After 2028 there are uncertainties about the sufficiency of oil in the world, and after 2050 “there is substantial uncertainty about the levels of future supply and demand for liquid fuels,” according to the EIA.
After 2028 there are uncertainties regarding the sufficiency of oil in the world
“This situation requires quick measures in the mobility activity, including the development of new types of biofuels“, states Dumitru Chisăliță, president of the Intelligent Energy Association (AEI).
According to him, while the classification of biofuel technologies varies in the literature, the products can generally be classified as first to fourth generation, depending on the type of feedstock and the conversion process that has been applied .
First generation biofuels
First generation biofuels are mainly divided into bioethanol and biodiesel. First-generation bioethanol production is based on the microbial fermentation of starch- and sucrose-rich edible feedstocks such as wheat, corn and sugarcane. Bioethanol production is not limited to first-generation biofuels; depending on the raw material and production strain, bioethanol can also be classified as second and third generation. Biodiesel is mainly obtained from food-grade rapeseed, soybean or palm oil. Although biobutanol production is also possible by fermenting sugar from sugar cane, corn, wheat and other food crops, it is limited by lower productivity and yields and high costs.
During the global food demand crisis of 2007/2008, biofuel crops became more important for food use, giving rise to the “food versus fuel” debate that persists to this day. In addition, an increased demand for crops (eg corn) for fuel production produced an increased market price for these foods. The models predict that massive agricultural areas would be required for fuel production and could still provide only limited amounts of fuel compared to total demand. It is estimated that more than twice the available arable land area globally would be needed to meet global market demand for biodiesel when produced from rapeseed oil. In addition, the increased market values of palm oil and other biofuel crops have led to extensive deforestation of tropical rainforests for plantations of biofuel crops, which release more CO2 than the emissions saved by these biofuels.
Second generation biofuels
As a result of the problems of the first generation, second generation biofuels have been developed using lignocellulosic biomass from agricultural and forestry residues as well as other waste streams (e.g. from the food industry such as wheat bran, animal fats or waste cooking and frying oil). Other non-food plants, such as the drought-resistant shrub or tree Jatropha curcas, which can also be grown in deserts, could still be another promising source for second-generation biofuels. Therefore, second-generation biofuels circumvent the need for agricultural land-use change and do not compete with food resources. However, second-generation waste streams often represent more complex feedstocks than sugarcane or palm oil, potentially containing compounds capable of reducing fermentation efficiency, such as lignin. Therefore, the application of additional treatment steps are frequent, increasing the time and costs of the process.
For the most part, first- and mostly second-generation biofuels are commercially produced.
Along with ethanol producers, second-generation biodiesel production is possible from microbial lipids produced by organisms, such as a yeast capable of producing up to 90% (w/w) lipids per biomass in a fermentation process, which can be grown on waste streams (eg hydrolyzed wheat bran medium). Second-generation biodiesel can also be obtained from waste oils through catalytic cracking and hydrogenation. Disadvantages of this process include incomplete conversion and coke formation, which leads to catalyst deactivation.
More than half of biologically stored carbon is linked to marine biomass, especially macroalgae and sea grass. Detached seagrass material is seasonally washed onto beaches and shorelines; due to low biodegradation and consumption by herbivores, an excess of it accumulates as waste. Estimates of up to 40 million tons of dry seagrass biomass that can be used for biofuel production are given. Through enzymatic hydrolysis, the carbohydrate content of sea grass can be used in a fermentation medium for microorganisms, additionally providing a low nitrogen and phosphorus content, which is usually required for lipid production.
Despite the highly favorable ability to valorize waste streams, second-generation biofuels will not be sufficient to provide energy for current global demand. As with first-generation biofuel food crops, the biomass used in these processes is available in limited quantities. Therefore, second-generation biofuels must be combined with other technologies to ensure a sufficient supply of fuels. This has prompted research into third-generation biofuels. However, scientific estimates predict that second-generation biofuels could provide up to 30% of the world’s transportation energy.
Third generation biofuels
Third-generation biofuels are mainly derived from microalgal and cyanobacterial biomass, which can be used to naturally generate alcohols and lipids to convert into biodiesel or any other high-energy fuel product. Algae exhibit photosynthetic rates 2 to 4 times higher than land plants, resulting in faster biomass formation. Algae does not require arable land or fresh water to grow. Many crops can be grown using wastewater, brackish or brackish water, which is cost-effective and circumvents competition with agricultural activity. Most importantly, efficient algal cultivation requires a direct supply of CO2, which can be derived from industrial emitters or through atmospheric carbon capture. In conventional cropping systems, about 70% of the supplied CO2 is used for photosynthesis and therefore biomass production. Therefore, algal biofuels could have a negative carbon footprint because they directly bind GHGs in their biomass. One of the most prominent third-generation processes is the production of biodiesel or other energy-dense biofuels such as biokerosene using oleaginous microalgae.
One of the most economically critical and versatile operations in algal biofuel production is algal cultivation. Algal bioreactors are independent of location and climate, so they can be operated almost regardless of these factors. For low-cost, high-volume products such as biofuels, algae are commonly grown in open ponds. Open pond reactors are significantly cheaper to build and operate, but have disadvantages such as high water loss through evaporation and lack of temperature control, which lowers biomass productivity. The preferred alternative for high-priced, low-volume products such as cosmetic ingredients is a closed photobioreactor, where process parameters can be precisely controlled, often resulting in higher productivity. These bioreactors also enable a three-dimensional mode of cultivation, significantly increasing productivity per area. Unlike second-generation biofuels, third-generation processes require agricultural land. In addition, oil production from algae is probably higher than that from higher plants, because lipids accumulate mainly in certain parts of the plant (e.g., in rapeseed), whereas in algae, each cell can it contains a large amount of lipids, which makes the process much more efficient. One hurdle in production is harvesting, as the size and low density of microalgae cells combined with the cells’ sensitivity to pH changes make it difficult. In addition, downstream processing for algal biofuels is typically more energy intensive than other biofuel production.
Fourth generation biofuels
The latest generation of biofuels, referred to as fourth generation biofuels, involves the use of genetic engineering to increase the desired traits of organisms used in biofuel production. This applies to a variety of traits from the use of more types of sugars (eg pentoses and hexoses), to greater lipid synthesis or increased photosynthesis and carbon fixation. Unfortunately, for most native biofuel producers, genetic engineering tools are much more limited.
Currently, two different approaches have been taken: pathway engineering in native producers (optimizing growth rates, using different carbon sources, directing metabolic flux towards biofuel production with increased yields) and reconstructing identified pathways in natural producers in a way more genetically accessible. A wide variety of microorganisms can be used as heterologous hosts for biofuel production, including bacteria, yeasts, and algae. Their metabolic versatility allows different substrates to be used to produce a wide range of biofuels. To allow increased accumulation of biofuels, the cellular stress response can be altered by genetic engineering, for example, with changes in the cell membrane.
With genetic engineering tools, the quantity and quality of biofuels can be controlled and increased, but will need political buy-in and support for widespread adoption. There is a controversial debate surrounding genetic engineering in agriculture and medicine, particularly in Europe; therefore, similar concerns can be anticipated regarding use in biofuel production. A European study concluded that genetically modified algae for biofuel production would be accepted by most consumers, when the safety of the systems can be guaranteed. However, with proper containment methods and carefully selected locations, such risks could be drastically minimized. Therefore, closed production systems with high security standards are expected to be built.
A new, more experimental approach that is likely to generate fifth-generation biofuels is the production of electrobiofuels. They are based on the approach of establishing new-for-nature hybrid systems that are able to use renewable electricity and carbon sources directly for the production of basic chemicals and biofuels, thereby enabling the conversion of solar energy into storable liquid fuel. Such a process could combine the higher photon efficiency of modern photovoltaic systems (compared to photosynthesis) with the sustainability of biofuel production, increasing the overall efficiency of the process.