QUESTION

 

                        What is the carbon footprint of DDG pellets as a fuel source to co-fire with coal?

 

SHORT ANSWER

 

This question can be answered only by making several assumptions, detailed below.  According to the assumptions we describe below, there are 0.0367 units of emitted CO2 associated with every 1 unit of DDG produced.  Using the assumptions presented, as a fuel source, DDG pellets would contribute approximately 38 to 77 times less CO2 on a per unit basis than coal.

 

DISCUSSION

 

The information presented in this memorandum frames the discussion related to the carbon footprint of dried distiller grains (³DDG²), but this memorandum is not a comprehensive technical report of DDG¹s carbon footprint.

 

As we understand it, DDG pellets are derived as a by-product of ethanol plants and can be used as both cattle feed and as fuel to be co-fired in a coal boiler, thereby displacing the amount of coal combusted.  As an energy fuel, DDG can be viewed as a renewable energy source because DDG derives from corn material.  Nevertheless, the ethanol process to derive DDG and pelletize the corn material requires several energy inputs, and such inputs must be included in any evaluation of the relative benefits of using DDG as an energy fuel. 

 

In order to perform an analysis of the ³carbon footprint² of DDG pellets, we have made several assumptions.  Assessing the ³carbon footprint² of a product or process generally means that one analyzes the various carbon inputs at each stage that are required to complete the product or process.  In the case of DDG pellets, such carbon inputs would chiefly come from the energy inputs to grow, harvest, process, and manufacture the DDG pellets.  For the purpose of the present analysis, we make assumptions only for a typical ethanol plant.

 

Further, there is an initial question about whether to include the carbon inputs, measured through energy inputs—e.g., diesel fuel used by the tractors, fuel for transport of corn—of growing and harvesting the corn.  Because the DDG pellets are derived as a by-product of ethanol plants, there are two ways to view the growth and harvest of corn for DDG pellets.

 

First, because ethanol plants would process the corn without any concern for DDG, it may be reasonable to assume that no carbon would be attributable to the growth and harvest stage of the corn for the DDG process.  The carbon input associated with corn growth and harvest is part of the ethanol plant¹s carbon footprint, regardless of DDG as a bi-product.  We assume, however, for purposes of this calculation that the carbon inputs of the energy related to DDG processing (separate from the ethanol production) is attributable to DDG production for two reasons.  First, some ethanol plants produce the bi-product mash without drying it as DDG product.  Second, separate from being used as a fuel source, there is a market for DDG as feed for cattle.  In this sense, DDG is not a necessary bi-product of ethanol production, but rather a product with commercial potential.  As such, the energy inputs associated with DDG production merit a separate carbon footprint calculation from the carbon footprint of ethanol production.

 

The chief carbon input for DDG pellets is the CO2 emitted by electric generation to supply ethanol plants with energy to process the corn.  The drying process to derive DDG consumes about one-third of an ethanol plant¹s energy requirements.[1]

 

According to EPA, of the 100 ethanol plants operational in 2006, all but 11 were fueled by natural gas (8 by coal plants and 3 by biomass facilities), and all but 10 were dry mill plants that produce DDG.[2]  Given this fact, we assume that the electricity provided to the ethanol facility comes from a natural gas plant.  The performance standard for emissions of CO2 by natural gas combined cycle turbines is generally stated as 1,100 lbs/MWh.[3]  Further, we assume that a typical dry mill ethanol plant demands approximately 4.3 MW of energy to operate and produces approximately 50 MMgal/yr (million gallons per year) of ethanol.[4]  We calculate that 6.4 lbs of DDG is produced for every gallon of fuel ethanol created at a dry grind plant.[5]

 

Therefore, we assume the following:

 

-       In one year, 320,000,000 lbs, or 160,000 tons, of DDG is produced

§       50 MMgal/yr * 6.4 lbs; 320,000,000 lbs / 2,000 for tons

 

-       4.3 MW average electricity demand at the ethanol plant

 

-       85% capacity factor for the combined cycle natural gas plant supplying the electricity

o      equals approximately 32,040 MWh of annual electricity demand

§       4.3 MW * 0.85 capacity factor * 24 hours in a day * 365.25 days in a year = 32,039.73 MWh

 

-       1,100 lbs/MWh of CO2 are emitted by a combined cycle natural gas plant

o      equals 35,244,000 lbs of CO2, or 17,622 tons of CO2, are emitted annually to supply the ethanol plant alone

§       32,040 MWh * 1,100 lbs/MWh = 35,244,000 lbs

§       35,244,000 lbs/2000  = 17,622 tons

 

-       35,244,000 lbs of CO2 per year / 3 (because 1/3 of the ethanol plant¹s electricity requirement is for drying to derive DDG)

o      equals 11,748,000 lbs of CO2, or 5,874 tons of CO2, emitted annually that can be attributed to DDG process of ethanol plants

 

-       Every 1 unit of DDG produced has a carbon footprint of 0.0367 units of CO2, whether in lbs or tons.

§       11,748,000 lbs of CO2/yr  /  320,000,000 lbs of DDG produced/yr, or

§       5,874 tons of CO2/yr  /  160,000 tons of DDG produced/yr

 

Additionally, it is possible that more energy inputs are required to pelletize the DDG material from the ³mash,² but the energy information related to this process is not available to us.  Also, the CO2 related to transportation of the pellets to the user, e.g., power plant, are not factored in this calculation.  However, there are countervailing CO2 inputs that would net against DDG¹s carbon footprint, such as CO2 that would otherwise be emitted from transportation of the waste mash produced at ethanol plants, if DDG were not derived as part of the process.  All of these factors would likely represent a somewhat smaller contribution to the carbon footprint of DDG pellets, and in the present calculation none of these components is factored.

 

Finally, combusting the DDG pellets as a fuel emits pollutants, including CO2.  We do not have any data on the amount of CO2 emitted by combusting DDG pellets.  In any case, many carbon control regimes consider the burning of sustainably harvested, renewable biological material to be ³carbon neutral² in that the amount of carbon emitted by combusting the material is ostensibly sequestered naturally by the regeneration of the plant fuel.[6]  In other words, burning corn releases CO2, but growing corn sequesters it.  Although the accuracy is uncertain of the claim that burning renewable, sustainably harvested biomass is carbon neutral, this calculation assumes that the actual combustion of the pellets is carbon neutral.

 

Given the calculation above, the carbon footprint of DDG pellets can be compared on a unit basis with other energy fuels, such as coal.  Emissions per 1 ton of coal range from 1.40 tons of CO2 to 2.84 tons of CO2, depending on the type of coal.[7]  Because DDG as a fuel source can be considered ³carbon neutral² as discussed above, the carbon footprint for DDG can be compared directly against the carbon emissions from using coal as a fuel source.  Furthermore, this factor for CO2 from coal does not figure in the CO2 inputs involved in mining and transporting the coal. 

 

DDG pellets contribute substantially fewer CO2 emissions than coal.  Because DDG can be considered a carbon neutral fuel source (as explained above), every 1 unit of DDG combusted as a fuel source to generate electricity has a carbon footprint of 0.0367 units of CO2.  On the other hand, every 1 unit of coal has direct carbon emissions of 1.4 to 2.84 units of CO2 (without even factoring the additional CO2 inputs from mining and transportation of the coal, which would add to coal¹s carbon footprint).  Using the calculations and assumptions detailed above, as a fuel source, DDG pellets would contribute approximately 38 to 77 times less CO2 on a per unit basis than coal.

 

 


[1]           See Pacific Northwest National Laboratory, 3.1.1.2 Feed Processing and Handling, DL2 Final Report, (prepared for the U.S. Dept. of Energy) (2006); see also, Steve Butzen and Doug Haefele, ³Crop Insights,² Dry-Grind Ethanol Production from Corn, at p 3, (2006) (stating that ³Drying distillers grains is energy-intensive, consuming about one-third of the energy requirements of the entire dry-grind plant.²).

[2]           See Environmental Protection Agency, US EPA CHP Partnership, Presentation to 2006 Fuel Ethanol Workshop and Expo, (dated June 22, 2006), available at http://www.epa.gov/chp/documents/2006_fuel_ethanol_workshop.pdf.

[3]           For example, this standard has been adopted by the California Public Service Commission in relation to the implementation of Assembly Bill 32, which regulates greenhouse gas emissions in California.

[4]           See Environmental Protection Agency, US EPA CHP Partnership, Presentation to 2006 Fuel Ethanol Workshop and Expo, (dated June 22, 2006), available at http://www.epa.gov/chp/documents/2006_fuel_ethanol_workshop.pdf.

[5]           Andrew McAloon, et al., ³Determining the Cost of Producing Ethanol from Corn Starch and Lignocellulosic Feedstocks,² U.S. Dept. of Agriculture and U.S. Dept of Energy, at 10 (2000), available at http://www.ethanol-gec.org/information/briefing/16.pdf.

[6]           For example, under the Regional Greenhouse Gas Initiative in the Northeastern United States, CO2 emissions deriving from co-fired biomass are not included in a source¹s CO2 calculation.

[7]           Wikipedia entry, available at http://en.wikipedia.org/wiki/Carbon_tax, (citing the U.S. Dept of Energy, Energy Information Administration statistics, available at http://www.eia.doe.gov/oiaf/1605/coefficients.html ).