Effects of equivalence ratio and iodine number on nitrogen oxide emissions from the flames of biofuels and hydrocarbons.

摘要

Increased energy consumption in the United States has led to a demand for the development of new bio-derived fuels. As biofuels are used more frequently in diesel and gasoline engines, it has become increasingly important to test the emissions resulting from the combustion of these fuels from internal combustion engines. This study was motivated by the need to test these fuels, predict the combustion characteristics of fuels used in engines, and provide quick feedback to fuel researchers on the combustion characteristics. Therefore, this dissertation presents a technique to characterize the combustion properties of liquid fuels based on the chemistry of the fuel alone. The first part of the dissertation describes the development of a method for the rapid characterization of combustion properties, such as emission index and flame radiation. The technique provided a way of comparing the particulate and pollutant emissions from flames of hydrocarbon fuels to those of new fuels such as biodiesel. Burner conditions were selected to make flame properties sensitive primarily to fuel chemistry. The technique was validated through a comparison of measured radiative heat release fraction and pollutant (NO and CO) emission indices available in literature. It was seen that the present values compared well with the emission indices documented during engine testing and in other flame configurations. Approximately a 10% increase was observed in NO pollutant when biofuels where burned compared to diesel as in engine studies. Findings showed that use of this technique can assist fuel researchers in the development of new fuels since pollutant and sooting tendency data obtained were similar to those from diesel engines. This technique in comparison to engine studies, however, requires only small amounts of fuel, time, and provides a method to compare fuels on a normalized basis.;Based on the observation that the biofuels produced more NO than diesel, it was desired to determine the cause for the increase in NO. For the second part of the dissertation, the equivalence ratio and iodine number were varied and their effect on the formation of NO was studied for four different fuels: canola methyl ester, soy methyl ester, diesel, and normal dodecane fuels. Measurements of intermediate species, flame temperatures, soot volume fraction, and global emissions were made for this purpose. At the lowest equivalence ratio of 1.2, the biofuel flames showed higher NO concentration values for in-flame measurements than diesel flames. NO production was primarily due to the Zeldovich mechanism for both biodiesel and diesel, since high temperatures were recorded, high concentrations of OH were observed, and NO concentration increased downstream of the burner, indicating a dependence on residence time. At higher equivalence ratios from 2 to 7, similar to those predicted to exist in diesel engines, NO production was much higher for the biofuel flames. The Fenimore mechanism was thought to be dominant at this condition, since the CH radical population was high in regions of peak measured NO concentration. A correlation between iodine number and peak NO concentration was also observed. Fuels with lower iodine number values (diesel and methyl stearate) produced less CH and NO concentrations, while fuels with higher iodine numbers (SME and CME) produced the highest CH and NO concentrations. It is thought that the double bonds present in unsaturated fuels, such as SME, facilitated the production of more CH. This coupled with the presence of the oxygen in the biofuels accelerated the formation of NO.