Thermal Performance Analysis and Process Optimization for Converting Marine Biomass (Seaweed) into Pyrolysis Oil

Environmental, economic, and political pressures have heightened interest in sustainable feedstocks for biodiesel production. Seaweed, encompassing green, brown, and red marine macroalgae, has gained significant attention as a sustainable biomass resource for producing food, biodiesel, and biochemicals. Among the various methods of conversion, thermochemical conversion, particularly pyrolysis, has been extensively researched for oil production from seaweed. Factors such as temperature, heating rate, particle size, residence time, and catalyst usage critically influence the yield and quality of pyrolysis products. 

This study investigated the pyrolysis of seaweed as a sustainable energy source, aligning with the United Nations Sustainable Development Goals (UN SDGs). It compared the performance of seaweed-derived pyrolysis oil to that from other organic wastes, namely macadamia nutshell (MNS), municipal green waste (MGW), and beauty leaf fruit husk (BLFH), and an inorganic waste high-density polyethylene (HDPE) which poses significant environmental challenges because of its non-biodegradability nature. While these feedstocks expand the scopes of this study, these feedstocks were chosen because of their availability, unique properties and potential for oil production for engine applications. The research employs thermal pyrolysis in a batch reactor, focusing on temperature, residence time, and stirring speed as key parameters. The process optimization was conducted using response surface methodology (RSM) and Box-Behnken design. 

Seaweed pyrolysis yielded 42.93% oil at 475°C, containing a range of oxygenated compounds, aromatics, and phenolic substances. This positions seaweed as a promising candidate for pyrolysis oil production due to its high carbohydrate content and rapid growth rate. However, the analysis revealed that seaweed-derived pyrolysis oil has higher viscosity, density, and lower calorific value compared to standard fuels, necessitating further refinement for engine compatibility. 

The comparison of seaweed pyrolysis with the pyrolysis of other organic wastes revealed that seaweed and BLFH peaked at 475°C with oil yields of 42.93% and 42.75%, respectively, while MGW and MNS yielded 44.72% and 45.09% at 447°C and 500°C, respectively. Despite their higher viscosity, density, and lower calorific value, these oils show potential as heat sources in boilers and furnaces. The distinct compositions of seaweed-derived oil, which include a significant presence of phenolic and oxygenated compounds, indicate its potential for producing valuable bio-chemicals in addition to oil. 

The research also optimized the thermal pyrolysis process for seaweed using RSM, achieving a yield of 42.94% oil at 463.13°C, a residence time of 65.75 minutes, and a stirring speed of 9.74 rpm. Temperature emerged as the most critical factor for maximizing oil yield. The pyrolysis oil, rich in phenols, aromatics, and alcohols, requires refining to reduce viscosity and water content for broader applications. These findings emphasize the potential of seaweed as a sustainable pyrolysis oil source, with further refinement and optimization necessary to enhance its applicability. 

Future research should focus on optimizing seaweed pyrolysis and upgrading techniques, such as hydrodeoxygenation, catalytic cracking, and esterification, to enhance the quality and usability of seaweed-derived pyrolysis oil. By improving the conversion technologies and conducting comprehensive life-cycle assessments, seaweed could play a crucial role in sustainable energy production, supporting carbon capture, nutrient recycling, and waste management aligned with the UN SDGs. This study lays the groundwork for future investigations into refining seaweed-derived pyrolysis oil and exploring its full potential as a versatile and sustainable energy resource.