Calorific value of firewood
The amount of heat released during the combustion of firewood depends on the type of wood and the moisture content of the wood. Humidity reduces the calorific value of firewood, as the evaporated water carries away part of the heat energy. Humidity losses slightly depend on the initial temperature of firewood (more precisely, the water in them) and are taken equal to 0.63 kWh per kilogram of water.
Absolutely dry hardwood firewood produces about 5 kWh of heat per kilogram of firewood.
Absolutely dry coniferous firewood gives about 5.2 kWh of heat per kilogram of firewood, due to the chemical difference of their wood.
In real conditions, it is impossible to achieve perfect dryness, therefore the table below shows the calorific value of firewood of different types of wood at a moisture content of 15% and for comparison of other types of fuel:
| Type of fuel | Calorific value of material, kWh/kg (MJ/kg) | Material density, kg/dm³ | Loading density, kg/m³ | Calorific value of firewood, kWh/m³ |
|---|---|---|---|---|
| Beech, ash | 4,2 (15) | 0,74 | 480 | 2016 |
| Oak | 4,2 (15) | 0,69 | 470 | 1974 |
| Birch | 4,2 (15) | 0,65 | 450 | 1890 |
| Larch | 4,3 (15,5) | 0,58 | 420 | 1806 |
| Pine | 4,3 (15,5) | 0,52 | 360 | 1548 |
| Spruce | 4,3 (15,5) | 0,44 | 330 | 1419 |
| fuel oil | 12 (43) | 0,84 | 840 | |
| Coal | 7,8-9,8 (28-35) | 0,6-1,9 |
Although the mass calorific value of coniferous wood is greater than that of hardwood, but due to the lower density of wood, the specific volumetric calorific value of coniferous firewood is lower than that of hardwood. Coniferous firewood takes up more space and burns faster.
A cubic meter of dry hardwood firewood can replace 200 liters of oil, 200 cubic meters of natural gas. The energy extracted from firewood is also of solar origin. But at the same time, unlike oil, coal and gas, wood is a renewable resource. Firewood as a type of biofuel is a source of renewable energy.
fuel energy. Specific heating value of fuel
Theory > Physics grade 8 > Thermal phenomena
In nature, there are many combustible substances that release heat when burned. These are firewood, coal, oil, alcohol, gas, etc.
However, only those combustible substances that have a high specific heat of combustion, a low ignition temperature, and the absence of harmful combustion products can be considered fuel.
The more heat released during the combustion of fuel, the better. This energy can be used to heat a home, drive various mechanisms or generate electricity.
To heat a house, coal is required several times less than dry firewood, since different types of fuel of the same mass emit different amounts of heat during complete combustion. It is possible to compare the amount of heat released during the combustion of different types of fuel using a physical quantity - the specific heat of combustion.
The physical quantity showing how much heat is released during the complete combustion of a fuel weighing 1 kg is called the specific heat of combustion of the fuel.The specific heat of combustion is denoted by the letter q. The unit of specific heat of combustion is 1 J/kg.
The specific heat of combustion is determined experimentally using rather complex instruments.
The results of the experimental data are given in .
The calculation formula for the amount of heat released during the complete combustion of fuel:
Q = q*m
where Q is the amount of released heat (J),
q is the specific heat of combustion (J/kg),
m is the mass of burned fuel (kg).
| EXAMPLES OF SOLVING PROBLEMS | ||
| Task number 1. How much heat is released when 15 kg of charcoal is burned? | ||
| Given: m \u003d 15 kgq \u003d 3.4 * 107 J / kg | Solution: Q \u003d q * mQ \u003d 3.4 * 105 J / kg * 15 kg \u003d 510000000 J Answer: Q \u003d 510000000 J | |
| Q=? | ||
| Task number 2.10 grams of ethyl alcohol burned in a spirit lamp in 10 minutes. Find the power of the spirit lamp | ||
| Given: m = 10 gq = 27,000,000 J/kg | SI0.01 kg | Solution: Let's find the amount of heat released during the combustion of alcohol: Q \u003d q * mQ \u003d 27,000,000 J / kg * 10 g \u003d 27,000,000 J / kg * 0.01 kg \u003d 270,000 J. Let's find the power of the alcohol stove: N \u003d Q / t = 270,000 J / 10 min = 270,000 J / 600 s = 450 W. Answer: N = 450 W |
| N-? | ||
| Task #3The Camel gold nugget has a mass of 9.3 kg and a temperature of 15 °C. What is the melting point of gold if 1892 kJ of heat would be required for remelting? | ||
|
Given: m = 9.3 kgt1 \u003d 15 ° Сλ \u003d 0.67 * 105 J / kg |
SI67000 J1892000 J |
Solution: Find the amount of heat received by gold for heating to the melting point: Q1 = c*m*(tmelt — t1); Q1 +Q2 = Qmelt |
| tmelt ? | ||
| Substance | q, J/kg | Substance | q, J/kg |
| Powder | 0,38*107 | Charcoal | 3,4*107 |
| Firewood dry | 1,0*107 | Natural gas | 4,4*107 |
| Peat | 1,4*107 | Oil | 4,4*107 |
| Coal | 2,7*107 | Petrol | 4,6*107 |
| Alcohol | 2,7*107 | Kerosene | 4,6*107 |
| Anthracite | 3,0*107 | Hydrogen | 12*107 |
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Chopping and harvesting firewood
Woodpile
firewood basket
Firewood transportation
Most of the firewood is log-shaped. The technology for making logs is simple: tree trunks and thick branches are sawn into segments 40-60 cm long () and then split with an ax in the longitudinal direction into smaller parts with an end area of up to 100 cm². For splitting, it is more convenient to use a special splitting ax, the blade of which has the shape of a wedge with an angle of about 30 °. A cleaver can split even the thickest chocks with less impact force. When chopping with an ax, especially thick chocks, there is a high probability of the blade getting stuck in the formed crack in the chock. Then, in order to pierce or release the ax, they turn the ax upside down with the stuck chock and hit the other chock with the butt.
The correct stance while chopping firewood is legs wider than shoulders. This will prevent injury in the event of a miss, an unsuccessful blow, or a breakage of the ax. Also, for convenience, it is best to place the chock at the height of a person’s belt, most often they use it as a stand - a very thick and thick piece of a round tree.
Birch firewood is considered the “hottest” and aspen firewood is considered the “coldest”.
A firewood basket is used to carry firewood.
