Process simulation, optimization and economic valuation of waste tyre pyrolysis for fuel production.
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Date
2023
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Abstract
Management of waste tyres is increasingly becoming a global challenge; the problem is only projected to get worse in future as worldwide tyre production is continually on the rise in response to the growing global population. Managing waste tyres through traditional landfilling approach has proven to be an unsustainable approach due to the associated environmental concerns. The said concerns include posing toxic gas emitting fire risks, contaminating ground water and housing disease carrying insets. Despite these challenges, a huge percentage of tyres are still managed through legally and illegally landfilling. Waste tyre recycling and retreating has proven to be a useful and sustainable management alternative, these technologies are however limited to only handle up to 13 % of global waste tyres.
The detrimental environmental problems associated with traditional waste tyre management has resulted in a significant rise in a number of studies focusing on investigating waste tyre management alternatives. Of the means investigated, waste tyre pyrolysis has emerged as the most manageable approach as it yields easy to handle products and has the ability to handle huge waste tyre volumes. In addition to the said benefits, tyre pyrolysis technology harnesses energy from tyres that would have been lost to the landfills. Tyre pyrolysis products are energy rich oil, char and gases. The process of tyre pyrolysis entails volatilization of tyres at temperatures above 400 oC in the absence of oxygen. There are a number of governing factors associated with tyre pyrolysis process, viz. type of pyrolysis rector used, operating temperature, heating rate, particle size, residence time, operating pressure, and a presence of the catalyst. The governing factors outlined are mainly focused on maximizing the oil yield and minimizing char and non-condensable gaseous products. Of the governing factors influencing tyre pyrolysis process, operating temperature has the biggest impact. According to the literature studies, the typical optimum oil yield is between 38 % and 60 % by weight of waste tyre, achievable between 425 oC and 720 oC.
On this study, a ASPEN Plus simulation computer software was used to develop a tyre pyrolysis process model. From the developed simulations a number of investigations were undertaken to understand the impact of the key process governing factors. Investigations conducted were on the operating temperature, reactor type, dimensions of the reactor, residence time, reactor operating pressure, heating rate, presence of the catalyst and tyre particle size
The reactor type investigations showed that the reactors with some level of mixing favoured higher oil yield. This premise was evident in the investigation between the CSTR and PFR. The CSTR presented oil yield increase from 50.93 % to 51.13 % while the PFR showed an increase from 44.02 to 44.13 % for the temperature range between 400 and 700 oC. To incorporate a kiln reactor comparison, the temperature was kept constant at 550 oC on all the reactors investigated. The CSTR and kiln reactor showed higher oil yields than PFR, with oil yields of 51.01 %, 50.93 % and 44.09 % respectively. The results obtained from the kiln reactor and CSTR showed to be relatively similar since the kiln reactor was modelled using CSTRs in series.
In the reactor size investigation which is linked to the residence time. For the PFR, no further improvements on the yield were noticed at the reactor diameters above 1m. The CSTR showed the yield to remain constant above a 1 hour residence time. The investigation on the operating pressure showed that the higher oil yields are achieved at lower operating pressure. When the pressure was increased from 0.1 atm to 1 atm, the CSTR oil yield decreases from 50.98 % to 50.97 %.
Increasing the reactor heating rate showed positive impact on the oil yield, an improvement from 50.75 to 50.89 % was achieved when the heating rate was increased from 5 to 15 K/min. However, at 20 K/min the oil yield decreased to 50.87 %. The use of the catalyst showed positive impact on the oil yield, an increase from 49.92 % to 51.7 % was noted from no catalyst basecase. Two size classes were considered in the particle size investigation, viz. 0.1 mm – 0.8 mm and 0.8 mm – 4 mm. The investigation showed that the lower particle size results in higher oil yield. The CSTR showed an increase with the increase in the residence time, an increase of 0.04 % points was noted when the residence time was increased from 0.1 to 1 hr.
A different tool was used to check the validity of the simulation findings, for this investigation a numerical model from literature was employed. The model was based on a laboratory study incorporating the particle size, temperature and feed rate. The un-optimised conditions of the model showed oil yield of 26.3 % while the optimum conditions showed oil yield of 47.9 %. The optimum conditions were identified be at a temperature of 400 oC, particle size of 1.0 mm and a feed rate of 0.78 kg/hr.
Based on the results of this study encompassing both numerical and simulation findings, conclusion were drawn that the optimum oil yield is obtainable from the CSTR operated at a temperature range between 400 oC and 550 oC, tyre particle size less than 1 mm, operating pressure below 1 atm, heating rate between 10-15 K/min and residence of time of less than 1 hour. The economic evaluations showed that the tyre pyrolysis plant only starts yielding positive ROI at least after 3.7 years of operating. This is relatively an acceptable ROI period pertaining to the investment decision making.
Description
Masters Degree. University of KwaZulu-Natal, Durban.