The combustion of wood

After choosing a stove, the case will look to turn the fuel used: wood, mostly, although some of the stoves in the preceding section may use alternative fuels. Heating with a wood stove is also a lifestyle choice, especially if you are used to adjust the temperature with a thermostat. Find, buy, carry, cut, wood store, as well as turn on and govern a fire and for ash disposal, are tasks that you will have to learn to love, they will become part of everyday life. Learn about wood as a fuel is essential to get good results from a stove, and help you assess whether your lifestyle will adapt.

 

 

 

 

HOW TO BURN WOOD

 

  The wood is, in essence, a warehouse of solar energy. The leaves of the trees act as small solar panels that absorb the energy radiated by the sun, using it to convert water, carbon dioxide and minerals in organic matter, the wood. Light a wood fire means to release this energy.

  Two things are necessary because the wood burns: oxygen and high temperature. Because a stove burning wood effectively has to keep the internal temperature and provide themselves with sufficient oxygen to consume as much firewood and gas possible.

 

  When wood burns, it goes through three phases of combustion:

1.L'umidità evaporates from the wood by the action of surrounding fire. Any wood contains a percentage of humidity. Because of the heat produced by the fire is used in its evaporation, it is much cheaper and less polluting, use seasoned wood (max 20% humidity) rather than green wood freshly cut (50% or more of moisture). This phase is complete when the timber reaches the temperature of 100 ° C (Boiling point of water)

2.Aumentando the temperature, the wood decomposes into volatile gases and coal. The wood catches fire at a temperature comprised between 260 ° C and 315 ° C, burning charcoal and a small percentage of gases. Most of the gas, however, will escape up the chimney, unless the temperature in the stove is sufficiently high to burn them. (Step 3) Once in the chimney, the gases combine with moisture to form creosote.

3.The gas and coal are burned. The coal begins to burn emitting heat between 540 ° C and 705 ° C, it is reduced to ash. In this phase it produces most of the heat exploitable. The volatile gases are switched on between 600 ° C and 650 ° C, provided they have sufficient oxygen. The gases seldom reach this temperature, unless they are somehow confined and redirected to the flame, or in an area of ​​the fire room where this temperature has been reached. The efficiency of the stove does catalytic depends on their ability to achieve this.

 

Calorific value of wood

  The calorific value of the wood interior is generally expressed in Kcal / Kg. and it represents the amount of heat given off by the complete combustion of one kilogram of wood. The calorific value depends on the quality of the wood and the moisture. On average, you can get the following values ​​per kg. Of dry wood: Leafy 4300 Kcal / Kg, Resinous 4600 Kcal / Kg. Faced with the same weight, softwoods have therefore a calorific value higher than other woods. This diversity is matched by increased density of leafy woods.

 

Excess air

  Like all fuels, the wood needs a certain amount of air (about 5 cubic meters of air per kg) to completely burn. Since, however, wood is a solid fuel, and mixes with difficulty with the air, it is necessary to provide an amount of excess air (up to a total of 8 cubic meters per kg.) To be certain that the combustion occurs regularly. Such excess air, of course, is not actually burned, and is discharged from the chimney, dragging with it, however, a part of the heat produced. This is one of the reasons why the yield of wood as fuel is lower than that of gaseous fuels, such as methane, or vaporizable, such as diesel oil, that it mixes intimately with the air, have less need to have in excess. Only in very sophisticated stoves, as already described stoves balls, you can obtain the gasification of wood, and great performance.

  In the open fireplaces the quantity of inlet air is in this excess (a fireplace with a mouth from 1 mx 1 m can suck up to 300 cubic meters of air per hour) that most of the heat produced by the combustion is wasted. In stoves, the ability to adjust, with the air register, the amount of oxygen supplied, allows to reduce this waste to a minimum

 

Combustion residues.

  The wood, whatever its origin, is made from an organic fuel (whose average chemical composition is Carbone 50%, oxygen 42.5%, hydrogen 6.5%, 1% nitrogen), on the one hand inert mineral, and water. The organic substances, that is, resins, tannins, and the polymers (cellulose, hemicellulose and lignin), by the action of high temperature oxidation during combustion, undergo deep chemical modifications, releasing energy and producing waste, which, depending on the quality of the combustion, can vary within certain limits. The major residues are:

 

The ashes constitute 2-3% of the total dry weight of the wood and are the residue of its mineral part (silicon, calcium, magnesium, potassium, phosphoric acid, etc.) constitute an excellent fertilizer, and can be excellently used also as a cleaning agent for glass with high thermal resistance of stoves and fireplace inserts. In fact, they were once used as a constituent of handmade soaps.

 

Oxides of Nitrogen. High temperatures favor the union between the molecules of oxygen and nitrogen present in the air. Their production does not depend, therefore, on the type of fuel used, and can not be completely eliminated, but only reduced by the control of the combustion air.

 

Sulfur oxides. The sulfur is present in minimal amounts in the wood, more 'in fossil fuels, such as coal and diesel oil. Sulphur dioxide, mixing with water vapor condensation especially in poorly insulated chimneys, produces small quantities of sulfuric acid, which is among those responsible for the early deterioration of the chimneys. Together with the nitrogen oxides, sulfur oxides are the main causes of acid rain.

 

Oxides of carbon. Constitute the main mass of the gases resulting of the combustion. The carbon monoxide is produced on the surface of the wood as the first result of the combustion, and then burns to carbon dioxide (carbon dioxide) for subsequent oxygenation. If, therefore, the combustion takes place in the absence of oxygen, it will not stop completely, but proceeds by producing carbon monoxide instead dioxide, which is the natural result of a proper combustion. This process, in addition to being uneconomical, since it greatly reduces the thermal power obtainable, is also very dangerous because the carbon monoxide is a powerful poison, by virtue of its affinity with the hemoglobin of the blood. It 'an insidious gas, odorless and colorless, noticeable only by the symptoms it produces: headaches, dizziness, drowsiness, then coma and death. Intoxication care iperossigenando in a hyperbaric chamber, but an advanced intoxication can still leave consequences. Since it is of slow metabolism, intoxication can occur by accumulation of successive exposures to the gas in the infested premises also occurred at intervals of time. The incidents of carbon monoxide poisoning caused by heating equipment for wood are very rare and are usually due to the poor state of the chimney, rather than the stove. Instead equipment fuel gas, are more frequently subject to accidents, but also in this case, the responsibility is however almost always be debited by failure to maintain or to the poor design of the flue. However, a well-designed system, made with quality products and skilled workforce, is absolutely safe.

The carbon dioxide (Carbon dioxide) is formed by the oxygenation of carbon monoxide, and is a harmless gas, whose production is implicated by the same process of combustion, which is said to complete just when each molecule of carbon in the fuel binds with a molecule of oxygen present in the combustion air to produce carbon dioxide. This entire process free most of the energy produced by combustion. It 'a gas essential to the life of plants, which absorb the carbon, releasing oxygen in the air. It can not therefore be considered in itself a pollutant gas.

 

Unburned hydrocarbon molecules that make up the organic part of the wood is extremely stable: we need a high temperature to break them through oxidization, as we have seen describing the third phase of the burning wood. In open fires or stoves in poor quality, this third phase of combustion does not take place, resulting in the formation of creosotes. The creosotes are aromatic hydrocarbons and polyaromatic highly oxygenated and constitute a good part of the condensate which forms in the flues to poor combustion. Mixed with soot deposits in the form of chimneys, especially traditional fireplaces, which can be due, if not periodically removed, of dangerous fires. The creosotes are formed especially when the flue gas temperature in the chimney is low. This allows the moisture to adhere to the walls. Burners burning in the fireplace that send very hot fumes will not produce many encrustations, because the temperature inside the barrel is too high to allow the formation of deposits, even if the creosotes are present in the gases. Creosote thickens more easily in the coldest parts of the barrel; in most installations this is done in the end portion. The steel chimneys have fewer problems with creosotes of those masonry, both for the inner walls more smooth, and because of their greater expansion and contraction due to changes in temperature tends to fracture deposits creosotes, causing their detachment and relapse into the fire.

 

Soot is another result of incomplete combustion. It 'basically consists of pure carbon (98%). Easily absorbs the condensation of creosotes, attaching it to the walls of the chimney.

 

Condensation occurs easily burning green wood in plants equipped with flue not well insulated. The first phase of the condensate is constituted mainly by water vapor result of the rapid drying of the wood in the fire room. Subsequently, with the progress of combustion, condensation forms by unburned hydrocarbons, which we have already spoken, and 'a dark liquid, oily, smelly, flammable and easy infiltration. You can avoid only good burning wood in plants built properly, with a good chimney.