影响静态箱检测开放式气体排放源N2O排放通量的关键因子

为研究影响静态箱检测开放式气体排放源氧化亚氮(N2O)排放通量的关键因子,以提高静态箱检测气体排放通量的准确性,该文在实验室条件下,探究了箱体配置(有无通气孔、有无风扇)和检测条件(不同密闭时间:30、40、50和60 min;不同排放源表面风速:0、0.5、1.0、1.5和2.0 m/s)对300 mm(直径)×300 mm(高度)(D300 mm×H300 mm)的静态箱检测N2O排放通量准确性的影响规律。结果表明,不同配置的静态箱测量结果偏差率随时间的变化趋势均相同,其中有通气孔和风扇的箱体在不同风速下的检测稳定性较好,检测准确性最高。当排放源表面风速为0~2 m/s时,风扇对静态箱检测准确性无显著性影响,排放源表面的风主要通过通气孔影响静态箱的检测准确性。静态箱检测的气体排放通量与实际排放通量的偏差率随排放源表面风速和箱体密闭时间的增加而显著降低。该试验推荐在排放源表面风速小于2 m/s的无粪便堆积的奶牛运动场以及排放源介质相似的开放式气体排放系统中使用有通气孔和风扇的静态箱对N2O排放通量进行检测,密闭50 min。

Key factors affecting the measurement of N2O emission from dairy farm using static-chamber method

Open gas emission sources, such as open dairy lot and manure stockpile, are still challenge to directly measure the gas emissions, due to their fully open nature and the relatively low flux of gas emission, particularly interfering by other emissions sources, such as barns and animals. The detection accuracy of the commonly used closed-chamber method depends on the chamber configuration and the different external environment. Four key parameters were evaluated, including the disturbing fan, vent holes, surface wind speed of emission(0.0, 0.5, 1.0, 1.5, and 2.0 m/s) and deployment time(0 to 60 min) in the 300 mm(diameter) × 300 mm(height)(D300×H300) closed chamber using nitrous oxide(N2 O) as reference gas. The experiment was carried out in a wind tunnel to adjust the wind speed in order to simulate the real environment of open dairy lots. A calibration system was designed to generate a reference flux, and the accuracy of chamber performance was defined based on the difference between the reference fluxes and the calculated fluxes in the closed chamber. The results showed that the deviation rates have the similar trends during the deployment time in the closed chambers with different configurations. The flux of gas emission that measured by the closed chamber was higher at the beginning of process, and then lower compared with that of the reference flux. The measurement accuracy of the closed chamber reached the maximum when the deployment time was 50 min, where the deviation rate of the closed chamber without the disturbing fan and vent was 1.02%--29.06%, 12.29%--47.92% without the disturbing fan and with the vent,-9.71%--40.92% with the disturbing fan and without the vent, and 4.42%--25.64% with the disturbing fan and vent. There was no significant difference in the deviation rates of the detected N2 O emission fluxes under different wind speeds(P>0.05) using the closed chamber with the disturbing fan and with/without vent, indicating these two types of chambers have better detection stability. However, the measurement accuracy of the D300 mm×H300 mm closed chamber with the disturbing fan and vent was significantly higher than that with the fan and without vent(P<0.05). When the emission speed of surface wind was 0-2 m/s, the disturbing fan had no significant influence on the measurement accuracy of the closed chamber(P>0.05), indicating the emission surface wind was affected by the Venturi effect through the vent. Both the deployment time and emission surface wind speed had significant negative correlation on the deviation rate of the closed chambers(P<0.05). However, the correlation between the deployment time and emission surface wind speed was not obvious(P>0.05). This study recommends to use a D300 mm×H300 mm closed chamber with the disturbing fan and vent to detect the N2 O emission flux in an open gas emission system, such as dairy open lots without manure and emission sources with similar media, with the speed of emission surface wind less than 2 m/s, and the deployment time of 50 min.