The use of biomass as a source of primary energy is carried out by a combustion process, since, if it cannot be used directly as fuel, it is sought to transform it into substances that can be used in this type of process.
Combustion leads to more or less the appearance of polluting products depending on the nature of the reagents and the technologies used, with a high risk of these products being released into the environment. On the other hand, it must be taken into account that in the various processes of transformation of biomass into other combustible substances, polluting substances are also produced which are released into the environment. Among them, particles, carbon dioxide and monoxide, sulfur compounds, nitrogen oxides and solid and liquid wastes stand out.
The particles obtained during a combustion process are fly ash. Currently, there are systems and equipment with fully developed and known technology that operate with very high efficiencies and therefore very high percentages of the total particles generated are retained. These are filters, cyclones and electrostatic precipitators, and their high level of development is due to their use with other fuels, especially coal, and in the chemical industry.
The risk of biomass is that, on many occasions, it is used in an uncontrolled way as a source of energy, since there are many small-scale combustion equipment for domestic use that do not incorporate particle retention systems .
Unlike fossil fuels, carbon dioxide from the biomass combustion process is returned to the atmosphere, where it was taken from when it was generated. According to this, the use of biomass as fuel does not increase the carbon dioxide content of the atmosphere and, therefore, does not attribute the greenhouse effect. The reality is that if the rate of said utilization becomes very high and is greater than the rate of biomass production, the amount of said compound in the air will increase.
In the case of carbon monoxide, the emissions produced when burning biomass are higher than when burning coal, although the technology used has a significant influence. Reduction in the formation of this compound is achieved when combustion equipment is functioning properly and is guaranteed to be complete.
The sulfur compounds that form during combustion are sulfur oxides, which can become sulfuric acid, being one of the substances that contribute to acid rain. Sulfur oxides can also form in fermentation processes and in the pyrolysis of biomass. Hydrogen sulphide is produced during the anaerobic digestion of animal waste, as well as during the pyrolysis of agricultural waste. But, in general, these compounds are not a problem in the case of biomass since it generally does not have sulphur, or if it does, it is in small quantities.
Nitrogen oxides are formed by the oxidation of biomass and air content, necessary for certain use and transformation processes. It appears that since biomass normally burns at lower temperatures than fossil fuels, the formation of nitrogen oxides is less than that which occurs with the latter. The way to go when it comes to reducing their emissions is to reduce their formation, which is achieved by rigorously controlling the combustion process, both in terms of the thermal level in which it develops and the percentage of air.
The incineration of urban waste can cause the emission of highly polluting substances. The presence of plastics and compounds containing chlorine in the waste gives rise to the formation of some very toxic ones, where dioxins, furans and hydrochloric acid stand out.
Wood combustion causes emissions of a wide variety of organic compounds, including substances classified as carcinogens, such as benzopyrene and polycyclic aromatic hydrocarbons. In the case of the latter, the problem that exists is that once outside and in contact with the ground, they are transformed into substances containing nitrogen and oxygen, and which have a more carcinogenic power. important than the first. In addition, locating the original hydrocarbons is difficult once abroad. In contact with the ground, they are transformed into substances containing nitrogen and oxygen, and which have a higher carcinogenic power than the initial substances. In addition, the location of the original hydrocarbons is difficult once they are emitted into the atmosphere, which can be found in water, on land or in plants. The formation of estos hidrocarburos is due to the incomplete combustion of the organic materials, keeping that signal that our sólo están asociadas a la biomasa, sino que también is produced in large cantidades in the incomplete combustion of the carbón and of the derivatives of the petróleo, as gasoline.
During the combustion of biomass, the ashes are obtained in the form of solid products. The retention, extraction and treatment of these substances can contaminate water and soil due to the presence of toxic substances among the compounds that make up the ashes, such as lead or cadmium, requiring exhaustive control in the different stages. which constitute the processing thereof.
Solid residues are also obtained in anaerobic management, but in this case the residues are not polluting, but rather are used as organic fertilizers. The tributaries are solids in suspension and with metallic compounds, which must be evacuated outside the facilities that work with biomass, they are the ones that can cause water contamination, requiring its control and treatment. There is a large amount of liquid tributaries with a high concentration of nutrients which are used as fertilizers or as raw materials in the chemical industry, but it must be taken into account that the quantities which cannot be used in such applications can damage the setting.
In gasification and pyrolysis processes, special attention should be given to the various compounds that are produced during the process, including acetic acid, phenol and water-insoluble oils such as benzene, toluene , benzopyrene, as well as a wide variety of non-aromatic organic compounds. Many of these compounds have a negative impact on human health, since some are still condemned as carcinogens.
To remove the tars obtained in the previous processes, wet scrubbers are used. At present, these are very expensive systems, which is why they are not included in many applications. What is being tried is to reduce the formation of those using catalysts.
In general, for the different types of biomass, in the different processes in which it is used and for the different types of pollutants produced by its use, the main problem in ensuring that it is not polluting is the lack of control of its use, since in most cases it is used in small installations, which makes it difficult to reduce and control emissions, while in large ones the adoption of measures and the implementation of systems cleaning is the usual standard