James G. Speight - Encyclopedia of Renewable Energy

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.

First-generation biofuels are biofuels produced from sugar, starch, vegetable oil, or animal fats using conventional technology. The oil is obtained using the conventional techniques of production. Some of the most popular types of first-generation biofuels are: biodiesel, vegetable oil, biogas, bioalcohols, and synthesis gas.

Biodiesel has a composition similar to fossil/mineral diesel except that components in biodiesel include animal fats and oils from soy, mustard, flax, and sunflower seeds. The oil or animal fat is reacted with an alcohol through a process called transesterification to create the fuel. Vegetable oil is most often used in the production of biofuels, but there are cases where straight vegetable oil is being used as a fuel.

Biogas is created when organic matter breaks down anaerobically (that means without any oxygen). It can be produced from gunk like manure, sewage, and municipal waste. Some types of biogas, such as landfill gas, contain something called volatile organic compounds that are restricted by environmental regulations. Synthesis gas (syngas) is a mix of carbon dioxide and hydrogen. It is created when biomass is combusted with a measured (limited) amount of oxygen. Syngas can be used to produce diesel and can also be converted into methane.

Bioalcohols are produced through the fermentation of starches and sugars. Ethanol is the most common bioalcohol, although there is also methanol, propanol, and butanol. Some bioalcohols can be used directly in gasoline-powered engines.

The basic feedstocks for the production of first-generation biofuels are often seeds or grains such as wheat, which yields starch that is fermented into bioethanol, or sunflower seeds, which are pressed to yield vegetable oil that can be used in biodiesel. These feedstocks could instead enter the animal or human food chain, and as the global population has increased, their use in producing biofuels has been criticized for diverting food away from the human food chain, leading to food shortages and price rises.

The most common first-generation biofuels are bioalcohols, biodiesel, biogas, and vegetable oil.

See also: Bioalcohols, Biodiesel, Biofuels From Synthesis Gas, Biofuels – Second Generation, Biofuels – Third Generation, Biogas, Methanol, Ethanol, Vegetable Oil.

Biomass can be converted into fuels and chemicals indirectly (by gasification to syngas followed by catalytic conversion to liquid fuels) or directly to a liquid product by thermochemical means. The process yields synthesis gas (syngas) composed primarily of hydrogen and carbon monoxide, also called biosyngas.

The production of high-quality syngas from biomass, which is later used as a feedstock for biomass-to-liquids (BTL) production, requires particular attention. This is due to the fact that the production of synthesis gas from biomass is indeed the novel component in the gas-to-liquids concept – obtaining syngas from fossil raw materials (natural gas and coal) is a relatively mature technology.

Gasification is actually thermal degradation of the feedstock in the presence of an externally supplied oxidizing (oxygen-containing) agent e.g., air, steam, oxygen. Various gasification concepts have been developed over the years, mainly for the purposes of power generation. However, efficient biomass-to-liquids production imposes completely different requirements for the composition of the gas. The reason is that in power generation, the gas is used as a fuel, while in biomass-to-liquids processing, it is used as a chemical feedstock to obtain other products. This difference has implications with respect to the purity and composition of the gas.

In contrast, for biomass-to-liquids production, the amount of carbon monoxide and hydrogen is only important (the larger the amount, the better), while the calorific value is irrelevant. The presence of other hydrocarbon derivatives and inert components should be avoided or at least kept as low as possible. This can be achieved in the following ways: (i) by adjusting the amount of the various constituents of the gas stream, (ii) choice of the oxidizing agent.

The amount of components other than carbon monoxide and hydrogen (primarily hydrocarbon derivatives) can be reduced via further transformation into carbon monoxide and hydrogen. This is, however, rather energy intensive and costly (two processes – gasification and transformation). As a result, the overall energy efficiency of syngas production and of biomass-to-liquids processing is also reduced, leading to higher production costs.

The amount of various components can be minimized via a more complete decomposition of biomass, thereby preventing the formation of undesirable components at the gasification step. The minimization of the content of various hydrocarbon derivatives is achieved by increasing temperatures in the gasifier, along with shortening the residence time of feedstocks inside the reactor. Because of this short residence time, the particle size of feedstocks should be small enough (in any case – smaller than in gasification for power generation) in order that complete and efficient gasification can occur.

In gasification for power generation, typically, air is employed as oxidizing agent, as it is indeed the cheapest among all possible oxidizing agents. However, the application of air results in large amounts of nitrogen in the product gas, since nitrogen is the main constituent of air. The presence of such large quantities of nitrogen in the product gas does not hamper (very much) power generation, but it does hamper biomass-to-liquids production. Removing this nitrogen via liquefaction under cryogenic temperatures is extremely energy intensive, reduces substantially the overall biomass-to-liquids energy efficiency and increases costs. Amongst other potential options (steam, carbon dioxide, oxygen), from a technical and economic point of view oxygen appears to be the most suitable oxidizing agent for biomass-to-liquids manufacturing. It is true that the oxygen-blown gasification implies additional costs compared to the air-blown gasification, because of the oxygen production. Nevertheless, the energy and financial cost of producing oxygen seems to be far lower than the renewable energy and financial cost of cleaning up the product gas from air-blown gasification from nitrogen. This is partly due to the fact that the production of high-purity oxygen (above 95% O2) is a mature technology.