Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 173

Currently, there is a focus on biofuels made from crops, such as corn, sugar cane, and soybeans, for use as renewable energy sources. Though it may seem beneficial to use renewable plant materials for biofuel, the use of crop residues and other biomass for biofuels raises many concerns related to major environmental problems, including food shortages and serious destruction of vital soil resources. Tropical countries have the highest potential to produce biofuel crops: higher energy yields, better greenhouse gas (GHG) balance if properly produced, lower costs, and in some countries, large reserves of uncultivated cropland. Sugar cane and oil palm are the highest-yielding tropical biofuel crops and consequently provide the greatest carbon offsets. Industrialized countries with biofuels targets (such as the United States and the European Union countries) are unlikely to have the agricultural land base needed to meet their growing demand for current production of biofuels, which are largely produced from food and feed crops (e.g., maize, oil palm, rapeseed, soy).

Bioethanol feedstocks can be divided into three major groups: (i) sucrose-containing feedstocks, such as sugar cane, sugar beet, sweet sorghum, and fruits, (ii) starchy materials, such as corn, milo, wheat, rice, potatoes, cassava, sweet potatoes, and barley, and (iii) lignocellulosic biomass, such as wood, straw, and grasses). In the short term, the production of bioethanol as a vehicular fuel is almost entirely dependent on starch and sugars from existing food crops. The drawback in producing bioethanol from sugar or starch is that the feedstock tends to be expensive and demanded by other applications as well.

Sugarcane as a bio-fuel crop has much expanded in the last decade, yielding anhydrous bio-ethanol (gasoline additive) and hydrated bio-ethanol by fermentation and distillation of sugarcane juice and molasses. The yield of ethanol per hectare, is on the order of 7,000 L per hectare. Brazil is the largest single producer of sugarcane with approximately 31% of the global production and millions of hectares (1 ha = 2.47 ac) of sugarcane under cultivation. Brazilian bio-ethanol is less expensive than that produced in the United States from corn or in Europe from sugar beet, because of shorter processing times, lower labor costs, and lower transport costs and input costs.

Another type of feedstock, which can be used for bioethanol production, is starch- based materials. Starch is a high yield feedstock for bioethanol production, but its hydrolysis is required to produce bioethanol by fermentation. Starch is a biopolymer, defined as a homopolymer consisting of only one monomer, D-glucose. The starch-based bioethanol industry has been commercially viable for approximately 30 years; in that time, tremendous improvements have been made in enzyme efficiency, reducing process costs and time, and increasing bioethanol yields. This type of feedstock is the most utilized for bioethanol production in North America and Europe. Corn and wheat are mainly employed with these purposes. Corn-based bioethanol production in most of the countries assessed is limited, especially compared to the United States.

There is also interest in a large amount of studies regarding the utilization of lignocellulosic biomass as a feedstock for producing fuel ethanol is being carried out worldwide. For countries where the cultivation of energy crops is difficult, lignocellulosic materials are an attractive option for the production of biofuels. Lignocellulosic materials serve as a cheap and abundant feedstock, which is required to produce fuel ethanol from renewable resources at reasonable costs. Producing bioethanol from lignocellulosic materials may allay many of the environmental and food-versusfuel concerns that are drawbacks of producing bioethanol from food crops like sugar or corn.

Biodiesel has been mainly produced from renewable oil crops such as soybean, rapeseed, mustard seed oil, sunflower oil, and jatropha as well as from recycled vegetable oils and animal fats. The benefits of biodiesel also depend on the type of oilseed used. Seeds of high oil content, such as sunflower (40 to 50% w/w oil), rapeseed (42 to 48% w/w oil), and soybean seeds (18 to w/w 20% oil) have gained much attention lately as renewable energy sources because of their relatively high yield per hectare.

Biodiesel from oil crops is being produced in increasing amounts as a clean-burning alternative fuel, but its production in large quantities is not sustainable. Microalgal biofuels are a viable alternative. Microalgae are photosynthetic microorganisms that convert sunlight, water, and carbon dioxide to algal biomass. Algae have the potential to dwarf all the other biodiesel feedstocks due to their efficiency in photosynthesizing solar energy into chemical energy. Many algae are exceedingly rich in oil, which can be converted to bio-diesel. In fact, the oil productivity of many microalgae often exceeds the best-producing oil crops. The oil content of some microalgae exceeds 80% of dry weight of algae biomass. Oil levels on the order of 20 to 50% w/w are quite common, and microalgae with high oil productivities are desired for producing biodiesel.