Conversion Technologies for Advanced Biofuels

Conversion Technologies for Advanced Biofuels

Preliminary Roadmap & Workshop Report

December, 6–8 2011

Arlington, VA

Conversion Technologies for Advanced Biofuels Workshop Executive Summary

Introduction

The appeal of developing renewable energy sources within the United States is largely coupled with the promise of attaining an elevated level of domestic energy security in the future. This idea is widely espoused throughout the entire renewable energy field, but it is specifically pronounced within the realm of biofuels development, where every day, engineers, researchers, and industry leaders are confronted with the significant task of displacing over 4.3 billion barrels of crude oil and petrochemical imports annually[1]—all the while maintaining price parity with foreign fossil imports. In a recent speech, the President affirmed:

“Biofuels are an important part of reducing America’s dependence on foreign oil and creating jobs here at home. But supporting biofuels cannot be the role of government alone… partnering with the private sector to speed development of next-generation biofuels will help us continue to take steps towards energy independence and strengthen communities across our country.”

–President Barack Obama, August 16, 2011

It was in this spirit that the Department of Energy’s (DOE) Office of the Biomass Program in the Office of Energy Efficiency and Renewable Energy (EERE) hosted the Conversion Technologies for Advanced Biofuels Workshop (CTAB) from December 6–8, 2011. The purpose of the conference was to engage industry, academia, and the national laboratories in defining the most important technical challenges and research activities that must be addressed to hasten the expansion of a domestic advanced biofuels industry. The primary focus was on developing hydrocarbon biofuels (renewable gasoline, diesel, and jet fuel) from lignocellulosic biomass-derived intermediates.

DOE’s Biomass Program was established to focus on the development and transformation of domestic, renewable, and abundant biomass resources into cost-competitive, high-performance biofuels, biopower, and bioproducts through targeted planning, research, development, and demonstration leveraging public and private partnerships. Focused originally on cellulosic ethanol, the Program is transferring knowledge gained through its cellulosic ethanol research, development, deployment, and demonstration (RDD&D) experience to accelerate advances in other advanced biofuel pathways.

Background

There are three types of challenges associated with pioneering a successful biofuels industry— technical, economic, and policy. Though the CTAB workshop primarily focused on technical challenges, it is impossible to refrain from mentioning the other two categories. The Biomass Program’s targets are framed by a host of federal laws and economic policy incentives, most notably the Energy Independence and Security Act of 2007 (EISA) which established the Renewable Fuel Standard (RFS2) and requires blending 36 billion gallons of renewable fuel by 2022 (21 billion gallons of which cannot be ethanol or corn-starch derived). Currently, the fledgling U.S. biofuels industry is just beginning to grapple with the reality of producing commercial-scale quantities of cellulosic ethanol (on the 20–25 million gallon per year plant capacity scale), which can only be used to displace light-duty vehicle fleet gasoline consumption at a 15% maximum blend wall. CTAB’s ultimate goal was to generate enough information necessary to help formulate technical targets for the Biomass Program looking ahead to 2022 to commercialize hydrocarbon biofuels technology and to update the Program’s existing technology roadmaps, which were published in conjunction with the DOE Office of Science in 2007.

Nearly 150 stakeholders with diverse subject matter expertise and backgrounds convened at CTAB to provide input to 10 technical breakout tracks over two days. The breakout tracks were organized into two groups dedicated to the following topics:

·  Production of carbohydrate derivatives from biomass and their subsequent upgrading to hydrocarbon biofuels.

·  Production of bio-oils from biomass and their subsequent upgrading to hydrocarbon biofuels.

Each breakout session was led by two co-chairs, one representing a national laboratory and the other from industry, academia, or government. Scribes were available in each session to capture notes and track group discussion. The aforementioned topic groups reflect existing Critical Technology Goals (CTGs) within the Biomass Program that revolve around producing and upgrading carbohydrates and bio-oils to “drop-in” fuels. Participants were encouraged to suggest, discuss, and prioritize technical barriers and research and development (R&D) activities. Crosscutting themes in barriers that emerged throughout all breakout sessions were:

·  Feedstock supply, logistics, and pre-processing considerations

·  Techno-economic and life-cycle analyses

·  Catalysis issues

·  Separation science needs

·  Process integration.

The following is a summary of the findings from the Workshop. Results are presented in terms of overarching themes and recommendations that emerged from the sessions. Preliminary findings from the Workshop indicate that additional years of basic and applied research are needed to fully realize a commercially successful advanced biofuels industry.

Bio-Oils

Production of bio-oil increases the energy density of raw biomass and converts it into a product that is amenable to additional processing en route to producing a liquid hydrocarbon fuel. Although there is no single composition for bio-oil and its chemical makeup is largely dependent upon the starting feedstock and process variables during production, bio-oils contain a variety of destabilizing components. The destabilizing components may be both inorganic and organic species, in either the vapor or liquid phase, which affect the stability of either the oil or the overall process and may:

·  Cause the condensed bio-oil intermediate to change physically and chemically over time and under various processing conditions

·  Cause operational problems in processing

·  Reduce catalyst performance during intermediate upgrading to biofuels

·  Impact existing fuel distribution infrastructure.

Bio-oil is an emulsion with suspended lignin solids, so physical instability may arise from agglomeration of lignin to form larger particles. If bio-oil is allowed to age, a phase separation can occur between the aqueous and organic fractions and large, agglomerated clumps will settle into a lignin-rich sludge at the bottom of the vessel. Chemical instability can arise from polymerization reactions. Species within the bio-oil that contain unsaturated, carbon-carbon and carbon-oxygen double bonds are especially susceptible to participating in polymerization reactions (e.g., aldehydes, aromatics, olefins, and organic acids). In addition to potential instability of the condensed bio-oil, there are components that can lead to instability in the chemical processes, which include degradation of materials and equipment, as well as deactivation of upgrading catalysts through chemical poisoning, fouling, or physical changes via mechanisms such as leaching.

To facilitate the use of bio-oil for production of hydrocarbon fuels, the removal of destabilizing components from the bio-oil is an essential activity. The removal of these components may occur by chemical/catalytic conversion of the unwanted species or by utilizing separation techniques. These removal processes can be implemented on either the vapor phase (e.g., in-situ or ex-situ vapor phase upgrading, hot gas filtration, cyclones) the whole condensed phase (filtration, membranes, liquid-phase catalysis), or either the aqueous or organic phases alone.

Fundamentally understanding how lignin, hemicellulose, and cellulose thermally depolymerize during biomass fast pyrolysis and how inorganic contents vary in different biomass materials (especially in terms of how they impact bio-oil production and upgrading) is crucial to engineering systems to produce bio-oils with desirable qualities. Also, attaining a better fundamental understanding of high-temperature solid-vapor separation was repeatedly noted as a research barrier, particularly as it applies to scale-up of bio-oil technologies. The key research needs that were identified for vapor-phase upgrading included; increased understanding of the catalyst interaction with oxygen functional groups (catalytic deoxygenation) and exploring the use of H2-donor molecules for in-situ hydrogenation and deoxygenation. Other large themes identified during the discussion on bio-oil production and upgrading were as follows:

Hydrogen cost and supply: The clear leading candidate for oil upgrading is catalytic hydrogenation (hydrotreating). Bio-oil contains oxygen that must be removed on the way to finished fuel. Hydrogen is the natural choice and is already used in refinery operations for upgrading, albeit with different heteroatom targets. The cost and logistical difficulties of a distributed hydrogen supply make hydrogen supply to a distributed biomass-based system a challenge. Production of hydrogen from in-field waste streams or improved processes for hydrogen reforming from biomass were both advocated. Internal generation of hydrogen (potentially from a hydrogen donor species) was also discussed. Donors that are in the existing fuel infrastructure that can be left in the process effluent were envisioned, but not defined.

Catalytic processing limitations: Heterogeneous catalysts were assumed to be the only practical solution for processes that have to be inexpensive and robust. Known systems are susceptible to fouling and deactivation. Development of fouling resistant catalysts and those with sufficient lifetime are required. Studies of fouling and deactivation fundamentals are proposed, as are new catalysts or new regeneration regimes.

Process intensification: The upgrading process has to be cost effective, implying that it must be simple. Effective integration with pyrolysis and refining must be examined to be successful. The ambiguity surrounding the integrated process must be clarified in order to develop specific R&D targets.

Oxygen removal without hydrogen addition: Radical new approaches to oxygen removal should be considered, even though well-defined options are lacking. Movement away from hydrogenation could result in process simplification and economic improvement.

Fundamental studies related to upgrading: Publicly available information on the catalyst performance and failure modes is lacking. The potential exists for improvement if better understanding of the mechanisms can be gained.

Carbohydrate Derivatives

Technical barriers to the generation of lignocellulosic sugars or saccharide-derived species include both feedstock properties and processing techniques. Processes for converting renewable resources into biofuels may be classified in terms of bioprocessing or thermocatalytic/ thermomechanical conversion techniques. Barriers are reflective of the feedstock, such as the carbohydrate content, type and amount of inhibitors, and structural integrity. These may be further classified as being ideal, acceptable, or unacceptable and are anticipated to be different for chemical/catalytic processes compared to biochemical or bio-based processes. Furthermore, barriers will be defined by the properties and robustness of catalysts (either biochemical or chemical) that are used.

Pretreatment and enzymatic saccharification: Pretreatment is needed to enhance the accessibility of lignocellulosic biomass to catalytic enzymes, microorganisms, and other types of catalysts during bioprocessing. Bioprocessing is defined as an enzyme or biological based technology for transforming pretreated lignocellulosic biomass to sugars (oligosaccharides and monosaccharides), followed by either biocatalytic or chemical catalytic transformation of oligosaccharides to monosaccharides, and monosaccharides to biofuel or biofuel precursors other than ethanol. A combined pretreatment and biobased (enzymatic) approach was identified as a key technology in need of focused research to develop an optimized process. Alternatively, if oligosaccharides are obtained, these may be processed to monosaccharides using chemical (catalytic) methods. The ideas feedstock for biofuels production would have reduced recalcitrance (through alterations of lignin or polysaccharides) to pretreatment methods and low inhibitor content such as acetyl groups, aldehydes and phenolics. Current research should be targeted at optimizing hydrolyzate quality while minimizing energy input.

Non-enzymatic routes to carbohydrates: Non-enzymatic sugar production from lignocellulose typically employs a mechanical system to deconstruct or fractionate an aqueous or solvent modified biomass slurry in the presence of acid, base or other reagents under varying temperature and pressure conditions. Biomass can be pre-processed to minimize recalcitrance beforehand or fed directly into a system, although unit operation intensification is typically preferred. One advantage to using non-enzymatic systems is the potential for rapid hydrolysis of biomass-based sugars, but a fundamental issue inherent to such processes involves economically recycling reagents and the technical challenges inherent to developing closed loop systems. Poor separation of biomass and solvent was identified as a major issue and area of active R&D. Difficulties in solubilizing high (greater than 85%) of the bulk sugar content from the biomass was also noted as a significant issue that can hinder process economics. Research on development of extraction techniques for targeting clean separation of organic and water layers, improving solids separations, development of acid inhibitor tolerant materials (e.g. membranes and mesopourous materials) and mechanical separation systems (e.g. screw extruders, supercritical fluid systems and shrinking bed reactors) were identified as crucial research targets in advancing the state of the art.

Microbial conversion of carbohydrates to biofuels: Desirable fuel precursors, including fatty acids, alcohols, esters, aldehydes, ketones, isoprenoids, polyketides, neutral lipids, and others, can be synthesized by specialized microbes from sugars released during pretreatment and hydrolysis. Microbes that can produce such compounds in sufficient quantities can be created through metabolic engineering or strain evolution. Central tasks to designing effective organisms are identification and overexpression of genes that encode for enzymes that synthesize precursors to fatty acids, other molecules containing fatty functional groups (??) and straight and branched alkanes. These precursors can then be extracted from either the host organism or the extracellular environment of the host (if excreted), and upgraded to produce hydrocarbon biofuel blends. Customization of a biofuels’ properties is based upon the functionalities catabolized by the production host. Key barrier issues are efficient carbon utilization during bioconversion (especially with regard to C5 sugar use), redox balance, lack of energy- and cost-efficient hydrocarbon product separation systems, identification and elucidation of biological conversion inhibitors and mechanisms, and prioritization of hydrocarbon molecules targeted for production.

Catalytic processes for converting carbohydrates to biofuels: Chemical conversions of carbohydrate derivatives represent new routes to hydrocarbon fuels that can use wide ranges of sugars and sugar-derived intermediates, including carbohydrate dehydration products and organic acids. The primary barriers to demonstrating technical and economic feasibility of these materials can be grouped by issues related to feedstocks, catalysts, carbohydrate processing, and fuel production. The co-design of upstream processes for biomass deconstruction with the downstream catalytic processes to convert biomass-derived intermediates to fuels is important. In particular, upstream processes determine the product slate of biomass-derived intermediates (including intermediates derived from lignin) and the potential introduction of contaminants, catalyst poisons, and fouling agents. The composition of the intermediate streams will significantly impact the final product slates, catalyst lifetimes, separations, and the operation of the downstream unit operations, all of which impact the economics of the processes.