Net energy of cellulosic ethanol from switchgrass

Schmer et al. 10.1073/pnas.0704767105.

Supporting Tables

Files in this Data Supplement:

SI Table 1
SI Table 2
SI Table 3
SI Table 4
SI Table 5
SI Table 6
SI Table 7

Table 1. Summary of total agricultural energy inputs for switchgrass fields grown for bioenergy for 5 years in Nebraska, South Dakota, and North Dakota

*A partial reseeding was done in areas at the Douglas farm. The Atkinson farm was reseeded in 2001 because of stand failure caused by drought in 2000.

Table 2. Agriculture and biorefinery inputs and respective energy values used to determine the energy balance of switchgrass grown for cellulosic ethanol

* Agricultural electricity use for switchgrass biomass production was not estimated in this study. EBAMM uses a value of 46 MJ Mg^-1 from GREET 1.6 that is based on estimated electrical rates for switchgrass production (1, 2),which are higher than electrical rates for corn grown in rain-fed regions (8, 9). The main use for electricity for corn production is for grain drying and/or in irrigation (9). Switchgrass will not require electricity for drying and projected land areas for switchgrass production will be non-irrigated fields. There is no direct agricultural application that would require electricity usage to vary by biomass yield (1).

† Farm Machinery Energy (MJ ha^-1) = [Total operational cost (dollar hectare^-1) ^´ (35.9 MJ

dollar^-1 iron and steel manufacturing + 7.95 MJ dollar^-1 farm machinery and equipment manufacturing)/15 years] (4, 5). Farm machinery energy inputs from this study were similar to estimates in previous reports (2, 3, 6, 10).

Table 3. Net energy values [output energy (MJ_Fuel liter_Fuel^-1) - input energy (MJ_Input liter_Fuel^-1)] for switchgrass fields

*NH, no harvest.

Table 4. Biomass yields from established (2 years after planting) switchgrass fields in the midcontinental US

Ethanol conversion efficiency was estimated at 0.38 liters kg^-1 (2).

Table 5. Summary of estimated greenhouse gas (GHG) emissions of ethanol derived from switchgrass

 

	Estimated GHG emissions by location and harvest year*
	Estab.†	2	3	4	5	Mean
Location	g CO2equivalent MJ-1 ethanol produced‡
Lawrence, NE	NH	17	16	4	7	11
Douglas, NE	NH	12	27	1	8	12
Atkinson, NE	7	37	8	9	-	15
Crofton, NE	NH	6	20	(2)	0	6
Ethan, SD	NH	31	11	(3)	13	13
Huron, SD	(11)	(12)	(3)	(6)	7	(5)
Highmore, SD	NH	NH	(1)	(4)	(11)	(5)
Bristol, SD	(7)	22	(2)	0	(13)	0
Streeter, ND	NH	(10)	3	6	13	3
Munich, ND	17	8	8	6	(2)	8
Mean	1	12	9	1	3	6

Estimated GHG emissions are based on switchgrass production inputs, cellulosic ethanol production requirements and ethanol distribution (2).

*Negative values (in parentheses) indicate displaced and sequestered CO₂ equivalents exceeded total CO₂ equivalents in the production of ethanol derived from switchgrass.

†NH, no harvest. All agricultural GHG emissions on nonharvested years were added to the first harvested year. The Atkinson, NE, farm was analyzed as having 2 establishment years.

‡GHG displaced by ethanol (Fig. 5) = [g CO_{2 equivalent} MJ^-1 ethanol (Table 5) - 94g CO_{2 equivalent} MJ^-1 petroleum]/ 94g CO_{2 equivalent} MJ^-1 petroleum (2),

where

g CO_{2 equivalent} MJ^-1 ethanol = [Net GHG emissions g CO_{2 equivalent} liter^-1/ethanol low heating value (21.2 MJ liter^-1)] - ethanol distribution (1.4 g CO_{2 equivalent} MJ^-1), where

Net GHG emissions (g CO_{2 equivalent} liter^-1) = [agriculture sector GHG emissions (kg CO_{2 equivalent} ha^-1)/switchgrass biomass yield (kg ha^-1) ´2.63 (kg biomass liter^-1 ethanol) ´ 1,000 g kg^-1] + [Biorefinery sector GHG emissions (124 g CO_{2 equivalent} liter^-1) - coproduct credit (106 g CO_{2 equivalent} liter^-1)] (2), where

Agriculture sector GHG emissions (kg CO_{2 equivalent} ha^-1) = [Nitrogen fertilizer emissions + herbicide emissions + transportation emissions + diesel emissions + packaging emissions + farm machinery emissions (2) - CO₂ sequestered (138.1 kg CO₂ per Mg of aboveground biomass; 11)].

Table 6. Energy inputs for switchgrass seed production using typical management practices for USDA-ARS Grain, Forage, and Bioenergy Research Unit (Lincoln, NE)

The seed field was assumed to be in production for 5 years.

*Switchgrass seed yields range from 220 to > 1,000 kg ha^-1 (13, 14). A seed yield of 220 kg ha^-1 was used to give a conservative seed energy value.

Table 7. Diesel use estimates for agricultural field applications in the production of switchgrass