An important component of TVA's current program is a project to develop complete "biomass refinery systems" Such systems are based on production and use of feedstocks including crops such as alfalfa, and crop residues such as corn stover, as raw materials for conversion to multiple products (food, feed, energy, and chemicals). This concept parallels the concept of wet milling corn into protein, oil, and starch, or the crushing of soybeans to yield oil and protein meal, but relies on biomass rather than a row crop as a feedstock. Integrating processing systems to yield several marketable products can increase both the value of the raw material and the flexibility of the production system.
Feedstocks with high protein contents are processed through a leaf/stem separator to provide a high-protein animal feed. Alfalfa fed through an air separation system resulted in good separation between leaves and stems and yielded about 400 kg of leaf fraction for an animal feed and 600 kg of alfalfa stem for subsequent processing per tonne of alfalfa.
In TVA tests, alfalfa stem, corn stover, sugarcane bagasse, and oak were hydrolyzed in the laboratory using concentrated acid hydrolysis to convert the hemicellulose and cellulose to sugars (Barrier et al. 1985). In this process, the feedstock was reduced in size and fed to the first-stage hydrolysis reactors where the hemicellulose was converted to pentose sugars (predominantly xylose). The remaining solids were then mixed with concentrated sulfuric acid and dried as a pretreatment to cellulose hydrolysis. The dried solids were fed to the second-stage hydrolysis reactors where water was added, and the cellulose was converted to glucose. The remaining solids (mostly lignin) were washed and allowed to air-dry. The hydrolyzate was recycled to the first-stage reactors to be used as makeup acid, resulting in a mixture of xylose and glucose in the final product stream. The product sugar stream was neutralized, and the resulting gypsum was filtered.
The hydrolyzate sugars can be processed to produce various products such as ethanol, lactic acid, furfural, or citric acid. Results of TVA's hydrolysis tests and fermentation of xylose and glucose to ethanol for each of the four feedstocks are shown in Table 2.
A combination of chemicals can be produced from the sugars if desired. For example, in an oak refinery system, about 70 kg of furfural can be produced per tonne of feedstock from the xylose sugars. The remaining glucose sugars can still be fermented to ethanol and will yield about 210 liters of ethanol per tonne of oak.
The solid residue, predominantly lignin, remaining after hydrolysis has an energy content of about 22,000 Btu/kg. The lignin residue can be burned to produce process steam and electricity. There are other higher value uses for lignin, such as adhesive production, which are being evaluated.
The liquid or stillage remaining after removal of the sugar products, such as ethanol still contains some organics and inorganics that can be used to produce coproducts. The stillage can be dried and burned with the lignin to produce process steam or electricity. Another alternative is to process the stillage in an anaerobic digester to produce methane. Future research of the microbial toxicity of stillage organic and inorganic components in anaerobic digestion systems is needed.
Successful commercialization of the refinery systems would integrate environmental resources development, conservation, economic development, and domestic energy production goals. Multiple agricultural issues are addressed while land-use flexibility is maintained. The biomass crop system could be used to segment land resources for traditional commodity production and hence serve as a supply control/price support mechanism; enhance utilization of resources in the agricultural sector; stimulate the rural economy and new agri-business; and challenge, develop, and sustain human and physical resources in the agricultural sector for the future.
Using biomass crops as a feedstock for an agricultural refinery need not compete with crops requiring prime farmland. Application of the concept could allow landowners to generate additional revenue from land in the conservation reserve program without influencing supply and price of commodities. The biomass refinery products would be entering different markets from crops included in government commodity support programs.
Production of biomass crops would encourage the use of erosive land for forage crop production rather than row crops. In addition, land considered marginally tillable could be used for the production of forage crops. Commercialization of refinery systems would also create opportunities for increased farm income; provide new markets for agricultural crops, wastes, and forestry products; and reduce dependence on imports for liquid fuel production. One of the primary products, ethanol can be used as an environmentally safe replacement for lead in gasoline.
The refinery system concept has a high probability of development and commercialization given appropriate research, demonstration, and technology transfer effort. The system, as envisioned by TVA, integrates many agricultural aspects already in existence and adds some new features and some new linkages. The investment in research and development with potential for long-term solutions would be extremely small relative to recent annual costs of supply control measures and soil conservation activities.
|Biomass composition (% dry matter)|
|Conversion efficiency (%)|
|Fig. 1. Refinery systems for alfalfa, corn stover, sugarcane bagasse, and oak wood.|