Torrefaction of empty fruit bunches (EFB) was conducted in the presence of mixed air and nitrogen to obtain desired oxygen concentrations between 2.5 and 10%. The effect of oxygen concentrations (0–10 vol.%) and temperatures (240–300°C) on the chemical properties and pyrolysis behaviors of torrefied products were investigated. The mass yield decreased significantly with increasing oxygen concentration and temperature. When the oxygen concentration rose from 0 to 10%, the carbon content in EFB torrefied at 300°C increased from 64.64 to 66.91 wt%. Simultaneously, the oxygen content in EFB decreased, reaching as low as 25.78 wt%. The higher heating value of the torrefied EFB at 300°C was remarkable, as high as 27–28 MJ/kg. The carbon and energy yields of torrefied EFB decreased as the oxygen concentration rose to 10%. However, these values remained above 45% and were higher than the torrefied mass yield. The response surface methodology revealed that low air was suitable for the proposed torrefaction. The pyrolysis behavior results showed that the presence of oxygen during torrefaction enhanced the biochar yield of torrefied EFB. The observed phenomenon might be attributed to the occurrence of cross-linking reactions during pyrolysis promoted by the presence of oxygen during torrefaction. The results of thermogravimetric and XRD analyses of cellulose suggested that oxygen played a vital role in modifying the cellulose structure in EFB, resulting in a higher char yield. These effects were essential for optimizing the torrefaction process under the limited air concentration and enhancing the biochar production from the torrefied biomass.

Graphical abstract

中文翻译：

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温和空气条件对空果早熟烘焙过程中化学性质和热解行为的影响

在空气和氮气混合的情况下对空果串进行烘焙 (EFB)，以获得所需的 2.5% 至 10% 之间的氧气浓度。研究了氧气浓度（0–10 vol.%）和温度（240–300°C）对烘焙产品的化学性质和热解行为的影响。随着氧气浓度和温度的增加，质量产率显着下降。当氧浓度从0%增加到10%时，300℃烘焙的EFB中的碳含量从64.64%增加到66.91%。同时，EFB中的氧含量下降，低至25.78wt%。烘焙后的 EFB 在 300°C 时的热值非常显着，高达 27-28 MJ/kg。当氧浓度升至 10% 时，烘焙 EFB 的碳产率和能量产率下降。然而，这些值保持在 45% 以上，并且高于烘焙后的质量产量。响应面方法显示低空气适合拟议的烘焙。热解行为结果表明，烘焙过程中氧气的存在提高了烘焙 EFB 的生物炭产量。观察到的现象可能归因于烘焙过程中氧气的存在促进了热解过程中交联反应的发生。纤维素的热重和 XRD 分析结果表明，氧气在改变 EFB 中的纤维素结构方面发挥着至关重要的作用，从而获得更高的炭产率。这些效应对于优化有限空气浓度下的烘焙过程以及提高烘焙生物质的生物炭产量至关重要。

图形概要