Novel high rate system for biogas production from energy crops

Methods

For a test run a laboratory scale ULB system was constructed and operated continuously other several months.





 Experimental design

The laboratory scale ULB system consisted of an ULB reactor and two fixed bed reactors FBR1 and FBR2. Main emphasis was put on analysing and improving the efficiency of the ULB reactor with a working volume of 26.5 liters. The working volume of the FBR with 78 liters in total was therefore designed generously.

Reactor feeding and withdrawal of the solid residues were carried out manually. In order to achieve an even distribution of newly feed biomass a magnetic stirrer at the bottom of the ULB reactor was used. Stirring was limited to the short periods of feeding and did not effect the leach-bed. For practical application the use of high pressurised hydraulic jets seams to be an appropriate solution.
For three phase separation the top of the ULB reactor was equipped with a filter in shape of a funnel with overlapping ring elements. Leachate circulation between the reactors was carried out by a peristaltic pump. The process temperature in each reactor was kept thermophilic at 55°C. As substrates two different types of maize silage were used: maize silage 1 (total solids TS = 33.1 %, volatile solids VS = 96.7 % TS) for the first 27 days of testing, followed by maize silage 2 (TS = 34.9 %, VS = 95.9 % TS). To maintain the texture of the leach-bed, barley straw was added to the maize silage at a mass based rate of 2 to 5 %. Batch fermentation tests in 2 litre bottles were carried out to reveal the methane yield potential for maize silage 1 (415 l kg^-1), maize silage 2 (364 l kg^-1) and straw (334 l kg^-1).
During the experiment reactor loading rates (OLR) of the ULB reactor were increased stepwise from 6.3 to 16 g l^-1d^-1. Besides other parameters special emphasis was giving to determinate the methane production of each reactor.

Results

The increase of the OLR from 6.3 to 16 g l^-1d^-1 led to higher total rates of methane production  ( Figure). The methane yield of pure maize silage decreased at the same time from 409 l kg^-1 at OLR 6.3 to 332 l kg-1 at OLR 16. Considering the different methane yield potentials of maize silage 1 and 2, the efficiency dropped only slightly from 98 to 91 %. After the feeding of substrate was stopped at day 60, methane production declined quickly.
The results show further that while the OLR was increased main production of methane shifted from the ULB reactor to the FBR. Regarding the total methane yield the share belonging to FBR rose from 10 % at OLR 6.3 to 75 % at OLR 16. Thus it can be concluded that the ULB reactor can handle at least a OLR of 6.3 l kg^-1 d^-1 on its own, while substantially higher OLRs induce an essential need for additional high rate systems. On basis of further results, it is assumed that the size of the high rate reactor can be reduced to 30 % of the size of the ULB reactor. Considering the calculated first order hydrolysis rate of 0.14 d^-1 degradation of maize silage was 5 times faster than in classic digesters at mesophilic temperatures (Linke and Mähnert 2005).

Testing