VOCs are light components of crude oil, which evaporate largely during loading operations or during the carriage of high-volatility crude oil cargoes. Accumulating VOCs need to be vented to the atmosphere or burnt off to prevent pressures in the tank reaching dangerous levels. While in the past VOCs have been treated as a waste product from crude oil handling and transport, heightened focus on new, less polluting fuel sources for ships has positioned them as a useable source of energy, the wastage of which represents both an environmental hazard and a loss of revenue to oil and gas producers and shippers. The recycling of VOCs is also much more beneficial for the environment. One of the VOC components is methane, a greenhouse gas with a high global warming potential. Heavier compounds within VOC vapour react with nitrogen oxide and UV radiation and form highly damaging ozone.
“Now, with the development of the X-DF engine’s new fuelling mode, VOC – when used in fuel share mode with natural gas – can be turned into a viable source of energy,” says Rudolf Wettstein, WinGD General Manager Sales & Application.
The use of a VOC recovery system can capture VOC emissions and turn them into a valuable fuel. Originally developed and tested to achieve the optimum combination of a high recovery rate and output composition to best suit the engine, VOC recovery systems use a two-stage condensation process. In the first stage, the heavier fractions from C7 and upwards are removed. The medium fractions from propane to hexane are then condensed to a liquid state in the second stage, forming liquefied VOC (LVOC). This is the energy source that can be added into the existing natural gas (NG) fuel source, such as LNG. In the final stage, the so-called ‘non-condensables’, which comprise methane and ethane plus the inert gases from the cargo hold, can be stored and used as an energy source in on-board processes such as burners.
The usable LVOC is a light hydrocarbon fuel which is kept in a pressurized storage tank on the deck of the vessel. Since the main components of LVOC are hydrocarbons that are heavier than methane, the fuel source has a significantly lower methane number (MN) than LNG. Therefore, WinGD recommend that it is advisable to blend LVOC with LNG fuel for use in their X-DF engines.
Apart from additional insulation around the gas manifold, the X-DF engine concept remains unchanged when operating with a NG/VOC fuel mix and as such no modified or additional parts in the engine are required for the use of VOCs as fuel. The engine is set with the same tuning as an engine running on LNG only.
“There are no significant changes in performance and emissions behaviour – the engines are fully IMO Tier III compliant burning the natural gas-VOC mix. In fact, aside from a few precautions according to ambient and boundary conditions, no hardware modifications and no application-specific control functions are required on the engine, and standard tuning can be retained. Likewise, the gas handling safety philosophy is identical with X-DF dual-fuel engines burning natural gas solely,” confirms Wettstein.