NO x Emissions of N-butanol and Methane in Low-speed Two Stroke Diesel Engines

: This article firstly structures a detailed combustion reaction mechanism including methane 、 N-butanol 、 nitrogen oxide. According to two-stroke low-speed diesel engine’s parameter with the characteristics of engine firing, homogeneous charge compression ignition is selected as simulation model. At the same the output power and the excess air coefficient is 1.5, by means of the simulation calculation and analysis of reaction path and reaction mechanism, when N-butanol and methane are mixed in different proportions, the exhaust temperature in the reactor reduces with the increase of methane proportion in reactants. At the same time NO mole fraction increases with the proportion of CH 4 in the mixed fuel decreases, but the NO 2 mole fraction will increase with the proportion of CH 4 in the mixed fuel increases.


Introduction
The International Maritime Organization Research Report pointed out that the total emissions of global sea transport CO2 in 2012 were 796 million tons, accounting for about 2.5% of the total global total. If no measures are taken, the discharge of the shipping greenhouse gas by 2050 will increase 50%-250% over 2012. [1] In 2020, my country proposed the development goal of "2030 carbon peaks and carbon neutrality in 2060", which provides a development direction for my country to respond to global warming and achieve green low-carbon emissions. [2] The shipping industry as an important part of the national economy has attracted more and more attention from various countries due to emissions. [3] In the face of energy and environmental crisis, and under the strict emission regulations of the International Maritime Organization， adopting clean energy instead of traditional fossil fuels as current research hotspots. [4][5] Methane is the least carbon-containing fossil fuel, but its reserves are abundant and the heat value is high. [6] Its combustion does not contain harmful substances such as sulfide, which can effectively reduce the emissions of nitride and sulfides. [7] Positive n-butanol is a relatively ideal biofuel. Alcohol fuel can partially replace diesel and can effectively reduce the emissions of diesel engine nitrogen oxides and carbon smoke. [8] The chemical preparation of N-butanol by ethanol can reduce the dependence on non-renewable resources and also effectively reduce greenhouse gas emissions. [9] And n-butanol and diesel can be completely dissolved without solvents. Its cetane number is higher and can better ignite diesel. [10] In order to reduce the emission of pollutants from internal combustion engines, dual fuel engines have developed rapidly. The type and proportion of fuel used in dual fuel diesel engine directly affect the emission of ship pollutants and the output performance of diesel engine. In this paper, starting from the mixing ratio of methane and n-butanol, the simulation method is used to study the changes of diesel engine combustion and pollutant content in different proportions of mixed fuels, which is widely used for dual fuel engines in ships and promotes the accelerated development of ' carbon neutralization ' in the shipping industry.

2.The technical parameters of the twostroke internal combustion engine
The model of the diesel engine selected in this study is MAN 6S35ME-B9 medium and low speed two-stroke diesel engine. The main parameters of the diesel engine required for simulation are shown in table 1. The simulation software of this study is CHEMKIN-PRO, and the model of the reactor is a homogeneous charge compression ignition, HCCI. The compression ratio of HCCI engine is higher than that of ordinary gasoline engine, and the non-uniform diffusion combustion of diesel engine is not used, so the fuel efficiency is improved. Tab 1. Basic technical parameter of 6S35 diesel engine [11] Parameter name Technical specification Nominal power /KW 3570 Rated speed /r·min -1 142 Compression ratio 21

3.Calculation model and chemical reaction kinetics model
Excess air coefficient, also known as the excess air coefficient, refers to the ratio of the air volume actually supplied to the fuel combustion to the theoretical air volume. It is an important parameter reflecting the ratio of fuel to air. In the simulation calculation, the ratio of reactants is calculated according to the ratio of fuel to air reaction (excess air coefficient ) of the two-stroke large engine of the overload medium and low speed ship. In order to make the fuel fully burn and reduce the formation of incomplete combustion products such as carbon deposition and CO and HC during ship operation, the mixing ratio of air and fuel is generally set to 1.5 : 1.0. In this study, the excess air coefficient is set to 1.5 to make the fuel of the two-stroke medium and low speed diesel engine fully burn during operation. CHEMKIN-PRO software was used to simulate the combustion of n-butanol and methane in different proportions under 75 % operating conditions. The reaction mechanism required for simulation includes detailed kinetic and thermodynamic data of reactants, products and intermediates. The reaction mechanism of n-butanol and methane comes from the mechanism model of the software. The mechanism includes the kinetic and thermodynamic data of all reactions in the low temperature combustion and high temperature combustion stages. However, the reaction mechanism only contains C, H and O elements. In the high temperature combustion stage, N and O elements will undergo oxidation reaction to produce NOx. Therefore, adding Zeldovich 's NOx reaction mechanism to the mechanism simulates the production and conversion of NOx during the combustion reaction.
The chemical reaction equation of complete combustion of n-butanol and methane is : (2) According to Formulas (1) and (2), the amount of oxygen and nitrogen required for complete combustion of nbutanol and methane with different mixing ratios can be obtained when the air excess coefficient is 1.5 and the total power is constant. As shown in Table 2, the mass of fuel and air required at different mixing ratios under 75 % load conditions. Tab 2. Quality of mixtures with the air serial number n-butanol / methane n-butyl (g) methane (g) oxygen (g) nitrogen (g)

4.Results and discussion
HCCI is a new combustion mode between gasoline engine and diesel engine. It injects a very uniform proportion of air and fuel mixture into the cylinder like traditional gasoline engine, but its ignition process is similar to that of diesel engine. It burns itself when the temperature of the mixture is increased to a certain extent by piston compression. Fig 1 shows the combustion temperature distribution of n-butanol and methane in two-stroke lowspeed HCCI. The combustion mode of HCCI can be well explained from the diagram. It can be seen from the diagram that the temperature in the HCCI reactor increases slowly with the increase of the crankshaft angle. When the piston moves near the top dead center, the temperature in the reactor rises abruptly to the maximum value, and then decreases until the exhaust port is opened. The main reason for the slow increase of temperature from the closing of the exhaust port to the occurrence of the reaction is that the piston is upward, the mixture is compressed, the pressure in the reactor is increased, and the temperature is increased. When the piston moves to the top dead center, the pressure and temperature rise to a certain value. The mixed fuel in the reactor burns and releases a lot of heat. The pressure in the reactor rises sharply and produces a large amount of gas. Due to the short combustion reaction time, the piston began to go down, and the temperature in the reactor decreased. From the simulation results, it can be seen that as the proportion of CH4 in the fuel continues to increase, the exhaust temperature decreases. When the sequence number 1, that is, the methane ratio is 0 %, the temperature when the exhaust port is opened is 1207.178 K. When the sequence number 2, that is, the methane ratio is 30 %, the temperature when the exhaust port is opened is 1196.53 K. When the sequence number 3, that is, the methane ratio is 70%, the temperature when the exhaust port is opened is 1190.81 K. When the sequence number 4, that is, the methane ratio is 100%, the temperature when the exhaust port is opened is 1184.36 K. In the case of the same output power, increasing the proportion of CH4 in the fuel can effectively reduce the exhaust temperature. When the crankshaft angle is about 20 °, the molar fraction of NO decreases slowly until the exhaust port is opened. It can also be seen from the figure that as the proportion of CH4 in the reactants increases, the mole fraction of NO decreases when the exhaust port is opened. This is because in the reactor, the higher the proportion of methane in the mixed fuel, the lower the temperature in the reactor during the reaction, the shorter the time of high temperature oxidation reaction of N2, and the less the amount of NO produced. Explain the phenomenon that the mole fraction of NO begins to decrease from the top dead center : The reaction equation involved in NO is analyzed. Absolute rate of production NO/(mol·cm -3 ·s -1 )

2NO
2NO O  + ,NO2 is the product. According to the total yield, the main reason for the increase of NO2 mole fraction is the production of NO.

5.Discussion
When the output power is the same and the air excess coefficient is 1.5, increasing the proportion of CH4 in the fuel will reduce the temperature when the exhaust port in the reactor is opened. Therefore, the increase of CH4 in the fuel can effectively reduce the exhaust temperature, and the mole fraction of NO in the exhaust gas increases with the decrease of the proportion of CH4 in the mixed fuel. Through the analysis of the reaction path and reaction mechanism under 75% operating condition, when the proportion of mixed fuel is 30% n-butanol and 70% methane, it can be seen that the decrease of NO mole fraction is mainly due to the conversion of part of NO into N2 and part of NO into NO2 .The decrease of the mole fraction of NO2 at the crankshaft angle of -2 º~20 º is due to the conversion of NO2 into NO, and the increase of the mole fraction of NO2 at the crankshaft angle of 20 ~98 º is due to the conversion of NO. The mixed fuel composed of n-butanol and methane in proportion not only does not change the nature of fuel combustion, but also can reduce the generation of nitrogen oxides and reduce pollution.