Municipal solid waste (MSW) is another potential feedstock. The municipal solid waste includes sewage sludge and industrial waste. Residential, commercial, and institutional post-consumer waste usually contains good amounts of plant-derived organic materials that can be used as potential source of biomass. The waste paper, cardboard, wood waste, and yard waste are examples of MSW. Typically MSW contains about 36% paper and paperboard products and 12 % yard trimmings. All these materials, after separation from other components such as metals and plastics can be converted to biofuels and bioproducts using similar processes used for conversion of lignocellulosic feedstocks. Municipal solid waste contains paper and paper products (37.8%), food waste (14.2%), yards waste (14.2%), wood wastes (3.0%) along with plastic (4.6%), rubber and leather (2.2%), textiles (3.3%), glass and ceramics (9.0%), metals (8.2%), and miscellaneous (3.1%) (14. 15).
Dedicated Crops (Terrestrial and Aquatic)
Both plants and residues have been identified as biomass sources. The most promising could be the following:
- • A few fibrous dicotyledon plants, already known for their textile or cordage bastfibre, i.e., flax (Linum usitatissimum), hemp (Cannabis scitiva), and kenaf (Hibiscus cannabinus);
- • Some fibrous monocotyledon plants, which are presently part of the natural vegetation in the area, i.e., reeds, common (Phragmites communis) or giant (Arundo donax), and esparto (Lygeum spartum) or alfa (Stipa tenacissima) grasses;
- • Several fibrous residues of either present or future crops: cereal straws, sorghum (Sorghum vulgare). Jerusalem artichoke (Helianthus tuberosus), and sunflower (Helianthus annus) stalks are the most prominent candidates in this class; other residues can also be considered on smallscale, local applications; Camelina grown on marginal lands. The assumption was that marginal crops for biofuel production could avoid competing with food crops for land and resources.
• Wood from short-rotation plantations, created on low-value agricultural or marginal land; most promising genera include softwoods like Pinus, Picea, Pseudotsuga, and Larix, and hardwood like Eucalyptus and Populus.
The energy crops are normally densely planted, high-yielding, and short-rotation crops. The crops are usually low cost and need low maintenance. These crops are grown dedicatedly to supply huge quantities of consistent-quality biomass for biorefinery. The energy crops mainly include herbaceous energy crops, woody energy crops, agricultural crops, and aquatic crops.
Herbaceous energy crops are perennials that are harvested annually. It takes 2-3 years to reach full productivity. These crops include grasses such sw'itchgrass, mis- canthus, bamboo, sweet sorghum, tall fescue, kochia, wheatgrass, reed canary grass, coastal bermuda grass, alfalfa hay, thimothy grass, and others.
Wood energy crops are fast growing hardwood trees that are harvested within 5-8 years of plantation. These crops include hybrid poplar, hybrid willow, silver maple, eastern cottonwood, green ash, black walnut, sweetgum, sycamore, etc. The short- rotation woody energy crops are traditionally used for manufacture of pulp and paper (16).
Agricultural crops comprise of oil crops (e.g., jatropha, oilseed rape, linseed, field mustard, sunflow'er, castor oil, olive, palm, coconut, groundnut, etc.), cereals (e.g., barely, wheat, oats, maize, rye, etc.), and sugar and starchy crops (e.g., sweet sorghum, potato, sugar beet, sugarcane, etc.). The crops are generally grown to produce vegetable oils, sugars, and extractives. The crops have potentials to produce plastics, chemicals, and products as w'ell.
Compared to other annual biomass production systems, short-rotation forestry (10-15 years) can be assumed as an extensive and most eco-efficient land use. In contrast to annual crops the production can contribute to different international conventions and commitments simultaneously (soil erosion, biodiversity, climate protection, and desertification). To optimize short-rotation forestry as an ecological and socio-economical sound land use, the different utilization techniques for the energetic use of dendromass have to be assessed. To minimize land consumption, different land-use-management systems for biomass production have to be compared and optimized. This needs an interdisciplinary approach of agricultural and forestry institutions of industrialized and developing countries. To guarantee ecological and socio-economic sound land use management systems international standards for the production of biomass have to be developed.
Sweet sorghum is a promising alternative crop for bioethanol production. Moreover, it is a ‘food fuel-energy/-industrial crop’ which ranks fifth among the world’s grain crops, requires low water/fertilizer input, has a high yield of grains and biomass (starch/sugars/lignocelluloses) for integrated multi-purpose processing, and grows well in marginal lands, in semi-arid and temperate regions, including Africa, India, Latin America, and Europe. A limiting factor for its widespread cultivation is the lack of varieties adapted to different growth conditions, including colder climate. Consequently, research should address the optimization of sweet sorghum as an energy crop through breeding. Besides biomass yield and relevant quality traits, genetic improvement/selection should concentrate on general agronomic traits (such as water and nutrient use efficiency) and, in particular, adaptation of sweet sorghum to colder climates. The project should also address agronomic practices and harvesting technologies leading to improved yield, quality, sustainability, and competitiveness of sweet sorghum production. Environmental and economic analysis of sweet sorghum cultivation, including energy balance and life cycle assessment, should also be carried out.
The accessible quantity of these resources could be increased by the following ways: (1) the improvement of existing or the development of new' cultivation practices; (2) the development of new crop rotation; (3) the rational management of natural vegetation; (4) the development of the appropriate harvesting technology; (5) the genetic manipulation of plants for the removal of undesired properties and/or the acquisition of desired ones; (6) the generation of viable multi-product or multi-use agricultural systems (17).