Study: greenhouse gas balance of different energy cropping systems

In a recent open-access study published in Ecological Applications, Paul Adler from the USDA's Agricultural Research Service (ARS) and colleagues compare the net production of carbon dioxide and two other greenhouse gases (GHG) associated with producing biofuels via different pathways from several bioenergy crops. Since a GHG balance includes the emissions from energy used during farming, transporting and converting the biomass into biofuels, the study also offers an energy balance of the different biofuels.

The results of the life-cycle analysis show that of all bioenergy pathways studied, the gasification of hybrid poplar and switchgrass for the production of electricity reduced GHG emissions most. A conventional-till corn–soybean–alfalfa rotation the biomass of which is converted into ethanol had the smallest potential. If the biofuels replace gasoline and diesel, the resulting reduction of GHG emissions for corn rotations was 50–65%, for reed canarygrass 120%, and about 145% and 165% for switchgrass and hybrid poplar, respectively (figure 1, click to enlarge).

Interestingly, the author briefly analyses the GHG-reduction potential of using biomass in so-called carbon capture and storage systems (CCS). Such 'Bio-Energy with Caron Storage (BECS) systems offer carbon-negative energy and could take historic GHG emissions out of the atmosphere (earlier post and references there).

Grasses and trees
Ethanol and biodiesel from corn and soybean are currently the main biofuel crops in the United States, but the perennial crops alfalfa, hybrid poplar, reed canarygrass, and switchgrass have been proposed as future dedicated energy crops. Rotations of annual and perennial crops are common and the diversity of individual crops will affect greenhouse gas (GHG) fluxes of the cropping system. Corn–soybean and corn–soybean–alfalfa rotations are common cropping systems in Pennsylvania. Crop residues have also been proposed as a current source of biomass for energy production including corn stover (leaves and stalks of corn), although this practice is not without controversy (earlier post).

Adler and his team looked at the GHG balance of the conversion of biomass into two types of energy. For the perennial grasses and hybrid poplar into liquid biofuels such as (celluosic) ethanol and biodiesel, and into electricity via the gasification of biomass. For the rotation crops corn, soybean and alfalfa into ethanol or biodiesel. They used the DAYCENT model to simulate the net GHG fluxes of bioenergy cropping systems in Pennsylvania for inclusion in a full assessment of GHG emissions associated with energy production from crops.

Five bioenergy cropping systems were compared:

1. switchgrass
2. reed canarygrass
3. corn–soybean rotation (2 years of corn followed by 1 year of soybeans),
4. corn–soybean–alfalfa rotation (3 years corn, 1 year soybeans, followed by 4 years of alfalfa)
   
5. hybrid poplar

Conventional and no tillage were compared within the corn–soybean and corn–soybean–alfalfa rotations. All simulations were for 30 years. Adler used two scenarios: a short term scenario based on the capacity of soils to naturally store soil organic carbon (SOC); and a long term scenario based on the observation that over the course of time, soils lose some of this capacity if farmed continuously.

Crop and biofuel yields
The results show that hybrid poplar and switchgrass had the highest harvested biomass yields. Corn scored lower because it is typically grown in rotation with soybean, which is much lower yielding.

Biofuel production is directly related to crop yield but not linearly because biomass composition affects conversion efficiency. Ethanol and biodiesel yields for the individual crops ranged from 1.8 to 7.5 MJ/m²/year; corn (grain plus 50% stover) had the highest biofuel yield, hybrid poplar and switchgrass were similar but about 10–15% lower than corn, reed canarygrass was around 40% lower, and alfalfa stems and soybean grain had about 75–85% lower biofuel yields (figure 2, click to enlarge):
bioenergy :: biofuels :: energy :: sustainability :: ethanol :: biodiesel :: biomass :: cellulosic :: transesterification :: gasification :: greenhouse gas emissions :: energy balance ::

The pattern between crop and biofuel yield among cropping systems was similar, with hybrid poplar comparable to switchgrass, and corn–soybean rotation, reed canarygrass, corn–soybean–alfalfa rotation having progressively lower yields. The electricity yields from gasification of biomass for cropping systems were highest for hybrid poplar and switchgrass, and reed canarygrass was around 20% lower.

The quantity of gasoline and diesel displaced by the production of ethanol and biodiesel from cropping systems followed the same pattern as ethanol/biodiesel yields, but values were lower because although the energy content of biodiesel and diesel are similar, ethanol has about two thirds the energy content of gasoline. The quantity of coal displaced by the production of electricity from gasification of biomass from cropping systems ranged from 14.7 to 18.4 MJ/m²/year for the perennial crops.


Greenhouse-gas sinks
So which factor and cropping system avoided most greenhouse gases? Displaced fossil fuel (Cdff) was the largest greenhouse gas (GHG) sink; hybrid poplar and switchgrass displaced the most fossil fuel. Hybrid poplar stored the most carbon followed by switchgrass, reed canarygrass, corn–soybean rotation, and corn–soybean–alfalfa rotation. No-till corn–soybean and corn–soybean–alfalfa rotations had higher carbon sink than conventional tillage. The amount of CO2 equivalents (CO2e) emitted from fossil fuels used in feedstock transport to the biorefinery, conversion to biofuel, and subsequent distribution was negative for the perennial grasses and hybrid poplar and positive for the grain crops when both biomass and grain were converted to ethanol or biodiesel.

Methane uptake (CCH4) was the smallest GHG sink. Hybrid poplar had the highest CCH4 at −3.98 CO2e-Cg/m²/year, the other cropping systems increased in CH4 uptake from −1.41 to −1.57 in the order of switchgrass, conventional tillage corn–soybean and corn–soybean–alfalfa rotation, reed canarygrass, and no-till corn–soybean–alfalfa and corn–soybean rotation. High CH4 uptake by hybrid poplar compared to the other systems is consistent with data from various global sites showing that mean CH4 uptake rates by deciduous forests exceed those in grasslands, cropped soils, and non-deciduous forests by a factor of 2 or more. Feedstock conversion to biofuel was a net source of energy for hybrid poplar and the perennial grasses.


Greenhouse-gas sources
The CO2e–C of N2O emissions estimated by the biogeochemical model DAYCENT were the largest GHG source. The corn–soybean rotation had the highest emissions followed by reed canarygrass, corn–soybean–alfalfa rotation, switchgrass, and hybrid poplar. As expected, estimated N2O emissions were driven largely by N inputs from fertilizers and fixation. Corn rotations under conventional tillage had slightly higher direct CN2O (CN2O Dir) than under no-till.

The relationship of direct soil N2O emissions between cropping systems calculated with the IPCC (Intergovernmental Panel on Climate Change, 2000) protocol differed from those predicted by DAYCENT. The N2O emissions calculated from IPCC were highest for the corn–soybean–alfalfa rotation, followed by the corn–soybean rotation and reed canarygrass; N2O emissions from hybrid poplar and switchgrass were much less. The difference between IPCC (2000)-calculated N2O emissions and DAYCENT were less than 20% for hybrid poplar, corn–soybean rotation, and reed canarygrass. However, the IPCC (2000)-calculated N2O emissions for the rotations that featured N fixers were significantly higher than DAYCENT (almost 40% and more than 50% for the corn–soybean–alfalfa rotation under conventional and no-till, respectively).

IPCC (2000) estimates of N2O emissions from switchgrass are around 35% lower than DAYCENT. Indirect N2O emissions differed widely among crops (combined with direct N2O emissions in Fig. 2d). NO3 leaching, the major source of indirect emissions in this case, ranged from ∼0.5 g N/m²/year for switchgrass, to ∼1 g N/m²/year for hybrid poplar, to more than 2 g N/m²/year for reed canarygrass and the corn rotations.

Emissions from chemical inputs were low for hybrid poplar and switchgrass and somewhat higher for the other cropping systems. Emissions from chemical inputs were high for reed canarygrass and the corn–soybean rotation largely because N fertilizer inputs are high for these crops.

Energy used for farming
The energy required for farm operations varied widely, with CO2 emissions ranging from 128 kg CO2-C ha/year for corn to to less than 20 kg CO2-C·ha−1·yr−1 for established alfalfa and switchgrass. Differences are a result of the frequency of farm implement use, the load the equipment was under during operation, and the required crop-specific equipment. These data are similar to those collected by others, but the integrated farm system model (IFSM) allowed comparison of current energy use from agricultural machinery between all farm operations under standardized conditions. (The exception was for hybrid poplar; since IFSM does not include forestry operations, data from separate sources were used.)

Perennial cropping systems can have lower agricultural machinery inputs than annual systems. The exception to this trend is hybrid poplar because energy costs of harvesting are high. Propane was used to dry corn and usually accounted for about one third of the C emissions for the corn rotations. Tillage accounted for almost 30% of the C emissions in the corn rotations but less than 10% in the switchgrass and reed canarygrass and less than 2% in hybrid poplar, where tillage was only used the first year. Harvesting was responsible for the majority of emissions for the hybrid poplar and perennial grass systems and at least 30% for the corn rotations.

Energy used for feedstock conversion
Feedstock conversion to biofuel was a net consumer of energy for all the corn, soybean, and alfalfa rotations and was also a net consumer when the grasses and hybrid poplar were gasified for electricity generation.


Net greenhouse-gas flux
Hybrid poplar and switchgrass provided the largest net GHG sinks with both systems having net CO2 e-C fluxes of less than −200 g/m²/year for the near term scenario when biomass and grain are converted to ethanol and biodiesel. The sink for reed canarygrass was about −120 g/m²/year and the sink for the conventional-till corn–soybean–alfalfa rotation was the smallest at about −50 g·m−2·yr−1 for the near-term scenario. Trends among the different cropping systems for the long-term scenario were similar, but the sinks were smaller because C storage in soil and belowground biomass was considered negligible in the long term.

The sinks were even greater when biomass was converted to electricity by gasification at the power plant, and there was a similar relationship among cropping systems. On a unit-area basis of crop production, gasification of the grasses and hybrid poplar yielded more than twice the GHG reduction than did converting these crops to ethanol. Net GHG emissions were from about −8 to −9 g CO2e-C/MJ ethanol for corn rotations, but about −18 g CO2e-C/MJ for reed canarygrass and less than −24 g CO2e-C/MJ for switchgrass and hybrid poplar. This resulted in a reduction of GHG emissions for corn rotations in the near term of about 50–65%, reed canarygrass ∼120%, and about 145% and 165% for switchgrass and hybrid poplar, respectively, compared with the life cycle of gasoline and diesel.

In the long term, where soil C sequestration was assumed to no longer occur, this resulted in a reduction of GHG emissions for corn rotations of ∼40%, reed canarygrass ∼85%, and ∼115% for switchgrass and hybrid poplar compared with the life cycle of gasoline and diesel. The GHGnet reduction from gasifying biomass instead of coal was about −64 to −70 g CO2e-C/MJ, an 85–93% reduction in greenhouse gases compared with the coal life cycle.


Conclusion
The near-term scenario used by Adler (one in which soil organic carbon (SOC) levels stay at their natural level) combined all the GHG sinks and sources evaluated in this study, and considered how using biofuels would reduce GHGnet compared to continuing to use fossil fuels in the near-term.

The displaced fossil-fuel C (Cdff) was the dominant factor in determining GHGnet. In general, switchgrass and hybrid poplar had higher yields, greater soil C sequestration, reduced GHG emission from feedstock conversion, reduced soil N2O emissions, and reduced GHG emissions from chemical input manufacture and agricultural machinery operation.

The long-term GHGnet assumed that SOC was zero because soils were equilibrated and no longer sequestering additional C. This scenario considers how using biofuels would reduce GHGnet compared to continuing to use fossil fuels in the long term. All cropping systems were still GHG sinks compared to their fossil fuel counterparts. Biofuels have been considered to have a near-zero net emission of greenhouse gases. However, coproducts such as lignin and protein, along with soil C sequestration, can reduce GHGnet, making these system sinks, and when compared with the life-cycle GHG emissions of the displaced fossil fuel, Adler's analysis shows biofuels having net GHG benefits.

Producing energy from crops is a land extensive approach to energy production. In addition to having metrics that allow easy comparison across technologies (such as GHG emissions per megajoule of fuel), to evaluate land-use implications of bioenergy cropping systems, a metric expressed in terms of policy impact per unit land area is needed. In the study cellulosic crops had higher biofuel yield and lower GHG emissions per unit land area than corn rotations. Cellulosic crops also had a greater reduction in GHG emissions per unit biofuel produced than corn rotations, resulting in greater reductions in GHG emissions associated with energy use compared with fossil fuels.

Carbon capture
Capture of CO2 from fuel production and energy generation would further increase the impact of biofuels on reducing GHGnet. Only a portion of biomass C is retained in ethanol and biodiesel. In an ethanol conversion facility for corn stover, about one third of the biomass C is converted to ethanol, the remainder of biomass C was emitted as combustion exhaust and fermentation-generated CO2. Similar proportions of biomass C were converted to ethanol in this study. Two thirds of the C could be captured at a biorefinery and nearly 100% could be captured at a biomass-gasification power plant. Spath and Mann have quantified the impact of CO2 capture for both coal and biomass-gasification systems. They found that even with CO2 capture, fossil-based systems still have greater GHG emissions per kilowatt-hour of electricity than for biomass power-generation systems without C capture.

Carbon credit markets associated with GHG mitigation strategies have been developed. Short-term strategies for mitigating greenhouse gases using biofuels include soil C sequestration. However, displacement of greenhouse gases associated with the use of fossil fuels is the only long-term mitigation mechanism when using biofuels and would be easier to track for carbon markets.

In short, the use of biofuel could reduce the net GHG flux of energy use, whether from production of liquid fuels, such as ethanol and biodiesel, or generation of electricity from gasification of biomass. The choice of crop and management practices will affect the net GHG fluxes of energy use from biofuel. Cellulosic energy crops such as switchgrass and hybrid poplar have the greatest potential to reduce net emissions of energy use in the near- and long-term.

Figure 1: Comparison of the life-cycle greenhouse-gas (GHG) emissions associated with the quantity of gasoline and diesel displaced by ethanol and biodiesel produced from the cropping systems (displaced fossil-fuel C [Cdff]) with the quantity of GHG emissions associated with the life cycle of biofuel (ethanol and biodiesel) production (feedstock-conversion C [CFC] + CCH4 + direct CN2O + indirect CN2O + chemical-inputs C [CCI] + agricultural-machinery C [CAgMa]); near-term includes change in system C [ΔCsys]). The percentage reduction in GHG emissions was calculated as the difference in the biofuel emissions and fossil-fuel emissions displaced from biofuel produced by a given crop expressed as a percentage of the displaced fossil-fuel emissions.

Figure 2: Crop and fuel yield from bioenergy cropping systems. Yields are expressed either as crop component (a, b) or system (c, d) yields. Corn yields assumed that only 50% of the corn stover (leaves and stalks) was harvested; alfalfa yields only contained stems, 50% of the total yield. (a) Component yields are presented; the 2-yr corn and 1-yr soybean (c2b1) rotation and 3-yr corn, 1-yr soybean, and 4-yr alfalfa (c3b1a4) rotation yields are from the conventional-tillage system. (b) All crop components were converted to ethanol except soybean grain, which was converted to biodiesel. (c) System yields were combined from crop rotations and annualized over the rotation cycle. (d) Crop component fuel yields of ethanol and biodiesel were combined to give system yields.

More information:
Paul R. Adler, Stephen J. Del Grosso, and William J. Parton, "Life-cycle assessment of net greenhouse-gas flux for bioenergy cropping systems", Ecological Applications, Volume 17, Issue 3 (April 2007), pp. 675–691.