Wood is one of the most precious materials offered by nature to man.
For the evolution of human civilization, the most important use of wood is linked to the discovery of fire: its use as a source of energy allowed primitive men to cook and heat themselves.
Wood and charcoal were the only fuels of practical use until the eighteenth century when the use of hard coal became widespread: this material allowed nineteenth-century England to develop the applications of the steam engine on a large scale and to start the “Industrial Revolution”. Even today, the most widely used energy sources are fossil fuels, which, however, are associated with numerous environmental effects.
Wood as an energy source has experienced ups and downs even in relatively recent years: during the last world war, for example, wood represented one of the main fuels for the united states. Subsequently, however, thanks to the increasing availability of fossil fuels at decreasing prices, the users of wood have decreased. The progressive decline of wood as an energy resource ends in correspondence with the oil crisis of the 1970s when Western countries – in order to reduce their dependence on oil from Arab countries – began to look with interest at alternative and renewable energy sources and to increase the tax burden on fossil fuels.
Wood is an almost neutral energy source with respect to greenhouse gas emissions as the amount of carbon dioxide (CO2) emitted during its combustion is equal to that absorbed through the photosynthesis process – a fundamental component of the carbon cycle. – during the vegetative life of the plant from which that wood derives, and is made ready for consumption through a production process that has a limited impact on the overall carbon dioxide balance.
In Italy, over 4.5 million households make use of this resource for domestic heating, albeit with considerable differences in the geographical distribution due to the different climates and different territorial typologies. Woody material is used to burn above all in mountain locations (and less so in hilly areas), in small inhabited centers (with less than 5,000 inhabitants), mainly in residential houses and isolated or terraced houses.
With respect to the type of material, the wood can be used in pieces or in the form of chips or pellets.
Types of Wood
For heating purposes, wood has different characteristics depending on the variety of plants from which it is obtained. Woods of excellent quality for combustion are, for example, oak, ash, beech, maple, fruit trees (except cherry); chestnut, birch, and alder are of fair quality; lime, poplar, and willow are of acceptable quality.
Wood is divided into “soft” or “hard” based on the weight in kg of a cubic meter of material.
- Softwood ignites easily, is consumed quickly and develops a long flame: it is used in ovens that require a long flame; examples are poplar, alder, chestnut and willow wood.
- Hardwood is more compact and has slower combustion, with short flames, and is, therefore, more suitable for domestic heating; examples are elm, oak, holm oak, beech, ash or locust wood.
To guarantee the efficiency of the heating, it is necessary to make sure that the characteristics of the wood meet some important requirements, the main one of which is undoubtedly the seasoning, i.e. the degree of drying: correct seasoning allows you to have a fuel with little humidity and excellent yield and low polluting.
The use of damp or even wet wood should be avoided as the yield decreases because part of the energy developed by combustion must be used to evaporate the water contained in it. To ensure good drying, the storage of the wood – already cut into pieces suitable for the firebox that is to receive it – must take place in sheltered and well-ventilated places.
Although wood is considered an “ecological” fuel, in certain circumstances its combustion can contribute to local air pollution.
It is, therefore, necessary to examine the relationship between wood and air quality.
WOOD AND AIR QUALITY
Although air quality has recorded clear improvements in the last twenty years due to the large-scale adoption of technological solutions as well as the better quality of fuels and fuels, atmospheric pollution is still an important environmental criticality.
At the same time, concern about climate change has progressively increased, as a result of which objectives have been defined – at international, and national levels – to reduce emissions of climate-altering gases and regulatory actions that provide for the promotion of biomass combustion for energy purposes. woody.
For some years now, interest has also grown in quantifying the extent of wood products used in domestic combustion, also in order to better evaluate their role as sources of polluting emissions into the atmosphere.
Numerous studies show that the relationship between domestic wood combustion and air quality has two sides: a positive side, thanks to which CO2 emissions into the atmosphere are reduced and climate change is fought, and a negative side due to the fact that domestic combustion – especially if badly conducted in small domestic systems – emits particulate matter and toxic compounds into the atmosphere.
The undoubted benefits in terms of reduction of CO2 emissions deriving from the use of wood must therefore be considered in the context of a more general strategy to reduce emissions of fine dust and toxic compounds: this can be achieved, for example, with the application of flue gas purification technologies on medium-high power biomass boilers used in condominiums and district heating networks. For medium and large wood-fired plants, the goal of reconciling the reduction of greenhouse gases with the improvement of air quality has therefore already been achieved.
WOOD AND CLIMATE CHANGE
Although the climate has always undergone changes due to natural causes, the climatic variations that took place in the twentieth century – and in particular in the last 40 years – are considered anomalous by the scientific community when compared with the variations of the last 1000 years.
The vast majority of scientists now agree that global warming is unequivocally and largely attributable to the influence of human activities, and is due to the presence of increasing concentrations of climate-altering gases that accentuate the natural “greenhouse effect” that occurs in the atmosphere thanks to the absorption of infrared rays irradiated from the earth’s surface and the consequent retention of heat.
If effective policies to reduce climate-altering emissions are not established, the climate changes expected for the future could lead to serious impacts on the natural environment; for this reason, it is therefore necessary to substantially reduce the emissions of greenhouse gases through strategic interventions planned at a global level, and implemented at a local level.
The combustion of woody biomass does not involve additional emissions of CO2 (carbon dioxide) – the main climate-altering gas – into the atmosphere as wood is a biogenic fuel, i.e. generated by photosynthesis starting from carbon already present in the atmosphere.
Firewood is therefore an interesting alternative fuel to fossil fuels as its use reduces greenhouse gas emissions, and is a renewable energy source.
In some cases, however, the contributions of wood combustion to climate change are not zero because they can derive from mechanisms that involve pollutants other than carbon dioxide; these contributions derive mainly from the emission of gaseous compounds and particulates, which have a heating effect.
In poor combustion conditions, firewood emits methane – one of the six gases considered by the Kyoto Protocol – and above all considerable quantities of soot, also called “black carbon” or “black smoke” or even elemental carbon. Black carbon is a very strong climate-altering agent: in the medium term (100 years) its average heating effect is about 500 times that of CO2 while in the short term (20 years) it is estimated to be over 2000 times that of CO2.
Lately, the scientific community has also paid great attention to “brown carbon”, an organic aerosol that originates from volatile organic substances (VOC) and humic substances. The effect of brown carbon on the climate is still uncertain and controversial: on the one hand, it is able to absorb ultraviolet radiation and therefore have a warming effect on the atmosphere, albeit significantly less than black carbon; on the other hand, it does not absorb infrared radiation and therefore leads to surface cooling.
Only if it burns well, wood is an energy source that fights climate change!
The combustion of 1 t of wood avoids the emission of about 80 kg of CO2 if burned in an open fireplace, and about 900 kg of CO2 if burned with an efficient stove. If we consider the black carbon and methane emissions of an open fireplace (or an inefficient stove), the combustion of wood also has a negative effect from the point of view of climate-altering emissions.
In other words, poor wood burning can lose the environmental advantage of not using fossil carbon. On the other hand, for pellet stoves – or wood stoves that burn in optimal conditions – the CO2 balance is largely favorable, to a greater extent the more the supply distance of the woody biomass is reduced.
WOOD AND PM10
The general expression “PM” (Particulate Matter) defines a “set of solid and liquid particles” which is suspended in the air. The terms PM10 and PM2.5 indicate the fractions of dispersed airborne particles having an aerodynamic diameter lower than 10 and 2.5 �m respectively. The small size allows PM10 to penetrate through the airways until it reaches the tracheobronchial tract, and PM2.5 to penetrate deeper to the alveolar region. For this reason, PM2.5 is often referred to as “fine particulate matter”.
PM can have primary or secondary origin
- Primary particulate matter is the part emitted into the atmosphere already in the solid or aerosol phase, originating in part from natural phenomena (such as soil erosion processes, forest fires and pollen dispersion) and largely from anthropogenic activities, in particular from vehicular traffic and combustion processes.
- Secondary particulate matter is formed in the atmosphere from other pollutants, such as NOx and SO2, which react with other substances present in the air – including ammonia – giving rise to the formation of sulfates and ammonium nitrates.
Meteorology is a factor that significantly affects the temporal trend of this pollutant: the accumulation of fine dust and the consequent increase in concentrations typically occur during the autumn and winter months, characterized by no wind and more stable weather conditions.
The most significant part of the emissions deriving from the use of wood is to be attributed to small domestic systems: to open fireplaces, characterized by low energy yields (and whose use is often linked to aesthetic and recreational reasons) and to traditional stoves, very often not very efficient. The more recently manufactured pellet and log stoves also contribute to emissions, albeit to a lesser extent.
As far as fine dust is concerned, the emissions of the best wood-fired domestic systems are in any case much higher than the average levels of natural gas systems.
In the combustion of wood in small domestic plants, the emissions of nitrogen oxides and sulfur (important precursors of secondary fine particles) are similar to those deriving from the combustion of conventional fuels (gas, diesel).
The importance of primary fine particulate emissions deriving from the combustion of wood in small domestic plants highlights the need for policies aimed at controlling this source. Undoubted advantages derive from the limitation of the use of the most obsolete appliances in the areas at greatest risk, or from their replacement with more efficient ones with lower specific emissions.
The challenge to be faced today to reduce fine dust pollution caused by small domestic wood heating systems also consists in ensuring in practice the correct operation and maintenance of new or existing systems, also avoiding the burning of waste or other products not allowed.
WOOD AND TOXICITY
Studies concerning the impact on human health of the use of wood as an energy source distinguish the problems that occur inside homes (indoor) from those that occur outside (outdoor). The former mainly concern developing countries in which biomasses are used intensively in appliances often without ventilation systems, which therefore lack the chimney for removing fumes from domestic environments.
All the studies on indoor pollution – conducted both in developing and industrialized countries – highlighting common aspects: the combustion of wood inevitably produces numerous toxic compounds and fine and ultra-fine dust, the quantity of which depends on the type of fuel, combustion techniques and the techniques used for the abatement of fumes.
As for outdoor pollution, the problem can be traced back to the more general one of the effects on human health of fine dust.
Wood smoke may contain at least five groups of chemicals classified as carcinogenic by the IARC (International Agency for Research on Cancer), and at least 26 chemicals listed as dangerous by the EPA (Environmental Protection Agency – USA). However, most epidemiological studies concern forest fires and fire extinguishing workers: the acute effects of exposure to smoke (at levels much higher than those usually found) affect the respiratory system and lung function.
Since the composition of the powders is extremely different in different combustion conditions, the particulate also has different toxicity characteristics.
A recent Swiss study compared the toxicity and mutagenic power (on lung cells in vitro) of dust from three different sources: a diesel car, a stove in a complete combustion regime, and a stove in an incomplete combustion regime. The results allowed for the classification of diesel dust as medium toxicity while the particulate from the incomplete combustion stove had a level of toxicity fifteen times higher and contained polycyclic aromatic hydrocarbons (PAHs) at concentrations twenty times higher than those of diesel particulate. On the other hand, the dust deriving from the complete wood combustion stove had five times lower toxicity than those deriving from diesel.
Although the epidemiological argument concerning the specific assessment of the particulate matter from the combustion of biomass cannot be considered exhaustive, some rules must absolutely be respected to protect health and the environment. In particular:
- DO NOT USE IN YOUR SMALL DOMESTIC PLANT wood waste impregnated, painted or coated with organo-halogen components (for example with PVC)
- DO NOT USE used wood, such as remains from construction sites or from the renovation or demolition of buildings, or wooden furniture and packaging in YOUR SMALL DOMESTIC PLANT
The combustion of wood waste used wood and problematic wood scraps in unsuitable boilers – such as domestic ones – causes high emissions of harmful substances such as carbon monoxide, hydrocarbons, nitrogen oxides, hydrochloric acid, dioxins, furans, formaldehyde and heavy metals. Waste in the stoves? No thanks!
Combustion is the chemical process through which a material reacts with the oxygen producing heat, light and hot gaseous substances.
For combustion to take place, the concomitant presence of three elements is required: the fuel, the comburent and the primer, which make up the so-called combustion triangle.
While the oxidizer is typically represented by oxygen present in the air, the fuel can be of various kinds: material of fossil origin (hydrocarbons or coal) or vegetable biomass; the trigger must instead be provided from the outside and represents the activation energy that allows the start of the reaction between the fuel and the comburent: it consists, for example, of a heat source or a spark; once combustion has started, the process continues by self-sustaining thanks to the energy released by the process itself.
If one of the elements of the combustion triangle is missing, the process does not develop or is extinguished: to extinguish a fire it is, therefore, possible to act by subtracting the fuel (for example, by exhaustion of the same), by suffocation (by canceling contact with the oxygen through a covering substance) or by cooling (i.e. stopping the self-sustaining reaction of the primer).
If the combustion – for example of an impurity-free hydrocarbon – were to take place perfectly, only carbon dioxide (CO2) and water would have to be produced.
In reality, the products of combustion often contain other substances – such as carbon monoxide, sulfur and nitrogen oxides, polycyclic aromatic hydrocarbons – which depend on the conditions of combustion itself (combustion temperature, mixing of air and fuel) and on the nature of the fuel (presence of impurities, i.e. unwanted substances).
In general terms, the combustion process can take place in two ways: as controlled combustion (for example that which occurs in small domestic wood-burning systems) and as uncontrolled combustion (for example during a fire).
THE CONTROLLED COMBUSTION OF WOOD
Wood combustion is essentially carried out in three phases, in relation to the temperature of the process: drying, degradation and combustion.
During the drying phase, the water contained in the wood begins to evaporate: this occurs from temperatures below 100 � C; evaporation lowers the temperature in the combustion chamber, slowing down the combustion process.
This is the reason why it is absolutely not recommended to use “fresh” wood, which contains a lot of water: it decreases the thermal efficiency, that is the ratio between the heat produced by the system and the energy content of the fuel.
In the thermal degradation phase – starting from a temperature of about 200 � C – the volatile component present in the wood begins to evaporate; in terms of weight, this component represents over 75% of the wood. The first components of the wood to be degraded are the hemicelluloses and then the cellulose; at 400 � C most of the volatile substances have been released and the evaporative process decreases rapidly.
The combustion phase – which begins between 500 and 600 � C and lasts up to about 1000 � C – consists of the complete oxidation of the gases.
Wood combustion is complete when all parts of the fuel have reacted with oxygen; in practice, the complete combustion of solid fuel such as wood is only a theoretical concept to strive for, as it is very difficult to achieve the correct degree of mixing between air and fuel in a limited period of time.
When the necessary conditions for the complete combustion of the wood are lacking, the harmful emissions in the fumes increase. The main causes of incomplete combustion are:
- the inadequate mixing of air and fuel in the combustion chamber;
- an overall lack of available oxygen;
- the combustion temperature is too low;
- too short residence times.
Incomplete combustion manifests itself in the incomplete oxidation of gases and in the increase of both organic and inorganic unburnt compounds: this results in an increase in the content of carbon monoxide (CO) and dust as well as in volatile organic compounds and nitrogen oxides and of sulfur in the exhaust fumes.
The technological innovation of recent decades has allowed the gradual increase in the efficiency of wood-fired boilers and the consequent substantial reduction of CO and other harmful emissions.
However, for a solid fuel it is difficult to reach the optimal conditions for completeness of combustion, as it can be done by using a gaseous fuel for which the contact between combustion air and fuel as well as turbulence is considerably facilitated. For this reason, the specific emissions of CO and VOC from the combustion of wood are, even for the most efficient appliances, much higher than those deriving from the combustion of natural gas.
When a material is burned outside a combustion appliance – and therefore outdoors – there is no possibility of controlling the temperature or mixing conditions: it follows that the emissions of pollutants increase significantly even up to 100 or 1000 times.
Dust containing soot and particulate matter makes the smoke visible, which also contains carbon monoxide and toxic organic substances such as dioxins and polycyclic aromatic hydrocarbons. In addition, in outdoor combustion, smoke is produced at ground level: this means that it is not dispersed at higher altitudes and that the health and environmental effects are substantially circumscribed around the area where uncontrolled combustion occurs.
Examples of uncontrolled combustion can be found in nature and in the home, as well as in agriculture and construction.
- Uncontrolled combustion – In forest fires
A forest fire represents an uncontrolled combustion of vast proportions from which the emission of numerous polluting compounds into the atmosphere derives. In large part they originate from the incomplete combustion processes of cellulose and lignin, as well as of the resins and oils contained in plants.
During a forest fire, substances such as CO2, CO, NOx, CH4, SO2, NH3, unburnt hydrocarbons, soot and fine dust are released into the atmosphere.
Even if the carbon dioxide emitted is of photosynthetic origin and not of fossil origin – and therefore does not involve an additional increase in CO2 for the earth’s atmosphere – the methane and soot emitted during the fires of the vegetation contribute to climate change.
Globally, the emissions of fires, both in the gaseous and particle phases, are transported into the atmosphere for thousands of kilometers: they therefore have an impact on air quality – and on the climate – both on a regional and global scale.
The formation of pollutants, and their release into the atmosphere, is inversely proportional to the intensity of the fire. Combustion with little flame and prevalence of embers emits much more pollutants than that with open flame: these are fast fires in which, after the passage of the flame, slow combustion persists for a long time. In slow fires, in which combustion is more complete, the production of pollutants is much lower.
- Uncontrolled combustion – In agriculture
The residues of agricultural activity – which include pruning residues, brushwood but also used packaging such as fertilizer bags, containers of plant protection products and polythene – are often eliminated in a very harmful way for the environment by stacking them and burning them directly on the fields. This generally constitutes an unauthorized activity of uncontrolled combustion of waste.
The atmospheric pollution that can be produced is very high, although it varies in relation to a number of factors such as the type of material that is burned and its humidity; however, it always burns incompletely because the temperature of the fire outdoors is not high enough and because the material remains in the fire for an insufficient time.
This type of combustion therefore produces dust and other substances that derive from incomplete combustion: carbon monoxide, hydrocarbons and toxic organic substances such as polycyclic aromatic hydrocarbons, dioxins and furans.
The national standard only provides for the possibility of burning small heaps of agricultural or forestry plant residues on site (up to 3 tons / hectare) for the purpose of recovering nutrients for land, with a ban in periods of maximum risk for forest fires and in compliance with any limitations introduced by the competent environmental administrations to protect health and air quality.
Italy has prohibited the open combustion of even small piles of this material of vegetable origin, in the territories located below the altitude of 300 m (200 m in mountain communities), in the winter semester due to the frequent establishment of meteorological conditions favorable to the formation and accumulation of pollutants in the atmosphere.
- Uncontrolled combustion – In household waste
In the United States, the “home” combustion of household waste – a prevalent practice in rural areas – has been the subject of great attention, which has been recognized as one of the major causes of dioxin air pollution
The pollution that results from the uncontrolled combustion of household waste is determined by the fact that they not only contain organic matter but also other artificial substances such as pigments or plastics; these substances, if burned at low temperatures, produce considerable quantities of toxic pollutants such as dioxins and polycyclic aromatic hydrocarbons.
To avoid the formation of these pollutants and strongly reduce emissions into the atmosphere, high temperatures and flue gas purification systems are required, which are guaranteed by waste-to-energy plants.
Dioxins are unwanted by-products of other chemical reactions; their formation in uncontrolled combustion is significantly favored by the presence of “precursors”, chemical substances (man-made) containing chlorine such as polychlorinated biphenyls (PCBs), chlorinated plastics (PVC), pentachlorophenol (PCP) or chlorinated plant protection products .
Polycyclic aromatic hydrocarbons (PAHs) represent a substantial class of over 100 organic compounds. They are released into the environment during incomplete combustion of fossil fuels, timber, fats, tobacco, incense and organic products in general, including municipal waste. These molecules are associated with dangers to human health and negative effects on the environment, such as acute and chronic toxicity for aquatic organisms and birds.
- Uncontrolled combustion – In construction
Within the construction sites, waste of different origins is produced, which is managed in a temporary deposit for which a mixing ban is in force. Waste present on construction sites are, for example, plastic and cardboard packaging used to bring materials to the construction site or pieces of wood that are no longer usable: burning this material directly on the construction site is obviously forbidden, but in many realities it is unfortunately done frequently.
It is important to remember that the unauthorized elimination of waste of any kind by burning is absolutely prohibited and punishable by law.
From the point of view of the production of environmental pollution, also in this case the combustion takes place in the worst conditions. Waste incineration is in fact in itself a process that produces toxic substances: dioxin is the best known but certainly not the only one; other toxic organic substances such as polycyclic aromatic hydrocarbons and heavy metals can also be emitted into the atmosphere.
Some of the pollutants produced have the characteristic of being persistent and bioaccumulative: this means on the one hand that they are difficult to destroy and therefore remain in the air, water and soil for long periods, and on the other hand they tend to accumulate at the same time. interior of living organisms.
Therefore, construction waste must not be burned on site but disposed of properly: if it is burned in modern incineration plants, polluting emissions are drastically reduced thanks to the combustion at very high temperatures and the adoption of extremely sophisticated control systems, including the dust collectors.
SMALL DOMESTIC WOOD-BURNING SYSTEMS
Small wood-fired heat generators are distinguished by the technology with which they are built, the size of the fuel they use, the combustion air draft and the heat distribution system.
The achievement of the optimal efficiency of a heat generator appliance, on the other hand, is achieved thanks to the simultaneous presence of five elements:
- the advanced technology of the device, which is the responsibility of the producers;
- the correct sizing of the generator with respect to the user, the responsibility of the designer or the end user;
- the correct construction and adjustment of the system, which is the responsibility of the installer;
- the good quality of the fuel used, which is the responsibility of the supplier;
- proper management of the device, which is the responsibility of the end user.
The innovative small wood-fired heat generators are appliances designed to produce heat easily and safely. They guarantee high management autonomy, both in terms of power supply and as regards the removal of ash. In terms of thermal efficiency, these values ​​are clearly higher than those characteristic of past types, thanks to the more complete combustion of wood.
SMALL DOMESTIC WOOD-BURNING SYSTEMS: TYPES
The household appliances for burning wood that are used today are, in fact, more advanced versions of the traditional appliances, used for many centuries. The different types can be divided into:
- Open fireplace
Open fireplace: it is certainly the simplest type of appliance and the one that has been least affected by technological evolution. It consists of a combustion chamber with a large opening towards the room in which it is located, which is directly connected to the chimney. Usually the heat produced by the fire is heated directly by radiation, without passing through water or hot air distribution pipes.
These are appliances with low energy efficiency (around 15%) and which usually produce higher polluting emissions than other appliances.
- Closed fireplace
Closed fireplace: these are appliances installed as separate structures, or placed inside a pre-existing open fireplace (the so-called ‘inserts’). Compared to the open fireplace, their characteristic is that the opening towards the room is closed by doors, in order to increase the temperature in the combustion chamber and energy efficiency. All appliances have openings that allow air to enter; these openings in more modern appliances may also have automatic adjustment valves.
Closed fireplaces currently in use have an energy efficiency that is often 55%. However, technological evolution is able to greatly improve performance, and today the best devices can reach efficiencies of 84% when fully operational, also greatly reducing polluting emissions.
Wood stoves: these are appliances with a closed hearth which, in some cases, do not send the smoke directly to the flue but pass it through the so-called ‘smoke passes’, i.e. pipes contained in the stove that serve to transfer the heat of the fumes to the ‘environment. Even stoves, like fireplaces, have openings that allow air to enter the combustion chamber; if the wood to be burned is too much compared to the air that enters, the stove burns badly and produces large quantities of pollutants.
It is precisely in optimizing the air intakes and the geometry of the combustion chamber that the greatest improvements in stove performance have been obtained in recent years: a traditional stove can have an efficiency of 45%, while the most modern appliances achieve 84%.
Pellet stoves: they are stoves that instead of burning wood use pellets, a fuel obtained from dried sawdust and then compressed in the form of small cylinders. In this way the fuel is less moist and more homogeneous, and therefore has a better chance of being burned well. Furthermore, the pellets are brought into the combustion chamber automatically and therefore efficiently by a loading device that adjusts itself according to the need for heat.
Pellet stoves are appliances that achieve much better performance than traditional stoves: the average efficiency of the appliances can be estimated at around 70%, while the best appliances can now reach 94%, producing much quantities of dust and other pollutants. low.
- Water heater
Boilers: these are devices of higher power, which are used not to heat the environment directly, but to heat the water that will then be used by the heating system. They can work both with wood logs, pellets or wood chips: in the latter two cases the feeding can be automatic and higher levels of efficiency can be achieved.
While wood-fired boilers are not subject to any type of mandatory certification, stoves and fireplaces are subject respectively to the mandatory certifications EN 13229 and EN 13240, and the flues to that of EN 1443. If these devices are not certified, there is no guarantee on the quality of the device and compliance with safety standards.
The appliances must also be equipped with a system booklet in which the first installation, ordinary and extraordinary maintenance operations as well as the combustion efficiency must be reported.
Energy efficiency expresses the ability of an appliance to use all the heat that the fuel (i.e. in this case the wood) can produce. It is expressed by the percentage ratio between the energy supplied by the appliance (for example the heat supplied to the room) and the energy burned in the combustion chamber (contained in the fuel).
A more efficient appliance, therefore, allows you to use less wood for the same heat produced: if it doubles the efficiency, for example, it means that half the fuel is used. The energy efficiency of a heat generator depends on several elements:
- The technology of the appliance
- The construction and adjustment of the system
- The quality of the fuel used
- Plant management
The energy efficiency of a good biomass appliance exceeds 80%, and can even reach 94% in the most modern pellet systems. An inefficient appliance, for example an open fireplace, has an average efficiency of 15% and even lower in some conditions. A big waste!
SMALL DOMESTIC WOOD-BURNING SYSTEMS: SPECIFIC EMISSIONS
As known, the products of “ideal” combustion are essentially water and carbon dioxide (CO2); in reality, various factors (for example the lack of the right amount of air necessary for complete combustion, or the presence of impurities) cause the fumes to contain many unburned elements including fine dust, non-methane volatile organic compounds (NMVOC), polycyclic aromatic hydrocarbons (PAHs) and carbon monoxide (CO).
Furthermore, for a solid fuel such as wood, the presence of unwanted products in the emissions is facilitated by the greater difficulty of a complete combustion of the solid material as well as by the presence of impurities in the fuel (for example sulfur or metals).
The presence of unburnt products among the combustion products – in addition to being a problem for air pollution – also indicates a lower efficiency of the appliance, i.e. the generation of a smaller amount of heat for the same fuel consumed.
The emissions of pollutants from small domestic wood-burning systems depend on numerous factors, and mainly:
- the type of appliances that ensure combustion
- from the completeness of combustion
- the type of wood and its conditions
Due to the dependence of emissions on these factors – and on others, such as the type of measurement systems – the available data on dust emissions from wood-burning appliances are highly variable, with values ​​that vary by more than 10 times for the same type of device.
The summary picture of the average levels of the emission factors of different types of appliances and fuels – used for the preparation of the INEMAR regional emissions inventory for the year 2014 – indicates that even the most advanced appliances that use wood biomass (e.g. for example, pellet appliances) have emissions values ​​that are significantly higher than those using natural gas for all pollutants except for CO2, whose emissions for wood can be considered nil.
In general, it can be considered that the most modern appliances (innovative wood stoves, pellet stoves) produce less fine dust, carbon monoxide and volatile organic compounds for the same amount of wood consumed; moreover, their “convenience” is not only environmental but also economic as they consume less wood to meet energy needs.
However, not everything depends on the device itself: the device must also work well.
Even in an innovative type of appliance, a malfunction – due for example to an excessive load of wood or a lack of air – can significantly increase the emission of dust. Furthermore, the dust that is created when wood burns badly contains substances such as polycyclic aromatic hydrocarbons and dioxins, which make them more toxic.
Since technological progress has significantly improved the performance of small biomass heating systems – now allowing the buyer to find on the market excellent stoves that reach lower PM10 emission levels than older appliances – to protect the quality of the air it becomes It is essential to replace obsolete systems with devices that represent the best technologies available, and to equip oneself to ensure the optimal use of such devices.
SMALL DOMESTIC WOOD-BURNING SYSTEMS: THE SAFETY OF THE SYSTEMS
Like most appliances, home heat generators can also be dangerous if – for whatever reason – they malfunction. The main consequences of the malfunction fall first on the performance of the system (and, consequently, on the costs for heating) and then on people, things and the environment.
Incorrect installation and insufficient maintenance of wood-burning appliances leads first of all to problems relating to the systems themselves, which often give rise to the fire of roofs and flues.
The statistics of the Fire Brigade estimate – at national level – the occurrence of about 10,000 roof fires each year resulting from the fire of flues. The first cause of these fires is the “not perfect” construction of the chimney, that is, the failure to comply with some precautions during construction. For example, if the wooden parts of the roof are not well insulated by the steel flue, the heat can be transmitted and trigger the fire of the roof. The second cause is poor maintenance: if you do not clean the soot that settles inside the flue, it can catch fire and ignite the fire.
Cleaning also serves to reduce polluting emissions into the atmosphere and to keep combustion efficient.
Recent regulations have subjected wood-fired heating appliances to the safety regulations of all heating systems. Consequently, the framework of obligations and controls is broader than in the past. Among others, the following prescriptions are in force today, still little known:
- it is mandatory to present a project drawn up by the technical manager of the installing company (or by a professional registered in the professional registers, in the case of collective branched flues)
- the installation, transformation, expansion and extraordinary maintenance works of the systems are to be entrusted to qualified companies, which must build the systems according to the rule of the art
- at the end of the works, the installing company must issue the client with the declaration of conformity of the installed systems
- the competent authority issues a certificate of fitness for use, after obtaining the declaration of conformity
- for the refurbishment or installation of new systems in buildings already equipped with a certificate of viability, the installing company deposits – at the end of the works – the declaration of conformity with the Municipality
SMALL DOMESTIC WOOD-FIRED PLANTS: BIOMASS COGENERATION AND DISTRICT HEATING PLANTS
Vegetable biomass can also be burned in real thermal plants, for example in cogeneration plants that produce both heat and electricity. In addition to wood, plants of this type can also burn other types of biomass such as residues from agriculture: for example, rice husks or hazelnut shells.
From an environmental point of view, it is certainly preferable to burn wood in a large system – such as a district heating plant that heats an entire neighborhood – rather than many small systems that heat a single house or a single apartment.
In fact, large plants are equipped with automatic regulation systems that allow them to burn much more efficiently and, above all, they are equipped with fume purification systems, which allow to eliminate most of the pollutants produced by combustion from the smoke emitted.
Proper management of small wood-fired heaters makes a valuable contribution to protecting the environment. Conversely, the use of obsolete systems and unauthorized waste materials, as well as the lack of maintenance of the appliances themselves, determine on the one hand an increase in the consumption of wood material – and therefore greater expense – and on the other a considerable worsening of emissions. in the atmosphere.
If you are using firewood for your home heating system, remember that much can be done to reduce pollutant emissions. Simple suggestions allow you to choose the type of appliance and wood, to carry out proper installation and maintenance of the system, to check the adequacy of combustion.
PRACTICAL ADVICE ON IMPLANTS
A correctly sized and positioned wood combustion appliance equipped with a system that allows adequate draft, reduces the consumption of wood and polluting emissions by producing a more usable amount of heat and reducing the need for maintenance.
It is often convenient to replace an old and inefficient appliance with a new concept one: however, it is necessary to turn to companies in the sector able to offer certified quality products.
- The appliances must be installed in a workmanlike manner by specialized technicians to ensure their safety and correct performance.
- For new systems it is mandatory that the qualified installer issues a certificate of conformity for the appliance (including the flue).
- The ventilation system and the flue are a critical part of the appliance: these represent the real driving force of the stove allowing the correct development of combustion. The intervention of a professional allows to ensure an adequate draft to the flue through a correct sizing (avoiding in particular oversized pipes), an adequate height (often higher than the minimum required by the specifications), a correct positioning (inside the home, where possible), a correct configuration (avoiding excessive horizontal sections and changes of direction) and protection from excessive cold.
- The intervention of experienced professionals allows you to size and arrange the appliances, obtaining a better and more efficient distribution of heat in the home.
- Any errors in the installation (often due to the intervention of a person other than the professional) may not be immediately visible and the consequent problems may not be found even for a considerable period of time.
- The safety of the home and of people depends on the complete understanding and implementation of the regulations of the manufacturers or manufacturers of the appliances. These include: an adequate distance between the appliance and the fire material, proper protection of people from the combustion area, the correct assembly and operation of the ventilation devices and the flue.
- It is important to keep the flue clean: it must be freed from soot and from invisible obstacles such as bird or hornet nests, dead birds. It is good practice to rely on a qualified chimney sweep to clean the flue.
- If the house is very thermally insulated, it is useful to improve the draft by providing an air intake with direct opening to the outside and located at the bottom of the room where the stove or fireplace is present; the socket must always be kept open during operation.
- The maintenance and control of the mechanical and electronic components of the appliance must be performed regularly by a qualified technician, while the regular cleaning of the flue and the generator should be carried out by the chimney sweep. This reduces polluting emissions, saves fuel, prevents the dirty flue from burning and allows damage and structural problems to be recognized in time.
PRACTICAL ADVICE ON WOOD
An indispensable premise concerns the fact that in the domestic system it is absolutely not necessary to use treated wood, waste wood from the demolition and renovation of buildings, that consisting of packaging (pallets) or wooden furniture, ant or plywood, because the combustion of these materials can release toxic substances.
Similarly, plasticized paper, artificial substances of any kind, packaging or containers (tetrapak) should not be burned because these materials also produce harmful gases and dust and, at the same time, damage the appliance.
- Burning dry seasoned wood. Dry wood ignites and burns easily, while the difficulty of ignition increases as the moisture content increases. If the wood is damp, part of the heat generated does not heat the house but is lost to evaporate the water.
- It is a good idea to buy wood during the summer period, between June and July.
- Never burn wet or green (unseasoned) pieces of wood. Properly seasoned firewood is darker, has cracks in the log, and sounds hollow when knocked against another piece of wood.
- Use clean wood: sand and mud make it less convenient.
- Use wood that comes from your local area to avoid fuel consumption, and therefore the pollution that comes from transport.
- Stack the purchased wood in an external but protected place, in an orderly and raised way with respect to the ground, with the upper part of the pile covered: in this way the seasoning process can continue.
- Burn wood of adequate size, avoiding pieces more than 40 cm long and more than 15 cm wide. Smaller pieces allow better storage of the wood before use, and generally burn better.
- If pellets are used, they must be of good quality and made from untreated wood. Pellets for which the manufacturer is able to declare compliance with technical regulations or quality standards is preferable. Check that there is not a lot of powdered wood inside the bags of packaged pellets. Indicatively, the lower the ash content in the pellets, the lower the environmental impact of the emissions produced by their combustion.
PRACTICAL ADVICE ON COMBUSTION
Even if it is not possible to burn the wood without unwanted emissions, it is important to take all known measures to try to obtain a combustion that is as complete as possible, and therefore “environmentally sustainable”. The main among these measures are:
- in the ignition phase, leave the air control completely open until the combustion chamber is full of flames and well heated
- to light the flame, use the right amount of finely split and very dry wood, or special lighting products or a minimum amount of newspaper (do not use coated paper, magazines or magazines)
- once the combustion has started, the wood should burn with a high flame until it is reduced to charcoal. If the fire languishes, split the wood into smaller pieces and use more than one piece for each load. Never load an excessive amount of wood: the maximum quantity is indicated in the appliance instruction booklet
- always keep the flame lively and hot. Blue, yellow-red or light red flames indicate good combustion; red or dark red flames mean bad combustion
- in good combustion, the smoke must be almost invisible: if you notice dense smoke at the exit of the chimney, yellow to dark gray in color, the combustion is not correct and checks must be carried out
- no odors must be generated from the combustion of wood: if they are smelled, it means that harmful substances are being formed in significant quantities
- the ash that comes from a good combustion is light gray or white: if you find dark and heavy ash, or the head of the chimney is dirty with black, it means that it is burning badly
- an efficient system involves low fuel consumption and little soot in the chimneys: if you see a lot of soot it means that there is a high consumption of fuel and therefore a greater expense
- it is necessary to keep the appliance doors closed when not loading or refueling the fuel
- to ensure the correct supply of air, the ash must be removed from the appliance using a metal container with a lid. The ash container must be left outside the house on a brick or concrete slab (never on a wooden roof or near the wood)
- it is recommended to install an anti-smoke alarm to alert you in the event of a fire; many victims in residential fires are caused by the inhalation of toxic fumes and gases
- It is also recommended to install a carbon monoxide (CO) identifier, an odorless, colorless and very toxic gas that derives from inadequate combustion
The issue has two aspects: on the one hand, the stimulation of the market for small wood combustion plants can contribute to reducing greenhouse gas emissions, on the other hand increasing the market for these same small wood combustion appliances risks generate significant non-climate-altering emissions, in particular of fine dust and polycyclic aromatic hydrocarbons.
Some areas of the territory are subject to particularly unfavorable weather conditions which determine the accumulation of atmospheric pollutants. In these areas, the control of emissions from small wood-burning plants is particularly important and can give results on improving air quality.
Since the territorial boundary of the different states is obviously not a limit for the spread of pollution, since 2007 the various territorial authorities have been dealing with the problem in a coordinated way. One of the objectives is precisely to define forms of regulation for the use of fuels, including wood for heating: for small wood-burning household appliances, we intend to require the dissemination and adoption of improving technologies.