Advantages of Cellulosic Ethanol
Cellulosic ethanol is the biofuel produced from many forms of lig - nocellulosic biomass such as grasses, wood, agricultural wastes, or
Table 2.2 A comparison of feedstocks; sugarcane and corn used for first generation ethanol production in Brazil and in the United States .
inedible parts of plants. The use of lignocellulosic biomass for the production of biofuels, and particularly the cellulosic ethanol, has a number of advantages, as shown below.
1. Cellulosic biomass is the most abundant form of organic carbon on earth. Unlike corn and sugarcane now used to make most ethanol, cellulose is not used for food; therefore cellulosic ethanol will not have adverse effects on food supply and prices. As there is a very wide range of plant materials that can be used for cellulosic ethanol production, it can be grown in all parts of the world. Cellulosic ethanol can be made of many agricultural wastes like corn stover, wheat straw, rice straw, grasses, farm residues, industrial wastes, sawdust, forest thinnings, waste paper, and municipal wastes. Fast-growing woody crops such as poplar and willow are also attractive options for cel - lulosic ethanol production.
2. Cellulosic ethanol achieves a significant reduction in greenhouse gases compared to other forms of ethanol. Figure 2.2 shows a comparison of greenhouse gas (GHG) emissions from ethanol produced from common feedstocks such as corn, sugar beet, wheat, sugarcane, and cellulosic, with gasoline. On a life-cycle
basis, all biofuels produce lower GHG emissions compared to gasoline. As this figure illustrates, corn-based bioethanol offers rather limited benefits, as it reduces GHG emissions by only 18% compared to gasoline. In contrast, cellulosic bioethanol results in almost 90% lower emissions . On a life-cycle basis, not all biofuels are equal in terms of environmental benefits. The net energy balance of biomass to bioethanol conversion is the key parameter that explains the interest in using bioethanol fuel instead of fossil gasoline. From a life-cycle assessment (LCA) viewpoint, the ratio of the energy content of bioethanol to the net non-renewable primary energy consumed in the whole production process must be taken into consideration. As the approach is LCA oriented, the energy input must be estimated in terms of primary energy . Studies have shown that corn-based bioethanol yields 20-30% more energy, typically fossil fuel energy, than is consumed in making it. On the other hand, sugarcane and cellulosic bioethanol yield renewable energy nine times worth the fossil energy used to produce them . The reductions in carbon dioxide emissions mean
that bioethanol is better for the environment. Using renewable resources-based bioethanol or bioethanol - gasoline blends as transportation fuels can significantly reduce gasoline use and exhaust greenhouse gas emission .
3. Land-use change (LUC) is another parameter used in evaluating the biomass-based renewable fuels. Dunn and coworkers from Argonne National Laboratory, USA, have recently published their results on a land - use change and greenhouse gas emissions from corn and cellulosic ethanol . Land-use change occurs when land is converted to biofuel feedstock production from other uses or states, including forests, nonbiofuel feedstock agricultural lands, and grasslands. This type of land-use change is at times called direct LUC. The resulting change in crop production levels like, for example, an increase in corn production, may cause a decrease in soybean production and in turn affect corn exports in one country, shifting the land uses in other parts of the world through economic linkages. This latter type of LUC is called indirect LUC and can be estimated through the use of economic models. A change in land use causes a change in carbon stocks above ground and below ground. As a result, a given LUC scenario may emit or sequester carbon. When a LUC scenario results in a net release of carbon to the atmosphere, it is debatable if biofuels result in GHG reductions at all [13, 14]. Of particular concern is the conversion of forests [15, 16], an inherently carbon-rich land cover that in some cases may be a carbon sink. Their conversion to biofuel feedstock production land could incur a significant carbon penalty . The estimation of LUC and the resulting GHG emissions is accomplished through the marriage of LUC data with aboveground carbon and soil organic carbon (SOC) data for each of the land types affected. The amounts and types of land converted as a result of increased biofuel production can be estimated with an agricultural-economic model, for example, a computable general equilibrium (CGE) model; several recent reports  provide an overview of CGE models and
their application to estimating LUC associated with biofuel production.
The researchers from Argonne National Laboratory investigated the effect of several key carbon content modeling parameters for the United States land-use change and greenhouse gas (LUC GHG) emissions. They used the international carbon emission factors from the Woods Hole Research Center, and the LUC GHG emissions were calculated from these LUCs and carbon content data with Argonne National Laboratory's Carbon Calculator for Land Use Change from Biofuels Production (CCLUB) model. Some of the key results of their study are summarized in Table 2.3, showing the range of land-use change and greenhouse gas (LUC GHG) emissions (g CO2e/MJ) for ethanol produced from switchgrass, miscanthus, corn stover and corn.
Argonne National Laboratory's study indicates that cellulosic ethanol production from miscanthus has the lowest LUC GHG emissions, whereas the
Table 2.3 Range of land-use change green house gas (LUC GHG) emissions (g CO2e/MJ) for ethanol produced from switchgrass, miscanthus, corn stover, and corn .
Values presented represent range of results generated at all combinations of surrogate CENTURY and CCLUB modeling parameters .
highest is from corn ethanol . In an earlier study, Scown et al. also reported comparable GHG sequestration (between -3 and -16 gCO2e/MJ) for ethanol production using miscanthus grass, but their study was limited to active cropland .
This study clearly demonstrated the advantage of cellulosic ethanol varieties over corn ethanol, as switch - grass, miscanthus, and corn stover produced lower LUC GHG emissions parameters in comparison to corn.
4. Processing lignocellulosic biomass for cellulosic ethanol through the saccharification-fermentation route leaves lignin as a byproduct, because only sugars can be converted to ethanol in this method. But, lignin can serve as an energy-rich boiler fuel for distillation of ethanol, or can be used as a raw material for lignin - based feedstock for the chemical industry. According to NREL analysis, there is enough lignin produced in these plants to provide all the energy needs of an ethanol production facility, and any excess lignin can be burned in thermal power plants to produce electricity . Lignin-based, value-added chemicals, like vanillin, are interesting polymer precursors or monomers. Furthermore, lignin can be used as a filler or copolymer as well. The use of byproducts from the cellulosic ethanol plants for chemicals, renewable resources-based polymer production and power generation is known as the integrated bio-refinery concept, which will be an integral part of the sustainable energy landscape of the future. This possibility of generating value-added products and electricity from byproducts is another advantage in cellulosic ethanol.
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