Fig.7 indicates that NOx emission decrease along with an increase in the absolute humidity under loads of both 50% and 75%, regardless of the constant speed and propeller law conditions. SFC showed little change.
3.2. Test of Engine A (34BLT)
Fig.8 shows the test resutts for the slow-speed engine A (34BLT). In principle, the tests were conducted under the same way as those for the above medium-speed test engine. However, a difference was detected in the absolute humidity of the air prior to steam addition, as the tests for constant speed and propeller law conditions were carried out on different days. In addition, because the measurement was conducted in June, the base of the absolute humidity was higher than during the period in which the medium-speed test engine was tested.
As indicated by the test results for the medium-speed engine, NOx emission was decreased through steam addition, and virtually SFC showed little change.
Fig.9 shows NOx levels against a horizontal axis that represents the amount of steam injection as a flow ratio to that of fuel, on a weight basis. In the tests on the medium-speed test engine, the largest steam/fuel flow ratio was obtained with a 50% load for propeller law condition, and the ratio was approximately 54% (steam flow at approximately 50 kg/h), providing an NOx reduction of approximately 36%. In engine A (34BLT), the largest steam/fuel flow ratio was obtained with a 75% load for propeller law condition, and the ratio was approximately 28% (steam flow at approximately 72 kg/h), and NOx emission was reduced by approximately 14%. The changes in the NOx emission level was not constant in engine A (34BLT), compared with that observed in the medium-speed test engine. This was partly due to the fact that there was a gap between humidity. bases, i.e., absolute humidity prior to steam addition under each load. Although the NOx emissions reduction was smaller in engine A (34BLT), this was probably due to the fact that its humidity basis was higher. Fig.10 shows the magnitude of the NOx emission reduction attained when the combination of engine parameters was optimized and steam was added. Eventually, NOx emission was reduced by 55%, and the IMO requirements were easily satisfied. It was also found that NOx emissions could be further reduced by increasing the amount of steam injection.
In the current tests, we examined the operation of the steam addition system and investigated the effects of amount of steam addition upon NOx emission and SFC. The absolute humidity goals were not changed on the system, but the input values of apparent ambient humidity were changed. Since the tests were conducted over a short period of time, the long-term effects of steam injection upon engine parts were not clarified.
In the future, it should be confirmed that the varied absolute humidity goal to be input into the control unit, and cheek whether the steam addition system can cope with the change. The influence of steam injection upon engine parts will also be investigated.
4. CONCLUSIONS
The design parameters were optimized through simulation in order to reduce NOx emission from medium- and slow-speed engines, and it is confirmed that the NOx reduction goals were attained in tests on actual engines. In addition, NOx emissions were further reduced by approximately 15% in engine A (34BLT), and approximately 55% in total, through steam addition using a steam addition system. Since it has been determined that NOx emission can be reduced through this steam addition system, it should be increased the reliability of the entire system, which consists of an engine and a steam addition system.
