|Renewable biological systems for alternative sustainable energy production. (FAO Agricultural Services Bulletin - 128) (1997)|
|Chapter 1 - Biological energy production|
|1.2 Photosynthesis and biomass|
Photosynthesis can be simply represented by the equation:
CO2 + H2O + light ® 6 (CH2O) + O2
Approximately 114 kilocalories of free energy are stored in plant biomass for every mole of CO2 fixed during photosynthesis. Solar radiation striking the earth on an annual basis is equivalent to 178,000 terawatts, i.e. 15,000 times that of current global energy consumption. Although photosynthetic energy capture is estimated to be ten times that of global annual energy consumption, only a small part of this solar radiation is used for photosynthesis. Approximately two thirds of the net global photosynthetic productivity worldwide is of terrestrial origin, while the remainder is produced mainly by phytoplankton (microalgae) in the oceans which cover approximately 70% of the total surface area of the earth. Since biomass originates from plant and algal photosynthesis, both terrestrial plants and microalgae are appropriate targets for scientific studies relevant to biomass energy production.
Any analysis of biomass energy production must consider the potential efficiency of the processes involved. Although photosynthesis is fundamental to the conversion of solar radiation into stored biomass energy, its theoretically achievable efficiency is limited both by the limited wavelength range applicable to photosynthesis, and the quantum requirements of the photosynthetic process. Only light within the wavelength range of 400 to 700 nm (photosynthetically active radiation, PAR) can be utilized by plants, effectively allowing only 45 % of total solar energy to be utilized for photosynthesis. Furthermore, fixation of one CO2 molecule during photosynthesis, necessitates a quantum requirement of ten (or more), which results in a maximum utilization of only 25% of the PAR absorbed by the photosynthetic system. On the basis of these limitations, the theoretical maximum efficiency of solar energy conversion is approximately 11%. In practice, however, the magnitude of photosynthetic efficiency observed in the field, is further decreased by factors such as poor absorption of sunlight due to its reflection, respiration requirements of photosynthesis and the need for optimal solar radiation levels. The net result being an overall photosynthetic efficiency of between 3 and 6% of total solar radiation.
Wastes and residues currently constitute a large source of biomass (1). These include solid and liquid municipal wastes, manure, lumber and pulp mill wastes, and forest and agricultural residues. With the exception of feedstocks of low water content, most of this biomass cannot be directly utilized, and must undergo some form of transformation, prior to being utilized as a fuel. Biological processes for the conversion of biomass to fuels include ethanol fermentation by yeast or bacteria, and methane production by microbial consortia under anaerobic conditions.
Wood wastes in the paper and pulp industries and bagasse from the sugar-cane industry are examples of biomass likely to accumulate at a single site. The cellulosic nature of these biomass materials, necessitates their hydrolysis to glucose, prior to ethanol fermentation. The net energy balance for the processes involved can, however, be problematic in that energy requirements for cellulose hydrolysis and distillation, must be lower than the energy in the output ethanol.
Unlike ethanol fermentation, anaerobic digestion for methane production, utilizes organic materials containing carbohydrates, lipids, and proteins. Many species of microbes work cooperatively in an anaerobic digester, in which these polymeric materials (i.e. carbohydrates, proteins and lipids) are first decomposed to organic acids, and then to hydrogen and carbon dioxide, from which methane is synthesized by methanogens. A variety of raw materials which include agricultural wastes, municipal solid wastes, market garbage, and waste water from food and fermentation industries, are applicable as substrates for this process. Waste products derived from animal husbandry are applicable to anaerobic digestion, with the added bonus of solving the environmental issues of unpleasant odors and eutrophication. Although small-scale digesters are popularly used at both the farm and village levels, large-scale operations are still in need of considerable technical improvement and cost reduction, and thus require both microbial and engineering studies.
Methane and ethanol can also be produced from cultured microalgal biomass through anaerobic digestion and microbial fermentation processes, respectively. The economics of fuel production from microalgal biomass is however largely dependent on a microalgal CO2 fixation step similar to that required for the production of H2 and algal oils.
One of the most serious environmental problems today is that of global warming, caused primarily by the heavy use of fossil fuels. In Japan, large amounts of CO2 are released into the atmosphere from electric power plants and industry. The CO2 generated by these large point sources could potentially be recovered with relative ease through the use of an established technology such as chemical absorption. The enormity of the amounts of potentially recoverable CO2 would however necessitate the development of technologies for sequestering or, more favorably, utilizing this CO2.
Photosynthetic microalgae are potential candidates for utilizing excessive amounts of CO2, since when cultivated these organisms are capable of fixing CO2 to produce energy and chemical compounds upon exposure to sunlight. The derivation of energy from algal biomass is an attractive concept in that unlike fossil fuels, algal biomass is rather uniformly distributed over much of the earth's surface, and its utilization would make no net contribution to increasing atmospheric CO2 levels. Although algal biomass is regarded as a low-grade energy source owing to its high moisture content, through biological processes, it may be converted to modem gaseous and liquid fuels such as hydrogen, methane, ethanol, and oils.
Hydrogen is regarded as a potential energy source of the future, since it is easily converted to electricity and bums cleanly. Hydrogen is currently produced by fossil fuel-based processes which emit large amounts of CO2, and relatively smaller amounts of other air pollutants such as sulphur dioxide and nitrogen oxides. Biological H2 production has thus recently received renewed attention owing to urban air pollution and global warming concerns (2). Biological hydrogen production methodologies incorporating artificial reconstitution systems with chloroplast, ferredoxin, and hydrogenase; a heterocystous cyanobacterial system with oxygen scavengers; and an algal system in a day-and-night cycle, have been studied in
Japan. From an engineering point of view, however, bacterial fermentation mechanisms for hydrogen production under either dark or light conditions is currently of importance in terms of environmental issues and the utilization of organic wastes such as waste effluent of the food and fermentation industries, pre-treated sewage sludge, and market garbage.
The use of microalgae as sources of liquid fuels is an attractive proposition from the point of view that microalgae are photosynthetic renewable resources, are of a high lipid content, have faster growth rates than plant cells, and are capable of growth in saline waters which are unsuitable for agriculture. While the lipid content of microalgae, on a dry cellular weight basis varies between 20 and 40 %, lipid contents as high as 85 % have been reported for certain microalgal strains. Botryococcus braunii, is a unique microalgal strain, having a long-chain hydrocarbon content of between 30 and 40% (dry weight basis), which is directly extractable to yield crude oil substitutes. Both physical and chemical processes are applicable in the production of liquid fuels from algal strains of high lipid content. These processes include direct lipid extraction in the production of diesel-oil substitutes, transesterification in the formation of ester fuels, and hydrogenation in the production of hydrocarbons (3). Oily substances are also produced via liquefaction of microalgal biomass through thermochemical reactions under conditions of high pressure and temperature.