Combustion: general aspects and analysis of by-products
Combustion is a complex phenomenon: the output of this violent reaction is, in the first instance, the heat and, simultaneously, a series of by-products and wastes that, in many cases, have got characteristics and properties harmful to human and to the environment.
Combustion takes place in dedicated plants: biomass boilers especially designed with devices that can handle the fuel characteristics and specificity.
The boilers feed with wood biomass require specific constructive philosophy that allows to have an efficient use of fuel from a variable calorific value, tendentially medium-low, characterized by a high moisture content (up to a relative humidity of 60% by mass) and by a discrete variability of the characteristics in the medium term and in seasonality (humidity, PCI, size, percentage of aggregates, etc.).
Feeding a boiler with solid fuel require:
We will not dwell here on aspects related to storage, handling and feeding, even if of fundamental importance for the quality of the final plant. We want to highline on combustion aspects and emission treatment.
We can outline the typical steps of solid biomass combustion process as follows:
It is important to emphasize that the main parameter so that byproducts and pollutants are limited, is the quality and stability of the combustion. This parameter, in general, depends on the level of control and stability that the boiler and the plant can maintain on the same reaction, or by the quality of the design of the boiler and by the use of proper fuel to the installation characteristics. The pollutants that are produced may be treated with a special line of flue gas treatment.
The main controls that a solid biomass combustion system must be able to carry out finely, to manage the quality of combustion reaction, are:
On a main plant level, also, it is good to consider:
Generally it is accepted that the wood is a clean and environmentally friendly fuel. In fact we can agree with the fact that it is neutral in terms of CO2 production, as well as sustainable (although some clarifications must be kept in mind, as the need to provide a suitable growth cycle and consumption and a “sourcing fuel” in areas adjacent to the plant).
Despite the high quality, also this form of fuel generates by-products and wastes (pollutants and not) dangerous for the environment and for human. For convenience, we divide them into:
The management of biomass ash is regulated differently ofr each country by internal environmental law and regulation. In Italy, for example, the management of the ashes follow Part IV of Legislative Decree no. 152/2006 (environmental regulations), which classifies them as “non-hazardous special waste ‘in the category inorganic waste from thermal processes.
As such, the landfill is only the last viable choice for their disposal; there are in fact simplified procedures that allow for the recovery of the ashes.
In fact, limited to the ash from combustion of virgin wood, under D.M 186/2006, there is the possibility of by-product recovery. In addition, implementing the EEC Regulation 2092/91, the Italian State allows the use of this waste, or better, by-product, as fertilizer and soil conditioner for organic farming (Legislative Decree no. 220/95).
Regarding the solid particles, it is good to remember that the combustion of biomass is a major source of inhalable particulate with size smaller than 10 microns (PM10). For the purposes of what follows, we will call these particles as aerosol, in virtue of the fact that are inhalable and, in a relatively short time, may enter the tissues (and from tissue into blood) of animals and humans.
These particles can be divided into:
The literature attributes the formation of the primary aerosols to three phenomena:
The secondary aerosols, however, are derived from the recombination of VOC into the atmosphere and carbonaceous compounds. Note that, contrary to what one may expect, the secondary aerosols (SOA Secondary Organic Aerosols – SIA Secondary Inorganic Aerosol) contribute significantly to the formation of inhalable particulate matter in the atmosphere and, due to an effect of reflection and self-heating, they contribute to the amplification of light pollution and air heating.
It is important to emphasize that the toxicity levels of above classes of pollutants are quite different from each other.
To this, must be added that since the pollutant categories that “emerge” in non-optimal combustion regimes, even the behavior of the most used abatement systems tends to deteriorate and not be uniform during such working conditions.
In addition to the above, each combustion reaction, typically, produces certain categories of molecules in the gaseous state:
In particular, the combustion of wood biomass, in the aspect of the gaseous pollutants, is relatively harmless. They are however to be noted emissions of acid gases, which usually have limited concentrations, but may vary substantially depending on the actual type of biomass into use. Among them we can find gas acids:
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Combustion: general aspects and analysis of by-products
Combustion: general aspects and analysis of by-products
Combustion: general aspects and analysis of by-products
Combustion: general aspects and analysis of by-products
Combustion: general aspects and analysis of by-products
Combustion: general aspects and analysis of by-products
Combustion: general aspects and analysis of by-products
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