Biogas in Denmark: A typical Centralized Biogas Plant
Main Results of the investigation
Conclusions
Location of Centralized biogas plants in Denmark
Typical centralized biogas plant
Ribe biogas plant
Full-scale investigation
Process stability
Process efficiency
Microbiology
In manure based biogas plants typically only 50-70 % of the organic matter is converted to biogas despite rather long average retention times. Some of the residual organic matter is recalcitrant and can not be digested. However, some degradable material is also lost with the effluent from the main digestion step, which is most often continuously stirred reactor tanks (CSTR’s). The reason for this loss of degradable matter is due to “short-circuit” of a portion of the feed which is staying in the reactor for a much shorter time than the nominal retention time.
Potential methods to improve recovery efficiency from manure
Increase stabilily of the digestion process
Pre-treatment of incoming substrate (to increase degradability)
Increase retention time of the manure reactor(s)
Arrange post-digestion systems (to increase degradation efficiency)
The last 5-8 years many biogas plants have installed gas collection systems in after-storage tanks, which in this way are functioning as post-digesters. However, temperature, retention time and mixing conditions as well as the degree of methane recovery experienced vary for each plant. Only a few plants have worked with extended retention time at full main process temperature in post-digesters.
Stability in Danish Biogas Plants
Correlation between ammonia and VFA.
Residual methane production
55oC
25oC
20oC
15oC
55oC 55oC
25oC
20oC
15oC
Residual methane production
Residual methane production
Restgastab i forhold til total produktion
>15% 10-15% < 10%
Temperature and residual methane
Distribution of the total methane potential
Main reactor residual methane loss versus retention time
Temperature effect on residual biogas potential
INCREASING TEMPERATURE Samples previously incubated at 10 -15C for a long period have been moved to process temperature (37-54C):
INNOVATIVE SOLUTIONS SITUATION:
The majority of the biogas plants have Reactors with good efficiency;
Many plants are loosing a lot and the gap between the theoretical and the practical potential is still wide; MAIN OBSTACLE: hydrolysis is the real rate limiting step for the further methanogenesis increase HRT to provide a better substrate hydrolysis SOLUTIONS: TRADITIONAL SOLUTIONS INNOVATIVE SOLUTIONS New process configurataion: Utilization of the After-Storage.
Increase the HRT in main reactor
Conclusions
Significant amounts of CH4 are lost (5-30%)
Plants with HRT< 15 days are lossing more CH4 from the main reactor
Post-digestion is highly influenced by the temperature
Post-digestion at low temperature are increasing the total methane potential of the material
Manure-plants are dominated by Methanosarcina, while sludge plants by Methanosaeta.
Acknowledgements
The study was funded by the Danish Energy Agency,“Development of Renewable Energy”
The operational staff of the Biogas Plants participating in the investigation is greatly acknowledged
Researchers:
Kanokwan Boe
Lars Ellegaard
Dimitar Karakshlev
Damien Batstone
Irini Angelidaki
Students:
Simone Labo
Lucía Fernández García
Eva Arler
He Zhen
Chao Pan
Troels Hilstrøm
Søren H. Laursen
Technicians:
Hector Garcia
Majbrit Staun Jensen
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