Chemical characterization of char derived from slow pyrolysis of microalgal residue

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Highlights

  • Pyrolysis temperature and holding time had obvious influence on the yield and nutrient properties of biochar, which was produced from a novel precursor–chlorella residue.

  • The resulting biochars contained the high concentrations of nitrogen (N) and inorganic elements.

  • The chlorella-derived biochar could be used as a high-N (>10%), rich-minerals and porous fertilizer.

Abstract

In this study, the slow pyrolysis (10 °C/min) of chlorella-based residue was investigated for the production of biochar in a thermogravimetric system under the different temperatures of 300–700 °C and holding times of 0–60 min. To evaluate their potential for soil amendment, their nutrient properties were obtained by means of elemental analyzer (EA), inductively coupled plasma–optical emission spectrometer (ICP–OES), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy–energy dispersive spectrometer (SEM–EDS). The results showed that the dried microalgal biomass and its resulting biochars contained the high concentrations of N and inorganic elements, including P, Fe, Ca, K, and Mg. The C content of biochar was increased from 56.3% (300 °C) to a maximal value of 66.2% (500 °C), and then slightly declined to about 65% (700 °C). On the other hand, the contents of hydrogen (H) and nitrogen (N) showed a decreasing trend with temperature. Based on the findings, it suggests that the chlorella-based biochar could be used as a high-N (>10%), rich-minerals and porous fertilizer.

Introduction

Biomass and its derived products (e.g., biofuels and biochar) have been recognized as carbon-neutral and renewable resources that can be used to replace fossil feedstocks with the benefits of mitigating global warming as a result of greenhouse gas emissions, and also improving soil quality for agricultural productivity [1]. In recent years, algae as a third generation biomass have received much attention because these waterborne plants have several advantages over terrestrial plants, including greater productivity in carbon fixation and less requirements of nutrients and arable land [2], [3]. On the other hand, because of the richness of nutritional contents like proteins, carbohydrates, and lipids [4], algae, especially in microalgae, are economically important, providing important biomass feedstocks for foods, biofuels (e.g., biodiesel), livestock/aquaculturel feeds, soil additives/nutrients, neutraceuticals, pigments, cosmetics, and so on [2], [4], [5], [6], [7], [8], [9]. However, the use of raw biomass residue as a fuel/soil amendment often faces several problems including large bulk volume, high moisture content, low heating value and energy density, hygroscopic nature, potential emissions of non-CO2 greenhouse gases (i.e., CH4 and N2O) and smoke during combustion. In view of overcoming some of the aforementioned limitations of biomass material, converting it into biochar or carbon-rich material has been considered as a successful approach to mitigate climate change [1].

Char from biomass, so-called biochar, is a carbon-rich material, which is produced by thermal decomposition of organic feedstock under limited supply of oxygen (O2) and at relatively low temperatures (<700 °C) [1]. As a consequence, the pyrolysis has been studied as a technically viable option for converting microalgal biomass into fuel and/or chemical products because this process typically refers to the thermal conversion of biomass at medium temperatures (350–700 °C) in the absence of air [10], [11]. Depending on the heating rate, temperature, and residence (holding) time, the pyrolysis process can be divided into three subgroups (including slow pyrolysis, fast pyrolysis, and flash pyrolysis), suggesting that these process parameters govern the properties of biochar [12]. However, as reported by Downie et al. [13], pyrolysis temperature is the most important factor. On the other hand, the slow pyrolysis process, which is generally performed at a lower heating rate (≤20 °C/min) and a longer holding time (≤60 min), is preferred to increase the biochar yield [12].

In spite of the advantages of using microalgae as feedstocks for the production of valuable materials/products, a great amount of the so-called post-extracted algal residue (AR) will be generated in the commercial-scale production of microalgae-derived commodities. As studied by Bryant et al. [8], the effective use of the microalgal residue would be critical to the economic feasibility of an algal production enterprise. However, recent studies focused on the utilization of AR for the production of biofuels using biochemical and thermochemical conversion processes [14], [15], [16], [17], [18], [19]. In addition, the dried AR was directly reused as a low-cost biosorbent for removal of cationic adsorbates (e.g., malachite green and methylene blue) from the aqueous solution because this biomass is mainly composed of polysaccharides with functional groups [20], [21].

Regarding the studies on the production of biochar from microalgal biomass or its post-extracted residue, only a few researches have been reported on its chemical characterization pertinent to its potential use as a soil amendment. In the pyrolysis of a diatom-based microalgae studied by Grierson et al. [22], the biochar produced at 10 °C/min up to a maximum temperature of 500 °C for an additional holding time of 20 min revealed a number of nutrient properties that are potentially valuable from an agronomic point of view. Torri et al. [23] investigated the yields and composition of the resulting products (including biochar), which were obtained from a hydrogen-producing microalgal residue (after lipid extraction) and low-temperature pyrolysis at 350 °C for 20 min. The spent algal biomass can be converted into nitrogen-rich biochar (i.e., the nitrogen content of 5.3%) with its mass yield of 44%. The authors, thus, suggested it could be used as a nitrogen-releasing fertilizer to the soil from the agronomical point of view. Chaiwong et al. [24] examined the chemical characterization of spirulina-based microalgal biomass and its bio-oil and biochar by slow pyrolysis heated at 8 °C/min up to a set temperature of 450–600 °C for a holding time of 60 min. The yields of biochars were about 31–32%, and their average C/H/N contents were 45.26, 1.24, and 2.57%, respectively. Using a fast pyrolysis for converting chlorella-based microalgae remnants into bio-oil and biochar at 500 °C [25], the authors studied the contents of organic elements and inorganic minerals for the resulting biochar, suggesting that it had good prospects as fertilizer. In the pyrolysis experiments examined by Gong et al. [26], two kinds of microalgae (i.e., Chlorella vulgaris and Dunaliella salina) were used as feedstocks for the production of bio-oil and biochar at the pyrolysis temperatures of 300–700 °C and a holding time of 20 min. The proximate, ultimate, and, main mineral elements analyses of the resulting biochars have been determined, showing that the biochar had a higher N content, and also contained higher concentrations of mineral elements such as Na, Mg, K, Ca, and Fe, as compared to the biochar from lignocellulosic biomass.

Although these studies, described above, have reported the utilization of microalgae biomass as a precursor for preparing biochar and other pyrolysis products at limited pyrolysis conditions [22], [23], [24], [25], [26], all of these researches mostly focused on the chemical characterization of bio-oil and gaseous products. Thus, the main goal of this research was to investigate the yields and chemical characterization of biochars from the slow pyrolysis of the chlorella-based algal residue at 10 °C/min. Under the different temperatures of 300–700 °C and holding times of 0–60 min, a series of biochars were thus produced, and their nutrient properties were further analyzed by means of elemental analyzer (EA), inductively coupled plasma–optical emission spectrometer (ICP–OES), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy–energy dispersive spectrometer (SEM–EDS).

Section snippets

Materials

The chlorella-based algal residue (AR) studied in this work was obtained from a local biotechnology workshop (Pingtung, Taiwan). This microalgal biomass was a derived residue generated from the patented cell disruption and spray-drying processes. The microalgal residue was first dried at about 100 °C for at least 24 h, and then closely stored in glass bottles. Its preliminary characterization has been described in detail in the previous work [20], [21], showing that this dried biomass is

Chemical characteristics of AR

As described above, the ash content of the dried microalgal residue is about 4.8%, indicating that the biomass comprised a large percentage of the organic constituents and should be rich in organic elements. In order to connect the biomass and its chars with their agricultural applications, the data in Table 1 shows the inorganic element contents of the dried microalgal residue. Obviously, the main inorganic elements in the ash included P, Fe, Ca, K, and Mg, suggesting that these elements could

Conclusions

The conclusions for the production of biochar from a chlorella-based residue via the slow pyrolysis (fixed heating rate: 10 °C/min; temperature range: 300–700 °C; and holding time: 0–60 min) in a thermogravimetric system were listed as follows:

  • The dried microalgal biomass contained the high concentrations of N and inorganic elements, including P, Fe, Ca, K, and Mg, which were indicative of reusing it as a soil fertilizer directly.

  • The biochar yields significantly decreased as the pyrolysis

Acknowledgements

Sincere appreciation is expressed to acknowledge the Instrumentation Centers at National Chung Hsing University, National Tsing Hua University and National Pingtung University of Science and Technology for the measurement assistances in the elemental analysis (EA), ICP–OES and SEM–EDS, respectively.

References (31)

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