A Study on NOx Emission Characteristics When Using Biomass-derived Diesel Alternative Fuels

Utilization of biofuels to vehicles is attracting attention globally from viewpoints of preventing global warming, effectively utilizing the resources, and achieving the local invigoration. Representative examples are bioethanol and biodiesel.

This study highlights biodiesel and hydrotreated vegetable oil (HVO) in view of reducing greenhouse gas emission from heavy-duty diesel vehicles. Biodiesel is FAME obtained through ester exchange reaction by adding methanol to oil, such as rapeseed oil, soybean oil, palm oil, etc. As already reported, FAME has fuel properties different from conventional diesel fuel, resulting in about 10% increase in NOx emission [1],[2],[3]. Suppression of such increase in the NOx emission during operating with biodiesel requires adjustment of the combustion control technology, such as fuel injection control and EGR, to the use of biodiesel. However, designing of vehicles and engines for dedicated use of biodiesel cannot be expected much because of the development cost. Besides, biodiesel suffers deterioration in low-temperature fluidity in cold climate, so that certain users use diesel fuel only in such cold climate. In this way, many issues must be solved before the vehicles and engines use only biodiesel. On the other hand, HVO is fuel produced by the hydrotreating reaction used in the oil refining process. It is possible for any fatty oil to be raw oil of this fuel. In addition, HVO is able to be used in substitution for conventional diesel fuel since it mainly consists of paraffinic hydrocarbon.

In this study, NOx emission was investigated when FAME and HVO were used for heavy-duty diesel engine under steady-state condition and JE05 driving cycle. From the results of this study, it was confirmed that the fuel injection volume had increased compared to the hydrocarbon fuels such as HVO and diesel fuel because the FAME had small lower heating value (LHV) per unit volume. It was thus confirmed that the combustion control state such as the EGR ratio and the fuel injection pressure had changed. This was presumed to be one of the factors causing the increase in NOx. It was revealed that FAME and the hydrocarbon fuel could suppress the increase in the NOx emission because the higher the H/C ratio of a fuel was, the more the flame temperature decreased. The above results showed that HVO having the LHV per unit volume equivalent to diesel fuel with the high H/C ratio was the biomass-derived diesel alternative fuel which could suppress the increase in NOx emission.