3.0 Emission Characteristics

Biodiesel is the only alternative fuel to have a complete evaluation of emission results and potential health effects submitted to the U.S.EPA under the Clean Air Act Section 211(b). These programs include the most stringent emissions testing protocols ever required by EPA for certification of fuels in the U.S. Emission results for pure biodiesel (B100) and mixed biodiesel (B20-20% biodiesel and 80% petrodiesel) compared to conventional diesel are given in Table-3.

Table-3: Biodiesel Emissions Compared to Conventional Diesel

*Estimated from B100 results. **Average reduction across all compounds measured. ***2-nitroflourine results were within test method variability.

The use of biodiesel in a conventional diesel engine results in substantial reduction of unburned hydrocarbons, carbon monoxide and particulate matter. Emissions of nitrogen dioxides are either slightly reduced or slightly increased depending on the duty cycle or testing methods. Biodiesel decreases the solid carbon fraction of particulate matter (since the oxygen in the fuel enables more complete combustion to CO2), eliminates the sulphur fraction (as there is no sulphur in the fuel), while the soluble or hydrogen fraction stays the same or is increased.

The life-cycle production and use of biodiesel produces approximately 80% less carbon dioxide and almost 100% less sulphur dioxide compared to conventional diesel. From Table-3 it is clear that biodiesel gives a distinct emission benefit almost for all regulated and non-regulated pollutants when compared to conventional diesel fuel but emissions of Nox appear to increase from biodiesel. Nox increases with the increase in concentration of biodiesel in the mixture of biodiesel and petrodiesel. This increase in Nox may be due to the high temperature generated in the fairly complete combustion process on account of adequate presence of oxygen in the fuel. This increase in Nox emissions may be neutralized by the efficient use of Nox control technologies, which fits better with almost nil sulphur biodiesel then conventional diesel containing sulphur. A comparative emission scenario with petrodiesel, biodiesel and biodiesel blends evolved from a real-life fleet study is presented in Figure-2.

CASE-I: Petrodiesel with 0.05% sulphur. CASE-II: 20% biodiesel with 3-degree injection timing adjustment. CASE-III: CASE-II + Catalytic Converter (Source: Twin Rivers Technologies, USA).

Fig-2: Comparative Emissions from petrodiesel & biodiesel

3.1 Nox and Biodiesel

Opinion regarding emissions of nitrogen dioxides varies from one study to another study. Some fleet tests concluded Nox emissions to have increased with the use of biodiesel as fuel while other studies proved that emissions of Nox can be controlled, if not decreased, by adjustments like retarding the injection timing or by adding heavy alkylate replacing 20% of the fuel of B20% blend biodiesel. Conclusions from different scientific studies are compiled below.

• "Adjustment of injection timing and engine operation temperature will result in reduction of Nox emission levels from biodiesel below that of petrodiesel levels"- Dr. Kerr Walker, Scottish Agricultural College (Journal of Royal Society of England).

• " Nitrous Oxides (Nox) are reported by several researchers to be increased with biodiesel. However, our own data shows a reduction in Nox, very consistently, throughout all our dynamometer tests. Nox started at 6.2 gm/mile for diesel and goes down to around 5.6 gm/mile with 100% biodiesel, with slightly more reduction with REE (rapeseed ethyl ester) than RME (rapeseed methyl ester)"- Toxicology, biodegradability and environmental benefits of Biodiesel; Charles L. Peterson and Daryl Reece, University of Idaho.

• " Fueling with biodiesel/diesel fuel blends reduced PM, THC and CO, while increasing Nox. Retarding the fuel injection timing reduced Nox emissions while maintaining the other emissions reductions"-DDC engine emission tests using methyl ester; L.G. Schumacher, W.G. Hires; University of Missouri.

• " As the concentration of biodiesel increased, Nox emissions decreased. The B20A20 fuel blend effectively reduced the oxides of nitrogen emissions below that of baseline diesel fuel. Retarding the timing was an effective way of reducing Nox. Moreover, Nox emissions from biodiesel can be successfully reduced below that of baseline diesel fuel either retarding injection timing or replacing 20% of the baseline diesel fuel of B20 blend with heavy alkylate"-Engine exhaust emissions evaluation of a Cummins L10E with biodiesel; William Marshall, Leon G. Schumacher; Society of Automotive Engineers, SAE paper # 952363.

• " Nitrogen Oxides (Nox) emissions from biodiesel increase or decrease depending on the engine family and testing procedures. Nox from pure biodiesel (100%) increased in this test by 13%. However, biodiesel’s lack of sulphur allows use of Nox control technologies that cannot be used with conventional diesel. So, biodiesel Nox emissions can be effectively managed and efficiently eliminated as a concern of the fuel’s use"- U.S. National Biodiesel Board, Biodiesel Report.

• " There are reliable proven methods for baselining or even reducing Nox produced when using biodiesel. I have certified emissions for the urban bus retrofit program with EPA using this technology. This package included use of an

3.2 Comparison of particulate composition: Diesel Vs. Biodiesel

Table-4 gives a comparison of particulate emissions of all forms (insoluble, fuel soluble, lube soluble and inorganic soluble) from petrodiesel and RSME (biodiesel from rapeseed methyl ester).

Table-4: Particulate Composition-Diesel Vs. Biodiesel

(Source: Concawe Report No. 2/95).

The study on mechanism of soot formation from diesel as well asbiodiesel (RSME) indicates reduction in total particulate matter. When the engine is operated on RSME, soot emissions (insolubles) are dramatically reduced, but the proportion of emissions composed of fuel derived hydrocarbons (fuel solubles), condensed on the soot, is much higher as can be seen from the Table given below. This implies that the RSME may not burn to completion as readily as diesel fuel. It should, however, be noted that gaseous HC emissions were reduced with RSME in the above tests. Since concern over particulates arises partly from the potential harmful effects of the soluble fraction, it might be suspected that emissions from RSME would be more harmful however data shows no tendency for the mutagenicity of exhaust gas to increase for a vehicle running on 20% RSME and 80% diesel blends.

3.3 Emissions of Greenhouse gas

Comparative emissions of greenhouse gases for diesel and biodiesel in various stagers of life cycle is depicted in Table-5. Life cycle analysis for various fuels including biofuels is diagrammatically represented in Figure-3, which shows that biodiesel (RSME) has the lowest Greenhouse emissions followed by ethanol from wood. Emissions of greenhouse gases during the production of diesel are about 32 g/km (Fig-5). These are hardly a half of the emissions from producing biodiesel even when straw rather than electricity is used to fire the processing. However, this difference is far outweighed by the emissions of CO2 during the combustion of the diesel itself (245 g/km).

Table-5: Emissions of Greenhouse Gases (g/km)

*Assumed mineral diesel oil used.

**Emissions of straw include those from transporting straw.