Biogas-to-Biomethane Conversion Technologies

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.

biogas-biomethane

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)

  PWS CA PSA MS CL Unit
Produced gas quality*2 98 99 97-99 99 99.5 CH4%
Methane slip 1 0.1-0.2 1-3 0.3-0.5 0.5 %
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 %
CAPEX Medium Medium Medium Low High  
Operation cost Low Medium Medium Low High  
Foot print Large Large Medium Small Large  
Maintenance needed Medium Medium+ Medium+ Low High  
Ease of operation Medium Medium+ Medium Easy Complex  
Consumables &

waste streams

AC*3/Water AC*3/amines AC*3/ absorbents AC*3/None AC*3/None  
References Many Many Medium Medium Very few  

*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.

Impression of a membrane system; Carborex ®MS – by courtesy of DMT

Impression of a membrane system; Carborex ®MS – by courtesy of DMT

Conclusions

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.

References

  • 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

http://www.bioenergyconsult.com/biomethane-utilization/

http://www.bioenergyconsult.com/pome-biogas/

Biomethane Utilization Pathways

Biogas can be used in raw (without removal of CO2) or in upgraded form. The main function of upgrading biogas is the removal of CO2 (to increase the energy content) and H2S (to reduce risk of corrosion). After upgrading, biogas becomes biomethane and possesses identical gas quality properties as  natural gas, and can thus be used as natural gas replacement. The main pathways for biomethane utilization are as follows:

  • Production of heat and/or steam
  • Electricity production / combined heat and power production (CHP)
  • Natural gas replacement (gas grid injection)
  • Compressed natural gas (CNG) & diesel replacement – (bio-CNG for transport fuel usage)
  • Liquid natural gas (LNG) replacement – (bio-LNG for transport fuel usage)

Prior to practically all utilization options, the biogas has to be dried (usually through application of a cooling/condensation step). Furthermore, elements such as hydrogen sulphide and other harmful trace elements must be removed (usually trough application of an activated carbon filter) to prevent adverse effects on downstream processing equipment (such as compressors, piping, boilers and CHP systems).

biomethane-transport

Although biogas is perfectly suitable to be utilized in boilers (as an environmental friendlier source for heat and steam production), this option is rather obsolete due to the abundance of alternative sources from solid waste origin.

Most Palm Oil Mills are already self-reliant with respect to heat and steam production due to the combustion of their solid waste streams (such as EFB and PKS). Consequently, conversion to electricity (by means of a CHP unit) or utilization as natural gas, CNG or LNG replacement, would be a more sensible solution.

The biogas masterplan as drafted by the Asia Pacific Biogas Alliance foresees a distribution in which 30% of the biomethane is used for power generation, 40% for grid injection and 30% as compressed/liquefied fuel for transportation purpose (Asian Pacific Biogas Alliance, 2015).

For each project, the most optimal option has to be evaluated on a case to case basis. Main decision-making factors will be local energy prices and requirements, available infrastructure (for gas and electricity), incentives and funding.

For the locations where local demand is exceeded, and no electricity or gas infrastructure is available within a reasonable distance (<5-10 km, due to investment cost and power loss), production of CNG could offer a good solution.

Moreover, during the utilization of biogas within a CHP unit only 40-50% of the energetic content of the gas is converted into electricity. The rest of the energy is transformed into heat. For those locations where an abundance of heat is available, such as Palm Oil Mills, this effectively means that 50-60% of the energetic content of the biogas is not utilized. Converting the biogas into biomethane (of gas grid or CNG quality) through upgrading, would facilitate the transportation and commercialisation of over 95%  of the energetic content of the biogas.

Within the CNG utilization route, the raw biogas will be upgraded to a methane content of >96%, compressed to 250 bar and stored in racks with gas bottles. The buffered gas (bottles) will be suitable for transportation by truck or ship. For transportation over large distances (>200km), it will be advised to further reduce the gas volume by converting the gas to LNG (trough liquefaction).

Overall the effects and benefits from anaerobic digestion of POME and utilization of biomethane can be summarized as follows:

  • Reduction of emissions i.e. GHG methane and CO2
  • Reduced land use for POME treatment
  • Enhanced self-sufficiency trough availability of on-site diesel replacement (CNG)
  • Expansion of economic activities/generation of additional revenues
    • Sales of surplus electricity (local or to the grid)
    • Sales of biomethane (injection into the natural gas grid)
    • Replacement of on-site diesel usage by CNG
    • Sales of bottled CNG
  • Reducing global and local environmental impact (through fuel replacement)
  • Reducing dependence on fossil fuel, and enhances fuel diversity and security of energy supply
  • Enhancement of local infrastructure and employment
    • Through electrical and gas supply
    • Through Fuel (CNG) supply

Co-Authors: H. Dekker and E.H.M. Dirkse (DMT Environmental Technology)

Note: This is the second article in the special series on ‘Sustainable Utilization of POME-based Biomethane’ by Langerak et al of DMT Environmental Technology (Holland). The first article can be viewed at this link

Biomethane – The Green Gas

Biomethane, also known as the green gas, is a well-known and well-proven source of clean energy, and is witnessing increasing demand worldwide, especially in European countries, as it is one of the most cost-effective and eco-friendly replacement for natural gas and diesel.

Advantages of Biomethane

The key advantage of biomethane is that it is less corrosive than biogas which makes it more flexible in its application than raw biogas. It can be injected directly into the existing natural gas grid leading to energy-efficient and cost-effective transport, besides allowing natural gas grid operators to persuade consumers to make a smooth transition to a renewable source of natural gas.

Biogas can be upgraded to biomethane and injected into the natural gas grid to substitute natural gas or can be compressed and fuelled via a pumping station at the place of production. Biomethane can be injected and distributed through the natural gas grid, after it has been compressed to the pipeline pressure.

The injected biomethane can be used at any ratio with natural gas as vehicle fuel. In many EU countries, the access to the gas grid is guaranteed for all biogas suppliers.

A major advantage of using natural gas grid for biomethane distribution is that the grid connects the production site of biomethane, which is usually in rural areas, with more densely populated areas. This enables biogas to reach new customers.

Storage of Biomethane

Biomethane can be converted either into liquefied biomethane (LBM) or compressed biomethane (CBM) in order to facilitate its long-term storage and transportation. LBM can be transported relatively easily and can be dispensed through LNG vehicles or CNG vehicles. Liquid biomethane is transported in the same manner as LNG, that is, via insulated tanker trucks designed for transportation of cryogenic liquids.

Biomethane can be stored as CBM to save space. The gas is stored in steel cylinders such as those typically used for storage of other commercial gases.

Applications of Biomethane

Biomethane can be used to generate electricity and heating from within smaller decentralized, or large centrally-located combined heat and power plants. It can be used by heating systems with a highly efficient fuel value, and employed as a regenerative power source in gas-powered vehicles.

Biomethane, as a transportation fuel, is most suitable for vehicles having engines that are based on natural gas (CNG or LNG). Once biogas is cleaned and upgraded to biomethane, it is virtually the same as natural gas.

Because biomethane has a lower energy density than NG, due to the high CO2 content, in some circumstances, changes to natural gas-based vehicle’s fuel injection system are required to use the biomethane effectively.

Biomethane Industry in Europe

Biomethane is a well-known and well-proven source of clean energy, and is witnessing increasing demand worldwide, especially in European countries. Between 2012 and 2016, more than 500 biomethane production plants were built across Europe which indicates a steep rise of 165 percent. The main reasons behind the growth of biomethane industry in Europe is increasing interest in industrial waste-derived biogas sector and public interest in biogas.  Another important reason has been the guaranteed access to gas grid for all biomethane suppliers.

Biomethane production in Europe has swiftly increased from 752 GWh in 2011 to 17,264 GWh in 2016 with Germany being the market leader with 195 biomethane production plants, followed by United Kingdom with 92 facilities. Biogas generation across Europe also witnessed a rapid growth of 59% during the year 2011 and 2016. In terms of plant capacities, the regional trend is to establish large-scale biomethane plants.

Sources of Biomethane in Europe

Landfill gas and AD plants (based on energy crops, agricultural residues, food waste, industrial waste and sewage sludge) are the major resources for biomethane production in Europe, with the predominant source being agricultural crops (such as maize) and dedicated energy crops (like miscanthus). In countries, like Germany, Austria and Denmark, energy crops, agricultural by-products, sewage sludge and animal manure are the major feedstock for biomethane production. On the other hand, France, UK, Spain and Italy rely more on landfill gas to generate biomethane.

A large number of biogas plants in Europe are located in agricultural areas having abundant availability of organic wastes, such as grass silage and green waste, which are cheaper than crops. Maize is the most cost-effective raw material for biomethane production. In many parts of Europe, the practice of co-digestion is practised whereby energy crops are used in combination with animal manure as a substrate. After agricultural biogas plants, sewage sludge is one of the most popular substrates for biomethane production in Europe.

Biomethane Utilization Trends in Europe

Biomethane has a wide range of applications in the clean energy sector. In Europe, the main uses of biomethane include the following:

  1. Production of heat and/or steam
  2. Power generation and combined heat and power production(CHP)
  3. Replacement for natural gas (gas grid injection)
  4. Replacement for compressed natural gas & diesel – (bio-CNG for use as transport fuel)
  5. Replacement for liquid natural gas – (bio-LNG for use as transport fuel)

Prior to practically all utilization options, the biogas has to be dried (usually through application of a cooling/condensation step). Furthermore, elements such as hydrogen sulphide and other harmful trace elements must be removed (usually trough application of an activated carbon filter) to prevent adverse effects on downstream processing equipment (such as compressors, piping, boilers and CHP systems).

biomethane-transport

Biomethane is getting popularity as a clean vehicle fuel in Europe. For example, Germany has more than 900 CNG filling stations, with a fleet of around 100,000 gas-powered vehicles including cars, buses and trucks. Around 170 CNG filling stations in Germany sell a blend mixture of natural gas and biomethane while about 125 filling stations sell 100% biomethane from AD plants.

Barriers to Overcome

The fact that energy crops can put extra pressure on land availability for cultivation of food crops has led many European countries to initiate measures to reduce or restrict biogas production from energy crops. As far as waste-derived biomethane is concerned, most of the EU nations are phasing out landfill-based waste management systems which may lead to rapid decline in landfill gas production thus putting the onus of biomethane production largely on anaerobic digestion of food waste, sewage sludge, industrial waste and agricultural residues.

The high costs of biogas upgradation and natural gas grid connection is a major hurdle in the development of biomethane sector in Eastern European nations. The injection of biomethane is also limited by location of suitable biomethane production facilities, which should ideally be located close to the natural gas grid.  Several European nations have introduced industry standards for injecting biogas into the natural gas grid but these standards differ considerably with each other.

Another important issue is the insufficient number of biomethane filling stations and biomethane-powered vehicles in Europe. A large section of the population is still not aware about the benefits of biomethane as a vehicle fuel. Strong political backing and infrastructural support will provide greater thrust to biomethane industry in Europe.