Fugitive methane emissions from a rice straw biogas facility using inverse-dispersion technique

Current practices of rice straw management in the Philippines are an alarming concern as soil incorporation and burning result to significant amounts of greenhouse gas emissions. One of the innovations that are being explored globally is the utilization of rice straw as a substrate to produce biogas through anaerobic digestion. However, given the high global warming potential and long atmospheric residence time of methane, it needs to be captured and stored to prevent unwanted emissions into the atmosphere. With this, methane losses in terms of fugitive emissions of the 917 m3 rice straw biogas facility utilizing rice straw and cow manure as co-substrates at Victoria, Laguna, Philippines were quantified through inverse dispersion method using an openpath laser system. Upon simulation of the gathered data, the resulting overall average emission rate is 1.25 kg CH4/hr, which varied from 0.03 to 22.09 kg/hr, and a total emission of 522.48 kg CH4 throughout the 120-day run of the anaerobic digester. This covers the periods where there were site activities and those without any activity being conducted inside the facility. Periods with no significant site activity recorded an average rate of 0.15 kg CH4/hr and a total CH4 emission of 427.48 kg, equivalent to 2,820.05 hours. On the other hand, site activities were conducted before and during the opening and closing of the digester, which ran for an accumulated period of 59.95 hours. These site activities include biogas collection, sludge withdrawal, rinsing of liquid digestate and draining of washings, removal of fittings, storing of biogas from gas bags to scrubbers, and opening and closing of the digester. Removal of fittings emitted the highest amount of CH4 at a rate of 8.22 kg/hr on average, while the withdrawal of sludge recorded the lowest average rate of 0.52 kg CH4/hr. The conduct of these activities logged an average emission rate of 1.60 kg CH4/hr and a total emission of 95.00 kg CH4. Furthermore, investigating the effect of the net concentration on emission rate generated a positive linear correlation between the two variables, with an R2 of 0.98 and an R of 0.99.