Pyrolysis of biomass residues into biochar is seen as a feasible way to mitigate climate change by biological carbon storage (carbon dioxide removal, CDR) and to substitute fossil fuel with sustainable biofuel. This study applies a combination of flash and ramp heating pyrolysis, and organic petrography to investigate the hydrocarbon (biofuel) potential and biochar stability and morphotypes of eight brown, red, and green macroalgal species of different tissue complexity. The carbon stability of biochar derived from macroalgae has not previously been assessed using organic petrography (reflectance measurements) and evaluated in the context of the geological carbon cycle. The biochar, hydrocarbon, and CO + CO yields vary due to different chemical composition of the macroalgal species, but the product yield variations are not related to the brown, red, or green macroalgal groups. The total biofuel yield shows an inverse trend with biochar yield. A slower heating rate produces more biochar and higher CO + CO and lower biofuel yields than the combined flash pyrolysis and faster heating rate. The morphotype composition of the biochar was qualitatively examined by reflected light microscopy while carbon stability was assessed by random reflectance (R) measurements. The diverse morphotype compositions observed in biochar formed under similar pyrolysis conditions likely stem from variations in the original algal composition. While some biochar samples show morphologies resembling the original macroalgal structure, porous morphotypes predominantly characterize the biochar samples overall. Despite a maximum pyrolysis production temperature (PT) of 650 °C, the highest mean R value among all biochar samples is 2.91%, corresponding to a carbonization temperature (CT) of 526 °C. This observation is tentatively related to the less lignocellulosic structure of the macroalgae compared to terrigenous biomass. Four biochar samples have their entire R distribution range above the inertinite benchmark (IBR2%) of R = 2% indicating high carbon stability. Conversely, the remaining four biochar samples exhibit R distributions extending below IBR2%, indicating the presence of a carbon fraction with lower long-term stability in soil. The statistically significant inverse relationship observed between the mean R values and the peak hydrocarbon generation temperature (T) can be attributed to the behavior of residual macromolecules within the biochar. When these macromolecules reach peak biofuel generation at a lower temperature, they undergo carbonization over a more extended time interval during pyrolysis. Consequently, this prolonged exposure to the pyrolysis process leads to higher degrees of carbonization, as reflected by higher R values. In conclusion, the findings from pyrolysis and organic petrography reveal: (1) Macroalgae demonstrate potential for biofuel production, although biofuel yields contingent upon both the species of macroalgal and the heating rate employed, and (2) This study documents for the first time that flash+ramp pyrolysis of macroalgae yields biochar suitable for long-term carbon storage. However, both the carbon stability inferred from R frequency distributions and biochar yields show variations across different macroalgal species and heating rate.

中文翻译：

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大型藻类的热解：深入了解产品产量以及生物炭形态和稳定性

将生物质残留物热解成生物炭被视为通过生物碳储存（二氧化碳去除，CDR）缓解气候变化以及用可持续生物燃料替代化石燃料的可行方法。本研究应用闪蒸和斜坡加热热解以及有机岩相学相结合，研究碳氢化合物（生物燃料）潜力和生物炭稳定性以及不同组织复杂性的八种棕色、红色和绿色大型藻类的形态类型。以前从未使用有机岩相学（反射测量）评估源自大型藻类的生物炭的碳稳定性，​​也未在地质碳循环的背景下进行评估。生物炭、碳氢化合物和 CO + CO 产量因大型藻类化学成分的不同而变化，但产物产量变化与棕色、红色或绿色大型藻类无关。生物燃料总产量与生物炭产量呈相反趋势。与快速热解和较快加热速率相结合相比，较慢的加热速率会产生更多的生物炭和更高的 CO + CO，但生物燃料产量较低。通过反射光显微镜定性检查生物炭的形态组成，同时通过随机反射率（R）测量评估碳稳定性。在相似的热解条件下形成的生物炭中观察到的不同形态类型组成可能源于原始藻类组成的变化。虽然一些生物炭样品显示出类似于原始大型藻类结构的形态，但多孔形态类型主要表征了生物炭样品的整体特征。尽管最高热解生产温度 (PT) 为 650 °C，但所有生物炭样品中的最高平均 R 值为 2.91%，对应于 526 °C 的碳化温度 (CT)。这一观察结果暂时与大型藻类与陆源生物质相比木质纤维素结构较少有关。四种生物炭样品的整个 R 分布范围均高于惰性基准 (IBR2%)（R = 2%），表明碳稳定性较高。相反，其余四个生物炭样品的 R 分布延伸至 IBR2% 以下，表明土壤中存在长期稳定性较低的碳组分。平均 R 值和峰值烃生成温度 (T) 之间观察到的统计上显着的反比关系可归因于生物炭内残留大分子的行为。当这些大分子在较低温度下达到生物燃料生成峰值时，它们在热解过程中会在更长的时间间隔内经历碳化。因此，长时间暴露于热解过程会导致更高程度的碳化，如更高的 R 值所反映的那样。总之，热解和有机岩相学的研究结果表明：（1）大型藻类显示出生产生物燃料的潜力，尽管生物燃料产量取决于大型藻类的种类和所采用的加热速率，(2) 本研究首次证明大型藻类的闪蒸+斜坡热解可产生适合长期碳储存的生物炭。然而，从 R 频率分布推断的碳稳定性和生物炭产量都显示出不同大型藻类和加热速率的差异。