The major drivers of biofuel promotion worldwide is the concern about climate change and the potential of biofuels to reduce GHG emissions. Although it is incontestable that the use of bioethanol is able to reduce GHG emissions significantly when compared to fossil fuels, assessments of quantified GHG reductions are useful and necessary. However, the GHG balance for bioethanol is highly variable and includes emissions of cultivation, transport, conversion process and distribution. Further, the GHG reduction potential depends on type of feedstock, agricultural practices, site productivity, conversion technology, and finally on the whole design of the study [1-2].
Detailed summaries of studies, indicating GHG reductions by using neat or blended bioethanol, are given by WWI and OECD/IEA. They show reductions of up to 96 % for anhydrous bioethanol in Brazil (MACEDO et al. 2003). Generally, ethanol produced from sugar cane grown in Brazil shows one of the greatest benefits. This is confirmed by several studies which all have found that emission reductions of sugar cane ethanol in Brazil far exceed those from grain-based ethanol produced in Europe and the United States. KALTNER et al. (2005) estimate that the total life-cycle GHG emissions reductions associated with the ethanol industry in Brazil are equivalent to 46.6 million tons annually. These are approximately 20 % of Brazil’s annual fossil fuel emissions. This is due to high site productivity in Brazil and its favorable climate for sugar cane, which is highly productive and only needs low inputs of fertilizer. Additionally almost all conversion plants use bagasse for process energy which also reduces GHG emissions. Many plants also cogenerate heat and electricity. Well-to-wheel CO2 emissions of sugar cane-ethanol are estimated to be, on average 0.20 kg per liter of fuel used, versus 2.82 kg for gasoline. These figures based on CO2 also take methane and N2O emissions into account (both mainly released from farming, from the use of fertilizers and from N2 fixed in the soil by sugar cane then released to the atmosphere).
Apart from sugarcane, other combinations of biofuel feedstock and conversion processes can reduce well-to-wheels CO2-equivalent GHG emissions to near zero, too. An example therefore is enzymatic hydrolysis of cellulose where ethanol is produced and biomass is used as process fuel.
In contrast, ethanol from corn shows very small GHG reductions within all potential feedstock options (WWI 2006 p. 153). Using commercial processes, the use of ethanol derived from grains, brings a 20% to 40% reduction in well-to-wheels CO2-equivalent GHG emissions, compared to gasoline .
For Europe, ethanol production from sugar beets is important, due to its high dominance in several European countries. Some European studies, which are summarized by OECD/IEA (2004 p. 58f), show that this feedstock and conversion process can provide up to a 56% reduction in well-to-wheels GHG emissions, when compared to gasoline.
Nevertheless, some results point out that using ethanol to make ETBE results in even greater GHG savings than blending ethanol directly with gasoline. This is because ETBE replaces MTBE, which has relatively high energy demand, whereas ethanol replaces gasoline, which requires less energy for production than does MTBE.
The major part of engine exhaust streams during ethanol combustion consists of the components nitrogen, carbon dioxide and water. All three components are non-toxic to human health. However, about 1.4% of petrol engine exhaust emissions are composed of more or less harmful substances to human health.
Apart from the above mentioned emissions, fuel combustion emits particulate matter (PM), volatile organic compounds (VOCs), nitrogen oxides (NOx), carbon monoxide (CO) and a variety of other toxic air pollutants. VOCs and NOx are precursors for tropospheric ozone. Momentary weather conditions and local geographic characteristics influence the impact of these air pollutants. Ozone formation e.g. occurs more easily during hot weather. Also toxic air pollutants are more evident under hot weather conditions. They can be emitted either by the engine exhausts or by evaporation from fuel storage and fuel handling since ethanol has high volatility and generally increases evaporative emissions of gaseous hydrocarbons. As opposed to this, carbon monoxide is a larger problem in cold weather and at high altitudes.
To asses the environmental impact of substituting petrol with ethanol, both fuels have to be compared regarding their emissions. Therefore a detailed comparison between emissions of ethanol and petrol combustion will be done.
Harmful engine exhaust emissions from combustion of ethanol are generally lower when compared to the tailpipe emissions of fossil petrol. Thus ethanol can reduce certain vehicle pollutant emissions which exacerbate air quality problems, particularly in urban areas.
Among the biggest benefits from using ethanol is the high reduction potential of carbon monoxide (CO) emissions. The use of E10 is reported to achieve a 25% or greater reduction in carbon monoxide emissions due to the increased oxygen content of ethanol (OECD/IEA 2004 p. 112). Ethanol contains approximately 35 % oxygen which promotes a more complete combustion of the fuel. Thus, in some countries, ethanol is used as oxygenate for fossil petrol and is increasingly replacing the oxygenate MTBE due to the high ground water contamination potential of MTBE.
On the other hand ethanol-blended petrol emits higher evaporative hydrocarbons (HC) and other volatile organic compounds (VOCs) than petrol. When ethanol is added to gasoline, evaporative VOCs can increase due to the higher vapor pressure, measured as Reid Vapor Pressure (RVP) of the ethanol mixture. Generally, adding the first few per cent of ethanol triggers the biggest increase in volatility. Raising the ethanol concentration further does not lead to significant further increases (and in fact leads to slight decreases), so that blends of 2%, 5%, 10% and more have a similar impact.
Impacts of ethanol on nitrogen oxides (NOx) are generally minor, and can either be increased or decreased, depending on conditions. NOx emission from combustion of ethanol blends range from a 10% decrease to a 5% increase over emissions from gasoline (OECD/IEA 2004 p. 114). However, if the full life cycle of ethanol is considered, NOx emissions can be significantly higher mainly due to emissions from feedstock production. NOx is released from fertilizers used to grow bioenergy crops, and is emitted mostly outside urban areas.
When gasoline is blended with ethanol, emissions of most toxic air pollutants decrease. This is primarily due to the dilution effect of ethanol which substitutes some part of gasoline, which emits toxic air pollutants. For instance, toxic emissions of benzene, 1,3-butadiene, toluene and xylene decrease when ethanol is added. Benzene is a carcinogen, while olefins and some aromatics which are emitted by the combustion of fossil fuels as well, are precursors to ground-level ozone. While few studies have looked at the impacts on pollution levels from high blends, it appears that impacts are similar to those from low blends.
The above mentioned toxics benzene, 1.3-butadiene, toluene and xylene, which are emitted by the combustion of fossil fuels, are considered to be more dangerous than emissions of ethanol combustion. During ethanol fuel combustion, emissions of the toxic air pollutants acetaldehyde, formaldehyde, and peroxyacetyl nitrate (PAN) increase relative to straight gasoline. Acetaldehyde is emitted most, but it is a less-reactive and less-toxic pollutant than formaldehyde. PAN is an eye irritant and is harmful to plants. No one of these pollutants is present in the unburned fuel, as they are only created as byproducts of incomplete combustion . Nevertheless, impacts of acetaldehyde and PAN seem to be minor as emissions are relatively low compared to other sources and as they can be efficiently removed by a vehicle’s catalytic converter.
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