## 1 Introduction  
Diesel engines are widely used on ships as the main power and power generation unit. In addition to CO\({}_{2}\) causing the greenhouse effect, ship exhaust gas also contains a large amount of pollutants such as sulfur oxides (SO\({}_{x}\)), nitrogen oxides (NO\({}_{x}\)), particulate matter (PM), and carbon monoxide (CO). [1,2] The International Maritime Organization (IMO) issued Convention of MARPOL 73/78 Annex VI, which lists relevant requirements for SO\({}_{x}\) and NO\({}_{x}\) emissions from ship exhaust. Marine diesel engines generally use heavy fuel oil with a 3.5% sulfur content. This usually results in a high concentration of PM in the exhaust gas, and causes sulfur poisoning of the selective catalytic reduction (SCR) catalyst. Low-sulfur fuel and scrubber technologies [3] are generally used to meet the Convention's requirements for SO\({}_{x}\) emissions. If a scrubber is used, the initial investment is relatively high, and wastewater and waste slag emissions need to be treated. IMO's survey of 2016 for a global, low-sulfur fuel supply chain showed that there will be enough low-sulfur fuel oil to meet the needs for global ships by 2020. Therefore, IMO will enforce the standard of 0.5% sulfur content of marine fuels for global ships in 2020. As a matter of fact, \(<\) 0.1% sulfur content fuel oil has already been enforced for emission control zones since 2015 [4].  
For NO\({}_{x}\) emissions, exhaust gas recirculation (EGR) [5] and SCR technologies are commonly used to meet the Convention's requirements. SCR is an effective post-processing technique with high efficiency, in which there are mainly vanadium-based, copper-based, and iron-based SCR catalysts. Iron-based catalysts have a higher denitration efficiency above 300 \({}^{\circ}\)C, and the production process is complex [6,7]. Copper-based catalysts have a high denitration efficiency in the range of \(\sim\)200-300 \({}^{\circ}\)C,while its thermal stability and sulfur tolerance are poor [8; 9]. Vanadium-based catalysts have a good thermal stability and sulfur tolerance and high denitration efficiency in a wide temperature range, and thus are more often used commercially. The issue with vanadium is that its price is relatively high. In addition, it can cause secondary pollution when expired, as V\({}_{2}\)O\({}_{5}\) is highly toxic. Therefore, it is of interest to find a catalyst with low cost and good denitration ability.  
Activated carbon (AC) has a high specific surface area and a variety of oxygen-containing functional groups, which can effectively adsorb NOx[10; 11; 12], and can be used as a SCR catalyst when NH\({}_{3}\) exists [13; 14; 15]. AC has been used in fossil power plants and steel plants for flue gas desulfurization, [16; 17; 18] denitration [12; 19; 20], and mercury control [21; 22]. Most previous researchers have used simulated gas to evaluate the desulfurization and denitration performance of AC. Activated carbon for contaminant removal in real exhaust of marine diesel engines has rarely been reported. However, simulated gas not only differs in composition from real exhaust, but also does not contain PM, which can adhere to the catalyst surface, resulting in a great influence on the denitration performance of the catalyst.  
In summary, low-sulfur fuel oil and activated carbon denitration together can be a possible path to control the emission of multiple exhaust pollutants from marine diesel engines. The goal of this paper was to study denitration capability of AC under real diesel exhaust gas conditions. As typical marine diesel engines are too large to study in a lab environment, a 15 kW non-supercharged direct injection diesel engine was used, although the PM concentration in land-based diesel engine exhaust is lower than that from a marine diesel engine. The feasibility of activated carbon in the removal of various exhaust pollutants from marine diesel engines and references for future applications are discussed.