Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 89

Biorenewable feedstocks can be converted into liquid or gaseous forms for the production of electric power, heat, chemicals, or gaseous and liquid fuels. Main biomass conversion processes are – alphabetically rather than by preference – (i) anaerobic digestion, (ii) direct combustion, (iii) fermentation, (iv) gasification, and (v) pyrolysis. Each process has its own particular aspects, and process application is dependent upon the type of feedstock and the desired product(s).

The amount of hemicellulose and cellulose in wood and the chemical products desired determine the general type of process that might be used to hydrolyze wood. Hardwoods yield more five-carbon sugars than softwoods. Since, at this time, only the six-carbon sugars from cellulose are readily fermentable, softwoods are desired for ethanol production, but they are not as widely available as hardwoods. Hardwoods are more widely available now, so considerable effort has been expended to develop processes to utilize their unique constituents.

The main components of wood cells are cellulose (an insoluble substance which is the main constituent of plant cell walls and of vegetable fibers such as cotton. It is a polysaccharide consisting of chains of glucose monomers.), hemicellulose (a class of substances which occur as constituents of the cell walls of plants and are polysaccharides of simpler structure than cellulose.), and lignin (a complex organic polymer deposited in the cell walls of many plants, making them rigid and woody), forming some 99 % w/w of the wood material. Cellulose and hemicellulose are formed by long chains of carbohydrates, whereas lignin is a complicated component of polymeric phenolics. Lignin is rich in carbon and hydrogen, which are the main heat producing elements. Thus, the calorific value of lignin is higher than that of cellulose and hemicellulose (both are carbohydrate derivatives). Wood and bark also contain so-called extractives, such as terpenes, fats, and phenols. The amount of wood extractives is relatively small compared to the amount of extractives from bark and foliage.

The nitrogen (N) content of wood is approximately 0.75 %, varying somewhat from one tree species to another. For example, nitrogen-fixing alder (Alnus sp.) contains twice as much nitrogen as most coniferous trees. Wood has practically no sulfur (S) and, compared to many other fuels, the wood has a relatively low carbon content (some 50% of the dry weight) and high oxygen content (some 40% w/w), which leads to relatively low heating value per dry weight.

The collection of solid wastes is usually organized on a communal basis; in developing countries though, it may be organized (to a greater or lesser extent) on an informal basis. The treatment and disposal of solid wastes are definitely connected. Treatment is applied to recover useful substances or energy, to reduce waste volume, or to stabilize waste remains to be dumped or disposed of in landfills. Wastes may be treated before disposal to reduce the volume or to alter the characteristics of the waste which can be achieved by various physical, chemical, and biological processes, while combustion can be used to destroy some toxic organic chemicals. Where a method of waste disposal is not specified, the choice of disposal route will typically depend on (i) the availability of facilities, (ii) volume of waste material, and (iii) hydro-geological characteristics; the influence of industrial and environmental lobby groups must also be taken into account.

Provided that there is no shortage of land with suitable geological formations, landfill remains the principal final disposal route for the majority of wastes, even in highly industrialized countries. Where there is treatment, it is usually designed to reduce the volume of waste to be landfilled and includes compaction, shredding, baling, and combustion. Most solid wastes will therefore directly be disposed of in sanitary landfills.

The prefix sanitary is mainly to be understood as providing some protection for the population against airborne dust and litter, stench, rodents, and insects. Most of these nuisances can be prevented by the prompt covering of freshly dumped waste with soil. However, to be able to call a waste tip or landfill sanitary from an environmental viewpoint requires more measures to be taken. The main environmental problem associated with landfilling is pollution of groundwater. Rainwater percolating through solid waste tends to carry large amounts of pollutants to groundwater aquifers if the underlying strata are pervious or fissured. Thus, wells drawing from the aquifers will be extracting groundwater contaminated by the leachate; such a situation is often difficult to remedy. Studies have shown that the leachate from solid wastes may have a pollution load up to 15 to 20 times higher than domestic wastewater.

Landfill tends to predominate as a waste disposal mode because it is regarded as an effective but low-cost method of disposal, also for hazardous waste. Even where other methods are more suitable for environmental reasons, the higher capital and (short-term) running costs mean that they cannot compete without government intervention. However, such cost calculations take no account of the longer term. In the long run, landfill of hazardous materials may impose a larger financial burden than other methods because of the high cost of ensuring that the site remains secure for the time it takes for the waste to be rendered harmless.