7.1 Previous study on conversion rate and IMO regulation value

　

 Considerable amount of studies were done with boiler combustion. However, the research is hardly done heretofore, about the conversion rate to NOx for the nitrogen in the fuel oil, in case of a low speed two cycle diesel engine. On the other hand, the nitrogen content in bunker fuel oil is increasing year by year along with the improvement of oil refinery system starting from atmospheric distillation to vacuum distillation and to FCC process. The CIMAC recommendation on NOx measurement published in 1991 is the only available paper that describes the figures of NOx conversion rate. It says the rate is 100% if the engine thermal efficiency is above 40%. The background of this figure is not explained in it.

　

 When we discussed NOx matters in IMO (1990), the main source of bunker fuel oil was residual oil from atmospheric distillation plant. The average value of nitrogen content in fuel was about 0.2%. But after that time, because of high demand on light products like gasoline, refining process was improved to vacuum distillation and to FCC process. And nitrogen was condensed deeply. At present the average nitrogen content in marine bunker fuel oil is estimated a round 0.4%. But in come cases even far higher nitrogen contents (0.8 to 1.0) were reported

　

 Under these circumstances, we should know the effects of nitrogen contents in bunker fuel oil to actually generated NOx from the engine to avoid the risk of monitoring process when it is used as verification measures of IAPP certificate.

　

 For example, if the conversion rate is 100% and fuel contains 0.8% of nitrogen, it means increase of 4g/kWh NOx from the engine. It will be not the negligible amount of NOx compared to IMO limit of 17g/kWh. This is 24% of IMO limit and far beyond the 10% that is approved by IMO as allowance when using residual fuel oil. This means even engines that passed shop test have the risk to over IMO limit if they monitor on board after entering commercial operation with residual fuel. And also the ship operators have to use diesel oil instead of residual fuel when requested to measure NOx by some reason.

　

　

7.2.1 Experimental apparatus and procedure

　

 To find the conversion rate from nitrogen in the fuel to the fuel-NOx, a constant volume combustion chamber (abbreviated to CVCC) was built and some tests were carried out. Fig. 7.2.1 shows the construction of CVCC. To test with CVCC has the following advantages.

　

 Influence of engine speed and fuel injection timing on NOx formation can be fully eliminated.

　

 Precise set of fuel injection condition is possible with an electronically controlled hydraulic actuated fuel injection system installed in CVCC.

　

 It is possible to eliminate the influence of humidity on NOx formation by charging absolutely dry air in the chamber.

　

 A single spray of bunker fuel heated to have a viscosity of 25-30 mm²/s is injected from a hole of 0.16 mm dia. with injection pressure of 66 MPa for 25 milliseconds into the chamber in which air at a pressure of 2.5 MPa and a temperature of 670℃ is charged. On such conditions, bunker fuel spray easily self-ignites. The reproducibility of injection patterns with this system was fully confirmed by testing many times beforehand. And no dependence of the injection pressure curve on the kind of fuel has been found.

　

 NOx measuring procedure is shown in Fig. 7.2.2. Fuel spray is injected and burned five times at a test without renewing the gas in the chamber. After that all the gas is discharged into a sampling bag as shown in Fig. 7.2.2 'Sampling'. During this process, sampled gas is mixed well and becomes homogeneous in the bag. After that gas is sent from the bag to the NOx analyzer as shown in Fig. 7.2.2 'Analysis'. At the same time oxygen % in the gas is measured and NOx data is converted to the state of 13% oxygen. In this experiment, the same zirconia sensor as on board and Shimadzu POT-101 type oxygen sensor were used.

　

7.2.2 Result of preliminary test

　

 At first five samples of fuels shown in Table 7.2.1 were tested with CVCC. MDO is a marine diesel oil which has very low nitrogen content (0.02%). 1-1 and 1-2 are bunker fuels taken from 'IKOMASAN'. A-1 is a bunker fuel from 'ANTARES'. As seen in this table, nitrogen contents of these three bunker fuels are 0.30〜0.39%. 'High N' is a bunker fuel with nitrogen of 0.66%, which was specially prepared for this experiment.

　

 Result of experiment with CVCC is shown in Fig. 7.2.3. Comparing three bunker fuels

from the monitored ships, I-1 and A-1 show higher NOx than I-2 though their nitrogen % are rather lower than I-2.

　

 If the conversion rate from nitrogen in the fuel to the fuel-NOx were 100%, for example, nitrogen of 0.39% in the fuel would form about 230 ppm NOx (with 13% oxygen) according to the calculation. On the other hand, most of the measured NOx data with CVCC are higher than 1000 ppm (with 13% oxygen). That means the thermal NOx has still higher share than the fuel-NOx in the total NOx even if bunker fuel is burned. Therefore, if there were a big change of thermal NOx in the data of experiments, it could hide the change of fuel-NOx.

　

 Fig. 7.2.4 shows the heat release rates of the sample fuels, measured by another smaller combustion chamber on the same fuel injection conditions. According to (a) of the figure, it is clear that I-1 and A-1 have longer ignition delay and larger scale of pre-mixed combustion than I-2. The reason that I-1 and A-1 show higher NOx data than I-2 could be the higher thermal NOx formation because of the larger amount of pre-mixed combustion. To catch the change of fuel-NOx with nitrogen content in the fuel clearly, the NOx data from such the fuel samples with almost similar heat release rate and with different nitrogen % must be compared.

　

 Fig. 7.2.4 (b) shows the heat release rates of MDO, I-2 and 'High N' sample. These three fuels show almost similar scale of pre-mixed combustion by chance. If it can be supposed that the thermal NOx of these three fuels has little difference, gap of NOx data could originate in the change of fuel-NOx.

　

 The solid line in Fig. 7.2.3 that represents the increase rate of measured NOx with increase of nitrogen in the fuel, is based on the data from above-mentioned three fuels with similar scale of pre-mixed combustion. According to it, the conversion rate from nitrogen in the fuel to the fuel-NOx can be counted at about 55%.