Encyclopedia of Renewable Energy. James G. Speight
Читать онлайн книгу.
while other plants store the energy as more complex starch derivative. These plants include grains like corn which are prominent food sources. Another type of plant matter – cellulosic biomass – is composed of complex sugar polymers, and is not generally used as a food source. Cellulosic feedstocks under consideration for biofuels include (i) agricultural residues which is the leftover material from crops, such as the stalks, leaves, and husks of corn plants, (ii) forestry wastes, which include chips and sawdust from lumber mills, dead trees, and tree branches, (iii) municipal solid waste, which includes household garbage and paper products, (iv) food processing and other industrial wastes such as black liquor, which is a paper manufacturing by-product (Table B-19), and (v) energy crops, such as fast-growing trees and grasses) developed just for this purpose.
Table B-19 Composition of black liquor.
| Element | % w/w |
|---|---|
| Carbon | 35.7 |
| Hydrogen | 3.7 |
| Nitrogen | ≥0.1 |
| Oxygen | 35.8 |
| Sulfur | 4.4 |
| Chlorine | 0.3 |
| Potassium | 1.1 |
| Sodium | 19.0 |
The main components of these types of biomass are (i) cellulose which is the most common form of carbon in biomass, accounting for 40 to 60% w/w of the biomass, depending on the biomass source and which is a complex sugar polymer, or polysaccharide, made from the six-carbon sugar, glucose – the crystalline structure of cellulose renders it resistant to hydrolysis, the chemical reaction that releases simple, fermentable sugars from a polysaccharide; (ii) hemicellulose which is also a major source of carbon in biomass, at levels of between 20% and 40% w/w of the biomass and is a complex polysaccharide made from a variety of five-carbon and six-carbon sugar derivative – it is relatively easy to hydrolyze into simple sugars, but the sugars can be difficult to ferment, and (iii) lignin which is a complex polymer that makes up 10% to 24% w/w of the biomass and provides structural integrity in plants – it remains as residual material after the sugars in the biomass have been converted.
In the simplest sense, biomass is carbon based and is composed of a mixture of organic molecules containing hydrogen, usually including atoms of oxygen, often nitrogen and also small quantities of other atoms, including alkali, alkaline earth, and heavy metals. However, because of the wide variations in the character and properties of biomass, it is anticipated and realized that the character and properties of the biofuels produced from biomass are very dependent upon the initial biomass. The exception is the fuels produced by gasification and Fischer-Tropsch synthesis.
In addition to the chemical composition, three properties of biomass that are significant to the performance of biomass as a fuel are (i) mineral matter content, manifested in thermal processes as mineral ash, (ii) susceptibility of the mineral matter to slagging and fouling, and (iii) the volatile matter content. The mineral matter content is the mass fraction of biomass composed of non-combustible inorganic material. Grasses, bark, and field crop residues typically have much higher content of mineral matter than wood. Systems that are designed to combust wood can be overwhelmed by the volume of ash if other biofuels are used. Slagging and fouling are problems that occur if ash begins to melt during combustion, forming deposits on combustor surfaces (fouling) or leaving hard chunks of glassy material in the bottom of the combustion chamber (slag, often referred to as clinkers).
Certain mineral components in biomass fuels, primarily silica, potassium, and chlorine, can cause these problems to occur at lower temperatures than normal. Dirt contamination also adds to the mineral content and associated slagging and fouling problems, so it is important that biomass feedstock be as clean (dirt-free) as possible. Slagging and fouling is minimized by keeping combustion temperatures low. Alternately, some biomass combustion equipment is designed to encourage the formation of clinkers (often referred to in the singular form, clinker) but is able to dispose of the hardened ash in an effective manner.
The content of volatile constituents (or volatile matter) in a fuel is a lesser-known property that refers to the fraction of the fuel that will readily volatilize (turn to gas) when heated to a high temperature. Fuels with high volatiles content will tend to vaporize before combusting, whereas fuels with low volatiles will burn primarily char. This affects the performance of the combustion chamber and should be taken into account when designing a combustor.
Other properties such as the particle size and density of biomass fuels are also important as they affect the thermal processing characteristics (especially the combustion characteristics) of biomass, especially the rate of heating and drying during the thermal process. The feedstock particle size also dictates the type of handling equipment required. For example, the incorrect size fuel will negatively impact process efficiency and may cause jamming or damage to the handling equipment. Smaller-sized fuel is more common for commercial-scale systems because smaller fuel is easier to use in automatic feed systems and allows for finer control of the processing rate by controlling the rate at which fuel is added to the reaction chamber. Fuel particle size and density are probably the most overlooked factors affecting fuel performance and should be given careful consideration when selecting a fuel type. Bulk density is the mass of a material divided by the volume it occupies. Bulk density of granular materials is dependent on the manner in which it is handled insofar as freely settled material has a lower bulk density than tapped or compacted materials.
Biomass – Conversion Technologies
There are many bioenergy routes which can be used to convert raw biomass feedstock into a final energy product. Several conversion technologies have been developed that are adapted to the different physical nature and chemical composition of the feedstock, and to the energy service required (heat, power, transport fuel). Upgrading technologies for biomass feedstocks (e.g., pelletization, torrefaction, and pyrolysis) are being developed to convert bulky raw biomass into denser and more practical energy carriers for more efficient transport, storage, and convenient use in subsequent conversion processes.
The production of heat by the direct combustion of biomass is the leading bioenergy application throughout the world and is often cost-competitive with fossil fuel alternatives. Technologies range from rudimentary stoves to sophisticated modern appliances. For a more energy-efficient use of the biomass resource, modern, large-scale heat applications are often combined with electricity production in combined heat and power (CHP) systems.
Different technologies exist or are being developed to produce electricity from biomass. Co-combustion (also called co-firing) in coal-based power plants is the most cost-effective use of biomass for power generation. Dedicated biomass combustion plants, including MSW combustion plants, are also in successful commercial operation, and many are industrial or district heating CHP facilities. For sludges, liquids, and wet organic materials, anaerobic digestion is currently the best-suited option for producing electricity and/or heat from biomass, although its economic case relies heavily on the availability of low-cost feedstock. All these technologies are well established and commercially available.
There are few examples of commercial gasification plants, and the deployment of this technology is affected by its complexity and cost. In the longer term, if reliable and cost-effective operation can be more widely demonstrated, gasification promises greater efficiency, better economics at both small- and large-scale and lower emissions compared with other biomass-based power generation options.