Encyclopedia of Renewable Energy. James G. Speight
Читать онлайн книгу.
the production of synthesis gas from biomass is indeed the novel component in the gas-to-liquids concept – obtaining syngas from fossil raw materials (natural gas and coal) is a relatively mature technology.
Gasification is actually thermal degradation of the feedstock in the presence of an externally supplied oxidizing (oxygen-containing) agent e.g., air, steam, oxygen. Various gasification concepts have been developed over the years, mainly for the purposes of power generation. However, efficient biomass-to-liquids production imposes completely different requirements for the composition of the gas. The reason is that in power generation, the gas is used as a fuel, while in biomass-to-liquids processing, it is used as a chemical feedstock to obtain other products. This difference has implications with respect to the purity and composition of the gas.
In contrast, for biomass-to-liquids production, the amount of carbon monoxide and hydrogen is only important (the larger the amount, the better), while the calorific value is irrelevant. The presence of other hydrocarbon derivatives and inert components should be avoided or at least kept as low as possible. This can be achieved in the following ways: (i) by adjusting the amount of the various constituents of the gas stream, (ii) choice of the oxidizing agent.
The amount of components other than carbon monoxide and hydrogen (primarily hydrocarbon derivatives) can be reduced via further transformation into carbon monoxide and hydrogen. This is, however, rather energy intensive and costly (two processes – gasification and transformation). As a result, the overall energy efficiency of syngas production and of biomass-to-liquids processing is also reduced, leading to higher production costs.
The amount of various components can be minimized via a more complete decomposition of biomass, thereby preventing the formation of undesirable components at the gasification step. The minimization of the content of various hydrocarbon derivatives is achieved by increasing temperatures in the gasifier, along with shortening the residence time of feedstocks inside the reactor. Because of this short residence time, the particle size of feedstocks should be small enough (in any case – smaller than in gasification for power generation) in order that complete and efficient gasification can occur.
In gasification for power generation, typically, air is employed as oxidizing agent, as it is indeed the cheapest among all possible oxidizing agents. However, the application of air results in large amounts of nitrogen in the product gas, since nitrogen is the main constituent of air. The presence of such large quantities of nitrogen in the product gas does not hamper (very much) power generation, but it does hamper biomass-to-liquids production. Removing this nitrogen via liquefaction under cryogenic temperatures is extremely energy intensive, reduces substantially the overall biomass-to-liquids energy efficiency and increases costs. Amongst other potential options (steam, carbon dioxide, oxygen), from a technical and economic point of view oxygen appears to be the most suitable oxidizing agent for biomass-to-liquids manufacturing. It is true that the oxygen-blown gasification implies additional costs compared to the air-blown gasification, because of the oxygen production. Nevertheless, the energy and financial cost of producing oxygen seems to be far lower than the renewable energy and financial cost of cleaning up the product gas from air-blown gasification from nitrogen. This is partly due to the fact that the production of high-purity oxygen (above 95% O2) is a mature technology.
In principle, the larger the carbon and hydrogen content in raw materials, employed in gas-to-liquids processing, is, the easier and more efficient the carbon monoxide and hydrogen. Hence, the natural gas pathway is the most convenient one since natural gas is gaseous and contains virtually carbon and hydrogen only. Solid raw materials (biomass, coal) involve more processing, because first they have to be gasified and then the product gas should be cleaned up from other components such as nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter to the extent of getting as high as possible purity of syngas.
Two basic types of biomass raw material are distinguished, viz, woody material and herbaceous material. Currently, woody material accounts for approximately 50% of total world bioenergy potential. Another 20% is straw-like feedstock, obtained as a by-product from agriculture. The dedicated cultivation of straw-like energy crops could increase the herbaceous share up to 40%.
See also: Bioalcohols, Biodiesel, Biofuels – First Generation, Biofuels From Synthesis Gas, Biofuels – Second Generation, Biofuels – Third Generation, Biogas, Gasification, Methanol, Ethanol, Vegetable Oil.
Biofuels – Liquid
Liquid biofuels, as their name suggests, are fuels derived from biomass and processed to produce a combustible liquid fuel. There are two main categories: (i) alcohol fuels, such as methanol and ethanol, and (ii) vegetable oils, which are derived from plant seeds, such as sunflower, sesame, linseed, and rapeseed.
Methanol is produced by a process of chemical conversion from any biomass with a moisture content of less than 60%. Potential feedstocks include forest and agricultural residues, wood, and various energy crops. As with ethanol, it can either be blended with gasoline to improve the octane rating of the fuel or used in its neat form. Both methanol and ethanol are often preferred fuels for racing cars.
Ethanol is the most widely used liquid biofuel and is produced by fermentation of sugars and starches or cellulosic biomass. Most commercial production of ethanol is from sugar cane or sugar beet, as starches and cellulosic biomass usually require expensive pre-treatment. Ethanol is used as a renewable energy fuel source as well as being used for manufacture of cosmetics, pharmaceuticals, and also for the production of alcoholic beverages.
Vegetable oils are used to produce biodiesel. The process of oil extraction from the biomass is carried out the same way as for extraction of edible oil from plants. There are many crops grown in rural areas of the developing world which are suitable for oil production – sunflower, coconut, cotton seed, palm, rapeseed, soy bean, peanut, hemp, and more. Sunflower oil, for example, has an energy content approximately 85% that of diesel fuel.
There are two well-established technologies for oil extraction: (i) the screw press and (ii) solvent extraction. The simple screw press, which is a device for physically extracting the oil from the plant - this technology is well suited to small-scale production of oil as fuel or as foodstuff in rural areas. The press can be motor-driven or hand-operated. On the other hand, solvent extraction is a chemical process which requires large, sophisticated equipment. This method is more efficient - that is, it extracts a greater percentage of the oil from the plant - but is less suited to rural applications.
Biodiesel production is not complex – the vegetable oil is converted to a useable fuel by adding ethanol or methanol alcohol along with a catalyst to improve the reaction. Small amounts of potassium hydroxide or sodium hydroxide (commonly called lye or caustic soda, which is used in soap making) are used as the catalyst material. Glycerine separates out as the reaction takes place and sinks to the bottom of the container. This removes the component that gums up the engine so that a standard diesel engine can be used. The glycerine can be used as a degreasing soap or refined to make other products.
See also: Bioalcohols, Biodiesel, Biofuels, Cellulosic Biomass, Methanol, Ethanol, Vegetable Oil.
Biofuels – Platforms
The technical platform chosen for second-generation biofuel production will be determined in part by the characteristics of the biomass available for processing. The majority of terrestrial biomass available is typically derived from agricultural plants and from wood grown in forests, as well as from waste residues generated in the processing or use of these resources. Much of the biomass being used for first-generation biofuel production includes agricultural crops that are rich in sugars and starch. Because of the prevalence of these feedstocks, the majority of activity toward developing new products in the United States has focused on the bioconversion platform.
Bioconversion isolates sugars from biomass, which can then be processed into value-added products. Native sugars found in sugarcane and sugar beet can be easily derived from these plants, and refined in facilities that require the lowest level of capital input. Starch, a storage molecule