Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 208

Consideration of biomass feedstocks for processes must go beyond such a simple representation as the van Krevelen diagram since the composition varies with the nature of the feedstock. For most agricultural residues, the heating values are even more uniform – approximately 6,450 to 7,300 Btu/lb, and the values for most woody materials are 7,750 to 8,200 Btu/lb. The moisture content is extremely important in determining the heating value of biomass as well as its suitability as a process feedstock. Air-dried biomass typically has approximately 15 to 20% moisture. The moisture content is also an important characteristic of coals, varying in the range of 2 to 30%. However, the bulk density (and hence energy density) of most biomass feedstocks is generally low, even after densification, approximately 10 and 40% of the bulk density of most fossil fuels. Liquid biofuels have comparable bulk densities to fossil fuels.

Most biomass materials are easier to gasify than coal because they are more reactive with higher ignition stability. This characteristic also makes them easier to process thermochemically into higher-value fuels such as methanol or hydrogen. Ash content is typically lower than for most coals, and sulfur content is much lower than for many fossil fuels. Unlike coal ash, which may contain toxic metals and other trace contaminants, biomass ash may be used as a soil amendment to help replenish nutrients removed by harvest.

Some biomass feedstocks stand out for their peculiar properties, such as high silicon or alkali metal contents – these may require special precautions for harvesting, processing, and combustion equipment. The mineral content can also vary as a function of soil type and the timing of feedstock harvest. In contrast to their fairly uniform physical properties, biomass fuels are rather heterogeneous with respect to their chemical elemental composition.

Among the liquid biomass fuels, biodiesel (vegetable oil ester) is noteworthy for its similarity to crude oil-derived diesel fuel, apart from its negligible sulfur and ash content. Bioethanol has only approximately 70% the heating value of crude oil distillates such as gasoline, but the sulfur content and the ash content are also low. Both of these liquid fuels have lower vapor pressure and flammability than their crude oil-based competitors – an advantage in some cases (e.g., use in confined spaces such as mines) but a disadvantage in others (e.g., engine starting at cold temperatures).

Plants offer a unique and diverse feedstock for chemicals. Plant biomass can be gasified to produce synthesis gas, a basic chemical feedstock and also a source of hydrogen for a future hydrogen economy. In addition, the specific components of plants such as carbohydrates, vegetable oils, plant fiber, and complex organic molecules known as primary and secondary metabolites can be utilized to produce a range of valuable monomers, chemical intermediates, pharmaceuticals, and materials:

Carbohydrates (starch, cellulose, sugars) are readily obtained from wheat and potato, while cellulose is obtained from wood pulp. The structures of these polysaccharides can be readily manipulated to produce a range of biodegradable polymers with properties similar to those of conventional plastics such as polystyrene foams and polyethylene film. In addition, these polysaccharides can be hydrolyzed, catalytically or enzymatically, to produce sugars, a valuable fermentation feedstock for the production of ethanol, citric acid, lactic acid, and dibasic acids such as succinic acid.

Vegetable oils are obtained from seed oil plants such as palm, sunflower, and soya. The predominant source of vegetable oils in many countries is rapeseed oil. Vegetable oils are a major feedstock for the oleo-chemicals industry (surfactants, dispersants, and personal care products) and are now successfully entering new markets such as diesel fuel, lubricants, polyurethane monomers, functional polymer additives, and solvents.

Plant fibers (lignocellulosic fibers) are extracted from plants such as hemp and flax can replace cotton and polyester fibers in textile materials and glass fibers in insulation products.

See also: Biofuels, Biomass.