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非甲烷總烴( NMHC) 表征VOCs濃度的若干問題
眾所周知,非甲烷總烴( NMHC) 作為表征廢氣中VOCs 濃度的綜合性指標仍是各類標準的主流,廣泛用于VOCs固定污染物總量控制、處理設施治理效率、末端排放達標等的評價。雖然2017年NMHC 的修訂稿中對非甲烷總烴的定義、標準氣體的選取、色譜柱的選擇有所更新,但NMHC受FID檢測原理的局限,存在無法完全準確地反映每種揮發性有機物的污染程度的問題。即如果用NMHC來表征廢氣中VOCs的濃度,會存在不準確的情況。

有一項研究選擇甲烷、甲苯、苯、甲醇、正己烷、乙酸乙酯、丙酮、乙醚、乙腈為代表性VOC物種開展研究,探討論證了這9種有機化合物在FID氫火焰離子檢測器上的相對響應值情況。用《固定污染源廢氣總烴、甲烷和非甲烷總烴的測定氣相色譜法》( HJ 38-2017) 監測方法分別測定上述9種有機化合物的響應值,以甲烷的單位摩爾碳原子響應值為基準計算各有機化合物相對單位摩爾碳原子響應因子。9 種揮發性有機物在FID 上響應結果如下所示。從中可以看出,各化合物相對單位摩爾碳原子響應因子有較大差異,其范圍在0. 36-1. 02。

由于《固定污染源廢氣總烴、甲烷和非甲烷總烴的測定氣相色譜法》( HJ 38-2017) 用甲烷作為標準氣體,因此各組分相對甲烷的響應因子差異直接導致此類化合物測定結果的差異。因此,NMHC并不能完全準確衡量固定污染源中VOCs的總量,尤其在分析含氧類揮發性有機物( OVOCs) 時測定結果普遍偏低。用非甲烷總烴指標來衡量OVOCs 排放為主的化工企業時,廢氣中揮發性有機物的濃度可能被低估。

從檢測原理分析,運用氫火焰離子化檢測器( FID) 檢測時,有機物通過氫火焰裂解為CH·等自由基,再氧化成CHO+正離子及自由電子,在儀器電場作用下,產生與摩爾碳原子濃度成正比的電流信號而被檢測器定量檢測。因此,有機化合物在FID中裂解產生CH·自由基的效率直接決定了其的單位摩爾碳原子響應值差異。在化合物中存在羥基、羧基等官能團時,會影響氫火焰裂解化合物產生CH·自由基,從而導致其相對響應值下降,使最終的監測結果偏低。

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