One of the advantages of biofuels is the possible reduction in engine emissions which contribute to global warming and atmospheric pollution. Here again most of the stud­ies have been carried out with the first-generation biofuels, ethanol and biodiesel.

Fig. 8.5. Effect of the addition of 20% ethanol (E20) and 20% methanol (M20) to unleaded petrol on brake torque at various engine speeds. (From Agarwal, 2007.)

Biodiesel

There have been a large number of studies on the exhaust emissions from engines using a variety of biodiesel types and concentrations (Graboski and McCormick, 1998; Willianson and Badr, 1998; EPA, 2002). However, it is difficult to compare results as different engines, conditions, and blends have been used. Figure 8.6 shows the mean of a number of studies on the effect of using 100% rapeseed biodiesel on the important engine emissions: hydrocarbons (HC), CO, nitrous oxides (NOx), and PM. Rapeseed biodiesel is the main biodiesel produced in the EU and the consensus shows a considerable reduction in the emission of HC and PM and a small increase in NO . The increase in NO was probably due to an increase in combustion tem­perature. The mean of the three studies on the effect of sunflower biodiesel on engine emissions is shown in Fig. 8.7. With sunflower biodiesel, the reduction in HC was

same as rapeseed biodiesel, and CO and PM were further reduced, but NOx emissions increased. The emissions from an engine fuelled with 100% waste olive oil biodiesel at different loads are shown in Fig. 8.8 (Dorado et al., 2003). As the load increases the reduction in CO, NOx and sulfur dioxide decreases to zero at the highest load. A different result was observed when a 50% sunflower biodiesel blend was used in a marine diesel engine (Fig. 8.9) (Kalligeros et al., 2003). With 50% sunflower biodiesel, the emissions decrease as for waste olive oil biodiesel but do not reach zero at the highest load. The advantages of using biodiesel to reduce emissions may therefore be eliminated when the engine is used at high loads. However, the reduction in emissions may depend on the test engine used.

Fig. 8.8. The effect of waste olive oil biodiesel (100%) on the percentage of changes in emissions from a diesel engine compared with diesel at various loads (Nm). (From Dorado et al., 2003.)

The effect of increasing concentrations of biodiesel on engine emissions is shown in Figs 8.10 and 8.11. As the concentration of commercial biodiesel in blends increased, the emission of CO was reduced and NOx increased (Fig. 8.10). When soybean biodiesel was tested in contrast to commercial biodiesel, CO was not reduced significantly but HC and PM were reduced and NOx increased.

In general, emissions from diesel engines running on blends or 100% biodiesel showed a reduction in CO, HC and PM, but an increase in nitrous oxide (NOJ levels. The reason for this change in emissions is thought to be the higher oxygen content of biodiesel, which gives a more complete combustion of the fuel and this reduces CO, HC and PM. The Environmental Protection Agency (EPA) has compiled the results of a number of studies on the effect of biodiesel content on emissions and the results were

similar to those observed in Figs 8.10 and 8.11. In a study using a MAN diesel bus engine, the fuel injection characteristics were different for diesel and rapeseed-derived biodiesel (Kegl, 2008). The biodiesel when injected into the engine forms a longer and narrower spray than mineral diesel, caused by a higher injection pressure, increased by low fuel vaporization and atomization due to higher surface tension and viscosity.

The reasons for the increased NOx production when using biodiesel may be the higher combustion temperature and injection characteristics. The increase in nitrous oxide (NOx) is probably due to the raised combustion temperature which is known to increase NOx formation. Advanced injection is caused by the higher bulk modulus of compressibility of biodiesel which allows the pressure wave from the pump to the nozzle to speed up, therefore advancing the timing. It has been observed that retard­ing the timing can in some way reduce NOx emissions. In order to reduce the emission of NOx with biodiesel, the injection timing was altered and the optimum setting was found to be 19° (°CA BTDC, degree of crankshaft angle before top dead centre) compared with 23° for diesel. The effect of altering the injection timing on emissions of CO and NO is shown in Fig. 8.12. The lowest NO emission was obtained at 21°, but the lowest CO emission was at 24°. In all the studies on emissions, no evidence has been given that the engines were optimized for biodiesel, and therefore modifica­tions such as altering the timing may reduce emissions of NOX.

Another way of reducing NOx production is to use exhaust gas recycling (EGR). Diesel engines fuelled with Jatropha oil biodiesel produce more NOX than diesel (Pradeep and Sharma, 2007). In this case exhaust gas recirculation was tested as a system to reduce NOX. The exhaust gases consisting of carbon dioxide and nitrogen are recirculated and injected into the engine inlet, reducing the oxygen concentration and combustion temperature which reduces NOX. The level of recirculation is critical because if the oxygen is reduced too far, incomplete combustion will produce higher levels of hydrocarbon, CO and smoke. In this case with a single cylinder diesel engine, the optimum recirculation was 15% as can be seen in Fig. 8.13.

Fig. 8.12. Effect of injection advance on emissions, the normal setting for diesel is 23°. (From Carraretto et al., 2004.)

100
80
60
0
Biodiesel + 15% EGR
Diesel	Biodiesel
□ Combustion time DNo*.

Fig. 8.13. Comparison of combustion duration (degrees) and NOx emissions for Jatropha sp.-derived biodiesel and diesel using exhaust gas recirculation. (From Pradeep and Sharma, 2007.)

Whatever biodiesel is used the scale of reduction in emissions will also be depend­ent on the engine characteristics such as combustion chamber design, injector nozzle, injection pressure, air-fuel mixture, load and other features. Therefore, the reduction in emission will vary from one diesel engine design to another.

Ethanol

Small quantities of ethanol (3-6%) have been added to petrol to increase the oxygen content to ensure complete combustion and reduce the emission of HC and PM. When high concentrations of ethanol are used such as the E85 fuel in a standard petrol engine, CO and NOx are reduced compared with petrol but there is an increase in HC (Fig. 8.14). In the flexible fuel engine where the conditions are optimized for E85 fuel, emissions of CO and NO were reduced but HC and methane were increased.

x

Fig. 8.14. The effect of E85 on the emis­sions from a standard engine and a flexible fuel engine. CO, carbon monoxide; NOx, nitrous oxides, CH4, methane. (From Wang et al., 1999.)

Dimethyl ether (DME)

A number of studies have been carried out on the emissions from a compression igni­tion engine (diesel) running on DME and DME blends. DME has been shown to pro­duce low noise, smoke-free combustion and reduced NO when used in an internal combustion engine (Huang et al., 2006). DME, because of its high cetane number and low boiling point, has been are developing truck and bus transport fuelled by DME. The emission levels from these development vehicles when run on DME show virtually no PM and low levels (0.5-2.0 g/kwh) of NOx.

Reduction in Carbon Dioxide Emissions used at 100% or as an oxygenated addition to diesel. When DME was used in a diesel engine, it reduced NOx and SOx emissions and was sootless (Semelsberger et al., 2006). Large motor manufacturers when Using Biofuels

Biomass

The carbon dioxide fixed during photosynthesis is released when the biological material is burnt which means that there is no net gain in atmospheric carbon dioxide. This indi­cates that biological materials are ‘carbon neutral’ in nature and therefore ideal for the mitigation of global warming. However, one of the main arguments against biofuels of all types is that fossil fuels are used and carbon dioxide released during the production of biofuels. Therefore, biofuels are not 100% carbon-neutral. Clearly fossil fuel will pro­duce the greatest amounts of carbon dioxide as they release carbon dioxide fixed millions of years ago, whereas biomass has fixed its carbon dioxide in the last 10 years. Table 8.2 gives some values for the carbon dioxide generated per megajoule (MJ) of energy during the combustion of fossil fuels. Coal and coke have the highest carbon content and pro­duce the highest levels of carbon dioxide. Natural gas (methane) has the lowest GHG emissions of the fossil fuels and is one of the reasons why electricity generation was switched to gas in the UK in the 1990s. To be suitable for carbon dioxide mitigation, biofuels will have to have GHG emissions much lower than those for fossil fuels.

The bioenergy crops and waste biological materials described in Chapter 4 if used for energy will clearly reduce the amount of carbon dioxide accumulating in the atmosphere. The carbon dioxide emissions from biomass crops were compared with those emitted from the solid fossil fuels coal and coke (Fig. 8.15). The carbon dioxide produced per megajoule when various biomass sources are burnt or used to generate electricity are compared with two solid fossil fuels coal and coke in

Table 8.2. Greenhouse gas emissions from fossil fuels. GHG emissions Fuel (g CO2/MJ) Reference Coal 107.1-110.4 Gustavsson et al. (1995) Matthews (2001) Coke 117.0-134.0 Gustavsson et al. (1995) Matthews (2001) Fuel oil 81.3-81.4 Gustavsson et al. (1995) Matthews (2001) Diesel 77.6-81.9 Gustavsson et al. (1995) Matthews (2001) LPG 73.6-80.8 Gustavsson et al. (1995) Matthews (2001) Natural gas 66.2-68.5 Gustavsson et al. (1995) Matthews (2001)

120-

100-

2 80­CM О 60-

40

Fig. 8.15. Carbon dioxide emissions from bioenergy crops when burnt or used to generate electricity. SRC, short rotation coppice. (From Gustavsson et al., 1995; Dubisson and Sintzoff, 1998; Matthews, 2001; Bullard and Elsayed et al., 2001; Heller et al., 2001; Keoleian and Volk, 2005.)

Fig. 8.16. Greenhouse gases saved when biomass is either gasified or combusted in g CO2 equivalents/MJ. (From Lettens et al. 2003.)

Fig. 8.15. The combustion of the biomass produces 20 times less carbon dioxide than coal. When short rotation coppice (SRC) and Miscanthus are used to generate elec­tricity more carbon dioxide is formed per unit of energy than simple combustion.

Biomass can also be gasified and the gas used as fuel for gas turbines to produce electricity. Combustion and gasification as a source of energy have been compared using three biomass sources in terms of the amounts of carbon dioxide saved (Fig. 8.16) (Lettens et al., 2003). The perennial grass Miscanthus sp. gives the least carbon dioxide and mixed coppice the greatest, and there appears to be little signifi­cant difference between combustion and gasification.