Second generation biofuels are also known as advanced biofuels. What separates them from first generation biofuels the fact that feedstock used in producing second generation biofuels are generally not food crops. The only time the food crops can act as second generation biofuels is if they have already fulfilled their food purpose. For instance, waste vegetable oil is a second generation biofuels because it has already been used and is no longer fit for human consumption. Virgin vegetable oil, however, would be a first generation biofuel.
Because second generation biofuels are derived from different feed stock, Different technology is often used to extract energy from them. This does not mean that second generation biofuels cannot be burned directly as the biomass. In fact, several second generation biofuels, like Switchgrass, are cultivated specifically to act as direct biomass.
Second Generation Extraction Technology
For the most part, second generation feedstock are processed differently than first generation biofuels. This is particularly true of lignocellulose feedstock, which tends to require several processing steps prior to being fermented (a first generation technology) into ethanol. An outline of second generation processing technologies follows.
The first thermochemical route is known as gasification. Gasification is not a new technology and has been used extensively on conventional fossil fuels for a number of years. Second generation gasification technologies have been slightly altered to accommodate the differences in biomass stock. Through gasification, carbon-based materials are converted to carbon monoxide, hydrogen, and carbon dioxide. This process is different from combustion in that oxygen is limited. The gas that result is referred to as synthesis gas or syngas. Syngas is then used to produce energy or heat. Wood, black liquor, brown liquor, and other feedstock are used in this process.
The second thermochemical route is known as pyrolysis. Pyrolysis also has a long history of use with fossil fuels. Pyrolysis is carried out in the absence of oxygen and often in the presence of an inert gas like halogen. The fuel is generally converted into two products: tars and char. Wood and a number of other energy crops can be used as feedstock to produce bio-oil through pyrolysis.
A third thermochemical reaction, called torrefaction, is very similar to pyrolysis, but is carried out at lower temperatures. The process tends to yield better fuels for further use in gasification or combustion. Torrefaction is often used to convert biomass feedstock into a form that is more easily transported and stored.
A number of biological and chemical processes are being adapted for the production of biofuel from second generation feedstock. Fermentation with unique or genetically modified bacteria is particularly popular for second generation feedstock like landfill gas and municipal waste.
Common Second Generation Feedstock
To qualify as a second generation feedstock, a source must not be suitable for human consumption. It is not a requirement that the feedstock be grown on non-agricultural land, but it generally goes without saying that a second generation feedstock should grow on what is known as marginal land. Marginal land is land that cannot be used for “arable” crops, meaning it cannot be used to effectively grow food. The unspoken point here is that second generation feedstock should not require a great deal of water or fertilizer to grow, a fact that has led to disappointment in several second generation crops.
A number of grasses like Switchgrass, Myscanthus, Indiangrass, and others have alternatively been placed in the spotlight. The particular grass chosen generally depends on the location as some are more suitable to certain climates. In the United States, Switchgrass is favoured. In Southeast Asia, Myscanthus is the choice.
The advantages of grasses are:
- They are perennial and so energy for planting need only be invested once
- They are fast growing and can usually be harvested a few times per year
- They have relatively low fertilizer needs
- They grow on marginal land
- They work well as direct biomass
- They have a high net energy yield of about 540%
The disadvantages of grasses are:
- They are not suitable for producing biodiesel
- They require extensive processing to made into ethanol
- It may take several years for switch grass to reach harvest density
- The seeds are weak competitors with weeds. So, even though they grow on marginal land, the early investment in culture is substantial
- They require moist soil and do not do well in arid climates.
Water demands are the biggest drawback to grasses and the factor that keeps them from becoming more popular as second generation biofuels. Despite this shortcoming, grasses do find a number of uses, particularly in the United States.
Jatropha and other seed crops
Seed crops are useful in the production of biodiesel. In the early Part of the 21st century, a plant known as Jatropha became exceedingly popular among biodiesel advocates. The plant was praised for its yield per seed, which could return values as high as 40 percent. When compared to the 15 percent oil found in soybean, Jatropha look to be a miracle crop. Adding to its allure was the misconception that I could be grown on marginal land. As it turns out, oil production drops substantially when Jatropha is grown on marginal land. Interest in Jatropha has waned considerably in recent years.
Other, similar seed crops have met with the same fate as Jatropha. Examples include Cammelina, Oil Palm, and rapeseed. In all cases, the initial benefits of the crops were quickly realized to be offset by the need to use crop land to achieve suitable yields.
Waste Vegetable Oil (WVO)
WVO have been used as a fuel for more than a century. In fact, some of the earliest diesel engines ran exclusively on vegetable oil. Waste vegetable oil is considered a second generation biofuels because its utility as a food has been expended. In fact, recycling it for fuel can help to improve its overall environmental impact.
The advantages of WVO are:
- It does not threaten the food chain
- It is readily available
- It is easy to convert to biodiesel
- It can be burned directly in some diesel engines
- It is low in sulphur
- There are no associated land use changes
The disadvantages of WVO are:
- It can decrease engine life if not properly refined
WVO is probably one of the best sources of biodiesel and, as long as blending is all that is required, can meet much of the demand for biodiesel. Collecting it can be a problem though as it is distributed throughout the world in restaurants and homes.
Municipal Solid Waste
This refers to things like landfill gas, human waste, and grass and yard clippings. All of these sources of energy are, in many cases, simply being allowed to go to waste. Though not as clean as solar and wind, the carbon footprint of these fuels is much less than that of traditionally derived fossil fuels. Municipal solid waste is often used in cogeneration plants, where it is burned to produce both heat and electricity.