Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 51

Agricultural residues, which are a convenient renewable energy source, include straw, manure, vegetables, as well as fruit waste and general garden waste (Table A-5). Other residues include potatoes and sugar beet tops as well as damaged fruit and nursery wastes, and use of agricultural waste reduces the possibilities for odor and water pollution by manure. Aside from their abundance and renewability, using agricultural residues will benefit farmers, industry, and human health and the environment.

Table A-5 Examples of the Types and Bulk Composition of Agricultural Residues.

Agricultural residue	Cellulose	Hemicellulose	Lignin
Bagasse (sugar cane)	32-48	19-24	23-32
Bamboo	41-49	24-28	24-26
Cattle manure	1.6-4.7	1.4-3.3	2.7-5.7
Coastal Bermuda grass	25	35.7	6.4
Corncobs	45	35	15
Corn stover	35	28	16-21
Grasses	25-40	35-50	10-30
Leaves	15-20	80-85	0
Nut shells	25-30	25-30	30-40
Pig waste	6.0	28	-
Rice straw	40	18	5.5
Sorghum (sweet)	27	25	11
Sorted refuse	60	20	20
Switch grass	30-51	10-50	5-20
Wastewater solids	8-15	NA	24-29
Wheat straw	33-40	20-25	15-20
Wood (hardwood)	40-50	24-40	18-25
Wood (softwood)	45-50	25-35	25-35

For example, until recently the excess straw produced was generally burned in the fields or ploughed back into the land. However, the environmental legislation put in place in many countries has restricted field burning and straw has been seen as a potential source of energy.

Dry residues include those parts of arable crops (such as corn stover, straw, and poultry letter) not to be used for the primary purpose of producing food, feed, or fiber, used in animal bedding and feathers. On the other hand, wet residues (which are residues and wastes that have a high water content as collected) include (i) animal slurry, (ii) farmyard manure, and (iii) grass silage.

Agriculture residues directly burned as fuel in the developing world include crop residues, forest litter, and also grass and animal garbage. Crop residues are more widely burnt than animal waste and forest litter. Crop residues encompass all agricultural wastes such as straw, stem, stalk, leaves, husk, shell, peel, lint, stones, pulp, stubble, etc. which come from cereals (rice, wheat, maize or corn, sorghum, barley, millet), cotton, groundnut, jute, legumes (tomato, bean, soy), coffee, cacao, olive, tea, fruits (banana, mango, coco, cashew), and palm oil.

Because agricultural residues are of a wide variety of types, the most appropriate energy conversion technologies and handling protocols vary from type to type. The most significant division is between those residues that are predominantly dry (such as straw) and those that are wet (such as animal slurry). Many agricultural crops and processes yield residues (Table A-6) that can potentially be used for energy applications, in a number of ways. Sources can include: (i) arable crop residues such as straw or husks, (ii) animal manures and slurries, and (iii) animal bedding such as poultry litter.

Table A-6 Examples of crops and crop wastes.

Crops	Crop wastes
Barley	Straw, bran
Coco	Hull, fiber
Coffee	Pulp, husk
Cotton	Stalk, lint, hull
Groundnut	Shell, stalk, leaves
Jute	Stem
Maize	Stalk, leaves
Mango	Peel
Millet	Straw, bran
Palm oil	Shell, fiber, fruit bunches
Rice	Straw, husk, bran
Sorghum	Straw, bran
Tomato	Stem
Wheat	Straw, husk, bran

Forest litter is mainly constituted of dry fallen leaves. Plantations of those species widely used for reforestation or soil conservation/sand fixation such as Eucalyptus and Casuarina equisetifolia, provide an important biomass litter yearly. Animal garbage is livestock manure. Generally, dry animal wastes directly used as fuel are cow and camel dung, and sometimes that of horse and sheep. The quantity of manure produced depends on the amount of fodder eaten, the quality of fodder, and the live weight of the animal. The excrement-yield values vary among the species.

In many developing countries, most agricultural residues burned as fuel are used in their natural state with some pre-treatment like drying, and cutting, and compacting in rare occasions. Crop residues are characterized by its seasonal availability and have characteristics that differ from other solid fuels such as wood, charcoal, and char briquette. The main differences are the high content of volatile matter and lower density and burning time.

The moisture content make wet residues energetically inefficient to use for combustion or gasification. It is preferable to process them close to production, and to use processes that can make use of biomass in an aqueous environment. Any moisture content must be driven off before processing (such as combustion) can take place, either in advance before storage or as part of the combustion process (which then uses part of the energy of the fuel). Gasification also requires relatively low moisture content (<10-15%).

Many of the above agricultural residues may have alternate uses or markets, and any decision to use them for energy must be made in the context of these renewable sources. In particular, many such residues are presently widely used for soil nutrient recycling and improvement purposes and may therefore be displacing significant quantities of synthetic fertilizers or other products. As the manufacture of many of these products entails significant carbon dioxide emissions and energy inputs, their substitution for agricultural residues should not be undertaken without caution.

See also: Agricultural Waste, Biomass, Residues, Rice Straw, Wheat Straw.