A data-driven study of IMO compliant fuel emissions with consideration of black carbon aerosols

Highlights

• A bottom-up ship emission inventory model is developed.

• GWP, non-GWP ship-resulted exhaust gases, fuel consumptions are calculated for 337 merchant ships.

• VLSFO has higher BCA emissions than HFO.

• The two LNG fuel options present great fuel saving potential and less environmental impacts.

• HFO option emits 38% more GWP gases than HPDF.

Abstract

The shipping industry has been facing increasing pressure to reduce environmental emissions. For instance, starting in 2020 the International Maritime Organization (IMO) required a reduction of the global sulfur limit for shipping fuels outside designated Sulfur Emission Control Areas (SECA) from 3.5% to 0.5% on a mass basis. Other pollutants are also being targeted for short-term and long-term reduction.

Aiming at providing sustainable fuel solutions for the stage of tank to propeller, this manuscript evaluates various shipping fuel options focusing on quantitatively comparing the fuel consumption and emissions of major air pollutants. A bottom-up emission inventory model is developed by analyzing and optimizing multiple sources of load factors, specific fuel consumption and emission factors. Rather than taking deterministic emission leading values, this work improves the model quality by considering external factors and the latest emission data and processing a detailed ship-by-ship calculation for the ship database collected by the shipboard Automatic Identification System (AIS). The amounts of carbon dioxide for the main engine, auxiliary engine and auxiliary boilier are firstly calculated, and then the fuel consumption per nautical mile are computed by integrating CO₂ intensity. Next, ship exhaust gases are calculated for two categories: (i) gases impacting the global warming potential (GWP) and (ii) the non-GWP gases. Besides sustainable solutions within one specific ship fleet, this work firstly establishes a nautical mile-based fuel consumption per unit of capacity bin for major merchant ships to serve as a basis for finding an optimized fuel solution among different ship types. In addition to addressing the typical ship-source pollutants, this work also focuses on negative environmental effects resulting from BCA because it has not been thoroughly considered in the literatures despite its contribution to GWP. To illustrate the value and applicability of the proposed approach, a case study is solved to estimate the cumulative fuel consumption and GWP-related gas emission inventory for a transoceanic trip from Houston to Rotterdam. The results demonstrate that the GWP gas emissions of heavy fuel oil with scrubber are 38% more than those of one LNG dual-engine option. Explicit consideration is also given to BCA emissions. The results indicate that the blended fuel aimed at satisfying the IMO 2020 Sulfur Cap does not perform well in BCA emission control.

Introduction

The world's economy relies heavily on maritime transportation since more than 90% of the global trade is facilitated by marine transportation (International Maritime Organization, 2019). Because of the extensive maritime transportation activities, there is a substantial impact on air pollution. The International Maritime Organization (IMO) has been working on reducing greenhouse gases (GHG) emissions and other pollutants such as sulfur oxides, nitrogen oxidizes. The latest Marine Environment Protection Committee (MEPC) regulation, Annex VI of the International Convention for the Prevention of Pollution from Ship (MARPOL), called “2020 Sulfur Cap”, is set to globally reduce the fuel sulfur content on board from 3.5% mass to 0.5% mass outside the emission control areas (ECA). The new ambitious convention brings many challenges and opportunities for each stage of the whole shipping fuel supply chain. Towards complying with the MARPOL convention, the shipping companies and oil refineries have made various improvements.

The current standard for shipping fuel is ISO 8217: 2017 and its latest revision, which was published on Sep. 18, 2019, named as ISO/PAS 23263:2019. It defines general requirements and serves to confirm the compliant fuels of ISO 8217:2017 with maximum of 0.50% sulfur content, and it addresses quality considerations and the range of marine fuels as well. A new category, very low sulfur fuel oil (VLSFO) with sulfur content range between 0.1% and 0.5%, was introduced to serve as the blended IMO fuel oil for non-ECAs in line with ISO 8217:2017. Besides, the proven IMO 2015 ECA grade distillate fuel, marine gas oil with maximum of 0.1% sulfur content is still an available option for shipowners to comply with the 2020 Sulfur Cap. The high sulfur fuel oil (HFSO), 3.5% maximum sulfur content, is considered as an attractive option for shipping industry because of the low fuel price, but the HFSO driven vessels should be equipped with pollutant reduction devices, mostly scrubbers, to continue serving for worldwide trade under the current MARPOL convention. As a clean fossil fuel, liquefied natural gas (LNG) has been applied to marine transportation for one decade. Many new vessels adopt this technique to meet the latest IMO rule and it is widely accepted as an effective way to comply with the IMO's long-term environment prevention strategy. So far, those four fuel solutions are the most common ones for decision-makers of the global commercial fleet.

The increasingly intense greenhouse effects have brought more concerns than ever before. Motivated by its mission of “safe, secure, clean and sustainable shipping”, IMO has implemented many strategies, regulations, and rules to reduce Greenhouse Gases (GHG) emissions from ships. The major Greenhouse Gases, CO₂, CH₄, N₂O, and O₃ have been studied for years to take effective measures to limit their emissions (Brynolf et al., 2014a; Comer et al., 2017; DNV GL, 2019a; IPCC Panel, 2014; Luo and Wang, 2017; Olmer et al., 2017; Pavlenko et al., 2020; Smith et al., 2014). The Global Warming Potential (GWP) was firstly proposed by Intergovernmental Panel on Climate Change (IPCC) and the 100-year GWP (GWP₁₀₀) was adopted by the Nations Framework Convention on Climate Change and its Kyoto Protocol (IPCC Panel, 2014). However, black carbon aerosol (BCA) has not been widely considered by the shipping industry and related research. Among all particulate phase species, BCA has its uniqueness: stable at high temperature, strong absorption of solar radiation, and insolubility in water, alcohol and other liquids (AMAP, 2015). It is worth noting that an individual soot particle can be aggregated to form external structures, which is subject to forming a mixture of coated particles to reduce the albedo of the surface and to contribute to a continuous warming effect (AMAP, 2015). As the dominant form of light-absorbing particulate matter in the atmosphere, BCA emission has been linked with negative influences on earth's climate change, especially on the vulnerable Arctic region (IPCC Panel, 2014; Sharafian et al., 2019). One recent study (Olmer et al., 2017) pointed out that BCA is the second largest contributor to ship-induced GWP emissions, larger than CH₄ and N₂O; Moreover, black carbon is a major contributor to the human heart and lung disease as well (Bond et al., 2013). As a result, there is no reason to ignore BCA emissions, and this paper considers it as a major pollutant for the global warming potential consequence.

In assessing the environmental impacts of shipping fuels, it is necessary to adopt a life cycle assessment (LCA) approach for the whole supply chain of alternative fuels. Several studies (Bengtsson, 2011; Bengtsson et al., 2012; Brynolf et al., 2014b) have considered the extraction of raw material, fuel production, fuel transportation and storage, and bunkering as the well to tank stage and ship operation with the produced fuel as the tank to propeller stage. An important segment of the LCA is the tank to propeller (TTP) stage which is responsible for the bulk of emissions for carbonaceous fuels and is under the direct control of the shipping industry.

This manuscript will focus on the TTP segment to provide ship fleet manager a reliable reference to estimate the fuel consumption and air pollutant emissions, including both GWP gases and non-GWP gases, during the cruising mode in non-SECAs by adopting IMO 2020 compliant fuel options.