by Dan Burden, content specialist, AgMRC, Iowa State University, djburden@iastate.edu.
Profile reviewed January 2008.
Overview
Section 9006 of the 2002 Farm Bill created the Renewable Energy Systems and Energy Efficiency Improvements Program. Several United States Department of Agriculture (USDA) programs are intended to promote renewable energy research and system development. Additionally, the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy is funding various research programs and has various systems in place to develop new renewable energy systems and work with private-sector cooperators.
A particular area of interest to both USDA and DOE is the biorefinery concept of energy and industrial material production. On its broadest scale, the bioreactor concept can be envisioned as a processing plant where various organic materials, probably ligninocellulytic (woody materials, corn or soy stovers, prairie grasses), are broken down into their most basic components: carbon and hydrogen. The hydrogen could be directly employed as fuel for fuel cells. Through complex chemical and mechanical engineering technologies, probably employing biotechnological tools, these carbon and hydrogen building blocks could be recombined into almost any product currently derived from the refining of petroleum.
The advantages over petroleum refining are numerous, some include: the products are derived from renewable resources, in some cases waste materials that would require landfill disposal or other underutilized resources; the biorefinery system is far safer for workers and environmentally friendly; the refinery stream can be tailored to produce only desired products, unlike petroleum refining where a set product stream producing a multitude of products produces a number of low-value or unwanted co-products; and biorefineries can be smaller and lend themselves to decentralized energy and industrial chemical production.
Biorefining also is applicable to small-scale systems where a single or limited range of feedstocks is used to produce a single product or a couple of closely related products. These systems also are applicable for use as a means to “bio-remediate,” or use a biological system, to breakdown unwanted waste products (paper-making sludge, sewage, livestock manure), oftentimes with the benefit of producing a value-added, market-saleable co-product.
Most biorefining makes use of closed-system reactors (heat and catalysts) or fermentors (fermentation systems using heat and enzymes). Currently, most “bioreactors” are actually specialized fermenters. Bioreactor research is taking many forms: there are bioreactors for production of bio-medical cell cultures in the weightless environment of space, small closed systems for home sewage and organic waste disposal, systems for production of “landfill gasses” (methane), the biological de-nitrification that is used in water treatment and specialized bioreactors for energy production from under-utilized biomass or industrial wastes.
Approaches to Reacting Biomass
Using algae to produce hydrogen
Scientists have learned how micro-algae and bacteria produce hydrogen. Thus an environmentally benign method for generating energy is possible. The challenge now is to develop a sustainable bio-hydrogen system that can operate continuously.
Hawaii Natural Energy Institute is exploring a process, dubbed the Hawaii Process, which uses water, cyanobacteria and sunlight. The algae use solar energy to accumulate carbohydrates, then release the stored energy as hydrogen in a two-stage process. Research will focus on design of a sustainable bioreactor system and identification and improvement of microbial strains that grow and then produce hydrogen effectively. This type of system may be applicable to agricultural manure remediation.
Polluted air “biomass” cleaner
Another system operates by bringing polluted air into contact with biomass selected for its high degradation efficiency on specific contaminants. This takes place in the bioreactor chamber containing a biocatalyst. The biocatalyst comprises a number of support beds with an inert polymer carrier, which provides a surface for the biomass to attach to and through which the contaminated air is passed. During the process of contaminant degradation, the biocatalyst is sprayed with a nutrient-containing solution. This irrigates the beds, provides the required nutrients to promote microbial activity and maintains the system pH and other conditions at optimum levels for biomass activity. When passing through the layers of the biocatalyst, the VOCs in the contaminated air are continuously degraded to water and carbon dioxide. The technology has applications in any industry that uses solvents, such as printing, painting, coating and gluing, or which produces VOCs in manufacturing. The process consumes little energy, is non-toxic, non-hazardous and environmentally friendly.
Bioconversion of corn starch
Corn starch is converted into ethanol through wet or dry milling processes followed by saccharification and fermentation. This is the foremost bioreactor fermentation technology in use today. Technology for the fermentation of starch was first developed many thousands of years ago for beer making, but today's processes are much more efficient. Enzymatic saccharification used for converting starch into fermentable sugars employs glucoamylases that operate at elevated temperatures. Fermentation of the resulting glucose is carried out within a few hours, and the ethanol is distilled and dehydrated using contemporary technology that consumes relatively little energy. In addition to ethanol, modern wet milling plants produce corn oil and animal feed products. These byproducts offset much of the grain price and retain the principal nutritive value. Fermentation of corn starch is an established but still evolving technology. Further improvements can be expected in the utilization of corn fiber byproducts and the implementation of even better distillation technology. For more information on ethanol production, please refer to the ethanol sections of the AgMRC site at http://www.agmrc.org/energy/eth.html.
Bioconversion of lignocellulosics to fermentable sugars
Bioconversion of lignocellulose to ethanol requires pretreatment, saccharification and fermentation. To utilize woody wastes by fermentation, it first is necessary to break down lignin into fermentable sugars. This has taken the form of a number of different “pretreatment” (pre-fermentation) strategies. Pretreatment requirements vary with the feedstock and are often substantially less in the case of various paper and hydrolysis waste streams. The basic systems, all of which are applicable to agricultural and wood-product waste streams, are as follows:
Cellulose saccharification
Cellulose saccharification is the process of turning polymeric lignocellulosic materials into fermentable sugars. This can be accomplished by a number of processes including acidic and enzymatic hydrolysis.
Pretreatment is essential for bioconversion of most lignocellulosic materials. Pretreatments include mechanical size reduction, heat, steam, steam explosion, autohydrolysis, acid hydrolysis, alkali treatment, ammonia, chemical pulping, solvent extraction and various combinations of these separate processes. The purpose of pretreatment is to maximize subsequent bioconversion yields and minimize the formation of inhibitory compounds. With heterogeneous waste streams such as municipal solid waste (MSW), it may be necessary to go through some sort of classification process such as air classification or magnetic separation before carrying out pretreatment.
In the case of agricultural residues, particle size reduction can often be done simply with grinding. For dry corn or soybean residues, density is often lower than what is desirable, and particle size reduction is not an issue. Municipal solid wastes present a particular problem because of their extremely heterogeneous nature. Large quantities of plastics, wood, metals and other materials are often present. If certain heavy metals are present in the mixture when acid is added, they can create severe downstream problems in fermentation and product formation. Batteries should not be a part of the waste stream.
Steam treatment in a tumbling reactor is a convenient way to facilitate separation of plastics and fibers while increasing MSW digestibility. In this process, garbage is introduced into a large, cylindrical, horizontal autoclave that is slowly rotated on its side while steam treatment takes place. The plastic materials collapse and the fibrous materials for a pulp. Metals and other non-fibrous materials (for example, old shoes) are readily separated on a grating after treatment.
Autohydrolysis
Autohydrolysis is the process of converting lignocellulose into fermentable sugars by exposure to high-temperature steam. Many lignocellulosic materials contain significant quantities of acetylated hemicellulose. Steam releases these in the form of acetic acid, which subsequently carries out a partial hydrolysis of the hemicellulosic and cellulosic sugars. The principal disadvantage of this approach is that sugar yields are generally very low.
Acid hydrolysis
Acid hydrolysis is often used as a pretreatment because it can be adapted to a wide variety of feedstocks. Except in the case of strong hydrochloric acid hydrolysis, it is generally carried out at elevated temperature (100-240°C) for various lengths of time. At higher acid concentrations, it can be carried out at temperatures as low as 30 degrees C. Generally an inexpensive process, acid hydrolysis may also produce large quantities of degradation byproducts and undesirable inhibitory compounds in higher temperature (low concentration, greater than 110°C processes).
Strong acid hydrolysis
Sulfuric acid can be used in concentrated form, but it is far more commonly used in a dilute solution of 0.5 to 5 percent sulfuric acid (on a w/w basis with dry solids). The concentrated form usually employs a method of separating and recycling the acid catalyst, limiting the total acid losses to approximately 3 percent, or the same as the dilute process. Use of the concentrated acid, however, allows lower temperature and pressure hydrolysis with fewer byproducts produced.
Concentrated hydrochloric acid (47%) is sometimes used for strong acid hydrolysis because it is relatively easy to recover. Hydrolysis with concentrated hydrochloric acid gives one of the highest sugar yields of any acid hydrolysis process. It is carried out at room temperature. The chief drawback is that it is highly corrosive, volatile, expensive and almost complete recovery is essential to make the process economical.
Weak acid hydrolysis
Sulfur dioxide is often used in combination with autohydrolysis because it gives better sugar yields and helps to modify lignin for subsequent extraction or recovery. Sulfur dioxide combined with steam is particularly effective as a pretreatment for enzymatic cellulose saccharification.
Dilute acid hydrolysis
Dilute acid hydrolysis with 1 to 5 percent sulfuric acid is generally considered the most cost-effective means of hydrolysing wood and agricultural residues. Yields of hemicellulosic sugars can be 80 to 95 percent of theoretical. Yields of glucose from cellulose are generally less than 50 percent but can approach 55 percent at elevated temperatures.
Synthesis gas (syngas)
Bioprocessing of synthesis gas (syngas) represents a departure from conventional approaches to breaking down lignocellulose in woody wastes and crop residues. Most biomass research over the last decade has focused on trying to enzymatically hydrolyze woody and herbaceous biomass into simple carbohydrates suitable for chemical synthesis. Currently, the high cost of enzymes and poor yields remain a major barrier to this approach, but research at several universities is working to correct this.
Syngas technologies rely on the gasification (750°-850°C) conversion of solid, carbonaceous fuels into flammable gas mixtures, sometimes known as synthesis gas or syngas, consisting of carbon monoxide (CO), hydrogen (H2), methane (CH2), nitrogen (N2), carbon dioxide (CO2) and smaller quantities of higher hydrocarbons. Syngas can be used for generation of heat and power, and serve as a feedstock for the production of liquid fuels and chemicals. This flexibility of production has been proposed as the basis for “energy refineries” that would provide a variety of energy and chemical products, including electricity and transportation fuels.
Gasification consists of several distinct processes: heating and drying of the fuel; pyrolysis of solid fuel to gases, condensable vapors, and char; solid-gas reactions that consume char; and gas-phase reactions that adjust the final chemical composition of the syngas. Pyrolysis, which begins between 300 degrees and 400 degrees C, may convert up to 80 wt-% of solid biomass into gases and vapors. The pyrolytic gases include CO, CO2, H2, H2O and CH4 while the condensable vapors include a variety of hydrocarbons and oxygenated organic compounds. The solid-gas reactions produce CO, H2 and CH4. Biotechnology can then be employed to identify and construct systems to produce various chemicals and fuels from these syngas reaction products. The advantages over a conventional petroleum refinery are numerous: (1) Biological conversion occurs at lower temperatures and pressures; (2) The reaction specificity of enzymes is typically higher than inorganic catalysts, leading to higher yields and better process control; (3) Biological catalysts are tolerant of trace sulfur gases, reducing an intermediary cleanup step before conversion to fuel or chemicals required by other systems; (4) Biological conversion does not have stringent requirements on the CO/H2 ratio, allowing for the use of various agricultural or woody biomass feedstocks; and (5) The chemical differences between the feedstocks are virtually eliminated when they are gasified.
Profile created May 2003 and reviewed January 2008.