Biochar from Slow Catalytic Pyrolysis of Spirulina platensis Residue: Effects of Temperature and Silica-Alumina Catalyst on Yield and Characteristics

The use of biochar varies on its ability as an adsorbent which adsorbs liquid or gas molecules. Biochar from Spirulina platensis residue (SPR) as an energy source, as its richness in nutrients, can be used as fertilizer and maintain water resources in plantations. Biochar can be used as an intermediary for the synthesis of nanotubes, activated carbon, carbon black, and carbon fiber. One of the essential things to be considered in the application of activated carbon from SPR is char’s characteristics. This study aimed to obtain data on the biochar and components from the pyrolysis of Spirulina platensis residue. The study was conducted in a fixed-bed reactor with electric heaters with a variety of temperatures (300-700 0C) and the amount of silica-alumina catalyst (0-20%). The biochar weight was obtained by weighing the char formed at the end of the pyrolysis. The char characteristics were obtained by the surface area, total pore volume, and pore size analysis. Based on the study results, the relationship between temperature and the amount of catalyst on the characteristics of biochar was studied. The higher the pyrolysis temperature, the less biochar. Also, the use of catalysts can reduce the amount of biochar. The higher the temperature, the higher the surface area and the total pore volume while the pore radius was reduced. The optimum condition for maximum biochar yield in non-catalytic pyrolysis at a temperature of 300 0C was 49.86 wt.%. The surface area, the total pore volume, and the pore radius at 700 0C catalytic pyrolysis with 5% silica-alumina was obtained as 36.91 m/g, 0.052 cm/g, and 2.68 nm, respectively.


Introduction
Pyrolysis is the most studied thermochemical technology to date and has proven to be one of the best techniques for producing biofuels and biochar from biomass feedstocks (Jamilatun et al., 2019;Li et al., 2016;Tripathi et al., 2016). Biomass sources influence biochar production through pyrolysis, biomass properties (e.g., particle size and moisture content), composition (e.g., cellulose, lignin, and ash content), and process parameters (e.g., temperature, heating rate, residence time) (Yu et al., 2017a). Dickerson and Soria (2013) explained the process parameters for slow pyrolysis; the heating rate is 0.1-1 °C/sec with residence time in the range of minutes to hours, and temperatures between 400-600 °C will produce around 33% char, 32% tar, and 35% gas. The intermediate pyrolysis at 400-500 °C, a heating rate at 1-1000 °C/sec, hot vapor residence at 10-30 seconds will produce 25% char, 50% tar, and 25% gas. In contrast, fast pyrolysis can provide 12% char, 75% tar, and 13% gas with a heating rate of 10 to more than 1000 °C/sec, a residence time of fewer than 2 seconds, and an optimum temperature between 400-650 °C (Jamilatun et al., 2017;Suganya et al., 2016).
Non-catalytic pyrolysis produces lowquality liquid products with a relatively high oxygenated compound content, which can cause corrosion to the engine. Reduction of oxygenate compounds can improve quality; another way is to use a catalyst during pyrolysis (Jamilatun et al., 2019). One of the catalysts commonly used for cracking hydrocarbons is silica-alumina, the solid acid catalyst most widely used in supporting the production of petrochemicals, chemicals, and renewable energy. High acidity (low Si/Al) can be used in the process of cracking petroleum; its function is to increase oxidation of CO (Wang et al., 2019). The silica-alumina catalyst is suitable for upgrading bio-oil, has a high melting point (1818 ºC) and surface area (Cheng et al., 2016;Duan et al., 2013). The catalytic pyrolysis results can improve bio-oil and biochar; it is essential to know the yield and characteristics (surface area, total pore volume, pore radius) of biochar produced in biochar application.
Microalgae is currently a third-generation raw material for biofuel production. It also produces several pharmacologically necessary and nutritious chemicals such as pigments and fatty acids. The simultaneous production of biofuel raw materials and fine chemicals in microalgae biorefinery can improve the economy (Elkhalifa et al., 2019).
Biochar from the pyrolysis of microalgae has a lower surface area and carbon content than biochar from lignocellulose. However, biochar has excellent characteristics such as higher pH, its ability to balance soil acidity, and higher nutrient content, including minerals such as nitrogen, ash, and inorganic elements compared to another biomass. Other characteristics of biochar from microalgae such as surface area, total pore volume, and pore radius are still rarely discussed; for this reason, it is necessary to identify with the BET method (Chen et al., 2018;Ido et al., 2019).  Distilled water was added to the homogeneous mixture and it was formed into pellets of 4 mm in diameter and 6 mm high.
The catalyst pellet was dried in a furnace at 500 °C for 2 hours, then cooled in a desiccator.

Procedures
Fifty (50) g SPRs were put into the reactor, then tightly closed, and heated with electricity. Temperature controlled with a NiCr-Ni thermocouple placed outside the reactor. Heating was carried out at a heating rate of 5-35 ºC/minute from room temperature (30 ºC) to the desired temperature (300-700 ºC). Pyrolysis gas was condensed, liquid products coming out of the condenser were collected in the accumulator, and the amount of gas production was measured. Biochar products were obtained after the experiment was completed. The amount of biochar was measured by weighing. The biochar yields were calculated by Equation (1).

Instrument and data analysis
The experiment was carried out in a fixed-  The liquid yield was collected in the accumulator, and the produced gas was measured. After the experiment finished, the remaining solid product (biochar) was taken and weighed. The bio-oil yields were calculated by Equation 1.
In this case, YC notation is the yield of charcoal products, while WM and WC are the initial SPR weighting and charcoal weight, respectively.

Silica-alumina was analyzed by Scanning
Electron Microscope-Energy Dispersive X-ray (SEM-EDX). The results are shown in Figure 2.

Biochar Yield
The biochar yield data of pyrolysis with fixed-bed reactors at various temperatures and the amount of silica-alumina is shown in

Biochar Surface Area
The effect of temperature and the amount of silica-alumina on the biochar surface area is shown in Figure 4. Based on Figure 4, the impact of temperature rise for both non-  The choice of pyrolysis temperature needs to be considered to obtain high biochar yield and surface area. According to Zheng et al. (2017), the pyrolysis of Chlorella sp. at a temperature of 600 ºC produces biochar with a surface area of 6.16 m 2 /g. Wang et al. (2013) reported that the biochar surface area of C.
Vulgaris was 2.40 m 2 /g, while Roberts et al. (2015) reported on biochar produced from macroalgae Eucheuma sp. has a much higher surface area (30.03-34.82 m 2 /g) than other species ranging from 1.29 to 8.87 m 2 /g.
Compared to this study, the biochar produced by SPR pyrolysis with silica-alumina at the same temperature, namely 600 ºC, has a much higher surface area, namely 36.91 m 2 /g.

Biochar Total Pore Volume
The effect of temperature and the amount of silica-alumina on the total pore volume are shown in Figure 5. The temperature and the amount of catalyst have significant effects on the total pore volume at 700 ⁰C. The optimum total pore volumes were obtained at a temperature of 700 ⁰C in non-catalytic and catalytic pyrolysis with 5, 10, and 20 % silicaalumina were 0.036, 0.052, 0.052, and 0.041 cm 3 /g, respectively. The total pore volume data of biochar from microalgae are not published as much as the data for the pore area. However, Chen et al. (2018) report that the pyrolysis of Spirulina platensis at 600 ⁰C with a barium (Ba) catalyst produces a total pore volume of 0.004 cm 3 /g. The effect of temperature and the amount of alumina-silica catalyst on the biochar radius can be seen in Figure 6. Based on this figure, the pore radius for temperatures of 300-400 ⁰C from non-catalytic and catalytic pyrolysis has the same trend. The rising temperature will increase the pore radius, but at a temperature of 700 ⁰C, the pore radius drops sharply. The correlation between surface area and pore radius is that the higher temperature increases the surface area; otherwise, the pore radius will decrease (Yu et al., 2017b (Jung et al., 2016), while the surface area is increasing (Bordoloi et al., 2016;Norouzi et al., 2016). Bordoloi et al. (2016) reported that an increase in temperature from 300-600 ⁰C affected the increase in surface area from 1.72 to 123 m 2 /g.

Conclusions
Non-catalytic pyrolysis produces bio-oil with a high content of oxygenated compounds, so the use of silica-alumina catalysts improves the bio-oil quality.
Catalytic pyrolysis will affect the quality of biochar, such as surface area, total pore volume, and pore radius. Based on the biochar characteristics, the optimum conditions were obtained at 700 ⁰C by catalytic pyrolysis with 5 % of silicaalumina, which obtained the surface area, total pore volume, and radius pore as 36.91 m 2 /g, 0.052 m 3 /g, 2.68 nm, respectively.