Cellulosics Conversion Gets a Boost

More and more attention is focusing on developing processes to make biofuels and biochemicals from non-food crops and cellulosic wastes. A leader in such efforts is Biofine Renewables, Waltham, Mass.

Since 2007, the company has operated a demonstration plant in Gorham, Maine, that's successfully converted cellulosic biomass feedstock into levulinic acid intermediates used in a variety of chemicals, plastics and fuels.

"Our product is data and our goal is to prove to our investors and potential clients that second-generation biofuels are worthwhile and profitable," says Steve Fitzpatrick, Biofine Renewables' president.

Many of these clients have immediate access to a low-cost feedstock that's high in cellulose, such as waste wood pulp. Today, though, such a material generally poses burdens not benefits.

The company's technology promises to change that. "Instead of having to pay to haul the waste away or just burning it in a boiler, they can convert it into chemical intermediates that have high values," Fitzpatrick notes. "The process also generates a byproduct called lignin that can be burned to provide energy to the plant or nearby consumers." (Academic and commercial research underway may lead to use of lignin as a biochemicals feedstock or as non-biodegradable soil amender.)

The Technology
The Biofine Renewables process involves high-temperature dilute-acid-catalyzed hydrolytic breakdown of cellulose to form levulinic acid. The company has developed a novel reactor configuration that promotes production of levulinic acid while reducing char formation.

The configuration consists of a plug-flow reactor followed by a lower-temperature continuously stirred tank reactor. Table 1 summarizes conditions in the two reactors. Conditions in the first stage favor the dominant fast first-order high-temperature acid-catalyzed hydrolysis of cellulosic and hemi-cellulose to soluble intermediates. Completely mixed conditions in the second-stage backmix reactor favor a first-order reaction sequence leading to levulinic acid. Calculations show the rate constant for glucose degradation at conditions in the second-stage reactor is at least an order-of-magnitude lower than that of cellulose degradation in the first stage. Additionally, reaction conditions in the first stage followed by vapor separation in the lower-pressure second stage favor high yields of furfural from the hemi-cellulose fraction of the feed.

The overall process leading to commercial-grade levulinic acid consists of five steps carried out continuously:

1. Feedstock preparation and mixing. Raw feedstock is ground to a particle size of around 0.5 cm and mixed with recycle dilute mineral acid.
2. Hydrolysis. The main conversion reactions occur and ligneous char is separated from the reaction mixture.
3. Product concentration. Water concentration is adjusted and formic acid and furfural, if present, are recovered.
4. Recycle acid separation. Product is removed from the acid, which then is recycled.
5. Product recovery. Product either is converted to derivative products or further purified, if necessary.

Process Advantages
The technology boasts several important advantages over other levulinic acid and cellulose conversion ones:

• Compactness. The reaction is fast, so residence time in the reactor system is short and a small reactor volume can provide a high throughput. The process is sufficiently compact that an ocean-going "Panamax" barge can accommodate a self-contained 1,000-ton/day unit, according to a feasibility study.

• Low cost. The only reaction catalyst is low-cost dilute mineral acid that's recycled within the process, eliminating the need for disposal of waste salts.

• Feedstock flexibility. The process can handle a wide range of low-grade variable-composition cellulosic feedstocks or proposed dedicated energy crops such as willow, poplar, miscanthus grasses or switchgrass.

• Robustness. The conversion chemistry only depends on dilute-mineral-acid-catalyzed hydrolysis of carbohydrate polymers and is unaffected by contaminants typically found in waste feedstocks.

• Ease of operation. Hydrolysis occurs continuously. This affords the potential for significant process energy integration and reduces equipment size and labor requirements as well as maintenance and energy costs compared with comparable batch operations.

• Byproduct flexibility. Byproduct credits don't significantly affect process economics. It's easy to convert the furfural byproduct from hemi-cellulose to levulinic acid (via hydrogenation to furfural alcohol), thereby markedly increasing yield of the main product. Formic acid can be sold or converted to derivatives.

• Energy self-sufficiency. The char byproduct contains sufficient energy to satisfy all the steam and electric power needs of the process. At larger scale (above 300 t/d), the process produces significant excess power that can be exported to the grid.