Ethanol fuel production from sweet sorghum using a MicroFueler

Ethanol fuel production from sweet sorghum using a MicroFueler

By Jon Cambron, Michael Bomford, Tony Silvernail, and Joni Nelson.

This material has been adapted from posters presented in October, 2012, at the Growing the Bioeconomy Conference in Banff, Alberta (Download 4.8 MB poster) and the Kentucky Academy of Science Meeting in Richmond, KY (Download 3.2 MB poster).


Introduction

MicroFueler

The MicroFueler is designed to produce up to 150 liters (40 gallons) of hydrous fuel ethanol weekly through fermentation and distillation of sugar solutions derived from biomass. It consists of a fermentation tank with an electric agitator; and an electric distillation and pumping unit (right).

During 2012, we evaluated the economic and energy return to on-farm fuel ethanol production from sweet sorghum (Sorghum bicolor) grown on organic land on the Kentucky State University Research and Demonstration Farm.

Sweet sorghum is a drought tolerant annual grass which grows well under low nitrogen conditions. It is well adapted to much of the US climate and shows potential as a low-input feedstock for bioethanol fuel production. Juice extracted from sweet sorghum stalks typically contains 15-20% sugar, and can be directly fermented without further processing. 


Methods

We grew sweet sorghum at three farm scales as part of our five-year farm scale study:

  1. Biointensive scale -- Plots (0.8 x 6.0 m) managed entirely with hand tools.
  2. Market Garden scale -- Plots (1.8 x 18 m) managed with walk-behind tractors and attachments.
  3. Small Farm scale -- Plots (5.5 x 38 m) managed with 4-wheeled tractors and attachments.

All metabolic and fossil energy inputs were recorded at each scale. Following harvest in 2012, the sorghum cane was pressed in an electric three-roller mill to extract the juice for fermentation (Video 1). Ten 15-liter (4 gallon) batches of juice were fermented to beer (~7.5% alcohol), which was distilled to 95% ethanol fuel using the electric MicroFueler.  An energy data logger was used to record all electricity inputs.  Energy output in the form of 95% hydrous ethanol fuel was assumed to be 20 megajoules per liter, or 95% of the lower heating value of pure ethanol.

A partial enterprise budget was constructed for 1,000 liters (264 gallons) of ethanol fuel production from sweet sorghum harvested and processed between September and November, which is the crop's approximate harvest window in Kentucky.

Video 1. Extracting sweet sorghum juice from cane using a century-old three-roller mill (Chattanooga #23) designed to be powered by draft animals, but now driven by an electric motor.


Results and Discussion

Average sweet sorghum yield was 45 tonnes per hectare (20 tons per acre), with no significant yield difference between farm scales. Energy inputs for farming differed between scales (Figure 1): The Market Garden and Small Farm plots, managed with machinery driven by internal combustion engines, used almost twice as much energy as the Biointensive plots, which were managed with hand tools only. Although Biointensive production required no fossil energy inputs, it consumed much more metabolic energy than the other farm scales.

Our sweet sorghum yield was similar to that reported by Montross et al. (2009), but our ethanol yield was much less than the 4,000 to 6,500 liters per hectare (420-700 gallons per acre) they estimated could be made from the juice. Some of the descrepency between these studies could be attributed to the lower juice extraction efficiency of our mill. Our mill extracted approximately 25% of the wet weight of the harvested cane as juice, while Montross et al. (2009) reported that 35% to 45% of the cane weight was readily extractable juice.

The MicroFueler consumed 1.1 kilowatt hours per liter of hydrous ethanol fuel produced. Actual ethanol fuel yield realized was 879 liters per hectare (94 gallons per acre) for an average energy output of 20 gigajoules per hectare (7.7 million BTU per acre). 

A modest net energy gain was realized from ethanol fuel production at each of the farm scales tested (Figure 1). The energy output was roughly three times the input for sweet sorghum produced at the Biointensive scale, and double the input for sweet sorghum produced at the market garden or small farm scales.


Energy inputs
            and outputs for small-scale ethanol production from sweet
            sorghum grown at three farm scales.

Figure 1. Energy inputs (negative values) and outputs for ethanol fuel production from sweet sorghum grown at three farm scales.

The net energy gain did not translate into a profitable enterprise (Table 1). The annual cost of on-farm ethanol production exceeded the value of the ethanol produced by $12,731, resulting in a net loss of $12.53 per liter of ethanol fuel made ($47.43 loss per gallon). The largest single cost was the MicroFueler itself, which was introduced at a price of $9,995 in 2008, but cost $40,000 when this analysis was conducted, in 2012. Other major costs included the value of the sweet sorghum feedstock, and the labor cost associated with milling the cane. The cost of electricity to power the mill and MicroFueler accounted for less than 1% of the total cost of ethanol fuel production.

Table 1. Enterprise budget for on-farm fuel production from sweet sorghum using a MicroFueler.


Fixed costs

Useful life (years)
Cost
($/unit)
Cost
($/year)
Microfueler

8
40,000.00
5,000.00
3-roller mill

25
2,500.00
100.00
Concrete pad

20
500.00
25.00
Wireless router/bridge

5
150.00
30.00
Total fixed cost



5,155.00






Variable costs

Amount

Unit
Cost
($/unit)
Cost
($/year)
Labor milling
254
hour
12.00
3,048.00
Labor fermentation
12
hour
12.00
144.00
Electricity
1,135
kWh
0.09
102.15
Sorghum cane
45.8
tonne
100.00
4,580.00
Yeast
2.5
kilogram
65.00
162.63
Antibiotic
0.04
kilogram
3.10
0.14
E-Fuel Global Network Fee
3
month
9.95
29.85
E-Fuel Organic Fuel Processing Fee
1016
liter
0.07
67.00
Internet access
3
month
30.00
90.00
Total variable cost



8,233.77





Total cost (Fixed + Variable) 13,378.77





Income

Amount

Unit
Income ($/unit)
Income ($/year)
Hydrous Ethanol fuel (95%)
1016
liter
0.64
648.21





Net return (loss)
(12,730.56)
Net return (loss) per liter



(12.53)
Variable return (loss) per liter
(7.46)
Net return (loss) per liter, excluding feedstock cost (5.81)


Conclusion

The MicroFueler is an energy-efficient distillation apparatus for small scale ethanol fuel production, capable of generating a modest energy return on energy invested. It may have value for educational or demonstration purposes, but it is not a cost effective tool for small farmers hoping to offset fuel costs by producing hydrous ethanol fuel from sweet sorghum. Using this technology, on-farm production of ethanol fuel from sweet sorghum harvested for three months each year would cost twenty times more than purchasing an equivalent amount of fuel from off-farm.


Acknowledgements

Thanks to:

  • Mike Mathews and Floyd Butterfield of the E-Fuel Corporation, for technical assistance with the MicroFueler
  • E. Pat Heist and Mike Heist of Ferm Solutions Inc., for assistance with fermentation, yeast selection, and antibiotic selection
  • Danny R. Townsend of Townsend Sorghum Mill, for supplying sweet sorghum seed and syrup


Updated 12/17/12


College of Agriculture, Food Science and Sustainable Systems