Transport sector is one of the major oil consuming sectors; it consumes 51% of the final oil consumption (International Energy Agency, 2010). There is a huge need of a sustainable fuel for the transport sector. Current trend of fossil fuel based transport sector only adds to the greenhouse gas crisis more and more. Lots of studies are going on and a lot of emphasis is given to other alternative fuel for transport sector which can provide fuel for a long term option and large scale as well. Biomethane Compressed Natural Gas (CNG) hybrid can be one of the options for transport sector. Biomethane is generated by anaerobic decomposition of organic material by bacteria or by biomass gasification and methane being its principal component. The biomethane gas has to be compressed to 200 bar to supply it to the vehicles. Biomethane CNG hybrid is one of the most promising fuels towards carbon neutral society and has lowest ‘Well to Wheel’ emission of any wheel. (Baldwin, 2008). It is also called as green energy equivalent of methane (Arnoersson, 2011).
Introduction
The use of biomethane CNG hybrid in the transport sector can reduce the greenhouse gas (GHG) emission by 80% as compared to that of use of gasoline (Boesel, 2009; Bordelanne et al., 2011) (Biomethane is basically the bio gas which has been purified to make it a quality natural gas. Biomethane is a bio gas without CO2 and chemically consists of 98% of methane and propane is added to it to increase its calorific value.).
The use of biogas-‐CNG blend in CNG vehicles is not possible without upgrading the quality of the biogas. Biomethane is upgraded biogas which can be used in CNG vehicles as a fuel. GHG emission from CNG vehicles is around 17% lower as compared to gasoline fuel. However, use of biomethane CNG blend vehicles can lower emission by 51% as compared to gasoline fuel. Toyota Prius CNG Hybrid prototype fuelled by biomethane (produced from waste) lowers emission by 87% in comparison to a gasoline vehicle (Bordelanne et al., 2011).
Solid waste, forest and agricultural residues, residential and industrial waste and energy crops can be used as a raw material for producing the biomethane; this is for short term production. In medium term production energy crops can be an option and for long term biomass gasification from lignocellulosic resources (Bordelanne et al., 2011). Biomethane requires some additional processes to be able to run a vehicle, if not it can corrode the engine.
Feasibility of technology and operational necessities
There are two processes for biomethane production classified by the type of feedstock as shown in Figure 1. Wet materials are suitable for Anaerobic fermentation or biomethanisation, while dry materials are appropriate with gasification process (Bordelanne et al., 2011).
The Biomethane production through biogas production process is sensitive to conditions: pH, temperature, digester technologies, moisture content of feedstock and/or feedstock type including the type of bacteria. Figure 2 provides the example of biogas yield from different feedstock. Biogas technology is considered a mature technology for the treatment of slurries and feedstocks with less than 12% dry matter. Technologies such as DRANCO© and KOMPOGAS© are in operation for high dry solids feedstocks (30-‐45%). Digestion can take place at either mesophilic (35–40oC) or thermophilic (55–60oC) temperature ranges. Theoretically, at thermophilic temperature ranges is appropriated for biogas generation than at mesophilic temperature ranges but more energy must be input to raise the temperature to the higher temperature range (Murphy & Power, 2009).
Biogas is most commercialized with electricity and heat generation as shown in Figure 3, however clean-‐up, compression and upgrading for vehicle fuel or direct use in the gas grid is mature technology, although this is an area still under active research (Adelt et al., 2011).
Up graded biogas is termed CH4-‐enriched biogas or biomethane. Various methods of scrubbing biogas (from about 55% CH4) to biomethane (97% CH4) are available. These include for dissolving the CO2 in water; CO2 is extremely soluble in water when compared to CH4. Pressure swing absorption (PSA) and membrane technology may also be used to remove CO2 from the biogas. H2S may be removed by air dosing (reduces volumetric energy density) or by addition of Fe to form precipitate (preferable when producing transport fuel).
Another possible future route for producing biomethane is gasification processes of woody biomass. There are two different low-‐temperature (800–1000oC) gasification technologies are currently now under development producing biomethane. The first is indirect gasification is deemed suitable and economic for small-‐ scale (<100MW). The second is circulating fluidized bed gasification (CFB) is usually the preferred choice for larger scale (>100MW) gasification plants (Åhman, 2010). The advantage of this process is more feedstock flexibility to use wet biomass streams than using it to produce ethanol, methane and/or Fisher–Tropsch diesel (Åhman, 2010).
Figure 4 shows a simplified scheme of the MILENA Bio-‐Methane process (Meijden, Rabou, Drift, Vreugdenhil, & Smit, 2011). The biomass needs to be dried to approximately 25 wt% moisture. Then it is fed into the riser. A small amount of superheated steam is added from below. Hot bed material (typically 925°C sand) enters the riser from the combustor through a hole in the riser (opposite and just above the biomass feeding point). The bed material heats the biomass to 850°C. The heated biomass particles are converted into gas, tar and char. After that the producer gas leaves the reactor from the top and is sent to the cooling and gas cleaning section (Meijden et al., 2011).
Methanation of the gas is done in catalytic reactors using nickel catalysts. The configuration shown in the figure assumes a pre-‐reforming step prior to the methanation step. In the pre-‐ reforming step the higher hydrocarbons (e.g. benzene) are converted into a mixture of CH4, CO, CO2, H2O and H2. Conversion of the higher hydrocarbons makes the removal of CO2 and the compression easier. After CO2 and H2O removal the gas is converted into Bio-‐Methane (Meijden et al., 2011).
Biomethane is a high quality energy carrier that can be used in a number of applications with high efficiency. In addition, to utilize Biomethane for transportation benefits on it can be integrated distribution into the existing natural gas distribution network (Åhman, 2010).
Status of the technology and its future market potential
Biomethane from wet biomass are available today (Åhman, 2010), but upgrading of the gas to a quality on par with, or better than pipeline gas, and the use in vehicles, is new (Boisen, 2008). Countries with ongoing commercial projects for supply of biomethane as a vehicle fuel are Korea, China, India, Pakistan, Germany, The UK, The Netherlands, Sweden, Spain, France, Switzerland, Austria, Norway, Iceland, Brazil and The USA (Boisen, 2008).
The market of NGV and biomethane is continuously rising. Natural & bio Gas Vehicle Association, Europe (NGVA) reported that in 2011 there are 14.5 million NGVs now running on natural gas and biomethane compare to 4 million at the end of 2004. This evidences a big shift of fuel substitution. It accounted 47.3 billion Nm3 of methane annually (39.1 Mtoe). A total of 20,600 filling stations worldwide as shown in Figure 5 (NGVA, 2011).
However, natural gas distribution network is limited in many countries therefore liquified biomethane (-‐160 oC) is an option for countries without an adequate NG pipeline system by using Cryogenic technology for up-‐grading and purification of biogas or landfill gas (Boisen, 2008). It would lead to achieve long term energy supply and environment mitigation.
How the technology could contribute to socio-economic development and environmental protection
Flexible of feedstocks for biomethane generation such as food waste, animal waste, municipal waste, landfill, biomass-‐agriculture residues, waste water from food industry etc. are available and have high potential. Therefore, it provides crucial benefits for all stakeholders. It enhances energy security by increasing alternative fuel for NGV substitution and utilizes domestic resources (MSW, solid waste, and waste water etc). It is easily connecting to the existing NG network or decentralize systems of Biomethane generation to the area where have high potential of feedstock. In addition, it could be transported in the form of liquefied biomethane.
It provides good performance of solid waste and waste water management in communities, cities and factories. It benefits on reduce energy cost, energy import dependency. Other benefits are: improved fertilizer, reduces the intensity of odour, reduces the air pollution through methane and ammonia, reduces the wash out of nitrate, hygienizes liquid manure, recycles organic residues (co-‐fermentation), can avoid costs for the connection to a central sewer etc.
In addition, biomethane could be utilized in many applications for example: for cooking in residential consumption, heat and power (both small and large scale), NGV substitution etc. As a result, it would be concluded that biomethane is widely feedstocks, widely production technologies, and wide range of utilization, wider contribution to environment (Boisen, 2008; Bordelanne et al., 2011; Chang et al., 2011; International Biogas and Bioenergy Competence Center(IBBK), 2007; Meijden et al., 2011).
Climate
LCA assessment of greenhouse gas emissions associated with biomethane cover emissions from the entire production chain, i.e. substrate production, harvesting and transportation, fermentation, fermenting residue management, upgrading of biogas to natural gas quality and provision of energy for the biogas plant (Adelt et al., 2011). Therefore, the potential of CO2 reduction is varied depend on the type of feedstock, technologies use etc. Table 1 presented that CO2 reduction by using landfill gas to CNG is accounted 88% compare to gasoline, while biogas to CNG is accounted 86% CO2 reduction. As a result, biomethane is environment sounds fuel.
Financial requirements and costs
Comparison of the potential cost and the assumptions for the different biofuels are shown in Figure 6. Biomethane current small scale farm based production is about 25 Euros/GJ compared to petrol 14 Euros/GJ (without EU tax). Therefore, in the short and medium-‐ term, the high taxation (average ~12 Euros/GJ) of petrol leaves the EU countries a room for introduce alternative fuels by allowing tax reductions. Interesting point, biomethane from large-‐scale gasification has a competitive to current production of 1st generation of ethanol, biodiesel and Fischer–Tropsh diesel (Åhman, 2010)
References
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