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).
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