This work is an extended summary of a recent publication by Ignacio Arias et al., aiming at hydrogen generation of onboard LNG vessel. At present, LNG ships with no reliquefaction plant consume the BOG (Boil Off Gas) generated in the engines, and the excess is burned in the GCU (Gas Combustion Unit) without any energy use. The need to improve the gas management system, therefore, is evident.
This paper proposes hydrogen production through a steam reforming plant, using the excess BOG as raw material and thus avoiding it being burned in the GCU. To test the feasibility of integrating the plant, an actual study of the gas management process on an LNG vessel with 4SDF (4 Stroke Dual Fuel) propulsion and with no reliquefaction plant was conducted, along with a thermodynamic simulation of the reforming plant. With the proposed gas management system, the vessel disposes of different fuels, including H2, a clean fuel with zero ozone-depleting emissions.
Developments in stringent maritime transport anti-pollution regulations have led this sector to be in the midst of a period of adaptation and change. The shipping industry is considered as one of the major contributors to global warming and air pollution. As consequence, the IMO (International Maritime Organization) developed Annex VI of the International MARPOL Convention for the Prevention of Pollution from Ships. The main restrictions apply to emissions of NOX (Tier Limits) and SOX (Global and ECA Limits), compelling companies to apply new market strategies in order to cut costs incurred from the need to use better quality fuels. Among such strategies, those most widely used are: an increased speed outside of ECA zones and a decrease inside; the avoiding of areas with restrictions, in spite of the longer distances; and the shortening of stays at port.
Ports are one of the main focal points with regard to contamination issues due to their close proximity to urban areas. Because of this, and so that vessels comply with the current regulations, di erent options are being looked into, such as: Cold Ironing, the use of clean and more e cient fuels such as hydrogen, or higher quality fuels and with a lower percentage of sulphur.
The high demand for NG (Natural Gas) worldwide is leading to signi cant increase in the number of LNG (Lique ed Natural Gas) vessels. The technology adopted by LNG vessel propulsion systems is not only closely linked to developments in the strict anti- pollution regulations, but also to the versatility of such systems, as they can consume di erent fuels depending on the BOG (Boil O Gas) management systems they comprise. For this reason, hydrogen generation on board these vessels could be a very appealing option, since NG is the most widely used raw material used for the extraction of hydrogen and its zero emissions makes its consumption possible in any area without any restrictions.
LNG vessel with DFDE propulsion, as a model for generating H2
Since early 2003, the number of newly built LNG vessels with a with dual fuel diesel electric engine (DFDE) propulsion system has increased considerably, reaching 159 units. This demonstrates that the propulsion system preference on LNG vessels steers towards the use of DF engines, which can consume both gas as well as liquid fuels.
DF engines are designed to use methane gas as fuel, thus it essential to separate the other NG components through a system called an Oil Mist Separator to ensure correct combustion and hence avoid Knocking. Upon leaving the gas separator, BOG pressure is increased in pitch and variable speed compressors, termed Low-Duty. An exchanger (BOG / Glycol) in installed out of the Low-duty compressors to stabilise the temperature prior to its consumption in the engines or, should there be any excess, burned in the GCU without any energy recovery, for the sole purpose of stabilising the pressure in the cargo tanks, as shown in Fig.1.
Fig.1 Gas management system in a DF (4S) engine system
The BOG produced is treated according to the current operational state of the vessel, this being namely when the vessel is at port/anchored or sailing.
When the vessel is at port or anchored, the propulsion system is out of service, and so only one power generator is required to provide power for on board services and auxiliary elements. In this situation, the BOG generated is used to feed the generator in operation, and the excess is burned in the GCU.
On the contrary, when navigating, the propulsion plant is in operation and so BOG consumption is greater, but always dependent on the demands/requirements of the vessel, with any of the following situations being possible:
– Forcing Vaporiser: Should more BOG be required than the amount generated naturally, BOG is forced in an LNG/Steam exchanger to perform the phase change and meet the demands of the propulsion plant.
– GCU: When propulsion plant demand is not great and the naturally generated BOG exceeds the amount consumed there is an excess of BOG, which is burned in the GCU to maintain the pressure inside the tanks, without any energy exploitation.
To follow is a case study, performed through the collection of data onboard, of an actual BOG management plant on an LNG ship with a DF 4S propulsion system.
Fig. 2 Relation of the BOG generated and that consumed in the propulsion plant and GCU in ballast conditions
Characteristics of the model LNG vessel under study
A 173400 m3 vessel propelled by four DFDE 4S engines is taken as a case study. The study of the gas management system revealed that BOG generation throughout the crossing was not constant, For this reason, data was collected through the vessel management and control system during the different operational states throughout the crossing, thereby minimising, as far as possible, any errors that could occur in the study.
The BOG produced daily on the vessel is used to fuel the DF engines and, should there be any excess, is burned in the GCU. Situations may arise in which propulsion plant consumption exceeds the BOG produced naturally, hence the use of the Forcing system to supply the required surplus. This process is illustrated in Fig.2, where both the naturally generated BOG as well as that consumed in the DF engines and that burned in the GCU is shown. The average naturally generated BOG is of 183 m3⁄day, thats consumed by the propulsion plant is of 170 m3/day, and the excess sent to the GCU is of 30 m3⁄day.
A standard practice that increases the BOG generated on these vessels is the spraying of LNG onto the cargo itself in order to cool it, through a phase change process, absorbing heat and so considerably increasing the temperature of the excess BOG at times, which has to be burned in the GCU.
In conclusion to the BOG management system study, low energy efficiency is witnessed due to the amount of m3/day of LNG burned in the GCU without any energy exploitation, along with the emissions that result. The strict antipollution regulations imposed by the IMO have led to the need for a new approach to gas management systems onboard LNG vessels in order to provide greater flexibility, better performance and to reduce ozone-depleting emissions.
Obtaining hydrogen from the excess BOG can be considered an interesting option, as a clean fuel would be available on board to use in ECA areas and ports, with the only exhaust discharge being water vapour.
Hydrogen generation on the model LNG vessel
Currently, the most widely employed method for generating H2 is natural gas steam reforming, reaching 50% of global H2 production. A system is proposed in order to exploit the excess BOG, which obtains hydrogen through steam reforming, given that this is a high-yield mature technology and guarantees production at low cost when compared with other methods.
Fig. 3 illustrates the reforming system layout on the model vessel, divided into three stages that must be performed in order to complete the H2 production process, and using the BOG as the raw material. The BOG that feeds the reformer is obtained from the excess generated on board once the pressure in the Low-Duty compressors has been increased and remains unconsumed in the power generation engines.
Fig.3 Steam reforming process using the BOG from the model vessel burned in the GCU without energy recovery.
This section analyses the efficiency of the hydrogen production plant via steam reforming in order to exploit the excess BOG generated on LNG vessels. The analysis of the system was performed by varying key parameters such as the temperature of the reforming BOG, of the fuel, combustion air and tail gases, using EES Software (Engineering Equation Solver), which offers the advantage of including fluid properties and optimisation tools.
Table 1 lists the parameters assumed for the simulation of the model plant. It is to be taken into account that the results obtained in the study are referred in kg/s of reforming BOG.
The study of the plant in the operational situation of maximum performance is achieved with the recirculation of reforming tail gases towards the combustion chamber, and the exploitation of the process waste heat in order to preheat the BOG, water, air and fuel. The results obtained are listed in Table 2, in which the hydrogen generated in conditions of maximum efficiency, the mass flow of each current, and the power consumed by the plant are demonstrated. An important fact to mention is that 0.37 kg/s of H2 are generated for every kg/s of BOG.
To obtain improved performance, management of the BOG generated on board LNG vessels needs to be improved because of the current energy wastage. This requires new systems that use the excess BOG produced on board which is burned in the GCU, without any energy exploitation. This paper proposes an alternative to current systems; hydrogen generation from the excess BOG through reforming.
The following conclusions were reached from the analysis:
– An LNG vessel without a reliquefaction plant generates an excess BOG of 30 m3/day in ballast, which is burned in the GCU without any energy recovery and produces air pollution.
– Hydrogen is considered as a clean fuel due to its zero ozone-depleting emissions and can be consumed inside areas with strict anti-pollution restrictions such as ECAs and ports.
– The hydrogen production system with greatest performance and market maturity is steam reforming, reaching 50% of the global production of hydrogen. This system is also of high efficiency, low cost and, when integrated with a PSA module, can obtain hydrogen currents with a purity of over 95%.
– In terms of results, a plant efficiency of 64% and a H2 production of 0.37kg/s per kg/s of available BOG is obtained.
The installing of a reforming plant is energetically viable and offers greater versatility to the vessel thanks to the availability of different fuels, as well as the improvement in BOG management. H2 storage onboard and the adapting of the system to consume the H2 generated is of utmost importance, and so will be the future lines of research of the authors, with the aim of attempting to complete the entire processes of the system.