Encyclopedia of Renewable Energy - James G. Speight

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indirect processes using both algae and photosynthetic bacteria have been proposed and tested. The simplest indirect process would use the same algal cells for carbon dioxide fixation, oxygen evolution, and hydrogen production, at separate times or even in different reactors. The hydrogen reactions would take place both in the dark and light. Light-driven hydrogen evolution requires suppression of the oxygen evolution process. The biophotolysis process must achieve the highest possible solar conversion efficiencies, which will require development of algal strains with reduced light-harvesting pigment content.

      See also: Biological Hydrogen Production, Hydrogen Production.

      Biohydrogen – Production

      The photosynthetic production of hydrogen employs microorganisms such as cyanobacteria, which have been genetically modified to produce pure hydrogen rather than the metabolically relevant substances. The conversion efficiency from sunlight to hydrogen is small, usually under 0.1%, indicating the need for large collection areas.

      Hydrogen build-up hinders further production, and there has to be a continuous removal of the hydrogen produced, by pipelines to e.g., a shore location, where gas treatment and purification can take place. Furthermore, if the bacteria are modified to produce maximum hydrogen, their own growth and reproduction are quenched. There presumably has to be a compromise made between the requirements of the organism and the amount of hydrogen produced for export, so that replacement of organisms (produced at some central biofactory) does not have to be made at frequent intervals.

      See also: Hydrogen.

      Biological Action

      Biological action is the action of biological organism on a substrate to produce a product, such as is envisioned in the production of biofuels by biological agents. In a scientific sense, a biological process is a method or means of changing one or more chemical reactions due to the activity of the biological agent which may result in a change in the composition of chemical(s) or material(s).

      Although biological actions may involve only one step, often, multiple steps are involved. In the case of multiple steps, the steps may be sequential in time or sequential in space. Also, for a given amount of a feedstock, an expected amount of material can be determined at key steps in the process.

      Thus, a biological process or bioconversion process involves the conversion of biomass into bioenergy, fertilizer, food, and chemicals through the biological action of microorganisms. One of the important biofuels obtained through bio-conversion is biogas (which is predominantly methane). In addition, and in the current context, biological processes are those processes that are vital for an organism to survive, and that shape the ability of the organism to interact with its environment. Biological processes involve many chemical reactions or other events that are involved in the persistence and transformation of life forms.

      In contrast, the non-biological processes such as (a) direct combustion, (b) conversion of biomass into liquid fuels such as fuel oil (e.g., by pyrolysis - a type of fertilization, liquefaction, etc. ), and (c) gasification.

      Biological Alcohol

      A biological alcohol is any alcohol that is provided through the action of a biological agent (such as a microbe, typically the biological agent is a colony of microbes) on a feedstock.

      As an example, the fermentation process is a complex biochemical process during which yeast converts sugars to ethanol, carbon dioxide, and other metabolic by-products that contribute to the chemical composition and sensorial properties of the fermented foodstuffs. Control of the fermentation process is generally considered as a prerequisite to determine the quality of the final product. In this context, fermentation monitoring is a growing need, which calls for fast, low-cost, and nondestructive methods providing real-time or online information in order to assure an effective control at all stages of the process.

      The production of the biological alcohol is accomplished by yeast, certain types of bacteria, as well as other microorganisms that result in the conversion of sugar derivatives into ethyl alcohol and carbon dioxide. In the process, yeast is mostly used as the biological agent and the yeast generally carries out the aerobic fermentation process, but it may also ferment the raw materials under anaerobic conditions. The process commences with the breakdown of sugars by yeasts to form pyruvate molecules (glycolysis) which, in the case of glucose, produces two molecules of pyruvic acid. The two molecules of pyruvic acid are then reduced to two molecules of ethanol and carbon dioxide.

      Under anaerobic conditions, the pyruvate can be transformed to ethanol, where it first converts into an intermediate product (acetaldehyde, CH3CHO) which further releases carbon dioxide and is converted into ethanol (CH3CH2OH).

      See also: Bioalcohol, Bioethanol, Fermentation, Fermentation Chemistry.

      Biological Conversion

      Biological conversion (also referred to as biochemical conversion) involves breaking down biomass to make the carbohydrates available for processing into sugars, which can then be converted into biofuels and bioproducts through the use of microorganisms and catalysts. Potential fuel blend stocks and other bioproducts include the following: (i) renewable gasoline, (ii) ethanol and other alcohols, (iii) renewable chemical products, and (iv) renewable diesel. Biochemical conversion uses biocatalysts, such as enzymes, in addition to heat and other chemicals, to convert the suitable portions of biomass (hemicellulose and cellulose) into an intermediate sugar stream. These sugars are intermediate building blocks that can then be fermented or chemically catalyzed into a range of advanced biofuels and value-added chemicals.

      Bioconversion processes generally take place in bioreactors, which may be operated in batch, continuous, or semi-continuous mode, among others. Moreover, different bioreactor configurations may be suitable depending on the specific application. The technology may range from solid-phase bioconversion processes to gas-phase ones, besides aqueous phase bioprocesses. In any case, a given amount of moisture is generally needed, as this is required, in most cases, for optimal microbial activity. For any given feedstock, biocatalyst and bioreactor configuration and operating conditions will need to be optimized, in terms of aspects such as residence time in continuous processes, pH, or media composition (such as, for example, the carbon-nitrogen ratio).

      Thus, bioconversion processes and biorefineries are environmentally friendly alternatives to common chemical processes and conventional oil refineries. They allow the production of a wide range of products with cheap biocatalysts, usually under mild conditions.

      See also: Biohydrogen, Biological Action, Biological Alcohol.

      Biological Conversion – Aerobic Digestion

      In general, aerobic digestion is a relatively simple process; there are

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