Baseline GHG Emissions
The GHG emissions for corn grain and corn stover ethanol, separate and combined, for the baseline scenario in which the energy demands of the stover ethanol process are met first and are summarized in Fig. 2. All of the CHP heat is consumed, and 5.5 MJ natural gas/liter of combined ethanol is imported. Corn grain and stover ethanol productions use 31.4 and 58.1 % of CHP-generated electricity, respectively. The total electricity demand of both feedstocks is met, and 10.5 % of the CHP electricity is surplus. This generated electricity is considered a co-product with a GHG burden of 25 g-CO2eq/MJ-electricity. For the baseline scenario, the net life cycle GHG emissions of corn grain ethanol are 57 g-CO2eq/MJ. The largest portion of these emissions comes from corn farming and transportation at 35 g-CO2eq/MJ, followed by ethanol production at 26 g-CO2eq/MJ. The displacement of animal feed with DGS provides a 15 g-CO2eq/MJ credit. The net GHG emissions for stover ethanol are 25 g-CO2eq/MJ. The largest contribution of these is from ethanol production (e.g., consumption of process chemicals and enzymes, imported energy) at 13 g-CO2eq/MJ, followed by stover collection and transport at 11 g-CO2eq/MJ. Corn grain ethanol and corn stover ethanol LUC GHG emissions are 7.6 and −0.6 g-CO2eq/MJ, respectively, based on previous analysis .
Also presented in Fig. 2 are the combined ethanol GHG emissions, which are the total emissions of a single ethanol stream coming out of the integrated plant. Results are presented as a combined liter of ethanol, of which 73 and 27 % come from corn grain and corn stover, respectively. The total GHG emissions of ethanol produced at the integrated plant are 48 g-CO2eq/MJ, with the largest portion of the emissions coming from corn farming and transportation at 26 g-CO2eq/MJ, then ethanol production at 23 g-CO2eq/MJ. When the integrated scenario results are compared to gasoline, the combined ethanol liter achieves a 49 % reduction assuming the GHG intensity of gasoline is 94 g-CO2eq/MJ . When corn grain and corn stover ethanol are treated in isolation, the associated GHG emission reductions are 40 and 74 %, respectively. The result of corn stover ethanol when based on integrated ethanol production is similar to previous studies with the GHG emissions of this fuel showing a marked emission reduction as compared to gasoline (>60 %)[13, 26, 34].
Alternative Allocation Scenarios
Figure 3 contains corn grain and stover ethanol life cycle GHG emission results for the baseline and two CHP allocation scenarios (Table 4). In scenario 1, ethanol production allocation, the stover ethanol GHG emissions increased from 24 to 43 g-CO2eq/MJ and the grain ethanol GHG emissions decreased from 57 to 50 g-CO2eq/MJ as compared to the baseline. Scenario 2 results mirror this trend with similar changes in results compared to the baseline. In this scenario, the life cycle GHG emissions of corn grain and corn stover ethanol are both 48 g-CO2eq/MJ. The GHG emissions for the combined liter from the integrated facility for scenarios 1 and 2 remain the same as the baseline. This result is expected because these scenarios only affect how the CHP-produced heat and electricity are shared between the grain and stover ethanol and the amount of materials and energy entering the total system remains the same.
For the baseline, farming energy is allocated based on attribution, meaning the only operations assigned to the corn stover are the energy to collect and transport the stover to the biorefinery and burdens associated with supplemental fertilizer production, application, and fugitive N2O emissions. Allocating the total farming emissions based on the energy content of the feedstocks results in a decrease of the grain ethanol GHG emissions from 57 to 55 g-CO2eq/MJ and an increase in the stover emissions from 25 to 40 g-CO2eq/MJ. The change in the emissions for each feedstock stems from a change in the farming emissions. When energy allocation is applied to corn farming with stover harvest, the GHG intensity of corn grain ethanol decreases by 9 %, while it increases by 140 % for corn stover ethanol. With the energy allocation scenario, both fuels provided a life cycle GHG emission reduction compared to gasoline of 42 and 57 % for grain and stover ethanol, respectively. If mass rather than energy allocation is applied to feedstock production, similar values, 55 and 38 g-CO2eq/MJ, result for grain and stover ethanol, respectively. It is important to note, however, that this treatment of the collection of corn grain and corn stover as an integrated system may not accurately represent the feedstock production stage. At present, farmers may view stover harvest less as a means to produce an additional product from their land, but rather as a technique to manage rising crop residue levels as corn yields increase. This viewpoint is likely more prevalent in areas with high corn yields with a corresponding substantial stover resource.
Treating CHP-Produced Heat and Electricity by Displacement
In this paper, CHP-produced heat and electricity are considered as a co-product of corn stover ethanol production with associated energy and emission burdens. When corn stover ethanol is produced in an individual plant, a common approach to handle CHP-produced heat and electricity is a displacement method where all GHG emissions are allocated to the ethanol and additional electricity is exported to the grid with the ethanol receiving a displacement credit [9, 33]. This methodology can also be applied to the integrated scenario.
When CHP-produced electricity and heat are treated with the displacement method, no GHG emissions are associated with the CHP-produced heat and electricity that is used internally in corn ethanol and corn stover ethanol production. As a result, when corn grain ethanol production consumes the CHP-produced energy, the ethanol receives the benefit of an emission-free energy source. All of the GHG emission burden associated with corn stover that serves as the CHP energy feedstock are assigned to corn stover ethanol production, regardless of the final fate of the CHP energy. In our analysis, we assume that the displaced electricity is produced from the national average grid mix, which is 0.5, 26.1, 41.5, 19.5, 0.3, and 12.2 % from residual oil, natural gas, coal, nuclear power, biomass, and others (e.g., hydro, geothermal), respectively . The displacement credits are assigned to corn stover ethanol. We generated results for when this displacement method along with the attributional approach to corn grain and stover agriculture is applied to calculation of the GHG emissions for the baseline scenario. Corn stover receives the CHP-produced energy first. Compared to a scenario with the same assumptions except the energy allocation method applied to co-produced CHP, the GHG emissions for stover ethanol increase from 25 to 27 g-CO2eq/MJ, while those for grain ethanol decrease from 57 to 54 g-CO2eq/MJ. The change in emissions is relatively small because 88 % of the heat and electricity is utilized by corn stover ethanol, meaning most of the emissions of the CHP co-product are assigned to the stover ethanol production in the baseline scenario. For this scenario, the GHG emission reduction is increased for corn grain from 40 to 42 % but decreased for stover from 74 to 71 %. These results would leave the status of corn grain ethanol and corn stover ethanol as conventional and cellulosic biofuels, respectively. The GHG emissions for the combined liter decrease when CHP is considered a byproduct from 48 to 47 g-CO2eq/MJ. Unlike when CHP-generated electricity is considered a co-product, a displacement credit can be taken for the generated electricity that goes unused at the biorefinery.
The effects of treating CHP as a co-product rather than a byproduct have a greater effect when we consider scenario 1, in which CHP energy is allocated based on ethanol production. It is important to note that burdening corn stover ethanol production with the energy consumption and emissions associated with converting corn stover to CHP-produced energy that is subsequently used by the corn grain ethanol process is likely not an appropriate allocation method for an integrated plant. We present this scenario, however, to highlight the pronounced effect on results that these types of choices in LCA can have. In this case, which essentially ties the burden associated with CHP-produced energy to corn stover ethanol, the GHG emissions in this scenario decrease for grain ethanol from 48 to 40 g-CO2eq/MJ, while they increase for stover ethanol from 48 to 69 g-CO2eq/MJ. The effect of the assumption about CHP energy is much greater because, in this scenario, stover ethanol only utilizes 57 % of the heat and electricity produced by CHP. Under these conditions, corn grain ethanol achieves a GHG emission reduction of 17 %. On the other hand, stover ethanol would only offer a 25 % GHG reduction compared to gasoline and would only qualify as a renewable fuel under RFS2.
Effect of Plant Size on GHG Emissions
We selected corn grain and stover ethanol plant capacities based on information about commercial facilities . As the industry expands and matures, plant capacities may change. We investigated the influence of plant capacity assumptions on integrated ethanol GHG emissions. In Fig. 4, we present GHG emissions of grain and stover ethanol as well as integrated ethanol when the stover ethanol plant size ranged from 0–210 MLY and the grain ethanol plant size was held constant at 210 MLY. The integrated and grain ethanol GHG emissions decrease with increasing plant size, but the emissions for stover ethanol are consistently 25 g-CO2eq/MJ. The result makes sense because, in the baseline scenario, energy and material use for stover ethanol production are independent of corn grain ethanol production. On the other hand, grain ethanol GHG emissions show the increasing benefits of heat and electricity integration with an increasing stover ethanol plant size. Figure 4 shows two slopes for grain ethanol GHG emissions. Between stover ethanol plant sizes of 0 and 60 MLY, the GHG emissions for grain ethanol decrease from 62–57 g-CO2eq/MJ. In this range, the corn grain ethanol plant benefits from CHP-produced electricity that increases as the stover ethanol plant capacity grows. Beyond a stover ethanol plant size of 60 MLY, corn grain ethanol GHG emissions see only a minor decrease from 57–56 g-CO2eq/MJ because, beyond this threshold, electricity consumption during grain ethanol production can be completely met by CHP-produced electricity. As corn stover ethanol plant capacity increases beyond 15 MLY, corn grain ethanol continues to benefit from increasing amounts of CHP-produced heat, which is reflected in the slow decline of corn grain ethanol GHG emissions beyond this point. The heat demand for grain ethanol (7.8 MJ/L) is more than ten times as large as the electricity demand (0.7 MJ/L). To produce enough energy to meet the heat and electricity demands for both grain and stover ethanol (with large amount of excess electricity), the corn stover ethanol production facility would need to be larger than 2700 MLY, an unrealistic size for a grain ethanol plant sized at 210 MLY.
Comparison of Life Cycle GHG Emissions from Integrated Versus Separate Analysis of Co-Produced Corn Grain and Corn Stover Ethanol
Figure 5 summarizes life cycle GHG emission results for corn grain and corn stover ethanol co-produced at one facility from integrated and separate analytical perspectives. The figure includes results for the baseline and the two alternative CHP scenarios (meeting the grain energy demands first and allocating CHP energy based on the share of the total ethanol produced). Included in this figure are lines that indicate the life cycle GHG emissions for corn grain ethanol and corn stover ethanol when they are produced at separate facilities . The corn grain ethanol GHG emissions are for a dry mill facility that produces only DDGS as a co-product, with natural gas as the process fuel. Additionally, the figure indicates the level of GHG emissions that would qualify biofuels as meeting the RFS standards for 20, 50, and 60 % reduction as compared to gasoline.
It is clear that regardless of whether analysis of corn ethanol considers it apart from or together with corn stover ethanol as an integrated liter of ethanol or as a separate liter of ethanol, this biofuel can achieve a 20 % reduction in GHG emissions as compared to baseline gasoline and qualify as a conventional biofuel under the RFS2. (Corn ethanol is not eligible for classification as an advanced or cellulosic biofuel, even if it achieves the GHG emission targets for these fuel categories.) Decisions about LCA methodology, however, strongly affect life cycle GHG results for corn stover ethanol, currently considered to be one of the most promising cellulosic biofuels in the USA. For the alternative CHP scenarios, created solely from changes in accounting technique rather than any physical, on-the-ground change in the life cycle of this biofuel, it no longer offers GHG emissions at a 50 or 60 % GHG reduction compared to gasoline to be classified as an advanced or cellulosic biofuel, respectively.