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

Abstract

BackgroundThe 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.ResultsMultiple 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.ConclusionsThis 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.

Evaluation of the environmental impacts of ethanol production from sweet sorghum

“…There are studies on the production of biofuels from sweet sorghum. Cai et al (2013) investigated the life-cycle energy use and GHG emissions from the production of ethanol from grain sorghum, forage sorghum and sweet sorghum, the results are summarized in Table 1. Köppen et al (2009) performed a screening assessment that analyzed the GHG emissions and energy use along the entire life cycle of the sweet sorghum ethanol process for different production and use scenarios.…”

Increased ethanol production from sweet sorghum juice concentrated by a membrane separation process

“…Global bioethanol production has risen gradually each year and was 88 billion liters in 2013 (Global Renewable Fuels Alliance, 2014). Sweet sorghum [Sorghum bicolor (L.) Moench] is a promising feedstock for bioethanol production (Zabed et al, 2014;Cai et al, 2013;Ratnavathi et al, 2011) due to its high biomass yield (20-30 dry tons ha À1 ), high concentration of fermentable sugars, short growing period (120-150 days), high tolerance to drought and salt compared to conventional bioethanol crops (e.g., sugarcane and corn), adaptability to climatic conditions (such as cold), low requirement for fertilizers, high efficiency of water usage (one third that of sugarcane and one half that of corn), and the possibility that all parts of the plant can be used for bioethanol production (Li et al, 2013;Thangprompan et al, 2013;Ratnavathi et al, 2011). Sweet sorghum juice usually contains 160-180 g L À1 fermentable sugars (sucrose, glucose and fructose), obviating the need for the enzymatic hydrolysis of polysaccharides to simple sugars (Ratnavathi et al, 2011;Laopaiboon et al, 2009).…”

Environmental and Energy Performance of Ethanol Production from the Integration of Sugarcane, Corn, and Grain Sorghum in a Multipurpose Plant

“…The authors considered two energy supply scenarios for the industrial stage [18]: (i) natural gas from fossil fuels as a source of thermal energy, as well as electricity from the grid; and (ii) renewable-origin natural gas (from anaerobic digestion of animal waste) to feed a cogeneration system, and produce heat and electricity. Thus, the system using fossil natural gas had EROI = 2.00 and the alternative adopting cogeneration from biogas achieved EROI = 4.90.…”

“…The authors considered two energy supply scenarios for the industrial stage [18]: (i) natural gas from fossil fuels as a source of thermal energy, as well as electricity from the grid; and (ii) renewable-origin natural The EROI of sugarcane ethanol produced in Brazil was estimated in different studies following the life cycle approach [11][12][13][14]. Some of the values described in the literature that were obtained from assumptions similar to ours were 7.52 [12], 8.84 [13], and 2.63 [14], albeit with a large energy input for infrastructure construction.…”