Biomass – Fermentation
Fermentation is an anaerobic process that breaks down the glucose within organic materials, and the process is a series of chemical reactions that convert sugars to alcohol or acid. Yeast or bacteria are added to the biomass material, which feed on the sugars to produce ethanol and carbon dioxide. The process is used commercially on a large scale in various countries to produce ethanol from sugar crops (sugar cane, beet) and starch crops (maize, wheat).
The biomass is ground down, and the starch is converted by enzymes to sugars. Yeast then converts the sugars to ethanol. Pure ethanol is produced by distillation which is a relatively energy intensive step. Approximately 450 L of ethanol can be produced per tonne of dry corn. The remaining solids can be used as cattle feed. In the case of sugar cane, the remaining bagasse can also be used as fuel for boilers or gasification processes.
Conversion of lignocellulosic biomass (such as wood and grasses) requires acid or enzymatic hydrolysis before the resulting sugars can be fermented to ethanol. Such hydrolysis techniques are currently at the pre-pilot stage.
See also: Fermentation, Fermentation Chemistry.
Biomass Fuel
Solid biofuels such as wood or dried dung have been used since man learned to control fire. On the other hand, liquid biofuels for industrial applications was used since the early days of the car industry.
Nikolaus August Otto, the inventor of the combustion engine, conceived his invention to run on ethanol, while Rudolf Diesel, the inventor of the diesel engine, conceived it to run on peanut oil. Henry Ford originally had designed the Ford Model T, a car produced between 1903 and 1926, to run completely on ethanol, and the desire to mass produce electric cars did not come to fruition at that time. However, when crude oil began being cheaply extracted from deeper in the soil (thanks to oil reserves discovered in Pennsylvania and Texas), cars began using fuels from oil.
Nevertheless, before World War II, biofuels were seen as providing an alternative to imported oil in countries such as Germany, which sold a blend of gasoline with alcohol fermented from potatoes under the name Reichskraftsprit. In Britain, grain alcohol was blended with gasoline by the Distillers Company Ltd under the name Discol. After the World War II, cheap Middle Eastern oil lessened interest in biofuels. Then, with the oil shocks of 1973 and 1979, there was an increase in interests from governments and academics in biofuels. However, interest decreased with the counter-shock of 1986 that made oil prices cheaper again. However, the variation in the price of crude oil continues and is subject to political pressures from oil-producing countries.
The supply of crude oil, the basic feedstock for refineries and for the petrochemicals industry, is finite, and its dominant position will become unsustainable as supply/demand issues erode its economic advantage over other renewable feedstocks. This situation will be mitigated to some extent by the exploitation of more technically challenging fossil resources and the introduction of new technologies for fuels and chemicals production from natural gas and coal.
However, the use of fossil resources at current rates will have serious and irreversible consequences for the global climate. Consequently, there is a renewed interest in the utilization of plant-based matter as a raw material feedstock for the chemicals industry. Plants accumulate carbon from the atmosphere via photosynthesis, and the widespread utilization of these materials as basic inputs into the generation of power, fuels, and chemicals is a viable route to reduce greenhouse gas emissions.
Thus, the crude oil and petrochemicals industries are coming under increasing pressure not only to compete effectively with global competitors utilizing more advantaged hydrocarbon feedstocks but also to ensure that its processes and products comply with increasingly stringent environmental legislation.
The production of fuels and chemicals from renewable plant-based feedstocks utilizing state-of-the-art conversion technologies presents an opportunity to maintain competitive advantage and contribute to the attainment of national environmental targets. Bioprocessing routes have a number of compelling advantages over conventional petrochemicals production; however, it is only in the last decade that rapid progress in biotechnology has facilitated the commercialization of a number of plant-based chemical processes. It is widely recognized that further significant production of plant-based chemicals will only be economically viable in highly integrated and efficient production complexes producing a diverse range of chemical products. This biorefinery concept is analogous to conventional oil refineries and petrochemical complexes that have evolved over many years to maximize process synergies, energy integration, and feedstock utilization to drive down production costs.
See also: Biofuels, Biomass.
Biomass Fuel – Characteristics
Biomass can be converted to thermal energy, liquid, solid, or gaseous fuels, and other chemical products through a variety of conversion processes. These processes include physical conversion to densified fuels (e.g., pellets or cubes), thermal conversion through combustion or pyrolysis, chemical conversion, and microbial conversion or fermentation. The abundance of plant organic constituents and other physical and chemical characteristics vary significantly by plant type and the proportions of plant components such as leaves, stems, bark, and twigs in the feedstock.
The design of a biomass power or ethanol facility requires careful consideration of the effects that feedstock characteristics and composition have on the conversion process. Generating electricity and useful heat energy is most frequently done by direct combustion of biomass in a boiler. Energy content, moisture content, and chemical makeup are among the most important biomass characteristics affecting combustion processes. Biomass gasification yields a combustible gas that can be used to generate electricity. Particle size, energy content, moisture content, and volatiles are among the biomass characteristics affecting gasification.
The technology that is closest to commercialization for converting biomass to ethanol involves extracting complex carbohydrates, primarily cellulose and hemicellulose, from the biomass feedstock and reducing these components to simple sugars (a process called hydrolysis).
Cellulose is a long polymer of glucose, and it serves as a structural component of plant cell walls. It is often found in a composite mixture with hemicellulose and lignin. Hemicellulose is a polymer containing a variety of simple sugars and is more soluble than cellulose. Lignin is a ring-type carbohydrate that acts as cement in plant walls. The lignin portion of biomass is not converted to simple sugars through hydrolysis, but lignin can be burned to generate process heat and electricity.
Extracting the carbohydrates may involve “steam explosion” of the cell walls or dissolving the organic constituents with acids, enzymes, or organic solvents. Sugars resulting from hydrolysis are then converted into ethanol through microbial fermentation. The bulk and biochemical composition of the feedstock largely determine ethanol yield because these traits affect the hydrolysis and fermentation processes.
Biomass – Gasification
Although gasification is old technology with respect to coal-based feedstock, it is a developing technology with respect to biomass, since it was never fully embraced on a larger commercial scale. Biomass materials differ from conventional solids fuels such as coal based on the volatile matter. The volatile matter in the coal is less than 20%, while it is up to 80% in biomass materials – the constituents of the gas are variable and are dependent upon the process parameters and the feedstock (
