**1. Introduction**

The International Maritime Organization assessed that international shipping accounted for about 2.2% of total carbon dioxide emissions in 2012, which is approximately 796 million tonnes of CO2, and forecasted that this amount will increase between 50% and 250% in the period to 2050 under a business-as-usual scenario [1]. However, when the Paris Agreement on climate change mitigation was adopted in 2015 to deal with the global-warming concerns, shipping was not included [2]. Instead, in April 2018, the IMO established a strategy to reduce the total amount of annual greenhouse gas (GHG) emissions from shipping by at least 50% by 2050 compared to 2008 [3]. To achieve this, several technical and operational measures that improve energy efficiency and reduce CO2 emissions in the shipping industry should be introduced, such as increasing energy efficiency of engines, adopting waste heat recovery systems, improving the hull form, implementing speed reduction and alternative

sea routes [4]. In addition to the above measures, using different propulsion systems, such as fuel cells, are also considered possible alternatives [5]. Hydrogen fuel cells emit no direct GHGs, but the emissions generated during hydrogen production should be considered. The emissions from hydrogen production are highly dependent on feedstock and primary energy sources [6]. In the shipping industry, fuel cell power generation can eliminate NOx, Sox, and particulate material (PM) emissions, and reduce CO2 emission compared to emissions from conventional diesel engines [2]. The advantage of fuel cells for maritime applications is the reduction of noise, vibrations, and infra-red signatures, along with their modular and flexible design, water generation, etc., although they may be application specific [5]. Seven fuel cell types were evaluated from the perspectives of relative cost, module power level, lifetime, tolerance for cycling, flexibility towards type of fuel, technological maturity, size, sensitivity to fuel impurities, emissions, safety, and efficiency. As a result of the evaluation, the low temperature proton exchange membrane (PEM) fuel cell and the high temperature PEM fuel cell (HT-PEMFC) received, respectively, the first and second highest score in the ranking, and this implied that those technologies are the most promising for marine use [7]. An LT-PEMFC has relatively higher power density than an HT-PEMFC; however, the HT-PEMFC has several other advantages [8]. The operating temperature of LT-PEMFCs is between 50 and 90 ◦C and that for HT-PEMFCs is between 120 and 200 ◦C. The lower operational temperature of LT-PEMFCs results in limited tolerance to fuel impurities such as sulfur and carbon monoxide (CO), which reduce its performance drastically. To be specific, LT-PEMFCs require fuels containing less than 30 ppm of CO and less than 1 ppm of sulfur, whereas HT-PEMFCs can work with concentrations of up to 3% of CO and 20 ppm of sulfur in the fuel without permanent degradation [7]. This higher tolerance to impurities of HT-PEMFC makes it possible to develop a simpler fuel reforming system [8]. Additionally, the water management of HT-PEMFCs is easier because the water produced in the fuel cell is in vapor, and the waste heat from HT-PEMFCs can be recovered and used for steam or hot water generation [9].

Since the gravimetric energy density of hydrogen is approximately 120 MJ/kg, 2.47 times higher than natural gas and 2.8 times higher than diesel, hydrogen provides higher gravimetric energy than other fossil fuels. However, volumetric energy density of liquid hydrogen is approximately 8.51 GJ/m3, which corresponds to 40.8% of natural gas and 23.7% of diesel [10]. The lower volumetric energy density could be a drawback for some vessels in those applications that cannot support a large volume of storage or higher frequency of refueling [11]. To overcome this, several maritime fuel cell studies have considered on-board reforming of methane and methanol to hydrogen, although the applied fuel cell types are different [5,12–20]. This is because methane and methanol stored at liquid state have higher volumetric energy density than hydrogen [10], and the operation expenditure (OPEX) can be reduced owing to the lower price of both fuels compared to that of hydrogen. In addition, the bunkering infrastructure for methane and methanol is not an issue, unlike that for hydrogen. The bunkering infrastructure for methane (namely LNG) is rapidly expanding, and that for methanol requires minimal modification from the existing conventional infrastructure [5,12].

There are several technologies for reforming carbon-based fuels to hydrogen, which include steam reforming, partial oxidation, and auto-thermal steam reforming. Among these processes, steam reforming provides higher efficiency, higher production yield, and lower rate of side reactions [21].

Steam methane reforming is one of the most proven and commercially available technologies for hydrogen production [22], and at present, 80% to 85% of global hydrogen production is derived via this technology [23,24]. Steam methane reforming technology has been widely investigated from the energy and exergy efficiency and economic and environmental perspectives in the past decades.

Simpson et al. evaluated the performance of steam methane reforming system using an exergy analysis. The results revealed that irreversibility of chemical reactions results in the largest amount of exergy destruction, and exergy loss through the exhaust gas stream was significant. Results of a parametric study show that the highest exergy efficiencies of 62.73% was achieved at operating temperatures of 974 K. The highest exergy efficiencies of 62.85% was attained at an operating pressure of 6.8 atm. The effect of steam carbon ratio (S/C ratio) was also investigated, and the result revealed

that the highest efficiencies were achieved at an S/C ratio of approximately 3.2 [23]. Welaya et al. evaluated the partial oxidation and steam reforming process to convert a carbon-based marine fuel, such as natural gas, gasoline, and diesel, into hydrogen-rich gases suitable for application to the PEMFCs on board ships. Among several options evaluated, the natural gas steam reforming system showed the highest fuel processing efficiency [14]. Authayanun et al. investigated the theoretical performance of an HT-PEMFC integrated with the steam reformer using various primary fuels, i.e., methane, methanol, ethanol, and glycerol. Results revealed that for the steam methane reforming, CO fraction lower than the acceptable limit for the HT-PEMFC can be attained with higher S/C ratios and lower temperature. For S/C ratios (3–6), operating temperature lower than 1000 K should be maintained in order to keep the CO fraction at an acceptable level for an HT-PEMFC or a water–gas shift (WGS) reactor should be included. Steam methanol reforming produces the lowest CO fraction among the studied fuels and can be directly fed to the HT-PEMFC for all of the studied cases (S/C ratio: 1–3, reformer temperature: 423–523 K). The steam methanol reforming system without a WGS reactor and steam methane reforming with a WGS reactor achieved the highest system efficiency, approximately 50%, among several options in the study [25]. Nerem et al. evaluated hydrogen, LNG, or methanol as PEMFC fuel on a cruise vessel, based on the space required on board, environmental impact, and life cycle cost (LCC) aspects. An external reformer other than hydrogen fuel was considered. Results show that the LNG system requires the smallest dimensions, whereas hydrogen and methanol require equal dimensions. From the perspective of environmental impact, LNG is a better solution than methanol for use in fuel cells. Further, LNG achieved the lowest LCC, 1.10 times higher than heavy fuel oil (HFO), while hydrogen and methanol are 1.14 and 1.15 times more expensive than HFO [12]. Arsalis et al. evaluated a micro combined heat and power system integrated with HT-PEMFC and steam methane reformer. They reported that the cogeneration and electrical efficiencies of the system are 55.46% and 27.62%, respectively [26].

Methanol is an advantageous fuel for mobile fuel cell applications since it has low boiling temperature (65 ◦C). Therefore, it can be stored in a liquid state at atmospheric pressure and normal environment temperature, unlike liquefied methane (−163 ◦C) [27,28]. In addition, as no carbon–carbon bond exists, methanol can be converted to hydrogen at lower temperature (150–350 ◦C) than other carbon-based fuels, and it can be activated at lower temperature than methane [28]. With these advantages, methanol steam reforming has been widely developed. Faungnawakij et al. has investigated the effect of varying S/C ratio (0–10), reforming temperatures (25–1000 ◦C), and pressures (0.5–3 atm) on the steam methanol reforming process. Results show that the optimized operating condition based on efficiency was the temperature range of 100–225 ◦C, S/C range of 1.5–3, and pressure at 1 atm. In addition, an operating temperature higher than approximately 150 ◦C and operating pressure varying from 0.5 to 3 atm did not affect the methanol conversion and hydrogen yield [21]. Herdem et al. modeled the methanol steam reforming system to produce power using a HT-PEMFC for portable power generation and examined performance variation of the HT-PEMFC with varying composition of reformate gas. The result reveals that lower S/C ratio and higher reforming temperature increase CO mole fraction in the reformed gas. However, higher fuel cell temperatures decrease the effect of CO mole fraction on the HT-PEMFC performance [27]. Mousavi Ehteshami and Chan analyzed the steam reforming of methanol, ethanol, and diesel in a technical and economical point of view. It was found that steam methanol reforming showed the easy conversion and the highest energy efficiency. Therefore, methanol is considered to be one of the promising fuels for hydrogen production by using steam reforming. However, the model used in the study did not take into account heat recovery and heat integration in the system. Therefore, it is possible that the efficiencies of fuels with higher reforming temperature than methanol can be increased when heat recovery and integration are applied [29]. Romero-Pascual and Soler investigated an HT-PEMFC-based CHP system integrated with a methanol steam reformer. The result reveals that 24% of system power efficiency and a CHP efficiency over 87% were achieved [9]. Table 1 summarizes the simulation parameters about steam methane and steam methanol reforming system from other works.


**Table 1.** Simulation parameters about steam methane and steam methanol reforming from the open literature.

Although steam reforming of methane and methanol on board ships offers several advantages as aforementioned, CO2 emission will be unavoidable because these are carbon-based fuels [15]. One of the future options for carbon-based fuels could be to install carbon capture and storage (CCS) systems on board ships [2]. If hydrogen production by reforming a carbon-based fuel is applied on board ships, the resulting CO2 can be captured by using a CCS, and then the produced hydrogen can be considered as a zero-emission fuel at the ship level [2]. The use of CCSs on board ships was investigated in an Eurostar project. On-board chemical capture, CO2 liquefaction, and a temporary storage system for ships were developed. The result shows that the concept is technically feasible and capable and can reduce CO2 emissions by 65% [35].

There are several technologies for CO2 capture in hydrogen production, which include adsorption, absorption, use of membranes, and cryogenic/low temperature processes [36,37]. Among them, the absorption process using monoethanolamine (MEA) is the most mature and promising process for CO2 capture [38,39]. Many authors have presented hydrogen production processes associated with the MEA absorption process [30,40,41]. The MEA absorption process comprises two major stages: Absorption of CO2 in the absorber and desorption of CO2 in the stripper to regenerate the amine solvent. One of the drawbacks of this process is the large amount of heat required to regenerate the amine solvent [42,43]. Typically, regeneration occurs at an elevated temperature (100–140 ◦C) and pressure not much higher than the atmospheric [38,44,45]. The heat required for regeneration is supplied to the re-boiler by a separate steam cycle [43].

The challenge in the marine environment is the handling and storage of any captured CO2. Storage of CO2 in gaseous form requires a huge space, whereas the storage of CO2 in liquid form requires a large amount of power consumption for its liquefaction. Considering the limited space of the vessel, the captured CO2 should be stored in a liquid form even if the energy consumption for liquefaction is high. The pressure and temperature of the liquid CO2 for storage should be higher than the triple-point pressure (5.2 bar) and lower than the critical-point temperature (31 ◦C). In the published reports, the pressure of liquid CO2 for shipping is mainly in the range from 6 to 20 bar, and in this range of pressure the temperature varies form −52 to −20 ◦C [46]. Feenstra et al. conducted a technical and economic evaluation of ship-based carbon capture on diesel or LNG fueled ships. For the LNG fueled ships, the cooling energy from evaporation of the LNG was used for liquefying the captured CO2. For diesel fueled ships, the ammonia-based refrigeration cycle was used for liquefaction of CO2 [47]. Berstad et al. evaluated the energy consumption and CO2 liquefaction ratio for CO2 phase separation from the flue gas. The result reveals that the specific power consumption and CO2 liquefaction ratio are significantly affected by CO2 concentration in flue gas. To maximize the CO2 capture ratio by liquefaction, the concentration of gases other than CO2 in the flue gas should be minimum and pressure should be as high as practically possible [48].

The authors of this study consider that to achieve zero emission from shipping alternative fuels such as green hydrogen from renewable energy should be applied. However, the storage volume for hydrogen, lack of infrastructure for hydrogen bunkering, and high cost of hydrogen, among other factors, are challenging at the current level of technology. Therefore, on-board methane and methanol steam reforming with CO2 capture could be one of the transition solutions.

Therefore, in this study, we present on-board methane and methanol steam reforming systems integrated with an HT-PEMFC system for power generation, along with a CO2 capture/liquefaction system for storage of the captured CO2 on board. The performance of the integrated systems was evaluated through an exergy and energy analysis. In addition, the spaces required for the captured CO2 and primary fuel storage were also compared. Those evaluations were carried out for a reference ship. The following features distinguish this study from previous works:


The main objectives of this study are as follows:

