Life-cycle energy use and greenhouse gas emissions of production of bioethanol from sorghum in the United States

Background The availability of feedstock options is a key to meeting the volumetric requirement of 136.3 billion liters of renewable fuels per year beginning in 2022, as required in the US 2007 Energy Independence and Security Act. Life-cycle greenhouse gas (GHG) emissions of sorghum-based ethanol need to be assessed for sorghum to play a role in meeting that requirement. Results Multiple sorghum-based ethanol production pathways show diverse well-to-wheels (WTW) energy use and GHG emissions due to differences in energy use and fertilizer use intensity associated with sorghum growth and differences in the ethanol conversion processes. All sorghum-based ethanol pathways can achieve significant fossil energy savings. Relative to GHG emissions from conventional gasoline, grain sorghum-based ethanol can reduce WTW GHG emissions by 35% or 23%, respectively, when wet or dried distillers grains with solubles (DGS) is the co-product and fossil natural gas (FNG) is consumed as the process fuel. The reduction increased to 56% or 55%, respectively, for wet or dried DGS co-production when renewable natural gas (RNG) from anaerobic digestion of animal waste is used as the process fuel. These results do not include land-use change (LUC) GHG emissions, which we take as negligible. If LUC GHG emissions for grain sorghum ethanol as estimated by the US Environmental Protection Agency (EPA) are included (26 g CO2e/MJ), these reductions when wet DGS is co-produced decrease to 7% or 29% when FNG or RNG is used as the process fuel. Sweet sorghum-based ethanol can reduce GHG emissions by 71% or 72% without or with use of co-produced vinasse as farm fertilizer, respectively, in ethanol plants using only sugar juice to produce ethanol. If both sugar and cellulosic bagasse were used in the future for ethanol production, an ethanol plant with a combined heat and power (CHP) system that supplies all process energy can achieve a GHG emission reduction of 70% or 72%, respectively, without or with vinasse fertigation. Forage sorghum-based ethanol can achieve a 49% WTW GHG emission reduction when ethanol plants meet process energy demands with CHP. In the case of forage sorghum and an integrated sweet sorghum pathway, the use of a portion of feedstock to fuel CHP systems significantly reduces fossil fuel consumption and GHG emissions. Conclusions This study provides new insight into life-cycle energy use and GHG emissions of multiple sorghum-based ethanol production pathways in the US. Our results show that adding sorghum feedstocks to the existing options for ethanol production could help in meeting the requirements for volumes of renewable, advanced and cellulosic bioethanol production in the US required by the EPA’s Renewable Fuel Standard program.

production are simulated separately in the GREET TM (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) model.

N 2 O emissions
Nitrogen fertilizer application, sorghum residue decomposition, and fertigation of vinasse result in N 2 O emissions for GS, SS, and FS farming, as estimated by Equations S2a, S2b, and S2c. estimated based on the stalk nitrogen content [8], which is 10,000 grams per tonne of grain harvested, and the amount of stalk left. We estimate the N 2 O emissions from fertigation of vinasse based on its composition. Vinasse contains 0.63 gram nitrogen per liter ethanol, 1.95 grams potash per liter ethanol, and 0.13 gram potassium per liter ethanol [9].

Sorghum feedstock transportation
We estimated the distance of sorghum feedstock transportation based on a supply-demand Ratio is the ratio of collected sorghum biomass to that received by the ethanol plant, as shown in Table 7.

Sorghum ethanol production processes
In this section, we outline the three types of ethanol production processes.

GS-based ethanol production
Figure S1(a) shows the grain-based dry-mill ethanol production processes. In the dry-milling process, the grain is cleaned by removing the debris and other contaminants and then ground into flour, which is slurried with water. A heat-stable enzyme (α-amylase) is added. This slurry undergoes liquefaction then is cooled to approximately 30°C, and a second enzyme (glucoamylase) is added for the saccharification step prior to the final fermentation, which uses yeast [10]. Next, ethanol is separated via distillation then dehydrated. The co-produced is processed into a livestock feed, the distillate grains with solubles (DGS). The sorghum grainbased dry-mill ethanol yield is similar to that of corn ethanol because the two grains have similar starch content as shown in Table S1.

SS sugar-based ethanol production
Figure S1(b) shows SS sugar-based ethanol production. SS is washed, chopped, and shredded by a set of mill combinations to extract the sucrose-rich juice. Bagasse is collected and burned in a CHP system to generate sufficient steam and electricity for process demands. Surplus electricity is exported to the grid. The extracted juice is then filtered and evaporated to produce molasses, which is sterilized to remove impurities and is then ready to be fermented into ethanol with the addition of yeast. After the process is complete, ethanol is recovered by distillation and subsequent dehydration.

FS-and SS bagasse -based ethanol
Figure S1(c) shows FS-and SS bagasse-based ethanol production. FS and SS bagasse that consist of cellulose, hemicellulose , and lignin are milled to reduce particle size and pretreated with chemicals and heat prior to enzyme hydrolysis processing. The subsequent fermentation step yields ethanol at a level that is affected by the composition of the feedstock. With similar cellulose and hemicellulose content of sorghum bagasse, as shown by Table S2, we assumed the same ethanol yield of 0.38 liters/dry kilogram of FS or SS bagasse for FS-and SS bagasse-based cellulosic ethanol production as for corn stover ethanol production [6].

Energy consumed during sorghum conversion to ethanol
For processes incorporating CHP, we developed a three-step procedure to estimate the net energy demand of the conversion step for each pathway. First, the steam and electricity demand per liter of ethanol in a scenario without process integration was estimated. Second, the potential steam and electricity supply from the CHP system was estimated. Finally the net electricity steam and electricity demand were determined. If CHP-produced steam did not suffice to meet process demands, we assumed 80% efficient natural gas-fired boilers provided the balance. If additional electricity was needed, it was sourced either from the regional central and southern plains generation mix (because feedstock is likely to be produced in this region) or from the U.S.
average generation mix.
We estimated total steam and electricity demand of GS ethanol production using fossil natural gas (FNG) as the process fuel based on a USDA ASPEN model. The model results predict that a grain sorghum ethanol plant uses 96.3% of the thermal process energy of a corn ethanol plant and 99.3% of the electrical energy [11]. Accordingly, we estimate that the total energy use for grain-based dry-mill GS ethanol production using FNG as the process fuel is 0.24 MJ of FNG per MJ of ethanol (86.4% FNG and 13.6% electricity). These estimates are based on GREET's modeling of corn ethanol production in a dry mill with wet DGS as the co-product. For GS ethanol production using renewable natural gas (RNG) as the process fuel, the RNG-fired CHP consumed 0.25 MJ of RNG per MJ of ethanol to provide sufficient steam and electricity at a power-to-steam ratio of 0.12 with a total efficiency of 79.6% , to meet the process energy demand.
This configuration of the RNG-fired CHP is feasible [12].
In the case of SS conversion to ethanol, steam demand was based on the unit-level steam demand of a Brazilian sugar and an ethanol co-production plant using sugarcane as the feedstock [13].
The steam demand was allocated between ethanol and sugar by mass (in Brazilian sugar mills, both ethanol and sugar are produced). The result is 0.29 MJ of steam per MJ of ethanol produced.
We assume the steam is produced by NG boilers with an efficiency of 80%, and therefore 0.37 MJ of FNG per MJ of ethanol is required. Based on our personal correspondence with Prof.
Jaoquim Seabra, the electricity demand averaged 0.066 MJ per MJ of ethanol. Therefore, the total energy use of sugar-based ethanol production is 0.43 MJ per MJ of ethanol.
We assumed conversion of FS in Pathway IV and SS bagasse to ethanol in Pathway V would resemble the production of cellulosic ethanol from corn stover as modeled by Humbird et al. [14].
This assumption is reasonable because corn stover has a similar composition to FS and sorghum bagasse (Table S3) Bagasse or lignin can be used as a feedstock for CHP in Pathways III, IV, and V. We estimate the steam and electricity generation by CHP based on the amount of biomass available for combustion. The lower heating values (LHV) of the feedstocks were also needed in these calculations. The LHV of SS bagasse is 16.8 GJ/tonne [15]; for lignin, it is 17.9 GJ/tonne [16].
CHP system parameters used were an 80% boiler efficiency for steam generation, an electricity generation efficiency of 20.8% [17], and a waste heat recovery of 65% [18].
For Pathways IV and V, enough feedstock is diverted to the CHP system such that process steam and electricity demand can be met without purchase of supplemental energy. This approach reduces fossil energy consumption and associated greenhouse gas (GHG) emissions. In this case, we estimated the mass fractions of forage sorghum biomass and sweet sorghum bagasse that must be diverted to CHP as 31% and 38%, respectively. Accordingly, an electricity surplus of about 0.56 kWh/liter and 0.32 kWh/liter for Pathways IV and V, respectively, is co-produced.
The default external electricity consumed for sorghum ethanol production is assumed to be supplied by the regional electricity generation mix in the central and southern plains, which consists of 50.5% from coal, 35.7% from FNG, 10.0% from nuclear, 0.2% from biomass, 0.3% from hydropower, and 3.2% from other renewable sources [19]. We also analyze each scenario with the average US grid.
6. WTW results of total energy use, petroleum use, fossil natural gas use, and coal use Figure S2 shows the WTW total energy use, petroleum use, fossil natural gas use, and coal use of sorghum-based ethanol, in comparison to gasoline.   Figure S1 Sorghum (a) grain-, (b) sugar-, and (c) cellulosic-based ethanol production processes.