Waste tires pose environmental and disposal problems. Waste tires are being disposed of in landfills and are recycled for reuse in composite asphalt or ground rubber. Since waste tires are composed of polymeric elastomers with a heating value of about 8,500 kcal/kg, carbon black and some other ingredients can be recovered as a fuel or as chemical feedstocks (carbon black, oil or gas). Numerous studies have been carried out to recover energy and chemical feedstocks from waste tires through direct incineration, pyrolysis and gasification. However, significant investments and operating costs are required for efficient tire incineration due to the sophisticated incinerator designs. Various pyrolysis techniques have been developed, but high-quality carbon black can not be obtained from these as a raw material for tire manufacturing. The gasification of waste tires is a very attractive means to recover fuel or chemical feedstocks in the gas phase without further processing.
The objective of this study is to determine the effects of process variables such as equivalence ratio, reaction temperature, fluidization velocity and steam/C ratio on the gasifier performance indices and to develop a mathematical model to predict the performance of a fluidized-bed waste tire gasifier.
The gasification of waste tires in a fluidized bed consists of three major reactions: devolatilization and gasification and combustion of pyrolyzed char. The devolatilization reaction of waste tire can be assumed to occur instantaneously because the heating rate in the fluidized-bed gasifier is high enough to decompose the volatile matter of waste tire in a few seconds. The combustion reaction of pyrolyzed char is also very fast. Therefore, the volume of the gasifier depends on the gasification rate of pyrolyzed char. The kinetics of tire-char gasification are important in the reaction mechanism and in designing gasifiers.
In the kinetic study of tire-char/$CO_2$, the effects of gasification temperature (850-1000℃) and partial pressures of $CO_2$ (0.3-1.0 atm) on gasification rate have been determined in a thermobalance reactor. The gasification rate is found to be independent of char size for sizes less than 0.65 mm and initial sample mass less than 1.0 g. Among the tested models, the modified volume reaction model is best for predicting the conversion data. The kinetic parameters (activation energy, pre-exponential factor and reaction order) are determined on the basis of the modified volume-reaction model. From the Arrhenius plot (km vs 1/T), the activation energy and the pre-exponential factor are found to be 57.06 kcal/mol and $1.49×10^8 1/min$, respectively. The reaction order is found to be 0.68 with respect to $CO_2$ partial pressures of 0.3-1.0 atm at 950℃. The gasification reaction rate of tire-char/$CO_2$ can be expressed as:
$\frac{dX}{dt}=1.49×10^8exp(-57,060 /RT)(P_{CO_2})^{0.68}(1-X)$.
In the steam gasification of tire-char, the experimental temperatures ranged 750-900℃ and the partial pressures of steam, 0.2 - 0.6 atm. Both the shrinking core and modified volume reaction models represent conversion data very well. The kinetic parameters are determined on the basis of a modified volume reaction model in the same way as $CO_2$ gasification. The activation energy and the pre-exponential factor are found to be 51.14 kcal/mol and $1.01×10^8 1/min$, respectively. The reaction order is found to be 0.78. Therefore, the steam gasification reactivity of tire-char can be expressed as:
$\frac{dX}{dt}=1.01×10^8\exp(-51,140/RT)(P_{H_2O})^{0.78}(1-X)$.
Shredded waste tires(1.4 - 2.2 mm) were gasified with air and steam in a fluidized bed reactor (0.076 m i.d. × 1.2 m-high) at atmospheric pressure. The effects of air feed rate (equivalence ratio: 0 - 0.33), reaction temperature (750-900℃), fluidizing velocity ($1.5u_{mf} - 3.0u_{mf}$) and steam/C ratio (0 - 1.19) on the product gas composition and the gasifier performance indices (product gas yield, carbon conversion, gross heating value and energy recovery) have been determined. The effect of air feed rate on the product gas contents with steam injection (800℃ , $2.0u_{mf}$) or without steam injection (800℃, $2.5u_{mf}$) has been determined. In the product gas, CO and $CO_2$ contents increase, and $H_2$ and hydrocarbons decrease with increasing the equivalence ratio since the combustion reaction of tire-char is enhanced by an increase of $O_2$ concentration in a fluidized bed. Gas yield and carbon conversion of product gas increase, but gross heating value and energy recovery decrease as equivalence ratio is increased.
The effect of reaction temperature on the gasifier performance has been determined at the equivalence ratios of 0.11 (with steam injection) and 0.25 (without steam injection). With an increase of reaction temperature, $H_2$, $CH_4$ and CO contents increase, while contents of $CO_2$ and light hydrocarbons decrease since the steam gasification reaction is enhanced. As the reaction temperature is increased, gas yield, carbon conversion and energy recovery of the product gas increase, whereas, gross heating value decrease. The fluidization velocity was varied in the range of 1.5 - $3.0u_{mf}$ to determine the effects of gas residence time and mixing pattern in a fluidized bed. The contents of $H_2$ and CO decrease slightly, and $CO_2$ and $C_3H_6$ increase slightly with increasing fluidizing gas velocity. As fluidizing velocity is increased, gas yield and carbon conversion of the product gas decrease slightly and gross heating value increase somewhat while energy recovery remained approximately constant. Steam gasification was carried out under an equivalence ratio of 0.25 at a gasification temperature 800℃ ($2.5u_{mf}$). In the range of steam/C ratio 0 - 1.19, $H_2$ content increase, CO and $CO_2$ decrease slightly, and the other hydrocarbons remained as steam/C ratio is increased. Gas yield, carbon conversion and energy recovery of toduct gas increase marginally and gross heating value is almost constant with increasing the steam/C ratio. A mathematical model for a fluidized-bed waste tire gasifier is proposed in a isothermal steady-state. In the model, hydrodynamic correlations of the bubbling bed model (Kunii and Levenspiel, 1968) are combined with the kinetic data of the gasification and combustion reactions. The devolatilization reaction of waste tires is assumed to occur instantaneously and the product compositions by devolatilization are added to the final product compositions as the pre-determined values. The model predictions for the product gas compositions are compared with the experimental data. The comparison shows a fairly good agreement so that the proposed mathematical model can be utilized as a guide line to design a fluidized bed tire gasifier.
