This study presents a quasi-online method for monitoring of dissolved volatile fatty acids (VFAs) in biogas fermentation processes with a carrier gas probe by use of thermo-cyclically operated metal oxide gas sensor arrays. Each of the two sensor arrays comprises a pure SnO₂ and three different SnO₂/additive-composites (additives: alumina, YSZ, NASICON) but differ by SnO₂ synthesis routes, namely Flame Spray Pyrolysis (FSP) and Sol-Gel (SG) technique, respectively. This allowed comparative studies of the influence of layer morphology on VFA sensing characteristics. For sensitive determination of the dissolved VFAs besides high concentrations of biogas components like CO or CH₄, first a pre-treatment routine of the fermentation sample was introduced to remove those physically dissolved gases without losing VFAs. The Conductance-over-Time-Profiles (CTPs) of eight different sensing layers were measured simultaneously at exposure to the gases extracted from the fermentation sample at different pH conditions. Almost all the investigated SnO₂/additive-composites show CTP-features clearly correlating with the undissociated VFA even at concentrations below 120 ppm as referenced by GC-analysis. The lower detection limit is well below inhibitory concentration for fermentation processes. As expected, most pronounced CTPs representing actual VFAs situation were measured at pH 3, well below the pKa of the VFAs. The FSP-layers highlighted clearly better sensitivity and CTP specificity of higher quality compared to SG-layers. Among the SnO₂/additives, the CTP-features of the SnO₂(FSP)/NASICON and SnO₂(SG)/NASICON layers showed the best specificity to acetic and propionic acid. For the first time, quasi-online analysis of VFAs using metal oxide gas sensors for early warning of VFA-development in biogas fermentation processes was demonstrated.

本研究提出了一种准在线方法，通过使用热循环操作的金属氧化物气体传感器阵列，使用载气探针监测沼气发酵过程中溶解的挥发性脂肪酸（VFA）。两个传感器阵列中的每一个均包含纯SnO ₂和三种不同的SnO _{2 /添加剂复合材料（添加剂：氧化铝、YSZ、NASICON），但SnO 2}合成路线不同，即火焰喷射热解（FSP）和溶胶-凝胶（SG） ）技术，分别。这使得可以比较研究层形态对 VFA 传感特性的影响。为了灵敏地测定除高浓度沼气成分（如 CO 或 CH _{4 ）}之外的溶解 VFA ，首先引入发酵样品的预处理程序以去除那些物理溶解的气体而不损失 VFA。在暴露于不同 pH 条件下从发酵样品中提取的气体时，同时测量八个不同传感层的电导随时间变化 (CTP)。几乎所有研究的 SnO ₂ /添加剂复合材料都显示出 CTP 特征与未解离的 VFA 明显相关，即使在 GC 分析所参考的浓度低于 120 ppm 的情况下也是如此。检测下限远低于发酵过程的抑制浓度。正如预期的那样，代表实际 VFA 情况的最明显的 CTP 是在 pH 3 下测量的，远低于 VFA 的 pKa。与 SG 层相比，FSP 层明显具有更好的灵敏度和更高质量的 CTP 特异性。在SnO _{2 /添加剂中，SnO 2} (FSP)/NASICON和SnO ₂ (SG)/NASICON层的CTP特征显示出对乙酸和丙酸的最佳特异性。首次展示了使用金属氧化物气体传感器对 VFA 进行准在线分析，以对沼气发酵过程中 VFA 的发展进行预警。