An open chamber technique for determination of methane emission from stored livestock manure

Abstract

The feasibility of an open chamber technique for measuring CH 4 emissions from farm scale pig slurry (PS) and pig solid manure (PSM) storages was examined. Emission rates were over-estimated considerably when the ratio between the area of the inlet and outlet ports was below 5 and 8 for chambers operated on PS and PSM, respectively. A first-order equation describing the chamber ventilation rate necessary to obtain steady state conditions showed high correlation with experimental data. For ventilation rates of 2–3 cm min −1 steady state was established after 40–54 min. The CH 4 emission from PS was controlled by the transfer resistance in the liquid phase leading to a low effect of wind speed. In contrast, the CH 4 emission from PSM was controlled by the gas-phase resistance and thus strongly dependent on wind speed. A very high spatial variability in CH 4 emissions was found for both PS and PSM storages. Coefficients of variation for emission rates during one day in March 1992 were 371% and 465% for PS and PSM, respectively. Daily CH 4 emission rates from PS ranged from 0.5 to 49.8 g CH 4 m −3 d −1 during a 19-day period in April–May 1992. The variation depended strongly on storage operations, such as loading and stirring prior to field application. Daily CH 4 emission rates from PSM ranged from 17.9 to 92.0 g CH 4 m −3 d −1 during a 31-day period in June–July 1992, the rate being correlated with PSM temperatures. Due to the large spatial and temporal variability in the CH 4 emission rate from PS and PSM, it is concluded that the accuracy of annual emission estimates is limited, when based on a few seasonal measurements by open chambers. It is recommended that annual CH 4 emission estimates as a minimum are based on weekly determinations of CH 4 emission rates.