Bio energy is produced from organic matter of animal or plant origin. Against the background of the gradual depletion of oil and gas reserves, bioenergy is gaining new positions.
Modern bioenergy technologies
Bioenergy technologies allow getting energy by physical, chemical or microbiological method. The main technologies can be represented as follows:
direct combustion (production of heat and electricity);
Synthesis (production of alcohols and oils);
gasification or pyrolysis (to produce syngas, hydrogen, heat and electricity). The feedstock is heated to high temperatures without oxygen, converted to solids and synthesis gas, which is used instead of natural gas to produce electricity;
Anaerobic fermentation is the destruction of organic raw materials by bacteria without oxygen. Biogas and digestate, an organic residue used as fertilizer, are produced. After removal of carbon dioxide and hydrogen sulfide the gas is thickened, after which it is converted into biomethane.
Bioenergy in matter
Plant biomass is formed through the process of photosynthesis. Its main components are carbon, hydrogen and oxygen. Other elements – nitrogen, potassium and phosphorus – are present in small amounts.
Photosynthesis is a chemical reaction represented by the following formula: CO2 + H2O + sunlight → CH2O + O2, where CO2 is carbon dioxide, H2O is water, CH2O is the generalized carbohydrate formula and O2 is oxygen.
Whereas the use of biomass for energy looks like this: CH2O + O2 → CO2 + H2O + heat.
Thus, solar energy is first stored by plants, and then released in the form of heat. If new plants are planted in place of the used biomass, the cycle will be closed and all the carbon dioxide will be absorbed again.
What does biomass refer to
Biomass has been used for energy since ancient times. Its modern application is based on obtaining fuel from special plantations, the use of any waste products of forestry and agricultural industry.
According to the source of origin there are substances of plant and animal origin, according to the stage of use – primary and secondary. Organic substances suitable for energy production mainly include:
wood and products of its processing (bark, chips, sawdust);
Agricultural waste (straw of cereals and corn, hay);
organic waste (waste paper, sewage sludge);
yields of energy plantations (poplar, cane, miscanthus, etc.);
biofuels (vegetable oils, bioethanol and biodiesel);
Biogas (obtained from manure and sewage sludge).
Despite the fact that coal and oil are also of biological origin, they cannot be called biomass due to the fact that their formation does not occur in the present.
The efficiency of biomass as an energy source is about half that of coal. For example, burning 1 kg of wood or straw releases 10 to 14 MJ, while hard coal releases 25 MJ.
Biomass – operating conditions
Resources are the main parameter that determines the efficiency of energy production.
Energy plantations are designed specifically for growing energy crops. Poor or depleted soils due to human activities are suitable for this purpose. One square meter of such a plantation can produce enough biomass to get up to 45 kWh of energy from it.
Since bioenergy also uses waste from agriculture and forestry, the cost of resources for energy production is not high, which makes the industry competitive.
Biochemical processing of organic waste
Organic waste is produced in large quantities. They are suitable for energy production and are therefore a valuable resource.
Wet organic waste produces biogas – a mixture of carbon dioxide (30%), methane (65%) and hydrogen sulfide (1%). Biogas burns with a blue flame and produces no smoke or smell, becoming an alternative to natural gas. It is produced in landfills as a result of anaerobic decomposition: organics decompose to digestible compounds. The gas is collected in the body of the landfill and is transported through a network of pipelines to sales.
Types of biofuels
Biofuels come in three forms – solid, liquid and gaseous.
Solid biofuel includes firewood, fuel pellets, pellets and briquettes. The latter are made from wood waste – sawdust, bark, wood chips, chippings and wood of poor quality, as well as agricultural waste – biomass, nutshells, etc.
Among liquid biofuels the leading bioethanol. It is an alcohol obtained from food raw materials (sugar cane, corn or grain) by fermentation. It is mostly produced in Brazil and the USA. In these countries it is blended with gasoline.
Biodiesel, or methyl esters of fatty acids, are made from vegetable oils from oil-bearing crops such as soybeans and rapeseed.
Biobutanol, also known as butyl alcohol, is made from biomass sugar, starch, wood or straw. In the latter case, it is called cellulosic butanol.
Liquid fuels have a good chance of replacing gasoline and diesel derived from oil in the long term.
The gaseous biofuels are biogas, biomethane and hydrogen. Biogas is a mixture of carbon dioxide, methane and various impurities. The source is domestic organic waste, animal by-products and waste water, which are decomposed without oxygen. Biomethane is a purified and compressed biogas.
Biohydrogen is produced from biomass by two methods – biochemical, i.e. with the help of special microorganisms, and thermochemical, in which the raw material is heated in the absence of oxygen.
It is likely that in the future these fuels will replace natural gas.
Biofuels for vehicles
The most common biofuel for vehicles is bioethanol. More than 6 million trucks and cars in the U.S. are fueled by a mixture of ethanol and gasoline, 85% and 15% respectively (E85). This fuel is less “energy-dense” than conventional gasoline. This means that the mileage of cars that run on E85 is about 75% of that of standard cars.
As a fuel, bioethanol is most used in Brazil, which even has a ban on the sale of pure gasoline to fill up cars. The fuel must be a mixture of gasoline and ethanol, and the latter must be present in an amount of 20% (E20). Since ethanol costs less than gasoline, it is often added in a higher percentage and can be as high as 40%.
All internal combustion engines run on E10, but only so-called “Flex-Fuel” engines can run on pure ethanol. They are also called “flex-fuel” due to the fact that they can be fueled with regular gasoline or a mixture of gasoline and ethanol in any proportion. It is believed that even a small addition of bioethanol significantly increases the octane number of fuel and reduces toxic emissions into the atmosphere.
Biodiesel is also popular. This is the second most common biofuel for vehicles. A mixture of diesel from oil and biodiesel is actively used.
Biogas is considered the most environmentally friendly type of fuel, because there are almost no harmful compounds in the exhaust gases. However, its storage and use requires heavy cylinders. Because of this, this type of fuel is more suitable for trucks.
Negative effects of biomass fuel production
Organic waste can be a great fertilizer. However, when biomass is used as fuel, the organic cycle is disrupted and soils have to be fertilized with mineral fertilizers, which have a negative impact on the environment. Special cultivation of energy crops also uses large amounts of pesticides, and forests are cut down to create new plantations.
Moreover, the burning of any fuel (and biomass in particular) releases carbon dioxide, bringing global warming closer. In theory, energy production should be “carbon-neutral,” because specially grown crops have already offset future carbon emissions into the atmosphere. But this is not the case, because growing these crops and then turning them into fuel is incredibly energy-consuming and comes with additional carbon dioxide emissions.
A – “carbon debt” (in Mg CO2 per hectare, 1 megagram = 106 g = 1 ton) – the amount of CO2 which should enter the atmosphere over 50 years. D – the time required to pay the carbon debt. Written at the bottom is the type of fuel and the ecosystem that was in place before the plantation became an energy crop (article by Fargione J. et al. in Science).
The future of biomass energy
The bioenergy sector has seen trends of digitalization and robotics. New technologies are being introduced in production. For example, scientists are already thinking about how to generate energy from algae.
Biomass is a renewable energy source, so in a world where fossil resources are limited, bioenergy has excellent prospects for development.