Sea Grape Waste Turned Into Coal-Like Fuel, With a Trade-Off

Researchers have turned a slimy byproduct of sea grape farming into a coal-like solid fuel, and mapped exactly how far the process can be pushed before the gains evaporate. Caulerpa lentillifera, the glossy green algae sold as sea grapes or green caviar, leaves behind piles of trimming residue after harvest. A team led by Kanit Manatura and Supachai Jadsadajerm reports in Cleaner Engineering and Technology that a mild heat treatment called torrefaction can upgrade this waste into a denser, more stable fuel, but the choice of gas surrounding the biomass during heating changes the outcome dramatically.

The study compared three atmospheres: inert nitrogen, ordinary air, and carbon dioxide. Each was tested across torrefaction temperatures of 250 to 300 degrees Celsius and residence times of 15 to 45 minutes in a laboratory fixed-bed reactor holding 5-gram samples. Rather than optimizing a single property, the researchers used response surface methodology with a desirability function that weighed five responses simultaneously: fuel ratio, ash content, atomic oxygen-to-carbon ratio, higher heating value, and an energy-mass co-benefit index that captures how much energy survives the mass loss inherent to the process.

The raw residue was a poor fuel by every measure. High volatile content drives rapid devolatilization and unstable flames during combustion, while the ash burden raises the risk of slagging and fouling in boilers. Aquatic feedstocks carry additional complications because alkali metals such as sodium and potassium can lower ash fusion temperatures, intensifying operational problems in combustion equipment.

Torrefaction changed the picture in every atmosphere tested. Heating drove off volatile matter and concentrated the carbonaceous fraction, raising fixed carbon and lowering the oxygen-to-carbon ratio in a trajectory the authors describe as coalification. Carbon dioxide behaved almost identically, reaching 58.70 percent carbon under the same conditions, which the authors interpret as the gas acting predominantly as an inert medium within this temperature window, with no evidence of appreciable carbon dioxide-carbon gasification below 300 degrees Celsius.

Air told a different story. Because oxygen accelerates devolatilization through exothermic surface oxidation, air torrefaction stripped mass far faster: solid yields dropped to a range of 42.33 to 62.67 percent, compared with 59.07 to 83.38 percent under nitrogen. Yet the same chemistry produced the fastest fuel upgrading. The highest fuel ratio measured, 0.93, came from air at 300 degrees Celsius for just 15 minutes, whereas nitrogen needed the full 45 minutes at the same temperature to reach 0.61.

The multi-objective optimization distilled these trade-offs into three distinct operating routes rather than a single winner. Air achieved its optimum at a much milder 279 degrees Celsius for 15 minutes, a threefold reduction in residence time, with a predicted fuel ratio of 0.81. Carbon dioxide landed in between, at 300 degrees Celsius for 34 minutes. Sensitivity analysis across four alternative weighting schemes shifted the optima by less than 2 degrees Celsius and 3 minutes, confirming the results were not artifacts of the chosen priorities.

Experimental validation exposed a critical nuance for the air route. The measured fuel ratio of 0.77 under air still substantially exceeded the validated values of 0.61 under nitrogen and 0.56 under carbon dioxide, confirming air as the fastest path to a coal-like product. But the validated energy-mass co-benefit index under air came in at 14.32 percent, below the 15 percent threshold the researchers had set as an economic floor, meaning the air condition technically fails their energy-retention criterion despite its speed advantage. Nitrogen delivered the highest validated energy retention at 25.80 percent, with carbon dioxide close behind at 17.82 percent.

Bomb calorimetry provided an independent check on the heating values used in the modeling. Directly measured higher heating values on a dry, ash-free basis were 27.18 megajoules per kilogram for the nitrogen-optimized char, 24.98 for air, and 24.83 for carbon dioxide, agreeing with correlation-based estimates within roughly 2.4 to 2.9 percent. When recalculated on a dry basis to reflect the actual ash-containing fuel, those values dropped to 17.35, 15.89, and 15.91 megajoules per kilogram respectively, against 14.24 for the raw residue. The gap between ash-free and dry-basis values quantifies how much the inorganic fraction dilutes the energy density even after upgrading.

Combustion testing added another layer to the comparison. Ignition temperature rose from 230.6 degrees Celsius in the raw material to between 283.8 and 291.9 degrees Celsius across the torrefied chars, suggesting improved resistance to spontaneous ignition during storage. Among the optimized chars, the air-derived sample showed the highest combustion reactivity, with a maximum combustion rate of 3.72 percent per minute and a comprehensive combustibility index of 0.33 times ten to the minus seven, likely aided by a more disrupted, cracked surface and the highest BET surface area at 7.098 square meters per gram. The authors caution this does not mean the treated fuel burns better than the untreated biomass, which ignited more readily and reacted faster overall.

The ash problem remains the central limitation. X-ray fluorescence analysis showed the ash was dominated by silicon, calcium, iron, manganese, and aluminum, with broadly similar elemental profiles before and after treatment, indicating torrefaction concentrated pre-existing minerals rather than creating new ones. The authors emphasize that the optimized conditions, derived from 5-gram fixed-bed experiments with poor heat-transfer scalability, should be treated as laboratory-scale operating windows requiring pilot-scale verification, particularly for air torrefaction, where temperature control and the risk of oxidative thermal runaway become more consequential at larger reactor sizes. Ash-fusion behavior, mineral-phase transformations, and deposition characteristics also need further study before any boiler application can be claimed. Potential mitigation strategies, including blending with low-ash biomass or deashing pretreatment, remain untested future work.

Subject of Research: Optimizing torrefaction of Caulerpa lentillifera residue under nitrogen, air, and carbon dioxide atmospheres to balance fuel upgrading, ash enrichment, and energy retention.

Article Title: Atmosphere-dependent torrefaction of Caulerpa lentillifera residue: multi-objective optimization for balancing fuel upgrading, ash enrichment, and energy retention

Article References: Manatura, K., Kaewtrakulchai, N., Loylom, J., Veerapasant, N., Wisetsai, A., Pimsamarn, J., Huang, C.-W., Laosiripojana, N., & Jadsadajerm, S. (2026). Atmosphere-dependent torrefaction of Caulerpa lentillifera residue: multi-objective optimization for balancing fuel upgrading, ash enrichment, and energy retention. Cleaner Engineering and Technology, Article 101347. https://doi.org/10.1016/j.clet.2026.101347

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101347

Keywords: torrefaction, Caulerpa lentillifera, sea grapes, biomass, solid fuel, response surface methodology, fuel ratio, ash content, energy yield, macroalgae, combustion, CO2