Investigating the influence of acid washing pretreatment and Zn/activated biochar catalyst on thermal conversion of Cladophora glomerata to value-added bio-products

Highlights

• Employing macroalgae simultaneously as feedstock and catalyst precursor to produce high efficiency upgraded bio-products.

• Promotion of bio-oil production and bio-oil quality via acid washing pretreatment of biomass.

• Evaluating the influence of biomass inorganic content on pyrolysis behavior of macroalgae.

• Optimizing the carbonization parameters for preparation of highly porous ZnCl₂-activated bio-char.

• Production of low oxygen and aromatic-rich bio-oil as well as H₂-rich biogas over activated bio-char supported Zinc catalyst.

Abstract

The production of sustainable energy resources are of great importance in the energy industry. As a result, studies have been extremely concerned with the conversion of coastal macroalgae into bio-products mostly through thermochemical processes such as pyrolysis. This study aimed to produce high efficiency value-added bio-products through the pyrolysis of the acid-washed Cladophora glomerata (ACG). It also attempted to upgrade the bio-products through the catalytic pyrolysis with the use of Zn/Activated biochar (Zn/C) catalyst. The catalyst was prepared via the carbonization of the ZnCl₂ impregnated ACG under the optimal reaction conditions and was characterized by BET, XRD, FT-IR, FESEM-EDS and ICP analyzes. The experimental results indicated that the acid washing pretreatment led to the removal of inorganic contents of the biomass, and consequently to the significant increase in the bio-oil yield. In addition to this remarkable change in the product distributions and according to the results of the catalytic experiment, the Zn/C catalyst decreased the acid and oxygen contents of the bio-oil. It also increased the aromatic contents of the bio-oil and promoted H₂ production. Accordingly, it favorably upgraded the quality of the bio-oil and pyro-gas. The study concludes that Cladophora macroalgae can be widely used as both feedstock and catalyst in order to produce value-added bio-products in the energy industry.

Introduction

Recent advances in industry have led to the dramatic development in human life. However, it has caused many devastating effects on the environment and energy resources [1]. Global warming, acidification, eutrophication, toxicity, and photo-oxidant formation factors are of great consequential concerns that stem from detrimental emissions to our air, terrain, and water [2], [3]. In recent years, the excessive eutrophication of coastal areas has raised many serious environmental issues and prompted the researchers to find a solution to both depletion of fossil fuel resources and the excessive blooming of macroalgae [4], [5], [6], [7]. To attain this goal, the industrial progress should be extended through finding sustainable energy resources that are capable of providing enough, clean and secure energy [8]. Over the past few years, researchers have been increasingly concerned with the production of fuel and value-added chemicals from coastal algae [9], [10]. Although some drawbacks have been reported regarding their corrosive nature and NO_x emissions during the combustion, some advantages of coastal macroalgae such as fast growth rate, high efficiency in CO₂ capture and photosynthesis, non-competitiveness with food crops, and lower temperatures required for thermal conversion, have made them still appropriate energy resources compared to lignocellulosic and crop biomass [7], [11]. Cladophora glomerata is a unique macroalgae distributed in the coastal regions of the Caspian Sea. Thermochemical conversion, especially pyrolysis, has been one of the most common techniques used to produce bio-fuel from Cladophera glomerata. For instance, Ebadi et al. [12] studied the production of bio-oil from Cladophora glomerata using fast pyrolysis and investigated the effects of reaction time, temperature and algae particle size on product yields. Moreover, Vu Ly et al. [10] investigated the effect of AW pretreatment on the pyrolysis characteristics and kinetics of Cladophora socialis. Maddi [13] carried out a comparative study on the pyrolysis of algae from freshwater blooms and lignocellulosic feedstocks. The study demonstrated that bio-oil yields obtained from algae and some lignocellulosic materials were similar. However, algal bio-oils contained higher N-content derived from protein degradation.

The techniques through which macroalgae has heretofore been converted into energy and chemicals were pyrolysis, hydrothermal gasification, and liquefaction [14]. In spite of the considerable significance of algal feedstock, there are still some limiting factors preventing the development of macroalgae-based fuel and chemicals such as the acids and the oxygenated and nitrogenous compounds in bio-oil product. These limiting factors are derived from the composition of macroalgae that is mainly composed of protein, lipid and carbohydrate [15], [16], [17]. Large amounts of oxygen lead to a decrease in bio-oil heating value, cause corrosive properties and chemical instability and also reduce bio-oil miscibility with hydrocarbon fuels. Therefore, upgrading the primary obtained bio-oils is of great importance [18], [19], [20], [21]. Carbon-based materials such as biochar have been extensively used as catalysts in the pyrolysis process in order to upgrade the bio-oil. The selection of such catalysts among others, including zeolite-based catalysts [16], [22], [23], silica-based catalysts [9], [17], waste metal based catalysts [11] and etc. is due to their low cost, high porosity, and modifiable surface properties [24]. The previous results of the catalytic pyrolysis of the Caspian Sea coast marine biomass using algal biochar, hydrochar, ZH-20 composite, and multi metal catalysts are presented in Table S1. Accordingly, the catalytic behavior of bio-char is comparable to that of conventional catalysts utilized in pyrolysis. However, algal bio-char contains significantly high amount of inorganics including alkali and alkaline Earth metals (AAEMs) and these impurities extensively advance both carbonization and gasification and reduce bio-oil yield [25].

Currently, engineered activated biochar, has been developed with its explicit advantages including low-cost, facile availability, environmental friendliness, desired thermal and chemical stability and tunable chemical nature and morphology [26], [27], [28]. In order to improve the catalytic performance of activated biochar on upgrading biomass pyrolysis bio-products, numerous reports were focused on the beneficial impact of metal (including transition metals such as Fe [29], Pt [30] and Ni [31], and some alkali metals such as Na, K [31]) loading on activated biochar through the conventional impregnation method. This method required four separate steps:1) preparation of biochar, 2) impregnation of the activating agent, 3) activation of biochar and 4) loading of the active phase. Moreover, the loading of the active phase through impregnation method can lead to the introduction of additional components to the support structure. On the other hand, the mature chemical activation process merges [32], [33], [34] the above mentioned impregnation and loading steps as well as preparation and activation steps, which can significantly reduce the cost, and time of the production. Thus, it is considered a simple and inexpensive fabrication method to produce activated biochar support for catalytic pyrolysis of biomass.

In this study, zinc nanoparticles supported on activated algal biochar (Zn/C catalyst), were prepared in one step through the carbonization of ZnCl₂ impregnated ACG. Then the prepared Zn/C catalyst was employed to catalyze the pyrolysis process of ACG. Zinc is a transition metal that is the most abundant element in the Earth’s crust and hence it is cost effective to purchase. Moreover, in other studies it has been confirmed that this element is efficient for the upgrading of bio-oils to hydrocarbon fuels through the deoxygenation, dehydrogenation and aromatization reactions [35], [36], [37]. During the catalyst preparation, the acid washing (AW) of algae was carried out as a pretreatment in order to reduce inorganic contents (AAEMs) [38] and to eliminate their interference effect on Zn. In addition, as suggested by our previous study [25], the use of acid-washed algae as the feedstock and the catalyst precursor was performed to produce higher yields of liquid products through the pyrolysis process. The effects of the AW pretreatment as well as Zn/C catalyst on the pyrolysis of Cladophora macroalgae were discussed in details through investigating the changes in the yields and composition of products, especially liquid products. Moreover, during the preparation of Zn/C catalyst the conditions of the carbonization process were optimized.