Daily Rules, Proposed Rules, and Notices of the Federal Government
As part of changes to the Renewable Fuel Standard (RFS) program adopted in a rule published on March 26, 2010,
Pursuant to 40 CFR 80.1416, EPA received a petition from Dakota Spirit AgEnergy (“Dakota”) on October 15, 2011, requesting that EPA evaluate a new fuel pathway's lifecycle GHG reduction and provide a determination of the renewable fuel category for which the new pathway may be eligible. Dakota is proposing to build a dry-mill corn ethanol plant in Spiritwood, North Dakota, with a nameplate production capacity of 65 million gallons of ethanol per year. Dakota's proposed process is unlike those used in pathways modeled for the 2010 RFS rule in that they plan to meet their process steam needs by importing steam from the adjacent Spiritwood Station coal-fired power plant, which would operate in a combined heat and power (CHP) mode.
EPA has not previously considered the treatment of steam from an offsite CHP plant in a lifecycle emissions accounting analysis under the RFS program. EPA is not aware of a previous regulatory context where an allocation approach has been applied to determine the emissions associated with process steam from an offsite facility. This notice describes the methodology EPA is considering to allocate emissions to the imported steam Dakota plans to use for biofuels production, as well as the Agency's rationale for selecting this methodology in the context of the RFS program and for the type of configuration being considered. EPA invites comment on the application of the GHG allocation methodology and on the feasibility and appropriateness of using this allocation methodology for other similar CHP configurations under the RFS program.
CHP is an efficient, clean, and reliable approach to generating power and thermal energy from a single fuel source. By installing a CHP system designed to meet the thermal and electrical base loads of a facility, CHP can greatly increase the facility's operational efficiency and decrease energy costs. CHP systems offer considerable environmental benefits when compared with purchased electricity and onsite-generated heat. By capturing and utilizing heat that would otherwise be wasted from the production of electricity, CHP systems require less fuel than equivalent separate heat and power systems to produce the same amount of energy.
In the 2010 RFS rule, EPA evaluated a corn ethanol biorefinery that utilized an onsite CHP system as part of the ethanol production process. The process evaluated a CHP system installed at the biorefinery which generated process steam and electricity for use in the process for producing ethanol. Dakota's proposed approach is different in that they plan to import process steam from the adjacent Spiritwood Station power plant that will operate in CHP mode.
The Spiritwood power plant combusts coal in a circulating fluidized-bed boiler that will generate steam at high temperature and pressure. This high pressure steam will be sent through a high-pressure steam turbine (HPST), where energy will be extracted to produce electricity. The steam will exit the HPST at lower pressure and temperature, at which point some of the steam will be diverted to the Dakota biorefinery plant to provide thermal energy for the ethanol production process. The remaining steam at Spiritwood will be sent through a low-pressure steam turbine (LPST) to produce additional electricity. The extraction steam diverted for use at the ethanol plant will result in a decrease in the amount of power to be generated from the power plant. Therefore, although the amount of electricity generated is reduced, the total fuel consumed and the resulting GHG emissions of the power plant remain unchanged.
To determine the emissions associated with the extracted steam, the total emissions of the Spiritwood power plant need to be allocated to the power plant's power production and to the steam extracted for use at the biorefinery. EPA analyzed the Dakota CHP configuration and reviewed several different allocation methods, including
EPA considers the work potential allocation approach to be most appropriate for CHP systems that use heat to primarily produce mechanical work or power, such as the case at the Spiritwood plant where the primary use for the steam is for power generation.
The Spiritwood power plant is designed for the primary function of generating electricity. The total emissions at the Spiritwood plant are constant, whether steam is diverted or not. When steam is diverted to the Dakota biorefinery, the emissions associated with the diverted steam and the resulting loss in electricity production is evaluated via the work potential method. We can determine an emission factor for the power plant when it is just generating electricity and not diverting steam to the Dakota biorefinery (i.e., operating in a “power only” mode). The GHG emissions attributed to the extracted steam is determined by estimating the amount of power not generated by the power plant because the steam was diverted from the turbine, and applying the power plant's “power only” emissions factor to that value. The emission factor is unchanged since the total emissions at the Spiritwood plant are unchanged and only a small portion of the steam energy generated at the power plant is diverted to the biorefinery. The process for determining the steam GHG emission factor using the work allocation approach is summarized by the following steps:
1. Calculate the GHG emission factor for the Spiritwood power plant without any steam extracted;
2. Determine the amount of electricity that is not generated due to the extraction of steam for the Dakota plant; and
3. Apply the Spiritwood emissions factor to the amount of electricity not generated due to steam extraction and calculate the associated emissions.
This following example illustrates how the work potential method allocates emissions based on useful energy produced. In Dakota's petition, they presented an example where the Spiritwood plant generates 92 MW of electric power in power-only mode, but only produces 82 MW of electric power in CHP mode due to the steam extraction. Thus, the steam extraction displaces about 11% of the total power production. Using the work potential allocation method, the extracted steam is allocated 11% of the total emissions from the Spiritwood plant, whereas the remaining 89% of emissions are allocated to electricity production.
EPA reviewed other allocation approaches to assess their appropriateness for allocating emissions for the Dakota petition. The other two most common methods to allocate emissions from a CHP system are:
The efficiency and energy content allocation approaches are based on assumptions, either of the efficiencies with which steam and electricity are generated, or on the relative values of energy outputs. As an example, the emission allocation of the efficiency method will vary based on how the electrical and thermal efficiencies are defined. Under these approaches, the emissions allocated to the remaining electricity generation (in terms of lbs/MWh) at the Spiritwood plant in CHP mode would be lower than the original emissions factor for electricity generated by Spiritwood operating in power-only mode, indicating an over-allocation of emissions to the extraction steam.
Since CHP system design and operating characteristics vary so widely, leading organizations in this field have not developed a consensus on one preferred allocation method. The California Air Resources Board issued a technical document as part of its Climate Change Reporting Requirements
The Western Climate Initiative received various recommendations on the treatment of combined heat and power in its initial draft design guidance for recording greenhouse gas (“GHG”) emissions since it has implications in both the industrial and electricity sectors. The
Under the RFS2 program, EPA is considering use of the work potential method for the configuration outlined in the Dakota petition because the primary purpose of the steam generated at Spiritwood power plant before extraction is to produce power. This method allocates the emissions to extracted steam based on the amount of power displaced (i.e., the electricity not generated).
A Memorandum to the Docket explains in more detail how the work potential methodology would be applied to the plant configuration proposed for the Dakota plant, resulting in a specific GHG emission factor per mmbtu of steam energy. This emissions factor would be used in analyzing the total GHG emissions per mmbtu of ethanol produced by the Dakota facility, as part of determining whether the ethanol produced by the facility would qualify under the lifecycle GHG thresholds established in the RFS program. For the configuration outlined in the Dakota petition, EPA's analysis finds that the process steam has an emission factor of 53,175 grams CO2-eq/mmbtu steam.
EPA invites comments on the proposed application of the work potential methodology to determine emissions associated with imported steam to the Dakota plant in the context of lifecycle emissions accounting. Furthermore, EPA invites comment on applying the work potential approach to other plants with similar CHP configurations under the RFS program. EPA also requests information on the appropriateness of applying alternative allocation approaches outlined in this notice to the Dakota plant, as well as any other approaches that could also be used to allocate emissions to steam for this specific CHP configuration under the RFS program.