Catalytic fast co-pyrolysis of bamboo sawdust and waste tire using a tandem reactor with cascade bubbling fluidized bed and fixed bed system
Graphical abstract
Introduction
Thermochemical conversion of fossil-based and bio-based wastes into alternative liquid fuels provides a promising approach to realize waste management and environmental protection [1], [2], [3], [4]. Among the fossil-based wastes, waste tires are one of the most important materials. The production of tires reaches to 1.5 billion tons each year, and a considerable amount of waste tire (800 million tons) needs to be disposed of [5]. Due to the nonbiodegradable and non-destructible properties of waste tire, post-treatment and/or recycling are difficult. In addition, landfill of waste tire is prohibited in many countries in Europe [6]. Therefore, efforts should be made to effectively convert waste tires into alternative fuels, such as thermochemical conversion to produce liquid products [7], [8], [9], gasification to generate flue gases [10], [11], and incorporating into cement concrete to replace some of the natural aggregates [12], [13].
Among the thermochemical techniques employed to convert bio-based and fossil-based wastes into liquid products, fast pyrolysis and catalytic fast pyrolysis (CFP) are the most commonly used ones [14], [15], [16], [17]. Studies on the pyrolysis of waste tires have been conducted extensively to investigate the effect of pyrolysis temperature, heating rate and reactor configuration on the product yields and chemical fractions in the pyrolysis oil [8], [18], [19], [20], [21], [22], [23], [24]. In addition, catalytic fast co-pyrolysis (co-CFP) of waste tire with lignocellulosic biomass also has attracted much attention as shown in Table 1. Various bio-based raw materials such as wheat straw, pine wood chips, and corn stalk, have been used in catalytic co-pyrolysis with waste tires (Table 1). Abnisa et al. [25] co-pyrolyzed palm shell and waste tire in a fixed bed reactor at 500 °C, and they found that the addition of scrap tire in the pyrolysis of palm shells improved both the quantity and quality of pyrolysis oil. Uçar et al. [26] conducted co-pyrolysis of pine nut shells and scrap tires at different blend ratios, and it was observed that compared to the bio-oil obtained from pyrolysis of pine nut shells alone, the pyrolysis oil derived from co-pyrolysis process had higher amounts of carbon and lower concentration of oxygen.
Concerning the reactor configurations used in the co-CFP of biomass and waste tire, a number of pyrolysis reactors such as fixed bed, microwave assisted reactor, auger reactor, and heinze reactor, have been used to study the co-pyrolysis process (shown in Table 1). Dai et al. [27] used a microwave assisted pyrolysis reactor to conduct the co-pyrolysis of soap stock and waste tire, and the effects of pyrolysis temperature, catalyst/feedstock mass ratio, soap stock/waste tire mass ratio on product distribution were studied. It was observed that waste tires could act as hydrogen donor which significantly promoted the production of pyrolysis oil and the formation of aromatic hydrocarbons. In addition, Martínez et al. [6] compared the pyrolysis oil characteristics obtained from co-pyrolysis of forestry wastes and waste tires using a fixed bed and a continuous auger reactor at 500 °C, and they found that co-pyrolysis experiments conducted by the auger reactor showed more significant synergistic effect than that of the fixed bed reactor. However, to the best of our knowledge, there is no study using a bubbling fluidized bed tandem with a fixed bed reactor to conduct the co-CFP of biomass and waste tire. Compared to the fixed bed system, the bubbling fluidized bed reactor provides better mass and heat transfers which are beneficial to biomass pyrolysis [28]. Huber et al. [29], [30] concluded that the production of aromatic hydrocarbons in pyrolysis oil obtained from CFP of biomass using fluidized bed is the highest at optimal reaction conditions.
On the other hand, catalysts used during the co-CFP of biomass and waste tire were focused on HZSM-5, SBA-15, and red mud with HZSM-5 being the most commonly used (Table 1). Nolte et al. [40] reported that acidic zeolites significantly promoted dehydration reactions in the deoxygenation of cellulose pyrolysis. However, base catalysts such as CaO or MgO favored the fragmentation reactions more which facilitated the production of lower molecular weight pyrolytic products [15], [41], [42]. Wang et al. [41] reported that the use of alkali metal oxides during the catalytic fast pyrolysis of lignocellulosic biomass could de-oxygenate the carboxylic acids, linear aldehydes and bulky oxygenates such as anhydrosugars (mainly levoglucosan), leading to the production of lower-molecular-weight oxygenates and light hydrocarbon compounds by fragmentation and CC bonding (such as ketonization and aldol condensation) reactions. Subsequently, the smaller oxygenates can enter the micropores of HZSM-5 zeolites to be converted into aromatic hydrocarbons. Therefore, a combination of acidic zeolite and base catalyst might result in dehydration reactions and low molecular weight pyrolytic products. To test this hypothesis, in this study, a bubbling fluidized bed connected with a fixed bed reactor was used to conduct the co-CFP of bamboo sawdust and waste tire, and HZSM-5 and MgO were used as catalysts. Two catalytic modes, namely HZSM-5 mixed with MgO (mixed mode) and HZSM-5 sequential with MgO (sequential mode) were investigated. The effect of bamboo sawdust/waste tire mass ratio and HZSM-5/MgO mass ratio on the pyrolytic product yields and distribution was studied. In addition, the additive effect between HZSM-5 and MgO in terms of promoting the formation aromatic hydrocarbons was also explored.
Section snippets
Materials
Waste tire powder with 100 screen mesh (∼0.15 mm) was purchased from a tire recycling plant in Guangdong province, China, and they were used as received. Bamboo sawdust was obtained from Zhejiang province, China, and they were ground and sieved with a 100 screen mesh (∼0.15 mm). Prior to the pyrolysis experiments, the feedstocks were dried at 105 °C for 24 h. The elemental analysis of the samples was conducted by Vario EL II elemental analyzer (Germany), and the results are summarized in Table 2
Effect of waste tire percentage on the yields of pyrolytic products
It’s well known that pyrolysis and catalytic upgrading temperatures generally play a determining role in the products yield and distribution during the thermal conversion of biomass. In this study, we first conducted fast pyrolysis of bamboo sawdust with/without HZSM-5 to explore the optimal reaction temperatures using the bubbling fluidized bed/fixed bed reactor, and it was observed that a pyrolysis temperature of 550 °C and catalytic upgrading temperature of 500 °C optimized the pyrolysis oil
Conclusions
In summary, compared to CFP of bamboo sawdust alone, co-CFP of bamboo sawdust with waste tire over pure HZSM-5 increased the pyrolysis-oil and char yields and the highest pyrolysis-oil yield was achieved when the waste tire percentage was 80 wt%. An addition of waste tire into bamboo sawdust resulted in a positive synergistic effect as the experimental relative concentrations of aromatic hydrocarbons and olefins were higher than the theoretical ones for all runs at different waste tire
Acknowledgements
The authors are grateful for the National Natural Science Fund Program of China (No. 51776042), the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1646), the Scientific Innovation Research Program of College Graduate in Jiangsu Province (KYLX16_0204) as well as the Financial Support from the Program of China Scholarships Council (No. 201706090032).
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