Raw biogas contains approximately 30-45% of CO2, and some H2S and other compounds that have to be removed prior to utilization as natural gas, CNG or LNG replacement. Removing these components can be performed by several biogas upgrading techniques. Each process has its own advantages and disadvantages, depending on the biogas origin, composition and geographical orientation of the plant. The biogas-to-biomethane conversion technologies taken into account are pressurized water scrubbing (PWS), catalytic absorption/amine wash (CA), pressure swing absorption (PSA), highly selective membrane separation (MS) and cryogenic liquefaction (CL) which are the most common used biogas cleanup techniques.
The Table below shows a comparison of performance for these techniques at 8 bar (grid) injection.
Table: Comparison of performance for various upgrading techniques (result at 8 bar) (Robert Lems, 2010) , (Lems R., 2012)
|Produced gas quality*2||98||99||97-99||99||99.5||CH4%|
|Electrical use||0.23-0.25||0.15-0.18||0.25||0.21-0.24||0.35||kWh/Nm3 feed|
|Thermal energy use||0,82-1.3||kWth/Nm3 prod.|
|Reliability / up time||96||94||94||98||94||%|
|Turn down ratio||50-100||50-100||85-100||0-100||75-100||%|
|Ease of operation||Medium||Medium+||Medium||Easy||Complex|
*2 If no oxygen of nitrogen is present in the raw biogas
*3 Activated carbon (AC) consumption is depending on the presence of certain pollutants (trace components) within the raw biogas.
From the above Table, it can be concluded that the differences between technologies with respect to performance seem to be relatively small. However, some “soft factors” can have a significant impact on technology selection. For example, water scrubber technology is a broadly applied technology. The requirement for clean process water, to make up for discharge and condensation, could be a challenging constraint for remote locations.
Moreover, PWS systems are prone to biological contamination (resulting in clogged packing media and foaming), especially when operated at elevated temperatures. Without additional preventative measures this will result in an increase of operational issues and downtime.
Amine scrubbers are a good choice when surplus heat is available for the regeneration of the washing liquid. The transport and discharge of this washing liquid could however be a burden, as well as the added complexity of operation. With respect to cryogenic Liquefaction (CL) one may conclude that, this technology has a questionable track-record, is highly complex, hard to operate, and should therefore not be selected for small-medium scale applications.
Both PSA and MS provide a “dry” system, both technologies operate without the requirement for a solvent/washing liquid, which significantly simplifies operation and maintenance. Distinctive factor between these technologies is that the membrane based system operates in a continuous mode, while the PSA technology is based on columns filled with absorption materials which operate in a rotating/non-continuous mode.
Moreover, the membrane based system has a more favourable methane slip, energy consumption and turndown ratio. The biggest advantage over PSA however, is that membrane systems do not require any transport of absorbents, its ease of operation and superior up-time.
Main disadvantage of membrane systems are that they are sensitive to pollution by organic compounds, which can decrease efficiency. However, by applying a proper pre-treatment (generally based on activated carbon and condensation) in which these compounds are eliminated, this disadvantage can be relatively easy nullified.
Based on membrane technology, DMT Environmental Technology, developed the Carborex ®MS. A cost-effective plug and play, containerized (and therefore), easy to build in remote locations) biogas upgrading system. The Carborex ®MS membrane system has relatively little mechanical moving components (compared to other upgrading technologies) and therefore, ensures stability of biomethane production, and consequently, the viability of the biogas plant operation.
Moreover, its design for ease of operation and robustness makes this technological platform perfectly suitable for operation at locations with limited experience and expertise on handling of biogas plants.
Capture of biogas through application of closed ponds or AD’s is not only a necessity for mitigation of greenhouse gas emissions, it is also a method of optimizing liquid waste treatment and methane recovery. Billions of cubic meters of biomethane can be produced on a yearly basis, facilitating a significant reduction of fossil fuel dependency.
Moreover, upgrading of raw biogas-to-biomethane (grid, CNG or LNG quality) provides additional utilization routes that have the extra advantage to be independent of existing infrastructure. To sum up, membrane based technology is the best way forward due to its ease of operation, robustness and the high quality of the end-products.
- Lems R., D. E. (2012). Next generation biogas upgrading using high selective gas separation membranes. 17th European Biosolids Organic Resources Conference. Leeds: Aqua Enviro Technology .
- Robert Lems, E. D. (2010). Making pressurized water scrubbing the ultimate biogas upgrading technology with the DMT TS-PWS® system. Energy from Biomass and Waste UK . London: EBW-UK .
Co-Authors: H. Dekker and E.H.M. Dirkse (DMT Environmental Technology)
Note: This is the final article in the special series on ‘Sustainable Utilization of POME-based Biomethane’ by Langerak et al of DMT Environmental Technology (Holland). The first two articles can be viewed at these links