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it would be a serious omission if recognition of some of the potential properties and effects of renewable fuels such as biogas and bio-oil and were omitted from this work. For example, biogas and bio-oil could cause harm to the environment is used without any form of treatment. The gas needs to be cleaned of objectionable environmentally harmful constituents and the bio-oil (from whatever the source) needs to be processed to provide useable products. Accordingly, it has been necessary to include options for gas cleaning and options for bio-oil refining (such as hydrotreating and hydrocracking) so that the reader may understand the conditioning of these products into environmentally benign use.

      The dynamics are now coming into place for the establishment of an alternate energy industry and it is up to various levels of government not only to promote the establishment of such an industry but to lead the way, recognizing that it is not only supply and demand but the available and variable technology. The processes for recovery of the raw materials and the processing options have changed in an attempt to increase the efficiency of energy production.

      In addtion, there are several interrelationships between conventional fuels and alternate fuels, especially in the areas of fuel production and fuel refining. Accordingly, it has been found necessary to include segments for the conventional fuel industries that are applicable to the alternate fuels industries. As ready reference, the articles in this encyclopedia have been assembled to assist the reader to understand the options that are available for the production of alternate energy, especially alternate fuels, and such processes from the conventional fuels industries and also from the unconventional fuels are, where they are applicable to renewable fuels, also included.

The Scrivener Encyclopedia of Renewable Energy was compiled because the need was recognized for an extensive work on the nature of renewable energy sources. This will be a convenient-to-use encyclopedia that will be a suitable companion for the researcher, teacher, and manager. Also, in order for help the reader understand the nature of renewable energy sources (Table 1), the encyclopedia contains segments that will allow the reader to compare renewable energy sources with the (to date) more conventional fossil fuels resources and how the conventional processing units that will be necessary to process renewable energy sources that are in the form of gases, liquid and solid.

Table 1 Simplified categorization and nomenclature of the various energy sources.

Conventional Energy Sources Natural gas Crude oil Heavy crude oil Coal*

Non-conventional Energy Sources Extra heavy crude oil Tar sand bitumen Coal* Coal gas Coal liquids Shale oil

Renewable Energy Sources Biomass Waste Hydrogen Hydroelectric energy** Geothermal energy Nuclear energy Ocean Energy** Solar energy Tidal energy** Wind energy

*When used as a solid fuel for combustion processes and without any modification other than cleaning, coal is often considered to be a conventional energy source. **Often grouped together under the name “hydrokinetic energy” – hydroelectric energy may also be included in this group.

The encyclopedia also contains a section on further reading for those readers who require further information on any of the subjects.

      Finally, the temperature scales used in this work are the Centigrade (Celsius) scale and the Fahrenheit scale. Generally, when the temperature is below 100°C the conversion to the exact temperature in degrees Fahrenheit is presented immediately following in parenthesis. On the other hand, when the temperature is above 100°C the conversion to the nearest 5° Fahrenheit is presented immediately following in parenthesis.

      Dr. James Speight

      Laramie, Wyoming, USA

February 2021

      A

      Absorption

      When gaseous products are produced from a source, the gas stream will invariably contain unwanted constituents that must be removed. This not only applies to natural gas streams and gas streams produced during the refining of the crude oil (as well as to gas streams produced from the other fossil fuels) but also to the alternative fuel industry where gas streams are produced from feedstocks such as biomass and waste. One such process for removing the unwanted constituents is the absorption process.

Absorption is a physical or chemical process by which molecules enter a liquid or solid bulk phase and is different to adsorption in which a chemical is subject to the surface forces of the adsorbent (Table A-1).

Table A-1 Examples of the types of absorption.

Solute*	Absorbent	Absorption type
Ammonia	Water	Physical
Carbon dioxide	Ethanolamine	Chemical, reversible
Carbon dioxide	Sodium hydroxide (aq.)	Chemical, irreversible
Hydrochloric acid	Sodium hydroxide (aq.)	Chemical, irreversible
Hydrochloric acid	Water	Physical
Hydrogen sulfide	Sodium hydroxide (aq.)	Chemical, irreversible
Hydrogen sulfide	Olamine**	Chemical, reversible
Nitrogen oxides	Water	Chemical, reversible
Sulfur dioxide	Water	Physical and chemical***
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