In order to present the results in a comprehensive way, the same structure as in the previ­ous sections is used. Default methodological choices include A-1 (i. e., energy allocation) regarding allocation methods (based on the recommendations of the EC, 2009) and LUC-1 (i. e., set aside to cultivated land) regarding land-use change. The default choice regarding fuels blends and vehicle/fuel performance is E5-1 (i. e., ethanol used as E5, with fuel perfor­mance based on actual vehicle tests), according to the most common situation in the EU. The results showing the effect of methodological choices on the WtW net GHG emissions of bioethanol are given in Table 5. The same results are illustrated in Figure 5.

The results are presented as net GHG emissions ( as net energy use, respectively) of fuel-ethanol, expressed in kg CO2 eq./km (in MJp/km, respectively). A positive value means that the system results in a net emission of GHG over the life cycle, whereas a negative value (only one case in the selected options below) means that the system is actually capturing GHG. These are then compared to gasoline in order to assess the actual balance and the potential for reducing GHG emissions and nonrenewable primary energy use. The net GHG emissions and net energy use of gasoline are 0.237 kg CO2 eq./km and 3.493 MJp/km, respectively. Any smaller (larger, respectively) score for ethanol means that the system actually results in a reduction (an increase, respectively) of environmental impact with respect to gasoline.

The base of thermochemical conversion is the pyrolysis process in most cases. The products of conversion include water, charcoal (carbonaceous solid), biocrude, tars, and permanent gases including methane, hydrogen, carbon monoxide, and carbon dioxide depending upon the reaction parameters such as environment, reactors used, final temperature, rate of heating, and source of heat.

2.1 Combustion

Combustion is the sequence of exothermic chemical reactions between a fuel and an oxi­dant accompanied by the production of heat and conversion of chemical species. During the combustion of lignocellulosic biomass, the heat is generated due to oxidation reaction, where carbon, hydrogen, oxygen, combustible sulfur, and nitrogen contained in biomass react with air or oxygen. By far the most common means of converting biomass to usable heat energy is through straightforward combustion, and this account for around 90% of all energy attained from biomass (http://www. esru. strath. ac. uk/EandE/Web_sites/06-07/Biomass/HTML/ combustion_technology. htm). It contributes over 97% of bioenergy production in the world. Combustion is a proven low-cost process, highly reliable technology, relatively well under­stood and commercially available. There are three main stages that occur during biomass combustion: drying, pyrolysis and reduction, and combustion of volatile gases and solid char.

Typically, the biomass contains high moisture and high oxygen content, which causes to have low heating values for biomass. The high moisture content is one of the most significant disadvantage features. Although the combustion reactions are exothermic, the evaporation of water is endothermic. As the moisture content increases, both the higher heating value (HHV) and lower heating value (LHV) decrease. HHV and LHV are used to describe the heat pro­duction of a unit quantity of fuel during its complete combustion. In determining the HHV and LHV values of a fuel, the liquid and vapor phases of water are selected as the reference states, respectively. The negative linear relationship exists between the moisture content and the heating value. Fouling (alkali and other elements) and corrosion (alkali, sulfur, chlorine, etc.) of the combustor are typical issues associated with biomass combustion. These are con­sidered to be detrimental because of the resulting reduction in heat transfer in the combustor.

There are a number of combustion methods/technologies/reactors available for biomass combustion and the main ones can be categorized under two headings: Fixed-bed combustion systems and fluidized-bed combustion systems.

In their largely discussed work, Fargione et al. (2008) and Searchinger et al. (2008) showed the importance of including land-use change emissions in the GHG balance of biofuels. Righelato and Spracklen (2007) have even questioned biofuels production as a strategy to mit­igate global warming. Direct land-use change concerns for example the case where produc­tion of energy crops for biofuels production leads to the conversion of land actually storing carbon (e. g., grassland, native ecosystems) to cultivated land for biofuels production. Missing to consider the previous storage of carbon will overestimate the reduction of GHG emissions of the biofuel chain. On the contrary, when the feedstock is produced on degraded soil, it can contribute to improve the soil carbon balance (Panichelli and Gnansounou, 2008). Conse­quently, the choice of the previous state of the land-use system can significantly affect the GHG balance of the biofuel. Direct land-use change is taken into account in a few recent stud­ies (e. g., ADEME, 2010; CONCAWE-EUCAR-JRC, 2008; EMPA, 2007a). In the three studies, the recommendations of IPCC (2003a) are used for this purpose.

Taking into consideration indirect land-use change (land-use changes due to displaced activities or biomass use) is more complex as the indirect conversion of land is a global and dynamic issue that is difficult to relate accurately to biofuels production, more research works are needed for improving the methodologies on this aspect.

Gasification is the conversion of solid raw material into fuel gas or chemical feedstock gas otherwise called as synthesis gas, which can be upgraded to liquid fuels (diesel and gasoline) by Fischer-Tropsch synthesis. Biomass gasification is a process that converts carbonaceous biomass into combustible gases (e. g., H2, CO, CO2, and CH4) with specific heating values in the presence of partial oxygen (O2) supply (typically 35% of the O2 demand for complete combustion) or suitable oxidants such as steam and CO2.

When air or oxygen is employed, gasification is similar to combustion, but it is considered a partial combustion process. In general, combustion focuses on heat generation, whereas the purpose of gasification is to create valuable gaseous products that can be used directly for combustion, or be stored for other applications. In addition, gasification is considered to be more environmentally friendly because of the lower emissions of toxic gases into the atmosphere and the more versatile usage of the solid byproducts (Rezaiyan and Cheremisinoff, 2005).

Gasification can be viewed as a special form of pyrolysis, taking place at higher temperatures to achieve higher gas yields. Biomass gasification offers several advantages, such as reduced CO2 emissions, compact equipment requirements with a relatively small footprint, accurate combustion control, and high thermal efficiency (Marsh et al., 2007;

Rezaiyan and Cheremisinoff, 2005). Gasification is normally carried out at temperatures over (727 °C)1000 K, but recently it has been demonstrated that H2 and CO can be produced through the aqueous phase reforming of glycerol at lower temperatures <347 °C (<620 K) (Simonetti et al., 2007; Soares et al., 2006) at which integration of syngas production with FT upgrading is feasible. The ratio of CO/H2 can be modified by the water gas shift reaction (CO + H2O! CO2 + H2).

The classification of gasification is based on several parameters such as types of gasifiers, gasification temperature, heating (direct or indirect), and gasification agent.

The results indicate a strong influence of the choice of allocation method, with net GHG emissions ranging from -0.016 kg CO2 eq./km (S-4, i. e., substitution with both straw and DDGS as fuel) to 0.151 kg CO2 eq./km (S-1, i. e., substitution with both straw and DDGS as animal feed), that is, from -107% to -36% with respect to gasoline, respectively. In all cases, however, the net GHG emissions of bioethanol are lower than those of gasoline (0.237 kg CO2 eq./km), with a percentage reduction of 36% in the "worst" case. The negative value in S-4 is explained by the fact that both the straw and the DDGS are replacing fossil energy agents for combined heat and power (CHP) applications. The electricity mix considered is that of Switzerland as reported in ecoinvent, while fuels for heat applications include 53% fuel oil and 47% natural gas (corresponding to the fuel mix in Switzerland). In S-1 and S-3 (i. e., substitution with straw as fuel and DDGS as animal feed), however, DDGS replace soybean meal (imported from Brazil and the US in equal shares, with a ratio of 0.82 kg of soybean meal per kg of DDGS based on dry weight protein content), where soybean oil is considered to be used as a feedstock for biodiesel production and to replace diesel fuel (substitution is applied over the global system). The con­sequence of using DDGS as animal feed in place of soybean meal is therefore unfavorable, showing on the net GHG emissions of bioethanol in S-1 and S-3.

As far as allocation methods are concerned, A-1 (energy), A-3 (carbon) and A-4 (dry mass) produce similar results, with net GHG emissions a lot more favorable than A-2 (economy). This is explained by the fact that wheat grains, straw, and DDGS have similar LHV and car­bon contents. In this particular case of wheat to ethanol, the net GHG emissions in A-2 are not particularly sensitive to ethanol and grain prices. Increasing both prices by 50% results in an increase of the net GHG emissions by only 3% (allocation to grains with respect to straw and ethanol with respect to DDGS being already close to 100%).

There are two prominent types of fixed-bed combustion: underfeed stokers and grate firings. With these methods of combustion, air is primarily supplied through the grate from below, and initial combustion of solid fuel takes place on the grate and some gasification occurs. This allows for secondary combustion in another chamber above the first where secondary air is added. Generally, fixed-bed combustion is used in small-scale batch furnace for biomass containing little ash. Typical examples of fixed-bed systems are manual-fed systems, spreader-stoker systems, underscrew systems, throughscrew systems, static grates, and inclined grates.

A high sensitivity to the allocation method has been reported for LCA results when evaluating carbon intensity and fossil energy consumption for bioethanol pathways (Beer and Grant, 2007; Kim and Dale, 2002; Malca and Freire, 2006). Allocation refers to the distribu­tion of environmental burdens between coproducts in the LCA of a multifunctional system. The issue of allocation is one of the weaknesses of biofuels LCA. The ISO 14040-series (ISO, 2006a, b) recommends avoiding allocation whenever possible either through division of the whole pro­cess into subprocesses related to coproducts or by expanding the system limits to include the additional functions related to them (often referred to as system expansion or substitution and treated in the literature as an allocation method of its own). A complete subdivision is not possible for joint production processes due to the dependence between coproducts’ flows. In fact, subdivision is only feasible when unit subprocesses are physically separate in space or time (combined production), so it is only on exceptional occasions that the allocation problem can be completely eliminated. Ekvall and Finnveden (2001), after screening a large sample of LCA case studies, did not find a case study where this was the case. In the present study, sub­division is applied to the stages downstream of the distillation process. According to Kim and Dale (2002), system expansion is based on the assumption that function-equivalent production systems have equal environmental impacts; that is rarely the case. Furthermore, this approach requires highly accurate data and can be subject to a high degree of uncertainty and/or inaccu­racy; its implementation is difficult as the result depends significantly on the substitute that is chosen in the reference system. Finally, estimating the impact of this substitute may lead to another allocation problem. If "avoiding" allocation is not possible, then the ISO series recommends using a method that reflects the physical relationship between the environmental burdens and the coproducts. In that sense, allocation can be carried out by mass (wet or dry), carbon content, energy content or volume. Allocation on a weight basis relates products and coproducts using a physical property that is available and easy to interpret. But some researchers claim that it cannot be a good measure of energy functions (Malca and Freire, 2006; Shapouri et al., 2002). Energy allocation is mostly used in US biofuels studies by the US Department of Agriculture (Shapouri et al., 2002) and the Argonne National Laboratory (Wang, 2005). It is also the methodology chosen in the European Union (EC, 2009) and conse­quently applied by ADEME (2010). However, an objection can be made against this approach in the case where the coproducts are not meant for energy purposes. When physical properties are not appropriate, ISO recommends the use of other basis for allocation such as the economic value of the products. The rationale for economic allocation is that environmental burdens of a multifunctional process could be allocated according to the share on sales value, because demand is the main driving force of the production system. Price variation, subsidies, and mar­ket interferences could however imply difficulties in its implementation (Bergsma et al., 2006; Shapouri et al., 2002; Wang, 2005). In an LCA carried out in order to determine the net energy value (NEV) of bioethanol production, Shapouri et al. (2002) do not recommend this method because prices are determined for a number of market factors that are not related to the energy content. Gurnee et al (2004) state that in spite of considerable price fluctuations, the shares on the total sales value remain quite constant, particularly in the longer term. According to some researchers (Weidema, 2003), attributional LCA requires market allocation, while consequential applications require system expansion. Allocation by mass is applied in ADEME-DIREM-PWC

(2002) . System expansion (or substitution) is used in CONCAWE-EUCAR-JRC (2008), GM-LBST (2002), and VIEWLS (2005). The latter is also tested in ADEME (2010) by means of a sensitivity analysis. Kim and Dale (2002) investigated an expanded system including ethanol production from dry and wet milling, agricultural corn production, soybean oil and soybean meal from soybean milling as well as the urea production system for animal feed. Economic allocation was used in EMPA (2007a) and Gnansounou and Dauriat (2004). Elsayed et al.

(2003) used alternatively economic allocation and substitution, depending on the biofuel pathway considered and the availability of data.

2.1.1.1 FIXED-BED GASIFIERS

Fixed-bed gasifiers generally produce low-heating-valued syngas. They are suitable for small or medium-scale thermal applications.

2.1.1.1.1 UPDRAFT (COUNTER-CURRENT) GASIFIERS The updraft gasifier is the simplest type of gasifier. The biomass is fed at the top while the air is injected at the bottom. Biomass and air move in a countercurrent direction. During its downward movement, biomass is firstly dried passing through a "drying zone." In the "distillation zone," biomass under­goes decomposition and is converted into volatile gases and solid char. The gases and char will be further converted into CO and H2 as they pass through "reduction zone". Since some of the char settles down in the bottom of the reactor, heat is generated through its combustion in the "hearth zone" and is transported upward by the upflowing gas to main­tain the pyrolysis and drying processes. In addition, CO2 and H2O vapor is also produced from char combustion.

Updraft gasifiers can accept biomass with relatively high moisture content (up to 60%). However, the resulting product gas has high tar content because the tar, newly formed during pyrolysis, does not have the opportunity to pass through the combustion zone.

2.1.1.1.2 DOWNDRAFT (CO-CURRENT) GASIFIERS The downdraft gasifier is currently one of the most widely used fixed-bed gasification systems. Different from the updraft gasifier, air in the downdraft gasifier is introduced into the reactor from the middle part. This design leads to the reversed order of the hearth zone and the reduction zone. In this gasifier, the injected air and biomass move cocurrently.

2.1.1.1.3 CROSS-FLOW GASIFIERS In a crossflow gasifier, biomass is added at the top of the reactor and moves downward. Air is introduced from one side of the reactor and the gas products are released from the other side of the reactor on the same horizontal level.

2.1.1.1.4 OPEN-CORE GASIFIERS Open-core gasifiers are generally employed to gasify biomass with low bulk density and high ash content. An example of this kind of biomass is rice husk. Instead of the narrow throat characteristic of other gasifiers, the open-core gasifier has a wide mouth for biomass injection to prevent fuel flow inhibition caused by bridging.

2.1.1.2 FLUIDIZED-BED GASIFIERS

Fluidized-bed reactors are widely employed as gasifiers. Fluidized-bed gasifiers can also be further classified into bubbling fluidized gasifiers and circulating fluidized gasifiers. In a bubbling fluidized gasifier, air is injected from the bottom of a grate, above which the moving bed is mixed with the biomass feed. The bed temperature is maintained at 700-900 °C. Biomass is pyrolyzed and cracked through contact with the hot bed material. In a circulating fluidized gasifier, the hot bed material is circulated between the reactor and a cyclone separator. During this circulation, bed materials and char go back to the reactor, while the ash is separated and removed from the system.

The results show a strong influence of the land-use change considered, with net GHG emissions ranging from 0.047 kg CO2 eq./km (LUC-5, i. e., grassland severely-degraded to cultivated land) to 0.249 kg CO2 eq./km (LUC-6, i. e., forested land to cultivated land), that is, from -80% to +5% with respect to gasoline, respectively. It comes out that the net GHG emissions of bioethanol are lower than those of gasoline in all cases except when growing energy crops leads to deforestation.

Generally suitable only for small-scale systems, underfeed stokers are a relatively cheap and safe option for biomass combustion. They have the advantage of being easier to control than other technologies, since load changes can be achieved quickly and with relative simplic­ity due to the fuel feed method. Fuel is fed into the furnace from below by a screw conveyor and then forced upward onto the grate where the combustion process begins. Underfeed stokers are limited in terms of fuel type to low ash content fuels such as wood chips. Due to ash removal problems, it is not feasible to burn ash-rich biomass as this can affect the air flow into the chamber and cause combustion conditions to become unstable.