Encyclopedia of Renewable Energy. James G. Speight
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Название: Encyclopedia of Renewable Energy
Автор: James G. Speight
Издательство: John Wiley & Sons Limited
Жанр: Физика
isbn: 9781119364092
isbn:
Biocatalysts are living (biological) systems that increase the rate of chemical reactions. In biocatalytic processes, natural catalysts, such as enzymes, perform chemical transformations of organic compounds. In fact, enzymes that have been isolated as separate molecular entities as well as enzymes still remain inside living cells are employed as catalysts that can catalyze novel small molecule transformations that may be difficult or impossible using classical synthetic organic chemistry. In addition, enzymes are environmentally benign insofar as they can be completely degraded in the environment.
Biocatalysts do not operate by different scientific principles from organic catalysts. The existence of a multitude of enzyme models including oligopeptide or polypeptide catalysts proves that all enzyme action can be explained by rational chemical and physical principles. However, enzymes can create unusual and superior reaction conditions such as extremely low pKa values or a high positive potential for a redox metal ion. Enzymes have increasingly been found to catalyze almost any reaction of organic chemistry. Moreover, the notion that biocatalysts are slow catalysts is false and optimized syntheses not only produce high selectivity or total turnover numbers but also satisfactory-to-high yields of products.
See also: Enzymes.
Biochar
Biochar is organic matter that has undergone combustion under low to no oxygen conditions (such as during pyrolysis) resulting in a recalcitrant, high carbon material specifically for use as a soil amendment. Recently, fervent interest in the production of biochar to address issues of fertility, water-holding capacity, remediation, climate change mitigation, etc., led to a much greater understanding of the complexities of this potential amendment in altering soil biological, chemical, and physical properties. Rather than assume the benefit of any biochar created from any feedstock added to any soil ecosystem, concepts of matching appropriate feedstock and pyrolysis condition to soil type to achieve specific goals associated with remediation, increasing yields, decreasing greenhouse gas emission, and/or climate change mitigation emerged.
This porous sponge-like property of biochar makes it useful for many things such as the production of activated carbon filters used to purify water. Industrial production of biochar employs pyrolysis, a means of combustion without much air or oxygen and that is more efficient in that it produces little ash.
The production of biochar is a sustainable option for waste management since the char contains 50% of the original carbon which is highly recalcitrant in nature; therefore, its production helps in carbon sequestration by locking the carbon present in the plant biomass. The elemental composition and structural configuration of biochar is strongly correlated with temperature, heating rate, and residence time maintained during its production. Along with the biochar, some amount of bio-oil and gases are also produced which can be used for generation of energy and various chemicals.
Soil pH and electrical conductivity (EC) increase in soil incorporated with biochar which may be due to the presence of ash residue that is dominated by carbonates of alkali and alkaline earth metals, and some amount of silica, heavy metals, and organic and inorganic nitrogen. With its large surface area, biochar helps in increasing water holding capacity, cation exchange capacity (CEC), and microbial activity (act as its habitat) and also reduces leaching of nutrient by providing nutrient binding sites. This reduces the total fertilizer requirement of biochar-amended soil and thereby reduces environmental pollution caused by leaching of inorganic fertilizer. It also plays a vital role in increasing crop productivity. Apart from improving soil quality, biochar provides various other benefits such as (i) mitigation of greenhouse gases (such as methane, CH4, nitrous oxide, N2O, and carbon dioxide, CO2), (ii) a decrease in the dissipation rate of herbicide in soil, and (iii) wastewater treatment. Due to large availability of biomass resources, biochar can be a prime product in many countries.
See also: Pyrolysis.
Biochemical Conversion
Biochemical conversion is the use of (i) fermentation or (ii) anaerobic digestion to produce fuels and chemicals from organic sources. In the general sense, fermentation refers to any chemical change of organic material that is accompanied by effervescence, normally without the participation of oxygen. The important differences between fermentation and anaerobic digestion are the nature of the product produced and the character of the biological contribution. Fermentation produces a liquid product in the presence of enzymes, while anaerobic digestion yields a gaseous product as a result of the metabolic activity of bacteria (Table B-3).
Ethanol is the principal product of the fermentation processes appropriate to biomass conversion, although other alcohols, as well as organic acids, ketones, and aldehydes, may be produced either as main products or as by-products. Anaerobic digestion is the decomposition of any organic material by the metabolic action of bacteria without the participation of atmospheric oxygen. Methane and carbon dioxide are the main products of the decomposition. The source of the oxygen in the carbon dioxide is the combined oxygen in the organic molecules and in the water.
Table B-3 Biomass conversion processes.
| Process | Biomass feedstock | Scale* | Product |
|---|---|---|---|
| Combustion | Wood, municipal solid waste, grasses, crop residue | Small, large | Heat, steam, electricity |
| Gasification | Wood, municipal solid waste (grasses, crop residue) | Large | Low-heat content gas, synthesis gas ethanol |
| Pyrolysis | Wood, sewage sludge | Large | Medium-heat content gas tar |
| Fermentation | Grain and sugar crops | Small, large | Ethanol |
| *Small implies domestic or farm application; large is industrial-scale processing of up to 1,000 t/d of biomass. | |||
Bacterial digestion is in effect accomplished by enzymes. Further, certain bacteria produce acids and alcohols as the principal degradation products. In some cases, it is not clear whether the degradation proceeds as a result of bacterial metabolism, or whether it can be achieved by non-growing cells. Nevertheless, the distinction between the two processes is convenient for presentation purposes, and should not cause confusion in classifying the important biochemical processes currently in contention.
Biomass fermentation to produce ethanol is СКАЧАТЬ