Thermal and Catalytic Co-pyrolysis of Rice Straw and Scrap Rubber Tires: Experimental Investigation and Techno-Economic Assessment

Abstract

The global economy, being highly dependent on energy, is threatened by the depletion and price volatility of fossil fuels. The increased global energy demand is speculated to consume entire fossil fuel reserves in near future ultimately leading to energy crises. Besides, extravagant usage of fossil fuels engenders harmful pollutants endangering the environment. Therefore, the world direly needs environmentally benign and sustainable resources and techniques such as pyrolysis to produce energy by valorizing the waste materials like rice straw (RS) and scrap rubber tire (SRT). Pyrolysis is a thermo-chemical process that produces bio-oil of high energy density from lower-grade solid waste along with invaluable by-products such as gas and biochar. However, the bio-oil derived from pyrolysis of sole biomass exhibits certain technical constraints due to elevated oxygenates and water content. This dissertation aimed to improve the quality of pyrolysis oil through various approaches such as co pyrolysis, pretreatments of rice straw and catalysts addition besides investigating the techno-economic analysis of commercial scale co-pyrolysis plants. During the co-pyrolysis of RS and SRT (1st objective), the impacts on products under broad range of feedstock ratio (20, 40, 60 and 80 wt.% of SRT into RS) in a fixed bed reactor at 550 °C were investigated. All experiments were performed in triplicate to ensure the reproducibility of data and standard deviation is provided where applicable. Oil yield, organic phase, pH, aromatics and olefins increased with an increase of SRT proportion in blend. Oil yield, aromatics and olefins reached to 45 area %, 42% and 30% at SRT/RS (80:20) compared to 36 area %, 2% and 4% in case of RS alone, respectively. Likewise, at the same blend ratio, 85% reduction in oxygenates was observed while higher heating value (HHV) of oil (41.40 MJ/Kg) was comparable to that of SRT (41.50 MJ/Kg). Hydrogen (H2) and methane (CH4) along with higher HCs were increased while oxides of carbon were reduced with the addition of SRT. s In 2 nd objective, the feedstock ratio was maintained at 1:1 to assess the impacts of acid washing and torrefaction of RS on co-pyrolysis with SRT in fixed bed reactor. To ensure brevity, oven-dried raw RS was labeled as R-RS while acid-washed RS was designated as W-RS. T-RS and WT-RS denote the torrefied raw RS and combined acid washed-torrefied RS sample, respectively. Acid-washing eluted 96% K, 74.73% Ca, 94.78% Mg and 98.33% Na metals from W-RS besides improving its higher heating value (HHV) and relative content of lignocellulose. The oil obtained from co-pyrolysis of WT-RS and SRT had 41% less oxygenates, 17% more hydrocarbons (HCs) and a significant amount of levoglucosan compared to combination of R-RS/SRT. The impacts of catalyst incorporation and pretreatments of RS were assessed in two phases in 3 rd objective. Two catalysts, Natural zeolite (NZ) and HZSM-5, were employed for pyrolysis of raw and pretreated RS during first phase while only NZ was used for co-pyrolysis with SRT during the second phase due to the comparable performance of NZ to that of HZSM-5. The selectivity of most valuable aromatic HCs e.g benzene, toluene and xylene (BTX) of HZSM-5 was superior to that of NZ in either of RS with the highest selectivity of 16%(B), 22%(T) and 30%(X) in W-RS/HZSM-5 due to its higher total pore volume, surface area and lower acidity and controlled reforming reactions. NZ selectivity for BTX was also improved from 7.69%, 13.04% and 8.69% during R-RS/NZ to 10.52%, 15.78% and 21.05% for W-RS/NZ, respectively. Moreover, HHV of W-RS/NZ bio-oil was 26.32 MJ/kg whilst that of W RS/HZSM-5 oil (30.95 MJ/kg) was highest among all. During the second phase, Incorporation of NZ increased the HCs by 28%, 31%, 30% and 40% during catalytic co-pyrolysis of R-RS/SRT, W-RS/SRT, T-RS/SRT and WT-RS/SRT compared to their respective non-catalytic co-pyrolysis. Selectivity of BTX with NZ addition was highest for WT-RS/SRT/NZ combination with sequence of WT-RS/SRT/NZ > W-RS/SRT/NZ > T-RS/SRT/NZ > R-RS/SRT/NZ. The comparative techno-economic modeling of three types of pyrolysis and co pyrolysis hypothetical plants of RS and SRT to produce oil and power with 30 tonnes/hour capacity is performed using SuperPro Designer software (4th objective). The RS plant has the lowest capital and operating costs of $ 53.730 million and $ 43.745 million respectively however, it is found to be unlucrative under current settings due to lowest quantity and quality of oil among all. The base cases of SRT and co-feed (RS and SRT) plants are found to be viable with capital costs of $ 66.913 million and $ 68.317 million, and operating costs of $ 77.186 million and $ 70.338 million respectively. Co-pyrolysis plant produces highest oil (main product) yield of 0.0740 million tonnes annually and power of 4801 KWe with lowest unit production cost of $ 950/Mt. Consequently, the co-pyrolysis plant offers highest economic performance with net present value (NPV), payback time (PBT), internal rate of return (IRR) and gross margin (GM) of $ 35.55 million, 5.080 years, 34.67% and 21.35% respectively. Sensitivity analysis suggest that NPV is more sensitive to oil selling price, feedstock (SRT and/or RS) cost, and capital investment for all plants.