Biomass fuels are exclusively combustion fuels and when they are burnt they generate carbon dioxide that enters the atmosphere in exactly the same way as fossil fuel would generate carbon dioxide. Biomass fuel is biomass converted directly to energy or converted to liquid or gaseous fuels such as ethanol, methanol, methane, and hydrogen. The following are considered biofuels:
- synthetic hydrocarbons;
- vegetable oils;
Generally speaking biofuels can be defined as all those substances capable of producing energy that derive directly or indirectly from a biomass. In other words they are all those solid, liquid and gaseous fuels coming from renewable sources such as plants and animals. For example, some biofuels are biodiesel, bioalcohols (bioethanol, biomethanol, etc.), biogas (methane), syngas (carbon monoxide + hydrogen), wood and coal. For the production of biofuels, the biomasses that can be used are many and include:
- Harvests of products rich in sugars (sugar beet, sugar cane, etc.)
- Harvesting of products rich in starch (corn, wheat, etc.)
- Harvesting of grass and lignocellulosic products
- Harvests of oil products (sunflower seeds, rape seeds, canola, rapeseed, soybeans, arichids, etc.)
- Agricultural wastes (straw, etc.)
- Low-rotation time forests (willows, poplars, eucalyptus, etc.)
- Wood wastes (forest residues, sawdust, industrial residues from wood processing)
- Municipal solid waste
- Industrial processing residues (food industry)
Biofuels can be distinguished between primary and secondary. Primary biofuels are those where the biomass is used as is without being processed, such as wood, wood chips and pellets. These biofuels are burned directly to produce energy. Secondary biofuels are derived from biomass that undergoes certain chemical, thermochemical or fermentation processes. These processes produce solid fuels such as coal, liquid fuels such as ethanol and biodiesel or gaseous fuels such as biogas, syngas and hydrogen.
Biofuels today represent an important source of energy with a low environmental impact. The availability of various renewable raw materials and the increasing investments being made in the sector could lead biofuels to compete with oil, decreasing energy dependence on fossil sources.
Although most biofuels are used for domestic purposes (for example, pellet stoves are now widespread), in recent years there has been a significant growth in the use of liquid biofuels for transport, with the aim of limiting the use of fuels of fossil origin. In particular, bioethanol and biodiesel are those that have had the greatest development and are those most present on the current market.
Types of biofuels
Biofuels are classified into three groups – first generation, second generation and third generation.
First generation biofuels are produced, using conventional technologies, from sugars, starches, vegetable oil or animal fats. Since these feedstocks are also food resources, the “food or fuel” debate is the main issue related to first-generation biofuels.
The production of first-generation biofuels is limited because it could have a negative effect on food availability and biodiversity. To meet the growing demand for biofuels, second-generation biofuels have been developed. These are produced from non-edible crops or portions of edible plants that cannot be used as food and are therefore discarded, such as stems, shells, wood chips and peels.
According to experts, second-generation biofuels allow for greater reductions in greenhouse gas emissions than first-generation biofuels. The production of second-generation biofuels, however, is more complex, as it is necessary to extract usable feedstocks from woody or fibrous biomass.
As the issues surrounding the production processes of first-generation biofuels are well known, parallel to them have been the development of what are called “second-generation” biofuels that are derived from sources not suitable for human consumption. Second-generation biofuel feedstocks include inedible energy crops, inedible crop oils, agricultural and municipal wastes, waste oils, and algae. The goal of second-generation biofuel processes is to expand the amount of biofuel that can be produced sustainably. For example, biomass consisting of the residual non-food parts of current crops, such as stems, leaves, and shells that are waste material after the crop has been grown, can be used.
The main problem with second-generation biofuel production processes is the difficulty of extracting useful materials from woody or fibrous biomass. In fact, it is necessary to make more accessible the cellulose and hemicellulose (containing the sugars) that are trapped by lignin, a structural constituent present in all plants. This has led to a slower development of second generation biofuels compared to first generation ones due to the difficulty of making large scale production feasible.
However, in recent years their availability on the market continues to grow thanks to both the progress of technology and a better understanding of the raw materials to be processed. The technology developed for second generation biofuels is different from that used for first generation biofuels. In fact, instead of increasing fermentation processes, thermochemical processes have been developed such as gasification and pyrolysis that do not require complex preliminary treatment of the biomass such as separating cellulose from lignin.
Third-generation biofuels are produced from algae. The production of “oilgae,” or algae-derived fuel, involves the fermentation of carbohydrates found in algae.
Second- and third-generation biofuels are also called advanced biofuels. An example of an advanced fuel still under development is Hydrogenation-Derived Renewable Diesel (HDRD). HDRD is produced from animal fats or vegetable oils that are refined in petroleum refineries. This fuel can be blended with petroleum.
Biofuels and CO2 carbon neutral
Biofuels are good alternatives to fossil fuels because, during combustion, they produce carbon neutral carbon dioxide. The carbon footprint of CO2 carbon neutral, not affecting the final concentration of carbon in the biosphere, is zero.
Biomass, during combustion, releases carbon dioxide, which is readily reabsorbed by plants – i.e., the source of the biomass. Through this cycle, plants remove carbon from the atmosphere, releasing it again when they are burned. Because of this balance, biomass is a carbon-neutral feedstock.
There are advantages to using biofuels for transportation, both in terms of their functionality and environmental impact, but there are advantages and disadvantages to their current production.
Among the advantages is the fact that, compared to fossil fuels, the production of biofuels can come from different sources and this guarantees a certain security of energy supply. This reduces dependence on imported fossil fuels. In addition, biofuels emit much less carbon dioxide than their fossil counterparts, since the carbon produced by their use is what the biomass absorbs for its growth. However, with regard to this last aspect, many studies have been conducted that have shown that the energy balance is actually negative.
The energy that has to be supplied for the production of the biofuel is greater than the energy that is released by the biofuel itself. In fact, it is necessary to consider the energy consumed and the pollution resulting from the processes of mechanical processing, fertilization, pesticide treatment, irrigation, harvesting, transport and processing.
Among the disadvantages, the most important one is that currently most of the liquid biofuels produced are of “first generation”, i.e. they derive from food crops grown on arable land. With this generation of biofuels, food crops are then explicitly grown for fuel production and not for anything else. This puts energy production in direct competition with food production. For example, in tropical countries more and more land is being used for biodiesel production. For this purpose, large areas are deforested for the cultivation of palm trees or for the cultivation of jatropha. The result is not only a high environmental impact, but also a decrease in the local food supply and an increase in the price of basic necessities.