Production of renewable jet fuel and gasoline range hydrocarbons from catalytic pyrolysis of soapstock over corn cob-derived activated carbons

Highlights

• Catalytic pyrolysis of soapstock over P-containing activated carbon was studied.

• The C8–C16 hydrocarbons and H₂-enriched bio-gas were obtained simultaneously.

• The high selectivity of jet fuel range hydrocarbons can reach to 98.78%.

• The gasoline range hydrocarbons can reach to 91.03%.

Abstract

Selective production of jet fuel and gasoline range hydrocarbons from waste soapstock was achieved for the first time by catalytic pyrolysis over activated carbon catalyst that was prepared via pyrolysis of H₃PO₄-impregnated corn cob pyrolysis. Experimental results exhibited that the concentration of H₃PO₄ played an important role in acid groups and porous properties of prepared activated carbon catalyst. The obtained catalyst had a remarkable catalytic performance for C8–C16 aromatics formation with the highest selectivity of 89.97% in the bio-oil. In the meantime, the selectivities of jet fuel and gasoline range hydrocarbons could reach up to 98.78% and 91.03%, respectively. The bio-gas yield was improved with the increase in H₃PO₄ concentration, pyrolysis temperature and feedstock/activated carbon catalyst ratio, and the highest concentration of H₂ (69.90 vol%) was achieved. The optimal reaction condition was at a pyrolysis temperature of 500 °C with a soapstock/ACC4 ratio of 1:1.5. In addition, a possible reaction mechanism was proposed for catalytic pyrolysis of soapstock over activated carbon catalyst. The current work might provide a novel, facile and efficient pathway to directly convert waste soapstock into valuable drop-in jet fuel and gasoline together with production of H₂-rich syngas.

Introduction

The mass exploitation of conventional fossil resources and consequent ecological environment deterioration have driven the exploring of renewable and environment-friendly energy resources to moderate worldwide energy and environment problems [1]. Biomass is recognized as one of the suitable alternatives to fossil resource by virtue of its abundance, environment-friendly and carbon neutrality [2]. Soapstock, a low-cost byproduct from waste oils/fats from vegetable oil and animal fat refining, is conventionally utilized to produce soap, fertilizer and acidic oil. More than 5 million tons of edible oil is produced every year in China, with 1.5 million tons of soapstock being generated [3]. Besides, with the development of society and the improvement of people’s living standard, the formation of waste kitchen oil and soapstock have seen a rapidly increasing trend in the world. Unfortunately, the present fate of these kitchen oil and soapstock is still discarded since a great majority of which cannot be retrieved. The soapstock ended up in the landfill and discharged into the environment system through an impertinent treatment, leading to serious environment contamination for underwater and soil along with the space scarcity and disposal costs [4]. In addition, the untapped soapstock causes an enormous energy dissipation, which is detrimental to the sustainable development of recent society. Therefore, developing an efficient, facile and cost-effective method to alleviate soapstock pollution and simultaneously convert these wastes into improvable energy products is of paramount significance.

The particular component inspires researchers to explore promising technologies to produce high value-added chemicals and renewable fuels from waste soapstock. To date, numerous studies have investigated the conversion of soapstock into biodiesel due to its high fatty acid and fatty acid salt content by fast pyrolysis (FP) technology [5]. FP has been widely regarded as an attractive technology to convert biomass into bio-oil and chemicals in a single-step process [6]. In the FP process, the feedstock is usually rapidly heated to target temperature (450–600 °C) with a quick heating rate (100–300 °C/s) under oxygen free condition. After FP process, the feedstock is always depolymerized into solid (bio-char), gaseous (bio-gas) and liquid products (bio-oil) simultaneously. The FP is characterized by flexibility and simple operation, which makes the biomass conversion more feasible and effective [7]. However, bio-oil is consisted of thousands of species and is highly oxygenated, which impedes its further application [8]. In this regard, soapstock pyrolysis is a promising pathway to produce high quality bio-oil. It has been demonstrated that the properties and components of bio-oil from soapstock pyrolysis are similar to the petroleum-based diesel [9]. In addition, due to its high H/C ratio, soapstock has been also widely employed as an excellent hydrogen donor in co-pyrolysis with renewable biomass to increase the aromatic hydrocarbon content of bio-oil [2]. Nevertheless, chemical profile of bio-oil from soapstock pyrolysis is still complex in terms of their application as traditional liquid fuels at an industrial scale, leading to the necessity of the subsequent downstream catalytic refinery. Alternatively, the utilization of catalysts in the FP process, namely catalytic fast pyrolysis (CFP), has been widely explored for upgrading bio-oil to produce green bio-oil and highly value-added chemicals [10]. The catalysis can dramatically improve the pyrolysis vapors via cracking, reforming and deoxygenation reactions, resulting in the formation of improved bio-oil with narrow distribution. Meanwhile, the energy consumption can be reduced by limiting the production of coke, water and water soluble organics [11].

The utilization of catalyst during CFP process leads to upgradation, reforming and cracking of biomass conversion products, depending on the operating conditions [12]. Initially, in-situ catalytic pyrolysis system is always employed to convert biomass into bio-oil. In this process, the catalyst is mixed with feedstock in the reactor, which results in the difficulty to recover the catalyst after reaction. This phenomenon inhibits the further application of catalyst, especially for some expensive catalyst, leading to the high cost of the technology. In the meantime, it also cause the catalyst inactivation due to the formation of coke and char during biomass pyrolysis process [13]. Another pathway, namely ex-situ catalytic pyrolysis system allows for separating catalyst from feedstock, which is more economic friendly and efficient to surmount above disadvantage. The catalyst is separated from feedstock, and the primary pyrolytic vapors can pass through smoothly and be reformed. The ex-situ process accelerates the separation of char and whittles the catalyst deactivation by inorganic material exist in biomass [14]. Many experiments have demonstrated that the ex-situ process can improve the selectivity of target compounds compared with in-situ process [15].

The selective production of desired gasoline, diesel or jut fuel can be achieved by applying a suitable catalyst [16]. Typically, catalysts with preferable deoxygenation property are utilized for direct pyrolysis of biomass into bio-oil. The special surface area, acid strength and porosity are critical factors on catalytic performance of catalysts [17]. In previous studies, the zeolites-based catalysts, such as HZSM-5, silica-alumina, MCM-41 and β-zeolite et al., have been demonstrated to be excellent in producing hydrocarbons lumped in gasoline [18]. Experimental results indicated that during soapstock pyrolysis process, HZSM-5 dramatically improved the selectivity of gasoline-ranged hydrocarbons. Meanwhile, the water and oxygen content in bio-oil decreased significantly, meaning the improvement of bio-oil quality [19]. Recently, the activated carbon catalyst (ACC) has attracted increasing attention due to its renowned physiochemical characterizations, such as tunable surface functional groups, large pore volume, high surface area and multiple pore size distribution [20]. The pore size, pore volume, acid site and strength of ACC are decisive factors on pyrolytic cracking and aromatization reactions [20]. Numerous studies mainly focused on themes of phenol generation from biomass pyrolysis with ACC, and most of them used commercial ACC [21]. To the best of our knowledge, rare studies were reported in regarding with catalytic pyrolysis of soapstock with ACC. In our previous study, the different types of catalyst (HZSM-5, ACC, Al₂O₃ and K₂CO₃) were applied to catalytic pyrolysis of soapstock. The results showed that the commercial ACC exhibited some catalytic performance in converting the soapstock into aromatic hydrocarbons [9]. As we all know, the commercial ACC is mainly produced from some high-cost agroforestry resource such as bamboo and wood. Besides, the contents of target compounds in bio-oil from commercial ACC catalyzed pyrolysis are not high enough to meet industrial requirement [22]. Interestingly, recent studies revealed a fresh pathway of applying microwave heating to convert renewable biomass into ACC via H₃PO₄ activation [23]. The biggest advantage of using microwave heating is that it can accelerate the activation rate and improve the product yield compared with traditional heating [23]. Besides, during H₃PO₄ activation process, the P-containing functional groups like P–OH, -O-P-O and PO can be introduced to ACC structure, leading to the conversion of long chain hydrocarbon into desired products via aromatization reaction [17]. Pioneer work from our group demonstrated that the ACC from corn stover activated by H₃PO₄ via microwave heating can dramatically improve the desired products selectivity in bio-oil [20]. Based on the aforementioned analysis, we anticipated that ACC can be used as proper catalysts in producing aromatic hydrocarbons directly from CFP process of soapstock. Therefore, in the current work, an investigation was carried out by using home-made ACC in ex-situ catalytic pyrolysis of soapstock for the first time. The ACC was synthesized from pyrolysis of H₃PO₄-activated corn cob under microwave irradiation. The goal of this work was to study the effects of reaction condition during production of ACC on their catalytic performance for selectivity of aromatic hydrocarbons. In addition, the distinct parameters including pyrolytic temperature and various catalyst/feedstock ratio were also investigated. The current study provided a green and efficient way to convert soapstock into aromatic hydrocarbon-enriched fuel and H₂-enriched biogas by using a low cost, abundant and renewable ACC.