Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 216

Herbaceous biomass is biomass from plants that have a nonwoody stem and which die back at the end of the growing season. This biomass includes most agricultural crops and grasses, including bamboo and wheat straw. Also, relatively young and essentially nonwoody parts of trees exhibit similar characteristics. In general, herbaceous biomass will have higher nutrient contents and lower lignin contents than wood. Herbaceous biomass is variable in composition depending on the time of year and on the type of tissue. Composition can also be strongly influenced by the availability of minerals or nutrients in the soil.

Herbaceous biomass is classified into cereal crops, pastures, oilseed crops, tubers and legumes, flowers, herbaceous biomass of gardens, parks, pruning, vineyards, orchards, and mixtures of all these. This biomass can be used raw (direct residues of the field) or processed (from the food industry).

Residues from cereal crops include the use of the following parts of the plant: dried stems, pods, and husks, as well as their mixtures. Of the grasslands are considered stems, shells, and their mixtures. In oil crops, stems, leaves, pods, rinds, and their mixtures are considered. The residues of the tuber crops are integrated by stems, leaves, roots, and mixtures of these. The stems, leaves, pods, and their mixtures can be harvested from legume crops, but it should be noted that the incorporation of legumes in an annual biomass cultivation system could reduce the nitrogen fertilizer requirements, which will reduce the inputs of bioenergy production. However, they also contribute to the anthropogenic emissions of nitrous oxide (NO2) which, in this case, nitrous oxide comes mainly from the decomposition of legume residues.

Finally, from the cultivation of flowers, the waste from the plant (when the quality of these is not adequate), the stems, leaves, as well as their mixture can be used. For its part, processed residues can come from herbaceous residues without chemical treatment, as well as chemically treated herbaceous residues and its mixtures. This type of biomass is used mainly in combustion and co-combustion systems, where the technology is well developed, has high efficiency, and low emissions; nevertheless, herbaceous crops that have a high content of potassium which may be detrimental for biomass combustion equipment. It is also used, albeit to a lesser extent, in gasification systems in mixtures with other sources of biomass.

The most abundant form of herbaceous biomass in the world, particularly in European countries, is straw, which is obtained during the annual production of cereals in a proportion of 0.6 to 0.8 tons of straw per ton of grain in the field. In countries where crop productivity is high (in the order of 8 to 10 tons of grain per hectare), the amount of straw per hectare can be between 6 and 8 tons and much of it has been removed for the next crop cycle.

Herbaceous biomass contains variable amounts of cellulose, hemicellulose, and lignin, and a small amount of other extractive, usually perennial, with looser fibers. The relative proportions of cellulose and lignin is one of the determining factors to identify the suitability of plant species for further processing as energy crops (Table B-21).

Table B-21 Approximate composition of herbaceous biomass gas from different sources.

Properties	Herbaceous biomass type
Wheat straw	Barley straw
Moisture (%, w/w)	16	30
Volatile material (%, w/w)	59	46
Fixed carbon (%, w/w)	21	18
Ash (%, w/w)	4	6
Net calorific value (MJ/kg)	18.6	16.1
C (%, w/w)	48.5	45.7
H (%), w/w	5.5	6.1
O (%, w/w)	3.9	38.3
N (%, w/w)	0.3	0.4
S (%, w/w)	0.1	0.1

Switchgrass, a type of fine-stemmed herbaceous biomass, has been used in the United States as a herbaceous cultivation model, has a calorific value comparable to that of wood, but with a lower moisture content; it also has a low content of ash and alkalis, indicating that it could be an adequate fuel for combustion. The main difficulty of this type of biomass is the production of slag at the time of gasification, associated with the ash content. To avoid slag production, the temperature can be reduced, but this results in the loss of carbon. The gas formed in the gasifier contains impurities such as particles, tar, nitrogen compounds, sulfur compounds, and alkaline compounds.

See also: Biomass.