(A) Feedstock Assumptions and Limitations
USDA has developed feedstock production scenarios based on dedicated crop feedstocks and waste wood biomass that come from timberland and agricultural lands that are a subset of the RFS2 definitions and assumptions. What is encouraging is that USDA considers these estimates as conservative, because the estimates do not include all possible feedstock sources that may eventually be eligible to achieve the 21 billion gallons of advanced biofuels as specified by RFS2. This is good news since qualified feedstocks identified by EPA but not included in the
current USDA analysis (e.g., tallow, municipal solid waste) could be counted toward meeting the RFS2 mandate. In addition, if technologies are developed in a timely manner to use an even wider variety of feedstocks, increase conversion ratios, or utilize waste or co-products technologies, while meeting greenhouse gas emission reduction targets, then the total volume of biofuels from approved feedstocks produced that meet emissions specified may exceed the 36 billion gallon RFS2 targets.
Feedstock Choice. In the RFS2 Final Rule, the EPA identifies a number of feedstock pathways and imports that would satisfy the RFS2 mandates. Feedstock pathways for advanced biofuels include switchgrass, soybean oil, corn oil, crop residues, woody biomass and other feedstocks.
The complement of feedstocks included in this USDA analysis and those identified by the EPA should not be considered an exhaustive list of all possible feedstock sources. The petition process exists to embrace the possibilities of technological advances and research evolution. In order to be successful in the implementation of a domestic biofuels industry this flexibility should be utilized in considering all the feedstock resources that America has to offer that meet the greenhouse gas requirements and other restrictions of the RFS2.
USDA scientists, intramural and extra-mural research, and collaborative work with other Federal agencies (i.e. Navy, Air Force) suggest that additional feedstocks such as biomass (sweet) sorghum, energy cane, and camelina may merit consideration under the RFS2. For the purposes of this analysis, USDA assumed that these feedstocks were sufficiently similar to those already eligible under the Renewable Fuels Standard and necessary to the regional approach. USDA recognizes that a formal review by the EPA of each of the feedstocks would be required to evaluate whether they qualify as an advanced biofuel.
Feedstock Definitions. EISA includes specific greenhouse gas reduction thresholds levels for renewable fuel mandates that feedstock pathways must meet. The EPA in its analysis of the feedstock pathways was required to carry out a complete greenhouse gases life-cycle-analysis, which considers reductions in carbon emissions and other greenhouse gas. In addition, EISA provides a specific definition of renewable biomass and places constraints on the types of land from which renewable biomass can be collected or harvested. For the sake of consistency USDA chose to use those definitions in the context of this report.
（一）原料假设与局限
美国农业部已经拟定了一些原料生产的设想情境，它们依据的是来自林场与农用土地的能源专用农作物原料及废木材生物质，这些原料与生物质都属于可再生燃料标准第二阶段所提出的定义与假设的一部分。美国农业部认为，这些设想情境的估计是比较保守的，因为在达到可再生燃料标准第二阶段制订的210亿加仑先进生物燃料目标方面，最终有可能合格的原料来源并未悉数囊括在这些估计中。这是鼓舞人心的好消息，因为在美国环境保护署指定的合格原料中，有些并没有包括在美国农业部当前的这份分析报告中（例如动物油脂、城市固体垃圾），这些原料也可以为达到可再生燃料标准第二阶段的强制性要求而派上用场。此外，如果能够及时开发技术来利用更多的原料，并且提高能源转化率，或是开发出利用废物或副产品的技术，同时达到温室气体减排目标，那么，以认可的原料生产出来的、能够达到指定减排量的生物燃料，就可能超过可再生燃料标准第二阶段所规定的360亿加仑。
原料选择。在《可再生燃料标准第二阶段最终规则》中，美国环境保护署指定了能够满足可再生燃料标准第二阶段强制性要求的一系列原料来源及进口原料。先进生物燃料的原料来源包括柳枝稷、大豆油、玉米油、农作物残余物、木质生物质等等。
美国农业部的这份分析报告中补充的原料，以及美国环境保护署指定的原料，都不应被视作是详尽地搜罗了所有的可用原料来源。申请程序的存在便是为了抓住技术进步与研究进展所创造的可能性。为了在发展国内生物燃料行业方面取得成功，那么在考虑美国必须提供的、满足可再生燃料标准第二阶段对温室气体减排的要求及其它限制条件的所有原料来源时，就应当运用这种灵活性。
根据美国农业部科学家的意见和部内部外所开展研究的结果，以及与其它联邦机构（例如海军部、空军部）所开展协作的结果，按照可再生燃料标准第二阶段的要求，象生物质高粱（甜高粱）、能源甘蔗和亚麻荠属这类另外的原料也值得考虑。从这份分析报告的角度而言，美国农业部假设，这些原料与可再生燃料标准规定的合格原料基本类似，并且在采用区域性方案来达到目标方面也是必不可少的原料。美国农业部承认，每一种这类原料都还需经过美国环境保护署的正式评审，以判定它们是否属于合格的先进生物燃料来源。
原料定义。原料来源必须满足对可再生燃料的强制性要求。《能源独立与安全法》为这些强制性要求规定了具体的温室气体减排量门限值。美国环境保护署受命在它的原料来源分析中对燃料生命周期内的温室气体排量执行完整的分析，它的意图在于考虑对碳和其它温室气体的减排。此外，《能源独立与安全法》还为可再生生物质提供了具体的定义，并为可以采集或收获可再生生物质的土地类型施加了限制。为一致性起见，美国农业部决定在这份报告中采用这些定义。
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