a study by the European Union in 2002, an estimate of 250Mill. tyres were disposed of and over 3 billion were stockpiled. Tyre is a composite mix of many chemical components,
including natural rubber, polypropylene, polyesterine, anti-oxidants, inorganic stabiliser, carbon black, silicate, fabric, nylon, aramid, and steel, in addition to the volcanising sulphur. Pyrolysis of a shredded and metal free/stripped tyre results in liquid oil and gas streams in addition to metal streams, as well as a reasonable grade of active carbon. Pyrolysis, however, still faces many technical challenges. The liquid cut, for example, owing to the low temperature, is a mixture of paraffin, aromatics, raisins, and asphaltine that require intense distillation and separation. Also, similar impurities exist within the gaseous stream, with the presence of sulphur dioxins in large proportions, methane, ethane, and propane beside syngas (CO and H2). The gaseous stream requires further distillation and gas separation, which are thermally driven processes that lower the overall process metrics. Alternatively gasification is conducted at higher
Mechanical and Materials Engineering Department, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates Corresponding author: Isam Janajreh, Mechanical and Materials Engineering Department, Masdar Institute of Science and Technology, Abu Dhabi, PO Box 54224, United Arab Emirates. Email: [email protected]
Downloaded from wmr.sagepub.com at UNIV OF GEORGIA LIBRARIES on May 28, 2015
2
Waste Management & Research
temperatures leading to a single stream of syngas that ensures complete feedstock conversion and a narrower syngas mix that delivers higher metrics process. Gasification is a core technology that can be utilised broadly to generate energy without polluting the environment. Because of its many applications, it is currently enjoying a considerable renaissance (DOE, 2007). Gasification is compatible with new applications in the area of: biomass conversion (when used with biomass, it is carbon neutral); coal-to-liquid; superior environmental performance especially with regard to CO2 capture and sulphur removal; and the prospect for substantial efficiency improvement when incorporated in integrated gasification combined cycle (IGCC) plants for electricity generation. Using the gasification process, a wide range of carbonaceous material can be converted into high value fuel. The range covers coal, biomass, and industrial/municipal/agricultural waste including discarded plastics and tyres. The product of gasification is normally composed of syngas, which is a mixture of hydrogen and carbon monoxide. The production of syngas using gasification is a complex process that depends on several factors, including the composition of feedstock, the gasifier temperature/pressure, and the type and amount of oxidiser and moderator (Abuadala et al., 2010). The process of gasification utilises the partial combustion of feedstock by carefully controlling the amount of oxidiser. The amount of oxidiser also controls the temperature of the gasification process, which is the most important parameter of the gasifier, as reported by Walawender et al. (1985). Details of the process cannot be captured by zero dimensional equilibrium models, which does not account for the gasifier dimensions or the detailed kinetics of the basic process reactions. The chemical and physical phenomena that occur during gasification of tyre crumbs are essentially similar to typical carbonaceous material thermal decompositions. These include drying, devolatilisation (which includes release of volatile flammable gases, flaming combustion of the volatiles, and glowing combustion of fixed carbon), heat conduction, fissuring, shrinkage, and fragmentation of solid particles (Blasi, 2000; Dogru et al., 2002; Kumar et al., 2006; Hsi et al., 2008; McKendry, 2002; Zainal et al., 2002). The chemical reactors used in the gasification process are distinguished by the concurrent flow of feedstock and gasifying agent. Previous research studies used systematic analysis, numerical simulation, and experimental studies at the Waste to Energy Lab at Masdar Institute and are available in the literature (Janajreh and Al Shrah, 2013; Janajreh et al., 2010, 2013; Shabber and Janajreh, 2013; Syed et al., 2012). Al Amoodi et al. (2013) studied the gasification process of polyethylene (PE) via modelling with a combination of various unit operation modules available in the Aspen Plus simulation package. It was found that at process conditions of equivalence ratio (ER) = 0.2 and steam-to-PE mass ratio = 0.4 to 0.6, the combined CO and H2 efficiency exhibited a maximum value of 40%. Pressure effects on gasification efficiency were also tested, and simulation results confirmed the production of a high temperature product gas that is useful for power generation using turbines.
Mitta et al. (2006) proposed a gasification model that will aid the understanding of the gasification of tyres and can be used as a predictive tool at the optimisation stage. Validation of this model was carried out using the gasification pilot plant located at the Chemical Engineering Department of Universitat Politecnica de Catalunya (UPC). In another study by Mtui (2013), the numerical modelling of the gasification and combustion of chipped scrap tyres in a twostage system was conducted. Their result indicated that the temperature in the primary chamber is uniformly distributed and consequently the concentration of syngas is nearly uniform along the axial and radial direction of the primary chamber. The effect of the equivalence ratio on the reactor operation was investigated, and the optimal value was found to be around 0.3. In this research a small scale gasifier was utilised to demonstrate the bench scale conversion metrics of tyre crumbs. The main aim of this study was to perform two levels of simulations: (i) systematic zero dimensional, and (ii) coupled reactive numerical simulation using computational fluid dynamics (CFD) to investigate the system conversion into syngas and the process metrics. In order to perform the desired simulation, an experimental study was conducted to measure the chemical composition of the tyres. These values are used to define the tyre particle in both the system approach model and CFD simulation. Results of the simulations are presented in the form of tyre conversion by means of volatile and char generation. The behaviour of the CO and H2 formation is also studied.
Material and methods Tyre composition To establish the theoretical framework for gasification of the tyre, both the thermal properties and chemical composition were determined. The thermogravimetric Q600SDT was used to determine the proximate composition of tyres in terms of its moisture, volatile, fixed carbon, and ash fractions. The analysis was based upon measurements of the mass change of the sample as a function of a controlled/specified temperature profile. Thermo scientific Flash 2000 Organic Analyser was used to evaluate the elemental composition of tyres by analysing the combusted phase in to an appropriate thermally conductive detection column. Also, the bomb calorimeter was used to evaluate the heating value of tyre. The result of proximate, ultimate, and heating value analysis are given in Table 1, beside the River Trading Company (RTC) of Kentucky coal for comparison.
Gasification modelling The study was performed to simulate the conversion behaviour of tyre crumbs into syngas using the stoichiometry of a tyre and ideal gasification process in which the tyre particles were completely converted into syngas without tendency of forming CO2, H2O, or any other energy losses. The chemical equation of this idealistic case is given as:
Downloaded from wmr.sagepub.com at UNIV OF GEORGIA LIBRARIES on May 28, 2015
3
Janajreh and Raza Table 1. Tyre proximate and ultimate analysis. Composition Proximate (Wt.%) Moisture Volatile matter Fixed carbon Ash (dry) Ultimate (Wt.%) (dry) C H N S O Ash HHV (MJ kg-1) MW (kg kmole-1) CH1.1057O0.0915N0.0035S0.0066
Tyre
Coal (RTC*)
1.0 ±0.025 68.0 ±0.55 23.2 ±0.24 8.8 ±0.03
4.0 ±0.027 36.66 ±0.45 52.92 ±0.22 8.42 ±0.25
73.8 ±0.50 6.8 ±0.19 0.3 ±0.01 1.3 ±0.01 9.0 ±0.26 8.8 ±03 36.0 ±0.56 14.83
73.15 ±0.55 5.31 ±0.17 1.53 ±0.01 1.01 ±0.1 10.58 ±0.31 8.42 ±0.25 33.25 ±0.61 22.7452 CH1.5708O0.5734
RTC: River Trading Company; HHV: High Heating Value; MW: Molecular Weight .
CH1.1057O0.0915 + 0.45425 ( O 2 + 3.75N 2 ) → CO + 0.55258H 2 + 1.70798N 2
(1a)
In comparison with the authors earlier work using Kentucky coal branded under RTC, the gasification stoichiometry was remarkably different, which is given as:
CH1.5708O0.5734 + 0.2133 ( O 2 + 3.75N 2 ) → CO + 0.2867 H 2 + 0.79987 N 2
(1b)
The first term on left-hand side of equations (1a) and (1b) represent the empirical formula of the tyre and RTC coal, respectively. It was calculated using the ultimate analysis presented in Table 1 for single tyre molecule. The number of mole of oxygen was calculated only for CO on the product side instead of CO2. Ideally, for any operating condition of gasifier if the above calculated amount of O2 is provided, it should result only in CO and H2. However, owing to the physical and chemical properties of tyre or coal, the desired ideal conditions are rarely guaranteed. The overall gasification process was a mildly exothermic process owing to the formation of CO2 and conversion of hydrogen present in the tyre into vapour (H2O). The formation of both CO2 and H2O during the process was unavoidable, yet their high yield existence at the gasifier was undesirable. The decomposition of tyres into volatile and char required a large sum of energy that could not be achieved if only the ideal gasification cycle was considered. Innovative design and configuration in the gasifier to maximise syngas metric is the subject of many researchers, including staged gasification, long reduction zone, and oxygen gasification. Overall, the conversion of some char into CO2 was necessary to provide the necessary thermal energy for the gasification system to proceed, similarly the partial combustion of H2 into H2O and heat.
Low fidelity/zero-dimension equilibriumbase system model approach Equation (2) and Table 2 show the overall stoichiometric reaction, such that 1.231 mole of the oxidiser was required to completely burn off one unit mole of tyre. This unit mole formula was inferred from the proximate analysis such that the tyres’ elemental composition was described as moisture ash free, then each was converted into molar value and finally normalised on the basis of unit carbon mole. Combustion:
CH1.1057O0.0915 + 1.231( O 2 + 3.76 N 2 ) → CO 2 + 0.5528H 2O + 4.52 N 2
(2a)
Gasification:
CH1.1057O0.0915 + 0.9085H 2O + m ( O 2 + 3.76 N 2 ) (2b)
Equation (2b) shows the gasification of one unit mole of tyre particles under steam moderator leading to nearly a comprehensive list of the possible gasification products (see Table 2). Therefore, Gibbs energy minimisation was utilised to determine their molar fraction (DOE, 2007). The addition of steam was necessary to control the high rising gasifier temperature when a high calorific value feedstock was subjected to gasification similar to liquid hydrocarbon fuel, anthracite coal, plastic, and rubber. Note also that air oxidation introduced another penalty to the gasifier metrics by the large volume of N2 diluent. Nevertheless, the details of the gasification process was very complex inside the reactor with chemically intrinsic, unsteady flow of multiplephase, highly turbulent, and multiple species of chemically reacting flow. Despite this, the process could be steered under equilibrium steady conditions given that the process was operating at high temperature, utilising small feedstock particle size (few tenths of a micron), and with short but sufficient residence time. These conditions were only operable within the entrained flow gasifier technology.
High fidelity coupled reactive flow approach The downdraft gasifier was modelled using the finite volume code coupled with a conjugate heat transfer with the bulk metal separators and insulation of the gasifier body. The computation was carried out on a two-dimensional axi-symmetric geometry of the gasifier. The segregated implicit double precision solver of Ansys Fluent was used to solve the transport equations. The flow was assumed to be two-dimensional axi-symmetric with steady state. A no-slip boundary condition was applied on reactor walls and separator/insulation bulk material. The modelled particles had a uniform distribution and were assumed to be spherical in shape, but at a much smaller size (
research-article2015
WMR0010.1177/0734242X15573656Waste Management & ResearchJanajreh and Raza
Original Article
Numerical simulation of waste tyres gasification
Waste Management & Research 1–9 © The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X15573656 wmr.sagepub.com
Isam Janajreh and Syed Shabbar Raza
Abstract Gasification is a thermochemical pathway used to convert carbonaceous feedstock into syngas (CO and H2) in a deprived oxygen environment. The process can accommodate conventional feedstock such as coal, discarded waste including plastics, rubber, and mixed waste owing to the high reactor temperature (1000 °C–1600 °C). Pyrolysis is another conversion pathway, yet it is more selective to the feedstock owing to the low process temperature (350 °C–550 °C). Discarded tyres can be subjected to pyrolysis, however, the yield involves the formation of intermediate radicals additional to unconverted char. Gasification, however, owing to the higher temperature and shorter residence time, is more opted to follow quasi-equilibrium and being predictive. In this work, tyre crumbs are subjected to two levels of gasification modelling, i.e. equilibrium zero dimension and reactive multi-dimensional flow. The objective is to investigate the effect of the amount of oxidising agent on the conversion of tyre granules and syngas composition in a small 20 kW cylindrical gasifier. Initially the chemical compositions of several tyre samples are measured following the ASTM procedures for proximate and ultimate analysis as well as the heating value. The measured data are used to carry out equilibrium-based and reactive flow gasification. The result shows that both models are reasonably predictive averaging 50% gasification efficiency, the devolatilisation is less sensitive than the char conversion to the equivalence ratio as devolatilisation is always complete. In view of the high attained efficiency, it is suggested that the investigated tyre gasification system is economically viable. Keywords Gasification, biomass, syngas, cold efficiency, reactive flow
Introduction Human development sounds positive, unfortunately it brings environmental and ecological stress and strain on energy–water– food resources, adding to the augmentation and stockpile of waste that continue to raise CO2, CH4, and other greenhouse (GH) gas emissions. Recycling, reusing, and reducing is an early intellectual practice that lessens the societal trans-developments side-effect, while concurrently being more considerate to the finiteness of earth resources. For example, the making of the wheel has carried civilisation forward, but less emphasis was put into its disposal until recently. Used tyres, besides being unsightly and taking up valuable land space, their illegal dumps and stockpiles raise many environmental concerns. It is estimated that the annual worldwide generation of discarded tyres is over 1.2 billion, with only a fraction being re-treaded, granulated, or subjected to energy recovery, while the majority being openly incinerated, dumped, or stockpiled. In the US, it is estimated that one tyre is generated per capita annually (i.e. over 300Mill.) and the current stockpiles is estimated to be more than 1.5 billion. In Japan around 100Mill. is generated annually and in Australia about 18Mill., with over 1 year of stockpile for both countries. In Europe, in a study by the European Union in 2002, an estimate of 250Mill. tyres were disposed of and over 3 billion were stockpiled. Tyre is a composite mix of many chemical components,
including natural rubber, polypropylene, polyesterine, anti-oxidants, inorganic stabiliser, carbon black, silicate, fabric, nylon, aramid, and steel, in addition to the volcanising sulphur. Pyrolysis of a shredded and metal free/stripped tyre results in liquid oil and gas streams in addition to metal streams, as well as a reasonable grade of active carbon. Pyrolysis, however, still faces many technical challenges. The liquid cut, for example, owing to the low temperature, is a mixture of paraffin, aromatics, raisins, and asphaltine that require intense distillation and separation. Also, similar impurities exist within the gaseous stream, with the presence of sulphur dioxins in large proportions, methane, ethane, and propane beside syngas (CO and H2). The gaseous stream requires further distillation and gas separation, which are thermally driven processes that lower the overall process metrics. Alternatively gasification is conducted at higher
Mechanical and Materials Engineering Department, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates Corresponding author: Isam Janajreh, Mechanical and Materials Engineering Department, Masdar Institute of Science and Technology, Abu Dhabi, PO Box 54224, United Arab Emirates. Email: [email protected]
Downloaded from wmr.sagepub.com at UNIV OF GEORGIA LIBRARIES on May 28, 2015
2
Waste Management & Research
temperatures leading to a single stream of syngas that ensures complete feedstock conversion and a narrower syngas mix that delivers higher metrics process. Gasification is a core technology that can be utilised broadly to generate energy without polluting the environment. Because of its many applications, it is currently enjoying a considerable renaissance (DOE, 2007). Gasification is compatible with new applications in the area of: biomass conversion (when used with biomass, it is carbon neutral); coal-to-liquid; superior environmental performance especially with regard to CO2 capture and sulphur removal; and the prospect for substantial efficiency improvement when incorporated in integrated gasification combined cycle (IGCC) plants for electricity generation. Using the gasification process, a wide range of carbonaceous material can be converted into high value fuel. The range covers coal, biomass, and industrial/municipal/agricultural waste including discarded plastics and tyres. The product of gasification is normally composed of syngas, which is a mixture of hydrogen and carbon monoxide. The production of syngas using gasification is a complex process that depends on several factors, including the composition of feedstock, the gasifier temperature/pressure, and the type and amount of oxidiser and moderator (Abuadala et al., 2010). The process of gasification utilises the partial combustion of feedstock by carefully controlling the amount of oxidiser. The amount of oxidiser also controls the temperature of the gasification process, which is the most important parameter of the gasifier, as reported by Walawender et al. (1985). Details of the process cannot be captured by zero dimensional equilibrium models, which does not account for the gasifier dimensions or the detailed kinetics of the basic process reactions. The chemical and physical phenomena that occur during gasification of tyre crumbs are essentially similar to typical carbonaceous material thermal decompositions. These include drying, devolatilisation (which includes release of volatile flammable gases, flaming combustion of the volatiles, and glowing combustion of fixed carbon), heat conduction, fissuring, shrinkage, and fragmentation of solid particles (Blasi, 2000; Dogru et al., 2002; Kumar et al., 2006; Hsi et al., 2008; McKendry, 2002; Zainal et al., 2002). The chemical reactors used in the gasification process are distinguished by the concurrent flow of feedstock and gasifying agent. Previous research studies used systematic analysis, numerical simulation, and experimental studies at the Waste to Energy Lab at Masdar Institute and are available in the literature (Janajreh and Al Shrah, 2013; Janajreh et al., 2010, 2013; Shabber and Janajreh, 2013; Syed et al., 2012). Al Amoodi et al. (2013) studied the gasification process of polyethylene (PE) via modelling with a combination of various unit operation modules available in the Aspen Plus simulation package. It was found that at process conditions of equivalence ratio (ER) = 0.2 and steam-to-PE mass ratio = 0.4 to 0.6, the combined CO and H2 efficiency exhibited a maximum value of 40%. Pressure effects on gasification efficiency were also tested, and simulation results confirmed the production of a high temperature product gas that is useful for power generation using turbines.
Mitta et al. (2006) proposed a gasification model that will aid the understanding of the gasification of tyres and can be used as a predictive tool at the optimisation stage. Validation of this model was carried out using the gasification pilot plant located at the Chemical Engineering Department of Universitat Politecnica de Catalunya (UPC). In another study by Mtui (2013), the numerical modelling of the gasification and combustion of chipped scrap tyres in a twostage system was conducted. Their result indicated that the temperature in the primary chamber is uniformly distributed and consequently the concentration of syngas is nearly uniform along the axial and radial direction of the primary chamber. The effect of the equivalence ratio on the reactor operation was investigated, and the optimal value was found to be around 0.3. In this research a small scale gasifier was utilised to demonstrate the bench scale conversion metrics of tyre crumbs. The main aim of this study was to perform two levels of simulations: (i) systematic zero dimensional, and (ii) coupled reactive numerical simulation using computational fluid dynamics (CFD) to investigate the system conversion into syngas and the process metrics. In order to perform the desired simulation, an experimental study was conducted to measure the chemical composition of the tyres. These values are used to define the tyre particle in both the system approach model and CFD simulation. Results of the simulations are presented in the form of tyre conversion by means of volatile and char generation. The behaviour of the CO and H2 formation is also studied.
Material and methods Tyre composition To establish the theoretical framework for gasification of the tyre, both the thermal properties and chemical composition were determined. The thermogravimetric Q600SDT was used to determine the proximate composition of tyres in terms of its moisture, volatile, fixed carbon, and ash fractions. The analysis was based upon measurements of the mass change of the sample as a function of a controlled/specified temperature profile. Thermo scientific Flash 2000 Organic Analyser was used to evaluate the elemental composition of tyres by analysing the combusted phase in to an appropriate thermally conductive detection column. Also, the bomb calorimeter was used to evaluate the heating value of tyre. The result of proximate, ultimate, and heating value analysis are given in Table 1, beside the River Trading Company (RTC) of Kentucky coal for comparison.
Gasification modelling The study was performed to simulate the conversion behaviour of tyre crumbs into syngas using the stoichiometry of a tyre and ideal gasification process in which the tyre particles were completely converted into syngas without tendency of forming CO2, H2O, or any other energy losses. The chemical equation of this idealistic case is given as:
Downloaded from wmr.sagepub.com at UNIV OF GEORGIA LIBRARIES on May 28, 2015
3
Janajreh and Raza Table 1. Tyre proximate and ultimate analysis. Composition Proximate (Wt.%) Moisture Volatile matter Fixed carbon Ash (dry) Ultimate (Wt.%) (dry) C H N S O Ash HHV (MJ kg-1) MW (kg kmole-1) CH1.1057O0.0915N0.0035S0.0066
Tyre
Coal (RTC*)
1.0 ±0.025 68.0 ±0.55 23.2 ±0.24 8.8 ±0.03
4.0 ±0.027 36.66 ±0.45 52.92 ±0.22 8.42 ±0.25
73.8 ±0.50 6.8 ±0.19 0.3 ±0.01 1.3 ±0.01 9.0 ±0.26 8.8 ±03 36.0 ±0.56 14.83
73.15 ±0.55 5.31 ±0.17 1.53 ±0.01 1.01 ±0.1 10.58 ±0.31 8.42 ±0.25 33.25 ±0.61 22.7452 CH1.5708O0.5734
RTC: River Trading Company; HHV: High Heating Value; MW: Molecular Weight .
CH1.1057O0.0915 + 0.45425 ( O 2 + 3.75N 2 ) → CO + 0.55258H 2 + 1.70798N 2
(1a)
In comparison with the authors earlier work using Kentucky coal branded under RTC, the gasification stoichiometry was remarkably different, which is given as:
CH1.5708O0.5734 + 0.2133 ( O 2 + 3.75N 2 ) → CO + 0.2867 H 2 + 0.79987 N 2
(1b)
The first term on left-hand side of equations (1a) and (1b) represent the empirical formula of the tyre and RTC coal, respectively. It was calculated using the ultimate analysis presented in Table 1 for single tyre molecule. The number of mole of oxygen was calculated only for CO on the product side instead of CO2. Ideally, for any operating condition of gasifier if the above calculated amount of O2 is provided, it should result only in CO and H2. However, owing to the physical and chemical properties of tyre or coal, the desired ideal conditions are rarely guaranteed. The overall gasification process was a mildly exothermic process owing to the formation of CO2 and conversion of hydrogen present in the tyre into vapour (H2O). The formation of both CO2 and H2O during the process was unavoidable, yet their high yield existence at the gasifier was undesirable. The decomposition of tyres into volatile and char required a large sum of energy that could not be achieved if only the ideal gasification cycle was considered. Innovative design and configuration in the gasifier to maximise syngas metric is the subject of many researchers, including staged gasification, long reduction zone, and oxygen gasification. Overall, the conversion of some char into CO2 was necessary to provide the necessary thermal energy for the gasification system to proceed, similarly the partial combustion of H2 into H2O and heat.
Low fidelity/zero-dimension equilibriumbase system model approach Equation (2) and Table 2 show the overall stoichiometric reaction, such that 1.231 mole of the oxidiser was required to completely burn off one unit mole of tyre. This unit mole formula was inferred from the proximate analysis such that the tyres’ elemental composition was described as moisture ash free, then each was converted into molar value and finally normalised on the basis of unit carbon mole. Combustion:
CH1.1057O0.0915 + 1.231( O 2 + 3.76 N 2 ) → CO 2 + 0.5528H 2O + 4.52 N 2
(2a)
Gasification:
CH1.1057O0.0915 + 0.9085H 2O + m ( O 2 + 3.76 N 2 ) (2b)
Equation (2b) shows the gasification of one unit mole of tyre particles under steam moderator leading to nearly a comprehensive list of the possible gasification products (see Table 2). Therefore, Gibbs energy minimisation was utilised to determine their molar fraction (DOE, 2007). The addition of steam was necessary to control the high rising gasifier temperature when a high calorific value feedstock was subjected to gasification similar to liquid hydrocarbon fuel, anthracite coal, plastic, and rubber. Note also that air oxidation introduced another penalty to the gasifier metrics by the large volume of N2 diluent. Nevertheless, the details of the gasification process was very complex inside the reactor with chemically intrinsic, unsteady flow of multiplephase, highly turbulent, and multiple species of chemically reacting flow. Despite this, the process could be steered under equilibrium steady conditions given that the process was operating at high temperature, utilising small feedstock particle size (few tenths of a micron), and with short but sufficient residence time. These conditions were only operable within the entrained flow gasifier technology.
High fidelity coupled reactive flow approach The downdraft gasifier was modelled using the finite volume code coupled with a conjugate heat transfer with the bulk metal separators and insulation of the gasifier body. The computation was carried out on a two-dimensional axi-symmetric geometry of the gasifier. The segregated implicit double precision solver of Ansys Fluent was used to solve the transport equations. The flow was assumed to be two-dimensional axi-symmetric with steady state. A no-slip boundary condition was applied on reactor walls and separator/insulation bulk material. The modelled particles had a uniform distribution and were assumed to be spherical in shape, but at a much smaller size (
