This section relates to the relevant consideration for the early adaption of Bio-methane as an alternative fuel.
Sustainable feedstock for bio-methane are waste streams of a biogenic origin such as manure, agricultural waste and food waste, which can be converted into bio-methane. According to current estimates, total feedstock potential for bio-methane is abundant, but it is challenged by high demand across multiple industries. At the same time, existing infrastructure, established practices, and current regulatory framework do not support large-scale collection of suitable waste streams.
A currently available and mature technology for producing biomethane is anaerobic digestion (AD), which yields biogas, a mixture of bio-methane and CO2. The CO2 can then be separated out of the biogas mixture or be reacted with green hydrogen to increase the biomethane yield. Special attention must be given to controlling methane slips and leakages to ensure a climate-positive impact. Operating supply chains with low emissions are possible, and there is a need to reach a common, high standard on fugitive emissions for new and old plants and processes.
Notice that processing certain biomass waste streams into biogas reduces methane emissions associated to incorrect waste management. In effect, this could make bio-methane a biofuel with negative GHG emission intensity.
As feedstock tends to have a low calorific value and may originate from distributed locations, feedstock collection and transportation may limit the scale of the biogas plants. This is a concern for further processing since upgrading and liquefaction have large economy of scale potential. Segregated / mass-balanced, bunkering of liquified biomethane can take advantage of a widespread gas grid that connects manufacturing zones to ports since large-scale liquefaction can be done close to ports. Segregated bunkering of liquified biomethane may be possible even without a gas grid in areas with large amount of stranded feedstock.
The main bottleneck for biomethane impact in the next decade is the rate of construction of new biogas plants. An increase in the rate of construction must be accompanied by regulation on methane fugitive emissions for the pathway to deliver climate benefits. Longer term, it is the biomass availability that will limit biomethane availability due to expected competition between sectors and their need for sustainable biomass feedstock.
Fuel storage, logistics & bunkering
Fuel supply logistics and bunkering are well established for liquefied natural gas (LNG). Given that methane is the main constituent of LNG, bio-methane does not represent any fundamentally new challenges in fuel storage, logistics and bunkering. If methane expands further as a shipping fuel, the port infrastructure for methane-based marine fuels must expand. Safety and all relevant operating procedures are already in place to deal with e.g., explosion risk. A remaining challenge at terminals and during bunkering of methane is the low boiling point resulting in a latent risk of boil-off. Well-to-wake emissions accounting will require strict control with methane venting or release in the entire supply chain.
Onboard energy storage & fuel conversion
Liquid storage of methane requires advanced cryogenic storage systems at -163C, but technologies for storing and converting methane onboard vessels are commercially available. Internal combustion engines (ICEs) using methane are currently adopted in the current liquified natural gas (LNG) fleet. Different ICE technologies including dual-fuel high-pressure and low-pressure two-stroke engines and four-stroke engines are currently used with varying cost, efficiency, and emissions. Furthermore, methane-fueled fuel cells are coming on the market with multiple smaller-scale demonstrators ongoing.
Onboard safety & operations
Currently, hundreds of LNG-fueled vessels are in commercial operation. If regulations and safety management practices are followed, no obstacles remain regarding safety and onboard operations for major scaling of methane as a maritime fuel. However, it will require attention to crew competence development for un-familiarized operators.
Methane combustion does release CO2 – but with biomethane providing significant negative value for well-to-tank emissions, a greenhouse gas emissions benefit is clearly obtained from a well-to-wake perspective. High-pressure two-stroke engines have very low methane slip (~0.2%). Other engine types (such as low-pressure two-stroke and 4-stroke engines) have higher methane slip (up 3-4%). Methane slip should be reduced through regulations that incorporate methane into a CO2-equivilant methodology combined with further development of onboard emission reduction technologies like catalysts.
Regulation & certification
Well-to-wake greenhouse gas quantification for bio-methane (or LNG) is not developed in appropriate regulatory bodies such as the European Union (EU) or International Maritime Organization (IMO). Well-to-wake methane emissions remain a concern to be properly managed and enforced.
Life cycle assessment policy needs to be developed. Regulating the climate impact of fuel use from a life cycle perspective offers the industry the opportunity to establish sustainable fuel production and consumption patterns. By regulating the upstream (well-to-tank) climate impact, fuel users can select fuels with solid sustainability credentials. Regulation from a life cycle perspective also reduces the risk of burden shift of climate impact from the downstream (tank-to-wake) part of the value chain to the upstream. This is an important consideration for alternative marine fuels whereby much of the climate impact resides in well-to-tank activities.
The well-to-tank contribution for bio-methane requires strong focus – including monitoring and enforcement of fugitive emissions up-stream as well as onboard combustion. Methane to be a regulated vessel emission with required levels set to promote further technology development and adoption of low-methane slip solutions. Vessel methane emission regulation to be implemented at the EU level and is currently under discussion at IMO.
LNG and methane-based marine fuels: Prospects for the shipping industry - Documentation of assumptions for NavigaTE 1.0 (2021)