Comparative Study on Biogas Yields of Plug Flow and Batch Reactors fed with Rice Straw at Mesophilic Temperature

Abstract

The present energy crisis has stimulated various research activities to evaluate energy potentials of renewable energy sources. Improper disposal of rice straw constitutes nuisance to our environment especially in most rice producing states of Nigeria. To this end, the biogas potential of rice straw was evaluated in Plug Flow Reactor (PFR) and Batch Reactor (BR) operated at mesophilic temperature (37^oC). A 140-litre capacity PFR was filled with 105 litres previously digested sludge as inoculum. The loading of the rice straw into the reactor was commenced at a low L_R of 0.5 goTS/l.d and was increased weekly by 0.5 goTS/l.d until a L_R of 4.5 goTS/l.d was attained. For the batch reactor, 10.15 g of rice straw was loaded into each of the two digestion vessels. Two additional vessels were filled with inoculum as control. For the PFR, the biogas produced was collected using gas bags and then measured with a multi-chamber rotor gas meter. The gas analyzes was done with an Infrared Gas Detector. The scale wet gas collectors were used for the batch reactor and the gas measurement done with the gas analyzer (Model GA 2000). It was established that the biogas yields increased with increase in L_R (PFR).  The highest biogas yields of 0.20 l/goS and 0.167 l/goS were recorded in PFR and BR, respectively. In all, continuous feeding using PFR (at optimum L_R) has been established to be better than batch feeding using BR at mesophilic temperature.

Keywords: Renewable energy; rice straw; Biogas; Loading rate; Plug flow reactor