| Energy research in developing countries |
|Volume 8: Bioenergy|
J. Lamptey, M. Moo-Young, and H.F: Sullivan
This paper discusses processes for biomass bioconversion and thermochemical conversion. The production of ethanol from lignocellulosics is described in some detail, whereas butanol-acetone and hydrogen production are reviewed briefly. Thermochemical processes to convert lignocellulosics to energy (for example, methanol from synthetic wood gas) are described. Other renewable energy sources (solar, ocean, and wind energy) are discussed briefly.
Production of Ethanol-Ethanol has a number of commercial applications as an energy source and as an industrial solvent and chemical. Chemical synthesis and fermentation are the two major techniques used to produce ethanol. Chemical synthesis is the catalytic hydrolysis of ethylene derived from petroleum. For fermentation, which can be carried out through different conversion processes, the four categories of feedstocks are sugar-containing materials (which are the most expensive to obtain), starch (including corn, potatoes, and cassava), lignocellulose (which is the major renewable carbohydrate source in the world), and urban and industrial wastes. There are problems associated with the collection and conversion of each feedstock to produce a useable substrate.
The structural features of cellulosic biomass make pretreatment necessary. This process, which removes lignin and increases the surface area available to enzymes, promotes hydrolysis and enhances the rate and extent of cellulose saccharification. Pretreatment techniques are classified as physical, chemical, biological, or combined. One physical technique is milling, which reduces crystallinity and the degree of polymerization and increases bulk density. High power requirements make this technique costly. Chemical pretreatment involves the use of strong acids, bases, or agents that swell or dissolve the cellulose. These expensive chemicals are corrosive and toxic, which makes this technique commercially unattractive. Steam explosion, in which cellulose materials are saturated with water under high pressure and temperature, is another physical technique. When pressure is released, the wood fibres separate and the acetic acid that is released catalyzes the hydrolysis of the hemicellulose.
The three principle conversion processes for lignocellulose are acid hydrolysis, enzymatic hydrolysis, and direct microbial conversion. Acid hydrolysis, which is suitable for any cellulose feedstock (but is most often used with wood), can be carried out with dilute or concentrated acids. Because the use of concentrated acids is expensive, most development work involves dilute acids. An increase in acid concentration or reaction temperature increases the production of glucose.
Lignin, a by-product of acid hydrolysis, is likely only useful as fuel. Although acid hydrolysis has many advantages (for example, the ease of use of wood as a feedstock, the short reaction time, and the low cost of the acid catalyst), its limitations include lower potential yield and sugar concentration, the production of degradation products that are toxic to fermentation yeast, and severe operating conditions that produce corrosion.
Unlike acid hydrolysis of cellulosic materials, enzymatic hydrolysis is not yet used commercially. Because the enzymes are reaction specific, this process could potentially produce higher yields of sugars under mild operating conditions that do not require corrosion-resistant equipment. The major disadvantage is the requirement for an aseptic environment for enzyme production because hydrolysis is susceptible to microbial contamination.
Microbial conversion is also not used commercially, but it has theoretically the highest yields of all techniques. This process seeks to convert cellulose and produce alcohol in a single step using one or two microbial organisms. The conversion is not easily applied because no efficient cellulosic microbe that produces ethanol has been found. Microbes have a low tolerance for ethanol and tend to produce unwanted by-products.
Fermentation systems for ethanol production require substantial cost reductions. To minimize costs, a balance must be struck between three goals for ethanol fermentation (high substrate utilization, high ethanol productivity, and high ethanol concentration). Research is currently under way to increase the rate of fermentation and to decrease costs. Rapid fermentation can be achieved by maintaining a high yeast population and removing rate-inhibitory ethanol from the fermenter as quickly as it is formed. Researchers have used both adsorption and entrapment techniques in laboratory experiments to immobilize yeast and to increase the rate of fermentation. Industrial applications have not yet been developed. Although there seems to be a promising future for ethanol production, the cost of the glucose substrate remains the main bottleneck for fermentation.
Production of Butanol-Acetone and Hydrogen- Biomass feedstocks can also be used to produce both butanol-acetone mixtures and hydrogen. Fermentation of products containing starch and sugar can produce butanol-acetone mixtures. However, research in improved production technology is required to make the process commercially attractive.
Excluding gasification, hydrogen can be produced from biomass feedstocks by
· Photochemical methods (including a technique to simulate plant photosynthesis in vitro and a technique that uses water solutions of biomass components in the presence of catalysts),
· Biological methods (the use of anaerobic bacteria in dark reactions to ferment the biomass),
· Photobiological methods (the use of photosynthetic algae or bacteria under anaerobic conditions in the presence of light), and
· Biomass-water electrolysis (which involves anode depolarization with a water-soluble, oxidizable material such as glucose).
These processes have not been demonstrated on a large scale, and they may be made obsolete by the direct photocatalytic decomposition of water to hydrogen and oxygen.
Biomass feedstocks offer a number of advantages for thermochemical conversion processes (for example, high volatility and char reactivity). Drawbacks to this process include the decentralized nature of the resource and the high moisture content of the fuel. Pretreatment processes include cleaning, drying, and size reduction. Drying is crucial.
Thermochemical conversion processes include
· Direct combustion (traditional biomass includes wood and bagasse, but recent projects involve municipal solid waste, hog fuel, and pelleted wood fuels),
· Gasification to produce low- or intermediate-energy fuel gas or synthesis gas (gasification is considered a commercial or near-commercial technology and can, under certain conditions, be used as a substitute for fossil fuels),
· Direct liquefaction to produce heavy oils or, with upgrading, lighter boiling liquid products (liquefaction remains one of the least commercially attractive conversion processes because of its high capital and energy costs), and
· Pyrolysis to produce a mixture of pyrolysis oils, fuel gases, and char (rapid pyrolysis using rapid heating rates has emerged as a promising alternative for producing liquid fuels).
Production of Methanol -Although natural gas is the preferred carbon source to make synthesis gas for the production of methanol, it is possible to generate the synthesis gas from biomass. The two broad categories of wood conversion technologies for methanol production are oxygen gasifiers and pyrolytic gasifiers. The gasification of wood yields hydrogen, carbon monoxide, carbon dioxide, and other gases. The raw gas is purified and processed to extract a 2: I ratio of hydrogen - carbon monoxide. The synthesis gas is then compressed and passed through a methanol synthesis reactor. The major disadvantages of this process are the size of plant and the amount of raw material required.
Other Energy Sources
Other sources of renewable energy are solar energy, ocean energy, and wind energy. The three categories of direct solar energy are solar heating and cooling, solar thermal, and photovoltaics. The most promising application of solar energy may be in heating domestic water and buildings. Few industrial applications are currently demonstrated or operational. Research in solar photovoltaics is focusing on cheaper fabrication methods and materials for photovoltaic cells. Eventually, photovoltaics may be used for central power generation in conjunction with a conventional backup system. The diffusion of solar technology depends on cost, the solution of technical problems, and the rate of adoption of the technology.
Research is currently being undertaken in ocean energy to exploit the incident solar energy absorbed by surface waters and the energy available in ocean waves. Wind energy is another research area. In locations with strong winds and high fuel costs, wind turbines are already economic.