Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.iii Executive SummaryThe U.S. Department of Energy (DOE) promotes the production of ethanol and other liquid fuels from lignocellulosic biomass feedstocks by funding fundamental and applied research that advances the state of technology in biomass collection, conversion, and sustainability. As part of its involvement in the program, the National Renewable Energy Laboratory (NREL) investigates the production economics of these fuels.This report describes in detail one potential biochemical ethanol conversion process, conceptually based upon core conversion and process integration research at NREL. The overarching process design converts corn stover to ethanol by dilute-acid pretreatment, enzymatic saccharification, and co-fermentation. Ancillary areas-feed handling, product recovery, wastewater treatment, lignin combustion, and utilities-are also included in the design. Detailed material and energy balances and capital and operating costs were developed for the entire process, and they are documented in this report and accompanying process simulation files, which are available to the public.As a benchmark case study, this so-called technoeconomic model provides an absolute production cost for ethanol that can be used to assess its competitiveness and market potential. It can also be used to quantify the economic impact of individual conversion performance targets and prioritize these in terms of their potential to reduce cost. Furthermore, by using the benchmark as a comparison, DOE can make more informed decisions about research proposals claiming to lower ethanol production costs.Building on design reports published in 2002 and 1999, NREL, together with the subcontractor Harris Group Inc., performed a complete review of the process design and economic model for the biomass-to-ethanol process. This update reflects NREL's current vision of the biochemical ethanol process and incorporates recent progress in the conversion areas (pretreatment, conditioning, saccharification, and fermentation), optimizations in product recovery, and an improved understanding of the ethanol plant's back end (wastewater and utilities). The major process updates in this design report are the following:• Feedstock composition is updated to a carbohydrate profile closer to the expected mean.• Pretreatment reactor configuration is revised with significant new detail.• Whole-slurry pH adjustment of the pretreated biomass with ammonia replaced the previous conditioning practice of overliming, eliminating a solid-liquid separation step.

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Off-gas 10% of total Carbon AHTL Oil 72% of total Carbon (77% algal carbon recovery) Natural Gas 3.5% of total Carbon in Water & Solids Recycle to Ponds 8% of total Carbon as dissolved CO2 (9% of algal carbon) Reformer & Heater Exhaust 23% of total Carbon (Includes 21% of algal carbon) Natural Gas Drier & Exhaust 3.5% of total Carbon in

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Microalgae biofuel production has been extensively evaluated through resource, economic and life cycle assessments. Resource assessments consistently identify land as non-limiting and highlight the need to consider siting based on combined geographical constraints of land and other critical resources such as water and carbon dioxide. Economic assessments report a selling cost of fuel that ranges between $1.64 and over $30 gal(-1) consistent with large variability reported in the life cycle literature, -75 to 534 gCO2-eq MJ(-1). Large drivers behind such variability stem from differences in productivity assumptions, pathway technologies, and system boundaries. Productivity represents foundational units in these assessments with current assumed yields in various assessments varying by a factor of 60. A review of the literature in these areas highlights the need for harmonized assessments such that direct comparisons of alternative processing technologies can be made on the metrics of resource requirements, economic feasibility, and environmental impact.

Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents are available free via www.OSTI.gov.

Costs, emissions, and resource availability were modeled for the production of 5 billion gallons yr(-1) (5 BGY) of renewable diesel in the United States from Chlorella biomass by hydrothermal liquefaction (HTL). The HTL model utilized data from a continuous 1-L reactor including catalytic hydrothermal gasification of the aqueous phase, and catalytic hydrotreatment of the HTL oil. A biophysical algae growth model coupled with weather and pond simulations predicted biomass productivity from experimental growth parameters, allowing site-by-site and temporal prediction of biomass production. The 5 BGY scale required geographically and climatically distributed sites. Even though screening down to 5 BGY significantly reduced spatial and temporal variability, site-to-site, season-to-season, and interannual variations in productivity affected economic and environmental performance. Performance metrics based on annual average or peak productivity were inadequate; temporally and spatially explicit computations allowed more rigorous analysis of these dynamic systems. For example, 3-season operation with a winter shutdown was favored to avoid high greenhouse gas emissions, but economic performance was harmed by underutilized equipment during slow-growth periods. Thus, analysis of algal biofuel pathways must combine spatiotemporal resource assessment, economic analysis, and environmental analysis integrated over many sites when assessing national scale performance.

One of the major challenges associated with algal biofuels production in a biorefinery-type setting is improving biomass utilization in its entirety, increasing the process energetic yields and providing economically viable and scalable co-product concepts.