Alternative Fuels User Facility

At the National Renewable Energy Laboratory's Alternative Fuels User Facility (AFUF) in Golden, Colorado, researchers develop and improve a number of the technologies used in converting biomass to fuels, chemicals, and materials. This research is carried out in the AFUF's state-of-the-art laboratories and in its process development unit (PDU) pilot plant, which can process up to one ton of biomass feedstock per day.

The laboratories in this facility are essential to NREL's research in biomass conversion. Although biomass includes all plant and plant-derived material, researchers at the AFUF focus on cellulosic material (the fibrous bulk of most plants) rather than the starch, sugar, and oil used now for food products and fuels.

The three primary components of biomass are cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are polymeric chains of sugar molecules. The challenge is to break down the polymers into their component sugars for processing to ethanol or other fuels, chemicals, or materials. This technology will enable the development of biorefineries that would produce a slate of different products, much as petroleum refineries do today with fossil resources.

The AFUF contains facilities that allow bench-scale development of the various steps in biomass conversion, testing of entire conversion processes at the "mini-pilot-plant" scale, and trials at the pilot scale. NREL's model process for deriving usable products from biomass involves thermochemical pretreatment of the biomass followed by enzymatic hydrolysis, fermentation, and product recovery. Researchers at the AFUF are also engaged in work involving process integration, compositional analysis, and process engineering.

Pretreatment

After it is mechanically milled, biomass is subjected to pretreatment—a process that typically uses heat, pressure, and chemical catalysts such as acid to break down hemicellulose to its component sugars (hydrolysis) and make the cellulosic fraction accessible to enzymatic hydrolysis. NREL has focused on pretreatment using dilute sulfuric acid, which hydrolyzes most of the hemicellulose to monomeric sugars (i.e., simple sugars) while solubilizing (making soluble in water) a limited amount of the lignin. Other pretreatment chemistries solubilize different amounts of the lignin and hemicellulose, but they are also effective for increasing the susceptibility of cellulose to hydrolysis by enzymes.

NREL's researchers also work with investigators in industry and other organizations to evaluate a variety of pretreatment alternatives. All the AFUF pretreatment equipment can be used to explore alternative pretreatment processes.

Bench-scale (less than a 10-liter working volume) AFUF pretreatment equipment includes the following:

• Stirred-tank, batch Parr reactors with optional hot separation and washing
• A batch steam gun
• A high-solids, stirred-tank Zipperclave reactor
• Various percolation and shrinking-bed flow-through systems.

Pilot-scale systems (with capacities greater than 10 kilograms of biomass per hour) include these:

• A continuous, one-ton-per-day-capacity Sunds hydrolyzer
• A two-stage, horizontal-vertical, shrinking-bed screw reactor system
• A Jaygo high-solids, steam-jacketed and steam-injected batch mixed reactor
• A pilot-scale Pneumapress pressure-belt filter, which can be used to perform NREL's hot-wash process to keep solubilized lignin from recrystallizing. (PDF 755 KB) Download Adobe Reader.

Enzymatic Hydrolysis

After pretreatment, residual solids are treated with enzymes to break down the cellulose to glucose sugar in a process known as enzymatic hydrolysis. NREL researchers have determined that enzymatic hydrolysis of cellulose has the greatest potential to achieve cost-effective biomass conversion.

To make this approach economical, NREL is pursuing a four-fold strategy: (1) determine the best mixture of enzymes to use; (2) via protein engineering, make the enzymes more effective so that fewer enzymes are needed; (3) lower the cost of producing the enzymes; and (4) reduce the cost of processing by operating at a higher solids level.

To succeed with this strategy, NREL has partnered with industry members that have established capabilities for making better enzymes and for reducing their production costs, as well as with universities to advance our knowledge of the structure and function of enzymes. All these partners have access to the AFUF, which has reactors for conducting and assessing performance at a wide range of scales.

Fermentation

After hydrolysis, the sugars obtained from both pretreatment and enzymatic hydrolysis are fermented to ethanol or otherwise processed to desired products. NREL scientists have developed an improved Zymomonas mobilis bacteria that is capable of fermenting at the same time both glucose (the six-carbon sugar derived from cellulose) and xylose (the primary five-carbon sugar derived from hemicellulose, which is more difficult to ferment than glucose).

This patented bacterium strain could reduce the cost of capital equipment and help to streamline process operations in a cellulose-based ethanol-production plant. For a biorefinery, however, it may be advantageous to ferment the sugar streams separately to produce different products. The AFUF fermentation reactors, ranging from bench scale to 9000 liters, can be used to test a variety of aerobic or anaerobic processes.

The PDU's fermentation and downstream separations and product recovery sections can also be operated as a Bioprocessing Pilot Plant for use by industrial partners developing processes that start with sugars instead of raw biomass. (PDF 404 KB) Download Adobe Reader.

Product Separation

The AFUF has a 10-meter distillation column for recovering ethanol. Researchers can use this system as well as a wide range of filters, centrifuges, chromatography systems, evaporators, and membrane devices to separate, purify, and concentrate a wide range of potential products.

Process Integration

NREL biomass researchers have expertise in integrating the various processing steps-i.e., the conversion of biomass to sugars and the fermentation of sugars to products-into cost-effective industrial processes. They are adept at integrating processes using a variety of biomass and sugar feedstocks. To test these prospective bioprocesses as they would be used in production, researchers most often use the AFUF's mini-pilot system rather than the PDU, which is not as suitable for such investigations and costs more to run.

AFUF researchers also have extensive experience working with industrial partners in support of specific R&D activities aimed at commercializing the next generation of bioconversion and bioprocessing technologies.

Process Analysis and Engineering

Process simulation and economic analysis underpin all experimental research. Along with market analysis, these factor heavily in strategic and tactical business planning. Therefore, the AFUF also houses substantial process engineering and analysis capabilities.

NREL process engineers use ASPEN Plus and a variety of other process simulation, techno-economic analysis, and life-cycle analysis software tools to model and assess the environmental and economic impacts of a wide range of prospective biochemical or thermochemical conversion processes. Process simulation and modeling help to determine the economic impacts of specific technology improvements; to understand the implications of particular process changes on overall system integration and performance; and to gauge the techno-economic feasibility of potential biorefining scenarios.

Compositional Analysis and Process Monitoring

AFUF researchers are well versed in biomass compositional analysis and process monitoring, and they can reliably assess mass and element balance closures across discrete process steps. NREL's analytical chemists have expertise in developing, applying and refining a portfolio of analytical methods to determine the composition of a wide range of biomass feedstocks and process intermediates. Components monitored include polymeric and monomeric sugars, lignin, organic acids, proteins, sugar degradation products, extractives, ash, and moisture.

A wide variety of instrumentation is available in the AFUF. It includes a range of spectrometers (infrared, visible, and ultraviolet), automated extraction units, and high-performance liquid chromatographs equipped with a variety of columns and detector systems.

Researchers have also developed "rapid analysis" methods that use multivariate techniques to extract compositional information directly from complex sample spectra. These methods enable the accurate, low-cost determination of multiple components in solid, liquid, and slurry samples, and they are being developed for on-line compositional analysis. The PDU and mini-pilot system include sophisticated data acquisition and control systems that permit effective real-time process control and monitoring during any mode of operation (e.g., batch-fed or continuous). Dedicated mass spectrometers permit the on-line monitoring of fermenter exhaust-gas compositions.

NREL Contacts

John Ashworth

Industrial Partner Use of AFUF Facilities
(303) 384-6858